Apart from imaging the residual imprint of an indentation, the integrated SFM which is mounted on the CSM Instruments Nano Hardness Tester (NHT) can also be used to check and quantify the tip defect and symmetry of the indenter used. The actual indenter tip, being fused to a vertical rod, can easily be removed from the NHT measuring head and positioned accurately under the SFM objective.
Tip Defects of Common Indenters
It has been shown that the tip defect of common indenters can be significant, especially in the case of the Vickers geometry, owing to the difficulties in grinding a square-based pyramid to an adequate tip. In contrast, the three-sided pyramidal geometry of the Berkovich naturally terminates at a point, thus facilitating the grinding of diamonds which maintain their sharpness to very small scales. Berkovich tip defects, as characterised by the effective tip radius, are frequently less than 50 nm in many of the better diamonds.
Figure 1. Scanning force microscopy images of Vickers (a) and Berkovich (b) diamond indenters.
Fig. 1 shows two SFM images of a Vickers and a Berkovich indenter. Although the SFM has a limited vertical range, most nanoindentations are less than 5 mm in depth, so the instrument is well suited to the measurement of the active portion of the indenter. In addition, the high resolution of the SFM allows shape information to be extracted directly from the images, as well as any deviation from a perfectly sharp pyramidal geometry. Previous work in this domain has shown that the area function of a pyramidal indenter can be directly calculated from information gained from an SFM image, after correcting for the effects of pixellation and SFM probe shape.
Pixellation occurs due to the fact that SFM data are acquired in digital form and so have a finite number of data points. For example, for a lateral scan size of 20 mm, and using the maximum number of points per line (512), the separation between each data point will be 20000/512 = 39 nm. This means that an image of a typical pyramidal indenter will in fact be made up of steps of lateral separation 39 nm.
If the contact area is to be calculated from such data then this may produce significant error, and so various correction routines must be used. Obviously, this effect is also present in SFM images of residual imprints and should be taken into account if pile-up volumes or contact areas are to be calculated.