The aim of this work is to study the influence of the cooling rate on the properties of the modified surface layer of AISI 304 steel after rf plasma nitriding.The nitrided samples were characterized by glow discharge optical spectroscopy, x-ray diffraction, optical microscopy, scanning electron microscopy and Vickers microhardness measurements.The results revealed that microstructure, nitriding rate and surface microhardness values were found to be cooling rate dependent.The treated layer is mainly composed of nitrogen expanded austenite (γ N ), iron nitride (γ'-Fe 4 N) and chromium nitride (CrN).A maximum thickness of treated layer (19.9 µm) is achieved for sample treated at medium cooling rate of 900 Cm 3 /min.It has a maximum surface hardness and nitriding rate of 1402 HV0.1 and 0.66 μm 2 /s, respectively.

AISI 304 austenitic substrates with different thicknesses were treated using rf plasma nitriding. This is to study the influence of the temperature gradient on the surface properties of the treated samples including nitrogen diffusivity and microstructure. X-ray diffraction, optical microscopy and scanning electron microscopy were used to characterize the treated samples. The results revealed that the surface temperature and temperature gradient of the nitrided substrate are substrate thickness dependent. It was found that the thickness of nitrided samples increases substantially as the temperature gradient increases from 1.93 x 10 5 to 10x10 5o C/m. The microstructure is characterized mainly by the nitride phases of nitrogen expanded austenite (γN) and chromium nitride (CrN). Furthermore, the iron nitride phase of Fe4N is detected at temperature gradient of 4.1x10 5o C/m and lower. A maximum value of the nitriding rate of 1.21 μm 2 /s have been observed with the temperature gradient of 10x10 5o C/m. Keywords—Temperature gradient, AISI 304 stainless steel, rf plasma nitriding, surface morphology.

Titanium oxynitrides combine the properties of metallic oxides and nitrides. In this presentation, titanium was oxynitrided using inductively coupled RF plasma in a gas mixture containing 80% N2 and 20% O2. The effect of plasma-processing power from 350 up to 550 W on microstructure, mechanical, tribological, wettability and electrochemical properties of the oxynitrided titanium was examined using different characterizations and testing techniques. The results demonstrated the formation of TiO, TiO2 rutile phase, TiNxOy and Ti2N as a result of plasma oxynitriding. The micro-hardness of the oxynitrided layers increases up to 766 HV0.1 as the plasma-processing power increases up to 550 W. The wear and corrosion resistance are improved for oxynitrided titanium in comparison with untreated samples. Moreover, the friction coefficient decreases from nearly 0.75 for the pure titanium to nearly 0.3 for oxynitrided titanium. The obtained data show an increase of surface energy and wettability of titanium oxynitride as the plasma power increases. The formation of hard oxide and oxynitride phases and the transformation of TiO2 from anatase to rutile structure at relatively high temperature are the main reasons for the good physical and electrochemical properties of titanium oxynitride.

Titanium nitrides have good tribo-mechanical and biomedical properties. They are employed to harden and protect cutting and sliding surfaces for industrial purpose and as a non-toxic outer-surface for bio-medical applications. In this study, pure titanium was nitrided using RF plasma technique. The microstructural, mechanical, tribological, electrochemical and biomedical properties of nitrided titanium were investigated. The X-ray diffraction demonstrates the formation of ε-Ti2N and the cubic δ-TiN phases after plasma nitriding. The microhardness of the nitride samples increases as the plasma-processing power increases up to 1300HV0.1. That represents approximately 7-fold increment in the microhardness in comparison with the untreated titanium. High nitriding rate of 0.17μm2/s was recorded for the sample that was treated at 650W. The wear and corrosion resistance are improved after plasma nitriding. Moreover, the friction coefficient is reduced from nearly 0.75 for the untreated titanium to 0.25 for the nitride one. An enhancement in the biocompatibility of the nitrided titanium has been achieved. The number of grown mesenchymal stem cells was higher for nitrided substrates compared to that of the untreated titanium. The improved tribo-mechanical and electrochemical performance of the nitrided titanium can be attributed to the formation of super-hard titanium nitrided phases.

Carbonitriding of AISI 304 austenitic stainless steel was performed at a plasma-processing power of 450 W using inductively coupled radio frequency (rf) plasma in a gas mixture of 50% N2 and 50% C2H2. The rate of carbonitriding, microhardness, phase structure of the compound layer, surface microstructure and cross-section morphology were studied before and after the annealing process. At the annealing temperature up to 800°C, the microhardness values of the compound zones decrease, while the associated values of the diffused zones increase. Little change was found in the thickness of the compound and diffused zones when the carbonitrided samples were annealed up to 400°C. However, at a higher annealing temperature, the thicknesses of both zones increase. The γ-Fe austenite is the main crystalline phase that can be detected by X-ray diffraction. As the annealing temperature increases up to 500°C, X-ray spectra show α-Fe and Fe5C2 phases. Nitrogen diffuses more deeply from the near surface to the interior of the treated sample as the annealing temperature increases up to 800°C and this might explain the extent of carbonitrided thickness and the enhanced microhardness of the diffused zone.

