As-deposited Coatings Research Articles

Enhancing the hardness and damage-tolerance of CoCrNiAlTi coatings through dual-phase and nanotwinned structures

A series of CoCrNiAlTi coatings were deposited, via direct current (DC) magnetron sputtering, onto M2 steel substrates employing a range of substrate bias voltages (−20 V ~ -120 V). Dual phase (fcc and hcp) microstructures, with high compositional homogeneity, were observed for all as-deposited coatings. Highly refined columnar grains, with a high density of twin boundaries that were oriented perpendicular to the coating growth direction, were identified. As the bias voltage increased, elements in the high entropy alloy exhibited different sputtering yields that led to different degrees of re-sputtering in the deposited films. This, in turn, played a critical role in determining the coating composition and hence stacking fault energy. Accordingly, the ratio between the hcp and fcc phases varied, with a maximum fraction of the hcp phase observed at a bias voltage of −80 V. The fraction of the hcp phase then decreased as the voltage increased to −120 V. An exceptional hardness of value of ~9.5 GPa, along with appreciable damage-tolerance, was exhibited in the coating deposited at −80 V. It is suggested that the presence of nanotwins, as well as the dual-phase microstructure, contributed to this excellent strength-ductility unity.

Microstructural Properties and Wear Resistance of Fe-Cr-Co-Ni-Mo-Based High Entropy Alloy Coatings Deposited with Different Coating Techniques

Materials can be subjected to severe wear and corrosion due to high temperature, high pressure and mechanical loads when used in components for the production of geothermal power. In an effort to increase the lifetime of these components and thus decrease cost due to maintenance High-Entropy Alloy Coatings (HEACs) were developed with different coating techniques for anti-wear properties. The microstructure, mechanical and tribological properties of CoCrFeNiMox (at% x = 20, 27) HEACs deposited by three different technologies—high-velocity oxygen fuel (HVOF), laser cladding (LC) and electro-spark deposition (ESD)—are presented in this study. The relationship between surface morphology and microstructural properties of the as-deposited coatings and their friction and wear behavior is assessed to evaluate their candidacy as coatings for the geothermal environment. The wear rates were lower for the HVOF coatings compared to LC and ESD-produced coatings. Similarly, a higher hardness (445 ± 51 HV) was observed for the HVOF HEACs. The mixed FCC, BCC structure and the extent of σ + µ nano precipitates are considered responsible for the increased hardness and improved tribological performance of the HEACs. The findings from the study are valuable for the development of wear-resistant HEAC for geothermal energy industry applications where high wear is encountered.

Thermal property and failure behaviors of Gd doped LaZrCeO coatings with feathery microstructure

LaZrCeO coatings are promising candidates to substitute Y2O3-stabilized ZrO2 in advanced gas turbine engines. In this study, Gd doped LaZrCeO coatings were deposited by electron beam physical vapor deposition. This study focuses on the phase, microstructure, thermal property, and thermal durability of (La1-xGdx)2(Zr0.7Ce0.3)2O7 coatings. The as-deposited coatings show relatively good thermal shock life and thermal cycling life. The broken regions are observed on the interface of thermal barrier coatings. The failure behaviors are relevant with crack evolution and thermally grown oxide growth. This study might guide the investigation of advanced coatings under high temperature.

Hot corrosion behavior of hybrid double-layered LZ/LZC + YSZ thermal barrier coatings made using powder and solution precursor feedstock

Lanthanum zirconate (La2Zr2O7; LZ) and cerium doped lanthanum zirconate (La2(Zr0.7Ce0.3)2O7; LZC) has gained immense attention as thermal barrier coating (TBC) materials because of their superior thermophysical properties, high-temperature phase stability and chemical inertness than yttria-stabilized zirconia (YSZ). In the present study, LZ and LZC double-layered TBCs are prepared by solution precursor plasma spray process (SPPS) as a top layer over the atmospheric plasma sprayed YSZ coatings, and their hot corrosion behavior is studied. The as-deposited SPPS coatings exhibited single phases, and the microstructural morphologies reveal a hierarchically structured surface with vertically cracked microstructure. Exposure to the high temperature corrosion process results in damage of the morphology and microstructure within the SPPS layer by forming lanthanum vanadate (LaVO4) and lanthanum cerium vanadate ((LaCe)VO4) in LZ and LZC coatings, respectively, while leaching out zirconia (ZrO2). In the case of conventional YSZ, the corrosive salts infiltrate entirely through the coating and result in complete densification and associated phase changes. While combining the above aspects, the hybrid double-layered architecture of LZ/YSZ and LZC/YSZ coatings can be beneficial to protect the underlying YSZ layer against the detrimental infiltration of molten corrosive salt.

