Elucidating the microstructure and wear behavior for multicomponent alloy clad layers by in situ synthesis

High-entropy alloys with a wide range of novel microstructures and properties are under extensive development. In this study, two equimolar powder mixtures of NiCrAlCoW and NiCrAlCoSi were prepared as cladding materials. They were clad by gas tungsten arc welding on AISI 1050 medium carbon steel substrate to form an in situ synthesized multicomponent clad layer. The microstructure, chemical composition and constituent phases of the clad layers were characterized by FESEM, EPMA and XRD, respectively. A rotating tribometer was employed to evaluate the tribological properties of the clad layers. Experimental results demonstrate that the NiCrAlCoW clad layer is composed of W (precipitates and residual particles), AlNi and Cr_15.58Fe_7.42C₆. The NiCrAlCoSi clad layer contains only a BCC phase. The two clad layers exhibit high hardness, caused by precipitation hardening with different precipitates. The NiCrAlCoW clad layer exhibits strong mechanical interlocking, which results from the complex phase and microstructure of the NiCrAlCoW clad layer. During rubbing, the in situ phase transformation of the NiCrAlCoSi clad layer induces the softening of the matrix. Therefore, the wear performance of the NiCrAlCoW clad layer is better than that of the NiCrAlCoSi clad layer.     

Elucidating the microstructural and tribological characteristics of NiCrAlCoCu and NiCrAlCoMo multicomponent alloy clad layers synthesized in situ

High-entropy alloys have received considerable attention owing to their unique structures and properties. In this work, two multicomponent alloy clad layers were synthesized from two equimolar powder mixtures of NiCrAlCoCu and NiCrAlCoMo via an in situ reaction on the surface of the AISI 1050 medium carbon steel using the gas tungsten arc welding (GTAW) method. The microstructure, constituent phases and tribological properties of the clad layers were examined under a field-emission scanning electron microscope (FESEM), an X-ray diffractometer (XRD) and a pin-on-disc rotating tribometer. Experimental results indicate that the microstructure of the NiCrAlCoMo clad layer comprises AlFe_0.23Ni_0.77, Co₆Mo₆C₂ and Fe₆₃Mo₃₇. The NiCrAlCoCu clad layer has a simple microstructure that consists of only BCC and FCC solid solution phases, with lattice constants of a = 0.288 nm and a = 0.362 nm, respectively. In the NiCrAlCoMo clad layer, the complex geometric effect caused by the vein-shaped reinforcement results in strong mechanical interlocking, which can prevent detachment of the reinforcement during rubbing. Therefore, the wear performance of the NiCrAlCoMo clad layer greatly exceeds that of the NiCrAlCoCu clad layer.     

Heat treatment effects on the corrosion resistance of some HVOF-sprayed metal alloy coatings

The present study evaluates the effects of a 600 °C, 1 h heat treatment on the corrosion resistance of three High Velocity Oxygen Fuel (HVOF) flame-sprayed alloy coatings: a Co-28Mo-17Cr-3Si (similar to Tribaloy-800) coating, a Ni-20Cr-10W-9Mo-4Cu-1C-1B-1Fe (Diamalloy-4006) coating and a Ni-32Mo-16Cr-3Si-2Co (similar to Tribaloy-700) coating. Electrochemical polarization tests and free corrosion tests were performed in 0.1 M HCl aqueous solution. The corrodkote test (ASTM B380-97R02) was also performed, to evaluate the coatings qualitatively. The heat treatment improves the corrosion resistance of the Co-28Mo-17Cr-3Si coating and of the Ni-20Cr-10W-9Mo-4Cu-1C-1B-1Fe coating by enhancing their passivation ability. The precipitation of sub-micron sized secondary phases after the treatment may produce galvanic microcells at intralamellar scale, but the beneficial contribution provided by the healing of the very small but dangerous interlamellar defects (normally present in thermal spray coatings but not detectable using ordinary scanning electron microscopy) prevails. The effect on Ni-32Mo-16Cr-3Si-2Co coatings is more ambiguous: its sensitivity to crevice corrosion is worsened by the heat treatment.     

Influence of parameters of the HVOF thermal spray process on the properties of multicomponent white cast iron coatings

High velocity oxi-fuel (HVOF) thermal spray process has been used in order to deposit a new alloy known as multicomponent white cast iron. The coatings were characterized in terms of macrostructure, phase composition, porosity and hardness. Coating characteristics and properties were found to be dependent on the particles size range, spray distance, gases flow rate and oxygen to propane ratio. For set of parameters utilized in this job a narrow particle size range between 20 and 45 μm with a spray distance of 200 mm and oxygen to propane ratio of 4.6 are the preferred coating parameters. Coating porosity of 0.9% and hardness of 766 HV were obtained under these conditions.     

