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mullite composite tested in creep
Materials Science and Engineering A368 (2004) 139–144
Microstructural characterisation of an alumina/mullite composite tested in creep L. Casas a,b , J.M. Mart´ınez-Esnaola a,b,∗ a b
CEIT-Centro de Estudios e Investigaciones Técnicas de Gipuzkoa, P. Manuel Lardizabal 15, 20018 San Sebastián, Spain Escuela Superior de Ingenieros, TECNUN, Universidad de Navarra, P. Manuel Lardizabal 13, 20018 San Sebastián, Spain Received 2 April 2003; received in revised form 8 October 2003
Abstract This paper presents the results of creep testing of a ceramic matrix composite (CMC) in which a matrix of mullite (Al2 O3 /SiO2 ) and SiOC (Umox) is reinforced with a cross-ply architecture [(0/90)3 ]s of alumina (NextelTM 610) fibres. Seven specimens have been tested under different loading (50–80 MPa) and temperature (1000–1200 ◦ C) conditions. The interfacial behaviour has been analysed through fibre push-in tests performed with a nanoindentation system. Fractographic and microstructural analyses have been performed in each specimen looking for correlations between measured parameters such as fibre and bundle pullout, crack density, crack path, oxidation, and interfacial mechanical properties. © 2003 Elsevier B.V. All rights reserved. Keywords: Creep test; Ceramic matrix composites (CMC); Alumina/mullite; Nanoindentation; Interfaces
1. Introduction A range of oxide and non-oxide ceramic fibres and matrices are being studied in the last two decades as potential candidates for high temperature applications [1–5]. A combination of low density, high strength and good oxidation resistance at high temperature is required, while avoiding the brittle nature of monolithic ceramics [6,7]. In particular, oxide/oxide systems have been proposed as an attempt to prevent high temperature oxidation damage, both in the fibres [8–11] and in the whole composite, such as in alumina-based systems [12,13]. Several experimental studies have been performed in order to simulate the operating conditions of these materials (monotonic, bending, creep, fatigue, etc.). Together with mechanical testing, the development of models able to reproduce the behaviour of materials under different test conditions is a helpful key to reduce the number of laboratory tests. Anyway, these global models require previous steps such as the development of statistical models involving dif-
∗
Corresponding author. Tel.: +34-943-212800; fax: +34-943-213076. E-mail address: [email protected] (J.M. Mart´ınez-Esnaola).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.11.014
ferent parameters both at the micro- and macro-levels such as fibre and bundle pullout, crack density, crack length, strain rate, interfacial properties, etc., and their relation with the tests conditions. From the micromechanical point of view, the interfacial properties (sliding stress, friction coefficient and debonding stress) have been demonstrated to be critical for the composite system behaviour [14–17]. In particular, detailed analyses of the evolution of the interface and its influence on the response of oxide/oxide systems have been reported in [18–20].
2. Material and experimental procedure NextelTM 610/Umox is an oxide-based ceramic matrix composite produced by DaimlerChrysler Forschung (DCXFo) that consists of a [(0/90◦ )3 ]s architecture of aluminafibres of NextelTM 610 (produced by 3 M) and a mullite (Al2 O3 /SiO2 ) and SiOC (Umox) based matrix developed by DCX-Fo. After the deposition of a thin carbon interface (to protect the fibres), the matrix is produced by using a slurry of mullite powder and a polymer which will change into SiOC during pyrolysis. After infiltration of the matrix,
L. Casas, J.M. Mart´ınez-Esnaola / Materials Science and Engineering A368 (2004) 139–144
Creep strain (%)
140 0.6 0.5 0.4 0.3 0.2 0.1 0
9.2A 17.2D 10.2C
0
0.5
Creep strain (%)
(a)
1 1.5 Time (h)
2
2.5
1.0 16.2B
0.8 0.6
14.2A
0.4 0.2 0.0
9.2A 17.2D 14.2C 16.2B 10.2C 7.3C 14.2A
Temperature (◦ C)
σ (MPa)
1200 1100 1100 1100 1000 1000 1000
50 70 70 50 80 75 70
Crack spacing (m) Longitudinal section 840 550 575 540 690 550 560
± ± ± ± ± ± ±
110 230 64 210 280 110 190
Transversal section 639 660 620 570 860 624 590
± ± ± ± ± ± ±
79 310 320 230 430 74 280
Pullout (m)
80 73 37 65 81
± ± ± ± ±
110 79 25 78 53
40 ± 31
7.3C
0
50
(b) Creep strain rate (%/h)
Table 1 Crack spacing (mean ± standard deviation) and fibre pullout (mean ± standard deviation) in NextelTM 610/Umox tested in creep
14.2C
100 150 200 250 Time (h)
1
T (˚C) 1000 1100 1200
0.1 0.01
0.001 40 50 60 70 80 90 100
(c)
Stress (MPa)
observed in the two nominally identical tests at 1100 ◦ C and 70 MPa. Nevertheless, and just for purposes of subsequent comparison with creep data of the NextelTM 610 fibres, the regions of approximately constant creep strain rates in the composite have been fitted to a power law of the form Q ε˙ c = Aσ n exp − (1) RT where A is a constant, n is the creep exponent, Q is an activation energy, R is the gas constant and T is the temperature in Kelvin. The best fit to the available data is given by
Fig. 1. Creep testing of NextelTM 610/Umox: (a) and (b) Creep strain vs. time; the conditions of temperature and stress of each test are given in Table 1. (c) Power law fit of the constant strain rate regions.
