Plasma-sprayed Yttria-stabilized Zirconia Coatings Research Articles

Friction and Wear Behaviors of Perfluoropolyether-Impregnated Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

Friction and Wear Behaviors of Perfluoropolyether-Impregnated Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

Influence of Surface Roughness of Aluminum Alloy Substrate on Tensile Adhesive Strength of Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

The adhesive mechanism of thermal sprayed ceramic coating is not clear because of the complicated profile of the blasted surface and the microstructure of the coating. In particular, little research has been carried out on the adhesive mechanism on light metal substrates. The objective of this study was to investigate the effect of surface roughness on the adhesive strength of yttria-stabilized zirconia coatings applied by atmospheric plasma spraying on an aluminum alloy (JIS A5052) substrate. Tensile adhesive and interfacial indentation tests were performed to evaluate their bonding properties. The substrates were blasted at three different pressures and seven different blast angles using fused alumina abrasives of three different particle sizes. Our measurements revealed that the tensile adhesive strength decreased with increasing substrate roughness (root mean square slope, RΔq). After the tensile strength tests, the fracture surfaces were observed using an FE-EPMA analyzer and analyzed using an X-ray fluorescence analyzer. It was found from elemental analysis that the fractured part shifted to the substrate with increasing RΔq. It was concluded that the tensile adhesive strength decreased with increasing surface roughness owing to the low strength of the substrate.

Tailoring wear and tribo-induced interaction of YSZ coating sliding against Si3N4 and Al2O3 counterparts

The friction and wear behaviors of plasma-sprayed yttria-stabilized zirconia (YSZ) coatings against Si3N4 and Al2O3 were evaluated from 25 °C to 800 °C in air. As the temperature increased, refined grains with severe lattice distortion and the phase transition from monoclinic to tetragonal were present within the wear track of the YSZ coating during friction. When the YSZ coatings were rubbed against Si3N4 and Al2O3 counterparts, all the worn samples showed common wear characteristics of fracture and fragmentation near the sliding surface. The wear discrepancy between the two tribo-pairs was due to the differences in tribo-induced interactions. In particular, TEM clarified the intense amorphization at the superficial regions of the YSZ coating when rubbed against Si3N4, which resulted in better wear resistance of the YSZ/Si3N4 sliding couple. The computational simulation using the semi-analytical model revealed higher contact temperatures and Von Mises stresses for the YSZ/Si3N4 sliding couple, which aided the amorphization of the regions near the sliding areas. Nevertheless, the higher normal stress between the YSZ/Al2O3 contacts caused cracking and localized fracture, leading to severe wear.

A comparative study of the optical and microstructural properties of suspension and atmospheric plasma sprayed thermal barrier coatings

Thermal radiation is an increasingly important issue for thermal barrier coating (TBC) applications in aircraft engine components. As operating temperatures increase, the radiative part of the heat load increases significantly faster than the conductive portion; as such, TBCs may not be as efficient in protecting their underlying heat-sensitive metallic components. To address this issue, new coating morphologies must be investigated. In this study, we compare the optical performance of atmospheric plasma sprayed (APS) and suspension plasma sprayed (SPS) yttria-stabilized zirconia (YSZ) coatings. First, we extract their absorption and scattering coefficients from spectrophotometry measurements via the inverse adding-doubling (IAD) method. The microstructure of the coatings is then analyzed and compared using scanning electron microscopy (SEM) and mercury infiltration porosimetry (MIP). SPS samples are found to offer higher radiation blocking properties due to a more uniform distribution of pores with an average smaller pore diameter than the APS samples. This results in an overall higher pore density, with each pore interface acting as a scattering site, and therefore a higher propensity for scattering events.

Controllable in-situ synthesis of MoS2/C in plasma-sprayed YSZ coatings: Microstructure, mechanical and tribological properties

Plasma-sprayed ceramic self-lubricating coatings are limited in their practical applications due to the degradation of mechanical properties caused by tribological design. Herein, a robust self-lubricating ceramic coating was fabricated by combining in-situ synthesis of molybdenum disulfide/carbon (MoS2/C) with the plasma spray technology process. Results show that MoS2/C grows in-situ in the pores and microcracks of plasma-sprayed yttria-stabilized zirconia (YSZ) coatings. Through the addition of MoS2/C, the hardness and cohesive strength of the coating could be significantly improved. Furthermore, the composite coating with the highest MoS2/C content exhibits the best tribological properties: the coefficient of friction is reduced by 79.6 % to 0.16, and the specific wear rate drops from 1.36 × 10−3 to 6.27 × 10−7 mm3 N−1·m−1. The improved tribological performance is attributed to a well-covered lubricating film composed of C, MoS2, YSZ, and Al2O3. The proposed strategy eliminates the most significant pain points of mechanical properties degradation of traditional ceramic self-lubricating coatings caused by tribological design and is believed to have great potential applications in wide engineering sectors.