Characterization and properties of the MgF2/ZrO2 composite coatings on magnesium prepared by micro-arc oxidation

Through micro-arc oxidation, the MgF₂/ZrO₂ composite coatings were prepared on magnesium at the different applied voltages (in the range of 400-550 V) in a zirconate electrolytic solution. The morphologies, phase components, microhardness, bond strengths, and corrosion resistances of the composite coatings were investigated. The effect of the applied voltages on the characteristics and properties of the composite coatings and the basic formation mechanism of the coatings were also discussed. The results indicate that the composite coatings are relatively dense and uniform in thickness, and predominantly composed of MgF₂, tetragonal ZrO₂ (t-ZrO₂) and monoclinic ZrO₂ (m-ZrO₂). The composite coatings exhibit a gradient distribution in phase component from the surface to the inner part. It is found that the applied voltage plays an important role in the characteristics and properties of the composite coatings. With the increase of the applied voltage, the thickness and the t-ZrO₂ content of the composite coatings increase, while the m-ZrO₂ content decreases and no significant variation is observed in the MgF₂ content. Moreover, the surface microhardness and bond strength of the coatings increases with the applied voltage increasing. The microhardness values display a gradient distribution in the cross sections of the coatings, and the maximum microhardness value and its corresponding position in the cross sections are related to the applied voltage. In addition, the corrosion resistances of the composite coatings on magnesium surface are obviously superior to the magnesium substrate in the NaCl solutions, and the effect is more remarkable at higher voltage.     

Thermal fatigue characteristics of gray cast iron with non-smooth surface treated by laser alloying of Cr powder

In order to enhance thermal fatigue resistance of gray cast iron with non-smooth surface further, researches on laser alloying of Cr powder with different scanning speed were performed to change both the composition and microstructure of non-smooth unit. The results indicated that there was an optimal scanning speed (2 mm/s) which can contribute maximal Cr element in alloyed zone, and increasing laser scanning speed resulted in smaller non-smooth unit and more pores in it. The microstructure in alloyed layer was composed of pre-eutectic phases, ledeburite eutectic and chromium carbides of Cr₂₃C₆. Thermal fatigue resistance of non-smooth sample is better than that of smooth sample, and among non-smooth samples, sample treated by laser Cr alloying has superior resistance to thermal fatigue compared with laser melting treated sample, in addition, the resistance of laser alloying non-smooth samples increases with scanning speed slowing.     

Effect of plasma surface treatment on mechanical and corrosion protection properties of UV-curable sol-gel based GPTS-ZrO2 coatings on mild steel

Hybrid sol-gel based nanocomposite coatings derived from hydrolysis and condensation of a photopolymerizable silane precursor 3-Glycidoxypropyltrimethoxy silane in combination with zirconium-n-propoxide were deposited on mild steel substrates by a dip coating technique. In some cases, substrates were subjected to an atmospheric air-plasma surface pre-treatment prior to coating deposition. The coatings were subsequently densified by exposure to ultraviolet radiation followed by a thermal treatment at 250 °C. Characterization of the coatings with respect to thickness, water contact angle, pencil scratch hardness, adhesion and abrasion resistance was carried out. Corrosion testing was carried out on the coatings for a 1 h exposure to a 3.5% NaCl solution by electrochemical polarization and impedance measurements. The hybrid sol-gel coatings were found to improve the mechanical properties and corrosion resistance of mild steel. Plasma surface pre-treatment was found to improve the adhesion of coatings significantly and decreased the corrosion rate from 0.2652 mpy obtained for coatings without any surface pre-treatment to 0.0015 mpy, which was nearly 600 times lower than that of bare mild steel.     

Effects of surfactants on electrodeposition of nickel-carbon nanotubes composite coatings

The influences of surfactants sodium dodecylsulfate (SDS) and hexadecyltrimethylammonium bromide (CTAB) on electrodeposition of Ni-carbon nanotubes (CNTs) composite coatings were studied, with a 0.6 g/L addition for SDS or CTAB in Watts bath. The results showed that SDS in the bath slightly decreased but CTAB increased the co-deposited CNTs content in the coatings. The coatings deposited with SDS or CTAB became coarser in low CNTs concentration baths. While as CNTs concentration in the bath increased, the coatings deposited with either SDS or CTAB became more homogeneous. XRD analysis showed that both SDS and CTAB decreased the grain sizes of deposited coatings. The surfactants also affected the preferred orientations of the deposited grains: (220) plane was the preferred orientation in coatings without surfactant, while the preferred orientations for coatings formed in solutions with SDS or CTAB were (200) plane and (111) plane respectively. The addition of SDS in depositing bath increased hardness of the composite coatings and improved adherence of the coating to the matrix. Corrosion resistance of the composite coatings was also slightly improved. However, CTAB was detrimental to both mechanical property and corrosion resistance of the composite coatings. The affecting mechanism of surfactants on the coating properties was discussed.     

