The Research On The Reinforcement Technology And Microstructure, Mechanical Properties Of Ti(C, N)-based Cermets

Ti(C,N)-based cermets are ideal candidate for high speed cutting tools because of their high hardness at elevated temperature, excellent wear-resistance and perfect oxidation resistance. However, the application of Ti(C,N)-based cermets is limited for its low transverse rupture. So the research on the strengthening and toughening of Ti(C,N)-based cermets is very important. In this paper, on the basis of preliminary study of our research group, the effects of forming agent types, particle size of raw WC powders, the addition of Ti(C0.5N0.5), (Ti,W)C solid solution and SiC whisker were studied by X-ray diffraction(XRD), scanning electron microscope(SEM), energy dispersive X-ray detector(EDS), bending strength tester, rockwell hardness and micro-hardness tester. And on the basis of previous study, gradient Ti(C,N)-based cermets were prepared with filling in layers by powder metallurgy method. The effects of chemical composition in each layer and gradient structure were also studied.In the first part of the dissertation, the development and properties of Ti(C,N)-based cermets have been critically overviewed. Based on the review, the purpose and significance of the dissertation have been set forth.The effects of three types of forming agents, such as SBP, GBY and PVA on the mechanical properties of TiC-TiN-20%Ni-Mo-WC-Cr3C2-C cermets were studied. And the results showed that the mechanical properties of the cermets with SBP addition are the best, and that of PVA are the worst.The effects of micro WC and sub-micron WC on the mechanical properties and microstructure were studied. The results showed that, compared with the Ti(C,N)-based cermets with micron WC addition, the TRS of the cermets with sub-micron WC addition is higher while the hardness is lower. The best mechanical properties the cermets with sub-micron WC addition were obtained when the cermets are sintered at 1445℃with the TRS of 1890MPa and hardness of 91.2HRA. The microstructure of two cermets exhibit typically "black core-gray rim" structure. While in the cermets with sub-micron WC addition, there exists "white core-gray rim" structure, meanwhile, the particle sizes of hard phase are smaller than that in the cermets with micron WC addition. The result of EDS analysis showed that the white cores and the rims contain the same chemical elements, but the heavy element content such as W and Mo in the white cores is higher. The fracture micrographs showed that the fracture modes in the two cermets are intergranular and transgranular fracture.Ti(Co.5No.5)-WC(sub-micron)-Mo-Ni system cermets were studied. The results showed that the microstructures of the cermets exhibit three kinds of grains:one has black core-grey rim structure, the second has white core-grey rim structure and the third has black core-white rim structure. The thickness of inner rim increases with the increasing of WC content. Additionally, the particle size of black core decreases with the increase of WC content in the range of 0 to 10wt.%, and then increases slightly with further increase of WC. The results of EDS analysis indicated that W content in the rim and binder increases with the increase of WC content, while Ti content in the rim decreases with the increase of WC content, but the change of Mo content in rim is not obviously. The fracture micrographs showed that both intergranular and transgranular fracture occur in these cermets. With the increase of WC content, intergranular fracture increases while transgranular fracture decreases. The results of mechanical properties showed that with the increase of WC content, the TRS increases due to the increase of relative density, while hardness decreases. When the WC content is up to 10%, the best mechanical properties of Ti(C0.5N0.5)-WC-Mo-Ni system cermets are obtained with the TRS of 1987MPa and hardness of 91.2HRA.The XRD results showed that the peaks observed in the sintered (Ti,W)C-Ti(C0.5N0.5)-Ni-Mo cermets are Ti(C,N) and Ni.. The microstructures of (Ti,W)C-Ti(Co.5No.5)-Ni-Mo cermets exhibit a typically core-rim structure:one has black core-grey rim structure, the second has white core-grey rim structure and the third has grey core- black rim structure. With the increase of (Ti,W)C content, the numbers of the grain with white core-grey rim structure increase. The results of EDS analysis indicated that the white core is the undissolved (Ti,W)C solid solution. And the gray core contains the same element as the rim phase, but the W content in gray core is lower than rim phase. The fracture modes of (Ti,W)C-Ti(Co.5No.5)-Ni-Mo-C cermets contain the intergranular fracture of small hard phase and the transgranular fracture of big hard phase. The results of mechanical properties of (Ti,W)C-Ti(Co.5No.5)-Ni-Mo-C cermets showed that, the TRS and hardness firstly increase and then decrease with the increase of (Ti,W)C content. And the optimal mechanical properties can be obtained when the content of (Ti,W)C is up to 36%. But the actual density and relative density increase with the increase of (Ti,W)C content. When the (Ti,W)C content is up to 36%, the best mechanical properties of Ti(Co.5No.5)-WC-Mo-Ni system cermets are obtained with the TRS of 1927MPa and hardness of 91.9HRA.The SiC whisker reinforcement Ti(C,N)-based cermets were fabricated by vacuum sintering. The results showed that the optimal mechanical properties can be obtained with the TRS of 1905MPa and KIC of 9.5MPa.m1/2 when the content of micron SiC whisker is up to 1vol.%. The XRD results showed that when the content of micron SiC whisker is over 1.6vol.%, the MoSi2 peak appeared. That indicated that a part of micron SiC whisker react with Mo. The fracture micrographs showed that there are some white small phases similar to strip shape in the Ti(C,N)-based cermets with the 6% micron SiC whisker addition. The EDS results that the white small phases similar to strip shape are the remnant of the raw micron SiC whiskers. The toughness mechanism are whisker pullout and crack deflexion.The gradient Ti(C,N)-based cermets were prepared with filling in layers by powder metallurgy method. The results showed that the mechanical properties of the symmetry gradient Ti(C,N)-based cermets with five layers are superior to that of dissymmetry gradient Ti(C,N)-based cermets with three layers. And the mechanical properties of the symmetry gradient Ti(C,N)-based cermets with the same binder content in each layer are superior to that of the cermets with different binder content in each layer. The best mechanical properties of the cermets with symmetry gradient Ti(C,N)-based cermets with the same binder content in each layer were obtained with the TRS of 2025MPa and hardness of 91.5HRA. In the symmetry gradient Ti(C,N)-based cermets with the same binder content in each layer, from outer layer to middle layer, the particle size and the numbers of black core decrease, at the same time, the thickness of rim phase increases. And the transgranular fracture of big hard phase decrease from out layer to middle layer. Furthermore, in the middle layer, the developed tear edges and the intergranular fracture of small hard phase were found.