Preparation Of Biomorphic SiC Ceramic Matrix Composites By Reactive Melt Infiltration From Rice Husks And Their Structure And Mechanical Properties

Biomass SiC ceramics possess some superior properties of the normal SiC ceramics, such as low density, high hardness, and good resistance to oxidation, corrosion and wear. Besides, they have the advantges of wide source of raw materials, low cost and bioactive. They are promising materials in many industry fileds such as mechanical eningeering, chemical engineering, catalysis and biomaterials. They are environmental-friendly materials which have been attached great attentions nowadays. In this thesis, rice husks (RHs) have been used as the main raw material to prepare SiC ceramic matrix composites with different structure by reactive melt infiltration using Si and FeSi2as infiltrates. The RHs were coked or/and further pyrolysed prior to be used to fabricate the preforms for the infiltration. SiC (with average size of0.5μm) and Mo powders were selected added in the preforms. The effects of the carbon thermal reduction parameters on the structure and morphologies of the pyrolysed RHs, and the effects of the infiltration temperatures, the addition of the SiC and Mo in the preform on the structure and mechanical properties of the SiC ceramic matrix composites were systematically investigated by means of X-ray diffraction/Rietveld, scanning electron microscopy, energy dispersive spectrometry, transmission electron microscope, elemental analysis, etc. and Vickers diamond pyramid indentation method, ultrasonic technique and three-point flexure strength method. The relation between the microstructure and mechanical properties was studied, and the fracture mechanisms were also discussed.The results show that RHs first coked at900℃for2h have been pyrolysed into β-SiC whiskers, particles and carbon mixture after carbon thermal reduction at1550℃for6h under flowing Ar. The average diameter of the as-obtained SiC whiskers was about200nm and length of several microns. The content of the free carbon in the pyrolyzed RHs estimated by a burning method was ca.65wt.%. Dense SiC ceramic composite with SiC whiskers and fine SiC particles (SiCw/SiC-Si) has been fabricated by liquid silicon infiltration (LSI) using the pyrolyzed RHs, which were milled and modeled to preform for the infiltration.The SiC whiskers in the preform were well maintained in the composite and acted as reinforcement for the composites. The free carbon in the preform reacted with the molten Si, forming newly fine SiC particles. Dense composite was obtained at an infiltration temperature of1550℃and an infiltration time of1h. The composite possesses superior mechanical properties. The Vickers hardness, flexure strength, elastic modulus, and fracture toughness of the composite were18.8±0.6GPa,354GPa,450±40MPa and3.5±0.3MPa m1/2, respectively. But the lower infiltration temperature of1450℃and the higher infiltration temperature of1550℃all lowered the mechanical properties, as the former resulted in an insufficient infiltration, and the latter resulted in the over evaporation of Si, which all led to the formation of unreacted carbon.The densification of the SiCw/SiC-Si composite has been improved by the addition of external SiC powders in the pyrolyzed preforms. With the increasing amounts of the addition of SiC in the preforms, the sizes of the SiC particles and the Si phase are reduced, and the distribution of the two phases is more homogenous. The bending strength of the composite decreases slightly with20wt.%SiC addition in the preform, and then the strength increases gradually with the addition of SiC, reaching a maximum value of590MPa when the amount of SiC addition is80wt.%. The fracture toughness of the composite decreases slightly as20wt.%of SiC is added, and then increases to4.0±0.3MPa m1/2when the amount of SiC addition raised to40wt.%. The fracture toughness dropped to3.4MPa ml/2as the the amount of SiC addition increases to60and80wt.%. The hardness of the composites approached the highest value of21.3GPa with20wt.%SiC addition and then decreased as the SiC addition further increases, being17.3GPa when the addition of SiC is80wt.%. The content of the SiCw, the particle size and the distribution of SiC and the content of residual Si all affect the mechanical properties of the composites.SiC ceramic matrix composites containing SiCw and MoSi2phase (SiCw/MoSi2-SiC) are successfully fabricated by LSI method from preforms of pyrolyzed RHs with Mo addition in contents of10-30wt.%. The Mo in the preform reacted with molten Si during infiltration, forming MoSi2in the composites. The present study provides an effective method to fabricate SiCw/MoSi2-SiC composite at comparatively low temperature. The introduction of MoSi2in the composite increases the elastic modulus and the fracture toughness, but lowers the flexure strength. The content of MoSi2in ca.11wt.%provides the highest value of4.1MPa m1/2for the fracture toughness of the composite. But too low amount of MoSi2provides no effective effect on the fracture toughness and too high amount of MoSi2caused too high residual stress in the composite, which also lowers the fracture toughness. RHs coked at900℃mixed with extra SiC powder in various amounts up to45wt.%were used to prepare preforms which were further pyrolysed at1550℃for6h and then were infiltrated with FeSi2alloy. SiC-based Fe-Si intermetallic composites (SiC/Fe-Si) composed of SiC, FeSi2and FeSi are obtained. The free carbon in the preform reacted with Si in the molten FeSi2, forming SiC. Due to the consumption of Si in the FeSi2alloy in the reaction process, part of the FeSi2alloy transformed into FeSi after the solidifaction of the molten infiltrate. With the increasing amounts of SiC addition in the preforms, the amount of FeSi2increases and that of FeSi decreases. Vickers hardness, elastic modulus, three-point flexure strength and fracture toughness of the composites all increase with SiC additions up to30wt.%in the preforms, reaching the values of18.2GPa,290GPa,213MPa and4.9MPa m1/2, respectively. Among them, the fracture toughness of the composite is more superior compared with that of the common reactive infiltration SiC ceramics. The SiC particles are fine and well dispersed in the composites. With the SiC addition further raised to45wt.%, the mechanical properties turned down probably due to high residual stress and hence the more intense induction of microcracks in the composite caused by the increased amount difference between FeSi2and FeSi, which have large difference in the thermal expansion coefficients between. Compared with Si infiltrate, the Fe-Si alloys possess higher toughness, resulting in the enhanced fracture toughness of the composites.For the SiCw/SiC-Si composite prepared from preforms with pyrolyzed RHs, the SiC whiskers reinforced the composites. The existence of the large amounts of the pullout of the SiC whiskers and the de-bonding of fine SiC from the Si phase all favor the improvement of the fracture toughness of the composites. However, transgranular fracture also exists in partial of the large SiC particles and Si area. For the SiCw/MoSi2-SiC composite, the introduction of suitable amount of MoSi2results in an appropriate residual stress, which plays an effective role on the improvement of the toughness. The mechanism of the enhanced toughness of the SiC/Fe-Si composite is suggested to be the de-bonding of the fine SiC particles from the Fe-Si alloys, the crack deflection along the surfaces of fine SiC particles and crack bridging of the Fe-Si phases.