Samples of pure titanium were laser nitrided by continuous wave CO2 laser irradiation in mixtures of nitrogen and argon gas with different ratios. In all cases, TiN formed in the surface. The properties and the characteristics of the processed samples were evaluated using a nanoindentation technique, optical microscopy, surface roughness measurements, x-ray diffraction and wear resistance measurements. It was found that the nitrogen content in the gas atmosphere has a massive effect on the microstructure and the mechanical properties of the laser nitrided samples. For all treated samples, the mechanical properties improve with the nitrogen content in the gas atmosphere. Moreover, the thickest TiN layers with high values of the microhardness and good wear resistance were obtained for the titanium sample that was treated in 80% N-2 and 20% Ar. In addition, the strain and the grain size of the coatings formed at the surface of the laser nitrided titanium samples were determined from x-ray data.

The rf plasma carbonitriding of AISI-304 austenitic stainless steel was examined as a function of plasma time. The properties of the carbonitrided layer were determined using optical microscopy, scanning electron microscopy, X-ray diffraction and microhardness testing. For the sample treated for 10 min, the rate of carbonitriding was calculated to be ∼0.5 μm2/s. The surface hardness of the carbonitrided layer processed for 10 min was 1715 Hv, 0.1 as compared to 228 Hv, 0.1 for the untreated surface. The high rate of carbonitriding and the excellent microhardness of the compound layer have been explained in terms of nitrogen and carbon concentration gradients, microcracks formed in the surface during the plasma process and temperature gradients.

The present work reports on the effect of input plasma processing power in the range of 350–650W on the microstructure and mechanical properties of plasma nitrided Ti. The plasma processing time was 20min and a gas mixture of 15% C2H2 and 85% N2 was used. The characteristics of the carbonitrided layer have been investigated by microhardness measurements, surface roughness measurements, optical microscopy, and X-ray diffraction. The measured surface hardness values of the compound layer shows a maximum of 2050HV0.1 for the sample treated at a plasma power of 550W. The thickness of the carbonitrided layer continuously increases as the plasma power increases. Moreover, the highest carbonitriding rate of 3.52μm2/s was observed when the input plasma power was adjusted at 600W. This high carbonitriding rate of treated titanium samples is ascribed to the high concentration of active carbon and nitrogen species in the plasma atmosphere and the formed microcracks in the near surface of the sample during the plasma processing.

The Ti–6Al–4V alloy was treated by inductively coupled rf plasma nitriding. The effects of plasma-processing time in the range of 5–35 min on the microstructure and the mechanical properties of the plasma-nitrided Ti–6Al–4V samples were studied. The plasma power input was adjusted at 450 W and pure N2 gas was introduced to establish a treatment pressure of 8.0–8.4 × 10−2 mbar. The characteristics of the nitrided layers have been investigated by microhardness testing, surface roughness measurements, optical microscopy, and x-ray diffraction. The results show that the surface microhardness increases as the plasma-processing time increases to reach 2000 HV0.1 at a plasma-processing time of 35 min. A high nitriding rate of 2.81 µm2 s−1 at a plasma-processing time of 25 min was achieved. The formation of the hard phases TiN, Ti2N, and Ti(N) in the Ti–6Al–4V surfaces are found to be the reason for the increased microhardness. Surface energy, yield strength and Young’s modulus for the nitrided Ti–6Al–4V alloy were calculated from the Vickers microhardness data.

Inductively coupled radio frequency plasma surface treatment was applied to commercially pure titanium sheets. The goal was to increase the efficiency of the carbonitriding process, i.e., to decrease the plasma treatment time to a few tens of minutes instead of several hours. The effects of different plasma-processing times on the microstructure and mechanical properties of plasma-carbonitrided Ti were examined. The characteristics of the carbonitrided layer were investigated by microhardness testing, surface roughness measurements, optical microscopy and X-ray diffraction. The surface microhardness and the thickness of the compound layer of carbonitrided Ti increase with the plasma-processing time. Surface energy, yield strength and Young’s modulus for carbonitrided titanium were calculated from the Vickers microhardness data. An attempt was made to interpret the high carbonitriding rate and the high microhardness values of carbonitrided titanium with respect to previously published results.

