Wear
Measurement and studies of wear and corrosion of materials
Measurement and determination of the resistance of materials to various types of wear is an important parameter for the development of new materials and the characterisation of products such as cutting tools, axles, guides, valves, blades etc in many applications.
The ways that materials can be worn down are varied:
1) Mechanical wear as a result of rubbing two surfaces against one another (abrasion wear), as happens on the surface of gears, ball-bearings, digging and cutting tools etc (2-body abrasion).If the two surfaces include a third materials (e.g. grains) then we have 3-body abrasion as happens in work gears, polishing of materials for metallography, drilling and coring of stone etc. In those cases where there is momentary adhesion between the materials (as a result of high temperatures generated on the materials surfaces during unlubricated friction) then we have fretting wear. Other abrasion wear modes are also possible.
Our laboratory has developed a number of simple techniques for measuring 2 and 3-body abrasion wear based on the grinding/polishing equipment used for metallography (right, top) with very reliable and reproducible results.
2) Mechanical wear as a result of air (or water)-born particle impingement on a surface, ie.e erosion wear. This happens in turbine blades, mud valves etc as well as on aircraft materials moving through dust or even clouds at high speed.
To measure erosion wear of hard-coated surfaces, we have designed and built a special erosion wear apparatus (right) which allows the measurement of air-born particle erosion using sand (or other particles as necessary) moving at speeds as fast as 100m/s. We have measured erosion in solid materials and hard flame and plasma-spray coatings with very good results. As the erosion testing chamber is large, the machine can also be used to measure erosion of whole products or sub-systems.
3) Cavitation erosion of materials occurs when microscopic bubbles on the surface of materials moving at high speed in liquids implode violently due to the sudden pressure gradient, as happens in pump valves, aircraft vanes and propeller blades. A typical wear evolution is shown in the micrographs below where the initial surface (left) becomes, after appreciable cavitation, the worn-out surface on the right. The mass change as a function of time is shown on the graph right.
In our lab we have built a special cavitation erosion rig (below right) based on an ultrasonic generator with titanium tips and we are able to measure cavitation erosion on most materials in various liquids at a range of temperatures (0 to about 60oC) and chemical environments.