part of the interface is oxidised then giving a fugitive carbon interface between fibres and matrix. Fibre diameters have been measured resulting in an average value of 11.8 ± 0.4 m (±standard deviation) [21]. Creep tests were carried out in air under stresses between 50 and 80 MPa, and temperatures between 1000 and 1200 ◦ C. Creep tests were performed in an experimental testing machine designed at the Department of Materials of CEIT. The strain was measured with a water cooled high temperature extensometer with SiC rods and a gauge length of 25 mm. A heating rate of 20 K/min was applied in all tests using a radiation furnace. Optical and scanning electron microscopy (SEM) were used for a detailed fractographic analysis and microstructural characterisation. Push-in tests were performed for the interfacial characterisation using a nanoindentation system for the interfacial study of the as-received and the creep-tested material.
Fig. 2. Broken fibres in loading direction.
3. Creep testing results Fig. 1 shows the results for creep strain (εc ) and time to rupture (tr ) obtained under different testing conditions of stress and temperature, which are detailed in Table 1. The very different lives resulting in the experiments (from 1 to 100 h) seem to indicate that the mechanisms leading to failure were also different. Considerable scatter is also
Fig. 3. Fibre pullout of specimen 14.2A.
L. Casas, J.M. Mart´ınez-Esnaola / Materials Science and Engineering A368 (2004) 139–144
A = 3.4 × 10−4 , n = 9.2 and Q = 5.9 × 105 J/mol (when ε˙ c is in percentage per hour and σ is in MPa), see Fig. 1. 4. Fractographic analysis of NextelTM 610/Umox The seven specimens tested in creep have been analysed after fracture. Three sections of each sample have been obtained, one corresponding to the fracture surface, the second one in the loading direction and the third one perpendicular to the loading direction.
141
A distribution of parallel cracks has been found both in longitudinal sections (parallel to the loading direction) and in transverse sections (perpendicular to the loading direction) (Table 1). Both in longitudinal and in transverse sections, the observed cracks do not cross the whole thickness of the specimens. One important feature observed in all the analysed specimens is that many broken fibres in loading direction appear in the crack paths (Fig. 2). Fibre pullout has been measured by SEM in each specimen. As shown in Table 1 and Fig. 3, the pullout length is
Fig. 4. EDS mapping, as-received material.
142
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very short and there is a large scatter in the measurements. Fibre pullout is negligible in this material. The electron dispersion spectroscopy (EDS) mapping technique has been used in specimens 10.2C (1000 ◦ C, 80 MPa), 16.2B (1100 ◦ C, 50 MPa) and 9.2A (1200 ◦ C, 50 MPa) to analyse the degree of oxidation in the specimens after being tested at different temperatures. Zones near the external face of the specimens have been analysed to determine which elements of the as-received material (Al, Si, O and C) are still present after testing (Figs. 4 and 5). Aluminium, corresponding to the fibres and the matrix, was detected in all the analysed specimens. As expected, the aluminium concentration in the fibres is higher than in the matrix. In sample 10.2C, the presence of Si and the oxy-
gen found in the matrix of the analysed area (Fig. 5) seem to indicate the presence of SiO2 . This is the most likely compound as the SiO2 is contained in the mullite itself and because the SiOC would oxidise to produce SiO2 and CO2 (or CO). The EDS analysis also indicates the presence of C in regions reach in SiOC in the as-received material (Fig. 4). However this is practically inexistent in the tested material (Fig. 5).
