J.Am. Ceran.So,89图1615-l620(2006) DOl:10.ll1551-2916.200600911.x o 2006 The American Ceramic Society ourn Role of Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior of Si3 N,/BN Fibrous Monolith Ceramics SigrunN. Karlsdottir'and John W.Halloran Materials Science and Engineering Department, University of Michigan. Ann Arbor, Michigan 48109-2136 rare-earth oxides ytterbium oxide(Yb2 O3)and lanthanum crystallize the grain-boundary phase. -Liu and Nemat-Nas- (La2O3)were used as additives in fibrous monolithic(FM) ser studied the microstructure of an in Si3N4BN tes to study their individual effect on flexural nitride. sintered with the rare-earth oxides lanthanum trength and oxidation behavior of the composite. Two LayO3) and Y,O3. They found crystalline grain-bot tives for the Si3,a and bn being either 8 wt%oYb,O3 or 8 wt% ases LasSiO12N and YssiO12N, formed at grain boundaries of the si, N,. Cinibul La,O3 Four-point flexural testing and static oxidation exper achieved good oxidation resistance and mechanical properties iments at 1400C in dry air for 10 h were performed. The ma- of monolithic Si3 N4 at high temperatures by sintering the sin terial with Yb2O3 showed a high flexural strength, graceful with various rare-earth oxides and silicon dioxide additives and failure, and comparable strength to reported Si3N4/BN FMs then heat treated it to form crystalline rare-earth silicate phase with 6 wt%Y2O3 and 2 wt% AlO3. The material with LazO Park et al.used Yb2O3 as a sintering aid to enhance the me- showed lower flexural strength and brittle failure in the majority chanical properties of Si3N4. They found that the amount of of the samples; this was believed to be related to the hydration of Yb O3 had considerable effects on the microstructural evolution LazO3 from the rare-earth apatite phase, LasSi3O12N, resulting and the composition of the secondary phase in the grain bound- in lanthanum hydrate crystals on the side surfaces of the sample ary. Different crystalline grain boundary phases were formed for nd disintegration of the material. The surface of the fmla amounts of Yb2O3, for 8 wt% Yb2O3 crystalline sample after oxidation showed severe oxidation. In contrast, the was formed at the grain boundary along with a glassy oxidation test of the FMs with Yb2O3 revealed a thin oxide scale he size of the Si3N4 grains also varied with the amoun Sio ng small Yb Si,0, on the SiN, cells but large Yb2 of Yb,O3. These changes influenced the mechanical properties of the material at room temperature and elevated temperature, showed 100 um recession in the bn cell boundary and an e the flexural strength increased with the increased amount of 4 um oxide scale on Si3N4 cells. Yb O3 used Other researchers have used sintering additives in Si3N4 to ncrease oxidation resistance. Lee and Readey increased the oxidation resistance of Si3 Na by using Y b2O3 as an additive and . Introduction then generating a protective ytterbium silicate (Yb2 Si2O7) skin F IBROUS MONOLITHIC(FM) ceramics are laminates with a 3D by a controlled oxidation process associated with the reaction structure. Because of their unique structure, they fail in between the Si3N4 oxidation products SiO2 and Yb2O3. Yb2 non-catastrophic way and thus have been considered as prom- Si2O, has also been reported as one of the main oxidation prod- ising materials for structural applications. FM ceramics have ucts formed on the surface of nanocomposite Si3NA-SiC with a fibrous texture and consist of a strong cell surrounded by a Yb2O3 as a sintering additive. weaker boundary phase. The most thoroughly investigated FM Just as rare-earth oxides have been shown to confer suitable ceramic system is the Si3N4/BN FMs. This system is con- grain boundary properties to Si N4 and enhance mechanical properties and oxidation resistance, rare-earth oxides can be ex strength at elevated temperatures and thermal shock resist- pected to confer suitable grain boundary properties to the si3n4 ance.6 The additives yttrium oxide(Y,0.)and aluminum ox- cell for the Si,Na/BN FMs and also possibly increase the sta- de(Al O)are conventionally used in the cells of the SigN4 in bility of the BN cell boundary phase at elevated temperatures the si3 N4/BN FMs as sintering aids It is well known that during However, so far, no research has been carried out on this aspect sintering of monolithic Si,N4(containing sintering additives), a In this paper, we investigated the mechanical properties and liquid phase forms when the sintering additives react with SiO2 oxidation behavior of SinA/BN FMs with the rare-earth oxide that coats the Si3N4 particles. After sintering, this liquid phase is additives, Yb203 and Laz O3, by flexural strength testing at room usually retained in a glassy intergranu lar phase. 