Kimiaki Tanihata

Gas pressure combustion sintering and hot isostatic pressing in the Ti-Si-C system

The processing of materials in the Ti-Si-C system produced by gas pressure combustion sintering (GPCS) and hot isostatic pressing (HIP) was studied. By variation of the raw material composition it was possible to produce different materials containing Ti5Si3, Ti3SiC2, TiSi2, TiC, SiC and titanium. The different phase compositions of samples with the same reactants obtained by GPCS and HIP are due to the duration of each of the processes; in GPCS the equilibrium state was not achieved. The properties of the specimen are strongly related to their chemical behaviour during processing at high temperatures. Not all phases built up at the high temperatures during the GPCS and HIP processes are stable at lower temperatures, and some reactions occur during cooling, which influence densification, microstructure and properties of the bodies finally obtained.

Toughening effects of WC/Co particles and compressive surface stress on (Al2O3–WC/Co)/TiC/Ni graded materials

A graded material of the (Al2O3–WC/Co)/TiC/Ni was prepared by using the self-propagating high-temperature synthesis-aided hot isostatic pressing process. The WC/Co particles were added to the outer Al2O3 layer in order to activate the heterophase toughening effects and control the thermal expansion mismatch between the outer and inner layers. SEM and TEM observations show a clustering structure of the WC/Co cermet in the Al2O3 matrix. The residual stress introduced by the compositionally graded structures was analysed using FEM calculation and X-ray diffraction. Measurements of the indentation toughness and R-curve behaviour indicated that the compressive surface stresses and WC/Co particles can significantly strengthen and toughen the Al2O3 ceramics. The indentation toughness of the (Al2O3–WC/Co)/TiC/Ni at the surface was 13.1 MPa m1/2, while those of the (Al2O3–WC/Co) uniform composite and the monolithic Al2O3 were 7.3 and 4.0 MPa m1/2, respectively. It was found by SEM observation of the crack propagation induced by indentation, thatthe crack deflection, bridging, blunting and arresting occurred at the WC/Co clusters.

Microstructure and property of (Si-MoSi2)/SiGe thermoelectric convertor unit

This research suggested a one-step process to fabricate the thermoelectric conversion cell with its electrode. Si0.78Ge0.22 alloy and (Si-MoSi2) composite were used as a thermoelectric conversion material and an electrode material, respectively. The HIP process was employed to prepare samples. The experimental and theoretical analyses on the thermal stress and electrical resistivity identified that this process has good prospects to substitute for jointing of SiGe with electrode.

Diffusion Bonding of A12O3 and Si3N4 Ceramics by Hiping

Diffusion bonding of Al2O3 and Si3N4 attempted using high pressure technology. The high pressure and temperature conditions were 100 MP a and 1650°C for HIPing and 3.0 GPa and 1650°C for high pressure experiments. The results of SEM observations and Vickers microhardness measurements for the interface of the bonded specimens suggested that the strong bonding between these sintered ceramic bodies was completed and that the high pressure technique is one of the useful methods for diffusion bonding of ceramics.

Effect of Microstructure and Internal Stress on Mechanical Properties Of WC/Co Doped Al2O3/TiC/Ni FGMs

The intrinsic low fracture toughness of ceramics limits their potential applications. Various processes have been developed to improve the fracture toughness of ceramics. One is to incorporate reinforcements, such as crack bridging, crack deflection and whisker or fiber pull-out, into the ceramic matrix to activate toughening mechanisms.1–3 Another well- recognized method to enhance crack propagation resistance is to introduce a compressive stress in the surface of ceramics. Lange4 introduced a compressive surface stress into Si3N4/ZrO2 ceramics by utilizing the volume expansion of the Zr oxynitride to monoclinic ZrO2 by oxidation. He found that the apparent surface fracture toughness was directly related to the oxidation kinetics and the initial volume content of ZrO2.

Fabrication of multifunctional gradient materials in the system of Al2O3/TiC/Ni/TiC/Al2O3 by SHS/HIP

Symmetrically arranged functionally gradient materials in the system of Al2O3/TiC/Ni/TiC/Al2O3 were fabricated by SHS/HIP. The compressive residual stress of 200 MPa appeared at the Al2O3 surface due to the thermal expansion mismatches between the surface and center layers. The symmetric FGMs are expected as multifunctional and toughened materials.

Gas Pressure Combustion Sintering of Materials in the Ti-Si-C-System

The processing of gas pressure combustion sintering (GPCS) to produce dense materials in the system Ti-Si-C was studied. By systematic variations of the reactants composition it was possible to produce different materials consisting of Ti5Si3, TiSi2, Ti3SiC2, TiC or SiC. The complete densification of the samples was not in all the cases successful. It is speculated that the time at high temperatures was too short to densify the materials with more refractory character. Besides, not all the phases built up by combustion at high temperatures are stable at lower temperatures and some reactions occur during cooling influencing densification and properties of the final material. Some properties of full densified samples produced by GPCS are also given. The ceramic-metal composite produced by higher addition of Ti showed better mechanical properties in bending strength and fracture toughness.

Gas-Pressure Combustion Sintering of TiC-Ni Composites Using Hip

Fully densified TiC-Ni composites with different Ni content from O to 65 vol.% and Ni were fabricated by the gas-pressure combustion sintering method using HIP. The composites were synthesized and densified simultaneously and rapidly under isostatic pressure of 100MPa Ar gas. Some mechanical and thermal properties such as flexural strength, Young’s modulus, thermal conductivity and thermal expansion were measured for different Ni content. The microstructure changes were also observed by using SEM.

Nitriding of Titanium Surface by the Self Propagating High Temperature Synthesis.

A new nitriding process titanium furface using the self propagating high temperature synthesis has been developed. A titanium plate was embeded in or coated with the self combustion agent of Nb or Ti metal powders, and the agent was ignited in a pressurized nitrogen gas atmosphere of 2.5Mpa. The nitriding reaction propagated through the agent and the surface of titanium was rapidly nitrided by the strongly exothermic heat over 2000K. The hardness, corrosion resistance and tensile strength of the surface nitrided titanium obtained by this method were comparable or superior to those of the surface nitrided titanium obtained by the conventional gas nitriding process.