Mechanical properties and high temperature oxidation resistance of (AlCrTiV)N coatings synthesized by cathodic arc deposition

High entropy alloy nitrides (AlCrTiV)N coatings were deposited by cathodic arc evaporation at various deposition bias and temperature. The mechanical properties of the as-deposited coatings and their oxidation resistance after annealing at various temperatures were analysed. X-ray analyses show in all coatings a FCC structure with a (111) preferred orientation up to 750 °C. Up to 600 °C, no oxygen penetration is measured in the films while at 800 °C, critical spalling occurs for all samples. The coating deposited at a temperature of 300 °C and negative bias of 100 V exhibits the most interesting compromise in both oxidation resistance and mechanical performances with a hardness of 40 GPa, a friction coefficient of 0.44 and a wear rate of 5.8 · 10−7 mm3·N−1·m−1. Annealing in air at 600 °C for 2h reduces the wear rate to 2.1 · 10−7 mm3·N−1·m−1 and the surface hardness to 17 GPa. Although annealing forms a thin oxide layer that reduces the hardness on the surface of the coating, it also averages the hardness inside the film to 33 GPa regardless of the deposition bias.

Electrodeposition and wear behavior of NiCoW ternary alloy coatings reinforced by Al2O3 nanoparticles: Influence of current density and electrolyte composition

Al2O3 nanoparticles incorporated NiCoW ternary alloy coatings were synthesized by direct current electrodeposition from sulphate-citrate electrolyte. The influence of applied current density and electrolyte composition on the evolution of surface morphology, element composition, microstructure, microhardness and sliding wear behavior of the as-deposited coatings was investigated. The as-deposited coatings exhibit colony surface morphology and the inter-colony boundaries lead a leaf vein-like cross-sectional structure. The composition of the as-deposited coatings is controlled by the induced co-deposition mechanism and the competition among the reductions of Ni, Co, Ni-W and Co-W. The as-deposited coatings change from crystalline to amorphous structure with the decrease of Ni content in coatings and the critical Ni content for the transformation is about 40 at.%. The incorporation of Al2O3 nanoparticles leads to a little increase of microhardness, degrades adhesive wear and oxidation wear, then significantly improve wear resistance. Variations in electrodeposition conditions lead the as-deposited coatings with different wear behavior, including abrasive wear, adhesive wear and oxidation wear, accompanying with plastic deformation. An attempt is made to establish the relationship between the electrodeposition conditions and wear behavior.

Development of SiC-ZrC-based ultra-high temperature ceramic coatings via composite method of polymer precursor pyrolysis plus gaseous reactive infiltration

SiC-ZrC-based composite coatings on carbon/carbon composites via ZrC polymer precursor pyrolysis (PPP) plus gaseous reactive infiltration (GRI) of Si or ZrSi 2 , separately denoted as GSIC and GZSIC, were comparatively studied to explore the feasibility and effectiveness of GRI of ZrSi 2 . This composite method by applying ZrSi 2 or Si was proven to similarly produce dense coatings with enhanced coating/substrate interfacial bonding strength for the formed zig-zag transition layer. Furthermore, the GRI of ZrSi 2 instead of Si further increased the UHTC (ZrC) content in the as-deposited coatings resulted from the reaction of infiltrated ZrSi 2 and pre-introduced graphite, enhancing the ablation resistance of the coatings for carbon/carbon composites due to the formed stable Zr-rich Zr-Si-O multiphase oxide. The average mass and linear ablation rates were −0.12 mg/s and 1.56 μm/s for GSIC and −0.3 mg/s and −0.53 μm/s for GZSIC specimens, respectively. The average linear ablation rate of the GZSIC specimen was decreased by ~134% when compared to the GSIC specimen. These results further indicate that the PPP + GRI method with metal silicide MeSi 2 (Me = Zr, Ti, Hf, Cr, etc.) will contribute to the optimal fabrication of dense composite coatings with controllable microstructure, adequate evenly-distributed ultra-high temperature ceramic (UHTC) phases, enhanced interfacial bonding strength and wide circumstance suitability for potential applications. The related work is ongoing in our laboratory. • SiC-ZrC-based coatings were made by precursor pyrolysis plus gaseous reactive infiltration. • Pre-introduced graphite reacted with infiltrated ZrSi 2 further improved ZrC content in the coatings. • The gaseous infiltrated ZrSi 2 instead of Si enhanced ablation resistance of the coatings. • The coating's linear ablation rate was reduced by ~134% as gaseous infiltrated with ZrSi 2 .

