Mechanical stability of DLC films on metallic substrates Part II — Interfacial toughness, debonding and blistering

"rhia~ Solid Fihns 312 (1998) 210-227

ELSEVIER

Mechanical stability of DLC films on metallic substrates Part lI Interfacial toughness, debonding and blistering X.L. Peng, T.W. Clyne I)el~#'lolu,I ot'M~tl~'ri~d.~ .~'~'i~',~'~'.~~#,1 M¢lallur.~y. (',mbrid.k,e I,~nirer.~ity, Pembroke .~'trecl. ('~#nln'id~c ('112 .¢QZ, I "K

Received 25 April 1007: :.L¢¢epled 3 Su'r~tenlhu'r 10'}7

Abstract DLC ¢oatjllgs Iiav¢ heell deposited onto four metallic suh,,~tlates, and :ll.,,o olltt~ silicon, with the hiterl'~t¢¢s bejllg prepared in ',:,,'thus ways. Films with a lq.lll~e I~ll" thicknes.ses have been pried(ICed and specimel~.,, have been subjected to cl'~anges il~ lemperature. Fron~ a knowledge Of the 0'enidtLal stress levels in these Iilnls ~.lsa It|llctjoll of tenll~eralure (see P~.lrt I), the strain etlel~) relea.,,e rate for it+tei't'acial debontlitlg has beell nlollitored tltli'jllg deposiliol~ illld stlbsetluenl lenlpel'~.ltLll'~ ellIs.lIlies, "File value of the Mt'aJI~ energy release rate at u point when intert'acial debondin~ occurred ha.', been taken :~,s tt]e intcrfucjal totighnens (l'raclure ellel'~y), It is concluded t~om lhQ,,,e ob.sei'vations that lhe jnterfacial Ioughness is low for DLC filnls deposited Onto mecharlically-polished surfaces of lilaniurru mild steel or stainless steel, hut quite high on atuminitmL Pre-ctealdng by bomb~lrdmenl with energelic ai'gon ions raises the inlerl'acial Iouglmess for the steels ~.|nd for ~dutlliniunL but Ilot for titaniuttl. "l'i]ese obsel'Vatjolis hWve been Iet~.|led to the TR|ILII'e of the oxide I]hl~s on these stlhsll'ales. A thin sPultel'ed interlayer of a.hllllinil.lnl w~.|s l'ourid I~ raise the interl'acial Iou~htlens sigrfil'ieanlly for tile steels ~.tlld I'or titanit, nt. Howevcl-. experinlcnln wilh silicon suhstlate specimens showed th[|t the good interlacial atlhesjon ohtaitlu'd ,,,.ill) AI jnterluyers is severely impaired by healiiL~ in v~.zctturn to IemPerattzi'es of th~ order of 50()GC. II hus been e~lablished that thin hllpairment of adhesioi~ is ~.1¢oltsetluel|ge ~1" ihe Ielea.,,e of.' r entrapped in tile AI dtlritt~ sputter deposili~m, which I'~rmuole.~the ftu'mation of blisters and delaehnletll of Ihe film. a~ 199N Elsevier Sek ;¢e S.A. Km'u'r.'d.L" Adhc,',ion: ('au'hon; I')]anumd; ell'e:',,.',,

I. Introduction D L C coatings have strLlettlr~.ll fe~.lttll'es whi¢ll give rise to various allt'[lclive I'~ror~erlies. Many of these features are dependenl oi'~ the bombardnler~t by ions and atoms which lakes place during deposilion This hon'd'~ui'drnent, however, also tends to give rise to siotlil'icaill levels or residu:d stress, which is normally compressive in tile approximate range of - 0 , 5 - 2.0 GPa ~.see Part I). These high .sirens levels have seVel'~.|l consequences ill terms o1' tile perils'.) rnance of the C{)ltlJllg, l rominellt aniong these is tile I]|L.'I that, partieLil:.lrly when ihe co~.ltirlg is relatively tilick, there is u strong drivillg force for it to becolnC detachetl, "i'lfis driving force is best expressetl as tile strain energy release n'ute for interl'a¢iul dehontlin~g. (;,, ~,llld,/or tile as,soot;areal stress intensity f;.|ctor, K,. Wr~erl these i~arameters reach :q]lm,)priate eritieul values, v,'hicl~ eharaclerise the Iotl,,hhess or the interface, then del~onding is exl'~eeted to Oek.'Ltr

