Laser-remelted Coatings Research Articles

Laser remelting of high-velocity arc-sprayed Fe-based amorphous coating for improving corrosion resistance in 3.5 % NaCl solution with varying Na2S concentrations

In this study, high-velocity arc-spraying (HVAS) was employed to produce Fe-based amorphous coatings, and subsequent to that, laser remelting (LR) technology was applied on the as-sprayed coatings (ASC) to prepare laser remelted coatings (LRC). The corrosion resistance of both coatings was tested in 3.5 wt% NaCl solutions containing varying Na2S concentrations (0, 20, 50, 100, and 200 ppm) to accurately simulate the corrosion environment of offshore components. The corrosion behavior was investigated using electrochemical workstation, scanning electron microscopy, energy dispersive spectroscopy and X-ray photoelectron spectroscopy. The experimental results revealed that LR resulted in the elimination of defects in Fe-based amorphous coatings, a substantial reduction in porosity, the disappearance of the laminar structure, and the achievement of a homogeneous composition. The corrosion tendency of ASC and LRC both exhibited a pattern of initially decreasing, then increasing and finally decreasing with increasing sulfide concentration. This was attributed to the deposition of corrosion products on the surface and the dissolution of the corrosion products. When the sulfide concentration exceeded 50 ppm, the corrosion current density of both coatings slowly and was lower than that at 20 ppm, indicating that both coatings possessed strong corrosion resistance at higher sulfide concentrations. Highly polarized anions in solution (e.g. HS−, S2−) occupied vacancies in the passivation film, destabilizing the coating's passivation film and consequently diminishing its corrosion resistance. From the electrochemical analysis, it was evident that LRC exhibited greater corrosion resistance than ASC at all sulfide-containing solution concentrations, which was attributed to its smoother surface, denser structure and more stable passivation film.

Effect of laser remelting on the microstructure and corrosion behavior of high-velocity arc-sprayed FeNiCrBSiNbW amorphous coating

Fe-based amorphous coating was produced using high-velocity arc-spraying (HVAS), and then laser remelting (LR) technology was used to treat the as-sprayed coatings (ASC) to prepare the laser remelted coatings (LRC). The morphology, phase composition, microhardness, porosity, and corrosion property of the Fe-based amorphous coatings before and after LR were investigated. The outcomes demonstrated that the internal defects of the ASC were eliminated and the porosity was significantly reduced after LR, which was significant in dramatically enhancing corrosion resistance. After the LR procedure, the ASC's free corrosion potential (Ecorr) in 3.5 wt% NaCl solution increased, and the corrosion current density (Icorr) almost reduced by a factor of ten. Therefore, LR helped to strengthen the corrosion characteristics of Fe-based amorphous coating and improve its microstructure.

Picosecond laser remelting of electrodeposited Ni[sbnd]P coating: Parameters optimization and electrochemical corrosion behavior

Excessive P content (≥ 10 wt%) easily leads to cracks on the surface of electrodeposited NiP coatings, causing rapid deterioration of corrosion performance. This study uses picosecond laser remelting (LR) technology to close cracks and improve corrosion performance. The influence of the laser parameters on the quality of laser-remelted (LRed) coatings in ambient air is discussed in detail, and the corrosion performance is evaluated through electrochemical corrosion tests. The results indicated that the scanning rate, laser power, and repetition frequency significantly affect the degree of crack closure and LRed defects. For cracks of similar sizes, both multiple-LR with a lower power (10.4 W, five times) and single-LR with a higher power (19.2 W, once) achieved an excellent crack closure. After single-LR, the surface roughness increased from 3.9 ± 0.3 nm to 33.4 ± 3.2 nm. In addition, the LRed layer crystallized from the initial amorphous state. Although the energy-dispersive spectroscopy results indicated that the oxidation behavior during the LR process was insignificant, X-ray photoelectron spectroscopy showed that the surface composition evolved from Ni/Ni–P compounds to Ni–POx after LR. Electrochemical corrosion tests indicated that the corrosion current density decreased approximately three times from 9.19 × 10−6 to 2.91 × 10−6 A/cm2 after LR. Moreover, the corrosion mechanism shifted from the original stress corrosion to pitting corrosion. Thus, the LR technology can effectively improve the corrosion performance and reduce the issues caused by cracks.

