Bonding Mechanism from the Impact of Thermally Sprayed Solid Particles
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INTRODUCTION
HIGH velocity oxy-fuel (HVOF) thermal spraying has been applied successfully in producing coating with higher density, superior bond strengths, and less decarburization due to its unique advantage of high momentum and low thermal output.[1–5] The experimental measurements of in-flight particles[6,7] show low-temperature profiles for HVOF-sprayed powder, which enables powder particles with a high melting point to remain in their solid state prior to impact, especially in liquid fueled HVOF systems. Such behavior is further confirmed by the simulation of HVOF-sprayed in-flight particles.[8–10] The impingement of liquid droplets including spreading, breakup, air entrapment, and solidification has been studied in References 11 and 12, while the solid particle impact and its subsequent bonding mechanism has not been well understood. Considering a normal deposition efficiency of ~50 pct for HVOF coating, it is important to have a good understanding of the intricate interaction between kinetic and thermal energy of particles and the bonding mechanism of coating, for effective control of the process. However, such quantitative analyses of HVOF coating have not been reported. The closest information is for the study of cold spraying, which gives more kinetic energy and a small amount of heat to the sprayed particles. Experimental observation[13–16] and numerical simulation[17–19] of the solid particle S. GU, Senior Lecturer, and S. KAMNIS, Research Fellow, are with the School of Engineering Science, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom. Contact e-mail: [email protected] Manuscript submitted January 16, 2009. Article published online August 22, 2009 2664—VOLUME 40A, NOVEMBER 2009
deformation in cold spraying shows that bonding is the result of extensive plastic deformation and related phenomena at the impact interface. Quantitative analyses[17–19] of the relationship between the deposition efficiencies and particle impact velocities indicate a critical particle velocity for successful bonding. Below the critical velocity, solid particles rebound from the substrate, which causes densification and abrasion similar to the shot pinning method. Above this critical velocity, particles deform plastically and bond with the substrate. Thermal softening due to plastic deformation needs to overcome the strain-hardening effect resulting in thermal-plastic shear instability,[20,21] which takes place in a thin region at the contact interface and is attributed to bonding.[19] The presence of both high kinetic energy and substantial thermal energy from HVOF-sprayed particles gives added complexity to quantify the critical impact parameter for successful bonding. The complexity means both particle velocity and temperature from HVOF spraying influence the bonding, instead of only particle velocity in the case of cold spray, or mainly particle temperature in arc, plasma, and other thermal spray processes. Thermally sprayed powder particles are varied in shape and size, according to the technology of powder p
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