A comparative study on high-temperature air oxidation of Cr-coated E110 zirconium alloy deposited by magnetron sputtering and electroplating

Chromium coatings were deposited either by magnetron sputtering and electroplating on E110 zirconium alloy. The as-deposited Cr coatings showed dense microstructure, but different mechanical properties and adhesion behavior depending on the deposition technology. High-temperature oxidation was performed in air at 1100 °C for 40 min. The magnetron-deposited coatings had a stable oxidation behavior which was improved by increasing coating thickness. Surface activation of E110 alloy prior to Cr electroplating resulted in formation of the interlayer with a thickness of 7–12 μm between the coating and the alloy containing hydride and fluoride phases. It was shown that cracks and defects in this interlayer of the electroplated Cr coatings affected the coating adhesion and resulted in lower oxidation resistance compared to the magnetron-deposited coatings.

Effect of microstructure on the thermal conductivity of thermal barrier coating deposited by chelate-flame spraying

To reveal the structure-property relationships of the microstructure and the thermal conductivities of the coatings, Er2O3 coatings were directly deposited on an aluminum alloy substrate without bond coating by chelate flame spraying (CFS). The thermal insulation capability, thermal conductivity, microstructure, porosity, and splats of each coating were studied as a function of thickness. The coating with a thickness of 300 μm exhibited a high ΔT value that was 43.1% higher than that of the 100 μm-thick coating deposited with the same spray conditions. The 100 μm-thick coating exhibited low thermal conductivity, which decreased from 4.56 Wm−1K−1 to 2.57 Wm −1K−1 in the semistable phase (500 °C and 600 °C); this was mainly due to its special microstructure, which contained more mesh splats and higher porosity (28.3%) than the 300 μm-thick coating. Moreover, the effects of the microstructure on the thermal insulation capacities of the CFS coatings were determined. The results showed that as-deposited thick coatings offered higher heat conduction in the studied temperature range (from room temperature to 600 °C) than thin coatings. To conclude, this study also provides a new test method to determine thermal conductivity and thermal insulation capacity.

Protective Cr coatings with CrN/Cr multilayers for zirconium fuel claddings

Cr coatings with CrN/Cr multilayers with a layer thickness of 100, 250 and 750 nm were deposited by a multi-cathode magnetron sputtering on E110 zirconium alloy in Ar and Ar + N2 atmosphere. The mechanical properties of the as-deposited coatings were measured by scratch testing and tribometry. A comparative study of crack and oxidation resistance of the Cr-coated E110 alloy was performed under thermal cycling (1000 °С) and high-temperature (1200–1400 °С) steam oxidation conditions. The influence of the CrN/Cr multilayers and their thickness on the mechanical properties and oxidation resistance of the coated E110 alloy is discussed. It is found that the CrN/Cr multilayers can limit interdiffusion between the coating and E110 alloy at high temperatures only for a short period.

