Availableonlineatwww.sciencedirectcom ScienceDirect E噩≈RS ELSEVIER Journal of the European Ceramic Society 27(2007)377-388 www.elsevier.com/locate/jeurceramsoc Degradation mechanisms of a sic fiber reinforced self-sealing matrix composite in simulated combustor environments Ludovic Quemard, francis rebillat", Alain Guette Henri Tawil. Caroline louchet-Pouillerie Laboratoire des Composites Thenmostructuraux, UMR 5801(CNRS-SAFRAN-CEA-UB1), 3 Allee de la boetie, 33600 Pessac, France b Snecma Propulsion Solide, Les Cing Chemins, 33187 Le Haillan, france Received 23 November 2005: received in revised form 10 February 2006: accepted 17 February 2006 Available online 26 May 2006 Non-oxide ceramic matrix composites are potential candidates to replace the current nickel-based alloys for a variety of high temperature applications in the aerospace field. The durability of a SiC(/Py Co/Si, C, Blm) composite with a multi-layered self-sealing matrix and Hi-Nicalon fibers was investigated at 1200C for exposure durations up to 600 h. The specimens are aged in a variety of slow-flowing air/steam gas mixtures and total pressures, ranging from atmospheric pressure with a 10-50% water content to l MPa with 10-20% water content. The degradation of the composite was determined from the measurement of residual strength and strain to failure on post-exposure specimens and correlated with microstructural observation of the damaged tows. The most severe degradation of the composite occurred at 1 MPa in an air/steam(80/20)gas mixture. Correlation between this degradation and the dissolving of the Sic fibers in the generated boria-containing glass, is discussed. 2006 Elsevier ltd. all rights reserved. Keywords: Ceramic matrix composites; Corrosion; Mechanical properties; Lifetime; Engine components; SiC fibers 1. Introduction self-sealing approach are to consume part of the incoming oxy- gen and limit access of residual oxygen to the PyC interphase Non-oxide ceramic matrix composites such as SiCon/PyCa/ by sealing the matrix microcracks with a Sio2-B2O3 oxide SiC(m) consist of SiC matrix reinforced with SiC fibers and pyro- phase. However, previous studies showed that B2034-6. 8. 12. 13 carbon(PyC)interfacial coating. These composites exhibit a and Sio2 5, 7 can volatilize, respectively, at 600 and 1100oC low density associated with high thermomechanical properties under water vapor-containing environments. This phenomenon and are potential candidates to replace the current nickel-based can cause the self-sealing capability to degrade, thus reducing alloys for a variety of long-term applications in the aerospace the lifetime of the SiC(/PyCo/[Si, C, b](m) field. In these applications, SiCon/Py Co/SiC(m) components The matrix layers are SiC, B4C and a phase noted Si-B-C. can be subjected to service conditions that include mechani- The efficiency of the self-sealing process under environments cal loading under intermediate to high temperatures and high containing both oxygen and water vapor, results from the com- pressure complex environment containing oxygen and steam. petition between the oxidation of the matrix layers and the The oxidation of the PyC weak interphase can occur under volatilization of the generated oxide phase dry air at a temperature lower than 500C and leads to inter- Under dry air, B4C undergoes oxidation and volatiliza- facial degradations of SiC(o/PyCo/SiC(m). SiC(n/PyCo/Si, C, tion reactions below 600 and 900C, respectively, as shown B(m) composites with a sequenced ealing matrix have been below 4-6,, 13 developed,and investigated-Ito protect the PyC interphase against oxidation effects up to 1400C. The principles of the B4C(s)+4O2(g)=2B2O3+CO2(g) (1) (2) Corresponding author. Fax: +33 5 5684 12 25. Under water vapor-containing environments, B2O301 may E-mail address: rebillat@lcts. u-bordeauxl fr(F. Rebillat) react significantly at 600C to form hydroxydes by the following 0955-2219/S-see front matter o 2006 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc. 2006.02.042
