Revista Latinoamericana de Metalurgia y Materiales, Vol. 13, N° 1 Y 2, 1993. 5

SURFACE ENGINEERING: ADVANCED MATERIALS FOR INDUSTRIALAPPLICATIONS

D.T.Gawne

Department ofMaterials Technology, Brunel, The University ofWest London, Uxbridge, Middlesex, UnitedKingdom.

AbstraetThe principles of surface engineering and the reasons for its emergence as a pervasive manufacturing

technology are outlined. A break:down of industrial activity in surface engineering in the UK and forces forchange are discussed, together with their implications for the role of future research in advanced materials forindustrial applications. Examples of current research studies are described involving plasma-assistedchemical vapor deposited Si02-Ge02 thin-film waveguides in integrated optics and electroplated chromiumpiston rings or cylinder liners in internal combustion engines.,

INTRODUCTION

Surface engineering may be defined as thedesign of engineering to improve their performancein service. This can be achieved by surfacetreatments, which can provide combinations ofsurface and bulk properties unobtainable in a singlematerial. A ceramic on a metal, for example, cangive extreme .hardness at the surface, whilemaintaining an acceptable fracture toughnessthroughout the section. The surface treatmentsmay involve coating, chemical modification orphysical processing. Coating processes, such aselectroplating processes, produce a discrete layer ofa new material with a sharp interface at thesubstrate. Chemical modification techniques, suchas nitriding, provide a diffuse layer of a foreignelement with no definitive interface. Physicalprocesses, such as grit blasting, change the surfaceprofile but not the chemical composition.

The properties of the surface layers areoften quite different from those of the bulkmaterial, because they are frequently formed undernon-equilibrium conditions. Many surfacetreatments involve rapid cooling or lowhomologous temperatures resulting in the creationof metastable structures such as new amorphous orcrystalline phases and extended solid solubilities,often with high levels of residual stress. Surface-sensitive properties are of particular interest sincethey are most readily affected by surface treatmentsand include friction, wear, corrosion, oxidationand opto-electronic behaviour.

The importance of surface treatments lies in

their ability to improve performance, create newproducts and conserve scarce materials bysubstitution. Respective examples are carburizinggears to extend component lifetimes, vacuum-evaporated cobalt-nickel thin films on PET as videotapes and electroless nickel-phosphorus coatingson plain carbon steel to replace certain chromiumstainless steel parts. Surface engineering is drivenby the engineering application and is primarily anapplied science. Surface engineering thus involvesa detailed knowledge of the end application, itsfunction and requirements, the property evaluationand quality control of the working surfaces,characterization of the surface structure and anunderstanding of the capabilities of the availablesurface treatment processes.

This paper is concerned with the use ofadvanced materials and the role of research in thesurface engineering industry. Research in surfaceengineering requires funding for labour, equipmentand .consumables. In the UK, this funding ismainly obtainable from government bodies, theCommission of the European Communities andindustrial companies, but the underlying primarymotivation of all three in this instance is to improvethe competitive position of UK industry. Themajority of research in surface engineeringtherefore has to be justified on the grounds oftangible industrial benefits. The direction ofresearch is thus closely linked to the current statusof the industry, the products required by the users,the forces for change and their effect on the futureneeds of industrial companies.

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PRODUCT APPLICA TIONS

The surface treatment industry is essentiallymarket-led and provides theproducts required byits customers, who aim to satisfy theapplicationsof the users. Surface engineering is an important,pervasive technology used by almost every part ofmanufacturing industry with the result that itsproduct applications are extremely diverse. Theuse of surface treatments has grown steadily overthe last two decades. Some of this growth is dueto the development of new techniques, such asplasma-assisted chemical vapour deposition andoion implantation. However, the less capital-intensive newer techniques, such as powderpolymer coating (e.g. electrostatic spray, fh.iidizedbed) and electroless deposition have to date mademuch more impact in economic terms; for example,the replacement of cadmium by epoxy resin coatingin suitable cases and the switch from anodizingaluminum to coating with polymers. The role ofelectroless nickel coating has been partlycannibalistic in the sense that they have replacedchromium plating in some cases, but in others havefound new applications due to the superior

.throwing power and unique properties.An example of the commercial benefit of

surface engineering can be found in high-speedtwist drills [1]. Steel drills have traditionally beensurface treated by a steam tempering process toproduce the familiar blue oxide finish. Acollaborative research program (SKF, Dorrner,MultiArc) has now develop a plasma-assistedphysical vapour deposition process for coating thedrills with titanium nitride. The use of this coatinginitially enabled the drill to .produce twice as manyholes before replacement com pared with theconventional drill. However, the greater cutting

.ability of the drill led to choking of the flutes byswarf. Subsequent re-design of the shape andhelical angle of the flutes aided swarf clearanceconsiderably and the resultant TiN-coated drillexhibited a tenfold improvement in life comparedwith the conventional product. Cost savingsaccruing from a longer drill life or faster speeds forthe same drill life are typically 30% per holedriIled.

