Kaiyrzhan Berikkhan

This study investigates the effect of low-temperature pulsed DC plasma nitriding on the surface properties of AISI 304 stainless steel for homogenizer valve-seat components. Plasma nitriding was performed in an ammonia atmosphere at 400, 440 and 480 °C for 8 h. The treatment led to the formation of expanded austenite at 400 °C, while higher temperatures promoted the formation of Fe-N and CrN-containing phases. The thickness of the modified layer increased from approximately 36 μm at 400 °C to 65 μm at 480 °C. Surface microhardness increased from 203 HV0.1 for the untreated steel to 652.6, 806.0 and 961.8 HV0.1 after nitriding at 400, 440 and 480 °C, respectively. The wear rate decreased markedly, reaching 1.92 × 10⁻⁵ mm³/(N·m) for DCPN480 compared with 30.65 × 10⁻⁵ mm³/(N·m) for the untreated sample. Among the nitrided samples, DCPN440 showed the most favorable corrosion behavior in 3.5 wt.% NaCl solution, indicating a balance between surface hardening and preservation of corrosion resistance. Numerical modeling confirmed that the strengthened surface layer can withstand equivalent homogenizer valve-seat loading without local plastic deformation. The results demonstrate that pulsed DC plasma nitriding can significantly improve the hardness and wear resistance of AISI 304 stainless steel while maintaining acceptable corrosion performance under optimized treatment conditions.

This paper presents a numerical simulation of the cold gas-dynamic spraying process of Al-Zn-TiO₂ composite powder onto a steel substrate. Using computational fluid dynamics (CFD) in COMSOL Multiphysics, the characteristics of the gas flow in a de Laval nozzle and the particle dynamics are obtained for various spraying parameters. Optimal conditions are determined: gas pressure of approximately 0.6 MPa, temperature of ~600°C, nozzle-to-substrate distance of 15 mm, spray angle of 90°. At these conditions, particles reach supersonic velocities of approximately 500-600 m/s, sufficient for their deposition on steel. It is shown that increasing gas pressure and temperature facilitates particle acceleration and increases their impact energy, while excessively small or large spraying distances reduce the efficiency of the process. Simulation of the impact interaction of particles with the substrate revealed intense plastic deformation of the powders under optimal parameters, ensuring strong adhesion of the coating to the base. The results of the numerical experiment are consistent with the observed characteristics of dense and adhesion-strong coatings. It has been established that the proposed CFD model can be used as an effective tool for optimizing the parameters of cold gas-dynamic spraying of composite powders.

Cold gas dynamic spraying (CGDS) enables the production of protective coatings without melting or oxidation. In this study, Al–Zn–TiO₂ composite powders were prepared by wet agglomeration with binders and by dry mechanical mixing, and deposited onto mild steel substrates. COMSOL simulations of gas dynamics and particle acceleration identified optimal parameters (0.6 MPa, 600 °C, 15 mm, 90°), which were then validated experimentally. Coatings formed under these conditions exhibited dense microstructures, minimal porosity (~0.5%), and continuous, defect-free interfaces with the substrate. SEM and XRD confirmed solid-state bonding without new phase formation. Corrosion tests in 3.5% NaCl revealed a tenfold reduction in corrosion current density compared to bare steel, resulting from synergistic sacrificial (Zn), barrier (Al), and reinforcing/passivating (TiO₂) effects. Tribological tests demonstrated reduced friction (CoF ≈ 0.4–0.5) and wear volume. Compared with reported Al- or Zn-based cold- and thermal-sprayed coatings, the optimized Al–Zn–TiO₂ system shows superior performance, highlighting its potential for industrial anti-corrosion and wear-resistant applications.

Cold gas dynamic spraying (CGDS) of multi-component agglomerated powders offers a promising method for producing protective coatings without melting or oxidation. This study investigates Al–Zn–TiO₂ composite powders prepared via both wet agglomeration with binders and dry mechanical mixing. Coatings were deposited on mild steel substrates using heated air at varying parameters. Computational modeling (COMSOL) was employed to simulate gas dynamics and particle velocities, identifying optimal conditions (6 atm, 600 °C, 15 mm, 90°) for effective deposition. Experimental validation showed that coatings formed under these conditions exhibited uniform microstructure, minimal porosity, and excellent adhesion. SEM and XRD confirmed dense solid-state bonding without new phase formation. Profilometry indicated increased surface roughness with higher powder feed rates. Corrosion testing in NaCl solution revealed a significant reduction in corrosion current density compared to bare steel, attributed to synergistic sacrificial and barrier mechanisms. Tribological tests demonstrated lower friction and wear due to embedded TiO₂. The combination of modeling and experimentation enabled successful process optimization for high-performance Al–Zn–TiO₂ coatings.