Investigation of nitrogen mass transfer within an industrial plasma nitriding system II: Application of a biased screen

Active screen plasma nitriding (ASPN) was conceived in order to reduce negative effects observed in direct current plasma nitriding arising from the application of bias to the components. The mechanism of nitrogen mass transfer in ASPN is still not fully understood. Here, we compare the microstructure, composition and hardness response of AISI P20 and H13 steels after nitriding. A set of samples was nitrided with sample bias applied directly and another set was nitrided at floating potential under an active screen. Similar nitrogen content and hardness profiles were obtained for the samples treated using a bias and under an active screen separated from the samples by 12 mm. When the sample-screen separation was increased from 12 to 70 mm the hardness response improved. The principle processes occurring during ASPN are proposed based on the experimental results. In ASPN, a flux of energetic nitrogen species is generated by the active screen which, provided that the samples are within the range of the energetic species, bombards the surface of the samples being treated. This flux is critical in establishing a nitrogen potential and a satisfactory response in the components.     

An in-situ study of plasma nitriding

Direct in-situ observation of phase generation and growth during heat treatment cycles gives information independent e.g. of effects resulting from cooling and atmospheric changes of properties. In this investigation time resolved in-situ X-ray diffraction (XRD) analysis of growing nitride layers during plasma nitriding was conducted to gain experimental data of growing compound layers for different plasma nitriding parameters. With two gas mixtures of 5% N₂-95% H₂ and 25% N₂-75% H₂. plasma nitriding of an AISI 1045 steel was performed in the temperature range of 450 °C < T < 560 °C. The in-situ XRD-observation consisted of series of 50 to 60 single runs of phase analysis during a 3-h plasma nitriding treatment. Nitriding with the formation of nitride phases starts at different times, depending on the nitriding temperature and the gas composition in the plasma for the given plasma parameters pressure, voltage and current density. The higher the nitriding temperature and the higher the nitrogen content in the process gas the shorter is the time for the first detection of the γ′-Fe₄N-phase. Single phase γ′nitride layers were detected for the 5% N₂-95% H₂ gas mixture in a temperature range 450 °C < T < 560 °C. For the highest temperatures 540 °C and 560 °C and the gas mixture 25% N₂-75% H₂ the ε-Fe_2-3N phase occurred later in the plasma nitriding process. Assuming that nitride layers in plasma nitriding also grow by nucleation of small γ′ particles up to a complete layer, the experimental data fitted in a reasonable way in plots calculated for the incubation time of the γ′-phase during gas nitriding.     

Negative effects of reactive sputtering in an industrial plasma nitriding

Plasma nitriding of a 3% Cr-Mo-V gear, a 310 SS PM slot, a 304 SS flat-disk, and an Inconel 718 fillet were performed to demonstrate the effects of reactive sputtering on case-depth uniformity. It was shown that the geometry of the sample in combination with the wrong processing pressure may have a negative influence on the distribution of the vapor-deposited phase, leading to the uneven concentration of a deposit from plasma on the nitrided surface. The presence of this deposit coincides with the disturbance of case-depth uniformity.     

On the formation of expanded austenite during plasma nitriding of an AISI 316L austenitic stainless steel

Plasma nitriding of an AISI 316L austenitic stainless steel at low (400 °C) and high temperatures (550 °C) was performed under different nitriding gas mixtures. Nitrided surfaces were characterized by XRD using the Rietveld method. Expanded austenite “γ_N” with a special triclinic (t) crystalline structure was formed during the low-temperature nitriding treatment. Minor volume fractions of Fe₃N, Fe₄N and Cr₂N nitrides were also found. The expanded austenite phase showed a distortion ε of the lattice angles due to a very high nitrogen content dissolved in austenite, supersaturating the solid solution and leading to a 10% lattice distortion and to high compressive residual stresses at the surface.After nitriding the specimens at 550 °C the case was composed primarily by a high volume fraction of Fe₄N, Cr₂N and CrN nitrides, leading to a low distortion of the parent austenitic phase, maintaining the original cubic lattice.     

