SiC composites with multilayered (PyC–SiC)n interphases at high temperatures in oxidizing atmosphere

Composites Part A 29A (1998) 1157–1164 1359-835X/98/$ - see front matter q 1998 Elsevier Science Ltd. All rights reserved

PII: S1359-835X(98)00093-1

Tensile static fatigue of 2D SiC/SiC composites with multilayered (PyC–SiC) n interphases at high temperatures in oxidizing atmosphere

S. Pasquier, J. Lamon* and R. Naslain Laboratory for Thermostructural Composites, UMR-5801 (CNRS-SEP-UB1), University of Bordeaux, 3 Alle´e de La Boe´tie, 33600 Pessac, France

The pyrocarbon (PyC) interphase, which is commonly used to monitor the fibre/matrix interactions in SiC/SiC and C/SiC composites, generally oxidizes in air at high temperatures. This phenomenon alters the performances and the load-carrying capability. Recently, SiC/SiC composites with multilayered interphases have been developed. The interphase consists of alternate sublayers of PyC and SiC deposited on the fibres by Chemical Vapour Infiltration. The composites exhibit mechanical properties (high strength, high toughness) and features (multiple crack deflection) that are interesting for high temperature applications. The mechanical behaviour and lifetime under tensile static loading (60–140 MPa) at high temperatures (7008C–12008C) in air are reported for a 2D SiC/ SiC composite with a multilayered interphase involving 4 PyC/SiC sequences ((PyC–SiC) 4). The lifetime of the composite with the multilayered interphase is significantly improved with respect to that of the composite with a homogeneous PyC interphase. q 1998 Elsevier Science Ltd. All rights reserved (Keywords: multilayered interphase; static fatigue test; oxidizing atmosphere)

INTRODUCTION In an oxidizing atmosphere, the pyrocarbon (PyC) interphase commonly used to control the fibre/matrix bonding in SiC/SiC composites is oxidized1,2. This oxidation results in an important damage of the composite performances in air at high temperatures due to (i) the formation of annular pores around the fibres which alters the load transfer, (ii) the weakening of the fibres, and (iii) the creation of brittle silica bridges between the fibres and the matrix. Many parameters influence the thermo-mechanical behaviour of composites at high temperature in air. In the present paper, the strain–time behaviour under tensile static loading in oxidizing atmosphere at various temperatures is investigated, on a 2D SiC f (Nicalon)*/SiC composite with single PyC and multilayered (PyC–SiC) 4 interphases3. The test procedure focuses on the preponderant parameters: the temperature, the maximum applied stress and the initial extent of damage characterized by the initially applied deformation.

EXPERIMENTAL All the specimens used in this study are SiC f/SiC twodimensional composites prepared from a fibre preform * Corresponding author. Tel: (33) 5 56 84 47 00; Fax: (33) 5 56 84 12 25. * NLM 202 (ceramic grade) from Nippon Carbon, Tokyo.

consisting of a stack of woven treated fabrics†, the fibres being in situ coated with an interphase and embedded in an SiC matrix by I-CVI4. Two sets of specimens with different interphase coatings are considered (Table 1): material R with a single homogeneous pyrocarbon interphase of 50 nm and material L with a multilayered interphase composed of 4 (PyC/SiC) sequences denoted (PyC–SiC) 4 (Figure 1). The first sublayer deposited by I-CVI on the fibre surface is pyrocarbon and the SiC of the last sequence is the matrix itself. The composite material has a porous texture in the asprocessed state, with small intrabundle pores and large interbundle cavities, arising from the architecture of the woven fibre preform and from the I-CVI process (Figure 2). Tensile specimens (8 3 200 3 3 mm 3) are cut with a diamond saw in rectangular plates (75 3 205 3 3 mm 3), one fibre direction being parallel to the loading direction, and protected after machining by a seal coat of I-CVI SiC matrix5. Batch characteristics, i.e. density, porosity, fibre and matrix volume fractions, are given in Table 2. The average fibre volume fraction per batch ranges from 35.1% to 38.2%. The open porosity is lower than 10.6% for each batch. The elastic modulus is evaluated under uniaxial tensile load at room temperature before each fatigue test. The average Young’s modulus for materials L and R is 216 (19.0) GPa and 200 (20.2) GPa, respectively. † Proprietary treatment performed by SEP, Le Haillan, France.

