Mechanical properties of hard W-C physically vapor deposited coatings in monolayer and multilayer configuration

ELSEVIER

Thin Snlid Films290~291 (1996) 232-237

Mechanical properties of hard W-C physically vapor deposited coatings in monolayer and multilayer configuration P. Juliet ~'*, A, Rouzaud a, K, Aabadi a, p. Monge-Cadet ~, Y, Pauleau a,~ ' C~A Grenoble, CEREM~DEM, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France i, TURBOMECA, 64511 Border, France

Abstract Tungsten-carbon based multilayer coatings obtained by reactive magnetron sputtering were shown to be very effective against sand erosion but their use needs optimization in order to avoid deformation of thin turbine blades due to the high internal stress level of such a coating while maintaining erosion resistance properties. By studying the intrinsic characteristics of each elemental layer (carbon content, stress level and hardness) vs, process parameters (working pressure, bias voltage, reactive gas flow), it was found that the stress level can be decreased without dramatic consequences on the hardness. The carbon content was found to be the leading parameter and a convenient stress level was obtained which allows multilayer deposition onto thin turbine blades. An analytical model able to predict the internal stress in n maltilayer arrangement from the stress of each elemental layer was used to optimize the coating. As a result, very good conformity of the coating on the sharp edge of the blade was obtained. Keywords: Magrmtronsputtering;Mullilayers;Mccl~tntcalproperties

!. Introduction In collaborative work with a manufacturer of turbine engines, promising results have been reported concerning the ability of W/W-C layers to resist erosion. It was shown that physical vapor deposition (PVD) magnetron multilayer coatings made from alternate tungsten layers and tungsten--carbon layers increvse the erosion resistance of a titanium alloy (TA6V) by a factor higher than 100 for both normal and transverse impinging sand particles [ 1], The best results were obtained using W-C films containing 14 at.% carbon in the form of a supersaturated solid solution, These coatings exhibited vet./ large compressive intrinsic stresses (about 1,4 GPa), The ultimata goal of this study being to protect the thin blades of the compressor, it was mandatory to optimize the coating by decreasing to as low as possible the stress level without destroying the erosion resistance, The aim of this work is to present the optimization task, consisting of a closed loop between the process paramct©rs on the one hand, and the physieochemical and mechanical characterization on the other hand. Finally an analytical model is presented which * Corresponding author, ' Ptesantaddress:InstitutNationalPolylnctmiquede Gvcnob[n.ENSEEG, BP 95, 38,tO2Saint M~in d'H/~t~s,France,

0040-6090/96/$15,00 @ 1996ElsevierScienceS,A, All fights re~ed PII S 0040.6090( 96 ) 0917 7-8

allows the forecast of the internal stressin a multilayer coating from knowk.dge of th© stress in each elemental layer. 2. Experimental procedure The W-C layers were deposited by a d,e, reactive magnetren sputtering process using a planar target (21 ×9 cm) of pure W. The fixed pro~ess parameters were a 7 cm working distance and a 5 W cm - 2sputtering power density. The sam. pie was d.c. biased from - 2 0 up to -250 V and the Ar working pressure was either 0.5 or 2 Pa, varied by adjusting the throttle valve apcrturs at a constant Ar flow. For deposition of W--C films, methane was added to the sputtering gas. The methane partial p r s s s ~ never exceeded 14% of the Ar partial pressure in the methane flow range investigated. During a typical run, the sample temperature was lower than 210 °C. The W/W-C muttilayers were obtained by switching on or off the methane flow as described in a previous paper [ 1]. The substrates investigated were single-crystal silicon, stainless steel and TA6V plates ( in a simulation arrangement close to the real turbine), The elemental thickness rPnged from 0.4 to 15 i~m according to the characterization method, The film thickness and sample curvaturs (for detemtination of the internal sm~ss) wer~ measured by profilamecy with a Tenter Pl system. The carbon ctntent was determinerl by nuclear

