Scientific instrumentation for hot microhardness measurements on amorphous solids

Abstract

A technique and a simple and convenient apparatus are described to carry out hot microhardness measurements on amorphous solids to enable the Vickers hardness number to be monitored as a function of temperature and heating rate. The apparatus consisted of modifying a commercially available microhardness test instrument so that the specimens could be mounted on a temperature-controlled aluminum platform, and the temperature of the indenting tip maintained equal to the that of the sample. A microcomputer and a data-acquisition system, with an appropriate algorithm and software, were used to ramp the temperature of the sample at any selected heating rate. A thermocouple and a heater arrangement attached to the indenter, with the same microcomputer and data-acquisition system, assured that the indenter tip temperature did not differ from that of the sample more than a fraction of a degree. The technique was applied to studying the influence of temperature on the Vickers hardness number (HV) of vacuum-deposited amorphous selenium-arsenic (a-Se0.997As0.003) alloy layers as used in x-ray medical imaging. It was found that the microhardness of such a typical chalcogenide glass decreases with the temperature, and in the glass transition region it undergoes a sharp fall, similar to the drop observed in the real part of the elastic constants of many glassy polymers. It is shown that an empirical glass transformation temperature Tg defined at the ‘‘break point’’ of the log HV-vs-log T plot depends on the heating rate r and that the Tg-r data can be adequately described by a Vogel–Tammann–Fulcher type of temperature dependence of the form exp[−333.4/(T−284 K)] similar to the general behavior of the viscosity of many chalcogenide glasses. It is expected that the variable heating rate technique with the modified microhardness apparatus described in this paper will be applied to study phase and glass transformations in many other amorphous solids. The technique is therefore aptly named ‘‘thermomicrohardness analysis’’ (TμHA).