Titanium alloy has been widely used in aerospace, shipbuilding, machinery, chemical industry and other fields due to its excellent characteristics such as light weight, high specific strength, and good corrosion resistance. However, its low surface hardness, poor wear resistance, and unsatisfactory corrosion resistance make it difficult for titanium alloys to meet the requirements of practical applications in many cases, which seriously hinders the further application of titanium alloys. At present, the surface treatment technologies to improve the wear resistance of titanium alloys mainly include ion implantation, chemical plating, laser cladding, plasma spraying, vapor deposition and micro-arc oxidation. Each single surface technology has certain limitations. In recent years, the composite treatment technology has been used to modify the surface of the titanium alloy, which has gradually improved its performance and solved the problem of titanium alloy surface strengthening. Therefore, this article describes the current single and composite strengthening treatment methods of several titanium alloy surfaces.
TC4 titanium rod

Titanium alloy wear-resistant surface modification and coating technology

  •  Ion implantation
    Ion implantation technology began in the 1960s. This technology quickly injected high-energy charged ions into the metal near the surface layer under vacuum and low temperature, which caused a series of complex reactions between ions and the matrix to form a new surface-modified alloy layer. The newly formed alloy layer has a strong binding force with the substrate, and the wear resistance is significantly improved. The outstanding advantage of this process is that it can not only maintain the performance of the metal matrix itself, change the macro size of the material, environmentally friendly and pollution-free, but also greatly improve the corrosion resistance and oxidation resistance of the material surface. The ion source can be either non-metal ions such as B, C, N, etc., or metal ions such as Zr, Mo, Re. In terms of non-metal ion implantation, when B, C, O, etc. are implanted into the surface of the titanium alloy, corresponding hard compounds (TiB, TiC, TiO) will be formed, so that the surface hardness and wear resistance of the material can be improved. Luo Yong et al. Implanted N3- into the surface of Ti6Al4V substrate to improve the mechanical properties of the material. The resulting TiN film significantly increased the microhardness of the titanium alloy surface. The average hardness was increased by about 25%, and the wear resistance was 2.5 times that of the titanium alloy substrate.
  •  electroless plating
    Electroless plating is also known as electroless plating or autocatalytic plating, that is, the use of metal self-catalysis under the premise of no external current, while using the reducing agent in the plating solution to reduce free metal ions to metal, and even A surface plating technique deposited on the surface of parts to be plated. At present, for the wear-resistant modification of titanium alloys, electroless plating has gradually developed from the initial single electroless Ni plating to a variety of metal and alloy and composite electroless plating surface treatment processes, such as electroless Cu, Ag, Au and Sn. Composite electroless plating is based on the addition of solid hard particles such as Al2O3, Cr2O3, SiC, etc. on the basis of the original plating solution, so that it co-deposits with the metal under external force, thereby obtaining better mechanical properties than the coating without adding particles.
    Zangeneh-Madar et al. Tried to make Ni-P-polytetrafluoroethylene (PTFE) composite coating on the surface of titanium alloy by electroless plating technology, and studied the influence of plating solution concentration, temperature and surfactant concentration on the formation of the coating. The friction and wear characteristics of the samples were also explored. The results show that the co-deposition of Ni-P and PTFE can significantly reduce the friction coefficient of the coating, reduce the amount of wear, and improve the lubrication performance.
    Compared to electroplating, electroless plating has the advantages of uniform density, no need for external current supply, simple operation process, and deposition of coatings on plastics and other non-conductors. Chemical plating has little pollution and low cost. At present, electroless plating is widely used in aerospace, automobile, machinery, chemical industry and other fields due to its ability to prepare films with good corrosion resistance and wear resistance.
  •  Laser cladding
