J.x. Zhang et al. /Ceramics International 30(2004)697-703 content(Fig. 5). Cracks are deflected in the interfacial layers rience a transition from graceful to catastrophic mode: de- until the Sic content is increased to 80 vol. lamination may still occur, but the failure of the subsequent The micrographs of SiC laminates are shown in Fig. 6. lamina will be determined by the delamination cracks across It is observed that some cracks in SiC layers initiate from the interface. In this case, the mechanical properties of Sic the growth of defects within the sic lamina, while other laminates are irrelevant to the thickness of the interfacial cracks from the growth of the delaminaton cracks propagat layers too. Further reduction in the thickness of interfacial ing along the interfacial layers. The latter would eliminate layers may result in a monolithic fracture mode. Clegg [6 the fracture toughness and WOF although graceful failure related this kind of behavior to the existence of gaps in the may still occur. During the failure process, frictional sliding interfacial layers. However, this explanation is not applica- at the debonded interface may also take effect [ 35], which ble to our case cannot be detected from the optical micrographs( Fig. 6) The influence of interfacial composition on the mechani- cal properties of SiC laminates can also be well correlated to 3.3. Diffusion effect the interfacial diffusion mechanism. The higher the content of c in interfacial layers, the weaker the interfacial layers As proposed by Cook and Gordon [2] and Clegg et al. after diffusion, consequently, the lower the strength. This 5, 61, the weak interfacial layers were deliberately intro- explanation is also applicable to the fracture toughness and duced as a toughening mechanism for deflecting growin work of fracture racks. Studies on the fracture behavior of laminated com- posites also showed that the critical ratio of strength and 4. Conclusion apparent toughness between interfacial layers and the adja cent matrix layers for crack deflection were relatively low The mechanical properties of Sic laminates depend on [2, 7, 8, 25, 26, 28, 31,32]. So it can be proposed that the inter- the thickness and composition of interfacial layers. For facial layers should be as thin as possible as long as these the interfacial composition of 50 vol % Sic +50 vol %C cracks could be deflected. However, specimens with the in- the strength, work of fracture and fracture toughness was terfacial thickness as 3 um did not show ure behavior (see Fig. 2).This might be due to the diffusion tively. Based on SEM and EDX observation, this depen- effects During sintering or other high temperature process, dence is mainly due to the diffusion of elements(Al,Y,si, elements in adjacent layers will diffuse to the interfacial etc. )from adjacent SiC layers layers due to the composition difference. In this case, the residual stress will not be a significant factor due to the very limited strength and thickness of interfacial layers Acknowledgements To observe clearly the diffusion phenomena, samples The authors were grateful to the Science and see Fig. 7(aH(e). A schematic illustration of the diffusion ogy Committee of Shanghai for providing support under the phenomena is shown in Fig. 7(a) contract number 02Dj14065 As shown in Fig. 7. the distribution of Al. Y. Si etc. el- ements across the interfacial layers exhibits a similar trend References suggesting some diffusion of these elements from adjacent SiC layers. However, due to the low content of sintering dditives(AlO3 and Y2O3), the EDX curves show a high [I A.G. Evans, Perspective on the development of high-toughness ce- ramics, J. Am. Ceram Soc. 73(2)(1990)187-206. undulation near the interface. It is difficult to characterize [2]J. Cook, J.E. Gordon, A mechanism for the control of crack prop- exactly the diffusion distance of elements(Al,Y, Si, etc. )in ion in all-brittle materials, Proc. R. Soc. Lond. A282(1964) nterfacial layers. Based on estimation from Fig. 7, the dif- 508-520. fusion distance should be less than 10 um. Assuming 10 um 3IV.J. Laraia, A.H. Heuer, Novel composite microstructure and me- as the distance for diffusion of si. al and y into the interfa chanical behavior of mollusk shell, J. Am. Ceram. Soc. 72(11) (1989)2177-2179 cial layers, then the proper interfacial layer thickness should [4]J D. Currey, A.J. Ko ture in the crossed-lamellar structure of be about 20 um. For thicker interfacial layers(>>20 um) us shells, J. Mat such diffusion has negligible effect on the mechanical prop- 5] WJ. Clegg, K. Kene alford T.W. Button, J.D. Birchall, A erties of interfacial layers, SiC laminates might exhibit a cs, Nature(London)347(1990) 455-457 pronounced interfacial delamination failure behavior though on and failure of laminar ceramic compos- the mechanical properties are largely reduced. In this case ites, Acta Metall. Mater. 40(11)(1992)3085-3093 he mechanical properties of Sic laminates is irrelevant to [7AJ. Phillipps, wJ T.W. Clyne, Fracture behavior of ceramic the thickness of the interfacial layers. On the other hand laminates in bending-l. Modeling of crack propagation, Acta Metall. for very thin interfacial layers(<20 um), the diffusion of Mater..41(3)(1993)805-817 elements(Al, Y, Si, etc. )will make the interfacial layers [8]AJ. Phillipps, WJ. Clegg, T.w. Clyne, The correlation of interfacial d macroscopic toughness in SiC laminates, Composites 24(2) strong, therefore, the failure of Sic laminates may expe (1993)166-176702 J.X. Zhang et al. / Ceramics International 30 (2004) 697–703 content (Fig. 5). Cracks are deflected in the interfacial layers until the SiC content is increased to 80 vol.