Why the ball to powder ratio (BPR) is insufficient for describing the mechanical ball milling process

The ball to powder ratio (BPR) is a processing parameter that is frequently used in both mechanical (ball) milling and mechanical alloying. A number of recent studies provided the BPR as a principal milling parameter while neglecting other parameters, such the vial volume, the diameter and quantity of milling balls and the powder mass. In this experiment, different batches of magnesium hydride powder were milled using varying ball size, powder mass, and other parameters and a constant BPR. The hydrogen desorption properties (i.e., differential scanning calorimeter) and phase evolution (i.e., XRD phase analysis) of the milled powders were subsequently investigated. The obtained results demonstrated that the BPR cannot be provided as a single processing parameter. The DSC curves obtained during decomposition with a scanning rate of 5 °C/min revealed significant differences in desorption peak temperature among the samples milled using the same BPR. Additionally, XRD patterns revealed that the crystallite size after milling varied, suggesting that differences existed in the effectiveness of the milling process.