Development and application of "soft" coated tools

1 Introduction

The tool coating combines the excellent properties of the tool base and the coating material to maintain the good toughness and high strength of the substrate, as well as the high hardness, high wear resistance and low coefficient of friction of the coating. Cutting performance is greatly improved. Since the advent of hard coated tools, chemical vapor deposition (CVD) technology and physical vapor deposition (PVD) technology have been developed, tool coating technology has developed rapidly, and the coating process is becoming more and more mature. The proportion of coated inserts used in Western countries in indexable inserts has increased from 26% in 1978 to 50% to 60% in 1985. About 80% of the cutting tools used in the new CNC machine tools use coated tools, and the proportion of coated blades from Swedish Sandvik and Kennametal has reached 80% to 85%. The proportion of coated carbide inserts on CNC machines in the United States is 80%. More than 70% of coated blades for turning in Sweden and Germany. In Japan, the total production of cemented carbide and ceramic blades accounts for more than 40% of coated blades. Coated tools will be an important tool in the field of machining in the future, and an important development direction of tool technology.

2 Types of coated tools

Depending on the coating method, coated tools can be divided into chemical vapor deposition (CVD) coated tools and physical vapor deposition (PVD) coated tools. Coated cemented carbide tools are generally CVD) with a deposition temperature of around 1000 °C. Coated high speed steel tools are generally PVD and have a deposition temperature of around 500 °C.
Depending on the material of the coated tool, the coated tool can be divided into carbide coated tools, high speed steel coated tools, and coated tools on ceramic and superhard materials (diamond and cubic boron nitride).

1

 Single coating

1

 Multi-coating
(with intermediate transition layer)

1

 Multi-coating
(Nano-structure) 		1 Gradient coating

1

 Super hard coating

1

 Soft/hard composite coating
Figure 1 Typical coating structure
Depending on the nature of the coating material, the coated tools can be divided into two broad categories: “hard” coated tools and “soft” coated tools. "Hard" coated tools pursue high hardness and wear resistance. The most widely used tool hard coating materials are TiC, TIN, TiC(N), Al 2 O 3 and TiAlN, and the like. The “soft” coated tool pursues a low coefficient of friction and has a low coefficient of friction with the workpiece material, which reduces bonding, reduces friction, and reduces cutting forces and cutting temperatures. The "soft" coated tool materials mainly include MO S 2 , WS 2 , TaS 2 and the like, and combinations thereof. The coating methods are: single coating, multi-coating, gradient coating, soft/hard composite coating, nano coating, super hard film coating, etc., as shown in Figure 1.

3 Development and application of “soft” coated tools

  1. Coating material of "soft" coated tool and its action mechanism
    2. At present, the "hard" coated tool technology has gradually matured and is the most widely used. However, many high-strength aluminum alloys, alloys or precious metal materials used in the aerospace industry are not suitable for processing with "hard" coated tools, and still mainly use uncoated high-speed steel or carbide tools.
    The development of "soft" coated tools can better solve the processing problems of such materials. The main component of the “soft” coating of the tool is a solid lubricating material with a low coefficient of friction, such as MoS 2 , WS 2 , BN, CaF 2 and T aS 2 , which have excellent tribological properties under special conditions of use, such as : Low friction coefficient, high load-bearing limit, good chemical stability at high temperature, small change in physical properties, can adapt to the operating temperature range above 1200 °C and a wide range of friction speeds, suitable for high temperature, high speed and large load. Use under ambient conditions.
    The coating forms a solid lubricating film on the surface of the tool, giving the tool material a very low coefficient of friction. Since the solid lubricant with layered structure has strong bonding ability to the friction surface and has low shear strength between the layers, the solid lubricating film existing on the surface of the tool is transferred to the workpiece during the cutting process. The surface of the material forms a transfer film, which causes friction during the cutting process to occur between the transfer film and the lubricating film, inside the solid lubricating film, thereby reducing friction, preventing adhesion, reducing cutting force and cutting temperature, and reducing tool wear. the goal of.
    Table 1 Adaptation range of MoS 2 /Ti "soft" coated tools
    Processing methods Cutting conditions result
    Turning V=150～250mm/min, fz=0.125～0.315mm/r, ap=1.5～2.5mm, intermittent turning Suitable
    V>150m/min, fz=0.125～0.315mm/r, ap=1.5～2.5mm, continuous turning Not suitable
    Milling V<250m/min, fz=0.25mm/r ap=1.5mm Suitable
    V>250m/min, fz=0.25mm/r ap=1.5mm Not suitable

