The main argument against the use of the internal HVOF process is the high thermal stress to which the substrates are subjected during the coating process. Traditional HVOF guns operate with a flame-stream energy level of 100–200 kW. Rendering HVOF technology usable for the application of internal coatings requires the reduction of the energy level of the flame stream to 20 kW, while safeguarding high particle velocity and sufficient temperature despite the reduced energy level. This requires an integrated process consisting of the HVOF gun, powder feeder, fuel control, and fine powder; the particle sizes of the powder are -25+5 µm, -15+5 µm, and -10+3 µm. Thermico’s ID CoolFlow M HVOF internal spraying gun comes equipped with a 5 mm acceleration nozzle and radial powder feed. It is suitable for internal diameters of 80 mm and above and eliminates the typical overheating problem. The ID CoolFlow M HVOF gun is suitable for internal coatings with Thermico 776 WC-CoCr powder, which comes in grain sizes of -15+5 µm and -30+15 µm. A comparison of both processes requires a number of specimen coatings with different parameters, which have to be compared to reference coatings. These reference coatings are produced using a Thermico CJS K4.2-776/G gun in combination with WC-CoCr 86 10 4 powder with a grain size of -30+15 µm, and a CJS K5.2-776 gun, using a finer powder with a grain size of -15+5 µm. The base material consists of heat-treated steel rings with a hardness of 45 HRC, an internal diameter of 130 mm and a wall thickness of 10 mm. Subsequently, the density, porosity, and structure of the specimen is assessed, and they are checked metallographically and with a scanning electron microscope, including EDX analysis. The specimen wear is monitored using the prototype of an internal coating test stand, developed by the Institute of Materials Science at the University of Applied Sciences Gelsenkirchen. It is essentially based on the same principle as the pin-on-disc tribometer for relative movements.

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