The relationship between indentation and uniaxial creep in amorphous selenium
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W. C. Oliver Nano Instruments, Inc., 1001 Larson Drive, Oak Ridge, Tennessee 37831
B.D. Fabes Department of Materials Science and Engineering, University of Arizona, Tucson, Arizona 85721 (Received 7 November 1994; accepted 17 April 1995)
Ultralow load indentation techniques can be used to obtain time-dependent mechanical properties, termed indentation creep, of materials. However, the comparison of indentation creep data to that obtained during conventional creep testing is difficult, mainly due to the determination of the strain rate experienced by the material during indentation. Using the power-law creep equation and the equation for Newtonian viscosity as a function of stress and strain rate, a relationship between indentation strain rate, kj = h/h, and the effective strain rate occurring during the indentation creep process is obtained. Indentation creep measurements on amorphous selenium in the Newtonian viscous flow regime above the glass transition temperature were obtained. The data were then used to determine that the coefficient relating indentation strain rate to the effective strain rate is equal to 0.09, or e = 0.09e/.
I. INTRODUCTION The development of ultralow load indentation has resulted in the ability to probe the mechanical properties of materials on the submicron scale. It is now relatively easy to obtain the hardness and modulus of bulk and thin film samples from ultralow load indentation experiments.1'2 With the continued development of mechanical properties microprobes (MPM), time-dependent properties of materials are also obtainable. It has been found that most materials, even ceramics, creep at temperatures well below half their melting temperature. Consequently, indentation creep experiments to determine the stress exponent for creep from ultralow load indentation experiments have been attempted.3""' Results from several direct comparisons between indentation and conventional creep tests on the same material5"9 indicate that the stress exponents and activation energies for conventional and indentation creep are similar. However, it is difficult to relate the experimental measurements of hardness and displacement, or displacement rate, to the stress and strain rate experienced by the material in a conventional creep test as there is no accepted, validated method for analyzing indentation creep data. For most crystalline materials at temperatures above 0.5 Tm, the time-dependent deformation mechanism (dislocation glide-plus-climb) is well described by powerlaw creep, for which the constitutive equation is € = 2024 http://journals.cambridge.org
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