Copper viscoelasticity manifested in scratch recovery

Abstract

We have investigated two thick film copper compositions used in thermoelectric device fabrication. Dynamic mechanical analysis, thermal mechanical analysis, tensile testing, Vickers microhardness, optical microscopy and scratch testing were performed. The small grain samples have much smaller microindentation areas and much higher hardness than large grain samples, a consequence of intergranular spaces and thus low cohesion in large grain materials. The small grain material without intergranular spaces has higher linear thermal expansivity α L up to 150 °C; above that temperature negative α L is seen, a consequence of orientation relaxation. The large grain material also exhibits α L < 0 but only above 275 °C or so, a consequence of sintering. The small grain material has a storage modulus 49% higher than the large grain material over a wide temperature range, again an effect of high cohesion in the former. The brittleness value for the large grain material is 3.5 times larger than for the small grains material. Both kinds of materials exhibit recovery in scratch testing in the overall range of 23–36%—a manifestation of viscoelasticity.