Papers by Author: Hitoshi Sumiya

Abstract: Wear characteristics of binder-less (single-phase) nanopolycrystalline diamond (NPD) and cubic boron nitride (BL-PcBN) were investigated by rubbing them against various ceramics such as SiO₂, Si₃N₄, Al₂O₃ and SiC. The wear rates of NPD and BL-PcBN against SiO₂ and Si₃N₄ at high speed rubbing (280-360 m/min) at a loading pressure of 55 MPa (starting condition) were considerably high, indicating the main wear process is a chemical reaction. BL-PcBN specimens were found to be worn at much higher (more than ten times) rates than NPD, suggesting that cBN highly reacts with these ceramics in comparison with diamond under the experiment condition.

Abstract: Binder-less nanopolycrystalline cBN (BL-PcBN) synthesized by direct conversion under high pressure and high temperature has a fine texture consisting of small cBN grains (<100nm), high hardness (40–50 GPa) and high transverse rupture strength (1.3–1.6 GPa). Their mechanical properties strongly depend on their starting materials and synthetic conditions. In this study, Ball-end mill tools prepared from several kinds of BL-PcBN synthesized under various conditions (8–20 GPa, 1500–2300oC) were examined for precision cutting of high-strength hardened steel comparing with conventional sintered cBN. A fine mirror-finish surface with Ra of 20 nm could be produced by precision BL-PcBN tools, while not by conventional sintered cBN tools. The tool wear of BL-PcBN synthesized from hBN at 10 GPa and 2200°C was four to five times less than that of conventional cBN. These results revealed that the BL-PcBN tools have excellent potential for precision cutting on ferrous materials.

Abstract: Single-phase (binder-less) nano-polycrystalline diamond (NPD) has been synthesized by direct conversion sintering from graphite under high pressure and high temperature. NPD is characterized by extremely high hardness compared with single crystal diamond (SCD), even at high temperature. In addition, NPD has high wear resistance, no anisotropic mechanical properties, no cleavages, and high thermal stability. These characteristics suggest that NPD has high potential for use in precision cutting tools for various hard works. In order to evaluate the cutting performance of NPD, cutting tests for various cemented carbides were conducted under various conditions and the results compared with those of single crystal diamond (SCD) and conventional polycrystalline diamond containing metal binder (PCD). The results revealed that NPD has outstanding potential for precision cutting and processing of diverse hard and brittle materials.

Abstract: Face cutting of tungsten carbide was conducted using two monocrystalline diamond tools and three polycrystalline diamond tools to investigate the wear characteristics in terms of the crystal structure and composition of the diamond. It was found that the wear of the monocrystalline diamond tool depends on the crystal planes that form the rake face and flank face of the cutting tool, and a cleavage fracture occurs when the cutting force acts as a shear force on the (111) crystal plane. The binderless nano-polycrystalline diamond tool exhibits excellent wear resistance beyond those of the sintered polycrystalline diamond tool and chemical vapour deposition polycrystalline diamond tool, as well as better wear resistance than the monocrystalline diamond tool.

Abstract: When cutting titanium alloy, the temperature of a cutting tool edge is easy to rise and the tool edge is tend to be worn away quickly because the titanium alloy has the characteristics of low thermal conductivity and high chemical activity. Therefore, it is difficult to achieve a balance between the productivity and the tool life in cutting the titanium alloy, namely, low-speed cutting must be carried out at present. To examine the possibility of the improvement in the cutting efficiency, a PCD (polycrystalline diamond) tool having high thermal conductivity was adopted to the cutting of titanium alloy and its cutting performance was investigated. The PCD tool was found to have excellent flank wear resistance compared with conventional cemented carbide tools. It was also revealed that unprecedented high speed cutting become possible by use of PCD tool with an application of high pressure coolant.

Abstract: High-purity, single-phase polycrystalline diamond and cBN have been successfully synthesized by direct conversion sintering from graphite and hBN, respectively, under static high pressure and high temperature. The high-purity polycrystalline diamond synthesized directly from graphite at ≧15 GPa and 2300-2500 °C has a mixed texture of a homogeneous fine structure (grain size : 10-30 nm, formed in a diffusion process) and a lamellar structure (formed in a martensitic process). The polycrystalline diamond has very high hardness equivalent to or even higher than that of diamond crystal. The high-purity polycrystalline cBN synthesized from high-purity hBN at 7.7 GPa and 2300 °C consists of homogeneous fine-grained particles (<0.5 μm, formed in a diffusion process). The hardness of the fine-grained high-purity polycrystalline cBN is obviously higher than that of single-crystal cBN. The fine microstructure features without any secondary phases and extremely high hardness of the nano-polycrystalline diamond and the fine-grained polycrystalline cBN are promising for applications in next-generation high-precision and high-efficiency cutting tools.