Preparation, characterization and wear behaviour of PACVD cermets

ELSEVIER

Surface and Coatings Technology 72 (1995) 37-42

* Preparation, characterization and wear behaviour of P A C V D cermets I. Endler ", E. Wolf", A. L e o n h a r d t a, V. Richter b a Institute of Solid State and Materials Science Dresden, Postfach, D-01171 Dresden, Germany b Fraunhofer Institute of Ceramic Technologies and Sintered Materials Dresden, Winterbergstrafle 28, D-O1277 Dresden, Germany

Received 29 April 1994

Abstract Commercial cermet inserts were coated with TiNx, TiCxNr and (Ti, Si)Nx by plasma-assisted chemical vapour deposition using a pulsed d.c. glow discharge. The influence of the coating parameters on the deposition rate, layer composition, layer-substrate interface, structure and microhardness of the layers was investigated within the deposition temperatures of 773-973 K. With optimized process parameters, layers with a low oxygen and chlorine impurity, high microhardness and a good adhesive strength are obtained. Higher microhardness values compared with that of TiN~ were observed for TiCxNy and (Ti, Si)Nx layers depending on the layer composition. Usually, the TiNx and also the TiCxNr and (Ti, Si)Nx layers with low carbon and silicon contents have a columnar structure with a (200) texture. The structure changes with rising carbon and silicon concentrations. The edge life of the insert tips is markedly increased for cast iron and steel machining. Keywords: Plasma deposition; Hard coatings; Wear; Cermets

1. Introduction In the last few years, titanium-carbonitride-based hard metals usually called cermets have proved superior in finish cutting on steel and cast iron [1,2]. However, most of them have been uncoated but, like the coated cemented carbides [3], an increase in performance can be expected if the cermet tool surface is protected with titanium nitride, titanium carbide or alumina layers. It was shown earlier [4] that conventional chemical vapour deposition CVD processes at high deposition temperatures are unsuitable for coating cermets because brittle intermetallic phases are formed. TiNx-coated cermets obtained at lower temperatures by physical vapour deposition (PVD) resulted in a much higher wear resistance in continuous cutting of unalloyed and alloyed steel [1,5]. Lower deposition temperatures can also be applied using the plasma-assisted chemical vapour deposition (PACVD) process. In this work TiNx-, TiCxNy- and (Ti, Si)Nx-coated cermets prepared by PACVD are presented. The deposition process has been studied and the samples characterized by chemical and structural analysis with following methods: metallography, scanning electron microscopy (SEM), energy* Presented at the 21st International Conference on Metallurgical Coatings and Thin Films, San Diego, CA, 24-29 April 1994. 0257-8972/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0257-8972(94)02338-7

dispersive X-ray spectroscopy, X-ray diffraction, and chemical analysis of nitrogen, oxygen, carbon and hydrogen. Some mechanical properties and the wear resistance for turning of steel and cast iron have been investigated.

2. Experimental TiN~ is deposited using a gaseous mixture of TIC14, N2, H2 and Ar, TiCxNy with additional methane or benzene and (Ti, Si)Nx with additional SIC14. The reaction chamber is a hot-wall reactor with an outer threezone resistance furnace. A pulsed d.c. glow discharge is generated between two parallel electrodes. The upper electrode which is used for introducing the gas mixture and the wall are the earthed anode. The substrates are held on the lower cathode. The substrate temperature is controlled by the furnace temperature and the plasma power and is measured by a thermocouple near the cathode surface. The apparatus has been described in detail in [-6]. Experiments were carried out in the temperature range 773-973 K, at a pressure of 330 Pa and mean d.c. power densities between 0.4 and 1.1 W cm -2. The total gas flow rate was 17-26 cm 3 s -1. The substrates were two commercial T i ( C , N ) (Mo, W ) C - N i , Co cermet inserts (Cerametal) with different binder contents. The binder-rich grade was favoured for wear investigations in order to combine

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L Endleret al./Surface and Coatings Technology 72 (1995) 37 42

high toughness of the substrate material with high wear resistance of the coatings. 3. Results and discussion

