Tribological Coatings Research Articles

Electrospark Deposition of Hard, Wear-Resistant and Anti-Friction Tribological Coatings on γ-TiAl Substrates in a Controlled Gas Environment

Electrospark Deposition of Hard, Wear-Resistant and Anti-Friction Tribological Coatings on γ-TiAl Substrates in a Controlled Gas Environment

Optimizing substrate bias voltage to improve mechanical and tribological properties of ductile FeCoNiCu high entropy alloy coatings with FCC structure

Conventional hard coatings are limited in their tribological applications due to their high brittleness. High-entropy alloy (HEA) coatings, which combine high toughness and hardness, represent a new class of tribological coatings with great potential. The microstructure and stress state of HEA coatings can be significantly affected by the deposition parameters of magnetron sputtering, which can regulate their mechanical and tribological properties. In this study, DC magnetron sputtering with varying substrate bias voltages was used to deposit FeCoNiCu HEA coatings with an FCC single phase structure on 316 stainless steel. The microstructure, mechanical, and tribological properties of these coatings were studied in detail. The residual tensile stress of the coatings decreases, while toughness and hardness increase as the bias voltage increases. The coatings exhibited optimal toughness, highest hardness (11.3 GPa), and the lowest wear rate (as low as 6.01 × 10−5 mm3/N·m) when the bias voltage was set to −200 V. The formation of a metal oxide adhesion layer during friction improved the tribological properties of the coatings. This study demonstrates that optimizing the deposition bias voltage is an effective way to improve the mechanical and tribological properties of high-entropy, face-centered cubic alloy coatings, which are intrinsically tough and ductile.

Tribological coatings for electric vehicle applications

Tribological coatings for electric vehicle applications

Influence of Multilayer Structure on the Structural and Mechanical Properties of TiAlN/CrN Coatings for Advanced Machining Applications

This study investigates single and multilayer TiAlN/CrN nanocomposite thin films developed using an RF magnetron sputtering system. The TiAlN and CrN layers showed a high degree of orientation, with the (200) peak being the strongest peak in both layers, and a multilayer structure was clearly observed. The surface roughness analysis using atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (TEM) revealed that the TiAlN/CrN coatings had a smoother surface than the single-layer coatings and minimal intermixing between the two layers. Depth-sensing indentation measurements were used to measure the hardness and Young’s modulus of the coatings, demonstrating that TiAlN/CrN coating had the highest hardness (~16.38 GPa) and elastic modulus (~3.82 GPa) among all the coatings studied. This indicates that the TiAlN/CrN multilayer coating possesses superior mechanical properties due to its interface strength. Our findings suggest that these multilayer coatings have potential applications in tribological and decorative coatings.

Electrodeposited MoxSyOz/Ni Tribological Coatings.

Deposition of molybdenum disulfide (MoS2) coatings using physical vapor deposition (PVD) and mechanical burnishing has been widely assessed for solid lubricants in space applications but still suffers from line-of-sight constraints on complex geometries. Here, we highlight one of the first demonstrations of electrodeposited MoxSyOz and MoxSyOz/Ni thin-film coatings from aqueous solutions of ammonium tetrathiomolybdate for solid lubricant applications and their remarkable ability to provide low coefficients of friction and high wear resistance. Characterization of the coating morphology shows amorphous microstructures with a high oxygen content and cracking upon drying. Even so, electrodeposited MoxSyOz can achieve low steady-state coefficients of friction (μ ∼ 0.05-0.06) and wear rates (2.6 × 10-7 mm3/(N m)) approaching those of physical vapor deposited coatings (2.3 × 10-7 mm3/(N m)). Additionally, we show that adding dopants such as nickel increased the wear rate (7.5 × 10-7 mm3/(N m)) and initial coefficient of friction (μi = 0.23) due to compositional modifications such as dramatic sub-stoichiometry (S/Mo ∼ 1) and expression of a NiOx surface layer, although doping did reduce the degree of cracking upon drying.