P, .talleret oA / Ybia 8olid Firms7,90,-291(1995) 2:~-~Y7

reaction analysis (NRA) using tbe:gC(d,p) t3C nuclear reaction, Elastic recoil detection analysis (ERDA) and Rutherford hackscattering (RBS) were used respectively for measurement of the hydrogen and argon contents. Some investigations were undertaken using X-ray photoelectron spectroscopy (XP$),

233

20

3. Results

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3.1, Pkysicochemicalcharacterlzationo~elementarylayers

t

0 o

All the studies were performed with a CH4 flow ranging from 0 up m 13 standard cm3 rain - 1in order to keep the layer in the o-W crystallographic structure as shown in our previous studies [ t,2], The deposition rate of W-C layers was determined at working presstn'es of 0,5 and 2 Pa, It was found to be slightly higher at 2 Pa (mean value 140 nmmin -t) than at 0,5 Pa (mean value 130 nm rain" t) and nearly independent of the CH4 flow in the range of CB4 partial pressure investigated. In this low partial pressure range, the rate of compound formation is slower at the subs~te than the rate of sputtering at the target and no poisoning effect is observed, The influence of sample bias on the deposition rate is shown in Fig. 1, A decrease in deposition rate with increasing substrata bias was observed. This behavior is mainly due to the resputtering effect of the growing layer by argon ions, Fig. 2 shows the carbon content vs, CH,, flow relationship as determined by NRA analysis for layers obtained at 0,5 and 2 1~ working pressures, The Cl-h flow range allows carbon contents of up to 24 at,% to be obtained. The rate of carbon incorporation seems to be slightly lower at 0.5 Pa working pressure (for - 20 V bias voltage). This effect is too small to attempt any correlation with process parameters. When varying the bias voltngc a much more significant effect on the carbon content is observed. As shown in Fig. 3, the incorporation of carbon increases markedly with increas1no "~ t20 t tO0

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Fig. 2. Carbon content vs. methane flow. M t g n e s o n power 1000 W, t i t s voltage - 20 V.

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Fig,3. Carboncontentvs, biasvoltage,Ma/pneU'enpowertO00W,methane flowrate S standardcm~rain-~ (2 Pa) end l0 smn0ardcm~rain-I t0.5 I~,). ing bias, This behavior is more p:onounced at low working

pressure, One of the main differer ¢es between high and low pressure conditions is the ionic a.,sistnnce during the dupe. sition when biasing the substrata The argon ion-to-metal atom ratio is greater at low presst:re and the ion energy is larger owing to the mean free path effect, The argon ion bombardment enhances pmbabty t~e carbon incorporation, either by elastic recoil implantation er by differentialresputtaring of the growing layer, This behavior is also currently observed for alloy deposition by eosputtering processes [3], Hydrogen incorporation in the W-C films was determined by ERDA for six samples with carbon content ranging from 0 up to 17 at.%. Apart from u surface contamination peak, no hydrogen in excess of the detection limit ( 1 at.% ) was found in the layers. This resu|t is in agreement with previous incus. tigations [4] and shows that hydrogen-free layers can be obtained using CFI4 reactive sputtering, The argon content was probed by RBS and found to be always lower than 0.5 at.%. whatever the process parameters (CH4 flow, working pressure, subslrate bias). This is probably due to the large atomic radius of the tungsten atom which prevents argon incorporation by recoil implantation.

