Analysis of nickel-clad ceramic powder materials

. The task of developing new powder materials for use in gas-thermal technologies is solved by cladding metal oxide particles with a refractory metal component. The results of studies of powders of domestic production based on aluminum oxide (corundum) and zirconium dioxide subjected to chemical nickel cladding (8 and 30 %) are presented. The morphology, granular and phase compositions, as well as the distribution of the elemental composition were studied using electron microscopy and X-ray diffraction analysis. It is shown that the formation of a dense and homogeneous metal shell around the particles of the matrix material occurs when 30 % of the metal is clad. According to X-ray diffraction analysis, powder materials have a multiphase composition: (Ni +  -Al 2 O 3 +  -Al 2 O 3 +  *-Al 2 O 3 ) – for aluminum oxide powder clad with 8 % Ni; (Ni +  -Al 2 O 3 +  -Al 2 O 3 ) - for aluminum oxide powder clad with 30 % Ni, (Ni + T-ZrO 2 ) - for dioxide powder zirconium clad with 30 % nickel. New powder materials of domestic production are competitive in relation to foreign analogues and due to cladding with a metal component will contribute to improving the plasticity of plasma coatings formed from them.


Introduction
Analysis of industrial development programs of economically developed countries shows that nowadays more than 80 % of manufactured mechanical engineering parts are used with coatings of various applications, which allows improving the quality, reliability, and durability of equipment [1,2]. Outer layers condition determines the wear resistance, corrosion resistance, conductivity for heat, and other physical and mechanical properties of the material and working capacity of parts as a whole [1,2].
Currently, the advancing of gas-thermal coating methods follows the path of the development of new composite powder materials [3]. The goal of the program of strategic development of Russian technologies is the development and production of commercially viable materials [4]. This goal was set because until recently, a significant part of Russia's need for powder materials was met through imports. Such corporations as H.C. Starck AMPERIT, Oerlikon Metco, Sulzer Metco are global leaders in the production of powders for gas thermal spraying. In modern economic conditions their product supply to Russia was stopped. Domestic producers were tasked with developing high-quality powder materials as part of the import phase-out program.
The most in-demand materials are metal oxide powder materials for producing multifunctional coatings. Nevertheless, coatings obtained from oxide powders are characterized by increased brittleness in comparison to metal powder mixtures. Therefore, domestic developers are following the path of cladding powder materials in order to improve the operational properties of coatings of metal oxide ceramics [5]. Cladding is the application of a coating layer on the surface of particulates involving chemical and physical processes [2]. The most effective and general method of applying a thin protective layer to powder particles is liquid-phase cladding. It provides such advantages as a wide range of sprayed materials and types of foundation, the absence of mechanical impact on the foundation, the possibility of mechanization of the technological process and high productivity.
This article presents the results of studies of new domestically produced powder materials based on aluminium oxide (corundum) and zirconium oxide chemically cladded with a refractory metal component (nickel).

Methods of research
The studies were carried out on the domestic pilot batches of new powder material based on aluminum oxide (Al2O3) with spherical morphology of particles (8 and 30 %) and different content of cladding metal (Ni).
The phase composition of the powders was examined by the «Dron-3M» X-ray diffractometer analysis (Cu-Ka-radiation, the symmetric shooting according to Bragg-Brentano).
Analysis of granular composition, morphology, microstructure, and distribution of elemental composition was conducted on a TESKAN VEGA scanning electron microscope. The quantitative element analysis was carried out with an energy dispersive spectrometer INCA Energy-250.

