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The adequacy of instrumented Vickers depth-sensing microindentation to determine Young’s modulus of alumina-based ceramics was analyzed. Monophase alumina materials and alumina + 10 vol% aluminium titanate composites, with different microstructures, were tested to determine the effect of microcracking. The load–depth penetration of the indenter curves together with the observation of the imprints by scanning electron microscopy were used to analyze the behavior of the materials. Maximum stiffness was determined from the derivatives of the load-depth curves during unloading. The areas of the imprints measured optically were more representative of the behavior of the materials than the areas calculated from depth-penetration measurements. The formation of microcracks affected the shape of the unloading portion of the curves. Significant differences between the values of Young’s modulus determined for different materials and definite relationships between the microstructural parameters of the materials and the Young’s modulus were found.

I. INTRODUCTION

Vickers indentation at the micrometer level is currently used to evaluate hardness of ceramics, and it is a very useful tool to determine toughness when only small volumes of material are available.1,2 Also, differences in the development of microindentation cracks in the different layers of layered materials, in which the thickness of the layers is in the order of hundreds of microns, are being investigated as a means to evaluate residual stresses through the layers.3,4 With the advent of instrumented indentation techniques, first developed as a depth-sensing microindenter by a group working at the Baikov Institute of Metallurgy in Moscow,5 much research has been devoted to the use of sub-micrometer and nanometer indentation to determine the behavior of materials, not only in the plastic but also in the elastic regime. In particular, data for hardness H and Young’s modulus E are obtained from one complete cycle of loading and unloading in the nanometer range, with sharp indenters of Vickers or Berkovich geometries6–18 and spherical indenters.8,18–20 To a lesser extent, indentation techniques in the micrometer level have also been proposed to analyze the mechanical behavior of ceramics and metals using spherical21–24 and Vickers23–25 indenters.

a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0007 J. Mater. Res., Vol. 21, No. 1, Jan 2006

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The schematic representation of load versus indenter penetration depth during the whole loading–unloading cycle is shown in Fig. 1(a). Deformation during loading is assumed to be both elastic and plastic in nature as the permanent hardness impression forms and during unloading it is assumed that only the elastic displacements are recovered.18 The key experimental parameters are the peak load Pmax, the depth at peak load hmax, and the elastic unloading stiffness S = dP/dh, defined as the slope of the upper portion of the unloading