R Venkatesh/Ceramics International 28 (2002)565-573 parameter(a), standard deviation and coefficient of gives 1375(17/50) 3.5=1010 MPa. Both values are variation of as-received and heat treated alumina fibres. lower than measured by the cited authors. The A room temperature strength of 1375 MPa was difference in results obtained could be explained by pre obtained for a gage length of 17 mm. This value is lower sence of voids. It can also be seen from Table 2 that the than that reported by romine [36] who obtained a strength decreases with increase in heat treatment tem strength of 2100 MPa for a gage length of 6.9 mm and perature. Pysher and Tressler [44, 45] have reported the Yang et al. [43] who obtained a strength of 1180 MPa presence of minor elements (0.01-0.1%)of Si, and P for a gage length of 50 mm. There are two effects which could form a SiO2-P2Os glassy grain boundary explaining such results: one is the different gage lengths, phase in PRD-166 fibres at high temperatures. These the other the presence of voids( Fig 4)in the as-received glassy phase could reduce the strength of the fibres. It the Weibull strength at a given has been shown in earlier works that the alumina fibres probability depends on the stressed volume as v-1, for undergo transgranular failure up to 800 oC beyond fibres of the same diameter tested on different gage which the failure mode is intergranular [44, 45 lengths L, L', the strength ratio should be a/o'=L/L Table 3 shows variation of tensile strength. Weibull the result of the present research to be compared with modulus and scale paremeters of SnO2 coated fibres the one by Romine is, accordingly, 1375(17/6.9)/3.5 Again, the tensile strength decreases with increase in 1779 MPa. The value to be compared with Yang et al coating thickness. Some loss in strength can be attrib uted to exposures at 500C during SnO2 deposition Another source of strength reduction could be thermal stresses generated after deposition and subsequent cool ing of the alumina/tin dioxide composite fibre. Thermal stresses at the fibre/coating interface were calculated using a two-element cylinderical model [46, 47, and the results are shown in Table 4. The radial stress is tensile while the axial and circumferential stresses are tensile in the fibre and compressive in the coating. The axial tensile stress and radial stress increases with coating thickness. This state of stress could reduce the strength of the fibres with increase in coating thickness. In order to obtain a measure of the effect of rough ne properties of the fibres, amplitude of 50μ roughness of as-received and SnO2 coated fibres was evaluated. The pea evaluated in the present study using SEM micrographs and tracings of roughness profiles. These results are shown in Table 5. It can be seen that the amplitude of roughness, A increases with increase in coating thick- ness. Under an axial load this roughness could act as a notch and decrease the strength of the fibres with increase in coating thickness The roughness of fibre nd coating hav matrix composites. In order to further study the effect of fibre roughne of fibre alumina fibre(PRD-166) and Saphikon fibre on the fracture characteristics of alumina fibre (PRD-166)/ glass(AG), Saphikon fibre/glass (SG), alumina fibre (PRD-166)/SnOz/glass (ASG) and Saphikon fibre SnO2/glass (SSG)composites were investigated. Evalu- 10 m ating compressive roughness strain, A/r(amplitude of roughness/radius of fibre) using tracings of interphase micrographs Fig. 5(a)and(b)it was observed that the A/r value of PRD-166/SnO, interface was about nine Fig. 7.(a and b) Fracture surface of Saphikon alumina fibre/Sno times that of the Saphikon/SnO interface(Table 6). I glass matrix composites. The SnO2 coating on a relatively smooth was also found that, in PRD-166/ SnO2/glass matrix saphikon fibre results in a neat a long fibre pull out composites, the compressive radial strain induced dueparameter (a), standard deviation and coefficient of variation of as-received and heat treated alumina fibres. A room temperature strength of 1375 MPa was obtained for a gage length of 17 mm. This value is lower than that reported by Romine [36] who obtained a strength of 2100 MPa for a gage length of 6.9 mm and Yang et al. [43] who obtained a strength of 1180 MPa for a gage length of 50 mm. There are two effects explaining such results: one is the different gage lengths, the other the presence of voids (Fig. 4) in the as-received alumina fibres. As the Weibull strength at a given probability depends on the stressed volume as V-1/b, for fibres of the same diameter tested on different gage lengths L, L0 , the strength ratio should be /0 =L/L0 ; the result of the present research to be compared with the one by Romine is, accordingly, 1375 (17/6.9)1/3.5 =1779 MPa. The value to be compared with Yang et al gives 1375 (17/50)1/3.5=1010 MPa. Both values are lower than measured by the cited authors. The difference in results obtained could be explained by pre￾sence of voids. It can also be seen from Table 2 that the strength decreases with increase in heat treatment tem￾perature. Pysher and Tressler [44,45] have reported the presence of minor elements (0.01–0.1%) of Si, and P which could form a SiO2-P2O5 glassy grain boundary phase in PRD-166 fibres at high temperatures. These glassy phase could reduce the strength of the fibres. It has been shown in earlier works that the alumina fibres undergo transgranular failure up to 800
C beyond which the failure mode is intergranular [44,45]. Table 3 shows variation of tensile strength, Weibull modulus and scale paremeters of SnO2 coated fibres. Again, the tensile strength decreases with increase in coating thickness. Some loss in strength can be attrib￾uted to exposures at 500
C during SnO2 deposition. Another source of strength reduction could be thermal stresses generated after deposition and subsequent cool￾ing of the alumina/tin dioxide composite fibre. Thermal stresses at the fibre/coating interface were calculated using a two-element cylinderical model [46,47], and the results are shown in Table 4. The radial stress is tensile while the axial and circumferential stresses are tensile in the fibre and compressive in the coating. The axial tensile stress and radial stress increases with coating thickness. This state of stress could reduce the strength of the fibres with increase in coating thickness. In order to obtain a measure of the effect of rough￾ness on the properties of the fibres, amplitude of roughness of as-received and SnO2 coated fibres was evaluated. The peak-valley roughness amplitude, A, was evaluated in the present study using SEM micrographs and tracings of roughness profiles. These results are shown in Table 5. It can be seen that the amplitude of roughness, A increases with increase in coating thick￾ness. Under an axial load, this roughness could act as a notch and decrease the strength of the fibres with increase in coating thickness. The roughness of fibre surface and coating have ser￾ious implications in the development of tough ceramic matrix composites. In order to further study the effect of fibre roughness, two different types of fibres namely, alumina fibre (PRD-166) and Saphikon fibre on the fracture characteristics of alumina fibre (PRD-166)/ glass (AG), Saphikon fibre/glass (SG), alumina fibre (PRD-166)/SnO2/glass (ASG) and Saphikon fibre/ SnO2/glass (SSG) composites were investigated. Evalu￾ating compressive roughness strain, A/r (amplitude of roughness/radius of fibre) using tracings of interphase micrographs Fig. 5(a) and (b) it was observed that the A/r value of PRD-166/SnO2 interface was about nine times that of the Saphikon/SnO2 interface (Table 6). It was also found that, in PRD-166/SnO2/glass matrix composites, the compressive radial strain induced due Fig. 7. (a and b) Fracture surface of Saphikon alumina fibre/SnO2/ glass matrix composites. The SnO2 coating on a relatively smooth saphikon fibre results in a neat a long fibre pull out. 570 R. Venkatesh / Ceramics International 28 (2002) 565–573