Fabrication and testing of oxide/oxide microcomposites with monazite and hibonite as interlayers

Oxide/oxide microcomposites were fabricated and tested to evaluate the effe ctiveness of monazite (LaPO4) and hibonite (CaAl12O19) as interlayers in sa pphire-reinforced Al2O3-matrix composites. For interlayer thicknesses of 0. 3-0.5 mu m, both interlayers showed evidence of crack deflection; however, debond lengths in hibonite-coated specimens were limited to just a small fr action of the fiber diameter. Monazite-coated specimens showed multiple mat rix cracks and extensive debonding at the coating/matrix interface. Composi te strengths were relatively high for both coatings, considering the fiber strength degradation during processing, The strengths were greater than the calculated matrix cracking stresses. However, the mean strengths were not significantly different from those of the control specimens, although coate d composites had higher Weibull moduli. The lack of difference in strength is attributed to porosity in the matrix, The results imply that matrix dens ity needs to be >85% to evaluate novel interface strategies reliably.