The effects of electrodeposition current density on properties of Ni-CNTs composite coatings

The effects of electrodeposition current density on the properties of Ni-carbon nanotubes (CNTs) composite coating were studied with methods of scanning electron microscopy (SEM), tensile testing, hardness testing and electrochemical testing. The results show that the composite coating becomes coarser when the depositing current increases. As the depositing current density increases, carbon content in the composite coating first increases then decreases, with the highest carbon content at 8 A/dm². The adherence, cracking resistance, micro-hardness and electrochemical resistance of the composite coatings all reach their peak values at 8 A/dm². The results indicate that the existence of CNTs in the composite coatings improves toughness, strength and corrosion resistance of the coatings.     

Influence of ion-bombardment-energy on thin zirconium oxide films prepared by dual frequency oxygen plasma sputtering

Thin ZrO₂ films were prepared using dual frequency oxygen reactive plasma sputtering for wear-resistance coating of ceramics products. Influences of ion-bombardment-energy E_i were investigated for improvement of film characteristics. The results revealed that the deposition rate and the hardness of the prepared ZrO₂ thin films gradually increased with increasing E_i for E_i < 220-250 eV and then decreased, whereas the water-contact-angle on ZrO₂ thin films was about 90 °, having a good water-repellent nature.     

Properties of the TiN coatings on previously Ti ion-implanted magnesium alloy substrate

To enhance the mechanical properties of TiN coating on magnesium alloy, metal vapor vacuum arc (MEVVA) ion implantation was performed to modify magnesium alloy substrate before TiN film deposition. Implantation energy was fixed at 45 keV and dose was at 9 × 10¹⁷ cm^− 2. TiN coatings were deposited by magnetically filtered vacuum-arc plasma source on unimplanted and implanted substrate. The microstructure composition distribution and phase structure were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The chemical states of some typical elements of the TiN coating were analyzed by means of X-ray photoelectron spectroscopy (XPS). The properties of corrosion resistance of TiN coatings were studied by CS300P electrochemical-corrosion workstation, and the mechanism of the corrosion resistance was also discussed.     

Development and characterization of single wire-arc-plasma sprayed coatings of nickel on carbon blocks and alumina tube substrates

A single wire-arc-plasma spray torch has been used to develop metal coatings on carbon and alumina substrates under argon atmosphere for various applications. Nickel coatings of around 1 mm thickness have been deposited on selected area (60 mm × 200 mm on each side) of large size carbon blocks by intermittent arc spraying and cooling to reduce thermal stresses and possibility of coating de-lamination from the base substrate. The same process is also used for depositing about 3 mm thick nickel metal coatings (8 mm dia. × 40 mm long) on alumina tubes for developing electrical feed throughs. The nickel coated alumina tubes were tested for the vacuum compatibility of the coated material with the base tube. The coated assemblies could withstand vacuum of the order of 1 × 10^− 6 Torr and the leak rate was found to be less than 1 × 10^− 9 Std. cc/s for Helium gas, indicating excellent bonding of the coated metal with alumina ceramics and no connected open porosity in the coatings. X-ray diffraction studies were conducted for identifying the phases and the optical microscope with image analysis technique was used for studying the microstructure and porosity in the coatings.     

Properties of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 coatings

TiO₂ coatings were manufactured by the High Velocity Suspension Flame Spraying (HVSFS) technique using a nanopowder suspension. Their microstructure, nanohardness, tribological properties and photocatalytic activity were studied and compared to conventional atmospheric plasma sprayed (APS) and HVOF-sprayed TiO₂ coatings manufactured using commercially available feedstock. The HVSFS process leaves a fairly large freedom to adjust coating properties (thickness, porosity, anatase content, hardness, etc…) according to the desired objective. Layers with higher anatase content and higher porosity can be produced to achieve higher photocatalytic efficiency, better than conventional APS and HVOF TiO₂. Alternatively, dense protective layers can be deposited, possessing lower porosity and pore interconnectivity and better wear resistance than as-deposited APS and HVOF layers. In all cases, HVSFS-deposited layers are thinner (20 µm-60 µm) than those which can be obtained by conventional spraying processes.     

