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Mechanical Characterization of Si3N4-BN(pl) Composites

The solid lubrication properties of BN led to the development of Si3N4BN composites, aiming the production of self-lubricated high performance materials. Composites containing 10, 18 and 25vol% of hexagonal BN were hot-pressed at 1650°C, 25MPa. The microstructure was characterized by SEM, X-ray diffraction and stereologic image analysis. Vickers indentations were performed to evaluate hardness and indentation fracture toughness. The hardness ranged from HV=12.9GPa to HV=2.1GPa with increasing BN amount. For a given amount of BN, hardness is higher in the surfaces parallel to the hot-pressing direction, as the basal planes of the BN platelets tend to align perpendicularly to this direction. The anisotropy in the microstructure also affects the values of KIc, since the cracks deflect, aligning themselves with the platelets, or, in some cases, they cannot even propagate. When the propagation direction of the cracks is not perpendicular to the basal planes of the platelets, the fracture toughness does not change significantly with the amount or orientation of BN, having values between KIc=3.5MPa.m 1/2 and KIc=5.9MPa.m 1/2 .