Tool coatings are produced by depositing a thin layer of wear-resistant compounds onto tough cemented carbide or high-speed steel substrates. Based on deposition methods, coated tools fall into two categories: CVD coated tools and PVD coated tools. CVD is generally adopted for coated cemented carbide tools, with deposition temperature around 1000°C. For a long time, researchers have worked to resolve the trade-off between toughness and wear resistance of cutting tools. This article discusses how to select coated tools matching practical demands.

Performance of Common Coatings

The overall performance of coated tools depends heavily on substrate materials and tool coatings. Common substrate materials include cemented carbide and high-speed steel. Typical tool coating materials cover TiC, TiN, TiCN, TiAlN, Al₂O₃, MoS₂ and diamond. Coatings can be single-layer or multi-layer structures. Coating thickness ranges from 2 μm to 18 μm. Thin coatings cope better with temperature variations under interrupted cutting, for they carry lower internal stress and resist crack formation. Thick coatings tend to crack under rapid heating and cooling, similar to glass subjected to sharp temperature shifts. Dry machining with thin-coated inserts can extend tool service life by 40%. Table 1 shows the performance of mainstream coatings.

Table 1 compares heat dissipation capacity of common tool coatings. Compared with TiN, TiAlN delivers the best heat dissipation, followed by TiC. Therefore, TiAlN is widely adopted for tools used in dry cutting (see Table 2).

table1

table2

TiN represents one of the most widely used tool coatings. It is commonly applied to high-speed steel substrates. TiN boasts higher hardness than high-speed steel, low friction coefficient, decent toughness and high hot hardness to sustain elastic deformation. Its thermal expansion coefficient is close to that of high-speed steel, generating low thermal stress during temperature fluctuation in cutting and maintaining strong bonding strength. TiN features good chemical stability, corrosion resistance and oxidation resistance, and barely reacts with workpiece metals. Besides, TiN films can be manufactured via nearly all PVD and CVD techniques.

Tools with TiN coatings apply to high-speed cutting or machining workpieces of medium hardness, such as normalized and quenched-tempered materials, hobbing of tin bronze, thread cutting and drilling.

Al₂O₃ coatings have sound mechanical properties, outstanding hot hardness and chemical stability. Inserts coated with Al₂O₃ resist crater wear effectively and achieve longer service life. In addition, Al₂O₃ has low thermal conductivity, which drops further as temperature rises. This characteristic blocks cutting heat from reaching cutting edges and prevents sudden tool failure caused by thermal plastic deformation.

Al₂O₃ coatings are mostly deposited on cemented carbide tools. Coated tools combine the wear resistance of ceramic tools and the strength of cemented carbide. They work in scenarios where ceramic tools easily chip due to brittleness, and are used for machining cast iron and high-speed steel.

For instance, when machining automotive cast iron brake discs and brake drums, inserts with Al₂O₃ coatings last 2 to 4 times longer than TiC coated inserts, cermet tools and ceramic tools, and 6 to 8 times longer than uncoated cemented carbide tools. At cutting speeds from 365 m/min to 550 m/min, its performance is comparable to ceramic tools. An intermediate layer of TiN, TiC or TiCN is usually added between Al₂O₃ coatings and substrates. This improves wear resistance and blocks diffusion of cobalt, tungsten and other elements from cemented carbide substrates under high temperature, strengthening bonding force. Nevertheless, TiC interlayers are brittle phases and will reduce coating strength.

TiAlN coatings fit well with high-speed dry cutting. TiAlN starts oxidation at 700–800°C, higher than TiC and TiN. It maintains higher hardness and better thermal stability than TiN at elevated temperatures. An oxidation film forms on TiAlN coatings under high temperature, easing friction between tools, workpieces and chips and reducing heat generation. Moreover, TiAlN has lower thermal conductivity than TiN, offering thermal insulation and enabling tools to withstand higher temperature during dry cutting.

TiAlN coatings with aluminum content above 50% are defined as AlTiN coatings to distinguish them from conventional TiAlN coatings. Higher aluminum content brings greater hardness and excellent wear resistance. AlTiN coatings retain high hardness even when the contact temperature between tools and workpieces reaches 800–900°C, making them advanced wear-resistant tool coatings. Multiple commercial AlTiN coatings contain over 65% aluminum. For example, coatings from Carboloy contain 67% aluminum, and products from Ion Bend reach 70%. In July 2002, Ceme Con developed a process for super nitride coatings with aluminum content up to 80%.

The ideal cutting speed for AlTiN coated tools ranges from 183 m/min to 244 m/min, paired with spindle speed of 20000–40000 r/min or above. The depth of cut should not be excessive. Under such conditions, AlTiN coated tools reduce spindle load, raise productivity and produce favorable surface finish.

AlTiN coated tools suit high-speed machining and dry high-speed machining of materials above 40 HRC with strong wear resistance. They can be used for milling, drilling and turning. Table 4-5 compares performance between AlTiN coatings and other tool coatings.

TiAlN outperforms TiN in oxidation resistance. TiN oxidizes at 600°C, while TiAlN remains stable until 800°C. A dense Al₂O₃ film forms during oxidation, improving diffusion resistance and oxidation resistance of TiAlN. Unalloyed Al₂O₃ has low microhardness at 1500 HV0.05, yet Al₂O₃ cutting materials achieve strong wear resistance, thanks to good oxidation stability and thermal stability of α-phase Al₂O₃. Nitrogen addition generates Al-O-N coatings. Adding titanium further creates oxygen-rich Ti-Al-O-N coatings with upgraded microhardness and wear resistance. TiAlON coatings produced via PVD process deliver high oxidation stability and higher microhardness, lifting wear resistance of tools in dry cutting.

Single-layer TiAlON coatings contain oxidation impurities at interfaces, leading to microhardness below 2000 HV0.05 and weak bonding. Multi-layer coatings based on TiAlN-TiAlON are adopted to guarantee sufficient bonding strength and high microhardness.

Researchers from Leibniz University Hannover studied TiAlON coatings applied in dry drilling. The results prove wear resistance varies with tool coating layers and element ratios inside tool coatings.

Oxide coatings feature high wear resistance and low friction coefficient, preventing heat transfer into tool substrates to the maximum extent. Common oxide coatings include AlTiN-Al₂O₃, TiAlN-ZrO₂ and TiZrN-ZrO₂. Their key parameters are shown in Table 3.

Example: Drilling blind holes of 6 mm diameter and 18 mm depth on SGI70 high-strength ductile iron. Cutting parameters stay consistent with wet cutting: cutting speed 80 m/min, feed rate 0.35 mm/r. All tools adopt cemented carbide substrates, and tool failure is judged when flank wear width reaches 0.3 mm. Oxide-based PVD multi-layer tool coatings exhibit outstanding wear resistance. Tools with TiZrN-ZrO₂ coatings achieve a cutting length of 7.2 m. Tools with AlTiN-Al₂O₃ coatings reach 16.2 m. TiAlN-ZrO₂ coated tools deliver the best performance with cutting length up to 18.1 m, four times the service life of regular TiAlN coated tools.

Coated tools balance tool toughness and wear resistance by covering substrates with wear-resistant tool coatings. Different coatings carry unique properties. TiN is widely used for general machining. Al₂O₃ fits cast iron machining. TiAlN works well for dry cutting. AlTiN with high aluminum content is selected for high-speed machining of hard materials. TiAlON and composite oxide multi-layer coatings achieve superior performance. No single coating satisfies all working conditions. Coating selection needs to consider workpiece materials, cutting modes (dry or wet) and cutting parameters.