Effect of Oxidation on Mechanical Properties of Fibrous Monolith Si 3 N 4 /BN at Elevated Temperatures in Air Young-Hag Koh,* ,† Hae-Won Kim,* and Hyoun-Ee Kim* School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Korea John W. Halloran* Materials Science and Engineering Department, University of Michigan, Ann Arbor, Michigan 48109-2136 The oxidation behavior and its effect on the mechanical properties of fibrous monolith Si 3 N 4 /BN after exposure to air at temperatures ranging from 1000° to 1400°C for up to 20 h were investigated. After exposure at 1000°C, only the BN cell boundary was oxidized, forming a B 2 O 3 liquid phase. With increasing exposure temperature, the Si 3 N 4 cells began to oxidize, forming crystalline Y 2 Si 2 O 7 , SiO 2 , and silicate glass. However, in this case, a weight loss was observed due to extensive vaporization of the B 2 O 3 liquid. After exposure at 1400°C, large Y 2 Si 2 O 7 crystals with a glassy phase formed near the BN cell boundaries. The oxidation behavior signifi- cantly affected the mechanical properties of the fibrous mono- lith. The flexural strength and work-of-fracture decreased with increasing exposure temperature, while the noncatastro- phic failure was maintained. I. Introduction S ILICON NITRIDE (Si 3 N 4 ) based composites have been regarded as one of the most promising materials for high-temperature structural applications. 1–4 Among these, fibrous monolith Si 3 N 4 / boron nitride (BN), consisting of a hexagonal arrangement of strong Si 3 N 4 cells surrounded by weak BN cell boundaries, has been found to exhibit noncatastrophic failure with high strength both at room temperature and at high temperatures due to exten- sive crack interactions (crack delaminations and crack deflections) through the BN cell boundaries. 4–7 As this material was intended for use at high temperatures, the oxidation behavior is one of the more important criteria that need to be clearly understood before it can be applied. The oxidation resistance of Si 3 N 4 containing sintering aids is strongly influenced by the composition and crystalline state of the secondary phase. 8 –12 However, when BN is exposed to an oxidizing atmo- sphere above 1000°C, it reacts with oxygen, forming a B 2 O 3 liquid or a gas phase. 13 Such oxidation behavior strongly affects the mechanical properties of the materials by forming an oxide scale on the surface. 12 In this paper, the oxidation behavior of a fibrous monolith in air at temperatures between 1000° and 1400°C was investi- gated. The oxidation behavior was measured by monitoring the weight changes of the specimens. The oxidation products that formed on the surface were identified by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). In addition, the mechanical properties, such as the strength and work-of-fracture (WOF), were measured using four-point bending tests and related to crack propagations. II. Experimental Procedure Fibrous monolith Si 3 N 4 /BN was fabricated by hot pressing using a method described elsewhere. 5 Sintered billets were cut into dimensions of 2 mm  4 mm  10 mm to measure the weight changes, and were machined with a 600-grit diamond wheel, and subsequently polished down to 3 m with a resin-bonded diamond wheel. Specimens with dimensions of 3 mm  4 mm  50 mm were prepared for the mechanical tests using a similar methodol- ogy. The tensile surface was polished down to 3 m and slightly chamfered to remove the existing defects. In addition, the side surfaces of each specimen were polished down to 30 m. Before oxidation, the surfaces were ultrasonically cleaned in acetone and ethanol. The oxidation test was conducted in a vertical alumina tube-furnace at temperatures ranging from 1000° to 1400°C for 20 h in laboratory air. The furnace was heated at a heating rate of 10°C/min and maintained at the exposure temper- atures. The polished specimens, suspended at the end of a platinum wire, were inserted into the hot zone from the top. Such a rapid heating process was selected to minimize oxidation during the heating stage, while a furnace cooling process was used to minimize the thermal stress. The weights of each sample were measured both before and after exposure using a digital balance with an accuracy of 0.1 mg. The oxidized surfaces were examined by SEM, XRD, and EDS. The flexural strength and apparent work-of-fracture were measured using a four-point flexural con- figuration with inner and outer spans of 20 and 40 mm, respec- tively, at a crosshead speed of 0.5 mm/min. In addition, crack propagation after the bending test was examined by optical microscopy. III. Results and Discussion The oxidation behavior was analyzed by monitoring the weight change of the specimens exposed to air between 1000° and 1400°C for up to 20 h, as shown in Fig. 1. When the specimens were exposed at 1000°C, the weight changes were found to be a function of the exposure time. At an early stage of oxidation, a weight gain was observed, implying that B 2 O 3 (l) was formed on the cell boundary. However, as the exposure time was increased, a weight loss was observed as a result of B 2 O 3 (l) vaporization. As the exposure temperature was in- creased to 1200°C, the B 2 O 3 (l) vaporization became more J. L. Smialek—contributing editor Manuscript No. 187162. Received February 5, 2002; approved September 23, 2002. *Member, American Ceramic Society. † Now with the University of Michigan. 3123 journal J. Am. Ceram. Soc., 85 [12] 3123–25 (2002)