Browsing by Author "Tsui, Ting Yiu"

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    Factors limiting the accuracy of mechanical-property measurement by nanoindentation
    (1997) Tsui, Ting Yiu; Pharr, George M.
    Nanoindentation techniques have been widely used to measure thin film mechanical properties. One of the most commonly used methods of analysis of nanoindentation load and displacement data was developed by Oliver and Pharr. The objective of this dissertation is to examine some of the limitations of this method and develop improvements so that a more accurate hardness and elastic modulus measurements can be made. Detailed experimental studies of bulk monolithic materials and soft films on hard substrates were performed to evaluate the validity of the Oliver and Pharr experimental technique and analysis procedures. Three different indenters were used. They were the Berkovich and Vickers pyramids, and a cone with a 70.3$\sp\circ$ included angle. It is shown that there are inherent limitations in the Oliver and Pharr indenter shape-function calibration method which means that it cannot be applied to the blunt Vickers and conical indenters used in this work. A new procedure was developed which avoids these problems. The pile-up behavior of monolithic and thin film materials was extensively investigated. Experimental results for monolithic materials show that materials with low elastic modulus to hardness ratios (E/H) such as ceramics are less likely to pile-up. On the other hand, monolithic materials which have high E/H ratios and low strain hardening coefficients or soft films on hard substrates are more likely to pile-up. The pile-up generated during the indentation process in these materials can create as much as 50% more contact area between the indenter and the specimen. The effects of pile-up on the hardness and elastic modulus measurements for monolithic and thin film materials were examined. It is shown that when pile-up occurs, Oliver and Pharr method overestimates both the hardness and the elastic modulus. Only if these extra contact area generated by the pile-up is included are the correct hardness and elastic modulus values obtained. The amount of pile-up is also found to depend on the indenter geometry. The Vickers indenter generates more pile-up at the indentation corners than the Berkovich indenter for both monolithic materials and soft films on hard substrates. The absolute amount of pile-up in monolithic materials for Vickers indentations is also more than the Berkovich.
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    The hardness and modulus of molybdenum silicide/niobium microlaminates and stressed materials determined by nanoindentation
    (1992) Tsui, Ting Yiu; Pharr, George M.
    Nanoindentation techniques have been used to explore the hardness and elastic modulus of Nb/MoSi$\sb2$ microlaminates and stressed aluminium and stainless steel alloys. The elastic moduli of the microlaminates fall on the high end of range set by the rule-of-mixtures and increase slightly above it at small wavelengths. The hardness of the microlaminates increases with decreasing wavelength in a manner consistent with Hall-Petch strengthening. It is shown that average mechanical properties of the microlaminates can be measured when the plastic penetration depths are as small as one bilayer thickness. Studies in the aluminium alloy and stainless steel show that while hardness is affected by applied stress, the standard methods by which hardness and modulus are determined from nanoindentation data may lead to inaccuracies when large stresses exist in the material.