Surface modification methodologies for polycrystalline alumina: effects on morphology and frictional coefficients

Several methodologies of surface modifications were applied to polycrystalline alumina (PCA) samples to study their effects on surface morphology and frictional coefficients. Modified surfaces were first tested in a specially designed frictional apparatus against wires of stainless steel (SS) and beta-titanium (β-Ti) alloys and then evaluated by scanning electron microscopy. Techniques included ion implantation of chromium ions (Cr⁺), ion beam assisted deposition of diamond-like carbon (DLC), coatings of gamma-irradiated polymers (PEO), and electroless nickel plating of a composite material of polytetrafluoroethylene dispersed in a nickel phosphorus matrix, Niflor NT^® (NF). Implanting ions into the bulk material had no effect on surface morphology. Although covering the surface, the DLC coating mimicked the underlying topography. The coatings of PEO and NF obliterated and smoothed over the normally rough and faceted PCA surfaces. When compared to control samples, neither the Cr⁺ or DLC process reduced the friction normally seen against SS and β-Ti wires. When tested in both the dry and wet states, the PEO coated samples retained their traditional levels of frictional resistance. Only the composite material, NF, successfully reduced the friction when compared with controls. Although this composite coating is not recommended for oral use, the results show that simply smoothing over the rough surface is inadequate for friction reduction; the surface must somehow also be made lubricious.