CONFOCAL RAMAN IMAGING OF (UNCOATED/COATED) HPZ FIBRES REINFORCING CELSIAN MATRIX COMPOSITES, BEFORE AND AFTER ALKALINE CORROSION.

Ph. Colomban, A. Attar and G. Gouadec N.P. Bansal LADIR-UMR7075 CNRS & UPMC 2 rue Henry Dunant 93420 Thiais France

NASA Glenn Research Center at Lewis Field Cleveland, OH, 44135 USA

ABSTRACT

Confocal Raman micro-spectrometry (lateral resolution ~ 1µm) and electron microscopy were used to identify and locate the phases present in HPZTM fibre-reinforced barium celsian matrix composites. Matrices were either pure or strontium-doped while HPZ fibres, a mixture of SiC, Si3N4 and C phases were either uncoated, BN/SiC-coated or BN/Si3N4-coated. The reaction of BN coating with the matrix gives rise to a liquid phase (B2O3-rich flux), which promotes the crystallisation of hexagonal celsian at the grain boundaries. SiC coating preserves the matrix from this detrimental reaction better than Si3N4. The corrosion of the composites by Na+ ions has also been investigated, in oxidising conditions. It turns out that BN coating and the surface of coated HPZTM fibres are easily corroded.

INTRODUCTION

The properties of a composite, as those of any heterogeneous material, depend on the constituents, on their distribution and on their bonding. For instance, the mechanical properties of a fibre-reinforced composite mostly depend on the properties of the fibres (and, to some extent, of the matrix), but also on the fibres alignment and relative proportion. Important parameters are the adhesion at the fibre-matrix interface and the ability of the interface (or more precisely, the interphase(s)) to transfer stresses from the matrix to the fibre (and conversely). For these reasons, different types of fibre coatings have been proposed. These are costly and long durability is mandatory. The recent developments of Raman micro-spectrometers make these instruments a unique tool providing information on the location of any phase, either crystalline or

amorphous, at the micron-scale. This offers the possibility to follow the evolution of the microstructure with thermal treatments, ageing or corrosion. The confocal setting optimises the vertical and lateral resolutions to the micron-scale while the in-depth resolution depends on the exciting wavelength 1.

In this paper, we will examine HPZTM fibre-reinforced celsian matrix composites. The HPZTM fibre (from Dow Corning, USA) is no longer available but this mixture of SiC and Si3N4 phases (mean composition %at Si: 57; N: 28, C: 10, O: 4) offers the possibility to study the stability of the most interesting phases for high temperature mechanical applications. Because these fibres are very reactive with aluminosilicates, only fibres coated with pure SiC and Si3N4 coatings, either associated or not to a BN inner coating, will be considered (in different celsian matrices). Among the corrosive agents encountered in turbines atmosphere, sodium ions are the most active. We will thus study the sodium corrosion of the above mentioned coated fibres.

EXPERIMENTAL PROCEDURE Composite Processing: Unidirectional fibre-reinforced celsian matrix composites were fabricated using a glass-ceramic approach to take advantage of viscous flow of the glass during hot pressing as described elsewhere2. Uncoated HPZ fibres or those having duplex layers of BN/SiC or BN/Si3N4 were used as the reinforcements. Glass powders of stoichiometric celsian compositions, BaAl2Si2O8 (BAS) or Ba0.75Sr0.25Al2Si2O8 (BSAS) were used as precursor to the matrix. Composites were fabricated by infiltrating the fibre tows with the glass slurry, winding the tows on a drum, cutting and stacking of the prepreg tapes followed by hot pressing. The monoclinic celsian phase in the matrix was produced in situ, during hot pressing.

Micro-configuration Raman spectroscopy was performed on polished sections that were analysed with an optical confocal microscope, focusing the laser spot and collecting the Raman scattered light. Pristine and corroded fibres were examined with a Scanning Electron Microscope (LEO 1530, Germany). They were set on metallic stubs and their tips were painted with silver lacquer. The surfaces could therefore be observed without any additional coating on them. Back illuminated Spex CCD matrices (2000 x 800 pixels) were used in both a XY and a Infinity spectrometers (Dilor, Jobin-Yvon-Horiba, Lille, France). The XY CCD was cooled by liquid nitrogen while the Infinity one was cooled down to –70°C by Peltier effect. The spectral resolution of the XY and Infinity Raman spectrometers was ca. 1 and 2 cm-1, respectively. The 457.9 nm line from an Ar-Kr ion laser, the 532 nm from a cw frequency doubled Nd:YAG laser and the 632.8 nm line from a HeNe laser were used as excitation sources. Short wavelength excitation gives larger Raman signals as compared to long

