Accurate measurement and evaluation of the nitrogen depth profile in plasma nitrided iron

Nitrogen depth profile of plasma nitrided pure iron was measured and evaluated by accurate experimental techniques. Plasma nitriding cycles were carried out on high purity iron substrate in an atmosphere of 75% H₂-25% N₂. Nitrogen concentration depth profiles in the compound layer and the diffusion zone were characterized by glow discharge optical emission spectroscopy (GDOES) and secondary ion mass spectroscopy (SIMS), respectively. Nitrogen diffusion depths were measured accurately by optical and scanning electron microscopy as well as SIMS technique at different nitriding times. Experimental results indicated good agreement between SIMS data and microscopic evaluations for various nitriding cycles. The results of SIMS showed the nitrogen diffusion depth of about 2000 μm in the diffusion zone for 10 h plasma nitriding at 550 °C. Such high depth had not been detected in previous investigations in which the conventional methods such as EDS, GDS, XPS, EPMA or ion probe techniques were used.     

Stress induced and concentration dependent diffusion of nitrogen in plasma nitrided austenitic stainless steel

The nitrogen transport mechanism in austenitic stainless steel during plasma nitriding at moderate temperatures (around 400 °C) is considered by stress induced diffusion model. The model involves diffusion of nitrogen in presence of internal stresses gradient induced by penetrating nitrogen as the next driving force of diffusion after concentration gradient. Furthermore, in the present work it was found that nitrogen diffusion coefficient vary with nitrogen concentration according to well-known Einstein-Smoluchowski relation D(C_N) = f(1/C_N). Nitrogen depth profiles in nitided AISI 316L steel at T = 400 °C for 1, 3 and 8 h calculated on the basis of this model are in good agreement with experimental nitrogen profiles. The dependencies of nitrogen flux and nitriding time on nitrogen concentration, nitrogen surface concentration and penetration depth are analyzed by proposed model. It is shown that, with the increase of nitriding time the compositionally-induced stresses and thickness of stressed steel layer increases.     

Ion-nitriding of an Fe-19 wt % Cr alloy

The ion-nitriding behaviour of an Fe-18.75wt% Cr alloy was investigated at 803 K under constant plasma conditions. Both a thin surface layer of-Fe₄N and an internal-nitriding layer were observed. The nitride formed in the internal-nitriding layer was found to be CrN, rather than Cr₂N. The hardness of the nitriding layer rises to Hv=1200 due to small CrN precipitates. The growth rate of the internal nitriding layer, in the present alloy is controlled by a nitrogen diffusion process in the matrix metal,-iron. Because such ion-nitriding behaviour is analogous to that of internal-oxidation, the growth rate of nitriding was discussed according to the rate equation to that of internal-oxidation. The nitrogen diffusion in the present alloy is scarcely affected by the CrN precipitates.     

Effect of plasma on nitriding of Fe-18Cr-9Ni alloy

The plasma nitriding behaviour of Fe-18Cr-9Ni alloy was compared with gas nitriding. The alloy was nitrided under the following conditions: specimen temperature: 823 K, nitriding time: mainly 108 ks, total pressure: 0.4–0.7 kPa, mixture ratio of N₂ and H₂∶ 0.25, discharge voltage: 350–450 V, current: 0.8–1.1 A. Formation of a surface layer of iron nitrides was not observed. Formation of a homogeneous internal nitriding layer, consisting of small precipitates of CrN and the γ-phase matrix, was, however, noted. The lattice constant at the specimen surface was smaller than that at greater depth. This may have been because the sputtering effect decreased the dissolved nitrogen content at the specimen surface. The sputtering of iron nitrides at the specimen surface by the plasma was experimentally confirmed through γ′-Fe₄N formation on Si beside an alloy specimen. The characteristics of the plasma nitriding mentioned above are discussed in relation to the sputtering.     

