Improve ceramic slurry flow rate stability for thermal spraying process (HVOF)

In the high velocity oxy-fuel (HVOF) thermal spraying process, ceramic powders are injected into a flame to impact with a target substrate. As a consequence, the substrate surface is coated with a material that will allow a higher surface hardness with better abrasion resistance, or a better chemical resistance. 

Whereas some HVOF processes employ the direct injection of dry powders into the flame, a process at the Faculty of Engineering of the University of Nottingham uses a liquid such as water to carry the powder into the flame, allowing to handle smaller particles down to the nanometric range. R&D of this university faculty occurs, for example, in the field of in-power generation, where application of these protective HVOF coatings allows combustion processes to occur at higher temperatures and pressures, thereby increasing the combustion efficiency and reducing the CO₂ footprint.

To improve the way in which the powder/liquid slurry is transported from the slurry-containing vessel to the flame, the university requested the help of Bronkhorst UK.

Process solution

The Bronkhorst solution consists of a few consecutive steps, starting with the purchase by the University of an ES-FLOW ultrasonic volume flow meter to improve their flow measurement ability. Indeed, the flow rate can be measured more repeatable. In addition to that, due to the straight-through sensor of this robust liquid flow meter, abrasion of the ceramic particles in the slurry is virtually not an issue. The slurry is about 20% powder in liquid, and in the old as well as the new setup, the vessel is stirred to keep the slurry homogenous.

As an extension to the solution, a Bronkhorst      EL-PRESS P-800 pressure controller is used in master-slave control with the ES-FLOW liquid flow meter. This configuration controls the pressure on top of the slurry vessel, to indirectly control the flow rate of the slurry being introduced to the flame. During the coating application, the operator is asking for a specific flow from the liquid flow meter, and this is being fed back to the P-800 pressure controller - which is, in effect, a pressure meter with two control valves. So the liquid flow meter is controlling the control valves, to indirectly control the flow rate by changing the pressure. For the current application, 40 ml/min of slurry is a typical flow rate. The pressure in the vessel is controlled in the range from 0 to 5 bar gauge, with a typical value of around 4-4.5 bar.