Titanium alloys have been widely used in aerospace, marine, machinery, chemical and other fields due to their excellent characteristics such as light weight, high specific strength, and good corrosion resistance. However, its low surface hardness, poor abrasion resistance, and unsatisfactory corrosion resistance make titanium alloys difficult to meet the requirements of practical applications in many cases, which seriously hinders further application of titanium alloys. At present, the surface treatment technologies for improving 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 its limitations. In recent years, the surface treatment of titanium alloys has been improved by adopting composite treatment technology, which has gradually improved its performance and solved the problem of surface strengthening of titanium alloys. Therefore, this article describes several current single and composite strengthening methods of titanium alloy surfaces.

1. Wear-resistant surface modification and coating technology of titanium alloy

1.1 Ion implantation

The ion implantation technology started in the 1960s. This technology quickly injected high-energy charged ions into the near surface of the metal under vacuum and low temperature, which caused a series of complex reactions between the ions and the matrix to form a new surface modified alloy layer. , The newly formed alloy layer has a strong bonding force with the substrate, and the wear resistance effect is significantly improved. The outstanding advantages of this process are that it can maintain the properties of the metal matrix itself, does not change the macro size of the material, is environmentally friendly and harmless, and can greatly improve the corrosion resistance and oxidation resistance of the material surface. The ion source can be 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 on the surface of titanium alloy, corresponding hard compounds (TiB, TiC, TiO) will be formed, so that the hardness and wear resistance of the surface layer of the material can be improved. Luo Yong et al. Injected N3- into the surface of the Ti6Al4V substrate to improve the mechanical properties of the material. The resulting TiN film significantly improved the microhardness of the titanium alloy surface, with an average hardness increase of about 25%, and abrasion resistance 2.5 times that of the titanium alloy substrate.

1.2 Chemical plating

Electroless plating is also referred to as electroless plating or autocatalytic plating, that is, the use of metal autocatalysis under the premise of no external current, and at the same time, the free metal ions are reduced to metal by the reducing agent in the plating solution and uniform. A surface plating technique deposited on the surface of a part to be plated. At present, for the wear resistance 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, so as to obtain better mechanical properties than the coating without particles.

Zangeneh-Madar et al. Tried to make a Ni-P-polytetrafluoroethylene (PTFE) composite coating on the surface of titanium alloy by electroless plating technology, and studied the effects 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 external current supply, simple operation process, depositing of plating on non-conductors such as plastic, etc., and less pollution and low cost of electroless plating. At present, electroless plating is widely used in aerospace, automotive, machinery, chemical and other fields because it can prepare a film layer with good corrosion resistance and wear resistance.

1.3 Laser cladding

Holmium laser cladding technology is a surface modification technology that combines laser technology with metal heat treatment technology. This technology sprays or adheres powder material on the surface of the substrate in advance, or transports the powder synchronously 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. During laser cladding, there is very little melting part of the substrate, which has almost no effect on the performance of the substrate. At present, few cladding materials have been used to 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 is brittle and does not match the thermal expansion coefficient of the titanium alloy, resulting in high residual stress, which easily causes the cladding layer to crack or even fall off. Therefore, ceramics / alloys are commonly used to improve the wear resistance of titanium alloys. Among them, self-melting NiCrBSi alloys are mostly used for alloys.

EngWeng et al. Laser cladding SiC with different contents on the surface of TC4 titanium alloy. During the whole process, SiC reacts with the matrix to form Si5Si3 and TiC. The formation of this reactant significantly improves the hardness and wear resistance of the matrix titanium alloy. The experimental results show that the hardness of the coating after laser cladding of SiC on titanium alloy reaches 1200 HV, which is more than three times the hardness of the substrate, and the wear resistance of the coating is also increased by 18.4-57.4 times; At 20% (mass fraction)), the coating hardness is gradually increased to 1300 ~ 1600 HV, and the wear resistance is also further improved.

1.4 Thermal spray

Thermal spraying is a processing method that uses a certain heat source to heat the spray material. After the sprayed material shows a flowable state, it is accelerated by the flame, and then sprayed on the surface of the pretreated substrate to deposit a coating with a specific function. The commonly used spray materials for wear-resistant modification of titanium alloys are generally non-metallic materials such as nickel-coated graphite, elemental metal materials Al, Ni and alloy materials TiN, NiCrAl, MCrAlY, etc. After the thermal spraying treatment, the interface between the coating and the substrate is straight and well bonded. In the subsequent high-temperature oxidation process, the sprayed material and the substrate mutually diffuse to form a metallurgical bonded diffusion layer, which greatly improves the wear resistance. Huang et al. Once introduced that thermal spraying an aluminum coating on the surface of a titanium alloy can deposit a protective layer on the surface of the substrate, but the protective layer is hard and brittle at low temperatures, and it is prone to peeling due to the mismatch of thermal expansion coefficients.

1.5 Physical Vapor Deposition

Physical vapor deposition technology is a technology that uses a physical method to vaporize a material source—solid or liquid surface—to gaseous atoms, molecules, or a portion of it into ions under vacuum conditions, and transports it to the surface of a substrate to form a solid-phase thin film. Physical vapor deposition technologies mainly include evaporation, sputtering, and ion plating, which can be used to prepare both metal and compound films.

Rhenium sputtering and ion plating are two common physical vapor deposition technologies, each with its own advantages. Ion plating has the advantages of good toughness, high ion energy, and large bonding strength. However, the prepared film is liable to contain defects such as molten droplets. The advantages of sputtering include: low operating temperature, controllable film composition, less material deformation, and a wide range of target materials that can be plated. However, the film deposition rate is slower. Yuntao Tao and others used magnetron sputtering and ion plating to prepare TiN film on the surface of TC4 titanium alloy, and compared its friction and wear performance. The results show that both the multi-arc ion plating and the magnetron sputtering TiN film layer have improved the wear resistance of the TC4 titanium alloy surface, and the multi-arc ion plating method has better film performance.

In summary, although the single titanium alloy surface wear-resistant modification technology can significantly improve the microhardness and wear resistance of titanium alloys, some disadvantages are unavoidable. For example, the thickness of the implanted layer of ion implantation technology is too shallow, only in the micron. Within the range of levels, the use is limited, and the sample size is also limited. The bonding strength between the electroless plating layer and the substrate is not high, the plating layer is thin, and hydrogen embrittlement is easy to occur. Laser cladding technology has complicated control of process parameters, and cracks and pores are easily generated 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 bonding force, large porosity, and poor uniformity. Some composite technologies introduced below can further improve the above defects.

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