The natural surface of titanium sets limits to its use in many engineering applications which require sliding or rubbing wear resistance. Similarly the performance of the metal in both hot and cold working and fabrication processes may be judged unacceptable if proper attention is not paid to surface pre-treatment and to die and tool lubrication.
The Co-Efficient of Friction of Titanium and Titanium Alloys
The Effect of Atomic Structure
The coefficient of friction of commercially pure titanium is given in the range .30 -.34, with lower figures in the range .25 - .30 for titanium alloys. Practical tests can indicate much higher figures, into the range .8 - .9 for rubbing contact of untreated surfaces without lubrication. Three fundamental factors combine to give titanium its high coefficient of friction and cause the poor tribological behaviour of the metal. The first arises from titanium’s atomic structure, and this can be marginally improved by bulk or surface alloying to form a harder and more wear resistant structure.
The Effect of Crystal Structure
The second factor comes from the crystal structure of titanium. Modification by alloying occurs naturally in the range of alloys, which to various extents offer slightly better resistance to wear as compared to pure titanium.
Properties of the Protective Oxide Film
The third problem is the relatively low tensile and shear strength of the titanium oxide film. When adhesive bonding occurs in rubbing contact with adjacent titanium or other metal surfaces, the fracture occurs in the oxide rather than at the interface, resulting in large amounts of material transfer, galling, and high wear rates. Improvement of strength and hardness by bulk alloying offers some improvement, but this problem and indeed the other two factors above can also be overcome by removing the titanium entirely from the tribo-system by coating with another material, metallic or otherwise.
Lubrication, or the combination of surface treatment or modification with effective lubrication makes it possible to work with titanium, within the limitations imposed by its surface characteristics. Surface treatment is the key to the successful fabrication of titanium and to the engineering use of the metal for its strength to weight ratio, corrosion resistance or other desired physical or mechanical characteristics.
Specifying Lubricants for Use with Titanium and Titanium Alloys
The coefficient of friction for titanium in rubbing contact with itself is up to twice the value for steel on steel, and four times the value for bronze and other ‘bearing’ metals. Separation of titanium surfaces by effective lubrication, using oil, grease or dry film lubricant is a first but minimal step to wear reduction. Standard hydrocarbon lubricants typically reduce the friction coefficient to 0.4 - 0.5 between moving titanium surfaces. Oils and greases will be specified by their suppliers as suitable for use with titanium, for specific processes or operating conditions and for the durability of their effectiveness. There are no standards for specification of lubricants for use with titanium. Lubricant suppliers should be consulted and their recommendation sought for the specific application.
The Importance of Lubrication
The tendency for titanium to gall in rubbing or sliding contact with tooling or other metal surfaces makes effective lubrication desirable for forming and working. High pressure oils, graphite or molybdenum disulphide based greases, plastic films and other such barrier lubricants may provide adequate separation of the tool and workpiece. Machining and fabrication lubricants, used for example in thread roll forming, or pressing or drawing are replenishable at the working surface and are normally removed after the working operation is completed.
Spray on Film Lubricants
A number of proprietary dip or spray on dry film lubricants based on molybdenum disulphide or PTFE are available which provide lubricity for severe forming operations such as heading of fasteners, cold forming of springs, thread rolling and deep drawing. Some of these temporary coatings are very stable, and may be left on titanium until all processing is completed. Those based on molybdenum disulphide which reduce the coefficient to 0.1 - 0.2 which are applied at the start of a manufacturing cycle may be retained for the service life of components such as fasteners.
Thermally Stable Lubricants
Thermally unstable lubricants such as oil, grease and plastics must be fully removed before any intermediate or final heat treatment is carried out.
Dry Film Lubricants and Surface Treatments
The combination of dry film lubricants with anodized and other surface treatments frequently provides the best longer term solution to adhesive wear of titanium.