Kemal I. Emurlaev

Introduction. Austenitic steels are widely used in mining, railway, and chemical industries due to their high toughness and corrosion resistance. However, their service life under severe wear conditions is limited, which necessitates the development of effective surface hardening techniques that preserve the ductility of the substrate. The purpose of this study is to investigate the microstructure, mechanical properties, and tribological behavior of Fe–Cr–B-based coatings fabricated by non-vacuum electron beam cladding on AISI 321 steel followed by combined treatment (quenching and hot rolling). Materials and methods. Cladding was performed using two powder mixtures with different boron contents: C1QHR (1 g B) and C2QHR (2 g B). After cladding, the samples were quenched from 1,100 °C and subsequently hot-rolled at 950 °C. The microstructure was examined using scanning electron microscopy, and the phase composition was determined by X-ray diffraction. Mechanical properties were evaluated by microhardness measurements, while tribological characteristics were assessed using a ball-on-flat dry sliding wear test. Results and discussion. The combined treatment was found to promote the precipitation of submicron (Fe,Cr)23C6 carbides within the austenitic matrix and the formation of (Fe,Cr)2(C,B) carboborides. The coating with the higher boron content (C2QHR) exhibited a hardness of 484 ± 20 HV, which is 2.4 times higher than that of AISI 321 steel. The wear resistance of the C2QHR coating is 5.3 times higher than that of the AISI 321 steel substrate. The main mechanisms enhancing the wear resistance of the coatings are precipitation hardening due to carbide precipitation, as well as the formation of borides and carboborides. Conclusions. The combination of quenching and hot rolling of the layers obtained by non-vacuum electron beam cladding forms a wear-resistant protective layer on the surface of AISI 321 steel.

In this study, Al₂₁Co₂₂Cr₂₂Fe₁₃Ni₂₂ high-entropy coatings were produced on steel substrates by non-vacuum electron beam cladding of Co, Cr, and NiAl powders. The high-temperature oxidation behavior of the coatings was studied by holding the specimens at 900 °C in air. The microstructure and phase constitution of the samples were studied both in the as-cladded state and after the heat treatment. The microstructure was characterized using light microscopy (LM) and scanning electron microscopy (SEM). Synchrotron X-ray diffraction (SXRD) and energy-dispersive X-ray spectroscopy (EDX) were used to study the phase constitution of the coatings and the “coating-substrate” interface. The coating consisted of disordered bcc (A2), ordered bcc (B2), and disordered fcc (A1) phases. Annealing the coatings for 50 h at 900 °C led to the formation of fcc precipitates in the bcc dendritic grains and a mixture of fcc and σ-phase particles in the interdendritic regions. Needle-like nanosized B2-precipitates were formed due to annealing in the fcc grains at the coating/substrate interface. The microhardness at the top of the as-cladded coating was 585 HV and gradually decreased towards the substrate. A more uniform distribution of the microhardness was obtained after the annealing. Its average value was 441 HV. Rhomboid Cr₂O₃, needle-like Al₂O₃, and spinels of a different morphology were found on the surface of the samples after oxidation at 900 °C.

Alumina-zirconia composites containing calcium hexaaluminate in the amount from 0 to 15 wt.% were investigated. The materials were obtained by water dispersion, granulation, axial pressing, and free sintering. Density and open porosity were determined by the hydrostatic weighing method. Phase analysis was performed using synchrotron radiation. Structural investigations were conducted using scanning and transmission electron microscopy. Vickers hardness was determined at a load of 10 kg. Fracture toughness was determined by the indentation method. With increasing CaAl12O19 content in the composites, the relative density decreased from 98.5% to 91.8%, and the open porosity increased from 0.2 to 1.4%. The lattice parameters of t-ZrO2 crystal lattice did not change up to 12 wt.% CaAl12O19, and the degree of tetragonality was 1.435. The degree of tetragonality decreased for the material with 15 wt.% CaAl12O19 and reached 1.420. The lattice parameters of CaAl12O19 decreased with increasing content. Platelet size increased with increasing calcium hexaaluminate content. For the materials containing up to 9 wt.% CaAl12O19, the average length of the platelets was 2 μm, the width was 0.4 μm, and the aspect ratio was 5. For the material with maximum calcium hexaaluminate content, the average length of the platelets was 4.2 μm, the width was 0.6 μm, and the aspect ratio was 7. With increasing CaAl12O19 content, the hardness decreased from 1700±25 to 1390±30 Hv, and the critical stress intensity factor increased by 34% to 6.7±0.3 MPa·m1/2.