Study On Surface Integrity Of TiAlN Tool Coating With Micro-blasting

Although a wide range of part forming and machining processes have been developed, there are still more than 90 percent of the mechanical parts made by cutting in the manufacturing industry. Statistical analysis shows that more than 80 percent of the cutting tools applied at present are coated. The coating tool materials with excellent performance can significantly improve tool life and cutting efficiency. With the rapid development of modern science and technology, it is put forward higher requirements on cutting performance for cutting tools especially with the rapidly increasing requirements of difficult-to-machine materials in aerospace, military, automotive and other manufacturing industries.It is a fast, efficient and low cost method to improve the cutting performance of the existing cutting tool coating with mature technology by post-treatment processes, comparing with developing new super hard coatings. At present, micro-blasting technology has been applied to production as post-treatment process of coated cutting tools, but the micro-blasting process parameters, especially the micro-blasting pressure and duration necessary to be optimized and the micro-blasting quality is difficult to control.In this thesis, the effects of micro-blasting on the surface integrity and wear resistance of the coatings were studied based on the objective of strengthening the mechanical properties of tool coatings.Firstly, the effects of micro-blasting pressure and duration on the surface integrity of the TiAlN coating were studied by the experiment of micro-blasting on TiAlN coating. The TiAlN coatings were subjected to a micro-blasting post-treatment at various micro-blasting durations and pressures. The surface roughness, topography, thickness, hardness, nano-indentation and nano-scratch depths of the as-deposited and blasted TiAlN coating as well as the adhesive strength between the tool coating and substrate were selected as technical indexes to study the effects of micro-blasting pressure and duration on the surface integrity of the TiAlN coating. The uniformity of the surface roughness of the blasted TiAlN coating and the adhesive strength between the coating and substrate were effectively improved. The defects such like droplet were effectively removed, the coating thickness is greatly influenced by micro-blasting pressure. Compared with that for the as-deposited coatings without micro-balsting, the maximum increase percentage of micro-hardness and the maximum decrease percentages of nano-indentation and nano-scratch depths of the blasted TiAIN coating were 19.7%,11.4% and 11.2% respectively.Then, the prediction models for the surface roughness and the hardening depth of the TiAlN coating were established basing on the quasi-static indentation theory, revealing the effects of micro-blasting pressure and duration on the surface roughness and the hardening depth in terms of the kinetic energy of abrasive particle and the coverage rate of abrasive to coating. The critical velocity of abrasive particle to deteriorate the mechanical properties was also predicted basing on the indentation fracture theory to obtain the maximum micro-blasting pressure. The effects of micro-blasting pressure on the surface roughness and hardening depth of coating were mainly reflected in the effect of the abrasive particle velocity. The higher micro-blasting pressure, the greater the kinetic energy of the abrasive particle, and the surface roughness Ra as well as the hardening depth was all in proportional to the cubic root of the kinetic energy of the abrasive particle. The effect of micro-blasting duration on the surface roughness of coating was mainly reflected in the effect of the coverage rate of abrasive to coating. The surface roughness Ra increased when the coverage rate increased from 0 to 100% and decreased, then increased to a stable value when the coverage rate was greater than 100%. The micro-blasting pressure should be lower than 0.4MPa in the micro-blasting post-treatment to TiAIN coating with the experimental conditions in this study.Finally, the effect of micro-blasting pressure on the friction coefficient of TiAIN coating was revealed and the improvement mechanism of the wear resistance of the coating was discussed by friction and wear test. As for steel-coating friction pair, the surface roughness of the TiAIN coating had a significant impact on coating damage mechanism. In regarding to ceramics-coating friction pair, the wear resistance of the TiAlN coating was extremely influenced by the hardness of the TiAlN coating. The minimum wear volume of the TiAlN coating was obtained at 0.4MPa which decreased 23% comparing with that for the as-deposited coatings.