Microstructure and antibacterial properties of microwave plasma nitrided layers on biomedical stainless steels

Nitriding of AISI 303 austenitic stainless steel using microwave plasma system at various temperatures was conducted in the present study. The nitrided layers were characterized via scanning electron microscopy, glancing angle X-ray diffraction, transmission electron microscopy and Vickers microhardness tester. The antibacterial properties of this nitrided layer were evaluated. During nitriding treatment between 350 °C and 550 °C, the phase transformation sequence on the nitrided layers of the alloys was found to be γ → (γ + γ_N) → (γ + α + CrN). The analytical results revealed that the surface hardness of AISI 303 stainless steel could be enhanced with the formation of γ_N phase in nitriding process. Antibacterial test also demonstrated the nitrided layer processed the excellent antibacterial properties. The enhanced surface hardness and antibacterial properties make the nitrided AISI 303 austenitic stainless steel to be one of the essential materials in the biomedical applications.     

The effect of surface nanocrystallization on plasma nitriding behaviour of AISI 4140 steel

A plastic deformation surface layer with nanocrystalline grains was produced on AISI 4140 steel by means of surface mechanical attrition treatment (SMAT). Plasma nitriding of SMAT and un-SMAT AISI 4140 steel was carried out by a low-frequency pulse excited plasma unit. A series of nitriding experiments has been conducted at temperatures ranging from 380 to 500 °C for 8 h in an NH₃ gas. The samples were characterized using X-ray diffraction, scanning electron microscopy, optical microscopy and Vickers microhardness tester. The results showed that a much thicker compound layer with higher hardness was obtained for the SMAT samples when compared with un-SMAT samples after nitriding at the low temperature. In particular, plasma nitriding SMAT AISI 4140 steel at 380 °C for 8 h can produced a compound layer of 2.5 μm thickness with very high hardness on the surface, which is similar to un-SMAT samples were plasma nitrided at approximately 430 °C within the same time.     

Improvement of corrosion resistance of nitrided low alloy steel by plasma post-oxidation

Post-oxidizing treatments can be performed to improve the corrosion resistance of nitrided steel samples. In this paper, plasma nitriding treatments were performed at 540 °C for 4 h using ammonia as the working gas, and plasma post-oxidizing treatments were carried out at temperatures ranging from 350 °C to 500 °C for 2 h in oxygen gas. The treated samples were characterized by using optical microscopy, SEM, XRD, and electrochemical polarization. The X-ray analysis revealed the formation of iron-nitride phases of ?-Fe_2-3N and γ′-Fe₄N during plasma nitriding and iron oxide phases of hematite (Fe₂O₃) and magnetite (Fe₃O₄) through the post-oxidizing treatment. In particular, it was found that the very thin magnetite layer 0.8-1.5 μm in thickness on top of the compound layer was obtained by plasma post-oxidized at 400 °C and 450 °C. It was also demonstrated that the corrosion characteristics of the nitrided compound layer were further improved by post-oxidation treatment.     

Surface engineering of biomedical metallic materials by plasma-based low-energy ion implantation

Plasma-based low-energy ion implantation, including plasma source ion nitriding/carburizing and plasma source low-energy ion enhanced deposition of thin films, for surface engineering of metallic materials was emerged as low-temperature, low-pressure surface modification technique. Plasma source ion nitriding onto AISI 316L austenitic stainless steel produced a high nitrogen face-centered-cubic phase (γ_N) layer about 10 μm thick at the temperature of 380 °C during 4 h with the high microhardness of HK_0.1 N 22.0 GPa. The microhardness of the nitrided surface from the titanium nitride phase [(Ti, Al, V)N] layer on Ti6Al4V alloy at 750 °C during 4 h achieved up to about HK_0.1 N 15.5 GPa. No pitting corrosion in the Ringer’s solution at 37 °C was detected by electrochemical polarization measurement for the nitrided AISI 316L stainless steel and Ti6Al4V alloy, respectively. Plasma source ion nitriding of the metallic materials provided the engineering surfaces with combined improvement in hardness and corrosion resistance.     

