Research On Microstructure And Properties Of Ti(C, N)-based Cermets Coated By Multivariant Nitrides

(Ti,W,Ta)C and Ti(C_0.7N_0.3) solid solution were used as hard phase stuff to prepare Ti(C,N)-based cermets by powder metallurgy technology in the present work, and the cermets with optimal mechanical properties were selected as substrate materials for depositing TiAlCrN and TiAlN coatings by multi-arc ion plating (AIP). The X-Ray diffractometer (XRD), optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), bending strength tester, rockwell hardness and microhardness tester were employed to observe and test the microstructures and room temperature mechanical properties of cermets and coatings as well as the oxidation resistance of coatings at high temperature.In the first part of the dissertation, the development of coatings has been summarized, which included the influence of manufacturing process and composition on the microstructures and properties of coatings; then, present research situation of hard coatings was introduced and the mechanism of protective coatings was concluded. Based on the above, the purpose and significance of the dissertation has been pointed out.The influence of Mo content on the microstructures and mechanical properties of (Ti,W,Ta)C-Ni system cermets has been studied. As Mo content increased, the core-rim structure of cermets changed from dark core-grey rim to dark core-white inner rim-grey rim. Mo had the effect on densification of the (Ti,W,Ta)C-Ni system cermets. With the increase addition of Mo, the shrinkage of cermets increased and the porosity reduced. In the fracture morphology of cermets, the transgranular fractures decrease and intergranular fractures increase when Mo has added in the system. As the increase of Mo content, the hardness of (Ti,W,Ta)C-Mo-Ni system cermets improved and the transverse rupture strength (TRS) increased initially and then decreased. When the Mo content was 15 wt.%, the optimal properties of (Ti,W,Ta)C-Mo-Ni system cermets was obtained: hardness HRA 90.2 and TRS 1661MPa, respectively.After the optimal Mo content has been determined, the element of N was added in the cermet system through the form of Ti(C_0.7N_0.3) solid solution, and the effect of Ti(C_0.7N_0.3) addition on the microstructures and mechanical properties of (Ti,W,Ta)C-Mo-Ni system cermets has been studied. The microstructure of (Ti,W,Ta)C-Ti(C_0.7N_0.3)-Mo-Ni system cermets consist of black core-grey rim structure and white core-grey rim structure. The black cores was the undissolved Ti(C_0.7N_0.3) particles and the white cores was undissolved (Ti,W,Ta)C particles. As the Ti(C_0.7N_0.3) addition increased, black cores increased and the rims became thinner. With the increase of Ti(C_0.7N_0.3), the hardness of (Ti,W,Ta)C-Ti(C_0.7N_0.3)-Mo-Ni system cermets almost remain unchanged but the TRS increased initially and then decreased. When the Ti(C_0.7N_0.3) content was 40 wt.%, the optimal properties of (Ti,W,Ta)C-Ti(C_0.7N_0.3)-Mo-Ni system cermets was obtained: hardness HRA 90.3 and TRS 1748MPa, respectively.The TiAlCrN and TiAlN coatings prepared by AIP were smooth and dense, without columnar crystals and pores and adhere to substrate well. The thickness of the coatings was 3μm and 2.5μm respectively. The island particles, which appear on the coating surface, were the droplets which usually formed in the AIP technology. The microhardness of TiAlCrN and TiAlN coatings were 31.9 GPa and 23.2 GPa, and the critical adhere strength with substrate were 67 N and 55 N. Both of the coatings were the B1 face-centered cubic structure, and had a tendency of preferred orientation on (111) crystal face.The high temperature (800¹000℃) oxidation products of TiAlN coating was TiO2,NiTiO3 and NiAl2O4, and the oxidation products of TiAlCrN coating at the same temperature was TiO2,Al2O3 and Cr2O2.4. Some large oxide particles, which appear on the coating surface, were the oxidation products of droplets. With the oxidation intensified, droplets grew rapidly in the form of columnar crystals. After the oxidation at 1000℃, there was a Al2O3-rich and Cr-rich oxide in the TiAlCrN oxide scale, which can reduce the inward diffusion rate of oxygen ion. Among the Ti(C,N)-based cermets substrate, TiAlCrN coating and TiAlN coating, TiAlCrN coating had the best oxidation resistance. When Cr was added into the TiAlN coating, the high temperature oxidation resistance of TiAlN coating has been improved.