Novel plasma nitriding-oxidizing duplex treatment of AISI 316 austenitic stainless steel

In this work, plasma nitriding and plasma nitriding-oxidizing treatment have been performed on AISI 316 austenitic stainless steel. In order to evaluate its response to this treatment, their microstructures and wear resistance have been compared with conventional plasma nitrided. The treatment of plasma nitriding was performed at temperature of 450 °C for 5 h with gas mixture of N₂/H₂:1/3 whereas plasma nitriding-oxidizing was performed with the same parameters of plasma nitriding and temperature of 450 °C with gas mixture of O₂/H₂:1/5 for 15, 30 and 60 min. The structural, mechanical and tribological properties were analyzed using XRD, SEM, microhardness testing and pin-on-disk tribotesting. The results showed that oxidation treatment reduces wear resistance of plasma nitrided sample under high loads. Furthermore the tribological evaluation indicates that by increasing the oxidation time further reduction of wear resistance can be occurred. In addition, it was found that oxidation treatment after plasma nitriding provides an important improvement in the friction coefficient against a AISI 52100 steel pin and reduces surface roughness.     

Comparative study of the corrosion behaviour of plasma nitrocarburised AISI 4140 steel before and after post-oxidation

In this study, the corrosion behaviour of plasma nitrocarburised AISI 4140 steel before and after post-oxidation was investigated. Plasma nitrocarburising was carried out at 530 °C, 570 °C and 630 °C for 5 h in an atmosphere consisting of 80 vol.% N₂, 17 vol.% H₂ and 3 vol.% CO₂. After nitrocarburising, plasma post-oxidation was performed at 450 °C for 1 h in a gas mixture of 50 vol.% O₂ and 50 vol.% H₂. The microstructure of the treated samples was characterized using X-ray diffraction, scanning electron microscopy and surface roughness techniques. Dynamic polarization test was also used to evaluate the corrosion resistance of the samples. The results indicated that the compound layer was composed of ε-Fe_2–3(N, C) and γ′-Fe₄(N, C) phases. The amount of ε-Fe_2–3(N, C) phase increased as the treatment temperature rose from 530 °C to 570 °C and decreased at 630 °C. The X-ray diffraction patterns of post-oxidized samples confirmed the formation of the highest amount of magnetite after post-oxidation of samples that had been nitrocarburised at 570 °C as compared with the samples that had been treated at 530 °C and 630 °C. Nitrocarburising at 570 °C followed by post-oxidation also provided the highest corrosion resistance among all treatment conditions.     

Effect of high temperature post-oxidizing on tribological and corrosion behavior of plasma nitrided AISI 316 austenitic stainless steel

In this research, the microstructure, tribological and corrosion properties of plasma nitrided-oxidized AISI 316 austenitic stainless steel at high oxidation temperature were studied and compared with conventional plasma nitride. The structural, tribological and corrosion properties were analyzed using XRD, SEM, microhardness testing, pin-on-disk tribotesting and electrochemical polarization. Plasma nitriding was conducted for 5 h at 450 °C with gas mixture of N₂/H₂ = 1/3 to produce the S-phase. The nitrided samples were post-oxidized at 500 °C with gas mixture of O₂/H₂ = 1/5 for 15, 30 and 60 min. X-ray diffraction confirmed the development of CrN, ? and γ′ nitride phases and magnetite (Fe₃O₄) oxide phase under plasma nitriding-oxidizing process. In addition, it was found that oxidation treatment after plasma nitriding provides an important improvement in the friction coefficient and the corrosion resistance. The optimized wear and corrosion resistance of post-oxidized samples were obtained after 15 min of oxidation.     

Effect of gas mixture of plasma post-oxidation on corrosion properties of plasma nitrocarburised AISI 4130 steel

Plasma post-oxidizing is a process to improve the corrosion resistance of nitrocarburised parts. In this study, the effect of gas mixture of post-oxidation process on corrosion resistance of AISI 4130 plasma nitrocarburised steel has been studied. Plasma nitocarburising was carried out at 520 °C for 5 h in an atmosphere containing 49 vol% nitrogen, 49 vol% hydrogen and 2 vol% carbon dioxide. The nitrocarburised samples were post-oxidized at 450 °C for 1 h under different O₂:H₂ ratios (5:1, 1:1, 1:3, 1:5 and 1:7). The treated samples were studied using XRD, SEM, surface roughness measurement, microhardness and potentiodynamic methods. X-ray diffraction patterns revealed that with decreasing the O₂:H₂ ratio from 5:1 to 1:5, the amount of magnetite phase and the corrosion resistance of the samples were increased. It was seen that with increasing the O₂:H₂ ratio, the thickness of oxide layer was increased and the surface roughness was decreased. Furthermore, a model for oxide formation during plasma post-oxidation is proposed.     

