CHAPTER 1
REINFORCEMENT OF CERAMIC MATRIX COMPOSITES: PROPERTIES OF SiC-BASED FILAMENTS AND TOWS

JACQUES LAMON,1 STÉPHANE MAZERAT,2 AND MOHAMED R'MILI3

1Laboratoire de Mécanique et Technologie (LMT), CNRS (Centre National de la Recherche Scientifique), ENS Cachan (Ecole Normale Supérieure), UPMC (Univesité Pierre et Marie Curie), Cachan, France

2Laboratoire des Composites Thermostructuraux, Université de Bordeaux, Pessac, France

3Laboratoire MATEIS, CNRS, INSA de Lyon, Université de Lyon, Villeurbanne, France

1.1 INTRODUCTION

Proper reinforcement of ceramics is aimed at increasing the resistance to crack propagation by introducing elements that can arrest the cracks. Only continuous fibers are able to arrest the cracks through deflection at fiber/matrix interfaces. Composite damage tolerance requires strong fibers and appropriate interfaces. Composite strength requires strong fibers and damage tolerant multifilament tows.

In continuous-fiber-reinforced ceramics, only those fibers that can withstand the high temperatures required by matrix processing (above 1000°C) can be used. Other high temperature requirements to be met include long-term stability, creep resistance, and oxidation resistance. A wide spectrum of continuous fiber-reinforced ceramic matrix composites (CMCs) can be foreseen owing to a wide variety of matrices and fibers. Non-oxide CMCs reinforced by non-oxide fibers have been the most studied. The reason for this is that carbon and silicon carbide fibers display the highest properties for use at high temperature. Second, for compatibility reasons, non-oxide fibers can be combined essentially to non-oxide matrices. However, carbon fibers degrade in oxidizing atmosphere at temperatures as low as 450°C, and they must be protected. SiC-based fibers are much more resistant to oxidation. Oxide fibers are inherently resistant to oxidation, but they have limited creep resistance and undergo grain growth at high temperatures, which causes strength degradation. Further, they display much higher densities than carbon and SiC-based fibers. Despite these drawbacks, alumina-based CMCs have been extensively studied.

The literature abounds in papers, book chapters, and books on microstructure/properties relationships for oxide and non-oxide fibers [1-4]. Carbon fibers are discussed in another chapter of this book. The present chapter focuses on a recent important issue for continuous SiC-based reinforcing ceramic fibers that has been ignored because potential use of CMCs was driven by their superior temperature resistance over metals. Use at very high temperatures above 1000°C was essentially foreseen. Recently, a growing interest was fostered by lightweight features of CMCs for use in the range of allowable temperatures for metals (below 1000°C in aeronautical engines).

The Nextel oxide fibers are the most widely used reinforcements for continuous fiber oxide-oxide composites [5-7]. Nextel 610 fiber has the highest strength and elastic modulus (3.1 GPa and 380 GPa, respectively), but it is limited by creep to temperatures <1000°C. Nextel 720 fiber has lower room temperature strength (2.1 GPa), but higher creep resistance, which allows use at higher temperatures (up to 1200°C). Sapphire (single crystal Al2O3) fibers are no longer available, their cost and diameter (>50 µm) limit their use in composites.

Non-oxide fibers exhibit superior tensile strength and creep resistance to the oxides. They possess comparable Young's moduli and diameters. Table 1.1 lists the main properties of SiC-based ceramic fibers [1, 2, 8-29]. Lower creep rates are observed at temperatures >1200°C, even under high stresses, whereas the oxide fibers can barely exceed 1000°C [1, 3, 30-35]. For example, Sylramic SiC fibers show less than 1% creep strain after 1000 hours at 1350°C and 100 MPa stress, or Hi-Nicalon type S fibers show less than 0.5% creep strain after 60 hours at 1350°C and 850 MPa stress. Creep strain of 10-8 per second is obtained at 1000°C and 100 MPa stress on the most creep-resistant oxide fiber (Nextel 720), at 1400°C and 300 MPa stress on Tyranno SA3, and at 1350°C and 850 MPa on Hi-Nicalon type S. Tyranno SA3 and Hi-Nicalon type S exhibit higher resistance to creep than Hi-Nicalon. The creep resistance is commensurate with low oxygen content, SiC grain size, and small amount of amorphous phase. It has been shown to be improved after high temperature treatment under various atmospheres. Creep behavior of Hi-Nicalon can be improved by using high temperature treatment that eliminates amorphous phase and organizes better carbon structure [3].

