Galling is a severe form of adhesive wear caused by the transfer of material between metallic sliding surfaces under load, especially if there is poor lubrication. Movement and plastic deformation lead to interaction between the two materials, sometimes transferring material from one sliding surface to another. Abrasive or corrosive media can increase the risk of galling by removing the passive oxide film and making the two metal surfaces more active. Galling often occurs in high load, low speed applications, but it also occurs in high-speed applications with very little load and results in significant increase of friction and damage to the mating surfaces.
Stainless steel and other corrosion-resistant alloys are particularly prone to galling. It is a common problem in engine bearings and pistons, actuators, hydraulic cylinders, gas turbine blades and vanes, screw threads and valves. The galling of parts screwed together can damage the thread and make dis-assembly impossible. On a valve actuator, galling can cause the valve to get stuck and on a gate valve it can cause leakage.
A common solution is to use two dissimilar materials as they are less likely to suffer from adhesive wear. Using a Hardide coating on one of the mating parts can help solve galling problems as the two mating surfaces will be dissimilar. They will have completely different compositions and properties which will reduce the adhesive surface forces. Hardide coatings are composed of tungsten (the most refractory metal with a melting temperature of 3410°C – the highest among all metals) and tungsten carbide (a refractory ceramic) and are thus less likely to ‘weld’. They also provide wear and corrosion resistance in addition to their anti-galling properties. This combination is important for applications where the risk of galling is combined with abrasive or corrosive media and where these factors can have a synergetic effect which accelerates the surface damage.
The coatings have been successfully tested for anti-galling resistance, contact us to discuss your application and find out more.