FUSED SILICA ROCKET NOZZLES

Plasma arc sprayed refractory coatings for rocket nozzle throats were evaluated. Apparatus and techniques were developed for arc-spraying. The spraying parameters investigated included powder particle size, moisture content, method of powder feed for uniform rate of feed, stand-off distance, and substrate temperature and preheating. Stresses set up by thermal expansion characteristics of the refractory coating materials were found to limit the size and shape which could be coated satisfactorily. Graphite was the most satisfactory material for the entrance and exit cones, with attention then directed to the evaluation of materials for arc-sprayed coatings on the throat insert where maximum heat and erosion effects are encountered. Investigations were carried out with nozzle throat insert coatings of Cr2C4, Al2O3, W, HfC, HfO2, and a composite coating of W and ZrO2. The composite coating of W-ZrO2 was found to give a uniform coating with a composition in close agreement with the composition desired and controlled by the feed rates of the 2 separate powders. (Author)