Evaluating Friction and Anti-Galling of Hardmetals in Severe Sliding Contact Applications

Hardmetals and hardfacings are the preferred material solutions for sliding contact applications where lubrication is impossible or provides only very little resistance against galling, which is a condition involving severe adhesive wear and material transfer.

Following are the applications that require sliding contact of high strength materials under unlubricated conditions:

• Gate valves used in high-pressure systems
• Mining and road construction
• Cutting-tool bits, drilling bits, or inserts
• Plain bearings used in stationary power generation

Hardfacings are often weld-deposited overlays containing hardenable (work-hardening or martensitic) steels and metal carbides. They are generally utilized for increased wear resistance. Hardmetals can be stronger than many of the hardfacings, and they include cemented carbides, carbonitrides, nitrides, and tungsten-based alloys. The hardmetals are commonly employed in areas that involve rigorous metal-on-metal sliding contact under conditions of limited or no lubrication, in high- temperature applications, or in cutting tools.

Rigorous conditions, similar to conditions in the in-service application, are needed to test hardfacings and hardmetals. An unlubricated plain bearing is an example involving high-speed sliding under conformal contact conditions. Figures 1 and 2 illustrate testing on the UMT using a ring-on-disk geometry. In this test, the intimate contact between the samples leads to notable thermal input caused by the sliding friction. This enables the assessment of the self-welding propensity.

Figure 1. Ring-on-disk setup on a UMT.

Figure 2. Ring-on-disk data showing behavior on startup of different hardmetal couples. Tests were conducted at 0.5MPa contact stress and 4.2m/s sliding speed.

Use of Various Tests for Hardmetal Testing

Every unique application may have a critical parameter that needs to be tested, e.g. coefficient of friction during startup, resistance to high temperatures caused by high sliding speeds, and galling resistance under low speed high stress sliding. In general, it is important to test the material changes for enhanced performance under the optimum conditions.

The UMT can apply high sliding speeds and high contact stresses under reciprocating or unidirectional motions while monitoring friction, temperature, and wear. It can also be programmed to terminate the test automatically upon satisfying predefined conditions. These versatile features of the UMT enable various kinds of hardmetals or hardfacings to be tested under application-specific conditions to aid various research and development studies.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.