As-sprayed Coatings Research Articles

Microstructure and tribological properties of plasma sprayed Cu-15Ni-8Sn coatings

Cu-15Ni-8Sn coatings were prepared using atmospheric plasma spraying. The influences of arc current and aging treatment on the microstructure, mechanical and tribological properties were investigated. The coating microstructure, the wear track and the wear debris were observed and analyzed. The melting state of the particles is improved by increasing of arc current. The hardness of the aged coatings is higher than the as-sprayed coatings. The wear resistance of the aged coatings is 3–4 times higher than the as-sprayed coatings. The continuous smooth tribolayer formed on the wear tracks contributes to the low wear rate of aged coatings. The dominant wear mechanism of the coatings is delamination coupled with spalling under dry sliding condition. This work provides an effective method for producing spinodal alloy coatings with enhanced mechanical and tribological properties.

Synthesis and characterization of nanostructured t′-YSZ spherical feedstocks for atmospheric plasma spraying

The properties of feedstocks have a significant effect on the quality of as-sprayed coatings. The 8YSZ feedstocks have been widely used in atmospheric plasma spraying (APS) for the preparation of thermal barrier coatings (TBCs). In this paper, the nanostructured APS 8YSZ spherical feedstocks were successfully synthesized by the nanopowder granulation method. The microstructures and physical properties of as-prepared 8YSZ feedstocks were characterized. By contrast, the phase composition of conventional 8YSZ feedstocks was also analysed. The results show that the nanostructured spherical 8YSZ feedstocks with the median particle size (d50) of 33.22 μm whose grain size is about 20 nm consist of the non-transformable tetragonal (t′) phase and no other phases. Furthermore, the density of the as-prepared feedstocks is high enough and the nanostructured 8YSZ feedstocks are free-flowing. The nanostructured t′-YSZ feedstocks have excellent combination properties, which makes them have a broader application prospect in APS TBCs compared with the conventional 8YSZ feedstocks in future.

Influence of water vapour on structural and thermal conductivity of post-heat treated plasma sprayed LZ and YSZ coatings

In this work, the role of water vapour content during the post-heat treatment on the microstructure and thermal conductivity of atmospheric plasma sprayed lanthanum zirconate (LZ) and yttria stabilized zirconia (YSZ) coatings were studied. For this purpose free standing LZ and YSZ coatings with thickness about 300–400 μm were prepared and post-heat treated at 1100 °C for 50 h and 100 h with and without water vapour content. Phase stability and microstructure were investigated using X-ray diffraction and scanning electron microscope respectively. Thermal conductivity of as-sprayed and post-heat treated coatings were measured by laser-flash thermal diffusivity method and results obtained show that the thermal conductivity of post heat treated coatings increased significantly in both coatings. Further, the thermal conductivity of the coatings increased with increasing heat treatment time. Overall observations show that the presence of water vapour during the post-heat treatment has significantly influenced the microstructures as well as thermal conductivity in both coatings. Thermal conductivity of post-heat treated lanthanum zirconate and yttria stabilized zirconia coatings with presence of water vapour is 2.12 W m−1 K−1 and 2.48 W m−1 K−1 respectively which is 46% and 41% higher than as-sprayed coatings.

Microstructural Analysis and Transport Properties of Thermally Sprayed Multiple-Layer Ceramic Coatings

Multilayer, graded ceramic/metal coatings were prepared by an air plasma spray method on Ti-6Al-4V, 4140 steel and graphite substrates. The coatings were designed to provide thermal barriers for diesel engine pistons to operate at higher temperatures with improved thermal efficiency and cleaner emissions. A systematic, progressive variation in the mixture of yttria-stabilized zirconia and bondcoat alloys (NiCoCrAlYHfSi) was designed to provide better thermal expansion match with the substrate and to improve thermal shock resistance and cycle life. Heat transfer through the layers was evaluated by a flash diffusivity technique based on a model of one-dimensional heat flow. The aging effect of the as-sprayed coatings was captured during diffusivity measurements, which included one heating and cooling cycle. The hysteresis of thermal diffusivity due to aging was not observed after 100-h annealing at 800 °C. The measurements of coatings on substrate and freestanding coatings allowed the influence of interface resistance to be evaluated. The microstructure of the multilayer coating was examined using scanning electron microscope and electron probe microanalysis.

