R-curve behavior of Si3N4 whisker-reinforced Si3N4 matrix composites

Three kinds of β-Si3N4 whisker-reinforced Si3N4 composites with different degree of whisker alignment were prepared by using tape casting, extrusion and hot pressing processes. Among these composites, one is obtained from a laminate with tapes, another is from a laminate stacked alternatively with tapes and extruded wires, and the last one is closely stacked with wires. The same whisker orientation was followed with all the tapes or wires. Four-point bending tests by stepped loading were employed to determine R-curves for the three materials. The results show that all the materials have better R-curve behaviors than composites with randomly oriented whiskers. The fracture toughness of the three materials are, respectively, 11.18, 11.44 and 10.60 MPa m1/2, and the corresponding threshold values for stable crack propagation are 9.14, 10.92 and 9.32 MPa m1/2, respectively.