plasma assisted surface treatment v1

verlag moderne industrie

Plasma-AssistedSurface Treatment

Nitriding, nitrocarburizing and oxidation of steel, cast iron and sintered materials

Thomas auf dem Brinke, Jürgen Crummenauer, Rainer Hans, Werner Oppel

++Metaplas_Umbruch_eng_5.0:++Metaplas_Umbruch_eng_5.0 03.07.2006 8:47 Uhr Seite 1

This book was produced with the technical collaboration of METAPLAS IONON Oberflächenveredelungstechnik GmbH.

Translation: Kevin Lossner, Hohen Neuendorf (Germany) on behalf ofSansalone Technische Übersetzungen, Cologne

© 2006 All rights reserved withsv corporate media GmbH, D-80992 Munich, Germanywww.sv-corporate-media.de

First published in Germany in the seriesDie Bibliothek der TechnikOriginal title: Plasmagestützte Oberflächenveredelung© 2006 by sv corporate media GmbH

Illustrations: No. 22 TRW Automotive GmbH, Tech Center Düssel-dorf; No. 23 KOKI TECHNIK Metallverarbeitung GmbH & Co KG,Niederwürschnitz; Nr. 24 HOERBIGER Antriebstechnik GmbH,Schongau; No. 29 Montanhydraulik GmbH, Holzwickede; all othersMETAPLAS IONON Oberflächenveredelungstechnik GmbH, Bergisch GladbachTypesetting: abavo GmbH, D-86807 BuchloePrinting and binding: Sellier Druck GmbH, D-85354 FreisingPrinted in Germany 889039ISBN-10: 3-937889-39-6ISBN-13: 978-3-937889-39-9

ContentsProtection from wear, corrosion and fatigue 4

Nitriding basics 6

The nitriding process ............................................................................... 6Layer structure of the nitrided zone ............................................................ 8Material property modification by nitriding............................................ 12Nitridable ferrous materials..................................................................... 16Dimensional characteristics..................................................................... 17

Nitriding in the plasma 18

Generating technical plasmas.................................................................. 18Heat treatment in the plasma................................................................... 20Partial nitriding........................................................................................ 24

Combined treatment 26

Nitriding and oxidation ........................................................................... 26Degreasing and plasma nitro carburizing of sintered materials ............... 31Nitriding and coating............................................................................... 33

Overview of applications 36

Automotive engineering.......................................................................... 36Hydraulics and fluid technology ............................................................. 47Chemical industry ................................................................................... 50Food industry........................................................................................... 51Engineering ............................................................................................. 52

Process technology 54

Pretreatment ............................................................................................ 54Treatment................................................................................................. 54

Conceptual design of plasma nitriding systems 59

Bell furnace systems................................................................................ 62Pit furnace systems.................................................................................. 63Chamber furnace systems........................................................................ 64Work safety and environmental protection ............................................. 66

Trends and future outlook 68

The company behind this book 71


Protection from wear, corrosion and fatigue 5

process variants (Fig. 1) and combined treat-ments, as well as the associated system tech-nology. Specific application examples fromvarious industries show which componentscan be treated by means of this method andhow surface treatment technology can be in-tegrated in existing production lines. Finally ,future developments in the field of plasma-based thermochemical surface treatment arediscussed.

4

Protection from wear, corrosion andfatigue

The steady increase worldwide of require-ments on the quality and performance of fer-rous materials as well as ever stricter environ-mental regulations make new developmentsnecessary to constantly improve the wear andcorrosion protection of components and tools.Furthermore, because the treatment of steelsurfaces can cut the need for valuable alloying elements in the base materials, thedevelopment of environmentally friendly andindustrially applicable methods for the modification and coating of ferrous materialsis one of the most important challenges insurface technology.Component or tool surfaces are subject to amultitude of conditions that may af fect them.The surface zone must protect the object fromcorrosion and wear , while the base materialneed only provide the necessary strength.This division of function between the surfacezone and the core enables high-performancematerials to be made from low-alloy steelsusing appropriate modification processes.Thermochemical methods offer cost-effectivesolutions for improving the useful propertiesof steels such as resistance to wear , frictionbehavior, fatigue resistance, corrosion resist -ance and fatigue characteristics.This book presents the current and environ-mentally friendly thermochemical treatmentprocesses of nitriding, nitrocarburizing andoxidation, in particular the plasma-based

Fig. 1: Gear wheel duringplasma nitriding

Surface zonetreatment

Thermochemicalmethod

Plasma-basedprocesses


The nitriding process 7

most layer on a nitrided zone. Such layeredcompound systems are characterized by ex-cellent corrosion resistance. The oxidation ofnitrided zones is also used in cases where thefriction and sliding properties or running- in behavior of ferrous materials need to beimproved.Enrichment with nitrogen or with nitrogenand carbon can be achieved by means of:

• a molten salt (salt bath nitriding)• a gas mixture (gas nitriding)• a low-energy plasma (plasma nitriding).

Salt bath nitridingSalt bath nitriding has been used for decadesin various branches of industry . The nitro-gen penetrating the surface zone is derivedfrom a liquid medium consisting of moltensalts. The temperature of this salt bath isusually between 400 and 600°C. Cyanate,which is used to this day , undergoes cat-alytic decomposition on the steel surfaces atthese temperatures to form cyanide, carbon-ate and adsorbed nitrogen. Due to the for-mation of the carbonates, this method is onlyable to produce nitride layers containingcarbon. Thus it is always a nitrocarburizingprocess. In order to avoid carryover of thehighly toxic bath constituents, the tools orcomponents must be washed thoroughly af-ter treatment. Used salt bath materials mustbe disposed of in an environmentally ac-ceptable manner.Salt bath nitriding will not be discussed fur-ther in this book. It is an old technology witha negative impact on the environment, whichis increasingly being replaced by gas orplasma nitriding in industrial use.

6

Nitriding basics

The nitriding processThe generic term nitriding refers to a ther-mochemical treatment with which the sur-face zone of ferrous materials is enrichedwith nitrogen (see DIN 17014). When nitro-gen diffuses into the surface zone, it is at first dissolved interstitially in the iron matrix.If the nitrogen concentration exceeds the solubility limit of 2.5 weight percent, a single- or multi-phase nitride layer is formed.This treatment is preferably carried out in thetemperature range between 400 and 600°C.The familiar nitriding processes provide notonly excellent corrosion protection but alsooutstanding protection against wear whilstalso allowing the dynamic characteristics ofcomponents made of ferrous materials to beimproved.If only nitrogen is incorporated in the surfacezone, the process is referred to as “nitriding”.If at the same time carbon dif fuses into thesurface zone as a result of the addition of acarbon source to the nitriding medium, theprocess is called “nitrocarburizing”. Bothmethods are used primarily for providingwear protection. With the selective addition of oxygen to the nitriding atmosphere, theprocess is referred to as “oxynitriding”. Thisprocess variant is used to produce a porousnitride layer in the nitriding zone, which isimportant for the adhesion of an oxide layerapplied subsequently.“Oxidation” in contrast is the selective oxida-tion of ferrous materials in which magnetite(Fe3O4) is preferably produced as the upper-

Nitriding andnitrocarburizing

Oxidation

Nitriding media

Carbonitridelayers


Layer structure of the nitrided zone 9

thick, hard and chemically resistant. Beneathit is the tougher dif fusion layer with a thick-ness of 0.1 to 0.8 mm (Fig. 2).The compound layer which consists of ironnitrides and/or carbonitrides determines theceramic character of the surface. The com-pound layer assumes one of the three follow-ing forms, according to the depth-dependentconcentration distributions of nitrogen andcarbon:

• a γ’-compound layer (γ’-nitride: Fe4N)• an ε-compound layer containing more ni-

trogen and/or carbon ( ε-nitride: Fe2-3N, ε-carbonitride: Fe2-3NC)

• a mixed-phase compound layer ( γ’-nitrideand ε-nitride).

γ’-compound layers are tougher than ε-com-pound layers, but they grow more slowly; at 2 to 6 µm thickness they are significantlythinner than typical ε-compound layers (10 to20 µm thick). The composition of the com-pound layer may also be modified by the pres-

8 Nitriding basics

Gas nitridingGas nitriding refers to nitriding or nitrocar-burizing in a stream of ammonia gas at at-mospheric pressure. Within this method, thecatalytic decomposition of ammonia providesthe required active nitrogen which is able to diffuse and form the dif fusion layer and the compound layer . To optimize the layerstructure, nitrogen and hydrogen are alsoused as additive gases, as are carbon-provid-ing additives such as carbon dioxide, methaneor carbon monoxide. The composition ofthese gas mixtures can be varied over wideranges to achieve reproducible layer proper-ties [see Chatterjee-Fischer , Ruth: Wärme -behandlung von Eisenwerkstoffen. Nitrierenund Nitrocarburieren (Heat Treatment of IronMaterials, Nitriding and Nitrocarburizing),Renningen: Expert-Verlag, 1986. (ISBN 3-8169-0076-3)].

Plasma nitridingPlasma nitriding or nitrocarburizing is alsoknown as ionitriding or glow nitriding. It in-volves the selective addition of nitrogen andin some cases carbon to ferrous materials in avacuum environment using a low-ener gyplasma. The incorporation of nitrogen andcarbon takes place in this case via an ionizedgas mixture consisting of nitrogen, hydrogenand an additive gas containing carbon, suchas methane or carbon dioxide.

