In order to improve the cutting performance of cemented carbide, more than 80% of cemented carbide tools in industrially developed countries have undergone surface coating treatment. Over the past few decades, double-coated, triple-coated and multi-coated composite blades have been developed at home and abroad, and some of them have even reached dozens of layers or hundreds of layers.

Cemented carbide coating technology can generally be divided into two categories: chemical vapor deposition (CVD) technology and physical vapor deposition (PVD) technology.

The choice of coating materials

The research on the tool wear mechanism shows that the tip temperature can reach 900°C during high-speed cutting. At this time, the tool wear is not only mechanical wear, but also bonding wear, diffusion wear and oxidative wear. Therefore, the cutting process can be regarded as a micro-zone physical and chemical change process. The choice of coating material has a great influence on whether the coating can play its due role on the tool.

Titanium carbide is a high-hardness and wear-resistant compound with good friction and wear resistance; titanium nitride has a slightly lower hardness, but it has higher chemical stability and can greatly reduce the difference between the tool and the processed workpiece. The coefficient of friction between. From the consideration of coating manufacturability, both are ideal coating materials, but no matter whether it is talking about titanium or titanium nitride, a single coating is difficult to meet the comprehensive requirements of high-speed cutting for tool coating.

Titanium carbonitride (TiCN) is a composite compound formed by nitrogen atoms (N) occupying the position of the original carbon atoms (C) in the lattice in a single TiC lattice. The ratio of carbon and nitrogen atoms in TiCxNy is Two more ideal models, namely TiC0.5N0.5 and TiC0.3N0.7. Because TiCN has the comprehensive properties of TiC and TiN, its hardness is higher than TiC and TiN, so it is an ideal tool coating material.

No material can compare with alumina (Al2O3) in terms of anti-oxidation and anti-diffusion wear properties. However, due to the large difference between the physical and chemical properties of alumina and the base alloy, a single alumina coating cannot produce an ideal coated tool. With the emergence of multiple coatings and related technologies, the coating can not only improve the bonding strength with the substrate, but also have the comprehensive performance of a variety of materials.

So far, the coating of cemented carbide inserts can be roughly divided into 4 series: TiC/TiN, TiC/TiCN/TiN, TiC/Al2O3 and TiC/Al2O3/TiN. The first two are suitable for ordinary semi-precision and fine cutting, and the latter two are suitable for high-speed and heavy-duty cutting.

Chemical vapor deposition (CVD) technology

Chemical vapor deposition (CVD) is an important technological breakthrough in the field of cemented carbide. It uses one or several compounds or elemental gases containing coating elements to act in the gas phase or in the reaction chamber where the substrate is placed. The coating is formed by the chemical reaction on the surface of the substrate. The common CVD technology uses C/N-containing organic cyanide (CH3CN) as the main reaction gas, and decomposes and chemically reacts with TiCl4, H2, and N2 at 700~900℃ Generate TiCN. The coating effectively improves the surface hardness and wear resistance of cemented carbide products, prolongs the service life of cemented carbide products, reduces losses, and improves machining efficiency.

Since the 1960s, CVD technology has been widely used in the surface treatment of cemented carbide indexable tools. In the mid and late 1980s, 85% of the cemented carbide tools in the United States had been treated with surface coating, of which CVD coating accounted for 99%. By the mid-1990s, CVD coated carbide blades were among the coated carbide tools. It still accounts for more than 80%.

At the end of the 1980s, the low-temperature chemical vapor deposition (PCVD) technology developed by Krupp.Widia reached a practical level. Its process temperature has been reduced to 450~650℃, effectively controlling the generation of η phase, and can be used for screw tools, TiN, TiCN, TiC and other coatings for milling cutters and molds, but so far, the PCVD process has not been widely used in the field of tool coatings.

In the mid-1990s, the emergence of new medium temperature chemical vapor deposition (MTCVD) technology revolutionized CVD technology. Using MTCVD technology can obtain dense fibrous crystal morphology coating. The coating thickness can reach 8~10μm. This coating structure has extremely high wear resistance, thermal shock resistance and toughness. MTCVD coated cemented carbide blades are suitable for use under high temperature, high speed, heavy load, and dry cutting conditions, and their service life can be about twice as long as ordinary coated cemented carbide blades.

my country began to study CVD coating technology in the early 1970s. Due to the strong specificity of this technology, there are not many domestic research units engaged in research. In the mid-1980s, the development of CVD tool coating technology in my country reached a practical level, and the level of process technology was comparable to the international level at that time, but the development was relatively slow in the following ten years. The research of low temperature chemical vapor deposition (PCVD) technology in my country began in the early 1990s. PCVD technology is mainly used for mold coating, and its application in the field of cutting tools is also very limited. At the end of the 1990s, my country started medium temperature chemical vapor deposition (MTCVD). Technology research and development work.