A study on failure of double-layer thermal barrier coatings subjected to uniaxial compression tests using acoustic emission analysis and digital image correlation

The failure behavior and fracture process of double-layer thermal barrier coatings under uniaxial compressive substrate loading has been investigated. Coating systems containing GZO with low and high porosity (LP, HP) were fabricated to examine the influence of microstructure on failure behavior and strain energy. An YSZ-HP single-layer system serves as a reference. All ceramic coatings were deposited via atmospheric plasma spraying (APS) on cylindrical rods and turbine blade-shaped specimens made from CoNiCrAlY (LCO-22) coated, nickel-based, single crystal superalloy (PWA 1483). Prior to compression tests, isothermal pre-oxidation at 1050 °C and dwell-times of 100, 500 and 1.500 hours, as well as cyclic annealing tests between 50 and 1050 °C up to 500 cycles were performed, to study the effects of thermal ageing on strain energy to failure. In-situ acoustic emission (AE) measurements provides quantitative information about the failure processes under compressive substrate loading. A stereo camera system monitors the three-dimensional displacements and the surface fracture processes. For as-sprayed coatings, strain to failure of the investigated GZO/YSZ systems is comparable to the referenced single-layer TBC. AE analysis indicates coating failure at earlier stages and less substrate loads after thermal ageing with increasing dwell-time. Consequently, the pre-oxidation leads to reduced strain to failure values in all investigated coating systems. Digital image correlations (DIC) suggests that the failure behavior of as-sprayed GZO/YSZ coatings is similar to the referenced YSZ system. However, a different behavior was observed for pre-oxidized coatings, where cracking and spallation of GZO occurs predominantly.

Structure and electrical insulation characteristics of plasma-sprayed alumina coatings under pressure

In this work, alumina coatings were fabricated on 316LN austenitic stainless steel by a plasma spray technique. The pressure dependence of the surface electrical resistivity of alumina coatings was investigated in detail. A combination of scanning electron microscopy and X-ray diffraction was employed to understand the microstructure and properties of the as-sprayed alumina coatings. The coatings can endure high pressures under a practical working environment. The surface electrical resistivity of the alumina coatings decreases continuously with an increase in pressure to 250MPa. Interestingly, the surface resistivity is still greater than 107Ω·mm for 250MPa, demonstrating that the coatings have good electrical insulation properties and can be fully utilized in the magnet support of ITER.

Influence of oxidation related structural defects on localized corrosion in HVAF-sprayed Fe-based metallic coatings

Oxidation occurred in thermal spray process is an important problem deteriorating corrosion performance of thermal-sprayed Fe-based metallic coatings due to the formation of Cr-depleted zones and structural defects around the spray formed oxides. To clarify the correlation between the oxidation related structural defects formed during thermal spray process (i.e. the interparticle regions and the connected pores) and the localized corrosion, a dense Fe-based metallic coating with high Cr-content was fabricated, the long term immersion tests which continued for 30days combined with electrochemical impedance spectroscopy measurements in 3.5wt% NaCl were performed. It was found that electrolyte solution diffused to the inner coating through the interparticle oxide layers, and subsequently the localized corrosion developed along the interparticle oxide layers. A number of corrosion pits and corrosion channels were observed along with the interparticle oxide layers for the coating after one month immersion. Through sealing the diffusion pathways in the as-sprayed coatings by using aluminum phosphate, the corrosion resistance of the coatings was enhanced with decreased corrosion current density and improved endurance in long term immersion tests.

Influence of laser re-melting and vacuum heat treatment on plasma-sprayed FeCoCrNiAl alloy coatings

FeCoCrNiAl high entropy alloy coatings were prepared by supersonic air-plasma spraying. The coatings were post-treated by vacuum heat treatment at 600 and 900 °C, and laser re-melting with 300 W, respectively, to study the influence of different treatments on the structure and properties of the coatings. The phase constitution, microstructure and microhardness of the coatings after treatments were investigated using X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. Results showed that the as-sprayed coatings consisted of pure metal and Fe-Cr. The AlNi3 phase was obtained after the vacuum heat treatment process. A body-centered cubic structure with less AlNi3 could be found in the coating after the laser re-melting process. The average hardness values of the as-sprayed coating and the coatings with two different temperature vacuum heat treatments and with laser re-melting were 177, 227, 266 and 682 HV, respectively. This suggests that the vacuum heat treatment promoted the alloying process of the coatings, and contributed to the enhancement of the coating wear resistance. The laser re-melted coating showed the best wear resistance.

