Abrasion is the progressive loss of material from a solid surface caused by relative sliding in forcible contact with hard, sharp particles or protuberances. The protuberances statement means that files and saws produce abrasion, but the damage is not done with particles, but rather, hard, sharp cutting edges that stand proud. There are four modes of abrasion generally accepted by people working in tribology:
Low stress High stress Gouging Polishing
These terms were defined in Chapter 2, and the different modes of abrasion have to do with the mechanics of the contact with the particles or protuberances that are causing the abrasion. The most commonly encountered abradants are components of the earth’s crust: soil, rocks, mineral deposits, sand, etc. Most of these materials are inorganic and harder than many engineering materials like construction steels. Table 4.1 is the famous Mohs hardness scale that ranks the relative hardness of common minerals. It is a table of what scratches what. This chart shows that talc is the softest of the common minerals and diamond is the hardest, but also that sand (quartz) is harder than many minerals and is in most soils. Consequently, most digging and earth-moving activities involve contact with hard substances. Also, corundum, which is aluminum oxide by chemical formula, is a significant part of the earth’s mantle. So, hard engineering materials frequently encounter minerals that tend to be harder than most materials, and therefore they are capable of causing abrasive wear.