OBRABOTKAMETALLOV Vol. 26 No. 2 2024 technology The phase composition of the coated experimental specimens was analyzed by X-ray diffraction with stepwise scanning of 2θ angles from 10 to 90° with a step of 0.05° on an ARL X’TRA diffractometer (Thermo Fisher Scientific, Switzerland) with Cu-Kα-radiation (λ = 0.1541744 nm). The phase composition was determined according to the standard technique in the PDXL program using the PDF-2 (JCPDS ICDD) powder radiographic standards database (2008). The infrared spectra were measured on an IRS 55/S IR Fourier spectrometer (Bruker, Germany) using a registration monochromator controlled by a personal computer (PC). Diffraction gratings of 300 and 150 shp/mm were used to extend the spectral range. The working spectral range of the gratings was 1.4–4.0 and 2.8–8.0 μm, respectively. Measurements were performed at a spectral slit width of 0.02 μm. The scanning step was chosen to be 10 nm. Due to the high absorption capacity of quartz lenses designed to focus radiation to the input slit of the monochromator in the range from 2.5 µm, the latter were removed and replaced by mirrors. The distance from the heated specimen (coated plate) to the monochromator slit was 60 cm. The focused radiation from the specimens was fed to the input slit of the monochromator using an aluminum mirror with focal length f = 150 mm. Mirror adapters (elliptical aluminum reflectors) were used behind the output slit of the monochromator, the use of which made it possible to collect the output radiation from the monochromator to the receiving area of the photodetector with minimal losses. In the 1.0–4.0 μm range, an automated turret with interference IR light filters switchable at wavelengths of 1.0, 1.6, and 2.0 μm was used to cut off higher-order radiation. In the 4.0–8.0 μm range, additional manually switchable IR light filters were used for a similar purpose. In the extended range of 1.0–10.0 μm, a module from ORIEL INSTRUMENTS (USA) was used as a photodetector (detector), the sensitivity of which did not depend on the radiation wavelength. During preliminary adjustment (debugging) of the recording system (signal search and optimization) in the near-infrared range (1.0–2.0 μm) we used more highly sensitive detectors: InGaAs-photodiodes IGA050-TE2-H (900–1,700 nm), IGA2.2-030-TE2-H (900–2,800 nm) and PbS-photoresistors PbS-050-TE2-H, (900–3,300 nm) of the company “ELECTRO-OPTICAL SYSTEMS INC” (USA-Canada); InGaAsP photodiodes PD24-20TEC1-PR (1,000–2,300 nm), PD25-20TEC1-PR (1,000–2,500 nm), PD36-05PR (1,200– 3,800 nm) of the company “IBSG Company Ltd” (St. Petersburg, Russia). The photodiodes and photoresistors were cooled to optimal temperatures. To increase the signal-to-noise ratio, registration was performed using radiation modulation at the input of the monochromator. The modulation frequency was 500 Hz. The pre-amplified signal from the detectors was fed to the main single-channel amplifier with a synchronous detector Lock-in nanovoltmeter type 232 B (Poland, USA). For spectral measurements of specimens in the temperature range from 100 to 500 °C, a method was developed and a thermoblock (mini oven) with heating of specimens and maintenance of its temperature (relative to the required temperature) with an error of ± 5 °C. The thermoblock consists of a heater, a heat conducting sleeve made of copper (d = 40 mm, thickness h = 8 mm) and a heat-resistant casing. A 20×20 mm specimen was pressed to the copper sleeve using screws. Temperature control was carried out using a calibrated constantan-copper thermocouple inserted into the hole of the copper sleeve. The required specimen temperature was maintained by selecting the heater current. The error in spectra measurements in the vast majority of cases did not exceed ± 5 %. In some cases, when the useful signal exceeded the background (noise) signal only 5–10 times, the error could reach ± 10 %. Thermal cycling of coated specimens was carried out in a muffle furnace. For each test, three specimens were placed on a tray. The tray could be moved in and out of the furnace chamber. An air-cooling system was attached to the outside of the chamber to cool the specimens. The oven temperature was set at 550 °C, as this is the maximum operating temperature of the hottest parts of the heat transfer surfaces of the baking oven. The specimens were kept in the muffle oven for 30 min. Then the moving tray with the specimens was removed from the oven and air cooling was applied to the for 10 min. One thermal cycle consisted of 30 min heating and 10 min air cooling. The specimens underwent 300 cycles to evaluate the effect on the coating.
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