Role of Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior of Si 3 N 4 /BN Fibrous Monolith Ceramics Sigrun N. Karlsdottir w and John W. Halloran Materials Science and Engineering Department, University of Michigan, Ann Arbor, Michigan 48109-2136 The rare-earth oxides ytterbium oxide (Yb 2 O 3 ) and lanthanum oxide (La 2 O 3 ) were used as additives in fibrous monolithic (FM) Si 3 N 4 /BN composites to study their individual effect on flexural strength and oxidation behavior of the composite. Two compo- sitions were prepared: 20 vol% BN/80 vol% Si 3 N 4 with addi- tives for the Si 3 N 4 and BN being either 8 wt%Yb 2 O 3 or 8 wt% La 2 O 3 . Four-point flexural testing and static oxidation exper- iments at 14001C in dry air for 10 h were performed. The ma- terial with Yb 2 O 3 showed a high flexural strength, graceful failure, and comparable strength to reported Si 3 N 4 /BN FMs with 6 wt% Y 2 O 3 and 2 wt% Al 2 O 3 . The material with La 2 O 3 showed lower flexural strength and brittle failure in the majority of the samples; this was believed to be related to the hydration of La 2 O 3 from the rare-earth apatite phase, La 5 Si 3 O 12 N, resulting in lanthanum hydrate crystals on the side surfaces of the samples and disintegration of the material. The surface of the FMLA sample after oxidation showed severe oxidation. In contrast, the oxidation test of the FMs with Yb 2 O 3 revealed a thin oxide scale containing small Yb 2 Si 2 O 7 on the Si 3 N 4 cells but large Yb 2- Si 2 O 7 on the BN cell boundary. Also, microscopic analysis showed B100 lm recession in the BN cell boundary and an B4 lm oxide scale on Si 3 N 4 cells. I. Introduction F IBROUS MONOLITHIC (FM) ceramics are laminates with a 3D structure. Because of their unique structure, they fail in a non-catastrophic way and thus have been considered as prom- ising materials for structural applications. 1–5 FM ceramics have a fibrous texture and consist of a strong cell surrounded by a weaker boundary phase. The most thoroughly investigated FM ceramic system is the Si 3 N 4 /BN FMs. 1–8 This system is con- sidered one of the most promising FMs because of its high strength at elevated temperatures and thermal shock resist- ance. 3–6 The additives yttrium oxide (Y 2 O 3 ) and aluminum ox- ide (Al 2 O 3 ) are conventionally used in the cells of the Si 3 N 4 in the Si 3 N 4 /BN FMs as sintering aids. It is well known that during sintering of monolithic Si 3 N 4 (containing sintering additives), a liquid phase forms when the sintering additives react with SiO 2 that coats the Si 3 N 4 particles. After sintering, this liquid phase is usually retained in a glassy intergranular phase. 9,10 For Si 3 N 4 / BN FMs with Y 2 O 3 and Al 2 O 3 as additives, the glassy phase forms and is known to migrate into the BN cell boundaries during hot pressing. 2,4 This intergranular glassy phase influences the mechanical properties of the Si 3 N 4 /BN FMs at elevated temperatures. 3 In an effort to enhance the mechanical properties of mono- lithic Si 3 N 4 at room temperature and at elevated temperatures, many researchers have added sintering additives to Si 3 N 4 to in- crease the refractoriness of the glassy intergranular phase and/or crystallize the grain–boundary phase. 11–15 Liu and Nemat-Nas- ser 12 studied the microstructure of an in situ reinforced silicon nitride, sintered with the rare-earth oxides lanthanum oxide (La 2 O 3 ) and Y 2 O 3 . They found crystalline grain–boundary phases La 5 Si 3 O 12 N and Y 5 Si 3 O 12 N, formed at grain pockets and two grain boundaries of the Si 3 N 4 . Cinibulk et al. 13,14 achieved good oxidation resistance and mechanical properties of monolithic Si 3 N 4 at high temperatures by sintering the Si 3 N 4 with various rare-earth oxides and silicon dioxide additives and then heat treated it to form crystalline rare-earth silicate phases. Park et al. 15 used Yb 2 O 3 as a sintering aid to enhance the me- chanical properties of Si 3 N 4 . They found that the amount of Yb 2 O 3 had considerable effects on the microstructural evolution and the composition of the secondary phase in the grain bound- ary. Different crystalline grain boundary phases were formed for different amounts of Yb 2 O 3 ; for 8 wt% Yb 2 O 3 crystalline Yb 2 Si 2 O 7 was formed at the grain boundary along with a glassy phase. The size of the Si 3 N 4 grains also varied with the amount of Yb 2 O 3 . These changes influenced the mechanical properties of the material at room temperature and elevated temperature, i.e. the flexural strength increased with the increased amount of Yb 2 O 3 used. Other researchers have used sintering additives in Si 3 N 4 to increase oxidation resistance. Lee and Readey 16 increased the oxidation resistance of Si 3 N 4 by using Yb 2 O 3 as an additive and then generating a protective ytterbium silicate (Yb 2 Si 2 O 7 ) skin by a controlled oxidation process associated with the reaction between the Si 3 N 4 oxidation products SiO2 and Yb 2 O 3 . Yb 2- Si 2 O 7 has also been reported as one of the main oxidation prod- ucts formed on the surface of nanocomposite Si 3 N 4 –SiC with Yb 2 O 3 as a sintering additive. 17 Just as rare-earth oxides have been shown to confer suitable grain boundary properties to Si 3 N 4 and enhance mechanical properties and oxidation resistance, rare-earth oxides can be ex- pected to confer suitable grain boundary properties to the Si 3 N 4 cell for the Si 3 N 4 /BN FMs and also possibly increase the sta- bility of the BN cell boundary phase at elevated temperatures. However, so far, no research has been carried out on this aspect. In this paper, we investigated the mechanical properties and oxidation behavior of Si 3 N 4 /BN FMs with the rare-earth oxide additives, Yb 2 O 3 and La 2 O 3 , by flexural strength testing at room temperature and static oxidation testing at 14001C in dry air for 10 h. II. Experimental Procedure (1) Material Fabrication FM Si 3 N 4 /BN samples were fabricated firstly by using coextru- sion to prepare green filaments. The filaments were then stacked to form a green billet. After a binder burnout step, the billets were hot pressed at 25 MPa for 1 h in a flowing N 2 atmosphere at 18001C. Detailed descriptions of the fabrication of FMs have been further described elsewhere. 1 Billets with two different compositions were prepared: 20 vol% BN/80 vol% Si 3 N 4 with additives for the Si 3 N 4 and BN being either 8 wt%- Yb 2 O 3 (REacton, Alfa Aesar, Ward Hill, MA) or 8 wt% La 2 O 3 . Lanthanum hydrate, La(OH) 3 (Alfa Aesar), was 1615 J ournal J. Am. Ceram. Soc., 89 [5] 1615–1620 (2006) DOI: 10.1111/j.1551-2916.2006.00911.x r 2006 The American Ceramic Society M. Cinibulk—contributing editor w Author to whom correspondence should be addressed. e-mail: nanna@umich.edu Manuscript No. 20810. Received July 27, 2005; approved November 29, 2005.