Layer structure of the nitrided zoneThe surface zones of nitrided ferrous materi-als are generally composed of two distinctparts. Directly on the surface is the com-pound layer which is typically 2 to 20 µm

Mic

roha

rdne

ss

2–20 µm 0.1–0.8 mmLayer thickness

Compoundlayer

Diffusion layer

Fig. 2: Layer composition ofa nitride layer withcharacteristic hard-ness gradient

Reproduciblelayer properties

Synonym: ionitriding orglow nitriding

Formation ofthe compoundlayer


10 Nitriding basics Layer structure of the nitrided zone 11

ence of special nitrides and a more or lesspronounced porous zone (Fig. 3).The considerable range of control options forthe plasma-based nitriding process enables thegrowth of the compound layer to be optimizedfor a specific application (Fig. 4). As a rule, amixture of both nitride phases is obtained. De-pending on the process control parameters,nearly single-phase γ’ (plasma nitriding) or ε-compound layers (plasma nitrocarburizing)can be produced. Both types of compoundlayer are characterized by high resistance towear. As the nitrogen content increases, hard-ness, corrosion resistance and ceramic charac-ter increase and ductility decreases.

Plasma nitriding is frequently used to createnitrided zones without a compound layer , i.e.pure diffusion layers. The metallic characterof surfaces having little or no compoundlayer provides good adhesion conditions forsubsequent coating processes, such as thephysical vapor deposition (PVD) process.Their hardness gradients provide protectionfrom rupture of the hard material layer withpoint or linearly distributed loads. With alter-nating loads, residual compressive stresses inthe surface lead to a significant improvementof fatigue performance, which can be furtherenhanced by combining with mechanicalprocesses such as shot peening or surfacerolling.To improve the corrosion resistance, frictionand sliding properties or aesthetics, nitrocar-burized surfaces can be oxidized subsequently.This requires a suf ficiently thick compoundlayer preferably comprising the more denselydiffused ε-carbonitride phase. The magnetitelayer (Fe3O4), only 1 to 2 µm thick, provides

Fig. 5: Influence of the alloying elementschromium (Cr) andmolybdenum (Mo) on surface hardness (1 kp ≈ 9.81 N)

1200

1000

800

600

400

1 2 5 10

Alloying element fraction (%)

Sur

face

har

dnes

s H

V2

(kp/

mm

2 )

20

Fig. 4: Surface layers afterplasma nitriding; a) 10 hours at 550°C b) 16 hours at 530°C c) 20 hours 510°C

ε-compound layer γ '-compound layer Diffusion layer

a b c

Fig. 3: Structure of the compound and diffusion layers

0.1–0.8 mm

Compoundlayer

Diffusionlayer

Surface

NitrogenCarbon

2–20 µm

Layer thickness

Special nitrides

Porous zone

Nitride precipitation zones

Special nitrides

Carbides

Nitrogen in “solution”

Layer structurecontrollable inthe plasma


Material property modification by nitriding 13

sible for enhancing the properties. Since thetreatment temperature is below 600°C, nochange in the microstructure due to austenitiz-ing occurs – in contrast to annealing. Duringcooling – regardless of the rate – martensiteis formed in only a few steels. The absence of a microstructure change results in minimaldistortion.The accumulation of nitrogen in the surfacelayer of components increases hardness andstrength, yet at the same time reduces the de-formability. The reduced malleability of thesurface layer increases the stiffness. Nitridingcan significantly increase the fatigue resistance,especially for flexural loads (Fig. 7). The extent

to which this increase occurs is determinedprimarily by the formation of the dif fusionlayer. Tensile stresses, which arise with flex-ural or torsional stress on components, arecounteracted by the formation of residualcompressive stresses. This increase instrength in the surface zone reduces the riskof crack for mation and thus also of premature

12 Nitriding basics

outstanding protection against corrosion inconjunction with the compound layer.Thick ε-compound layers can be producedquickly and economically through the appli-cation of sensor-controlled gas nitrocarburiz-ing, which in combination with subsequentplasma activation provides ideal conditionsfor a well-bonded oxide layer with superiorcorrosion resistance.Alloying elements in ferrous materials such aschromium, vanadium, molybdenum and alu-minum form special nitrides which af fect thesurface hardness (Fig. 5) and the depth of thediffusion layer. Typical nitriding hardnessdepths (Fig. 6) are between 10 µm and 0.8 mm.

Material property modification bynitridingNitriding and nitrocarburizing improves notonly the resistance to static and dynamicloads, but also the corrosion resistance. Thespecific structure of the compound layer andthe underlying dif fusion layer are respon-

Fig. 6: Influence of the alloying elementschromium (Cr) andmolybdenum (Mo) on the nitridinghardness depth

1 2 5 10

Alloying element fraction (%)20

0.5

0.6

0.4

0.3

0.2

0.1

Nitr

idin

g ha

rdne

ss d

epth

(m

m)

Str

engt

h

Number of load cycles

Fatigue resistance

Plasma nitriding

Fatigueendurancelimit

Fig. 7: Increase in com -ponent durability dueto plasma nitriding

No microstruc-ture change, low distortion

Increasedstrength


Material property modification by nitriding 15

duces the tendency to react with materials inthe environment, including lubricants for ex-ample (tribo-oxidation). Nitrided steels havea somewhat lower resistance to corrosionthan nitrocarburized steels. The latter canendure 24 hours in salt spray fog testing according to DIN 50021SS. Subsequent ox-idation produces a layered compound struc-ture made of ε-carbonitride and magnetite,which increases the corrosion resistancemore than twenty-fold. Durability results ofover 500 hours in salt spray fog are possiblewith this combined process.When processing to enhance the corrosion resistance, a distinction is made betweenplasma nitrocarburizing and oxidation on theone hand, and the combination of gas nitro-carburizing, plasma nitrocarburizing and oxi-dation in a single process sequence on theother hand. The latter combined process ispatent-protected (IONIT OX®) and combinesthe advantages of nitrocarburizing with gasand plasma. This combined plasma processcan achieve corrosion endurance values com-parable to those of electroplating (Cr, Ni-Cr).

Characteristics of nitride layers

Compound layer• hardness: 800 to 1400 HV• low abrasion• reduced adhesion• reduced tribo-oxidation• improved corrosion resistance.

Diffusion layer• hardness gradient to the base material• improved fatigue resistance against flex-

ural and torsional stress.

14 Nitriding basics

failure (Fig. 8). The strength characteristicscan be in fluenced by specific selection of theprocess parameters.The formation of a compound layer duringnitriding significantly enhances the wearproperties of components. This layer resultsin a reduction of the friction coef ficient andthe adhesion tendency with metallic wearpartners. Abrasion resistance and fatiguestrength are greatly improved.Furthermore, diffusion of nitrogen in the sur-face zone causes a hardness gradient that ex-tends into the base material. As a result of theincrease in the strength of the component,lower -strength materials can be used in thedesign of components. It must be noted thatthese heat treatments result in an increase in dimensions and roughness on finishedparts.The addition of carbon leads to the formationof hard, wear-resistant compound layers thatare more chemically resistant than the sur-faces of untreated steels. Nitrocarburizing re-

Har

dnes

s

Distance from the surface

Increasingalloy content

Increasingnitriding timeand temperature

Fig. 8: Increase of surfacelayer hardness resulting from specific selection ofprocess parameters

Hardness gradient

Advantages ofnitrocarburizing

Corrosion resistance

Plasma-basedtreatmentprocesses

Improved componentcharacteristics


Dimensional characteristics 17

Depending on the alloying elements present inthe treated steels, the same treatment parame-ters can achieve a range of surface hardness upto 1300 HV and nitriding hardness depths totenths of a millimeter (T able 1). Alloying ele-ments with a high af finity for nitrogen, such aschromium, molybdenum, aluminum, titaniumor vanadium, yield especially hard surface lay-ers with a high resistance to wear from slidingfriction. With carbon steels and low-alloy steels,the increase in resistance to abrasive wear andadhesion is also achieved together with aboveall improved corrosion resistance.

Dimensional characteristicsComponent dimensions generally increasesomewhat as a result of nitriding (Table 2). Pre-cision-fitting parts must therefore be manufac-tured to a dimension smaller than specified

before processing. Generally speaking, compo-nents made of unalloyed or low-alloy steels experience an increase in length of 40 to 50percent of the compound layer thickness dur -ing nitrocarburizing. For components made ofsteels with higher degrees of alloying, the di-mensional behavior depends on the chromiumcontent and the nitriding hardness depth. Steelsthat form martensite while cooling down fromthe nitriding temperature shrink.

16 Nitriding basics

Nitridable ferrous materials

In principle, all steel, cast iron and sinteredmaterials can be nitrided. However , limita-tions may be encountered, for example withtoo much copper in sintered materials. Thechoice of the nitriding process or combinedtreatment depends on the component require-ments. Typical steels for nitrocarburizing areconstruction steels, carbon steels and low-al-loy steels. If, in addition to wear protection,good corrosion resistance is also required, theprocess combination of nitrocarburizing andoxidation is selected.The properties of nitrided surface layers aredetermined by the material and the treatmentparameters. Due to the formation of special nitrides, the nitriding of alloyed steels canachieve a greater surface hardness than casehardening; however, the decrease in hardnessin the interior of the component is more abruptdue to the lower dif fusion depths. Since thetreatment temperatures used for nitriding are300 to 400°C lower than the carburizing tem-peratures used in case hardening, the increasein hardness occurs with less distortion.