13.1. Preparation and characterization of TiNx coatings The most important parameters influencing the deposition rate, the composition, the structure and the properties of the layers are the TiC14 flow rate, the partial pressure ratio (PI~2/PN2)o in the input gas, the substrate temperature, the power density and the pulse-to-pause ratio of the pulsed d.c. discharge [6]. The deposition rate increases with rising TiCI4 flow rate and hydrogen-to-nitrogen partial pressure ratio up to constant values dependent on temperature. The TIC14 flow rate influence points to a transition from a mass transport-controlled to a kinetically controlled process [7]. Increasing plasma power density in the range from 0.4 to 0.8 W cm -1 gives a nearly linear rise for the deposition rate. Oxygen-poor and nearly stoichiometric layers have been obtained in the kinetically controlled deposition range with TIC14 flow rates above 1 x 10 -2 mol h -1 and (PH2/PN~)Opartial pressure ratios above 9. Partial pressure ratios higher than 13 lead to an increase in the hydrogen content in the layers, which is probably hydridically bounded [8,9]. The higher oxygen contents observed in the mass-transport-controlled deposition range are probably due to residual gas influence and hot-reactor-wall desorption effects. It is necessary for the deposition of stoichiometric oxygen-poor TiNx layers to maintain pause periods below 50 gs in the pulse-to-pause ratio if the pulse period is in the range between 10 and 40 las. The chlorine content is influenced by the substrate temperature and the plasma power density. It is lowered from 1.9-2.2 at.% at 773 K to 0.9 at.% at 973 K. If the plasma density is too low, the chlorine content increases markedly. The structure analysis shows that homogeneous TiNx layers without intermetallic phases in the layer and interface range have been obtained up to 973 K (Fig. 1). Normally, a columnar structure with a (200) preferred orientation has been observed (see Fig. 4(a) later). A change from a columnar to a granular equiaxed structure can be achieved if the TiC14 partial pressure is higher than 1.6 Pa at 793 K. This is advantageous for the reduction in crack extension and the improvement in the wear resistance. Lattice constant determinations by X-ray °analysis resulted in values between 4.250 and 4.270 A, which are generally higher than in the data literature for stoichiometric TiNx (a=4.240A [10] deposited by conventional CVD. Oxygen impurities lead to lower lattice constants, chlorine and hydrogen [9] as well as superstoichiometrically solved nitrogen and compressive stress to higher values [11,12]. Because of the low chlorine and hydrogen contents (less than 2 a t . ° ) in optimized coat-

Fig. 1. Cross-section polish of a TiNx-coated cermet deposited at 773 K. ings, stress and additional dissolved nitrogen are probably responsible for the higher lattice constants. The adhesion was estimated by scratch test investigations. The adhesion strength clearly increases with rising temperature, deposition rate (TIC14 flow rate and (Pn2/PN2)o ratio) and plasma power density. The temperature influence is due to faster diffusion processes in the interface range, whereas the increase in the adhesion strength with increasing deposition rate and plasma power density should be caused by the decreasing oxygen and chlorine concentrations in the layers and stronger interaction between the activated gas-phase species and the solid surface. Furthermore, a suitable plasma pretreatment of the substrate directly before starting the deposition process can also improve the adhesion. Critical loads between 40 and 75 N can be reached at 773 K and 85-113 N at 973 K. The influence of the substrate temperature on the microhardness and the friction wear is shown in Fig. 2. The friction wear was investigated with a pin-on-disk tribometer. The microhardness of nearly stoichiometric coatings decreases with rising substrate temperature. PVD layers prepared at low temperatures also have a relatively high microhardness. After a tempering procedure at 973 K a Vickers microhardness reduction to values of nearly 2500 HV is observed according to our microhardness at 973 K [ 11]. One explanation is that the higher microhardness is caused by layer stresses and lattice imperfections but the effect of the grain boundary hardening cannot be excluded. The friction wear is strong correlated with the microhardness. The higher the microhardness is, the lower the friction wear will be.

3.2. Preparation and characterization of TiCxNy coatings If the TiC14 flow rate is fixed, titanium carbonitride layers can be prepared by changing the molar ratio [nc/(nc + nN)]o in the input gas.