9, I For SisN BN FMs with Y2O3 and Al2O3 as additives, the glassy phas 10h. forms and is known to migrate into the bn cell boundaries during hot pressing. This intergranular glassy phase influences the mechanical properties of the Si3N4/BN FMs at elevated II. Experimental Procedure temperatures. (1) Material Fabrication effort to enhance the mechanical properties of mono- Na at room temperature and at elevated temperatures. FM Si N/BN samples were fabricated firstly b by using coextru- esearchers have added additives to Si3N4 to in- sion to prepare green filaments. The filaments were then stacked le refractoriness of the tergranular phase and or to form a green billet. After a binder burnout step, the billets were hot pressed at 25 MPa for I h in a flowing N2 atmosphere at 1800oC. Detailed descriptions of the fabrication of FMs have M. Cinibulk--contributing editor been further described elsewhere Billets with two different compositions were prepared: 20 vol% BN/80 vol% Si3N4 with additives for the Si3 N4 and bn being Manuscript No 20810. Received July 27. 2005: d November 29. 2005. either 8 wt%o-Yb2O3(REacton, Alfa Aesar, Ward Hill, MA)or Author to wt%La2O3. Lanthanum hydrate, La(OH)3(Alfa Aesar). 1615Role of Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior of Si3N4/BN Fibrous Monolith Ceramics Sigrun N. Karlsdottirw and John W. Halloran Materials Science and Engineering Department, University of Michigan, Ann Arbor, Michigan 48109-2136 The rare-earth oxides ytterbium oxide (Yb2O3) and lanthanum oxide (La2O3) were used as additives in fibrous monolithic (FM) Si3N4/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% Si3N4 with addi￾tives for the Si3N4 and BN being either 8 wt%Yb2O3 or 8 wt% La2O3. Four-point flexural testing and static oxidation exper￾iments at 14001C in dry air for 10 h were performed. The ma￾terial with Yb2O3 showed a high flexural strength, graceful failure, and comparable strength to reported Si3N4/BN FMs with 6 wt% Y2O3 and 2 wt% Al2O3. The material with La2O3 showed lower flexural strength and brittle failure in the majority of the samples; this was believed to be related to the hydration of La2O3 from the rare-earth apatite phase, La5Si3O12N, 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 Yb2O3 revealed a thin oxide scale containing small Yb2Si2O7 on the Si3N4 cells but large Yb2- Si2O7 on the BN cell boundary. Also, microscopic analysis showed B100 lm recession in the BN cell boundary and an B4 lm oxide scale on Si3N4 cells. I. Introduction FIBROUS 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 Si3N4/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 (Y2O3) and aluminum ox￾ide (Al2O3) are conventionally used in the cells of the Si3N4 in the Si3N4/BN FMs as sintering aids. It is well known that during sintering of monolithic Si3N4 (containing sintering additives), a liquid phase forms when the sintering additives react with SiO2 that coats the Si3N4 particles. After sintering, this liquid phase is usually retained in a glassy intergranular phase.9,10 For Si3N4/ BN FMs with Y2O3 and Al2O3 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 Si3N4/BN FMs at elevated temperatures.3 In an effort to enhance the mechanical properties of mono￾lithic Si3N4 at room temperature and at elevated temperatures, many researchers have added sintering additives to Si3N4 to in￾crease the refractoriness of the glassy intergranular phase and/or crystallize the grain–boundary phase.11–15 Liu and Nemat-Nas￾ser12 studied the microstructure of an in situ reinforced silicon nitride, sintered with the rare-earth oxides lanthanum oxide (La2O3) and Y2O3. They found crystalline grain–boundary phases La5Si3O12N and Y5Si3O12N, formed at grain pockets and two grain boundaries of the Si3N4. Cinibulk et al. 13,14 achieved good oxidation resistance and mechanical properties of monolithic Si3N4 at high temperatures by sintering the Si3N4 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 Yb2O3 as a sintering aid to enhance the me￾chanical properties of Si3N4. They found that the amount of Yb2O3 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 Yb2O3; for 8 wt% Yb2O3 crystalline Yb2Si2O7 was formed at the grain boundary along with a glassy phase. The size of the Si3N4 grains also varied with the amount of Yb2O3. 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 Yb2O3 used. Other researchers have used sintering additives in Si3N4 to increase oxidation resistance. Lee and Readey16 increased the oxidation resistance of Si3N4 by using Yb2O3 as an additive and then generating a protective ytterbium silicate (Yb2Si2O7) skin by a controlled oxidation process associated with the reaction between the Si3N4 oxidation products SiO2 and Yb2O3. Yb2- Si2O7 has also been reported as one of the main oxidation prod￾ucts formed on the surface of nanocomposite Si3N4–SiC with Yb2O3 as a sintering additive.17 Just as rare-earth oxides have been shown to confer suitable grain boundary properties to Si3N4 and enhance mechanical properties and oxidation resistance, rare-earth oxides can be ex￾pected to confer suitable grain boundary properties to the Si3N4 cell for the Si3N4/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 Si3N4/BN FMs with the rare-earth oxide additives, Yb2O3 and La2O3, 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 Si3N4/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 N2 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% Si3N4 with additives for the Si3N4 and BN being either 8 wt%- Yb2O3 (REacton, Alfa Aesar, Ward Hill, MA) or 8 wt% La2O3. Lanthanum hydrate, La(OH)3 (Alfa Aesar), was 1615 Journal 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