Thomas F. Juliano

Measuring tip shape for instrumented indentation using atomic force microscopy

Atomic force microscopy (AFM) was used to determine the three-dimensional geometry of instrumented indentation probes. From the AFM image data, the cross-sectional area, A, was determined as a function of the distance, hc, from the tip apex for a number of indentation probes, including Berkovich pyramidal tips and several rounded conical tip geometries. Nonlinear behaviour of the vertical AFM scanner caused significant uncertainties in the A(hc) data above a given image size, limiting the range of hc for accurate calibration to approximately 1 µm. The A(hc) data averaged from multiple AFM images taken within this range were similar to the values determined from fused silica indentation. Deviations from the ideal tip shape were quantified by measuring the values of the tip radius and tip angle from the AFM images. Large deviations were observed for the rounded conical tips; for example, the values of the tip radius deviated from manufacturer values by 16% to 97%. In contrast, the Berkovich probes had tip angles within 4% of the ideal value (65.3°) and with only minimal tip rounding (tip radius <300 nm). This method is useful for characterizing indentation tips, particularly those with significant rounding, to larger contact depths than is possible using indentation of a reference material, which is important for probing compliant materials.

Aspects of Tip Shape Characterization for Nanoindentation of Compliant Materials

The application of nanoindentation methods to compliant materials, such as polymeric and biological materials, often requires the use of instrumentation designed with enhanced force sensitivity, which limits its maximum force level. Because tip geometry is normally characterized using indentation of fused silica, the maximum contact depths achieved by low-force instruments during this calibration are often less than 300 nm. However, penetration into more compliant materials can be several micrometers or more. Extrapolation of tip shape data from fused silica indentation can lead to significant uncertainties in the indentation measurements for compliant materials. In this paper, atomic force microscopy (AFM) is used to provide tip shape information for a Berkovich tip and a series of conical tips. Use of AFM allows a larger range of depth to be calibrated and provides three-dimensional tip information.