In this work, duplex treatment has been carried out using radio frequency (rf) plasma nitriding process and direct current (dc) magnetron sputtering of titanium. Nitriding of AISI 304 stainless steel, using rf plasma technique, created a thick modified layer of approximately 20 mu m for short plasma processing time of 10 min. After nitriding process, a thin titanium nitride film has been deposited using de magnetron sputtering of titanium for different nitrogen/argon gas pressure ratios. The treated samples were characterized via glow discharge optical spectroscopy, X-ray diffractometry, scanning electron microscopy, profile meter and Vickers microhardness tester. The elemental composition, thickness and microhardness values of the duplex treated layers are found to be gas composition dependent. The data shows that the microhardness of the duplex treated layer increases to 1.42 fold relative to the associate value of the nitrided one. Moreover, high deposition rate of 110 nm/min is obtained. (c) 2007 Elsevier B.V. All rights reserved.

Nitriding of AISI 304 austenitic stainless steel was prepared using inductively coupled radio frequency (ICRF) plasma with different H2/N2 ratios. Thickness of nitriding layer, microhardness, and structural phases of the compound layer, surface microstructure and cross-section morphology were studied before and after annealing treatment. Little change is found in the microhardness values and thickness of the compound layer when the treated samples are annealed up to 400°C. With further increase in the annealing temperature, the microhardness values decrease while the thicknesses increase. Due to the low temperature plasma processing for the 50% N2+50% H2 sample, post-annealing over 500°C has enormous influence on the structural phases of the sample. However, the 25% N2+75% H2 sample shows little variation on the structural phases due to annealing process.

Austenitic stainless steel AISI 304 has been nitrided by radio frequency (rf) plasma containing various nitrogen–hydrogen gas mixtures, in order to study the effect of hydrogen on structure and magnetic properties of the formed compound layer. The thermal temperature has been measured at the vicinity of the samples. The compound layer thus produced has been characterized using, X-ray diffractometer and vibration sample magnetometer. Providing the total pressure of nitrogen and hydrogen is held constant, the addition of hydrogen up to 50% gives new structural phases. The magnetization values of the plasma treated samples are strongly dependent on the percentage of H2 in the gas phase. An excessive amount of hydrogen (∼75%) on the other hand, retards the nitriding process. The surface temperature of the sample and plasma condition is crucial factors for nitriding process.

Radio frequency (RF) plasma nitriding using different input plasma processing powers (250–600 W) improves the surface of titanium by forming hard phases of TiN, Ti2N, and Ti (N) into the surface. The characteristics of the compound layer have been investigated by optical microscopy, microhardness measurements, and X-ray diffraction. The effect of plasma power on the sample temperature, electron temperature, and plasma density was studied using Langmuir double probe. The measured surface hardness value of the compound layer is 2190 HV 0.1 for treated sample at plasma power 500 W. The compound thickness continuously increases as the plasma power increases. The highest nitriding rate of 5.88 μm2/s was recorded when the input plasma power was adjusted at 550 W. This high nitriding rate of treated titanium samples is ascribed to the high concentration of active nitrogen species in the plasma atmosphere and the formed microcracks near to the surface of the sample during the plasma processing. We have proposed that at low input plasma power (low temperature) the interstitial diffusion is the main mechanism. However, vacancy-controlled diffusion for high input plasma power (high temperature) is probably the one needed to surmount the energy barrier.

Plasma immersion ion implantation (PIII) has been employed for nitrocarburizing 304 stainless steel. The sample was treated at relatively low gas pressure of nitrogen and acetylene in the range of 10−3 mbar. The microstructure variation, the austenite lattices spacing and the phase transformations were studied in-situ during heating up to 800 °C and after cooling, using synchrotron X-ray diffraction. Glow discharge optical spectroscopy (GDOS), optical microscopy and hardness profile measurements have been used before and after thermal treatment to analyse the nitrocarburized layer.

A systematic study was undertaken with surface of titanium nitrided by Rf inductively coupled plasma. The continuous plasma processing time was changed from only 3 to 40min in steps of 5min or less. The other plasma parameters were fixed. The effect of cyclic plasma processing time on titanium sample was investigated too. The single period of plasma processing was adjusted to be 25min and the nitriding process was repeated twice, triplet, and quadruple. X-ray diffraction (XRD), optical micrograph (OM), and Vickers microhardness (HV) were employed as analytical techniques. The results clearly show that the surface hardness and nitriding rate increase incessantly as the continuous plasma processing time increases to reach the maximum values of 2150 HV0.1 at 40min and 3.38μm2/s at 30min, respectively. This behavior may be attributed to formation of new hard phases α-Ti(N), δ-TiN, and ε-Ti2N in the treated layer. In this interval, the active nitrogen species penetrate faster through surface grain boundaries and through the formed surface microcraks of titanium sample. For relatively long nitriding time more than 30min of continuous plasma processing, the rate of nitriding decreases. The formed nitrided phases might block the previously formed microcracks in the treated layer. Therefore, the penetration of nitrogen species rate through these microcracks decreases and the rate of nitriding process decreases consequently. Using cyclic nitriding method yielded an increase in the microhardness of titanium sample to be 2650 HV0.1 with relatively high rate of nitriding. These investigations show that the short Rf plasma processing time treatment and cyclic nitriding method play an important role for producing hard surface titanium with high rate of nitriding.