wavelengths. The excitation powers on the samples were kept to a few milliwatts or less, in order to avoid any detrimental thermal effect. The laser spot diameters were about 5 or 1.5 µm for the measurements (back-scattering configuration with x50 or x100 long focus Leitz objectives, the total magnification being 500 and 1000, respectively). To account for the structural phase distribution over a large area of the glaze, Raman spectra were recorded at various points of each sample. Fibres and polished sections of the composites were immersed in molten sodium nitrate, as described in ref. 3.

Peak fitting and data processing The experimental data were computed using Origin (Microcal Software, Inc.)

and Labspec (DILOR) softwares. Spectra baseline was subtracted prior to any curve fitting.

RESULTS AND DISCUSSION Matrices Figure 1 compares the polished sections of three HPZTM fibre-reinforced celsian matrix composites. The HPZTM fibres are i) free of any coating ii) coated with BN (thickness: 0.4µm)/SiC(0.2 µm) iii) coated with BN(0.4 µm)/Si3N4 (0.2 µm). Figure 2 compares the Raman spectra of the different matrices. Celsian polymorphism is well-established 4,5 and formation of the tectosilicate structure, the monoclinic form, is favoured by partial substitution of Ba2+ ions by Sr2+ or by adding Li+ ions 4-6. All phases are easily recognised from their Raman spectrum 6-8. Pure (BAS) and Sr-doped (BSAS, Ba/Sr = 3) barium aluminosilicate matrices will be studied here. HPZTM - BSAS BN / SiC / HPZTM -BSAS BN / Si3N4 / HPZTM - BAS

Figure 1: Optical microphotographs of polished sections of composites

associating un-coated, BN/SiC-coated or BN/Si3N4-coated HPZTM fibres with celsian matrices (BAS if un-doped, BSAS if strontium-doped).

The two regions visible on the optical microphotograph of HPZTM

BN/ Si3N4 - BAS sample (Fig. 1) have been analysed by Raman micro-spectroscopy. All matrices contain carbon traces. The strong doublet at ca. 1350-1600cm-1 is

characteristic of sp2/3 and sp2 C-C bonds in disordered carbon (B-N stretching modes occur in the same range) 9,10. The spectrum of the light grey region of the HPZTM BN/ Si3N4 – BAS composite corresponds to the phyllosilicate hexacelsian phase (main peak at ca. 405cm-1 6,7). The darker region is the same monoclinic form as that found in the other two composites (main peak at ca. 508cm-1).

500 1000 1500 2000

Si3N4/BN/HPZTM

SiC/BN/HPZTM

HPZTM

BAS

BSAS

white

grey

405

508

Ram

an In

tens

ity (A

rb.u

nit)

Wavenumber / cm-1

600 800 1000 1200 1400 1600 1800

(60)(120)

(35)(83)

1604

1351

1602

1356

(37)

1606

(85)

1356

Ram

an In

tens

ity (a

rb. u

nit)

Wavenumber / cm-1

Uncoated

BN / SiC

BN / Si3N4

Figure 2: Left, Raman spectra recorded on the matrix of the composites shown in Figure 1. Representative spectra of the white and grey regions of the

BN/ Si3N4 /HPZTM -BAS composite are given. Right, Raman spectra recorded on the core of HPZTM fibres, either un-coated, BN/ Si3N4 – coated or BN/ SiC-

coated. The numbers in bracket are the Full Widths at Half Height.

Fibre Cores We shall first consider the fibres prior to any matrix-embedding. Spectra will

be recorded on fractures and the typical Raman fingerprints for the expected phases are summarised in Table 1. Peak fitting of the bands (Figure 3) shows that the wavenumber of the sp2/3 band is almost similar for uncoated and SiC coated fibres (ca. 1356cm-1, Full Width at Half Height ~83cm-1) but shifted, and broadened, for the Si3N4-coated fibre (1351cm-1 ; FWHH~120 cm-1). The sp2 band wavenumber remains rather constant (ca. 1604cm-1) but the band broadens in the Si3N4-coated fibre (FWWH ~60 cm-1 instead of 35cm-1). This broadening indicates that the wavenumber shift is not due to a stress imposed by the coating 11, but has a chemical nature (change in the SiC nanostructure during the coating). Note a band at ca. 1535 cm-1 is present in all spectra, as usually observed in SiC fibres 7,9,10. It can be concluded that the coating by Si3N4 involves some transformation of the fibre nanostructure. Note, we do not detect any Raman peak characteristic of Si3N4 phase on the fibre spectra.

