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The purpose of this paper is to present the results of a study on friction characteristics of plasma, salt‐bath and gas nitrided layers produced in AISI 304 type austenitic and AISI 420 type martensitic stainless steels.

Plasma nitriding processes were carried out with DC‐pulsed plasma in 80% N₂+20% H₂ atmosphere at 450°C and 520°C for 8 h at a pressure of 2 mbar. Salt‐bath nitriding was performed in a cyanide‐cyanate salt‐bath at 570°C for 1.5 h. Gas nitriding was also conducted in NH₃ and CO₂ atmosphere at 570°C for 13 h. Characterization of all nitrided samples has been carried out by means of microstructure, microhardness, surface roughness measurement and friction coefficient. The morphologies of the worn surfaces of the nitrided samples were also observed using a scanning electron microscope. Friction characteristics of the nitrided samples have been investigated using a ball‐on‐disc friction and wear tester with a WC‐Co ball as the counterface under dry sliding conditions.

The plasma nitrided and salt‐bath nitrided layers on the 420 steel surfaces were much thicker than on the 304 steel surfaces. However, there was no obvious and homogeneous nitrided layer on the gas nitrided samples' surface. The plasma and salt‐bath nitriding techniques significantly increased the surface hardness of the 304 and 420 samples. The highest surface hardness of the 304 nitrided samples was obtained by the plasma nitrided technique at 520°C. On the other hand, the highest surface hardness of the 420 nitrided layers was observed in the 450°C plasma nitrided layer. Experimental friction test results showed that the salt‐bath and 450°C plasma nitrided layers were more effective in reducing the friction coefficient of the 304 and 420 stainless steels, respectively.

The relatively poor hardness and hence wear resistance of austenitic and martensitic stainless steels needs to be improved. Friction characteristic is a key property of performance for various applications of austenitic and martensitic stainless steels. This work has reported a comparison of friction characteristics of austenitic 304 and martensitic 420 stainless steels, modified using plasma, salt‐bath and gas nitriding processes. The paper is of significances for improving friction characteristics, indirectly wear performances, of austenitic and martensitic stainless steels.

The purpose of this paper is to consider the corrosion properties of laser nitrided Ti‐6Al‐4V alloys that have been reported previously by several researchers.

Different kinds of surface nitriding methods of titanium alloys, such as plasma nitriding, ion nitriding, gas and laser nitriding, are introduced. Microstructure changes, such as phase formation and the influence of laser processing parameters in laser nitriding layers of Ti‐6Al‐4V alloys, were investigated using scanning electron microscope, transmission electron microscope, X‐ray photo‐electron spectroscopy, and X‐ray diffraction. Based on investigations presented in the literature, the effect of laser nitriding on the corrosion behavior of Ti‐6Al‐4V alloy was reviewed.

By regulating the laser processing parameter, the microstructure of the nitrided layer can be controlled to optimize corrosion properties. This layer improves corrosion behavior in most environments, due to the formation of a continuous TiNxOy passive film, which can retard the ingress of corrosive ions into the substrate and can maintain a constant value of a current density. Therefore, the laser gas nitrided specimens have a relatively noble corrosion potential and a very small corrosion current, as compared to untreated specimens.

This paper comprises a critical review, and its collection of references is useful. It summarizes current knowledge in laser surface treatment research.

The laser nitriding process is involved with high temperature heating and high cooling rates. This, in turn, results in high levels of thermal stresses in the heated region. Moreover, the residual stress in the heated region remains high after the completion of the heating process, which limits the application of the laser nitriding process. The purpose of this paper is to investigate thermal stresses development and residual stress levels in the nitrided region.

The microstructural changes and residual stress development in the laser gas‐assisted nitrided zone are examined. Finite element modeling is carried out to predict temperature and stress fields in the laser nitrided layer. The indentation tests and X‐ray diffraction (XRD) technique are used to determine the residual stress levels while previously derived analytical formula is used to predict the residual stress levels in the nitrided region.

The residual stress predicted attains values within 230 MPa, which remains almost uniform in the nitrided layer, except in the surface region. In this case, residual stress reduces slightly due to the low temperature gradient developed in this region and the unconstrained expansion of the free surface. When comparing the residual stress predicted with the measurement results as obtained from the XRD technique as well as the indentation tests, all the results are in reasonably good agreement. The small discrepancies between the experimental data and predictions are attributed to the assumptions made in the model study and the measurement errors.

The depth of nitrided layer is limited 60 μm. This limits the applicability of the coating for high wearing rates.