Research on the Microstructure of Laser Remelted As-sprayed TiB2-TiC0.3N0.7 based Composite Coating

Ceramic coatings with excellent properties of wear resistance, corrosion resistance, oxidation resistance, stability in acid and alkaline environments, and good bond strength with a metallic substrate, are widely used in different industries. Air plasma spraying has high energy and efficiency for depositing ceramic materials with a high melting point. Due to intrinsic porosity and mechanical bond strength between coating and substrate, laser remelting technology can be applied to the coating surface for improving coating properties. As-sprayed TiB2-TiC0.3N0.7-based composite coating was surface remelted by laser. The surface roughness of as-sprayed and laser-remelted coatings was measured. The phase composition and the microstructure as well as the Vickers microhardness of coatings were studied by X-ray diffractometer (XRD), scanning electron microscopy (SEM), and Vickers indentation instrument. It is found that the phase composition does not change after laser remelting. However, the surface roughness of laser-remelted coating is smoother than that of as-sprayed coating. And the microhardness on the cross-section is increased after laser remelting. There is a micro-crack in the laser-remelted coating due to the fast heating and cooling of the laser. The microhardness of the laser-remelted coating is increased with bimodal Weibull distribution.

Tribological properties of plasma sprayed zircon-alumina powder mixture with and without laser re-melting

ABSTRACTThis investigation focusses on the wear mechanism of as-sprayed and laser treated mullite based coatings, produced by plasma spraying, under sliding wear condition. First, an alumina powder and zircon sand mixture was plasma sprayed to produce a mullite coating. Selected as-sprayed coatings were subsequently laser treated. The tribological performances of both as-sprayed and laser re-melted coatings were assessed using a pin-on-disc wear apparatus. Hardened steel and WC-Co balls served as rubbing counterparts. A plasma sprayed alumina coating was used for bench marking purposes. Plastic deformation was the dominant wear mechanism under low load-low speed condition for both as-sprayed and laser treated coatings. However, at higher loads and speeds the coatings were found to undergo micro-fracture followed by pulverisation. Wear resistance of the coatings improved following laser treatment.

Erosion–Corrosion Resistance of Laser Surface Alloying of NbC Thermal Spray Coatings on AISI 304L Steel

NbC coatings were flame sprayed on AISI 304L substrates and further remelted using a 400-W power ytterbium-doped fiber laser (YLR-500-MM-AC-Y11), inserted into a chamber for argon atmosphere control. For the selection of the remelting parameters, isolated beads with remelting of the coatings were performed using three focal lengths and three remelting speeds. Using the selected laser parameters, layers were remelted with eight passes and 40% superposition. Erosion–corrosion tests were performed in a solution of NaCl in distilled water with SiO2. Surfaces and cross sections of the coatings were characterized by optical and scanning electron microscopy with dispersive energy microanalysis and x-ray diffraction. Microhardness measurements were taken in the cross section of the coatings. Modified surfaces 800 μm dense without pores or other defects and interfaces with the substrate of excellent metallurgical bonding were obtained. It was found that the dilution of the coating with the substrate formed a gradient of chemical composition and mechanical properties. The erosive–corrosive wear resistance of the laser-remelted coatings was higher for the eroding impact angle of 90°. When the impact angle was 30°, the weight loss was higher and the ductile wear mechanism was conditioned by the formation of pitting on the modified surface.

Hot Corrosion Behavior of YSZ Thermal Barrier Coatings Modified by Laser Remelting and Al Deposition

Thermal barrier coatings that are widely used to protect the turbine blades of gas turbines from failure are subjected to hot corrosion conditions. The present study aims to improve their hot corrosion resistance by modifying the plasma-sprayed yttria-stabilized zirconia topcoat by laser remelting and Al deposition. First, a dense remelted layer with some vertical cracks was obtained by laser remelting. Then, Al film was deposited on the surface of laser-remelted coatings by magnetron sputtering, and the dense α-Al2O3 overlay was in situ synthesized through the reaction of Al and ZrO2 during vacuum heat treatment. Hot corrosion tests of all coatings were conducted in a mixture of 55 wt.% V2O5 and 45 wt.% Na2SO4 corrosive salts at 1000 °C for 30 h. The defects such as pores and microcracks in the as-sprayed coatings provided paths for molten salt penetration, which led to the worst hot corrosion resistance of the as-sprayed coatings. The minimal surface roughness of the dense laser-remelted coatings was the main reason for improving the hot corrosion resistance. However, the resistance of laser-remelted coatings to hot corrosion was significantly improved when they were modified by Al deposition, which is mainly due to the reduced reaction surface between molten salts and yttria stabilizer. The corresponding hot corrosion mechanism was discussed.