Coatings

Figure 3 compares the Raman spectra recorded at the fibre-matrix interface for Si3N4 and SiC-coated HPZ fibres. A line scan from the surrounding matrix to the fibre core (across the coating) is shown in Figure 4 for the Si3N4 coated sample. The increase of the intensity in the 1000-1800cm-1 range is straightforward. This is due to BN contribution. Similar intense scattering has been observed for BN-coated Hi-NicalonTM fibres 8. The spectra neither correspond to that of hexagonal BN (peak at ca. 1366cm-1) 8 nor to that of cubic (1057-1309cm-1 doublet 12) BN, but to a more disordered form. The strong intensity of the scattering has been assigned to the change of the electronic absorption because of C dissolution in the BN framework 8. Yet, deconvolution of the band (Table 2) shows broad components at ca. 1040-1070 cm-1, which correspond to a cubic-like form. The main intensity at ca. 1355cm-1 arises from BN and superimposes to that of carbon bonds. The small component at ca. 790 cm-1 could correspond to the Raman fingerprint of Si3N4

13. The slightly lower wavenumber measured in free-standing pristine fibres

(Figure 2) is due to the poor thermal transfer between the fibre and the air. For the same condition of laser illumination, the slight increase of the temperature under the laser spot lowers the wavenumber 11. A compressive stress of the embedded fibre is also a possible explanation for the observed up shift within the composites.

1602.0

1545.6

1357.6

1259.4

1183.8

1074.0

1030.0

859.7

796.5

1592.4

1536.0

1371.5

1256.8

wavenumber

Figure 3: Raman spectra of BN / Si3N4 - (left) or BN / SiC- (right) coatings

on HPZTM fibres (457.94nm, 0.9mW) recorded on polished sections shown in Fig. 1. See Table 2 for wavenumber, bandwidth and assignment of the different

components.

Coating-matrix reaction

The high intensity of the scattering of BN-rich phases can be used to map the area of reaction between the fibre coating and the matrix. Example is given in Figure 5 with the dissolution of SiC/BN ring in celsian matrix. This illustrates the interest of Raman microscopy to image the diffusion of a phase constituted of light elements, hardly detectable by other techniques.

Corrosion by sodium nitrate

SEM micrographs of pristine (un-coated), BN-SiC- or BN-Si3N4 coated fibres immersed for 100 hours in molten nitrate are shown in Figure 6. Although the surface of un-coated corroded fibres remains safe, pores are formed in overcoatings. The Si3N4/BN coating cracks and goes out in many places.

Table 1: Raman assignment of the fibres main phases. Phases Wavenumber

cm-1 Hybridation type

Remarks

Carbon 1100-1200 sp3 C-C bond with C-H branch

1331 sp3 Diamond 1340-1365 sp2/3 surface of C grains 1530-1560 sp2 C-C bond with C=O

or C=N branch 1450 sp2 Fullerene 1580-1600 sp2 graphitic grain 1580,1620 sp2 Graphite

BN 1366 sp2 Hexagonal form 1057 & 1308 sp3 Cubic form 1000-1700,

broad, strong sp2/3 BNC solid solution

SiC 794-966 TO-LO Cubic, 3C form 765, 860, 960 6H and other polytypes

β Si3N4 1190, 1045, 940-930, 865, 730, 450

α Si3N4 1140,1110, 1030, 975, 950, …

Figure 4: 3D map of the Raman intensity in the 800-1800cm-1 range recorded across the BN/ Si3N4 coating of the polished section shown in Fig.1, right

(457.9nm; 0.9mW, 240 spectra). The section visible on the left side corresponds to the Raman spectrum on Fig 3 (left).