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[ I - 5 ] . For ex~.mlple, delwmdilv-,e IS expected Io occur when (; I reache.~ the eriti¢:.d interl'aciul strain e n e t g y t¢lcas¢ rate, or il'lterf~lcia.I ftacture erlergy, GI,:. hi practice, Ihct'e may be ,~Olne barrier Io imti~.ltiorl o f tile iiltet-I'a¢iul ¢t'a~ck [ 6 - ~ ] , ,,,o thai a hi~her dri,,in~, force Ihatl that pt'cdided tni.~hl in f a d be rlecessarv. Ho\;'ever, inilJlllJOn o f debol',.lirlg may occur reatlil~,, rronl ed,,',es v.llere tlav,,n are inevilablv presetll, I~arlieulal'ly for rehltively thin fihlls [8]. There are also \'~Lriotis issues collcerlljll.~ llle exact nlechanisms hy \vhich tIle I'ilm ma\' r~ecnme detached, including the t'racttz|'e i)'teeh:,lnit.'s of tile buckling r, roces.,, [ 0 - 1 2 ] altd tile possit',ilJty of gus accumulation at Ihe interl~ce iJromotJn~ the initial debontling [I 1.13-16). DI.C films have also been o~Jserved [t 7] to buckle when expo.sed to air for prolonged period~, There has also been intere.,,1 11~-201 ill the de~clol'unent of tlu'ough-thickne.s.', ct'ackn ,,,,ithitl ¢oathl~,s as u rCsLIII o1" resid[l~,ll sIren,sen. The fornlaljoll of these mu3 al'fe¢! the onset of dehonding. There has been \ e l \ little ,~)Melllatj¢ ~,Vork oll the re.sjdual str¢.,,n levels, cortesl'~olldin ~ ,,,truth enet'~) release

22(1

X.L Pc/~g. 7: W. ('l[+~'tl(' / T h i , Solid Film.v 312 ¢ I(~
rates and interracial toughnesses of substrate/DLC coating systems. There have. however, been various attempts to explore in a qualitative or senti-quantitative way the factors which affect interlitcial adhesion. For example, il ha:, been shown thal adhesion of DLC film,~ to certain sul~strafes can be improved by surface pretrcatmenls [21] and by using thin interlayers such as silicon [22] or ,'fluminium [23,24], or by generating DLC/merdi mullilayer structures [251.

DLC films (particularly if hydrogenated) lend to be themlally unstable [26], since heating can rest, It in the release of hydrogen and translbrmalion of sp ~ carbon to sp -~carbon (graphilization). Furthemmre, it has been noted [27.28] thai inerl gases present iq the atmosphere during deposition, which become entrapped within the DLC structure, can be released during healing so as to collect at the interface and prornote the formation of blisters or even fracture of the films. The question of lhermal stability is particularly important if the DLC films are being employed as wear-resistant protecti,,e coatings, since lhe frictional heat generated during exposure to abrasive or erosive conditions might cause significant increases in temperature. Complete thermal stability is very difficult to achieve, since most DLC structures are thermodynamically unstable and have an inherent tendency to revert to graphite. Hydrosen-free DLC, lbrmed using solid carbon sources (ionbeam. magnelmn sputtering, arc-discharge, laser ablation, etc.) often have higher sp ~ contents and greater thermal stability [29] (up to 700°C). However. formation of snn, II graphitic particles lends to occur during deposition of solid carbon source DLC films, leading to degradation of surface smoothness and of certain properties. Although magnetic filtering [30] and pulsed laser deposition [29,3 I] can reduce or eliminate the formation of such micro-particles. RF glow di,;charge using hydrocarbon gases is still the most widely used approach, since it ix well suited to the deposition of homogeneous DI.C films over large areas. The current pair of papers concerns the nlechanical stability of DLC films prepared by RF methane glow discharge. In this second paper, the debonding and blistering behaviour of DLC films is studied over a range of temperature and as a function of the residual stress levels and interracial structure.

