Measurement of scratch-induced residual stress within SiC grains in ZrB 2 –SiC composite using micro-Raman spectroscopy Dipankar Ghosh a , Ghatu Subhash a, * , Nina Orlovskaya b a Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA b Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816, USA Received 2 April 2008; received in revised form 13 July 2008; accepted 14 July 2008 Available online 26 August 2008 Abstract An analytical framework for determination of scratch-induced residual stress within SiC grains of ZrB 2 –SiC composite is developed. Using a ‘‘secular equation” that relates strain to Raman-peak shift for zinc-blende structures and the concept of sliding blister field model for scratch-induced residual stress, explicit expressions are derived for residual stress calculation in terms of phonon deformation poten- tials and Raman peak shift. It is determined that, in the as-processed composite, thermal expansion coefficient mismatch between ZrB 2 and SiC induces compressive residual stress of 1.731 GPa within the SiC grains and a tensile tangential stress of 1.126 GPa at the ZrB 2 – SiC interfaces. With increasing scratch loads, the residual stress within the SiC grains becomes tensile and increases in magnitude with scratch load. At a scratch load of 250 mN, the calculated residual stress in SiC was 2.6 GPa. Despite this high value, no fracture was observed in SiC grains, which has been rationalized based on fracture strength calculations from Griffith theory. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Residual stress; Raman spectroscopy; Borides; Scratch test; Ceramic matrix composite 1. Introduction Ultra-high-temperature materials with low density, good mechanical strength and high oxidation resistance at elevated temperatures (>2000 °C) are potential candi- dates for applications in future hypersonic vehicles, kinetic energy interceptor missiles, reusable launch vehicles, etc. [1–6]. Specific examples include wing leading edges, engine cowl inlets, nose-caps, etc. These components have sharp aero-surfaces that are subjected to reactive environments at temperatures >2000 °C. Currently, materials used in high-temperature aerospace structural components are mostly carbon–carbon composites and silicon carbide- based composites [4]. Although, these composites have high-temperature structural capabilities, their oxidation resistance is known to be poor. Ultra-high-temperature ceramics (UHTC) such as borides of transition metals (e.g., zirconium (Zr) and hafnium (Hf)) and their compos- ites are identified as the next generation materials for high- temperature aerospace applications [1–4]. Among UHTC, zirconium diboride–silicon carbide (ZrB 2 –SiC) composites have been receiving significant attention in recent years [3,7–19] owing to their low density (6.0 g cm 3 ), high melting point (>3000 °C) and good oxidation resistance >1500 °C. In the above-mentioned aerospace applications, the UHTC may be subjected to impact by atmospheric debris that can lead to wear of the structural components. This abrasive action of the atmospheric debris particles causes inelastic deformation within the material. Therefore, it is important to study the wear behavior and the associated damage mechanisms in UHTC subjected to such loads. However, most of the available literature on ZrB 2 –SiC composites is focused on processing and their oxidation behavior [3,7–19]; knowledge on wear behavior of these materials is literally non-existent. 1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2008.07.031 * Corresponding author. Tel.: +1 352 392 7005; fax: +1 352 392 7303. E-mail address: subhash@ufl.edu (G. Subhash). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 56 (2008) 5345–5354