Corrosion behavior of nitride layer obtained on AISI 316L stainless steel via simple direct nitridation route at low temperature

Newly developed low-temperature nitride synthesis route was used to introduce interstitial nitrogen into the passive layer of as-received and as-polished 316L stainless steel. The new thermochemical route is based on treating the stainless steel samples in potassium nitrate melt in an ultra pure nitrogen atmosphere at 450 °C. Electrochemical impedance spectroscopy (EIS) and dc polarization measurements have been used to evaluate the nitride layer performance in 3.5% NaCl solution. Results showed a marked increase in the corrosion resistance of nitrided stainless steel even after maintaining two weeks in NaCl solution. The effect of the treatment temperature was also studied. Data showed that the as-polished samples nitrided at 450 °C have the highest corrosion resistance. The polarization resistance (R_p) for the as-polished and as-received blank stainless steel samples was estimated by EIS were approximately 4.0 × 10⁴ Ω cm² and 2.0 × 10⁴ Ω cm², respectively. The R_p increased by a factor of 2.5–5 for the nitrided samples. Increasing the nitriding temperature from 450 to 600 °C affects negatively the corrosion resistance of stainless steel in NaCl solution. The R_p of the samples nitrided at 600 °C decreased sharply being almost 1/30 of the R_p of the samples nitrided at 450 °C. Linear polarization measurements showed that the lowest corrosion rates and highest polarization resistances obtained from the as-polished nitrided samples at 450 °C. It has been found from the potentiodynamic measurements that the E_corr of the as-polished nitrided samples at 450 °C is nobler than that measured from the other groups. The surface morphology was analysed by optical microscope and SEM-EDS under different nitriding conditions.     

Effect of treating atmosphere in plasma post-oxidation of nitrocarburized AISI 5115 steel

In this study quenched and tempered AISI 5115 steel was plasma-nitrided and nitrocarburized at 550 °C for 5 h in atmospheres of 80% N₂ balanced with various amounts of CO₂ and H₂ gases. The amount of CO₂ varied from 0 to 10 vol%. The highest amount of ε phases was formed in the compound layer after treating in atmosphere containing 7 vol% CO₂. Optimized compound layer was post-oxidized for 1 h at 450 °C under O₂/H₂ volume ratios of 1/1 and 3/1 as well as 100% oxygen. The treated samples were characterized using metallographic techniques, XRD, SEM, roughness measurement and potentiodynamic methods. The results showed that the growth rate of the oxide layer increased with increasing O₂ in the oxidizing gas mixture. X-ray diffraction analysis of oxidized layers confirmed the formation of highest amount of magnetite at post-oxidation in an atmosphere with the O₂/H₂ volume ratio of 1/1. Electrochemical polarization tests proved the enhancement of corrosion resistance by plasma post-oxidation and the highest corrosion resistance obtained after oxidizing under an O₂/H₂ volume ratio of 1/1.     

Comparison of ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel

This paper compares the ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel carried out to improve the surface corrosion resistance. The gas composition for plasma nitriding was 85% N₂–15% H₂ and that for plasma nitrocarburizing was 85% N₂–12% H₂–3% CO₂. Both treatments were performed for 5 h, for different process temperatures of 570 and 620 °C for ferritic and austenitic plasma treatment, respectively. Optical microscopy, X-ray diffraction and potentiodynamic polarization technique in 3.5% NaCl solution, were used to study the treated surfaces. The results of X-ray analysis revealed that with increasing the treatment temperature from 570 to 620 °C for both treatments, the amount of ε phase decreased and γ′ phase increased. Nitrocarburizing treatment resulted in formation of a more amount of ε phase with respect to nitriding treatment. However, the highest amount of ε phase was observed in the ferritic nitrocarburized sample at 570 °C. The sample nitrided at 620 °C exhibited the thickest layer. The potentiodynamic polarization results revealed that after plasma nitriding and nitrocarburizing at 570 °C, corrosion potential increased with respect to the untreated sample due to the noble nitride and carbonitride phases formed on the surface. After increasing the treatment temperature from 570 to 620 °C, corrosion potential decreased due to the less ε phase development in the compound layer and more porous compound layer formed at 620 °C with respect to the treated samples at 570 °C.     

