Low temperature nitriding is an effective method of surface modification of austenitic stainless steels, which allows to improve their surface hardness and tribological properties, but the treatment conditions depend on steel composition. In this research the influence of nitriding conditions on the microstructural, mechanical and corrosion resistance characteristics of the modified surface layers has been evaluated for a Ni-free steel, UNS S29225, and, for comparison, for AISI 316L and AISI 202 steels. The modified surface layers consist mainly of the so called S phase. Ni-free steel is more sensitive to nitride precipitation than AISI 316L, so that treatments should be carried out at temperatures lower than 400 degrees C. By choosing the appropriate nitriding temperature and duration it is possible to obtain a significant increase of corrosion resistance in a 5% NaCl solution, in comparison with the untreated alloy.

The nitriding conditions used for low alloy steels or tool steels cannot be employed for stainless steels, since the treatment temperatures (approx. 500 degrees C or higher) cause the formation of large amounts of chromium nitride, CrN. As a consequence, together with an increase of surface hardness, a marked decrease of corrosion resistance in usually observed, due to the depletion of Cr atoms from the matrix. CrN precipitation can be inhibited with the low temperature nitriding process. For austenitic stainless steels nitriding at temperatures lower than 450 degrees C produces modified surface layers with a double layer structure, having a thicker outer layer, in which a supersaturated solid solution of nitrogen in the expanded and distorted f.c.c. austenite lattice, known as S phase or expanded austenite, is present, and a thinner inner layer, which consists of a solid solution of interstitial atoms (nitrogen, carbon) in austenite. The S phase has a nitrogen content up to about 10 wt. %, and it shows very high hardness (up to 1500 HV) and improved corrosion resistance in chloride-ion containing solutions. Treatment temperature and time are known to influence the characteristics of the modified surface layers, in particular regarding nitride precipitation; however, also treatment pressure plays an important role, especially when the glow-discharge process is employed for nitriding. In the present research the influence of treatment parameters (temperature, pressure, time) on the microstructural, microhardness and corrosion resistance characteristics of low temperature glow-discharge nitrided AISI 304L was studied. Prismatic samples (30x17x3 mm) were cut from an annealed bar (diameter: 60 mm) and then they were ground and polished up to 6-pm diamond suspension. Before the nitriding treatment the samples were heated up 380 degrees C by means of a cathodic sputtering performed at 1.3 mbar with 80 vol. % N-2 and 20 vol. % H-2. After this step temperature and pressure were increased up to their nominal value. Nitriding treatments were carried out at temperatures chosen in the range 400 - 500 degrees C, at pressures in the range 2.5 -10 mbar and for times from 1 to 8 h, using a gas mixture of 80 vol. % N-2 and 20 vol. % H-2. The characteristics of the modified surface layers depend on treatment conditions. The surface of the nitrided samples has an etched appearance, delineating the austenitic microstructure with the characteristic twins; moreover, shear lines are observable within the grains and reliefs are present at grain boundaries (Fig. 1). These features are due to both the sputtering and nitriding processes and local plastic deformations caused by the formation of the modified surface layers, and they are more noticeable as the treatment temperature and time are higher, or the pressure is lower. The modified surface layers have a double layer microstructure (Fig. 2), consisting of a thicker outer layer, in which the S phase is detected, and a thinner inner layer, in which a solid solution of interstitial atoms (nitrogen, carbon) in f.c.c. austenite lattice, gamma(N,C), is present. In the outer layer, the presence of further phases together with S phase depends on treatment conditions (Figs. 3, 4; Table 1). Local plastic deformations cause the formation of a solid solution of nitrogen in h.c.p. martensite, epsilon(N).. Precipitation of chromium, CrN (c.f.c.), and iron-based, epsilon-M2-3 N (hex.), gamma'-M4N (c.f.c.) (M = Fe, Cr, Ni, Mn), nitrides tends to increase as the treatment temperature and time are higher, or the pressure is lower. The thickness of the modified layers as a whole tends to increase as the treatment temperature and time increase, due to larger nitrogen diffusion, or the pressure decreases, since at lower pressures higher discharge voltage and mean free path occur, and they cause the increase of ion and fast neutral energy and thus a more efficient nitriding process (Table 1). All nitrided sample types have higher surface microhardness in comparison with the untreated steel, and the hardness values tend to increase as the modified layers are thicker and the amount of nitride precipitates is larger (Table 1). In the modified layers microhardness values are very high, and then they steeply decrease to matrix values (Fig. 5). The thickness of the hardened layers is in accordance with morphology observations. Corrosion behaviour of untreated and nitrided samples, tested in a 5 % NaCl aerated solution using the potentiodynamic method, is typical of passive materials subjected to localized corrosion when potential value is higher than a threshold (E-pit) (Fig. 6). All the nitrided sample types have corrosion potential values higher than that of untreated AISI 304L, but the passive potential range and surface damage depend on the thickness of the modified surface layers and the amount of nitride precipitates, which are influenced by treatment conditions (Fig. 7). Thin modified surface layers, as those obtained when nitriding is performed at 400 degrees C for 5 h or at 430 degrees C for 1 h, do not allow a marked increase of c in comparison with that of the untreated steel. Large nitride amounts, as those observed when the treatment temperature is 450 degrees C or higher, or the pressure is 5 mbar or lower, cause a decrease of corrosion resistance. On the other hand, modified surface layers, which have an adequate thickness and a fairly small amount of nitride precipitates, as those of samples nitrided at 430 degrees C, 10 mbar for 5 and 8 h, allow to significantly increase the corrosion resistance in comparison with that of untreated AISI 304L.