1157

Tensile static fatigue of 2D SiC/SiC composites: S. Pasquier et al. derived from S according to the following equation6 t¼

Figure 1

Material L with 4 sequences PyC/SiC interphase

High-temperature tensile tests are performed with a hydraulic machine equipped with a resistance-heated furnace designed for 200 mm long specimens and a cold grips loading system (only the central part of the specimen is heated, whereas the ends are held by water-cooled grips). Temperature is uniform along gauge length. Strain is measured by a water-cooled extensometer with 2 alumina contact rods. Acoustic emission from the specimen is recorded using an acoustic sensor fixed on the top grip. The testing procedure is shown in Figure 3. All the specimens were pre-cracked at high temperature under a tensile load at a constant stress rate (300 MPa/min). Just after the precracking loop, the material is reloaded to the constant fatigue stress, j m. The evolution of the measured strain is recorded versus time. Unloading–reloading cycles were performed at various time steps to provide the following data: elastic modulus, E, which is the slope of the linear part of the loop, the loop area, S, the interfacial shear stress, t, and the permanent strain, « res. The interfacial shear stress is

b2 N(1 ¹ a1 Vf )2 Rf 3 jm 12Em Vf2 S

(1)

where b 2 and a 1 are Hutchinson coefficients, N is the number of matrix cracks, R f is the fibre radius, E f and E m are the fibre and matrix Young’s moduli, and V f is the fibre volume fraction. Tensile tests were performed at room temperature on the specimens which did not fail during the static fatigue tests, to determine the residual tensile behaviour and failure properties of aged specimens. The static fatigue tests were performed in air at 7008C, 8508C, 10008C and 12008C, under various applied stresses (40–140 MPa). Pre-cracking loads corresponded to the following strains (« endo) 0.06%, 0.25%, 0.75%. These strain levels characterize the initiation of different families of cracks identified at room temperature in SiC/SiC composites with PyC interphase7. In addition, the fracture surfaces for most tested specimens and the interphase zone in polished cross-sections were also examined by High Resolution Scanning Electron Microscopy, (HR-SEM), Optical Microscopy (OM) and Transmission Electron Microscopy (TEM).

RESULTS AND DISCUSSION The static fatigue behaviour results can be grouped into three families depending on the applied tensile stress and temperature (Figure 4).

Table 1 Material composition Material

Composition

L Thickness R

F SiC/ (nm) F SiC/ (nm)

Figure 2

1158

PyC/ 50 PyC/ 50

Tensile test specimen (SEM image)

SiC/ 50 Matrix SiC

PyC/ 50

SiC/ 100

PyC/ 50

SiC/ 150

PyC/ 50

Matrix SiC

Tensile static fatigue of 2D SiC/SiC composites: S. Pasquier et al. Table 2 Characteristics of the batches of L and R materials Material

Batches

da

V po (%)

V f (%)

V m (%)

L (number of specimens)

1 2 1 2

2.62 (0.07) a 2.64 (0.01) 2.34 (0.03) 2.51 (0.04)

9.4 (0.8) 10.6 (0.9) 15.6 (0.8) 11.0 (1.4)

35.1 (1.6) 37.2 (0.6) 38.2 (0.6) 37.8 (0.7)

46.6 (2.8) 45.5 (0.9) 42.9 (1.2) 48.5 (1.7)

R

(52) (13) (13) (16)

Key: d a, apparent density; V po, total porosity; V f, fibre volume fraction; and V m, matrix volume fraction. a Standard deviation j n-¹1 in brackets.

