Surface Morphology Of Coatings Research Articles

Effects of calcium glycerophosphate concentration on micro-arc oxidation coating of Mg alloy

Due to its good biocompatibility and compatible hardness with human bones, magnesium (Mg) alloy is considered as a promising material for bone nails and implants. However, Mg and its alloys are characteristics of active chemical properties and high corrosion rate, which is detrimental to the recovery of patients implanted with Mg alloy bone scaffold. In the present work, to retard the corrosion of the Mg alloy, calcium glycerophosphate (C3H7CaO6P•H2O) was added in the electrolyte to form the micro-arc oxidation (MAO) coatings. Various experimental techniques such as XRD, SEM and EDS have been used to characterize the surface morphology and phase composition of MAO coatings. MAO coatings of different CaP content were studied to explore the synergetic effect between the biocompatibility and corrosive properties by means of electrochemical measurements, hemolysis and pyrogen test. The results showed that higher CaP content can improve the biocompatibility and corrosion resistance while decelerating the hemolysis rate.

Preliminary exploration of the electrical insulation performance of APPJ SiO2 coating on 304 stainless steel pipes

The atmospheric pressure plasma jet (APPJ) technique is used to prepare SiO2 insulation coatings onto the outer surface of 304 stainless steel pipes (304-SSPs). The diameter and surface roughness of 304-SSPs increase from 18 to 32 mm and 0.1 to 1.0 μm. The results show that APPJ scanning methods have little influence on the chemical composition, chemical bond structure, crystal structure, and surface morphology of SiO2 coatings, which display Si/O = 1/(1.95 ± 0.02), dominant Si-O bond, amorphous structure, and dense layer structure. However, APPJ scanning methods have obvious influence on the uniformity and electrical insulation resistance (REI) of SiO2 coatings. Only when APPJ is parallel to the normal direction of 304-SSPs outer surface, the balance between APPJ impingement, gravity, and coating surface tension could reach the optimal condition, which improves the uniformity and REI of SiO2 coatings. With the optimal scanning method, REI of SiO2 coatings decrease with the increase of surface roughness of 304-SSPs, which are caused by the decrease of effective thickness of SiO2 coatings. Most importantly, with the other parameters as constant, REI of SiO2 coatings reaches to ∼101 MΩ and displays the better uniformity when the diameter of 304-SSPs is about 5 times the diameter of APPJ tube.

Characteristics and corrosion resistance of hard-anodised 6061-T6 aluminium alloy

ABSTRACT Hard anodic coatings were formed on 6061-T6 aluminium alloy samples using direct current. Surface morphology and cross-section of coatings, as well as their hardness and corrosion behaviour, were studied. It was found that decreasing the applied current density from 55 to 35 mA cm−2 resulted in more uniform coatings with fewer surface defects, lower porosity, and higher hardness values. The optimum electrolyte concentration and hard-anodising time, where less defective coatings were achieved, were found to be 180 g L−1 and 50 min, respectively. The hardness of the optimum film was around 500 HV, and its corrosion current density was 3.2 × 10−9A cm−2. All other samples that were produced at different current densities (30-55 mA cm−2), times (40-100 min), and electrolyte concentrations (25-220 g L−1) had lower hardness and corrosion resistance than the optimum coating. The polarisation resistance of the optimum coating was about 1100 times higher than the bare substrate.

Effect of parallel magnetic field on growth and properties of jet electrodeposited Ni-Co-SiC composite coatings

Ni-Co alloy and composite coatings were fabricated by a parallel magnetic field-assisted jet electrodeposition (PMAJE), and the effect of nano SiC particles on the properties of Ni-Co alloy coating was studied. Crystal structure, micromorphology and 3D profile, microhardness, adhesion and corrosion resistance of the Ni-Co alloy and composite coatings were studied respectively. The deposition mechanism of Ni-Co alloy and composite coatings in PMAJE was analyzed. It was found that the parallel magnetic field (PM) significantly affected the surface morphology and flatness of Ni-Co alloy and composite coatings in jet electrodeposition. The microprotrusions of Ni-Co alloy and composite coating gradually increased with the magnetic flux density. PM refined the grain growth of Ni-Co alloy and composite coating when the magnetic flux density was 50 mT. Compared with jet electrodeposition, the Ni-Co-SiC composite coating exhibited a high mechanical properties and corrosion resistance in PMAJE. The microhardness of Ni-Co-SiC composite coating was 582.4HV, the adhesion was 27.2 N, the corrosion current density of Ni-Co-SiC composite coating was as low as 5.824 μA·cm−2 and the polarization resistance reached 6471 Ω·cm2, which was better than that of jet electrodeposition.