2. Experimental procedures

2. I. Spt't'imetl I.'e/~#~llio/I

(tIlt/,vl/'llt'ftet~d

(5~l//tillaljol(

jetted to bombardmenl tiorn 1.111Ar or (Ar + I(Y;~ CH~) plasma for 10 rnin and/or deposition of :m AI bonding layer, using a DC rnagnelron sputten'ing source in an Ar atmosphere. The sputtering target was alurnir|ium, of 99.9e,4 purity, cleaned by in-situ pre-sputlering for I0 rain. Some of the Al-ccmted substrates were then exposed to air. The deposition chamber was pulnped down to below Ill r, Pa and a pure ineth:me atmosphere introduced for DLC deposition. Full details of the DI.,C deposition conditions are given in P,'u't !. The DLC fihn thickness was measured direclly OI1 the cross-section, t,sing irnage display software on a JOEl. 58(XILV SEM. Specimens were also examined at various stages using optical inicroscopy, SEM, Raman and FTIR spectroscopy. Mosl of the DI,C deposition in Ihis study was carried OUt al --11)5 V negative bias and !0 Pa pressure, since these pmamelers generated the highest intrinsic stress (around - 2 GPa) ~u DI.C films and therefore the highest driving force Ibr dcbonding at 1.1given fihn thickness. Details are given in Part I.

2.2. Measurement o.f imerfir'ial lott,ghm,s.~' 2.2.1. Pm'l delmsilio, in.vw~'liofl Residual stresses, and hence strain energy release rates for fllterfacial debonding, are generated dkning deposition ai!d also during subsequent temperature changes, provided the coating and subs(rate have different then'real expansivities. The strain energy release rate. (;,. is also dependent on the thickness o1" tile fihn (and of the substrate, unless it can be taken as beillg infinitely thick). The following expression [32] can be used.

(;,=f

,,

#(y)~ ~ , / : " (d y ) , ' - - J '"

h ~r,.(y): , ~/ :d " ( ,y ")

(l)

where h and tt 1.|re the thicknesses of I'ilm and substrate. E'(r) is the effective Young's modulus as a function of depth and ~r(y). ~r,(y) are Ihe stress distributions before and after debonding. In the absence of comprehensive data on lhis, the effvclive Young's nnoduli of all the DI.C films produced in the present work have been taken here as 174 GPa. which is the value reported by Blech and Wood [33]. However. it should he recognised thai this is an approximarion, since the Yotmg's modulus of DI.C films has i~een shown [34j to vary significantly with bias voltage. For an effectively infinite subslrate (h << tt), in which the varialion with depth of tile stress in the I'i1111C~.lll be neglected. this expression reduces to: h~r7~

Substrates of silicon (8 × 30 x 0.38 0rim), titanium (8 X 30 X 2 ram). Munliniun~ (8 x 30 x 1.5 illnl)o 316 stainless steel and mild steel (both 8 × 30 × 1.5 ram) were polished to a mirror finish (0.25 u.m diamond) and ultrasonically cleaned for 21) rain in acetone, blown dry and placed in tile deposition chamber. Selectcl stlbstrates were then sub-

('J71 =

-,

(2)

assuming thai the strain energy stored in Ihe dehonded film is negligible. Since the detached fihn i'ragmenls invariably exhibited lillle or no curvature, this approximalion has been used in tile present work.

X,I., P,,ng, T, W. Clym" / 771in Solid I.'ilms 312 (1998) 219-227

Provided the generation of residual ,;tress can be predicted with confidence (i.e., the deposition stress and deposition ternperature are known, and the system ren, ains elastic during cooling), information about the critical strain energy release rate, G,~. (interfacial fracture energy) can be obtained by observing whether a fihn with a particular thickness has debonded by the time it cools to room temperatt, re. It" the assumption is inade that G,~. is given by the wdue of G i when debonding occurs, then an observation that a particular ,. ~ating remains attached to the substrate provides a iower bound on Gi~,, whereas if detachment has occurred, an upper bound can be deduced. A series of experiments was, therefore, carried out by simply depositing films of various thickness and observing whether or not they had debonded when the chamber was opened after deposition and cooling.

