Influence of alumina content on the nucleation crystallization and microstructure of barium fluorphlogopite glass-ceramics based on 8SiO2·y Al2O3·4MgO·2MgF2·BaO Part II Microstructure, microhardness and machinability

The influence of alumina content, heat treatment time and temperature on the microstructure, hardness and theoretical machinability of barium fluorphlogopite glass-ceramics were investigated. The glass-ceramics were based on glasses of the general composition 8SiO₂-yAl₂O₃-3.75MgO-2.25MgF₂-BaO. Glasses with high alumina contents with y > 2.0 were found to give fine feathery microstructures that did not coarsen readily to give the classic house of cards microstructure associated with machinability. The glass with the lowest alumina content (y = 1.5) that gave a house of cards microstructure was therefore investigated in detail. The Vickers hardness decreased slightly on formation of the barium fluorphlogopite phase, but decreased significantly on forming an interconnected house of cards microstructure. Machinability was highly dependent on the formation of an interconnected microstructure.