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Tuesday, May 15, 2007 - 3:40 PM

Analysis of In-Flight Particle Properties of Ultrafine Powders

J. Zwick, F. Ernst, M. Brühl, Surface Engineering Institute, RWTH Aachen University, Aachen, Germany; K. Bobzin, Surface Engineering Institute (IOT), RWTH Aachen University, Aachen, Germany; G. Matthäus, Thermico GmbH & Co KG, Castrop-Rauxel, Germany

Demands on functional coatings with high dimensional accuracy and high surface quality has led to increasing interest in processing of very fine powder grades in a particle size range < 25 µm in the past years. Fine powders are not only showing a distinct potential for application of thin and dimensionally accurate coatings, but are also very promising for the production of dense and homogeneous coatings with improved mechanical properties. The large specific surface of fine powders is allowing for relatively low thermal energy levels that are introduced into the process. Nevertheless it also requires a very sensitive temperature control, to prevent overheating of the particles. The reduction of the thermal energy level is resulting in significant advantages particularly for the usability of the HVOF process for coating of inner diameters. Within this work in-flight particle properties of an ultrafine carbide powder < 15 µm for HVOF spraying were analysed and compared to a coarser feedstock powder. The studied HVOF process allows the adjustment of a broad parameter range by utilisation of a hydrogen stabilised liquid fuel combustion process. A conventional straight nozzle type as well as curved nozzles for internal spraying are studied.

Summary: In this work an optimised HVOF process for coating of internal diameters has been used for spraying of ultrafine carbide materials. Two different particle size distributions have been sprayed and in-flight particle properties measured. The evolution of temperature and velocity in the spray jet was compared.