surface engineering advanced materials for industrial application

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

RFACE ENGINEERING: ADVANCED MATERIALS FOR INDUSTRIAL

partment ofMaterials Technology, Brunel, The University ofWest London, Uxbridge, Middlesex, United

The principles of surface engineering and the reasons for its emergence as a pervasive manufacturingnology are outlined. A break:down of industrial activity in surface engineering in the UK and forces forange are discussed, together with their implications for the role of future research in advanced materials forustrial applications. Examples of current research studies are described involving plasma-assistedemical vapor deposited Si02-Ge02 thin-film waveguides in integrated optics and electroplated chromiumton rings or cylinder liners in internal combustion engines.,

Surface engineering may be defined as theign of engineering to improve their performanceservice. This can be achieved by surfaceatments, which can provide combinations offace and bulk properties unobtainable in a singleterial. A ceramic on a metal, for example, cane extreme .hardness at the surface, whileintaining an acceptable fracture toughnessoughout the section. The surface treatmentsy involve coating, chemical modification orysical processing. Coating processes, such asctroplating processes, produce a discrete layer ofnew material with a sharp interface at thestrate. Chemical modification techniques, suchnitriding, provide a diffuse layer of a foreignment with no definitive interface. Physicalcesses, such as grit blasting, change the surfaceile but not the chemical composition.The properties of the surface layers areen quite different from those of the bulkterial, because they are frequently formed undern-equilibrium conditions. Many surfaceatments involve rapid cooling or lowmologous temperatures resulting in the creationmetastable structures such as new amorphous orstalline phases and extended solid solubilities,en with high levels of residual stress. Surface-sitive properties are of particular interest sincey are most readily affected by surface treatmentsd include friction, wear, corrosion, oxidationd 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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RODUCT APPLICA TIONSThe surface treatment industry is essentiallyarket-led and provides theproducts required bys customers, who aim to satisfy theapplicationsf the users. Surface engineering is an important,ervasive technology used by almost every part of

anufacturing industry with the result that itsroduct applications are extremely diverse. These of surface treatments has grown steadily overe last two decades. Some of this growth is duethe development of new techniques, such asasma-assisted chemical vapour deposition andoimplantation. However, the less capital-ve newer techniques, such as powderolymer coating (e.g. electrostatic spray, fh.iidizeded) and electroless deposition have to date madeuch more impact in economic terms; for example,e replacement of cadmium by epoxy resin coatingsuitable cases and the switch from anodizinguminum to coating with polymers. The role ofess nickel coating has been partlynnibalistic in the sense that they have replacedromium plating in some cases, but in others haveund new applications due to the superiorrowing power and unique properties.An example of the commercial benefit ofrface engineering can be found in high-speedist drills [1]. Steel drills have traditionally beenrface treated by a steam tempering process to

roduce the familiar blue oxide finish. Allaborative research program (SKF, Dorrner,ultiArc) has now develop a plasma-assistedhysical vapour deposition process for coating therills with titanium nitride. The use of this coatingitially enabled the drill to .produce twice as manyoles before replacement com pared with thenventional drill. However, the greater cuttingility of the drill led to choking of the flutes byarf. Subsequent re-design of the shape andelical angle of the flutes aided swarf clearancensiderably and the resultant TiN-coated drillhibited a tenfold improvement in life comparedith the conventional product. Cost savingscruing from a longer drill life or faster speeds fore same drill life are typically 30% per hole

There are numerous examples in otheral sectors. Yttria-stabilized zirconia coatingsroduced by plasma spraying provide higherrcraft engine efficiencies by acting as thermalarrier layers on critical cornponents such asrbine blades. Other industries include automotiveearings, 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 engineeringindustry 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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chnology ValueUS$(million) % share Technology Value1000 40 US$(million) % sharerface heat treatment 600 23 Physical vapour 10 36300 12 depositionwder coating 200 8 Chemical vapour 6 22osphating, chromating 200 8 depositionennal spraying 70 3 Plasma nitriding 5 1845 2 Plasma spraying 5 18her (enamelling, etc.) 40 2 Ion implantation 0.5 2

w technology 40 2 Laser/electron beam 0.5 22495 100 Other (e.g. friction) 0.5 2TOTAL 27.5 100

al UK manufacturing output. However, theue placed on such intermediate products canly be very approximate.

BLE 1. Estimated value of the UK surfaceengineering industry

r!. heattment Other(enamel, etc)AnOdizingT New tech'lologieshermal spr avrnq

Galvanizing powder coating

Total value 2.5 billion US $

URE 1. Estimated value of the UK surfaceengineering industry.

e overall growth of the surface engineeringor between 1986 and 1989 was about 10% perr but has now slowed to 5% per year. Aakdown of the new technologies is given inle 2 and figure 2; these processes contribute2% to the total sector but their growth rate hasn almost double that of the established sector.

e new technologies are particularly strong incialized 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 tm orant auo n

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 mostimportant 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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ompanies with a workforce per company ofpically 20-30. The total number of employees ine surface engineering sector is approximately0,000. The business tends to consist of workat is subcontracted from other (often muchigger) companies. For example, about 1000mall electroplating firms serve the UK automotivedustry and a similar number o smallubcontractors service the UK aerospace industry.he electroplating industry is generally a highrnover, low profit margin and small order-bookusiness where custorners tend to demand shortad times. Investment is inhibited by a lack ofnancial resources and a reluctance to write offisting facilities.The surface engineering industry is veryproximately evenly split between subcontractingy mostIy small companies and in-house. by often much larger companies. Therge companies have tended to subcontract workan increasing extent over the last five yearsecause this provides production flexibility,bviates investment and offsets risks. However,ere are now some indications that this trend maye reversed due to concern of the larger companiesthe commercial viability of some of theontractors, the dangers of single-source supplynd the need for closer technological control. Overlonger period, however, it is possible that thealance of subcontracting to in-house processingill oscillate from one to the other due tompeting technological and economic factors.