PEO protective coatings on inner surface of tubes

Plasma electrolytic oxidation (PEO), also called micro-arc oxidation (MAO) or anode spark deposition (ASD), is a novel technique to produce hard ceramic coatings on metals, such as aluminum, titanium, magnesium (called valve metals) and their alloys. Up to now, many researchers focused on the PEO process for workpieces of regular shapes such as cube, cake or stick samples, etc. But no one paid attention to the irregular samples such as tubular materials. This research emphasized the PEO process of long tubes, especially the way to obtain uniform thickness of ceramic coatings on inner surface of tubes. Furthermore, the PEO kinetic behaviors of aluminum tubes were also investigated. The potential difference between the electrodes indicated a linear relationship with the coating thickness. A central accessory electrode was axially used to eliminate the shielding effect of electric field and was effective to obtain axially uniform coating on the inner surface of the tubes. The combination of hot-dipping aluminum and PEO process to obtain ceramic coating on 45# steel tubes was also performed in this work.     

Effect of the suspension composition on the microstructural properties of high velocity suspension flame sprayed (HVSFS) Al2O3 coatings

Seven different Al₂O₃-based suspensions were prepared by dispersing two nano-sized Al₂O₃ powders (having analogous size distribution and chemical composition but different surface chemistry), one micron-sized powder and their mixtures in a water + isopropanol solution. High velocity suspension flame sprayed (HVSFS) coatings were deposited using these suspensions as feedstock and adopting two different sets of spray parameters.The characteristics of the suspension, particularly its agglomeration behaviour, have a significant influence on the coating deposition mechanism and, hence, on its properties (microstructure, hardness, elastic modulus). Dense and very smooth (Ra ~ 1.3 μm) coatings, consisting of well-flattened lamellae having a homogeneous size distribution, are obtained when micron-sized (~ 1-2 μm) powders with low tendency to agglomeration are employed. Spray parameters favouring the break-up of the few agglomerates present in the suspension enhance the deposition efficiency (up to > 50%), as no particle or agglomerate larger than ~ 2.5 μm can be fully melted. Nano-sized powders, by contrast, generally form stronger agglomerates, which cannot be significantly disrupted by adjusting the spray parameters. If the chosen nanopowder forms small agglomerates (up to a few microns), the deposition efficiency is satisfactory and the coating porosity is limited, although the lamellae generally have a wider size distribution, so that roughness is somewhat higher. If the nanopowder forms large agglomerates (on account of its surface chemistry), poor deposition efficiencies and porous layers are obtained.Although suspensions containing the pure micron-sized powder produce the densest coatings, the highest deposition efficiency (~ 70%) is obtained by suitable mixtures of micron- and nano-sized powders, on account of synergistic effects.     

X-ray photoelectron spectroscopy analysis of zirconium nitride-like films prepared on Si(100) substrates by ion beam assisted deposition

Thin zirconium nitride films were prepared on Si(100) substrates at room temperature by ion beam assisted deposition with a 2 keV nitrogen ion beam. Arrival rate ratios ARR(N/Zr) used were 0.19, 0.39, 0.92, and 1.86. The chemical composition and bonding structure of the films were analyzed with X-ray photoelectron spectroscopy (XPS). Deconvolution results for Zr 3d, Zr 3p_3/2, N 1s, O 1s, and C 1s XPS spectra indicated self-consistently the presence of metal Zr⁰, nitride ZrN, oxide ZrO₂, oxynitride Zr₂N₂O, and carbide ZrC phases, and the amounts of these compounds were influenced by ARR(N/Zr). The chemical composition ratio N/Zr in the film increased with increasing ARR(N/Zr) until ARR(N/Zr) reached 0.92, reflecting the high reactivity of nitrogen in the ion beam, and stayed almost constant for ARR(N/Zr) ≥ 1, the excess nitrogen being rejected from the growing film. A considerable incorporation of contaminant oxygen and carbon into the depositing film was attributed to the getter effect of zirconium.     