Physical vapor deposition (PVD) technology

Physical vapor deposition is mainly divided into 3 categories: evaporation coating, ion coating and sputtering coating. Vacuum evaporation coating is an earlier and most widely used PVD coating technology. It still occupies 40% of the world's market, but the scope of applications is shrinking. This technology uses resistors, electron beams, etc. to heat the coating material under vacuum conditions to melt, evaporate and then deposit on the surface of the alloy substrate to form a coating.

Ion plating is to inject Ar gas under vacuum conditions, use glow discharge to separate the gas and the coating material, and make ions bombard the target to shoot out the material ions on the target, and deposit them on the surface of the alloy substrate. Multi-arc ion plating is a more successful technology used in the coating of superhard materials for cutting tools.

Sputter coating is a technique that uses charged ions to bombard the surface of the target in a vacuum chamber, bombards atoms and other particles in the target through the momentum transfer of the ions, and deposits them on the surface of the alloy substrate to form a coating. Sputtering coating can realize large-area rapid deposition.

PVD technology appeared in the late 1970s. Because its process temperature can be controlled below 500°C, it can be used as a final process for coating high-speed steel tools. Since the use of PVD technology can greatly improve the cutting performance of high-speed steel tools, this technology has been rapidly promoted since the 1980s.

Industrial developed countries have been committed to the research of PVD coating technology for cemented carbide tools since the early 1990s, and made breakthrough progress in the mid-1990s. PVD coating technology has been widely used in cemented carbide milling cutters, drills, Coating treatment for step drills, oil hole drills, reamers, taps, indexable milling inserts, special-shaped tools, welding tools, etc.

my country's PVD coating technology research and development work has developed a cathodic ion coating machine, and developed high-speed steel tool TiN coating technology. At the end of the 1990s, the domestic successful development of cemented carbide TiNTiCNTiN multi-component composite coating technology reached a practical level. However, compared with the international development level, the PVD coating technology of cemented carbide tools in my country is still lagging behind by about 10 years. At present, the PVD coating technology of foreign tools has been developed to the fourth generation, while the domestic is still at the second generation level, and single-layer TiN coating is still the mainstay.

PVD, CVD coating technology

In comparison, about 80% of cemented carbide tools currently use CVD technology for superhard material coating. Since TiNPVD coated high-speed steel tools were put into industrial application in the early 1980s, people have been exploring whether PVD can be used to replace CVD process to coat carbide inserts. Compared with CVD coating technology, PVD coating technology has the following advantages: (1) PVD technology has a low deposition temperature and can deposit superhard coatings such as TiN at about 500℃, so it will not reduce the original material of the base material. It has flexural strength, and there is no η phase between the coating and the substrate, which expands the scope of application; (2) The coating has a fine structure, generates compressive stress inside the coating, and has strong crack propagation resistance; (3) Coating The surface is smooth, which is more effective than CVD coating in preventing the propagation of transverse cracks on the rake face, and at the same time it can reduce the coefficient of friction; (4) Sharp-edged tools can be used as the substrate, which is very important for high-speed cutting.

Although PVD coating has the advantages that CVD coating is difficult to compare, practice shows that the performance of TiC/Al2O3 or TiC/Al2O3/TiNCVD coating of general turning (partial milling) inserts is still better than PVD coating, except for CVD The technology can be used for αAl2O3 coating, and the bonding strength between the coating and the substrate is higher than that of PVD coating, which is also an important factor for its performance better than PVD technology. The scratch test of coated carbide blades shows that the critical load of PVD coating is generally 30~40N, while the critical load of CVD coating can be >90N; the thickness of CVD coating can reach 8~0μm, while PCD coating The thickness of the layer must be controlled within 3~5μm, otherwise the coating is likely to peel off. In addition, the industrialization cost of CVD coating of cemented carbide blades is lower than that of PVD coating, which is one of the reasons why CVD technology is more widely used.

The two technologies of CVD and PVD will still coexist and complement each other in the coating of carbide tools, and will occupy their respective shares in the proportion of tool coatings due to their own advantages. Generally speaking, it is ideal for high-speed steel and other steel tools, sharp cemented carbide precision cutting blades and cemented carbide integral multi-edged tools to adopt PVD technology coating. Most of the remaining carbide blades can be coated with CVD technology. Moreover, CVD coatings are also constantly developing. At present, in addition to the use of medium-temperature CVD coatings to reduce the reduction in the strength of cemented carbide, computers can also be used to precisely control the thickness of single-layer coatings to avoid the formation of columnar crystals in the coating to meet Coating requirements for fine-cut carbide blades.