P. Juliet ef at / ~in Solid Fitm~v290-291 (19P6) 2.]2-237

234

to ~tttlt4

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Fig, 4, XRD pa~ems with increasing carbon coa~en~.Ma~elroo power 1000 W, bias voltage - 20 V,

XPS investigationswere undertakenboth on apure W layer and a 14 at,% C content layer. Surface examinationshowed in both cases a shift of the W 4f level characteristic of the we2 oxide. At a depth of 50 am, :m oxygen trace is detected. For the carbon doped layer, the 4f level binding energy is different from those of the pure W and from those of standard WC and W2C carbides, This electronic structure is specific to the process and to the deposited compound. In Fig. 4, X-ray diffraction (XRD) patterns of the W-.C films show the b.c.c, structure of a-W from 0 up to 14.3 at.% carbon content. The predominantgrowthdirection was found to be in the (110) direction. This preferred orientation is particularly visible for highly doped films and/or low working pressure films. For doping levels over 17 at.%, the (110) line becomes progressively asymmetricand the structurecan be described approximatelyas a mixture of the b,c.c. W phase and of hexagonalW2C. In the a-W range, the slight shift in the line position towards small diffraction angles cannot be attributed to the internal stress level. As a matter of fact, the stress level was found to be independent of the carbon content at a given working pressure (see below). Hence the lattice parameter expansion is mainly dee to carbon incorporation in the W lattice. Assuming the sample curvature to be negligible (for a thin W--C layer on a single crystal silicon substrate), and taking into account the instrumentalfactor, the crystallitesize was calculated from the peak broadening using the Scberrer formula (Fig. 5), 3.2. Mechanicalproperties of elementary layers 3.2.1. lntrinsic stresses Stresses in different ebmentary layers were estimatedby means of the substrate deflectionmeasurementsaccordingto Stoney's formula, Experimental results presented hereafter proceed from measurements of thin W or W--C films (0.4 p,m) deposited onto silicon substrates. Fig. 6 presents the evolution of the intrinsic stress of the layer as the carbon content is increased, in the standard depositionconfiguration we used (P= 1000 W, Ub= - 2 0 V for both high and low

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Pig. 6. Internalstressvs. carboncontent,Magnelmnpower1000W,bias v o l t ~ - 20 V, working pressu~s), It follows that the pressure remains the main parameterdriving the magnitudeof the stress: W atoms impinging the substrata with high energies ([.e low pressure corresponding to a large mean free path) induce larger stresses than the 2 Pa pressure [5]. The influence of bias voltage on the stress field was also investigatedas reported in Fig. 7. For both pressures, a large stress increase is observed for bias voltages higher than 150 V, induced by high energy At* ion peening t6]. It must be pointed out that the control of intrinsic stresses is of paramount importance for the foreseen application, Indeed, large compressivestresses (basically higherthan 800 MPa) decrease the adhesion of thick films on the TA6V substrates. 3,2,2, Hardness The hardness of the layers was investigated by means of classical Vickers tests nn thick (greater than 15 tLm) W or W--C layers, The penetration depth was always kept below 10% of the layer thickness in order to measure only the mechanical properties of the film, This last point has been confirmedexperimentallyby indenting films deposited onto different kinds of substrate (TA6V, stainless steel, cop-

P, ,lulierezaL I Thin Solid FHrcs 290-291 ( l ~ ) 232-237

235

thickness of the plate is small compared with its transverse

lo. ,o..ly

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dimensions); (it) the intrinsic stress is assumed to be isotropic in the plane of the plate; (iii) it is supposed that the intrinsic stress is uniform within a monolayer. Finally, the total strain in a point of the system is

~=~+~ I -11~0

..... where ~ is the elastic deformation and ~ is the thermal deformation given by eu,~= a A T g~ 0

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200

rials to the system, the total strain is

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flow rate 8 standardcm~rain-' (2 Pa) an(l l0 standardr.m~rain"x (0.5 P~),

Since the deflection w in the (Oz) direction is negligible compared with transverse dimensions of the sample, non-

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linear effects of deformation ate not taken into account, The curvatuse K (K = i Jr) of the multilayer plate is related to the deflection w by

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Fig, S.Vicken mtctohlrdness~s, carboacontent,Magnetronpower1000 W, bias voltage- 20 V, per...). A complete set of hardness measurements of layers deposited at a working pressure of 2 Pa is presented in Fig. 8. The hardness maximum observed for aW-14,3at,%Ccoating is in agreement with previous results for layers obtained with 0.5 Pa working pressure [2], A decrease in hardness is con-

firmed for higher carbon content, while layers still remain harder than pure W.

