Temperature Dependence of Interfacial Shear Strength in SiC‐Fiber‐Reinforced Reaction‐Bonded Silicon Nitride

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Abstract
The interfacial shear strength of AVCO SCS‐6 SiC‐fiber‐reinforced reaction‐bonded Si3N4 (RBSN) composites was studied as a function of temperature. Fiber “push‐through” experiments were conducted with a diamond indenter and a high‐temperature microhardness tester. The interfacial shear strength was variable and depended mostly on interfacial bonding at low temperatures (5 to 18 MPa at room temperature) and frictional forces at high temperatures (12 to 32 MPa at 1300°C). The frictional component is attributed to the surface roughness of the fibers. The interfacial shear strength increased with temperature, because of the relief of residual stresses arising from the thermal expansion mismatch between fiber and matrix. Because of the composite nature of these fibers, a number of interfaces were tested in each experiment. The interface which debonded and slid was not always the same. Interfacial fracture took place either between the two outermost carbon layers of the SCS‐6 fibers, or between the SiC core and the innermost of the two outer carbon layers. The outermost carbon layer of the fiber always stayed bonded to the Si3N4 matrix.