ANNOTATJ,ON In the presew investigation the dry rolling-sliding wear rate of the Ti-6Al-4V alloy against itself and the influenc,e of an optimized ion-nitriding treatment were investigated. The ion-nitriding treatment was found t() be effective in red~.cing wear. at the load of 100 N. In fact, as long as the compound layer remained in its place wear was negligible. After the we~ ,°.f the compo.un,q !ayer the wear rate increased but it remained lower _than that displayed by the untreated alloy since it involved the inner diffusion layer. At 200 N the removal of the compound layer started very soon and the w~ar rate was higher than that shown by the untreated alloy fc;>r the whole duration of the test.

Austenitic stainless steels are employed in many industrial fields due to their very good resistance to general corrosion in several environments. However, their use is limited due to the fact that they suffer localised corrosion in specific environments, particularly in chloride-ion rich solutions. Low temperature nitriding treatments can improve both corrosion resistance in chloride-ion containing media and surface hardness, due to the formation of a metastable phase known as expanded austenite or S phase; this phase can be outlined as a supersaturated interstitial solid solution of nitrogen in the expanded and distorted gamma-Fe f.c.c. lattice. We review the main experimental results obtained in our research on low temperature glow-discharge nitriding of austenitic stainless steels. By means of proper treatment parameters, low temperature glow-discharge nitriding is able to markedly improve the corrosion resistance of austenitic stainless steels, such as AISI 316L and AISI 202 in chloride-ion rich solutions in comparison with the untreated alloys.

In the last years an increasing interest has been taken in AISI 200 series austenitic stainless steels for use in industrial applications. In these steels nickel is partly replaced by other austenite stabilising elements, like manganese and nitrogen. However, their chromium content is usually lower than that of the largely used Fe–Cr–Ni based AISI 300 series stainless steels, so that their corrosion resistance may be lower. For AISI 300 series steels low temperature nitriding treatments have been successfully employed to increase their corrosion resistance, due to the formation of modified surface layers consisting of the so called S phase. In this research, a low nickel AISI 202 austenitic stainless steel was subjected to glow-discharge nitriding treatments in the range 653–773K, for 5h, at 10hPa, and the microstructural, mechanical and corrosion resistance characteristics of the nitrided samples were investigated and compared to those of the untreated steel. The treatments produce modified surface layers having a double layer structure. When the treatments are carried out at temperatures up to 673K, the outer modified layer consists mainly of the S phase and a nitrogen induced h.c.p. martensite; in the inner modified layer a solid solution of nitrogen in γ-Fe is present. As the treatment temperature increases, also chromium and iron nitrides are observed in the outer modified layer, and their amount increases as the treatment temperature is higher. High surface hardness values (from 940 to 1750 HK0.025 depending on treatment temperature) have been observed on the treated samples. Corrosion resistance tests, performed in 5% NaCl aerated solution with the potentiodynamic method, show that, with the used treatment parameters, glow-discharge nitriding at temperatures up to 703K allows to increase significantly the corrosion resistance in respect of the untreated alloy, reducing the anodic current values up to about 5 orders of magnitude.

