Materials Science Forum Vols. 534-536

Abstract: Cr2AlC, one of the ternary carbide families, was synthesized by a reactive hot pressing of CrCx (x=0.5) and Al powder mixture used as starting materials at the temperature range of 1200 oC~1400 oC under 25 MPa in Ar atmosphere. Fully dense Cr2AlC with high purity was synthesized by hot pressing CrCx and Al powder mixture at the temperature as low as 1200 oC. After synthesizing the bulk Cr2AlC, it was found that it has a typical layered structure as found in other ternary carbides such as Ti3AlC2 and Ti3SiC2. The average grain size of synthesized bulk Cr2AlC was varied in the range of 10-100 ㎛ depending on hot pressing temperatures. The Vickers hardness of bulk Cr2AlC was less than 4 GPa under the loading of 10N and this decreased with increasing indentation load. The maximum flexural strength of synthesized bulk Cr2AlC exceeded 600 MPa. Also, it was found that synthesized Cr2AlC was readily machinable by a conventional WC tool bit due to its low hardness as well as damage tolerance properties upon fracture.

Abstract: The effect of oxygen addition on oxidation behavior of the β-Si3N4 ceramics with 5 mass% Y2O3 and 2 or 4 mass% Al2O3 was investigated by performing oxidation tests in air at 1300° to 1400°C and cutting performance tests. These tests were intended to clarify their ware resistance as cutting tools. The results of mass change, SEM observation and composition analysis of the specimens before and after oxidation test showed that as the Al2O3 content in the β-Si3N4 ceramics increased, mass changes resulted higher oxidation during which process pores and cracks formed due to the release of N2 gas. The values of hardness and bending strength of the specimens with relatively small amount of 2 mass% Al2O3, which formed solid solution in the Si3N4 structure [Si6-zAlzOzN8-z (z = 0.1)], showed larger than those of the specimen with 4 mass% Al2O3 (z = 0.2). The specimens group added with Al2O3 of 2 mass% (Z = 0.1) also showed high wear resistance. From this, we could conclude that the mechanical properties of β-Si3N4 ceramics depending on oxygen introduction is much effective on cutting performance improvements of the cutting performance of β-Si3N4 ceramics.

Abstract: In this paper, the fundamental attributes, phase composition of three pre-alloyed powders for diamond tools by water atomization were investigated. The density, hardness, bend strength and bending modulus of their sintered samples by hot pressing were examined under various temperatures. The results showed that the three pre-alloyed powders have excellent low temperature sintering characteristics. The physical and mechanical properties of the samples were found to be nearly the same as those of ultra-fine cobalt powders.

Abstract: In order to improve wear resistance of atmospheric thermal plasma sprayed molybdenum (Mo) coating, diamond deposition on the atmospheric plasma sprayed molybdenum coating by the combustion flame chemical vapor deposition (CFCVD) has been operated. Although diamond particles could be deposited on the molybdenum coating, the coating was fractured and peeling off due to thermal influences
in our previous study. In this study, to diminish the thermal damage of the substrate during operation,
a thermal insulator was equipped between substrate and water-cooled substrate holder. Consequently, diamond particles could be created on the Mo coating without fracture and peeling off. From these results, it was found that this process had a high potential in order to improve wear resistance of thermal sprayed coating.

Abstract: Although diamond tools have been used for over a century, the diamond grits distribution in the matrix is not uniform. This is because the large and light diamond grits tend to segregate from the small and heavy metal powder during the mixing process, hence diamond distribution in the diamond tools is intrinsically heterogeneous. As a result, the cutting performance of the diamond tools cannot be optimized. In 1997, Dr. James Chien-Min Sung applied two historical patents that can allow the design of diamond distribution according to a predetermined pattern. As the result, the life of diamond tools may be doubled; and the cutting speed, may also be twice as high. The three-dimensional saw segments with arrayed diamond grits were made back in 1999 with the improved performance as predicted. The Sung invention can allow the diamond tools industry to make ideal saw segment that has variable diamond size and diamond separation at different regions. Conventional diamond saws contain diamond grits that are distributed randomly in a metal matrix, as a result, their cutting speeds are slow and their sawing lives are short. In 1997, Dr. James C. Sung applied new patents that revealed revolutionary technology for making diamond tools with diamond grits set in a predetermined pattern. The diamond placement design was first appeared in a series of DiaGrid® products, such as wire saws and grinding wheels. In 1999, DiaGrid® pad conditioners was introduced and it has since become the world's standard for dressing pads, particularly those used for chemical mechanical planarization of semiconductor devices. In 2005, Shinhan adapted the idea and produced saw segments with diamond grits set in a predetermined pattern, their results confirmed that the sawing speed and the life were significantly improved over conventional designs.

