Table 1 Wear resistance of magnetron sputtering coatings under different substrate biases
2.2. Ion plating technology
Ion plating is an emerging technology that has the characteristics of fast deposition rate, simple process, and no environmental pollution, and has been rapidly developed and widely applied. The working principle of ion plating is as follows: under the action of negative bias of the substrate, a plasma zone is formed between the target material and the workpiece. The target material generates metal particles by heating and evaporating, and the metal particles collide with the working gas (argon atoms) and electrons through the plasma zone, producing positive ions and neutral atoms, which deposit on the surface of the substrate to form a coating. The significant advantages of ion plating coatings are high bonding strength, high density, good uniformity, good diffraction, high ionization rate, and uniform thickness. The main process parameters include substrate bias, cathode arc current, and deposition pressure.
2.2.1. Substrate bias voltage
In the process of ion plating, a negative bias voltage is generally applied to the surface of the substrate to form a negative electric field, which accelerates the electron velocity in the plasma and forms a sheath layer on the surface of the substrate; The sheath accelerates the movement of positively charged ions evaporated from the target material, thereby increasing the deposition rate. Research has shown that increasing the substrate bias voltage enhances the impact of high-energy particles on the substrate surface, helping to remove large-sized particles from the substrate surface and improve coating density. Liu Lingyun et al. found that as the substrate bias increases, the deposition rate accelerates, the ion bombardment effect increases, and the density and erosion resistance of CrAlN coatings improve. Research by YAO et al. found that as the substrate bias increases, the friction coefficient of TiSiN coating increases from 0.2 to 0.4, and the wear mechanism gradually shifts from adhesive wear to plow groove wear. Wang Di et al. found that with the increase of substrate bias voltage, the grain size of CrAlN coating is refined, the surface roughness is reduced, and the wear resistance is enhanced. The substrate bias has a significant impact on the surface smoothness, density, and tribological properties of ion plating coatings, and the impact pattern is complex. In practical applications, comprehensive consideration is needed.
2.2.2. Cathodic arc current
The cathodic arc current determines the particle energy of the cathode target material evaporated during ion plating. Increasing the cathodic arc current will raise the temperature of the target material, increase the energy of the particles evaporated from the cathodic target material, increase the number of evaporated particles, and increase the plasma density. However, at the same time, the size of the evaporated particles also increases, and impurity phases are generated, which is not conducive to improving the wear resistance. Research on bangs has found that as the cathodic arc current increases, the hardness of ion plated chromium nitride coatings first increases and then decreases, and the friction coefficient first decreases and then increases. Zheng Chenchao and others found that as the cathodic arc current increases, the surface smoothness of CrN coating decreases. From the above results, it can be seen that although the increase in cathodic arc current improves the mechanical properties of the coating to a certain extent, the synchronous increase in particle size evaporated often hinders the improvement of wear performance. On the premise of ensuring mechanical properties such as hardness, small cathode arc current should be reasonably used to reduce the generation of large-sized droplets and high-energy ion clusters, thereby reducing the friction coefficient and improving the wear resistance of the coating.
2.2.3. Sedimentary pressure
Sedimentary pressure refers to the partial pressure of nitrogen gas introduced into the vacuum chamber. When the deposition pressure is low, the number of gas molecules involved in the discharge is small, resulting in a lower ion density generated from the surface of the target material, a longer average molecular free path for ions, and a higher energy for metal ions to reach the substrate surface. Research has shown that increasing the nitrogen flow rate within a certain range can increase the density of nitrogen plasma, make its reaction more complete, form more nitrides on the substrate surface, and improve coating density. However, excessive nitrogen flow rate can also lead to high target temperature, droplet splashing, and increased surface roughness. CHANG et al. found that as the nitrogen partial pressure increases, the growth direction of CrN phase gradually changes from (111) orientation to (220) orientation; The orientation of (220) is towards the cylindrical direction of NaCl structure, with fewer slip systems and greater hindrance to dislocation movement, indirectly improving the surface hardness of the coating. Liu Shuangwu et al. found that with the increase of nitrogen partial pressure, the wear resistance of TiSiN coating is enhanced. In summary, while avoiding the weakening of the bombardment ability of particles involved in film formation, which leads to a decrease in coating density, a higher deposition pressure should be chosen to improve the wear resistance of the coating.
2.3. Ion beam assisted deposition technology
The principle of ion beam assisted deposition technology is to obtain carbon ions from graphite cathodes under the action of gas high-voltage discharge and gas ion bombardment, and deposit them on negatively charged substrates through electric field acceleration. The process parameters that affect the wear resistance of ion beam assisted deposition coatings mainly include ion source discharge current and ion bombardment energy.