Modeling of nitrogen penetration in polycrystalline AISI 316L austenitic stainless steel during plasma nitriding

The nitrogen depth profile in polycrystalline AISI 316L austenitic stainless steel after plasma nitriding at temperatures around 400 °C is analyzed by the “trapping-detrapping” model. This model considers the diffusion of nitrogen under the influence of trap sites formed by local chromium atoms. Nitrogen depth profiles in polycrystalline AISI 316L steel simulated on the basis of this model are in good agreement with experimental nitrogen profiles. The enhanced nitrogen diffusivity as well as a plateau-type shape of nitrogen depth profile can be explained. The nitrogen diffusion coefficient at 400 °C is found to be D = 4.81 × 10⁻¹² cm²/s and the diffusion pre-exponential factor D₀ (0.837 × 10⁻³ cm²/s) and detrapping activation energy E_B (0.28 eV) were deduced from fitting experimental data. It is known that the nitrogen penetration depth (and nitrogen diffusivity) depends on the crystalline orientation and a tentative to take into account this anisotropy effect and describe nitrogen depth profiles in polycrystalline AISI 316L steel is proposed by using different diffusion coefficients characteristic for each crystallite orientation.     

The influence of surface microcracks and temperature gradients on the rf plasma nitriding rate

Rf plasma nitriding processes have been used recently for surface treatment. 304 Austenitic stainless steel has been nitrided using an inductively coupled rf plasma. High rates of nitriding and relatively high microhardness have been achieved. The effect of plasma power and plasma processing time on the rate of nitriding and the microstructure was investigated. A high nitriding rate of 3 μm²/s and a microhardness value of 1800 HV0.1 were achieved under optimum plasma conditions. The interpretation is based on a new approach that the microcracks in the surface, at a certain ambient temperature and plasma, can create traps for the active nitrogen species. The nitrogen concentration and temperature gradient can also draw some of the nitrogen species towards the bulk side. The microhardness can be explained in terms of the high concentration of nitrided phases created under the surface of the treated samples.     

Triode plasma nitriding and PVD coating: A successful pre-treatment combination to improve the wear resistance of DLC coatings on Ti6Al4V alloy

Diamond-like carbon (DLC) coatings have found great applicability in the automotive industry because of their low friction coefficient and high wear resistance. Nevertheless, their tribological performance can be greatly reduced on soft substrates such as titanium alloys. The hard DLC coating cannot usually follow elastic and plastic deformation of the substrate without failing. In order to overcome this property mismatch between hard coating and soft substrate, triode plasma nitriding was applied as a pre-treatment to improve the mechanical properties of the Ti6Al4V alloy and further enhance the load support for the DLC coating. DLC and multilayered TiN/DLC, CrN/DLC CrAlN/DLC coatings were deposited onto “standard” and plasma nitrided Ti6Al4V substrates. Triode plasma nitriding increased the load-bearing capacity of the coating/substrate system, as higher critical adhesion loads were recorded for DLC coatings on plasma nitrided Ti6Al4V substrates. This treatment also reduced the wear rate of the DLC coating/substrate. Further load support and lower wear rates were achieved by using TiN, CrN and CrAlN as intermediate layers on plasma nitrided Ti6Al4V substrates.     

Nanocomposite coatings of Ti/C:H plasma polymer particles providing a surface with variable nanoroughness

Nanocomposite metal plasma polymer films were prepared by deposition of C:H plasma polymer particles using a Haberland type cluster source and overcoating of these particles by titanium layer by means of planar magnetron. This study is focused mainly on characterization of surface roughness of such films and its influence on other properties, such as surface chemistry and wettability. Selected samples were also subjected to biotests, namely to evaluate adhesion of human osteoblast-like MG63 cells.     

Electropolishing of 304 stainless steel: Interactive effects of glycerol content, bath temperature, and current density on surface roughness and morphology

In this study, surface roughness of 304 stainless steel (304SS) varying from ca. 7 to 45 nm can be controlled by changing electropolishing variables in glycerol-containing baths. On the basis of the adsorbate-acceptor mechanism and the model for preferential adsorption of shielding molecules, incorporating the molecular interaction concept is helpful to explain the interactive effects of glycerol content, bath temperature, and current density on the surface roughness (Ra) and morphologies of 304SS. In order to easily understand how to control the surface roughness of 304SS in the glycerol-containing electrolyte, a simplified scheme integrating the above two mechanisms was proposed. From the atomic force microscopy (AFM) analyses, a rough surface is easily obtained by electropolishing at high bath temperatures, which endows the 304SS substrate with pores.     

