Further Studies on Environment‐Sensitive Hardness and Machinability of Al 2 O 3

Studies were continued on the effects of surface‐active environments on the hardness and the rates of drilling and abrasive wear of mono‐ and polycrystalline Al 2 O 3 . Changes in hardness induced by n ‐alcohols and various aqueous environments were correlated with changes in ζ‐potential, hardness being maximum at the point of zero charge (pzc). Diamond‐core drilling rates exhibited a similar relation. Studies of the environment‐sensitive abrasive wear of both mono‐ and polycrystalline Al 2 O 3 on a diamond‐impregnated metallographic wheel revealed more complicated behavior. The wear rate of Al 2 O 3 under n ‐alcohols is markedly dependent on the viscosity of the environment, with little or no correlation with ζ‐potential being evident. Studies using methanol‐water mixtures indicated, on the other hand, a slight viscosity dependence and the converse correlation with ζ‐potential to that observed in diamond‐core drilling, i.e. the rate of material removal was lowest at the pzc. Tests in aqueous solutions revealed no dependence on viscosity, but a strong inverse dependence on ζ‐potential, wear rate again being lowest at the pzc. These and other results are interpreted in terms of the various influences of adsorbed species on the near‐surface flow and fracture behavior of Al 2 O 3 .