《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Effect of thermomechanical loads on microstructural damage and on the resulting thermomechanical behaviour of silicon carbide fibre-reinforced glass matrix composites

MATERAL CHARACIERAAION ELSEVIER Materials Characterization 54(2005)75-83 Effect of thermomechanical loads on microstructural damage and on the resulting thermomechanical behaviour of silicon carbide fibre-reinforced glass matrix composites A.R. Boccaccinia,*. A.M. Torre. C.R. oldanib D.N. Boccaccinic Department of Materials, Imperial College London, Prince Consort Rd, London SW7 2BP UK DEpartamento de Materiales, Universidad Nacional de Cordoba, argentina Dipartimento di Ingegneria dei Materiali e dell'Ambiente, Universita di Modena e, Reggio Emilia, Modena, itah Received 23 July 2004; received in revised form 26 October 2004: accepted 5 November 2004 Abstract The development of microstructural damage in SiC fibre(Nicalon )reinforced glass matrix composites subjected to different mechanical and thermal loads was investigated by assessing the change of the thermal expansion coefficient and the resistance to impact loads of the composites. The thermal expansion coefficient, measured by dilatometry after thermal shock tests from 650C to room temperature, was found to be 'insensitive to microstructural damage such as matrix microcracking, matrix softening or matrix/fibre interface degradation. The impact resistance of the composites, measured by a pendulum-type apparatus, was high even after subjecting the samples to different thermomechanical loads, h as repetitive thermal shocks from 650C for up to 20 cycles. However, the samples showed an appreciable degradation of their impact resistance after thermal aging in air at 700C for 100 h. This was shown to be due to extensive porosity formation due to softening of the glass matrix and due to oxidation of the carbon-rich fibre/matrix interfaces C 2004 Elsevier Inc. All rights reserved Keywords: Glass matrix composites; Thermal expansion; Microstructural damage; Impact behaviour; Thermal shock; Thermal aging 1. Introduction Silicate glasses possess a combination of attractive properties as: high hardness, high resistance to Corresponding author. Tel. +44 20 7594 6731; fax: +44 20 che low density, high wear resistance, 75843194. insulating electrical properties and optical transpar E-mail address. aboccaccini@imperial ac uk ency. The main disadvantages that glasses present for (A.R. Boccaccini) technical applications are their extreme fragility, their 1044-5803/S- see front matter e 2004 Elsevier Inc. All rights reserved doi:10.1016 j. matcha.2004.11.0

Effect of thermomechanical loads on microstructural damage and on the resulting thermomechanical behaviour of silicon carbide fibre-reinforced glass matrix composites A.R. Boccaccinia,*, A.M. Torreb , C.R. Oldanib , D.N. Boccaccinic a Department of Materials, Imperial College London, Prince Consort Rd., London SW7 2BP, UK b Departamento de Materiales, Universidad Nacional de Co´rdoba, Argentina c Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Universita` di Modena e, Reggio Emilia, Modena, Italy Received 23 July 2004; received in revised form 26 October 2004; accepted 5 November 2004 Abstract The development of microstructural damage in SiC fibre (NicalonR) reinforced glass matrix composites subjected to different mechanical and thermal loads was investigated by assessing the change of the thermal expansion coefficient and the resistance to impact loads of the composites. The thermal expansion coefficient, measured by dilatometry after thermal shock tests from 650 8C to room temperature, was found to be dinsensitiveT to microstructural damage such as matrix microcracking, matrix softening or matrix/fibre interface degradation. The impact resistance of the composites, measured by a pendulum-type apparatus, was high even after subjecting the samples to different thermomechanical loads, such as repetitive thermal shocks from 650 8C for up to 20 cycles. However, the samples showed an appreciable degradation of their impact resistance after thermal aging in air at 700 8C for 100 h. This was shown to be due to extensive porosity formation due to softening of the glass matrix and due to oxidation of the carbon-rich fibre/matrix interfaces. D 2004 Elsevier Inc. All rights reserved. Keywords: Glass matrix composites; Thermal expansion; Microstructural damage; Impact behaviour; Thermal shock; Thermal aging 1. Introduction Silicate glasses possess a combination of attractive properties such as: high hardness, high resistance to chemical attack, low density, high wear resistance, insulating electrical properties and optical transpar￾ency. The main disadvantages that glasses present for technical applications are their extreme fragility, their 1044-5803/$ - see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2004.11.001 * Corresponding author. Tel.: +44 20 7594 6731; fax: +44 20 7584 3194. E-mail address:a.boccaccini@imperial.ac.uk (A.R. Boccaccini). Materials Characterization 54 (2005) 75 – 83

