Erosion wear of materials

Abstract

Wear is damage to a surface as a result of relative motion with respect to another substance. One key point is that wear is damage and it is not limited to loss of material from the surface. However, loss of material is definitely one way in which a part can experience wear. Another way included in this definition is by movement of material without loss of mass. An example of this would be the change of geometry or dimension of a part as a result of plastic deformation (e.g., from repeated hammering). There is also a third mode implied, which is damage to a surface that does not result in mass loss or dimensional changes. An example of this third mode might be development of network of cracks in a surface. This might be of significance in applications where maintaining optical transparency is a prime engineering concern. Lens and aircraft windows are examples where this is an appropriate definition of wear. In the older definitions of wear there used to be a greater stress on the “loss of material”, however now-a-days the newer and more general definitions of wear is very natural to the design or device engineer , who thinks of wear in terms of a change to a part that effects its performance. The focus is on the change which may be translated to damage. The implication of this generalization will be further explored in the discussion of wear measures. Previously wear was defined as damaged to a surface. The most common form of that damage is loss or displacement of material and volume can be used as a measure of wear—volume of material removed or volume of material displaced. For scientific purposes this is frequently the measure used to quantify wear. In many studies, particularly material investigations, mass loss is frequently the measure used instead of volume. This is done because of the relative ease of performing a weight loss measurement. However there are some problems in using mass as primary measure of wear. Direct comparison of materials can only be done if their densities are same. For bulk material this is not a major obstacle, since the density is either known or easily determined. In the case of coatings however, this can be a major problem. The other problems are more intrinsic ones. A mass measurement does not measure displaced materials. In addition it is sensitive to wear debris and transferred material that becomes attached to the surface and can not be removed. This material does not necessarily have to be from the same surface; it can from the counter face as well. From the above it can be seen that volume is the fundamental measure for wear when wear is calculated with loss or displacement of material. However, in engineering applications, is generally with the loss of a dimension, the increase in clearance or change in contour not the volume loss. Volume, mass loss and a dimension are not the only measures for wear that are used in engineering. Life, vibration level, roughness, appearance, friction level, and degree of surface crack or crazing are some of the operational measures that are encountered.