~Warm Dry Air Perspex Viewing Windows

I

-

[]

I."

i

I,

i,

2.2.3. Viewing during i~ost-deposititm cooling For specimens which had not debonded after deposition and cooling to room temperature, further information about the interfacial toughness can be obtained by observing the specimen during subsequent cooling below room temperatare. Since in most cases the thermal expansivity of the substrate is greater than that of the DLC coating, such cooling will generate compressive stress in the DLC. enhancing the deposition stress and raising the value ot' G,. Cooling through 200°C will, lot a typical expansivity inislnatch of 10 x 10 ~' K i, generate a film stress (on an infinite substrate) of about - 0 . 3 5 GPa ( = A a ~ T E'Dtx,), This is significant compared with typical stress levels at room temperature (see Part !). so that pronmtion of debonding by this melhod should in principle be possible. at least lor most metallic substrates. The cooling operations were carried out using the apparatus ,drown in Fig. I. Typically. the specimen cooled from rOOlll temperature down to about - 193°C over a period of about 30 rain. This is convenient for continuous obserw~tion and for accurate recording of the temperature at which any debonding tnighl occur. 2.3. Amu,aling e.vlwriments Experiments were carried out to explore the effect of heating on the adhesion of I)I~C fihns. Silicon substrates were used for this work to minilnise the thermal stress and the possible interfaciat oxidisation in a rough w.icuunl

Aluminium Observation Chamber

i

J

Cooling Block

I" x ? " :

2.2.2. Viewing &o'ing d~Tmsition For some experiments, the sputtering target flange (see Fig. I in Part I) was replaced by a glass viewing window. This allowed direct in-situ observation of debonding during deposition. When debonding occurred, deposition was stopped and the film thickness was measured as outlined in Part I. This allowed the value of G,¢ to be determined from a knowledge of the deposition stress (see Part !) under the condilions concerned.

221

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IL x

'

t I,

ii!

Thermocoupleconnectionsii[:i.........

~ Rotary Pump

}J lLiquid

Nitrogen

Fig. I. Sd~emutic repre.~enlation of the cryostatic cooling chamber and viewing s: stein.

during heat treatment. Specimens were heated under vacuum (5 Pa) from morn temperature to 550°C. at a constant heating rate of about 20°C per rain. This was carried out in a chamber with a viewing window. These specimens were inspected visually during heating and in the optical microscope/SEM after they had cooled to room temperature. Deductions were made about the conditions under which any observed debonding had ~x:curred.

3. Debonding behaviour 3. i. Low temperature th't~onding

Observations on tile conditions under which debonding occurred are summarised in Table l. which gives inferred values of, or bounds on. G,,. (taken as equal to G L at the point of debonding). These results are also presented in Fig. 2. Several features are immediately apparent. For the titanium and lbr both types of steel, the toughness of the interface formed by depositing DLC directly onto the substrate is low (G,,. ",-7 J m :). It is very difficult to deposit films of more than about 0,3 /am in thickness

without them spontaneously debonding. On aluminium.

X.I,. i'vm.,. 7: W. ( ' h ' o e / Thin S o l i d I"ilm.+ 312 ( I<#).+0 _"~ 1 It- _~ , 7

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Re+,ittual +qf'e,e, lexels (.'lnd herlee strain etlt:rg.'+ reJeil~,e I';.lldh) Weft,' calcltl;lled using depoxilioll xtr'e,,s values (see Part 1} ]till1 tile folhweJl|g therlllal e\pun,,ixilydilta[371:

+~1~t.<.=2.3.'< 1(1 n K

~. c t l , = R . 5 ×

The l ) l . ( ' tlepmitiu,~ eundilitms lhr all o f th¢5¢ .,.,pl:t.'illl¢ll:.,

111 " K %l,rl,.'re

r. ++,q~= 1 2 . 2 x

however, the adhesion is much better, with a value <.>f G,~. in excess or 101) J nl 2. This is probably due It) the different nature of the oxide film on aluminium, wl+ich is normally very thin and coherent. Tile DLC might be expected to bond well to a clean, thin and adherent alun l i n a layer. In tl)e case of titanit, m. tills low toughness was not xignilicantly affected by introduction of an Ar bombardmerit pre-cleaning ot" the substrate. However, for the two steels, and especially Ibr the stainless steel, such pre-cleaning effceted a large increase in G,,. This observation is r~resurnably related to the nature of the surface oxide filnls I'ornled in these systems. Such it lihl) could net oe elirnisated i'rtml titanium in it tccllnicat \'acuunl by a simple 400