    Laser cladding technology is a surface modification technology that combines laser technology with metal heat treatment technology. This technology sprays or bonds powder materials on the surface of the substrate in advance, or synchronously conveys the powder with the laser beam, and then irradiates the surface of the material with a high energy density laser beam to melt the powder material and form a good metallurgical bonding layer on the substrate metal. Since the laser cladding, the melting part of the base material is very small, which basically has no effect on the performance of the substrate. At present, there are not many cladding materials that can improve the wear resistance of titanium alloys. Commonly used are hard ceramics (SiC, TiC, Al2O3, TiN and TiB2, etc.), nickel-based self-fluxing alloys and ceramics / alloys. Among them, the single hard ceramic laser cladding layer has high brittleness and does not match the thermal expansion coefficient of the titanium alloy, resulting in high residual stress, which is easy to cause the cladding layer to crack or even fall off. Therefore, ceramics / alloys are commonly used to improve the wear resistance of titanium alloys, among which self-melting NiCrBSi alloys are mostly used.
    Weng et al. Laser cladding SiC4 with different contents on the surface of TC4 titanium alloy. During the entire process, SiC reacts with the substrate to form Si5Si3 and TiC. The formation of the reactant significantly improves the hardness and wear resistance of the titanium alloy. The experimental results show that the hardness of the titanium alloy after laser cladding SiC reaches 1200 HV, which is more than 3 times the hardness of the substrate, and the wear resistance of the coating is also improved by 18.4 ~ 57.4 times; and with the increase of SiC content (low At 20% (mass fraction)), the coating hardness is gradually increased to 1300 ~ 1600 HV, and the wear resistance is further improved.
  •  thermal spray
    The thermal spraying is to use a certain heat source to heat the spray material. The material to be sprayed is flowable and then accelerated by the flame stream, and then sprayed onto the surface of the pre-treated substrate to deposit a coating with a specific function. The commonly used spray materials for titanium alloy wear-resistant modification are generally non-metallic material nickel-coated graphite, elemental metal materials Al, Ni and alloy materials TiN, NiCrAl, MCrAlY, etc. After the thermal spraying process, the interface between the coating and the substrate is straight, and the combination is good. In the subsequent high-temperature oxidation process, the sprayed material and the substrate diffuse with each other to form a metallurgical diffusion layer, which greatly improves the wear resistance. Huang et al. Have introduced that thermal spraying aluminum coating on the surface of titanium alloy can deposit a protective layer on the surface of the substrate, but the protective layer is hard and brittle at low temperature. Due to the mismatch of thermal expansion coefficient, it is prone to peeling.
  •  Physical vapor deposition
    Physical vapor deposition technology is a technology that uses a physical method to vaporize the material source-solid or liquid surface into gaseous atoms, molecules or parts into ions under vacuum conditions, and transport it to the surface of the substrate to form a solid film. Physical vapor deposition techniques mainly include evaporation, sputtering, and ion plating, etc., which can prepare both metal films and compound films.
    Sputtering and ion plating are two common physical vapor deposition techniques, each with its own advantages. Ion plating has the advantages of good toughness, high ion energy, and high bonding strength. However, the prepared film is easy to contain defects such as droplets. The advantages of sputtering include: low operating temperature, controllable film composition, less material deformation, and a wide range of target materials for plating; etc .; but the film deposition rate is slow. Xi Yuntao et al. Prepared the TiN film on the surface of TC4 titanium alloy by magnetron sputtering and ion plating, and compared the friction and wear performance. The results show that the multi-arc ion plating and magnetron sputtering TiN film layer both improve the wear resistance of the TC4 titanium alloy surface, and the performance of the film layer obtained by the multi-arc ion plating method is better.
    In summary, although the wear resistance modification technology of a single titanium alloy surface can significantly improve the microhardness and wear resistance of the titanium alloy, some shortcomings are inevitable, such as the ion implantation technology, the thickness of the implanted layer is too shallow, only Within the level range, the use is limited, and the sample size is also limited. The bonding strength of the chemical plating layer and the substrate is not high, and the plating layer is thin, which is prone to hydrogen embrittlement. Laser cladding technology process parameter control is more cumbersome, and cracks and pores are easy to occur in the cladding layer. Thermal spraying technology is not suitable for processing substrates that are not resistant to high temperatures, and the sprayed coating has low binding force, large porosity, and poor uniformity. Some of the composite technologies described below can further improve the above defects.
    Titanium alloy flange.

