On the Formation of a Diffusion Bond from Cold-Spray Coatings
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ent years, adapting toward a trend for lowtemperature processes, cold-gas dynamic spraying (otherwise simply known as cold spray [CS]) has drawn increasing attention in surface engineering. A high pressure gas, nitrogen or helium, is used to accelerate particles, in the solid state, toward a substrate to form metallic or composite coatings.[1–4] Initially, the impact between particles and substrate could break up the oxide layers of both particle and substrate through severe plastic deformation at a high strain rate, which permits impinging particles to bond with the newly exposed, oxide-free metal surface.[5–8] Subsequently, bonding between particles results from extensive plastic deformation during impact; thus, the coating is built up layer by layer. Compared with thermal spray that involves either complete or partial melting of powder particles, CS eliminates thermal defects, such as high residual stress caused by solidification shrinkage, high-temperature oxidization, and potential damage of the substrate caused by molten metal impact. Hence, CS is particularly suitable for coating thermal-sensitive materials, such as QIANG Wang, PhD Candidate, and MING-XING Zhang, Associate Professor, are with the School of Mechanical and Mining Engineering, The University of Queensland, St Lucia, QLD 4072, Australia, are also with the CRC for Alloy and Solidification Technology (CAST), and the Australian Research Council (ARC) Centre of Excellence for Design in Light Metals, Australia. Contact e-mail: [email protected] NICK BIRBILIS, Associate Professor, is with the School of Material Engineering, Monash University, Clayton, VIC 3800, Australia, is also with the Australian Research Council (ARC) Centre of Excellence for Design in Light Metals, Australia. Manuscript submitted November 9, 2011. Article published online March 8, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
nanocrystalline and amorphous materials,[9,10] and for oxygen-sensitive and low-melting-temperature materials, such as aluminum, magnesium, and titanium alloys.[3,8,11] For durable service in application, bonding strength is a major concern for CS coatings. Generally, it is considered that there is a mixture of mechanical and metallurgical bonding between the coating and the substrate.[8,12–14] However, considerable work has been undertaken to improve the bond strength. One example includes the incorporation of ceramic particles into the powder feed, leading to substrate surface roughening and higher localized deformation of metal powder, which results in better mechanical bonding.[3,15] In addition, because of faster atomic diffusion at increased temperatures, postspray annealing can be employed to promote a diffusion bond, therefore increasing the bond strength between the coating and the substrate (and also between the particles within the coating). Compared with as-sprayed coatings, several recent works observed both higher electrical conductivity and better interfacial bonding between particles within the coating,[12,16–18] in addition to recrystall
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