Pulsed microwave plasma polymerization of silicon oxide ?lms: Application of efficient permeation barriers on polyethylene terephthalate

A microwave driven low pressure plasma reactor is developed based on a modi?ed Plasmaline antenna for plasma processing of polyethylene terephthalate (PET) foils and bottles. It allows for the treatment of thermolabile packaging materials, e.g. plasma sterilization and permeation barrier coating. Silicon oxide ?lms are deposited on PET foils as a permeation barrier coating. A pulsed hexamethyldisiloxane:oxygen plasma is ignited under various conditions and the oxygen permeation is investigated. A criterion for the homogeneous deposition of SiO_x coatings is described depending on the residence time of process gases. Additionally, the composition of the coatings is analyzed by means of Fourier transform infrared spectroscopy regarding carbon and hydrogen content. A strong relation between barrier properties and ?lm composition is found: good oxygen barriers are observed as carbon content is reduced and ?lms become inorganic, quartz-like. A residual permeation as low as J = 1.0 ± 0.3 cm³ m^− 2 day^− 1bar^− 1 for SiO_x coated PET foils is achieved. The dependencies of important plasma parameters, such as gas mixture, process pressure, power and pulse conditions on oxygen permeation through packaging foil are shown to optimize the coating process.     

Investigation on the microstructure and cracking susceptibility of laser-clad V2O5 /NiCrBSiC alloy coatings

The effect of V₂O₅ on the refinement of microstructures and the reduction of cracking susceptibility of laser-clad NiCrBSiC hardfacing coatings are investigated. It is shown that high volume fraction and inhomogeneous distribution of the coarse brittle phases of chromium borides and carbides in NiCrBSiC layer are the dominating origins for hot cracks. The addition of V₂O₅ has an apparent effect on enhancing the toughness, refining the microstructure and reducing the cracking sensitivity of the coating. This is attributed to the generation of vanadium borides and carbides. The vanadium borides generate prior to and consequently inhibit the formation of chromium borides during rapid solidification because of lower standard free energy of formation of the former. They also restrain the growth of chromium carbides and make them distribute homogeneously. The reasons for crack initiation and distribution are discussed based on the microstructures of the cladding layers. It is demonstrated that the generation of cracks is determined by the microstructure of the cladding layers and the occurrence of cracking is governed by heat stress.     

Microstructure and cavitation erosion characteristics of Al-Si alloy coating prepared by electrospark deposition

With a high-power electrospark deposition (ESD) processing unit, an Al-Si coating was prepared on ZL101 aluminum alloy substrate. The Al-Si coating was composed of many thin deposited layers with the thickness of 30-40 μm and showed peculiar microstructural characteristics. The eutectic Si phase, which was homogenously distributed in the coating, existed in the form of latticed morphology with characteristics of being composed of many fine (~ 50 nm size) spherical Si particles. Hardness tests showed that the microhardness of the coating was in the range of 100-110 HV with a small fluctuation, which implied that the deposited coating possess a uniform strength through the whole coating. After 4 hours' cavitation erosion tests, the cumulative volume loss of Al-Si coating (9.03 mm³) was only 31.5% of the substrate (28.66 mm³), which indicated that the Al-Si coating possessed better cavitation erosion resistance than ZL101 substrate. The excellent performance of Al-Si coating was mainly attributed to the peculiar microstructure and the formation of fine spherical Si particles.     

Plasma polymerized allylamine films deposited on 316L stainless steel for cardiovascular stent coatings

Coronary stents are metallic (316L stainless steel) medical devices used during balloon angioplasty to scaffold diseased arteries and prevent their reblockage. To reduce the restenosis rate, bare metal stent coating is a promising solution. The coating can protect the metallic surface of the stent from corrosion attack caused by the biological environment. In addition, according to Food and Drug Administration (FDA) the coating properties must be guaranteed even after stent expansion. The aim of this study was to develop a dry process to coat the metallic surface from the biological environment by depositing an ultra-thin, stable, cohesive and adhesive plasma polymerized allylamine (CH₂=CH―CH₂―NH₂) coating with high selectivity towards primary amine groups. Plasma polymerized allylamine (PPAA) coatings were deposited on electropolished 316L stainless steel (316L SS) samples using a low pressure plasma reactor (70 kHz). XPS (X-Ray Photoemission Spectroscopy) and FTIR-ATR (Fourier Transform Infrared-Attenuated Total Reflectance) spectroscopy measurements were used to investigate the chemical composition of the coatings. A chemical derivatization technique was employed in order to quantify the amine retention rate of the deposited films. Morphology of the films was evaluated by FE-SEM (Field Effect-Scanning Electron Microscopy) imaging. Furthermore, special attention was devoted to study the stability of the coating and its adhesion properties after plastic deformation up to 25%. The effect of the power discharge and treatment time on these properties was also investigated. Our results showed that coatings present the required adhesion and cohesion properties to be stable upon deionised (D.I.) water immersion and to resist to a stent expansion.