Oxidation of ZrB 2 –SiC: Influence of SiC Content on Solid and Liquid Oxide Phase Formation Sigrun N. Karlsdottir w,z and John W. Halloran Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48104 The effect of SiC concentration on the liquid and solid oxide phases formed during oxidation of ZrB 2 –SiC composites is investigated. Oxide-scale features called convection cells are formed from liquid and solid oxide reaction products upon ox- idation of the ZrB 2 –SiC composites. These convection cells form in the outermost borosilicate oxide film of the oxide scale formed on the ZrB 2 –SiC during oxidation at high temperatures ( 15001C). In this study, three ZrB 2 –SiC composites with different amounts of SiC were tested at 15501C for various du- rations of time to study the effect of the SiC concentration par- ticularly on the formation of the convection cell features. A calculated ternary phase diagram of a ZrO 2 –SiO 2 –B 2 O 3 (BSZ) system was used for interpretation of the results. The convection cells formed during oxidation were fewer and less uniformly distributed for composites with a higher SiC concentration. This is because the convection cells are formed from ZrO 2 precipi- tates from a BSZ oxide liquid that forms upon oxidation of the composite at 15501C. Higher SiC-containing composites will have less dissolved ZrO 2 because they have less B 2 O 3 , which results in a smaller amount of precipitated ZrO 2 and conse- quently fewer convection cells. I. Introduction T HE boride-based composites, ZrB 2 –SiC and HfB 2 –SiC, are considered among the most promising ceramic composites for high temperature and structural applications due to their unique properties. 1 The ZrB 2 –SiC and HfB 2 –SiC composites are ultra-high-temperature ceramics, that are oxidation resistant at high temperatures due to the presence of a complex multilayer oxide scale that slows down inward diffusion of the oxygen. 1–4 It is well known that SiC improves the oxidation behavior 4 by adding silica to the oxide film, but the details of the role of SiC in the amounts and compositions of the phases in the film have not been described in detail. This paper compares the oxide film microstructure after oxidation at 15501C for three different SiC concentrations. During high-temperature oxidation (412001C) of the ZrB 2 – SiC composite, a liquid oxide film, borosilicate (SiO 2 –B 2 O 3 ), forms on the outer surface. 5,6 Solid zirconium oxide (ZrO 2 ) is also formed along with the liquid oxide film above the unreacted ZrB 2 –SiC material. Because of the high vapor pressure of boria (B 2 O 3 (l )) at these temperatures, compared with silica (SiO 2 (l )), the B 2 O 3 is preferentially evaporated from the borosilicate liq- uid. The liquid oxide film at the outer surface then becomes a predominantly viscous SiO 2 -rich borosilicate liquid. 1,2,7–9 A novel oxidation mechanism involving flow of the liquid oxide has been proposed. 10 Flow of a boria–silica–zirconia (ZrO 2 –SiO 2 –B 2 O 3 (BSZ)) liquid was used to explain the distinc- tive microstructural features on the external oxide surface and in cross section. 11 These features, called convection cells, consist of ZrO 2 ‘‘islands’’ located in larger SiO 2 -rich ‘‘lagoons’’ with B 2 O 3 - rich patterns surrounding the islands. 10–12 These features are shown in Fig. 1, which shows a surface of a ZrB 2 -15 vol% SiC composite oxidized at 16001C for 30 min. The area around the convection cells consists of a SiO 2 -rich glass with small microm- eter-sized zirconia dispersoids. The B 2 O 3 -rich flower petal-like patterns are visible in backscattered electron (BSE) imaging, and in stronger contrast in cathodoluminesence imaging, but are not observed in secondary electron imaging scanning electron mi- croscopy (SEM). We previously suggested that ZrO 2 cores form by precipitation during the evaporation of boria (B 2 O 3 ) from a BSZ liquid that rises through the outer SiO 2 -rich borosilicate layer and flows laterally, forming the B 2 O 3 -rich regions (the petals) around the ZrO 2 islands. 10,11 The BSZ liquid is formed when a borosilicate liquid rich in B 2 O 3 dissolves some ZrO 2 . We estimate the compositions of these phases with the aid of a calculated isothermal section of a B 2 O 3 –SiO 2 –ZrO 2 system at 15001C to describe the equilibrium between a BSZ liquid and crystalline ZrO 2 . 11 A distinction is made between two zirconia morphologies formed during oxidation. The ‘‘primary’’ zirconia (porous underlying ZrO 2 (s)) is formed directly by the oxidation of ZrB 2 by oxygen diffusion through the SiO 2 -rich borosilicate primary surface layer. The ‘‘secondary’’ zirconia is precipitated from the BSZ liquid. At the surface, the B 2 O 3 evaporates from the BSZ liquid and secondary zirconia precipitates. The surface is then covered with the less volatile phases i.e. the silica-rich liquid and secondary zirconia, which are located near the site of the B 2 O 3 evaporation (near the B 2 O 3 petals). The secondary zirconia precipitates form the zirconia island and dispersed zir- conia. 11 What we observe at room temperature are patterns of crystalline zirconia and glasses formed after the silicate liquid cools. 10–12 The convection cell theory suggests that the formation of the convection cells is dependent on the composition of the BSZ liquid, which in turn is dependent on the composition of the ZrB 2 –SiC composite. Here, three ZrB 2 –SiC composites with Fig. 1. Backscattering electron image of a surfaces of a ZrB 2 -15 vol% SiC composite tested at 16001C for 1 2 h, showing an example of convec- tion cells and their patterns. M. Cinibulk—contributing editor z Current address is at the Department of Materials, Biotechnology and Energy, Inno- vation Center Iceland, IS-112 Reykjavik, Iceland. w Author to whom correspondence should be addressed. e-mail: nanna@umich.edu Manuscript No. 24488. Received March 31, 2008; approved November 5, 2008. J ournal J. Am. Ceram. Soc., 92 [2] 481–486 (2009) DOI: 10.1111/j.1551-2916.2008.02874.x r 2009 The American Ceramic Society 481