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Surface treatment and tribology

A construction-machine part is rubbed by soil containing quartz sand grains (hardness about 1000 HV). Which is the correct way to choose a material that reduces abrasive wear?

  1. ARaising the surface hardness to about that of the sand grains or higher makes it hard for the grains to dig into the surface and reduces wear greatly
  2. BMartensite in hardened steel is harder than quartz, so simply hardening the steel makes wear almost disappear
  3. CThe amount of abrasive wear is set by the sliding distance alone and does not depend on the surface hardness
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Answer: A. Raising the surface hardness to about that of the sand grains or higher makes it hard for the grains to dig into the surface and reduces wear greatly

Empirically, a grain must be about 1.2 times harder than the surface (Ha ≥ about 1.2Hs) to scratch it, and once the surface hardness Hs is raised so that Hs/Ha exceeds about 0.8, abrasive wear drops sharply. The grains can no longer dig into the surface, and their corners are crushed instead. For this reason, high-chromium cast irons containing much Cr carbide (about 1200–1800 HV), which is harder than quartz, hardfacing welds containing WC, and ceramic coatings are used. Martensite in hardened steel is around 900 HV even at high carbon, about as hard as quartz or softer, so it raises wear resistance but does not eliminate wear. The amount of wear is proportional to the sliding distance, but hardness also has a large effect, so it is not independent of hardness.

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