| Abstract:The work temperature of machining region rapidly increases due to the friction between coated cutting tools and metal materials during machining. Therefore, the thermal stability and high-temperature oxidation resistance of coating become very important for the coatings applied in cutting tools. Aluminum based ternary transition metal nitrides MeAIN hard coatings (Me=Zr, Ti, Cr, etc.) with cubic structure, where Al substitutes for Me in the MeN based lattice, are in favor for such industrial applications due to their high hardness and melting point together with high oxidation resistance. Among them, Ti-Al-N and Cr-Al-N are the most widely used in cutting tools due to more excellent anti-oxidation performance. However, the hardness of Ti-Al-N coating reduces while exceeding1000℃due to thermal decomposition, and the anti-oxidation temperature of Ti-Al-N coating is below850℃. Although the anti-oxidation temperature exceeds1000℃, the mechanical properties of Cr-Al-N coating decrease due to thermal decomposition above900℃. During severe industrial applications with working temperature reaching or exceeding1000℃, new coating materials with the optimized oxidation resistance and thermal stability are needed.Here, a comparative research on magnetron sputtering and arc evaporation deposition of Ti-Al-N coatings is investigated in order to choose coating technology. And then, an improvement of the properties for Ti-Al-N and Cr-Al-N coatings is achieved by incorporation of the forth alloy elements. This thesis is comprised of the following five parts:1. Evaporated Ti-Al-N coating exhibits the typical growth defects originated form incorporated macro-particles and roughened surface, compared with the non-defect surface of sputtered Ti-Al-N coating. However, high atom mobility of arc evaporation process arising from the high ion to neutral ratio results in a more dense structure, and thus higher oxidation resistance. Simultaneously, the thermal decomposition temperature of evaporated Ti-Al-N coating with low stress shifts to higher temperature, and the coating exhibits better thermal stability. At last, evaporated Ti-Al-N coated tools exhibit better machining performance.2. The alloying with Si into Ti-Al-N coating promotes the growth of w-AIN phase, and the coating behaves hardening behavior. Ti-Al-Si-N coating exhibits higher thermal hardness due to the retarded thermal decomposition process with incorporation of Si into Ti-Al-N. Additionally, addition of Si element significantly improves the oxidation resistance of Ti-Al-N coating. The oxidation temperature of Ti-Al-Si-N coating is elevated to1100℃from800℃of Ti-Al-N coating.3. Incorporation Cr into Ti-Al-N coating results in an increase in hardness from～31.2GPa for Ti-Al-N coating to～34.4GPa due to the solid solution strengthening and decreased grain size. Therefore, Cr-containing coating behaves higher thermal hardness although alloying with Cr promotes the thermal decomposition process. The oxidation temperature of Ti-Al-Cr-N coating is elevated to900℃from800℃of Ti-Al-N coating.4. The N-loss of Cr-N coating at elevated temperature occurs, and thus results in the formation of h-Cr2N and finally Cr due to the instability of Cr—N bond. Addition of Al into Cr-N coating strengthens the stability of N in the CrN phase and thus promotes its thermal stability. Correspondingly, the oxidation resistance of Cr-N coating is improved by incorporation of Al.5. The thermal stability of Cr-Al-N coating is significantly improved with incorporation of Zr, where the thermal decomposition temperature is increased up to1100℃from900℃for Cr-Al-N coating. Nevertheless, alloying with Zr into Cr-Al-N coating leads to a drop in oxidation resistance.The results show that addition of the forth alloy element into Ti-Al-N and Cr-Al-N is beneficial to their application at high temperature. Alloying with Cr and Si into Ti-Al-N coating improves its mechanical and thermal properties. And incorporation of Zr into Cr-Al-N coating retards its thermal decomposition process, and thus improves the thermal stability of Cr-Al-N coatings. |
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