Journal of the European Ceramic Society 27 (2007) 377–388 Degradation mechanisms of a SiC fiber reinforced self-sealing matrix composite in simulated combustor environments Ludovic Quemard a, Francis Rebillat a,∗, Alain Guette a, Henri Tawil b, Caroline Louchet-Pouillerie b a Laboratoire des Composites Thermostructuraux, UMR 5801 (CNRS-SAFRAN-CEA-UB1), 3 All´ee de la Bo´etie, 33600 Pessac, France b Snecma Propulsion Solide, Les Cinq Chemins, 33187 Le Haillan, France Received 23 November 2005; received in revised form 10 February 2006; accepted 17 February 2006 Available online 26 May 2006 Abstract Non-oxide ceramic matrix composites are potential candidates to replace the current nickel-based alloys for a variety of high temperature applications in the aerospace field. The durability of a SiC(f)/PyC(i)/[Si, C, B](m) composite with a multi-layered self-sealing matrix and Hi-Nicalon fibers was investigated at 1200 ◦C for exposure durations up to 600 h. The specimens are aged in a variety of slow-flowing air/steam gas mixtures and total pressures, ranging from atmospheric pressure with a 10–50% water content to 1 MPa with 10–20% water content. The degradation of the composite was determined from the measurement of residual strength and strain to failure on post-exposure specimens and correlated with microstructural observation of the damaged tows. The most severe degradation of the composite occurred at 1 MPa in an air/steam (80/20) gas mixture. Correlation between this degradation and the dissolving of the SiC fibers in the generated boria-containing glass, is discussed. © 2006 Elsevier Ltd. All rights reserved. Keywords: Ceramic matrix composites; Corrosion; Mechanical properties; Lifetime; Engine components; SiC fibers 1. Introduction Non-oxide ceramic matrix composites such as SiC(f)/PyC(i)/ SiC(m) consist of SiC matrix reinforced with SiC fibers and pyrocarbon (PyC) interfacial coating. These composites exhibit a low density associated with high thermomechanical properties and are potential candidates to replace the current nickel-based alloys for a variety of long-term applications in the aerospace field. In these applications, SiC(f)/PyC(i)/SiC(m) components can be subjected to service conditions that include mechanical loading under intermediate to high temperatures and high pressure complex environment containing oxygen and steam. The oxidation of the PyC weak interphase can occur under dry air at a temperature lower than 500 ◦C and leads to interfacial degradations of SiC(f)/PyC(i)/SiC(m). SiC(f)/PyC(i)/[Si, C, B](m) composites with a sequenced self-sealing matrix have been developed1,2 and investigated3–11 to protect the PyC interphase against oxidation effects up to 1400 ◦C. The principles of the ∗ Corresponding author. Fax: +33 5 56 84 12 25. E-mail address: rebillat@lcts.u-bordeaux1.fr (F. Rebillat). self-sealing approach are to consume part of the incoming oxygen and limit access of residual oxygen to the PyC interphase by sealing the matrix microcracks with a SiO2–B2O3 oxide phase. However, previous studies showed that B2O3 4–6,8,12,13 and SiO2 15,17 can volatilize, respectively, at 600 and 1100 ◦C under water vapor-containing environments. This phenomenon can cause the self-sealing capability to degrade, thus reducing the lifetime of the SiC(f)/PyC(i)/[Si, C, B](m). The matrix layers are SiC, B4C and a phase noted Si–B–C. The efficiency of the self-sealing process under environments containing both oxygen and water vapor, results from the competition between the oxidation of the matrix layers and the volatilization of the generated oxide phase. Under dry air, B4C undergoes oxidation and volatilization reactions below 600 and 900 ◦C, respectively, as shown below4–6,8,12,13: B4C(s) + 4O2(g) = 2B2O3(l) + CO2(g) (1) B2O3(l) = B2O3(g) (2) Under water vapor-containing environments, B2O3(l) may react significantly at 600 ◦C to form hydroxydes by the following 0955-2219/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2006.02.042
