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 2 In fl uence of the initial particle size distribution and chemical composition on the powder after sintering Chemical composition of powders, masses.% Initial size of powders, μ m Powder yield after spraying, % Granulometric composition after sintering, % 0..40, μ m 40..63, μ m 63..100, μ m >100 μ m Ni-17Cr-10Al-1Y 0…100 – 29.2 36.0 28.1 5.6 Ni-22Cr-16Al-1Y 0…100 – 24.0 37.6 27.4 11.0 0…40 35.8 61.4 32.3 6.6 – 40…100 64.2 7.7 35.1 44.1 1.3 Table 2 presents the granulometric composition of the sintered powders. The bulk of the resulting powder has a size greater than 40 μ m. The yield of the 0...40 μ m fraction is less than 25 %. Obtaining the powder used for spraying in a dynamic vacuum requires sintering a smaller fraction of 0...40 μ m. The obtained results demonstrate that sintering a powder with a grain size of 0 ... 40 μ m leads to a signi fi cant powder grain size increase due to partial sintering of conglomerates among themselves, which deteriorates the morphology of the fi nal product. This is due to a larger contact area than in the case of 40 ... 100 μ m fraction sintering. The yield of fractions after sintering is also affected by the composition of the powder. Thus, sintering a powder of the Ni-17Cr-10Al-1Y composition yields a smaller amount of a large fraction than sintering a powder of the Ni-22Cr-16Al-1Y composition, which is associated with a lower Al content and, as a result, less heat released during the sintering reaction and a lower sintering temperature (Table 2). After sintering the powder, it is divided into fractions and recovery-annealed. The sieving of 40 ... 100 μ m fraction is carried out using a standard set of sieves, and the separation of the fraction with a size of 10 ... 40 μ m was carried out by washing the powder three times in distilled water, followed by drying for 2...3 hours at a temperature of 373...383 K. Finishing treatment of the powder is recovery annealing in a hydrogen atmosphere at 873 K for half an hour to remove oxides present in the initial powders and formed during the preparation of conglomerates. To assess the quality of the obtained powders, their resistance to oxidation in the air was measured at 1323 K. The test results can be described by a regression equation:          73, 3 3, 63 2, 65 5, 95 5, 65 17, 05 8, 03 14, 95 , ìã/ã. M V X P VX VP XP XVP (3) According to the experimental data, to improve the quality of the powder, the amount of solvent ( V ) in the sprayed suspension should be reduced; X and the excess pressure in the chamber ( P ) should be increased. In practice, as a rule, the volume of the solvent tends to be reduced. Thus, to obtain powders for plasma spraying in a dynamic vacuum, we determined the following technological regulations: V = 360 ml/1 kg of powder; X = 2.0 mm; P = 1.28 atm. Powders obtained with the sprayed air heating are characterized by lower heat resistance, for example, for 363 K, it is 170.9 mg/g, and for 523 K, it is 208.6 mg/g. This indicates that the accelerated drying of conglomerates occurs during the spraying process, leading to deformation and increased porosity of the particles. We conducted a comparative properties study of the powders obtained using spray drying and spraying of melt in a vacuum (Table 3). Bothmethods, applied to plasma spraying, give powders that are similar in properties. However, the spray drying method is cheaper and more universal in terms of the powders produced. The chemical composition of the powder obtained by the spray drying method practically does not differ from the composition of the initial components.