Analysis of Thermal History and Residual Stress in Cold-Sprayed Coatings Z. Arabgol, H. Assadi, T. Schmidt, F. Ga¨rtner, and T. Klassen (Submitted May 13, 2013; in revised form July 23, 2013) Residual stress in coatings has significant effect on their performance. In cold-sprayed coatings, in which particles impact the substrate at high velocity in solid state, in-plane residual stresses are usually con- ceived to be compressive. In this research, analysis of residual stresses in cold-sprayed deposits is per- formed by analytical and numerical modeling. The influence of various parameters such as the dimensions and elastic properties of the coating and the substrate on the residual stress are analyzed. In addition, the amount of heat input as a key parameter in the build-up of the residual stress is examined. It has been found that the heat input and the associated thermal history have a major influence on the final distortion and the residual stress, to an extent that the in-plane stress can in some cases change from compressive to tensile. Based on these results, a simple model is put forward for the prediction of the final state of the stress and distortion in cold-sprayed flat components. Keywords coating, cold spray, distortion, finite element modeling, residual stress 1. Introduction Cold spraying is a rapidly developing technology for depositing materials in the solid state. In this process, the particles of the feedstock material are accelerated to high velocities of up to 1200 m/s. Beyond a critical velocity defined by the material properties and process conditions metallurgical bonding is obtained (Ref 1-4). Unlike con- ventional thermal spraying methods, in this method the particles are in solid state upon impact on the substrate. Because of the relatively low temperature in cold spray- ing, oxidation, phase transformations, compositional changes, and other problems associated with conventional thermal spraying are mostly alleviated (Ref 5, 6). The primary application of cold-spray coatings is for the sur- face enhancement of metals to improve properties such as wear and corrosion resistance, electrical/thermal conduc- tivity, etc. (Ref 7, 8). In all such applications, the integrity of the whole component is important. This may be defined as the quality of bonding between particles within the coating, and between the coating and the substrate (Ref 7). The properties of the coating influence the engineering appli- cability, the operation behavior and the component life- time (Ref 9). The coating integrity is influenced by the residual stresses present in the coating. This important character- istic of a coating influences its adhesion, wear, fatigue life, resistance to cracking, and overall performance in service. Residual stresses may also lead to peeling and delamina- tion of the coating. (Ref 9-14). Therefore, understanding, prediction, and control of residual stresses can contribute to improved coating performance. There have been several investigations on residual stress evaluation of the coatings in thermal spraying (Ref 12, 15, 16) and cold spraying (Ref 7, 11, 17, 18). In thermal spraying, residual stresses develop in two stages: (i) during the deposition where the quenching stress arises due to rapid cooling and solidification of molten particles upon impact, and (ii) during cooling down after the Nomenclature k Curvature of the substrate H Substrate thickness  h Ratios of the coating/substrate thickness  E Ratios of the coating/substrate YoungÕs modulus De Misfit strain d A Distortion De def Deformation-induced misfit strain De th Thermal misfit strain Da Difference in thermal expansion DT Difference between room and maximum temperature T measured Hot junction temperature of the thermocouple T real Real temperature of the specimen This article is an invited paper selected from presentations at the 2013 International Thermal Spray Conference, held May 13-15, 2013, in Busan, South Korea, and has been expanded from the original presentation. Z. Arabgol and H. Assadi, Department of Materials Engineering, Tarbiat Modares University, Tehran, Iran; and T. Schmidt, F. Ga ¨ rtner, and T. Klassen, Institute of Materials Technology, Helmut Schmidt University, Hamburg, Germany. Contact e-mail: ha10003@yahoo.com. JTTEE5 23:84–90 DOI: 10.1007/s11666-013-9976-x 1059-9630/$19.00 Ó ASM International 84—Volume 23(1-2) January 2014 Journal of Thermal Spray Technology Peer Reviewed