Development of cemented carbide coated tools

In recent years, tool manufacturers have introduced medium temperature (MT) CVD TiCN coatings. When using Ethyl as an organic C/N source, TiCN deposition occurs at approximately 850 ° C, while high temperature CVD TiCN coatings are heated above 1000 ° C. . The MT-TiCN coating has excellent wear resistance for turning and milling. It has a stable C/N ratio and reduces the tendency of the interface to form an eta phase at the interface between the cemented carbide substrate.

More than a decade ago, physical vapor deposition (PVD) has been applied to cylindrical carbide tools, including intermittent cutting and/or metal cutting inserts that require sharp edges. Initially PVD coatings were limited to TiN, and there are now commercially available PVD TiCN and TiAlN coatings using a variety of different PVD techniques such as electron beam evaporation, sputtering, arc evaporation, and the like.

The CVD diamond coating uses a number of diamond synthesis techniques, the most common being the hot wire method, the microwave plasma method, and the d, c plasma jet method. Diamond coated carbide tools have been produced by improving the coating method and coating adhesion, and play an important role in the processing of non-ferrous and non-metallic materials. Recently diamond coated tools have been used industrially.

Hard coating of cemented carbide tools

Hard coating properties

The success of the hard coating of the cutting tool base is due to the combined effect of the physical and mechanical properties of the coating. From the point of view of use, the coating should have stable chemical stability, thermal hardness and strong bonding properties with the substrate. Optimized coating thickness, fine microstructure and residual compressive stress can further improve coating performance.

Chemical stability

The standard for chemical inertness of a coating material is its formation standard, the negative energy of the free energy is very high or its solubility in the workpiece material is very low at the cutting temperature. To date, CVD Al2O3 hardcoats have met these requirements in the processing of materials. Amorphous PVD Al2O3 coatings are soft and unstable, so they are not as crystalline as CVD Al2O3; PVD TiAlN coatings have higher stability than TiN or TiCN and are therefore likely to be used in high speed machining. Diamond coated tools are suitable for processing non-ferrous alloys (such as silicon-aluminum alloys) containing second phase abrasive particles and non-metallic materials (such as metal matrix composites and fiber reinforced plastics) that do not react with sulfur.

hardness

The back of the cutting tool is subjected to abrasive grinding. At the cutting temperature, as long as the hardness of the hard coating is higher than the hardness of the substrate, it helps to enhance the wear resistance. Although the cutting is mainly controlled by chemical wear, the coating is high. The hardness will increase the anti-crater wear resistance of the front of the tool at higher temperatures. This view is still controversial.

Microstructure and morphology

The coating method and process parameters affect the microstructure of the hard coating. Conversely, the microstructure (such as particle size, particle structure, particle boundaries, and phase boundaries) affects the mechanical properties and metal cutting properties of the hard coating. It is well known that PVD TiN coatings have finer particle sizes and have higher microhardness than CVD TiN coatings. PVD coatings with high lattice defect density also have high residual stress, which also helps to improve. Its microhardness.

The Al2O3 coating of cemented carbide tools is usually deposited by CVD. Al2O3 has many crystalline forms; the most prevalent polymorphic forms are stable α-Al2O3 and metastable K-Al2O3. The particle shape of α-Al2O3 is columnar, which has a larger dislocation density and pores than K-Al2O3, and pores are often present at the grain boundaries. The K-Al2O3 coating is 2 to 0.5 μm of fine particles and has no dislocations.

Bonding

In order to achieve satisfactory cutting performance, the bond of the tool coating to the substrate must be strong. The nucleation of the coating on the substrate should be the interdiffusion of the coating and the matrix atoms at the interface, which can be achieved in the thermal CVD method. The plasma-assisted deposition method produces lattice defects even with high energy bombardment at lower temperatures. It can also improve the rapid diffusion of the coating sample at the interface. For the diamond coating, before the coating, the cobalt on the surface of the substrate to be coated is the key to measure the bond strength between the coating and the substrate. Surface corrosion and heat treatment are used. Another method to promote adhesion of a diamond film to a cemented carbide substrate.

Coating thickness

In order to achieve the maximum metal removal rate, the thickness of the coating must be optimized: too thin, too short to hold during cutting; too thick, it acts as a whole material, losing the combination with the substrate Superiority. It has been determined that new tool coating thicknesses range from 2 to 20 μm. The thickness of the coating deposited by CVD depends on the application, typically in the range of 5-20 μm, while the thickness of the PVD coating is typically less than 5 μm. The thickness of the diamond coating is generally thicker than that of the CVD or PVD coating, and is comparable to the polycrystalline diamond coating in the range of 20 to 40 μm.

