R Venkatesh/Ceramics International 28(2002)565-573 omposites was only 3%. It should be pointed out that roughness plays a major role in the fracture resistance SSG composites were fabricated using a small quantity of ceramic matrix composites Interfacial roughness in of fibers only to verify the importance of fiber CMCs can be controlled through the use of smooth fibres roughness. The bend strength increased with the volume or reducing the coating thickness fraction of the fibers. The strength of AG was slightl reater than AsG, possibly due to the strong chemical bonding at the fiber/matrix interface leading to better Conclusion load transfer in AG as compared to ASG composites The work of fracture evaluated from the area under the 1. The strength of alumina-ZrO, fibres(PRD-166 load-displacement curve for both coated and uncoated decreased with increase in heat treatment tem- composites is shown in Table 7. This parameter perature of fibres. This could be because of the increased with volume fraction of fibers in both uncoa- presence of a glassy second and or due to pro ted and coated fiber composites. The work of fracture of cessing defects in the fibre ASG composites was larger than that of AG composites 2. The strength of the fibres decreased with increase due to contributions from modulus mismatch, crack in SnOz coating thickness. It was observed that deflection, fiber bridging and fiber pullout. Fracture as the coating thickness increased, roughness of toughness as a function of volume fraction of fibers for the coating increased. This acted as a notch in both AG and AsG composites is shown in Table 7. The order to decrease the strength of the Al,O3/SnO toughness of AG composites obtained in this study is in composite fibre close agreement with that obtained by Michalske and 3. In CMCs, as rough ness Increa sed. fibre debond Hellmann [48]. The toughness of AG and ASG compo- ing decreased and toughness of composite sites increased with volume fraction of fibers. The decreased. PRD-166/SnO2/glass matrix compo- toughness of ASG composites was larger than that of sites exhibited non-planar failure with fiber brid- AG composites. The main contributors to the increase ging and debonding as major toughening in toughness of ASG as compared to AG composites mechanisms. Saphikon/ SnO/glass matrix com- are crack deflection, partial debonding, fiber bridging posites failed in a tough manner with extensive and partial fiber pullout fiber pullout Fracture surfaces of ASG composites are non planar, 4. The difference in the failure mode between prd. Fig. 6. Fig. 6 also shows that the predominant mechan- 6/SnO/glass and Saphikon/SnO2/glass matrix ism of toughening is fiber bridging and fibre debonding composites could be attributed to roughness of Partial pullout of fibers can also be seen. A higher the fibres magnification micrograph of the fracture surface of 5. It is important to control the roughness at the ASG composites along the fiber, [Fig. (6c)), shows fibre/coating and coating/matrix interfaces in clearly the partial removal of the coating and the rough order to develop tough ceramic matrix compo- fiber surface. Hence the primary toughening mechan sites. methods g roughness at the isms in ASG composites are crack deflection, fiber interfaces consist in the incorporation of smooth bridging, partial fiber debonding and pullout. It has fibres and in decreasing coating thickness been shown that as the roughness of interphase increa- es, the compressive clamping stress increases thereby affecting the fracture resistance of ceramic matrix com- posites [49-58]. This increase in the compressive clamp ing stress due to fibre roughness causes an increase in References the shear stress transfer at the interface beyond matrix cracking from fiber to matrix. This causes a reduction in [1 I.w. Donald, P w. McMillan, J Mater. Sci. 11(1976)146 2R. W. Rice, Ceramic matrix composite toughening mechanisms: the debond length, i.e., fibers break rather than debond an update, in Ceramic Science and Engineering Series, Vol. 6, as the matrix crack grows, resulting in a composite fracture surface in ASG with little or no fiber pullout on 3A.G. Evans, Perspective on the development of high-toughness the fracture surface Fracture surface of SSG compo- ceramics, J. Am. Ceram Soc. 73(1990)187 sites, Fig. 7(a and b), showed neat fiber debonding and 4W.B. Hillig, Strength and toughness of ceramic matrix compo- fiber pullout at the fiber/SnO2 interface, as confirmed [D.B. Marshall, J E. Ritter, Reliability of advan also by EDs on the pulled out saphikon fiber in Fig 8 ceramics and ceramic matrix composites-a review, In the region marked as a, the EDs analysis showed 66(1987)309 only Al and no Sn since SnO, and Al, O, have no [6 H.G. Sowman, DD. Johnson, in: K.s. Mazdiyani (Ed. ), Fibre mutual solubility. Hence fibre debonding and fibre pullout with lengths over 100 um take place in saphikon Technology, Noyes Publications, USA, 1990, pp l-/sand [7 G. Das, Ceram. Eng. Sci. Proc. 16(5)(1995)977-986 fibre/SnO2/glass matrix composites. Thus interfacial [8 G. Das, Ceram. Eng. Sci. Proc. 18(6)(1997)25-33composites was only 3%. It should be pointed out that SSG composites