1. The coating composition and microstructure of TiAlN
Since Al atoms and Ti atoms have similar sized atomic radius, Al atoms can be replaced by Ti atoms in the binary coating TiN to form a new type of ternary composite coating (Ti, Al) N with face-centered cubic structure. Al makes the lattice of TiN distorted, the number of grain boundaries increases accordingly, and the relative slip between crystals becomes more difficult, so the hardness of the coating is further increased.
Zhu Xinfa et al. use the element composition of TiAlN coating prepared by cathodic arc technology (as shown in Table 1). The composition of the coated elements in Table 1 shows that the ratio of the Ti element to the Al element is close to 1: 1, and further confirms that the Al in the coating is in the form of replacing Ti, forming a compound of TiN similar to AlN.
Tab 1 TiAlN coating composition
Content / (%)
The addition of Al elements not only plays the role of solid solution strengthening, but also the lattice constant become smaller, which makes the surface of the prepared TiAlN coating dense and smooth. Fig. 1 is the surface topography and cross-sectional image of the TiAlN coating prepared on the WC-CO cemented carbide substrate by cathodic arc technology.
Fig. 1 The surface topography and cross-sectional image of the TiAlN coating
2. Properties of TiAlN coating
The TiAlN ternary composite coating is based on the addition of Al element to the TiN coating for alloying, thereby improving the hardness, wear resistance and high temperature stability of the coating, and methods for improving the other properties of the coating. The hardness of TiN coating prepared by cathodic arc technology is about 23GPa. However, with the increase of Al elemental composition in the TiAlN coating, the hardness of the coating increases first and then decreases. This is due to the difference in the proportion of Ti and Al elements in the coating. The difference in the content of Ti and Al in the coating prepared by the Ti target and the Al target causes the hardness of the coating to be different (as shown in Table 2).
The traditional TiN coating has poor high-temperature oxidation resistance and wears resistance. However, the TiAlN coating prepared by cathodic arc technology can achieve a high temperature oxidation temperature of 900°C, which is greatly improved compared with 550°C of TiN coating. As the Al element in the coating reacts with the O element in the air under high temperature conditions and form a dense Al2O3 phase, which hinders internal oxidation.
In addition to the above properties, TiAlN coating has good corrosion resistance. The TiAlN coating substrate was immersed in the acid solution and the salt solution for the corrosion test. Observation under the scanning electron microscope (SEM) revealed that the surface condition of the coating substrate was good and there was no obvious corrosion peeling.