Hitoshi Tokura

Grinding induced damage in ceramics

Abstract Although grinding is widely used as a productive technique for finishing ceramic components in the manufacturing industry, it often causes damage to the machined components. The exact nature, the manner and the penetration depth of grinding-induced damage are, however, still not clear, leaving many uncertainties and sometimes danger in using ceramics for structural applications. This paper presents research results on diamond grinding of advanced ceramics, including hot-pressed silicon nitride, hot-pressed alumina, slip-cast zirconia, and pressureless sintered silicon carbide. Grinding induced damage in these ceramics is assessed and characterized using three destructive inspection techniques and progressive lapping technique combined with scanning electron microscopy (SEM), and transmission electron microscopy (TEM). As a result, two types of grinding damage are identified, pulverization and microcracking. Damage depth is found to be related to the properties of ceramic materials, especially their brittleness. For a given grinding condition, damage penetrates deeper in less brittle materials than in more brittle materials. In addition, two types of grinding-induced microcracks are identified, scattered and clustered. The former is observed on all four types of materials tested under various grinding conditions, while the latter is only associated with less brittle materials subjected to relatively aggressive grinding conditions. The mechanisms of damage nucleation and propagation are discussed. The results provide valuable insights into the dependence of grinding-induced damage on the properties of workpiece materials, and on the grit size of grinding wheels.

Properties of hydrogels synthesized by freezing and thawing aqueous polyvinyl alcohol solutions and their applications

The hydrogel synthesized by freezing a polyvinyl alcohol (PVA) aqueous solution and thawing it slowly has high water content, excellent mechanical properties of high tensile strength, elongation and good shape recovery by elasticity. The PVA used had a degree of polymerization of 2500 and a degree of saponification of 99.5 mol%. The solution was obtained by dissolving 7.5 g of PVA in 80 g of water, this was frozen at −50°C for 3 h and then warmed up to room temperature over 10h. This freezing–thawing process was repeated once again and a hydrogel was synthesized. The hydrogel had a water content of about 90 wt%. Its tensile strength was 0.6 MPa and the elongation at break was 130%. The shape of the hydrogel which was deformed by an external force recovered in a short time when it was released from the force. This recovery had good persistence and repeatability. Applying these properties a strain sensor and a gas pressure sensor were tested. Furthermore, a PVA hydrogel rod containing polyacrylic acid was used as a bending actuator. This hydrogel had the ability to deform when direct current was applied.

Smoothing of chemically vapour deposited diamond films by ion beam irradiation

Abstract In this paper, smoothing of chemically vapour deposited diamond films by ion beam irradiation is described. Diamond films were synthesized by arc discharge plasma jet chemical vapour deposition (CVD) and microwave plasma CVD. Argon and oxygen gases are used as an ionized gas. As a result, by setting the incident angle at 0° and 80°, smooth surfaces of microwave plasma CVD diamond films were obtained. The peak-to-valley (p-v) surface roughness is reduced from 3 μm to 0.5 μm at 80°. In addition, we discuss the smoothing mechanism of diamond films and describe it using a simple model.

Study on surface cracking of alumina scratched by single-point diamonds

Scratching experiments have been carried out on hot-pressed alumina by using single-point diamonds of conical shape (conical angle 85, 108, 128 and 65° and nose radius 1.6, 1.1, 1.9 and 45.0μm, respectively). With an increase of the scratching depth, the material exhibits the following behaviour: macroscopic plastic deformation → scale-like cracking → cracking or chipping. Cracking or chipping is absent only if the groove depths are less than certain values which vary with the diamond shape. The crack penetration depth is approximately in proportion to the depth of cut. For a depth of cut less than 2μm, the penetration depth of crack induced by the diamond of nose radius 45.0μm is almost 8 times those produced by the other diamonds. For a depth of cut more than 2μm, the former is more than twice the latter. The coefficients of pile-up increase as the depths of cut decrease.

Study on the polishing of chemically vapour deposited diamond film

Abstract Using the reaction between diamond and metal at high temperature, diamond film deposited by chemical vapour deposition (CVD) was polished. The effects of polishing conditions on the polishing rate of CVD diamond film were estimated and the optimum polishing conditions determined experimentally. The polishing mechanism is discussed. The polishing rate mainly depends on the diffusion of carbon atoms from the CVD diamond film into a metal polishing plate. The formation of methane as a result of the reaction of carbon with hydrogen, and the etching by the oxygen remaining in the polishing atmosphere are also effective in the removal of CVD diamond film. A mechanism for this polishing is proposed.