The effects of AISI 316L austenitic stainless steel, tested in untreated state or subjected to glow-discharge nitriding (at 10 or 20 hPa) and nitriding + post-oxidizing treatments, on human umbilical vein endothelial cells (HUVEC) and on peripheral blood mononuclear cells (PBMC) were evaluated. All the treated samples showed a better corrosion resistance in PBS and higher surface hardness in comparison with the untreated alloy. In HUVEC put in contact for 72 h with the sample types, proliferation and apoptosis decreased and increased, respectively, in the presence of the nitrided + post-oxidized samples, while only slight differences in cytokine (TNF-alpha, IL-6, and TGF-beta1) release were registered. Intercellular adhesion molecule-1 (ICAM-1) increased in HUVEC incubated with all the treated samples, while vascular cell adhesion molecule-1 (VCAM-1) and E-selectin increased in the presence of all the sample types. PBMC incubated for 48 h with the samples showed a decrease in proliferation and an increase in apoptosis in the presence of the untreated samples and the nitrided + post-oxidized ones. All the sample types induced a remarkable increase in TNF-alpha and IL-6 release in PBMC culture medium, while only the untreated sample and the nitrided at 10 hPa induced an increase in ICAM-1 expression. In HUVEC cocultured with PBMC, previously put in contact with the treated AISI 316L samples, increased levels of ICAM-1 were detected. In HUVEC coincubated with the culture medium of PBMC, previously put in contact with the samples under study, a noteworthy increase in ICAM-1, VCAM-1, and E-selectin levels was always registered, with the exception of VCAM-1, which was not affected by the untreated sample. In conclusion, even if the treated samples do not show a marked increase in biocompatibility in comparison with the untreated alloy, their higher corrosion resistance may suggest a better performance as the contact with physiological environment becomes longer.

Il trattamento di nitrurazione in scarica ionica puo modificare la durezza, la resistenza a usura e a corrosione degli acciai inossidabili austenitici. Le caratteristiche degli strati superficiali modificati dipendono dai parametri di processo, in particolare dalla temperatura di trattamento. Nel presente lavoro viene studiata la variazione della resistenza ad usura dell’acciaio inossidabile austenitico AISI 316L in funzione della temperatura di nitrurazione quando questa e variata nell’intervallo da 400 a 500 °C. I test di usura sono stati effettuati per mezzo di un tribometro in configurazione block-on-ring. I campioni nitrurati alle temperature piu elevate mostrano un notevole incremento della resistenza all’usura rispetto al materiale tal quale grazie alla formazione di strati modificati contenenti nitruri dotati di elevata durezza. Tali campioni sono tuttavia notoriamente poco resistenti alla corrosione a causa dei fenomeni di sensibilizzazione causati dal trattamento. I campioni nitrurati a temperature piu basse mostrano strati modificati composti essenzialmente da fase S, che presenta un’elevata resistenza alla corrosione. Tali campioni, fino a che lo strato modificato e integro, presentano una resistenza ad usura comparabile a quella dei campioni nitrurati a temperature piu elevate. La nitrurazione in scarica ionica a bassa temperatura consente di incrementare la durezza superficiale, la resistenza a usura ed a corrosione degli acciai inossidabili austenitici.

The reactive plasma spraying (RPS) of titanium powders in a nitrogen containing plasma gas produces thick coatings characterised by microdispersed titanium nitride phases in a titanium matrix. To increase the cohesion between the coating particles and decrease the coating porosity, an air oxidation is proposed as a post-coating treatment, and its influence on the microstructure, sliding wear resistance and corrosion resistance of Ti–TiN RPS coatings obtained on carbon steel substrates is studied. The oxidising treatment, carried out at 973 K for 2 h, promotes both the growth of titanium oxide on the coating pore walls, allowing a partial pore sealing, and interdiffusion processes, which increase the coating internal cohesion. After the post-coating treatment the average microhardness of the coating increases from about 650 to 800 HK0.2. Wear tests, performed in dry sliding conditions, show that for the sprayed and oxidised samples the wear volumes are lower, particularly at low sliding velocities (0.4, 0.8 m s−1), in comparison with the as-sprayed samples. Moreover, corrosion resistance tests, carried out in 0.5 M NaCl and 0.1 M HCl deareated solutions, show that a reduction of the anodic current values is achieved for the sprayed and oxidised samples. This simple post-coating treatment is effective in improving the wear and corrosion protection of Ti–TiN RPS coatings on carbon steel substrates.