    1
    Figure 2 Material volume that can be cut by the tool

    1
    Figure 3 Comparison of the amount of flank wear on the tool

  2. Application effect of "soft" coated tools
    Renevier and other studies have shown that MoS 2 /Ti "soft" coated tools are not suitable for continuous high-speed turning, because the high temperature generated by continuous high-speed turning often causes severe oxidative wear of the MoS 2 /Ti coating; in low-speed milling, The life of MoS 2 /Ti “soft” coated tools is 1.5 to 2 times higher than that of uncoated tools. In general, MoS 2 /Ti "soft" coated tools are suitable for low-speed interrupted cutting, as shown in Table 1. Figure 2 shows the workpiece material volume that can be milled for each tool when MoS 2 /Ti “soft” coated tool, MoS 2 “soft” coated tool and uncoated tool are used to mill 34CrNiMo6 workpiece, where fz=0.1mm/r , ap = 1.5mm. It can be seen that the life of the MoS 2 /Ti "soft" coated tool is much higher than that of the MoS 2 "soft" coated tool and the uncoated tool.
    Figure 3 shows the Wr/Co/MoS 2 “soft” coated tool and the WC/Co cemented carbide tool. When machining 302 stainless steel, the tool flank wear amount is compared, where V=103～180m/min, f=0.1mm /r, ap=0.25mm. It can be seen that when the WC/Co cemented carbide tool is cut at 1800m, the tool flank wear amount VB reaches 0.32mm, and when the WC/Co/MoS 2 “soft” coated tool cuts 4000m, the tool flank wear amount VB is only 0.3. Mm. Table 2 shows some of the effects of some soft coated tools.

    Table 2 Effect of “soft” coated tools

    Tool Test conditions test results
    Uncoated carbide indexable face milling cutter
    MoS 2 "soft" coated carbide indexable face milling cutter     		Al-Cu-Mg alloy "Soft" coated milling cutters can increase productivity by up to 3 times compared to non-layered milling cutters
    Uncoated tap
    TiAlN coated tap
    MoS 2 "soft" coated tap     		Al-Cu-Mg alloy Uncoated taps can process 20 holes, TiAlN coated taps can process 1000 holes, and MoS 2 “soft” coated taps can process 4000 taps
    Single coated (TiCN) carbide milling cutter
    TiCN/MoS 2 /Ti "soft" coated carbide milling cutter     		V=150m/min, f=0.04mm/r, ap=4mm, workpiece: AlSi304 stainless steel The soft-coated milling cutter has a significantly reduced cutting force and the surface quality is significantly improved.
    Ordinary high speed steel
    TiCN/MoS 2 /Ti "soft" coated high speed steel drill bit     		V=6000r/min, workpiece: AlSi15 silicon aluminum alloy “Soft” coated drill bit has no wear after drilling 938 holes, and ordinary high speed steel drill can only drill 100 holes.
    Uncoated high speed steel drill bit
    TiCN/MoS 2 /Ti "soft" coated high speed steel drill bit     		V=2200r/min, workpiece: Ti6Al4V alloy The "soft" coated drill bit has a drilling power reduction of approximately 33% compared to the uncoated drill bit, and the drilling performance is significantly better than the uncoated drill bit.
    Uncoated Si 3 N 4 tool
    Si 3 N 4 /MoS 2 “soft” coated tool uncoated Ti(CN) tool
    Ti(CN)/MoS 2 “soft” coated tool     		V=103～180m/min, f=0.1mm/r, ap=0.25mm, workpiece: 1045 carbon steel "Soft" Xu layer tool life is more than 50% higher than uncoated tools

4 Conclusion

The “hard” coating tool technology at home and abroad is becoming more and more mature, and its application has been very extensive. The "soft" coated tool technology has just started in foreign countries, and there is no research report in this area in China. The research and development of “soft” coated tools has opened up new avenues for improving tool performance, providing new ideas and new research fields for the design of new coated tools. "Soft" coated tools can be applied to dry cutting, which has great theoretical and practical significance for preventing bonding, reducing friction, reducing wear, improving tool life and reducing processing cost, and has broad application prospects.

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