5. Interface characterisation The nanoindentation technique has been used for the micromechanical characterisation of the fibre-matrix interface.
Fig. 5. EDS mapping, specimen 10.2C.
L. Casas, J.M. Mart´ınez-Esnaola / Materials Science and Engineering A368 (2004) 139–144
143
The fibre push-in test consists of applying a load to the end of a fibre within a composite, that is oriented parallel to the direction of the applied load, to produce debonding and sliding. The fibres have been subjected to three loading-unloading cycles up to a maximum load of 500 mN. The load and the displacement of the indenter are recorded simultaneously during the experiments. One test piece has been obtained from the shoulders of specimen 14.2A, which has been considered representative of the as-received material. Three pieces from samples 9.2A, 16.2B and 14.2A have been obtained after creep testing (one test piece from each temperature, see Table 1). All the indentation tests (consisting of loading, unloading and reloading) were carried out on fibres surrounded only by matrix. In the as-received material, a maximum load of 300 mN has been sufficient to debond all tested fibres (Fig. 6(a)). A typical curve obtained from the nanoindenter for the as-received material is shown in Fig. 6(b). Debonding occurred in a sudden manner in all cases, as shown in Fig. 6(b). In sample 14.2A (1000 ◦ C, 70 MPa) loading cycles up to maximum loads of 300 mN (10 fibres), 400 mN (10 fibres) and 500 mN (30 fibres) have been performed. As in the as-received material, all the tested fibres appeared debonded. However, in this case, about 60% of the tested
Fig. 7. Fibre push-in tests on tested material: (a) tested fibres in sample 14.2A; (b) typical applied load vs. tip displacement in fibres of sample 14.2A.
fibres presented particles of matrix on the fibre surface (Fig. 7(a)). These particles could have been produced because of the enbrittlement of the matrix with temperature. Different debonds and types of debond (gradual and sudden debonds) have been observed in all tests. A typical curve (with several debonds) of a push-in test in sample 14.2A is shown in Fig. 7(b). In sample 16.2B (1100 ◦ C, 50 MPa), the fibres have been subjected to cycles up to 400 mN (10 fibres) and 500 mN (20 fibres). Only two of the fibres
Fig. 6. Fibre push-in tests on the as-received material: (a) example of tested fibres; (b) typical applied load vs. tip displacement in a fibre push-in test in the as-received material.
Fig. 8. Tested fibres in sample 9.2A.
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tested at 400 mN appeared debonded and both presented a dirty surface. In the rest of the fibres no debond occurred. 20% of the fibres tested at 500 mN presented no debond. Up to 50% of the fibres tested in sample 9.2A (1200 ◦ C, 50 MPa) presented no debond and only one fibre presented its perimeter completely debonded. Most of them presented only partial debonds (Fig. 8). As in samples 14.2A and 16.2B, some fibres presented particles of matrix on their surface after testing. 6. Discussion and conclusions The interfacial study of NextelTM 610/Umox has pointed out a clear degradation with temperature of the interfacial properties. In the as-received material a load of 300 mN was high enough to cause the debond in all tested fibres (sudden debond in all cases). However, in sample 9.2A (1200 ◦ C, 50 MPa) 50% of the fibres did not debond after applying a load of 500 mN. The higher the temperature, the larger the number of fibres that remain bonded and the stronger the fibre-matrix adhesion. The absence of carbon in the EDS analysis shows the oxidation of the whole material due to the oxygen ingress through matrix cracking. The oxidation of the “fugitive carbon” interface produces a strong bonding between the fibre and the matrix (probably due to the subsequent oxidation of matrix and fibre layers next to the interface), which causes the enbrittlement of the material. This observation correlates well with the existence of broken fibres on the crack path (instead of being deflected along the interface), and the short fibre pullout length observed. A creep exponent in the range 2.8−3.1 has been reported for single fibres of NextelTM 610 [22] at temperatures between 1000 and 1200 ◦ C, which is much lower than that obtained in this work (∼9). This seems to indicate that the creep behaviour of the material is not controlled by the creep of the fibres. It can be concluded that, at the investigated temperatures (1000–1200 ◦ C), NextelTM 610/Umox might be used in applications below its matrix cracking stress because the oxidation of the material produces its enbrittlement and catastrophic failure. Acknowledgements This work is part of the Brite-EuRam project BE97-4020 with financial support from the European Commission,
co-ordinated by Rolls-Royce plc, in collaboration with Rolls-Royce Deutschland, MTU, ITP, Ansaldo Ricerche, Qinetiq, IE-JRC Petten and CEIT. The authors would also like to thank the valuable comments received during the refereeing process.