L.R. Boccaccini et al. Materials Characterization 54(2005)75-8 low resistance to thermal shock and their low ical loads resulting in microstructural damage. Meas- mechanical strength, which makes them unsuitable urements of the thermal expansion coefficient were as structural materials. The introduction of reinforce- utilised for the first time to assess the sensitivity of ments in the form of fibres in glass matrices, forming this parameter to microstructural changes, as a composites, can change these characteristics of possible method for detection of microstructural glasses making it possible to develop materials damage in its early stages of developI tolerant to microstructural defects or cracks. with evaluation of the mechanical behaviour of the high mechanical resistance and a very good resistance composite material after thermomechanical loads to thermal shock [1-4]. The composite materials thus was carried out through impact tests. Scanning obtained constitute advanced lightweight structural electron microscopy (SEM) was used to verify the materials, which can be utilised at high temperatures occurrence of energy-dissipation mechanisms, such as and under mechanical loads in oxidant atmospheres fibre pull-out, during composite fracture Therefore, these materials may find applications in energy-conversion systems, aerospace vehicles and structures, as well as in the building industry 2. Experimental fireproof components or thermal protection elements that must work under mechanical stresses [3-5]. This 2. 1. Material favourable flaw-tolerant behaviour of the composites is not only due to the high modulus of elasticity and The material investigated was a commercially mechanical resistance of the fibres but also due to th available unidirectional SiC-Nicalon&(NL 202) presence of a thin carbon-rich fibre/matrix interfacial fibre-reinforced borosilicate (duraN)glass-matrix layer(20-30 nm thickness). This layer, produced composite fabricated by Schott-Glaswerke (Mainz, during the high-temperature fabrication step, origi- Germany)[14. The composite was prepared by the nates energy-dissipation mechanisms during fracture, ethod. Details of this technique can be such as crack deflection and fibre"pull-out, provid read in previous publications [15, 16]. Nominal ing the appropriate matrix/fibre bonding strength properties of the matrix, fibre and composite are leading to favourable"pseudo-ductile"fracture behav iven in Table 1 [17, 18]. The material was received in iour [3, 5]. To achieve the satisfactory use of these the form of prismatic test bars of nominal dimensions materials in the applications above mentioned and to 4.5X3.8X100 mm. The density of the composites was extend their application to other technical areas, it is 2.4 g cm and the fibre volume fraction.4. This necessary to have a broad knowledge of their material is being considered for applications in the thermomechanical behaviour. This explains the grow- glass and non-ferrous metallurgy industries, for the ing quantity of studies published in the specialised fabrication of tools for handling of hot glassware and literature in the last decade about the behaviour of metal parts, respectively [14] glass-matrix composite materials when submitted to thermal gradients and combined thermomechanical stresses [6-13]. There is, however, further need in Table developing techniques to characterise microstructural Properties of composite constituents and of the composite [15-17] damage in composites after thermomechanical dam Property latrix DURAN Fibre sic age loads is required to assess the expecte (borosilicate Nicalon(40 voL% lifetime of components in service. The objective of (NL202) fibre content) this study is to investigate the influence of mechanical Density (g cm") 2.23 and thermal loads on the development of micro- Young's structural damage and in the resulting thermomechan- modulus(GPa) ical behaviour of silicon carbide(Nicalon)fibre- 0.22 0.200.2 Thermal expansion 3. 25x10 3×10-63.10×10-6 reinforced glass matrix composite materials. Thermal oefficient(K-) shock, thermal cycling, thermal aging and mechanical Tensile strength 60 600700 re-stressing were studied as typical thermomechan (MPa)

