Research On The Ti(C,N)-based Cermets Of High Performance And Cutting Properties Of Cermets Cutters

In order to improve the strength and toughness of Ti(C,N) cermets efficiently and make cermets win more and more wide applications, wettability of nickel on various carbides in cermets, relationship between valence electron structure (VES) and wettability, the microstructures and mechanical properties of Ti(C,N)-based cermets materials with nano-TiN modifications and cutting properties of cermets cutters are investigated systematically in this dissertation by means of powder metallurgy, materials science and empirical electron theory of solids (EET theory).In the first part, the development process, microstructures and properties, present research situation, problems presently existing and application areas as cutters have been outlined and a special attention is paid to the role and effect of grain growth inhibitors on the microstructures and properties of cermets and research status in the nano reinforcing Ti(C,N)-based cermets and ultra-fine grained Ti(C,N)-based cermets. Simultaneously, the present research progress of EET theory is also outlined and the development trends of cermets pointed out. Grounded on the above work, the purposes and significance of this dissertation have also been pointed out. Preparation techniques for wetting test samples was briefly given and results have shown that ceramic substrates with high density (above 97 %) can be prepared by using hot press technology (pressure being 30MPa, temperature being 1800℃ for 1 h at an argon atmosphere).Furthermore, wettability of nickel on various carbides in cermets was investigated and wetting tests reveal that contact angle (θ) reduces continuously and wettability of Ni/(Ti,Me)(C,N) are improved efficiently with the addition of various carbides such as Mo₂C, TaC, WC, VC as well as NbC and contact angle ( 9) decreases definitely with the increase of carbides content, the impact order of the carbides on θis Mo₂C>TaC>WC>VC >NbC. It is also found that wetting mechanism of Ni/(Ti,Me)(C,N) system is virtually a process in which diffusion and dissolution of elements are characterized. On the other hand, it can be deduced from EET calculations that the covalent electrons on the strongest bond (n_a) in ceramic phases increase with the content of carbides, and the impact order is VC>Mo₂C>NbC>WC>TaC. The author have also posed the regression relationship among the chemical composition (x), contact angle (θ) and valence electron structure (n_A). In this section, the author have also found that addition of M02C can efficiently improve the wettability between (Ti,Me)(C,N) and Ni and the interfacial binding and strength are thereforeimproved, and cermets with high strength and toughness can be obtained.In the second part, microstructures and mechanical properties of TiC based cermets with nano-TiN modifications are presented. Results reveal that addition of nano-TiN particles or compound addition of nano-TiN and micro-TiN can to some extent strengthen the cermets and a finer microstructure and higher mechanical properties have been obtained with the increase of content of nano-TiN. Investigations have also shown that optimally comprehensive properties are achieved when the content of nano-TiN reaches 6⁸wt%.TEM characterizations reveal that dispersive distribution of residual nano-TiN particles at/among TiC interfaces results in the fining effect and improvements of mechanical properties of cermets. On the other hand, the effects of Mo on the microstructures of 5Co-5Ni cermets and lOCo-lONi cermets are not identical. Mechanical tests show that bending strength and fracture toughness decline definitely with addition of Mo and reach a maximum value when Mo content is about 4 wt%. In general, the reinforcing mechanism of nano-particles on cermets can be summarized as grain fining strengthening, dispersion strengthening and solid solution strengthening.Next, the cutting characteristics and wearing mechanism of cermets cutters with nano-TiN modifications are discussed in detail. Results show that in machining normalized medium carbon steel, gray cast iron, quenched steel and stainless steel cermets cutters with nano-TiN modifications definitely exhibit better cutting properties and longer tool life in comparison with cemented carbide cutters (YT15 and YG8) and conventional cermets cutter. Grain wear, diffusion wear and oxidation wear are predominant wear mechanism when the cutter is cutting at a higher speed (200 m/min 400m/min). Comparably, the main wear mechanism changes to adhesion wear, diffusion wear and oxidation wear when the cutter is utilized in cutting gray cast iron. Investigations also show that in cutting quenched steel cermets cutter readily fails in the form of "chipping".Finally, thermal shock resistance of cermets with nano-modifications is investigated. Results reveal that the quantity and size of the voids plus the size of micro-cracks in cermets increases with thermal shock cycles (N) and cermets with nano-modifications possesses a better thermal shock resistance in comparison with conventional cermets materials.