Crystallization of Amorphous Metastable Ceramic Pvd-Coatings
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621 Mat. Res. Soc. Symp. Proc. Vol. 321. ý1994 Materials Research Society
COATINGS IN THE AL-O SYSTEM Due to the extremly high thermal gradient in the R.F. magnetron sputter ion plating process and the fact that ceramic materials need a long crystallization time combined with a high energy level [11, the sputtered Al-O coatings are deposited amorphous. This is in contrast to the crystalline CVD coatings [2,3]. In figure 1, which gives X-ray diffraction pattern measured with a Co-tube, is shown, that at higher temperatures, the coating starts to crystallize in several low temperature alumina modifications, e.g. -y, 6- or O-A120 3 [4]. Above 800'C, the coating starts to form the ce-phase. Due to this crystallization, the volume contraction is causing very high stresses in the coating so that cracks and finally peel off effects are destroying the substrate-coating composite. Therefore a stabilization of the amorphous structure at higher temperatures is needed. The stabilization can be done in several ways. There is a number of variable technical and process parameters like power source, power, partial gas pressures, target materials, bias etc. which have an influence on the structure and morphology of the coatings. Most parameters are not suitable for a successful increase of the thermal stability, only small improvements were achieved. 0
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a) b) Figure 1: Crystallization of A120 3 (a) and A1-O-N (b) coatings
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Another way for the stabilization of the amorphous phase structure is a small change in the coating composition, for instance with an integration of a small element, like nitrogen, oxygen and carbon which can be solved at interstitial places. This is possible for the here used MSIP (Magnetron Sputter Ion Plating) process by the addition of a gas during the deposition. Remarkable improvements in the thermal stability are achieved with nitrogen, shown in figure 1. where the X-ray diffraction patterns for the systems, AI-O and Al-O-N, are given. The left side is showing the X-ray diffraction pattern of an A120 3 coating, the right side shows an AI-O-N coating after several annealing steps. No crystalline phases are observed in A120 3 below 800'C. Several A12 0 3 phases are formed at 800 °C, 1000 °C and 1200 'C. At 1400 'C, no phases are given. This caused by the alumina substrate, which has got also the a-phase,. The proof is not directly possible. Examinations on other substrates like cemented carbide confirm this result. The AI-O-N coatings show a similar crystallization behaviour with one important difference: the starting temperature for the crystallization is 200 'C higher and at 1400 'C is not only the a-phase formed. Further investgations show that this temperature difference is a function of the nitrogen
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