Designation AISI DIN Surface Nitriding CompoundSAE hardness hardness layer thick-

(HV2) depth (mm) ness (µm)

Construction steel 1020 1.0037 150–350 0.3–0.8 4–10Carbon steel 1045 1.0503 350–500 0.3–0.8 4–15Case-hardened 5115 1.7131 550–700 0.3–0.7 6–10steelTempering steel 4140 1.7225 550–650 0.2–0.6 4–8Nitrided steel A355 1.8550 900–1100 0.2–0.5 2–10Hot-working steel H13 1.2344 900–1200 0.1–0.3 2–10Cold-working steel D2 1.2379 900–1250 0.1–0.2 –High-speed steel M2 1.3343 1000–1250 0.005–0.1 –Rust- and acid- 316 1.4571 950–1300 0.05–0.1 –resistant steel

Chromium Steel type Material Increase in diametercontent with reference to nitrid-(weight %) ing hardness depth (%)

0.4 Carbon steel C45 21 Case-hardened steel 16MnCr5 33 Nitrided steel 31CrMoV51 45 Hot-working steel X40CrMoV51 6

13 Rust- and acid- X40Cr13 10resistant steel

Table 1: Characteristic data for plasma- nitrided steels

Table 2: Increase in diameterdue to plasma nitriding

Special nitrides

Surface hard-ness and nitrid-ing hardnessdepth

Dependent onthe material


Generating technical plasmas 19

are referred to as DC, pulsed DC, high-fre-quency or microwave plasmas.

Self-sustaining gas dischargeThe characteristic curve for a self-sustaininggas discharge shows a number of distinguish-ing segments. In the region of normal glowdischarge, the cathode is only partially cov-ered by the discharge. The increase in current– with the voltage nearly unchanged and thecurrent density constant – is coupled with agrowth of the glowing surface. For surfacetreatment, the subsequent range of the char-acteristic curve for abnormal glow dischar geis used (Fig. 9): the plasma covers the entiresurface of the cathode and the current densityincreases linearly with voltage. The segmentfor abnormal glow dischar ge, however, does

18

Nitriding in theplasma

Generating technical plasmas

Once a particular ener gy threshold (tempera-ture) is exceeded, a gas in a state of thermo-dynamic equilibrium will enter the ionizedstate. This plasma state is defined as thefourth physical state of aggregation, the firstthree being solid, liquid and gas.Technical plasmas are – in contrast to solarplasmas, for example – not completely ion-ized as a rule. They are a hot, electrically con -ductive mixture of freely moving negative andpositive charge carriers (electrons and ions),electrically neutral particles (atoms and mol -ecules) and photons (light particles), whichcontinuously interact with one another.Low-energy plasmas for surface treatment areactivated by a self-sustaining gas dischar ge.The naturally occurring positive ions andelectrons in the gas mixture used are acceler-ated in an electric field along the lines of fluxbetween the anode and cathode. Ener geticcollisions of the ions with neutral gas mole-cules produce a cascade of ions by impactionization. The temperature of the char gedparticles can be many times that of the gas mixture, hence these plasmas are non-isothermal.Due to losses, energy must be added continu-ously to maintain the plasma state. In techni-cal plasmas, this is accomplished using elec-tric fields with a constant or changing fieldstrength. Depending on the frequency andwave form of the field, the plasmas produced Current

Vol

tage

Biascurrent

Normaldischarge

Abnormaldischarge

Transitionto arcing

Arcdischarge

Countertubes

Glowdischargestabilizers

Plasma heat treatment Fluorescentlamps

Welding

Fig. 9: Self-sustaining gasdischarge at lowpressures

Plasma state of matter

Low-energyplasma

Normal and …

… abnormal gasdischarge


Heat treatment in the plasma 21

• chemical reaction of the element in themetal, formation of precipitation zones.

In plasma-based heat treatment, the transportof the ionized particles in the reactionmedium takes place via an electric field, innon-plasma heat treatment processes, it oc-curs via convection and diffusion.Since the internal ener gy of a technicalplasma is significantly higher than that of anequally hot gas mixture in thermodynamicequilibrium without plasma activation, chem-ical reactions requiring higher activation en-ergies can occur in the plasma. This energeticeffect of the plasma is used not only inplasma heat treatment for thermochemicalmodification of the surface zone of ferrousmaterials, but also in plasma chemical vapourdeposition (CVD) for depositing hard layerson steel materials.Gas ions arising on the cathode can initiatethe following processes:

• warming of the cathode• knock-out of atoms, molecules or clusters

of atoms and molecules by transfer of themomentum of the colliding particle to theatoms of the metal matrix

• activation of dif fusion processes by an in-crease of the void density in the region nearthe surface

• activation of reactions leading to layer for-mation, and an increase in temperature dueto the heat released by the reactions.

Interaction of the plasma with the solid sur-faces are of particular importance in plasmadiffusion treatments. Heating of the solidbody by the ion bombardment is a secondaryeffect which should be minimized as much as

20 Nitriding in the plasma

not extend indefinitely. Once the voltage ex-ceeds a critical value, local arc dischar gesoccur. The voltage drops precipitously , thehigh current density produced by glow emis-sion of electrons from the cathode leads tolocal overheating and melting of the cathode.

Abnormal glow dischargeThe abnormal glow dischar ge for plasma- based thermochemical surface treatment occursat low pressures (10 to 1000 Pa) and at rela-tively low potential dif ferences (electric volt-ages of 300 to 800 V) between the anode andcathode in the transition zone of the negativeglow light near the cathode (the componentsto be treated). The visible light emissions are produced by excited atoms, ions and mol-ecules.

Heat treatment in the plasma

Heat treatment in a technical plasma for pur-poses of incorporating a chemical element inthe metal matrix can always be divided intothe following partial stages:

• chemical reaction with the formation of avolatile compound containing the elementwhich is to be diffused into the metal

• diffusion of the volatile compound in thereaction medium, transport to the surface ofthe substrate, removal of the reaction prod-ucts from the phase boundary reaction

• adsorption of the compound at the surfaceof the material, interfacial reaction with theformation of the element capable of dif fu-sion in the metal matrix, desorption of reac-tion products

• diffusion of the element into the metal matrix

Plasma nitridingin the transitionzone

Energetic effectof the plasma

Cathodeprocesses


Heat treatment in the plasma 23

The positively char ged nitrogen ions acceler-ated toward the component provide nitrogenwhich is able to dif fuse due to its high kineticenergy. It is incorporated into the surface and,depending on the duration of the treatment andthe temperature, it dif fuses into the surfacezone of the component. The plasma nitridingprocess is regulated by precise specification ofthe voltage, gas composition and temperature.

Typical process parameters• primary gases used in nitriding: nitro-

gen, hydrogen• primary gases used in nitrocarburizing:

nitrogen, hydrogen, carbon dioxide ormethane

• additive gas: argon• temperature: 350 to 600°C• gas pressure: 50 to 500 Pa• gas consumption: from 20 l/h (lab scale)

to 500 l/h (industrial plant)• treatment time: 0.5 to 60 hours• plasma power: 500 A at 0 to 800 V.

The environmentally friendly plasma nitrid-ing process offers some key advantages overtraditional nitriding in the salt bath or withgas. In particular the layer structure, the depthof the hardness gradient and the homogeneityof the surface layers produced can be selec-tively controlled in a manner largely indepen-dent of each other through control of the dis-charge parameters (power, voltage, pulse fre-quencies and mark-to-space ratio), the processgas conditions (gas composition, pressure andflow rate) and the component batch parame-ters (temperature, time, heating and coolingrate).


possible. The component temperatures re-quired for the dif fusion processes should bekept as homogeneous as possible within abatch and be maintained with additional en-ergy sources (resistance heating).Solid body sputtering prior to the actual startof treatment facilitates cleaning and depassi-vation in addition to activation of the compo-nent surface. Not only are atoms removedfrom the cathode surface, but ions are also im-planted in the solid body.In order to increase the concentration of ele-ments able to dif fuse on the surface of themetal (the interphase region), the treatment iscarried out in special gas discharge chambers.With plasma nitriding, treatment of the com-ponents takes place in a vacuum chamber ,which is evacuated during treatment. Low quantities of nitrogen and hydrogen (20to 500 litres per hour) are required as processgases for plasma nitriding. The hydrogen isused for depassivation or reduction of oxidelayers on the surface. With plasma nitrocar-burizing, a carbon source such as methane orcarbon dioxide is also added (3 to 15 litresper hour). With its high atomic mass the no-ble gas ar gon is used as an additive gas forsputtering the surface.An electrical voltage of appropriate polarity isapplied between the components, which areusually placed on fixtures, and the chamberwall. This creates an abnormal glow dischar gewith a high current density in the process gases.The state of the art is represented by pulsed di-rect current voltages with pulse frequencies upto 25 kHz. Pulsing not only decouples the ther-mal from the chemical process control, but italso suppresses the development of light arcsby the controlled toggling of the plasma power.

Solid body sputtering

Process gases

Diffusible nitrogen

Advantages ofplasma nitriding


Partial nitriding 25

suming and expensive, because to be ef fec-tive, the compounds must be dried well be-fore the treatment, and afterward they mustbe removed.With plasma nitriding, areas that do not re-quire treatment can be masked mechanically .It is necessary to ensure that the coverings aresufficiently stable, because the residualstresses induced by nitriding might deformthin metal covers. If such deformation doesoccur during treatment, the surface under-neath the covering could be nitrided uninten-tionally. Drilled holes or tapped blind holesare best covered by screws.The plasma treatment requires an electriccontact from the component to the cathodi-cally connected loading fixture. The contactareas cannot be nitrided. However , this limi-tation can be turned into an advantage in thatthe batching aids or receptacles for compo-nents are designed to serve as coverings at thesame time.