2. Titanium alloy wear-resistant composite treatment technology

At present, driven by the growing industrial demand, composite coating technology will gradually replace the single coating technology. Micro-arc oxidation is also called micro-plasma oxidation. This technology can prepare ceramic layers with good metallurgical properties on the surfaces of light metals Al, Mg, Ti and corresponding alloys with the help of high voltage, high current and instantaneous high temperature. The main component of the ceramic layer is the oxide grown in situ in the matrix, and the electrolyte component will also participate in the micro-arc oxidation film layer. The electrical parameters (such as solution formulation, voltage and current, duty cycle, pulse frequency, etc.) in the micro-arc oxidation process have a great influence on the preparation and microstructure of the micro-arc oxidation film. The method is safe in process, easy to operate, and environmentally friendly in solution. It also has the advantages of simple technology, uniform and dense film layer, and less restrictions on the size of the workpiece.
The titanium alloy micro-arc oxide film has the advantages of high hardness, high bonding strength of the film base, corrosion resistance, wear resistance, etc., but its film surface is rough, loose and porous, and has a large friction coefficient, thereby reducing the wear resistance of the film , Shorten the service life of the oxide film, which is not conducive to the application of titanium alloy in the wear environment. At present, in order to improve the defect of the titanium alloy micro-arc oxide film, some institutes of the Russian Academy of Sciences have carried out a lot of work, and domestic attention to this problem has just begun. This article mainly introduces the related technologies of composite treatment based on micro-arc oxidation technology, combined with sealing method, aluminum plating method, pulsed electron beam method, hydrothermal method and electrophoretic deposition method.