Microstructure and corrosion resistance of the layers formed on the surface of precipitation hardenable plastic mold steel by plasma-nitriding

Plasma-nitriding is used to improve the wear resistance and corrosion resistance of plastic mold steels by modifying the surface layers of these steels. In this study, a precipitation hardenable plastic mold steel (NAK80) was plasma-nitrided at 470, 500, and 530 °C for 4, 8, and 12 h under 25% N₂ + 75% H₂ atmosphere in an industrial nitriding facility. The microstructures of the base material and nitrided layers as well as the core hardness were examined, and various phases present were determined by X-ray diffraction. The corrosion behaviors were evaluated using anodic polarization tests and salt fog spray tests in 3.5% NaCl solution.The results had shown that plasma-nitriding does not cause the core to soften by overaging. Nitriding and aging could be achieved simultaneously in the same treatment cycle. Plasma-nitriding of NAK80 mold steel produced a nitrided layer composed of an outer compound layer constituting a mixture of ?-nitride and γ′-nitride and an adjacent nitrogen diffusion layer on the steel surface. The amount of ?-nitride and total nitrides increased with an increase in nitriding temperature and nitriding time. Corrosion study revealed that plasma-nitriding significantly improved the corrosion resistance in terms of corrosion potential, corrosion and pitting current density, and corrosion rate. This improvement was found to be directly related to the increase in the amount of ?-nitride at the surface, indicating the amount of ?-nitride controlling the corrosion resistance.     

Surface improvement and biocompatibility of TiAl24Nb10 intermetallic alloy using rf plasma nitriding

The present work describes the surface improvement and biocompatibility of TiAl₂₄Nb₁₀ intermetallic alloy using rf plasma nitriding. The nitriding process was carried out at different plasma power from 400 W to 650 W where the other plasma conditions were fixed. Grazing incidence X-ray diffractometry (GIXRD), Auger electron spectroscopy (AES), tribometer and a nanohardness tester were employed to characterize the nitrided layer. Further potentiodynamic polarization method was used to describe the corrosion behavior of the un-nitrided and nitrided alloy. It has been found that the Vickers hardness (HV) and corrosion resistance values of the nitrided layers increase with increasing plasma power while the wear rates of the nitrided layers reduce by two orders of magnitude as compared to those of the un-nitrided layer. This improvement in surface properties of the intermetallic alloy is due to formation of a thin modified layer which is composed of titanium nitride in the alloy surface. Moreover, all modified layers were tested for their sustainability as a biocompatible material. Concerning the application area of biocompatibility, the present treated alloy show good surface properties especially for the nitrided alloy at low plasma power of 400 W.     

Plasticity and ab initio characterizations on Fe4N produced on the surface of nanocrystallized 18Ni-maraging steel plasma nitrided at lower temperature

18Ni-maraging steel has been entirely nanocrystallized by a series of processes including solution treatment, hot-rolling deformation, cold-drawn deformation and direct electric heating. The plasma nitriding of nanocrystallized 18Ni-maraging steel was carried out at 410 °C for 3 h and 6 h in a mixture gas of 20% N₂ + 80% H₂ with a pressure of 400 Pa. The surface phase constructions and nitrogen concentration profile in surface layer were analyzed using an X-ray diffractometer (XRD) and the glow discharge spectrometry (GDS), respectively. The results show that an about 2 μm thick compound layer (mono-phase γ′-Fe₄N) can be produced on the top of the surface layer of nanocrystallized 18Ni-maraging steel plasma nitrided at 410 °C for 6 h. The measured hardness value of the nitrided surface is 11.6 GPa. More importantly, the γ′-Fe₄N phase has better plasticity, i.e., its plastic deformation energy calculated from the load-displacement curve obtained by nano-indentation tester is close to that of nanocrystallized 18Ni-maraging steel. Additionally, the mechanical properties of γ′-Fe₄N phase were also characterized by first-principles calculations. The calculated results indicate that the hardness value and the ratio of bulk to shear modulus (B/G) of the γ′-Fe₄N phase are 10.15 GPa and 3.12 (>1.75), respectively. This demonstrates that the γ′-Fe₄N phase has higher hardness and better ductility.     