Results of research
The powder material based on aluminum oxide (Al2O3) is designed for plasma spraying of wear-resistant coating. A corundumbased powder material was selected as the matrix material. The granular composition of the analysed powder materials ranges from 20 to 50 m with a base fraction size of about 40 m ( fig. 1). The whole particles of the powders under investigation are mainly spherical, but there are also irregularly shaped and chipped particles ( fig.1а).
At greater magnification, the rough surface of the clad coatings of the analysed powder material is visible ( fig. 2). Uneven cladding of the particles of the powder material with 8 % of Ni is observed ( fig. 2b). Most of the microspheres are partially nickel-clad ( fig. 1b, 2b), although there are particles completely coated with a homogeneous over its thickness layer. Nickel microparticles have a size of less than 1 мкм in separate depositions. The thickness of the whole coatings is 3-5 m, while the microstructure of Al2O3 grains of 2-3 m in size is visible ( fig. 3b). Nickel coatings do not have a dense package ( fig. 3b), as a result, they have a reduced adhesion strength with corundum particles.
In the powder with 30 % of Ni, nickel forms a dense coating about 3 мкм thick on the surface of ceramic particles ( fig. 3a). On this coating, it is possible to see some settled insolated microparticles 0.5-2 m in size, as well as conglomerates of them, that are about 5 m in size ( fig. 2a).    The study of the elemental composition distribution confirmed that Al and O elements are concentrated in the powder particle body and Ni is localised in the clad coating ( fig. 4). The synchronous arrangement of the distribution curves for aluminum and oxygen indicates a bound state in the form of aluminum oxide. When passing through the coating, an upsurge in the intensity of the nickel reflex is observed. The X-ray diffraction analysis showed that powder materials in the initial state have a multiphase composition: -(Ni + -Al2O3 + -Al2O3 + *-Al2O3) -for aluminum oxide powder clad with 8 % Ni ( Table  1). The highest intensity reflex on the diffractogram corresponds to the aluminum oxide faze (corundum) ( Table 1); -(Ni + -Al2O3 + -Al2O3) -for aluminum oxide powder clad with 30 % Ni ( Table 2). The highest intensity reflex on the diffractogram corresponds to nickel, a cladding metal component ( Table 2). This is due to the passage of X-rays through the cladding coating of the powder particles.
The combination of several oxide phases in the body of the particles is due to the powder technology factors (sintering heating temperature, pressing pressure and grinding). According to the findings, when cladding aluminium oxide powder material with 8 % nickel, the formation of a whole coating does not occur, as a consequence, isolated nickel coatings do not have high adhesion strength. The cladding of the aluminium oxide powder material with 30 % nickel makes it possible to successfully aply Ni as a dense metal coating on microspheres of the matrix material with high adhesion strength and uniform distribution of the main components along the inner dimension of the particles.
Further studies were continued on a pilot batch of a new for dioxide powder zirconium clad with 30 % nickel grade powder material based on zirconium dioxide ZrO2 stabilized with (71) % yttrium oxide, with a spheroidal form of 30 % nickel-clad particles. This powder material belongs to the group of ceramics for plasma spraying of heat-protective coatings, designed to increase the durability of parts that operate at high temperatures. A Z7Y40-60S grade material was chosen as the initial matrix powder. It is obtained by powder metallurgy (main technological operations are high sintering, grinding, spheroidization treatment). The chemical cladding process was carried out by the method of reduction (from salt solutions using sodium hypophosphite).
The powder material is characterized by a homogenous consistency and gray particle color. Granular composition and particle morphology of the analyzed powder material are shown in figure 5. The particle size is in the range of 40 -60 m. Powder particles are of a solid structure and spherical in shape, but individual particles of irregular shape are also found ( fig. 5a). With a larger magnification, the rough surface of the clad coating is visible ( fig. 5б), on which the settled individual nickel microparticles (0,2 -0,5 m) are noticeable, as well as particle clusters ( fig. 6). Mechanical milling of powder particles exposes the internal microstructure of the core and the coating (fig. 7). Nickel particles form a metal coating around the matrix material. This coating is dense and homogeneous over its thickness, which is about 2 m (fig. 7b).
The chemical analysis, which was performed in the clad coating of the powder particles, revealed high concentrations of nickel ( fig. 8, table 3). A small amount (4.08 wt %) of phosphorus was also indicated. The essential feature of chemical cladding is the parallel reduction of the metal and of this impurity settling throughout the clad coating. Therefore, it is practically impossible to obtain a pure nickel coating by using this method.    Analysis of the distribution of the main elements showed a synchronous spurt of zirconium and oxygen reflexes. It confirms the presence of a zirconium oxide compound. When moving from the core to the clad coating, a spurt of nickel is observed ( fig. 9). Figure 10 shows the distribution of the main elements in the particles of the powder material by means of the color visualization method. Atoms of oxygen and oxide-forming elements are evenly spaced along the core body. Nickel is deposited in the form of a dense coating on the spherical surface of ceramic particles.  Based on the results of the X-ray diffraction interpretation ( fig. 11), it can be stated that the analyzed powder composite material has a two-phase composition (Т-ZrO2 + Ni) (table 4). The highest intensity reflex on the diffractogram corresponds to the tetragonal phase of zirconium Т-ZrO2. Oxidic particles have a tetragonal lattice, where yttrium atoms partially replace zirconium atoms in the lattice of zirconium dioxide. Thus, stabilization of the tetragonal lattice is achieved, which has a positive effect    The research has revealed that according to the structural-phase composition, the new powder material is a domestic equivalent of the Metco 204NS powder material manufactured by Sulzer Metco. This powder material will help improve the plasticity of the spray coating and prevent oxidation of the sublayer at high temperatures on account of cladding with nickel.

Conclusion
The cladding with 30 % nickel of aluminium oxide and zirconium oxide powder materials allows Ni to be successfully coated as a dense metal shell on the microspheres of the matrix material with high adhesion strength and uniform distribution of the main components over the inner cross-section of the particles.
The studied domestic powders are capable of competing with foreign analogues due to their structural-phase state. They are recommended for plasma spraying of multifunctional coatings and can be in demand in such industries as energy, metallurgy, mechanical engineering and defense industries to increase the durability and reliability of the most critical parts.
The work was carried out within the Russian state task for fundamental scientific research (FFUF-2021-0025).