Influence of microstructure on hardness of plasma sprayed Al2O3–TiO2–MgO coatings with interface diffusion by heat treatment

The effect of heat treatment on the microstructure and mechanical properties of Al2O3–TiO2 coatings doped with 5 wt% MgO was investigated in this paper. The composite coatings were prepared by atmospheric plasma spraying (APS) and heat treated at 1000 °C for 24 h in Ar. The coatings were analyzed using scanning electron microscopy with electron probe x-ray microanalysis and x-ray diffraction. The hardness was determined using a Vickers hardness test on the as-sprayed coatings and after heat treatment. The results showed that the interface diffusion between the Al-rich and Ti-rich layers resulted in mutual pinning within the coating during the heat treatment. The newly formed MgAl2O4 phase promoted cracking-healing behavior within the coating. We conclude that increase of the hardness of the coatings was mainly caused by the mutual pinning interface and crack healing.

Predictive modelling of dry sliding wear in sealed plasma-sprayed Mo coating using response surface methodology

Plasma spraying is used to produce wear resistant coatings. However, the primary problem is the poor bonding strength between the coating and the substrate. The secondary problem is the high porosity in the as-sprayed coatings, which reduces the wear resistance of coating. In order to overcome these problems, the sealing of plasma-sprayed coating by electrodeposition has been used. The sealing of plasma-sprayed coatings alters the wear mechanism and wear resistance. The wear mechanism and wear resistance largely depends on the applied load, sliding speed and sliding distance. Hence, an effort has been made in the present work to study the effects of these parameters on wear volume loss using response surface methodology (RSM)-based mathematical models. The experiments were conducted as per Central Composite Design (CCD). It reveals that the applied load was the most predominant factor affecting the wear volume loss of the coating material. The sliding speed is the next most important parameter influencing the wear volume loss. The wear volume loss of the sealed plasma-sprayed molybdenum coating occurs mainly due to the formation of grooves, surface tribo films, fracture of splats and delamination of the coating.

High efficiency chemical assisted nanofinishing of HVOF sprayed WC-Co coating

Thermally sprayed coatings are widely used to improve surface hardness and wear resistance of components. However, surface roughness of the as-sprayed coatings is higher than the acceptable limit for many applications in aerospace, automobile and printing industries. In the present study, a multi-step finishing strategy is adopted to improve the surface finish of high velocity oxy-fuel spraying (HVOF) sprayed WC-Co coating. Shape adaptive grinding (SAG) is performed using a zirconia-alumina abrasive polishing pad which resulted in differential finishing of WC and Co binder due to their significant hardness difference. To overcome this situation, chemical assisted SAG is performed using Murakami's reagent. A thin passivation layer is formed on the top of coating due to chemical interaction which has hardness many times lesser than the bare coating and facilitates high and uniform material removal rate. X-ray photoelectron spectroscopy measurements confirm the presence of WO3 in the passivation layer. No fracture or WC fragmentation is observed on the surface processed using chemical assisted SAG. Areal surface roughness (Sa) is reduced from 5.04μm to 53nm using the above strategy.

Synthesis of thick photocatalytic titania surface layers by solution plasma spraying and subsequent treatment by pulsed laminar plasma jet

The two-stage method of synthesis of TiO2 photocatalytic surface layers with improved adhesion and cohesion, and photocatalytic properties based on vortex atmosphere solution precursor plasma spraying (SPPS) of thick anatase-rutile mixed coatings and their subsequent melting by pulsed laminar argon-helium plasma jet followed by auto-quenching and solidification is offered and approved for the first time. The titania coatings with increased rutile/anatase ratio characterized by high degree of crystallinity, increased specific surface area and permeability, and also advanced cohesion and bond strength with metal substrate, are sufficiently obtained for practice. The properties of the coatings were studied using X-ray diffraction (XRD) analysis, scanning electron and optical microscopy, BET- assisted measuring specific surface area and methylene-blue (MB) decoloration testing in an ultra-violet (UV) light room. Since the experimental determination of conditions of the post-treatment of the as-sprayed coatings by pulse plasma jet is rather labor- and power-consuming procedure, preliminary computer-aided evaluation has been carried out.

