Wear and Fatigue of MMC's in Rolling-Sliding Contact

Metal matrix composites (MMC's) can be used for high strength, low weight structural components. An aluminium matrix with hard silicon carbide particles is a form of MMC used commonly in the aerospace industry. Its use in tribological applications, where it may be subjected to localised contact stresses and surface wear, is less common. This objective of this project was to determine how MMC's would perform in rolling contact.

Steel or copper alloys are normally associated with rolling contacts. Depending on the operating conditions, they can fail by the initiation of cracks at the contact region (rolling contact fatigue) or by wear from the surface. Where there is plastic deformation the surface strain can accumulate (ratchet) leading to wear or cracking. 

Test disks were machined from MMC and the base aluminium alloy. These disks were run in a twin disk simulator. Tests were carried out under conditions of pure rolling and with 1% slip. The tests were run either dry or with water.

The wear rate under pure rolling was very low (comparable with what might be expected from a steel disk pair). When the contact had some sliding (1%) the wear rate was greatly increased. Wear debris was in the form of a fine oxidised powder and appeared to be by an adhesive process. There was no evidence that the hard silicon carbide particles led to a two body abrasive wear. The wear rate of the MMC and the aluminium disk were of the same order.

Sections through a dry run MMC surface.
Sections through a wet run MMC surface.

 

It proved difficult to initiate contact fatigue cracks in dry rolling-sliding. Only when water was introduced (by drip feeding into the disk inlet region) did cracks start to initiate. This is comparable to rolling contact fatigue in steels, where the liquid can penetrate into an initiating crack and become hydrostatically pressurised thus driving the crack in mode I growth.