Table 2: Wavenumber, bandwidth and assignment for Raman fingerprint of SiC/BN and Si3N4/BN coatings Coating # Wavenumber

cm-1 FWHH cm-1

Assignment

Si3N4/BN 1 1357.6 173 h-like BN & sp2/3 C-C bond

2 1602 48 Graphite-like C-C

3 1545.6 121 O-bonded C-C 4 1259.4 122 BNC ? 5 1183.8 75 BNC or H bonded C-C 6 1074 132 c-like BN 7 789.4 39 Si3N4 ?

SiC/BN 1 1358.7 123 h-like BN & sp2/3 C-C bnd 2 1542.8 82 O-bonded C-C 3 1602.2 40 Graphite-like C-C 4 1283.1 190 BNC ? 5 - - - 6 1038.6 73 c-like BN 7 - - - 8 796 26 (TO) SiC

Note the corrosion of the fibre outer region just below its surface, as evidenced for the BN/SiC HPZTM fibre (Figure 5).

Figure 5 : Raman imaging of the BNC-rich region formed around a fibre which SiC / BN coating has reacted with BSAS matrix (457.9nm; 0.9mW, 200

spectra). See Fig. 1.

HPZTM ------- BN-COATED HPZTM -----

BN/SiC-COATED HPZTM --- SI3N4/BN-COATED HPZTM ---

Figure 6: Scanning electron microphotographs of fibres after 100h immersion in

molten sodium nitrate (bar = 1 µm).

Raman fingerprints of Si3N4 phases are not observed on pristine HPZTM fibres. After the oxidising attack of sodium nitrate, broad components appear between 700 and 1200cm-1. They are consistent with the fingerprint of β Si3N4 13. The large broadening could arise from the very small size of the crystals. Comparison of the Raman spectra of (MgO-doped) sintered and RBSN Si3N4 ceramics confirms the poor Raman intensity of the α and β-Si3N4 phases (Figure 7). After sodium nitrate attack, the new bands on the fibre spectra are consistent with an assignment to Si3N4 phases.

The intensity of the Raman fingerprint of disordered, C-doped BN phase decreases with corrosion duration, which is likely to result from the dissolution of BN, as suggested by pore formation. We do not observe clear spectrum of SiC phase in SiC overcoating, neither before nor after free standing fibre corrosion but, rather, a strong fingerprint of disordered carbon. This is consistent with the elimination of carbon as observed for Hi-NicalonTM fibres 3 .

500 1000 1500 2000

(β ) RBSN - Si3N4

(β + α) MgO : Si3N4

(100h)BN / HPZTM

HPZ (1h)

(1h)SiC / BN / HPZTM - BSAS

(100h)SiC / BN / HPZTM - BSAS

Ram

an In

tens

ity (a

rb. u

nit)

Wavenumber / cm-1

80370 760 1005

15891360

409 555 795

1598

15871337

11601010555420

16031360

12301030

1230850

770725

1190

1045940925860

730615450

Figure 7: Raman spectra recorded at the fibre/matrix interface on polished sections of composites corroded 1 h or 100 h in molten sodium nitrate.

Comparison is made with the spectra of RBSN and MgO doped- Si3N4 ceramics and with the spectra of BN - coated and uncoated HPZTM fibres (457.9nm;

0.75mW).

CONCLUSION

The reactivity of fibers coatings could be very detrimental to composites stability. Thus, a Si3N4 ring does not avoid the reaction between a BN undercoating and a celsian matrix. This leads to the formation of a liquid phase, which promotes the formation of hexacelsian around the fibres and at the matrix grain boundaries. The protective effect of a SiC overcoating is greater than that of Si3N4 but some reaction with the matrix is still observed. Regions containing BNC phase are easily imaged from the mapping of the Raman intensity. Imaging of this B-rich region is easily obtained because of the strong intensity of the BNC fingerprint.

HPZTM fibres surface is not attacked in molten sodium nitrate but a strong corrosion of the BN ring is observed. This involves the cracking of the Si3N4 or SiC overcoating. The SiC coating is pitted in many places, as observed in the corrosion of NLM NicalonTM fibres. This could be related to the presence of carbon in it. The under-surface region of the HPZ fibre appears to be more sensitive to corrosion than the surface. This may be related to composition or nanostructure changes.

Acknowledgements

The authors wish to thank M. Hajjaj for his contribution to the corrosion study.

REFERENCES

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13 N. Wada and S.A. Solin, " Raman and IR Absorption Spectroscopic Studies on α, β and Amorphous Si3N4", J. Non-Cryst. Solids, 43 7-15 (1981).