Technology of obtaining composite conglomerate powders for plasma spraying of high-temperature protective coatings

OBRABOTKAMETALLOV Vol. 23 No. 1 2021 TECHNOLOGY Fig. 1. Technological scheme for obtaining composite powders Traditionally, a set of sieves with different cell sizes is used to determine the granulometric composition of the powder. An aerodynamic classi fi er of the gravity type was designed and manufactured to automate the process of sifting the powder, (Fig. 3). It is a system of axially arranged pipes. The air fl ow rate and the dimensions of the pipes are selected in such a way that there is a double classi fi cation of the powder into three fractions. Depending on which fraction needs to be used in the further powder preparation process, the remaining parts are sent to the head of the process, which ensures almost 100% powder use in the spray drying. An important role in the spraying process is given to the choice of the spray nozzle type (Fig. 4). An analog of the nozzle we used was the one for dispersed fuel oil to the furnaces combustion chamber [22]. Such a nozzle allows obtaining the size of the sprayed fuel oil particles within 0 ... 100 μ m. Since the vis- cosity of fuel oil and the suspensions we used is approximately the same, this nozzle design was applicable for our purposes. The total performance of the plant for conglomerating powders is 20 ... 50 kg of dried material per hour. The fi rst stage of the developed manufacturing process deals with preparing the suspension to be subsequently spray-dried. A solvent and binders for the suspensions are selected based on the properties of the resulting compositions, as well as granulation conditions and requirements for the purity of the resulting product. There are certain requirements for the solvent and binder: they should easily and, if possible, completely evaporate from the powder conglomerates under appropriate temperature conditions. The solvent evaporates from the suspension droplets during spraying and drying in the air stream. The binder is removed during the subsequent sintering of the composite particles. According to the authors [23], the binder should be stable at conglomeration temperatures and evaporate at approximately 50 K above these temperatures