Error sources of nano-indentation

The nano-indentation facility is able to provide the results of mechanical properties, such as hardness, elastic modulus at micrometer or sub-micrometer scale. The errors in these results are mainly from three different sources, which include the measurement of force, depth and misalignment between the indenter and the sample surface. For example, the function of hardness when measured through nano-indentation method is: 2 F H kD = (6-1)

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constant factor of area function (Berkovich is 24.5). These three parameters are able to contribute to the total uncertainty in hardness. Consequently, it is necessary to examine the main source of the errors resulted from the force and depth uncertainties as well as the misalignment between the tip and the sample surface. This work would contribute to better understand the measurement of mechanical properties at nano scale.

The uncertainty in applied force influences the hardness result directly. Generally, the force resolution is between 1µN to 1nN in nano-indentation method. Nevertheless, according to the traceable force is about 5µN. The regulation is similar to the atomic force microscopy, where it specified that the force resolution is smaller than the actual minimum traceable force. This is because in practice there will be surface force (Van de Waals, adhesion, etc.) that would lead to a gap between the force applied to the contact zone and the value measured from the force sensor. In addition, the tilt angle between the applied force and surface normal vector is another contribution of error in force. The misalignment will result in an error of cosine the angle.

The depth error is measured and limited by the displacement sensor in the facility. The uncertainty in depth is determined by a traceable displacement interferometer or capacitive sensor with sub-nanometer resolution. Similarly to force uncertainty, the misalignment between the drive and measurement axis would lead to uncertainty in displacement result.

The errors in force and displacement are considered scrutiny in this research area. However, other factors may exhibit more impact on the result of nanoindentation. The area function of the indenter is one of such uncertainties which own a direct influence

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on the nanohardness. This uncertainty is generated from the manufacturing of the ideal geometry for the tips. For instance, the typical errors in Berkovich tip are face and base angle errors as well as tip rounding. For the measurement of the tip shape, the scanning white light microscopy is not feasible due to the steep angle and spatial resolution. While, this error can be investigated by atomic force microscopy to map the profile of the tip shape.

Another important source of error is due to the misalignment between the axis of the tip and the normal vector of the sample surface, which is not dependent on the manufacturing of the tip. It is defined in ISO 14577 [108] that the error in angle on the axis should not be more than 1.0 degrees. It is not mentioned the detail information about the values for different types of indenters (Vicker, spherical or Berkovich). In addition, there is no clearly definition of whether this value is from the central axis in a spherical coordinate system or if the indenter and the tilt angle should be within a degree (larger than 1 degree in spherical coordinates).

Generally, for the tips of indentation, a diamond is adhered onto a stiff shank and polished to an appropriate dimension in order to eliminate the error resulted from manufacturing. The geometry of the tips will be altered due to diamond tip would be wear with the usage of the indenter increased. It is difficult to avoid this type of variation in actual measurement. Moreover, extra error can be produced by the thermal variation in the indenter of sample. The calibration of these misalignments for every measurement is time consuming and not practicable. Therefore, it is important to examine the uncertainties in these parameters, which are directly related to the results

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of nanoindentation. The work in this chapter mainly focuses on the misalignment between the tip and sample impact on the results of hardness and elastic modulus. During the process of nano-indentation, the applied force and the depth of penetration are recorded at the same time. The Oliver-Pharr method [31] is widely used in the instrumented indentation test to work out the hardness and elastic modulus. More specifically, the hardness is defined as the ratio between the maximum indented force and the projected contact area.

max c P H A = (6-2)

The elastic modulus can also be derived based on the elastic contact theory of Sneddon’s analysis [38], which is shown in Equation (6-3):

2 r c S E A

π

β

Where E_r is the reduced modulus, β is a correction factor which is accounted for the tip geometry. S is contact stiffness, which is equal to the slope of unloading force-depth curve. In the actual experiment, a Berkovich tip was selected, the value of

β is 1.072 in this study. The deformations of the indenter and the sample are considered in the reduced modulus. Therefore, the reduced modulus can be defined as:

2 2 1 1 1 s i r s i v v E E E − − = + (6-4)

Where E_s and ν_s represent the elastic modulus and Poisson rate of testing sample, respectively. E_i and ν_i represent the elastic modulus and Poisson rate of the Berkovich tip. The project area is the major factor in the determination of the

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mechanical properties on the basis of nano-indentation method. The projected area is calculated based on an analytic function of the contact depth, A= f h( )_c . For a Berkovich tip, A=24.5h_c2. The contact depth is derived by Sneddon equation below:

max max c P h h S ε = − (6-5)

Where h_max is the depth at the peak load (P_max). ε is another correction factor which depends on the indenter geometry as well. The value of ε is set to be 0.75 according to the Oliver-Pharr method [31].