B Saruhan et al. /Composites: Part A 32(2001)1095-1103 the fibres at 1300.C for 2 h [3]. The calculated composite extended heat-treatment (170 MPa against 135 MPa, Youngs modulus of 121 GPa should be considered as the respectively) indicates that the load transfer between the maximum value which can be achieved if there is a perfect fibre and the matrix is not due to the substantial sintering load transfer between the fibre and the matrix. If the modu- contact points, which impair the fibre strength by notch-like lus is fully dominated by either fibre or the matrix, then the defects, but more due to dissipative effects in the fugitive composite modulus would have upper bounds, respectively layer(Fig. 5(a) and(b). The maintenance of the fracture where Vm =0 and Ve=0 in 1). This applies to the strength in the composite at all temperatures indicates that composites tested under tensile loading. Since we used the oxide layer has a positive effect on fibre strength by redu- three-point-bending tests to characterise our composites, cing the damages caused by formation of the sintering necks the upper bound, where Vm=0, cannot be applied. This On the other hand, the stress/strain curve of the continuously describes the theoretical case that no matrix contribution heat-treated composite exhibited brittle fracture, indicatin exists for the load transfer. In such a case. the fibres cannot that there was some change in the interfacial relations. This transfer the bending load alone. We can, however, assume that is also supported by the SEM-observation of the composite, there exists no load transfer from the matrix to the fibre, in displaying partial closure of the fugitive layer(Fig. 4(c). other words the fibre does not contribute to the stiffness of the Occurrence of a fibre-contribution into the load transfer composite in bending, then with Ve=0 the composites should become noticeable as the Youngs modulus of the should have a lowest Youngs modulus of around 30 GPa. composite approaches to that of the calculated modulus The experimentally determined Youngs modulus of the (120 GPa). This case is more obvious with carbon(fugi reference composite showed that in the as-hot-pressed refer- tive)/LrO2-composites, where the experimentally deter- ence composite and in the composite where the carbon layer mined composite modulus lies around 106 GPa with the was removed at 1200C(ECMC 42 and 27 GPa, respec- as-hot-pressed and reduces to 87 GPa with the 1200C tively ), there existed almost no interaction between the fibre heat-treated sample. Removal of carbon layer retards the and the matrix. Extended heat-treatment of the reference effectiveness of load transfer in these composites. On the composite at 1300.C yielded a modulus of around 84 GPa, other hand, prolonged heat-treatment at 1300C gives a indicating that a certain load transfer through the sintering- reduced strength compared with the as hot-pressed sample related contact points at the fibre/matrix-interface occurred. as a result of severe fibre damage. The increase of the The higher load transfer between the fibre and the matrix Youngs Modulus to 117 GPa(cyclic heat-treated compo- caused also a simultaneous increase of the strength after the site)indicates strong interfacial bonding. The Youngs 1300.C heat-treatment. Thus, the load transfer is likely due modulus and strength of the continuously heat-treated to the formation of substantial sintering necks(Figs. 4(a) composite could not be compared, since the sample was and 6(a)). The fracture behaviour of the 1300C prolonged from a different batch but brittle fracture behaviour was, heat-treated composite is brittle in the case of continuously in this case too, obvious. This result corresponds with the heat-treated composite. Some remaining load capability in microstructural observation of a totally closed gap between the case of the cyclic heat-treated composite is believed due the fibre and the oxide layer. In the cyclic heat-treated to changes in matrix porosity distribution and formation of sample, presence of a limited degree of damage tolerance less effective sintering points between the fibre and the can be observed(Fig. 6(b))which is believed due to a less matrix which may break easily providing some damage tight bonding between fibre and matrix(Fig. 5(b)) tolerant behaviour By three-point-bending tests, it is diffi- observations can be explained by the repeated ve cult to distinguish clearly, if maintenance of some load (length) change in the ZrOz-layer during cooling and capability is due to dissipation at the fibre/matrix-interface, ing periods of the cyclic heat-treatment so called damage tolerance, or merely due to the delamina In a previous study, Keller et al. [4] have used 20 and tion fracture within the matrix of the composite. Since, 40 nm thick carbon coated Nextel 720-fibers in composites Youngs modulus and strength of the as-hot-pressed and having either a dense CAs(calcium aluminium silicate) 1200.C/2 h heat-treated reference composites are almost glass-matrix or a porous alumina/mullite-matrix. They the same as those of the matrix material we infer that the have demonstrated that the strength retention in porous apparent damage tolerant stress/strain behaviour was merely matrix composites was not dependent on the presence and thickness of a fugitive layer. In other words, presence of a The Youngs modulus of the carbon(fugitive)/Al2O3- fugitive layer in porous matrix composites makes little composites in the as-hot-pressed and 1200.C heat-treated difference to those without an interphase. 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