I. Endler et al./Surface and Coatings Technology 72 (1995) 3 7 -42 8 T .................................................... i .................... Y

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The deposition rate is continuously lowered from 4 pm h-1 on the N-rich side to 2.5 lam h-1 for TiCx at 773 K and from 6 to 4.8 ktm h-1 at 973 K. This is due to the higher negative value for the free enthalpy of TiN formation. The composition of the TiC~N~, layers is influenced not only by the molar ratio [nc/(n c + nN)]o but also by the d.c. power density, pulse-to-pause ratio and the partial pressure ratio (PH2/PAr)0 in the input gas. As observed for the TiN~ coatings a higher d.c. power density leads to a lower chlorine impurity concentration as well as a lower (P.2/PAr)O ratio. However, with rising carbon content in the layers deposited at 773 K the chlorine level also increases from 2 at.% for TiN~ to 4 at.% for TiC~. The reason may be a change in the reaction mechanism. Similar values were found in the literature [ 13 ]. Higher pause times than 100 ~ts of the pulsed d.c. discharge leads to a reduction in the carbon content and to increasing oxygen values in the layers. This is probably caused by the short lifetime of many hydrocarbon radicals. Therefore pause times only below 40 ~ts were used. A main problem in the TiCxNy deposition process is to avoid the formation of free carbon, which is a function of the molar ratios (nc/nxi)o and (nN/nTi)O in the gas mixture. For preparing nearly stoichiometric TiCx, (nc/nTi)O must not be higher than 2 if benzene is used as carbon source and 4 in the case of methane. The deposition of titanium carbonitride without free carbon is possible only if these (nc/nTi)O values are reduced according to the rise in (nN/nTi)O. As mentioned for the TiN= coatings, no intermetallic phases are observed up to 973 K. The TiCxNy layers with a stoichiometric factor x < 0.5 show also a columnar (200)-textured structure as TiNx. Above 0.5 the preferred orientation (200) is lowered and (111> increases.

Finally, the dominant orientation for TiCx is (111>. An increase in the (111 > peak intensity in the X-ray diffraction is also observed in the literature [ 14]. The lattice constant rises continuously from a =4.25 A for TiN x to a = 4.36 ]k for TiCx. The higher lattice constant deviation of TiCxNy on the carbon-rich side can be explained by stronger chlorine incorporation (literature data; a(TiC) = 4.33 ~,) as experimentally were found. The microhardness progress in dependence on the layer composition shows a maximum at x around 0.5 (Fig. 3). This curve for TiCxNy coatings will be flatter at 973 K because the microhardness of TiC is higher and reaches 4000 HV [0.01 ]. Such observations are also

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Fig. 3. Dependence of the microhardness for TiN~N~, layers deposited at 773 K on the stoichiometric factor ~.

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I. Endleret al./Surface and Coatings Technology72 (1995) 37-42

described in the literature for TiCxN r layers prepared by PVD as well as PACVD [14,15]. The adhesion strength of TiCxNy layers decreases with rising carbon content. Critical loads like those for the TiNx coatings are measured on the nitrogen-rich side: 50-60 N at 773 K and 93-106 N at 973 K. The critical loads for nearly stoichiometric TiC~ layers were 25 N at 773 K and 40 N at 973 K.

3.3. Preparation and characterization of ( Ti, Si ) N~ coatings We have investigated the dependences of the deposition rate, layer composition, structure, topography, electrical resistivity, microhardness and adhesion on the ratio R x = [nsJ(nsi+nTi)]o in the input gas at substrate temperatures of 873 K (Table 1 and Fig. 4) and 773 K. The deposition rate at 873 K continuously decreases from R~ = 0 to Rx = 0.6, which means that the deposition rate of TiN~ is higher than that of SiNx under these deposition conditions. Up to R~ = 0.2 the silicon content and also the electrical resistivity are low. The change in the layer structure is insignificant (see Figs. 4(a) and 4(b)). Above R~=0.2 the silicon content increases rapidly. If the atomic ratio

of silicon to titanium in the layer is higher than 1 (at Rx=0.6), the isolating silicon nitride determines the resistivity. This is connected with a transition from a columnar (200)-textured structure to an equiaxed and finally an amorphous structure (Fig. 4) as well as a decrease in the surface roughness. The Vickers microhardness is nearly constant in this range of Rx. The measured values are 2970, 2950 and 2740HV [0.02] at R~=0, 0.2 and 0.6 respectively. (Ti, Si)Nx layers deposited at 773 K show a weak microhardness maximum of 3400 HV [0.02] at Rx = 0.2. These values show a difference to the data in the literature [ 16] where a strong microhardness maximum of 6350 HV [0.05] was found but, in contrast, steel substrates were used and the microhardness was measured on the layer surface and not on the cross-section polish of the layer. The critical loads of the scratch test as the degree of adhesion reached values of 40-70 N.

