Studies on the Fabribation, Microstructure and Mechanical Properties of Mo/Al2O3 Ceramic Composites.

• Main Authors: Wang, Sheng-Chang, 王聖璋
• Other Authors: Wei Wen-Cheng J.
• Format: Others
• Language: zh-TW
• Published: 1997
• Online Access: http://ndltd.ncl.edu.tw/handle/24473470669565779790

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 85 === AbstractThis study employed a process starting with MoO3 powder which was dissolved in ammonia solution to produced ammonium molybdate. The molybdate ions was adsorbed on the Al2O3 powder surface in suspension. Then the suspension was spray-drying to form composite powder, followed a reduction in H2 atmosphere and a hot-pressing to get the composites with various volume fractions of Mo. The phases and Mo grain size of the spray-dried powders and sintered specimens were characterized by X-ray and measured. The microstructure and atomic interface of the specimens were studied by using electron microscopes (including SEM, TEM and HRTEM). The fracture strength, toughness and hardness of densified composites were measured and reported. Results revel that the suspensions could produce the sphere shape ammonium molybdate/alumina granules with the spray-drying technique. The size distribution of the granules is between 1~20 mm. Using 700(C reducing temperature in H2 atmosphere for 3 hr or 900(C for 1 hr could completely reduce molybdate phase to metallic Mo. The Mo grains grow during H2 reduction. The grain growth kinetic index of Mo and the activation energy of the grain growth was determined to be n= 8 and Q=200 kJ/mol, respectively.After hot pressing at 25MPa, 1400(C for 1 hr in vacuum , the relative density of the composites could achieve 99.4% T.D.. Intergranular Mo grains in micrometer-size are dispersed at the grain boundary of Al2O3, compared with those with nanometer-size are found at the intragranules of matrix Al2 O3. There was no any grain boundary phase found at the Mo/Al2O3 interface from the characterization of high resolution atomic image. Nearly coherent, (332)Mo//(012)Al2O3, and non-coherent interfaces were also observed on the interface of intragranular Mo and Al2O3. The intergranular Mo grains retarded the movement of the grain boundaries of Al2O3 during densification, and finally reduced the grain size of Al2O3. The relation between the grain size (G) of Al2O3 and volume fraction of Mo could be expressed as G= f^ -0.085, different from the prediction by a Zener modal or topological modal. The fracture strength of Mo/ Al2O3 composites increased by the increase of Mo content. The maximum strength of the composites was 26% greater than a monolithic alumina. The strengthen mechanism of this composites was mainly a refining of matrix grain due to the addition of Mo grains. The relationship of the fracture strength and grain size (d) can be expressed as : (f=384 d^ -0.35 (MPa) . The toughness of the composites increased by the addition of Mo, and had 32% improvement. The toughening mechanism of the composites are the combination of cracks deflection, crack bridge and the pull-out of metallic grains. The hardness of the composites was higher than the prediction by the rule of mixture. The maximum hardness is the composite with 1 vol% Mo and is higher than that of a monolithic alumina for about 17% increment. The reasons for a harden composite could be the effects of thermal stresses and the refinement of matrix grain size.The sintered density and strength of the ceramic composites made by mechanically mixing or hot-plate drying are lower than that made by the chemical co- precipitation. But the toughness of the composite made by mechanical mixing or hot-plate drying is better. Keywords: Alumina, Molybdenum, composites, grain growth, microstructure, mechanical properties.