Surface Coating Of Ultrafine Ti(C,N) Powders For Preparation Of Ti(C,N) Based Cermets

The heterogeneous precipitation-thermal reduction process was applied to preparation of metal coated titanium carbonitride composite powders for the first time in this paper. Two new technologies of heterogeneous precipitation-thermal reduction process for synthesizing Ni coated Ti(C,N) composite powders and Ni&Mo coated Ti(C,N) composite powders had been developed,then the Ti(C,N) based cermets were prepared from the as-prepared composite powders after vacuum sintering. The preparation technologies of the coated powders were studied by applying various analysis techniques such as SEM, EDS, XRD, TG-DSC. The microstructure, micro-area chemical composition and properties of the as-prepared cermets were also analyzed.Ni/Ti(C,N) coated powders were synthesized by heterogeneous precipitation-thermal reduction process using nickel nitrate, ammonium bicarbonate and Ti(C,N) powders as the main materials, The optimum process parameters were determined through our experiments, i.e., 0.5μm of particle size of Ti(C,N), 15g/L of Ti(C,N) powders, 3mL/min of feeding rate of ammonium bicarbonate solution, 800r/min of stirring rate, 5~10g/L of PEG6000, 8.0~8.5 of final pH value, then calcing and reducing at 550℃in H2 atmosphere. The Ni/Ti(C,N) coated powders were pressed and vacuum sintered,finally the cermet was obtained. The results show that the distribution of hard phase and binder phase in the cermet is uniform, but the phenomenon of growth of grain still occurs; fracture micrographs show that main failure mode of the cermet is a mixed one, i.e., trans-granular and inter-granular fractures are both exist; the phase constituents of the cermet are pure Ti(C,N) phase and Ni phase; the cermet show better comprehensive properties compared to the cermet which prepared by traditional process.Combing the theory of coordinated precipitation with the mechanism of heterogeneous nucleation,Ni-Mo-C/Ti(C,N) coated powders were also synthesized by heterogeneous precipitation-thermal reduction process using nickel nitrate, ammonium molybdate, ammonium hydroxide and Ti(C,N) powders as the main materials, The optimum process parameters are as follows: 15g/L of Ti(C,N) powders, 800r/min of stirring rate, 8 g/L PEG6000, 1~1.5 times dilution with water, 55~60℃reaction temperature, then calcing and reducing at 850℃in H2 atmosphere. The futher experiments show that an ultrafine and uniform microstructure of (Ti,Mo)(C,N)-Ni cermet was obtained and there is no obvious surrounding structure can be found in the microstructure of the cermet; The fracture micrographs of the cermet show that the fracture is mainly characterized by inter-granular fractures and dimple fractures; the main toughening mechanisms of the cermet are crack deflection and crack bridge; the Ni3Ti phase is founded in the cermet; with the exception of fracture toughness, the mechanical properties of the cermet are better than cermet which perpared by traditional process; because excessive Ti&Mo atoms dissolved in binder phase, the magnetic properties of the cermets are not detected.