Laila Sulyubayeva

Laser hardening is normally applied in the solid-state regime, and the behaviour of chromium steels treated at a line energy high enough to melt the surface is far less documented. Here 40Cr steel (AISI 5140) was laser hardened at 3000 W and 5 mm/s (600 J/mm) with one, two and three passes and compared with the untreated state by roughness measurement, microhardness profiling, quantitative porosity analysis, X-ray diffraction, dry ball-on-disc sliding against a 100Cr6 ball (6 N, 3 cm/s, 200 m, n = 3) and abrasive testing to GOST 23.208-79. One pass produced a hard (530–590 HV) but porous remelted layer, with 6–9% porosity in the top 300 µm and a dislocation density of 1.6 × 10¹⁵ m⁻². Repeated remelting reduced the porosity to below 1%, while the near-surface hardness fell to 280–310 HV, the dislocation density decreased four- to five-fold, and the hardened region extended to about 1300 µm, indicating tempering of the previously hardened material. After grinding and polishing, the specific wear rate of the steel in sliding fell significantly (p < 0.001) from 6.5 × 10⁻⁵ to 2.9, 4.3 and 1.7 × 10⁻⁵ mm³/(N·m), i.e., by 54, 34 and 74%, whereas the friction coefficient, 0.62–0.66, did not differ between the states. Under loose abrasive, the mass loss fell by 8.4, 25.5 and 14.3%. Three passes are recommended for sliding contacts and two passes for abrasive conditions; a single pass is not recommended because of the porosity of its remelted layer.

This study investigated the effect of Ultrasonic Nanocrystalline Surface Modification (UNSM) on the microstructure, local mechanical properties, surface condition, and tribological behavior of a laser-welded butt joint of AISI 304 austenitic stainless steel. Laser welding was performed using ER308LSi filler wire at approximately 560 W (28% of 2000 W) and a frequency of 3000 Hz. UNSM treatment was subsequently applied using a Designmecha VM200A system under a static load of 10 N, a scanning speed of 1000 mm/min, a frequency of 20 kHz, and an amplitude of 20 μm. The joint was characterized by SEM/EDS, surface profilometry, instrumented indentation, and tribological testing. Laser welding produced a heterogeneous structure consisting of the base metal, heat-affected zone, and a cellular-dendritic fusion zone. After UNSM, the specimen surface exhibited severe plastic deformation and structural refinement. Surface roughness increased in all zones, while microhardness rose from 212 to 411 HV in the base metal, from 194 to 422 HV in the heat-affected zone, and from 244 to 496 HV in the fusion zone. The cross-sectional area of the wear track decreased from 63,241 to 29,648 μm². These results demonstrate that UNSM effectively strengthens laser-welded AISI 304 joints and significantly improves their wear resistance.

This study investigated the influence of detonation spraying parameters on the structure and tribological and mechanical properties of self-fluxing coatings of the Ni–Cr–Fe–B–Si–C system. The investigated parameters included the barrel filling level (48–68%), the molar ratio of the working gases (O/C = 1.026–1.856), and the inter-shot delay (0–1 s). Morphological analysis of the coatings revealed a characteristic lamellar structure and a dependence of coating thickness and defect formation on the spraying conditions. A lower barrel filling level resulted in a denser coating structure with minimal porosity, whereas medium and high filling levels promoted the formation of microcracks, presumably due to increased residual stresses. Tribological tests showed that the minimum coefficient of friction was obtained at a barrel filling level of 48% (μ = 0.674 ± 0.136) and an O/C ratio of 1.026, but these conditions were also associated with increased wear. The highest wear resistance was obtained at an O/C ratio of 1.856 and an increased inter-shot delay of up to 1 s, resulting in a decrease in the specific wear rate to 0.000189 mm³/(N·m). Roughness analysis showed that increasing the inter-shot delay reduced Ra from 5.00 to 3.60 μm, which may be attributed to a reduction in thermal load and more favorable coating formation conditions. The maximum hardness, up to 905.6 HV, was obtained for a combination of parameters that promoted the formation of a dense structure and strengthening phases. The results indicate that optimizing the detonation spraying parameters requires a trade-off between reducing the coefficient of friction and increasing wear resistance. The findings can be used to develop tech nologies for the restoration and surface strengthening of machine components.