Effect of post-coating technique on microstructure, microhardness and the mixed lubrication regime parameters of thermally-sprayed NiCrBSi coatings

The purpose of this paper is to understand and to quantify the effect of the re-melting technique on the microstructure and tribological behaviour of oxy-fuel (OF) thermally sprayed NiCrBSi coatings in the mixed lubrication regime. Two different re-melting techniques have been used: Surface Flame Melting (SFM) and laser re-melting processes. The microstructure of the obtained coatings was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Deposited coatings demonstrated similar phases but manifested notable differences in their morphology, size, distribution and relative proportions of the observable phases. These differences in microstructure may explain the differences in coating properties. Tribological experiments were carried out using a pin-on-disc approach at different sliding speeds and contact pressures. Wear and indentation tests have also been conducted. Laser re-melted coatings achieve higher hardness, less friction and lower accumulated wear volume in lubricated conditions. Test data were fitted with regression models. The fitted equations allow a robust assessment of the effect of the re-melting technique and working conditions on the friction coefficient in components that operate in the mixed lubrication regime. Therefore, these equations may help to select the adequate coating and tribological parameters for specific applications.

Microstructures and High-Temperature Friction–Wear Performance of Laser-Remelted WC-12Co Coatings by HVOF

ABSTRACTA WC-12Co coating prepared by high-velocity oxygen fuel (HVOF) was remelted with a CO2 laser, and the surface–interface morphologies, plane energy spectrum, and phases of the coating were analyzed by means of field emission scanning electron microscopy (FESEM), energy-dispersive spectrometry (EDS), and X-ray diffraction (XRD), respectively. The friction and wear behaviors of the WC-12Co coating were investigated at high temperature with a wear test, and the morphologies and the changes in chemical elements on the wear scar after the wear test were analyzed with SEM and EDS, respectively. In addition, the influence of high temperature on the coefficient of friction (COF) and wear performance is discussed. The results show that the substrate is closely bonded with the substrate after laser remelting (LR), which includes mechanical bonding accompanied by metallurgical bonding. The average coefficient of friction (COF) at 600, 700, and 800°C is 0.6832, 0.3957, and 0.1922, respectively. The wear mechanisms of WC-12Co coating at 600 and 700°C are adhesive wear, abrasive wear, and oxidative wear, respectively, and the wear mechanism of the coating at 800°C is serious oxidative wear.

Effect of plasma sprayed and laser re-melted Al2O3 coatings on hardness and wear properties of stainless steel

Commercially available austenic stainless steel substrate was coated with commercially available, raw Al2O3 powder applied by means of plasma spraying method and then re-melted with CO2 laser beam of various parameters. Tribological and mechanical properties of the 120J/mm and 160J/mm laser re-melted coatings were compared with the tribological and mechanical properties of the “as-sprayed” coating. The influence of the laser beam of various parameters on the microstructure, phase constituents, and mechanical and tribological properties of the ceramic coating was investigated by means of scanning electron microscopy, light microscopy, computer tomography, X-ray diffraction technique and nanoindentation tests. The micro sliding wear performance of the coatings was tested using a nanoindenter. The study showed an improvement of the mechanical and tribological properties caused by the laser treatment. The best results were achieved for coating re-melted with 120J/mm laser beam.

