Cutter coating

Certain films are formed on the surface of cutters by chemical or physical methods and in this way the cutting performance get improved for high speed cutting technology. Chemical vapour deposition (CVD) and physical vapour deposition (PVD) developed greatly since the 1970s when hard coating cutters are invented. Compared with uncoated cutters, coated cutters have longer lifetime, high cutting efficiency and precision as well as nice surface quality; consumption of cutter materials are saved for low cost; fewer cooling liquids are used which is environmental friendly.

Characteristics of cutter coating

1. Coating technology improves the surface hardness of cutters while maintains the cutters’ strength; the present level of hardness obtained by experiments is about 100 GPa.

2. Chemical stability and High temperature oxidation resistance of the film are outstanding, thus making the high speed cutting possible.

3. Nice solid phase lubricant performance improves the processing quality; suitable for dry cutting process.

4. Coating technology, as the final procedure of cutter manufacturing, has little effect on cutters’ precision, and coating process is repeatable.

Common coatings

1. TiN coating: TiN is a general PVD coating, it can improve cutters’ hardness with high oxidation temperature. It is widely used for high speed steel (HSS) cutting tools and forming tools.

2. CrN coating: CrN has nice adhesion resistance which is the first choice for processing in which fragment tend to accumulate. It is widely used for HSS cutters, cemented carbide cutters and forming tools.

3. Diamond coating: CVD diamond coating is used for nonferrous metal materials; it can be applied on graphite, metal matrix composites, SiC-Al alloy and other corrosion wear materials. (Notice: cutters with pure diamond coating cannot be used to process steel parts since large amount of heat are generated during cutting and thus bring about chemical reaction, destroying the adhesive layer between the coating and the cutter.

4. TiCN coating: carbon is added into TiCN coating and thus improves hardness of cutters as well as surface lubricant property.

5. TiAlN/AlTiN coating: alumina layer formed in the TiAlN/AlTiN coating can prolong the lifetime of cutter at high temperature. This coating is used for cemented carbide cutters for dry or semi-dry cutting. AlTiN coating can provide higher surface hardness than TiAlN coating as per Al/Ti proportion.

Coating technology

1. Due to the higher content of carbon, hardness of the TiCN coating increase by 33%, the varying range of hardness is Hv3000-4000; much higher than that of TiN.

2. CVD diamond coating: surface hardness is as high as Hv9000, its lifetime improves 10 to 20 times longer than uncoated cutters, and the cutting speed improves 2-3 times.

3. Requirements for hard films on the surface of cutters is as follow: high hardness, wear resistant; nice chemical stability, no reaction with workpiece; heat resistant and oxidation resistant; low friction coefficient; adhesive with matrix.

Single coating material can hardly fulfill the above requirements and compound coatings such as TiC—A12O3一TiN coating and TiCN, TiAlN coatings gradually replace the single TiN, TiC coatings. Now new film materials of TiN／NbN, TiN／CN are developed.

4. For the manufacturing of coated cutters, parameters of hardness, wear resistance, high temperature oxidation resistance, lubricant property and adhesion property are often considered. Oxidation property of the coating relates directly to the cutting temperature. Oxidation temperature refers to the temperature value at which the coating begins to decompose. With high oxidation temperature, the cutting process performs better.

5. From the viewpoint of applied technology, depth of cutting, speed of cutting and cooling liquid all affect the cutter coating.

Development of common coating materials and superhard coating technology

TiN is the most widely used hard coating material. TiN coated HSS cutters in developed countries reach to 50%-70% among the HSS cutters, and un-regrinding cutters reach to 90%. Due to the poor oxidation resistance, when at 500 ℃ or even higher, the film begins to oxidize and gets ablated. And hardness of TiN is also not very high. As a result, TiN can hardly fulfill the increasing demand by modern metal cutting technology. TiC has higher micro-hardness and abrasion resistance is nice, it can stick to matrix well. Therefore TiC is usually used as the underlying membrane with matrix.

TiCN and TiAlN further improve the coated cutters. TiCN can reduce internal stress, strengthen toughness and increase thickness of the coating; prevent diffusion of the cracks and cutting edge chipping. As the main abrasion resistant layer for coated cutters, TiCN can significantly prolong the lifetime of cutting tools. TiAlN has good chemical stability and abrasion-oxidation resistance. It can prolong the lifetime 3-4 times than TiN coated cutters when processing high-alloy steel, stainless steel, Titanium alloy and nickel alloy. With high content of Al in the TiAlN coating, a thin film of Al2O3 is formed on the coating surface, acting as a hard inert protection membrane. TiAlN coated cutters usually are used for high speed cutting process. TiCNO added with oxygen has good micro-hardness and chemical stability, similar to TiC/A12O3 coating.

Among the above hard membrane materials, diamond film, CBN and CN show micro-hardness more than 500GPa. Deposition temperature for diamond film is about 600℃一900℃, and it is used for cemented carbide cutting tools. Commercialization of diamond cemented carbide tools represents the great achievement of the coating technology in recent years.

CBN shows good hardness and thermal conductivity, just following diamond. Its thermal stability is nice and no oxidation occurs even at 1000℃. Unlike diamond that cannot process steel parts, CBN shows nice chemical stability against ferrous metals, and can widely be used to finish or grind steel parts. Beside good abrasion resistance, CBN coating is also used at high cutting speed to process heat resistant steel, titanium alloy, hardened steel and to cut chilled roll of high hardness and Si-Al alloy.