Kazunori Umeda

Low-pressure plasma-sprayed ZrO2-CaF2 composite coating for high temperature tribological applications

Abstract The microstructure and tribological behavior of low-pressure plasma-sprayed (LPPS) ZrO2–CaF2 composite coatings were studied. Optimum spray parameters were obtained to produce a less porous and strongly adherent ZrO2–CaF2 composite through carefully selecting the powder feed rate, primary gas pressure and spraying distance. The as-sprayed composite coating exhibited a typical lamellar structure of ZrO2 and CaF2 constituents, with a lot of microcracks in the splats. The resolidified interfacial structure featured by fine columnar grains were observed at the boundaries of ZrO2 lamellae and were considered to have formed due to the local temperature and compositional variations during the solidification process of the molten splats. Small amounts of discontinuous oxides distributed at the interface region between the coating and substrate were demonstrated to be a mixture of the complicated oxidized products of iron, chromium, nickel and calcium, ZrO2(Y2O3) particles, and independent Al2O3 and SiO2 particles located within the rough surface of the substrate. The ZrO2–CaF2 composite surface exhibited a distinct improvement in wear resistance and frictional characteristics in comparison to Y2O3-stabilized ZrO2 (YPSZ) coating at elevated temperatures. At 600 and 700°C, the composite exhibited a lower friction and wear than at room temperature, 400 and 800°C. CaF2, acting as a solid lubricant at 600°C, effectively reduces friction and wear. Different tribological behaviors were observed on the worn surfaces, with different microstructural features after the 600°C wear test. In the individual ZrO2 splats, microcracking and microfracture dropping led to material removal. However, in CaF2 splats smooth CaF2 surface films containing fine ZrO2 hard particles was formed to reduce the friction and wear. Brittle fracture and delamination of ZrO2–CaF2 composite were demonstrated to be the dominant wear mechanisms at room temperature and 400°C. Plastic deformation, the continuous formation of CaF2 transfer films, adhesive wear and viscous flow appeared as the dominant wear mechanisms at the higher temperature used in this investigation.

The friction and wear characteristics of low-pressure plasma-sprayed ZrO2-BaCrO4 composite coating at elevated temperatures

Abstract The friction and wear characteristics of low-pressure plasma-sprayed (LPPS) ZrO 2 -BaCrO 4 (ZB) coating at elevated temperatures have been studied using a high-temperature reciprocating wear tester. The ZB coating exhibits a less porous, alternative lamellar structure and a better splat spreading as contrasted to the porous yttria partially stabilized zirconia (YPSZ) coating. The hardness of the ZB coating is 276–435 HV, which is much lower than that of the YPSZ coating (650–727 HV). The ZB coating shows distinct improvements in wear resistance and frictional characteristics at elevated temperatures, as contrasted to the YPSZ coating. At room temperature, the friction coefficient of the ZB coating against sintered Al 2 O 3 is quite high and exhibits an increase in trend with increasing load. At above 300 °C, the ZB coating exhibits low friction and mild wear. But for the YPSZ coating, the situation is reversed with increasing temperature. Brittle fracture and delamination featured by large wear sheets are considered as the dominated wear mechanisms of the ZB coating at low temperatures. However, plastic deformation, formation and transfer of BaCrO 4 lubrication films appears as the main wear mechanisms at elevated temperatures. BaCrO 4 , acting as an effective solid lubricant at above 300 °C, reduces the friction and wear of the coating.

Friction and wear of boride ceramics in air and water

Abstract The tribochemical response of hot-pressed boride ceramics, ZrB 2 , B 4 C, ZrB 2 + B 4 C and ZrB 2 + B 4 C + SiC , has been investigated in air and in de-ionized water. In dry air, the coefficients of friction for all materials tested were about 0.9–0.95, while in humid air of relative humidity (r.h.) 95% they ranged from 0.2–0.3, except for that of ZrB 2 , which was 0.4. Hertzian fractures were noted on the worn surfaces of monolithic boride and ZrB 2 + B 4 C composite, but not on those of SiC-toughened boride or SiC-toughened ZrB 2 + B 4 C . In water, ZrB 2 underwent very severe tribochemical degradation, as suggested by the following reaction: ZrB 2 +10 H 2 O → Zr ( OH ) 4 + 2 H 3 BO 3 + 5 H 2 , while B 4 C, ZrB 2 + B 4 C and ZrB 2 + B 4 C + SiC underwent less tribochemical degradation, and showed lower specific wear rates than ZrB 2 in water.

High Temperature Tribology and Solid Lubrication of Advanced Ceramics

The high-temperature friction and wear characteristics of different ceramics and ceramic matrix composites (CMCs) incorporated with various solid lubricants have been investigated from room temper- ature to 1000oC. The solid lubricants considered in this paper include representative precious metals, hexagonal boron nitride, graphite, fluorides, soft oxides, chromates, sulfates, and combinations of various solid lubricants. General design considerations relevant to solid lubrication were proposed on the basis of friction and wear data of self-lubricating CMCs. The self-lubricating composites incorporated with SrSO4 or/and CaSiO3 exhibits low and stable friction coefficients of 0.2 to 0.3 and small wear rates in the order of 10-6 mm3/Nm from room temperature to 800oC. The optimized composites appear to be promising can- didates for long-duration, extreme environment applications with low friction and small wear rate.

The Spark Plasma Sintering Method Using Laminated Titanium Powder Sheet for Fabricating Porous Biocompatible Implants

☆Rate of shrinkage ・46-52 % (Total of SPS process) ・5-11 % (By sintering) ☆Rate of expansion ・4.7-5.0 % (φ14.31±0.02→φ15 mm) ・Sintering temperature-independent ☆Porosity ・14 % (Sintering temperature: 650°C) ・8 % (Sintering temperature: 700°C) The proposed method is a combined technology of SPS with rapid prototyping or layered manufacturing. A sheet of titanium powder is formed as thick as about 150 μm by the mask printing method. The sheet is processed to form a two-dimensional shape by laser cutting, and then laminated to build a threedimensional object. Finally, a stack of the titanium layers is packed into a carbon mold and sintered by the SPS process. Implant shape by rapid prototyping Pore structure by spark plasma sintering

High-temperature tribological properties of barite-type-sulfate-coated substrates with different isoelectric points

In the present study, Al2O3, (ZrO2-3mol% Y2O3)-39.6mass% Al2O3, Si3N4 and MgO substrates coated with SrSO4 particles were prepared, and their high-temperature tribological properties were investigated. It was clarified that SrSO4 worked as solid lubricant efficiently against Al2O3 and (ZrO2-3mol% Y2O3)-39.6mass% Al2O3 substrates in the temperature range of room temperature to 1073K in air. In addition, thin SrSO4 films were observed on the wear scars formed on the Al2O3 and (ZrO2-3mol% Y2O3)-39.6mass% Al2O3 substrates.