Using optical emission spectroscopy in atmospheric conditions to track the inflight reduction of plasma sprayed TiO2−x feedstock for thermoelectric applications

Thermal spray deposition (specifically Atmospheric Plasma Spraying, APS) is a well-established surface coating technology with a broad scope of applications (i.e., insulative coatings, tribological coatings, anti-corrosion coatings, etc.). In addition, there is a constant drive to introduce the APS process into new and emerging fields. One such niche application for APS would be sub-stoichiometric TiO2−x coatings with enhanced thermoelectric performance (compared to the bulk material). The APS process in this context has a unique ability—given the use of hydrogen as a plasma gas—to reduce TiO2−x material during processing. However, to this point, there is neither a reliable nor self-consistent method to assess (nor control by parametric optimization) the inflight reduction of molten oxide particles during processing. This study shows that using Optical Emission Spectroscopy (OES), it can be possible—even in atmospheric conditions—to identify characteristic emission peaks associated with the inflight reduction of TiO2 during APS. Using this OES data, the input spray processing parameters and their influence on coating microstructure and the degree of inflight reduction of the material will be shown. Results suggest under equilibrium conditions only a minimal amount of hydrogen gas is needed in the plasma to fulfill the TiO2 reduction.

Tribological Coatings for Open Space

Tribological Coatings for Open Space

Simple Deconvolution Models for Evaluating the True Microhardness of Thin Nanostructured Coatings Deposited via an Advanced Physical Vapor Deposition Technique

This article discusses the micromechanical properties and true microhardness determination of nanostructured tribological coatings (NTCs) based on a multilayered alternating nitride/carbonitride bilayer substructure for transition metals. The constituent nitride/carbonitride bilayers in the superlattice structure of the NTC were alloyed with refractory metals, denoted as Me = Me1 or Me2= Cr, Hf, Nb, W, and Zr. The resulting NTC coatings were deposited onto 100Cr6 steel substrates using an advanced physical vapor deposition (PVD) technique, referred to here as high-power ion-plasma magnetron sputtering (HiPIPMS). The comprising crystalline nanometer-scale TiAlSiMe1-N/TiMe2-CN nanoparticles strengthened by Me additives significantly increased the NTC microhardness to over 3200 HV. The primary focus of this research was to determine the true microhardness of the NTC film samples. The apparent microhardness (Ha) of the film/substrate system for various NTC samples was measured during microindentation testing using the Vickers method. Nine NTC samples were tested, each generating a corresponding microindentation dataset containing between 430 and 640 imprints, depending on the specific NTC sample. These datasets were analyzed using three distinct empirical approaches: (i) the inverse power-law model (IPL-Model), (ii) the sigmoid-like decay model (SLD-Model), and (iii) the error function model (ERF-Model). The observed solid correlation between the proposed models and experiments suggests that the true microhardness estimates (Hf) obtained through the empirical mathematical modeling approach are reliable.

Thermal and solutal slip impacts of tribological coatings on the flow and heat transfer of reiner-philippoff nanofluid lubrication toward a stretching surface: The applications of Darcy-Forchheimer theory

Numerous non-Newtonian fluid models are formulated as an alternative to Newton's Law of viscosity, which describes absolute viscous sources as a function of shear rate rather than constantly. In this paper, the Reiner-Philippoff fluid (RPF) model is considered out of many existing models since it can capture certain non-Newtonian characteristics in certain cases. An adopted underlying mathematical model employs a partial differential system, which is reduced to the system of ordinary differential system with the help of suitable similarity variables. The resultant system is computed with the Runga-Kutta-Fehlberg (RK-F) method to boost-up the confidence of its accuracy. It is found that the porosity and Darcy-Forchheimer parameters improve the flow rate of RPF. On the other hand, heat transfer is positively affected by thermal slip in contrast to the outcomes of radiation parameter, which, however, are inauspicious. Furthermore, the mass diffusion rate is noticed to be significantly influenced by the Schmidt number along with the chemical reaction parameter. In conclusion, all the results are expected to benefit numerous transport phenomena of industrial, biomedical, and thermal sciences.