The effect of solution temperature on the microstructure and tensile properties of Al–15%Mg2Si composite

The effect of different solution temperatures has been investigated on the microstructure and tensile properties of in situ Al–Mg₂Si composite specimens were subjected to solutionizing at different temperatures of 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C and 580 °C for holding time of 4 h followed by quenching. The microstructural studies of the polished and etched samples by scanning electron microscopy (SEM) in the solution condition indicated that the increase in the temperature changes the morphology of both the primary and secondary Mg₂Si phases. Solutionizing led to the dissolution of the Mg₂Si particles and changed their morphology. Tensile test results indicated that ultimate tensile strength (UTS) gradually decreased upon solutionizing from 300 to 550 °C while further increase in the temperature followed by a sharp decrease in UTS up to 580 °C solutionizing temperature. It was found that the elongation has become three times greater in comparison to the as-cast state. Elongation results showed an increase up to 500 °C and then reduced temperatures of 550 and 580 °C. Fractographic analysis revealed a cellular nature for the fracture surface. On the cellular fracture surface, the features of both brittle and ductile fracture were present simultaneously. As a result of solution treatment the potential sites for stress concentration and crack initiation areas were reduced due to softening of the sharp corners and break up of eutectic network respectively, while increase in the number of fine dimples rendered the nature of fracture to ductile and also increased elongation.     

Phases in copper-gallium-metal-sulfide films (metal=titanium, iron, or tin)

The incorporation of metal impurities M (M = Ti, Fe, or Sn) into CuGaS₂ films is investigated experimentally as a function of impurity concentration. Films are synthesized by thermal co-evaporation of the elements onto glass/Mo substrates heated to 400 °C-570 °C. The compositions of the resulting films are measured by energy-dispersive X-ray spectroscopy and the structures of the present phases are studied by X-ray diffraction. The formation of Cu-M-S ternary phases is observed in a wide range of conditions. Films of Cu-Ga-Ti-S, synthesized at 500 °C, show the presence of a cubic modification of CuGaS₂ and Cu₄TiS₄. Alloying of CuGaS₂ and tetragonal Cu₂SnS₃ is observed for substrate temperatures of 450 °C. A miscibility gap opens at 500 °C and above with separate Sn-rich and Ga-rich phases. Similarly, alloys of CuFeS₂ and CuGaS₂ are only found in Cu-Ga-Fe-S films synthesized at lower substrate temperature (400 °C), whereas at 500 °C a miscibility gap opens leading to separate Fe-rich and Ga-rich phases.     

Fine Fe16N2 powder prepared by low-temperature nitridation

α-Fe was prepared by reduction of a fine γ-Fe₂O₃ powder under hydrogen at 500 °C for 8 h. The α-Fe fine powder, about 100 nm in crystallite size, was then nitrided under an ammonia flow at 130 °C for 100 h. X-ray single-phase Fe₁₆N₂ was obtained with a magnetization value of 225 emu/g at room temperature under a magnetic field of 15 kOe. The Mössbauer spectrum at room temperature could be resolved into three sets of hyperfine fields with an average magnetic moment of 2.52 μ_B. An additional paramagnetic component was present in the spectrum with an area ratio of 19%.     

Microwave-assisted synthesis of barium molybdate by a citrate complex method and oriented aggregation

BaMoO₄ powders, which have scheelite type structure, were successfully synthesized at low temperatures by a modified citrate complex method assisted by microwave irradiation. The citrate complex precursors were heat-treated at temperatures from 300 to 500 °C for 3 h. Crystallization of the BaMoO₄ powders were detected at 350 °C, and completed at a temperature of 400 °C. TEM image of the BaMoO₄ product obtained above 400 °C revealed spindle-rods-like or flake-like morphology. The anisotropic growth habit of BaMoO₄ leads to the oriented aggregation, which is attributed to the high chemical potentials of the intrinsic structure of BaMoO₄. The BaMoO₄ powder prepared at 500 °C showed the strongest photoluminescent intensity.     

Forming behavior and workability of 6061/B4CP composite during hot deformation

Compression tests of 6061/B₄C_P composite have been performed in the compression temperature range from 300 °C to 500 °C and the strain rate range from 0.001 s⁻¹ to 1 s⁻¹. The flow behavior and processing map have been investigated using the corrected data to elimination of effect of friction. The processing maps exhibited two deterministic domains, one was situated at the temperature between 300 °C and 400 °C with strain rate between 0.003 s⁻¹ and 0.18 s⁻¹ and the other was situated at the temperature between 425 °C and 500 °C with strain rate between 0.003 s⁻¹ and 0.18 s⁻¹.The estimated apparent activation energies of these two domains, were 129 kJ/mol and 149 kJ/mol, which suggested that the deformation mechanisms were controlled by cross-slip and lattice self-diffusion respectively. The optimum parameters of hot working for the experimental composite were 350 °C - 0.01 s⁻¹ and 500 °C - 0.01 s⁻¹. In order to exactly predict dangerous damaging mechanism under different deformation conditions exactly, Gegel’s criterion was applied to obtain processing map in the paper. The result showed that the processing map used Gegel’s criterion can be effectively to predict the material behavior of the experimental composite.     

