The surface of titanium rods and titanium alloy rods has poor abrasion resistance. Especially in sliding conditions, titanium surfaces in contact with each other or with other metals can be quickly welded together, and even in the case of light loads and small relative movements, the entire surface of the weld can also occur. If the welded surfaces are separated by force, the surfaces can be seriously damaged. Uncoated titanium is therefore unsuitable for use in threads, gears and gear rods, where it is not possible to design them to avoid moving contacts, and these titanium surfaces must therefore be protected.
There are a number of protective coatings applicable to titanium which can be generally categorized as follows:
(1) Thickening oxide film The thickness of the naturally occurring oxide film can be artificially increased to provide a protective layer which prevents galling under light loads, low weeping speed contact, and has been used to protect a particular type of connector against thread galling. Thick oxide films produce a porous structure in which general solid lubricants such as molybdenum disulfide can be stored, thus further improving support properties. Thickened oxide films can also be used for pressure loads that are essentially perpendicular to the surface, such as retaining caps on engine valve stems, contact surfaces of titanium valves, and automotive springs for racing cars. The advantage of the thickened oxide film treatment is that it can be carried out at room temperature, thus giving good dimensional accuracy.
(2) replacement coating some substances can be diffused into the titanium surface through higher temperature heat treatment, so that it produces a high surface hardness. Some of these treatments have been industrialized. Probably the most effective of these is the Tiduraii treatment. In this method, the material is immersed in a cyanide molten salt bath at a controlled temperature of about SOOt, but this high temperature may cause some problems in distorting the part. Most replacement coatings cause a significant reduction in the fatigue properties of titanium, but subsequent shot peening can restore fatigue properties to more than 90% of those prior to treatment.
There are also some in the test or production process used in titanium replacement coating forms available, they are nitriding, boriding, carbonizing, cyanide treatment, as well as aluminum, copper, chromium, tin diffusion treatment.
(3) flame, plasma and explosion gun spray coating with the traditional flame spraying method of titanium flame spraying so far can not be very satisfactory a although there has been sprayed with molybdenum to the titanium alloy linkage with the racing car on the rod, but the adhesion is not good. People on the use of explosive gun and plasma spraying method in the titanium surface coating has done a lot of work. This technique has been widely used in the aerospace industry in the United Kingdom and the United States, and the adhesion between the substrate and the coating is satisfactory. The technique can be used on a wide range of materials, from pure metals to ceramics.8 Generally, the tensile properties and the fatigue strength are only slightly reduced by the properties of the coated surface, i.e., the properties of the sprayed material. Due to the variety of coating processes, different suona materials can be selected for specific applications.
(4) electroplating and chemical plating although the metal to be plated on titanium is difficult, but to provide some adhesive coating technology is possible. This includes silver triggered or treated with a variety of organic compounds, followed by ordinary plating methods, a chromium plating technology with a solid chromium coating is being successfully applied to high-strength titanium alloys, according to the more guided, the fatigue performance of the treatment is not significantly reduced? The chemical nickel plating method, known as the Kanigeri method, can be used instead of electroplating. The method is carried out at relatively low temperatures, so it can significantly improve wear resistance, but the tensile strength, plasticity and fatigue strength decreased. As long as the mechanical properties are not important, this method provides titanium with a coating that has good wear resistance in light sliding contact.
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