low resistance to thermal shock and their low mechanical strength, which makes them unsuitable as structural materials. The introduction of reinforce￾ments in the form of fibres in glass matrices, forming composites, can change these characteristics of glasses making it possible to develop materials tolerant to microstructural defects or cracks, with high mechanical resistance and a very good resistance to thermal shock [1–4]. The composite materials thus obtained constitute advanced lightweight structural materials, which can be utilised at high temperatures and under mechanical loads in oxidant atmospheres. Therefore, these materials may find applications in energy-conversion systems, aerospace vehicles and structures, as well as in the building industry as fireproof components or thermal protection elements that must work under mechanical stresses [3–5]. This favourable flaw-tolerant behaviour of the composites is not only due to the high modulus of elasticity and mechanical resistance of the fibres, but also due to the presence of a thin carbon-rich fibre/matrix interfacial layer (20–30 nm thickness). This layer, produced during the high-temperature fabrication step, origi￾nates energy-dissipation mechanisms during fracture, such as crack deflection and fibre bpull-outQ, provid￾ing the appropriate matrix/fibre bonding strength leading to favourable bpseudo-ductileQ fracture behav￾iour [3,5]. To achieve the satisfactory use of these materials in the applications above mentioned and to extend their application to other technical areas, it is necessary to have a broad knowledge of their thermomechanical behaviour. This explains the grow￾ing quantity of studies published in the specialised literature in the last decade about the behaviour of glass-matrix composite materials when submitted to thermal gradients and combined thermomechanical stresses [6–13]. There is, however, further need in developing techniques to characterise microstructural damage in composites after thermomechanical dam￾age loads. This is required to assess the expected lifetime of components in service. The objective of this study is to investigate the influence of mechanical and thermal loads on the development of micro￾structural damage and in the resulting thermomechan￾ical behaviour of silicon carbide (NicalonR) fibre￾reinforced glass matrix composite materials. Thermal shock, thermal cycling, thermal aging and mechanical pre-stressing were studied as typical thermomechan￾ical loads resulting in microstructural damage. Meas￾urements of the thermal expansion coefficient were utilised for the first time to assess the sensitivity of this parameter to microstructural changes, as a possible method for detection of microstructural damage in its early stages of development. The evaluation of the mechanical behaviour of the composite material after thermomechanical loads was carried out through impact tests. Scanning electron microscopy (SEM) was used to verify the occurrence of energy-dissipation mechanisms, such as fibre pull-out, during composite fracture. 2. Experimental 2.1. Material The material investigated was a commercially available unidirectional SiC-NicalonR (NL 202) fibre-reinforced borosilicate (DURANR) glass–matrix composite fabricated by Schott-Glaswerke (Mainz, Germany) [14]. The composite was prepared by the sol–gel slurry method. Details of this technique can be read in previous publications [15,16]. Nominal properties of the matrix, fibre and composite are given in Table 1 [17,18]. The material was received in the form of prismatic test bars of nominal dimensions 4.5
3.8
100 mm. The density of the composites was 2.4 g cm3 and the fibre volume fraction ~0.4. This material is being considered for applications in the glass and non-ferrous metallurgy industries, for the fabrication of tools for handling of hot glassware and metal parts, respectively [14]. Table 1 Properties of composite constituents and of the composite [15–17] Property Matrix DURANR (borosilicate glass) Fibre SiC NicalonR (NL202) Composite (40 vol.% fibre content) Density (g cm3 ) 2.23 2.55 2.4 Young’s modulus (GPa) 63 198 119 Poisson’s ratio 0.22 0.20 0.21 Thermal expansion coefficient (K1 ) 3.25
106 3
106 3.10
106 Tensile strength (MPa) 60 2750 600–700 76 A.R. Boccaccini et al. / Materials Characterization 54 (2005) 75–83

