S-phase Layer Research Articles

Deformation behavior of the hybrid S-phase layer formed on a stainless-steel substrate by thermochemical heat treatment under mechanical loading

The hybrid S-phase layer was formed on the surface of an austenitic stainless steel by the thermomechanical heat treatment, which was carried out at a low temperature (475 °C). The thickness of the layer depends on the methane (CH4) gas content in the heat treatment chamber. The microstructure of the layer contains plenty of microstructural defects, such as slip bands, twins, stacking faults, and entangle dislocations, as confirmed by detail electron microscopy investigations. The S-phase layers were subjected to nanoindentation and nanoscratching to investigate their micro-mechanical properties and deformation aspects; and compared against the substrate (bulk austenitic) material. This layer exhibit about 6 folds increase in hardness (11.65–14.65 GPa) to that of bulk austenitic (2.33 GPa), whereas the Young's modulus increased by 1.70 times (124.84–142.34 GPa for the S phase layers and 83.25 GPa for bulk material). This increase in hardness caused relatively less plastic deformation in the layer, compared to bulk austenitic under identical loading conditions. This induced different sub-surface deformation aspects, not only in terms of deformation depth, but also in microstructural aspects, as reviled through a transmission electron microscopy investigation on the residual nanoindentation imprint and scratch tracks.

Tribological and corrosion behaviour of medical grade 316LVM steel by low temperature hybrid gaseous nitriding and carburizing

This study approached a low temperature hybrid gaseous nitriding and carburizing at varying temperature and gas composition to develop S-phase layer on 316LVM steel. The hybrid gaseous treatment at 475 °C and 10 % CH4 developed a distinct S-phase layer that is harder (1444 ± 32 HV0.025) than untreated surface. The S-phase layers are effective in reducing specific wear rates by 67 % to 1.19 ± 0.05 × 10−3 mm3/Nm, coefficients of friction by 26 % to 0.49 ± 0.02, and corrosion rate by 23 % to 6.02 ± 0.32 µm/y. Both untreated and treated surfaces had dominant abrasive wear mechanisms but on treated surface spalling holes were observed. This research highlights low-temperature hybrid gaseous nitriding and carburizing is promising for improving 316 LVM steel surface for orthopedic.

High-Efficient Gas Nitridation of AISI 316L Austenitic Stainless Steel by a Novel Critical Temperature Nitriding Process

To improve the surface properties of austenitic stainless steels, a thick S-phase layer was prepared by using a novel critical temperature nitriding (CTN) process. The properties of the thick S-phase layer were optimized by controlling the process parameters. The microstructures and phase compositions of CTN-treated layers were characterized by the optical microscope, scanning electron microscope and X-ray diffraction, respectively. The surface properties, including corrosion and wear resistance, were systematically investigated by the electrochemical workstation, micro-hardness tester and ball-on-disk tribometer, respectively. The results showed that a thick S-phase layer with a thickness of 18 to 25 μm can be fabricated in a short time by critical temperature nitriding, which represented higher efficiency than conventional low-temperature nitriding. Although the most top surfaces of CTN-treated layers contain massive iron nitrides, there are no precipitates in the inner nitrided layer. The electronic work function calculated by first-principles method has confirmed that those iron nitrides had a slight influence on the corrosion resistance of nitrided layers. The optimized CTN-treated layer exhibited a comparable corrosion resistance and wear resistance as the low-temperature nitrided layer. The CTN process is considered a potentially highly efficient surface modification method for austenitic stainless steels.

Effect of low-temperature nitriding on impact-sliding wear behavior of an austenitic stainless steel at room and sub-zero temperatures

The role of the S-phase layer formed after low-temperature nitriding on the wear resistance of a 17Cr-10Ni-2Mo stainless steel at room and sub-zero temperatures was investigated by using an impact-sliding wear tester, simulating the progress of surface degradation on engineering components under dynamic loading. While the as-received samples suffered from plasticity-dominated wear, surface cracking was dominant for the nitrided samples. Both of the samples exhibited lower wear rates with decreasing test temperature. The highest contact pressure employed (3 GPa) caused spallation of the ∼3 µm thick S-phase layer at room temperature, unlike sub-zero temperature. Under this highest contact pressure, the thicker S-phase layer (∼6 µm) remained intact with the substrate and provided good protection against wear at room temperature.