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[17] [18] [19] [20]
[21]
[22]
D.M. Wilson, J. Mater. Sci. 32 (1997) 2535. A.R. Bunsell, M.H. Berger, J. Eur. Ceram. Soc. 20 (2000) 2249. B. Wilshire, F. Carreño, J. Eur. Ceram. Soc. 20 (2000) 463. C.G. Papakonstantinou, P. Balaguru, R.E. Lyon, Composites Part B 32 (2001) 637. L. Casas, J.M. Mart´ınez-Esnaola, Acta Mater. 51 (2003) 3745. M.R. Elizalde, Efecto de la intercara en materiales compuestos de matriz cerámica y fibras continuas, Doctoral thesis, Escuela Superior de Ingenieros, TECNUN, Universidad de Navarra, San Sebastián, 1997. H. Ohnabe, M. Masaki, M. Onozuka, M. Miyahara, T. Sasa, Composites Part A 10 (1999) 489. K. Jakus, V. Tulluri, Ceram. Eng. Sci. Proc. 10 (1982) 1338. D.D. Johnson, A.R. Holtz, M.F. Grether, Ceram. Eng. Sci. Proc. 8 (1987) 744. V. Lavaste, M.H. Berger, A.R. Bunsell, J. Besson, J. Mater. Sci. 30 (1995) 4215. F. Deléglise, M.H. Berger, A.R. Bunsell, J. Eur. Ceram. Soc. 22 (2002) 1501. K.K. Chawla, H. Liu, J. Janczak-Rusch, S. Sambasivan, J. Eur. Ceram. Soc. 20 (2000) 551. M. Holmquist, R. Lundberg, O. Sucre, A.G. Razell, L. Molliex, J. Benoit, J. Adleborn, J. Eur. Ceram. Soc. 20 (2000) 599. E. Lara-Curzio, Composites Part A 30 (1999) 549. A.M. Daniel, Interfacial properties of fibre reinforced ceramic matrix composites, Ph.D. thesis, University of Warwick, 1994. M.R. Elizalde, A.M. Daniel, I. Puente, A. Mart´ın-Meizoso, J.M. Mart´ınez-Esnaola, Anales de Mecánica de la Fractura 14 (1997) 355. R.J. Kerans, T.A. Parthasarathy, Composites Part A 30 (1999) 521. C.G. Levi, J.Y. Yang, B.J. Dalgleish, F.W. Zok, A.G. Evans, J. Am. Ceram. Soc. 78 (1998) 2077. K.A. Keller, T. Mah, T.A. Parthasarathy, E.E. Boakye, M.K. Cinibulk, Ceram. Engng. Sci. Proc. 22 (2001) 667. R.J. Kerans, R.S. Randall, E.E. Boakye, K.A. Keller, T. Mah, T.A. Parthasarathy, M.K. Cinibulk, in: W. Krenkel, R. Naslain, H. Schneider (Eds.), High Temperature Ceramic Matrix Composites, HT-CMC4, Wiley-VCH, Weinheim, 2001, pp. 129– 135. L. Casas, Fluencia de CMCs reforzados con fibras largas, Doctoral thesis, Escuela Superior de Ingenieros, TECNUN, Universidad de Navarra, San Sebastián, 2003. D.M. Wilson, D.C. Luenburg, S.L. Lieder, Ceram. Eng. Sci. Proc. 14 (1993) 609.
