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

Abstract

Introduction. For parts of gas turbines operating under conditions of corrosion-erosion and intense wear, the most acceptable are compositions containing Ni, Co, Cr, Al, B, Y both in pure form and in the composition of compounds applied on the contact surface during thermal spraying. Modern integrated complexes obtained by combining dissimilar substances in the form of a single composition are promising. Such powders are obtained either by cladding or by conglomeration of finely dispersed starting components into a larger particle. The problem of developing and manufacturing plants for conglomeration of powders is urgent and practically important, since it makes it possible to obtain material for thermal spraying of coatings for high-temperature purposes. The aim of the work is to develop a technological scheme for obtaining powders of the required chemical composition with a given particle shape and size, intended for spraying high-temperature protective coatings. Materials and research methods. A technology is developed for the production of integrated powders for spraying coatings using the method of spray drying and subsequent sintering in vacuum or in an argon-hydrogen atmosphere, which avoids the loss of feedstock due to the return of fine and coarse fractions. A technology for preparing materials for spray drying and granulation is proposed. A gravity type aerodynamic classifier is designed and manufactured, which makes it possible to select automatically the powder fraction necessary for spraying the coating, as well as return the unwanted fraction for recycling. The morphology of the granular powder is determined using a TESCAN scanning electron microscope. The chemical composition of the resulting integrated complexes is determined by X-ray microanalysis on an OXFORD attachment. Results and discussion. The technological conditions for obtaining powders of a given size (40…100 µm) are established. It is shown that the shape of the conglomerate particles after spray drying is close to spherical. On the basis of a multifactor experiment, the optimization of the technological process for obtaining powders Ni-17Cr-10Al-1Y and Ni-22Cr-16Al-1Y with sizes up to 100 µm is performed. It is shown that when conglomerating powders with increased aluminum content (Ni-22Cr-16Al-1Y), it is necessary to take into account the exothermic reaction of nickel aluminide formation and dilute the mixture of initial components before sintering with the finished sintered powder. The resulting integrated complexes are characterized by high heat resistance; therefore they are designed and successfully used for plasma spraying of protective coatings for high-temperature purposes. Conclusions. A technology is developed for obtaining composite conglomerated powders Ni-17C-10Al-1Y and Ni-22Cr-16Al-1Y with particle sizes up to 100 µm and a shape close to spherical. A distinctive feature of this technology is that it avoids the loss of feedstock by returning fine and coarse fractions.