The nitrided surface improves the surface properties of steel, which can be used in industry more efficiently.

The paper describes an original model study on stress formation, an experiment for surface characterization and estimation of residual stress formation and contains new findings.

This paper aims to investigate the structure and scratch resistance properties of gas nitrided pure iron samples.

The effects of material strain hardening and amount of grain boundaries exposed on nitriding surface were evaluated by cold rolling the starting samples to different reduction levels before gas nitriding.

The study finds that nitriding without any prior cold rolling produced a comparatively wide compound layer with a large fraction of porous zone featuring low scratch hardness values but no evidence of damage. On the contrary, cold rolling before nitriding led to a more irregular and thinner compound layer with reduced amount of porous zone and much finer nitrides in the diffusion zone. Scratch hardness was increased but failure mechanism changed by generation of conformal cracks within the track groove and the appearance of discontinuous spallation at high loads.

One of the issues of great industrial importance concerning nitriding of steels is the need to predict the extent of the nitrided layer in products showing small variations in microstructure or in extent of cold working due to complex manufacturing cycles. Despite the practical importance, relatively little information is available in literature about these issues. The present paper is therefore aimed at investigating the structure and mechanical properties of pure iron samples, gas nitrided with different amounts of cold working and microstructural conditions.

This paper aims to investigate the tribology, corrosion resistance and biocompatibility of high-temperature gas-nitrided Ti6Al4V alloy.

The tribological properties were studied by reciprocating wear tester. The corrosion resistance was evaluated by using potentiodynamic polarization test. The purified mouse leukaemic monocyte macrophage cells are used to investigate the biocompatibility.

The results show that the nitriding treatment leads to a significant improvement in the hardness and tribological properties of Ti6Al4V alloy. Specifically, compared to untreated Ti6Al4V, the hardness increases from 3.24 to 9.02 GPa, while the wear rate reduces by 12.5 times in sliding against a Ti6Al4V cylinder and 19.6 times in sliding against a Si3N4 ball. Furthermore, the nitriding treatment yields an improved corrosion resistance and a biocompatibility similar to that of untreated Ti6Al4V.

The nitrided Ti6Al4V alloy is an ideal material for the fabrication of load-bearing artificial implants.

The purpose of this paper is to investigate the morphological and metallurgical changes of laser gas‐assisted nitriding of titanium implants.

Laser gas‐assisted nitriding of titanium implant is carried out and the metallurgical as well as the morphological changes in the nitride layer are examined using optical microscopy, SEM, XRD, and X‐ray photoelectron spectroscopy. Temperature and thermal stress fields are computed during the laser heating process adopting the finite element method. The residual stress formed in the nitride layer is measured using the XRD technique while micro‐indentation tests are carried out to determine the fracture toughness of the surface after the laser treatment process.

It is found that nitride depth layer extends to 40 μm below the surface and it is free from the cracks and micro‐voids. The residual stress formed on the surface region is higher than at some depth below the surface in the nitride layer, provided that the maximum residual stress is less than the elastic limit of the substrate material.

The paper contains original findings and the findings are not submitted any other journal for publication.

The purpose of this study is to select a proper surface treatment to enhance wear resistance of engine camshafts. The camshaft is a relevant part of a diesel engine which works under torsion, fatigue and wear efforts. They are usually manufactured by casting, forging or machining from forged bar of low alloy steels, and in most cases, the machined surfaces are quenched and tempered by induction heating. After that, in many cases, to withstand the efforts imposed on the active surfaces and improve tribology and fatigue properties, the industry used for decades, thermochemical technologies such as salt bath or gaseous nitriding and nitrocarburizing processes.

This paper studied the effects of plasma nitriding and plasma nitrocarburizing, on the tribological behaviour of the steel SAE 1045HM3 proposed to produce camshafts. After the plasma treatments, the change in surface roughness was measured; the modified layers were studied by X-ray techniques and its thickness by optical microscopy. The diffusion zone was evaluated by Vickers microhardness determinations. Tribology tests were performed by pin-on-disc configuration using WC ball as a counterpart.

Results show that plasma nitrided samples present the best tribological behaviour compared with the nitrocarburized ones; also, the influence of the roughness produced by the thermochemical processes appears to be important.

Although both the plasma treatments have been applied for many years, and also reported separately in the scientific literature, there was no information comparing these two treatments for carbon steels, and also, there is not much about tribology in lubricated conditions of nitrided and nitrocarburized carbon steels. In fact, it is not proved that the porosity of the nitrocarburized layer is beneficial for wear resistance in lubricated conditions. In this paper, it was proved that at least in the tested conditions, it is not.