Nitriding behaviour of maraging steel: experiments and modelling

The microstructure and the kinetics of growth of the nitrided zone of a Mo-containing maraging steel were investigated by performing gaseous nitriding at temperatures between 713 K (440 °C) and 793 K (520 °C) and at nitriding potentials up to 0.5 atm^?1/2 for both solution-annealed and precipitation-hardened specimens. The microstructure of the nitrided zone was investigated by means of X-ray diffraction (phase constitution; crystal imperfection). Fine, initially largely coherent Mo₂N-type precipitates developed in the nitrided zone. The elemental concentration-depth profiles were determined employing glow discharge optical emission spectroscopy (GDOES). The nitrogen content within the nitrided zone exceeds the nitrogen content expected on the basis of the molybdenum content and the equilibrium solubility of nitrogen in a (stress-free) ferritic matrix: excess nitrogen occurs. A numerical model was applied to predict the nitrogen concentration-depth profile within the nitrided layer. The model describes the dependence on time and temperature of the nitrogen concentration-depth profiles with, as fit parameters, the surface nitrogen concentration, the diffusion coefficient of nitrogen in the matrix, a composition parameter of the formed nitride and the solubility product of the nitride-forming element and dissolved nitrogen in the matrix. Initial values for the surface nitrogen concentration and the composition parameter were determined experimentally with an absorption isotherm and fitted to the measured nitrogen concentration-depth profiles. The results obtained revealed the striking effects of the amount of excess nitrogen and the extent of precipitation hardening on the developing nitrogen concentration-depth profile.     

Nitride precipitation in salt-bath nitrided interstitial-free steel

Nitride precipitation and its effect on microstrain in salt-bath nitrided interstitial-free steel were investigated using transmission electron microscopy and neutron diffraction. As the cooling rate after nitriding decreased, two nitrides, γ′-Fe₄N and α"-Fe₁₆N₂, were identified in diffusion zone. Combined analyses using Rietveld whole-profile fitting and size–strain analysis revealed that the microstrain in the nitrided specimen increased due to nitrogen supersaturation and then decreased after nitride precipitation, whereas the effective particle size continuously decreased. It was found that microstrain is the dominant factor in peak broadening of the nitrided specimen.     

Tailoring the stress-depth profile in thin films; the case of γ’-Fe4N1-x

Homogeneous γ’-Fe₄N_1-x thin films were produced by gas through-nitriding of iron thin films (thickness 800 nm) deposited onto Al₂O₃ substrates by Molecular Beam Epitaxy. The nitriding parameters were chosen such that the nitrogen concentration within the γ’ thin films was considerably lower (x ≈ 0.05) than the stoichiometric value (x = 0). X-ray diffraction stress analysis at constant penetration depths performed after the nitriding step revealed the presence of tensile stress parallel to the surface; the tensile stress was shown to be practically constant over the entire film thickness. For further nitriding treatments, the parameters were adjusted such that nitrogen enrichment occurred near the specimen surface. The depth-dependent nitrogen enrichment could be monitored by evaluating the strain-free lattice parameter of γ’ as a function of X-ray penetration depth and relating it to the nitrogen concentration employing a direct relation between lattice parameter and nitrogen concentration. The small compositional variations led to distinct characteristic stress-depth profiles. The stress changes non-monotonously with depth in the film as could be shown by non-destructive X-ray diffraction stress analysis at constant penetration depths. This work demonstrates that by a specific choice of a first and a subsequent nitriding treatment (employing different nitriding potentials and/or different temperatures for both treatments) controlled development of residual stress profiles is possible in thin iron-nitride surface layers.     

