Effect of angled indentation on mechanical properties

Indentation on a smooth surface, perpendicular to the indenter tip, is critical to obtaining accurate mechanical property values with nanoindentation. However, for some materials, achieving such a scenario may not always be feasible. To investigate the effect this may have, angled indentations were made on flat, sintered hydroxyapatite samples individually mounted so as to produce indentation angles of 10°, 20°, 30°, 40° and 50°, as well as leading contact with either the face or edge of the Berkovich tip used. While significant scatter in results reinforced the importance of perpendicular penetration, two phenomena were found to serve as potential indicators of angled indentation, and hence unreliable data. It is recommended that topographical profiles are obtained on any material of uncertain roughness prior to indentation.