78 L. Quemard et al. Journal of the European Ceramic Sociery 27 (2007)377-388 reactions 46,8,12,13 H3B3O Tranverse 3B2030+2H2O(g=H3 BO3e fiber tows Bulk porosity +5H2O HBO Multi-layered The competition between the oxidation()and volatilization reactions(2)(5)can lead to the recession of B4C In high temperature environments and under dry air, the active oxidation of Sic occurs above 1000C and below Po,=100 Pa Fig. 1. Polished cross-section of the as-received C410 material. according to(6). At a higher Po,, a paralinear oxidation of Sic 4 occurs to form a protective Sio2 scale according to(7) In environments containing O2 and H2O, the formation of Sioz different matrix layers are crystallised SiC, amorphous B4C and is dramatically enhanced by reaction(8)4-16 a SiC-B4C phase named Si-B-C which can be described as a SiC(s)+O2(g)=Siog)+CO( (6) mixture of Sic nanocrystals in a B, C amorphous phase.7The sequenced matrix is reinforced by Hi-Nicalon "SiC fibers and 2SiO(s)+2C0 (7) plane multi-layer reinforcement is used to eliminate delamina- tion sensitivity that is common for 2D ceramic matrix compos SiO2(s)+CO(g)+3H (8) ites Fiber volume fraction, material density and mainly closed Under steam-containing environments, the SiO2 scale may bulk porosity, as reported by the composite manufacturer, are, volatilize. by the main following reaction respectively, 34%0, 2.25+0.05 and 13+1%. The interphase is pyrocarbon SiO2(s)+2H2 O(g)= Si(oh)4(g) The test specimen geometry used in this study has a reduced gauge section(Fig. 2). It is 200 mm long, with a grip sec- The volatilization rate of SiO2 5, 7 is much lower than for tion width of 24 mm, a reduced gauge section width of 16mm B2034-6. 8, 12,13 However, as for boron matrix layers, the com- and a thickness of 4.4 mm. Two different material batches petition between the oxide formation(7)and (8)and the oxide manufactured in similar conditions, are used for these works phase volatilization(9)reactions can lead to the recession of Test samples are machined from composite plates using dia- mond grinding and then are seal-coated with CVI layers of The Si-B-C matrix layer can be described as a mixture Sic, B4C and Si-B-C. The sequenced seal-coat thickness, of Sic nanocrystals in an amorphous B4C phase. A previous with a SiC final layer, is 120 um on the composite surface Idy investigated the oxidation of Si-B-C coatings under oxy- and about 40 um on the machined edges. Corrosion tests gen and steam-containing environments by thermogravimetric are also performed on Si-B-C coating and SiC/SiC coupons analysis. 4.5 It has been shown that SiC nanocrystals can oxidize for comparison purposes. The Si-B-C coating, with a thick significantly at 600C and at atmospheric pressure, simultane- ness of 30+3 um, is deposited on Sic chips(diameter of ously to the B2 O3 formation, to form silica according to reaction 8 mm and thickness of 2 mm)via chemical vapor deposition Previous works 9 focused on the corrosion behavior of the (CVD) SiC(o/PyCo/Si, C, Blm)at high pressure and at 600C. The 3. Test procedures aim of this study is to evaluate, at a higher temperature, the effects of both oxygen and water vapor on the self-sealing 3. 1. Pre-damaging process of SiC(o/PyCo/Si, C, B](m) composites subjected to high-pressure environments. Corrosion tests are conducted for The dog-bone specimens are loaded in tension monotonically long periods of time on SiC(o/PyC(/Si,C, B](m)composites at at room temperature to a tensile stress of 150 MPa(the stress cor- 1200C in oxygen and steam-containing environments at atmo spheric pressure and high pressure. Post-exposure mechanical tests are performed at room temperature(rT)to investigate the effects of corrosion phenomena on the retained mechanical prop- erties 2. Materials and test specimens The material investigated is the CERASEPA41010, I (C410) manufactured by Snecma Propulsion Solide(France) via chemical vapor infiltration(CVD). It is a woven-SiC-fiber Fig 2. C410 specimen geometry used in this study. Dim re in millime. reinforced [Si, C, B] sequenced matrix composite(Fig. 1). The ters