Effect of laser remelting on the microstructure and corrosion resistance of plasma sprayed Fe-based coating

Fe-based amorphous and nanocrystalline coatings were fabricated by air plasma spraying. The coatings were further treated by laser remelting process to improve their microstructure and properties. The corrosion resistance of the as-sprayed and laser-remelted coatings in 3.5wt% NaCl and 1 mol/L HCl solutions was evaluated by electrochemical polarization analysis. It was found that laser-remelted coating appeared much denser than the as-sprayed coating. However, laser-remelted coating contains much more nanocrystalline grains than the as-sprayed coatings, resulting from the lower cooling rate in laser remelting process compared with plasma spraying process. Electrochemical polarization results indicated that the laser-remelted coating has great corrosion resistance than the as-sprayed coating because of its dense structure.

Influence of columnar grain microstructure on thermal shock resistance of laser re-melted ZrO2-7 wt.% Y2O3 coatings and their failure mechanism

In this study, influence of columnar grain morphology on thermal shock resistance of laser re-melted ZrO2-7wt.% Y2O3 thermal barrier coatings (TBCs) has been researched. The influence of variation in columnar grain microstructures on the fatigue mechanism of laser re-melted coatings in thermal shock experiments has been investigated. The thermal shock experiments have confirmed the positive effect of small grain size on thermal fatigue life. This is thought to have resulted from an increase in the number of grain boundaries that restrict the grain motion during deformation. Further, “void effect” could be responsible for promoting peeling of the laser re-melted coatings when spallation is less than 10% because it can cause a relatively large tensile stress between the columnar grains and the underlying splats. However, it is found to have negligible effect on the shedding of LRCs shed when the spallation percentage is 20% or more because the remaining LRCs are relatively homogeneous having less or no voids. The presence of separate columns and large intergranular clearance may have allowed better compliance of stress through free-movement of the individual columnar grains. There are many more restrictions on stress relaxation. An additional moving resistance is found to be present at the undulate intergranular interfaces for the irregular gear-like columnar grains with small inter-crystalline space and large intergranular contact area. The one-piece structure with pseudo-columnar grains was found to improve the thermal cycle lifetime slightly. It is believed to be due to its relatively lower strain tolerance as well as lower fracture toughness as compared to that of the structure with separate columnar grains.

Microstructure and Tribological Properties of Laser-Remelted Ni-Based WC Coatings Obtained by Plasma Spraying

Ni-based WC coatings were successfully fabricated on Ti6Al4V titanium alloy substrate by the air plasma spraying (APS) process. The as-sprayed coatings were remelted with a continuous wave CO2 laser. The coatings were characterized using a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS), an X-ray diffractometer (XRD), a Vickers microhardness tester, a ball-on-disc friction and wear tester, and a 3D measuring laser microscope. The results indicated that the laser-remelted coatings possessed a more homogeneous microstructure and lower porosity, were more dense, and bonded well with the substrate. The lamellar structure and microcracks of the as-sprayed coatings were eliminated after laser remelting. The average porosity markedly decreased to 1.3% from 9.7% of the as-sprayed coating. Many needle dendrites grew outward from the dissolution surface of the WC, while massive precipitates abundantly appeared around and away from the WC particles. The microhardness of the laser-remelted coating was enhanced to 884.7–1363.3 HV0.3, which was much higher than that of the as-sprayed coatings and nearly three times higher than that of the substrate. Under the same load, the wear volume of the substrate was nearly 13 times higher than that of the laser-remelted coating and five times higher than that of the as-sprayed coating. The wear mechanism of the laser-remelted coating and the as-sprayed coating was mainly abrasive wear; however, the wear mechanisms of the substrate were a combination of adhesive wear, micro-cutting, and delamination wear.

Effects of laser remelting on microstructure and solid particle erosion characteristics of ZrO2–7wt%Y2O3 thermal barrier coating prepared by plasma spraying

In this study, the influences of laser remelting on microstructural characteristics, microhardness, and solid particle erosion behavior of MCrAlY/ZrO2–7wt%Y2O3 double-layer structure thermal barrier coatings prepared by plasma spraying were examined. The erosion failure mechanisms of the as-sprayed and laser-remelted coatings were investigated. Results show that the plasma-sprayed coating has a typical lamellar stacking characteristic with high porosity. The surface lamellar defect of the as-sprayed coating was erased, and the compactness of the coating improved significantly after laser remelting. The laser-remelted coating consisted of a remelted layer with a dense column-like crystal structure, a heat-affected layer with a more dense structure than that of the as-sprayed coating, and a remaining plasma-sprayed layer. The microhardness and erosion resistance of the laser-remelted coating are considerably better than the as-sprayed one. The spalling of the sprayed splats from the lamellar interface and crushing of brittle ceramic coating are mainly attributed to the erosion failure of the plasma-sprayed coating. By contrast, cracks occurred in the region near the surface of the laser-remelted layer, and the erosion occurred mainly because of the removal of these small blocks. The laser-remelted coating has evident micro-cutting marks and some ductile erosion characteristics.