Development of nitride-layer of AISI 304 austenitic stainless steel during high-temperature ammonia gas-nitriding

Ammonia-gas nitriding of AISI 304 austenitic stainless steel was studied at temperatures higher than 800 °C using SEM and X-ray diffraction. The result showed that S-phase, an expanded austenite, was formed even at such high temperatures due to a high nitriding potential of ammonia gas. The equilibrium phase, CrN was formed through a decomposition of S-layer in two different modes; the one was through continuous precipitation of particles at the surface-side of S-layer due to a higher nitriding potential; the other through a discontinuous(-like) precipitation at the austenite interface-side, producing a fine lamellar structure of austenite and CrN. The γ-phase in the surface-side resulting from the precipitation of CrN particles subsequently transformed into Fe₄N because of a fast enrichment of N atoms and a limited mobility of Cr atoms at the surface-side. A coarse lamellar structure made of austenite and Cr₂N was developed in front of fine lamellae composed of austenite and CrN by the decomposition of supersaturated austenite through a discontinuous precipitation via grain boundary movement.     

Cr-Ni-Mo-Co surface alloying layer formed by plasma surface alloying in pure iron

Using double glow plasma alloying technique, a multi-elements alloyed layer containing elements of Cr, Ni, Mo and Co was formed on the surface of pure iron. After undergoing suitable aging treatment followed solid solution treatment, the formed alloying layer keeps a good combination of corrosion resistance and wear resistance. The relationship between the process parameters of heat treatments and the properties of the formed Cr-Ni-Mo-Co alloying layer, such as the chemical composition, hardness, corrosion resistance and wear resistance, was investigated in this study. It was revealed that the formed alloying layer exhibits a better behavior than that of 304 stainless steel and pure iron by employing a suitable heat treatment system. The temperature employed in solid solution treatment is 1453 K (1180 °C) followed by water quenching and the aging temperature is 813 K (540 °C) followed by water cooling.     

Evolution of magnetic phase at low aging temperature in a heavily cold-drawn stainless steel fiber

The evolution of the magnetic phase upon aging at 300–520 °C in a heavily cold-drawn AISI 316L austenitic stainless steel fiber was studied using thermomagnetic analysis (TMA) and magnetic force microscopy with a heating stage. An increasing trend of magnetization from 50 °C to around 470 °C in the heating curves of TMA in austenitic stainless steels after a cold-drawing process was observed. No significant M_s temperature signal in the TMA curve at cooling indicated an increase in magnetization upon cooling period without significant phase transformation. A series of in situ magnetic force microscopy observations reveal a growth of the magnetic domain structure after aging at 300 °C for 2.5 h. Results show that the ferromagnetic increase during aging at lower annealing temperature resulted from the growth of martensite.     

Corrosion behaviour of AISI 304 stainless steel with Cu coatings in H2SO4

The work addresses the influence of cementation and electrodeposition of copper coatings on the corrosion resistance of AISI 304 stainless steel immersed in 30 wt.% H₂SO₄ at temperatures of 25 and 50 °C. Corrosion process was evaluated by gravimetric tests, DC measurements and electrochemical impedance spectroscopy (EIS). The specimen surfaces were analysed by scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction. The corrosion performance of AISI 304 stainless steel in sulphuric acid solution was greatly improved by copper coatings. The amount of copper deposited by the cementation process was sufficient to protect the stainless steel of corrosion. A greater amount of copper obtained by electrodeposition treatments does not supply further improvement in the corrosion behaviour. The improved corrosion resistance is related to copper dissolution at the initial stages of immersion tests and the presence of Cu²⁺ in the solution, which makes the medium more oxidizing, increasing the stability of the passive layer. In addition, the presence of copper at the surface reduces the overpotential of cathodic reaction, enabling the transition from an active region to the passive one.     

Surface modification of 30CrNiMo8 low-alloy steel by active screen setup and conventional plasma nitriding methods

In this paper, we report on a comparative study of active screen plasma nitriding (ASPN) and conventional dc plasma nitriding (CPN) behavior of 30CrNiMo8 low-alloy steel that has been examined under various process conditions. The process variables included active screen setup parameters (screen and iron plate top lids placed on the screen setup with 8 mm of hole size), treatment temperature (550 and 580 °C), gas mixture (75/25 and 25/75 of N₂/H₂) and treatment time (5 and 10 h) in 500 Pa pressure. The structure and phases composition of the diffusion zone and compound layer were studied by X-ray diffraction (XRD), microhardness tests, light optical microscopy and scanning electron microscopy (SEM). It was observed that treated sample surfaces in both CPN and ASPN methods consist of γ′ and ? phases, and while the nitriding time and/or temperature increases, the intensity of ? phase in the compound layer will increase for ASPN and decrease for CPN method. Results show that the amount of nitrogen transferred from holes of screen toward the sample surface via sputtering and re-condensation mechanism can be affected due to the hardness and thickness of the layer.     

