Sprayed Yttria Stabilized Zirconia Coatings Research Articles

Fracture toughness of freestanding plasma sprayed yttria stabilized zirconia coatings via in situ tensile experiments

Tensile response and fracture toughness of the freestanding atmospheric plasma spray 8YSZ coatings was investigated using in situ high resolution optical full-field strain measurement. The elastic properties obtained directly from the tensile experiments were employed in computing the critical mode-I stress intensity factor. Comparison between the fracture toughness values computed from specimen geometry and J-integral via displacement field around the crack tip showed small differences on single edge notched tensile (SENT) specimens to extract their intrinsic mode-I fracture toughness. Two kinds of coatings having different porosity were studied to obtain stress-strain response and compute effect of porosity on mode-I fracture toughness. The samples were fabricated by depositing the coatings on, un-notched and pre-notched substrate templates, and then carefully separating the coatings by acidic dissolution of the substrate. High resolution in situ deformation of the coatings was captured for both tensile and SENT specimens using digital image correlation method combined with an in-house designed microtensile stage. The deformation from the specimen surface was used to obtain elastic material properties under uniaxial tension, which are then used to compute the energy release rate by the path independent J-integral. The critical stress intensity factors calculated from SENT geometry for the low and high porosity coatings were found to be 1.23 ± 0.21 MPa-m0.5 and 0.67 ± 0.12 MPa-m0.5, respectively. In addition, considering the experimental observation of nucleation and propagation of a crack from the notch tip before catastrophic failure, the J-integral based calculation of the critical fracture toughness values were 1.37 ± 0.19 MPa-m0.5 and 0.5 ± 0.1 MPa-m0.5, for the low and high porosity coatings, respectively. As the crack always nucleates from the notch before complete fracture, it can be concluded that the J-integral based method is accurate and suitable for finding the fracture toughness in porous coatings.

Understanding the Sintering Behavior and its Effect on the Thermal Conductivity of YSZ Coatings for Gas Turbine Applications

Thermal barrier coatings are essential coatings to protect the combustion chamber liner material from harsh environments in modern gas turbines that are used in aerospace and land-based power generation facilities. As there are several different materials to produce thermal barrier coatings, the conventional thermal barrier coating is yttria stabilized zirconia (YSZ). The most common method to manufacture YSZ coating is plasma spraying method due to its flexibility and rapid production capacity. Using plasma spraying, often requires understanding the process parameters effect on the coating structure. As there are many parameters to control coating process the main outcome of all parameters is the particle temperature and velocity during the spraying process hence the coating properties such as hardness, porosity ratio and deposition rates. Furthermore, not only produced microstructure but also during the service conditions sintering behaviour also be considered. Sintering behaviour of thermal barrier coatings results declining of their thermal insulation properties. Therefore, in this study we have evaluated sintering effect of on the thermal conductivity of the plasma sprayed yttria stabilized zirconia coatings. To achieve this objective, we produced free-standing coatings and subjected them to heat treatment, followed by measurements of their thermal conductivities. The results of this study will contribute to a better understanding of the sintering behaviour and its impact on the thermal performance of thermal barrier coatings.

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.

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.

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.

Effects of TiO2 on microstructural, mechanical properties and in-vitro bioactivity of plasma sprayed yttria stabilised zirconia coatings for dental application

One of the most compatible coatings, known as yttria-stabilised zirconia polycrystal (YZP) is deposited on metallic Ti alloys due to its excellent hardness and aesthetic value as well as its low affinity for plaques. However, poor bioactivities of YZP and the existence of micro crack propagations due to the aging of YZP may result in spontaneous implant failure thus limiting its clinical use. In this work, YZP coating reinforced titania (TiO2), which is formed via a plasma spray technique was investigated in order to enhance the bioactivity and the mechanical properties of YZP coatings for dental implants. Based on microstructural studies performed on the deposited coating, a distinguished lamellar structure comprising YZP and TiO2 was observed. It was found that the reinforcement of TiO2 in YZP coating significantly reduced the crack due to the improved densities and the lamellar structure. The mechanical properties were also found to improve with 90% of hardness, 45% of adhesion strength and 54% of Young's Modulus with TiO2 addition, which is desirable for dental implants. An in-vitro bioactivity test was then conducted by immersing the coatings in a simulated body fluid (SBF). As a result, an apatite formation was found on the YZP/TiO2 coating surface after 3 days of immersion. Besides, it was verified in an XRD analysis that the crystalline TiO2 was found in a rutile phase which was highly effective in generating apatite (natural mineral in human bones) on YZP coatings, proving that the bioactivities of the coating were significantly improved. Further studies were also performed on the SBF treatment, which took up to 14 days also demonstrated that only a small decrease in hardness was noted, indicating that YZP/TiO2 coatings had reached an excellent mechanical stability.

Microstructure and phase stability of suspension high velocity oxy-fuel sprayed yttria stabilised zirconia coatings from aqueous and ethanol based suspensions

Two commercial 7–8wt.% Yttria Stabilised Zirconia (YSZ) suspensions were sprayed by Suspension High Velocity Oxy Fuel (SHVOF) thermal spraying for advanced high temperature coatings. Heat treatments of the free-standing coatings were conducted at 800°C and 1000°C for 72h. The SHVOF coatings using two liquid carriers: water and ethanol, behaved differently in terms of micro-structure and phase stability. The ethanol coatings retained a fully tetragonal composition after heat treatments; while the aqueous coatings, however, underwent the undesirable tetragonal to monoclinic phase transformation at 1000°C, which is lower than previously reported temperatures (>1200°C) in thermal sprayed YSZ coatings. The heat treatments not only resulted in densification of both coatings, but also caused excessive crystallite growth in aqueous coatings promoting the undesirable phase transformation. On the contrary, the ethanol suspension improved the phase stability by favouring the homogenization of yttrium during spraying.