A.R. Boccaccini et al. Materials Characterization 54(2005)75-83 2.2. Thermal shock tests applied loads investigated were both below and above the fracture load, which was taken from results in the The thermal shock tests involved heating the literature on the same composite [12] amples in air in a muffle furnace at a pre-determined temperature(650C) for 10 min and then dropping 2.6. Microstructural characterisation them abruptly into an unstirred water bath maintained at room temperature(20C). The thermal gradient SEM of polished sections was conducted in an was chosen on the basis of previous studies [12, 18] in attempt to characterise the microstructural damage order to ensure that the microstructural damage induced in the specimens. Since the main energy induced in the samples could be attributed only to dissipation mechanism that occurs during fracture of the effect of thermal shock and not to other environ- these materials is the phenomenon of fibre pull-out mental influences such as, for example, fibre/matrix [10, 12], whose occurrence an nsion can be interface or SiC fibre oxidation. After each thermal appreciated visually, SEM observations of fracture shock test, the samples were dried in an oven at 100 surfaces were carried out C for I h. They were carefully inspected, visually, for the appearance of any macroscopic damage, such as 2.7. Impact test delamination, chipping or fibre protrusion. Samples in form of test bars were impacted with an 2.3. Thermal cycling energy of 4 J. This value of energy was adopted on the basis of impact test data obtained in previous For the thermal cycling tests, the samples were studies [19]. a pendulous apparatus with a mass and alternated quickly between high temperature(700C) arm especially designed to deliver the necessary and room temperature for different number of cycles energy to ensure the same operative conditions was (up to 1000 cycles). A computer program controlled used. A custom-made data acquisition system was the movement of the sample holder. Details of the adapted to the impact tester for observing if signifi equipment used for this test can be found elsewhere cant variations of energy consumed by the samples [10]. The time at high temperature was set at 15 min, occurred during impact. More detailed information of with 5 min being allowed for the sample to reach the these experiments is given elsewhere [19] target temperature of 700C, while the time at room temperature was fixed at 5 min. Thus, a complete 2.8. Measurement of the thermal expansion coefficient thermal cycle lasted 20 min. After a determined number of cycles, the samples were inspected care The thermal expansion coefficient (ax) of compo- fully for evidence of any macroscopic damage, such ite samples before and after thermomechanical as a change of the surface colour, delamination, loading was determined using a high temperature protrusion of fibres, spalling and/or chipping dilatometer (NETZSCH GmbH TMA 402). The samples were cut with a diamond saw to an 2. 4. Thermal aging approximate length of 23 mm. Before carrying out the experiments, the dimensions of the samples were The thermal aging experiments involved heating measured using a vernier with a precision of 0.05 samples at 600 and 700C in a fumace in an air mm. The measurements were carried out in normal atmosphere for long periods of time (up to 100 h). atmosphere between room temperature(20C)and After the thermal exposure, the samples were cooled 750C with a heating rate of 5 K min. At least two down slowly inside the fumace measurements were performed for each specimen and the results were averaged. The values were calculated 2.5. Mechanical pre-stressing for temperature intervals: 20-300, 20-400 and 20- 00C. The accuracy of the measured values was Selected samples were subjected to mechanical Ax=+lx10K. A high reproducibility of results loading in a three-point bending configuration. The was confirmed

2.2. Thermal shock tests The thermal shock tests involved heating the samples in air in a muffle furnace at a pre-determined temperature (650 8C) for 10 min and then dropping them abruptly into an unstirred water bath maintained at room temperature (20 8C). The thermal gradient was chosen on the basis of previous studies [12,18] in order to ensure that the microstructural damage induced in the samples could be attributed only to the effect of thermal shock and not to other environ￾mental influences such as, for example, fibre/matrix interface or SiC fibre oxidation. After each thermal shock test, the samples were dried in an oven at 100 8C for 1 h. They were carefully inspected, visually, for the appearance of any macroscopic damage, such as delamination, chipping or fibre protrusion. 2.3. Thermal cycling For the thermal cycling tests, the samples were alternated quickly between high temperature (700 8C) and room temperature for different number of cycles (up to 1000 cycles). A computer program controlled the movement of the sample holder. Details of the equipment used for this test can be found elsewhere [10]. The time at high temperature was set at 15 min, with 5 min being allowed for the sample to reach the target temperature of 700 8C, while the time at room temperature was fixed at 5 min. Thus, a complete thermal cycle lasted 20 min. After a determined number of cycles, the samples were inspected care￾fully for evidence of any macroscopic damage, such as a change of the surface colour, delamination, protrusion of fibres, spalling and/or chipping. 