Abstract: Diamond segments were fabricated by cold pressing and sintering under pressure at the temperature up to 750 °C. It was obtained from this study that if we increase the cobalt ratio, the hardness of the matrix increases and the wear rate of the segments decreases. But the optimum matrix composition with less wear rate must ensures that the acceptable matrix toughness. Because the harder matrix wears more slowly than the diamonds, the space between the cutting edges and the matrix is constantly reduced. Because the swarf cannot be carried away properly, and the segment will continuously lose its ability to cut. From this point of view, the cobalt ratio in matrix composition for this formulation should not be higher than the 60 wt.% in order to have a good cutting ability.

Abstract: Effect of phase transformation and grain-size variation
of hot-pressed cobalt on its dry sliding wear was investigated. The sintered cobalt specimens were heat treated under different conditions and their tribological characteristics were examined. The sliding wear test was carried out against glass (83% SiO2) beads at 100N load using a pin-on-disk wear tester. A constant sliding speed of 0.38m/s and sliding distance of 600m were adapted. Worn surfaces, cross sections, and wear debris were examined by a scanning electron microscopy (SEM). X-ray diffraction (XRD) was utilized to identify phases of the specimen and wear debris. The cobalt specimens exhibited low friction coefficients of around 0.2. Thermal transformation of the cobalt from the hcp ε phase to the γ (fcc) phase during the wear was detected, which was deduced as a wear mechanism of the sintered cobalt. Typical wear characteristics of the cobalt including a thin detaching surface layer and fine wear debris were explained by the transformation. A correlationship between the grain size and the transformation was also explored.

Abstract: The performances of metal-bond of diamond cutting tools were investigated by changing the cobalt and bronze ratio conversely as the matrix material. Diamond tools were fabricated by cold pressing and sintering under pressure at the temperature up to 750 °C. Investigation of the microhardness behavior of the segments was showed that increasing the cobalt ratio causes the increase of the hardness of the matrix material. This caused to decrease of the wear rate of the matrix. Because the matrix wears more slowly than the diamonds, the space between the cutting edges and the matrix is constantly reduced. The swarf cannot be carried away properly, and the segment will continuously lose its ability to cut with higher cobalt contents.

Abstract: PCBN tools are used worldwide for machining of hardened steel parts in automotive industries. Because cutting by PCBN tools has many advantages, which are high-precision, highefficiency, high-flexibility and low total machining cost, compared with conventional grinding methods. But in heavy interrupted cutting of hardened steel, the tool life is not so stable by sudden breakage of the cutting edge, and total cost of cutting by PCBN is not so economical compared to the grinding. So the conventional grinding is still main process in such applications. Recently, demand for decreasing machining cost has grown in heavy interrupted cutting and much tougher PCBN material is required. To solve this problem, new PCBN has been developed. New PCBN has very fine and homogeneous microstructure to increase the toughness of sintered body that it provides a reliable tool life for heavy interrupted cutting. The features and cutting performances of new PCBN are described in this report.

Abstract: The methods of making diamond tools have undergone a fantastic development since the invention of the synthetic diamond in the mid-1950’s. Over time, new production techniques based on diamond tooling have been implemented into various areas of industrial activity enabling to do the job faster, more accurately and at less cost. The recent statistics indicate that the consumption of diamond abrasives reached an impressive volume of billion carats in 2000, as compared with 380 million carats in 1990 and 100 million carats in 1980. In the new millennium the market for diamond tools continues to grow rapidly. The present decline in the price of industrial diamond makes it a commoditised product capable of competing with conventional abrasives. In terms of production volume, by far the largest group of diamond tools comprises the metal-bonded diamond impregnated tools, such as circular and frame sawblades, wire saws, and core drills for cutting natural stone and construction materials, and core bits for drilling in various rock formations. The objective of this article is to provide a compendious coverage of the PM diamond tool-making routes, and to identify the recent trends towards changing the tool design and composition to render it cheaper and more efficient.