Glow-discharge nitriding treatments can modify the hardness and the corrosion resistance properties of austenitic stainless steels. The modified layer characteristics mainly depend on the treatment temperature. In the present paper the results relative to glow-discharge nitriding treatments carried out on AISI 316L austenitic stainless steel samples at temperatures ranging from 673 to 773K are reported. Treated and untreated samples were characterized by means of microstructural and morphological analysis, surface microhardness measurements and corrosion tests in NaCl solutions. The electrochemical characterization was carried out by means of linear polarizations, free corrosion potential–time curves and prolonged crevice corrosion tests. Nitriding treatments performed at higher temperatures (>723K) can largely increase the surface hardness of AISI 316L stainless steel samples, but decrease the corrosion resistance properties due to the CrN precipitation. Nevertheless nitriding treatments performed at lower temperatures (⩽723K) avoid a large CrN precipitation and allow to produce modified layers essentially composed by a nitrogen super-saturated austenitic metastable phase (S-phase) that shows high hardness and very high pitting and crevice corrosion resistance; at the same polarization potentials the anodic current density values are reduced up to three orders of magnitude in comparison with untreated samples and no crevice corrosion event can be detected after 60 days of immersion in 10% NaCl solution at 328K.

The vacuum plasma spray (VPS) technique is a useful tool for designing the characteristics of the coatings and, thus, the tribological properties of coated components. In the present paper, the wear properties of iron boride coatings produced by means of VPS technique on AISI 1040 steel samples were evaluated as a function of their microstructural characteristics. One coating type was obtained by using Fe2B pure powder, the other with differentiated FeB+α-Fe blends, with the FeB content increasing and α-Fe content decreasing from the matrix to the surface. Wear tests were performed by means of a tribometer in block-on-ring configuration, without lubricant and in air, by using 40- and 60-N coupling loads and 0.8- and 1.6-ms−1 sliding velocities. On Fe2B coated samples, wear is essentially oxidative until the failure of the coating, the fragments of which cause a third body abrasion. On the FeB+α-Fe coated samples the wear mechanism is mainly oxidative and the coating totally wears out without spalling as a consequence of its graded structure, which succeeds in both improving the adhesion of the coating to the substrate and reducing the residual stress at the coating–substrate interface.

The reactive plasma spraying (RPS) of titanium powders in a nitrogen containing plasma gas produces thick coatings characterised by microdispersed titanium nitride phases in a titanium matrix. In this paper, the wear resistance properties of Ti–TiN coatings deposited on carbon steel substrates by means of RPS technique are studied. Wear tests were performed in block-on-ring configuration and dry sliding conditions, at different applied loads (45 and 100N) and sliding velocities (in the range 0.4–2.0ms−1) by using hardened and stress relieved AISI O2 disks as counterpart. At low applied load the wear volumes are low, and tend to slightly increase as the sliding velocity increases. At high applied load and low sliding velocities the highest wear volumes for the coated samples are observed, due to adhesion in the contact area with the tool steel counterpart and decohesion of coating particles. As the sliding velocity is increased, the wear volume of the coated samples tends to decrease owing to oxidation phenomena.

The influence of treatment pressure on the characteristics of the modified surface layers produced by low-temperature d.c. glow discharge nitriding on AISI 316L austenitic stainless steel samples is investigated. Glow discharge nitriding treatments were performed at 703 K for 5 h at working pressures in the range of 1.5–20 hPa. Morphology and microstructure of the untreated and nitrided samples were studied by means of microscopy techniques, energy dispersion spectroscopy and X-ray diffraction analysis; microhardness measurements and corrosion resistance tests were also performed. The nitriding treatments produce a hardened surface layer consisting mainly of the so-called S phase. The presence of nitrides and the thickness of the modified layer depend on the used treatment pressure. When treatments are performed at 2.5 hPa, a fairly large amount of nitrides is observed in the modified layer, while when the nitriding pressure is lower or higher than 2.5 hPa, the nitride amount decreases and the layer becomes thinner. When the treatments are performed at 10 or 20 hPa, only a very small amount of chromium nitride is present as small surface precipitates. Metallographic analysis shows that many slip lines are present both at the surface and in the cross-section of the modified layer, presumably due to high stresses occurring during the formation of the layer. X-ray diffraction analysis of the S phase shows that its diffraction peaks are shifted from those of a perfect f.c.c. lattice; the observed shifts may be explained assuming that the S phase has an f.c.c. structure with a high density of stacking faults. Corrosion resistance tests, performed in 5% NaCl aerated solution with the potentiodynamic method, show that with the used treatment parameters nitriding at a pressure of 10 hPa or higher allows to obtain a significant improvement of the corrosion resistance in respect of the untreated alloy, reducing the anodic currents up to about 4 orders of magnitude.

