Research progress on PVD coating and preparation technology for wear-resistant titanium alloy surface

Titanium and titanium alloys have the advantages of low density, high specific strength, and good corrosion resistance, and are widely used in aerospace, energy and chemical, biomedical and other fields. However, titanium alloys have low hardness and poor tribological properties, with high and unstable friction coefficient, severe adhesive wear, and strong sensitivity to micro motion wear, which limits their application in the field of wear.
Surface modification can effectively improve the tribological properties of titanium alloys. Common surface modification techniques include micro arc oxidation, chemical plating, laser cladding, thermal spraying, chemical heat treatment, ion implantation technology, electroplating, physical vapor deposition (PVD), etc. The oxide layer prepared by micro arc oxidation technology has good corrosion resistance, certain load-bearing capacity and wear resistance under high load, but this technology has problems such as high noise and energy consumption. The chemical coating has uniform thickness and high density, but the solution cost required for this technology is too high. Laser cladding technology has the advantages of small heat affected zone and high energy density, but there are problems such as coating cracking and structural defects. Thermal spraying has the advantages of lightweight equipment, flexible process, and controllable coating thickness, but the coating has problems such as micropores and low strength. Chemical heat treatment has the advantages of simple equipment, easy operation, and low cost, which can significantly improve surface hardness and wear resistance. However, its infiltration rate is slow, the infiltration layer is difficult to control, and the efficiency is low, which has certain limitations. Ion implantation technology can accurately control the type, dosage, and depth of injected ions to obtain specific surface properties, but it has shortcomings such as high equipment cost, low processing efficiency, limited ion implantation depth, and difficulty in processing complex three-dimensional shaped workpieces. Electroplating can form a uniform, continuous, and thickness controllable metal coating on the surface of conductive materials, with a relatively simple process and low cost. However, for non-conductive materials such as plastics and ceramics, special pretreatment is required before electroplating, and some electroplating processes can cause environmental pollution. PVD technology uses glow discharge, arc discharge, or heating evaporation methods in a vacuum chamber to evaporate the target material into gas molecules, which are then deposited onto the substrate surface to form a coating. It has the characteristics of uniform and dense coating, good adhesion, controllable thickness, high repeatability, no need for thermal activation, and good adhesion. It is suitable for large-scale industrial assembly line production and is a highly promising titanium alloy surface modification technology. At present, although there are numerous studies on PVD coatings on titanium alloy surfaces, there are relatively few systematic summaries and generalizations. In order to provide reference for relevant researchers, the author introduces common types of PVD coatings, systematically elaborates on the characteristics of commonly used PVD technology and the influence of process parameters on the mechanical and tribological properties of coatings, points out the problems in the preparation of PVD coatings on wear-resistant titanium alloy surfaces, and looks forward to their development direction.
1. Common Types of PVD Coatings
1.1. Diamond like Coating
Diamond like carbon (DLC) is a new type of coating material that has emerged in recent years, combining the excellent characteristics of diamond and graphite. Its sp2 hybrid bonds (graphite structure) have self-lubricating properties, endowing DLC coatings with excellent wear resistance, making them widely used in fields such as machinery, aerospace, and marine industries. MANHABOSCO et al. found that the chemical inertness and barrier effect of DLC coating can reduce the wear volume loss of TC4 alloy by three orders of magnitude. HE et al. deposited micro pit textured DLC coating on the surface of titanium alloy and found that the wear resistance of the alloy was significantly improved due to the storage effect of micro pit structure on wear debris and the induction of graphitization by pit morphology.