A comparative study of corrosion behavior of S-phase with AISI 304 austenitic stainless steel in H2S/CO2/Cl- media

S-phase was introduced on stainless steel surface by low-temperature liquid oxy-nitriding and its influence on the corrosion behavior has been explored through surface analysis and weight-loss method. The results indicated that the S-phase layer can significantly improve the corrosion resistance of austenitic stainless steels in H2S/CO2/Cl− simulated solutions, which was proved by lower corrosion rate, surface, and cross-sectional topography after corrosion. This improvement can be attributed to the fact that the Fe3O4 layer blocking the sulfur-containing medium and the consumption of H+ by the active nitrogen atoms, making it difficult for the corrosive medium to penetrate the steel.

Comprehending the role of the S-phase on the corrosion behavior of austenitic stainless steel exposed to H2S/CO2-saturated liquid and vapor environments

The corrosion behavior of austenitic stainless steel after low-temperature liquid oxy-nitriding (LON) was investigated by exposing in H2S/CO2-saturated liquid and vapor environments up to 720 h at 60 °C. The corrosion rates before and after LON were compared by the weightlessness method, and the microstructure as well as the corrosion scales were characterized using surface analysis methods. The results indicated that the composite S-phase layer with the outer Fe3O4 layer and the inner nitrogen-rich sublayer could improve the corrosion performance in H2S/CO2-saturated environment. The base material (BM) suffered local corrosion first, which then transformed into uniform corrosion. As a comparison, The LON sample, covered with a thin corrosion product layer, indicated slight local corrosion. The excellent corrosion resistance of the S-phase should be attributed to the blocking effect of the continuous Fe3O4 film as well as the suppression of the atomic mobility by the nitrogen-containing supersaturated solid solution.

ВЛИЯНИЕ МЕТОДА И ТЕМПЕРАТУРЫ ИОННО-ПЛАЗМЕННОЙ ОБРАБОТКИ НА ФИЗИКО-МЕХАНИЧЕСКИЕ СВОЙСТВА ПОВЕРХНОСТНЫХ СЛОЕВ В АУСТЕНИТНОЙ НЕРЖАВЕЮЩЕЙ СТАЛИ

Ion-plasma saturation with interstitial atoms (nitrogen or carbon) is a promising method for enhancing the surface strength and wear resistance of austenitic stainless steel parts and products. The paper considers the influence of method and temperature of ion-plasma treatment (IPT) on phase composition, thickness, and strength properties (microhardness) of the surface layers in 01H17N13M3 austenitic stainless steel specimens. Steel specimens with a coarse-grained structure were nitrided in the arc and glow discharge plasma at different temperatures (400 °C, 550 °C, and 700 °C). Regardless of temperature and IPT-method, ion-plasma nitriding leads to the formation of hardened surface layers in steel specimens. In this case, the thickness and phase composition of IPT-hardened layers depend on both the method and temperature of nitriding. Nitrogen saturation of specimen surfaces in the glow discharge at a temperature of 400 °C promotes the formation of a thin S-phase layer (nitrogen-expanded austenite, 4 μm in thickness). At the same IPT temperature in the arc discharge plasma, the authors observed the formation of a heterophase (Fe-γN, Fe4N, CrN, and Fe-α) surface layer with a significantly greater thickness (40–45 μm). Regardless of the IPT-method, a saturation of specimens at temperatures of 550 °C and 700 °C is accompanied by the formation of thick heterophase hardened layers (40–60 μm). In this case, the IPT method has a negligible effect on the phase composition of layers but significantly affects the ratio of the volume content of the hardened phases. After being IPT-processed in different modes, the microhardness distribution profile for all specimens has three typical zones: a composite layer (or S-phase at the IPT in a glow discharge at Ta=400 °C), a diffusion zone, and a matrix. With an increase in the saturation temperature, the thickness of the transition diffusion zone increases regardless of the IPT method.

Properties of stainless-steel surface after hollow cathode assisted plasma nitriding

AISI 304 stainless steel was nitrided at different temperature in the plasma equipment using a new auxiliary device. The proposed hollow cathode auxiliary device improves the plasma density, thereby accelerating the nitriding process. The modified surface was characterized by x-ray diffraction, scanning and transmission electron microscopies, atomic force microscopy, surface microhardness testing. Electrochemical corrosion test was used to measure the localized corrosion resistance of the samples. Single S-phase layer without the CrN precipitation was obtained by hollow cathode plasma nitriding at 450 °C, which improved the microhardness, wear and corrosion resistance of the steel surface. Nitriding at higher temperature of 550 °C resulted in substitution of the S-phase towards layer composed from CrN and α-Fe phases, improving the microhardness but decreasing the corrosion resistance.