378 L. Quemard et al. / Journal of the European Ceramic Society 27 (2007) 377–388 reactions4–6,8,12,13: 3 2B2O3(l) + 3 2H2O(g) = H3B3O6(g) (3) 3 2B2O3(l) + 1 2H2O(g) = H3BO3(g) (4) 1 2B2O3(l) + 1 2H2O(g) = HBO2(g) (5) The competition between the oxidation (1) and volatilization reactions (2)–(5) can lead to the recession of B4C. In high temperature environments and under dry air, the active oxidation of SiC occurs above 1000 ◦C and below PO2 = 100 Pa according to (6). At a higher PO2 , a paralinear oxidation of SiC14 occurs to form a protective SiO2 scale according to (7). In environments containing O2 and H2O, the formation of SiO2 is dramatically enhanced by reaction (8)14–16: SiC(s) + O2(g) = SiO(g) + CO(g) (6) 2SiC(s) + 3O2(g) = 2SiO2(s) + 2CO(g) (7) SiC(s) + 3H2O(g) = SiO2(s) + CO(g) + 3H2(g) (8) Under steam-containing environments, the SiO2 scale may volatilize, by the main following reaction17: SiO2(s) + 2H2O(g) = Si(OH)4(g) (9) The volatilization rate of SiO2 15,17 is much lower than for B2O3. 4–6,8,12,13 However, as for boron matrix layers, the competition between the oxide formation (7) and (8) and the oxide phase volatilization (9) reactions can lead to the recession of SiC.18–20 The Si–B–C matrix layer can be described as a mixture of SiC nanocrystals in an amorphous B4C phase.7 A previous study investigated the oxidation of Si–B–C coatings under oxygen and steam-containing environments by thermogravimetric analysis.4,5 It has been shown that SiC nanocrystals can oxidize significantly at 600 ◦C and at atmospheric pressure, simultaneously to the B2O3 formation, to form silica according to reaction (7). Previous works9 focused on the corrosion behavior of the SiC(f)/PyC(i)/[Si, C, B](m) at high pressure and at 600 ◦C. The aim of this study is to evaluate, at a higher temperature, the effects of both oxygen and water vapor on the self-sealing process of SiC(f)/PyC(i)/[Si, C, B](m) composites subjected to high-pressure environments. Corrosion tests are conducted for long periods of time on SiC(f)/PyC(i)/[Si, C, B](m) composites at 1200 ◦C in oxygen and steam-containing environments at atmospheric pressure and high pressure. Post-exposure mechanical tests are performed at room temperature (RT) to investigate the effects of corrosion phenomena on the retained mechanical properties. 2. Materials and test specimens The material investigated is the CERASEP®A41010,11 (C410) manufactured by Snecma Propulsion Solide (France) via chemical vapor infiltration (CVI). It is a woven-SiC-fiber reinforced [Si, C, B] sequenced matrix composite (Fig. 1). The Fig. 1. Polished cross-section of the as-received C410 material. different matrix layers are crystallised SiC, amorphous B4C and a SiC–B4C phase named Si–B–C which can be described as a mixture of SiC nanocrystals in a B4C amorphous phase.7 The sequenced matrix is reinforced by Hi-Nicalon® SiC fibers and plane multi-layer reinforcement is used to eliminate delamination sensitivity that is common for 2D ceramic matrix composites. Fiber volume fraction, material density and mainly closed bulk porosity, as reported by the composite manufacturer, are, respectively, 34%, 2.25 ± 0.05 and 13 ± 1%. The interphase is pyrocarbon. The test specimen geometry used in this study has a reduced gauge section (Fig. 2). It is 200 mm long, with a grip section width of 24 mm, a reduced gauge section width of 16 mm and a thickness of 4.4 mm. Two different material batches, manufactured in similar conditions, are used for these works. Test samples are machined from composite plates using diamond grinding and then are seal-coated with CVI layers of SiC, B4C and Si–B–C. The sequenced seal-coat thickness, with a SiC final layer, is 120m on the composite surface and about 40 m on the machined edges. Corrosion tests are also performed on Si–B–C coating and SiC/SiC coupons for comparison purposes. The Si–B–C coating, with a thickness of 30 ± 3m, is deposited on SiC chips (diameter of 8 mm and thickness of 2 mm) via chemical vapor deposition (CVD). 3. Test procedures 3.1. Pre-damaging The dog-bone specimens are loaded in tension monotonically at room temperature to a tensile stress of 150 MPa (the stress corFig. 2. C410 specimen geometry used in this study. Dimensions are in millimeters.