Effect of annealing on the properties of plasma-enhanced atomic layer deposition grown HfO2 coatings for ultraviolet laser applications

HfO2 monolayer coatings are grown by plasma-enhanced atomic layer deposition (PEALD), and the effects of the thermal annealing atmosphere and temperature on the surface morphology, crystal structure, chemical composition, optical absorption, and laser-induced damage threshold (LIDT) of the HfO2 monolayer coatings are investigated. The experimental results show that annealing in an oxygen atmosphere is more beneficial to the decomposition of impurity compounds (carbonates and amines) and the elimination of oxygen deficiency in HfO2 coating than in a nitrogen atmosphere. Compared to the as-deposited coating, the HfO2 coating annealed under an oxygen atmosphere and specific temperature has lower C and N impurity contents, a better stoichiometric ratio, and thus lower absorption and higher LIDT. The highest LIDT (at 355 nm) of 6.3 J/cm2 is obtained at an annealing temperature of 700 ℃, which is approximately three times that of the as-deposited coating. We believe that this experimental result can help improve the LIDT of PEALD-grown oxide coatings or other oxide coatings containing similar impurities.

Investigation of Tribological Characteristics of PEO Coatings Formed on Ti6Al4V Titanium Alloy in Electrolytes with Graphene Oxide Additives.

Coatings with a thickness from ~40 to ~50 µm on Ti6Al4V titanium alloys were formed by plasma electrolytic oxidation (PEO) in a silicate-hypophosphite electrolyte with the addition of graphene oxide. The PEO treatment was carried out in the anode-cathode mode (50 Hz) at a ratio of anode and cathode currents of 1:1; their sum density was 20 A/dm2, and the treatment's duration was 30 min. The effect of the graphene oxide's concentration in the electrolyte on the thickness, roughness, hardness, surface morphology, structure, composition, and tribological characteristics of the PEO coatings was studied. Wear experiments, under dry conditions, were carried out in a ball-on-disk tribotester with an applied load of 5 N, a sliding speed of 0.1 m·s-1, and a sliding distance of 1000 m. According to the obtained results, the addition of graphene oxide (GO) into the base silicate-hypophosphite electrolyte leads to a slight decrease in the coefficient of friction (from 0.73 to 0.69) and a reduction in the wear rate by more than 1.5 times (from 8.04 to 5.2 mm3/N·m), with an increase in the GO's concentration from 0 to 0.5 kg/m3, respectively. This occurs due to the formation of a GO-containing lubricating tribolayer upon contact with the coating of the counter-body in the friction pair. Delamination of the coatings during wear occurs due to contact fatigue; with an increase in the concentration of GO in the electrolyte from 0 to 0.5 kg/m3, this process slows down by more than four times.

Effect of boron carbide reinforcement on surface properties of electroless Ni–B and Ni–B–W coatings

Today, composite coating of nickel-boron with an electroless method is considered as an innovative, attractive, and trend topic. It is known that the hardness and tribological properties of electroless nickel-boron (EN-B) coating can be improved with co-deposition of ceramic particles. In this study, boron carbide (B4C) was added to binary (Ni–B) and ternary (Ni–B–W) EN-B coatings: (i) Ni–B–B4C, and (ii) Ni–B–W–B4C. Surface morphology, phase structure, tribological characteristics, and electrochemical behaviors for all coatings were investigated and compared. The surface morphology of composite coatings was transformed from cauliflower to cloudy-shaped with the addition of B4C particles. Characteristic amorphous structure with some degree of crystallinity was obtained at all types of coatings. Experimental test results revealed that the boron carbide particles provide a noticeable enhancement in hardness. The hardnesses of composite coatings were improved by approximately 52% in Ni–B–B4C and 62% in Ni–B–W–B4C compared to EN-B coatings. Boron carbide reinforcement and poly-alloying with tungsten caused an increase of hardness at Ni–B from 673 HV0.025 up to around 1096 HV0.025 (Ni–B–W–B4C) which is a noticeable increase in hardness. With boron carbide addition binary and ternary coatings wear factors showed a discernible diminishment by several times. Besides, electrochemical analyses indicated a minimal decrease in the corrosion resistance of composite coatings compared to that of Ni–B and Ni–B–W coatings.

