Wear
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| Mechanical failure modes |
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| Buckling |
| Corrosion |
| Creep |
| Fatigue |
| Fracture |
| Melting |
| Thermal shock |
| Wear |
In materials science, wear is the erosion of material from a solid surface by the action of another solid. The study of the processes of wear is part of the discipline of tribology. There are four principal wear processes:
- Surface fatigue :
- Adhesive wear
- Abrasive wear
- Corrosive wear
The definition of wear does not include loss of dimension from plastic deformation, although wear has occurred despite no material removal. Also this definition fails to include impact wear where there is no sliding motion or cavitation (also corrosion) where counterbody is a fluid..
Wear can possibly be better defined as a process in which interaction of surface(s) or bounding face(s) of a solid with the working environment results in the dimensional loss of the solid, with or without loss of material. Wear environment includes loads(types include unidirectional sliding,reciprocating, rolling, impact),speed, temperatures, counterbodies(solid, liquid, gas), types of contact (single phase or multiphase in which phases involved can be liquid plus solid particles plus gas bubbles) . In standard wear test (e.g., those formulated by respective subcommittees under ASTM Committee G-2) results, the loss of material during wear is expressed in terms of volume. The volume loss gives a truer picture than weight loss particularly when comparing wear resistance properties of materials with large variations in density. For example, weight loss of WC+Co (density = 14g/cc) as 14g and that of Al alloy (density = 2.7g/cc) as 2.7g shall result in same wear in terms of volume loss of 1cc. For engineering components the working life is over when the dimensional losses exceed the specified tolerance limits.
Wear along with other aging processes, such as, fatigue, creep and fracture toughness cause progressive degradation of materials with time leading to failure of material at an advanced age. Under normal operating parameters, the property changes during usage normally occur in three different stages as follows:- • Primary or early stage or run-in period, where rate of changes can be high. • Secondary or mid-age process where a steady rate of aging process is maintained. The useful or working life is comprised mostly of the life span at this stage. • Tertiary or old-age stage, where rapid rate of aging leads to early failure.
With increasing severity of environmental conditions, such as, higher temperatures, strain rates, stress and sliding velocities, the secondary stage is shortened and the primary stage tends to merge with the tertiary stage, thus drastically reducing the working life. Surface engineering processes are used to minimize wear and extend working life of material. References 1.R.Chattopadhyay; Surface Wear- Analysis, Treatment, and Prevention, ASM-International, OH, USA, 2001, ISBN no 0-87170-702-0.. 2. R.Chattopadhyay, ’Advanced Thermally Assisted Surface Engineering Processes’ Kluwer Academic Publishers, MA, USA (now Springer, NY), 2004, ISBN 1-4020-7696-7, E-ISBN 1-4020-7764-5.
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[edit] Adhesive wear
Adhesive wear is also known as scoring, galling, or seizing. It occurs when two solid surfaces slide over one another under pressure. Surface projections, or asperities, are plastically deformed and eventually welded together by the high local pressure. As sliding continues, these bonds are broken, producing cavities on the surface, projections on the second surface, and frequently tiny, abrasive particles, all of which contribute to future wear of surfaces.
[edit] Abrasive wear
When material is removed by contact with hard particles, abrasive wear occurs. The particles either may be present at the surface of a second material or may exist as loose particles between two surfaces. Abrasive wear can be measured as loss of mass by the Taber Abrasion Test according to ISO 9352 or ASTM D 1044.
[edit] Corrosive wear
Often referred to simply as “corrosion”, corrosive wear is deterioration of useful properties in a material due to reactions with its environment.
One form of high temperature corrosive (oxidative) wear can lead to the formation of compacted oxide layer glazes, which under certain circumstances reduces wear.
[edit] Surface fatigue
Surface fatigue is a process by which the surface of a material is weakened by cyclic loading, which is one type of general material fatigue.