2.4. Thermal aging The thermal aging experiments involved heating samples at 600 and 700 8C in a furnace in an air atmosphere for long periods of time (up to 100 h). After the thermal exposure, the samples were cooled down slowly inside the furnace. 2.5. Mechanical pre-stressing Selected samples were subjected to mechanical loading in a three-point bending configuration. The applied loads investigated were both below and above the fracture load, which was taken from results in the literature on the same composite [12]. 2.6. Microstructural characterisation SEM of polished sections was conducted in an attempt to characterise the microstructural damage induced in the specimens. Since the main energy￾dissipation mechanism that occurs during fracture of these materials is the phenomenon of fibre pull-out [10,12], whose occurrence and extension can be appreciated visually, SEM observations of fracture surfaces were carried out. 2.7. Impact test Samples in form of test bars were impacted with an energy of ~4 J. This value of energy was adopted on the basis of impact test data obtained in previous studies [19]. A pendulous apparatus with a mass and arm especially designed to deliver the necessary energy to ensure the same operative conditions was used. A custom-made data acquisition system was adapted to the impact tester for observing if signifi￾cant variations of energy consumed by the samples occurred during impact. More detailed information of these experiments is given elsewhere [19]. 2.8. Measurement of the thermal expansion coefficient The thermal expansion coefficient (a) of compo￾site samples before and after thermomechanical loading was determined using a high temperature dilatometer (NETZSCH GmbH TMA 402). The samples were cut with a diamond saw to an approximate length of 23 mm. Before carrying out the experiments, the dimensions of the samples were measured using a vernier with a precision of 0.05 mm. The measurements were carried out in normal atmosphere between room temperature (20 8C) and 750 8C with a heating rate of 5 K min1 . At least two measurements were performed for each specimen and the results were averaged. The values were calculated for temperature intervals: 20–300, 20–400 and 20– 500 8C. The accuracy of the measured values was Da=F1
107 K1 . A high reproducibility of results was confirmed. A.R. Boccaccini et al. / Materials Characterization 54 (2005) 75–83 77

A.R. Boccaccini et al. Materials Characterization 54(2005)75-83 3. Results and discussion Negligible differences exist between the values measured for the different samples, except for the 3.I. Microstructure of the investigated material specimen subjected 20 times to thermal shock (AT=630C), indicating that this sample may have A SEM micrograph showing the microstructure of suffered a higher degree of microstructural damage a polished section of an as received sample is shown than the rest. The experimental values of the thermal in Fig. 1. It can be seen that the matrix material is expansion coefficient obtained for the temperature dense, without residual porosity, and the composite ranges 20-400 and 20-500C were very similar to exhibits a fairly regular fibre distribution wn In Fig Fig 3 shows the thermal expansion curve obtained 3.2. Evaluation of the thermal expansion behaviour expansion that characterises the glass transition Considering the values of thermal expansion temperature (Ty) of the glass matrix (525C for efficient pertaining to the duran)glass matrix dURAN glass)[17] is not apparent due to the and to the silicon carbide fibres(am and dominant influence of the fibres respectively ), and taking into account that the fibre ig. 4 shows the thermal expansion curves of both volume fraction is -0.4, the theoretical value of the the first and second measurements performed on the thermal expansion coefficient of the composite can be same sample which had been subjected to thermal calculated by a simple rule of mixtures: sho oc= aflf + amM measurement a“wave" between300and400°Ccan be observed. this is characteristic of materials that where Pr and Vm are the fibre and matrix volume possess internal stresses. These stresses are generated fractions, respectively. With the values of ar and om as result of the progressive number of thermal shocks given in Table 1, the thermal expansion coefficient [ 18]. Concerning the second measurement on the obtained from Eq. (1)is: ac-3. 