The different behaviour of hexagonal and cubic metals in their friction, wear and work hardening during abrasion

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Abstract
The mass wear per centimetre M and the friction coefficient μ of metals during abrasion on a relatively rigid hard rough surface are studied experimentally at 1 kg load and a speed of a few centimetres per second. `Smooth-cut' steel files were used as the abrasive surface, cleaned carefully by scratch-brushing rather than by etching, between consecutive experiments. In the mean, linear relations were observed, as expected theoretically, between the reciprocal of the volume-wear rate and the microhardness HD of the filed surface (thus work-hardened to the maximum extent), but the loci were widely different for the cubic metals (Pb, Ca, Al, Ag, Cu, Pt, Fe, Ni, Mo, Cr) and the hexagonal metals (Cd, Mg, Zn, Zr, Ti). For a given surface hardness, the volume-wear rate for the hexagonal metals was about half that for the cubic metals, hence for the hexagonal metals a larger proportion of the metal is displaced by plastic flow (slip) from the groove volume instead of being removed as wear; and this is evidently because of easier slip, mainly on a single slip plane (0001), instead of multiple slip and the associated heavy work hardening as occurs in cubic metals. A single linear locus was obtained when the reciprocal of the volume-wear rate was plotted against the hardness of the annealed metals. For the cubic metals μ decreased mainly linearly with increasing HD of the (work-hardened) filed surface, but for the hexagonal metals μ was practically constant, independent of HD. The relatively rapid surface oxidation of Mg and Ca caused a decrease in μ relative to that of the other metals of similar structure and hardness. These results establish clearly that there is a fundamental difference in wear and friction behaviour of hexagonal and cubic metals.