The depth dependence of elemental composition, phase distribution, and cross-sectional morphology of rf plasma nitrocarburized 304 austenitic stainless steel were investigated using glow discharge optical spectroscopy (GDOS), grazing incidence X-ray diffraction (GIXRD), and optical microscopy, respectively. A step-wise mechanical polishing method was used to remove successive sublayers of the treated surface. It was found that the properties of the nitrocarburized layer depend critically on the plasma gas composition, which controls the supersaturation of nitrogen and carbon through the compound layer depth. Iron nitride phases and/or nitrogen-expanded austenite (γN) were detected in the nitrocarburized layer prepared at high plasma nitrogen (N2) content. In the compound layer processed at high plasma carbon (C2H2) content, besides the carbon-expanded austenite phase (γC), carbide phases were found predominantly in the top-layer, in which the carbon concentration has a maximum value of ∼2 wt.%. The lattice expansion of the expanded austenite phases changes with sampling depth, depending on local variations in nitrogen and carbon content. The applied rf plasma processing power influences significantly nitrogen and carbon distribution in the treated sublayers.

Plasma immersion ion implantation (PIII) has been used to modify the surface properties of 304 austenitic stainless steel (AISI). The influence of working gas pressure, 0.2–1.0 Pa, and substrate temperature, 300–500 °C, on the microstructure, treating rate, nitrogen/carbon concentration depth profile, and surface microhardness was investigated. A gas composition of 25% C2H2, 75% N2, r.f. plasma power input of 350 W, and a negatively biased potential of 30 kV were fixed during the experiment. The experimental results show that the substrate temperature and the diffusion process of nitrogen and carbon depend on the gas pressure inside the plasma chamber. The thickness of the modified layer has been found to be more than 30 μm for samples were treated in the plasma for 60 min. The results show also that the values of diffusion coefficient and surface microhardness of the treated samples are high to be 3.4 × 10−1 μm2/s and 1880 kg/mm2, respectively.

Adding C2H2 gas to nitrogen gas during r.f. plasma processing has significant influence on the properties of 304 austenitic stainless steel. The elemental depth profiles of the compound layer were investigated by glow discharge optical spectroscopy. Different treatment gas compositions of N2/C2H2 mixtures (0–100% in steps of 10 or 5% in some cases) have been investigated. The total gas pressure during the plasma processing was 8.4×10−2 mbar. The sample temperature was approximately 550 °C with an insignificant variation for different gas compositions. A high rate of diffusion of carbonitriding has been obtained without initial sputter removal of the surface oxide layer, which demonstrates that the surface oxide layer does not play a crucial role during r.f. plasma carburizing and carbonitriding of stainless steel. The nitrogen concentration and the compound layer thickness increase, when 10% of C2H2 is added. With further increase in the C2H2 ratio, they decrease again gradually. The carbon depth profiles for the same samples show lower concentration and opposite tendency with respect to the nitrogen content. X-ray diffraction was used to characterize the microstructure of the compound layers. Fe2N, Fe3N, CrN, nitrogen-expanded austenite (γn) and carbon-expanded austenite (γc) phases are detected. The relative proportions of the phases are critically dependent on the N2/C2H2 ratio. Depending on the N2/C2H2 ratio, the microhardness value increases by a factor between 2 and 6. The carbonitrid layer exhibits a corrosion resistance better than pure nitrided and carburized layers.

Radio frequency (rf) inductively coupled plasma was used for carbonitriding the surface of AISI-304 austenitic stainless steel (ASS) samples at different input plasma powers (300–650 W). The morphology and the microstructure of the treated surfaces of carbonitrided austenitic stainless steel samples (CNASS) were examined by optical microscopy (OM) and scanning electron microscopy (SEM). The surface hardness of the treated surfaces as well as the microhardness profile of the samples was measured. X-Ray diffraction was employed to investigate the structure of product phases as a result of carbonitriding process. The results indicate that by using the rf plasma technique, we can greatly improve the tribomechanical properties of these materials. The high rate of carbonitriding (1 μm2/s) and the high hardness of CNASS (1375 Hv, 0.1) at plasma input power 550 W have been interpreted in terms of nitrogen and carbon concentration gradients, and microcracks formed in the surface during the plasma process in the compound layer.