Hot corrosion behavior of intermixed layered LTA/YSZ thermal barrier coatings

In present work a unique combination of hybridized intermixed five layered coatings structure is developed by using Arc plasma spray method. A Lanthanum-Titanium-Aluminum Oxide (LaTi2Al9O19) that is LTA powder is used as a ceramic top coat material along with YSZ. A bond coat of nickel dominant NiCrAlY having thickness of 90 µm (µm) is applied on the substrate Inconel 718 post shot blasting. A preceding layer of YSZ with 50% and remaining 50% of 50 µm bond coat is sprayed. After that a layer of YSZ having thickness of 100 µm is applied. Then a layer of 50 µm thickness having 50% YSZ and 50% LTA is applied. LTA layer of thickness 100 µm is applied at the end. Cyclic and isothermal hot corrosion is performed using salts Na2SO4 & NaCl in 3:1 mass proportion at 900 °C. The mass increment by as-sprayed intermixed coatings is less than 0.05 mg/cm2 and it is 0.02 mg/cm2 for annealed coating. An annealed intermixed coating shows negligible mass increment and has survived upto 100 h in case of isothermal hot corrosion. In case of cyclic hot corrosion annealed intermixed coatings shown no mass increment and is survived till 70 h. Mass increment of as-sprayed intermixed coatings is less than 0.05 mg/cm2 but after 50 h severe spallation of coating occurred. The coating surface morphology is evaluated through field emission scanning electron microscopy (FESEM) and x-ray (XRD) diffraction.

Heat Transfer Characteristics of Pool Boiling with Scalable Plasma-Sprayed Aluminum Coatings

To ensure adequate reliability in two-phase cooling systems involving boiling, it is essential to enhance the heat transfer coefficient and maximize the critical heat flux (CHF) limit. A key technique to avoid surface burnout and increase the CHF limit in pool boiling is the frequent coolant supply to the probable dry-out locations. In the present work, we have explored the plasma-spray coating as a surface modification technique for enhancing heat transfer coefficient and CHF value in pool boiling applications. Three plasma-coated aluminum surfaces (C-15, C-20, and C-25) are fabricated on a copper substrate at three different plasma powers of 15, 20, and 25 kW, respectively. Detailed surface morphologies of the plasma-sprayed coatings are presented, and their roles in pool boiling heat transfer mechanisms are analyzed. Plasma-coated surfaces exhibit wickability characteristics and enhanced wettability compared to the plain copper surface. Saturated pool boiling experiments are performed with DI (deionized) water at atmospheric pressure. Plasma spray-coated surfaces show favorable boiling incipience with less wall superheat and more active nucleation sites than the plain copper surface. Compared to the plain copper surface, enhancement values of nearly 68, 60.7, and 55.5% in the heat transfer coefficient are observed for C-15, C-20, and C-25 plasma-coated surfaces, respectively. Experiments could not be performed beyond the heat flux of 197 W/cm2 due to repeated failure of the cartridge heaters. Based on the experimental measurement of wickabilities, the CHF values of plasma-coated surfaces have been theoretically calculated. Compared to the plain copper surface, a maximum 2.39 times higher CHF value is observed for C-15 plasma-coated surface. Improved wettability and wickability are responsible for CHF enhancement in the case of plasma-coated surfaces. At higher heat flux, capillary wicking and frequent rewetting of the dryout locations delay the burnout phenomenon, enhancing CHF in plasma-coated surfaces. The plasma-spray coating is a robust and scalable process, which can be a potential candidate for high heat flux dissipation in various industrial applications.