Effect of Carbide Dissolution on Chlorine Induced High Temperature Corrosion of HVOF and HVAF Sprayed Cr3C2-NiCrMoNb Coatings

Highly corrosion- and wear-resistant thermally sprayed chromium carbide (Cr3C2)-based cermet coatings are nowadays a potential highly durable solution to allow traditional fluidized bed combustors (FBC) to be operated with ecological waste and biomass fuels. However, the heat input of thermal spray causes carbide dissolution in the metal binder. This results in the formation of carbon saturated metastable phases, which can affect the behavior of the materials during exposure. This study analyses the effect of carbide dissolution in the metal matrix of Cr3C2-50NiCrMoNb coatings and its effect on chlorine-induced high-temperature corrosion. Four coatings were thermally sprayed with HVAF and HVOF techniques in order to obtain microstructures with increasing amount of carbide dissolution in the metal matrix. The coatings were heat-treated in an inert argon atmosphere to induce secondary carbide precipitation. As-sprayed and heat-treated self-standing coatings were covered with KCl, and their corrosion resistance was investigated with thermogravimetric analysis (TGA) and ordinary high-temperature corrosion test at 550 °C for 4 and 72 h, respectively. High carbon dissolution in the metal matrix appeared to be detrimental against chlorine-induced high-temperature corrosion. The microstructural changes induced by the heat treatment hindered the corrosion onset in the coatings.

Elevated Temperature Solid Particle Erosion Performance of Plasma-Sprayed Co-based Composite Coatings with Additions of Al2O3 and CeO2

In this paper, investigation into solid particle erosion behavior of atmospheric plasma-sprayed composite coating of CoCrAlY reinforced with Al2O3 and CeO2 oxides on Superni 76 at elevated temperature of 600 °C is presented. Alumina particles are used as erodent at two impact angles of 30° and 90°. The microstructure, porosity, hardness, toughness and adhesion properties of the as-sprayed coatings are studied. The effects of temperature and phase transformation in the coatings during erosion process are analyzed using XRD and EDS techniques. Optical profilometer is used for accurate elucidation of erosion volume loss. CoCrAlY/CeO2 coating showed better erosion resistance with a volume loss of about 50% of what was observed in case of CoCrAlY/Al2O3/YSZ coating. Lower erosion loss is observed at 90° as compared to 30° impact angle. The erosion mechanism evaluated using SEM micrograph revealed that the coatings experienced ductile fracture exhibiting severe deformation with unusual oxide cracks. Reinforced metal oxides provide shielding effect for erodent impact, enabling better erosion resistance. The oxidation of the coating due to high-temperature exposure reforms erosion process into oxidation-modified erosion process.

HVOF- and HVAF-Sprayed Cr3C2-NiCr Coatings Deposited from Feedstock Powders of Spherical Morphology: Microstructure Formation and High-Stress Abrasive Wear Resistance Up to 800 °C

Chromium carbide-based coatings are commonly applied to protect surfaces against wear at high temperatures. This work discusses the influence of feedstock powder and spray torch selection on the microstructure and high-stress abrasion resistance of thermally sprayed Cr3C2-NiCr coatings. Four commercial feedstock powders with spherical morphology and different microstructures were deposited by different high-velocity spray processes, namely third-generation gas- and liquid-fueled HVOF torches and by the latest generation HVAF torch. The microstructures of the coatings were studied in the as-sprayed state and after various heat treatments. The high-stress abrasion resistance of as-sprayed and heat-treated coatings was tested at room temperature and at 800 °C. The study reveals that the selection of the spray torch mainly affects the room temperature abrasion resistance of the as-sprayed coatings, which is due to differences in the embrittlement of the binder phase generated by carbide dissolution. At elevated temperatures, precipitation and growth of secondary carbides yields a fast equalization of the various coatings microstructures and wear properties.

Thermo-electrical properties of the alumina coatings deposited by different thermal spraying technologies

Abstract Thermal and electrical properties of alumina (Al 2 O 3 ) coatings produced via Atmospheric Plasma Spraying (APS) and Detonation Gun Spraying (DGS) were investigated and compared in this study. A combination of Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) analysis, image analysis and X-ray Diffraction analysis (XRD) were used to better understand microstructural characteristics of as-sprayed alumina coatings. Microstructure formation of coating layers and its main features such as splats, pores, and boundaries were experimentally observed for both deposited coatings. The measurement of splats average thickness indicated that splat sizes, as building blocks of coatings were approximately similar for both coatings. The XRD showed that feedstock was pure α-Al 2 O 3 , but both deposited alumina coatings contained the mixture of α-Al 2 O 3 and γ-Al 2 O 3 phases. However, the amount of α-Al 2 O 3 phase was two times higher in DGS deposited coating. The DGS deposited coatings were also had a higher hardness compared to APS deposited one. The electrical properties of alumina coatings were measured using the pseudo two-contact impedance spectroscopy method that allowed separation of the electrical resistivity of splats and their boundaries within the coating microstructure. The obtained results showed significant difference between these two features in terms of electrical resistivity, namely splats had 5 orders of magnitude smaller resistivity compared to the splat boundaries. In addition, DGS deposited coating was characterized with higher electrical resistivity of splats and splat boundaries compared with APS coating. Finally, the experimentally measured thermal conductivities of alumina coatings showed higher thermal conductivity in DGS deposited coatings compared to the APS ones. These values were validated using analytical models and Finite Element Analysis (FEA).