Microstructural characterisation of an alumina/mullite composite tested in creep L. Casas a,b , J.M. Mart´ınez-Esnaola a,b,∗ a b
CEIT-Centro de Estudios e Investigaciones Técnicas de Gipuzkoa, P. Manuel Lardizabal 15, 20018 San Sebastián, Spain Escuela Superior de Ingenieros, TECNUN, Universidad de Navarra, P. Manuel Lardizabal 13, 20018 San Sebastián, Spain Received 2 April 2003; received in revised form 8 October 2003
Abstract This paper presents the results of creep testing of a ceramic matrix composite (CMC) in which a matrix of mullite (Al2 O3 /SiO2 ) and SiOC (Umox) is reinforced with a cross-ply architecture [(0/90)3 ]s of alumina (NextelTM 610) fibres. Seven specimens have been tested under different loading (50–80 MPa) and temperature (1000–1200 ◦ C) conditions. The interfacial behaviour has been analysed through fibre push-in tests performed with a nanoindentation system. Fractographic and microstructural analyses have been performed in each specimen looking for correlations between measured parameters such as fibre and bundle pullout, crack density, crack path, oxidation, and interfacial mechanical properties. © 2003 Elsevier B.V. All rights reserved. Keywords: Creep test; Ceramic matrix composites (CMC); Alumina/mullite; Nanoindentation; Interfaces
1. Introduction A range of oxide and non-oxide ceramic fibres and matrices are being studied in the last two decades as potential candidates for high temperature applications [1–5]. A combination of low density, high strength and good oxidation resistance at high temperature is required, while avoiding the brittle nature of monolithic ceramics [6,7]. In particular, oxide/oxide systems have been proposed as an attempt to prevent high temperature oxidation damage, both in the fibres [8–11] and in the whole composite, such as in alumina-based systems [12,13]. Several experimental studies have been performed in order to simulate the operating conditions of these materials (monotonic, bending, creep, fatigue, etc.). Together with mechanical testing, the development of models able to reproduce the behaviour of materials under different test conditions is a helpful key to reduce the number of laboratory tests. Anyway, these global models require previous steps such as the development of statistical models involving dif-
∗
Corresponding author. Tel.: +34-943-212800; fax: +34-943-213076. E-mail address: [email protected] (J.M. Mart´ınez-Esnaola).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.11.014
ferent parameters both at the micro- and macro-levels such as fibre and bundle pullout, crack density, crack length, strain rate, interfacial properties, etc., and their relation with the tests conditions. From the micromechanical point of view, the interfacial properties (sliding stress, friction coefficient and debonding stress) have been demonstrated to be critical for the composite system behaviour [14–17]. In particular, detailed analyses of the evolution of the interface and its influence on the response of oxide/oxide systems have been reported in [18–20].
2. Material and experimental procedure NextelTM 610/Umox is an oxide-based ceramic matrix composite produced by DaimlerChrysler Forschung (DCXFo) that consists of a [(0/90◦ )3 ]s architecture of aluminafibres of NextelTM 610 (produced by 3 M) and a mullite (Al2 O3 /SiO2 ) and SiOC (Umox) based matrix developed by DCX-Fo. After the deposition of a thin carbon interface (to protect the fibres), the matrix is produced by using a slurry of mullite powder and a polymer which will change into SiOC during pyrolysis. After infiltration of the matrix,
L. Casas, J.M. Mart´ınez-Esnaola / Materials Science and Engineering A368 (2004) 139–144
Creep strain (%)
140 0.6 0.5 0.4 0.3 0.2 0.1 0
9.2A 17.2D 10.2C
0
0.5
Creep strain (%)
(a)
1 1.5 Time (h)
2
2.5
1.0 16.2B
0.8 0.6
14.2A
0.4 0.2 0.0
9.2A 17.2D 14.2C 16.2B 10.2C 7.3C 14.2A
Temperature (◦ C)
σ (MPa)
1200 1100 1100 1100 1000 1000 1000
50 70 70 50 80 75 70
Crack spacing (m) Longitudinal section 840 550 575 540 690 550 560
± ± ± ± ± ± ±
110 230 64 210 280 110 190
Transversal section 639 660 620 570 860 624 590
± ± ± ± ± ± ±
79 310 320 230 430 74 280
Pullout (m)
80 73 37 65 81
± ± ± ± ±
110 79 25 78 53
40 ± 31
7.3C
0
50
(b) Creep strain rate (%/h)
Table 1 Crack spacing (mean ± standard deviation) and fibre pullout (mean ± standard deviation) in NextelTM 610/Umox tested in creep
14.2C
100 150 200 250 Time (h)
1
T (˚C) 1000 1100 1200
0.1 0.01
0.001 40 50 60 70 80 90 100
(c)
Stress (MPa)
observed in the two nominally identical tests at 1100 ◦ C and 70 MPa. Nevertheless, and just for purposes of subsequent comparison with creep data of the NextelTM 610 fibres, the regions of approximately constant creep strain rates in the composite have been fitted to a power law of the form Q ε˙ c = Aσ n exp − (1) RT where A is a constant, n is the creep exponent, Q is an activation energy, R is the gas constant and T is the temperature in Kelvin. The best fit to the available data is given by
Fig. 1. Creep testing of NextelTM 610/Umox: (a) and (b) Creep strain vs. time; the conditions of temperature and stress of each test are given in Table 1. (c) Power law fit of the constant strain rate regions.