system is a linear function of the z coordinate, and is governed by two parameters ~o and K, provided that we conserve ideal adhesion between the layers in the stacking, Considering that the two-dimensional isotropy conditions in layer i in terms of the stress-strain relation gives

and expressing the general equilibrium of force and momentum in a cross-section of the composite plate: N,.,= f ~r(z)dz=O s~tlon

M,ot= f z•(z)dz=O re:finn

4, Multilayer modeling The stress level is of prime importance when the coating has to be applied on a thin subs!rate. This problem has been addressed by several authors [7-9] who have developed different models describing the mechanicsofa film/substrata system, The multilayer coatings need a specific approach to forecast the stress level, i,e, the subsUnte deformation, The basic knowledge of intrinsic stresses in each elementary layer may allow one to predict the general behavior of the composite (suhstrate+n elemenlary layers) provided that the following assumptions are satisfied: (i) the system follows the thin plate theory (availab[e as long as the total

This generates two independent equations:

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from knowledge of the multilayer structure (layer number and thickness) and monolayer properties (Young's modulus, Poisson's ratio, thermal expansion coefficient) and n

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236

P, Juliet etal, / Thin Solid Films 2~291 (1996) 232-237

The substrata corresponds to the layer zero in the sum formulae. Two applications follow directly from our central

equations (Eq. (In), F-~I,(Ib)). The first corresponds to the simple configuration in whioh a monolayer is bonded on an initially stress-free substrata (it = 2). In this situation, the (2,2) linear system allows us to determine the intrin sic stress o',~xin the deposited layer by measuring the radius of curvature R of the sample. The plate deflection is due to both the intrinsic stress in the layer and the temperature difference, and the model incorporates these two effects to calculate o's,t, This case is treated by the Stoney equation, but Stoney's approach does not model the thermal contribution to curvature, and is only appropriate for the ease that the coating thickness can be neglected in comparison with the substrata thickness. The proposed model does not make any preliminary assumption concerning the coating thickness; its range of application is thus wider. The second application corresponds to a general configuration, with n layers. The intrinsic stress level ~,t of every single layer is supposed to be previously known. From this start point, the resolution of the (2,2) linear system gives us the equilibrium values of the deformation parameters 8° and R, Afterwards, we can determine the stress distribution o'(~) inside the composite stacking, as a function of the z coordinate, The model was tested for differentkinds of (substrate +layers) arrangements. The option calculating o'i.t in an n = 2 configuration fits Stoney's formula for A T= 0. We calculated and evaluated the o'(z) function for a tungsten multilayer bonded on a TAOV substrate. The intrinsic stresses in the individual layers were first determined experimentally, after measurements of the radius of curvature of the sample. The proposed example corresponds to a six-layer stack on a 0.5 mm thick substrata, with h,T= - 200 aC. The layers are 5 Itm thick, anti their intrinsic stresses arc alternatively - 400 MPa and -750 MPa. The ~'(z) curve from Fig.9 gives information about the stress level inside the substrata after deposition. This model has been particularly useful to define the maximum conditions allowable for deposition of canyon-

iently stressed thick multilayers onto 1 mm thick TA6V turbine blades. The model was tested by comparison with experimental results in different configurations (from 6 up to 24 layers, stress between 0,4 and 2.5 GPa, elemental thickness from 1 up to ]0 ltm). As an example for a six-layer stack (33 ;zm total thickness) deposited onto a !mm thick TA6V plate, u radius of curvature of 0.47 m was measured while the thee. retical calculations predict 0.53 m.

