L. Zhang, V.D. Krstic/Theor Applied Fracture Mechanics 24(1995)13-19 Fig. 7. Change of apparent fracture toughness with graphite layer thickness. Fig. 5. Polished section of the laminated SiC ceramics showing the graphite layer to form the Sic according to the crack path This maximum in fracture toughness, at particu SiO+2C→SiC+CO (4) lar SiC layer thickness, occurs in all sample Clearly, the extent of graphite conversion to where the ratio of SiC layer thickness to graphite Sic depends on the relative amount of carbon in layer thickness is around 30. It is believed that the graphite and the amount of Sio liberated this behaviour is associated with the extent of from the Sic layer. The amount of Sio formed graphite conversion to SiC which in turn controls depends, in turn, on the SiC layer thickness the strength of the interface. Because of the Thus, there will be some optimum SiC layer afinity between the carbon from the silicon car- thickness capable of producing sufficient amount bide and oxygen from the alumina sintering aid of Sio to completely convert graphite into SiC volatile Al,O component is formed [10] and form an optimum interfacial strength. Based SiC Al2O3+ Al20+ Sio + CO on this argument, one can predict that the higher As it forms, the Sio component diffuses the graphite layer thickness, the thicker the SiC through the sample and reacts with carbon from layer thickness is required to achieve an optimum interfacial strength. Typical micrograph of graphite layers before and after conversion is shown in Fig. 8. Inspection of Fig. 8. shows that before sintering, the graphite layers are in the form of network while after sintering the inter face is full of large and uniform size hexagona a-SiC crystals. X-ray analysis confirmed that these were indeed a-SiC crystals( Fig 9). Another important characteristic of this new class of SiC materials is the notch width insensi- tivity seldom found in brittle materials. Fig. 10 shows the variation of apparent fracture tough ness with notch width for monolithic and lami- nated SiC. As expected, a continuous decrease in Fig. 6. Measured variation of apparent fracture toughness acture toughness with decrease in notch width with average Sic layer thickness. was observed in monolithic SiC, whereas in the16 L. Zhang, V.D. Krstic / Theoretical and Applied Fracture Mechanics 24 (1995) 13-19 ] • t : l,r.! ] ,~,l i i :: t •1 ] Fig. 7. Change of apparent fracture toughness with graphite layer thickness. Fig. 5. Polished section of the laminated SiC ceramics showing the crack path. This maximum in fracture toughness, at particu￾lar SiC layer thickness, occurs in all samples where the ratio of SiC layer thickness to graphite layer thickness is around 30. It is believed that this behaviour is associated with the extent of graphite conversion to SiC which in turn controls the strength of the interface. Because of the afinity between the carbon from the silicon car￾bide and oxygen from the alumina sintering aid, a volatile Al20 component is formed [10]: SiC + A1203 "-~ A]20 -t- SiO + CO. (3) As it forms, the SiO component diffuses through the sample and reacts with carbon from L ;? i: : i i : 1 i S ; Fig. 6. Measured variation of apparent fracture toughness with average SiC layer thickness. the graphite layer to form the SiC according to the reaction: SiO + 2C ~ SiC + CO. (4) Clearly, the extent of graphite conversion to SiC depends on the relative amount of carbon in the graphite and the amount of SiO liberated from the SiC layer. The amount of SiO formed depends, in turn, on the SiC layer thickness, Thus, there will be some optimum SiC layer thickness capable of producing sufficient amount of SiO to completely convert graphite into SiC and form an optimum interracial strength. Based on this argument, one can predict that the higher the graphite layer thickness, the thicker the SiC layer thickness is required to achieve an optimum interracial strength. Typical micrograph of graphite layers before and after conversion is shown in Fig. 8. Inspection of Fig. 8. shows that, before sintering, the graphite layers are in the form of network while after sintering the inter￾face is full of large and uniform size hexagonal a-SiC crystals. X-ray analysis confirmed that these were indeed a-SiC crystals (Fig. 9). Another important characteristic of this new class of SiC materials is the notch width insensi￾tivity seldom found in brittle materials. Fig. 10, shows the variation of apparent fracture tough￾ness with notch width for monolithic and lami￾nated SiC. As expected, a continuous decrease in fracture toughness with decrease in notch width was observed in monolithic SiC, whereas in the