The effects of heat treatment and coating roughness on the strength of alumina-zirconia fibres

The strengths of Al2O3–20 wt.% ZrO2 fibres as-received, heat treated and coated with SnO2 have been determined. The mean strength of as-received fibres was 1380 MPa for a gage length of 17 mm. The strengths of fibres heat treated at 500, 600 and 900 °C decreased by 5, 7 and 21%, respectively, relative to the as-received fibres. This could be because of a secondary glassy phase formation at the grain boundaries. It was found that the accuracy of the tensile strength distribution increased with the number of samples tested and at a sample number of 80, the variation in the tensile strength was about 2%. The thickness and roughness of the SnO2 coating had a profound effect on the strength of the coated fibres. It was observed that as the coating thickness increased, roughness of the coating increased. This acted as a notch in order to decrease the strength of the fibres. Control of roughness at the fibre/coating and/or coating/matrix interphase is very important in the development of tough ceramic–matrix composites. As the interphase roughness increases, fibre debonding and pull-out decrease because of the increased compressive clamping stress. Hence for increased toughness in CMCs through fibre debonding and pull-out mechanisms, a relatively smooth interphase is necessary. The interphase roughness can be controlled either by selecting smooth fibres or by decreasing coating roughness.