For bonding silicon carbide optics, which require extreme stability, hydroxide catalysis bonding is considered [Rowan, S., Hough, J. and Elliffe, E., Silicon carbide bonding. UK Patent 040 7953.9, 2004. Please contact Mr. D. Whiteford for further information: D.Whiteford@admin.gla.ac.uk]. This technique is already used for bonding silicate-based materials, like fused silica and Zerodur. In application with silicon carbide, the technique is highly experimental and the aim is to test the strength of the bond with silicon carbide. The silicon carbide is polished to λ/10 PV flatness and then oxidized at 1100 • C in a wet environment prior to bonding to form a necessary layer of SiO 2 on the surface. The bonding is performed in clean room conditions. After bonding the pieces are sawed into bars to determine the strength in a four-point bending experiment. The oxidization process shows many different color changes indicating thickness variations and contamination of the oxidization process. The bonding has been performed with success. However, these bonds are not resistant against aqueous cooling fluids, which are used during sawing. Several bars have survived the sawing and a maximum strength of 30 N mm −2 has been measured.

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Calcium hydroxyapatite bioceramic was toughened by preparing composites with silver particles as reinforcements. The composites were fabricated from hydroxyapatite and silver oxide raw powders. The sintering behaviour was investigated using dilatometry. An optimized sintering programme was designed to promote densification and to suppress the decomposition of the hydroxyapatite matrix and the evaporation of silver. High density was achieved on both small cylinder samples and large block samples by pressureless sintering. The density of the composites is over 92.4% theoretical with silver inclusions up to 30 vol %. The strength of the composites is greater than 80 MPa as tested by four-point bending. Silver inclusions improve the toughness consistently, from 0.70 MPa m 1/2 for the monolithic hydroxyapatite to 2.45 MPa m 1/2 at 30 vol % silver. Studies on the toughening mechanism indicate that crack bridging and subsequent plastic work of silver are mostly responsible for the toughening, whereas crack deflection also makes some contribution.

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