In sliding contact applications where lubrication is not possible or can provide only limited resistance to galling (severe adhesive wear combined with material transfer), material solutions include both hardfacings and hardmetals.
Applications requiring sliding contact of high strength materials under unlubricated conditions include:
- Gate Valves in high pressure systems
- Drilling bits, cutting-tool bits or inserts
- Mining and road construction
- Plain bearings for stationary power generation
Hardfacings are often weld-deposited overlays and include hardenable (Martensitic or work-hardening) steels and metal carbides, and are usually applied for increased wear resistance. Hardmetals, on the other hand, can be of even higher strength than many hardfacings, and include cemented carbides, nitrides, carbonitrides, and tungsten-based alloys. Hardmetals are often used for cutting tools, in high temperature applications, or where severe metal-on-metal sliding contact occurs under conditions of limited or no lubrication.
Testing of hardmetals or hardfacings requires similarly severe conditions as those of the in-service application. One example is high-speed sliding under conformal contact conditions, such as those of an unlubricated plain bearing. Testing on the UMT is accomplished using a ring-on-disk geometry, as shown below. Since the samples are always in intimate contact, significant thermal input results from the sliding friction in this test, and therefore self-welding propensity also can be assessed.