part of the interface is oxidised then giving a fugitive carbon interface between fibres and matrix. Fibre diameters have been measured resulting in an average value of 11.8 ± 0.4 m (±standard deviation) [21]. Creep tests were carried out in air under stresses between 50 and 80 MPa, and temperatures between 1000 and 1200 ◦ C. Creep tests were performed in an experimental testing machine designed at the Department of Materials of CEIT. The strain was measured with a water cooled high temperature extensometer with SiC rods and a gauge length of 25 mm. A heating rate of 20 K/min was applied in all tests using a radiation furnace. Optical and scanning electron microscopy (SEM) were used for a detailed fractographic analysis and microstructural characterisation. Push-in tests were performed for the interfacial characterisation using a nanoindentation system for the interfacial study of the as-received and the creep-tested material.
Fig. 2. Broken fibres in loading direction.
3. Creep testing results Fig. 1 shows the results for creep strain (εc ) and time to rupture (tr ) obtained under different testing conditions of stress and temperature, which are detailed in Table 1. The very different lives resulting in the experiments (from 1 to 100 h) seem to indicate that the mechanisms leading to failure were also different. Considerable scatter is also
Fig. 3. Fibre pullout of specimen 14.2A.
L. Casas, J.M. Mart´ınez-Esnaola / Materials Science and Engineering A368 (2004) 139–144
A = 3.4 × 10−4 , n = 9.2 and Q = 5.9 × 105 J/mol (when ε˙ c is in percentage per hour and σ is in MPa), see Fig. 1. 4. Fractographic analysis of NextelTM 610/Umox The seven specimens tested in creep have been analysed after fracture. Three sections of each sample have been obtained, one corresponding to the fracture surface, the second one in the loading direction and the third one perpendicular to the loading direction.
141
A distribution of parallel cracks has been found both in longitudinal sections (parallel to the loading direction) and in transverse sections (perpendicular to the loading direction) (Table 1). Both in longitudinal and in transverse sections, the observed cracks do not cross the whole thickness of the specimens. One important feature observed in all the analysed specimens is that many broken fibres in loading direction appear in the crack paths (Fig. 2). Fibre pullout has been measured by SEM in each specimen. As shown in Table 1 and Fig. 3, the pullout length is
Fig. 4. EDS mapping, as-received material.
142
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very short and there is a large scatter in the measurements. Fibre pullout is negligible in this material. The electron dispersion spectroscopy (EDS) mapping technique has been used in specimens 10.2C (1000 ◦ C, 80 MPa), 16.2B (1100 ◦ C, 50 MPa) and 9.2A (1200 ◦ C, 50 MPa) to analyse the degree of oxidation in the specimens after being tested at different temperatures. Zones near the external face of the specimens have been analysed to determine which elements of the as-received material (Al, Si, O and C) are still present after testing (Figs. 4 and 5). Aluminium, corresponding to the fibres and the matrix, was detected in all the analysed specimens. As expected, the aluminium concentration in the fibres is higher than in the matrix. In sample 10.2C, the presence of Si and the oxy-
gen found in the matrix of the analysed area (Fig. 5) seem to indicate the presence of SiO2 . This is the most likely compound as the SiO2 is contained in the mullite itself and because the SiOC would oxidise to produce SiO2 and CO2 (or CO). The EDS analysis also indicates the presence of C in regions reach in SiOC in the as-received material (Fig. 4). However this is practically inexistent in the tested material (Fig. 5).
5. Interface characterisation The nanoindentation technique has been used for the micromechanical characterisation of the fibre-matrix interface.
Fig. 5. EDS mapping, specimen 10.2C.
