Gelson Biscaia de Souza

This study evaluates nitrogen plasma immersion ion implantation (PIII) effects on the microstructure, mechanical response, and tribological performance of duplex coatings on AISI H13 steel, produced by nitriding and AlCrN physical vapor deposition (PVD). Results were compared with conventional plasma nitriding (PN) performed at the same temperature (375 °C) and with an industrial duplex coating. PIII formed a thinner modified layer than PN (15 µm versus 55 µm); both mainly produced nitrogen-expanded martensite (α′_N). PN yielded higher nitrogen contents and substantial γ′-Fe₄N and ε-Fe_2–3N formation in the compound layer. During PVD, this nitride-rich layer partially decomposed into low-nitrogen martensite due to chemical-potential gradients, impairing tribological resistance. PIII-treated surfaces developed much smaller depletion zones, attributed to the thinner nitride-layer and enhanced nitrogen mobility in α′_N at the PVD temperature. Consequently, PIII + PVD improved wear performance by 20% under the severest condition compared with PN + PVD. The microwear tests employed here, still underexplored, impose contact pressures one order of magnitude above those typically encountered in conventional tests. Controlling the load enabled evaluation of wear-severity regimes and the hardness-to-elastic-modulus ratio as a wear-resistance indicator. The findings demonstrate that interface design and nitrided case stability govern load-bearing capacity in duplex-coated tool steels.

This study aimed to optimize the nitriding parameters for Plasma Immersion Ion Implantation (PIII) of stainless steels. UNS S32750 super duplex stainless steel, widely employed in the petrochemical industry, was subjected to PIII under varying nitriding atmospheres (mixtures of H₂ and N₂) and treatment pressures. The fixed PIII nitriding parameters included a temperature of 300 °C, a duration of 3 h, a bias voltage of approximately −10 kV, a frequency of 500 Hz, and a pulse width of 30 μs. Following the treatments, the phases were characterized by X-ray diffraction (XRD), while the hardness and elastic modulus of the modified surfaces were evaluated via nanoindentation. Regarding the nitriding atmosphere, gas mixtures approaching a 60% N₂/40% H₂ (vol.) ratio yielded a higher volume fraction of nitrogen-rich expanded phases in solid solution. Furthermore, higher treatment pressures promoted the formation of these expanded phases, consequently enhancing the surface hardness up to 2.7 times the hardness value of the untreated sample. These findings stand in contrast to those found for low-energy plasma nitriding (PN) processes.

The growing need to utilize superalloys in challenging environments is driving ongoing advancements in effective coating systems. Within this framework, plasma nitriding can be evaluated as either a standalone solution or integrated with other methods. Applied to nickel-based Hastelloy X alloy, this surface processing represents an original aspect of the research, providing comprehensive analysis through compositional (via EDS), structural (XRD and micro-Raman), and microstructural (SEM) characterizations of both the surface and bulk material. Nitrogen diffusion and retention in the lattice compete with chromium depletion, resulting in various layer thicknesses (6.5 μm to 3.2 μm) and structures depending on the treatment temperatures: mainly expanded austenite γN at 400 °C; nitride phases (notably CrN, Cr2N) atop γN at 600 °C; and predominantly nitrides at 700 °C. Heat-induced carbide precipitation in the bulk has a limited impact on mechanical properties. Accordingly, the hardness-to-elastic modulus ratio ([[EQUATION]]), measured by nanoindentation, increases by a factor of 5.1 at the 400 ºC condition and 2.8 in the 700 ºC one. The unique features of the nitrided layers notably influence the material´s thermal diffusivity, determined by using the rear photoacoustic signal technique (sensitive to micrometer-scale layer contributions), achieving up to a 2.4-fold enhancement at 700 ºC. These findings indicate that plasma nitriding enables the tailoring of mechanically resistant surfaces with distinctive thermal properties on Hatelloy X.

Plasma nitriding is a thermo-chemical treatment widely used to improve the tribological properties of austenitic stainless steel, due to the formation of an expanded austenite layer, which presents increased hardness. Although low-temperature plasma nitriding of austenitic stainless steels has been extensively studied in recent years, the reported results consistently show similarly high nitrogen concentrations, owing to the diffusive nature of the process. Excess nitrogen in the expanded austenite can impair the integrity of the treated surface and thus compromise the overall viability of the treatment. In such a context, the present work evaluated the use of intermittent nitrogen flow during plasma nitriding of ISO 5832-1 steel to control nitrogen concentration in the layer formed. Throughout the treatments, alternated cycles were applied between nitrogen flow periods and periods of interruption, which were repeated throughout the process. Different pulse conditions were used, in which the nitrogen flow corresponded to 10%, 15% and 50% of the cycle time. The results indicated that, in the intermittent flow condition with 50% time of nitrogen offer, the layer thickness, nitrogen concentration, and hardness values were very close to those observed in treatments carried out with continuous flow. On the other hand, in the conditions where the nitrogen flow was kept at 10% and 15% of the cycle, more significant differences were observed compared to the results obtained in continuous flow treatments. Nevertheless, those samples presented a substantial improvement in the surface hardness compared with the untreated steel. Therefore, the intermittent nitrogen flow can be used during plasma nitriding to control expanded austenite properties, enabling the design of surface properties.