Figure 3 Tensile static fatigue test procedure: evolution of the applied stress with time

• At 12008C and 10008C, failure occurs at the end of phase I, as shown in Figure 4a. This type 1 behaviour consists of one short phase of decreasing stiffness until failure, which may occur within 1 or 2 min. Lifetime is always less than 1 h. • For specific static fatigue test conditions, the composite shows a dual behaviour8. In the first phase (1–3 h duration), the stiffness decreases, whereas the hysteresis loop areas increase. The interfacial shear stress is proportional to 1/S, (eqn (1)). Then, as the loop area increases, t decreases continuously. The most dramatic drop is observed under 100 MPa at 12008C: t drops from 120 MPa at the first loop to 30 MPa after 30 min, and reaches the minimum value of 7 MPa after 1 h (Table 3). The acoustic emission increases drastically during stage I and permanent and total longitudinal deformations also occur. • In the second phase, the Young’s modulus (E) increases, and the loop area (S) and the interfacial shear stress (t) remain constant whatever the temperature (Table 3). No acoustic emission is detected during this stage II; while strain–time curves become flat. Figure 4b shows this type 2 behaviour, when elastic modulus reaches the minimum value of 0.5 E fV f. The type 3, behaviour shown in Figure 4c corresponds to the same dual behaviour with a minimum modulus larger than 0.5 E fV f. This dual behaviour (type 3) has already been observed on uncoated SiC/SiC composites9. The trends revealed by Figure 4 depend on two parameters: temperature and applied stress. The maps shown on Figure 5 give the temperature and applied stress

Figure 4 The 3 categories of static fatigue behaviours observed during fatigue tests performed at 12008C, 100 MPa in air

1159

Tensile static fatigue of 2D SiC/SiC composites: S. Pasquier et al. Table 3 Interfacial shear stress for the material L during static fatigue tests in oxidizing atmosphere under 100 MPa j ¼ 100 MPa

Ageing time under load (hours)

T (8C) 700 N ¼ 30 850 N ¼ 23 1000 N ¼ 30 1200 N ¼ 25

S (kJ/m 3) t (MPa) S (kJ/m 3) t (MPa) S (kJ/m 3) t (MPa) S (kJ/m 3) t (MPa)

0 (initial state) 1.2 3 10 ¹3 243 1.3 3 10 ¹3 172 1.9 3 10 ¹3 154 1.9 3 10 ¹3 128

0.5 (phase I) 2.2 3 10 ¹3 132.7 3.9 3 10 ¹3 57 5.2 3 10 ¹3 56 8 3 10 ¹3 30

Transition I/II 32 3 10 ¹3 t mini ¼ 9.1 32 3 10 ¹3 t mini ¼ 7

10 (phase II) 26.4 3 10 ¹3 11 31 3 10 ¹3 7.2

30 (phase II) 30 3 10 ¹3 9.7 18 3 10 ¹3 12.4

Table 4 Influence of damage on the lifetime in static fatigue under air, material L Pre-cracking strain (%)

0.06 (cracks of the 1st family)

0.25 (families 1 and 2)

0.75 (families 1, 2 and 3)