%. The micrographs of SiC laminates are shown in Fig. 6. It is observed that some cracks in SiC layers initiate from the growth of defects within the SiC lamina, while other cracks from the growth of the delaminaton cracks propagat￾ing along the interfacial layers. The latter would eliminate the fracture toughness and WOF although graceful failure may still occur. During the failure process, frictional sliding at the debonded interface may also take effect [35], which cannot be detected from the optical micrographs (Fig. 6). 3.3. Diffusion effect As proposed by Cook and Gordon [2] and Clegg et al. [5,6], the weak interfacial layers were deliberately intro￾duced as a toughening mechanism for deflecting growing cracks. Studies on the fracture behavior of laminated com￾posites also showed that the critical ratio of strength and apparent toughness between interfacial layers and the adja￾cent matrix layers for crack deflection were relatively low [2,7,8,25,26,28,31,32]. So it can be proposed that the inter￾facial layers should be as thin as possible as long as these cracks could be deflected. However, specimens with the in￾terfacial thickness as ∼3
m did not show the graceful fail￾ure behavior (see Fig. 2). This might be due to the diffusion effects. During sintering or other high temperature process, elements in adjacent layers will diffuse to the interfacial layers due to the composition difference. In this case, the residual stress will not be a significant factor due to the very limited strength and thickness of interfacial layers. To observe clearly the diffusion phenomena, samples with thick interfacial layers are characterized by EDX, see Fig. 7(a)–(e). A schematic illustration of the diffusion phenomena is shown in Fig. 7(a). As shown in Fig. 7, the distribution of Al, Y, Si, etc. el￾ements across the interfacial layers exhibits a similar trend, suggesting some diffusion of these elements from adjacent SiC layers. However, due to the low content of sintering additives (Al2O3 and Y2O3), the EDX curves show a high undulation near the interface. It is difficult to characterize exactly the diffusion distance of elements (Al, Y, Si, etc.) in interfacial layers. Based on estimation from Fig. 7, the dif￾fusion distance should be less than 10
m. Assuming 10
m as the distance for diffusion of Si, Al, and Y into the interfa￾cial layers, then the proper interfacial layer thickness should be about 20
m. For thicker interfacial layers ( 20
m), such diffusion has negligible effect on the mechanical prop￾erties of interfacial layers, SiC laminates might exhibit a pronounced interfacial delamination failure behavior though the mechanical properties are largely reduced. In this case, the mechanical properties of SiC laminates is irrelevant to the thickness of the interfacial layers. On the other hand, for very thin interfacial layers ( 20
m), the diffusion of elements (Al, Y, Si, etc.) will make the interfacial layers “strong,” therefore, the failure of SiC laminates may expe￾rience a transition from graceful to catastrophic mode: de￾lamination may still occur, but the failure of the subsequent lamina will be determined by the delamination cracks across the interface. In this case, the mechanical properties of SiC laminates are irrelevant to the thickness of the interfacial layers too. Further reduction in the thickness of interfacial layers may result in a monolithic fracture mode. Clegg [6] related this kind of behavior to the existence of gaps in the interfacial layers. However, this explanation is not applica￾ble to our case. The influence of interfacial composition on the mechani￾cal properties of SiC laminates can also be well correlated to the interfacial diffusion mechanism. The higher the content of C in interfacial layers, the weaker the interfacial layers after diffusion, consequently, the lower the strength. This explanation is also applicable to the fracture toughness and work of fracture. 4. Conclusion The mechanical properties of SiC laminates depend on the thickness and composition of interfacial layers. For the interfacial composition of 50 vol.% SiC + 50 vol.% C, the strength, work of fracture and fracture toughness was 580 MPa, 4282 J·m−2, and KIC = 10.8 MPa·m1/2, respec￾tively. Based on SEM and EDX observation, this depen￾dence is mainly due to the diffusion of elements (Al, Y, Si, etc.) from adjacent SiC layers. Acknowledgements The authors were grateful to the Science and Technol￾ogy Committee of Shanghai for providing support under the contract number 02DJ14065. References [1] A.G. Evans, Perspective on the development of high-toughness ce￾ramics, J. Am. Ceram. Soc. 73 (2) (1990) 187–206. [2] J. Cook, J.E. Gordon, A mechanism for the control of crack prop￾agation in all-brittle materials, Proc. R. Soc. Lond. A282 (1964) 508–520. [3] V.J. Laraia, A.H. Heuer, Novel composite microstructure and me￾chanical behavior of mollusk shell, J. Am. Ceram. Soc. 72 (11) (1989) 2177–2179. [4] J.D. Currey, A.J. Kohn, Fracture in the crossed-lamellar structure of conus shells, J. Mater. Sci. 11 (1976) 1615–1623. [5] W.J. Clegg, K. Kendall, N. McAlford, T.W. Button, J.D. Birchall, A simple way to make tough ceramics, Nature (London) 347 (1990) 455–457. [6] W.J. Clegg, The fabrication and failure of laminar ceramic compos￾ites, Acta Metall. Mater. 40 (11) (1992) 3085–3093. [7] A.J. Phillipps, W.J. Clegg, T.W. Clyne, Fracture behavior of ceramic laminates in bending-I. Modeling of crack propagation, Acta. Metall. Mater. 41 (3) (1993) 805–817. [8] A.J. Phillipps, W.J. Clegg, T.W. Clyne, The correlation of interfacial and macroscopic toughness in SiC laminates, Composites 24 (2) (1993) 166–176