Mikrostrukturelle und röntgenographische Analysen an einem plasmanitrierten Schnellarbeitsstahl

Microstructural analysis of a plasmanitrided tool steel by means of metallography and X‐ray diffraction Nitriding leads to improved tribological and corrosive properties of iron alloy components. In order to study the effect of plasma nitriding parameters on the structure of compound layer and diffusion zone, a systematic variation of process parameters, temperature and process gas atmosphere has been carried out. Metallographic inspection, X‐ray diffraction and Glow Discharge Optical Spectroscopy analysis (GDOES) were used in this investigation. The results clarified that depending on the amount of nitrogen in the gas atmosphere nitrided layers with and without compound layer can be generated in the surface of M2 tool steel for temperatures from 350°C to 500°C. For plasma nitriding in 5 vol.% Nitrogen and 95 vol.% Hydrogen no compact compound layer was formed. The gas mixture of 76 vol.% Nitrogen resulted in compound layer formation for all temperatures from 350°C to 500°C. X‐ray phase analysis indicated an almost 100% ε‐(carbo)nitride phase but the existence of the γ′‐(carbo)nitride could not be excluded completely from the X‐ray phase diagrams. After corrections to account for the nitrogen gradient, high compressive surface residual stresses have been measured in the diffusion zone. They increased with temperature. After a qualitative correction for chemical composition gradients high tensile residual stresses were found probably existing in the ε‐(carbo)nitride phase for the investigated plasma nitrided tool steel samples.     

Investigation of the formation of Al,Fe, N intermetallic phases during Al pack cementation followed by plasma nitriding on plain carbon steel

Plain carbon steels are not suitable for nitriding as they form an extremely brittle case that spalls off readily, and the hardness increment of the diffusion zone is small. In this research, the effect of plasma nitriding time and temperature variation on the microstructure of the pack cemented aluminized plain carbon steel is investigated. All samples were aluminized at 900 °C for 2 h; the aluminized samples were subsequently plasma nitrided at 500 °C, 550 °C and 600 °C for 2.5, 5, 7.5 and 10 h. The phases formed on the sample surface were detected by X-ray diffraction (XRD). The cross section and samples surface were investigated by optical and scanning electron microscopy (SEM). Microhardness test was conducted to determine hardness change from the surface to the sample core. Results showed that by aluminizing the steel, Fe₃Al phases as well as Fe–Al solid solution were formed on the surface and some aluminum rich precipitates were formed in solid solution grain boundaries. Plasma nitriding of the aluminized layer caused the formation of aluminum and iron nitride (AlN, Fe₄N) on the sample surface. Consequently, surface hardness was improved up to about eight times. By increasing the nitriding temperature and time, aluminum-rich precipitates dissociated. Moreover, due to the diffusion of nitrogen through aluminized region during ion nitriding, iron and aluminum nitrides were formed in aluminized grain boundaries. Increasing nitriding time and temperature lead to the growth of these nitrides in the grain boundaries of the substrate. This phenomenon results in the increment of sample hardness depth. Plasma nitriding of aluminized sample in low pressure chamber with nitrogen and hydrogen gas mixture reduced surface aluminum oxides which were formed in aluminizing stage.     

Nitriding behaviour in Fe-Al-Mn-Cr-C alloys at 1000–1100°C

Nitridation in alloys with compositions Fe-31 Mn-9Al-0.87C-xCr (x = 0, 3, 6) was investigated. The alloy developed a needle-like nitriding product of AIN in a nitrogen atmosphere at 1000–1100°C. The measured penetration depth and the evaluated nitriding rate increased with temperature and the chromium content in the alloy. A quasi-steady state diffusion model of nitrogen migration through the alloy has been employed to describe the kinetics of nitridation. The activation energy of the nitriding rate was also evaluated.     

Kinetics and wear behaviour of M50NiL steel plasma nitrided at low temperature

The quenched M50NiL steel was plasma nitrided at 460°C for different time to investigate the effects of the duration time on the microstructure, microhardness and wear resistance of the nitrided layers. The results show that the plasma nitrided layer depth increases with increasing nitriding time. The plasma nitrided layer includes only the diffusion layer without compound layer. The main phases in the nitrided surface layer are nitrogen expended α′-Fe and γ′-Fe₄N. The microstructure of the nitrided layer is refined. The wear resistance of the nitrided samples can be improved significantly by plasma nitriding. The sample nitrided for 4?h possesses the highest wear resistance, due to its relatively smooth surface and ultra-fine grains in the nitrided layer.     