2.3.1. Ion source discharge current
The discharge current of the ion source determines the number of atoms reaching the surface of the substrate. As the discharge current of the ion source increases, the number of atoms reaching the substrate surface increases, the atomic activity increases, the coating density improves, and the material's re sputtering ability and ion etching effect are enhanced. Ren Yi's research found that the larger the discharge current of the ion source, the fewer defects in the TiN coating, the stronger the bonding strength, the higher the hardness, and the better the wear resistance; But when the discharge current of the ion source is too high, some nitrogen molecules enter the coating, causing the TiN coating to deviate from the ideal stoichiometric ratio and form pores, which in turn reduces the density of the coating and leads to a decrease in wear resistance. Feng Dan's research found that with the increase of ion source discharge current, the hardness of Ti-Cu-N coating first increases and then decreases. When the current is 30A, the hardness * can reach 39.24GPa, and the wear resistance *. The increase in ion source discharge current leads to an increase in atomic activity, promoting the diffusion of various atoms and making the coating structure denser; But when the current is too high, the ion etching effect is enhanced, which can cause back sputtering of the coating.
2.3.2. Ion bombardment energy
The energy of ion bombardment has a certain impact on the growth rate of coatings. High bombardment energy can affect the quality of coating growth, while low energy cannot achieve interfacial mixing. Liu Gang et al. found that as the energy of ion bombardment increases, the hardness of DLC coatings first increases and then decreases. Tan Ming et al. found that with the increase of ion bombardment energy, the friction coefficient of ZrN/TiAlN coating first decreases and then increases. When the bombardment energy is 200eV, the friction coefficient * is small, which is 0.22; This is because when the bombardment energy increases within the range of 100-200eV, N+transfers energy to atoms through collision, promoting crystal nucleation and growth, which helps to improve the tribological properties. However, when the bombardment energy is too high, the atomic ordering is chaotic, the integrity of the interface is damaged, and the coating density decreases under the influence of splashing, resulting in poorer tribological properties.
2.4. Vacuum evaporation plating technology
Vacuum evaporation plating, abbreviated as evaporation plating, was proposed by Faraday in 1857 and is an early developed technique in PVD. Its principle is to heat the target material in a vacuum chamber to convert solid particles in the target material into gas-phase molecules, which deposit onto the substrate to form a solid coating. Vacuum evaporation coating consists of three processes: target evaporation, transport of vaporized atoms to the substrate surface, and aggregation of evaporated atoms to the substrate surface. It has the advantages of simple operation, high efficiency, fast film formation rate, and large-area coating. However, it also has disadvantages such as short coating life, difficult control of uniformity, and poor process repeatability. Chen Xiaoming et al. deposited a titanium coating on the surface of Ti6Al4V alloy using vacuum evaporation plating. The results showed that vanadium element would accumulate on the coating surface at 1000 ° C, leading to a decrease in mechanical and tribological properties. This indicates that excessively high temperatures are not suitable for vacuum evaporation plating of titanium alloys. Due to the use of low melting point target materials in vacuum evaporation plating, the deposited coatings are mainly used for decoration and are generally less used to prepare wear-resistant coatings with good density and high hardness. Therefore, research on vacuum evaporation plating on titanium alloy surfaces is limited.
3. Conclusion
Titanium alloys have low hardness, high and unstable friction coefficient, severe adhesive wear, and strong sensitivity to micro motion wear, which limits their application in the field of wear. PVD technology is one of the important surface modification techniques for improving the wear resistance of titanium alloys. Common PVD coatings include diamond-like carbon coatings, modified nitride coatings, composite nitride coatings, and gradient coatings. The commonly used PVD techniques include magnetron sputtering, ion plating, ion beam assisted deposition, vacuum evaporation plating, etc. The process parameters include substrate bias, sputtering power, deposition pressure, deposition temperature, cathode arc current, etc. At present, there are some issues with the preparation of wear-resistant PVD coatings, such as the lack of research on the influence of preparation process parameters on the wear resistance of coatings, and the need to explore the effects of deposition time, evaporation power, target material composition, and other factors on the frictional properties of coatings; When titanium alloys experience friction and wear, they are generally not only affected by a single factor, and the frictional properties of PVD coatings under the synergistic effect of multiple factors still need further research; Research on surface modification of titanium alloys mainly focuses on TC4, TC11, and TC18 titanium alloys, with less research and insufficient data on other series of alloys.