The friction and the mechanical properties of solid krypton

An experimental study has been made of the frictional behaviour and of the hardness of solid krypton. It is found that the friction is nearly independent of load over the range investigated. At temperatures close to the melting point the friction is very low and the frictional behaviour is similar to that of ice. When the temperature is well below the melting point the friction is much higher, about $\mu $ = 0$\cdot $7, although the surface forces of the van der Waals crystal must be weak. An explanation of this high friction is suggested by the measurements of hardness. These show that the solid is extremely soft at temperatures near the melting point. Even at liquid-hydrogen temperatures, where the creep becomes negligible, the yield pressure is only about 0$\cdot $3 Kg/mm$^{2}$. Thus large areas of contact are formed between two surfaces which are placed together under load. It is suggested that, as with other plastic solids, the friction is primarily determined by the shearing of these contact areas formed by local plastic flow. An estimate is made of the activation energy for flow, deduced from creep measurements, giving the low value of about 1 to 2 kcal/gmol., and an approximate value of 4 $\times $ 10$^{6}$ dyn/cm$^{2}$ is computed for the critical shear stress of undeformed krypton at liquid-hydrogen temperature.