General Overview Of Tool Coatings
Coated cutting tools refer to the tools that have a thin layer of refractory metal or nonmetallic compounds on their cemented carbide or high-speed steel (HSS) surface, the film can be also deposited on ceramic, diamond, cubic boron nitride (CBN) and other superhard material blades). As a chemical barrier and thermal barrier, the film can reduce the diffusion and chemical reactions between the tool and the workpiece, thereby reducing the wear of crescent. Coated cutting tools have the characteristics of high surface hardness, good wear resistance, good chemical stability, great heat and oxidation resistance, small friction coefficient and low thermal conductivity and so on, these advantages can improve the tool life by 3 ~ 5 times than the uncoated tool when cutting, also, the cutting speed can be improved by 20% to 70%, the machining accuracy can be improved by 0.5 to 1, and the tool consumption costs can be reduced by 20% to 50%. As a result, coated cutting tools have become the symbol of modern cutting tools and the use ratio of it in cutting tools is more than 50%. At present, all kinds of tools used in cutting include turning tools, boring tools, drills, reamers, broaches, screw taps, thread chasers, milled heads, milling cutters, forming tools, gear cutter hobs and slotting cutter all can use coating process to improve the performance.
There are four types of coated tools: coated HSS tools, coated cemented carbide tools, coated ceramic tools, and coated superhard materials (diamond or cubic boron nitride) blades. But coated HSS tools and coated Cemented carbide tools are most used. Coatings on ceramic and super-hard blades are materials with a lower hardness than the substrate. The purpose is to increase the fracture toughness of the blade surface (the fracture toughness can be improved by 10%), which can help reduce the flaking and breakage of the blade and expand the range of applications.
At present, there are two kinds of coating methods commonly used in production: physical vapor deposition (PVD) method and chemical vapor deposition (CVD) method. The deposition temperature of PVD is 500 ℃, with the coating thickness is 2 ~ 5μm. The deposition temperature of CVD is 900 ℃ ~ 1100 ℃, with the coating thickness is 5 ~ 10μm, also, the equipment of CVD is simple and CVD coating is very uniform. HSS tools commonly used PVD method because the deposition temperature of PVD does not exceed the tempering temperature of high-speed steel itself. Most of the cemented carbide uses CVD method, because when coating, a brittle layer of decarburization (η phase) will be formed between the coating and the substrate due to its high deposition temperature. In recent ten years, with the development of coating technology, cemented carbide can also use PVD method. As a composite coating process, the combination of PVD and CVD is called PACVD method (plasma CVD method). PECVD method uses plasma to promote chemical reactions, and the coating temperature can be reduced to below 400 ℃ (currently the coating temperature has been reduced to 180 ℃ ~ 200 ℃), which ensures there are no spread, phase change or exchange reaction between the cemented carbide substrate and the coating material and then to maintain the blade's original toughness. This method is particularly effective for diamond and cubic boron nitride (CBN) superhard coatings.
When adopting the CVD method, the cutting edge needs passivating treatment in advance (the radius of the blunt circle is usually 0.02-0.08mm, and the strength of the cutting edge increases with the enlargement of the radius of the blunt circle). So, the cutting edge is not as sharp as the uncoated blade. As a result, the cutting tools that require sharp cutting edge should use PVD method. The coating can not only deposit on the general cutting blade, but also on the solid tool, and now, it can be coated on welded cemented carbide tools. Adopting the PCVD method on cemented carbide drills can make the life of drills 10 times longer than the high-speed steel drill, the efficiency can be improved to 5 times.
Coating materials must meet the requirement of high hardness, good wear resistance, good chemical stability, no chemical reaction with the workpiece material, good heat and oxidation resistance, low friction coefficient, good adhesion with the substrate and so on. Clearly, a single coating material is difficult to meet the above requirements. Therefore, the hard coating material has entered a new stage that develops thick film, composite coating and multi-component coating from the beginning of a single TiC, TiN, Al2O3. The composition of newly developed TiCN, TiAlN, TiAlN multiple, ultra-thin, super multi-layer coating and TiC, TiN, Al2O3 coatings as well as new anti-plastic deformation matrix make significant progress on improvement of the coating toughness, the bonding strength of coating and substrate, wear resistance of the coating, etc.
At present, the technology that coating diamond film on the cemented carbide substrate reached a breakthrough, it can comprehensively improve the performance of the tool. The most mature and most widely used hard coating material is TiN, but the bonding strength between TiN coating and substrate is less than TiC coating, the TiN coating is easy to peel off, and the hardness is lower than TiC. When the cutting temperature is high, the TiN coating is easy to be oxidized and ablated. TiC coating has a higher hardness and wears resistance, as well as good oxidation resistance, but it is brittle with poor impact resistance. TiCN has the advantages of both TiC and TiN, it can control the TiCN properties by continuously changing the composition of C and N during the coating process and then form a multi-layer structure with different compositions, thereby reducing the internal stress of the coating, improving the toughness, increase the thickness of the coating, preventing the growth of crack and reduce flaking. The TiCN-based coating is suitable for processing ordinary steel, alloy steel, stainless steel and wear resistant cast iron, etc. The material removal rate can be increased by 2 to 3 times when machining by it.
TiAlN, CrN, TiAlCrN are newly developed hard coating material in recent years. TiAlN coated blades have been commercialized. With good chemical stability and oxidation wear resistance, the tool life is 3-4 times higher than TiN coating when machining high alloy steel, stainless steel, titanium alloy, and nickel alloy. In addition, if there is a suitable aluminum concentration in TiAlN coating, a layer of hard inert protective film will be formed between cutting tool face and chip interface, with good thermal insulation, this film can effectively be used for high-speed cutting. CrN is a kind of titanium-free coating which is suitable for cutting titanium, titanium alloys, copper, aluminum and other soft materials with good chemical stability and no stickiness. TiAlCrN is a gradient structure coating with high toughness and hardness, as well as small friction coefficient, it is suitable for milling cutter, hob, tap, and other tools, the cutting performance is significantly better than TiN.