Although titanium alloys have high strength and excellent corrosion resistance, their poor wear resistance is the main drawback for their application in engineering. Currently, the mature surface treatment technologies in the industrial field are mainly the following types, each with its own advantages and disadvantages.
Wet coating method
The coating mainly consists of Cr and Ni-P alloys. On titanium rods and wires, due to the dense titanium surface passivation film, direct chromium plating has poor adhesion. In industry, it is common to first chemically deposit Ni or pre-deposit a Ni layer on the titanium substrate, and then electroplate chromium. The electrolytic method forms the film quickly, with a coating thickness of several micrometers, and the decorative coating is only about 1 μm, which can meet general wear resistance and decorative requirements. It is one of the most cost-effective surface treatment methods at present. However, here are shortcomings. The coating is mechanically bonded to the substrate. It is prone to peeling under impact and heavy load conditions. Moreover, the electroplating process involves chromium-containing wastewater treatment, which brings significant environmental pressure.
Thermal diffusion method
Drawing on the mature carburizing, nitriding and boronizing processes of steel materials, the interstitial atoms are diffused into the interior of the titanium matrix at high temperatures to form a hardened layer. This method can obtain diffusion layers ranging from several tens to several hundred micrometers in thickness, with a gentle hardness gradient and strong bonding with the base material, making it suitable for applications subjected to contact stress. However, the processing temperature is usually above 800℃, and the titanium alloy structure is prone to coarsening. Subsequent heat treatment is required for regulation, and the process window is narrow. Moreover, at high temperatures, titanium absorbs oxygen and hydrogen, and strict protection atmosphere control is necessary to address these issues.
Welding method
The hard alloy or intermetallic compound coating is welded onto the titanium surface using the plasma transfer arc (PTA). The dilution rate is controllable, the bonding strength is high, and the wear resistance is excellent. The main advantage is that it allows for local treatment without the entire base material being heated, thus avoiding the deterioration of mechanical properties in large-sized workpieces. The disadvantage is that the surface of the weld layer is rough, and secondary machining is required to achieve dimensional accuracy. It is also limited by the reachability of the PTA welding gun, and is mainly used for large flat plates, shafts, and other regular workpieces. It is difficult to handle complex-shaped parts.
Vapour deposition method (CVD/PVD/PCVD)
Physical vapor deposition (PVD), chemical vapor deposition (CVD), and plasma-enhanced chemical vapor deposition (PCVD) are a group of technologies that have developed rapidly in recent years. They can be used to prepare hard coatings such as TiN, TiAlN, and DLC on the surface of titanium, with a thickness typically ranging from 2 to 10 μm. The hardness can reach 2000 to 4000 HV, and the friction coefficient is significantly reduced. They are commonly applied to titanium target materials. Compared with wet coating, the adhesion of gas-phase deposition coatings is better (especially with the assistance of bias voltage), and the process temperature gradually decreases - PVD can be controlled below 400°C, having little impact on the microstructure of the titanium alloy substrate, and suitable for the final processing of precision parts.
When comparing the three types of coatings, CVD coatings are dense and have good over-coating performance, but they require high temperatures (>800℃) and are prone to generating harmful by-products; PVD coatings have low stress and moderate temperature, but their over-coating performance is poor, and it is difficult to uniformly cover the complex inner hole surfaces; PCVD combines plasma activation and chemical vapor deposition, and can deposit coatings with better bonding strength than PVD at 500-600℃, and has better over-coating performance than PVD. It is currently the leading direction for surface strengthening of titanium alloys, and the equipment investment and process control difficulty are also the highest among the three.
Technical selection suggestions:
For parts with cost priority and loose dimensional accuracy, wet chromium plating or Ni-P chemical plating can be given priority.
For heavy-load and high contact stress conditions, the thermal diffusion method or hardfacing method is recommended to obtain a thick diffusion layer.
For precision parts with strict dimensional tolerances and low service temperature, PVD/TiN coating is a mature solution.
For high-end applications that consider adhesion, adhesion properties, and microstructure control (such as components for aircraft engines), PCVD technology has the greatest potential.
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