Effect of MoSi2 addition on densification and properties of ultra high temperature ceramics produced by pressureless sintering

摘要

Ultra high temperature ceramics are a class of material of great interest thanks to their combination of engineering properties such as high melting point, good thermochemical and thermomechanical properties in aggressive environment. Potential applications of these materials are forecast in aerospace industry and in industrial sectors like metallurgy, energy and power production.. To ensure that good properties are attained, control of densification and microstructure is necessary. Because of the high melting point of these compounds (ZrB2: 3245?C, HfB2: 3250?C, ZrC: 3540, HfC: 3890?C), these materials need pressure-assisted sintering procedure and temperature above 2000?C. It has been proved that the addition of ceramics like Si3N4, HfN and SiC can help the densification and allows less hard sintering conditions. This work aims to evaluate the influence of the addition of 5-20 vol% of MoSi2 on sinterability and properties of ultra high temperature ceramics, namely HfB2 and HfC. Sintering behaviour, microstructure and mechanical properties are examined and discussed. Dense composites were obtained by pressureless sintering at 1950?C. The final microstructures were fine and uniform (mean grain size: 1.5- 3 m) and a small amount of residual porosity was observed. By means of scanning electron microscope and energy dispersive spectroscopy secondary phases were detected: in the HfB2-system traces of HfC and HfO2; in the HfC-system a mixed product containing Hf, Si, Mo, indicating mutual solubility between the starting phases. The following mechanical properties were measured: Vickers hardness, Youngu27s modulus, fracture toughness by chevron notched beam method in flexure and 4-pt bending strength at room temperature, 1200 and 1500?C. Hardness ranged from 15 to 18 MPa; Youngu27s modulus values were in the range of 400-480 GPa thanks to the stiffness of the present phases; values of toughness were in agreement with those found in literature. Flexural strength values at room temperature were in the range of 380-570 MPa. The boride composites retained excellent strength values (~580 MPa) up to 1500?C. High temperature behaviour in aggressive environment was evaluated by plasma torch tests at temperature up to 2000?C on 5vol% MoSi2 containing sample