Yukihiko Yamauchi

Effects of heating rate and particle size on pulse electric current sintering of alumina

Two Al2O3 powders with different particle sizes were sintered by pulse electric current sintering method at different heating rates. Rapid heating reduced grain growth rate, and the level of reduction depended on the initial powder size and sintering temperature. Under certain conditions, rapid heating could enhance densification.

Tribological properties of dimpled silicon nitride under oil lubrication

It is known that under lubricated conditions, dimpled surfaces can show improved tribological properties under planar/planar contact. This study investigates the effect of dimpling on the tribological properties of a silicon nitride ceramic against hardened bearing steel under oil lubricated, planar to curved surface contact conditions. Block-on-ring friction tests under oil lubrication were carried out over a range of sliding velocities, with a silicon nitride ceramic whose surface was dimpled by abrasive jet machining (AJM). The dimpled surface showed better tribological properties in terms of reduced friction coefficient compared with a lapped surface.

Material response to particle impact during abrasive jet machining of alumina ceramics

Abstract Abrasive jet machining (AJM), a specialized form of shot blasting, attracts much attention as a hopeful micro-machining method for hard, brittle materials such as structural ceramics. The present paper attempts to identify the material response of alumina ceramics to the abrasive particle impact in the AJM process. Three kinds of commercial abrasive particles were utilized to dimple the sintered alumina samples, and it was found that the material response to particle impact depends drastically on the employed abrasives. The softest abrasive, aluminum oxide (WA), leads to roughening of the alumina surface but causes no engraving, due to the lack of the abrasive hardness against that of the workpiece. When silicon carbide (GC) abrasive is employed, a relatively smooth face can be produced, principally as a result of ductile behavior under the elevated temperature caused by the abrasive impacts. The impingement by synthetic diamond (SD) abrasive tends to cause large-scale fragmentation, and therefore the impacted surface becomes rough.

Effect of workpiece properties on machinability in abrasive jet machining of ceramic materials

Abrasive jet machining (AJM), a specialized form of shot blasting using fine-grained abrasives, is an attractive micro-machining method for ceramic materials. In this paper, the machinability during the AJM process is compared to that given by the established models of solid particle erosion, in which the material removal is assumed to originate in the ideal crack formation system. However, it was clarified that the erosion models are not necessarily applicable to the AJM test results, because the relative hardness of the abrasive against the target material, which is not taken into account in the models, is critical in the micro-machining process. In contrast to conventional erosion by large-scale particles, no strength degradation occurs for the AJM surface, which is evidence that radial cracks do not propagate downwards as a result of particle impacts.

Wear properties of Y–α/β composite sialon ceramics

Abstract The tribological properties of yttrium containing α/β composite sialon ceramics have been studied under non-lubricated conditions by means of block-on-ring and ball-on-disk type experiments against a commercial silicon nitride material. The sialon ceramics were produced by hot pressing powder mixtures of Si3N4, AlN, Al2O3 and Y2O3, resulting in composite ceramics containing different amounts of the α/β phases. The effects of microstructural differences on the mechanical properties of the ceramics, and their wear characteristics under a range of testing conditions have been assessed. It was found that Vickers hardness decreased whilst both fracture toughness and bending strength increased with increasing amount of β phase in the composite. Under mild testing conditions, material removal was considered to occur by polishing of the surface, and in this case the high α-sialon composites exhibited the highest wear resistance, reflecting their higher hardness. Under severe testing conditions, the wear behaviour was characterised as microcracking caused by the higher Hertzian stress levels, and resulted in grain removal or “dropping” from the surface of the materials. Under these conditions, the elongated microstructure and higher fracture toughness of the low α-sialon composites hinder the crack propagation and result in better wear characteristics when compared to the fine equiaxed α-sialon materials.

Tailoring the mechanical properties of silicon carbide ceramics by modification of the intergranular phase chemistry and microstructure

Abstract Liquid-phase-sintered SiC has attracted increasing interest for its ability to form an in-situ toughened material and its potentially superior mechanical properties relative to the solid-state-sintered SiC. In the present work, a submicron-size β-SiC powder was densified with additives of various combinations of rare-earth oxides (RE 2 O 3 ; RE=La, Nd, Y and Yb) and alumina by hot-pressing, and the hot-pressed materials were further annealed at higher temperatures. The phase compositions, microstructures and mechanical properties of the hot-pressed and the annealed materials were characterized. It was found that the mechanical properties were strongly influenced by the type of the sintering additives. The additives also affected the microstructural development during annealing. While the fracture toughness of all the annealed materials continuously increased with increasing annealing temperatures, however, the flexural strength either increased or decreased with annealing temperatures, depending on the kinds of the RE 2 O 3 additives. By using appropriate rare-earth oxides, e.g. La 2 O 3 or Nd 2 O 3 , fracture toughness and flexural strength synergetically improved after an annealing treatment.