Effect of laser remelting on copper‑nickel alloy coating prepared by extreme high-speed laser cladding

Copper‑nickel alloy is an important corrosion resistant material for marine environment. Advanced extreme high-speed laser cladding (EHLA) technology can realize a metallurgical bonding of copper‑nickel alloy and obtain better surface and structural characteristics. However, because of the high heat conductivity and high reflectivity of copper, copper-alloy coatings prepared by EHLA have defects such as segregation, blowhole, and cracks. In this study, EHLA copper‑nickel alloy coating was improved by in-situ laser remelting process. A single pass finite-element-model of the remelting process was built to analyze the effect of remelting speed on the solidification of molten pool. The surface morphology, microstructure, element distribution, and phase composites of coatings at different remelting speeds were characterized, and the anti-corrosion of the coatings was further discussed by constant potential polarization and EIS. The results show that heat input increases the convection strength of liquid in molten pool, promotes the uniformity of the coating microstructure and reduces the defects. Although the columnar crystals to equiaxed crystals transformation of the coatings did not obviously changed at different remelting speed, the high heat input during remelting speed of 5 m/min promoted the alloying of copper‑nickel coating and eliminated the defects, obtaining the best corrosion resistance. The research will provide some technical guidance and theoretical support for the practical application of copper‑nickel alloy coating with EHLA-LR process in marine equipment.

A study of Ni–P electroless coating modified by rare earth oxide (La2O3, Y2O3 and Ce2O3) on pre-phosphorized zirconium alloy

Electroless Ni–P coatings are successfully deposited on the surface of pre-phosphorized zirconium alloy from plating baths containing various concentrations of rare earth oxide (Re Y2O3, Ce2O3 and La2O3). The surface morphology, elementary composition, and structure of coatings are characterized by scanning electron microscopy, energy dispersive X-ray spectrometer and X-ray diffraction, respectively. Gravimetrical method is used to calculate the deposition rate. Electrochemical corrosion resistance of coatings is analyzed using electrochemical workstation. Results revealed that a small amount addition of rare earth oxides does not change the original cauliflower-like morphology and amorphous structure of coatings. When the amount of addition is moderate, the deposition rate and microhardness increase with the increment of the amount of addition, while P content decreases. However, excessive addition can lead to poisoning of the plating bath. Electrochemical corrosion data indicates that the addition of rare earth oxide in the plating solution has little influence on the corrosion resistance of the coating.

Growth mechanism of 2024 aluminum alloy micro-arc oxide layer in cobalt-containing electrolyte

The coatings were prepared on the surface of 2024 aluminum alloy by micro-arc oxidation. The effect of the addition of cobalt salt on the growth mechanism of the coating was investigated. The addition of cobalt salt prolonged the spark discharge phase of microarc oxidation and achieved the co-film formation of the solution component and the substrate. XRD, SEM, EDS, and XPS were used to investigate changes in the physical composition, surface morphology, and chemical composition of coatings prepared at different times. The hardness and corrosion resistance of the coatings were investigated as oxidation time increased. The surface of the coatings can be divided into two regions: the loose “cobalt-rich zone” and the dense “aluminum-rich zone”, and with the increase of oxidation time, the surface of the coating becomes more dense and flat, and the pores are mostly closed. The time-voltage curve shows that the growth of the micro-arc oxide coating in cobalt-containing electrolyte can be divided into three stages. The coating grows rapidly in the first 15 min, and a black coating is obtained, with the color-revealing substance being CoO; the coating turns from black to blue after 15 min, as CoO transforms to blue CoAl2O4. XPS and XRD analyses showed that the addition of cobalt salt significantly promoted the formation of alumina in the coating, and the conversion of CoO to CoAl2O4 and γ-Al2O3 to α-Al2O3 occurred at the later stage of micro-arc oxidation, when the hardness of the coating would obtain a large enhancement. The corrosion resistance test results showed that the coating prepared by treatment in the cobalt-containing electrolyte for 60 min showed the best corrosion resistance with a corrosion potential 100.3 mV higher than that of the substrate and a corrosion current reduced by 4 orders of magnitude to 6.20 × 10−9 A·cm−2.

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.