The DLC coating deposited on the surface of titanium alloy has high internal stress and is prone to detachment from the surface of the titanium alloy. By doping alloying elements such as zirconium and chromium, a nanocrystalline/amorphous composite structure is formed inside the coating, which can release internal stress through grain boundary diffusion, improve bonding strength, and enhance anti peeling properties. MOHAN et al. found that zirconium doping improved the adhesion of DLC coatings on Ti-13Nb-13Zr alloy substrates, increased the disorder of DLC coatings, and enhanced their tribological properties. TOTOLIN et al. deposited tungsten doped DLC coating on the surface of TC4 titanium alloy and found that the coating has strong chemical inertness and is prone to form a low shear strength transfer film, resulting in minimal wear volume loss and excellent corrosion and wear resistance. HATEM et al. deposited a titanium doped DLC coating on the surface of TC4 titanium alloy, and the open circuit potential of the coating surface was significantly higher than that of the substrate, indicating good corrosion and wear resistance. Doping some non-metallic elements into the coating can not only passivate the friction interface, but also bond with carbon and hydrogen atoms in the coating, thereby significantly improving the friction and wear performance. Dai Mingjiang deposited a silicon doped DLC coating on the surface of TC4 titanium alloy and found that the coating generated a SiO2 film during friction, which played a certain lubricating role. The average friction coefficient of the coating was 0.133, which was significantly reduced compared to the undoped silicon DLC coating (0.302). Tang Yongtao deposited a nitrogen doped DLC coating on the surface of TC4 titanium alloy. Compared with the undoped coating, its higher nitrogen content increased the film substrate bonding force, reducing the cracks in the coating during wear and making it difficult to propagate, providing stable protection for the substrate.
1.2 Modified nitride coating
The early research on nitride coatings mainly focused on binary coatings such as TiN, CrN, etc. In the 1980s, TiN coatings were widely used in practical engineering due to their high hardness and excellent wear resistance. The hardness of CrN is around 19GPa, slightly lower than TiN (21GPa), but it has good toughness, strong film substrate adhesion, and lower friction coefficient. By adding metal elements such as aluminum, chromium, silver, or non-metal elements such as boron, carbon, and silicon to modify the coating, new characteristics can be imparted to the coating, making it more suitable for the needs of actual industrial production.
Due to the different radii of nitrogen and aluminum atoms, adding aluminum elements to nitride coatings can cause lattice distortion, leading to an increase in coating hardness and thus improving wear resistance. Liu Rong et al. deposited TiAlN coating on the surface of TA19 titanium alloy and found that Al2O3 induced by frictional heat acted as a solid lubricant, significantly reducing the volume wear rate of TiAlN coating compared to the substrate. G Ó MEZ-OVALLE et al. found that the face centered cubic structure Al0.66Ti0.33N coating prepared by cathodic arc PVD technology has a friction coefficient of only 0.2 at 700 ℃. CHEN et al. deposited a silver doped AlCrN coating on the surface of Ti-6Al-4V alloy and found that the coating had high bonding strength with the substrate. Moreover, silver formed "rod-shaped debris" under the shear force during friction, acting as a lubricant and significantly reducing the friction coefficient of the nitride coating. Li Tong's research found that the magnetron sputtering deposition of Ag TiN coating formed a face centered cubic structure TiN, and the grain size of TiN decreased compared to undoped silver. Zhuang Chenqi's research found that the volume wear rate of TiCrN coating is 3.2 × 10-5mm3 · N − 1 · m − 1, which is significantly lower than that of TC4 titanium alloy substrate (5.4 × 10-4mm3 · N − 1 · m − 1). This is attributed to the fact that hard phases such as TiN and TiCrN2 in the coating can reduce the adhesion tendency between the coating surface and the wear pair. TiCN is a solid solution of TiN and carbon, belonging to the face centered cubic structure of TiN. Therefore, carbon can be doped into TiN coatings to significantly increase the hardness of the coating through solid solution strengthening, thereby improving wear resistance. Wan Qiang introduced silicon element into TiN coating and found that the addition of silicon can refine grain size, improve hardness, and reduce friction coefficient. SHAN et al. found that silicon doping can also improve the hardness and density of CrN coatings, and reduce the wear rate.
