Wear of fluorapatite single crystals. II. Frictional behavior.

It is a convenient approximation to regard the frictional forces between two solids as made up of two components: the force required to shear adhering junctions in the region of surface contact and the force required to deform the underlying bulk material.' With an elastic solid, the deformation component may be due mainly to an elastic hysteresis loss; but if plastic flow occurs, the deformation component will represent the force required to produce bulk deformation or to drag surface irregularities through the solid and to plow out a track.2 Although the coefficient of friction is not necessarily indicative of wear phenomena, it is traditional to attempt to correlate it with wear data. Steijn,3 in his examination of polycrystalline and single crystal copper, observed that the plowing component of friction was increased by indentors of smaller diameter and by higher loads for a given indentor. Bowden and Brookes4 reported for MgO single crystals that depth of penetration (from static indentation tests) was related directly to the magnitude of the coefficient of friction, with deeper penetration correlating with higher friction. Both low and high friction regimes were observed depending on the load and slider design used.