PARTICLE COAGULATION AND ITS COATING ON BALL SURFACES AND MILL CONTAINER WALL DURING TUMBLER-BALL MILLING OF CU POWDER

A milling experiment was carried out to investigate the particle coagulation and coating and its spalling-off, which are characteristic to mechanical alloying of ductile powders, using Cu powder as a model material. The milling process was studied by microscopic observations of the morphological evolution of the milled powder and kinetic and dynamic considerations. The coagulation and coating during milling were analyzed by the Gillespie-Langstroth equation. The analysis showed the rate equation consisted of the coagulation and coating rate terms, and the contribution of the coating increased with milling time. An analysis on the basis of the Hertz and Timoshenko ball impact theory showed that disk-shaped particles, which are the main constituents in the intermediate stage of milling, were formed by breaking-off of the coated films due to tensile stress generated by the impacting action of the milling balls. The experimental observations on the particle diameter and the thickness of the disk-shaped particles were in good agreement with the theoretical estimations.