How Aerospace Surface Treatments Protect Titanium Parts
It is worth noting that although Titanium is a corrosion resistant metal, it does not mean it does not need any surface treatment.
Summary
Titanium forms its own oxide layer, but that layer does little to stop galling or fretting fatigue, so aerospace parts are often given a surface treatment as well. Anodising adds colour for identification and a little fretting resistance, HVOF tungsten carbide coatings handle wear zones, and PVD or ion vapour deposition adds thin, hard films such as titanium nitride. Because coatings add thickness, threads, bearing faces and other mating surfaces are often masked to hold tight tolerances.
Why Titanium Still Needs Treatment
Titanium naturally forms a stable oxide layer when it is exposed to air. However, the oxide layer formed naturally by titanium offers little protection against galling, fretting fatigue, etc. Surface treatment to the titanium part addresses the above specific weaknesses without compromising the base material.
Anodising for Identification and Mild Protection
Titanium can be anodized in various colours by altering the voltage to produce a thin layer of oxide to approximately 10 microns. Titanium parts can be identified between different alloy grades or even be used to identify individual parts within a batch of similar parts. Although there is only mild corrosion resistance, there is some improvement in fretting resistance.
Thermal Spray for Wear Zones
High-velocity oxy-fuel (HVOF) spray coatings made of tungsten carbide are often used for wear situations on titanium parts. The resulting surface provides a hard and dense wear surface which cannot be achieved by the substrate material alone. Common applications are bearing housings and sliding contact surfaces.
PVD and Ion Vapour Deposition
PVD coating types such as Physical vapour deposition and ion vapour deposition produce extremely thin hard films which can be used to increase surface hardness of the part and reduce surface friction. Titanium nitride coating is typical of this type of coating and is used for components such as aerospace components that require close tolerance machining.
Masking and Dimensional Control
The precision machining of titanium components means that parts such as those with threads, bearing faces or other mating surfaces can need to be masked prior to surface treatment. Even a thin layer of coating can increase the dimension of a part and such parts are often made to very close tolerances on the titanium itself.
Treating parts as early as possible in their production can save costly rework of expensive parts.
FAQs
What is galling?
Galling is a type of wear where two metal surfaces sliding against each other under load stick together and tear material from one another. Titanium is especially prone to it, which is why sliding and threaded titanium parts are often coated or lubricated.
What is fretting fatigue?
Fretting is damage caused by very small, repeated movements between two parts that are clamped or pressed together. Over time it can start tiny cracks at the contact surface, and under repeated loading those cracks can grow and shorten the life of the part.
Where do the colours in anodised titanium come from?
The colours are not dyes. They come from the way light reflects through the thin oxide layer, so a different oxide thickness gives a different colour. That is why altering the voltage changes the result, and why the colour can wear away where the surface is rubbed.
Why does titanium nitride look gold?
Titanium nitride has a natural gold colour, so parts coated with it are easy to spot. The colour is a useful visual check that the coating is present, alongside the extra hardness and lower friction it provides. Our article on six finishing treatments for metal parts covers some of the other options.