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

OBRABOTKAMETALLOV Vol. 23 No. 1 2021 TECHNOLOGY Introduction In modern gas turbine construction, sprayed coatings of the Ni-Co-Cr-Al-B-Y system are considered the most effective for protecting parts operated under corrosion and erosion conditions and intensive wear [1–8]. The use of plasma spraying as a method of applying high-temperature protective coatings allows for local or additional hardening and protection of individual parts of the surface to give them special service properties. Certain technological dif fi culties arise when spraying dissimilar metal powders with signi fi cantly different melting temperatures and thermophysical properties. The resulting coatings are characterized by increased porosity, heterogeneous chemical composition, a tendency to crack formation, and poor adhesion to the surface of the part. The use of the so-called integrated complexes, which are a combination of dissimilar substances in the form of a single composition in each powder particle solves the problem of differences in thermophysical properties. The integrated complexes are created by either cladding one or several layers of other materials on the initial matrix particle, or by conglomerating fi nely dispersed initial components into a larger particle [11, 12]. The maximum particle size, their shape, and the stability of the granulometric and chemical composition are particularly important for obtaining integral compositions, so many researchers pay special attention to determining these characteristics. Systematic long-term studies have revealed the most acceptable compositions to be the ones that crystallize to form eutectic structures based on nickel and/or cobalt, and that combine high protective properties with satisfactory plasticity due to changes in the content and concentration of alloying elements [13,14]. Previous studies, including fi eld tests, have shown that the best results were obtained in the case of using the developed integral composition of the composition Ni-22Cr-16Al-1uA [15-17]. The accumulated experience shows that for plasma spraying of high-temperature coatings, the particle size of integral complexes should correspond to the interval of 20 ... 80 μ m. An important issue is the ability of powders not to create congestion in transport pipelines, their uniform supply to the plasma jet and free movement with the gas fl ow. This can be achieved with a spherical or similar particle shape [6, 11, 12]. The spray drying method used to intensify the drying and granulation processes of materials has proved effective [19-21]. In other countries, variations of this method are used to obtain powders for plasma spraying, for example, Al 2 O 3 -Mo, WC-Co [22-25]. 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 coatings. Research Methodology The authors proposed a fundamentally new technological scheme for obtaining composite powders for spraying coatings, which includes several sequential operations (Fig. 1). The developers designed and manufactured a plant for spraying suspensions, its scheme is shown in Fig. 2. The body of the plant’s chamber 1 has the following dimensions: diameter 850 mm, height 3000 mm. Such dimensions of the chamber ensure complete drying of the powder and prevent it from sticking to the walls. This in turn enables obtaining fi ne powders in a shape close to spherical. The presence of the upper 2 and lower 3 inputs of the sprayer 4 in the chamber allows for the process of spraying suspensions to work in two directions: “top-down” and “bottom-up”. According to the second option, the drying time of the drop in the air stream is signi fi cantly higher. However, there is signi fi cant turbulence of the air-drop fl ow, which increases the probability of liquid droplets’ interaction with the walls of the chamber and increases the number of deformed and destroyed granules. The fl ow rate of the suspension is set by the value of the air pressure in the feeder and the diameter of the nozzle outlet 1.3...1.2 mm. The fl ow rate of the spraying air is set similarly. The performance of the plant and the granulometric composition of the powder are largely determined by the spraying mode; with the set nozzle size, the mode depends on the pressure and temperature of the air that feeds and sprays the suspension. The chamber has a door with a viewing glass 5 and a classi fi er 6 . It is installed on a stand 8 and is equipped with a feeder for suspensions 7 , a distribution panel 9 , and a spray air heater 10 .