A comparative study of the tribocorrosion behaviour of low temperature nitrided austenitic and duplex stainless steels in NaCl solution

This work studied and compared the tribocorrosion behaviour of low temperature nitridedaustenitic (316L) and duplex (2205) stainless steels. After gaseous nitriding at 450 °C for10 h, the S-phase layer formed on the duplex stainless steel is thicker and harder than that onthe austenitic stainless steel. The formation of S-phase layers reduces both mechanical wearand chemical wear of stainless steels but the duplex type exhibits better tribocorrosion performance. Wearing through of the S-phase layer during tribocorrosion leads to asignificant drop in open circuit potential, friction instability, and total material loss from the nitrided specimens. The work concludes that low temperature gaseous nitriding is effective in improving the tribocorrosion behaviour of these materials to avoiding catastrophic surface failures.

Tribocorrosion response of duplex layered CoCrMoC/CrN and CrN/CoCrMoC coatings on implant grade 316LVM stainless steel

The objective of this work was to improve the tribocorrosion performance of biomedical grade 316LVM stainless steel via the application of duplex layered coatings. Two different PVD coatings were deposited via magnetron sputtering: (1) an underlying CoCrMo carbon S-phase layer followed by an outermost CrN layer designated as CrN/S and; (2) an underlying CrN layer followed by an outermost CoCrMo carbon S-phase designated as S/CrN. Tribocorrosion of the coated and uncoated 316LVM was studied against an alumina ball which was made to slide in a reciprocation motion in Ringer's solution. The contributions of wear, corrosion and their synergy were elucidated by performing tests under open circuit, cathodic and anodic potential conditions. The resultant wear scars were analysed via SEM, EDS, optical microscopy and contact profilometry. Both coatings displayed superior tribocorrosion response to AISI 316LVM with CrN/S exhibiting the best performance. CrN/S displayed lowest coefficient of friction under all electrochemical conditions (~0.10–0.15) and a material loss under anodic condition with a 93% decrease when compared to 316LVM. The CrN/S scars exhibited a smooth morphology under all test conditions in contrast with the other two test materials which displayed shearing marks oriented along the sliding direction under OCP and anodic conditions. By considering the resultant scar morphologies, a hypothesis is being proposed to explain the tribocorrosion degradation mechanisms encountered.

S-Phase Layer Development on 316 LVM Using Low Temperature Hybrid Thermochemical Treatment Process

This investigation focuses on the improvement of surface properties of medical grade austenitic stainless steel (AISI 316LVM). The aim is to develop a homogenous supersaturated hard layer of expanded austenite (s-phase) at the surface of AISI 316LVM using low temperature hybrid thermochemical heat treatment process. The s-phase layer produced by this process is able to improve the surface properties of AISI 316LVM, overcoming its drawback of low surface hardness and wear resistance, without impairing the corrosion resistance of the steel. During the heat treatment process, ammonia (NH3) and methane (CH4) gasses were introduced into the furnace with temperatures of 425°C and 475°C, at 6 and 12 hours with gas composition of 75% of NH3, 10% of CH4, and 15% of Nitrogen (N2). Characterization on the microstructure showed the formation of the S-phase layer with variation of thickness according to parameters used. The S-phase formation was confirmed with phase analysis using XRD. Besides, the surface hardness also significantly increased from 210.9 HV to 1170.0 HV. In conclusion, low temperature hybrid heat treatment process is able to produce a homogenous hard s-phase layer.

Dry Sliding Wear Behavior of AISI310 Stainless Steel with Low Temperature Salt Bath Nitriding and Gas Nitriding Processes

AISI 310 is an austenitic stainless steel that accomplished in high thermal applications like turbines, boiler parts etc. In this study AISI310 was treated with salt bath nit- -riding for 60min, 120min and 150min and gas nitriding for 6hrs, 12hrs and 18hrs at the temperature of 5700c respectively. Comparison study of nitrided specimens were performed under various metallographic tests like scanning electron microscope, X-ray Diffraction, pin on disc apparatus. Experimental results shown that when salt bath nitrided sample at 150min showed a white layer called “S-phase” layer which was detected. In gas nitriding also “S-phase” layer, an expanded austenite was observed, after 18 hrs, CrN phase was discovered after the decomposition of s-phase layer gas nitrided sample of 18hrs which showed the best corrosion resistance .Salt bath specimen 150 min showed minimum wear loss and gas nitrided sample of 18hrs showed more hardness, minimum wear and improved corrosion resistance compared to untreated sample.