Microstructure evolution of laser remelted Al2O3–13wt.%TiO2 coatings

Abstract Laser remelting of plasma sprayed Al 2 O 3 –TiO 2 coatings has been extensively investigated. However, little work on the microstructural evolution of laser-remelted Al 2 O 3 –TiO 2 coatings has been reported. This study presents a detailed microstructural investigation of laser-remelted Al 2 O 3 –TiO 2 coatings. Nanostructured feedstock has been reconstituted from nano-sized starting powders. A compound process of plasma spraying and laser remelting has been applied to fabricate the Al 2 O 3 –TiO 2 coatings on titanium alloy. The evolution of microstructures of laser-remelted Al 2 O 3 –TiO 2 coatings was investigated by X-ray diffractometer (XRD), scanning electron microscope (SEM), energy-dispersive spectrometer (EDS) and transmission electron microscope (TEM). Microstructural observations revealed microstructure correlation throughout the compound process. It was found the α-Al 2 O 3 in feedstock was transformed to γ-Al 2 O 3 after plasma spraying, which was completely transformed into stable α-Al 2 O 3 phase after laser remelting. Bi-modal microstructure features are found for the laser-remelted coatings. For nanostructured coating, the net structure of its remelted coating resembles that of its feedstock and as-sprayed coating. For conventional coating, the strip-like structure of its remelted coating also resembles that of its feedstock and as-sprayed coating.

Laser remelting of plasma-sprayed conventional and nanostructured Al 2O 3–13 wt.%TiO 2 coatings on titanium alloy

Plasma-sprayed microstructured and nanostructured Al 2O 3–13 wt.%TiO 2 coatings were successfully deposited on Ti–6Al–4V titanium alloy substrates with commercial Metco 130 powder and as-prepared nanostructured feedstock, respectively. The as-sprayed coatings were remelted by a CO 2 laser to further enhance their compactness and bonding strength. The effects of laser remelting on the microstructure, phase constituents and mechanical properties of the ceramic coatings were investigated by scanning electron microscope, X-ray diffractometer and Vickers microhardness tester. The results indicate that the laser-remelted coatings exhibit more compact and homogenous structure as well as strong metallurgical bonding to the substrates. The dominating metastable γ-Al 2O 3 phase in the as-sprayed coatings transforms to stable α-Al 2O 3 during laser remelting. The microhardness value of the as-sprayed Metco 130 and nanostructured Al 2O 3–13 wt.%TiO 2 coatings is in the range of 700–1000 HV 0.3, while the microhardness values of the corresponding remelted coatings are enhanced to 1000–1350 HV 0.3 and 1100–1800 HV 0.3, respectively. With the decrease of laser scanning velocity, the microhardness is increased.

Laser surface remelting of plasma sprayed nanostructured Al 2O 3–13wt%TiO 2 coatings on titanium alloy

Plasma sprayed nanostructured coatings were successfully fabricated on a titanium alloy (Ti–6Al–4V) substrate using the as-prepared nanostructured Al 2O 3–13wt%TiO 2 feedstock. A CO 2 laser was used to remelt the plasma sprayed coatings. The effects of laser remelting on the phase constituents, microstructure and properties of the ceramic coatings were investigated. The laser remelted coatings (LRmC) possessed a much denser and more homogenous structure and excellent metallurgical bonding to the substrate. The average porosity of the LRmC was reduced to 0.9%, compared with 6.2% of the as-sprayed coatings. The net-like structure in the as-prepared feedstock remained in the coatings before and after laser remelting. The metastable γ-Al 2O 3 phase in the as-sprayed coatings transformed to stable α-Al 2O 3 during laser remelting. The LRmC could remain nanostructure. The microhardness of the coatings was enhanced to 1000–1400 HV 0.3 after laser remelting, which was much higher than that of the plasma sprayed coatings and 2–3 times higher that of the substrate. Significant decreases in surface roughness were also found in the LRmC.

