The coefficients of sliding friction of
single crystals of commonly used high explosives pentaerythritol tetranitrate
(PETN), cyclotrimethylene trinitramine (RDX) and beta-cyclotetramethylene
tetranitramine (β-HMX) under several rubbing configurations and at a relative
sliding speed of 0.22 mm s−1 were measured. The sliding configurations were (1)
crystal–polished steel pairs, (2) like-crystal pairs and (3) unlike-crystal
pairs. For every rubbing configuration the friction force showed oscillations,
which are thought to be caused by the formation and shearing of the adhesive
junctions formed at the surface of the rubbing crystals. This shearing of the
adhesive junctions led to the formation of microscopic and sub-microscopic
particles, which were confirmed by an environmental scanning electron
microscope study. For every rubbing configuration and for relatively high
normal loads pressing the rubbing crystals together, the coefficient of
friction was generally in the range 0.2–0.25 and it has been concluded that the
coefficient of friction is controlled by the adhesion with almost negligible
contribution from the ploughing component. From a knowledge of the coefficient
of friction and the uniaxial yield stress values of single crystals of RDX and
β-HMX, the shear strength of these crystals were determined to be ~13.4 MPa and
~16.8 MPa, respectively.
Source:IOPscience
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