Effect of DLC film on the mitigation of white etching areas (WEAs) under dynamic loading

The premature failures in wind turbine gearbox bearings are primarily caused by the decay of subsurface microstructure called white etching areas and white etching cracks. Severe operating conditions of wind turbine gearbox bearings necessitated for tribological coatings to enhance the system performance. The current study investigated the performance of hydrogenated diamond-like‑carbon (DLC) coating against white etching areas formation under sliding-dynamic conditions. The dynamic loading experiments were carried out in the dynamic pin-on-disc tribometer with Polyalphaolefin base stock oil under a boundary lubrication regime. Prior to the dynamic load tests, detailed tribological characterisation of the deposited DLC coating was carried out. The post-test analysis results were compared with the uncoated AISI 52100 bearing steel tribopair to evaluate the performance of DLC-coated bearing steel against white etching areas formation under given conditions. The results obtained show the capability of the proposed coating to delay the formation of white etching areas in the bearing steel.

Friction Coefficient Dynamics of Tribological Coatings from Engine Lubricants: Analysis and Interpretation

Even today, there is no full understanding of the relationship between the physical, chemical, and mechanical properties and the behavior of the lubricating films formed in tribological systems. Most of the published scientific research measures and reports the overall values of friction and wear, but the information given via statistical signals in the tribological tests is, in general, dismissed, leaving a hole in the study of the dynamics of tribological systems. In this work, an experimental study of statistical friction data, coating characteristics, and tribological performance is carried out using a pin-on-disk tribometer to test some metallic samples coated with lubricant films under several experimental conditions. The results indicate that long deposition times at high deposition temperatures of coatings from engine oil develop low-friction intervals, which fall until 20% of the uncoated coefficient friction. However, an unexpected and unfavorable behavior of the coatings was observed for the short deposition times and high temperature. In these conditions, the developed friction profiles reached a maximum level of friction that was 20% more than the uncoated coefficient friction. Moreover, it was found that the proper analysis and interpretation of the statistical data, in combination with the characterization of the coatings, describes the dynamics of the interactions between the bodies in contact and reveals the transitions of the surfaces with non-homogeneous properties throughout their depth.

Tribological Properties Laser-Cladded Spherical WB-Reinforced Co-Based Coatings under Low-Temperature Friction.

Three groups of spherical WB-reinforced Co-based coatings (Co coating, Co + 15%WB coating, Co + 45%WB coating) were fabricated by laser-cladded technology. The microstructure and constituent phase of spherical WB-reinforced Co-based coatings were examined through scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). The low-temperature tribological properties were analyzed by coefficient of friction, 2D and 3D profiles across the wear track, and wear surface morphology, respectively. The results showed that the phases in the WB-reinforced Co-based coatings are mainly γ-Co, carbides Cr23C6 and Cr7C3, WB, and WO3. Under dry sliding friction at -20 °C, the more spherical WB, the lower the friction coefficient. The wear rate of Co + 45%WB coating was as low as 3.567 × 10-4 mm3/N·m-1, indicating the outstanding wear resistance. Abrasive wear was observed on the rough surface of the WB-added coatings. Compared with dry sliding, due to the plastic deformation of micro-convexes and lubrication function in the 3.5 wt.% NaCl solution, the wear tracks on the surfaces of the three tested coatings were shallower, exhibiting distinct elongated plough grooves.

Role of graphene concentration on electrochemical and tribological properties of graphene-poly(methyl methacrylate) composite coatings.

This study aims to investigate the influence of graphene nanoplatelet (GNP) concentration on the electrochemical and tribological properties of GNP-poly(methyl methacrylate) (PMMA) composite coatings. GNP-PMMA coatings were prepared with varying GNP concentrations (0.5, 1.0, 3.0, and 5.0wt %) using the drop-casting method onto AA6061 aluminum alloy substrates. Results showed that the addition of 1.0wt % GNP increased the tensile strength of PMMA but further increase reduced the tensile strength and fracture strain of the composites. Permeability studies indicated that 1.0GNP-PMMA had the lowest water vapour transition rate. All GNP-PMMA coatings showed a higher coating resistance and impedance modulus at the lowest frequency compared to neat PMMA with 1.0GNP-PMMA having the highest |Z|0.01 Hz value in comparison to the composites with higher GNP concentrations. According to Raman mapping, an increase in the concentration of GNP in the composite resulted in the agglomeration of graphene, which caused the debonding of the graphene-PMMA interfaces and also resulted in a higher number of shear fronts and other defects on the fracture surface that reduced barrier properties of graphene. The specific wear rate of 1.0GNP-PMMA was lower than that of neat PMMA, indicating improved wear resistance. The coefficient of friction was lowest for 5.0GNP-PMMA, although this was due to a higher amount of material being transferred to the counterface. Accordingly, optimizing the GNP concentration enables the development of high-performance PMMA coatings with enhanced strength, improved barrier properties, and reduced wear rates, making them well-suited for applications such as corrosion protection and tribological coatings.