19x10-6K- same sample, a different value of thermal expansion Fig. 2 shows the experimental values of the coefficient was obtained, which is very close to that thermal expansion coefficient measured in the temper- calculated by Eq. (1). Moreover, no"wave"is ature range of 20-300C for all samples investigated. observed in the curve, as shown in Fig. 4. This As can be this graph, and taking into suggests that during heating the sample up to 750C, account the average error of the measurements during the first measurement of the thermal expansion (+lx10K), the measured values are very close coefficient, an internal relaxation of stresses was to that calculated by the rule of mixtures(Eq(1). induced. Previous studies on the same material [12 have shown that thermal shocks of At=570C do not cause microstructural damage in this type of compo site, at least up to 21 cycles. However, thermal shocks with△T≥690° C produce serious microscopic degra- dation of the material in the form of microcracks delamination and oxidation of the carbon-rich inter face and of Nicalon SiC fibres. In the same previous study [12], microstructural damage was confirmed on samples subjected to thermal shock for 20 cycles with a thermal gradient equal to that utilised in this work (AT=630C). In that study, a forced-vibration technique and flexural strength tests were used to 50m detect microstructural damage. The damage detected was in the form of small microcracks In the Fig. 1 SEM micrograph of the polished section of the composit glass ial investigated. A regular distribution of Sic fibres in the matrix, without major delamination or mass changes matrix is observed and absence of porosit samples. This precludes the

3. Results and discussion 3.1. Microstructure of the investigated material A SEM micrograph showing the microstructure of a polished section of an as received sample is shown in Fig. 1. It can be seen that the matrix material is dense, without residual porosity, and the composite exhibits a fairly regular fibre distribution. 3.2. Evaluation of the thermal expansion behaviour Considering the values of thermal expansion coefficient pertaining to the (DURANR) glass matrix and to the silicon carbide fibres (am and af, respectively), and taking into account that the fibre volume fraction is ~0.4, the theoretical value of the thermal expansion coefficient of the composite can be calculated by a simple rule of mixtures: aC ¼ afVf þ amVm ð1Þ where Vf and Vm are the fibre and matrix volume fractions, respectively. With the values of af and am given in Table 1, the thermal expansion coefficient obtained from Eq. (1) is: aC=3.19
106 K1 . Fig. 2 shows the experimental values of the thermal expansion coefficient measured in the temper￾ature range of 20–300 8C for all samples investigated. As can be seen in this graph, and taking into account the average error of the measurements (F1
107 K1 ), the measured values are very close to that calculated by the rule of mixtures (Eq. (1)). Negligible differences exist between the values measured for the different samples, except for the specimen subjected 20 times to thermal shock (DT=630 8C), indicating that this sample may have suffered a higher degree of microstructural damage than the rest. The experimental values of the thermal expansion coefficient obtained for the temperature ranges 20–400 and 20–500 8C were very similar to those shown in Fig. 2. Fig. 3 shows the thermal expansion curve obtained for the as-received sample. The quick increase of expansion that characterises the glass transition temperature (Tg) of the glass matrix (525 8C for DURANR glass) [17] is not apparent due to the dominant influence of the fibres. Fig. 4 shows the thermal expansion curves of both the first and second measurements performed on the same sample which had been subjected to thermal shock for 20 cycles (DT=630 8C). In the first measurement a bwaveQ between 300 and 400 8C can be observed. This is characteristic of materials that possess internal stresses. These stresses are generated as result of the progressive number of thermal shocks [18]. Concerning the second measurement on the same sample, a different value of thermal expansion coefficient was obtained, which is very close to that calculated by Eq. (1). Moreover, no bwaveQ is observed in the curve, as shown in Fig. 4. This suggests that during heating the sample up to 750 8C, during the first measurement of the thermal expansion coefficient, an internal relaxation of stresses was induced. Previous studies on the same material [12] have shown that thermal shocks of DT=570 8C do not cause microstructural damage in this type of compo￾site, at least up to 21 cycles. However, thermal shocks with DTz690 8C produce serious microscopic