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.

Influence of Surface Roughness of Aluminum Alloy Substrate on Tensile Adhesive Strength of Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

Influence of Surface Roughness of Aluminum Alloy Substrate on Tensile Adhesive Strength of Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

Elastic constants of plasma-sprayed yttria-stabilized zirconia coatings determined via resonant ultrasound spectroscopy coupled with laser-Doppler interferometry

Plasma-sprayed yttria-stabilized zirconia (YSZ) coatings exhibit five independent elastic constants with transverse isotropy owing to splat-based layered structures, including numerous preferentially oriented defects. In this study, we measure all the elastic constants of free-standing YSZ coatings in both as-sprayed and thermally treated conditions from 800 °C to 1300 °C for 2 h. The YSZ coatings were produced via atmospheric plasma spraying under one typical processing condition. The elastic constants are determined using resonant ultrasound spectroscopy coupled with laser Doppler interferometry . This approach allows us to overcome the difficulty of resonant-mode identification caused by the high internal friction of plasma-sprayed YSZ coatings; hence, reliable elastic constant data are obtained. We discovered that the splat-based layered structures provide strong elastic anisotropies in both the shear and longitudinal moduli. In addition, our measurements in the as-sprayed condition suggest that the as-sprayed YSZ coatings exhibit degenerate (elliptic) transverse isotropy with inter-splat pores that tend to be perpendicular to the spraying direction. The dependence of the elastic constants on the heat-treatment temperature shows that the elastic anisotropy of the plasma-sprayed YSZ coatings increases slightly and then decreases as the heat-treatment temperature increases. This finding is supported by the different effective sintering temperatures of the inter-splat pores and intra-splat cracks. • All the elastic constants of free-standing YSZ coatings were determined precisely. • YSZ coatings exhibited degenerate transverse isotropy with inter-splat pores. • Elastic constants provided appropriate information on the coating microstructures.

EBSD analysis of microstructure changes in YSZ coatings during thermal cycling

Failure of atmospheric plasma sprayed (APS) yttria-stabilized zirconia (YSZ) coatings is complicated. Therefore, determining the microstructural changes that occur in the top coat during thermal cycling is beneficial to figure out the dominant failure mechanism in thermal barrier coatings. In this study, thermal cycling tests were conducted on YSZ coatings sprayed by APS. For comparison, coatings with the same spray parameters were held at 1100 °C for 25 h. Areas of interest in the cross-sections of the coatings were chosen prior to thermal cycling, in which ex-situ observation and analysis were performed via scanning electron microscope and electron backscatter diffraction. Residual stress values of coatings with different thermal cycling times were measured by X-ray diffraction. The growth of quasi-columnar crystals, which were related to a depletion in Al and an increase in stress, was observed. During thermal loading, the vertical cracks propagate comparatively easier leading to the spallation. The residual stress varies continually during the thermal cycling test and the changes are divided into several stages. The monoclinic phase transformation tends to occurred inside grains, creating compressive stress inside the coating, which improve the longevity of the coating.

Influence of Residual Blast-materials on Adhesion of Plasma-sprayed Yttria-stabilized Zirconia Coatings

Influence of Residual Blast-materials on Adhesion of Plasma-sprayed Yttria-stabilized Zirconia Coatings

Effects of Splat Interfaces, Monoclinic Phase and Grain Boundaries on the Thermal Conductivity of Plasma Sprayed Yttria-Stabilized Zirconia Coatings

Microstructure has a significant influence on the thermal conductivity of thermal barrier coating (TBC) systems. In this work, the microstructures including splat interface, monoclinic phase and grain boundaries in the YSZ air plasma spraying (APS) TBC systems are investigated. A finite element simulation model based on electron backscatter diffraction (EBSD) images is established. It is found that the simulation results of thermal conductivity are in good agreement with the experimental results. Using this model, the effect coefficient of splat interface, monoclinic phase and grain boundaries on thermal conductivity are calculated. Results show that the splat interface influences the thermal conductivity of the TBCs. Those results provide important guidance for reducing the thermal conductivity of thermal barrier coatings.