Compared to conventional hardening proces -ses, plasma nitriding treatments are distin-guished by the following manufacturing char-acteristics:

• high dimensional accuracy• little increase in surface roughness• low, predictable increase in volume• repair welding is possible• simple partial treatment.

Partial nitridingLayer types and thicknesses which can beproduced selectively during nitriding and ni-trocarburizing are chosen to meet the opera-tional requirements of the part. Often thechanges in properties associated with heattreatment are only desired on certain areas ofthe components. In particular cases, the sur-face treatment may need to be applied to a lo-calized area (Table 3). In order to protect thecomponents from the nitriding medium inthese areas and prevent nitriding, maskingcompounds are applied. Copper -based paintcompounds are suitable for nitriding and ni-trocarburizing processes. Copper itself doesnot form compounds with nitrogen or carbonand thus provides a good dif fusion barrier.However, this method is relatively time-con-

Production Welding after nitriding; machining after treatment, such as thread cutting

Function Nitride layer undesired, for examplethreads or sharp edges are easilynitrated through and embrittled.

Distortion Thin cross-sections can become distorted after large treatment depthsdue to residual compressive stresses.

Table 3: Reasons for partialnitriding

Productioncharacteristics

Paint compound

Mechanical covering


Nitriding and oxidation 27

cellent corrosion protection results, togetherwith very good friction and sliding propertiesfor the tools or components treated. Further-more, this combined treatment represents anenvironmentally friendly alternative to theusual corrosion protection processes, such as

26

Combined treatment Nitriding and oxidationGas nitriding or gas nitrocarburizing andoxidation (Fig. 10), plasma nitriding orplasma nitrocarburizing and oxidation (Fig.11), and the combination of conventionaland plasma nitriding or nitrocarburizingwith subsequent oxidation (Fig. 12) are

among the most important thermochemicalcorrosion protection processes in industrialuse. They also meet the increasing demandsfor treatments which have a minimal environ-mental impact. Combining conventional nitriding processessuch as gas nitriding or gas nitrocarburizingwith the plasma processes and a controlledoxidation process enables the layer and com-ponent characteristics which can be achievedwith the different methods to be bundled. Ex-

Time

575°C

Heating Gas nitrocarburizing

Pre

ssur

e/te

mpe

ratu

re

Cooling Oxidation Cooling

Eva

cuat

ion

Temperature

Pressure

1 hr3 hrs

500°C

1030 mbar

Fig. 10: Process sequence for the combinedtreatment with gasnitrocarburizing andoxidation

Time12 hrs 1 hr

Eva

cuatio

n

Pre

ssur

e/te

mpe

ratu

re

Plasma nitrocarburizingHeating Oxidation Cooling

500°CTemperature

1030 mbar

Pressure

575°C

Fig. 11 (top): Process sequence for the combinedtreatment withplasma nitrocarbu-rizing and oxidation

Time

Pre

ssur

e/te

mpe

ratu

re

GasnitrocarburizingHeating

Plasmaactivation

Oxi-dation Cooling

Eva

cuat

ion

1030 mbar Pressure

TemperatureEva

cuat

ion

Fig. 12 (opposite): Process sequence forthe plasma combin -ation treatment withgas nitriding, plasmaactivation and oxi -dation (IONIT OX®)


Nitriding and oxidation 29

ward the component surface. The partial reac-tions from solid body sputtering, which leadsto removal of surface dust and unevenness inthe nitride from the previous gas nitrocarbu-rizing step, are especially important for lateroxidation. Plasma interactions such as parti-cle adsorption and ion implantation are usedto prevent a loss of nitrogen in the compoundlayer formed.In the oxidation treatment subsequent to that,a cohesive, homogeneous layer of iron oxideis formed (Fig. 13). The resultant significantincrease in corrosion protection (Fig. 14) isbased on the combination of the plasma-basednitriding technology (plasma activation) withthe nitriding process at normal pressure (gasnitrocarburizing) and oxidation in a singleprocess cycle.The nitrocarburized and oxidized compoundlayer structure provides resistance to corro-sion and wear with improved dynamic prop-

28 Combined treatment

galvanic and chemical deposition procedures(nickel-plating, chrome-plating, etc.), as wellas an alternative to conventional salt bath nitriding.During gas nitrocarburizing, the ε-compoundlayer with a porous zone (which ensures bet-ter adhesion of the oxide layer) is formedquickly. Layer formation occurs as a result ofchemical reactions between the process gasesammonia and carbon dioxide. It can be con-trolled selectively via the nitriding index, i.e.the nitriding potential of these reactions, and itoccurs faster than would be possible withplasma nitrocarburizing. Gas nitrocarburizingforms a more pronounced porous zone thanplasma nitrocarburizing.The nitride surfaces are activated subse-quently by plasma nitrocarburizing. A gasdischarge – similar to that in plasma nitriding– ignites, and the positively charged nitrogen,hydrogen and carbon ions are accelerated to-

Fig. 13: Layer structure after the plasma combination treat-ment IONIT OX®

a) Cross-section b) Scanning electron

microscope image(SEM)

Hardchrome 20 µm

Chemicallyplatednickel20 µm

Plasmacombination

treatmentIONIT OX®

0

100

200

300

400

500

600

Res

ista

nce

(hr)

Fig. 14: Comparison of corrosion resistanceafter the plasmacombination treat-ment IONIT OX®

Oxidelayer

Compoundlayer

1–3 215–30 200–500

Layer thickness in µm Layer thickness in µm

Diffusionlayer

Oxidelayer

Compoundlayer

a b

Fast formationof the compoundlayer

Cohesive, homogeneousmagnetite layer


Degreasing and plasma nitro carburizing 31

Layer structure with plasma combination treatment

Oxide layer• Fe3O4 (magnetite)• fine-grained, dense• chemically resistant• low-friction coefficient• reduces tribo-oxidation (frictional corro-

sion) • no contact corrosion with aluminum• very good adhesion even with flexural or

shear stress• color: anthracite to black.Compound layer• high proportion of ε-nitride• selective open-pored formation• hardness 800 to 1400 HV• high resistance to wear.Diffusion layer• interstitially dissolved nitrogen (Fig. 16)• hard special nitrides• hardness gradient to the base material• improved fatigue resistance due to resid-

ual compressive stresses.

Plasma combination treatments provide allunalloyed and low-alloy steels with excellentfunctional properties. Specially developedprocess variants provide optimal wear andcorrosion resistance even with sintered steeland cast iron.

Degreasing and plasma nitro -carburizing of sintered materialsSintered materials are nitrided or nitrocarbu-rized primarily to decrease component wear .


erties. The effect of the oxide layer on the ε-compound layer can be compared with asealed chromium dioxide layer (CrO 2) onpassivated rust-resistant steels. An unal-loyed steel treated in such a way shows a wide range of passivity (Fig. 15) with low corrosion current and high breakdownpotential.

Fig. 15 (top): Comparison of corrosion character-istics of C15 after theplasma combinationtreatment IONIT OX®

Cur

rent

(m

A)

Voltage (V)

Test conditions: 12.5 mV/sElectrolyte: 0.05 n H2SO4

C 15

–1 10 2

–1.0

1.0

2.0

3.0

C 15 nitrocarburized

C 15 IONIT OX®

X 8 CrNiTi 18.10

10

5

Depth (µm)

0 10 15 20 25 30 35 40 45

20

30

40

50

60

70

80

90

100

Con

cent

ratio

n (m

ass

%) Iron

Nitrogen

Oxygen Carbon

Fig. 16 (opposite): Element distributionafter the plasmacombination treat-ment IONIT OX®

Optimal protectionagainst wearand corrosion


Nitriding and coating 33

the vacuum pump. For series-production com-ponents, the process is adjusted to ensure removal of the test oil in order to keep thetreatment times as short as possible.Sintered parts nitrided on a lar ge scale in-clude synchronization components in vehicletransmissions such as synchronizer hubs (Fig.17) and rings, sprocket wheels from the valve

train assembly – which are sometimes com-bined with components for the adjustablecamshaft timing – oil pump rotors, steeringarms or even components from hydraulicpumps.

