Silicon nitride granule friction measurements with an atomic force microscope: effect of humidity and binder concentration

This study introduces the atomic force microscope (AFM) for direct measurement of internal and external friction in ceramic powder pressing. The friction measurements were performed between two single granules, and a granule and a hard metal substrate as a function of granule binder concentration, relative humidity and sliding velocity. We found that the friction coefficient decreased with increasing humidity for a specific binder concentration, the effect being more pronounced for low concentrations. The friction coefficient also decreased with increasing binder concentration. Evaluation of the adhesion force showed a steady increase with both humidity and binder concentration. A substantial difference, more than an order of magnitude, was seen for the highest binder concentration at low and high humidities. We attribute these findings to the hygroscopic nature of the binder, poly(ethylene glycol) (PEG). Softening of the PEG at increasing humidities lowers the friction coefficient but increases the adhesion force. The results are consistent with flowability and angle of friction measurements.