Relation between Adhesion and Interfacial Roughness Parameters of Plasma-sprayed Yttria-stabilized Zirconia Coatings

Relation between Adhesion and Interfacial Roughness Parameters of Plasma-sprayed Yttria-stabilized Zirconia Coatings

Effects of laser surface remelting on the molten salt corrosion resistance of yttria-stabilized zirconia coatings

In the present study, atmospheric plasma-sprayed yttria-stabilized zirconia coatings were post-remelted by a continuous diode laser to improve the hot corrosion resistance for as-sprayed coatings. The coating surfaces were covered with a salt mixture (V2O5 and Na2SO4) and then subjected to a hot corrosion test at 1100 °C in air. The influence of laser parameters including power and scanning rate on the coating microstructure and corrosion resistance was investigated. Results showed that the hot corrosion resistance can be improved by producing a dense and smooth surface and reducing the coating permeability to the molten salt. The transformation of the hot corrosion mechanisms was clarified on the basis of the observed corrosion behaviors. A laser power of 1500 W and scanning rate of 9 mm/s can produce minimal surface roughness with few segmented cracks, which can provide improved performance of the hot corrosion resistance.

Influence of Lamellar Interface Morphology on Cracking Resistance of Plasma-Sprayed YSZ Coatings

Splat morphology is an important factor that influences the mechanical properties and durability of thermal barrier coatings (TBCs). In this study, yttria-stabilized zirconia (YSZ) coatings with different lamellar interface morphologies were deposited by atmospheric plasma spraying (APS) using feedstocks with different particle sizes. The influence of lamellar interface roughness on the cracking resistance of the coatings was investigated. Furthermore, the thermal shock and erosion resistance of coatings deposited by two different powders was evaluated. It was found that the particle size of the feedstock powder affects the stacking morphology of the splat that forms the coating. Coatings fabricated from coarse YSZ powders (45–60 μm) show a relatively rough inter-lamellar surface, with a roughness about 3 times greater than those faricated from fine powders (15–25 μm). Coatings prepared with fine powders tend to form large cracks parallel to the substrate direction under indentation, while no cracking phenomena were found in coatings prepared with coarse powders. Due to the higher cracking resistance, coatings prepared with coarse powders show better thermal shock and erosion resistances than those with fine powders. The results of this study provide a reference for the design and optimization of the microstructure of TBCs.

The Influence of Heat Treatments on the Porosity of Suspension Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

Suspension plasma-sprayed coatings are produced using fine-grained feedstock. This allows to control the porosity and to achieve low thermal conductivity which makes the coatings attractive as topcoats in thermal barrier coatings (TBCs). Used in gas turbine applications, TBCs are exposed to high temperature exhaust gases which lead to microstructure alterations. In order to obtain coatings with optimized thermomechanical properties, microstructure alterations like closing of pores and opening of cracks have to be taken into account. Hence, in this study, TBC topcoats consisting of 4 mol.% yttria-stabilized zirconia were heat-treated in air at 1150 °C and thereafter the coating porosity was investigated using image analysis (IA) and nuclear magnetic resonance (NMR) cryoporometry. Both IA and NMR cryoporometry showed that the porosity changed as a result of the heat treatment for all investigated coatings. In fact, both techniques showed that the fine porosity decreased as a result of the heat treatment, while IA also showed an increase in the coarse porosity. When studying the coatings using scanning electron microscopy, it was noticed that finer pores and cracks disappeared and larger pores grew slightly and achieved a more distinct shape as the material seemed to become more compact.

Mueller matrix polarimetry on plasma sprayed thermal barrier coatings for porosity measurement

Yttria-stabilized zirconia (YSZ) is the most widely used material for thermal plasma sprayed thermal barrier coatings (TBCs) used to protect gas turbine engine parts in demanding operation environments. The superior material properties of YSZ coatings are related to their internal porosity level. By quantifying the porosity level, tighter control on the spraying process can be achieved to produce reliable coatings. Currently, destructive measurement methods are widely used to measure the porosity level. In this paper, we describe a novel nondestructive approach that is applicable to classify the porosity level of plasma sprayed YSZ TBCs via Mueller matrix polarimetry. A rotating retarder Mueller matrix polarimeter was used to measure the polarization properties of the plasma sprayed YSZ coatings with different porosity levels. From these measurements, it was determined that a sample's measured depolarization ratio is dependent on the sample's surface roughness and porosity level. To this end, we correlate the depolarization ratio with the samples' surface roughness, as measured by a contact profilometer, as well as the total porosity level, in percentage measured using a micrograph and stereological analysis. With the use of this technique, a full-field and rapid measurement of porosity level can be achieved.

