Interfacial Diffusion Behavior Between TiN_0.3 And Refractory Compounds And Strengthening And Toughening Of Composites

In order to improve sinterability and fracture toughness, nonstoichio metric TiN_0.3 was added to the refractory compound matrixes, such as Al N, Ta C and Al₂O₃, respective ly. The diffusion behavior in binary systems, as well as the consequent microstructure, sinterability and mechanical property of binary composites, was researched.A micro-region diffusion phenomenon was found at the Al N-TiN_0.3 interface during sintering. N atom from Al N diffused to fill N vacancy in Ti N 0.3 lattice; simultaneously, due to the existence of O2 as impurity, a part of Al atoms released combined with O2 to form Al₂O₃. During the process, TiN_0.3, Al N and O2 took place reaction and diffus ion behavior, forming a series of Al ON. The phase evolut ion of Al N- TiN_0.3 composites was investigated using X- ray diffractometer（XRD）. The phase composition of Al N- TiN_0.3 composites is controlled by the proportions and particle size of raw materials, sintering temperature, and introduction of a third phase material. The number and type of phases in the Al N- TiN_0.3 system is controlled by Al content at the same sintering temperature. The amount of dissolved Al N in Al ON increased with raising the sintering temperature; and there were different compositions of Al ON solid solution, among which Al8O3N6（1700 ℃） or Al9O3N7（1800 ℃） was main. Under the same experimental conditions, Al₂O₃ was much easier to form as the particle size of Al N was decreased from 0.5 μm to 0.05 μm. There are many similarities between VC and Ti N. O wing to introducing VC into Al N- TiN_0.3 composite(VC /TiN_0.3/Al N mole ratio 1:1:1), N vacancy concentration in nonstoichiometric compound was decreased and it was accelerated to re- form Al N, so the reaction between Al₂O₃ and Al N was enhanced.（Ti, V）（N, C）y and Al9O3N7 was appeared in the composite sintered at 1700 ℃.A diffusion phenomenon was also found in TiN_0.3-Si₃N₄ composite sintered via Spark Plasma Sintering（SPS）. There was main phase Ti1-y Siy Nx having face-centered-cubic structure, as well as a little Si or Ti-Si compounds, in TiN_0.3-Si₃N₄ composites sintered at 1500 ℃.The diffusion behavior and solid solution formation in the binary system Ta C- TiN_0.3 were investigated. Ta C-TiN_0.3 and Ta C- Ti N diffusion couples were used for the study of diffus ion behavior with an energy dispersive spectrometer（EDS）. The thickness of the diffusion layer in Ta C- TiN_0.3 diffusion couple increased obviously with the increase of temperature; and under the same condit ions, it was thicker than that in Ta C- Ti N diffus ion couple. The interdiffusion coeffic ient of Ta, Ti, and N was calculated using the Boltzmann-Matano method. At the same heat treatment temperature, the interdiffusion coefficient of Ti and N is closer to ea ch other and smaller than that of Ta in the Ta C- TiN_0.3 system. In the Ta C- Ti N system, the interdiffus ion coefficient of N and Ti is larger than that of Ta, and the interdiffus ion coefficient of N is larger than that of Ti. The interdiffusion coeffic ient of the three atoms is larger in the Ta C-TiN_0.3 system than in the Ta C- Ti N system. Solid solut ion formation in the Ta C- TiN_0.3 system with varying compositions and heat treatment temperatures was characterized by XRD. Ta1-x Tix（C, N）m solid solut ion was formed when less than 50 vol.% TiN_0.3 was added into Ta C. Both Ta C-based and TiN_0.3-based solid solut ions appeared in the XRD patterns when more than 50 vol.% Ti N 0.3 was added into Ta C. Ta, Ti, C, and N atoms show faster diffus ion and higher dissolut ion rate in the Ta C- TiN_0.3 system than in the Ta C- Ti N system because of the vacancy effect.When Al₂O₃-TiN_0.3 composites were sintered via SPS, a micro-region diffusion phenomenon was observed at the Al₂O₃- TiN_0.3 interface. Ti atoms from TiN_0.3 diffused into Al₂O₃ to occupy Al sites, which led to the formation of Al vacancies that enabled the transport of aluminum by a vacancy mechanism. The optimal densification temperature of the Al₂O₃-30 vol.% TiN_0.3 composite was approximately 1400 °C.The mechanical properties of the composites were enhanced by adding TiN_0.3 into refractory compounds, especially for fracture toughness. For the Al₂O₃-30 vol.% TiN_0.3 composite sintered at 1400 °C, the fracture toughness, relative density, hardness and bending toughness are 6.60 MPa·m1/2, 98.7%, 18.8 GPa and 528 MPa, respectively; the maximum fracture toughness measured is 6.91 MPa·m1/2, from the composite with 30 vol.% TiN_0.3 sintered at 1500 °C. When TiN_0.3 and Al N were mixed as 4:1 molar ratio and the composite was sintered at 1600 ℃, the fracture toughness, hardness and bending toughness are 7.43 MPa·m1/2, 17.7 GPa and 507 MPa, respectively. For the Ta C-30 vol.% TiN_0.3 composite sintered at 1500 °C, the maximum fracture toughness and hardness measured are 5.46 MPa·m1/2 and 19.3 GPa, respectively.