A novel synthesis method for functionally graded alloy coatings by induced electrodeposition

In this paper, a novel method for synthesis of functionally graded alloy coatings by using induced co-deposition has been proposed. The graded NiCoW alloy coatings were synthesized by one step, galvanostatic, direct current electrodeposition from a sulphate-citrate electrolyte. Scanning electron microscope, energy dispersive spectroscopy and X-ray diffractometer were applied to characterize the as-deposited coatings, which possess the composition and structure gradually varying crossing the coating thickness. During the electrodeposition, the competition between the formation of Ni complexes and Co complexes as well as the competition between induced deposition of W and direct deposition of Ni, are responsible for the formation of graded NiCoW coatings. This easy synthesis method by using induced electrodeposition should be a promising route for the preparation of functionally graded alloy materials.

Effect of post coating processing on the morphological and mechanical properties of plasma Spray-reinforced hydroxyapatite coating

The biocompatible Hydroxyapatite (H.A.) coating helps titanium alloy recover and insulates implants from the body's environment. FDA recommends plasma spray to cover H.A. on body implants. H.A.'s poor mechanical properties limit its use in weight-bearing orthopaedic applications like hip and knee joints. This study examines the influence of H.A. reinforcement on plasma-sprayed coatings' microstructure and mechanical characteristics. The influence of thermal post-treatments on as-deposited (pure H.A. & reinforced H.A.) coatings was also examined. SEM images of as-deposited (pure H.A. & reinforced H.A.) coatings were taken before and after thermal processing (S.E.M.). The use of reinforcement improves as-sprayed coatings' microhardness and bond strength. After two hours at 700 °C in air, both substrates had ultrafine particles that may connect to bone and promote bone development.

Role of heat treatment on the friction and wear behavior of duplex electroless nickel deposits

In the present investigation, duplex electroless Ni-P/Ni-Cu-P and Ni-P/Ni–W-P coatings are deposited on mild steel specimens using the dual bath technique. Both the coatings are developed with Ni-P as the inner layer. The scanning electron microscope (SEM), X-ray diffraction techniques (XRD) and energy dispersive X-ray analysis (EDX) are used to characterize the duplex coatings at pre and post-heat treatment state. Heat treatment is carried out at a temperature range between 200 °C and 800 °C. The surface morphology of the as-deposited coatings is nodular. Grain growth and grain boundaries are clearly visible in heat treated coatings. The EDX of Ni-P/Ni-Cu-P and Ni-P/Ni–W-P coating showed the phosphorus content as 12% and 9% respectively indicating high-P deposits. The copper and tungsten content of the coatings are 10% and 3% respectively. XRD analysis of both the as-deposited coatings indicate amorphous phase while heat-treated ones have crystalline phase structure. The crystalline structure increases the hardness of coating along with its wear resistance. The friction and wear behavior are assessed by a pin on disc tribometer. Overall, it can be seen that Ni-P/Ni–W-P system performs better compared to Ni-P/Ni-Cu-P in terms of friction and wear. However, heat treatment at higher temperature has negative impact on both the coatings as far as the tribological behavior is concerned.

Protecting the EBE coatings from vacuum-air-shift by ion assistance or ALD capping layer

Vacuum-air-shift causes trouble to conventional electron beam evaporation (EBE) coatings. This study compares the protection effect of ion assistance and the ALD (atomic layer deposition) Al 2 O 3 capping layer. The vacuum-air-shift was reproduced on an EBE Ta 2 O 5 single-layer film, a Ta 2 O 5 /SiO 2 double-layer coating, and a high-reflectance mirror. Similar samples were prepared by EBE assisted with ion beam bombardment. Part of the as-deposited coatings were capped by ALD Al 2 O 3 film with a thickness of about 100 nm. The vacuum-air-shift of the samples was tested with an in-situ spectrophotometer. The cross-section of the coatings was imaged by a scanning electron microscope. The ion assistance was found to reduce the vacuum-air-shift to a limit extent, while the ALD capping film was found to eliminate the vacuum-air-shift. The mechanism of the two protection techniques was analyzed, and their usage was compared and discussed. The ALD capping technique may be used in high precision EBE coatings with convenience and low cost.

Icephobic and Anticorrosion Coatings Deposited by Electrospinning on Aluminum Alloys for Aerospace Applications.