This study presents a comparative investigation of the structure, adhesion and corrosion properties of homogeneous and gradient Cr₃C₂-NiCr detonation coatings formed on the surface of 12Kh1MF steel. A Cr₃C₂-NiCr composite powder with a composition of 75/25 and a particle size of 10–45 µm was used to apply the coatings, using a CCDS2000 detonation spraying system. A homogeneous coating was formed at a constant gas mixture filling ratio of 57 per cent, whilst a gradient coating was obtained by sequentially varying the filling ratio from 57 to 64 to 73 per cent, which ensured the formation of layers with different structural and phase states. An investigation of the microstructure and elemental composition revealed a uniform distribution of components in the homogeneous coating and a regular variation in the Cr and Ni content across the thickness of the gradient coating. X-ray phase analysis identified the presence of the Cr₃C₂, Cr₃Ni₂ and CrNi₃ phases. Corrosion resistance was assessed using the potentiodynamic polarization method in a 3.5 per cent Na₂SO₄ solution and through long-term tests in hydrochloric acid mist with the addition of CuCl₂·2H₂O, lasting up to 1000 h. The polarization curves of the gradient coating were shifted towards the region of lower current densities, indicating a slowing down of both anodic and cathodic electrochemical processes. In the tested specimens, visible corrosion was first observed on the homogeneous coating after 456 h, whereas no visible corrosion damage was observed on the gradient coating after 1000 h. Mass loss amounted to 1.14 per cent for the homogeneous coating and 0.17 per cent for the gradient coating. The enhanced corrosion resistance of the gradient coating is due to its denser layered structure and the barrier effect of the interlayer boundaries, which prevent the propagation of through-cracks and the penetration of the aggressive medium to the ‘coating-substrate’ interface. The results obtained demonstrate the potential of forming a Cr₃C₂-NiCr gradient structure to enhance the protective and operational characteristics of detonation coatings.

The aim of this study was to comparatively evaluate the effects of electrolytic-plasma surface hardening (EPSH) and laser surface hardening (LSH) on the microstructure, phase state, microhardness, and friction behavior of low-carbon Steel 20. EPSH was performed using a two-stage regime of 320 V for 3 s followed by 200 V for 40 s and a thermocyclic regime of 320 V for 2 s, 200 V for 2 s, and 50 V for 2 s repeated for five cycles. LSH was carried out at a scanning speed of 7 mm/s and 75% of the maximum laser power (approximately 2250 W) using single- and double-pass treatments. The modified layers were characterized by SEM, XRD, microhardness measurements, and dry sliding tribological tests. EPSH produced the strongest hardening response, reaching approximately 600 HV with a hardened-layer depth of 70–100 μm. Double-pass LSH increased the near-surface microhardness to approximately 460 HV. Phase analysis revealed Fe_1−xO after EPSH, Fe₃O₄ after single-pass LSH, and Fe_1−xO, Fe₃O₄, and Fe₂O₃ after double-pass LSH. All treatments reduced the coefficient of friction compared with untreated steel. Overall, EPSH provided deeper and stronger hardening, whereas LSH produced more localized surface modification and oxidation.

This study investigates the influence of detonation spraying parameters on the microstructure, mechanical properties and tribological performance of self-fluxing Ni–Cr–Fe–Si–B–C coatings deposited on AISI 321 stainless steel substrates. The effects of the barrel filling ratio (48–68%), the oxygen-to-fuel molar ratio (O/C = 1.026–1.856), and the delay time between detonation shots (0–1 s) were systematically evaluated. The coatings were characterized using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), microhardness measurements, instrumented indentation, and ball-on-disk tribological testing. Microstructural investigations revealed that the spraying parameters significantly influence coating thickness, porosity and defect formation. The lowest porosity (0.306 per cent) and the most homogeneous microstructure were obtained at a barrel filling ratio of 48 per cent, an O/C ratio of 1.026 and a shot delay of 1 s. XRD analysis identified a multiphase structure consisting of a Ni₃Fe matrix reinforced by Cr₇C₃ carbides, Ni₃B and CrB borides, and Ni₃₁Si₁₂ silicides. Tribological tests demonstrated that increasing the delay between shots significantly improved wear resistance, reducing the wear rate to 1.89 × 10⁻⁴ mm³/(N × m). The optimised coating exhibited an average coefficient of friction of 0.578 ± 0.093 and a wear rate of 1.03 × 10⁻⁴ mm³/(N × m). Instrumented indentation revealed a hardness of 1049.1 ± 43.4 HV and a Young’s modulus of 215.9 ± 8.5 GPa. The wear mechanism was predominantly abrasive–adhesive, whilst the wear rate of the 100Cr6 counter-body remained low at 1.20 × 10⁻⁵ mm³/(N × m). The results obtained demonstrate that appropriate optimisation of detonation spraying parameters enables the formation of dense Ni–Cr–Fe–B–Si–C coatings with superior mechanical and tribological properties, making them promising candidates for wear-resistant engineering applications.