There are numerous examples in otherindustrial sectors. Yttria-stabilized zirconia coatingsproduced by plasma spraying provide higheraircraft engine efficiencies by acting as thermalbarrier layers on critical cornponents such asturbine blades. Other industries include automotive(bearings, shafts, carns, tappets, etc.), chemical

plant (catalyst support beds, pumps, valve seals),textile machinery (rollers, bearings, thread guides)and machine tool (cutting, boring, drilling,slideways).

Manufacturing industry is moving awayfrom the use of surface treatment as a mean ofcompensating for poor material properties towardsthe incorporation of surface treatrnent specificationsat the design stage, particularly in cases wherecost-effective benefits are identified. Coatingtechnologies are being increasingly adoptedindustry to improve its competitive position byadding value to components through their superiorperformance,

SURFACE ENGINEERING INDUSTRYIN THE UK

(a) Market ValueProduction in the UK surface engineering

industry expressed in terms of value added, or thevalue of output net of inputs, is estimated in table 1and figure 1. The figures are very approximatepartly because Government statistical data onlyrelates to companies having more than 50employees, which excludes a considerableproportion of the UK surface engineering base.The figures apply to the so-called functionalcoatings (non-decorati ve with specific propertyrequirements such as wear, corrosion or oxidation,although the division between functional anddecorative coatings is somewhat arbitrary) andexcludes paints. The paint industry in the UK isvalued at approximately 1200 US$ and isdominated by large companies such as lC!. Thetotal value of the functional coatings business isapproximately 2.5 billion US$ per year, which isabout 1% of the total UK manufacturing activity.However, surface engineering is often only oneunit process in a chain of processes making up theintegrated manufacturing route of an engineeringproduct, the surface engineering content of whichis typically 5-10% of the sale price [2]. The truevalue of surface engineering can be estimated interms of the value of the interrnediate products soldthat are critically dependent on surface engineeringprocesses. These intermediate products areprimarily components supplied to larger assembliesas, for example, zinc-plated carburettors forautomobiles. The sale of intermediate products thatare critically dependent on surface engineeringprocesses in the UK has been estimated [2] at 20billion US$ per year or the order of 10% of the

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Technology ValueUS$(million) % share Technology Value

Electroplating 1000 40 US$(million) % shareSurface heat treatment 600 23 Physical vapour 10 36Galvannizing 300 12 depositionPowder coating 200 8 Chemical vapour 6 22Phosphating, chromating 200 8 depositionThennal spraying 70 3 Plasma nitriding 5 18Anodizing 45 2 Plasma spraying 5 18Other (enamelling, etc.) 40 2 Ion implantation 0.5 2New technology 40 2 Laser/electron beam 0.5 2TOTAL 2495 100 Other (e.g. friction) 0.5 2

TOTAL 27.5 100

total UK manufacturing output. However, thevalue placed on such intermediate products canonly be very approximate.

TABLE 1. Estimated value of the UK surfaceengineering industry

Sur!. heattreatment

Other(enamel, etc)AnOdizing

T New tech'lologieshermal spr avrnq

Galvanizing powder coating

Total value 2.5 billion US $

FIGURE 1. Estimated value of the UK surfaceengineering industry.

The overall growth of the surface engineeringsector between 1986 and 1989 was about 10% peryear but has now slowed to 5% per year. Abreakdown of the new technologies is given intable 2 and figure 2; these processes contributeonly 2% to the total sector but their growth rate hasbeen almost double that of the established sector.The new technologies are particularly strong inspecialized sectors such as machine tools and

aerospace but there is little evidence to date thatthey are replacing the established processes fortraditional product applications to any significantextent.

TABLE 2. Estimated value of new surfaceengineering technologies in the UK

CVD

OtherLaser /electron beamIon tmorantauon

Plasma nitriding Plasma spraying

Total value 30 mili ion US $

FIGURE 2. Estimated value of new surfaceengineering technologies in the UK.