The study of tribological and corrosion behavior of plasma nitrided 34CrNiMo6 steel under hot and cold wall conditions

This paper reports on a comparative study of tribological and corrosion behavior of plasma nitrided 34CrNiMo6 low alloy steel under modern hot wall condition and conventional cold wall condition. Plasma nitriding was carried out at 500 °C and 550 °C with a 25% N₂ + 75% H₂ gas mixture for 8 h. The wall temperature of the chamber in hot wall condition was set to 400 °C. The treated specimens were characterized by using scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness and surface roughness techniques. The wear test was performed by pin-on-disc method. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were also used to evaluate the corrosion resistance of the samples. The results demonstrated that in both nitriding conditions, wear and corrosion resistance of the treated samples decrease with increasing temperature from 500 °C to 550 °C. Moreover, nitriding under hot wall condition at the same temperature provided slightly better tribological and corrosion behavior in comparison with cold wall condition. In consequence, the lowest friction coefficient, and highest wear and corrosion resistance were found on the sample treated under hot wall condition at 500 °C, which had the maximum surface hardness and ε-Fe_2–3N phase.     

Hot corrosion behavior of powder metallurgy Rene95 nickel-based superalloy in molten NaCl–Na2SO4 salts

Hot corrosion behavior of powder metallurgy (PM) Rene95 Ni-based superalloy in molten 25%NaCl + 75%Na₂SO₄ salts at 650 °C, 700 °C and 750 °C are investigated by weight loss measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). Experimental results show that hot corrosion kinetics follow a square power law at 650 °C and linear power laws at 700 °C and 750 °C. The corrosion layers on the surface of PM Rene95 superalloy are detected to be mainly composed of Cr₂O₃, NiO, and Ni₃S₂ at each temperature. Besides, small amounts of NiCr₂O₄ at 700 °C and NaCl at 750 °C are observed respectively. Cross-sectional morphologies and corresponding elemental maps indicate that corrosion layers near scale/alloy interface are composed of oxides at 650 °C while duplex oxides and sulfides at 700 °C and 750 °C. According to these results, a cooperating mechanism of oxidation and sulfuration for hot corrosion of PM Rene95 Ni-based superalloy is confirmed.     

Isothermal oxidation behavior of reactive hot-pressed TiN–TiB2 ceramics at elevated temperatures

Isothermal oxidation behavior of reactive hot-pressed TiN–TiB₂ ceramics with various TiN/TiB₂ molar ratios of 2/1, 1/1 and 1/2 was evaluated in the temperature range of 500–800 °C in air. TiN–TiB₂ ceramics have a relative density of 97–98.6%. The oxidation weight gains of TiN–TiB₂ ceramics depend upon the composition, oxidation temperature and exposure time. The structure and morphology of oxidized layers of TiN–TiB₂ ceramics were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). During isothermal oxidation of TiN–TiB₂ ceramics, anatase and rutile-TiO₂ form as the oxidized products at 500 °C. However, phase transformation from anatase to rutile occurs at temperatures between 500 and 600 °C, and therefore rutile-TiO₂ becomes the only crystalline phase after oxidation at temperatures of 600–800 °C for 10 h. The oxidation mechanism was proposed with reference to thermodynamically feasible oxidation reactions. The influence of composition on oxidation behavior of TiN–TiB₂ ceramics varies with temperature.     

The effect of temperature on plasma nitriding behaviour of DIN 1.6959 low alloy steel

A series of experiments have been conducted on DIN 1.6959 low-alloy steel using a 5 kVA DC plasma nitriding apparatus with the aim of elucidating the role of treatment temperature in plasma nitriding process. Treatments were carried out in 75%N₂-25%H₂ atmosphere of 4 mbar for 5 h at temperatures ranging from 350 to 550 °C. Optical microscopy, scanning electron microscopy, X-ray diffraction, along with surface roughness and microhardness measurements were utilized to characterize the treated samples. The depth, microstructure, hardness profile and phase constituents of the nitrided layers as well as the surface roughness of the samples were assessed as a function of treatment temperature. The results suggested that the compound layers were mostly dual phase consisting of gamma prime and epsilon iron nitride phases. Increasing treatment temperature increases compound layer and diffusion layer thicknesses. However, maximum surface hardness and roughness were found on the samples treated at 500 and 550 °C, respectively.     