Hot corrosion behavior of nanostructured Gd2O3 doped YSZ thermal barrier coating in presence of Na2SO4 + V2O5 molten salts

Nickel-based superalloy DZ125 was first sprayed with a NiCrAlY bond coat and followed with a nanostructured 2mol% Gd2O3−4.5mol% Y2O3-ZrO2 (2GdYSZ) topcoat using air plasma spraying (APS). Hot corrosion behavior of the as-sprayed thermal barrier coatings (TBCs) were investigated in the presence of 50wt% Na2SO4 + 50wt% V2O5 as the corrosive molten salt at 900°C for 100h. The analysis results indicate that Gd doped YVO4 and m-ZrO2 crystals were formed as corrosion products due to the reaction of the corrosive salts with stabilizers (Y2O3, Gd2O3) of zirconia. Cross-section morphology shows that a thin layer called TGO was formed at the bond coat/topcoat interface. After hot corrosion test, the proportion of m-ZrO2 phase in nanostructured 2GdYSZ coating is lower than that of nano-YSZ coating. The result reveals that nanostructured 2GdYSZ coating exhibits a better hot corrosion resistance than nano-YSZ coating.

Thermo-physical behavior of atmospheric plasma sprayed high porosity lanthanum zirconate coatings

Lanthanum zirconate (La2Zr2O7) is one of the potential thermal barrier top coat material due to its high melting point, phase stability and low thermal conductivity in comparison to the conventional yttria stabilized zirconia (YSZ) coatings. In this work, an attempt was made to further reduce the thermal conductivity of lanthanum zirconate from its bulk value by fabricating a highly porous microstructure. For this purpose, lanthanum zirconate top coating with different porosities (16, 21 and 28%) was prepared by atmospheric plasma spraying under selective operating conditions and their microstructure, permeability and thermo-physical properties were investigated. SEM micrographs of the porous lanthanum zirconate show the features of the coating microstructures and porosity percentage. The bond strength of the as-sprayed coatings linearly decreases while increasing the porosity in the coating microstructure. The gas permeability of the porous coating microstructure significantly increased with increase in the porosity. An increase in coating porosity, results in significant reduction of both thermal diffusivity and conductivity of the coating microstructure.

Investigation of the crystallinity of suspension plasma sprayed hydroxyapatite coatings

Hydroxyapatite coatings were deposited on stainless steel substrates. The arc current was varied to study its effect on the coating crystallinity. The crystallinity was calculated according to the XRD patterns via Jade 6.0 software and the full width at half maximum (FWHM) of Raman peak at 962cm−1. The FE-SEM images showed that HA coatings had rod-like nanostructures and agglomerated into microspheres. The XRD patterns indicated that the as-sprayed coatings were composed of HA and some decomposition phases. Micro-Raman spectroscopies demonstrated that the main phase in the coatings was HA. The results showed that the crystallinity was increased from 68.68% to 76.84% while the FWHM varied from 9.74 to 6.38cm−1, when the arc current increased from 400A to 600A. The selected area electron diffraction (SEAD) patterns were used to analyze the crystallinity qualitatively, and the results agreed with the conclusions of XRD and FWHM of Raman peak.

Relationship between cracks and microstructures in APS YSZ coatings at elevated temperatures

Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were prepared by plasma spraying. Thermally grown oxide (TGO) was formed between the YSZ layer and the bond coat during heat treatment of the YSZ TBCs. The generation and extension behavior of cracks before and after heat treatment were characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) combined with 3D reconstruction technology. The characterization results give a direct evidence that the generation and extension of cracks is closely related to other microstructures such as columnar grains, TGO morphology, TGO composition, and TGO orientation. For example, large interlaminar cracks in as-sprayed YSZ coatings often appeared along with columnar grains, which can be explained by their similar formation conditions. The formation of new cracks such as vertical cracks, plane cracks and other cracks along the TGO grain boundary after heat treatment often resulted from TGO growth stresses. Therefore, the generation and extension of cracks can be controlled by regulating the formation of columnar grains and TGO.