Available online at www.sciencedirect.com Journal of the European Ceramic Society 30 (2010) 2365–2374 In situ microscopy observation of liquid flow, zirconia growth, and CO bubble formation during high temperature oxidation of zirconium diboride–silicon carbide Sindhura Gangireddy a , Sigrun N. Karlsdottir b , S.J. Norton a , J.C. Tucker a , John W. Halloran a,∗ a Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA b Department of Materials, Biotechnology and Energy, Innovation Center Iceland, IS-112 Reykjavik, Iceland Available online 20 February 2010 Abstract The oxidation of ZrB 2 –SiC composites at 1450–1650 ◦ C was directly observed with in situ optical microscopy. Video frames showed the flow of silicate liquids, the formation of zirconia deposits, and the growth and collapse of gaseous bubbles on the oxide surface. Contrast in the incandescence of in situ images is analyzed as spatial variations in hue and intensity and related to differences in emissivity of the oxide scale surface features by comparing these hot images with room temperature images. Above 1450 ◦ C, gaseous bubbles were observed to grow and collapse causing perturbations in the liquid oxide on the surface. The bubbles are associated with the evolution of CO from SiC oxidation and the onset is related to the critical temperature where the partial pressure of CO under the oxide scale exceeds atmospheric pressure. © 2010 Elsevier Ltd. All rights reserved. Keywords: Optical microscopy; Borides; Refractories; Composites; In situ 1. Introduction 1.1. Ribbon method Ultra-high temperature ceramics (UHTCs) have recently gained interest as potential materials for a reusable thermal pro- tection system and other components in hypersonic vehicles. 1,2 Typically oxidation experiments have been conducted inside fur- naces where it is not practical to directly observe the material while it undergoes high temperature oxidation. The other tech- nique used in studying the high temperature oxidation behavior, arc-jet testing, simulates reentry environment and can allow viewing of the specimen. However, arc-jet testing is very expen- sive, not easily accessible and not typically instrumented for in situ studies. 3 Consequently, oxidation processes have been inferred from post-test analysis of the oxide scale quenched to room temperature, regardless of the oxidation method. 4,5 On the other hand, this study attempts to directly observe the UHTC specimen during high temperature oxidation using the Ribbon ∗ Corresponding author. Tel.: +1 7347631051; fax: +1 734763478. E-mail address: peterjon@umich.edu (J.W. Halloran). Method. 6 It has been verified that the ribbon method reproduces the complex oxide scales which form during high-temperature testing of UHTC. Owing to the special geometry of ribbon spec- imens and design of the process, only about ∼100 W of power is required to reach 1500–2000 ◦ C. As a result the heat flux from the sample is also small, enabling an optical microscope to approach the hot ribbon and image the surface at magnifica- tions as high as ∼100×. An added advantage of ribbon method is that the fast heating rate (∼450 ◦ C/min) and free cooling rates (∼700 ◦ C/s) 6 allow for samples to be tested easily in cyclic heat- ing conditions while minimizing the effects of pre-oxidation caused by slower heating methods. The video images taken dur- ing oxidation showed gradual microstructural changes in the oxide scale due to liquid oxide flow and sudden changes caused by gas bubbles. In situ observations are compared with room temperature observations made after successive heating cycles. 1.2. Liquid convection hypothesis of oxide scale formation on ZrB 2 –SiC Our observations are interpreted based on the dynamic evo- lution of crystalline and liquid features in the oxide in terms of the convective flow mechanism proposed by Karlsdottir et 0955-2219/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2010.01.034