Transformation toughening of MoSi_2 with incorporation of PSZ by means of CVI

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This study was intended to fabricate high strength ZrO_2/MoSi_2 composites, which took advantage of transformation toughening. Chemical-Vapour-Infiltration(CVI) method was adopted for incorporation of ZrO_2 into partially sintered MoSi_2. In order to obtain fundamental knowledge for fabrication process and the microstructure of the composites, ZrO_2 were deposited with no additive oxides. Microstructures of post-annealed ZrO_2 were observed and the optimum annealing conditions for realizing transformation toughening were discussed. The following results were obtained.1) Fine MoSi_2 grains were well-covered with CVI-ZrO_2 by using Metal-Organic(MO) precursor.2) The infiltrated ZrO_2 were all amorphous, therefore post-annealing treatment was required. The post annealed ZrO_2 at 1073K for 3.6ks in air consisted of equi-axed grains of several 10nm in diameter. Existence of monoclinic ZrO_2 was observed and this microstructure suggests the limitation of annealing conditions.3) Hardness increase was realized by increase of ZrO_2 into MoSi_2 and maximum hardness of 20GPa was obtained in the composites. High toughness was recognized by Vickers indentation and occurrence of transformation toughening was realized.