Effects of various nitriding parameters on active screen plasma nitriding behavior of a low-alloy steel

Active screen plasma nitriding (ASPN) is a novel surface modification technique that has many capabilities over the conventional DC plasma nitriding (CPN). In this study, 30CrNiMo8 low-alloy steel was active screen plasma-nitrided under various nitriding parameters such as active screen set-up parameters (different screen hole sizes, mesh sheet and plate top lids) and treatment temperature (520, 550 and 580 °C), in the gas mixture of 75% N₂+25% H₂ and chamber pressure of 500 Pa for 5 h. The properties of the nitrided specimens have been assessed by evaluating composition of phases, surface hardness, compound layer thickness and case depth using X-ray diffraction (XRD), microhardness measurements and scanning electron microscopy (SEM). It was found that the screen hole size and top lid type (mesh or plate) play an important role in transition of active species (nitrogen ions and neutrals) toward the sample surface, which in turn can affect the nitrided layer hardness and thickness. Treatment at higher temperature with bigger screen hole size resulted in a thicker compound layer and higher layer hardness. The compound layers developed on the samples treated under different conditions were dual phase consisting of γ′-Fe₄N and ε-Fe_2-3N phases.     

Sol-gel derived TiO2 coating on plasma nitrided 316L stainless steel

Titania film is coated on plasma nitrided 316L stainless steel by sol-gel method. Crystallization of titania as well as N loss and formation of Fe₂O₃ occurs during the annealing heat treatment. The titania film has short cracks within the grooves of plasma etched grain boundaries. With the increase of annealing temperature and duration, surface hardness of the samples is increased, but the toughness is decreased due to oxidization of the surface layer. The coating sample heat treated at 350 °C for 10 min and 450 °C for 10 min has better corrosion resistance than the nitrided stainless steel tested by the potentiodynamic polarization in 0.9% NaCl solution. Water contact angle of the titania film on the rough nitrided steel substrate decreases with UV irradiation treatment, reaching 17° after 3 h treatment.     

Electrochemical behavior of a lead-free SnAg solder alloy affected by the microstructure array

The aim of this study is to evaluate the electrochemical corrosion behavior of a Sn–Ag solder alloy in a 0.5 M NaCl solution at 25 °C as a function of microstructural characteristics. Different microstructure morphologies, which can be found in Sn–Ag solder joints and that are imposed by the local solidification cooling rate, are evaluated and correlated to the resulting scale of the dendritic matrix and the morphology of the Ag₃Sn intermetallic compound. Cylindrical metallic molds at two different initial temperatures were employed permitting the effect of 0.15 °C/s and 0.02 °C/s cooling rates on the microstructure pattern to be experimentally examined. Electrochemical impedance spectroscopy (EIS) diagrams, potentiodynamic polarization curves and an equivalent circuit analysis were used to evaluate the electrochemical parameters. It was found that higher cooling rates during solidification are associated with fine dendritic arrays and a mixture of spheroids and fiber-like Ag₃Sn particles which result in better corrosion resistance than coarse dendrite arrays associated with a mixture of fibers and plate-like Ag₃Sn morphologies which result from very slow cooling rates.     

Behavior of N atoms after thermal nitridation of Si1 − xGex surface

Behavior of N atoms after thermal nitridation of Si_1 − xGe_x (100) surface in NH₃ atmosphere at 400 °C was investigated. X-ray photoelectron spectroscopy (XPS) results show that N atomic amount after nitridation tends to increase with increasing Ge fraction, and amount of N atoms bonded with Ge atoms decreases by heat treatment in H₂ at 400 °C. For nitrided Si_0.3Ge_0.7(100), the bonding between N and Si atoms forms Si₃N₄ structure whose amount is larger than that for nitrided Si(100). Angle-resolved XPS measurements show that there are N atoms not only at the outermost surface but also beneath surface especially in a deeper region around a few atomic layers for the nitrided Si(100), Si_0.3Ge_0.7(100) and Ge(100). From these results, it is suggested that penetration of N atoms through around a few atomic layers for Si, Si_0.3Ge_0.7 and Ge occurs during nitridation, and the N atoms for the nitrided Si_0.3Ge_0.7(100) dominantly form a Si₃N₄ structure which stably remains even during heat treatment in H₂ at 400 °C.