Metallic ceramics for cutting tools began with the experimental research on TiC compounds in the 1920s. TiC-Mo-Ni metallic ceramics were used as tool materials for high-speed precision cutting of steel in the 1950s. Some excellent properties of TiC-based metallic ceramics (such as high hardness, low specific gravity and chemical stability at high temperature) made it possible to be an alternative material to WC-based hard alloys. However, at that time, TiC-based metallic ceramics were not widely used as industrial cutting tool materials, because although they had high strength and hardness comparable to hard alloys, their toughness was relatively poor.

In the 1970s, in order to improve the toughness of metal ceramics and improve their machinability, a lot of research was carried out, and a kind of fine particle TiC-TiN based metal ceramic with good toughness was developed. Since then, the application of metal ceramics in tool development has become more and more extensive.

Metallic ceramic materials are very suitable for precision machining and near net shape processing, and their application is expanding, and they have become a basic cutting tool material. Nowadays, this material shows excellent cutting performance in precision machining.

WC is the main component of cemented carbide, which is considered a strategic raw material. With the sharp rise in global tungsten prices, as a tool material that can partially replace cemented carbide, metal ceramics is receiving increasing attention.

The characteristics of metal ceramic materials

Industrial metal ceramic materials have complex chemical compositions. This cutting tool material is made by adding secondary carbides (such as WC, Mo2C, TaC, and Co-Ni) as a binder to the TiC-TiN matrix. Commercialized metal ceramic materials have very high hardness and strength and toughness comparable to those of cemented carbides. As the performance of metal ceramics continues to be improved, it has become a very popular material for metal cutting tools.

Metallic ceramics are suitable for medium load machining, semi-fine machining and fine machining of various steel parts and cast iron parts. When the cutting depth (ap) is below 2.5mm, the feed per revolution (fn) is below 0.25mm/r and the feed per tooth (fz) is below 0.20mm/tooth, the metallic ceramic cutting tools have excellent cutting performance.

Metallic ceramics can be machined to process all kinds of steel materials (such as carbon steel, alloy steel, low carbon steel, cast steel, cast iron, etc.), and are also suitable for some processing methods (such as turning, milling and end milling). According to its material properties, metallic ceramics are very suitable for high-speed cutting, and are not easy to produce built-up chips, which will ensure that users can obtain very precise machining results.

When machining with metal ceramic cutting tools, the user must take the following points into account in order to achieve better performance: for turning operations, a chip breakers of appropriate height must be used in relation to the cutting amount; for milling or end milling operations, the selection of the appropriate tool geometry is essential.

Compared with other knife materials, the larger advantage of metal ceramic is its stable chemical properties, which can inhibit chemical reactions between the knife and the workpiece in cutting processing.

The chemical stability of metal ceramics enables them to achieve very high levels of machining accuracy and surface finish. When users wish to complete their machining cycle with finish or medium-duty machining, the stability of the metal ceramic can ensure a high level of machining quality. In valve machining or interrupted internal machining, where the cutting temperatures are high and machining accuracy is critical, metal ceramic tools perform very well.

Metallic ceramic materials themselves have very good heat resistance and wear resistance, so, compared to cemented carbide cutters that can maintain their physical shape for a long time, metallic ceramic cutters can relatively easily maintain the sharpness of the cutting edge for a long time. The surface of the workpiece cut by a metallic ceramic cutter is smoother than that cut by a cemented carbide cutter.

To date, metal ceramics have been widely used for machining a variety of steel parts, and in recent years, it has become a preferred cutting material for precision machining of cast iron and ductile iron. High frequency vibration is easily generated during machining cast iron, and its side effect is the generation of chip breakage, because high frequency vibration can cause chipping or damage of the cutting edge of metal ceramic. However, today's metal ceramic grades are reinforced with toughness, and its toughness and strength are sufficient to maintain the stability of the cutting edge during machining cast iron.

In addition, whether cutting ordinary workpiece materials or machining various alloy steels and quenched steels with hardness below HRC45, metal ceramics can provide stable tool life and good surface finish. As the current development trend of cutting processing, economic feasibility and high-speed cutting are becoming more and more important, and the tool life of metal ceramic tools can be increased by more than 15 minutes at a cutting speed of about 500m/min, and the tool life is more balanced.

Correct selection of metal ceramic cutting tools

When machining, the user must carefully select the appropriate metal ceramc cutting tool, since its machining characteristics are quite different from those of coated cemented carbide. Unlike coated cemented carbide, metal ceramc is not suitable for rough machining. When the generated chips are large, the shearing action and cutting resistance change greatly, and this continuous impact will cause the machined dimensions to be out of tolerance and the tool to break.

