•Ti Grade 2 and Ti6Al7Nb were nitrided at cathode (CP) and plasma potentials (PP).•Layers formed by PP were characterized by lower roughness and also twice as thin.•CP layers were characterized by ...higher wear resistance, but also higher CoF.•Nitriding didn’t affect very good corrosion resistance of Ti6Al7Nb alloy.
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Plasma nitriding of titanium Grade 2 and Ti6Al7Nb titanium alloy at 730 °C was used to produce nitrided surface layers with an outer zone consisting of nanocrystalline titanium nitride - TiN, and a Ti2N zone right above the αTi(N) diffusion area. The paper presents a comparison of structure (TEM, SEM), phase composition (XRD, SAED) surface topography (AFM, optical profilometer), corrosion (potentiodynamic method), tribological properties (‘ball-on-disc’), and adhesion (scratch-test) of TiN + Ti2N + αTi(N) type nitrided surface layers produced on Grade 2 titanium and Ti6Al7Nb titanium alloy using conventional glow-discharge nitriding (so called glow discharge nitriding at the cathode potential) and with the use of an ‘active screen’ (glow discharge nitriding at plasma potential). A reduction of cathodic sputtering in the plasma potential process made it possible to produce surface layers maintaining a high smoothness of the titanium surfaces. Due to the low surface roughness and the high compressive residual stress of the sample after nitriding at plasma potential, ‘ball-on-disc’ test results showed the lowest coefficient of friction and the lowest penetration depth for this layer and very good adhesion of the layer to the substrate – i.e. Ti6Al7Nb titanium alloy. Nitriding of titanium alloy at cathode potential increased the corrosion potential value and slightly decreased corrosion current density. The corrosion current density of the sample nitrided at plasma potential was similar to initial state Ti6Al7Nb titanium alloy. The nitriding process type (cathode potential or plasma potential) used to produce TiN + Ti2N + αTi(N) surface layers on Ti6Al7Nb titanium alloy has an influence on the microstructure of the layer, its residual stress, in particular its TiN outer titanium nitride zone, its thickness, and on its properties such as wear and corrosion resistance.
The Ti6Al7Nb alloy has been elaborated as a substitute for Ti6Al4V alloy especially for applications leading to its direct contact with human body fluids as the former totally eliminates the ...carcinogenic vanadium present in the latter. The Ti6Al7Nb presents so much needed high specific strength, but its aluminum addition is also harmful and will have to be taken care of. Nitriding could improve wear resistance of these alloys and form a diffusion barrier at least at first stage of their use. However, knowledge of phase composition of near surface area of Ti6Al7Nb after such treatment is still limited. Therefore, these experiments were planned to describe the effect of the plasma nitriding of Ti6Al7Nb alloy performed at 680 °C and 740 °C for 6 h. The transmission electron microscopy observations revealed that the plasma nitriding of this alloy performed at both of specified temperatures allows to produce a zone consisting of only three layers i.e. δ-TiN, α″-Ti martensite and (α-Ti + Ti3Al) phases. The only difference between both treatments was that plasma nitriding at 740 °C caused a porosity of upper part of the δ-TiN layer, while the one formed at 680 °C was only slightly thinner, but fully dense. Penetration of the nitrogen into sub-surface areas during plasma nitriding of Ti6Al7Nb alloy pushes down both the aluminum and niobium deep into the substrate freeing the near surface β-Ti from alloying additions. It allows to transform the latter into α″-Ti martensite during cooling down. The last layer in the nitrided zone was found to be formed by a mixture of α-Ti and Ti3Al phases.
•Phase composition of plasma nitrided Ti6Al7Nb and Ti6Al4V alloys differs.•Plasma nitriding of Ti6Al7Nb at ~700 °C forms δ-TiN/α″-Ti/α-Ti + Ti3Al phases.•Lowering nitriding temperature to 680 °C eliminates the porosity of δ-TiN layer.•Diffusion of N into sub-surface areas pushes away the Al and Nb.•β-Ti, freed from alloying additions, turns on cooling into α″-Ti martensite.