S, Application: deposition onto turbine blades

As mentioned earlier, three major problems, linked to the high internal stress level of mnltilayers, have made the coating of turbine blades challenging. (i) High residual stresses may strongly decrease the adhesion of thick coatings on TAfiV. (it) They may deform the initial shape of the blades. (iii) They may induce local delaminntion, particularly on the front end of blades where the curvature is very large (3 ram- i). Moreover, this part of the blades is the area where the sand erosion is maximum. The first problem has been partially solved by optimizing chemical precleaning and sputtereleaning fordepositiononto titanium alloys, and has been described elsewhere [ 10]. The second difficulty has been overcome with the help of the previous calculations to estimate the maximum deflection induced by the coating. The third point ban been investigated experimentally by coating simulated mountable blades which were cross-sectioned and analyzed by scanning e)ee~on microscopy. Fig. I0 shows the morphology obtained at the sharp end of such blades. Excellent coating conformity without any trace of delamination is observed which confirms the promising capabilities of such a multilnyer to protect turbine engines. It has to be quoted that this low stress muttilayer design suitable for turbine blade coatings has been tested on an erosion bench. When compared with the performance of our previous high stress arrangement, the erosion resistance remains satisfactory.

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Fig. 10, t ' ~ s s , s ~ t l ~ o f t l ~ m~ilaye: (~all~g on the s l ~ p blad~ edge.

P. loller etal,/T~n Solid Firm 2g0,-291 ( ! ~ ) 232-237

6. Cenelusion

Three main parameters influence the physical pruperties of W and W..C layers, (i) The carbon content does not affect the internal stress level but is of prime importance for the layer hardness, This parameter seems to act independently of the sirens level which was found to be roughly constant for different carbon contents. Moreover, the crystalllte size decreases regularly nsthe carbon content increases.Therefore interstitial carbon atoms and more generally related defects (dislocations, stacking faultt) seem to be responsible for the hardness increase, (ii) The effect of working pressure on the residual stress level agrees with the suggested correlation between the energy of impinging atoms and the stress field [5]. The smaller mean free path in high pressure conditions induces a larger loss in kinetic energy, (iii) According to the former result, the bias voltage contribution satisfies the ion peening model [6]. The increase in carbon content when biasing could have a synergistic: effect, Practically, it is thus possible to obtain layers presenting both high hardness andeonvenient moderate stress level (0,5 OPa). This allows stacking of a large number of layers with different morphologies and hardnesses which are effective against erosion, The moderate stress level also allows dope-

237

sition onto very thin titanium alloy blades without affecting their geometry. Lastly, the mechanical model predicting the stress level in a multilayer arrangement appears to be useful for layer engineering. Such a model, efficient for mechanical applications, could also be applied each time a multilayer system is foreseen, such as in VLSI technology.

Refereneet l l] E. Quesneland Y. Panteaa,gu~. Coat, T~clmol. 62 (1993) 4"/4, [2] Y. Pauleau,P. Goay.Palllerand S. Pa~dassi,gu~ Coat TeclmoL,$4-" $5 (1992) 324. [3] Z,F, ghou,Q,G,21muand Y,D, Fan,Mater, Re~. S~e. Syrup. Prec., /88 (1990) 91, [41 M, Wang,K, Schmid!and K, Reiehelt,J. Mater, Re~,, ? (6) (1992) 1465, [5l H, Windiechmana,Crit, Re.v. Solid State Mater, St'/,, 17 (6) (1992) 547,

[6] J,A.ThoA'oaand D,W.Hoffman,Thin Solid Films, I71 (1989) 5. [7] H.C.LIe and S.P. Mumrka,2. Appl. Phys., 72 (8) (1992) 3458, [8] M,lgnat,A,Chouafend P, Normandon,Ma~er,Rez,5'oc,.~mp,Prec., 188(1990) 97. [9] C,B,Ma~ter~N,J,Salamonend D,E FalmtiM.MeWr. Res, So¢, Syr~, Prec,, 188(1990) 21. [I0] A. Ro~zaud,K. K~ki and J, Holsa,r~MRSSpring Meet., 81rasboar&, Y~ne 1996, in pro~,