Abrasive resistance and corrosion properties of AISI 316 sieve via low-temperature gaseous nitriding

AISI 316 austenitic stainless steel samples were treated via gaseous nitriding at low temperature (430 °C) to obtain a single expanded austenite layer (S-phase). The structural phases were characterized via both optical microscopy and X-ray diffraction. Microhardness and dry-sliding wear behavior were investigated via scanning electron microscopy, electron probe of microanalysis, Vickers indentation tester, and ball-on-flat wear tester, respectively. The wear of the substrate AISI 316 steel was severe and characterized by strong adhesion, abrasion, and spallation, while the wear of the S-phase layer was mild and dominated by slight abrasion. Furthermore, the corrosion properties were investigated via potentiodynamic polarization testing in 3.5% NaCl solution. The results show that the S-phase layers was produced at low nitrided temperature with improved wear resistance as well as improved corrosion resistance.

Effect of Gas Content and Treatment Temperature on the Characteristic of Surface Layers of Low Temperature Plasma Nitrided 316L Austenitic Stainless Steel

The effect of gas content and treatment temperature on the surface characteristics of hardened layers of low temperature plasma nitrided 316L Austenitic Stainless Steel was investigated. The process was performed at fixed processing time at 15Hr and changing the N2 content from 10% to 25%, changing the treatment temperature from 370 °C to 430 °C. The surface hardness and the thickness of expanded austenite (S-Phase) layer increase with increasing both temperature and nitrogen content. At 430°C processing temperature and 25% N2 content, the S-phase layer thickness increased up to 13 μm and surface hardness increased about 3.5 times of that of untreated sample which is 880 HV0.1 (before treatment 250HV0.1 hardness). However, the XRD pattern of the this sample shows the presence of γ'-(Fe,Cr)4N precipitation on the surface which is the reason for the increased corrosion current density that results in increased corrosion rate. Thus the corrosion resistance degrades and presents even worst behavior comparing that of before treatment. Therefore, for increasing the corrosion behavior, further research was conducted by fixing the N2 content at 10% with changing the CH4 content from 0% to 5%. The best results were found when treated at 430°C with 5% CH4 content. At this condition the S-phase thickness increase to around 17μm and surface hardness (980 HV0.1) is improved about 3.9 times of the hardness of untreated one. This sample also showed superior corrosion resistance than the other treated samples and the untreated one due to much higher pitting potential.

Phase Stability and Microstructure Evolution of Solution-Hardened 316L Powder Feedstock for Thermal Spraying

A solution-hardening of AISI 316L stainless-steel powder was conducted. The expansion of the crystal lattice and a strong increase in the nanoindentation hardness confirm the successful diffusion of carbon and nitrogen in the interstices. A multiphase state of the powder feedstock with phase fractions of the metastable S-phase (expanded austenite) mainly at the particle’s edge, and the initial austenitic phase within the core was found. Thermal spraying using high velocity oxy-fuel (HVOF) and atmospheric plasma spraying (APS) prove the sufficient thermal stability of the Sphase. Microstructural investigations of the HVOF coating reveal the ductility of the S-phase layer, while the higher heat load within the APS cause diffusion processes with the initial austenitic phase. The lattice expansion and the nanoindentation hardness decrease during thermal spraying. However, the absence of precipitates ensures the sufficient heat stability of the metastable S-phase. Even though further efforts are required for the thermochemical treatment of powder feedstock, the results confirm the feasibility of the novel powder treatment approach.

Wear and Corrosion Properties of Cold-Sprayed AISI 316L Coatings Treated by Combined Plasma Carburizing and Nitriding at Low Temperature

Cold-sprayed AISI 316L stainless steel coatings are treated to form an austenite phase with excessive dissolved nitrogen (known as the S-phase) by plasma nitriding at temperatures below 450 °C. The S-phase is a hard and wear-resistant layer with high corrosion resistance. However, the S-phase layer formed after only nitriding is thin and the hardness abruptly decreases at a certain depth; it lacks mechanical reliability. We examined two types of combined low-temperature plasma treatment to enhance the mechanical reliability of the S-phase layer: (i) sequential and (ii) simultaneous. In the sequential plasma treatment, the carburizing step was followed by nitriding. In the simultaneous treatment, the nitriding and carburizing steps were conducted at the same time. Both combined plasma treatments succeeded in thickening the S-phase layers and changed the hardness depth profiles to decrease smoothly. In addition, anodic polarization measurements indicated that sequential treatment involving carburizing followed by nitriding for 2 h each resulted in high corrosion resistance.