ORCES FOR CHANGEA number of new factors have arisencently which will force major changes in theurface engineering industry in the next ten years.he nature of the industry at the end of this decadeary much on how it deals with the opportunities

nd threats brought about by these externalNew environmental legislation to controlealth and safety of the workers, effluent andaste disposal, and protection of the environmentutside the factory is causing many processingompanies to examine the viability of their existingchnology base. The pressure comes fromational and particularly supra-national bodies,uch as the European Community and the Montrealrotocol on CFCs. For example, the use ofexavalent chromium salts and nickel sulphate is

ow 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.

RESEARCHThe 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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achine tool). These companies often have theirn research departments but also interact with thest of the UK science base. The surfacegineering science base in the UK is significantd resides in industrial research centres,vernment research establishments, universitiesd contract research companies.The research in the science base is criticallyependent upon funding from industry,ovemment bodies (Science and Engineeringesearch Council, Department of Trade anddustry, Ministry of Defense) and the Europeanommunity (e.g. BRITE-EURAM). However, allese funding sources in this context are directed atproving the competitive position of UKanufacturing companies. Surface engineeringsearch in the UK is thus driven by the needs ofUp to the present time, the subcontractingmpanies have interacted comparatively little withe UK science base and generally respond only toe demands of their current customers. However,ture threats and opportunities, together with theossibility of company mergers into larger unitsight possibly lead to a change in their approachresearch.

XAMPLES OF RESEARCH STUDIES

The final section in this paper provides twoa~ple~ of current research studies in surface

) Integrated optic waveguidesThe recent development of optical fibresaveguides as a mean of transmitting informationas now penetrated into virtually every area offormation transmission, processing and storage.ptical fibres are also used to link computereripherals, are likely to link processors, and aresed as sensors. Planar waveguides play anportant role in optical transmission asterconnections, beam splitters, mixers,avelength multiplexers, etc. [4]. In its simplestrm, aplanar' optical waveguide consists of aansparent thin film of refractive index nj , slightlyreater than of its substrate n2. For shallowcident angles, Snells law of refraction cannot betisfied, total internal reflection occursand theterface acts as a perfect mirror (Sin8>n2/nl)igure 3). Under this condition, the film operatesan 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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mperature of 18000C is required in conventionalhemical vapour deposition for this reaction andis work concerns the use of plasma-assistance toeduce the reaetion temperature. The depositionpparatus is shown in figure 5 and consist of aeated reaction zone maintained at a low pressure-1 mbar) into whieh are fed reaetant gases (SiC4.eCI4. 02) and a readily ionizable gas (e.g.rgon). A microwave generator (500W, 2.45Hz) is used to excite a plasma in the reaction zonend faeilitate the deposition of a thin film on thebstrate. 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 theolecular species by the plasma has enabledeposition to take place at temperatures as low as

50C compared with 18000C in the absence of

02~5iC!4~ ,02~-

Ar_vocuumpump

sIICQsubstrote

IGURE 5. Schematic of PACVD apparatus forbrieation of planar waveguides. MFC = Massow Controller.

The quality requirements of the films centreound their mechanical and optieal properties.he mechanical properties of the films (thiekness5~m) were assessed by indentation fractureigure 6) and the data in figure 7 indieate that thehesion (interfacial crack length, C), integrity andsidual stress (radial crack length, Cr) deterioratedpidly at low deposition temperatures. Optiealaracterization showed that high qualityrformance was obtained in waveguides deposited10QOoCin 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 aerueial arrangement in the internal combustionengine for the conversion of chemical energy tomechanical energy (figure 11). The contactbetween the pistn 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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Revista Latinoamericana de Metalurgia y Materiales. Vol. 13. N 1 Y 2. 1993. 11wn schematically in figure 13 with theirresponding material ratio curves (figure 14).reas only 10% of the metal surface is exposedbear the load after lurn wear on the nodularface, as much as 70% is exposed in the etchedface. The high material or bearing area of thehed surface is beneficial in reducing temperatureld-up and contact pressures, but is also veryective in trapping oil in its deep valleys in anerwise fairly flat surface. The oil retentionume Vo as shown in figure 15, is the volume ofleys below the core roughness and is anication 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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E:J 90'-'::zw 6 01: , : : SiOz-GeOz(Cr)U


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10c o"'O 8.'-~8 . 6

"'O( . .~ 4'O: . . , ( 2~o; . .

O 0.50 2 . 0 0. 0 0 1 . 5 0oroooaolion lenalh c.m 2 . 5 0

FIGURE 10. Attenuation measurements of six moded Si02-Ge02waveguide deposited at 100C 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 ce n -, -

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

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

IGURE 12. SEM micrographs 0 1 ' the srfaces 0 1 'mium plating: (a) conventional, (b) nodular,back-etched.

70%

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

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

O.A.J. Vaughan, "Future of Surfaceineering in the UK", Centre for Exploitation ofnce and Technology, London, 1991.. T. Gawne, Trans. Inst. Met. Fin. 67 (1989)

.T. Gawne, N. Nourshargh, I. KandasarnyE.M. Starr, Surface Engineering, 6 (1990)

F.G. Arieta and O.T. Gawne, Brunelsity. To be published..O. Arieta and O.T. Gawne, J. Mater. Sci.,(1986) 1973.

surface engineering advanced materials for industrial application

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