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bombardnlenl process. Tile same is role Ibr aluminium, hul the high value o1" G,~ observed in the absence of pre-eleaning was increased significandy by argon bombardment to over 350 ,I m :, Tllis is preslmlahly associated with renloval of gerteral contanlination SllCJl its hyth'ated layers (Itl the surface of the itlunlill;l. For tile steels, tile stlr|'~tce t)xide I'illns were presumably removed by the bornbardm e r i t prucess. In view of the excellent adhesion exhibited on aluminium, an AI interlayer offers promise as a method of improving, the bonding. It was indeed fotmd tl)at a lllit! (80 rim) layer of AI deposited on the substrate prior to DLC deposition raised the interfacial toughness substantially for titanium and for both steels. The importance of surface oxide in contact with the DLC was highlighted for this case aim by introducing a stage in which tile aluilliniunl interlayer was exposed to air. allowing formation of a thin surface oxide. (Ill fact, there would be ;ll least +.1monolayer of oxide present in any event, bt~t this would tend to thicken appreciably and also to form weak hydrated layers on exposure to air). Introduction of the oxid:ition stage resulted in the measured interfacial I
223

X.L. t'eng, 7+,W. Clym, / 77+i. Solid I,'ilms 312 1 ItJg,'O 2 I~- 22 7 Table 2 Deposition conditions and debonding behaviour of Si substrate specimens subjected to heat I:,~ ,~tments Specimen details

Detmnding

Su~trate ( m s )

Pre-cleaning condititms

DLC prelayer (p.m)

AI (/J.nO

Oxidise

DLC ( p r o )

Temp. ('~C) G,. (J m- : )

Appearance

St. St, St. St, St, St, St,

Ar, I Pa Ar. I Pa Ar-II)C], CH~, 1 l"~ Ar-1()9~ CH.~, 2 Pa Ar, I Pi, Ar. t P+'l

(I 0 0 0,02 0.05 0 0

O i) 1) 0 0 0.08 0.(18

¢ ¢

0.8 5.7 4,5 t,0 1,0 0.1 -

41) 41) 541) 50+,) 451) 450 4(XI

D L C / S i debond Fructured within Si D L C / S i del'amd Isolated blisters tFig, 6at Linked blister,'+ (Fig. 6b) Wrinkles (Fig. 7) Isolated blisters (Fig. 8)

0.38 0.38 0.38 0.38 0.38 0.38 0,38

17,8 127 97 19.2 19.5 2.0 ~

Residual stress levels (and hence strain energy release rates) were calculated usi,g deposition stress values (see Part !) and the following values [371 fur the thernml expansivity of DLC and silico.:
that debonding occurred between the interlayer and the titanium substrate with the unoxidised AI interlayer, but that the adhesion between the interlayer and the DLC film was weakened by the oxidation and the interlayer was left adhering to the titanium substrate in this case. The measured toughness was thus one between the DLC and an

T

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oxide layer, as for the cases of metal substrates not subjected to pre-cleaning or interlayer deposition.

3.2. High temperature debomting Before considering the behaviour observed on heating, the expected effects should be briefly outlined. Since at)l. c < a,,,,, in all cases, tensile stress will develop in the fihn during heating. However. |br the DLC-Si system, this change is less than +0.1 GPa for a 500°C temperature rise. This will have only a small effect on the deposition stress ('-- - 2 . 0 GPa for the cases considered here). In principle, therefore, the driving force lbr inteffacial debonding should ~ecrease slightly during heating. In practice, heating may effect a more substantial reduction in this driving force, because the DLC will tend to undergo structural changes in which sp .+ bonds are convened to sp -+, with an associated reduction in stiffness and hence in the residual stress level [35]. It follows from this that an observation of dcbonding taking place oa heating must be

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Fig, 3, F.I)X spectra li'oll| Irt+e ntl,rlitees produced with tilanium substrutes and aluminium interlayers (at without, and (b) wifll prior oxidaliol| of tile i,tcrtaycr prior io DLC del~Ositiol|,

No heating

NO layers DLC pre.layer AI interlayer Interface

I:i~+ 4, l-xperhl~entat data siloxvin~ the effect o|1 the illterfaciat tougl|lle~s of variotts llVatmellls It+ th¢ sltbslr~it¢ prior It+ deponilion o f DI(" on Si alid s,ubseqttettt hcatht+ Its ;.WoUnd 51~)'("

