Hardness versus wear

Hardness versus Wear Resistance. The wear processes that are usually mitigated by the use of hard surfaces are low-stress abrasion, wear in systems involving relative sliding of conforming solids, fretting wear, galling, and to some extent, solid-particle erosion (Ref 3). Unfortunately there are many caveats to this statement, and substrate/coating selection should be... 

Graphitar versus tool steel and Mo versus Rex AA or Stellite 90 have shown the best results of the hard versus soft combinations tested. The results have been good in that the wear has been very smooth however, the wear has been excessive. The use of these combinations would be limited to very low-load applications. 

The main differences in the SteUite aUoy grades of the 1990s versus those of the 1930s are carbon and tungsten contents, and hence the amount and type of carbide formation in the microstmcture during solidification. Carbon content influences hardness, ductUity, and resistance to abrasive wear. Tungsten also plays an important role in these properties. 

For erosive wear. Rockwell or Brinell hardness is likely to show an inverse relation with carbon and low alloy steels. If they contain over about 0.55 percent carbon, they can be hardened to a high level. However, at the same or even at lower hardness, certain martensitic cast irons (HC 250 and Ni-Hard) can out perform carbon and low alloy steel considerably. For simplification, each of these alloys can be considered a mixture of hard carbide and hardened steel. The usual hardness tests tend to reflect chiefly the steel portion, indicating perhaps from 500 to 650 BHN. Even the Rockwell diamond cone indenter is too large to measure the hardness of the carbides a sharp diamond point with a light load must be used. The Vickers diamond pyramid indenter provides this, giving values around 1,100 for the iron carbide in Ni-Hard and 1,700 for the chromium carbide in HC 250. (These numbers have the same mathematical basis as the more common Brinell hardness numbers.) The microscopically revealed differences in carbide hardness accounts for the superior erosion resistance of these cast irons versus the hardened steels. 

Figure 8.13 Materials selection chart of dry sliding wear rate constant [Eq. (8.21)] versus hardness. Reprinted, by permission, from M. F. Ashby, Materials Selection in Mechanical Design, 2nd ed., p. 60. Copyright 1999 by Michael F. Ashby.

Figure 3. Wear versus the modulus and hardness of polymeric substrates.

Figure 7.45 Wear factor versus unlubricated countermaterial hardness in thrust bearing tester of DuPont Engineering Polymers Vespel SP-21—15% graphite-filled PI [10].

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