Nitriding and coatingIn coating, new material is applied to the basematerial under input of thermal ener gy to thecomponent surface. In nitriding, one alsorefers to “layers”, but these layers are pro-


The formation of a uniform compound layerthus assumes particular significance. Thegrowth and phase composition of the com-pound layer is determined by process para-meters such as temperature, treatment time,gas composition and surface condition of thecomponents. The condition of sintered materials is not idealfor nitriding. The surfaces are often oxidized;the components contain test oils, waxes andcorrosion protection agents, and they are alsoporous. Since a plasma cannot penetrate intopores, plasma dif fusion processes are bettersuited for thermochemical treatment than gasand salt bath nitriding techniques. They arecharacterized by the best dimensional stabilityand deformation resistance. Plasma nitrocar-burizing generally results in thicker , harderlayers than plasma nitriding. Whereas copperin the base material inhibits layer growth, ni-tride formers such as chromium or aluminumresult in harder nitride layers.Impurities in the nitriding atmosphere due tooutgassing test oils can inhibit nitriding justlike copper. Since the requirement for sinteredparts to be free of oil, grease and wax is notalways met, the components must be cleanedto remove such materials before nitrocarburiz-ing. This can either be done separately in anadditional furnace or as part of the process cy-cle in the plasma nitriding system.Deoiling and/or dewaxing, plasma nitrocar-burizing and oxidation in a single treatmentcycle is economical and also very environ-mentally compatible. During a pause in theheating phase to the nitrocarburizing temper-ature, oils and waxes can be removed undervacuum, leaving no residue. They are col-lected in a cold trap located downstream of

Fig. 17: Synchronizer hubmade of sintered material

Cleaning, nitro-carburizing andoxidation in asingle cycle

High-volumeproduction applications


Nitriding and coating 35

crease in the mechanical efficiency of movingsystems and reduces the need for lubricants.Such a combination of coating and thermo-chemical surface treatment is thus well suitedto modifying contact surfaces in lubricatedsystems, such as transmission gears and hy-draulic components made of alloyed steel,cast iron, special and sintered materials.The nitriding and coating process steps cantake place in two separate systems. However ,a combined treatment of this type can be car-ried out in a single cycle in a system specifi-cally designed for that purpose. The combina-tion provides a highly ef fective method forprotecting components and tools from wearand fatigue.


duced by a phase change of the base material.Since the nitride layer and the bulk materialare based on the same chemical element,bonding of the layer is generally no problem.Nitriding increases the fatigue resistance. Theincrease in hardness in the surface zone is accompanied by a reduction in toughness.These characteristics are prerequisites for anadditional coating and ideal for thin func-tional layers applied by means of physical va-por deposition (PVD) (Fig. 18). The diffusionlayer, responsible for fatigue resistance of thecomponent and support of the PVD layer , is

usually 0.05 µm to 0.3 mm thick, and thePVD functional layer on top of it is only afew micrometers.Layer systems such as tungsten carbide layers(W-C:H) have very low coef ficients of fric-tion and show excellent resistance to wear .Reduced friction leads to a significant in-

Fig. 18: Typical PVD hardmaterial layers a) TiCN b) AlTiN c) TiN d) CrN

a b

c d

… possible inone system

Nitriding andPVD coating …


Automotive engineering 37

valve springs and pump covers), transmissioncomponents (synchronizer rings, synchro-nizer hubs, plate supports, clutch disks, pin-ion shafts and shift shafts), chassis compo-nents (ball pivots and steering pinions), brakecomponents (brake cylinders, lining carriers),wiper shafts and interior equipment (compo-nents for seat adjustments). Many of thesecomponents require additional corrosion pro-tection or need only be partially nitrided, forwhich plasma nitriding with re-usable solidmasking may be suitable.Forming dies (Fig. 19) for body panels areusu ally manufactured from EN-GJS-700-2(according to DIN EN 1561) globular graycast iron. Adhesions and excessive wear on thebending edges can cause problems if the requirements for fitting accuracy are demand-ing with respect to gap dimensions. Nitridingtherefore finds increasing use with these heavytools (up to as much as 30 tons). The best re-sults can be achieved with a combination of in-

36

Overview of applicationsThe first applications of the plasma nitridingprocess were limited to tools and devicessuch as extrusion screws. The objective wasto make use of the improved wear resistanceresulting from the greater surface hardnessand of the reduced tendency to cold weld due tothe ceramic character of the compound layer .The lower process temperatures, comparedwith other surface hardening and nitridingprocesses, also permit low-distortion nitridingof tool steels and steels with low temperingtemperatures. Variation of the process gasmixture even of fers the possibility of reduc-ing the passive layer on stainless steels, thusmaking these nitridable for the first time. Thisopens up completely new fields of applica-tion, among others in the valve industry.

Automotive engineering

There is a great demand in the automotive in-dustry for parts that are both cheap and me-chanically robust. The plasma heat treatment ofcomponents manufactured from inexpensivecommodity steels is ideally suited for meet-ing this demand. Plasma nitriding in particu-lar meets the requirements for a processlargely free of dimensional distortion with re-producible tolerances. This is of fset by therelatively high costs for charging fixtures andmasks, which, however , amortize over theirservice life in high-volume production. Therange of applications is very broad and includes engine parts (crankshafts, camshafts,

Fig. 19: Forming die for body construction

Plasma nitridingopens up newfields of application

Forming diesfor body panels



at least as good, and the waste disposal prob-lems associated with electroplating are elimi-nated. The sub-surface corrosion found withthe chromium coatings, which are susceptibleto microcracking, cannot occur . This elimi-nates the need to regrind the brake cylindersafter electroplating. A particular advantageworth mentioning is the corrosion protectionthat is achieved in the interior of the brakecylinder; a feature not possible with electro-plating. Processing brake cylinders in a saltbath is not satisfactory as too much salt is lostin the subsequent washing, causing contami-nation of the waste water and environmentalissues. Long-term testing shows particularlyfavorable wear characteristics for the squareseal on the nitrided and oxidized surface. Theadvantage gained by an additional plasma activation of the nitrided surface prior to oxi-dation is the superior corrosion protectioncompared to straight gas nitrocarburizing- oxidation proces ses and the outstandingbonding strength of the oxide layer.Ball pivots (Fig. 21) are used primarily in thesuspension and constitute the main part ofjoints that transmit steering movements andhandle wheel location. For a long time, thesurfaces of ball pivots were not treated. Thegreatly increased service life of vehicles andthe constant increase in driving performancehave increased the demands placed on thesesafety-critical components so much that sur-face treatment is now a necessity . A nitridedsurface decreases the wear as well as the fric-tion between the ball and plastic bearingbushing, and therefore also decreases thecounterbody wear. And because high corrosionprotection must also be ensured, combinationwith an oxidation process is recommended.

38 Overview of applications

duction hardening of the wearing edges – asso-ciated with an increase of base hardness andmicrostructure refinement – and subsequentplasma nitriding. Since high hardness and alow adhesion tendency is required, carbondioxide is added to the process gas so that thecompound layer consists mainly of ε-nitride.Such tools can be repaired without dif ficultyby grinding and re-surfacing damaged areas.Subsequent plasma nitriding to restore thewear-resistant surface is also no problem withthe selec tion of an appropriate filler material.The significantly denser compound layer com-pared with other nitride layers increases theservice life of the tool by up to a factor of 10before rework, thus saving cost- and time-con-suming set-up work for the tool.Today, chromium on brake cylinders (Fig. 20)has already been replaced in lar ge-volumeproduction by a combination of nitrocarburiz-ing and oxidation. The corrosion protection is

Fig. 20: Brake cylinder

Brake cylinders

Ball pivots



sensitive areas against nitriding while still al-lowing atmospheric oxidation and thus tem-porary corrosion protection of these areas.The shift shaft (Fig. 23) is used to transfermovements of the gearshift lever to the trans-mission. The connection to the gearshift link-age extends out of the transmission and istherefore subject to corrosive attack by media

in the environment. Furthermore, there is arisk of contact corrosion between the stubshaft and the transmission housing made oflight alloy. The stops on the bell housing orshift drum must function precisely even aftermany thousands of shifting motions, and re-quire corresponding protection against wear .Reduced friction is also desirable to minimizethe force required for shifting. All these re-quirements can be met – without subsequentmachining – by a plasma combination treat-ment on the finished component with gas ni-


This also further reduces the friction coefficientof the surface. Plasma combination treat-ments with gas nitrocarburizing, plasma acti-vation and oxidation have become establishedfor large-scale production, and even permitthe covering of threads to reduce the risk of cracking. Corrosion protection of thethreads is accomplished by subsequent lu -brication with oil or lacquering or sealing (Fig. 22). A process variant for plasma nitrocar -burizing and oxidation has been developedwhich enables solid masking to protect crack-

Fig. 21: Ball pivots in saltspray fog testing

Fig. 22: Ball pivot as installed

Fig. 23: Shift shaft for passenger vehicletransmission

Shift shafts



secure bonding of the metallic surface in thisregion. This high-quality solution can only beachieved through the use of the plasma nitrid-ing process.The pinion shafts (Fig. 25) installed in dif -ferential gears are precision components with exacting requirements for concentricity ,diametric tolerance (DIN k6: approx. 10 to 13 µm) and for the unit pressure resulting from the pinion running directly on the shaft.A plasma combination treatment consisting ofgas nitrocarburizing, plasma activation and oxidation fulfills all these requirements. Thewell-bonded oxide layer reduces friction andensures emergency operation if there is insuffi-cient oil; a property which cannot be achievedwith conventional gas nitriding. The costs aresignificantly less than those for a thermalspray coating of the areas subject to friction;this type of coating requires regrinding and


trocarburizing, plasma activation and oxida-tion. Compared to the older procedure oftreating the shift shaft by case hardening, par-tial chrome-plating and regrinding, there isalso a considerable reduction of cost.Synchronizer rings (Fig. 24) in manual trans-missions ensure speed equalization between thetransmission gears and the drive shaft whenchanging gears. During the gear shiftingprocess, the external teeth engage with the in-ternal teeth of the sliding sleeve. The friction

lining on the inner cone in contact with theclutch body accelerates or slows the gear wheelto match the speed of the drive shaft. High pro-tection against wear is therefore required forthe teeth, while the inner cone requires a se-curely bonded metallic surface for applying thefriction lining. A hard, wear -resistant surfacecan be achieved on the teeth by plasma nitro-carburizing. Solid masking of the inner cone,which reliably shields the transition zone, pro-tects the non-nitrided area and thus ensures

