Adapting of the Background-Oriented Schlieren (BOS) Technique in the Characterization of the Flow Regimes in Thermal Spraying Processes

Adapting of the Background-Oriented Schlieren (BOS) Techniq

The Figure A illustrates the setup of the tribometer in the large chamber SEM. A disc-shaped specimen was fixed on a rotating table, which was powered by a stepping motor. A pin with an Al2O3 ceramic ball was pressed onto the surface of the specimen with a constant normal force The specimen rotated with a rotational speed of 60 rpm in the large chamber SEM and stopped once per rotation. At every stop a SEM picture of the wear track was taken. All these pictures were joined and built into a video stream. At the beginning of the video, a small channel with a round smooth bottom, which was generated by abrasive wear of the Al2O3 ball, can be found on the surface of the coating (Figure C). After about 90 rotations, two small scratches appeared on both sides of the channel with a distance of about 100 µm (Figure D). Obviously, these two scratches were not generated by the Al2O3 ball. After the first 50 rotations, there are many loose particles, which were separated from the surface of the coating due to abrasive wear and were taken up and adhered onto the surface of the Al2O3 ball. These particles contain many sharp edged WC cores and thus scratched the surface of the coating. After some more Rotations, some pits appeared on the scratch (Figure E). They were clearly created by surface fatigue.

Journal Reference

Journal of Thermal Spray Technology, January 2014, Volume 23, Issue 1-2, pp 21-30.

W. Tillmann, M. Abdulgader, H. G. Rademacher, N. Anjami, L. Hagen.

Dortmund University of Technology, Dortmund, Germany and

Dortmund University of Technology, Institute of Materials Engineering, Dortmund, Germany.

Abstract

In thermal spraying technique, the changes in the in-flight particle velocities are considered to be only a function of the drag forces caused by the dominating flow regimes in the spray jet. Therefore, the correct understanding of the aerodynamic phenomena occurred at nozzle out let and at the substrate interface is an important task in the targeted improvement in the nozzle and air-cap design as well as in the spraying process in total. The presented work deals with the adapting of an innovative technique for the flow characterization called background-oriented Schlieren. The flow regimes in twin wire arc spraying (TWAS) and high velocity oxygen fuel (HVOF) were analyzed with this technique. The interfering of the atomization gas flow with the intersected wires causes in case of TWAS process a deformation of the jet shape. It leads also to areas with different aero dynamic forces. The configurations of the outlet air-caps in TWAS effect predominantly the outlet flow characteristics. The ratio between fuel and oxygen determine the dominating flow regimes in the HVOF spraying jet. Enhanced understanding of the aerodynamics at outlet and at the substrate interface could lead to a targeted improvement in thermal spraying processes.

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