Preparation And Properties Of SiC/MoSi₂ Composites By Infiltration Method

The silicon carbide/molybdenum disilicide（SiC/MoSi₂）composites combine excellent properties with SiC and MoSi₂ materials,such as high temperature resistance,high temperature strength and excellent oxidation resistance.It is a high-temperature structural material with good application prospects.SiC/MoSi₂ composites are prepared by direct infiltration method,which utilize the good physical and chemical compatibility of MoSi₂ and SiC.This method has a short process cycle,low preparation cost,and can economically prepare large-scale products.Studying the process for preparing SiC/MoSi₂composites by infiltration method and the relationship between its properties and process have important guiding significance for the development and application of SiC/MoSi₂composites.In this paper,SiC particles（100⁵00μm）,SiC fine powder（≤65μm）,SiC micro-powder(D₅₀=2.5μm),MoSi₂ micro-powder(D₅₀=3.0μm)were used as raw materials;phenolic resin（PF）was used as binder and Polymethylmethacrylate（PMMA）was used as pore-forming agent.Different series of formulas were designed by regulating the particle composition of SiC raw materials and adding PMMA（E series:no SiC micro-powder;W series:SiC fine powder fixed to 20 wt%;P series:adding PMMA）.SiC preforms with different pore structures were prepared via mixing,mechanical forming,drying and high temperature heat treatment,and then SiC preforms were infiltrated MoSi₂at 1970℃to prepare SiC/MoSi₂ composites.The pore structure of the SiC preforms,infiltration process of MoSi₂,the phase composition,pore structure,micro-structure,flexural strength,thermal shock resistance and oxidation resistance（600℃）of SiC/MoSi₂composites were analyzed.The influence of pore structure on the infiltration behavior of MoSi₂ was analyzed carefully.The main results are as follows:（1）The apparent porosity and pore size distribution of the SiC preforms have an important influence on the infiltration amount of MoSi₂ and the density of the infiltrated sample.The infiltration amount of MoSi₂ which is increasing with the increase of the apparent porosity is closely related to the apparent porosity of the SiC preforms.The density of the infiltrated samples and the filling rate of MoSi₂ are mainly affected by the pore size distribution of the SiC preforms.The pores with the pore size of 1¹0μm in SiC preforms are filled with MoSi₂ melt,while pores with the pore diameter larger than 10μm are difficult to fill with melt.The addition of SiC micro-powder is beneficial to reduce the average pore size of the pores of the SiC preforms and increase the density of the infiltrated samples.In the W series formulation samples,when the SiC micro-powder content is 25 wt%,the infiltrated sample which apparent porosity is 7.3%was the most dense.（2）The high temperature flexural strength of the infiltrated samples of E,W and P series at 1400℃i s significantly higher than the flexural strength at room temperature.A large number of cracks appear in the infiltrated phase MoSi₂ as matrix and some cracks had spread into the SiC aggregates,which damages the normal temperature flexural strength of the SiC/MoSi₂ composites.At 1400℃,MoSi₂ shows metal plasticity,which is beneficial to the healing of cracks and improve the high-temperature flexural strength of SiC/MoSi₂composites.（3）The thermal shock resistance of SiC/MoSi₂ composites is closely related to the thermal shock test temperature and the MoSi₂ brittle-ductile transition temperature（1100℃）.When the experimental temperature of air-cooled thermal shock was1100℃,the residual flexural strength and strength retention rate after thermal shock is better than the thermal shock test temperature of 900℃.（4）Under the oxidation condition of air atmosphere at 600℃,the oxidation resistance of SiC/MoSi₂ was the worst among SiC/MoSi₂,MoSi₂ and R-SiC,while R-SiC is the best.Under the oxidation condition of 600℃air atmosphere,MoSi₂ can form volatile MoO₃ after oxidation,resulting in a small amount of powder on the surface of high-density MoSi2 sample and a large degree of powdering of SiC/MoSi2 sample.It is concluded that the SiC/MoSi₂ composites prepared by the infiltration method can’t be used for a long time under oxidizing atmosphere at 600℃.（5）The apparent porosity of the SiC preforms and the infiltration amount of MoSi₂can be significantly improved by adding the pore former PMMA.The apparent porosity and the infiltration amount of MoSi₂ of the infiltrated samples increase with the increase of the pore former content.The normal temperature flexural strength of the infiltrated samples significantly reduces by the addition of pore former PMMA.When the PMMA content is≤9 wt%,the normal temperature flexural strength of the infiltrated samples decreases significantly as the PMMA content increases.The effect of adding PMMA on the thermal shock resistance of the samples isn’t significant.