224

A'.L, IYng, "I~W ('lyric/lhin Srdid ~"i/ms 312 ¢1~9,~'~ 219 227

duct() a redtnction in the effective intcrlitcial toLnghness. Therefore. whil,- values for G,,. can be estimated l'rom an observed ten|pcrattlre of debonding, using the same procedtlres as in Section 3. I. it should be noted that these values may be overestimates a:; a consequence of the fi=ll in film stiffness and hence in f i l n i stress. Results of the lleating experiments are st~mmarised in Table 2 and Fig. 4. The first point to note is that the interfacial toughness of ~ 'standard' (pro-cleaned) Si-DLC interface is relatively high. In experiments of the type described in Section 3.1, it was found that when large driving forces lor debonding were generated, they aCttl,'flly caused fr,'~cture wifllin the Si substrate, rather Ihar! at the interface. Such observations allowed a lower bound to bc established on G,, o f about 130 J nl -~. Heating o f specinlens which were intact ;n room temperature produced debonding at about 540°C. corresponding to a G,, value of about 100 J m -'. Such heating evidently has only a minor

effect on the interfacial toughness. However, this situation changes when iI1¢ pre-cleaning is carried out with methane present in the bor;~barding gas. When this is done, some DLC is deposited during the cleaning process and this is sometimes thought to inlprove the interti|cinl strength. In fact, it is clear ill',~t this process leads to a ,veak,:r interface on subsequent heating. Blisters tend to form, followed by extensive detachment, and the apparent inlerfacial toughhess falls to below 20 J nl "~. It seems likely that debonding can be promoted by the nucleation and growth or a gas bubble at Ihe interface. The effect of this is Io reduce she apparent interfacial toughness. The initiation and subsequent arrest of the growth of blisters is depictcd schematically in Fig. 5. A potentially important factor is the decrease in crack opening (mode i) stress intensity l~lctor at the crack tip as the blister are;~ cxpands [I I]. On almealing, hydrogen (unbonded and bonded), and possibly hydrocarbon gas and argon intro-

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-i Fig. 7. SEM micrograph showing wrinkling after heating (ff a O.I p m DLC fihn on a Si subs|rate, which had been pre-eleaned in :m atmosphere of Ar under a pressure of I Pa and an AI interlayer |lien introduced by sputtering, in an atmosphere ot" Ar at 0,6 Pa,

A further point to note concerns the possibility of argon entrapment in an inter-layer deposited to improve adhesion. Even with a thin DLC layer (low G~), uebonding can

Fig. 6. SFM micrographs showing blister tk~rmatio,i and film detachment •trier heating of DLC films on Si. which had been pre-cieaned with an atmosphere of At-I(FT~ CH.~ under pressures of (:0 I Pa. and (b) 2 Pa.

dueed during bombardment, may be released from within the DLC. This may promote local interfacial decohesion. !o facl., D[,C fih'ns tend [26,36] to be highly permeable to molecular hydrogen, making it unlikely that arty build-up will occur. This is, however, unlikely to he true for most hydrocarbons or for most of the inert gases. Such gases could, therefore, accurrmh, e at the interface, generating significant gas pressures. In the present case, ti~e important effect is clearly that the argon becomes entrapped within the thin pre-layer of DLC formed during the pre-cleaning process, since no entrapment apparently takes place during normal DLC growth (when the atmosphere is pure methane). Such entrapment is more pronourkced with the higher gas pressure or higher methane concentration in Ar during precleaning, because this will resul, in higher DLC growth arm thus more Ar entraprnent (less time for At" to diffuse outwards before it will bc buried by the growing DLC film). This is clear from the micrographs shown in Fig. 6. which show how extensive blistering and detaehntent o|" the coating ha,~ occurred. Evidently, an argon bombardrnent cleaning process in which a pre-layer of DLC forms is likely to be unsatisfactory in terms of the n~echanical stability ot' the l'ilm on heating.

I:iL~. 8. SEM micmgraph,~ showing bli,~ter I'ormatiun al~er he~lin~ ~t" a Si substrate with ail ~t) 11111AI co,~.ilin~, "|'i~e suhslrate had been pre-c]caned hi an atmO~l~hcre of Ar under a pre.~.,,ure of I Pa and Ihc AI tl~en introduced I~y sputtelin~, in ai~ aimt~.,,pilere ~|" Ar nt l).(~ Pa.

