Titanium and titanium alloys are widely used in various fields such as aerospace, petrochemical, food and medical because of their non-toxicity, light weight, high specific strength and good biocompatibility. However, performance defects such as low hardness, poor wear resistance and insufficient resistance to high-temperature oxidation have limited the further development of titanium alloys. To overcome these shortcomings, chemical heat treatment (or chemical modification) has become an effective means.
Chemical heat treatment is the use of chemical reactions, and sometimes physical methods, to change the chemical composition of the surface layer of the metal parts and organizational structure, so as to obtain better performance than homogeneous materials metal heat treatment process. For titanium and titanium alloys, the most commonly used chemical heat treatment methods include nitriding, carburizing, boriding and metallizing.
Nitriding produces high hardness nitrides (e.g. TiN and Ti2N) on the surface of titanium alloys, which provide excellent corrosion and wear resistance. Common nitriding techniques include salt bath nitriding, gas nitriding, ion implantation nitriding, double-layer glow plasma nitriding, surface laser nitriding and vacuum nitriding.
Carburizing treatment is used to obtain carbides on the surface of titanium alloys to improve their hardness and wear resistance. Due to the dense passivation film on the surface of titanium alloys and the low atomic diffusion coefficient, the carburizing process is more complicated and requires higher temperature and finer technical control. Titanium and carbon can form TiC reinforced phase, carburizing technology includes solid carburizing, ion carburizing, gas carburizing and laser carburizing. Among them, solid carburizing technology is simple, easy and low cost, but it is difficult to control the oxygen concentration, the carburized layer is not uniform and the thickness is small.
Boronizing treatment can form borides on the surface of titanium alloys to further improve hardness and corrosion resistance, suitable for applications with very high hardness and wear resistance requirements. The main compounds formed by titanium and boron are TiB and TiB2. Boron penetration techniques also include solid, liquid and gas methods, with a wide range of specific techniques.
Metallurgy is used to improve the properties of titanium alloys by infiltrating other metallic elements into the surface to form composite materials. There is a wide choice of pre-infiltrated metal elements, but they need to have good solid solubility with the titanium alloy. Factors affecting the solid solubility of metals mainly include atomic size, chemical affinity, crystal structure, and relative atomic valence.
In summary, the chemical heat treatment of titanium alloy has its own advantages, and the appropriate process should be selected according to different needs. At present, nitriding and carburizing technologies are more widely used. With the continuous development of titanium and titanium alloy technology, the surface treatment of titanium alloy will usher in a greater space for development, giving higher performance for titanium and titanium alloy products.