L Quemard et al. Journal of the European Ceramic Society 27(2007)377-388 Table 1 Summary of the test conditions for C410 composites exposed at 1200C in various environments PTot(MPa) Air/steam Po,(kPa) PH,o(kPa) PH,o/Po, v(cms) Furnace ABCDEF Furnace 1200±15 1220±25 0.56 HP furna 1225±30 1 HP furnace 8020 responding to twice their elastic limit) then unloaded before the 3.2. Corrosion tests corrosion exposures. The aim of this pre-damaging is to generate a controlled crack network in the matrix, which facilitates the The corrosion test conditions are reported in Table 1. Two ingress of the corrosive species. The residual strain is very low corrosion test equipments are used for these tests. High pressure (0.001%)and can be neglected for post-exposure mechanical corrosion tests are conducted in the high pressure-high tem- tests. The pre-damaging microcracks are mainly located in the perature furnace-(Fig 3). High pressure air is provided by a seal-coat of the gauge section of the specimens. At room tem- pressurized gas supply system then mixed with water in an evap- perature, their mean spacing distance is 230+ 30 um and their orator. The air and water flows are independently controlled by width is 0.5-3 um. In addition, few microcracks with a width mass flow meters and the air/H2o gas mixture is injected in the lower than 1 um at RT are present at the edge of the macrop- alumina test tube (i d: 34 mm, purity: 99.7%, OMG, France) porosities of the furnace. A system of pneumatically driven back pressure allv driven hack nn val HyOn mass now me Pneumatically driven PRESSURIZED GAS let tuhe Pneumatically driven Dry air mass nkm meter SIFAM GENERAT(N Fig 3. Schematic of the high temperature-high pressure corrosion test equipment(a)and view of the fumace(b)
L. Quemard et al. / Journal of the European Ceramic Society 27 (2007) 377–388 379 Table 1 Summary of the test conditions for C410 composites exposed at 1200 ◦C in various environments No. Exposure T ( ◦C) PTot (MPa) Air/steam PO2 (kPa) PH2O (kPa) PH2O/PO2 v (cm s−1) A Furnace 1210 ± 10 0.1 90/10 18 10 0.56 5 B Furnace 1210 ± 10 0.1 80/20 16 20 1.25 5 C Furnace 1200 ± 15 0.1 50/50 10 50 5 10 D HP furnace 1220 ± 25 0.45 90/10 81 45 0.56 8 E HP furnace 1225 ± 30 1 90/10 180 100 0.56 8 F HP furnace 1225 ± 30 1 80/20 160 200 1.25 8 responding to twice their elastic limit) then unloaded before the corrosion exposures. The aim of this pre-damaging is to generate a controlled crack network in the matrix, which facilitates the ingress of the corrosive species. The residual strain is very low (∼=0.001%) and can be neglected for post-exposure mechanical tests. The pre-damaging microcracks are mainly located in the seal-coat of the gauge section of the specimens. At room temperature, their mean spacing distance is 230 ± 30m and their width is 0.5–3m. In addition, few microcracks with a width lower than 1 m at RT are present at the edge of the macroporosities. 3.2. Corrosion tests The corrosion test conditions are reported in Table 1. Two corrosion test equipments are used for these tests. High pressure corrosion tests are conducted in the high pressure–high temperature furnace21 (Fig. 3). High pressure air is provided by a pressurized gas supply system then mixed with water in an evaporator. The air and water flows are independently controlled by mass flow meters and the air/H2O gas mixture is injected in the alumina test tube (i.d.: 34 mm, purity: 99.7%, OMG, France) of the furnace. A system of pneumatically driven back pressure Fig. 3. Schematic of the high temperature–high pressure corrosion test equipment (a) and view of the furnace (b).
380 L. Quemard et al Journal of the European Ceramic Sociery 27 (2007)377-388 Bubbling exhaus presaturator Flow meter Fig. 4. Schematic of the high temperature furnace and the water vapor saturator at atmospheric pressure. reducers maintains a slight difference of pressure between the After exposures, the test specimens are cut perpendicularly tube interior and the metallic vessel(Ptube -Pvessel=-3 kPa). and parallel to the loading axis then polished for examina This permits to minimize stresses on the tube and increase its tion using an optical microscope. Moreover, the fractured sur- airtightness. The uniform heating zone of the furnace is approx faces are analyzed by scanning electron microscopy(SEM) imately 120 mm long which is longer than the gage length of the The chemical composition of the oxide phase formed during the corrosion tests is determined on polished cross-sections of A high temperature furnace associated with a water saturator the composites using electron probe micro analysis(EPMA is used to run the corrosion tests at atmospheric pressure( Fig 4). CAMECA