Microstructure and Mechanical Properties of Plasma Assisted CVD Ti(C,N) Films as a Function Of Carbon Content
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Mat. Res. Soc. Symp. Proc. Vol. 403 0 1996 Materials Research Society
cut into a thin foil which was bent automatically under residual stresses, and the curvature was measured by optical comparator. The microhardness was measured according to the method proposed by Joensson and Hogmark [2]. The adhesion of the coating to the substrate was evaluated by both scratch test and contact fatigue test. Scratch test critical loads were determined by accousitic emission. The details of the contact fatigue test are described elsewhere [1,3]. The resulting interfacial fatigue strength used here is the contact stress amplitude which will cause coating to spall in 5 x 106 contact cycles. RESULT AND ANALYSIS Figure 1 shows X-ray diffraction patterns of the coatings with different carbon concentrations. For the sample with low carbon content, both (200)-Ti(C,N) and (I Il)-Ti(C,N) peeks represent the structure of TiN in nature. This implies that within the resolution of X-ray diff-raction, the coatings can be considered as a single solid solution. Nevertheless, with the increase of carbon concentration, the (200) peek takes the first step to split into two parts, and the (111) peek follows as the carbon content becomes higher. The split indicates that the single phase of Ti(C,N) solid solution tends to become mixture of two phases TiN and TiC during deposition. The similar result has been reported for the same plasma assisted CVD Ti(C,N) coatings[4]. But it was emphasized that a complete solid solution was maintained in ion-plating PVD process for all of tested Ti(C,N) coatings[5]. Another difference lies in the fact that the PACVD results in (200)-type texture, whereas the PVD Ti(C,N) coatings have a strong (111)-type texture. Nevertheless, in both cases, the addition of carbon does not give rise to changes in crystal orientation. An observation of SEM implies that the coated surface and the particle size appear to be smooth and small as the carbon content increases. The comparison of morphology between 1000'
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Figure 1: X-ray diffraction pattern of Ti(C,N) coatings with carbon content of 0, 22, 34 and 0 % atomic percent respectively
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coatings with low and high carbon content is shown in Figure 2a. An examination of TEM on the coatings revealed very fine grains from which only electron diffraction rings can be formed. It is evident from Figure 2b that crystal grains in the coating with higher carbon content are smaller than that with lower carbon content. This implies that the formation of fine grains goes concurrently with the development of separate phase of TiN and TiC. Moreover, an intrinsic columnar structure tends to disappear with the increase of carbon content as shown in Figure 2c. The changes in microstructure must result in the changes in mechanical properties. Residual
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