Fig. 24: Synchronizer rings

Fig. 25: Pinion shaft asinstalled

Synchronizerrings

Pinion shafts



compressive stresses and with it an increasein fatigue resistance by approximately 30 per-cent. The demands on the nitriding processare similarly high, since on one hand theremust be no compound layer created on thesurface of the spring subject to vibrationalstress – this would inevitably lead to rapidcrack initiation and breakage of the spring –and on the other hand, treatment temperaturesmust be relatively low to avoid loss ofstrength in the spring due to a tempering ef-fect. Plasma nitriding processes can beadapted to these requirements, but handlingof the components and particularly the batchloading – many spring geometries require themaking of special fixtures to avoid overheat-ing effects – are challenging, costly and time-consuming. A sensor-controlled gas nitridingprocess based specifically on the require-ments of the spring industry (MET ANIT®)permits not only the manufacture of a surfacewith almost no compound layer , but alsobatch loading in baskets.With oil resources becoming scarcer , therapeseed methyl ester mixture referred to as“RME biodiesel” is blended in increasingmeasure with diesel fuel from fossil sourcesor is also used straight as a fuel for suitablediesel vehicles. Due to its high water con-tent of up to more than 20 percent, phaseseparation may occur when the vehicle isparked for longer periods of time, produc-ing an electrolyte which promotes contactcorrosion between the light-alloy housingand the steel cover of the diesel injectionpump. The rust particles produced have adisastrous effect on the components, whichare sensitive due to the high demandsplaced on them. This is particularly the case


also exhibits problems with bonding strength.Since plasma nitriding is a dif fusion processthat modifies the surface but does not coat it,there are no problems with the bondingstrength.The requirement to increase the performanceand reduce the weight of engines also placescorresponding demands on the valve train as-sembly and power transmission. Peak RPMsand torques increase, while the design be-comes more compact to give more ef fectiveload space. Springs used in the engine (valvespring) and the drive train (torsional dampersprings, Fig. 26) are already optimized withrespect to the material and heat treatment. Upto now, the requirement for reducing the vi-brational masses and compact design couldbe met by vapor blasting the surface to createresidual compressive stresses. Howeve r, steelhas reached the limit of improvements regard-ing the degree of purity , heat treatment andsubsequent machining that can be expectedfrom conventional means. Additional thermo-chemical heat treatment (nitriding), however ,can achieve a further increase in residual

Fig. 26: Torsion dampersprings for the drivetrain

Springs in theengine anddrive train

Pump cover resistant tobiodiesel


Hydraulics and fluid technology 47

to that is the fact that the coatings producedwith the current processes are prone to mi-crocracking and are not gas-tight – the pneu-matic springs lose pressure over time and fi-nally lose their function – and are thereforean unsatisfactory solution. For this reason,gas- and plasma-based nitrocarburizingprocesses with post-oxidation are used forthe majority of piston rods. These combinereproducible results with the desired charac-teristics in an environmentally friendlyprocess.

Hydraulics and fluid technologyA victim of the stricter environmental re-quirements is the process of chrome-platingused to coat components. Combined treat-ments with plasma nitrocarburizing and oxi-dation have meanwhile become established asan economical, tech nically competitive andenvironmentally friendly alternative. The par-ticular requirements for the bonding strength,friction coefficients and corrosion resistanceof such layers make high-quality processes anecessity. Optimum results can be achievedonly by adapting the sealing system to thenew surface. Long-term properties in parti c -ular show that after an initial run-in phase, nitrided and oxidized surfaces have moreconsistent coefficients of friction and lesswear than conventional coatings. There arealso processing advantages for components ofparticularly complex geometry.A good example of this is the cylinder headof a pump (Fig. 28) with its numerous boreholes and complex shape. Other surfaceprocesses cannot ensure complete coating andprovide inferior bonding of the oxide layer


for the common rail direct injection systemsused today. Thus nearly all manufacturers ofdiesel vehicles rely on pump covers (Fig.27) which have been nitrided and oxidizedin gas. The frequently thin-walled design(approximately 1 mm) and the final shapeproduced by die cutting and cold formingmean that the covers are a great challengefor batch management, process control andquality assurance. In untreated condition,insuffi cient corrosion protection (oiling) orrough handling during transport can renderthe pump covers useless for further process-ing.Many convenience functions are based onthe use of pneumatic springs. These includeraising the tailgate, trunk lid and hood. Thisis also true of the seat adjustments. The pis-ton rods must move with little friction, resistwear and, depending on where they are lo-cated and the application, be more or lesswell protected from corrosion. Chrome-plat-ing, which was previously widely used, hasbecome uncommon primarily as a result ofenvironmental issues and rising costs. Added

Fig. 27: Cover of a diesel injection pump(biodiesel resistant)

Pneumaticsprings

Cylinder headsfor pumps


Hydraulics and fluid technology 49

proven itself in the field of metal working. The dimensions of the piston rods arelimited only by the size of the vacuumchamber.Figure 30 shows a body used as a clamp inoil production which holds the drill pipe (upto 350 tons) while the drill head is extendedor replaced. Low friction is required for lowbreakaway torque in the chuck cone and ar-ticulation. Corrosion protection poses a par-ticularly demanding requirement, since thework is often of fshore and – in volcanicallyactive zones – the material is subjected tovery corrosive flush water from the well.These high-stress components can achieve the


and/or less corrosion resistance or are muchmore expensive. Since the magnetite layerexhibits no contact corrosion with other met-als, nearly any material combination can beused in hydraulics. Subsequent varnishing,lacquering, sealing is also no problem, al-though it is not necessary for corrosion pro-tection (Fig. 29).Piston rods are the most common actuatorsin hydraulics. The primary requirements forthis component include low wear , low fric-tion with the seals used on the piston rodand high corrosion protection in environ-ments where it is required. Suitable sealingsystems for nitrided and oxidized surfaceshave been tested and frequently constitute abetter combination than conventional sealson chrome-plated surfaces. Damaged pistonrods can be repaired by grinding the dam-aged spot, repair welding, precision-grinding and renewed plasma nitrocarburizing and oxi -dation. This method of repair has already

Fig. 28: Cylinder head of apositive displacementpump

Fig. 29: Cylinder head of apositive displacementpump as installed

Piston rods

Components foroil production


Food industry 51

spindles and wear on the valve flaps with-out reducing the resistance to corrosive media.

Food industryThe surfaces produced by nitriding or nitriding and oxidation in gas and/or plasmahave been approved for the food industry .An exception to this is when the environ-ment is strongly acidic (pH < 4.5), since thiscan dissolve the protective magnetite layeron non-stainless steel. In porous compoundlayers, which are generally produced in apure gas nitriding process, residues can re-main in the pores after cleaning, making theuse of such layers in the food sector imper-missible. A solution for these cases is to useacid-resistant stainless steels which achievehigh surface hardness with acceptable cor-rosion protection by plasma nitriding at lowtemperature.


required properties by using plasma combina-tion treatment with the sub processes gas nitro-carburizing, plasma activation and post-oxi-dation.

Chemical industryThe field of chemistry uses primarily high-quality materials. High-strength steels aremade into guide bars and extrusion screws(Fig. 31) for plastics processing, withplasma nitriding providing the requiredprotection against wear. Component distor-tion can be prevented by suspending com-ponents and careful process control withslow heating and cooling cycles. Dimen-sions of more than 20 meters in length havealready been handled in this manner . In thevalve industry, rust- and acid-resistantsteels are used which must maintain theircorrosion protection. Plasma nitriding atlow temperatures inhibits the erosion of

Fig. 30: Body for oil exploration

Fig. 31: Extrusion screws

Components for plastics processing

ValvesTreating com p on ents for acidic environments


Engineering 53

plasma nitriding and a PVD coating hasproven advantageous for the service life ofgear cutters. It significantly reduces the riskof edge fracture and increases the service in-tervals for regrinding and recoating of thesurface.


Engineering

In the field of engineering, fasteners, devices,fittings, bolts, toothed gears, cold and hotforming tools and a number of other machineparts are nitrided as a matter of tradition. These are mostly custom jobs or small lots. The re-quirements are as diverse as the number ofapplications. The advantage of nitriding isthat finished components can be surface-treated and usually require no subsequent ma-chining.The toughest requirements for wear protec-tion can be met by plasma nitriding and sub-sequent PVD coating in a combined process.The PVD layer, with a hardness up to 2500 HV,provides secure protection in conjunctionwith the hardness gradient of the dif fusionzone even against point or linearly distributedloads, which are a problem for a simple PVDcoating on softer substrates. Aside fromformed parts, this layered compound struc-ture is also used for heavy-duty componentsmade of inexpensive base materials that donot have suf ficient intrinsic hardness for aPVD coating. The economic advantage ofthis approach has already been demonstratedin high-volume production applications.Machining tools are often made of high-speedsteels (HSS). The tendency to form coldwelds or built-up edges can be reduced by ni-triding (Fig. 32). The significantly greatersurface hardness leads to an increase in wearresistance. Grinding stresses are relieved bythe tempering effect. The combination with anoxidation process enables a firmly bondedoxide layer to be formed, which gives the tooleven greater corrosion resistance and im-proved sliding properties. The combination of

Fig. 32: Gear cutter duringplasma nitridingMachine parts

Formed parts

Machining tools


Treatment 55

more detail and compared with gas nitriding.After that, the combined treatment of gas ni-trocarburizing, plasma nitriding and nitrocar-burizing and subsequent oxidation mentionedpreviously will be described.