In addition, in the deep cutting process, the impact of the deep chips on the back of the tool will aggravate the scoring wear, thus damaging the metal cermet tool. This serious scoring wear is more likely to occur when cutting difficult-to-cut materials such as nickel-based, iron-based, and cobalt-based heat-resistant alloys. Therefore, metal cermet tools are not very suitable for cutting difficult-to-cut materials. For this reason, coated cemented carbide tools are more suitable than metal cermet tools for cutting difficult-to-cut materials and roughing. Table 1 compares the tool life and chip removal of cemented carbide, coated cemented carbide, and metal cermet when cutting carbon steel. It can be seen that it has better economy because of the larger chip removal in high-speed cutting. Like CVD coated cemented carbide tools, metal cermet tools are also a kind of economical high-speed cutting tool.

Dry cutting and wet cutting

As the cutting speeds and feed rates of roughing operations continue to increase, metal ceramic cutting tools are suitable for dry cutting. Wet cutting with the use of coolants can easily produce thermal cracks, which can damage the cutting edge of the metal ceramic cutting tool.

Compared to hard alloy cutting tools, metal ceramic cutting tools exhibit better high-temperature stability and machining performance in dry cutting. Generally, when metal ceramic cutting tools are used for wet cutting, it is necessary to adopt smaller feed rates and depths of cut to avoid excessive machining temperatures. However, in the harsh cutting conditions where the tool wear rate is very fast, the machining temperature will rise sharply due to the wear of the cutting edge. In such cases, dry cutting is conducive to delaying the generation of thermal cracks, thus obtaining stable machining performance.

The level of cutting temperature is first of all determined by the cutting conditions, and in addition, the thermal conductivity, toughness and hardness of the workpiece also influence the generation of thermal cracks. In order to make the right selection of cutting conditions for metal ceramic cutting tools, the user must be familiar with all the available machining alternatives. If the thermal expansion or thermal deformation has no significant influence on the machining accuracy, it is better not to choose wet cutting.

In milling operations, the temperature increases as the tool cuts into the workpiece and drops as it mills through the air, and this temperature fluctuation is repeated, causing a thermal shock to the tool. Metal-ceramic tools are less resistant to thermal shock than hard alloy tools. If wet cutting is used, the tool cutting edge is rapidly cooled by the coolant, which is the main cause of shortened tool life, as it results in the formation of thermal cracks perpendicular to the cutting edge.

The use of coated metal ceramic cutting tools has increased in recent years. Now, PVD coated metal ceramic cutting tools with TiAlN, TiN and AlCrN coatings are widely used in the machining operations. TiN coating helps to prevent the build-up of chips; while TiAlN coating can improve the thermal shock resistance and wear resistance of the tool, thereby extending the tool life.

Improve the toughness and wear resistance of metal ceramics

In the automotive and aviation industries, precision machining has become a development trend, and the requirements for surface finish of workpieces are also increasing. The use of metal ceramic cutting tools in these industries is increasing day by day. In order to extend the life of metal ceramic cutting tools, the focus is on how to improve their toughness and wear resistance.

The research and development of micro-structured metal ceramics (solid solutions and high nitrogen metal ceramics) has made progress. For example, the CT3000 metal ceramic grade developed by Korea Tektur Company can be used for turning and milling operations, and it has significantly improved cutting performance and tool life in turning operations compared to traditional metal ceramic grades. This grade is prepared using fine grain TiCN powder and optimized alloy design technology, which has excellent alloy performance and stable microstructure. The higher nitrogen content helps to improve the uniformity of the fine grain microstructure and optimizes the cutting performance of the tool.

In order to make the metal ceramic cutting tool have good toughness and wear resistance, it is necessary to optimize the surface and internal structure of the cutting edge. To this end, reducing the content of cobalt and nitride binder on the surface is conducive to improving the hardness and wear resistance of the cutting edge. On the other hand, increasing the content of the binder in the internal structure below the surface can improve the toughness, thus extending the tool life.

By applying powder metallurgy technology and sintering technology to the preparation of PVD coated metal ceramics, the bonding content of the surface and interior can be optimized to form a gradient functional structure with low bonding content on the surface and high bonding content in the interior.

Compared with traditional metal ceramic cutting tools, the stability of gradient functional metal ceramic cutting tools is significantly improved and the tool life is longer. However, the toughness and thermal conductivity of existing metal ceramic cutting tools are still slightly inferior to those of cemented carbide cutting tools, so their application is only limited to cutting processing under intermittent cutting or thermal shock conditions.