226

X.L. i ¥ . g , 7: W. (.Tyro'~ Thin Solid Films 312 (i~)t~,~) 2 I~)- 227

be promoted by heating a system containing an AI interlayer. The result of heating a specimen with a I00 nm DLC layer and an 80 nm AI interlayer (oxidised) is shown in Fig. 7. It is clear that extensive wrinkling of the film has occurred. This is apparently a consequence of the build-up of Ar released from the AI interlayer, reducing the apparent interracial toughness to a very low value ~ of about 2 J m -~. That this debonding is associated with problems :arising in the AI layer is apparent from the observation that blistering occurred on heating when only the AI layer was present. This can be seen in Fig. 8. This is an important observation, since it means that conditions during the production of such an interlayer may be critically important if the danger of debonding on heating is to be avoided. For example, entrapment of Ar during sputter deposition of the interlayer could in principle be reduced by using a lower Ar pressure a n d / o r a higher substrate temperature (allowing the any entrapped At' to diffuse out of the layer). In practice, such entrapment may be diMcult to avnid. This is an area requiring further work.

4. Conclusions The following conclusions can be drawn from this work. (1) When DLC coatings are deposited under standard conditions onto mechanically polished substrates of titanium, mild steel or stainless steel, debonding tends to take place readily. The interfacial toughness lbr all of these cases has been estimated at about 7 J m--~. This low value is attributed to the presence of thick, friable oxide layers. For aluminium, on the other hand, the toughness was quite high, at over !00 J m - ' . In this case, the oxide layer was presumably relatively thin and coherent, so that it did not readily promote debonding. The DLC apparently adheres well to such an alumina layer. (2) For the two steels, an in situ pre-cleaning process applied to the substrate, involvi,~g bombardrnenl by argon ions, raised the interfacial ~'oughne,,,s considerably, to values in excess of 200 J m -'~. This is presumed to be associated with the effective removal of the oxide layers. in the case of aluminium, the toughness was raised to a very high value of over 350 J m ~-~. This may have been caused by a small reduction in oxide layer thickness and general removal of surface contamination such as hydrated layers. For the titanium substrate, on the other hand, the

J The concept of ascri' *rig ~,n ¢t'fecliVe inlerfacial tqmghness on the ha.,,i,, ~1' the residual stres.,,e~ pre~ent when debtmdin~. ~ccurs clearly bet'omes very dcmhtful when the dcb~mding is larL~cly due i~ [zas evolution and btinler Iornmti~,n. wilh the residual strenses p~ssihly playing m~ r~l¢ at all. F,vcn in this siluuli~n, h~wevcr, the procedure is still a ¢OltVellJen! way ¢~i highlighlh~g Ihe I'acl Ihal ~as ev~)lution is dramatically r;fi~in~ Ihe likelihu~d ~1' dehemdin~.

pre-cleanihfg process did not signilic:mtly affect the interfacial toughness. Apparently a relatively thick and fi'iable oxide film persists in this system, inhibiting good bonding. (3) For the titanium and tn~, two steels, the introduction of an aluminium interlayer by sputter deposition has been fi~und to el'feet a significant increase in the interfacial toughness, to around 4 0 - 5 0 J m -~. This is consistent with the good bonding ob:,~erved between DLC and AI substrates and with a sputtered AI layer adhering well to a cleaned metallic substrate. However, when the sputtered AI layer was e~posed to air before the DLC was deposited, then the inlerfilcial toughness was reduced to values fairly close In those in the absence of any pre-cleaning or interlayer deposition. (4) Experiments have been carried out in which DLC coatings on silicon substrates have been heated in vacuum to about 550°C. DLC adheres well to Si, provided a pre-cleaning process is carried out. It was found that, while good adhesion (-,-, 100 J tn ~2) was retained on heating for the standard interface, it became severely impaired in certain other cases as a consequence ol' the release of argon entrapped within the structure, and the promotion of debonding by blister lbrmation. In particular, when precleaning is carried out with methane in the atmosphere, so that a thin pre-lt,~yer HI" DLC is formed, this pre-layer contains entrapped Ar which is released on heating. Furthermore, it appears thai Ar becomes entrapped in the A! interlayer when this is formed by sputter deposition. Thus, DLC coatings with AI interlayers also debonded readily on heating.

Acknowledgements Financial support has been provided for one ol' tile authors (XLP) by Cambridge Overseas Trust (COT) and ORS.

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