This paper describes several different physicochemical mechanisms that produce an indentation size effect on microhardness. The mechanisms are illustrated with reference to published data. A simple quantitative model for surface layer effects is presented, and its relevance to chemomechanical effects on ceramics is discussed.

A well-behaved empirical function describing some indentation size effects is introduced, and its advantages are discussed. It is used in a new method of graphically presenting microhardness data which is presented here for the first time. This “error-ellipsoid” diagram not only is visually compact and statistically rigorous but is based on a new physically meaningful parameter, the “10-μm hardness.”