Influences of laser remelting on microstructure of nanostructured Al 2O 3–13 wt.% TiO 2 coatings fabricated by plasma spraying

The effects of laser remelting on microstructure of nanostructured Al 2O 3–13 wt.% TiO 2 ceramic coatings prepared by plasma spraying with agglomerated powders were studied. The microstructure of the feedstock, as-sprayed and laser-remelted coatings were investigated by using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffractometry (XRD). The results indicate that the plasma-sprayed ceramic coating consists of both fully melted regions and partially melted regions. The totally ceramic coating, especially the fully melted regions, has a typical plasma-sprayed lamellar-like structure as the conventional coating, and has some pores. According to the difference of microstructures, the partially melted regions can be divided into liquid-phase sintered regions (a three-dimensional net or skeleton-like structure: Al 2O 3-rich submicron particles embedded in the TiO 2-rich matrix) and solid-phase sintered regions (remained nanoparticles). The lamellar defect of the as-sprayed coating is erased, and the compactness of the coating is improved significantly after laser remelting. The laser-remelted region composed of fine equiaxed grains, which are different from the conventional column-like crystals along the direction of the heat current. Due to the rapid solidification of laser remelting process, there are still some nanoparticles in the remelted region because of an insufficient time for grains growth.

Erosion-Corrosion of As-Plasma-Sprayed and Laser-Remelted NiCrAlY Bond Coats in Working Conditions of a Coal-Fired Boiler

Abstract Ni-22Cr-10Al-1Y plasma spray coating has been formulated on boiler tube steels, namely, low-carbon steel ASTM SA210-Grade A1, 1Cr-0.5Mo steel ASTM SA213-T-11, and 2.25Cr-1Mo steel ASTM SA213-T-22. The coated steels also have been laser-remelted using a Nd:YAG laser. The degradation behavior of as-sprayed and laser-remelted coatings have been evaluated in actual conditions in a coal-fired boiler for 1,000 h at 755°C. The laser remelting has been found to be effective to increase the degradation resistance of plasma-sprayed boiler steels. ASTM SA213-T-22-coated and laser-remelted steel has proved to be most effective in resistance to degrading species.

Corrosion Behavior of HVOF-Sprayed and Nd-YAG Laser-Remelted High-Chromium, Nickel-Chromium Coatings

Thermal spray processes are widely used to deposit high-chromium, nickel-chromium coatings to improve high temperature oxidation and corrosion behavior. However, despite the efforts made to improve the present spraying techniques, such as high-velocity oxyfuel (HVOF) and plasma spraying, these coatings may still exhibit certain defects, such as unmelted particles, oxide layers at splat boundaries, porosity, and cracks, which are detrimental to corrosion performance in severe operating conditions. Because of the process temperature, only mechanical bonding is obtained between the coating and substrate. Laser remelting of the sprayed coatings was studied in order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties. The coating material was high-chromium, nickel-chromium alloy, which contains small amounts of molybdenum and boron (53.3% Cr, 42.5% Ni, 2.5% Mo, 0.5% B). The coatings were prepared by HVOF spraying onto mild steel substrates. A high-power, fiber-coupled, continuous-wave Nd:YAG laser equipped with large beam optics was used to remelt the HVOF-sprayed coating using different levels of scanning speed and beam width (10 or 20 mm). Coating that was remelted with the highest traverse speed suffered from cracking because of the rapid solidification inherent to laser processing. However, after the appropriate laser parameters were chosen, nonporous, crack-free coatings with minimal dilution between coating and substrate were produced. Laser remelting resulted in the formation of a dense oxide layer on top of the coatings and full homogenization of the sprayed structure. The coatings as sprayed and after laser remelting were characterized by optical and electron microscopy (OM, SEM, respectively). Dilution between coating and substrate was studied with energy dispersive spectrometry (EDS). The properties of the laser-remelted coatings were directly compared with properties of as-sprayed HVOF coatings.