Metallic tubes are widely used in industrial applications. However, the internal smoothness and chemical resistance of metallic tubes are often major concerns. In contrast to the widely studied large-diameter metallic tubes (typically > 4 cm internal diameter) commonly investigated for surface modifications in industrial applications, we have focused on modifying the internal sur-faces of much smaller SS304 tubes (0.6 cm internal diameter) using nitrogen and argon PIII, as well as acetylene PIII&D. This study demonstrated that high-quality internal surface modifica-tions can be achieved with a straightforward setup and appropriate analysis techniques, even in tubes of industrial relevance with such small dimensions. In addition to gaining insights into the characteristics of hollow cathode (HC) and dielectric barrier (DB) discharges in small-diameter tubes, we found that Ar-PIII can partially mitigate the poor surface quality of as-received tubes. The addition of an N-PIII step further enhanced the internal surface, resulting in a smoother fin-ish with fewer trapped impurities. Moreover, acetylene PIII&D enabled the rapid deposition of a thick (>300 μm) Glassy Carbon film. These findings highlight the potential of these techniques for various plasma-related applications and industrial processes, offering promising avenues for future development.

The possibility of inducing structural crosslinking and densification of plasma-deposited SiO_x networks by controlling low-energy reaction mechanisms was investigated. For this, films were deposited for 300 s from HMDSO (2%), O₂ (86%) and Ar (12%) mixtures at a working pressure of 15.7 Pa. A radiofrequency signal was used to excite the plasma in a configuration so as to not deliberately induce ion bombardment of the growing layers. The plasma excitation power was varied (100 to 300 W) to promote changes in the deposition mechanisms, which were investigated from deposition rate and layer thickness, chemical structure, elemental composition, topography, roughness, hardness, elastic modulus, corrosion potential, corrosion current density and porosity of the films. Under the experimental conditions studied, inorganic SiO_x thin films (x = 1.8–1.9) with a low carbon content were deposited. The increase in the applied power during the deposition process reduced the number of silanol groups in the coatings, due to dangling bonds recombination by structural crosslinks, which avoided hydroxyl incorporation and silanol formation. As a consequence, the structure became harder, more compact and corrosion resistant.

The Plasma Immersion Ion Implantation (PIII) nitriding was used to form a modified layer rich in expanded austenite (γ_N) and expanded ferrite (α_N) phases in super duplex steel. The thermal stability of these phases was investigated through the in situ synchrotron X-ray diffraction. All the surfaces were analyzed by SEM, EDS, and nanoindentation. During the heating stage of the thermal treatments, the crystalline structure of the γ_N phase expanded thermally up to a temperature of 350 °C and, above this temperature, a reduction in the lattice parameter was observed due to the diffusion of nitrogen into the substrate. During the isothermal heating, the gradual diffusion of nitrogen continued and the lattice parameter of the γ_N phase decreased. Increasing the treatment temperature from 450 °C to 550 °C, a greater reduction in the lattice parameter of the γ_N phase occured and the peaks related to the CrN, α, and α_N phases became more evident in the diffractograms. This phenomenon is associated with the decomposition of the γ_N phase into CrN + α + α_N. After the heat treatments, the thickness of the modified layers increased and the hardness values close to the surface decreased, according to the diffusion of the nitrogen to the substrate.

The possibility of inducing structural crosslinking and densification of plasma-deposited SiOx networks, by controlling low energy reaction mechanisms was investigated. For this, films were deposited for 300 s from HMDSO (2%), O2 (86%) and Ar (12%) mixtures at a working pressure of 15.7 Pa. A radiofrequency signal was used to excite the plasma in a configuration to not deliberately induce ion bombardment of the growing layers. Plasma excitation power was varied (100 to 300 W) to promote changes in the deposition mechanisms, which were investigated from deposition rate and layer thickness, chemical structure, elemental composition, topography, roughness, hardness, elastic modulus, corrosion potential, corrosion current density and porosity of the films. Under the experimental conditions studied, inorganic SiOx thin films (x=1.8-1.9) with low carbon content were deposited. The increase in the applied power during the deposition process reduced the number of silanol groups in the coatings, which became more compact, and consequently, harder and more corrosion resistant.