Research of On-Line Monitoring Technology Based on Laser Triangulation for Surface Morphology of Extreme High-Speed Laser Cladding Coating

This work aims to develop a novel method for on-line monitoring of coating quality during the Extreme High-speed Laser Cladding (EHLA) process. JG-11 coating was prepared by EHLA, and microstructure, microhardness, corrosion performance, and scratch resistance were investigated. To analyze the influences of fluctuations in processing parameters on coating quality, a single-factor experiment scheme was designed and an on-line monitoring system based on laser triangulation was built. Furthermore, a new forming method for the surface profile of EHLA coating was proposed, and a new comprehensive evaluation index of surface morphology was accordingly designed. Benefitting from the extremely high cooling rate, EHLA JG-11 coating had fine grains, high hardness, and better corrosion resistance and scratch resistance than those of Electroplating Hard Chromium (EHC). The results revealed that the surface morphologies presented different characteristics due to the fluctuations of process parameters, such as high surface flatness, deep pits, small peaks, poor directionality, etc. The comprehensive evaluation index composed of Sa, Ssk, and Str could effectively characterize the surface morphology of EHLA coating, which proved that the monitoring system and evaluation method could realize on-line monitoring of the process parameters during the EHLA process.

Contact Angle and AFM Analysis of Glassy Carbon Coatings after an Extended Stay on the International Space Station (ISS)

The proposed work presents the results for contact angle and atomic force microscopy (AFM), which are applied to characterize the surface of two types of samples, “reference” sample and “space” sample, both made of the same material – graphite covered with glassy carbon coating, but stored for 28 months in different environments (conditions). One “space” sample was mounted outside the International Space Station, and the other “reference” sample was stored on Earth.
 The technique applied to measure the contact angle of wetting of the surface of the samples (hydrophilicity-hydrophobicity) is using water droplets. Values of the energy surface area of the sample surfaces were determined by measuring the contact angle with the substances formamide and diiodomethane. Atomic force microscopy (AFM) was applied to characterize the glass surface morphology of carbon coatings.

Influence of the Biological Medium on the Properties of Magnesium Doped Hydroxyapatite Composite Coatings

In this paper, the stability of magnesium-doped hydroxyapatite/chitosan (MHC) suspension obtained with the sol-gel approach was evaluated using nondestructive ultrasound measurements. The MHC coatings obtained by the spin-coating technique were characterized before and after immersion for 7 and 14 days, respectively, in Dulbecco’s modified eagle medium (DMEM) by scanning electron microscopy, equipped with an EDAX detector. Also, the functional groups present on the MHC coatings surface were analyzed with the aid of attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy. The surface microstructure was evaluated using two commentary techniques, namely atomic force microscopy (AFM) and metallographic microscopy (MM). The influence of immersion in DMEM on the biological properties was studied with in vitro studies using primary osteoblast and HCT-8 cell lines. Our results revealed that both surface morphology and chemical composition of the MHC coatings allow rapid development of a new apatite layer on their surface after immersion in DMEM. Preliminary in vitro biological studies underlined the noncytotoxic effect of the studied samples on the proliferation of primary osteoblast and HCT-8 cell lines, which makes them a promising candidate for applications in fields such as orthopedics or dentistry. The antifungal assay of the MHC coatings was assessed using Candida albicans ATCC 10231 and their results showed a good inhibitory effect. The coatings made on the basis of the MHC composite could contribute to increasing the degree of success of implants by decreasing the risk of infections and postoperative inflammation.

Tribological Properties of Cr2O3, Cr2O3–SiO2-TiO2 and Cr2O3–SiO2-TiO2-Graphite Coatings Deposited by Atmospheric Plasma Spraying

In this study, Cr2O3, Cr2O3-SiO2-TiO2 and Cr2O3-SiO2-TiO2-graphite coatings were formed by atmospheric plasma spraying. The influence of SiO2-TiO2 and SiO2-TiO2-graphite reinforcements on the surface morphology, elemental composition, structure and tribological properties of chromia coatings was determined. The friction coefficients and specific wear rates were investigated by a ball-on-flat configuration using 1 N and 3 N loads under dry-lubrication conditions. The addition of SiO2-TiO2-graphite resulted in the lowest surface roughness and the most homogenous surface of the coatings. The X-ray diffraction (XRD) measurements demonstrated that all as-sprayed coatings consisted of an eskolaite chromium oxide phase. The results showed that the Cr2O3-SiO2-TiO2 coating demonstrated the lowest friction coefficient values. The SiO2-TiO2 and SiO2-TiO2-graphite additives reduced the specific wear rates of Cr2O3 coatings by 30% and 45%, respectively. Additionally, the wear resistance was improved almost 45 times in comparison to the steel substrate.