degra￾dation of the material in the form of microcracks, delamination and oxidation of the carbon-rich inter￾face and of NicalonR SiC fibres. In the same previous study [12], microstructural damage was confirmed on samples subjected to thermal shock for 20 cycles with a thermal gradient equal to that utilised in this work (DT=630 8C). In that study, a forced-vibration technique and flexural strength tests were used to detect microstructural damage. The damage detected was in the form of small microcracks in the glass matrix, without major delamination or mass changes of the samples. This precludes the possibility of a Fig. 1. SEM micrograph of the polished section of the composite material investigated. A regular distribution of SiC fibres in the glass matrix is observed and absence of porosity. 78 A.R. Boccaccini et al. / Materials Characterization 54 (2005) 75–83

A.R. Boccaccini et al. Materials Characterization 54(2005)75-83 a), b), c)Samples in the as-received condition. d), e) Samples thermally aged for 100 hs at 600 and 700 C, respectively 20-300°C ample mechanically pre-stressed above the fracture load j, k) Samples mechanically pre-stressed above the fracture load subjected to thermal shock. 4 3,5 a) b) c) d) e) f g) h) i) 1) k) Fig. 2. values obtained for the thermal expansion coefficient in the range 20-300C for all composites investigated (as-received and after thermomechanical loading, as indicated in the legend): (O) first measurement, (O)second measurement, (O) third measurement. The error average in the measurements was±1×10-7K-1 significant oxidation of the interface at those inter-wave"in the thermal expansion curve recorded mediate temperatures. Matrix microcracking occurs during the first measurement of ac(Fig. 4) because in a quenching test, the sample surface is Regarding the samples thermally cycled in air,a brought to the temperature of the cooling medium previous study [10] has shown that microstructural rapidly, whereas the interior of the samples remains at damage can be mainly attributed to partial oxidation high temperature. This temperature gradient creates of the interfaces and to softening of the glass matrix internal stresses, which are tensile at the surface and during the exposure to a high-temperature oxidising compressive in the interior. With an increasing environment. In the previous study [10, thermal number of quenching cycles, the tensile stresses at cycling in air from 700C to room temperature the surfaces can reach a critical value sufficient to resulted in the generation of microstructural damage microcracks in the matrix. In this investigation, which was detected after 77 cycles by the simulta- nulated residual stresses are detected by the neous decrease in Youngs modulus and increase 2 200300 Temperature(C) Fig. 3. Curve representing the thermal expansion behaviour of the composite material investigated in the initial state(as-received). Heating rate=s K min

significant oxidation of the interface at those inter￾mediate temperatures. Matrix microcracking occurs because in a quenching test, the sample surface is brought to the temperature of the cooling medium rapidly, whereas the interior of the samples remains at high temperature. This temperature gradient creates internal stresses, which are tensile at the surface and compressive in the interior. With an increasing number of quenching cycles, the tensile stresses at the surfaces can reach a critical value sufficient to create microcracks in the matrix. In this investigation, the accumulated residual stresses are detected by the bwaveQ in the thermal expansion curve recorded during the first measurement of ac (Fig. 4). Regarding the samples thermally cycled in air, a previous study [10] has shown that microstructural damage can be mainly attributed to partial oxidation of the interfaces and to softening of the glass matrix during the exposure to a high-temperature oxidising environment. In the previous study [10], thermal cycling in air from 700 8C to room temperature resulted in the generation of microstructural damage, which was detected after 77 cycles by the simulta￾neous decrease in Young’s modulus and increase in Fig. 2. Values obtained for the thermal expansion coefficient in the range 20–300 8C for all composites investigated (as-received and after thermomechanical loading, as indicated in the legend): (o) first measurement, (n) second measurement, ( R ) third measurement. The error average in the measurements was F1
107 K1 . Fig. 3. Curve representing the thermal expansion behaviour of the composite material investigated in the initial state (as-received). Heating rate=5 K min1 . A.R. Boccaccini et al. / Materials Characterization 54 (2005) 75–83 79