OBRABOTKAMETALLOV technology Vol. 26 No. 2 2024 Introduction Ceramic composite materials have been actively used during the last decade for protection against abrasion and thermal corrosion. There have also been a number of works [1–6] devoted to the study of the emissivity of ceramic composite materials in order to use it as coatings on the heat-transferring surfaces of industrial furnaces to increase energy efficiency. The heat treatment process is one of the most common technological operations in all industries. The heat energy transfer in the furnace occurs through convection and radiation mechanisms, but radiation heat transfer becomes dominant with increasing temperature [7]. The main criterion that characterizes the efficiency of radiation heat transfer is the emissivity of the heat-emitting surfaces, in industrial heating furnaces. Such surfaces are internal walls, gas ducts and coils, depending on the design and type of furnace. Increasing the energy efficiency of industrial heating furnaces is currently considered as one of the promising ways to overcome the ever-growing energy crisis, because it is in heating processes that a large amount of energy is wasted [8]. To this end, ceramic composite coatings with high emissivity and thermal stability during operation consisting of powder compositions of Fe2O3, Al2O3 + 10 % Fe2O3, Ti + + 10 % Fe2O3 were developed and investigated. The effect of the presence of iron oxide and aluminum oxide in the coating on increasing emissivity was shown by other researchers [9–11]. Previously developed coatings were applied in different ways on the heat-transmitting walls of the furnace, which significantly improved the energy efficiency of heat energy transfer [9–12]. The emissivity coefficient of a material is the ability of its surface to radiate energy through radiative heat transfer; numerically this characteristic can be expressed as the ratio of the energy radiated by a particular material to the radiated energy of a absolutely black body at the same temperature, where a absolutely black body will have a value equal to 1, and for a comparable material the value is in the range from 0 to 1 [13]. At present, many methods have been studied for applying high-emissivity coatings to metal surfaces, for example: sol-gel method, glazing, magnetron sputtering, electron beam vapor deposition, plasma spraying, etc. [14–19]. In the presented work, the possibility of forming coatings with high emissivity coefficient on the heat-emitting surfaces of industrial baking ovens using detonation gas-dynamic spraying is investigated. This method makes it possible to apply coatings with low porosity (1 %) and high adhesion to the base [20], which will ensure the resistance of the coating to thermal cycling. The coating process is carried out by heating and accelerating powders by detonation combustion products of combustible gas mixture of propane, oxygen and air with a frequency of 20 Hz and above, the sprayed particles velocity using this method reaches 1,200 m/s, and the materials utilization rate for oxide ceramic powders is not less than 67 % [21, 22]. The purpose of the work is to obtain coatings with high emission indices in the infrared range for further recommendation of its use in baking ovens of Shebekinsky machine-building plant. To achieve the purpose the following tasks were solved: 1. The compositions were determined and powder compositions of Fe2O3, Al2O3 + 10% Fe2O3, Ti + + 10 % Fe2O3 were prepared. 2. Technological parameters for applying powder compositions using the detonation gas-dynamic method were determined. 3. The structure and phase composition of the obtained coatings were investigated. 4. The emissivity of the obtained coatings was determined. 5. The thermal stability of the obtained coatings was studied. Research methodology As raw components for the creation of coatings, powders Ti (PTS-1, purity 99 %), Al2O3 (ChDA, purity 98.5 %), Fe2O3 (extra-pure 2–4, purity 99.7 %) were purchased. The characteristics of the purchased powders are given in Table 1. Mixing of powder compositions of Al2O3, PTS-1 and Fe2O3 was carried out mechanically in a Fritsch Pulverisette 6 planetary mill at a mass ratio of balls to mixture of 2:1 at a speed of 200 rpm for 5 min.
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