(b) StructureElectroplating has rernained the most

important process within the industry for manyyears. It consists [3] of a limited number (-110) ofmedium-sized chemical supply companies and amuch larger (-1000) of small processirig

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companies with a workforce per company oftypically 20-30. The total number of employees inthe surface engineering sector is approximately20,000. The business tends to consist of workthat is subcontracted from other (often muchbigger) companies. For example, about 1000small electroplating firms serve the UK automotiveindustry and a similar number oí smallsubcontractors service the UK aerospace industry.The electroplating industry is generally a highturnover, low profit margin and small order-bookbusiness where custorners tend to demand shortlead times. Investment is inhibited by a lack offinancial resources and a reluctance to write offexisting facilities.

The surface engineering industry is veryapproximately evenly split between subcontractingby mostIy small companies and in-houseprocessing. by often much larger companies. Thelarge companies have tended to subcontract workto an increasing extent over the last five yearsbecause this provides production flexibility,obviates investment and offsets risks. However,there are now some indications that this trend maybe reversed due to concern of the larger companiesin the commercial viability of some of thecontractors, the dangers of single-source supplyand the need for closer technological control. Overa longer period, however, it is possible that thebalance of subcontracting to in-house processingwill oscillate from one to the other due tocompeting technological and economic factors.

FORCES FOR CHANGE

A number of new factors have arisenrecently which will force major changes in thesurface engineering industry in the next ten years.The nature of the industry at the end of this decadevary much on how it deals with the opportunitiesand threats brought about by these externalpressures.

New environmental legislation to controlhealth and safety of the workers, effluent andwaste disposal, and protection of the environmentoutside the factory is causing many processingcompanies to examine the viability of their existingtechnology base. The pressure comes fromnational and particularly supra-national bodies,such as the European Community and the Montrealprotocol on CFCs. For example, the use ofhexavalent chromium salts and nickel sulphate isnow restricted under health and safety regulations.

These chemicals are used widely to producecoatings in the electroplating industry andsubstitution is proving difficult. Sorne of thepossible alternatives such as those based on nickelchloride and sulphamate may themselves becomefurther restricted in the future. Charges for thedisposal of trade effluent to sewer and of wastesludges to landfill are expected to rise substantiallyin the next few years, particularly if the proposedEC directives become operative. Further concerncentres around the anticipated rise in cost of amajor raw material, water, stimulated partly by theprivatization of water authorities in the UK. Morestringent restrictions on air emissions from ovens,heat treatment furnaces, spraying booths, etc. willalso result in increased production costs.

The principal industrial competitors of theUK are Germany, France, Japan and the USA.International competition will be enhanced afterDecember 1992 with the creation of the singleEuropean market, which will facilitate free tradethroughout the member states. The principalcompetitors are investing significantly in thesurface engineering industry and the requisiteexploitation infrastructure. For example, theGerman Ministry of Research and Developmenthas invested 90 million US$ per year for the nextfive years on plasma-related surface treatments.International competition will have a major impacton the industry in the coming decade.

Another consequence of the formation ofthe single European market in 1992 is the movetowards the rationalization of standards withinEurope in order to remove the technical barriers totrade. Once a European Standard has been agreed,all Be members are under a legal obligation toadopt it as a national standard. Europeanstandardization will affect many processes andproducts within the surface engineering industry.In the extreme, the processing in some companiesmay be inadequate and they may lack the resourcesto upgrade in order to manufacture their products tothe new standards.

RESEARCH

The research activity in industry is generallyconfined to the larger companies in volved inchemical supply (e.g. electroless nickelformulations, phosphates), process equipmentmanufacture (e.g. advanced thermal spraying,chemical vapour deposition) and the usercompanies (e.g. automotive, aerospace, electronic,

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machine tool). These companies often have theirown research departments but also interact with therest of the UK science base. The surfaceengineering science base in the UK is significantand resides in industrial research centres,government research establishments, universitiesand contract research companies.

The research in the science base is criticallydependent upon funding from industry,govemment bodies (Science and EngineeringResearch Council, Department of Trade andIndustry, Ministry of Defense) and the EuropeanCommunity (e.g. BRITE-EURAM). However, allthese funding sources in this context are directed atimproving the competitive position of UKmanufacturing companies. Surface engineeringresearch in the UK is thus driven by the needs ofindustry.

Up to the present time, the subcontractingcompanies have interacted comparatively little withthe UK science base and generally respond only tothe demands of their current customers. However,future threats and opportunities, together with thepossibility of company mergers into larger units,might possibly lead to a change in their approachto research.

EXAMPLES OF RESEARCH STUDIES

The final section in this paper provides twoexa~ple~ of current research studies in surfaceengineenng.

