Fundamental properties and tribological wear behaviour of sputtered CrCrN coatings on light metals

Sur~a~vand Coatn~ Technology. 57119931

77- 80

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Fundamental properties and tribological wear behaviour of sputtered Cr-CrN coatings on light metals* W. Herr, B. M a t t h e s a n d E. B r o s z e i t lflstitute of Malt,rtat~ Science. Tcchericat Unit,er.sit.r of Dannsladl. GrqfeetstraJk, 2. D ~ IOl~ Darmstadt ( Germany/

M . M e y e r a n d R. S u c h e n t r u n k AfBB Deut.*x'ta. Acrospat'e. Ceofrat l~lborator),. Oel,artment of Sllr~lee I;tmshing. P.O Box ,*¢01109. D.KfMHIMunich 80 (Gt'rman)'t

tRcccivcd S¢gtcmber 30, 19~2: acecptcd in final lama October 30.19921

Abstract CrN hard coating~ v.cre deposited by sputtering onto pu.e titanium and titanium alloy TIAIfV4 in order to tmpro',c the wear protection and the oxidation resistance of the substratc matcnals. To obtain dense coatings with high hardness and good adhesion to the subslrate, the bias vohag,,.'~ (Lf., d.c.} were varied systematit.-ally and different thin intcr'mt."diatc layers {chromium. NICr) were deposiled prior to sputtering at the C r N ctmtings. Furthcrrne, r,~. the influcnc~ of thermoshtxi treatment (fast coohng from ?ll~) (" to 10 C in ~.~,atcr)on tfic Iilm prt~Ixrlies v,a.,,analysed, l"hc wear bchaviour of the coatings v,as es aluatcd asing a plate on cylinder model wear tc~t apparatus.

I. Introduction

2. Experimental details

Light metals and light alloys are increasingly used in mechanical engineering, automotive and aerospace industry. Today their lifetime is being more and more reduced by harder a orking conditions leading to increasing friction and wear, more corrosive environments and higher temperatures. Titanium and its alloys can be used up to 500 C. This limitation is due to the increasing tendency for the diffusion of hydrogen, nitrogen and oxygen into the surface at higher temperatures, which leads to embrittling effects and therefore premature failure of the components. With vacuum deposition techniques, these diffusion effects cannot occur because of the absence of these gases. Investigations of CrN coatings on steel substrates have shown that this hard coating can be deposited with excellent corrosion and ,.sear properties [I]. We have therefore investigated CrN hard coatings by sputtering onto pure titanium substrates as well as onto titanium alloy TiAI6V4, which is commonly used in aerospace industry. The Cr'q films ,'.,ere optimized with regard 'o high hardness. ~, (lense microstructure and good cohesion adhesion properties in order to obtain maximum wear protection and oxidation resislance for the substrafe material.

The substratcs were deposited i:'~a d.c. magnetron sputtering plant 7. 700 from Lcybold Hcraeus. with r.f. and d.c. bias. Prior to the deposition, the substrates ,acre chemically cleaned in HI-" HNO~ solution, folloxx.ed by intensive ion etchin~ in the *..3CtlOmchamber to Fcdnec the oxide layer of the substratcs l'hc ('rN coatingx ~crc deposited by varying systematically, the bias ~oltagc. After film deposition, sen'to of the coaled ~amptcs wctc removed into a heating chamber and heated up to 700 C under atmospheric conditions. Following the heat treatment they were fast cooled in an ice water mixtnre. This last step is called "'thermosho2k-treatment "'. In order to increase the adhesion of the CrN film. in some cases thin intermediate la:. "s of -nromium and NiCr were sputtcrcd. The s0bst~atc temperature during deposition was less than 250 C, the thickness of the CrN-coatings was in all cases 14_+ I p.m. The deposition rate was about 15 pm h t for chromium and 10 ~.tm h t for C r N Table I shows the deposition parameters The Vickers microhardness of the coatings xsa~- measured by a I.eitz microhardness te:,ter with loads of 15' and 50 g. To evaluate the coating cohesion and adhe,,ion. a CSEM scratch tester was used with a Rockwell C d i a m o n d The critical loads L,t (cohc~,'ive failure) and L,,, (adhesive failure) were determined b? measuring the acoustic emission and tangential force and verifying by optical microscopy.

*PalX:r presented at the lath International Conference on Metallurgtt:al(oating'~andlhin I-ilms. San l)lcgo,('A, llSA, April6 10. 1992.

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Thc structures or the Iilms were analysed by exanlination using fracture section analysisand X-ruy diffraction with Cu Kx radiation by a thin lilm Guinier goniorneler. T h e chemical c o m p o s i t i o n was analysed by Auger Sl',cctroscopy and vcritied by WDX-analysi:,. For evalualiou ,,if the v,.car re~,istancc, the l'llale on cylinder lesl ,A.ax chosen [Fig. I I. A coaled plate ~sa,, picsscd v, lth a dclitlcd load L agaim, I a rotating c.', lindcr. [)ttrmg lhe ~ c a r tc~,t the [riclion co¢t|icicnt v.as dclccicd b} a slrilitl gtll.lge ,~l'lcr Ihc tc.,t rfftlci~dllfC the plate illld c y l i n d e r wcrc l e m o ' , c d iilid tile UCill" ~,t'Jl[lll~C I1]elin[lli2t]

b3 cl r~rolilonlt.'lci. "Flit.' c} Iintlcr', x~¢tu' ~l'Otiild..-10 lllnl it: d i u m e l e r , made o f c o l d w o r k e d slcel. T h e s l i d i n g sp,,:cd was 0.4 m s t, the s l i d i n g distance 30(i m. T h e l l o r m a l l o a d in the conta¢l area was 100 N. ri~c a t l / b i o n t temperart.ire was 2(l C £ 2 (7. the humidity 50%, ± 3'!.i,,