3.4. Wear behaviour of coated cermets The wear behaviour of the TiNx- and TiC~Ny-coated cermets was investigated under different conditions in continuous and interrupted cutting of steel and cast iron. (Ti, Si)Nx-coated cermets will be tested, especially

b

Fig. 4. SEM fractographs of (Ti, Si)Nx-coated cermets at different molar ratios R~ [-nsi/(nsiq- nxi)]o in the input gas (substrate temperature, 873 K): (a) R~=0; (b) Rx=0.2; (c) Rx=0.4; (d) R:,=0.6. =

L Endler et aL/Surface and Coatings Technology 72 (1995) 37-42

41

Table 1 Deposition rate, composition, electrical resistivity and roughness of the (Ti, Si)Nx layers in dependence on the ratio Rx at a deposition temperature of 873 K and constant flow rate (SiCI4 + TIC14) of 1.2 x 10 -2 mol h 1 and a substrate roughness R==0.81 0.85 gm Gas-phase ratio Rx = [nsi/(nsi + Hri)]o

0 0.1 0.2 0.3 0.4 0.6

Deposition rate (p,m h 1)

Content (at.%) Ti

Si

5.0 4.5 4.1 3.5 2.9 2.1

46.2 46.5 45.1 33.8 29.8 13.9

0 0.2 0.9 8.0 10.3 17.7

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Electrical resistivity (~)

Surface roughness R: (gm)

1.1 0.7 0.7 0.5 0.7 0.9

0.31 0.30 0.34 0.66 2.95 90400

1.34 1.23 1.19 1.03 0.80 0.81

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Fig. 5. Continuous turning of steel C60N: dependence of the edge life of PACVD-TiNx-coated cermets (substrate temperature T,=973 K) on the layer thickness (uncoated cermet, 100%) at a constant flank wear of 0.3 mm.

coating 1

coating 2

Fig. 7. Continuous turning of steel C60N: dependence of the edge life of PACVD-TiCxNy-coated cermets (substrate temperature T, = 973 K) on the cutting parameters feed and cutting depth for a 3 gm homogeneous (coating 1) and multilayer (coating 2) at a constant flank wear of 0.15 mm (uncoated cermet, 100%).

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in machining of cast iron in the next future. Coating of cermets with titanium nitride and carbonitride caused a substantial higher cutting performance in continuous (Figs. 5-7) as well as interrupted turning.

It was found that for the optimal performance of the coated tools the deposition temperature as well as the composition and thickness of the layer has to be carefully adjusted to the workpiece and the working conditions. In continuous turning, the higher substrate temperature of 973 K leads to the best results. In the case of steel cutting, an optimal layer thickness of about 3-4 gm is observed whereas, with cast iron, thicker layers up to 10 lam are advantageous (Figs 5 and 6). In interrupted cutting, lower deposition temperatures can lead to a higher edge life especially for a greater cutting depth than 1.0 mm. The lifetime increases up to more than 200% in machining steel and up to more than 400% for machining of grey cast iron with the always optimal layer thickness at feeds f~>0.3 mm rev -~. At lower feeds (f~<0.2 mm rev-1) the gain in lifetime that arises from coating cermets seems to be lower but still significant (see Fig. 7). Further, Fig. 7 shows that a TiCxN r multilayer gives the best results in steel cutting according to results of multilayers on cemented carbides in the literature [17].