Researchers also attempted to improve the wear resistance of nitride coatings by co doping with two or more elements. Wu Yiruo et al. found that replacing chromium atoms in Cr2N with aluminum atoms can form a (Cr, Al) N hard solid solution phase. At the same time, adding silicon to refine the grains can increase the hardness of the coating to 3222HV and reduce the friction coefficient to 0.22. CHANG et al. found that the hardness of AlCrTiSiN coating is as high as 41.14GPa, with a friction coefficient of 0.22. The composite addition of silicon, titanium, and chromium elements refines the grain size and enhances the wear resistance of the coating. Zhai Li's research found that the volume wear rate of CrAlSiN coating is 5.42 × 10-7mm3 · N − 1 · m − 1. Compared to the titanium alloy matrix, it has decreased by three orders of magnitude. This is because the coating contains amorphous silicon, which can inhibit the growth of large particles and reduce plastic deformation. At the same time, the addition of aluminum forms an Al2O3 oxide film, which can hinder the diffusion of external oxygen elements into the coating and act as a protective layer.
In summary, doping metal elements in nitride coatings can refine grain size and improve density, while doping non-metal elements can promote the formation of amorphous coated nanocrystalline composite structures, thereby improving the mechanical and tribological properties of the coating.
1.3 Composite nitride coating
The single-layer nitride coating prepared by PVD usually has defects such as pinholes and large particles due to the columnar growth of nitride crystals, resulting in a decrease in the coating's ability to resist crack propagation. To solve this problem, researchers have adopted a multi-layer structure design to prepare composite nitride coatings, which can integrate the advantages of each single-layer coating to achieve better tribological properties.
TiAlSiN/TiN multilayer coatings typically have a three-dimensional network structure formed by the embedding of nanocrystals and amorphous phases, in which dislocations are difficult to form. The amorphous phase can block grain boundary slip, and the interface between the two phases can enhance the resistance to microcrack propagation. Therefore, the coating has high hardness and wear resistance. MA et al. prepared TiAlSiN/TiN multilayer and TiAlSiN monolayer coatings on TC18 titanium alloy surface using multi arc ion plating. It was found that under a load of 2mN, the indentation depths of the multilayer and monolayer coatings were 83.0 and 96.8nm, respectively. The friction coefficient curve of the multilayer coating was relatively stable and had a volume wear rate of *. Huang Xueli et al. found that the average hardness of TiN/CrN nano multilayer coatings increases with the shortening of the modulation period, and the volume wear rate decreases accordingly. When the modulation period is 12nm, the coating hardness * reaches up to 31GPa, and the volume wear rate * is small, only 1.18 × 10-7mm3 · N − 1 · m − 1. WIECINSKI et al. deposited Cr/CrN nano multilayer coatings on the surface of titanium alloys and found that: the top layer of the multilayer coating is CrN, and the first layer is chromium. CrN as the outer layer can ensure high resistance to plastic deformation, and the wear volume loss of the coating is reduced compared to the substrate. Chromium, as the contact layer with the substrate, can reduce interfacial stress and improve adhesion; When the modulation ratio from chromium layer to CrN layer is 0.81, the coating is dense, without defects such as pores and cracks. YONEKURA et al. deposited Cr/CrN multilayer coatings on Ti-6Al-4V alloy substrate using arc ion plating method. The study found that when fatigue cracks occur and propagate to the Cr/CrN interface, the top layer of CrN falls off, the bottom layer of chromium wears, and the high hardness CrN debris acts as abrasives, causing severe abrasive wear; As the number of coating layers increases, the thickness of the top layer of CrN becomes thinner, the hardness of the coating increases but the increase is not significant, and the wear resistance of the 3-layer Cr/CrN coating *.
In summary, compared to single-layer nitride coatings, multi-layer structure design can improve the hardness and mechanical properties of nitride coatings. Meanwhile, the modulation period and modulation ratio have a significant impact on the structure and properties of multilayer coatings, and multilayer coatings with appropriate modulation periods and modulation ratios exhibit superior mechanical and tribological properties.
