SiC Nicalon fibre-glass matrix composites: interphases and their mechanism of formation

Abstract

The ® bre± matrix reaction that occurs during the processing of a Nicalon SiC ® bre± Pyrex glass matrix composite is analysed. Interphases are characterized by means of various complementary techniques: electron di raction, HRTEM, EDX, EELS, SIMS and XPS. Neither the results of the present study nor those previously obtained for Nicalon SiC ® bre± LAS (Li 2 O± Al 2 O 3 ± SiO 2) glass or LAS + MAS (MgO± Al 2 O 3 ± SiO 2 ) glass± ceramic matrix composites support the available model of reaction. An alternative reaction mechanism is suggested whereby the dissolved and non-bridging oxygen atoms of the matrix di use and oxidize SiC and SiO x C y in the ® bre. The reaction yields carbon and silicon oxycarbide in the ® bre and SiO 2 which dissolves into the matrix. When the oxygen in excess in the matrix is consumed, the reaction stops and the phases undergo a reorganization in the reaction layer which generates two interphases, one carbon rich and the other silicon oxycarbide rich. These interphases are observed at the ® bre periphery in all glass or glass± ceramic matrix composites. § 1. Introduction Glass or glass± ceramic matrix composites are developed for thermostructural applications such as in aircraft parts submitted to severe thermomechanical stresses. Glass± ceramic is reinforced by means of long ® bres that are essentially stronger and less brittle than the matrix. This reinforcement yields composites with high strength and a`pseudoplastic' strain su cient to alleviate the catastrophic rupture mode of a brittle material. Amongst commercial ® bres, the SiC Nicalon 202 ® bre has been successfully used in the last years. The mechanical properties of Nicalon± glass matrix composites depend on the nature of the interphases that develop during material processing (Brennan 1988, Lancin 1991. A good characterization of the reaction between the ® bre and the matrix together with the understanding of this reaction are essential in order to elaborate composites with required speci® cations.Studies of the interphases produced in the course of the ® bre± matrix reaction have been conducted in a number of composites based on various matrices (Brennan 1988, Chen et al. 1989, Bonney and Cooper 1990. The most currently used are Li 2 O-Al 2 O 3 -SiO 2 (LAS) or CaO± Al 2 O 3 -SiO 2 (CAS) containing some additives such as niobium, arsenic or yttrium. In order to explain the formation of interphases, one usually refers to the model proposed by Cooper and Chyung (1987) which

Frequency Effects on Fatigue Behavior of a Silicon Carbide Fiber‐Reinforced Glass–Ceramic Composite (SiC/MAS)

“…A previous study also suggested the existence of a silicon oxicarbide phase (SiO x C y ) instead of SiO 2 in TL. 28 Surface temperatures on the specimen measured during fatigue tests were not much (B51C) in the present study, even at a higher frequency of 900 Hz. However, Halbig and Eckel have suggested that an internal temperature close to 11001C could be generated due to the internal friction in the C/SiC CMC system.…”

Transmission electron microscopy of interfaces in structural ceramic composites

“…While the existence of carbon-rich layers in tough Si-C-O fibre-reinforced lithium aluminosilicates has been known for some time (Brennan, 1986;Brennan, 1987), it is only recently that a relatively unambiguous identification of the structure and chemistry of interfacial layers in as-processed and oxidized Si-C-O fibre-reinforced magnesium aluminosilicates (MAS) has been made by transmission electron microscope techniques (Kumar & Knowles, 1996a, b). The recent work by Le Strat et al (1998) on Nicalon fibre-Pyrex glass matrix composites has shown the complexity of the reaction products at the fibre-matrix interfaces using a variety of spectroscopic techniques, highlighting in particular the difficulty in distinguishing via electron energy loss spectroscopy (EELS) between amorphous silica-rich and amorphous oxygen-rich silicon oxycarbide interphase products.…”

“…In addition to the diphasic layer, there can be a thin diffusion layer of elements diffusing from the matrix into the fibres, and there can also be a separate carbon layer, depending on the processing conditions used (Kumar & Knowles, 1996a). Le Strat et al (1998) have shown via X-ray photoelectron spectroscopy (XPS) that the silica-rich interfacial reaction product between the fibre and the carbon layer adjacent to the matrix in the composites they examined consisted of some silicon carbide, some carbon and at least two oxygen-rich silicon oxycarbides. It is likely that other systems exhibit equally complicated reaction products, for which XPS, rather than EELS, would appear to be the preferred experimental technique for spectral deconvolution into the various chemical species present.…”