were fabricated using a small quantity of fibers only to verify the importance of fiber roughness. The bend strength increased with the volume fraction of the fibers. The strength of AG was slightly greater than ASG, possibly due to the strong chemical bonding at the fiber/matrix interface leading to better load transfer in AG as compared to ASG composites. The work of fracture evaluated from the area under the load-displacement curve for both coated and uncoated composites is shown in Table 7. This parameter increased with volume fraction of fibers in both uncoa￾ted and coated fiber composites. The work of fracture of ASG composites was larger than that of AG composites due to contributions from modulus mismatch, crack deflection, fiber bridging and fiber pullout. Fracture toughness as a function of volume fraction of fibers for both AG and ASG composites is shown in Table 7. The toughness of AG composites obtained in this study is in close agreement with that obtained by Michalske and Hellmann [48]. The toughness of AG and ASG compo￾sites increased with volume fraction of fibers. The toughness of ASG composites was larger than that of AG composites. The main contributors to the increase in toughness of ASG as compared to AG composites are crack deflection, partial debonding, fiber bridging and partial fiber pullout. Fracture surfaces of ASG composites are non planar, Fig. 6. Fig. 6 also shows that the predominant mechan￾ism of toughening is fiber bridging and fibre debonding. Partial pullout of fibers can also be seen. A higher magnification micrograph of the fracture surface of ASG composites along the fiber, [Fig. (6c)], shows clearly the partial removal of the coating and the rough fiber surface. Hence the primary toughening mechan￾isms in ASG composites are crack deflection, fiber bridging, partial fiber debonding and pullout. It has been shown that as the roughness of interphase increa￾ses, the compressive clamping stress increases thereby affecting the fracture resistance of ceramic matrix com￾posites [49–58]. This increase in the compressive clamp￾ing stress due to fibre roughness causes an increase in the shear stress transfer at the interface beyond matrix cracking from fiber to matrix. This causes a reduction in the debond length, i.e., fibers break rather than debond as the matrix crack grows, resulting in a composite fracture surface in ASG with little or no fiber pullout on the fracture surface Fracture surface of SSG compo￾sites, Fig. 7 (a and b), showed neat fiber debonding and fiber pullout at the fiber/SnO2 interface, as confirmed also by EDS on the pulled out saphikon fiber in Fig. 8. In the region marked as a, the EDS analysis showed only Al and no Sn since SnO2 and Al2O3 have no mutual solubility. Hence fibre debonding and fibre pullout with lengths over 100 mm take place in saphikon fibre/SnO2/glass matrix composites. Thus interfacial roughness plays a major role in the fracture resistance of ceramic matrix composites. Interfacial roughness in CMCs can be controlled through the use of smooth fibres or reducing the coating thickness. Conclusion 1. The strength of alumina–ZrO2 fibres (PRD-166) decreased with increase in heat treatment tem￾perature of fibres. This could be because of the presence of a glassy second and/or due to pro￾cessing defects in the fibres. 2. The strength of the fibres decreased with increase in SnO2 coating thickness. 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 Al2O3/SnO2 composite fibres. 3. In CMCs, as roughness increased, fibre debond￾ing decreased and toughness of composite decreased. PRD-166/SnO2/glass matrix compo￾sites exhibited non-planar failure with fiber brid￾ging and debonding as major toughening mechanisms. Saphikon/SnO2/glass matrix com￾posites failed in a tough manner with extensive fiber pullout. 4. The difference in the failure mode between PRD- 166/SnO2/glass and Saphikon/SnO2/glass matrix composites could be attributed to roughness of the fibres. 5. It is important to control the roughness at the fibre/coating and coating/matrix interfaces in order to develop tough ceramic matrix compo￾sites. Methods of decreasing roughness at the interfaces consist in the incorporation of smooth fibres and in decreasing coating thickness. References [1] I.W. Donald, P.W. McMillan, J. Mater. Sci. 11 (1976) 146. [2] R.W. Rice, Ceramic matrix composite toughening mechanisms: an update, in Ceramic Science and Engineering Series, Vol. 6, American Ceramic Society, Columbus, OH, p.589. [3] A.G. Evans, Perspective on the development of high-toughness ceramics, J. Am. Ceram. Soc. 73 (1990) 187. [4] W.B. Hillig, Strength and toughness of ceramic matrix compo￾sites, Ann. Rev. Mater. Sci. 17 (1987) 341. [5] D.B. Marshall, J.E. Ritter, Reliability of advanced structural ceramics and ceramic matrix composites–a review, Ceram. Bull. 66 (1987) 309. [6] H.G. Sowman, D.D. Johnson, in: K.S. Mazdiyani (Ed.), Fibre Reinforced Ceramic Composites: Materials Processing and Technology, Noyes Publications, USA, 1990, pp. 122–138. [7] G. Das, Ceram. Eng. Sci. Proc. 16 (5) (1995) 977–986. [8] G. Das, Ceram. Eng. Sci. Proc. 18 (6) (1997) 25–33. 572 R. Venkatesh / Ceramics International 28 (2002) 565–573