42

L Endler et al./Surface and Coatings Technology 72 (1995) 37-42

4. Conclusions TiNx, TiCxNy and (Ti, Si)Nx layers on cermets were deposited using pulsed d.c. PACVD in a hot-wall reactor system. The most important process parameters which influence the deposition rate, layer composition, structure, properties and wear behaviour are the partial pressures of the reactants, substrate temperature, the power density and the pulse-to-pause ratio of the pulsed d.c. glow discharge. The results obtained are the following. (1) The formation of brittle intermetallic phases can be avoided up to a substrate temperature of 973 K. (2) Nearly stoichiometric TiNx layers with low oxygen and chlorine impurities can be prepared at high deposition rates characterized by a sufficient TiC14 flow rate, (PrI2/PN~)o>9, a plasma power density between 0.4 and 0 . 8 W c m -2 and substrate temperatures of 773-973 K. Growth of TiCxNy and (Ti, Si)N~ layers is performed by variation in the molar ratios [nc/(nc + nN)]o and [nsi/(nsi + nTi)]o respectively. (3) The highest TiN~ microhardness values above 3000HV [0.05] are observed for layers prepared at 773 K. The hardness increases up to a maximum value if carbon or silicon is incorporated, to 4200 HV for the titanium carbonitride system and 3400 HV for (Ti, Si)Nx deposited at 773 K. (4) TiNx, nitrogen-rich TiCxNy and silicon-poor (Ti, Si)N~ layers have a columnar structure with a (200) texture. A transition to the (111) preferred orientation is observed with increasing carbon content in the TiC~Ny system. With increasing silicon content of (Ti, Si)Nx layers the structure changes from a (200) preferred orientation to an equiaxed and finally to an amorphous structure. (5) The adhesion strength of the layers is the best for coatings with low oxygen and chlorine contents and therefore it depends on the initial gas phase and plasma parameters. Carbon-rich coatings have a lower adhesion than nitrogen-rich coatings do. The adhesion can be improved with an increase in the substrate temperature and suitable plasma pre-treatment. (6) The wear resistance of the cermets is markedly

increased by coating with TiNx and TiCxNr This has been found for the continuous turning of steel and cast iron as well as for the interrupted turning of steel. The lifetime increases by more than twofold for the continuous turning of steel and the fourfold for the turning of cast iron.

Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft. The authors are indebted to Dr. W. Gruner (Institute for Solid State and Materials Science Dresden) for the measurement of the nitrogen, carbon and oxygen contents in the coatings.

References [1] K. Malle, VDI-Z, 134 (1992) 61. [-2] P. Ettmayer and H. Kolaska, Metall, 43 (1989) 742. [-3] J.E. Sundgren and H.T.G. Hentzell, J. Vac. Sci. Technol. A, 4 (1986) 2259. [4] K. Bartsch, A. Leonhardt, E. Wolf, M. Sch6nherr and M. Seidler, J. Mater. Sci., 22 (1987) 3032. [-5] M. Kato, H. Yoshimura and Y. Fujiwara, Proc. 12th Int. Plansee Semin., Vol. 3, Metallwerk Plansee, Reutte, 1989, p. 93. [-6] I. Endler, E. Wolf, A. Leonhardt, A. Beget and V. Richter, Preparation characterization and wear behaviour of TiNxcoated cermets obtained by PECVD, J. Mater. Sci., in press. [-7] H. Arnold, Chemische Dampfablagerung, Akademie, Berlin, 1982, p. 34. [-8] G. Meunier, J.P. Manaud and P. Grall, Mater. Sci. Eng. B, 18 (1993) 303. [9] B. Arnold and I. Endler, TEM investigations on PECVDtitanium carbide layers, Fresenius J. Anal. Chem., 349 (1994) 249. [-10] C.C. Jiang, T. Goto and T. Hirai, J. Alloys Comp., 190 (1993) 197. [-11] V. Valvoda, R. Cerny, R. Kuzel, L. Dobiasova, J. Musil, V. Poulek and J. Vyskocil, Thin Solid Films, 170 (1989) 201. [12] A.J. Perry, J. Vac. Sci. Technol. A, 6 (1988) 2140. 1-13] T. Arai, H. Fujita and K. Oguri, Thin Solid Films, 165 (1988) 139. [14] L. Shizhi, S. Yulong, X. Xiang, Y. Hongshun and Z. Cheng, Plasma Surf Eng., 1 (1989) 155. [15] B.E. Jacobson, C.V. Deshpandey, H.J. Doerr, A.A. Karim and R.F. Bunshah, Thin Solid Films, 118 (1984) 285. [16] L. Shizhi, S. Yulong and P. Hongrui, Plasma Chem. Plasma Process., 12 (3) (1992) 287. [17] R. Porat, Surface Eng., 8 (4) (1992) 292.