Among the titanium alloys employed as implant materials, the Ti–6Al–4V alloy is still widely used. Ti–6Al–4V titanium alloy samples, in untreated state and subjected to treatments in air by furnace or glow-discharge processes, were put in contact with human umbilical vein endothelial cells (HUVEC) in order to evaluate their effects on biocompatibility. In HUVEC kept for 48 h in the presence of the three sample types neither cell proliferation nor protein content nor lactate dehydrogenase release in the culture medium are affected, while apoptosis is induced after 48- and 96-h contact of the cells with the untreated sample type, and after 96-h contact with the plasma treated one, the furnace treated sample type being ineffective. The expression of two adhesion molecules, intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) was also studied. The incubation of HUVEC with the three sample types for 48 or 96 h induces a significant increase in ICAM-1 protein levels, in comparison with control cells, while VCAM-1 expression is not detectable. In the same way, TNF-α release in the culture medium, assayed after 48- and 96-h contact of the cells with the three sample types, is significantly higher, in comparison with control, even if the highest values are registered in the presence of the untreated samples. Taken together, these data indicate that, although Ti–6Al–4V alloy samples, and in particular the treated ones, show a good biocompatibility, attention must be given to the first signs of inflammation.

Post-oxidising treatments can be performed to improve the corrosion resistance of nitrided steel components. The glow-discharge process appears particularly attractive, since it allows to carry out both nitriding and post-oxidising treatments in a single technological operation, simply varying the treatment atmosphere and the working conditions. In the present paper the effects of using air as the treatment atmosphere of glow-discharge post-oxidising treatment have been evaluated. Nitriding treatments were performed at 773 K for 5 h, and post-oxidising treatments were carried out at 623 and 773 K for 1 h. The nitriding and nitriding+post-oxidising treatments produce modified surface layers consisting of an outer compound layer and an inner diffusion layer. When the post-oxidising treatments are performed, the compound layer consists of an oxide layer on the top of the nitride layer. With a 623 K post-oxidising treatment the oxide layer consists mainly of magnetite, Fe3O4, while when post-oxidising is carried out at 773 K it consists essentially of hematite, Fe2O3. In the inner nitride layer iron (γ′-Fe4N, ε-Fe2–3N) and chromium (CrN) nitrides are present. In all the treated samples high hardness values are obtained in the modified layers. The hardness profiles of the post-oxidised samples are only slightly affected by the 773 K post-oxidising treatment. The corrosion resistance of the treated samples was tested and the results show that the nitriding+623 K post-oxidising treatment is able to significantly improve the corrosion resistance of the untreated and treated AISI H11 samples.

The surface hardening of sintered stainless steel components is a major goal, in order to extend the applications of these materials. The aim of the present study was to evaluate the influence of the treatment atmosphere on the microstructure and mechanical properties of AISI 316L austenitic and AISI 410 martensitic sintered stainless steels by performing glow-discharge nitriding and nitrocarburizing treatments. The treatments, performed at 773 K for 8 h, produced modified surface layers. The use of a nitrocarburizing atmosphere is able to promote the formation of epsilon carbonitride [Fe2-3(N, C)], especially on AISI 410 martensitic samples. The microhardness profiles of the treated samples show high hardness values in the modified layers and a steep decrease to matrix values. The AISI 316L samples have thinner hardened layers and lower hardness values, in comparison with AISI 410 ones. Thinner hardened layers with a steeper decrease to matrix values are obtained using a nitrocarburizing atmosphere, if compared with those of nitrided samples. On the modified layers of both the nitrided and nitrocarburized AISI 316L samples the metastable S phase is detected. The X-ray diffraction analysis has shown that its lattice parameter values, evaluated by fitting it with a face centred tetragonal lattice, are influenced by the treatment atmosphere and that the amount of this phase increases when a nitrocarburizing atmosphere is used. (C) 2002 Elsevier Science B.V All rights reserved.

The aim of this work is the study of the corrosive behaviour of glow discharge nitrided Ti-6Al-4V alloy, using electrochemical techniques.Potentiodynamic polarisation and electrochemical impedance measurement show the excellent corrosive resistance of titanium alloy after thermochemical treatment, as well as different behaviour between the alloy before and after the nitriding treatment.Also, in very hostile environments, such as 5 wt.% HCl, where the titanium alloy is heavily corroded, the nitrided samples show good resistance. Nevertheless, 750 degreesC nitrided samples have worse behaviour than those treated at 900 degreesC. This is probably due to the lower corrosion resistance of the nitride epsilon present in the nitrided layer created at the lower temperature than the nitride 8, and the limited thickness of the modified layer obtained at this temperature. (C) 2002 Elsevier Science Ltd. All rights reserved.