Interparticle Bonding of Cu Powder under Repetitive Unidirectional Friction

Unidirectional friction experiments on uniaxially compressed pure Cu powder were performed to clarify how the solid-phase interparticle bonding proceeds during a powder molding method by applying biaxial force, termed as a compression shearing method at room temperature. Relations among the applied normal load, number of sliding cycles, and microstructural changes of the powder particles were investigated by morphological and cross-sectional observations of the samples after the friction experiments. The structural observations revealed some layered regions with different microstructures within the sample cross-section. All regions increased in size at a higher applied normal load, but their size did not change as the number of the sliding cycles increased. This phenomenon was quantitatively explained by Hamilton and Goodman's model, which showed that the applied normal load correlated with the stress distribution applied to the sample. Tensile stress applied to the powder particles along the sliding direction appeared to be most effective for interparticle bonding. In addition, it was suggested that the bonding process of the powder particles proceeded through the steps: (1) plastic deformation; (2) initially crystal grain refinement to micrometer size; (3) bonding by local sliding between powder particles; and (4) crystal grain refinement to sub-micrometer size.