Investigation of ZnCoP coating electrodeposited on Q235B steel and its corrosion resistance in simulated acid rain solution

In order to further improve the corrosion resistance of Q235B steel used for sports equipment, the Zn coating, ZnCo coating, and ZnCoP coating are electrodeposited respectively on the Q235B steel. The relationship between corrosion resistance, surface morphology, structure, and surface roughness of different coatings is studied. All the electrodeposited coatings are crystalline with typical diffraction peaks. Three diffraction peaks of Co(111),Co(200), and Co(220) can be observed on the XRD pattern of ZnCoP that indicate that the addition of sodium hypophosphite in the plating solution is conducive to the electrodeposition of cobalt. The Zn coating shows strip-like surface morphology with the roughest surface (1.431 µm) while the ZnCo coating and ZnCoP coating are both composed of granular structures with compact surfaces. Compared with all the coatings, the corrosion current density of Zn coating is about 345.78 μA/cm2 which is the largest indicating poor corrosion resistance. The corrosion current density of ZnCo coating and ZnCoP coating is 47.83 μA/cm2 and 15.63 μA/cm2 respectively which are both lower than that of Q235B. The ZnCoP coating has the best corrosion resistance due to phosphorus and cobalt doped in the coating that refine the grain size and improve the surface compactness.

Properties of TiAlN Coatings Obtained by Dual-HiPIMS with Short Pulses

The paper focuses on the dual high-power impulse magnetron sputtering of TiAlN coatings using short pulses of high power delivered to the target. The surface morphology, elemental composition, phase composition, hardness, wear resistance, and adhesive strength of TiAlN coatings with different Al contents were investigated on WC-Co substrates. The heat resistance of the TiAlN coating was determined with synchrotron X-ray diffraction. The hardness of the TiAlN coating with a low Al content ranged from 17 to 30 GPa, and its wear rate varied between 1.8∙10-6 and 4.9∙10-6 mm3·N-1·m-1 depending on the substrate bias voltage. The HF1-HF2 adhesion strength of the TiAlN coatings was evaluated with the Daimler-Benz Rockwell C test. The hardness and wear rate of the Ti0.61Al0.39N coating were 26.5 GPa and 5.2∙10-6 mm3·N-1·m-1, respectively. The annealing process at 700 °C considerably worsened the mechanical properties of the Ti0.94Al0.06N coating, in contrast to the Ti0.61Al0.39N coating, which manifested a high oxidation resistance at annealing temperatures of 940-950 °C.

Mechanical properties, electrochemical performance and interfacial contact resistance of as-deposited and annealed NbxAlyC and CrxAlyC coatings on stainless steel 316L substrates

NbxAlyC and CrxAlyC coatings (where x is the Nb or Cr target current and y is the Al target current) were deposited on stainless steel 316L substrates using a radio frequency unbalanced magnetron sputtering system. The sputtered NbxAlyC and CrxAlyC coatings were vacuum annealed at 800°C and 750°C for 1 h, respectively. The cross-sectional and surface morphologies of the as-deposited coatings showed a columnar structure and a loose surface, respectively. The X-ray diffraction results showed that the annealed NbxAlyC and CrxAlyC coatings contained Nb2AlC and Cr2AlC MAX crystalline phases, respectively, The MAX crystalline phase improved the hardness, corrosion resistance, and electrical conductivity of the coatings. The annealed coatings also formed an Al2O3 layer with a small cluster size and a dense surface, which significantly improved the corrosion resistance compared to that of the as-deposited coatings. The electrochemical test results showed that the corrosion current density (Icorr) and interfacial contact resistance (ICR) values of all the annealed coatings were lower than those of the as-deposited coatings and were consistent with the standard for the bipolar plates in commercial proton exchange membrane fuel cells. Among all the coatings, the annealed Nb2.5Al0.5C-H coating showed the lowest Icorr and ICR values.