SX100) The dry air flows through a heated water column in order to be saturated in steam before its introduction in the alumina tube 4. Results (i.d. 34 mm, purity: 99.7%, OMG, France)of the furnace. The temperature of the water in the column is slightly higher than the 4.1. Post-expo chanical results dewpoint corresponding to the desired water vapor partial pres sure. For example, an air/steam(90/10)gas mixture is obtained The post-exposure mechanical properties of the C410 pi for a column temperature of 48C(dewpoint for PH20= 10kPa damaged specimens are determined using cyclic tensile tests at is 46C). Water content in the gas stream is monitored by mea- RT. The results are shown in Fig. 5 and reported in Table 2. suring the condensate in the gas exhaust daily and the amount Three C410 specimens are used to determine the mechanical of water in the stock which supplies the heated column. The properties up to failure of the two batches of the as-re uniform heating zone of the furnace is approximately 220 mm material using cyclic tensile tests at RT. The failures of all the specimens occurred in the gauge In both corrosion test equipments, the C410 specimens are section. Small brittle rupture areas and wide non-brittle rup- oriented parallel to the gas flow and placed on alumina sample holders(purity: 99.7%, OMG, France)specially designed. The heating and cooling rates, used at atmospheric pressure under ambient air, are, respectively, 150 and 100Ch-. The expe scale(Precisa Instruments AG, Switzerland)with an accuracy oo t ures are regularly interrupted to weigh the specimens using a ofl×10-mg 3.3. Characterization of specimens after exposur 9四苏 Post-exposure cyclic tensile tests are performed at RT on the 5 40 dog-bone specimens. A spring-loaded clip on-gauge is attached to the 25 mm long of the straight section of the samples to record displacement. The composites are tested up to failure in a servo controlled testing machine(INSTRON 1185)equipped with self Fig. 5. The effect of corrosion environments on the ultimate strength and on the aligning grips at a cross-head speed of 0.40+0.05%o min strain to failure of the C410 specimens exposed for 600h
380 L. Quemard et al. / Journal of the European Ceramic Society 27 (2007) 377–388 Fig. 4. Schematic of the high temperature furnace and the water vapor saturator at atmospheric pressure. reducers maintains a slight difference of pressure between the tube interior and the metallic vessel (Ptube − Pvessel = −3 kPa). This permits to minimize stresses on the tube and increase its airtightness. The uniform heating zone of the furnace is approximately 120 mm long which is longer than the gage length of the specimens. A high temperature furnace associated with a water saturator is used to run the corrosion tests at atmospheric pressure (Fig. 4). The dry air flows through a heated water column in order to be saturated in steam before its introduction in the alumina tube (i.d.: 34 mm, purity: 99.7%, OMG, France) of the furnace. The temperature of the water in the column is slightly higher than the dewpoint corresponding to the desired water vapor partial pressure. For example, an air/steam (90/10) gas mixture is obtained for a column temperature of 48 ◦C (dewpoint for PH2O = 10 kPa is 46 ◦C). Water content in the gas stream is monitored by measuring the condensate in the gas exhaust daily and the amount of water in the stock which supplies the heated column. The uniform heating zone of the furnace is approximately 220 mm long. In both corrosion test equipments, the C410 specimens are oriented parallel to the gas flow and placed on alumina sample holders (purity: 99.7%, OMG, France) specially designed. The heating and cooling rates, used at atmospheric pressure under ambient air, are, respectively, 150 and 100 ◦C h−1. The exposures are regularly interrupted to weigh the specimens using a scale (Precisa Instruments AG, Switzerland) with an accuracy of 1 × 10−2 mg. 3.3. Characterization of specimens after exposure Post-exposure cyclic tensile tests are performed at RT on the dog-bone specimens. A spring-loaded clip on-gauge is attached to the 25 mm long of the straight section of the samples to record displacement. The composites are tested up to failure in a servo controlled testing machine (INSTRON 1185) equipped with self aligning grips at a cross-head speed of 0.40 ± 0.05% min−1. After exposures, the test specimens are cut perpendicularly and parallel to the loading axis then polished for examination using an optical microscope. Moreover, the fractured surfaces are analyzed by scanning electron microscopy (SEM). The chemical composition of the oxide phase formed during the corrosion tests is determined on polished cross-sections of the composites using electron probe micro analysis (EPMA, CAMECA SX100). 