Hardness and deformation properties of solids at very high temperatures

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Abstract
This paper describes an experimental study of the deformation and strength properties of refractory solids at temperatures up to 2000$^\circ$C. It is difficult to make direct stress-strain measurements at elevated temperatures on small specimens of these materials, and consequently the strength property measured was indentation hardness. The problem of obtaining an indenter that is sufficiently hard and stable at these temperatures was overcome by indenting a cylinder or prism of the material with an identical specimen of the same material. Mutual indentation measurements obtained in this way show that at low temperatures most of the refractory solids are very hard but brittle. Above a critical temperature they become ductile and the hardness falls as the temperature increases. With sintered porous specimens the fall in hardness is augmented by collapse of the structure and compaction of the material. At higher temperatures the behaviour is dominated by the intrinsic properties of the solid and the hardness properties of the sintered material resemble those of fully compacted crystalline specimens. For most of the refractory solids, as for pure metals, the most rapid fall in hardness occurs at temperatures above about one-half of the absolute melting point, T$_m$. By contrast the carbides of titanium and tungsten fall off rapidly in hardness at temperatures between 0.2 and 0.4 T$_m$, probably on account of the increased mobility of the carbon atoms. At any one temperature the indentation hardness decreases as the loading time is increased. Semi-empirical relations may be used to describe this and it is possible to deduce an activation energy for the creep processes involved. The results show that the only materials having an indentation hardness above 100 Kg/mm$^2$ at 1500$^\circ$C are the nitrides and carbides of silicon.

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