Effects of rare-earth oxide and alumina additives on thermal conductivity of liquid-phase-sintered silicon carbide

SiC ceramics were prepared from a β-SiC powder doped with two different sintering additives-a mixture of La 2 O 3 and Y 2 O 3 and a mixture of Al 2 O 3 and Y 2 O 3 -by hot pressing and annealing. Their microstructures, phase compositions, lattice oxygen contents, and thermal conductivities were evaluated. The SiC doped with rare-earth oxides attained thermal conductivities in excess of 200 W/(m K); however, the SiC doped with additives containing alumina had thermal conductivities lower than 71 W/(m K). The high thermal conductivity of the rare-earth-oxide-doped SiC was attributed to the low oxygen content in SiC lattice, high SiC-SiC contiguity, and lack of (3- to α-SiC polytypic transformation. The low thermal conductivity of the alumina-doped SiC was attributed to the point defects resulting from the dissolution of Al 2 O 3 into SiC lattice and the occurrence of polytypic transformation.

Wear behaviour of α-Si3N4 ceramics reinforced by rod-like β-Si3N4 grains

Abstract The microstructure of silicon nitride ceramics has a significant affect on their mechanical and tribological properties. The grain morphology, in particular, is a significant factor for controlling wear resistance. In this study, α-Si 3 N 4 with various amounts of in-plane aligned rod-like β-Si 3 N 4 grains were fabricated by hot pressing the tape cast and stacked green bodies with a small amount of rod-like β-Si 3 N 4 seeds. The effect of anisotropy in microstructure and β-Si 3 N 4 content on tribological properties was investigated by means of block-on-ring tests without lubricant using commercially supplied Si 3 N 4 as ring specimens. Three sliding directions were set for the texured Si 3 N 4 used as the block specimens in the friction test; directions perpendicular to and parallel to the casting direction on the casting plane (referred to as C-perp and C-para, respectively), and a direction parallel to the casting plane on the stacking plane (referred to as S). The effect of sliding direction on wear rate was small for the textured Si 3 N 4 with less than 20% β-Si 3 N 4 content, but it became larger with the increasing β-Si 3 N 4 content. The C-para specimen exhibited larger wear rate than the other specimens of textured Si 3 N 4 with more than 30% β content. The worn surface of the C-para specimen was irregular owing to grain pull-out, in contrast to the other specimens whose surfaces were quite smooth. In addition, for the textured Si 3 N 4 with more than 30% β content the wear resistance of the S specimen was higher than of the C-perp specimen. The wear behaviour of the textured Si 3 N 4 with high β content can be explained by the toughness anisotropy, assuming that wear proceeds through generation of microcracking, followed by their propagation mainly parallel to the sliding surface.

Tribological behaviour of uni-directionally aligned silicon nitride against steel

Abstract Textured silicon nitride, where the β-Si 3 N 4 grains were uni-directionally aligned, was fabricated and the effect of anisotropy in microstructure on tribological properties was investigated, compared with a conventional Si 3 N 4 . The wear tests were carried out for the tribopair of textured silicon nitride ceramic and steel using a block-on-ring tester without lubrication. For the textured Si 3 N 4 , tribological properties were evaluated in three directions with respect to the grain alignment; the plane normal to the grain alignment and in the directions parallel and perpendicular to the grain alignment in the side plane. The friction coefficient values of each specimen were of the same level under the same sliding conditions. The values of specific wear rate for the plane normal to the grain alignment were lower than those of the other specimens for all sliding conditions. It is considered that the high wear resistance of this plane was caused by restricted microfracture, such as grain dropping and minimal abrasion by wear debris. Both the friction coefficient and specific wear rate were decreased with increasing sliding speed and normal load because of the formation of lubricative FeO between the sliding surfaces.

Grain growth in microwave sintered Si3N4 ceramics sintered from different starting powders

Abstract Silicon nitride ceramics have been produced by microwave sintering at 28 GHz with Y 2 O 3 , Al 2 O 3 and MgO as sintering additives. The effect of initial β content of the Si 3 N 4 starting powder on the microstructural development has been assessed by scanning electron microscopy (SEM) and quantitative image analysis. Phase transformation behaviour was assessed by X-ray diffraction. Mechanical properties of the sintered bodies were determined through assessment of hardness and fracture toughness. It was found that the samples sintered from powders with lower initial β content developed larger grains than those from higher β content powders, due to fewer nucleation sites during the α→β transformation. However, attempts to develop a more bimodal microstructure by using a mixture of the two grades of powder, in an effort to increase both fracture toughness and fracture strength, were unsuccessful. In this case the microstructure was similar to that developed in the materials produced from higher β content powders. The mechanical properties of the sintered bodies were very similar, despite differences in microstructure. This was attributed to the strong bonding between the grains and grain boundary phase resulting in crack paths in all the materials that were predominantly transgranular, with little debonding or crack deflection. Under these circumstances the effect of larger grains is eliminated.