Sequence of plasma nitridingTreatment in the plasma nitriding system oc-curs in a series of individual process steps.The achievement of predefined process condi-tions automatically switches the system con-trol to the next step. The sequence can be de-scribed in a simplified manner as follows.The cleaned parts are individually fixed in the vacuum chamber where they become thecathode for the gas dischar ge (Fig. 33). Thechamber wall serves as the anode. It is impor-tant to realize that the contact areas of the

54

Process technology

Pretreatment

In general, components to be nitrided arecompletely finished and can be used withoutfurther machining after the plasma treatment.However, a physically clean, grease-free,bare metallic surface on the parts to betreated is important for the quality of the heattreatment. The components must therefore becleaned and degreased prior to treatment. Un-til a few years ago, solvent baths were usedfor this purpose. Today, the baths used con-tain aqueous alkaline solutions. Stubborn dirtand grime can be removed with ultrasound.Subsequent drying with hot air prevents theformation of rust and spots.The cleaning quality is determined by themethod (ultrasound, immersion or spraying),the bath composition, the bath temperatureand the cleaning time. In fully automatic in-line washing systems with heated cleaningbaths, the components travel in baskets or onpallets down the cleaning line and thenthrough a dryer. The baskets or pallets are de-signed to be used afterward as a batchingrack during plasma nitriding. Individualzones on the parts which are not to be hard-ened (such as threads) can be mechanicallymasked after cleaning (for partial nitriding).

TreatmentIn the following section, the treatment se-quence for plasma nitriding, the most widelyused plasma-based heat treatment process forindustrial applications, will be explained in

Fig. 33: Batch of ball pivotsprior to loading inthe system

Cleaning anddegreasing …

… in the batching rack

Loading of the vacuum chamber


Treatment 57

glow discharge at constant regulated tempera-ture for up to 20 hours (Fig. 34). The vacuumpump continuously removes the used gas andthis is replenished with fresh gas. The lowworking pressure keeps the gas consumptionlow (100 to 400 litres per hour).After the end of the treatment, the chamber isflooded with nitrogen and the circulation fancarries the heat to the chamber wall. The wallis cooled by externally mounted fans. Once thebatch temperature is below 100°C, the cham-ber can be opened and the parts removed.

Comparison of gas nitriding and plasmanitridingSince gas nitriding takes place in an atmos-phere with a slight positive pressure, thechamber need not be vacuum-tight and novacuum pump is necessary . However, be-cause hydrogen and ammonia are used inhigh concentrations, the chamber must be prop-

56 Process technology

parts will not be nitrided. Care has to betaken with bore holes and hollow spaces,since the components can become overheatedby hollow dischar ges that occur . Optimal setting of the gas pressure prevents hollowdischarges.After the chamber is closed, it is first evacu-ated to under 10 Pa with a vacuum pump. The chamber is then flooded with nitrogen tojust below atmospheric pressure. The partialvacuum keeps the closed chamber air -tight,so the parts cannot be oxidized by air enter-ing the system. An integrated heater and cir-culation of the nitrogen ensures uniform heat-ing of the batch to approximately 500°C. Withlarge parts, a dwell time at this temperaturemust be provided to allow the parts to heatthrough properly.Afterward, the chamber is evacuated againand the treatment gas (a nitrogen-hydrogenmixture) is introduced. At a pressure of 10 to500 Pa the voltage is slowly increased. Thegas discharge ignites at approximately 500 V(abnormal glow dischar ge) and spreadsevenly over the entire surface of the parts.Modern voltage generators use a pulsedunipolar or bipolar direct current voltage. Inthis phase, an additional cleaning of the sur-face takes place as a result of the intense ionbombardment (sputtering). The increased ki-netic energy of the heavy ions such as nitro-gen, argon and carbon dioxide striking thesurface atomizes the uppermost atomic layersof the components and ensures optimal clean-ing of the surface in preparation for the sub-sequent diffusion process.The final treatment temperature selected(usually about 570°C) depends on the mater-ial. The parts are exposed to the abnormal

Fig. 34: Batch during theplasma treatment

Evacuation,flooding, heating

Evacuation, introduction of the processgases, gas discharge

Plasma formation

Flooding, cooling, batchremoval


59

Conceptual design of plasma nitridingsystems

The present-day plasma nitriding systemsconsist of a vacuum chamber with externalheating (furnace), a pump unit, gas supply , apower element and a system control. The vac-uum chamber is the lar gest component of theplasma nitriding system. It can be heated intwo ways:• by the plasma only• by the combination of resistance heating

and plasma.With very large, densely packed batches con-sisting of several thousand parts, or veryheavy, solid parts, heating can be accom-plished with the glow dischar ge alone. Thelarge surface area of the batches requires highplasma power. The temperature of the com-ponents increases very rapidly with this kindof heating. The chamber wall is thermally in-sulated and provided with an external aircooler for selective cooling.With the combination of the plasma nitridingand oxidation methods and with many applica-tions of straight plasma nitriding, the chamberwall is also heated externally with a multi-zoneresistance heater (Fig. 35). This enables theplasma power to be set to a lower valve. Thelower the plasma power setting, the more ho-mogeneous the temperature distribution in thebatch. However, there is a minimum plasmapower required to achieve good nitriding re-sults. High-alloy steels require, for example,

58 Process technology

erly sealed. The components can be handledas bulk materials. Even parts that are coveredwill be nitrided by the reactive gas. Specialsensors are needed for regulating the gas at-mosphere. These measure the hydrogen andcarbon concentration in the chamber and reg-ulate the chemical equilibrium of the ammoniadissociation (nitriding potential).

Sequence of the combined treatment withgas nitrocarburizing, plasma nitriding ornitrocarburizing and oxidationHeat treatment systems developed especiallyfor this combined treatment operate with low-pressure processes such as plasma nitrid-ing and plasma nitrocarburizing as well as thegas nitrocarburizing and oxidation processesat normal pressure. The individual processescan also be carried out without having to adaptthe system. The high degree of automation,low consumption of energy and process mate-rials as well as the consistent elimination ofmanual process steps provide a considerablecost advantage.The process sequence (see Fig. 12, p. 27) alsoconsists of a series of steps with a number ofprocess parameters that can be programmedindividually. Gas nitrocarburizing uses a mix-ture of ammonia, nitrogen and carbon diox-ide. The plasma treatment steps can be fine-tuned by variation of the plasma voltage,plasma current, pulse frequency and pulse/pause ratio. Oxidation takes place after that,and as the final step, cooling takes place withnitrogen circulation. An external gas/waterheat exchanger can be used for faster cooling.

Regulating thenitriding index

Heating withplasma only

Multi-zone resistance heating


Conceptual design of plasma nitriding systems 61

controls the gas composition, can recognizemalfunctions and react immediately with amessage. The devices are combined with pres-sure reducers, pressure switches and solenoidvalves in a compact gas control station. Prop-erly positioned sensors for hydrogen, ammo-nia and other process gases monitor the air inthe plant and trigger an alarm if the thresholdlimit values (TLV) are exceeded.Plasma generators are available as pulse gen-erators for unipolar and bipolar operation withpower up to 800 V at 320 A. The pulse fre-quency is 1 to 25 kHz. Compared with pulsedDC generators, pulse generators have the ad-vantage that most of the arc dischar ges aresuppressed as they occur. Extremely fast over-current shut-off prevents the arc dischar gesthat occur from causing damage to the parts.Another advantage of pulse generators is theoption for continuous regulation of the plasmacurrent at constant voltage via the pulse/pauseratio. This facilitates exact temperature con-trol of the parts, leads to more even tempera-ture distribution and saves energy.The system control is equipped with state-of-the-art electronics and is designed for fullyautomated processing. It provides a well-or-ganized display that facilitates operation. Theuse of industrial PCs makes networking sim-ple and enables remote diagnosis and mainte-nance to be performed. The system controland power element are constructed compactlyand located in cabinets to protect the electriccomponents from dust and spray water.Plasma nitriding systems are available as bell,pit or chamber furnace systems. Dependingon the system design selected and the capaci-ties required, single, tandem or multiple sys-tems can be built.

60 Conceptual design of plasma nitriding systems

more plasma power than low-alloy steels at thesame treatment temperature.The vacuum pump station should be dimen-sioned to allow evacuation of the chamber ina reasonable period of time compared withthe overall treatment time. Thus for chamberslarger than 1000 litres volume, pumping sta-tions with a two-stage rotary vane pump com-bined with a rotary piston pump (Roots pump)are used frequently. Pressure regulation in thechamber is handled by controlling the speedof the Roots pump. Gas metering today is accomplished predomi-nantly with electronically controlled gas flowregulators. These devices communicate di-rectly with the system control to determine setvalues and report actual values. The computer

Gas circulation

Cooling

Vacuum pump

Vacuumchamber

Batch

3-zoneheater

Fig. 35: Plasma nitriding system with resistance heater

Vacuum pumpstation

Gas metering

Plasma generator

System control


Pit furnace systems 63

system, it nonetheless has the advantage thata batch can be nitrided on one base while anew batch is prepared on the other platform.This saves a lot of time.