8508C 100 MPa 7008C 100 MPa

. 48 h 78

71.9 . 24

60 40

conditions required to obtain each type of behaviour for materials L and R. Material R presents only two types of behaviour (types 1 and 3). In consequence, lifetime is stress- and temperaturedependent (Figure 6). But it does not seem to be tremendously affected by the degree of pre-cracking (Table 4). Use of multilayered interphase increases lifetime compared to single PyC interphase composite. For example, material R fails after 2 h under 100 MPa at 7008C, whereas the material L lifetime exceeds 24 h. It appears clearly that the type 1 behaviour specimens have the shortest life. Lifetime exhibits a certain scatter and these preliminary results should be ascertained by further data to confirm these trends (Figure 6). The fatigue behaviour is not significantly influenced by the number of cracks in the matrix and the crack type (Figure 7), for « endo of 0.06%, 0.25% and 0.75%. Residual behaviour after static fatigue The residual behaviour under tension at room temperature has been examined on 5 specimens of material L after static tensile test at 8508C, 100 MPa for 30 min, 1 h, 10 h, 20 h and 48 h (Table 5, Figure 8). The same procedure was applied to 3 specimens of material R after static tensile test at 8508C, 40 MPa for 1 h, 20 h and 63 h (Table 6). Failure of as-received material L is 370 MPa and it falls to 105 MPa after 1 h of fatigue under 100 MPa at 8508C. It remains at this level after 10, 20 and 48 h (Figure 9). After 1 h of fatigue under 40 MPa at 8508C, the mechanical behaviour of material R remains the same as that of the asreceived material until failure which occurs at 266 MPa (instead of 300 MPa). After longer times, the mechanical behaviour is degraded. The ultimate stress drops to 96 MPa after 63 h (Figure 10). Internal degradation mechanisms The SiC/SiC composite possessing PyC-based interphases is prone to oxidation reactions1,2. First of all, the

1160

Figure 5 Evolution domains in static fatigue of SiC/SiC composites in oxidizing atmosphere, as a function of the applied constant stress and of the temperature: (a) material L with a (PyC/SiC) 4 multilayered interphase and (b) material R with a single homogeneous PyC interphase

Tensile static fatigue of 2D SiC/SiC composites: S. Pasquier et al.

Figure 7 Influence of the damage level on the evolution of the Young’s modulus for material L (static fatigue under 100 MPa) Table 5 Static fatigue at 8508C/100 MPa followed by a residual tensile test at room temperature, material L Fatigue phase

Ageing time (h) j res (MPa)

« res (%)

E (GPa)

I I II II II

0.5 1 10 20 48

0.43 0.37 0.25 0.17 –

121 42 60 80 –

215 105 115 120 100

Figure 6 (a) Evolution of the lifetime in static fatigue for material L in air, with temperature and applied stress (the arrow shows that failure did not occur and that the lifetime is higher than indicated): W, 7008C; A, 8508C; S, 12008C. (b) Evolution of lifetime in air with the damage level, « endo, for material L (static fatigue test at 7008C/100 MPa and 8508C/100 MPa): W, 7008C; A, 8508C.

oxidation of carbon results in the formation of carbon gaseous oxides and a degradation of the fibre/matrix bond (eqn (2) and (3)) C(s) þ O2(g) → CO2(g)

(2)

2C(s) þ O2(g) → 2CO(g)

(3)

At low temperatures, the carbon consumption is controlled by the kinetics of these reactions. At higher temperatures, it becomes controlled by the oxygen diffusion through the cracks and the interfacial porosity created by the carbon combustion1,2,10. This carbon removal has been observed by TEM in a transverse section of material L. The specimen tested in tensile static fatigue for 48 hours under 100 MPa at 8508C in air shows SiC sublayers without carbon layers between them (Figure 11).

Figure 8

Static fatigue at 8508C under 100 MPa. Material L

Table 6 Static fatigue at 8508C/40 MPa/0.06% followed by a residual tensile test at room temperature, materials L and R L

R

t (h)

j res (MPa)

« res (%)

Es(tT) (GPa)

E o RES (GPa) Phase

j res (MPa)

« res (%)

Es(t) (GPa)

E o TA (GPa)

Phase

1 20 60

– – 190

– – 0.47

– – 142

– – < 170

266 144 96

0.7 0.48 0.22

180 133 141.7

170 120 150

I II II

I II II

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Tensile static fatigue of 2D SiC/SiC composites: S. Pasquier et al.

