The contact fatigue tests were first carried out using Ni-Mo pre-alloyed sintered steel (46F4H) spur gears with different densities in the range of 7.30~7.54 Mg/m³. The sintered gear specimens were machined from sintered steel packs made from the single-press single-sinter route and some were surface-rolled using CNC form rolling machine. All test gears were case-carburized. The experimental results showed that the surface-rolled sintered gears with a density of 7.40 Mg/m³ or more had sufficiently high contact fatigue strength to replace gears made of typical Cr-Mo case-carburized wrought steel. These experimental results were then analytially investigated by examining the contact stress distributions around the operating pitch point of P/M pinion using a simplified two-dimensional finite element model considering voids. The peak of the maximum shear stress τ_max increased with the decrease of density of the as-sintered gears and decreased as the amount of fully densified depth δ increased. There was only a slight difference in the distribution profile of τ_max for every P/M pinion when δ reached to a specified depth of approximately 0.3 mm. These simulation results appear to explain the gear-running test results well and reveal the appropriate surface densification level.

The recycling process of the machined chips from the commercial Ti-6%Al-4%V (Ti-64) alloys to raw powders was established through the combination of brittle TiH₂ compounds formation via heat treatment in hydrogen gas atmosphere and fragmentation by ball milling technique. TG-TDA and XRD analysis obviously suggested the hydride and dehydride behavior of pure Ti and TiH₂ powders. The suitable heat treatment temperature at 873 K or more in H₂-Ar mixed gas successfully caused the formation of TiH₂ compounds, and resulted in the fragmentation of Ti-64 machined chips to powders with a median particle size of 120 μm, which were completely consolidated by pressing. In addition, the original machined chips never remained in the ball-milled machined chips after heat treated over 873 K. The green compact after vacuum sintering at 1273 K showed a relative density of about 93%, larger than that of the sintered material in using the commercial Ti-64 powders.

Piezoelectric performance can be improved by using crystal-axis oriented materials because piezoelectric properties of the ferroelectric materials are dependent on the crystal-axis direction. In this study, we tried to fabricate Ba_1−xCa_xTiO₃ (BCT) oriented ceramics by reactive template grain growth (RTGG) method using H_1.07Ti_1.73O₄ layered titanate (HTO) plate-like particles as the reactive template. BCT ceramics with [110]-orientation were prepared using HTO-BaCO₃-Ca(OH)₂ (HBC) and HTO-BaCO₃-Ca(OH)₂-TiO₂ (HBCT) reaction systems. The degree of orientation and the relative density of the BCT ceramics are dependent on the green sheet preparation conditions, such as dispersion condition and added binder amount, calcination temperature and starting material composition. In these reaction systems, firstly oriented BCT particles are formed by a topotactic reaction of HTO template particles with Ba and Ca matrix components, and then, BCT oriented ceramic is formed by the growth of the oriented BCT particles. BCT ceramics with higher density and higher orientation can be fabricated using HBCT reaction system than HBC reaction system, where the orientation degree is dependent strongly on the ratio of HTO template particle to TiO₂ matrix particle.

Microstructural and mechanical properties of powder metallurgy (PM) copper (Cu) composites dispersed with carbon nanotube (CNTs) were investigated in detail. A pure copper powder was coated with un-bundled CNTs by using the zwitterionic surfactant solution containing CNTs. The powder rolling process was applied to increase the powder surface area to be coated with CNTs. The total rolling reduction of Cu-CNT composite powder by 5 times rolling was about 75%. With increasing the rolling number, the content of CNTs coated on the Cu powder surface increased because of the increment of the flat surface area of flaky Cu rolled powder. As a result, the CNT content was 0.67 mass% after 5 times powder rolling. It was about twice as that of as-coated Cu-CNT composite powder without rolling process. The grain size of PM extruded Cu-CNT composite powder material was about 1/5 of that of the extruded monolithic Cu material without CNT. Yield stress of the extruded material of CuCNT composite via the rolling processing was 192 MPa, and about twice compared to the extruded monolithic Cu material (88 MPa). CNTs distributed at primary particle boundaries were effective to prevent the grain coarsening by their pinning effects, and this grain refinement was a main strengthening factor of the Cu-CNT composite material via rolling process.