Direct measurement of surface shape for validation of indentation deformation and plasticity length-scale effects: a comparison of methods

It is ironic that recent developments in instrumented indentation, such as the drive to obtain tensile properties from indentation data and to understand length-scale effects in plasticity, have seen a return to direct imaging of indentations. Significant uncertainties in contact size arise when using contact mechanics calculations that do not take into account the lateral dilation of elastic recovery (Hou et al 2008 J. Phys. D: Appl. Phys. 41 074006) and important sink-in and pile-up contributions to the contact response (Lim and Chaudhri 1999 Phil. Mag. A 79 2979-3000). High resolution, direct measurement avoids these problems. Accurate wear volume and coating thickness measurements obtained by cap grinding methods also depend on high accuracy and low uncertainty direct measurement methods. The use of metrological atomic force microscopy to measure and certify the shape of indenters is well established (Aldrich-Smith et al 2005 Z. Metallk. 96 1267-71) and is essential for valid mechanical property measurement by instrumented indentation. In this paper, we consider indent measurement and compare three direct measurement techniques: optical microscopy, metrological atomic force microscopy (AFM) and confocal laser scanning microscopy (CLSM). We compare the relative merits and uncertainties of various 2D and 3D analysis methods with a new analysis method of differentiating 3D data obtained from AFM and CLSM. This new method has the lowest uncertainty (2.8% for a 50 mu m diameter indent at the 95% confidence level). Better still, it enables objective measurements of indent size that avoid the issues caused by difficult-to-standardize parameters (such as illumination angle, contrast and brightness settings), which strongly affect manual estimates of the edge position of an indentation/crater (Gee et al 2002 NPL Measurement Good Practice Guide No 57).