sentences:  For the situation where the asperities on the two surfaces have a Gaussian height distribution and the peaks can be assumed to be spherical, the average contact pressure is sufficient to cause yield when formula_129 where formula_130 is the uniaxial yield stress and formula_131 is the indentation hardness.Greenwood and Williamson defined a dimensionless parameter formula_132 called the plasticity index that could be used to determine whether contact would be elastic or plastic.The Greenwood-Williamson model requires knowledge of two statistically dependent quantities; the standard deviation of the surface roughness and the curvature of the asperity peaks.