Pit furnace systemsPit furnace systems are used particularly inseries manufacturing and for long compo-nents such as spindles, extrusion screws orshafts. Mixed batches can also be treated. Pitfurnaces are loaded from the top using acrane (Fig. 38). They have the advantage offast batch change. Single, tandem or evenmultiple furnace systems can be constructed.Tandem systems reduce the downtime forbatch loading and removal. Multiple systemsmaximize the cost advantage.


Bell furnace systemsBell furnaces (Fig. 36) are used if the com-ponent or the batch has to be easily accessi-ble for the system operator . It is advanta-geous to be able to access the batch fromseveral sides. This makes it much easier , forexample, to place thermal elements in thebatch to achieve a homogeneous temperaturedistribution.

Bell furnace systems are used particularly for single-part production and for mixedbatches, but also for lar ge parts. They arealso suited for processing series-productionparts, particularly with the integration of au-tomated batch loading systems and robots.They can be designed as single and tandemsystems.Figure 37 shows a typical bell furnace sys-tem, consisting of two platform bases, a fur-nace bell and a hydraulic lifting device. Although this design is not a true tandem

Bat

ch

Fig. 36: Loading diagram fora bell furnace system

Fig. 37: Bell furnace systemwith dual base platforms

Single-part production Series manu -

facturing andlong components


Chamber furnace systems 65

The components in a chamber furnace system(Fig. 40) are the vacuum chamber, heating unit,charging fixture, locking chamber door , vac-uum system, controller, gas supply and plasmagenerator. Integrated forced cooling reduces thecooling times, and fast cooling with a heat ex-changer accelerates cooling even more.


The main components in a pit furnace systemare the vacuum chamber, heating unit, charg-ing fixture, cover lifting and pivoting mecha-nism, vacuum system, controller , gas supplyand plasma generator . Pit furnace systemsusually have integrated forced cooling to re-duce the cooling times. Additional quickcooling systems designed as heat exchangersshorten the cooling times even more.

Chamber furnace systemsChamber furnace systems (Fig. 39) are usedparticularly if there is little space available forthe installation, for example when the ceiling islow. They are suited for plasma nitriding oflarge single parts such as cutting or formingdies as well as for the production of seriesparts. Chamber furnace systems can be integrated comparatively easily in existing pro-duction lines. Heat treating operations in partic-ular often have automated batching and con-veyer systems that can be integrated very well.

Bat

ch

Fig. 38: Loading diagram fora pit furnace system

Batch

Fig. 39: Loading diagram fora chamber furnacesystem

Fig. 40: Chamber furnacesystem for completebody working tools

Little space required

Simple integration inproduction lines


Work safety and environmental protection 6766 Conceptual design of plasma nitriding systems

Incoming goods

Cleaning

Screwing/batch loading

Polishing

Oiling

Packaging

Outgoing goods

Plasma heat treatment

Quality assuranceFig. 41 (opposite): Integration of aplasma nitriding system in the surfacetreatment of ball pivots for steeringsystems in auto -motive engineering

Work safety and environmentalprotection

Working safety regulations dictate how thoseworking in a production facility are protectedfrom the risks associated with the workprocesses. Environmental protection in thiscase is the protection of the environment of asystem or a production facility from hazardousmaterials produced in solid, liquid or gaseousform during manufacturing or processing,which may escape into the environment. InGermany, the Chemicals Act (ChemG) and theassociated Ordinance on Hazardous Substances(GefStoffV) are among the cornerstones ofwork safety. In addition, there are the TechnicalRules for Hazardous Substances (TRGS). Par-ticularly worth mentioning is TRGS 900 (expo-sure limit values in workplace atmosphere),also known as the “MAK” list (MAK is theGerman acronym for “maximum workplaceconcentration“). The Federal Immission Con-trol Act (BImSchG) as well as the associatedimplementation ordinances (BImSchV) and ad-ministrative regulations (BImSchVwV) are ofprimary importance in controlling emissionsand are better known as the Technical Instruc-tions on Air Quality Control (TA-Luft).Encapsulated extraction systems minimize theex posure of operating personnel using plasmanitriding systems. In the combination pro cessof plasma nitriding or plasma nitro carburizingwith oxidation, exhaust air is passed through apilot flame and then vented into the atmospherevia an exhaust duct above the roof of the plant.Due to the envir onmental friendliness of theprocess, plasma nitriding systems can be inte-grated directly into the production processwithout problems (Fig. 41).

Statutory regulations

Environmentallyfriendly plasmanitriding systems


Trends and future outlook 69

ing more expensive. The layer combinationof iron nitride with magnetite of fers effectivecorrosion protection for all constructionsteels, carbon steels, low-alloy steels, case-hardened and nitrided steels, temperingsteels, cast iron and sintered materials. Thenitriding of cheaper steels makes it possibleto save on raw materials in the form of alloy-ing elements. Component properties can beimproved several-fold by the incorporation ofnitrogen. Furthermore, low-alloy steels areeasier to process.Surface treatment in the plasma, in gas or bycombined processes such as gas nitrocarbu-rizing, plasma nitriding and oxidation or ni-triding and coating is the final processing stepin the manufacture of components. Subse-quent machining is not necessary . Compo -nent dimensions increase in a controlled andcalculable manner. Undersize manufacturingenables finishing steps such as regrinding tobe eliminated.In combination with a subsequent oxidation,one obtains a product that not only satisfiesrequirements for wear and corrosion in opera-tion, but which can also be recycled withoutdifficulty. Such components can be remeltedin the blast furnaces of the steel industry with-out the need for additional processing steps.The process variant involving the hardeningof stainless steel in plasma at lower tempera-tures without loss of the corrosion resistancealso has considerable application potential.Systems with plasma activation can be integrated in existing production lines with-out difficulty due to the flexibility of the system technology and its proven environ-mental safety. Today there are plasma nitrid-ing systems in service companies for long

68

Trends and futureoutlook

In the past 30 years, the plasma nitriding ofiron-based alloys has become a recognizedtechnique for industrial production along withsalt bath nitriding and gas nitriding. The estab-lished advantages of the plasma process causeit to be used with increasing frequency. In thefuture, conventional nitriding processes suchas salt bath nitriding will be subject to in-creasing regulation by environmental authori-ties. However, plasma technology is availableas an alternative which with respect to emis-sion values, can be classified as environmen-tally friendly.Above all the combination of plasma nitridingand gas nitriding in a single process cycle offers definite advantages. This allows thediffusion zone to be produced by the plasmaand the compound layer in the gas step. Thissaves gases during long treatment periods and allows the advantages of the compound layer produced in the gas atmosphere, for example the strongly developed porous zoneand fast layer growth, to be exploited.Combined treatments without processing in-terruptions such as nitrocarburizing with subsequent oxidation or plasma nitriding withsubsequent coating are very promising andhave already led to a significant increase ofapplications in the surface treatment of low-alloy steels.The prices of zinc and chromium on theworld market are rising steadily . As a conse-quence, the methods for galvanizing, chromiz -ing and also hard chrome-plating are becom-

Combination treatment and …

… process variants all the rage


The company behind this book

METAPLAS IONONOberflächenveredelungstechnik GmbHAm Böttcherberg 30–38D-51427 Bergisch Gladbach, GermanyPhone: +49 (0) 22 04/2 99-0Fax: +49 (0) 22 04/2 99-2 66E-mail: [email protected]:www.sulzer.com

The original MET APLAS IONON company was established in 1984 in the field of PVD coating technology . With the pur-chase of Klöckner IONON in 1993, MET APLAS IONON ex-panded its activities to the field of plasma-based heat treatment.In 2001 the company was purchased by Sulzer Metco, a leadinginternational supplier of surface technology . Sulzer Metco (sur-face technologies and service) is one of four core divisions of theSulzer Group along with Sulzer Pumps (pumps and associatedservice), Sulzer Chemtech (components and service for separa-tion columns and static mixing) and Sulzer Turbo Services (re-pairs and service for thermal turbo machinery). METAPLAS IONON is active in three areas of business on themarket: its service centers for contract treatment of fer state-of-the-art system technology and operate fully automatically with in-dustrial robots in some areas. The experience gained in the ser-vice centers is applied to the ongoing ef forts in system engineer-ing: METAPLAS IONON develops and builds complete turnkeysystems covering the range from pretreatment to coating and heat treatment systems, including posttreatment systems and theinfrastructure equipment required for operation. This includesknowledge transfer in the form of licenses and patents. The high-est standard of quality – certified according to ISO/TS16949:2002 – is guaranteed. Furthermore, MET APLAS IONONoffers shop-in-shop solutions. We operate our own systems andmake them available on-site to customers.The two basic technologies of MET APLAS IONON are of feredunder different brands. The MAXIT® brand covers the field ofPVD coating technology. The IONIT® and IONIT OX® brandscover plasma heat treatment. The combination of both technol -ogies has meanwhile become established on the market.

70 Trends and future outlook

or bulky components that could not beprocessed before.PVD coating technology is used not only fortool coating but also increasingly for the coat-ing of components. This requires system de-signs that are suitable for highly auto-mated, large-scale production. System de-signs that enable nitriding as well as coatingin a single cycle will become established.Low-alloy steels are particularly well suitedfor this technology . Low concentrations ofparticular alloying elements (Cr , Al, V, Moand Mn) enable high surface hardnesses to beachieved with nitriding and consequently pro-vide an excellent base for the subsequent ap-plication of a hard material or low-frictionfunctional layer.

METAPLAS IONON

Nitriding andPVD coating inone system


plasma assisted surface treatment v1

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