Ceramic material processing by electrical discharge in electrolyte

New ceramic materials receive a great deal of attention as machine components, but they are hard to work. So a hybrid processing which combines electrical discharge processing with grinding is proposed. In this study, in order to examine the possibility of this hybrid processing, the electrical discharge processing on four kinds of ceramics was carried out with a needle electrode. The ceramic materials were alumina and three kinds of silicon nitride series to which are added alumina (ASN), magnesia (MSN), yttria and alumina (YASN). The results obtained showed that a pit can be formed on any ceramic and the pit depth apparently varies with the ceramic material. The removal rates of ASN, alumina, YASN and MSN become low in turn, and are independent of their mechanical properties. In the case of the silicon nitride series, the removal rates depended on their sintering additives, and the higher the applied voltage, the greater was the volume removed. High removal rate and low electrode loss are obtained when the needle electrode is negative. The ceramic materials are mainly removed by etching the grain boundary in a high-temperature electrolyte during the discharging process.

Metal particle manipulation by laser irradiation in borosilicate glass

We propose a new technique of manipulating a metal particle in borosilicate glass. A metal particle that is heated by laser illumination heats the surrounding glass by radiation and conduction. A softened glass enabled metal particle migration. A 1-µm-thick platinum film was deposited on the back surface of a glass plate and irradiated with a green CW laser beam through the glass. As a result, the platinum film was melted and implanted into the glass as a particle. Platinum particles with diameters of 3 to 50 μm migrated at speeds up to 10 mm/s. In addition to platinum particles, nickel and austenitic stainless steel (SUS304) particles can be implanted.

Heat treatment of diamond grains for bonding strength improvement

The heat treatment of synthetic cubo-octahedral diamond grains to produce tight bonding by making their surface rough is reported. In order to obtain a rough surface, a thermal etching technique at atmospheric pressure in air at 700 to 1100°C is applied. The shape, surface features and surface area are investigated, then in order to examine the effect of the roughened diamond surface on bonding strength, peeling and bending tests were carried out. The results obtained showed that: rough surfaces may be obtained by thermal etching at atmospheric pressure in air; on etching at 700 to 1000°C, the etch rate of the {111} face is higher than that of the {100} face, and in particular, diamonds etched at 700 to 1000°C have a hollow {111} face; the surface area of one grain can be increased by etching — on etching at 900°C for 15 min, surfaces become fully covered with clear etch pits and the surface area shows maximum value; the surface area seems to have an influence on bonding strength, and when diamond grains are bonded with phenol resin or electroless plating nickel, bonding strength improves by about 10%.

Polishing of CVD Diamond Film

Abstract Using the reaction between diamond and metal, CVD diamond film has been polished. The optimum polishing conditions is determined experimentally. Simultaneously the polishing mechanism is discussed. Stock removal depends on the diffusion of carbon from diamond to a metal plate and the formation of methane as a result of the reaction between carbon and hydrogen, simultaneously depend on the etching by rest oxygen.

CW-laser induced modification in glasses by laser backside irradiation (LBI)

We describe modification of various glasses by a CW Laser Backside Irradiation (CW-LBI) method. In this method, an absorbent which was attached on one side of a glass sheet, was irradiated from the other side through the glass with CW laser beam. Ar ion laser was used and copper foil was chosen as an absorbent. Silica, Pyrex and soda-lime glasses were tested as sample glasses. When the absorbent was irradiated, heated spot appeared in the glass adjacent to the absorbent and it ran to the other side of the glass in the path of laser beam. Then cylindrical modified zone that was transparent and crack-free was produced. Diameter of the modified column was 30~100 μm and the cylinder could grow up to 30 mm. The growth rate was approximately 200 mm/s. The dimension of modified zone depended on irradiation conditions, such as laser fluence or beam profiles. The threshold fluences for modification of the glasses were 1~2.5 × 105 W/cm2. Then the characteristics of modified zone were examined by etching in 5 wt. % HF solution. As a result, it turned out that modified zone consisted of two layers and etching rate of the inner part was lower than that of the outer part. The Raman spectroscopy revealed that the density of the inner part increased by the modification.