Hard coating deposition method

According to the volatilization of the original material and the energy required for the reaction, there are thermal deposition and plasma-assisted deposition.

Today, metal-cutting tools use titanium-based hard coats TiN, TiCN, TiC, and TiAlN, as well as ceramic hard coat Al2O3. Many different thermal deposition techniques and plasma deposition techniques are used to make coatings with the same composition. Several different coating methods can be used in most cases. The microstructure of the coating and the properties of the coated tool are determined by the coating method and the coating process parameters. Generally, the volatilized form and the deposition temperature distinguish different coating methods. The hot high temperature CVD method (HT-CVD) occurs at 900 to 1100 ° C, which is higher than the 300 to 600 ° C taken by the PVD method.

The operating temperature of the hot intermediate temperature CVD (MT-CVD) method is between the HT-CVD and PVD methods. When using MT (CN) as an organic C/N source to deposit the MT-TiCN coating, MT-CVD The coating temperature is reduced to 750~900 °C. The CVD hard coat is plasma assisted, and it is possible to further reduce the deposition temperature to below 750 °C. In the PA-CVD process, a catalytic low-pressure glow discharge catalyzed reaction gas can cause a chemical reaction at a lower temperature. Unlike the CVD method, the PVD deposition temperature is relatively low, so the PVD plasma produces a metastable structure. The use of a negative bias to introduce ion bombardment on the substrate improves the adhesion of the PVD coating and grows a fine-grained anti-wear layer. Intense ion bombardment also introduces high internal stresses in the PVD coating.

Due to the different volatilization forms of the metallic hard components, the PVD method and the plasma conditions employed are also different.

The PVD sputtering method is that the metal vapor is directly volatilized from the metal target without passing through the liquid phase, and the main advantage is that it can evaporate metals of different melting points (such as TiAlN). The PVD arc evaporation method uses higher input energy than the PVD sputtering method. The high-energy arc quickly passes through the volatilized metal surface, causing a small, limited area to evaporate, and the resulting plasma is composed of highly ionized metal vapor.

The PVD method often uses a high-energy electron beam. The advantage of this method is that it has good process control and balanced plasma ionization.

At present, there are three methods for diamond deposition: microwave plasma method, hot wire method and plasma jet method. High quality diamond films should have a large number of hydrogen atoms to stabilize the SP3 diamond bond and reduce the amount of graphite in the film.

The microwave plasma method uses a microwave energy source to produce a glow discharge having a deposition rate of 2 to 3 μm/h. The hot wire method heats the refractory wire to 2000 to 2500 ° C to produce a sufficient amount of atomic hydrogen and diamond raw particles, and the deposition rate can reach 0.5 to 1.5 μm / h. The plasma spraying method includes d, c, and r, f plasmas, and their gas temperatures range from 5000 to 8000 °C. Thermal plasma promotes the decomposition of gas particles to achieve extremely high deposition rates (up to 400 μm/h).

All of the above thermal and plasma assisted coating methods require expensive equipment, and in addition, the complexity of the coating process (technically or economically) is difficult to combine different coating methods.

Coating technology is basically divided into two important process parameters: deposition temperature and working pressure. These parameters have a great influence on the deposition conditions and have a great influence on the performance of the coated products.

Today, more than 60% of metal cutting inserts in the US and Western Europe are CVD coated due to their resistance to abrasive wear, crater wear resistance and the use of higher cutting speeds. The brittleness of the cemented carbide matrix is ​​related to the η phase formed by the early CVD coating deposition technique. Now, due to the better carbon control of the matrix and the improvement of the CVD method, the formation of the η phase has been greatly reduced or has been eliminated. Layered carbide tools are used in a wide range of applications, including turning, boring, shredding, grooving, cutting and milling. These tools are suitable for processing sulfur, alloys, stainless steel, grey cast iron, ductile cast iron and superalloy materials.

5μm HT-CVD coating for anti-wear in the milling process, the first layer is TiN, which reduces the tendency to form the η phase; the coating used for turning, the first layer is 13μm thick TiC (HT-CVD). The main active layer grown on top of the first layer is TiCN, which optimizes hardness, resistance to crater wear and resistance to back wear, and the surface layer is TiN.

