Evaluation of Parameters for Assessment of Inter-Splat Bond Strength in Cold-Sprayed Coatings
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G. Sundararajan, Naveen M. Chavan, G. Sivakumar, and P. Sudharshan Phani (Submitted September 18, 2009; in revised form April 28, 2010) The quality and performance of cold spray coatings are largely determined by the extent and strength of bonding between the adjacent splats. Usually, the extent of inter-splat bonding is only qualitatively estimated by examining the polished and etched sectioned surfaces of the coated sample. Thus, there is a clear need for indirect techniques to quantitatively estimate the extent of inter-splat bonding so that they can serve as quality control tools. In this study, elastic modulus, electrical conductivity, and critical load for inter-splat debonding as determined using a scratch test are considered as possible parameters for the estimation of the extent of inter-splat bonding using four different cold spray coatings (Ag, Cu, 316 Stainless Steel, and Zn) as model coatings. It is demonstrated that all the three parameters are capable of quantifying the extent of inter-splat bonding.
Keywords
cold spray coating, critical load for inter-splat debonding, elastic modulus, electrical conductivity, inter-splat bond strength
1. Introduction Cold spray coatings, belonging to the family of thermal spray coatings, are formed through repeated impacts of powder particles onto the substrate at high velocities (500-1200 m/s), and as a result, exhibit very low porosities. Since the powder particles are heated only to maximum temperature of 500-600 C in the cold spray technique, the composition and phases present in the powder feedstock are essentially retained in the final coating formed. In particular, the oxidation of powder particles during their flight (prior to impact onto the substrate) is minimal leading to near-zero oxide content in the coating with the attendant advantages like high electrical and thermal conductivity in the coating. Cold spray coatings with attractive properties have been obtained in the case of pure metals such as Cu, Al, Ti, Ni, Fe, and Ta (Ref 1-9) and also in the case of metallic alloys such as stainless steels, Inconel, Ni-Cr alloys, Ti-6Al-4V, and steels (Ref 10-17). In all these coatings, the coating property and its performance are substantially influenced by the extent of bonding between the splats (powder particles which flatten on impacting the substrate are defined as splats) that make up the coating. It is believed that the bonding between the splats is essentially G. Sundararajan, Naveen M. Chavan, G. Sivakumar, and P. Sudharshan Phani, International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, India. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
the result of high pressure generated in the contact surface, which helps in breaking and pinching out the oxide layer on splat surfaces in contact and thus enhancing the bonding, and also adiabatic shearing effects which concentrate the deformation on impact to the contact region causing a substantial temperature rise in the contact area, which again a
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