(a) Integrated optic waveguidesThe recent development of optical fibres

waveguides as a mean of transmitting informationhas now penetrated into virtually every area ofinformation transmission, processing and storage.Optical fibres are also used to link computerperipherals, are likely to link processors, and areused as sensors. Planar waveguides play animportant role in optical transmission asinterconnections, beam splitters, mixers,wavelength multiplexers, etc. [4]. In its simplestform, aplanar' optical waveguide consists of atransparent thin film of refractive index nj , slightlygreater than of its substrate n2. For shallowincident angles, Snells law of refraction cannot besatisfied, total internal reflection occursand theinterface acts as a perfect mirror (Sin8>n2/nl)(figure 3). Under this condition, the film operatesas an optical waveguide: a ray of light is ret1ected

from side to side along the film until it emerges atthe far end. For example, figure 4 shows thetransmission of light in a silica-germania thin filmwaveguide on a silica substrate.

substrote

FIGURE 3. Ray schematic of light propagation inthin film optical waveguide.

FIGURE 4. Transmission of light in Si02-Ge02planar waveguide on silica, showing couplingprism on left and guided light from left to right incentre of field. X1.6.

No consensus has been reached on a singleideal material fabrication technique for planar opticwaveguides. Popular materials inc1ude lithiumniobate, III-V semiconductors, glass and a varietyof polymers. Typical fabrication techniquesinc1ude diffusion, epitaxial, ion exchange anddeposition. This research concerns the use ofplasma-assistedchemical vapour deposition as afabrication method, in which germania-doped silicais deposited as a glass layer on silica. A

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temperature of 18000C is required in conventionalchemical vapour deposition for this reaction andthis work concerns the use of plasma-assistance toreduce the reaetion temperature. The depositionapparatus is shown in figure 5 and consist of aheated reaction zone maintained at a low pressure(-1 mbar) into whieh are fed reaetant gases (SiC4.GeCI4. 02) and a readily ionizable gas (e.g.argon). A microwave generator (500W, 2.45GHz) is used to excite a plasma in the reaction zoneand faeilitate the deposition of a thin film on thesubstrate. The overall reactions are:

SiCI4(g) + 02(g) = Si02(S) + 2Ch(g)GeC14(g) + 02(g) = GeOS + 2Cl2(g)

The high exeitation levels induced in themolecular species by the plasma has enableddeposition to take place at temperatures as low as250C compared with 18000C in the absence ofplasma.

02~

5iC!4~ ,

02~-

Ar

_vocuumpumps¡IICQsubstrote

FIGURE 5. Schematic of PACVD apparatus forfabrieation of planar waveguides. MFC = MassFlow Controller.

The quality requirements of the films centrearound their mechanical and optieal properties.The mechanical properties of the films (thiekness- 5~m) were assessed by indentation fracture(figure 6) and the data in figure 7 indieate that theadhesion (interfacial crack length, C¡), integrity andresidual stress (radial crack length, Cr) deterioratedrapidly at low deposition temperatures. Optiealeharacterization showed that high qualityperformance was obtained in waveguides depositedat 10QOoCin terms of a sharp step index profile

(figure 8) and low attenuation (figure 9). A majordeterioration in optical properties oecurred at lowdeposition temperatures and the data in figure 10shows that the attenuation is well above themaximum acceptable value of ldBcm-l. Theresearch has thus shown that Ge02-Sj02 planarwaveguides can be produced by plasma-assistedchemical vapour deposition at temperatures wellbelow those for conventional chemical vapourdeposition but that an optimum reactiontemperature is required for acceptable mechanicaland optical properties.

(b) InternaI Combustion En~ineThe piston-cylinder combination is a

erueial arrangement in the internal combustionengine for the conversion of chemical energy tomechanical energy (figure 11). The contactbetween the pistón rings and inside surfaee of anengine cylinder must be as light as possible inorder to seal the annular gap with a lubrieant.However, the piston should still be able to slidefreely and not wear its pisto n rings por the cylinderbore too rapidly, The dominant requirement withprecision components is to provide low friction andwear throughout the life of the engine.Electroplating the piston rings or the cylinder linerwith chromium may reduce friction and wear dueto its high hardness (HV - 1000Kg mm-2) •. butsuffer from a relatively poor oil wettability. Underadverse conditions, the oil does not spreadadequately over the cylinder walls leading tomarginal lubrieation, metal-to-metal contact andsubstantial wear. It study investigates the influenceof surface .topography on the wettability ofelectroplated chromium coatings [6].

