Green state joining of silicon carbide using polymer precursors
Green state joining of structural ceramics has shown its potential for forming large, complex components. In this work, methods of green state joining of silicon carbide using polymer precursors are explored. Pastes containing either polycarbosilane (PCS) or Allylhydridopolycarbosilane (AHPCS) are used in the joining process. It has been demonstrated that joints formed under external pressure using PCS containing pastes are indistinguishable from the matrix. The Flexural strength of joined samples having a theoretical density (TD) of 94% is measured to be 312 MPa, which is comparable to that of the control sample with similar density. Unfortunately, applied pressure is necessary for achieving thin, defect-free joints, which might limit wider application of the method. In order to eliminate applied pressure in the joining process, a method of joining SiC using AHPCS containing pastes is investigated. It is discovered that, using AHPCS containing pastes with appropriate SiC addition, crack-free joints can be obtained without applied pressure. Optimum SiC loading in the paste was determined to be in the range of 25 to 35vol%. Joints formed by AHPCS+(SiC+5wt%B) paste are essentially indistinguishable from the matrix and have an average strength of 323 MPa, comparable to that of the control sample. For better control over the quality of the joints, x-ray tomography has been used to investigate the density variations in SiC joints. The results indicate that the green density of the joints, formed by PCS containing pastes, increases from 54% to 66%TD as the applied pressure increases from ambient to 138 MPa. Also, a minimum density difference between the matrix and joint in the green state is achieved when the PCS content in the pastes is around 50vol%. Furthermore, regions with higher densities are observed at the joint/matrix interfaces, suggesting that the polymer precursor infiltrates into the matrix during the joining process using AHPCS containing pastes. In this work, a method for producing SiC whisker in situ in monolithic mixture of AHPCS and SiC is also explored.