Effect of Particle Size on the Thermal Shock Resistance of Plasma-Sprayed YSZ Coatings

In this study, yttria-stabilized zirconia (YSZ) coatings were deposited by atmospheric plasma spraying (APS) using feedstocks with two different particle sizes. The effect of particle size on the pore structure and failure mechanism of the coatings was investigated. The evolution of the pore structure of the two kinds of coatings during cyclic thermal shock test was described by quantitative metallography. The influence of pore orientation on the thermal stress of the coating system was analyzed by the finite element method. It was found that the coatings deposited using coarse particles show a high thermal shock life time. The orientation of the pores in the coatings prepared by different particle sizes was different. A structural parameter was proposed to effectively characterize the pore orientation of the coatings. Coatings prepared by coarse YSZ powder tend to form almost the same number of horizontal and vertical pores, while coatings prepared by fine powder tend to form horizontal ones parallel to the direction of the substrate. The simulation results revealed that the vertical pores can reduce the thermal stress in the coating. The results of this investigation are a benefit to the design and integrity of TBCs.

Effect of Particle Size on the Micro-cracking of Plasma-Sprayed YSZ Coatings During Thermal Cycle Testing

The failure of plasma-sprayed thermal barrier coatings (TBCs) during service or thermal cycle testing usually results from internal cracking in the top coat, erosion and CMAS (calcium-magnesium-alumina-silicate)-induced damage, etc. The microstructure of ceramic coatings affects their durability and other properties of TBCs. In the present study, yttria-stabilized zirconia (YSZ) coatings were deposited by atmospheric plasma spraying (APS) using feedstocks with different particle sizes. In addition, the effect of particle size on damage evolution in the top coat was investigated. It is found that the coatings deposited using coarse particles show the higher thermal cycle life. Crack length grew with increasing numbers of thermal cycles. The faster crack growth rate can be found for the coatings deposited from fine particles. The porosity of the coating made from the coarse powder is larger than the porosity of the coating made from fine powder both in the as-sprayed condition and after thermal cycling. The changes in crack growth rate and the porosity are related to the effect of sintering and stress evolution in coatings during the thermal cyclic tests.

Computational image analysis of Suspension Plasma Sprayed YSZ coatings

The paper presents the computational studies of microstructure- and topography- related features of suspension plasma sprayed (SPS) coatings of yttria-stabilized zirconia (YSZ). The study mainly covers the porosity assessment, provided by ImageJ software analysis. The influence of boundary conditions, defined by: (i) circularity and (ii) size limits, on the computed values of porosity is also investigated. Additionally, the digital topography evaluation is performed: confocal laser scanning microscope (CLSM) and scanning electron microscope (SEM) operating in Shape from Shading (SFS) mode measure surface roughness of deposited coatings. Computed values of porosity and roughness are referred to the variables of the spraying process, which influence the morphology of coatings and determines the possible fields of their applications.

HOT CORROSION OF YTTRIUM STABILIZED ZIRCONIA COATINGS DEPOSITED BY AIR PLASMA SPRAY ON A NICKEL-BASED SUPERALLOY

In this research, the electrochemical impedance spectroscopy (EIS) and Tafel analysis were utilized to study the hot corrosion performance at 700[Formula: see text]C of air plasma-sprayed (APS) yttria-stabilized zirconia (YSZ) coatings with a NiCrAlY bond coat grown by high velocity oxygen fuel spraying (HVOF), deposited on an INCONEL 625 substrate, in contact with corrosive solids salts as vanadium pentoxide V2O5 and sodium sulfate Na2SO4. The EIS data were interpreted based on proposed equivalent electrical circuits using a suitable fitting procedure performed with Echem AnalystTM Software. Phase transformations and microstructural development were examined using X-ray diffraction (XRD), with Rietveld refinement for quantitative phase analysis, scanning electron microscopy (SEM) was used to determinate the coating morphology and corrosion products. The XRD analysis indicated that the reaction between sodium vanadate (NaVO[Formula: see text] and yttrium oxide (Y2O[Formula: see text] produces yttrium vanadate (YVO[Formula: see text] and leads to the transformation from tetragonal to monoclinic zirconia phase.