This paper reviews the developments achieved to date in cBN abrasive technology and explores progress expected in the near future. Recently there are increasing needs for environment-friendly grinding process. In spite of cBN's great advantage in this respect over conventional abrasive aluminum oxide, cBN abrasive poses a significant obstacle to be widely adopted as a substitute for aluminum oxide due to its lower cost performance ratio. A significant improvement in grinding performance or manufacturing costs is thought to be necessary for a breakthrough in extensive use.
Ultra fine particle cBN composite compacts are manufactured at ultra high pressure and high temperature. Fine structure of cBN composite compacts consist of two elements, those are fine cBN grain and fine binder structure. cBN grain suppliers with an effort achieved fine cBN grain. Fine binder structure isn't obtained. Blanks suppliers effort is necessary to obtain it. cBN composite compacts made by ultra fine cBN particles produce smoother surface roughness on hard turning. cBN composite compacts made by ultra fine cBN particles reduce the ware rate on sintered alloy turning.
Amorphous boron nitride (aBN) powders were prepared for planetary milling of hexagonal BN and phase transformation behavior under high pressure and high temperature was studied. Since aBN was extremely reactive with humidity in the ambient atmosphere, two types of aBN powders were prepared: one was hydrated and the other was free from humidity, prepared in a dry nitrogen atmosphere. Phase transformation to cBN from aBN with humidity took place under similar conditions previously reported for aBN, while the formation of cBN from dry aBN was more difficult compared to that starting from other crystalline phases of BN. It was suggested that fine-grained cBN powders and their sintered texture could be obtained by appropriate HP/HT conditions by using aBN.
Properties of polycrystalline materials strongly depend on their microstructural features such as the grain size distribution, the boundary condition and so on. In this paper, we provide our works concerning microstructural features and some properties of high-purity polycrystalline cBN synthesized from hBN by the direct conversion method. A fine-grained (<0.5 μm) and homogeneous high-purity polycrystalline cBN (>99.9%) with the highest mechanical properties can be obtained by controlling temperature conditions within 2200-2400°C at 7.7 GPa. Many cBN grains rapidly become larger at temperatures above these conditions, and a considerably inhomogeneous microstructure which causes the deterioration of mechanical properties is formed.
A submicron-sized diamond under 50 nm was prepared by ball-milling synthetic single crystalline diamonds. The average particle size from the crushed of the diamond exhibited a linear relation with its specific surface area as plotted on a double logarithmic coordinate, and the resultant diamond could be easily distinguished from a polycrystalline diamond made by dynamic compression. The submicron-sized diamond surface was chemically active and easily attacked by oxidizer or hydrogen under heating. Also TEM observation showed a thick non-diamond carbon layer on the surface of the submicron-sized diamond heated in nitrogen. Its formation apparently was triggered by the removal of oxygen from the diamond surface.
The synthesis of commercially available polycrystalline diamonds (hereafter PCD) with metallic and non-metallic sintering agents and their physical properties were briefly introduced. The PCD with non-metallic sintering agents such as alkaline earth carbonates had excellent heat-resistant properties, but because of the higher viscosity of molten carbonate, it is very difficult to infiltrate into the diamond layer with a grain size below 5 μm. In this article, recent advances in the synthesis of a fine-grained PCD under a static high pressure of 7.7 GPa were reviewed. Especially, experimental techniques such as sample assembly, pretreatment of natural diamond powder, etc, and sintering agents for the synthesis of PCD with a grain size from submicron to 100 nm were described.
Shock compaction methods for diamond powder can produce nanocrystalline diamond ceramics consisting of particles a few tens of nanometers in diameter. However, by using these methods, it is difficult to consolidate nanocrystalline diamond powder of only a few nanometers in diameter because heat localization, which is essentially important for the shock compaction process, can not be achieved. On the other hand, the SCARQ (shock compression and rapid quenching) method can produce transparent, nanocrystalline and/or amorphous diamond films 5-μm thick by controlling the nucleation and growth process in the phase transition from fullerene to diamond. The stability of the diamond phase dependent on their size is also discussed.
A fine-grained diamond compact synthesized using a mixed sintering agent of magnesium carbonate and oxalic acid dihydrate was evaluated through precision machining of aluminum alloys in comparison with natural single crystals of diamond. The flank wear of the tool made of polycrystalline compact was half that of single crystal. And the tool made of polycrystalline compact achieved as excellent a surface finish as that of single crystal.
The recent development of the high-pressure synthesis of cubic, spinel-type silicon nitride (cSi3N4) by static and shock compressions was summarized in this review. The structural, thermal, and mechanical properties of cSi3N4 have been investigated experimentally and theoretically. The shock compression technique is the only way to produce a large amount of cSi3N4.
Practical toroidal-type continuously variable transmissions (CVT) for automobile are subject to extremely severe operating conditions, namely, the maximum contact pressure is very high at 3 GPa or higher, the rolling speed exceeds over 30 m/s, and the lubricant temperature goes up over 120°C. The objective of this research is to grasp the traction characteristics of extremely severe EHL (Elastohydrodynamic Lubrication) contact conditions. The bulk moduli of a synthetic traction oil at quasi-static conditions were evaluated using a phase diagram. Next, the EHL traction was measured by a two-disc machine and a four-disc machine. It was verified that the traction coefficients were based on the phase diagram of traction oil.
In static high-pressure experiments, pressures measured by high-pressure X-ray diffraction are regarded to be most reliable. In principle it is true but in reality it is not always true. The pressure values become reliable only when enough care is paid to various technical problems associated with the X-ray diffraction. Various sources of problems are discussed.