Glow discharge optical emission spectroscopy for accurate and well resolved analysis of coatings and thin films

In the last years, glow discharge optical emission spectrometry (GDOES) gained more and more acceptance in the analysis of functional coatings. GDOES thereby represents an interesting alternative to common depth profiling techniques like AES and SIMS, based on its unique combination of high erosion rates and erosion depths, sensitivity, analysis of nonconductive layers and easy quantification even for light elements such as C, N, O and H. Starting with the fundamentals of GDOES, a short overview on new developments in instrument design for accurate and well resolved thin film analyses is presented.The article focuses on the analytical capabilities of glow discharge optical emission spectrometry in the analysis of metallic coatings and thin films. Results illustrating the high depth resolution, confirmation of stoichiometry, the detection of light elements in coatings as well as contamination on the surface or interfaces will be demonstrated by measurements of: a multilayer system Cr/Ti on silicon, interface contamination on silicon during deposition of aluminum, Al₂O₃-nanoparticle containing conversion coatings on zinc for corrosion resistance, Ti₃SiC₂ MAX-phase coatings by pulsed laser deposition and hydrogen detection in a V/Fe multilayer system. The selected examples illustrate that GDOES can be successfully adopted as an analytical tool in the development of new materials and coatings. A discussion of the results as well as of the limitations of GDOES is presented.     

Microstructure and mechanical properties of copper–titanium–nitrogen multiphase layers produced by a duplex treatment on C17200 copper–beryllium alloy

In this study, a copper–titanium–nitrogen multiphase coating was fabricated on the surface of C17200 copper–beryllium alloy by deposition and plasma nitriding in order to improve the surface mechanical properties. The phase composition, microstructure and microhardness profiles of the as-obtained multiphase coating were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and Vickers microhardness measurements, respectively. Pin-on-disk tribometer and SEM equipped with energy dispersive spectrometer (EDS) were applied to measure tribological properties and analyze wear mechanisms involved. The XRD results show that the phase composition changes with nitriding temperature. The Ti₂N layer is replaced by a Cu–Ti intermetallic layer when the nitriding temperature is higher than 700 °C. The Cu/Ti ratio in the multiphase coatings remains at a constant value of 2:1 due to the incorporation of nitrogen atoms. The surface hardness achieves a maximum value of 983 HV at 650 °C, and decreases as the nitriding temperature increases. The increased hardness corresponds to the improved wear resistance and decreased frictional coefficient and the surface hardness is proportional to the wear rates. The wear mechanism depends on the phase composition of the multiphase coatings. With the nitriding temperature increasing, the oxidative wear mechanism changes to adhesive and abrasive mode.     

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.     

Research on new rapid and deep plasma nitriding techniques of AISI 420 martensitic stainless steel

Cyclic plasma oxynitriding and cyclic plasma nitriding catalyzed by rare earth La of AISI 420 martensitic stainless steel were performed and compared with conventional plasma nitriding. The nitrided layers were investigated by means of an optical microscope, microhardness tester, Auger electron spectroscopy (AES), X-ray diffraction (XRD), wear machine, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The results show that the wear resistance of AISI 420 martensitic stainless steel is improved significantly by the two new rapid and deep plasma nitriding techniques. The new techniques increase the surface hardness of the nitrided layers and make the microhardness profiles gentler, which are consistent with the nitrogen concentration depth profiles. Meanwhile, the nitrided effect improves with increasing cycles. It was also found that the optimum phase compositions of nitrided layers with more γ′ phases and less ? phases for long-term service conditions can be obtained by the two new techniques, which is in agreement with the microstructure. In addition, traces of Fe₃O₄ were found in the cyclic plasma oxynitrided sample. Combining the SEM and EDS analysis indicated the existence of La in the nitrided layer of the sample under cyclic plasma nitriding catalyzed by rare earth La.     