1.4 Gradient coating
Gradient coating refers to a composite structure coating with a gradient change in composition and structure along a certain direction. Gradient coating replaces sharp interfaces with gradient interfaces, achieving a smooth transition from one performance to another. The elastic modulus, Poisson's ratio, shear elastic modulus, thermal expansion coefficient, etc. all change continuously along the design direction. In the direction perpendicular to the coating, the ratio of different components in the gradient coating is gradual, which can effectively reduce stress concentration at the inner and outer interfaces, improve the bonding ability between the coating and the substrate, and thus enhance the tribological performance. CAI et al. deposited gradient nano TiN coatings using direct current reactive magnetron sputtering. The adhesion between the coating and the titanium alloy substrate was as high as 81N, far higher than that of single-layer TiN coatings. The friction coefficient was only 0.24, indicating good wear resistance. Shan Xiangheng's research found that the Nb2O5/Nb2O5 Ti/Ti gradient coating had no obvious delamination phenomenon and had good interlayer bonding. The addition of an intermediate layer hindered the continuity of columnar crystal growth and improved the density of the coating. Its volume wear rate was 2.56 × 10-5mm3 · N − 1 · m − 1, which was reduced by 90.42%, 89.28%, and 86.28% compared to the substrate, Nb2O5 single-layer coating, and Nb2O5/Ti double-layer coating, respectively. The ceramic industry used magnetron sputtering technology to deposit hydroxyapatite/TiN gradient coating on the surface of TC4 titanium alloy. It was found that the addition of a dense and wear-resistant TiN intermediate layer improved the bonding strength between the coating and the substrate, thereby enhancing the wear resistance.
At present, the hardness and thickness of gradient coatings still need to be improved. Insufficient hardness makes the coating more prone to wear during friction, thereby shortening its service life. Insufficient thickness leads to insufficient support and protection, especially when subjected to heavy loads or impacts. The relationship between the composition, microstructure, and tribological properties of gradient coatings needs further investigation.
2. Common PVD technology
2.1. Magnetron Sputtering Technology
Magnetron sputtering technology uses a film material as the cathode and a substrate as the anode. Under the action of glow discharge, the argon gas introduced into the vacuum chamber is transformed into argon ions, which continuously bombard the film material, transforming it into gas-phase molecules and depositing them on the surface of the substrate. The history of magnetron sputtering has gone through the process from traditional magnetron sputtering, non-equilibrium magnetron sputtering to closed field non-equilibrium magnetron sputtering. The plasma region of traditional magnetron sputtering is relatively narrow, while non-equilibrium magnetron sputtering broadens the range that plasma can reach. The substrate of closed field non-equilibrium magnetron sputtering has dense plasma around it, which improves the sputtering efficiency of target ions. The process parameters of magnetron sputtering technology mainly include sputtering power, target substrate distance, deposition temperature, and substrate bias. The following will provide a specific introduction to them.
2.1.1 Sputtering power
The sputtering power determines the kinetic energy of the target atoms. Increasing the sputtering power will increase the energy of particles leaving the target, accelerate the deposition rate, and increase the coating thickness. Wu Bi investigated the effect of sputtering power on the tribological properties of titanium diboride coatings and found that when the sputtering power was 500W, the coating had a fine columnar crystal structure and lower hardness. This combination can improve the coordinated anti deformation ability with the substrate and the ability to hinder crack propagation, reducing the wear rate of the coating. Research has found that as the sputtering power increases, the energy of deposited atoms and atomic clusters increases, which is beneficial for crystal growth, promotes uniform distribution of grains, and increases the hardness of the coating. The friction coefficient first decreases and then increases, with a minimum value of 0.42. Sputtering power not only affects surface hardness, but also affects the adhesion between the coating and the substrate. Liu Hui's research found that as the sputtering power increases, the adhesion of the coating first increases and then decreases, and the wear resistance first increases and then decreases. Increasing the sputtering power within an appropriate range can increase the number of ionized ions, allowing them to transfer more energy during collisions with sputtered particles. Therefore, the particles are tightly bound to the substrate and have fewer defects; But when the power is too high, ion implantation will occur, causing the coating to become rough and the adhesion to decrease.
