Thermal spraying under control
Improving process control of thermal spray technologies to enable better application of nanostructured coatings is the aim of a research project at the University of Stuttgart in Germany.
The team has created a numerical model of high-velocity oxide fuel (HVOF) and high-velocity suspension flame spraying (HVSFS). ‘Increasing efforts have been made in recent years toward submicron and nanostructured layers promising large potentials in functional and structural coating properties,’ explains Dr Andreas Killinger, a researcher on the project. ‘These potentials have been encouraging researchers to aim for an improved understanding and optimisation of HVOF and HVSFS systems to be able to control coating properties.’
These techniques spray the suspensions with hypersonic speed to deposit thin and dense coatings. However, the heat and mass transfer processes and interactions between the process boundary conditions are complex, making experimental process optimisation a cost and time intensive procedure.
A numerical model of the system, including plasma source, helps to analyse and visualise these processes. The analysis requires 3D computational fluid dynamic calculations of models for plasma jet formation, gas combustion and supersonic
gas flow.
‘Detailed understanding of thermal spray processes is necessary for their integration in complete manufacturing process chains and for improved process efficiency and cost reduction’, says Killinger.
The results suggest that thermal heating from the plasma jet contributes to an increase in gas temperature and expansion as well as fuel and oxidant mixing due to a higher turbulence level.
‘When the plasma was switched on, the gas temperature increased from 300K to 13,000K in the first part of the spray gun, and then it decreased when the plasma jet mixed with the nitrogen gas just before it entered the combustion chamber,’ says Killinger. ‘Shock diamonds were observed due to the increase in temperature and velocity. Supersonic gas flow from the nozzle is therefore slightly over or under expanded, resulting in a loss of pressure. This suggests that designs for HVOF and HVSFS need to be improved’.
However, John Gent, a specialist in surface engineering at Rolls-Royce in London, UK, believes that the research needs to go further. ‘The reaction scheme does not make it possible to predict the role of different chemical species,’ he explains. ‘The first step of future work should be to use a more developed model that takes into account minor species and product dissociation.’
Killinger concludes that these simulation results are the first step towards improved process control as well as adaptation of combustion chamber design to the trajectories and dwell time of spray particles for heat transfer optimisation.Materials World Magazine, 01 Apr 2010
The team has created a numerical model of high-velocity oxide fuel (HVOF) and high-velocity suspension flame spraying (HVSFS). ‘Increasing efforts have been made in recent years toward submicron and nanostructured layers promising large potentials in functional and structural coating properties,’ explains Dr Andreas Killinger, a researcher on the project. ‘These potentials have been encouraging researchers to aim for an improved understanding and optimisation of HVOF and HVSFS systems to be able to control coating properties.’
These techniques spray the suspensions with hypersonic speed to deposit thin and dense coatings. However, the heat and mass transfer processes and interactions between the process boundary conditions are complex, making experimental process optimisation a cost and time intensive procedure.
A numerical model of the system, including plasma source, helps to analyse and visualise these processes. The analysis requires 3D computational fluid dynamic calculations of models for plasma jet formation, gas combustion and supersonic
gas flow.
‘Detailed understanding of thermal spray processes is necessary for their integration in complete manufacturing process chains and for improved process efficiency and cost reduction’, says Killinger.
The results suggest that thermal heating from the plasma jet contributes to an increase in gas temperature and expansion as well as fuel and oxidant mixing due to a higher turbulence level.
‘When the plasma was switched on, the gas temperature increased from 300K to 13,000K in the first part of the spray gun, and then it decreased when the plasma jet mixed with the nitrogen gas just before it entered the combustion chamber,’ says Killinger. ‘Shock diamonds were observed due to the increase in temperature and velocity. Supersonic gas flow from the nozzle is therefore slightly over or under expanded, resulting in a loss of pressure. This suggests that designs for HVOF and HVSFS need to be improved’.
However, John Gent, a specialist in surface engineering at Rolls-Royce in London, UK, believes that the research needs to go further. ‘The reaction scheme does not make it possible to predict the role of different chemical species,’ he explains. ‘The first step of future work should be to use a more developed model that takes into account minor species and product dissociation.’
Killinger concludes that these simulation results are the first step towards improved process control as well as adaptation of combustion chamber design to the trajectories and dwell time of spray particles for heat transfer optimisation.Materials World Magazine, 01 Apr 2010
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