1 Introduction <br> <br> modern cutting technology is toward high-speed, high precision and high efficiency of the direction of development, at the same time, the technical requirements of the cutting tool performance have become more sophisticated. Polycrystalline Cubic Boron Nitride (PCBN) is a new tool material that is synthesized by artificial methods and is second only to natural diamond. It is the most suitable tool material for high-speed cutting of iron-based metals.
Polycrystalline cubic boron nitride (PCBN) was first developed by GE in the last century. Due to its high hardness and wear resistance, high thermal stability and high temperature hardness, excellent chemical stability and good PCBN materials. The thermal conductivity and low coefficient of friction, so the PCBN tool has been valued by the developed countries such as the United States, Germany, Britain, and Japan since its inception. After more than 30 years of development, foreign research on PCBN tools has basically entered a mature stage, and products are also developing in a diversified and serialized direction. There are also many units and scholars in China who are engaged in the development and research of PCBN tools, but compared with foreign countries, there is a lag in the research of PCBN material synthesis and tool cutting performance.
As one of the major replacement tools in the 21st century, PCBN tools account for a growing proportion of the total use of tools in industrialized countries. US GE believes that in industrialized countries, PCBN tool materials will have a market of 620 million US dollars. The focus of the “10th Five-Year Plan†of China's machine tool industry is to develop CNC machine tools and their supporting components. In 2005, the domestic PCBN blade demand has reached 662,000 pieces (equivalent to 30mm pieces). According to market research and scientific predictions, the total value of PCBN tool consumption in China will reach 540 million yuan by 2010.
2 PCBN blade development
From 1957, the United States GE company developed cubic boron nitride (CBN) single crystal powder, the successful development of PCBN tools in the 1970s, after more than 30 years of development, PCBN tools have gradually matured. At present, PCBN tools are mostly welded by PCBN blades and tool holders or indexable inserts. In recent years, there have been more and more types of integrated PCBN blades. The PCBN insert is a composite whole formed by directly sintering a PCBN layer of about 0.5 mm on a cemented carbide substrate. This material not only has the high hardness, high thermal stability and high chemical stability of PCBN, but also has the advantages of good strength and good weldability of the cemented carbide, so that the PCBN tool can not only cut workpieces of various hardnesses, but also Easy to produce. The quality of the PCBN blade directly affects the cutting performance of the tool.
2.1 PCBN blade sintered PCBN blade is mainly produced in two ways: one is a single polymerization method, that is, a hexagonal boron nitride (HBN) is directly polymerized into a high-density sintered body; the other is a secondary polymerization method. High-density cubic boron nitride (CBN) is first synthesized, and then sintered into high-concentration cubic boron nitride by high temperature and high pressure. At present, the secondary polymerization method of adding a binder is more common:
(1) There are many types of CBN sintered metal binders to which metal binders are added, and there are mainly Ni, Co, Ti, Ti-Al and the like. At the sintering temperature, the metal binder becomes a liquid phase, and the appearance of the liquid phase promotes the sintering of CBN, and the sintering can be carried out at a slightly lower temperature and pressure. In addition, since CBN is insoluble in the metal binder, a dissolution precipitation process which promotes shrinkage and crystal growth does not occur. During the sintering process, the cobalt in the cemented carbide penetrates into the CBN grain boundary in a liquid state, and the cemented carbide and the CBN sintered body are closely bonded together. PCBN tools with metal binders have lower hardness than pure CBN sintered tools, but have good toughness. The AMB90 of Element Six of the United Kingdom, the BZN6000 of GE of the United States, and the MB710 of Mitsubishi of Japan are all sintered from a metal binder.
(2) The CBN sintered ceramic binder to which the ceramic binder is added mainly includes TiN, TiC, Al2O3 and the like. Since cracks are easily generated during the sintering process, iron elements (iron, cobalt, nickel) and Mo or Mo2C are added to the ceramic binder to increase the strength between the ceramic ions, thereby acting as a fluid pressure transmitting medium, which is advantageous. A sintered body having no deformation inside is formed, and molybdenum or the like can also improve the wettability between the ceramic particles and the iron element. The content of binder in ceramic binder PCBN is generally higher, and the toughness is worse than that of metal binder tools, but its high temperature resistance is better. Such products include DBA80 and DBC50 of Element Six of the United Kingdom, BZN7000S and BZN8200 of GE of the United States, NBX300 and NBX3200 of Sumitomo of Japan.
(3) Pure CBN sintering can also be directly sintered into a polycrystalline cubic boron nitride without adding a binder in a CBN polycrystal. The CBN undergoes recrystallization and grain growth during sintering, and the crystal grains of the sintered CBN polycrystal are not the original CBN single crystal particles. This PCBN tool has good wear resistance and long life, but it is brittle and the temperature is harsh. It is difficult to make a blade with high toughness and large size.
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