Ti-6Al-4V alloy was glow discharge nitrided at 680°C/4h and 750°C/3h at 2.5mbar nitrogen pressure. Surface diffusive layers formed after such processes were characterized with TECNAI transmission ...microscope equipped with EDAX microanalysis system. Thin foils were cut out with Quanta 200 FIB. The X-ray phase analysis was performed using Bruker D2 Phaser. The performed experiments proved, that the diffusive layers obtained after these treatments are formed of four distinct sub-layers, i.e. nano-crystalline δ-TiN, fine-crystalline δ′-Ti2N, medium-coarse columnar grains filled with plates of α″-Ti martensite, to be followed by medium-coarse roughly equiaxed Ti3Al intermetallic bordering with coarse α-Ti substrate grains. The δ-TiN and δ′-Ti2N nitride sub-layers are much thinner than the subsequent metallic ones and therefore the nitrides are minor phases, while martensite and the Ti3Al are the major phases of the diffusive layer. The plasma nitriding process – promoting nitrogen diffusion into the metallic substrate – “pushes out” aluminium from the surface area. The depletion in aluminium of the upper part of the diffusive layer stimulates β-Ti→α″-Ti martensite transformation in the metallic middle sub-layer, while enrichment in this element of the sub-layer nearest the substrate causes α-Ti→Ti3Al diffusion driven transformation. Therefore, the aluminium is pushed out of most of the diffusive layer including α″-Ti martensite.
•New findings on phase composition of a diffusive layer on nitrided Ti6Al4V alloy.•Glow discharge nitriding of Ti6Al4V causes formation of four distinct sub-layers.•Diffusive layer on nitrided Ti6Al4V consists of δ-TiN, δ′-Ti2N, α″-Ti and Ti3Al.•The α″-Ti martensite was never considered part of diffusive layer in the past!
The oral cavity due to its temperature fluctuations, changing pH, high humidity, action of mechanical forces and the presence of microorganisms is a favorable environment for degradation of dental ...materials. The paper presents comparative results on orthodontic arch-wires AISI304 steel before and after low temperature plasma nitriding carried out at cathodic potential (conventional) and at plasma potential, i.e. in a process incorporating an active screen. Corrosion resistance test on nitrided layers produced on stainless steel were carried out via electrochemical impedance spectroscopy (EIS) and the potentiodynamic method in non-deaerated artificial saliva solution at 37°C. The results were complemented with analysis of the structure, surface topography and microhardness. The results showed an increase in corrosion resistance of AISI304 steel after conventional glow-discharge nitriding.
The key issue in bone implants biomechanics is – besides using biocompatible materials - the optimization of tribological properties of friction pairs occurring in joint implants e.g. knee and hip ...endoprothesis. Increasingly important role in these systems, alongside ceramics-on-ceramics and metal-on-metal, have metal-on-polymer friction pairs. Commonly used in orthopedics ultra-high molecular weight polyethylene (UHMWPE) does not meet the requirements of modern bone implants, mainly because of its harmful wear products which can cause inflammation and osteolysis of surrounding tissues after several years of using the endoprosthesis. Therefore, other polymer materials have been developed to replace UHMWPE, as well as several surface engineering methods are used for enhancing biocompatibility and tribological properties of applied materials. One of the materials to replace UHMWPE is increasingly used in medicine polyetheretherketone (PEEK) – a polymer material with a high biological indifference and mechanical properties.
The article presents characteristics of TiN+Ti2N+αTi(N) nitrided layer produced on Ti6Al4V titanium alloy using glow discharge assisted nitriding process at the plasma potential, also known as the active screen plasma nitriding process and hydrogenated amorphous carbon doped with nitrogen layer a-C:N:H produced via RFCVD process on PEEK-based composite consisting of 10% graphite, 10% carbon fibers and 10% PTFE. Tribological properties of a-C:N:H – TiN+Ti2N+αTi(N) friction pair using “ball-on-disc” and “block-on-roll” tests were examined in correlation with microstructure (TEM, SEM, Raman spectroscopy) and surface morphology and topography (SEM, AFM, optical profilometer).
The goal of this work is to present a new possibility of material solution for a ‘head-acetabulum’ friction pair in hip joint endoprosthesis using PEEK (PEEK T) as a replacement for commonly used UHMWPE and nitrided layer produced on Ti6Al4V titanium alloy using active screen plasma nitriding process.
Amorphous carbon layer, 200nm thick, produced on PEEK composite significantly improved tribological properties of PEEK composite – TiN (nanocrystalline)+Ti2N+αTi(N) layer friction pair, decreasing the friction coefficient by 2 times and minimizing wear of both used materials in the point contact (“ball-on-disc” method) and in surface contact (“block-on-roll” method).