Effect of nitriding time on secondary recrystallization behaviors and magnetic properties of grain-oriented electrical steel

The effect on secondary recrystallization behaviors and magnetic properties of grain-oriented electrical steel of nitriding time from 0 to 240 s in the acquired-inhibitor method has been studied. It was found that the volume fraction of nitride precipitates increased with increasing nitriding time. However, the average diameter of the nitride precipitates decreased with increasing nitriding time. Two kinds of nitride precipitates were found to have formed after primary recrystallization annealing. A fine rod-shaped precipitate was found to be Si₃N₄ and and a coarse, lozenge-shaped precipitate was MnSiN₂. Moreover, primary grain size decreased with increasing nitriding time due to retarding of the grain growth by precipitates. After secondary recrystallization annealing, the specimen that was nitrided for 30 s obtained the largest volume fraction of abnormal growth grains and largest area percentage of Goss grains. Conversely, specimens that were nitrided more or less than 30 s demonstrated poor secondary recrystallization and obtained low area percentage of Goss grains. Furthermore, the optimum nitriding time to obtain the best magnetic properties was 30 s. In addition, the optimum nitrogen content was 150 ppm.     

Elevated temperature dry sliding wear behavior of nickel-based composite coating on austenitic stainless steel deposited by a novel central hollow laser cladding

In order to improve the high-temperature wear resistance of austenitic stainless steel, a wear resistant composite coating reinforced with hard (Cr,Fe)₇C₃ carbide and toughened by ductile γ-(Ni,Fe)/(Cr,Fe)₇C₃ eutectic matrix was fabricated by a novel central hollow laser cladding technique. The constituent phases and microstructure as well as high-temperature tribological behaviors of the Ni-based coating were investigated, respectively, and the corresponding wear mechanisms were discussed. It has been found that the composite coating exhibits superior wear resistance than substrate either at ambient or high temperatures. The coating shows better sliding wear resistance at 600 °C than 300 °C owing to high-temperature stability of the reinforced carbide and polishing effect as well as formation of continuous lubricious films, which implied it has large potential industrial applications at relatively higher temperatures.     

Comparative study of titanium carbide and nitride coatings grown by cathodic vacuum arc technique

Titanium nitride (TiN), titanium carbide (TiC) thin films and TiC/TiN bilayers have been deposited on AISI 304 stainless steel substrates by plasma assisted physical vapor deposition technique—reactive pulsed vacuum arc method. The coatings were characterized in terms of crystalline structure, microstructure and chemical nature by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. Tribological behavior was investigated using ball on disc technique. The average coefficient of friction was measured, showing lower values for the TiN/TiC bilayer. Dynamic wear curves were performed for each coating, observing a better wear resistance for TiN/TiC bilayers, compared to TiN and TiC monolayers. On the other hand, the TiCN formation in the TiN/TiC bilayer was observed, being attributed to the interdiffusion between TiN and TiC at the interface. Moreover, the substrate temperature influence was analysing observing a good behavior at T_S = 115 °C.     

Microstructure and wear behaviors of laser clad NiCr/Cr3C2-WS2 high temperature self-lubricating wear-resistant composite coating

The high temperature self-lubricating wear-resistant NiCr/Cr₃C₂-30%WS₂ coating and wear-resistant NiCr/Cr₃C₂ coating were fabricated on 0Cr18Ni9 austenitic stainless steel by laser cladding. Phase constitutions and microstructures were investigated, and the tribological properties were evaluated using a ball-on-disc wear tester under dry sliding condition at room-temperature (17 °C), 300 °C and 600 °C, respectively. Results indicated that the laser clad NiCr/Cr₃C₂ coating consisted of Cr₇C₃ primary phase and γ-(Fe,Ni)/Cr₇C₃ eutectic colony, while the coating added with WS₂ was mainly composed of Cr₇C₃ and (Cr,W)C carbides, with the lubricating WS₂ and CrS sulfides as the minor phases. The wear tests showed that the friction coefficients of two coatings both decrease with the increasing temperature, while the both wear rates increase. The friction coefficient of laser clad NiCr/Cr₃C₂-30%WS₂ is lower than the coating without WS₂ whatever at room-temperature, 300 °C, 600 °C, but its wear rate is only lower at 300 °C. It is considered that the laser clad NiCr/Cr₃C₂-30%WS₂ composite coating has good combination of anti-wear and friction-reducing capabilities at room-temperature up to 300 °C.     

