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

OBRABOTKAMETALLOV Vol. 23 No. 1 2021 TECHNOLOGY Fig. 4. Suspension spray nozzle and at least 50 K below the sintering temperature of subparticles in conglomerates. These requirements allow the use of a wide range of substances as a binder, for example, polyvinyl alcohol, stearic acid, paraf fi ns, polyethylene glycol, and various resins. In this work, for spray drying at room temperature, rubber and gasoline or polyvinyl acetate (PVA) and an equi-volume mixture of butyl acetate and acetone were used as a binder and solvent, respectively. The latter pair is preferred due to the higher evaporation of the solvent during the spraying process and the formation of strong conglomerates, which facilitates the classi fi cation of the obtained powders. The indisputable advantage of using PVA, unlike rubber, is the possibility of its almost complete removal during the sintering of subparticles in conglomerates. The amount of binder introduced into the suspension signi fi cantly affects its stability before being fed to the spray nozzle, as well as the strength of the resulting granules of composite materials. In each speci fi c case, we experimentally determined the amount of the binder introduced into the charge from the initial fi ne powders, its value varying within 0.5...1.5% of the powder weight. The amount of solvent should be minimal for the mobility of the suspension to be preserved; it is also determined experimentally. The granulometric composition of the powders ultimately depends on the content of the binder and solvent in the suspension, as well as the type of spraying device and the conditions of spraying. The initial suspensions used for spraying have a solid component concentration of 70...85% depending on the density and dispersion of the integrated complexes. The introduction of additives (surfactants or electrolytes) is recommended in case of insuf fi cient stability and mobility of the suspension [24]. To obtain conglomerates corresponding to the given chemical composition, the suspension was prepared by thoroughly mixing the initial charge with a solvent and a binder in mixers of various designs for a suf fi cient time, varying depending on the composition and the characteristics of the starting materials from 2 to 8 hours. The suspensions with small amounts of alloying additives (up to 2%), e.g. tantalum, yttrium, and niobium were mixed for a longer time. The morphology of the integrated complexes was determined using a TESCAN scanning electron microscope, whose software capabilities allow to automatically determine the particle size. The chemical composition was determined by the X-ray microanalysis of powder particles using an OXFORD energy dispersion spectrometer. Averaging was performed on 20 particles in each of the fi ve samples. The resistance of the obtained integrated complexes to high-temperature oxidation was determined by the changes in the speci fi c mass ( Δ M ) of the powder sample after exposure in an alundum crucible in an electric resistance furnace at a temperature of 1324 K for 25 hours. The fl uidity of the conglomerated powders was determined according to the requirements of the state standard GOST 20899–98. Results and discussion The dispersion of the initial materials is important for conglomerating powders using the spray drying method. The size of the subparticles of these powders should not exceed 1/5 of the conglomerates’ diameter [25]. Reducing the size of the subparticles, fi rstly, increases the mobility and stability of suspensions. Secondly, since very small starting materials are used, a more uniform distribution of all the constituent components in the fi nished powder is achieved, especially during micro-alloying. Thirdly, conglomerates with a close to spherical shape are formed from small subparticles which are less susceptible to mechanical