Gas or plasma nitrocarburizing is usually recommended for this kind of applications, although the modified layer is porous. This paper attempts to prove that nitriding could be better than nitrocarburizing, even with a thinner white layer.

This paper aims to investigate the effects of plasma nitriding and Ti-C:H coating deposition on AISI 316L and to find the best tribological performance of various specimens.

An experimental investigation is performed into the effects of plasma nitriding and Ti-C:H sputtering on the tribological properties of AISI 316L biomedical stainless steel. Five samples are prepared, namely, original AISI 316L stainless steel (code: 316L), nitrided 316L (code: N316), 316L and N316 sputtered with Ti-C:H (codes: D316 and DN316, respectively) and polished N316 sputtered with Ti-C:H (DN316s). The microstructure, mechanical properties and coating adhesion strength of the various samples are investigated and compared. The tribological properties of the samples are then evaluated by means of reciprocating wear tests performed in 8.9 Wt.% NaCl solution against three different counterbodies, namely, a 316L ball, Ti₆Al₄V ball and Si₃N₄ ball.

It is shown that plasma nitriding followed by Ti-C:H deposition (DN316s) improves the tribological properties of AISI 316L; the sample provides the best tribological performance of the various specimens and has a wear rate approximately 156 times lower than that of the original 316L substrate.

The results suggest that nitriding followed by polishing and Ti-C:H sputtering provides an effective means of improving the service life of AISI 316L biomedical implants.

It is well known that alloys, based on iron, were exposed to steam oxidation environment producing thick and non-protective oxide scale. More expensive stainless steels contain more Cr and are able to form more protective scales. The purpose of this research was to show ability to employ nitride coating on different alloys (T23, T91, E1250, 347HFG and HR3C) in order to enhance steam oxidation resistance.

The alloys were exposed to steam oxidation rig. Before the test, furnace was purged by nitrogen in order to remove moisture and oxygen. Di-ionised water was pumped from the reservoir using a peristaltic pump into the furnace. System was kept in the closed circle. To reduce solubility of oxygen, di-ionised water was constantly purged by nitrogen. The total exposure time was 2,000 h at 650°C under 1 bar pressure.

Due to the research, it was found that plasma nitriding process is detrimental for the protection of high-temperature structured materials; the high concentration and high activity of Cr produced a CrN phase. This phase is not stable in steam environment and underwent oxidation to Cr₂O₃ and further into volatile phase (CrO₂(OH)₂). Therefore, austenitic steels (E1250, 347HFG and HR3C) coated with nitride coating deposited by plasma nitriding process suffered similar degradation as the uncoated low Cr ferritic steel.

The main limitation of the research conducted in this study was corrosion resistance of the exposed materials.

To the best of the authors' knowledge, this report is the first of its kind to present nitrided alloys (ferritic and austenitic) exposed in steam oxidation.

The aim of this work is the study the tribological behaviour and tribocorrosion resistance of newly developed multilayer PVD coatings Cr/CrN and CrN/ZrCN applied on nitrided F1272 steel for gear applications.

Tribological characterization has been completed by several tribological tests performed under ball‐on‐disc configuration, extreme pressure tests to determine the maximum load before the films failure and rolling‐sliding tests under line‐contact conditions (35‐40 per cent of sliding). The response of the different coatings to sodium chloride aggressive environment has been simulated by accelerated tribocorrosion tests, combining simultaneously chemical and mechanical factors. The synergistic effect of wear on corrosion behaviour and vice versa, has been studied in order to compare the protective properties of the different PVD coatings developed.

Cr/CrN PVD coating improves wear in almost a 90 per cent compared to the nitrided substrate, presenting a similar behaviour to this one under extreme pressure conditions. CrN/ZrCN coating also improved substrate wear and especially good behaviour for this coating was observed under extreme pressure conditions. Cr/CrN coating strongly decreases micropitting and scuffing effect when it is tested under rolling‐sliding configuration. Under micro‐pitting conditions, coating protects the substrate and reduces the fatigue of uncoated discs. When adhesive wear (scuffing) is studied also Cr/CrN improves notable the nitrided steel performance. Under simultaneously corrosion‐wear conditions, Cr/CrN coating registered the lowest material loss because in this case only corrosion effect contributed to the coated surface degradation being the mechanical contribution inappreciable.

New multilayer coatings with improved wear performance and tribocorrosion resistance have been developed and comprehensively characterized. These coatings can be used in advanced gears for corrosive environmental conditions as well as with biodegradable lubricants.