Interlayer effect on the characterization of the La-Cr-O coatings with post-sputtering annealing treatment

In this study, the deposition of La-Cr-O coatings on AISI 316 stainless steel was conducted by using a DC magnetron sputtering process. Three types of La-Cr-O coatings were designed, one type without interlayer, and another two types with an interlayer of Cr and CrN film, separately. This study aims to explore the interlayer effect on the properties of La-Cr-O coatings after post-annealing treatment. The coatings were characterized using the X-ray diffractionmeter (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and thermal gravimetric analysis (TGA)/differential scanning calorimetry (DSC).The results showed that the three as-deposited La-Cr-O coatings had an amorphous structure. After annealing at 800 °C for 1 h in the air, the structure was transformed to the LaCrO₃ perovskite phase. Compared with the coatings without an interlayer, the double-layered coatings did not show the Cr₂O₃ phase in the structure. It implies that the use of the interlayer in the coatings could efficiently prohibit the diffusion of Cr to form oxide on the out-layer. Based on the TGA/DSC analysis and the electrical resistance measurement, it is understood that the annealed La-Cr-O coating with the CrN interlayer turns in the best performance in both chemical stability at high temperature and electrical resistance.     

Behavior of N atoms in atomic-order nitrided Si0.5Ge0.5(1 0 0)

Behavior of N atoms in atomic-order nitrided Si_0.5Ge_0.5(1 0 0) by heat treatment in Ar at 600 °C was investigated by X-ray photoelectron spectroscopy (XPS). For thermal nitridation by NH₃ at 400 °C, nitridation of surface Si atoms tends to proceed preferentially over nitridation of surface Ge atoms. It is also clear that, with the heat treatment, nitridation of Si atoms proceeds by transfer of N atoms from Ge atoms. Angle-resolved XPS results show that Ge fraction beneath the surface nitrided layer increases significantly at 600 °C compared to the initial surface. These results indicate that preferential nitridation of Si atoms at surface over Ge atoms induces Ge segregation beneath the surface nitrided layer at higher temperatures above 400 °C.     

Effect of a small increase in the Ni content on the properties of a laser surface clad Fe-based alloy

Two Fe-based alloys with a small variation in the Ni content, Fe-15.2Cr-5.1Ni and Fe-15.7Cr-7.1Ni (wt.%), were fabricated on a martensitic stainless steel 1Cr13 substrate by laser surface cladding (LSC) using a CO₂ laser and Ar shielding gas that was blown into a molten pool. Both LSC alloys exhibited typical rapid directional solidification structures. However, 2 wt.% Ni increase led to ∼9% increase in the weight fraction of austenite, and ∼5% increase in the area proportion of interdendritic regions, which contained the higher Cr contents. These microstructural changes caused a great reduction in the microhardness and great improvements in the resistance to electrochemical corrosion in 0.5 M H₂SO₄ solution and high temperature oxidation in air at 900 °C. The reasons for these differences are discussed in detail.     

Influence of ternary addition of transition elements (Cr,Si and Mn) on the microstructure and magnetic properties of nano-structured CuCo alloy

The current state of studies presents the effect of ternary addition of transition elements such as Mn, Cr and Si (10 wt%) on the mechanically driven non-equilibrium solubility of 40 wt% Co containing Cu–Co alloy. X-ray powder diffraction analysis indicates that addition of Mn has been found to be the most effective in enhancing the solubility and formation of a complete solid solution between Co and Cu in a short duration (30 h) of ball milling. The microstructure of the ball milled CuCoMn alloy was found to be stable after the isothermal annealing up to a temperature of 450 °C for 1 h. The magnetic properties such as magnetic saturation, coercivity and remanence of ball milled CuCo alloy in the presence of Mn significantly altered after annealing in the temperature range 350–650 °C for 1 h. The best combination of magnetic properties of CuCoMn alloy has been found after annealing at 550 °C for 1 h.