Functional magnetic Maxwell viscoelastic nanofluids for tribological coatings- a model for stretching flow using the generalized theory of heat-mass fluxes, Darcy-Forchheimer formulation and dual convection

A theoretical and computational study of an improved Fourier and Fickian model for locally non-similar magnetohydrodynamic Maxwell (non-Newtonian) nanofluid convective flow under thermo-solutal buoyancy forces in a non-Darcian (Darcy-Forchheimer) porous medium is presented. Heat sink/source is included. Buongiorno’s two-component nanofluid model is used to simulate nanoscale characteristics featuring thermophoresis and Brownian diffusions. The problem under consideration is formulated utilizing fundamental relations of fluid dynamics. The primitive partial differential expressions (PDEs) are transfigured to ordinary ones with apposite transformations. The emerging locally non-similar boundary value problem is solved via series expansions utilizing a homotopy algorithm. Characteristics of sundry parameters on velocity, temperature, nanoparticle concentration and skin-friction coefficients are interpreted graphically. Convergence results acquired via homotopy algorithm are presented. Comparison results are included for the authentication of the homotopy solutions. The velocity distribution is increased with greater mixed convection and non-Newtonian material parameters whereas velocity distribution is reduced with increment in Hartmann (magnetic) number, porosity and buoyancy ratio parameters. The temperature distribution is reduced when Prandtl number and thermal-relaxation (non-Fourier) parameter are augmented whereas temperature distribution is increased for larger Brownian diffusion and thermophoresis. Additionally, it is observed that increment in the heat absorption variable diminishes temperature whereas an enhancement in the heat generation variable augments temperature. Nanoparticle concentration is enhanced subjected to higher values of thermophoresis factor whereas it reduces with larger Schmidt number, Brownian movement and solutal-relaxation (non-Fickian) parameters. Furthermore, it is noticed that elevation in Hartmann number, porosity and inertial (non-Darcy) coefficient parameters increase the skin friction coefficient whereas elevation in Deborah number and buoyancy ratio is found to suppress skin friction. The simulations are relevant to hydromagnetic nano-materials processing operations for coatings deployed in multi-functional tribological systems and surface protection.

Tribological behavior of different doped ta-C coatings for slip-rolling contacts with high hertzian contact pressure

PurposeThe curved and tribologically highly stressed surfaces of bearing components pose a major challenge for steel alloys or tribological resistant coatings like tetrahedral amorphous carbon (ta-C) coatings which in particular have an increased risk of delamination due to the significantly increased residual stresses. A possibility to prevent coating failure is the use of dopants while maintaining or even increasing tribological properties. This study aims to compare the tribological behavior of several doped diamond-like-carbon coatings with an undoped ta-C coating under varying slip conditions and Hertzian pressure up to 1800 MPa.Design/methodology/approachFor this purpose, the tribological behavior was studied using of a ball-on-disc tribometer and a two-disc test rig under mixed/boundary conditions. The tests were conducted with coated specimens against uncoated 100Cr6 steel. Additionally, the influence of lubrication additives was studied due to the use of two fully formulated PAO-based oils, one without and one with molybdenum containing additives. The friction was measured in situ, and the wear was analyzed trough laser scanning microscopy and tactile measurement.FindingsIt was shown that the use of doped ta-C coatings exhibited a tendency for a more favorable tribological behavior compared to undoped ta-C coatings, with no general dependence on the lubricants used. The use of the most suitable coatings reduced the wear of the steel counter-body considerably.Originality/valueTo the best of the authors’ knowledge, this is the first approach of testing the tribological behavior of these doped ta-C coatings, developed for friction efficiency, in dependency on lubrication additives under the given load collective. The approach is relevant to determine whether the friction reduction and the wear inhibition of these coatings are suitable for higher contact pressures and load cycles.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-11-2022-0336/