Hydrophobic over-recovery during aging of polyethylene modified by oxygen capacitively coupled radio frequency plasma: A new approach for stable superhydrophobic surface with high water adhesion

In this work, a new approach for the stable superhydrophobic surface with a high water adhesion has been found from an aging process at a high aging temperature of 90 °C during an aging time up to 24 h for the low density polyethylene (LDPE) modified by the oxygen capacitively coupled radio frequency plasma (CCP) under a radio frequency (RF) power of 200 W for an exposure time of 5 min. The plasma nanotexturing produced the nanofibrils array on the LDPE surface showing the contact angle of approximately 0°. During the aging process, the as-modified superhydrophilic LDPE surface underwent a hydrophobic over-recovery into the superhydrophobicity with a pinned water droplet showing the contact angle higher than 150°. A time-dependent contact angle model on the nanotextured surface was developed from the Cassie's equation for the heterogeneous surface containing different wetting patches, to describe the hydrophobic over-recovery due to the surface restructuring on the nanofibrils array. The stable superhydrophobicity with the high water adhesion was attributed to the CCP modified LDPE surface with the nanofibrils composed of the heterogeneous chemical compositions of polar and nonpolar chains/groups after the aging process.     

Effect of carbon fiber surface treatment on Cu electrodeposition: The electrochemical behavior and the morphology of Cu deposits

The relationship between surface functional groups and electrochemical behaviors of polyacrylonitrile (PAN)-based carbon fibers (CFs) differentiated by oxidation treatment in air was studied. The chemical character of the CFs surface was estimated by X-ray photoelectron spectroscopy (XPS) and the electrochemical behavior of treated CFs in CuSO₄ plating solution was studied by electrochemical setup. The influence of functional groups on the morphology of copper deposits was characterized by field-emission scanning electron microscopy (FESEM). It was found that the O/C atomic ratio rose rapidly from 23.05% (as-received carbon fibers) to 42.83% as the oxidation temperature was increased to 400 °C and the content of -CO was the highest. Concentrations and types of the functional groups on CFs surface showed a close connection with the electrochemical response of CFs in CuSO₄ plating bath. It was showed that Cu electrodeposition was the interaction of applied voltage and the reduction of surface functional groups. With the functional groups increased, the quantities of the Cu nuclei increased, further the morphology of deposited Cu was affected.     

The nanostructure and microstructure of steels: Electrochemical Tafel behaviour and atomic force microscopy

The influence of chemical composition and heat treatment on a low-carbon steel, chromium steel and high speed steel has been examined by polarisation curves and electrochemical parameters deduced from the Tafel plots. The electrochemical corrosion resistance, which is small between the as-received steels become greater after heat treatment, following the order: carbon steel < chromium steel high speed steel. To explain these differences, the nano- and microstructure of the steels has been characterized by the ex situ techniques of atomic force microscopy and optical microscopy, before and after surface etching with Nital (a solution of 5% HNO₃ in ethanol). This causes preferential attack of the ferrite phases showing the carbide phases more clearly. From these nanostructural studies it was possible to better understand why the passive films formed on chromium steel and high speed steel have superior protective properties to those formed on carbon steel.     

Radio frequency plasma polymers of n-butyl methacrylate and their controlled drug release characteristics: I. Effect of the oxygen gas

The effect of oxygen gas on the chemical structure, surface property and controlled drug release characteristics of radio frequency (RF) plasma poly-n-butyl methacrylate (PPBMA) thin films was investigated. The ATR-FTIR and XPS spectra of the resultant PPBMA films showed relatively higher concentration of C-H, C-C and CC groups, but less intense peaks of CO and C-O functionalities, which imply that the oxygen gas had no significant influence on the chemical structure of the plasma films. Results of the SEM experiment revealed that a dome-like structure was observed in the case of deposition without oxygen, but in the case of deposition with oxygen, a smooth and dense surface was produced. Moreover, the hydrophilicity of PPBMA obtained from the deposition with oxygen was higher than that without oxygen. Drug release from PPBMA deposition coating without oxygen had biphasic patterns, a fast release followed by a slow release, but the one with oxygen exhibited a slow Higuchi release.     

Electrospark alloying of titanium and its alloys: The physical, technological, and practical aspects. Part I. The peculiarities of the mass transfer and the structural and phase transformations in the surface layers and their wear and heat resistance

A detailed analysis of the works concerning the electrospark alloying of the surface of titanium and its alloys to improve their physicochemical properties and performance characteristics has been carried out. It has been shown that the electrospark alloying method makes it possible to enhance some important features of titanium such as its wear resistance, heat resistance, and corrosion stability.