L. Casas, J.M. Mart´ınez-Esnaola / Materials Science and Engineering A368 (2004) 139–144
143
The fibre push-in test consists of applying a load to the end of a fibre within a composite, that is oriented parallel to the direction of the applied load, to produce debonding and sliding. The fibres have been subjected to three loading-unloading cycles up to a maximum load of 500 mN. The load and the displacement of the indenter are recorded simultaneously during the experiments. One test piece has been obtained from the shoulders of specimen 14.2A, which has been considered representative of the as-received material. Three pieces from samples 9.2A, 16.2B and 14.2A have been obtained after creep testing (one test piece from each temperature, see Table 1). All the indentation tests (consisting of loading, unloading and reloading) were carried out on fibres surrounded only by matrix. In the as-received material, a maximum load of 300 mN has been sufficient to debond all tested fibres (Fig. 6(a)). A typical curve obtained from the nanoindenter for the as-received material is shown in Fig. 6(b). Debonding occurred in a sudden manner in all cases, as shown in Fig. 6(b). In sample 14.2A (1000 ◦ C, 70 MPa) loading cycles up to maximum loads of 300 mN (10 fibres), 400 mN (10 fibres) and 500 mN (30 fibres) have been performed. As in the as-received material, all the tested fibres appeared debonded. However, in this case, about 60% of the tested
Fig. 7. Fibre push-in tests on tested material: (a) tested fibres in sample 14.2A; (b) typical applied load vs. tip displacement in fibres of sample 14.2A.
fibres presented particles of matrix on the fibre surface (Fig. 7(a)). These particles could have been produced because of the enbrittlement of the matrix with temperature. Different debonds and types of debond (gradual and sudden debonds) have been observed in all tests. A typical curve (with several debonds) of a push-in test in sample 14.2A is shown in Fig. 7(b). In sample 16.2B (1100 ◦ C, 50 MPa), the fibres have been subjected to cycles up to 400 mN (10 fibres) and 500 mN (20 fibres). Only two of the fibres
Fig. 6. Fibre push-in tests on the as-received material: (a) example of tested fibres; (b) typical applied load vs. tip displacement in a fibre push-in test in the as-received material.
Fig. 8. Tested fibres in sample 9.2A.
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tested at 400 mN appeared debonded and both presented a dirty surface. In the rest of the fibres no debond occurred. 20% of the fibres tested at 500 mN presented no debond. Up to 50% of the fibres tested in sample 9.2A (1200 ◦ C, 50 MPa) presented no debond and only one fibre presented its perimeter completely debonded. Most of them presented only partial debonds (Fig. 8). As in samples 14.2A and 16.2B, some fibres presented particles of matrix on their surface after testing. 6. Discussion and conclusions The interfacial study of NextelTM 610/Umox has pointed out a clear degradation with temperature of the interfacial properties. In the as-received material a load of 300 mN was high enough to cause the debond in all tested fibres (sudden debond in all cases). However, in sample 9.2A (1200 ◦ C, 50 MPa) 50% of the fibres did not debond after applying a load of 500 mN. The higher the temperature, the larger the number of fibres that remain bonded and the stronger the fibre-matrix adhesion. The absence of carbon in the EDS analysis shows the oxidation of the whole material due to the oxygen ingress through matrix cracking. The oxidation of the “fugitive carbon” interface produces a strong bonding between the fibre and the matrix (probably due to the subsequent oxidation of matrix and fibre layers next to the interface), which causes the enbrittlement of the material. This observation correlates well with the existence of broken fibres on the crack path (instead of being deflected along the interface), and the short fibre pullout length observed. A creep exponent in the range 2.8−3.1 has been reported for single fibres of NextelTM 610 [22] at temperatures between 1000 and 1200 ◦ C, which is much lower than that obtained in this work (∼9). This seems to indicate that the creep behaviour of the material is not controlled by the creep of the fibres. It can be concluded that, at the investigated temperatures (1000–1200 ◦ C), NextelTM 610/Umox might be used in applications below its matrix cracking stress because the oxidation of the material produces its enbrittlement and catastrophic failure. Acknowledgements This work is part of the Brite-EuRam project BE97-4020 with financial support from the European Commission,
co-ordinated by Rolls-Royce plc, in collaboration with Rolls-Royce Deutschland, MTU, ITP, Ansaldo Ricerche, Qinetiq, IE-JRC Petten and CEIT. The authors would also like to thank the valuable comments received during the refereeing process.
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