Effect of Grain Growth on Hot‐Pressed Optical Magnesium Fluoride Ceramics

An optical MgF2 ceramic was hot-pressed at 838 to 983 K with 241-MPa pressure. The maximum mechanical strength (>180 MPa) developed in the resulting ceramic within the grain-growth range μ/μ0= 7 to 10, where μ is the grain size of the hot-pressed article and μ0 that of the starting powder. In the early stages of grain growth (μ/μ0<7), however, mechanical strength was directly proportional to grain size, which appeared to contradict the expectations of the Petch and Knudsen relations. Optical transmission was inversely proportional to grain size, especially in the visible wavelength. This study thus demonstrates a particular case in which increasing grain size promotes higher mechanical strength but lower optical transmission.