The high-velocity oxygen fuel (HVOF) thermal spray is a particulate deposition process in which micro-size particles of metals, alloys or composite materials are propelled and heated in a sonic/supersonic combusting gas stream and are deposited on a substrate at high speeds to form a thin layer of lamellar coating. The thermal spray coatings prepared by HVOF have been widely used in automotive, aerospace and chemical industry. Representative examples include WC/Co based wear resistant coatings for drilling tools, YSZ based thermal barrier coatings for turbine blades, and Ni based corrosion resistant coatings for chemical reactors. In a previous work [1], we developed a multiscale modeling framework for the HVOF process; however, in this work, we did not explicitly account for the effect that the complex flow profiles have on the coating film.

In the present work, a computational modeling framework is developed for the multiphase flow in an HVOF thermal spray coating process with steel powders as the feedstock [2]. Our modeling framework explicitly accounts for the effects of radial velocity of the gaseous carrier and injection location (of the particles) on important particle properties such as particle temperature and velocity at the point of impact on the substrate. Specifically, the numerical model includes continuum-type differential equations that describe the evolution of gas dynamics and multi-dimensional tracking of particle trajectories and temperature histories in the turbulent reacting flow field. The numerical study shows that the particle temperature is strongly affected by the injection position while the particle velocity is less dependent on this parameter. Moreover, both particle velocity and temperature at impact are strongly dependent on particle size, although the spatial variation of these two variables on the substrate is very small. It is also found that not all the particles are deposited on the substrate perpendicularly (even if the spray angle is 90 degrees), due to substantial radial fluid velocities near the stagnation point. A statistical distribution of particle velocity, temperature, impinging angle and position on the substrate as well as particle residence time is obtained in this work through numerous independent random tracking of particles by accounting for the distributed nature of particle size in the feedstock and injection position as well as the fluctuations in the turbulent gas flow.

References:

[1] Li, M.; Christofides, P. D. Multiscale Modeling and Analysis of an Industrial HVOF Thermal Spray Process, Chem. Eng. Sci., 60, 3649-3669, 2005.

[2] Li, M.; Christofides, P. D. Computational study of particle in-flight behavior in the HVOF thermal spray process, Chem. Eng. Sci., 61, 6540-6552, 2006.