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•200nm thick a-C:N:H layers were deposited on PEEK and PEEK-based composite.•Sp3/sp2 bonding concentration in amorphous carbon layers was about 50%.•Nitrided layer was produced on the surface of Ti6Al4V using active screen process.•a-C:N:H layers significantly decreased friction coefficient in contact with TiN.•PEEK T+a-C:N:H-TiN can be a new friction pair in hip joint replacement (head-cup).
AISI 316L steel was subjected to nitrocarburizing under glow discharge conditions, which was followed by DLC (diamond-like carbon) coatings deposition using the same device. The coatings were applied ...under conditions of direct current and pulsed glow discharge. In order to determine the influence of the produced nitrocarbon austenite layer and the type of discharge on the microstructure and mechanical properties of the coatings, the following features were analysed: surface roughness, coating thickness, structure, chemical composition, adhesion and resistance to frictional wear. For comparison purposes, DLC coatings were also deposited on steel without a nitrocarburised layer. The obtained results indicate a significant influence of the type of glow discharge on the roughness, hardness, nitrogen content and of the nitrocarburised layer on the resistance to wear by friction and adhesion of the produced coatings.
The paper presents a novel spectral algorithm EVSA (eigenvector structure analysis), which uses eigenvalues and eigenvectors of the adjacency matrix in order to discover clusters. Based on matrix ...perturbation theory and properties of graph spectra we show that the adjacency matrix can be more suitable for partitioning than other Laplacian matrices. The main problem concerning the use of the adjacency matrix is the selection of the appropriate eigenvectors. We thus propose an approach based on analysis of the adjacency matrix spectrum and eigenvector pairwise correlations. Formulated rules and heuristics allow choosing the right eigenvectors representing clusters, i.e., automatically establishing the number of groups. The algorithm requires only one parameter-the number of nearest neighbors. Unlike many other spectral methods, our solution does not need an additional clustering algorithm for final partitioning. We evaluate the proposed approach using real-world datasets of different sizes. Its performance is competitive to other both standard and new solutions, which require the number of clusters to be given as an input parameter.
The paper presents an analysis of tribological wear of an assembly composed of cylinder liner and piston rings in a piston aircraft engine. In place of the previously used technology of gas nitriding ...of the cylinder liner (made out of 38CrAlMo6‒10 steel), formation of multi-component or composite layers by nitriding in plasma discharge environment was proposed. Also, a modification of the previously used chromium coating of the piston rings by plasma nitriding was proposed. A study on the structure and properties of surface layer diffusion manufactured by utilizing the phenomenon of cathode sputtering was carried out. Wear resistance tests were performed on a specially designed bench, with the use of isotope markers. Verification of each of the friction couples was based on using the wear of the ring as a function of the distance trip, and the roughness of the cylinder liner and ring.
This paper describes a new method of classification based on spectral analysis. The motivations behind developing the new model were the failures of the classical spectral cluster analysis based on ...combinatorial and normalized Laplacian for a set of real-world datasets of textual documents. Reasons of the failures are analysed. While the known methods are all based on usage of eigenvectors of graph Laplacians, a new classification method based on eigenvalues of graph Laplacians is proposed and studied.
Studies on biocompatibility of AISI 316LVM steel indicate the need to eliminate the nickel from the surface and replace it with other elements of improved biocompatibility. Therefore, in the ...presented work selected physicochemical and mechanical properties of the diffusive nitrocarburized layer formed by plasma potential by means of an active screen made of the Fe-Cr-Ni were studied. In the paper we present results of microstructure and phase composition of the layers, roughness, and surface wettability, potentiodynamic pitting corrosion resistance, penetration of ions into the solution as well as mechanical properties. The studies were conducted for the samples of both mechanically polished and nitrocarburized surfaces, after sterilization, and exposure to the Ringer’s solution. Deposition of the nitrocarburized layer increased the contact angle, surface roughness, surface hardness, and corrosion resistance with respect to the polished surfaces. The nitrocarburized layer is a barrier against the ions release into the solution and sterilization and exposure to Ringer solution. The obtained results showed beneficial increase of both mechanical and electrochemical properties of the deposited layer, and thus the applicability of the proposed method of surface treatment of the 316LVM steel for short-term implants after sterylization.
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