This study examines how electrolytic plasma treatment can improve the wear resistance of 30CrMnSi bucket teeth used in loaders operating under severe abrasive conditions. Samples were subjected to various processing methods, including electrolytic plasma hardening, nitriding, carburizing, nitrocarburizing hardening, and quenching. Multiple analytical techniques, including scanning electron microscopy energy dispersive spectroscopy, X-ray diffraction, and microhardness testing, were used to comprehensively characterize the structure, phases, and mechanical properties of the samples. The tribological properties of the materials were evaluated using a TRB3 tribometer, and abrasive wear testing was conducted according to GOST 23.208–79. The results show that electrolytic plasma treatment results in the formation of diffusion layers with a thickness of 15 to 25 μm. This process is accompanied by the formation of carbide, nitride, and carbonitride phases (Fe₄C, Fe₇C₃, Fe₄N, Fe₂N, and Fe₃(CN)). This process significantly increases surface hardness (up to 810 HV) and improves wear resistance. The study also showed that electrolytic plasma nitriding provides an optimal balance between hardness and toughness, resulting in low friction and increased resistance to abrasive wear. The development of this technology can extend the service life of earthmoving and mining equipment components, reducing maintenance costs and downtime.

The work investigated the formation mechanisms, structure, and properties of the modified layer obtained during electrolytic-plasma nitroboriding of Steel 20 in the temperature range of 650–850 °C. A comprehensive approach, including numerical modeling and experimental methods, was applied to analyze diffusion processes, phase formation, and performance characteristics. A one-dimensional diffusion model was developed, taking into account the coupled transport of boron, nitrogen, and carbon, as well as the movement of phase boundaries. It was shown that a gradient layer is formed, characterized by boron enrichment in the near-surface zone, deeper nitrogen penetration, and carbon redistribution. The calculated layer thickness at 850 °C (~70–75 μm) is in good agreement with the SEM data (~73.4 μm, taking into account the transition zone). SEM and EDS analysis confirmed the formation of a multilayer structure with a pronounced transition region. A significant increase in microhardness up to ~950–1000 HV at 850 °C was established, with a gradual decrease to the matrix level (~200–250 HV) at a depth of 70–90 μm. Tribological tests showed a decrease in the coefficient of friction and an increase in wear resistance, with the best characteristics achieved at 850 °C.

This study analyses the effect of electrolytic plasma treatment on improving the wear resistance of 30CrMnSi steel used under conditions of high abrasive and impact-abrasive loads. The samples were processed using various technological regimes, namely electrolytic plasma quenching, nitriding, nitrocarburising, and carburising. A range of analytical methods were employed to comprehensively characterise the structure, phase composition, and mechanical properties, including SEM/EDS, XRD, and microhardness testing. The tribological properties of the materials were evaluated using a TRB³ tribometer, and abrasive and impact-abrasive wear tests were performed in accordance with GOST 23.208–79 and GOST 23.207–79 standards. The results show that electrolytic plasma treatment leads to the formation of diffusion layers with a thickness of 50–150 μm, accompanied by the formation of carbide, nitride, and carbonitride phases (Fe₄C, Fe₇C₃, Fe₄N, Fe₂N, Fe₃(CN)). This process results in a significant increase in surface hardness (up to 610–930 HV) and improved wear resistance. The study indicates that electrolytic plasma nitrocarburising provides a favourable combination of hardness and tribological behaviour, leading to a low friction coefficient (0.25–0.35) and enhanced resistance to abrasive and impact-abrasive wear. The obtained results demonstrate the potential of this technology for improving the performance of components made of 30CrMnSi steel operating under severe wear conditions.

High-entropy coatings based on CoCrFeNiMn obtained by thermal spraying have demonstrated the potential to improve the wear resistance of traditional materials used in extreme conditions. The aim of the work was to study the effect of the oxygen/fuel ratio when using kerosene as fuel in the HVOF process on the microstructural characteristics of CoCrFeNiMn coatings, including phase composition, microhardness, elastic modulus, and wear resistance. Phase and microstructural transformations in gas-atomized powder during HVOF spraying were analyzed using XRD, SEM, and EDS methods. The tribological and mechanical properties of the coatings obtained were also evaluated. The results obtained are consistent with thermodynamic predictions based on the Scheil model for non-equilibrium conditions. The data obtained indicate the high potential of high-entropy CoCrFeNiMn alloys for use as protective coatings for industrial purposes. In addition, the results of the study emphasize the promise of using thermodynamic prediction of high-entropy alloys using Thermo-Calc software. The best mechanical and tribological properties were obtained in the HVOF 1 regime, which provided a maximum microhardness of 783.8 HV and a minimum wear rate of 7.45 × 10⁻⁵ mm³ × N⁻¹ × m⁻¹.