412 V. Bianchi et al Table 1. Main characteristics of P25/YMAS and T400H/YMAS composites, interfacial debonding(od)and shear(r)stresses Ref. Fiber Densification Crystallization Crack spacing fracture Fracture nterfacial Interfacial shear conditions(C/h) conditions(C/h) strength(MPa) debonding stress t(M stress, od(MPa) 950 245⊥65 440土60 P3P25 P4P25 105005 PS P25 1050/1 455±7 178±48 050/1.5 P7P25 1050/6 P8P25 l100/ P9P25 1150f P10P25 10500.5 12500.5 士180 Pl1 P25 10501 1250/15 ±270 59±33 1-0±0-7 TI T400H 950/l 95±25 300士 controlled 1322±258 108±337 T2 T400H 520±70 lcss controlled I137±167 363±8-0 T3 T400H 190±35 670±35 brittle T4T400H100/1 330±85 770±10 TS T400H 450±1201100±30 769±285 167±8-8 T6 400H 1050/1 1250/1.5 550±l50570±35 ess brittle 846+162 204±111 apitch-based carbon fibers from thorne PPAN-based carbon fibers from toray (a)and(b) present the relative variation of Indccd, since the coefficient of thermal expansion of the the apparent longitudinal Youngs modulus of a P25/ glass is slightly higher than that of the ceramic matrix, YMAS composite. During the heat treatment, thethe thermal stresses induced by this thermal expansion modulus decreases linearly, This regular and reversible mismatch are greater for a smaller temperature variation evolution is observed for the major part of the materi Is.fRom 860 C, the softening of the glass allows 3. 3 Coefficients of thermal expansion of the carbon matrix microcracks to close and explains the apparent fibers increase of the modulus. As the temperature is increased In order to evaluate the residual thermal stresses in the again [Fig. 2(b)], it is the crystallization of the glass composites from the model that is developed above, the which causes the second rise of the Young,s modulus. coefficients of thermal expansion of the fibers must be The temperature was maintained for I h at 1050C to determined The CTEs of the matrix' and the compo- reproduce the sintering therinal cycle of the coinposite, sites were measured in a vertical dilatometer(Set but the growth of crystals is observed to be a slow pro- TMA 92), under a flow of argon, with a linear heating cess compared with the crystallization of the glass On and cooling rate of 3 C min- and were fitted with cooling of the crystallized composite [Fig. 2(b)] the polynomial laws. Two identical thermal cycles were modulus is stable until a temperature of 625C, after carried out successively in order to verify the accuracy which it decreases rapidly, this temperature seemingly of the measurements corresponding to the onset of microcracking. The end of igure 4(a)and(b) present the relative expansion the curve is drawn as a dotted line: the progressive curves for the composites P5 and T3(crystallized matri decohesion between the sample and the wave guide 000c and show the differenc cannot be avoided. For the vitreous composite existing between the longitudinal and transverse CtEs cracks are closed and the glass becomes rigid again. a(5.0x10 6C and a,=10.4x10Cn values are [ Fig. 2(a)), the modulus increases on cooling: micro For the range 50 to 1000C, the m From 700 C, the modulus decrease suggests the begin Because of the presence of matrix microcracks in the ning of microcracking composites, on heating, the measurement of the long The same phenomena are observed with T400H/ itudinal CtE of the composites is likely to correspond YMAS composites [Fig. 3(a) and (b)]. A variation of to the longitudinal CTE of the fibers. When the sinter- about 30C is noted and can be attributed to variations ing temperature(1050 C) is reached, the microcracks in the fabrication of the matrix On cooling, the mod- should be a priori closed up, so that, on cooling down to ulus is observed to be stable until 300C for the crystal- the temperature at which thermal stresses induce lized composite and until 575@C for the glassy sample. microcracking in the matrix, the CTEs of the fibers can For the P25 and T400H composites, microcracking be calculated from those of the matrix and the compo appears in the glass matrix at higher temperature. sites. This suggests that, in this temperature range which412 V. Bianchi et al. Table 1. Main characteristics of PZS/YMAS and T4OOH/YMAS composites,2 interfacial debonding (nd) and shear (t) stresses Ref. Fiber Densification Crystallization Crack spacing Fracture Fracture Interfacial lnterfacial shear conditions (“C/h) conditions (“C/h) (w) PI P25” 95oi I P2 P25 970/l P3 P25 lOOO/ 1 P4 P25 lO50/0~5 P5 P25 1050/l P6 P25 lO5Ojl lO5Ojl~S P7 P25 lO5Ojl 1050/S P8 P25 I loo/l P9 P25 1150/l PI0 P25 lO5OjO.5 125OjO.5 PI1 P25 1050/l 125Oil.5 Tl T400Hh T2 T400H T3 T400H T4 T400H T5 T400H T6 T400H T7 T400H 950/I 1 ooo/ I 1050/l 1100/1 ll5Ojl 12001 I 1050/l 1250/1.5 “Pitch-based carbon fibers from Thornel. ‘PAN-based carbon fibers from Torayca. 