Wear-resistant surface modification and coating technology of titanium materials such as TC4 titanium alloy and TC11 titanium alloy

               
Update: 15-06-2021
               
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Titanium alloy has been widely used in aerospace, shipb […]

Titanium alloy has been widely used in aerospace, shipbuilding, machinery, chemical and other fields because of its 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, electroless plating, laser cladding, plasma spraying, vapor deposition and micro-arc oxidation. Each single surface technology has its limitations. In recent years, the use of composite treatment technology to modify the surface of titanium alloys has gradually improved its performance and solved the problem of surface strengthening of titanium alloys. Therefore, this article focuses on the current single and composite strengthening treatment methods of several titanium alloys.
  TC4 titanium rod
   1. Wear-resistant surface modification and coating technology of titanium alloy
  1.1.ion implantation
Ion implantation technology started in the 1960s. This technology rapidly injects high-energy charged ions into the near surface of the metal under vacuum and low temperature, causing a series of complex reactions between the ions and the substrate to form a new surface modified alloy layer , The newly formed alloy layer has a strong bonding force with the matrix, 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 macroscopic size of the material, is environmentally friendly and has no pollution, 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., but also metal ions such as Zr, Mo, and Re. As far as non-metal ion implantation is concerned, when B, C, O, etc. are injected into the surface of the titanium alloy, the corresponding hard compounds (TiB, TiC, TiO) will be formed, which improves the hardness and wear resistance of the material surface. 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, and its average hardness increased by about 25%, and the wear resistance was 2.5 times that of the titanium alloy substrate.
  1.2. electroless plating
Electroless plating is also called electroless plating or autocatalytic plating, that is, under the premise of no applied current, the autocatalysis of metal is used, and the reducing agent in the plating solution is used to reduce free metal ions to metal and uniformly A surface coating technology that is deposited on the surface of the part 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 original plating solution by adding solid hard particles such as Al2O3, Cr2O3, SiC, etc., to make it co-deposit with the metal under external force, so as to obtain better mechanical properties than the coating without particles.
Zangeneh-Madar et al. tried to use electroless plating technology to make Ni-P-polytetrafluoroethylene (PTFE) composite coating on the surface of titanium alloy, and studied the influence of bath 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 lubricating performance.
   Compared with electroplating, electroless plating has the advantages of uniformity and compactness, no need for external current supply, simple operation process, deposition of plating on plastics and other non-conductors, and low pollution and low cost. At present, electroless plating is widely used in aerospace, automotive, machinery, chemical and other fields because it can prepare films with good corrosion resistance and wear resistance.
  1.3. laser cladding
  Laser cladding technology is a surface modification technology that combines laser technology and metal heat treatment technology. In this technology, powder materials are sprayed or bonded on the surface of the substrate in advance, or the powder is synchronized with the laser beam, and then the surface of the material is irradiated with a high-energy density laser beam to melt the powder material and form a good metallurgical bonding layer on the substrate metal. Since there is very little melting part of the base material during laser cladding, there is basically no effect on the performance of the base material. 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 is brittle and does not match the thermal expansion coefficient of the titanium alloy, resulting in high residual stress, which is easy to cause cracks or even fall off in the cladding layer. Therefore, ceramics/alloys are commonly used to improve the wear resistance of titanium alloys, and the alloys are mostly self-fluxing NiCrBSi alloys.
  Weng et al. laser cladding SiC with different contents on the surface of TC4 titanium alloy. During the whole process, SiC reacts with the substrate to form Si5Si3 and TiC. The formation of this reactant significantly improves the hardness and wear resistance of the substrate titanium alloy. The experimental results show that the hardness of the coating after titanium alloy 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 increased by 18.4 to 57.4 times; and with the increase of SiC content (low At 20% (mass fraction)), the hardness of the coating gradually increases to 1300~1600 HV, and the wear resistance is further improved.
  1.4. thermal spraying
   Thermal spraying is a processing method that uses a certain heat source to heat the spray material. After the sprayed material is in a flowable state, it is accelerated by the flame, and then sprayed on the surface of the pre-treated substrate to deposit a coating with specific functions. The commonly used spray materials for wear-resistant modification of titanium alloys are generally non-metallic materials nickel-coated graphite, elementary 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 the combination is good. In the subsequent high temperature oxidation process, the sprayed material and the substrate diffuse each other to form a metallurgical bonding diffusion layer, which greatly improves the wear resistance. Huang et al. once 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 temperatures, and it is prone to peeling due to the mismatch of the thermal expansion coefficient.
  1.5.physical vapor deposition
  Physical vapor deposition technology is a technology that uses physical methods to vaporize the material source-solid or liquid surface into gaseous atoms, molecules or part of the ionization into ions under vacuum conditions, and transport them to the surface of the substrate to form a solid-phase film. Physical vapor deposition technology mainly includes evaporation, sputtering and ion plating, etc., which can prepare metal films as well as 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 prone to defects such as droplets. The advantages of sputtering include: low operating temperature, controllable film composition, small material deformation, wide selection of target materials that can be plated, etc.; but the film deposition rate is slow. Xi Yuntao et al. used magnetron sputtering and ion plating to prepare TiN film on the surface of TC4 titanium alloy to compare its friction and wear properties. The results show that both the multi-arc ion plating and magnetron sputtering TiN film improve the wear resistance of the TC4 titanium alloy surface, and the film performance obtained by the multi-arc ion plating method is better.
In summary, although a single titanium alloy surface wear-resistant modification technology can significantly improve the microhardness and wear resistance of titanium alloys, some shortcomings are inevitable. For example, the thickness of the implanted layer of ion implantation technology is too shallow, only in microns. Within the level range, 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, and the plating layer is thin, which is prone to hydrogen embrittlement. The process parameter control of laser cladding technology is cumbersome, and cracks and pores are prone 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 bonding force, large porosity, and poor uniformity. Some of the composite technologies introduced below can further improve the above-mentioned defects.

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