| MadSci Network: Engineering |
Shawn, Any material will wear But metals have some idiosyncratic mechanisms. This website outlines them: http://www.alloysteel.net/english/techlib_factors.asp The properties of metals that commonly cause wear are their softness, corrosion especially reaction with oxygen, and metal to metal bonding. The common mechanisms are: Abrasion-an abrasive material comes in contact with the metal and wears the metal away an example is sandpaper or a file. This is solved by cleanliness or by using a harder metal and sometimes by lubrication or coatings but coatings can also wear and cause their own problems. Removal and reformation of a surface oxide layer, fretting corrosion- This is common in aluminum and stainless steels which form a hard protective oxide layer that can break from the surface exposing metal that oxidizes etc. The removed oxide can then act as an abrasive. This can be reduced by lowering stress, lubrication and removing oxygen and using dissimilar metals. This is sometimes controlled by surface treatment such as nitriding or case hardening. Metal to metal bonding- http://www.chemguide.co.uk/atoms/bonding/metallic.html#top Metal bonds are such that pure metals when forced into contact will bond together. Throw in some corrosion and fretting and havoc ensues. These bonds form and break and eventually form wear particles. This can be reduced by good lubrication, reduced pressures, coatings, dissimilar metal surfaces and good bearing design and surface finish. Lubricants must not be forced from the interfaces by pressure. That's why certain slippery materials such as silicones and WD-40 can be ineffective under stress. The choice of a grease or lubrication system can be critical.
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