245 * 65 600& 180 lOOO*270 95*25 lOOi 190*35 33Oi85 450* 120 4ooZt 110 550* 150 Figure 2(a) and (b) present the relative variation of the apparent longitudinal Young’s modulus of a P25/ YMAS composite. During the heat treatment, the modulus decreases linearly. This regular and reversible evolution is observed for the major part of the materi￾als.” From 860°C the softening of the glass allows matrix microcracks to close and explains the apparent increase of the modulus. As the temperature is increased again [Fig. 2(b)], it is the crystallization of the glass which causes the second rise of the Young’s modulus. The temperature was maintained for 1 h at 1050°C to reproduce the sintering thermal cycle of the composite, but the growth of crystals is observed to be a slow pro￾cess compared with the crystallization of the glass. On cooling of the crystallized composite [Fig. 2(b)] the modulus is stable until a temperature of 625°C after which it decreases rapidly, this temperature seemingly corresponding to the onset of microcracking. The end of the curve is drawn as a dotted line; the progressive decohesion between the sample and the wave guide cannot be avoided. For the vitreous composite [Fig. 2(a)], the modulus increases on cooling: micro￾cracks are closed and the glass becomes rigid again. From 700°C the modulus decrease suggests the begin￾ning of microcracking. The same phenomena are observed with T400H/ YMAS composites [Fig. 3(a) and (b)]. A variation of about 30°C is noted and can be attributed to variations in the fabrication of the matrix. On cooling, the mod￾ulus is observed to be stable until 300°C for the crystal￾lized composite and until 575°C for the glassy sample. For the P25 and T400H composites, microcracking appears in the glass matrix at higher temperature. strength (MPa) type debonding stress r (MPa) stress, od (MPa) (Push-in-test)’ 440 & 60 controlled 300*45 controlled 520 * 70 less controlled 670* 35 brittle 77oi 105 1100*300 760 * 65 570135 less brittle 455 f 73 17.814-8 59*33 I .o * 0.7 1322*258 108 * 33.7 11371167 36.3 Z!Z 8.0 769 f 285 16,7+8.8 846i 162 20.4 i- I 1. I Indeed, since the coefficient of thermal expansion of the glass is slightly higher than that of the ceramic matrix, the thermal stresses induced by this thermal expansion mismatch are greater for a smaller temperature variation. 3.3 Coefficients of thermal expansion of the carbon fibers In order to evaluate the residual thermal stresses in the composites from the model that is developed above, the coefficients of thermal expansion of the fibers must be determined. The CTEs of the matrix’ and the compo￾sites were measured in a vertical dilatometer (Setaram TMA 92) under a flow of argon, with a linear heating and cooling rate of 3”Cmin’ and were fitted with polynomial laws. Two identical thermal cycles were carried out successively in order to verify the accuracy of the measurements. Figure 4(a) and (b) present the relative expansion curves for the composites P5 and T3 (crystallized matrix composites) up to 1000°C and show the difference existing between the longitudinal and transverse CTEs. For the range 50 to lOOO”C, the mean values are c$ = 5.0x 10-6”C-’ and CX; = 10.4x 10-6”C-‘. Because of the presence of matrix microcracks in the composites, on heating, the measurement of the long￾itudinal CTE of the composites is likely to correspond to the longitudinal CTE of the fibers. When the sinter￾ing temperature (105O’C) is reached, the microcracks should be a priori closed up, so that, on cooling down to the temperature at which thermal stresses induce microcracking in the matrix, the CTEs of the fibers can be calculated from those of the matrix and the compo￾sites. This suggests that, in this temperature range which