Effect of current density on the mechanical properties and defects of nanocrystalline Cr[sbnd]Al solid solution alloy coatings

Hard chromium coatings electrodeposited are highly valued in multiple industries for their excellent mechanical characteristics and resistance to corrosion. The general process for preparing these coatings includes Cr6+ and Cr3+ process, the Cr6+ process is highly toxic and strictly regulated. The Cr3+ process is less hazardous but can produce coatings with defects. We propose an alternative process of electroplating nanocrystalline CrAl alloy coating with high hardness by using anhydrous Cr2+ electrolyte. The impact of current density on mechanical properties (i.e., hardness H, elastic modulus E, fracture toughness H/E and resistance to plastic deformation H3/E2) and coating defects has been systematically investigated. As current density increases from 1 mA/cm2 to 5 mA/cm2, the coating surface morphology changes from polyhedral to nodular clusters with grain size decreasing from a few microns to several nanometers. The nanoindentation test was performed on the cross-section of the coating. The nanohardness H of the coating reaches a high of 8.26 GPa, attributed to both solid solution strengthening and grain refinement. A significant decrease in elastic modulus E of the coating compared to bulk Cr is observed and can be explained by the grain boundary mediated process and a grain coalescence model. High values H/E and H3/E2 indicate improved fracture toughness and resistance to plastic deformation of the coating. As current density exceeding 5 mA/cm2, laminar cracks develop due to co-deposition of CrCl2 particles. This is attributed to the electrolyte acidity transition, caused by depletion of Al2Cl7− at the interface. This work provides a deeper understanding of the microstructural evolution, mechanical properties and defects of CrAl alloy coatings and also provides a new method to fabricate high-quality coatings with enhanced properties. The defect-free coatings achieved in this study have great potential for practical applications in various fields such as tribological and corrosion resistance coatings.

Development of Tribological Coatings for Aerospace and Missile Applications

Tribological coatings play a key role in enhancing the performance and service life of engineering systems and components. Over the years, DMRL has worked extensively in studying various aspects of tribological coatings used in aerospace and missile propulsion systems where harsh and hostile environments prevail. The coatings have been deposited mostly by thermal spray processes for these applications. The studies have been done to understand the relationship between microstructure, mechanical properties and tribological behaviour of coatings. In this work, a cermet coating based on carbide to resist wear and an abradable coating based on cobalt alloy to control clearance between the blade tip and shroud for gas turbine aeroengine application and a low friction coating developed for a missile application will be discussed.

Special Issue: Tribological Coatings—Properties, Mechanisms, and Applications in Surface Engineering

Tribological coatings are found on machine elements (e [...]

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Poly-adamantane as precursor for low temperature chemical vapor deposition of nanocrystalline diamond films

Nanocrystalline diamond (NCD) films are technologically important tribological coatings and their large area, uniform growth is needed for a wide range of applications. The conventional growth of NCD requires the seeding of the foreign substrates by detonation nanodiamond particles. In terms of diamond seeds for subsequent NCD film deposition, the use of chemical precursor offers advantages in producing higher purity and higher crystallinity nanodiamond particles than that by detonation method. However, the easy sublimation of chemical precursor in plasma is a serious drawback. Here we report that the poly-adamantane layer formed by the photochemically induced polymerization of adamantane serves as a promising precursor for the growth of wafer scale NCD film using linear antenna plasma-assisted chemical vapor deposition at 500 °C. TEM cross-section analysis of the interface reveals the formation of well-faceted diamond nuclei within the amorphous carbon matrix (sp 3 % ~ 40 %) decomposed from poly-adamantane. The influence of plasma chemistry on the nucleation process was also discussed. Our work provides guidance for low-temperature deposition of high-quality NCD film by chemical nucleation. • Deposition of 4-in. nanocrystalline diamond wafer using 230 nm-thick poly-adamantane precursor layer • Cross-sectional TEM study of diamond nuclei derived from poly-adamantane precursor • Study of plasma chemistry on nucleation process by poly-adamantane precursor • Film quality control by poly-adamantane precursor at low temperature