Novel wear-resistant anti-bacterial stainless steel surfaces

ABSTRACTTo generate a long-lasting antibacterial surface on stainless steels for demanding applications involving rubbing and wear, a novel triple-glow plasma (TGP) technology was employed to generate Ag/N co-alloying layers on AISI 316 austenitic stainless steel. The mechanical, chemical, antibacterial and tribological behaviour of the samples treated under different settings and parameters were fully characterised. It is identified that a durable antibacterial surface has been achieved using the novel TGP technology under an optimal parameters, as proved by the high hardness, low wear rate and high antibacterial efficacy. A bilayer structure composed of a thin top layer containing about 20–25% Ag embedded in N supersaturated austenite followed by a relatively thick (∼10 µm) S-phase layer was formed after Ag/N co-alloying by TGP. The silver embedded in the hard S-phase provides bactericidal activity against Gram-positive Staphylococcus aureus (S. aureus) with a reduction rate of 95% compared with the untreated stainless steel.

Development and characterisation of novel anti-bacterial S-phase based coatings

It is well-known that biologically active Ag/Cu ions are strong bactericides and silver or copper nanoparticles have been used in polymer-based antibacterial coatings. However, their poor durability has limited their use in tribological applications. This problem has been largely addressed recently by developing novel plasma co-alloying of austenitic stainless steel surfaces with both nitrogen and Ag/Cu to form wear resistant antibacterial S-phase. However, this technology is only applicable to austenitic stainless steel as the S-phase cannot be formed to other materials.In this study, S-phase based anti-bacterial coatings have been, for the first time, developed using magnetron sputtering through co-deposition of austenitic stainless steel with Ag/Cu to form hard S-phase doped with Ag, Cu or both in monolayer and multilayer structures. These coatings were fully characterised using multiple techniques such as SEM, TEM, XRD, GDOES and anti-bacterial tests. It has been found that it is possible to produce dense Ag and Cu doped S-phase layer with significant anti-bacterial efficacy. This was achieved while preserving the advantageous properties of the S-phase microstructure. As opposed to the popular diffusion based S-phase production such as plasma nitriding, this technology can also be applied on all kinds of surfaces, including low-cost steel surfaces, polymers and ceramics.

Stress Corrosion Behavior of Low-temperature Liquid-Nitrided 316 Austenitic Stainless Steel in a Sour Environment

Low-temperature nitridation is a widely used surface heat treatment. Low-temperature liquid nitridation was applied to 316 austenitic stainless steel and an S-phase (expanded austenite) layer was achieved on the alloy surface. The effect of the S-phase layer on corrosion resistance and stress corrosion cracking was investigated in a sour environment. When a bending stress of 164 MPa (80 pct yield stress, YS) was applied, no macroscopic corrosion cracking and pits were observed on the nitrided samples and the S-phase layer stayed intact. Although no macroscopic corrosion cracking was observed on the non-nitrided samples under 205 MPa (100 pct YS), some pits were formed on the alloy surface. This could be attributed to the high stresses and hardness, and the excellent corrosion resistance of the S-phase layer introduced by low-temperature nitridation. Supersaturated nitrogen atoms in the S-phase layer can effectively prevent the decrease in pH of the corrosive medium and accelerate the alloy repassivation kinetics. However, when the bending stress was increased to 205 and 246 MPa (100 pct YS, 120 pct YS), macroscopic cracks were observed in the presence of both tensile stress and a corrosive medium.

The effect of applied compressive stress on the diffusion of carbon in carbon supersaturated S-phase layer

In this study, hydrostatic compressive stresses are applied on the prior-carburized samples of 316 ASS to discover the relationship between interstitial diffusion process and the applied stresses. Carbon diffusion profiles are compared and concentration dependent diffusion coefficients are discussed to further advance scientific understanding of diffusion process in supersaturated S-phase. The experimental results demonstrate that the applied compressive stress can retard the fast diffusion process comparing to the unstressed one. Based on theoretical modelling, the effect of the applied hydrostatic compressive stresses on the diffusion of carbon in carbon supersaturated 316 austenitic stainless steel is discussed.