Increasing performance requirements of IC engines often leads to higher combustion chamber temperatures that cause premature failure of exhaust valves. This paper presents a tribological assessment of hollow head & sodium filled valves (HHSVs) produced using a new manufacturing process. Tests were conducted using bespoke bench-top wear and fatigue apparatus, and the HHSV specimens survived defined durability tests. When compared to traditional solid valves, the highest temperature of the hollow stem & sodium filled valves decreased from 745 °C to 590 °C. It was established that the new process did not adversely affect the wear mechanisms (oxidation accelerated adhesion) and the material loss magnitude when compared to solid valves. The design of HHSVs tested gives a 16.1% reduction in mass.
Gradient Zirconia toughened alumina (ZTA) ceramics with high fracture strength and toughness were prepared using centrifugal slip casting method. Aqueous Al2O3-20vol% ZrO2 slurries with different solid contents were prepared and the rheological characteristic of the slurries were investigated. The effect of solid loading of slurries on green density difference of ZTA ceramic compacts was observed. Mass segregation of Al2O3 and ZrO2 particles were studied. Microstructure and bending property of the sintered ZTA ceramics were investigated. The results show that the segregation phenomenon of Al2O3-ZrO2 slurries comes form the difference of settling velocity of Al2O3 and ZrO2 particles. With the increase of solid content of slurries, the gradient distribution of green density becomes unconspicuous. After sintered at 1550○C for 2 h, the gradient ZTA ceramics centrifuged with 20vol% slurries have high sintered density (99.2% TD) and continuous gradient distribution of Al2O3 and ZrO2 particles. The fracture strength and toughness of gradient ZTA ceramic are 732MPa and 5.4MPam1/2, respectively.
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