  •  Micro-arc oxidation + sealing method
    Because the surface of the titanium alloy micro-arc oxide film is loose and porous, some researchers have tried to fill the pores of the micro-arc oxide film with physical, chemical or electrochemical methods to achieve the effect of self-lubrication. Among them, polytetrafluoroethylene (PTFE) has good thermal stability and excellent chemical stability in various environments, making it an ideal self-lubricating material.
    Zhao Hui, etc. filled PTFE particles into the pores of the titanium alloy micro-arc oxidation film layer for curing treatment, and prepared a PTFE composite self-lubricating film. After sealing, the scanning electron microscope (SEM) observation results showed that the pores of the composite film were significantly reduced, and the surface morphology was smoother; in the subsequent friction and wear experiment, the friction coefficient of the micro-arc oxide film was about 0.4, and after sealing The friction coefficient of the treated composite membrane was only 0.15. Du Nan et al. Added a small amount of Cr2O3 particles to the micro-arc oxidation electrolyte, and also improved the wear resistance of the titanium alloy micro-arc oxidation composite film through the hole sealing method. At present, most of the sealing agents used in the sealing method are insulating organic compounds, so there are many restrictions on the application of conductive materials. At the same time, when the hole sealing method is used to process parts with complex structure and large size, it takes a long time and is difficult to apply in place.
  •  Micro-arc oxidation + hydrothermal method
    The hydrothermal method is to contain a reaction medium in a closed container, and use a heat source to continuously heat the container to bring the inside of the container to a high temperature and high pressure state. Under the action of high temperature and high pressure, the insoluble or insoluble substances will dissolve and further heavy crystallization. Vangolu and others explored the wear resistance of TC4 titanium alloy composite film modified by micro-arc oxidation and hydrothermal technology. Compared with Ti6Al4V treated by micro-arc oxidation, the composite film is a TiO2 layer containing hydroxyapatite. When friction is carried out at a load of 1 N and a speed of 6.5 cm / s, the composite treatment reduces the friction coefficient of the film from 0.6 to 0.4, and the wear rate is also reduced from 0.25 to 0.18, which significantly improves the wear resistance of the film . However, the wear resistance of the composite membrane layer is weakened under high load, and it is not suitable for application under high load.
  •  Micro-arc oxidation + pulsed electron beam
    The pulsed electron beam (HCPEB) surface treatment technology uses high-speed electrons as a carrier to apply incident energy to the surface of the material in a very short time, and causes a series of phenomena, including melting, condensation, vaporization, enhancement, diffusion, etc. Physical, chemical and mechanical properties difficult to achieve with other heat treatments. Using the HCPEB method to prepare a composite modified layer on a titanium alloy micro-arc oxide film belongs to the category of coating remelting. That is, a micro-arc oxidation coating is prepared in advance, and then HCPEB is applied to the oxide film to perform electron beam remelting, thereby improving The uniformity of the film layer, the degree of grain refinement, the bonding strength of the film base, wear resistance, corrosion resistance and other characteristics.
    Du Chunyan treated the titanium alloy micro-arc oxide film with HCPEB method. SEM observation showed that the treated micro-arc oxide film had pores and particles obviously disappeared. The maximum hardness reached 1695 HV, and the signs of abrasive wear were reduced. However, there are certain cracks on the surface and cross section of the composite membrane, which in turn leads to a decrease in the bonding strength of the membrane layer. At the same time, the HCPEB method requires high roughness of the micro-arc oxide film. At present, there are not many examples of combining HCPEB with MAO, and there are few related studies.
  •  micro-arc oxidation + aluminum plating
    Aluminum alloys have many similar properties compared to titanium alloys. For example, they all belong to valve metals. They have similar applications, low density, and high specific strength. They are also widely used in the field of valve metals. However, the oxide properties of titanium alloy and aluminum alloy after micro-arc oxidation treatment are very different. The main composition of the oxide film formed by titanium alloy micro-arc oxidation is TiO2 (rutile and anatase); the main product of aluminum alloy micro-arc oxidation is Al2O3 (α-Al2O3 and γ-Al2O3). The hardness value of Al2O3 is between 1200 and 1800 HV, which is significantly higher than that of TiO2 (550 to 1050 HV). At the same time, TiO2 has insufficient toughness, and γ-Al2O3 has high structure and toughness. Therefore, in terms of hardness and wear resistance, the titanium alloy micro-arc oxide film is not as good as the aluminum alloy micro-arc oxide film.
    If the aluminum coating technology and the micro-arc oxidation technology can be combined in an appropriate method to prepare a composite coating, and the advantages of the two technologies can be fully utilized, the wear resistance and corrosion resistance of the titanium alloy after micro-arc oxidation can be significantly improved . At the same time, broaden the further application of titanium alloys in the aerospace field.
    Because of its excellent performance, various preparation methods, considerable economic benefits, and abundant resource reserves of raw material Al, it has always been a hot spot in coating modification technology research. At present, there are various methods of aluminum plating on the surface of the titanium alloy micro-arc oxide film. The common aluminum plating technologies combined with micro-arc oxidation are: hot dip plating, multi-arc ion plating and magnetron sputtering aluminum plating.
  • micro-arc oxidation + magnetron sputtering aluminum plating.
    Ouyang Xiaoqin et al. First sputtered aluminum on the surface of TC4 titanium alloy, the sputtering time was 2.5 h, and then the coating was subjected to micro-arc oxidation for 30 min, current density was 5 A / dm2, TC4 substrate micro-arc oxide film and sputtering The mechanical properties of the aluminum-coated micro-arc oxide film show that the hardness of the TC4 titanium alloy is generally about 360 HV, the hardness of the titanium alloy after micro-arc oxidation reaches 1.69 times that of the substrate, and the hardness of the composite coating after micro-arc oxidation treatment Reaching more than 1700 HV, the friction coefficient has also dropped from 0.38 to 0.25. In addition, adhesion analysis shows that the adhesion of the MSD / MAO composite coating is better than that of a single titanium alloy micro-arc oxide layer.
  •  micro-arc oxidation + hot dip aluminum plating.
    The surface of the titanium alloy is hot-dip aluminized. In molten aluminum, a series of reactions occur on the surface of the titanium alloy substrate, including the diffusion and interaction of liquid Al to the titanium alloy substrate. After high temperature thermal diffusion treatment, a titanium alloy layer with high hardness and high temperature resistance can be obtained on the metal surface. If combined with micro-arc oxidation technology.

3. Summary and Outlook

  •  Among the surface treatment technologies to improve the wear resistance of titanium alloys, micro-arc oxidation technology has obvious technology because of its advantages of low preparation temperature, simple equipment, environmental protection of the solution, uniform and dense film, and less restrictions on the size and shape of the workpiece. Advantage.
  •  The combination of micro-arc oxidation and other technologies can improve the performance of the film prepared by a single micro-arc oxidation technology, regardless of wear resistance and corrosion resistance. Therefore, composite technology is the future development direction of titanium alloy wear-resistant technology.