Table 1.1 Physical Properties and Composition of Various SiC-Based Fibers [1, 2, 8-29]

Composition (at. %) Fiber Diameter (µm) Fibers/fil Curing Tpyrolyse (°C) Si C O Hetero-element Clibre (at. %) <SiC-ß> (nm) s (S/m) a × 10-6 Density (g/cm3) NL101 15 [23] 500 ox 1200 36.4 41.1 46.8 43.3 16.7 15.6 -[22] -[1] 18.8 [22] 10.0 [1] 1.2 [22] 1.1 [23] 0.02 [28] 3.1 [29] 2.57 [21] NL102 15 [23] 500 ox 1200 33.9 35.0 48.8 51.4 17.3 13.6 -[22] -[1] 23.6 [22] 23.3 [1] 1.7 [23] 3.1 [29] 2.61 [21] NL207 14 [11] 500 ox >1200 [1, 8, 10] 38.6 37.4 48.6 49 12.8 13.6 -[22] -[1, 10] 16.4 [22] 18.4 [1, 10] 1.9 [22] ~2 [10, 11, 13] 0.006 [28] 0.1-0.01 [15, 25, 27] 0.005 [17] 3.2 [1, 15] 2.55 [1, 10, 11, 13, 15, 17] 2.58 [21] Hi-Ni 14 [8] 12 [1] 500 e- 1300 [1] 1500 [12] 39 41.6 41.9 60.4 57.8 57.2 0.6 0.6 0.9 -[1, 8, 13] -[21] 21.7 [1, 8, 13] 15.8 [22] 5 [8, 12, 13] 23 [28] 71 [25] 80 [27] 3.5 [1] 2.74 [1, 10, 11, 13, 15] Hi-Ni-S 13 [14] 500 e- >1500 48.2 48.7 50.8 51.0 1.0 0.3 -[22] -[1, 9, 13, 14] 3.1 [22] 2.6 [1, 9, 13] 50 [11-13] 0.14 [2, 8, 27] 3.05 [1, 13, 14] 3.10 [9] T-S 11 [13] 8.5 [13] 800 1600 ox 31.6 31.8 41.7 48.1 26.0 19.4 (Ti) 0.7 [21] (Ti) 0.7 [22] 23.0 [21] 26.0 [22] 1.2 [22] 0.09 [28] 3.1 [24] 2.35 [21, 24] Lox-M 8.5 [13] 11 [13] 800 ox 1200 [1] 1300 [12] 35.4 35.8 36.9 53.3 49.0 50.4 10.4 14.4 11.9 (Ti) 0.9 [22] 0.8 [1, 10, 13] 0.8 [17] 23.1 [22] 20.4 [1, 10, 13] 19.5 [17] 1.9 [22] 1.4 [18] 2.6 [28] 3.3 [17] 3.1 [1] 2 37 [10, 11, 13] 2.48 [1, 9, 17, 19, 24] Lox-E 11 [1] 800 e- 1300 [12] 35.7 36.4 57.0 57.8 6.6 5.8 (Ti) 0.7 [1, 10, 17] -[9] 24.6 [1, 10] 24.3 [9] 2 [10] 125 [17] 40 [27] 2.39 [10, 11, 13] 2.55 [9, 17] ZMI 11 [24] 800 ox 1300 35.5 37.3 54.3 53.7 10.0 8.8 (Zr) 0.2 [22] 0.2 [10] 23.9 [22] 20.7 [10] 2.2 [22] 2.0 [18] 16 [28] 50 [25] 4.0 [19, 24] 2.48 [10, 18, 21, 24] ZM 11 [1] 800 ox 1300 [1] 1550 [17] 34.7 37.7 56.5 53.4 8.6 8.7 (Zr) 0.2 [21] 0.2 [1, 17] 26.0 [21] 20.5 [1] 2.48 [22] ~2 [17] 47 [28] 50 [17] 2.48 [1, 13, 17] ZE 11 [1] 800 e- 1300 [1] 1580 [17] 36.3 38.7 61.9 59.1 1.7 2.0 (Zr) 0.1 [21] 0.2 [1, 13, 17] 26.4 [21] 21.4 [1, 10] 3.5 [22] 380 [28] 330 [17, 25] 2.55 [1, 10, 17] ZX 11 [19] 800 ox 1500 [19] 37.0 34.8 53.7 52.9 9.1 12.1 (Zr) 0.2 [19] 0.2 [19] 21.2 [19] 24.1 [19] ~2 [19] 2.48 [19] 2.42 [19] AM 11 [18] 8.5 800...