Anti-icing or passive strategies have undergone a remarkable growth in importance as a complement for the de-icing approaches or active methods. As a result, many efforts for developing icephobic surfaces have been mostly dedicated to apply superhydrophobic coatings. Recently, a different type of ice-repellent structure based on slippery liquid-infused porous surfaces (SLIPS) has attracted increasing attention for being a simple and effective passive ice protection in a wide range of application areas, especially for the prevention of ice formation on aircrafts. In this work, the electrospinning technique has been used for the deposition of PVDF-HFP coatings on samples of the aeronautical alloy AA7075 by using a thickness control system based on the identification of the proper combination of process parameters such as the flow rate and applied voltage. In addition, the influence of the experimental conditions on the nanofiber properties is evaluated in terms of surface morphology, wettability, corrosion resistance, and optical transmittance. The experimental results showed an improvement in the micro/nanoscale structure, which optimizes the superhydrophobic and anticorrosive behavior due to the air trapped inside the nanotextured surface. In addition, once the best coating was selected, centrifugal ice adhesion tests (CAT) were carried out for two types of icing conditions (glaze and rime) simulated in an ice wind tunnel (IWT) on both as-deposited and liquid-infused coatings (SLIPs). The liquid-infused coatings showed a low water adhesion (low contact angle hysteresis) and low ice adhesion strength, reducing the ice adhesion four times with respect to PTFE (a well-known low-ice-adhesion material used as a reference).

Microstructure and wear properties of atmospheric plasma-sprayed Cr3C2-NiCr composite coatings

Gas turbines and other critical power generation components in coastal areas are susceptible to corrosion and erosion due to solid particle erosion by air-borne particles, humidity, and sea salt, that severely damage the parts. The synergistic effect of both erosion and corrosion can accelerate the material degradation process compared to the individual effects, which might thus lead to less safety, economic loss, and in extreme cases, rapid failure of the components. Cr3C2-NiCr coatings around 150 to 250 μm are commonly used to safeguard these high-valued components. To further enhance the wear resistance of the base coating's secondary particles, B4C and Cr2O3 were added (5 and 10% weight fraction). These coatings were deposited on SS 304-L substrate material by using the atmospheric plasma spray process and Metco TriPlex Pro-210 gun. The microstructure, microhardness, contact stiffness, fracture toughness, sliding wear and dry erosion resistance of the as-deposited coatings were investigated. The results showed that there was a slight phase change Cr3C2-NiCr composite powder after the spraying process. The developed coating has a dense microstructure with relatively low defects such as micro-cracks and pore; high hardness, and toughness. Among all the tested coatings, Cr3C2-NiCr coating, with a 10% weight fraction of Cr2O3 exhibited high wear resistance under all conditions. Cr3C2-NiCr with 10% Cr2O3 exhibited an almost 3.12 times lower wear rate and almost 1.15 times higher erosion resistance than the base coating (Cr3C2-NiCr). This improvement in the wear resistance could be attributed to the improvement in hardness and fracture toughness of the coating material after the addition of hard secondary phases. Morphology of wear samples was analyzed using SEM to investigate the damage mechanism.

Microstructure characteristics and properties of NiCrMoFeCoAl HVOF coating for T22 boiler tube steel

This paper demonstrates the characterization of HVOF sprayed NiCrMoFeCoAl coating on SA213-T22 bare alloy to improve surface properties for corrosion applications. The microstructure of the sample was examined using a range of characterization methods including XRD and SEM/EDAX methods. The as-deposited coatings are characterized as relating to microstructure & mechanical properties. The porosity, microhardness, coating thicknesses, and density of coating have been also evaluated. It has been found that microhardness values have declined from coating to bare alloy.

Effect of Annealing on Surface Morphology and Structure of Nickel Coatings Deposited from Deep Eutectic Solvents