Figure 9 Residual tensile curves at room temperature for material L after static fatigue in air (8508C, 100 MPa). The as-received material failed at 370 MPa and 1%

Figure 10 Residual tensile curves at room temperature for material R which has been pre-strained at 0.06% then aged in static fatigue at 8508C/ 40 MPa

Figure 11 TEM micrograph for material L after static fatigue in air during 48 hours at 8508C under a constant applied stress of 100 MPa (matrix pre-cracking of 0.06%), showing the removal of PyC layers in the interphase and the formation of a silica layer on the fibre surface

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Tensile static fatigue of 2D SiC/SiC composites: S. Pasquier et al.

Figure 12 HR-SEM (a) and TEM (b) micrographs of a specimen aged in static fatigue in air (8508C/100 MPa/40 h)

fibre : SiCO(s) þ O2(g) → SiO2(s) þ CO, CO2(g)

(4)

90 80 70

(a) oxidised zone

60

carbon

40

silicon

30 20

oxygen

10 0·2

0·4

CONCLUSION The behaviour under static fatigue in air at high temperature was evaluated for SiC/SiC composites with multilayered interphases of 4 (PyC/SiC) sequences and a monolayered interphase of 50 nm PyC. The use of a multilayered interphase improves significantly the lifetime and the fatigue behaviour. This material also shows an interesting resistance to oxidation at temperatures below 10008C, under stress lower than 120 MPa in the presence of small and large densities of cracks. Three types of static fatigue behaviour have been observed. They are temperature and stress dependent. Type 1 corresponds to one single-stage behaviour.

0·6

0·8

1·0

1·2

1·4

Depth (µm)

(5)

The silica formation has been observed by TEM and analysed by Auger electron spectroscopy (AES) on the fibre, matrix and interfacial sublayers (Figures 12 and 13). some local bridges which make contact between fibres and matrix, and/or SiC sublayers may explain the stage II evolution during static fatigue test, when E and t increase. These contacts permit a progressive and partial matrix reloading.

Fibre Si-C-O

50

0 0

Atomic concentration (%)

matrix : SiC(s) þ O2(g) → SiO2(g) þ CO, CO2(g)

100

Atomic concentration (%)

The interfacial pyrocarbon layer oxidation and the resulting reloading of fibres explain the E decrease during the first stage of static fatigue. E decreased to a minimum value of about 30 MPa ( < 0.5 E fV f), which indicates that the load is carried only by the fibres and that the destruction of the carbon bond is complete. The carbon bond destruction results in an interfacial shear stress decrease. The second type of chemical reactions concerns the passive oxidation of the matrix and fibres according to eqn (4) and (5)1,2,11

120 100 80

(b) SiC

SiC

SiC

carbon

60

silicon

40 20 0 0

oxygen 1000

2000

3000

4000

Time (sec.) Figure 13 AES atomic concentration profiles recorded on a tensile test specimen aged in static fatigue (8508C/100 MPa/48 h): (a) fibre side; (b) matrix side

During this stage 1, which is one to three hours long, the elastic modulus and the interfacial shear stress t decrease. Types 2 and 3 are dual behaviours with 2 stages. Stage I is the same as for type 1 behaviour. Stage II corresponds to an increase of E and t. The E minimum value, 30 GPa, is obtained with type 2 behaviour and corresponds to a complete degradation of interphases. The internal mechanisms which have been observed by optical microscopy,

1163

Tensile static fatigue of 2D SiC/SiC composites: S. Pasquier et al. HR-SEM and TEM, are ruled by oxidation phenomena in the interphase. Oxidation of SiC sublayers (stage II) may restore load transfers affected by the PyC degradation (stage I).

4.

5.

ACKNOWLEDGEMENTS This work has been supported by the French Minister of Education and Research and by SEP through a grant to S.P. The authors are indebted to C. Robin-Brosse, E. Inghels from SEP and to P. Carre`re and B. Humez from LCTS for assistance in specimen fabrication and mechanical testing, as well as for valuable discussion.

6. 7. 8.

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