Three types of chromium coatings wereproduced:( a) conventional, (b) nodular by meansof a low electrolyte temperature during plating and(e) etched reversing the polarity at the end ofplating (figure 12). A technique [7] was used toevaluate the rate of spreading of oil on a surface bymeans of a wettability exponent, W:

(1)

for wich A is the area of spread of the oil after atime t while W and C are constants for the system.The wettability exponent of the conventionalchromium of 0.139 was increased by the presenceof nodules to a value of 0.253 and particularly byback-etching to a value of 0.351. The surfaceprofiles of the nodular and etched chromium are

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shown schematically in figure 13 with theircorresponding material ratio curves (figure 14).Whereas only 10% of the metal surface is exposedto bear the load after lurn wear on the nodularsurface, as much as 70% is exposed in the etchedsurface. The high material or bearing area of theetc.hed surface is beneficial in reducing temperaturebuild-up and contact pressures, but is also veryeffective in trapping oil in its deep valleys in anotherwise fairly flat surface. The oil retentionvolume Vo as shown in figure 15, is the volume ofvalleys below the core roughness and is anindication of the oil retained by a cylinder bore

surface after it has been scraped by a piston ring.The Vo value is much larger for the etched surfacethan the nodular surface (figures 13 and 14) withthe result that the etched surface will be moreadequately fed with oil under sliding contact. Boththe nodular surface and the etched surface thusprovide improved wettability but the etched surfacehas the additional advantage of higher oil retentionand a larger bearing area. This work shows thecritical importance of surface topography inlubricated sliding contact and how it may becontrolled during surface treatment.

a

b_,...

FIGURE 6. Ole micrographs in Si02-Ge02 waveguides deposited at given temperatures.

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;20

E:J

90'-'

¡::üzw 60-1

:,:: SiOz-GeOz(Cr)U<o::::u

30Siq,-GeOz (Ci)

0t------r-----,------.-----~----~~o 200 400 600 800 lOCODEPOSITION TEMPERATURE, ( e)

FIGURE 7. Effect of deposited on the interfacial C¡ and C, crack lengthsin Si(h-Ge02 waveguides on silica substrates.

0.015

e<J

0010,r--------..--

0005

OOO!:-_--:--_~ __ ~ ~_~o 2 4 6 8

dE'plh microrrs,10·_ 12

FIGURE 8. Refractive index profile for Si02-Ge(h film deposited at ll000C on silica: differencein refractive index between film and substrate (Dn)is plotted as a function of depth.

5

CD'O

1.00 2.00 2.50

'O 2v11I

CII>o~

•...4••~ ..n r--~~-~~••-~.~--~.~.~~~----~ 3 •••••• • • _- . --..-.-CII

° o.so 1.50

propogolion lE'nglh , cm

FIGURE 9. Attenuation measurements of fourmoded Si02-Ge02 waveguide deposited at 11000Con silica.gradient = -0.21 dB cm-1

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10

co"'O 8.'-(¡¡~8. 6

"'O(¡¡•..~ 4'O:..,¡

(¡¡

2~oñ;•..

O 0.50 2.001.00 1.50oroooaolion lenalh c.m

2.50

FIGURE 10. Attenuation measurements of six moded Si02-Ge02waveguide deposited at 100°C on silica: gradient.

15

1. Cylinder head2 .. Piston3. Piston rings4. Cylinder block5. Piston pin or gudgeon pin6. Piston red7. Connectlng rod8. Crank or crankshaft9. Crank case

la. Sump11. Exhaust valve12, Inlet valve13. Spark ;llug14. Valve spring, 5. Camshaft16. Pushrod

FIGURE 11. Schematic of the reciprocating internal combustion engine.

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~I~~~~k~ __~/ ... ~:.~_.. ,.~ ,..--

o+o+e-L-

-IOOPt"r cen-, -

FIGURE 14. Derivation of material ratío(or bearing area) curve.

FIGURE 15. The oil retentíon volume, v-; shownby the shaded area.

FIGURE 12. SEM micrographs 01' the sürfaces 01'chromium plating: (a) conventional, (b) nodular,(e) back-etched.

70%

FIGURE 13. Schematic 01' surface profiles and material ratio curvesfor etched (top) and nodular chromium surfaces.

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REFERENCES1. R. Goodwin, "The Hole Story" in "A CuttingEdge for the 1990s", Institute of Metal, London,1990.

2. O.A.J. Vaughan, "Future of SurfaceEngineering in the UK", Centre for Exploitation ofScience and Technology, London, 1991.

3. D. T. Gawne, Trans. Inst. Met. Fin. 67 (1989)15.

4. D.T. Gawne, N. Nourshargh, I. Kandasarnyand E.M. Starr, Surface Engineering, 6 (1990)107.

5. F.G. Arieta and O.T. Gawne, BrunelUniversity. To be published.

6. F.O. Arieta and O.T. Gawne, J. Mater. Sci.,21 (1986) 1973.

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