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

OBRABOTKAMETALLOV Vol. 23 No. 1 2021 TECHNOLOGY Ta b l e 3 Comparative characteristics of powders obtained by spray drying and spraying of melts Powder Property Method of obtaining Size of powders, μ m 0…40 40...50 50...63 63...100 100...200 >200 Content, mass.% Spray drying 61.4 16.3 16.0 6.6 0 0 Spraying of melts 50.5 24.2 13.4 9.1 1.8 0.9 Fluidity, sec Spray drying 69.9 34.2 28.8 23.5 – – Spraying of melts 57.8 25.3 28.0 31.5 – – To give the sprayed conglomerates suf fi cient strength and density, they are sintered, which includes the stages of removing the organic binder and sintering the subparticles in the conglomerate. The process of sintering the sprayed powder has a signi fi cant impact on the properties of the resulting product. The morphology, strength of the particles, and their granulometric composition largely depend on the conditions of the process. After spray drying, the powder is placed in a free fi lling in a vacuum oven. Next, the furnace is degassed and fi lled with argon to the pressure of 0.1 MPa. Then the temperature is gradually raised at a speed not higher than 3...4 K  min -1 to remove the binder. If polyvinyl acetate (PVA) is used as a binder, the temperature of its complete removal from the powder mass is approximately 820 K. Then the working space of the furnace is degassed to a residual pressure of 0.1 Pa and heated to sintering temperatures. The sintering process can also be carried out at atmospheric pressure, for example, in an argon-hydrogen gas medium. In this case, the degree of powder oxidation is signi fi cantly reduced. There are certain differences in the sintering process of Ni-Cr-Al-Me powders, where Me is the alloying additives of one or several rare elements, such as Y, Ta, Nb. These conglomerate powders are exothermic due to the reaction of nickel aluminides formation. The synthesis reaction begins when the powders are heated to 900...950 K, with the temperature in the reaction space of the furnace rising to 1500 K. In these conditions, conglomerates are strongly sintered among themselves, and this is unacceptable according to the technology. To reduce the sintering temperature of the powders, the sprayed powder was diluted in a certain ratio with a ready-made sintered powder of the same composition before placing it in the furnace. The degree of dilution ultimately determines the heating temperature of the powder and, as a result, the density and mechanical strength of the sintered conglomerates. To obtain strong and dense powder granules of the Ni-Cr-Al-Me composition with different content of chromium (20...28 %) and aluminum (12...20%), the optimal amount of diluent powder was determined to be within 20 to 40 %. For example, a powder of the composition (mass. % ): Ni-28Cr-20Al-Me is best sintered with a dilution of up to 40 %. After the exothermic reaction, the temperature in the furnace is set to 1020 K and maintained for half an hour until the subparticles are completely sintered in conglomerates. The result of sintering is a briquette that is easily crushed into individual granules. Control sieving is performed to separate small powder particles formed as a result of the individual granules’ destruction and large conglomerates sintered together. The powder fraction suitable for spraying is subjected to recovery annealing in a hydrogen atmosphere at the