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3.1. k uudamental propertien 3.1./, llardue.Ys ,NIoJIhcF the d c p o s i l h m p 3 r a m c l e r bias v i i l l a ~ c n o r I l l c lhcrmoshtlck tre;llnlcnl h;t~,'c a ~.iguifieani ~.,ystcmali¢ influence on the hardness IPig. 21. F o r all C r N coatings, hardness values of a b o u t 2000 __ 200 H V 0.05,'0.015 were measured. Thesc m e a s u r e m e n l s agree very well v,,hh h,lrdnes,,~ valucs of C r N c o a t i n g s investigated by oilier a u t h o r s [2, 3]. 3.1.2. Stru(ture A l l c o a t i n g s s h o w a v e r y dense, n o n - c o l u m m l r s t r u t lure. l-'igurc 3 shows a S E M pictunc o f a C r C r N coating. A crack, probably caused by the fracture treatincm, was slopped in the i n t c r m e d i a l c c h r o m i u m layer. which points t:~ the good t o u g h n e s s of this c h r o r n i u m hlyer. W h e n treated w i l h I h c r m o s h o c k (7110 ( . ' ~ 10 ('). tl'lc

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trace could be sccn at very 1o~ loads of about I0 N. ('oatings deposited with d.c. bias did not sho~ thi,, behaviour in the scratch test The first crack,; appeared shortly bcl~)re adhesive |ailurc (dclamination of the c~"ating). Notable for all L'rN coating,, was ;t ,igrulicant

3,1.3, Chcmic,d t'~m]p(,xiti,,:~ For the analysed CrN coatings, which were deposited using a nitrogen flow of 50 standardcm 3 r a i n ~. a composition of [N],,'[Cr]=0.47 was determined by Auger spectroscopy and verified by WDX-analysis. The nitrogen content in the coating could not be raised further by increasing the nitrogen flow rate to 20 standard cm "~ rain t. At the surface of the coatings, a thin oxide layer with a thickness between 50 and 1(10nm ,,,as dctc,:ted ('or titanium substrates. For CrN coatings, deposited onto stccl substrata, this oxide layer was not dctccled by Augcr analysis.

inurea.-.c in the cohesv.e and adhe,-,r.c [mLar¢ alter th¢~+ moshoek treatment. After thcrmoshock treatmem, high critical loads of 45 N (Led and greater than 60 N (I,,:1 were measured, which were not influenced b,v the bias voltage Id.c. or r.D. The reason for this increase was a reduction in internal strcsses, which was confirmed by SEM analysis of the scratches. Thermoshock treated samples reveal only small delammations, compared with untreated ~mples (Fig. 6). The adhesion of the coatings was also influenced by the substrat¢ material and could b¢ increased ~,y the deposition of intermediate layers. Figure 7 shows that all coatings deposited onto the titanium alloy TiAI6V4 show higher adhesion v;dues, compared with pure titanium. [-'or the coating deposited with an NiCr intermedi-

3.1.4. Adhesion The adhesion and cohesion of the coaling were strongly influenced by the bias voltage, substratc material and the thermoshock treatment. Figure 5 shows the inflnence oftbe bias vohagc and thermoshock treatment. For all layers, deposited by r.f. bias, cracks in the scratch

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Fig. 7. Adhes.ion of (. rN coating ~)'stcrns on Ti64 and Ticp

ate layer, the highest L~ values of about 80 N were measured, ?t.2. Wear properties The wear bchavioar was tested with a plate on cylinder test apparatus. The CrN coatings show excellent wear properties, i.e. no measurable wear could be detected by a profilometer, neither on the plates nor on the cylinders. even under thesc severe test conditions. Yet there was local delamination of wear particles from coatings deposited onto Ticp (Fig. 8). This adhcsivc failure of the

4. Conclusions The investigations show that CrN hard coatings can be deposited onto Ticp and Ti64 substrates with the followin 8 properties. High hardness values of about 2000 HV are obtained, which are hardly influenced by the deposition parameter bias voltage and thermoshock treatment. Very dense structures are obtained. The morphology changes from well crystalline to amorphous by Ihermoshock treatment. The coatin~;s had a Cr2N structure. No CrN phase could bc deposited, evcn at high nitrogen flows. Good adhesion values, which were strongly influenced by the bias voltage, were found. D.C.-sputtered coatings show bettcr cohesion values. Thermoshock treatment increased the adhesion of all coatings. For coatings deposited onto Ti64 substratcs, higher critical loads, comparcd with Ticp, were mcasured. By depositing intermediate layers of NiCr and chromium the adhcsion could be increased. The coatings had excellent wear bchaviour, tested with a plate on cylinder tribomeler under severe test conditions.

Aeknm, v l e d g m e n l

This work ss;t~, supt'~or(e,,| 1)~ the I~l~.mdcsmmisleruna ffir Forschuag ulld Tcclmologie under Grant 13N5823

References

: i

L~ I- tg ,'.; Lt.,cal dam;i.~c tff a '.,.cat t,csted CrN coaling t~n Ticp r.ul.strate

I A Aubcrt. R. Gdlct. A. G,:lticher atld J. Tcrrat. 7h01 Sohd fih~l~. Ilh~ (19R3) 165 172. 2 W D Mdnz. Rcacti','c Sputtering o1 ,iznde~, ~md calrhidcs, Rt'p 11-50 7.2. Lcybold-lletaeur, tTil'r,bH. Hanau. ItJ85 ) S Konli)a, S. (Jn~. N. t:n.ct.l and T. Naru~cs~a. ]hipl Solid I.'ihux. 45 (1977)433 445