Coating titanium

Materials World magazine
,
1 May 2009
A cylinder coated with Hardide

A process for imparting wear, abrasion and chemical resistance to titanium components has been developed, say researchers at Hardide Coatings, based in Bicester, UK.

The Hardide-T coating, made from nanostructured tungsten carbide, is applied using low temperature chemical vapour deposition (CVD). This could enable titanium to be used in new engineering applications, such as Formula 1 engine parts, which are exposed to extreme friction.

‘Titanium demonstrates an exceptional combination of mechanical strength, chemical resistance and light weight, but suffers from intensive galling,’ explains Dr Yuri Zhuk, Technical Director at Hardide. ‘Titanium metal is quite active and forms a protective oxide layer almost immediately. When two titanium parts are moving against each other under load, this oxide layer is damaged, which results in severe adhesive wear, high and unstable friction, and surface damage.’

Traditional techniques used to deliver improved properties to metals, such as surface oxidisation, spray coating and plating, do not work on titanium, as its oxide layer also forms a passive barrier against chemical or metallurgical bonds.

The researchers have adapted Hardide’s coating to achieve a stronger adhesion bond. ‘This bond is not dependent on surface roughness, [so] the coating can be used on parts which require a good finish,’ notes Zhuk (he could not reveal details of the coating’s development).

Researchers have also created a CVD process for films of between 50-100µm to be applied. ‘This is exceptional for CVD coatings, which are usually restricted to a few microns,’ adds Zhuk.

The process is carried out in a vacuum furnace heated to 500ºC. ‘We have a controlled mixture of reactive gases passing through the reactor. As a result of a series of chemical transformations, the coating is crystallised atom-by-atom on all surfaces in contact with the gas media.’ This allows internal surfaces and complex shapes to be evenly coated.

The resulting hardness properties of the coating are 1,100-1,600Hv, and it can withstand deformations and chemicals such as mineral acids. Zhuk says it is ideal for oil and gas industry drilling tools, as well as automotive parts that continually move against each other, such as bearings, steering mechanisms, pins and bushes.

Dr Andrew Bloyce, Technical Manager at coating technology firm OC Oerlikon Balzers, based in Milton Keynes, UK, says this combination of properties, including the ability to be applied in 100µm thick layers, ‘makes the process very interesting and different from others used on titanium alloys’.

Hardide has initially focused on the Ti6-Al-4V alloy often used for aerospace applications, but Zhuk says it could adopt the process for other grades of titanium. The company aims to begin mass production of the coating within the next 12 months.

Further information: Hardid Coatings