If hardness is unreliable, then how is wear measured?

It depends on the type of wear involved, but in the case of abrasive wear—by far the most predominant wear mechanism—the ASTM Intl. G65 Dry Sand Rubber Wheel Test is used extensively. This essentially is a test in which the sample is weighed before and after the test, and the result usually is expressed in grams of weight loss or volume loss.

A sample is held against a spinning rubber wheel with a known force for a number of revolutions. A specific type of sand, which is sized carefully, is trickled down between the sample and rubber wheel. This simulates pure abrasion, and the numbers are used as guidelines in material selection.