Especially for intermittent cutting applications (milling), MT-CVD coatings can further improve the toughness of CVD coated carbide tools, lower deposition temperatures (~850 ° C) and shorter deposition times for MT-CVD methods. The tendency to form a brittle η phase at the interface between the coating and the substrate increases the performance of the coated tool in intermittent cutting applications. The higher deposition rate of the MT-CVD method produces a columnar coating structure. MT-CVD coatings can be used in the processing of all HT-CVD coated tool applications.

The CVD Al2O3 coating method has also been greatly improved to produce a thick and uniform Al2O3 coating with a certain crystal structure. Due to the high temperature properties of Al2O3, HT-Al2O3 coated tools can be used for high speed machining of steel and cast iron. Al2O3 coatings can be deposited as single or alternating layers of alpha-Al2O3 or K-Al2O3 structures, which can achieve nearly the same high cutting speeds of ceramic cutting tools.

At present, the high-temperature performance of the Al2O3 coating is excellent, and the high-toughness composite CVD coating of MT-CVD TiCN coating has been developed. These MT-TiCN-Al2O3 composite coatings have been successfully used for milling and turning. K-Al2O3 and α-Al2O3 can be deposited under controlled conditions.

Progress in PVD coating

PVD TiN coatings have been widely used in cemented carbide tools and because of their low deposition temperature, smooth, fine-grained crack-free coating and good residual internal stress on sharp edges. Other substrates (such as cermet substrates that are sensitive to coating temperature).

Advances in PVD technology have led to the emergence of commercially available new compounds such as TiCN, TiAlN, and TiZrN and CrN.

In coated blade applications, CVD coated tools account for the majority. In the machining of milling, drilling, threading, grooving and cutting, the tool PVD coating has exceeded the CVD coating. PVD coatings work well in the processing of difficult-to-machine materials such as superalloys and austenitic stainless steels.

Diamond coating

The diamond coating has the characteristics of high hardness, low friction coefficient, high thermal conductivity and low thermal expansion coefficient. However, diamond reacts with elements of Groups IIA to VIIA of the periodic table, so the diamond coated tool is only suitable for processing non-ferrous and non-ferrous materials. Metal workpiece material. Diamond tool wear methods include oxidation, chemical reaction with workpiece materials, microcracking and severe fracture.

The diamond coating has a highly planar, planar structure which results in a micro-rough surface in front of the blade. This rough diamond facet acts as a microchip breaker, and behind the blade, this facet can result in poor surface finish of the workpiece.

Today, the automotive industry uses diamond-coated tools when processing silicon-aluminum alloys (especially 300 series). Diamond coated tools are also expected to find applications in the processing of metal matrix composites (MMC), carbon-carbon composites and the village processing industry.

Cubic boron nitride (CBN) coating

Cubic boron nitride (CBN) is a high temperature and high pressure phase of boron nitride. It is the second hardest material (up to 60 GPa) and its structure is similar to diamond, but CBN is chemically inert to hot iron, hot steel and oxidizing environments. When oxidized, a thin layer of boron oxide is formed, which provides chemical stability to the coating, so that when processing hard iron (50-65HRC), gray cast iron, superalloy and sintered powder metal Has obvious advantages.

Many researchers have attempted to deposit cubic boron nitride films using CVD and PVD techniques. The test results show that some progress has been made in the synthesis of CBN phase, good adhesion to cemented carbide matrix and suitable microhardness. At present, the thickness of cubic boron nitride deposited on the cemented carbide substrate is only 0.2~0.5μm. If it is to be commercialized, reliable technology must be used to deposit high-purity and economical CBN film. It is 3~5μm and its effect is confirmed in actual metal cutting. Future potential

The use of CVD coated carbide tools has been rapidly developed. The toughness of MT-CVD coatings exceeds that of HT-CVD coatings. However, in addition to the deposition of TiCN coatings, it is still impossible to expand this coating technology. . Plasma-assisted CVD coatings have similar advantages, but coating compositions are also limited. It is expected that a new coating composition can be produced by a low temperature deposition method.

The development of new PVD coating materials, including PVD Al2O3 and PVD multi-coating, will expand the range of applications for PVD coated tools, which will be a challenge for CVD coatings.

A composite coating of CVD and PVD is fully achievable. TiN/NbN, TiN/Ni and TiN/NiCr superlattice coatings have higher hardness than those of single-phase nitrides, and they are expected to find applications in metal cutting. Improvements in process economics will increase the likelihood of using diamond coated tools, however their range of applications is limited to non-ferrous metals. The breakthrough development potential is pinned on cubic boron nitride (CBN) coated carbide tools, which can be used to process more than 75% of today's materials.