Nanocrystalline nickel coatings deposited on a copper base material from DES made of choline chloride and ethylene glycol in 1:2 molar ratio containing 1 mol dm−3 NiCl2⋅6H2O were modified through post-deposition heat treatment at the temperatures from 100 to 400 °C. As-deposited coatings were composed of spheroidal agglomerates with the size of several hundred nanometers interspersed with lamellar crystals, but after annealing at 300 °C and 400 °C only single nano-sized plates embedded in a granular and porous layer remained. As the temperature of the heat treatment increased from 100 °C to 400 °C, the mean crystallite size increased from 13 to 35 nm. The change in crystallite size was accompanied by a change in microhardness, the maximum value of which was measured for the annealed coating at 200 °C. As a result of heat treatment, coatings were gradually covered by a layer of oxidized nickel species. XPS analyses showed that NiOOH and Ni(OH)2 dominated among them. Above 200 °C the share of these compounds began to decline in the face of the increasing share of NiO. This, in turn, clearly translated into a deterioration of the corrosion resistance of Ni coatings annealed at 300 °C, and especially at 400 °C, during exposure in 0.05 mol dm−3 NaCl solution.

Tailoring microstructure of double-layered thermal barrier coatings deposited by suspension plasma spray for enhanced durability

Gadolinium zirconate (GZ)-based TBCs comprising GZ as the top layer and yttria stabilized zirconia (YSZ) as the base layer, are attractive double-layered thermal barrier coatings (TBCs) for high temperature gas turbine engine application. This work attempts to understand the influence of individual layer microstructure on the durability of GZ/YSZ double-layered TBCs processed by suspension plasma spray (SPS). Two different spray parameters were chosen to obtain a combination of three microstructurally distinct GZ/YSZ double-layered TBCs i.e. GZ porous (P)/YSZ porous (P), GZ dense (D)/YSZ porous (P) and GZ dense (D)/YSZ dense (D). Thermal diffusivity of the as-deposited coatings was measured using Laser Flash Analysis (LFA) technique and the thermal conductivity of the TBCs was calculated. The GZ/YSZ double-layered TBC specimens were subjected to two different durability tests, i.e. thermal cyclic fatigue (TCF) and burner rig test (BRT). Sintering behavior of the individual layer TBC microstructures was evaluated by comparing the porosity evolution in as-deposited and TCF tested TBCs. Fracture toughness measurements performed on each layer of the double-layered TBCs were correlated with the durability results. Thermal cycling results amply demonstrate that the individual layer microstructure of GZ/YSZ double-layered TBC influenced its durability. Detailed failure analysis of the TCF and BRT failed specimens revealed similar failure modes for GZ (P)/YSZ (P), GZ (D)/YSZ (P) and GZ (D/YSZ (D) TBCs under identical thermal cyclic test conditions. However, failure modes differed when subjected to different thermal cyclic test conditions (TCF and BRT) and the probable causes are discussed. Findings from this work provide key insights on designing durable GZ/YSZ double-layered TBCs.

Investigating load-dependent wear behavior and degradation mechanisms in Cr3C2–NiCr coatings deposited by HVAF and HVOF

Wear resistant coatings that comply with non-toxic environment goals are highly desirable. Cr3C2–NiCr is a promising alternative to the toxic, ‘Co’- containing WC–Co coatings to mitigate wear. The purpose of this study was to examine the suitability of Cr3C2–NiCr coatings for automotive brake disc application by systematically investigating their dry sliding wear behavior at different test conditions. Therefore, High Velocity Air Fuel (HVAF) and High Velocity Oxy Fuel (HVOF) were employed to deposit Cr3C2–NiCr coatings. The powder feedstock and as-deposited Cr3C2–NiCr coatings were characterized for their microstructure and phase composition using SEM and XRD. Mechanical properties (hardness, fracture toughness), porosity and surface topography of the as-deposited coatings were evaluated. The coatings were subjected to sliding wear tests at different normal loads (5 N, 10 N and 15 N) using alumina ball as the counter surface. Coefficient of friction (CoF) evolution of HVAF and HVOF deposited coatings, along with their wear performance, was obtained for different normal load conditions. The wear performance ranking of HVAF and HVOF processed coatings was influenced by the test conditions, with HVAF coatings demonstrating better wear resistance than HVOF coatings at harsh test conditions and the HVOF coatings performing better under mild wear test conditions. Detailed post-wear analysis of worn coatings, the alumina ball counter-body and the resulting debris was performed to reveal the degradation mechanisms at different test conditions. Findings from this work provide new insights into the desirable microstructural features to mitigate wear, which can be further exploited to deposit wear-resistant coatings.