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Titanium has a tendency to gall when in rubbing contact with itself and other surfaces. A number of techniques have been developed to engineer the titanium surface and overcome this problem:
• Lubrication
• Anodising
• Electroplating
• Ion Implantation
• Nitriding
• Shot Peening
Plasma Nitriding
Plasma nitriding is the simplest and most widely available process. Able to generate coatings uniformly on the exposed surfaces of components of any configuration. Plasma nitriding poses two problems for titanium, the process temperature of 700°C or above delivers most alloys in the fully annealed or averaged condition, secondary problems are surface roughening and a reduction of fatigue limit, particularly with thick layers of nitride. At this temperature the titanium oxide film is removed, and nitrogen is used to produce a harder substrate by diffusion before deposition of the titanium nitride is initiated.
Lower Temperature Nitriding Processes
Lower temperature processes (450°C) are available which deposit very thin films (3µm), and cause no surface roughening. A problem encountered with these thin layers is the deformation of the underlying titanium and consequent failure of the coating as surface loadings increase. Stiffening and strengthening of the substrate can be achieved by laser surface melting with nitrogen, (sometime referred to as laser nitriding), or electron beam melting of preplaced alloying additions. A ten fold increase in the depth of the hardest layer can reduce overall wear rates by two orders of magnitude, placing titanium performance close to that of hardened tempered steel (En19). Techniques and equipment exist to nitride both relatively small and larger components. Nitrided titanium alloy hydraulic shafts up to 4 metres long (manufactured and coated by Permascand AB) were used for the Snorre Tension Leg Platform subsea tensioning system. The layer thickness was 5-12µm with an average hardness of 900HV.Fluidised bed nitriding (Nobleizing), equipment, currently able to handle larger parts up to 750mm diameter produces thicker films without risk of spalling or cracking. Hardness levels above 650HV are reported for titanium.
Applications of Nitrided Titanium Components
The motor racing industry has been a major user of nitrided titanium components, with successful applications on valves, and valve gear, connecting rods, bearings, hubs and other sliding and wearing surfaces. Ball valve bodies represent another successful application for sea water and chemical plant duties. Parts for implantation may require polishing after nitriding to reduce surface roughness.
Nitride and nitrocarburised coatings are an effective treatment against scuffing. In one case, the life of Grand Prix car front and rear hubs was extended from a single race to a full season.
Problems Associated with Nitrided Components
One of the problems associated with nitride coatings is reduction of component fatigue life due to surface layer cracks developing and propagating through the parent material. This effect increases film thickness as a second and associated problem is chipping of heavier films (6µm plus).
Characteristics of Nitrided Titanium Surfaces
A wide variety of processes are commercially available for achieving a hard (1100-2500HV) but usually very thin (typically no more than 4µm thick) gold coloured titanium nitride layer on titanium. Titanium nitride, TiN, is inert, biocompatible and will tolerate temperatures of up to 480°C in air. Against other nitrided surfaces the coefficient of friction is 0.05-0.15.
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