Ammonia gas nitriding of Fe-18Cr-9Ni alloy at lower than 823 K

An Fe-18Cr-9Ni alloy, which it had not previously been possible to nitride at temperatures below 873 K, was found to form nitrides in an ammonia gas atmosphere at temperatures as low as 823 K after annealing at low hydrogen pressure at 1473 K. Microstructure and hardness were examined on cross-sections of the nitrided specimens. An internal nitriding layer had formed beneath an external nitriding layer on the specimen surface. Vickers hardness was above 1000 throughout the internal nitriding layer. The nitrides formed at the specimen surface and in the internal nitriding layer were identified using grazing incidence X-ray diffraction and ordinary X-ray diffraction methods, respectively. The external nitriding layer, which was about 6 to 10 m thick, formed on the surface, which consisted of -Fe_2–3N, -Fe₄N, and CrN. Two types of chromium nitride were precipitated by ammonia gas nitriding of the present alloy: CrN in the external nitriding layer and Cr₂N in the internal nitriding layer.     

Ion-nitriding behaviour of Fe-Ti alloys in theα-phase region

The ion-nitriding behaviour of four iron alloys containing between 0.11 and 1.48 wt% titanium was investigated in the-phase region to discuss kinetics of the growth of the nitriding layer. The ion-nitriding experiments have been made at 823 K. Two nitriding layers were observed: a thin surface layer which mainly consists of Fe₄N; an internal nitriding layer beneath the surface layer, where the nitride formed was found to be TiN. The growth of the internal nitriding layer is controlled by a diffusion process of nitrogen in the matrix metal. The apparent diffusion coefficient of nitrogen in the nitriding layer, evaluated using the rate equation proposed for internal oxidation, increases linearly with the volume fraction of titanium nitride. Furthermore, by excluding the effect of the titanium nitride from the apparent diffusion coefficient, the diffusion coefficient of nitrogen in-iron was calculated, being in good agreement with that reported so far. In addition, the increase in hardness in the internal nitriding layer has been discussed.     

Nitriding kinetics of Fe-Al-Mn-Cr-C alloys at 1000° C

A needle-like structure of AlN is observed in Fe-31 Mn-9Al-0.87C-xCr (x=0, 3 and 6) alloys at 1000° C in a nitrogen atmosphere. The reaction front of nitriding moves parabolically with time. The nitriding rate is evaluated on the basis of the penetration depth of the nitriding layer at various lengths of time, and is found to be increased with increasing chromium content in the alloy. A quasi-steady state diffusion model is employed to investigate the nitriding kinetics by nitrogen migration through the alloy matrix. The nitriding rate depends on the solubility of nitrogen as well as the diffusivity of nitrogen in the alloy system. It is argued that the solubility of nitrogen predominates the nitriding, and thus the growth of AlN prevails in the austenitic phase instead of ferrite, due to the higher nitrogen solubility in austenite. In addition, the chromium alloying into the Fe-Al-Mn system increases the lattice parameter, which leads to a higher solubility of nitrogen, and hence a higher nitriding rate is obtained.     

Pearlite in hypoeutectoid iron–nitrogen binary alloys

In this study, hypoeutectoid Fe–N binary specimens have been prepared by gas nitriding pure iron in austenite domain at 840 °C. The slow cooling of these specimens led to the α-ferrite + γ′-Fe₄N pearlitic microstructure which is similar to the pearlite in Fe–C binary system. This pearlitic microstructure has been characterized by electron microscopy. The crystal structure of the γ′-Fe₄N nitride has been identified by electron microdiffraction and the Nishiyama–Wassermann (N–W) and near Kurdjumov–Sachs (K–S) orientation relationships have been found between the α-ferrite and the γ′-Fe₄N.