Effect of La2O3 addition to modification of grain-boundary phase in MoSi2

MoSi2 with additions of 1-4 wt.% La2O3 were prepared by spark plasma sintering (SPS). The influence of La2O3 addition on the crystallization and chemical composition of the grain-boundary phase has been studied. It is found that the siliceous grain-boundary phase in MoSi2 is eliminated completely when the La2O3/SiO2 ratio is approximately 3.2 by weight. The siliceous grain-boundary phase reacts with La2O3 addition forming a single crystalline La2Si2O7. In the case of excess addition of La2O3, there are two polymorphs of lanthanum silicates at trigonal grain boundaries. The main difference of the two polymorphs is the content of elemental La and Si. (c) 2008 Elsevier B.V. All rights reserved.