In summary, within a certain range, as the sputtering power increases, the thickness, deposition rate, adhesion, and surface hardness of the magnetron sputtering coating also increase. However, when the sputtering power is too high, not only will severe ion implantation occur, but the target material will also melt, causing the substrate temperature to rise or even burn out.
2.1.2. Target base distance
Target substrate distance refers to the vertical distance between the center of the target material and the center of the substrate. The increase in the distance between the target and the substrate will increase the collision frequency between particles leaving the target and gas molecules during their flight towards the substrate, causing the energy carried by the particles to be carried away by the gas molecules. At the same time, the target atoms will slow down due to the impact, resulting in a slower deposition rate. Qi Feng et al. found that when the target substrate distance was 70mm, TiN coating did not experience large-scale failure during 10000 cycles of wear, while when the target substrate distance was 140mm, it had already failed as early as 3000 cycles; Reducing the distance between the target and substrate can enhance the ion bombardment effect, improve the density and wear resistance of the coating. Ding Jia's research found that as the distance between the target and substrate decreases, the hardness and elastic modulus of the silver coating increase. Research has shown that although reducing the target substrate distance can improve the wear resistance of coatings, when the target substrate distance is too small and the energy of sputtered particles is too high, it can cause the coating to undergo re sputtering, thereby reducing wear resistance. Therefore, while ensuring the required wear resistance, a larger target base distance should be used as much as possible. LENIS et al. evaluated the effect of target substrate distance on the 200 ° C tribological properties of hydroxyapatite/silver coatings and found that the target substrate distance had almost no effect on the high-temperature tribological properties, which is inconsistent with other research results and may be related to the lower hardness, elastic resistance, plastic deformation resistance, and other properties of the coatings.
2.1.3. Sedimentation temperature
The deposition temperature determines the average kinetic energy of particles, which in turn affects the morphology, mechanical properties, and tribological properties of the coating. Zhang Yintan's research found that when the substrate is not heated, the grain boundaries of the deposited WTi alloy coating are blurred, and the structure is loose and porous. After heating the substrate, the coating structure becomes dense. Ma Jianjun's research found that compared to room temperature deposition coatings, DLC coatings deposited at 200 ° C have a lower wear rate. Liu Zaobao et al. found that as the deposition temperature increases, the mechanical properties of the coating first enhance and then weaken, and at 200 ℃ *. The average kinetic energy of particles is directly proportional to the deposition temperature. When the deposition temperature is low, atomic indentation occurs on the surface of the coating, resulting in lower adhesion and hardness; As the temperature gradually increases, the deposition rate accelerates, the kinetic energy of particles reaching the substrate surface increases, the coating density improves, and the adhesion, hardness, and wear resistance improve; But when the temperature is too high, the average kinetic energy of particles is too large, making it difficult for atoms to grow in their initial positions, increasing the surface roughness of the coating, and reducing the adhesion, hardness, and wear resistance. In the process of magnetron sputtering deposition, a higher deposition temperature should be selected within a reasonable range.
2.1.4. Substrate bias voltage
Substrate bias is one of the parameters that affect the structure and performance of coatings in PVD technology, and can directly control the migration rate of adsorbed atoms during the coating growth process. Table 1 lists recent studies on the wear resistance of magnetron sputtering coatings under different substrate biases. According to Table 1, the frictional properties are highly sensitive to substrate bias voltage. As the substrate bias voltage increases, the friction coefficient first decreases and then increases. This is because as the substrate bias voltage increases, the migration rate of adsorbed atoms increases, which inhibits the growth of columnar crystals and transforms the coating structure into a fine crystalline structure, increasing density and reducing surface roughness; When the negative bias voltage is too high, excessive ion bombardment can cause defects such as inclusions and cracks in the coating, weakening its wear resistance.

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.