The influence of graphite coatings applied before pressing on the mechanical strength after pressing

Abstract

Composite samples containing 80% and 85% HMX (cyclotetramethylene tetranitramine) in a polymer-plastizer binder were produced by mixing, extruding, cutting, drying and pressing. Before pressing the extruded material was in some cases coated with a thin layer of graphite (≈ one micro meter) for ease in pressing. As part of a general study of these composites the compressive strength was determined as a function of temperature, strain rate and the amount of the graphite added. Without graphite the compressive strength increases with decreasing temperature and increasing strain rate. With graphite this strength has the same behavior above approximately -10° C, but is insensitive to both temperature and strain rate below this temperature for a strain rate of 1.0/Sec. In addition, the low temperature value of the compressive strength appears to decrease with increasing graphite thickness. The cracking and fracture patterns are temperature and strain rate dependent and are different with and without graphite. These results indicate that the bond produced by pressing the graphite containing material is stronger than the composite above about -10° C and weaker below this temperature so that failure initiates in the composite above about -10° C and in the bond below -10° C. With increasing strain rate this transition temperature increases. The results also indicate that the bond produced by pressing the material without graphite is at least as strong as the extruded material itself.