Two major categories of tool coatings
Tool coating technology can be divided into chemical vapor deposition (CVD) technology and physical vapor deposition (PVD) technology, respectively reviewed as follows.
I. Development of CVD technology
Since 1960s, CVD technology has been widely used in the surface treatment of carbide indexable cutting tools. Due to the relatively easy preparation of metal sources for CVD process, the deposition of TiN, TiC, TiN, TiBN, TiB2, Al2O3 single-layer and multi-layer composite coatings can be realized. The bonding strength between coatings and matrix is high, and the thickness of films can reach 7 ~ 9μm. Therefore, in the mid and late 1980s, In the United States, 85% of cemented carbide tools are coated, 99% of which are CVD coated. Up to the mid 1990s, CVD coated cemented carbide inserts still account for more than 80% of coated cemented carbide tools.
Although CVD coating has good wear resistance, but CVD process also has its congenital defects: one is the process of high temperature, easy to cause the bending strength of the tool material decline; The second is that the film inside the state of tensile stress, easy to lead to the use of the tool microcrack; Third, the exhaust gas and waste liquid discharged by the CVD process will cause greater environmental pollution, which is contrary to the green manufacturing concept strongly advocated at present. Therefore, the development and application of high-temperature CVD technology has been restricted to a certain extent since the mid-1990s.
At the end of 1980s, the low temperature chemical vapor deposition (PCVD) technology developed by Krupp.Widia has reached the practical level. The process temperature has been reduced to 450 ~ 650℃, which effectively inhibits the production of η phase. PCVD process is not widely used in the field of tool coating. In the mid 1990s, the emergence of new technology of medium temperature chemical vapor deposition (MT-CVD) revolutionized CVD technology. MT-CVD technology is a new technology which uses the organic matter containing C/N acetonitrile (CH3CN) as the main reaction gas, decomposes with TiCl4, H2 and N2 at 700 ~ 900℃ and reacts to form TiCN. The dense fibrous crystalline coating was obtained by MT-CVD technique, and the thickness of the coating was 8 ~ 10μm. This coating structure has high wear resistance, thermal shock resistance and toughness, and can be used to deposit Al2O3, TiN and other materials with good high temperature oxidation resistance, low affinity with the processed material and good self-lubricating performance on the blade surface by HT-CVD process. MT-CVD coated blade is suitable for high speed, high temperature, heavy load, dry cutting conditions, its life is about twice as long as that of ordinary coated blade. At present, CVD (including MT-CVD) technology is mainly used for the surface coating of carbide turning tools, coating tools are suitable for medium and heavy cutting of high speed rough machining and semi-finish machining. The use of CVD technology can also achieve α-Al2O3 coating, which is difficult to achieve PVD technology at present, so in the dry cutting process, CVD coating technology still plays a very important role.
Second, PVD technology development
PVD technology appeared in the late 1970s, because its process temperature can be controlled below 500℃, so it can be used as the final treatment process for high speed steel tool coating. Since the PVD process can greatly improve the cutting performance of HSS tools, the technology has been rapidly promoted since the 1980s. By the end of the 1980s, the proportion of PVD coating on complex HSS tools in developed countries has exceeded 60%.
The successful application of PVD technology in the field of high speed steel tools has attracted great attention from the manufacturing industry of all countries in the world. People are competing to develop high-performance and high reliability coating equipment at the same time, but also the expansion of its application field, especially in the application of cemented carbide and ceramic tools have been more in-depth research. The results show that compared with the CVD process, the PVD process has no effect on the bending strength of the tool material when the temperature is lower than 600℃. The internal stress state of the film is compressive stress, which is more suitable for coating on cemented carbide precision and complex cutting tools. The PVD process has no adverse effect on the environment and conforms to the development direction of modern green manufacturing. With the advent of the era of high speed cutting and high speed steel cutting tool application proportion gradually decline, carbide cutting tools and application of ceramic cutting tool rise has become a inevitable trend, therefore, the industrial developed countries since the early ninety s began to committed to the research of carbide cutting tools PVD coating technology, breakthrough progress has been made in the mid - ninety - s, PVD coating technology has been widely used in the coating treatment of carbide end mills, drill bits, step drills, oil hole drills, reamers, taps, indexable milling blades, profiled cutting tools, welding tools, etc.