4. Results 4.1. Post-exposure mechanical results The post-exposure mechanical properties of the C410 predamaged specimens are determined using cyclic tensile tests at RT. The results are shown in Fig. 5 and reported in Table 2. Three C410 specimens are used to determine the mechanical properties up to failure of the two batches of the as-received material using cyclic tensile tests at RT. The failures of all the specimens occurred in the gauge section. Small brittle rupture areas and wide non-brittle rupFig. 5. The effect of corrosion environments on the ultimate strength and on the strain to failure of the C410 specimens exposed for 600 h
L Quemard et al. Journal of the European Ceramic Society 27(2007)377-388 Table 2 Summary of C4 10 composites post-exposure tensile properties at RT Test condition Exposure time(h) Weight change(%) Failure UTS (MPa) Strain to failure(%) E(GPa) +44 BBB 6245 170 As-received( 21212112 304±45 0.46±0.1 252±4 368±11 0.73±0.02 86±21 315±20 a Presence of brittle rupture areas. b Brittle rupture. n-brittle rupture ture areas characterized by fiber pull-out, are observed on the tested in low steam-pressure environment are similar to the as- fractured surfaces(Fig. 6)of the specimens exposed in low received ones(Fig. 7). Thus, a non-linear stress-strain behavior steam-pressure environments(conditions A and B). Brittle rup- without a plateau is observed up to the ultimate failure of the ture areas are located at the edge of the bulk porosities filled specimens. According to this behavior induced by matrix crack with an oxide phase. The stress-strain curves of the specimens ing, a continuous damaging occurred in the composite up to its failure. Moreover, the width of the hysteresis loops is narrow and the residual strains after unloading are very low. This ind cates a high fiber-matrix load transfer, thus a strong interfacial shear stress. The retained mechanical properties of samples aged at low steam-pressure are similar to the as-received materials and C410 data base, indicating that the fibers are not dam- aged significantly(Fig. 5). Many brittle rupture areas, located A1MPa. Air sen(Nigo) MB·A3am(20 Strain ( Borosilicate Glass MP· Air Skans门0 MPa- Air Slar ISss0 Fig. 6. SEMfracture surfaces of C410specimens in an air/ steam(80/20) b o gas mixture for 616 h at 1200C and 0.1 MPa(a) and for 603 h at 1200 C and Fig. 7. Stress-strain curves of the C410 specimens of batch 1(a)and batch 2 I MPa(b) (b)obtained at RT after exposure at 1200C for 600 h in various envi
L. Quemard et al. / Journal of the European Ceramic Society 27 (2007) 377–388 381 Table 2 Summary of C410 composites post-exposure tensile properties at RT Test condition Batch Exposure time (h) Weight change (%) Failure UTS (MPa) Strain to failure (%) E (GPa) A 2 611 +3.4 B areasa 296 0.68 170 B 1 616 +2.4 B areas 310 0.65 220 C 2 601 +3.3 Bb 195 0.22 220 D 1 606 +4.4 B 229 0.34 210 E 2 609 +3.9 B 240 0.45 170 F 1 603 +5.9 B 97 0.08 180 As-received (average) 1 – – NBc 304 ± 45 0.46 ± 0.12 252 ± 4 As-received (average) 2 – – NB 368 ± 11 0.73 ± 0.02 186 ± 21 C410 data base15,16 – – NB 315 ± 20 0.5 220 ± 25 a Presence of brittle rupture areas. b Brittle rupture. c Non-brittle rupture. ture areas characterized by fiber pull-out, are observed on the fractured surfaces (Fig. 6) of the specimens exposed in low steam-pressure environments (conditions A and B). Brittle rupture areas are located at the edge of the bulk porosities filled with an oxide phase. The stress–strain curves of the specimens Fig. 6. SEM fracture surfaces of C410 specimens exposed in an air/steam (80/20) gas mixture for 616 h at 1200 ◦C and 0.1 MPa (a) and for 603 h at 1200 ◦C and 1 MPa (b). tested in low steam-pressure environment are similar to the asreceived ones (Fig. 7). Thus, a non-linear stress–strain behavior without a plateau is observed up to the ultimate failure of the specimens. According to this behavior induced by matrix cracking, a continuous damaging occurred in the composite up to its failure. Moreover, the width of the hysteresis loops is narrow and the residual strains after unloading are very low. This indicates a high fiber–matrix load transfer, thus a strong interfacial shear stress. The retained mechanical properties of samples aged at low steam-pressure are similar to the as-received materials and C410 data base,10,11 indicating that the fibers are not damaged significantly (Fig. 5). Many brittle rupture areas, located Fig. 7. Stress–strain curves of the C410 specimens of batch 1 (a) and batch 2 (b) obtained at RT after exposure at 1200 ◦C for 600 h in various environments