Cavitation erosion of plasma sprayed YSZ coatings produced by feedstocks with different initial sizes

Cavitation erosion of plasma sprayed yttria stabilized zirconia coatings produced by feedstocks with different initial sizes was investigated according to the main guidance of ASTM-G32. Prior to thermal spray, substrates were preheated to 150 and 300°C. The size distribution of pores on the surface of coatings (as-sprayed and eroded) was estimated specifically to investigate its influences on the surface degradation during cavitation erosion. The results indicate that high feedstock size leads to high porosity of coating and low microhardness along with low resistance against cavitation erosion. Additionally, preheating processes could improve the coating resistance against cavitation erosion. The pore size distribution analysis results reveal that initial pores grow up and coalesce and cavitation pits format during cavitation erosion tests.

A finite element assessment of influential factors in evaluating interfacial fracture toughness of thermal barrier coating

It was previously reported that the adhesion strength of thermal sprayed yttria-stabilized zirconia coatings, as measured by the modified tensile test, decreased with increased coating thickness. Results from this method include the effects of both residual stress introduced during the spraying process and the adhesive glue used for the specimen preparation. This work quantified these effects by means of finite element analysis. The model also considered friction factors for interfacial crack closure induced by compressive residual stress in the coatings. It is concluded that friction factors affect interfacial fracture toughness slightly, but the most significant effect is caused by a change in the crack propagation path in the case of the thinnest coatings.

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.

Investigating the anisotropic mechanical properties of plasma sprayed yttria-stabilised zirconia coatings

The adhesion and cohesion bond strengths of plasma sprayed yttria-stabilised zirconia (YSZ) coatings were measured by performing the tensile adhesion test (TAT) and the tubular coating tensile test (TCTT). The TAT allowed assessment of adhesive/cohesive bond strength of a coating microstructure perpendicular to the substrate. In contrast, the TCTT quantifies the strength of a thermal spray coating parallel to the substrate without the use of any adhesive. The failure strength of the coatings from the respective tests can be approximated to a Weibull distribution and indicated the anisotropic behaviour of plasma sprayed coatings. The average coating strength parallel to the substrate is approximately 1.5 times greater than the bond strength perpendicular to substrate. The anisotropic behaviour of the plasma sprayed YSZ coatings were also probed using Knoop hardness measurements that were orientated at a well-defined geometry with respect to the lamellar microstructure. In addition, uniaxial compression tests evaluated the Poisson's ratio of these anisotropic YSZ coatings when loaded with respect to the different microstructural orientations.

Correlation of thermal conductivity of suspension plasma sprayed yttria stabilized zirconia coatings with some microstructural effects

Yttria-stabilized zirconia (3YSZ) coatings were successfully obtained by suspension plasma spraying. The coatings present generally two-zones-microstructure comprising nanostructured zones contributed by unmolten, partially sintered nanoparticles surrounded by lamellar splats formed from molten and agglomerated in-flight fine solids. In addition, different types of cracks inside the coating microstructure were classified and quantified by image analysis such as e.g. inner microcracks and segmentation cracks associated with quenching process as well as horizontal interlamellar cracks. Thermophysical properties of sprayed coatings were tested with a thermal diffusivity set up basing onto light flash principle. Subsequently, the thermal conductivity was determined with the use of literature data of density and specific heat. The calculations showed very low thermal conductivities values. The values did not correlate with coatings porosity data. Consequently, the analysis of variance (ANOVA) test allowed evaluating the possible impact of the various types of cracks on thermal conductivity. By this analysis, a good correlation between vertical cracks, which include microcracks and segmentation cracks, and thermal conductivity was found. The findings also confirmed the increase of thermal conductivity associated with this type of cracks.

Corrigendum to “Phase transformation and wear studies of plasma sprayed yttria stabilized zirconia coatings containing various mol% of yttria” [Mater. Charact. 62 (2011) 697–705

Corrigendum to “Phase transformation and wear studies of plasma sprayed yttria stabilized zirconia coatings containing various mol% of yttria” [Mater. Charact. 62 (2011) 697–705

Phase transformation and wear studies of plasma sprayed yttria stabilized zirconia coatings containing various mol% of yttria

Plasma sprayable grade zirconia powders doped with various mol% of yttria (0, 2, 3, 4, 6, 8 and 12 mol%) were synthesized by a chemical co-precipitation route. The coprecipitation conditions were adjusted such that the powders possessed good flowability in the as calcined condition and thus avoiding the agglomeration step like spray drying. Identical plasma spray parameters were used for plasma spraying all the powders on stainless steel plates. The powders and plasma sprayed coatings were characterized by X-ray diffractometry, Scanning Electron Microscopy and Raman spectroscopy. Zirconia powders are susceptible to phase transformations when subjected to very high temperatures during plasma spraying and XRD is insensitive to the presence of some non crystalline phases and hence Raman spectroscopy was used as an important tool. The microstructure of the plasma sprayed coatings showed a bimodal distribution containing fully melted and unmelted zones. The microhardness and wear resistance of the plasma sprayed coatings were determined. Among the plasma sprayed coatings, 3 mol% yttria stabilized zirconia coating containing pure tetragonal zirconia showed the highest wear resistance.

Microstructure and thermo-physical properties of yttria stabilized zirconia coatings with CMAS deposits

Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are used to protect hot-components in aero-engines from hot gases. In this paper, the microstructure and thermo-physical and mechanical properties of plasma sprayed YSZ coatings under the condition of calcium–magnesium–alumina–silicate (CMAS) deposits were investigated. Si and Ca in the CMAS rapidly penetrated the coating at 1250 °C and accelerated sintering of the coating. At the interface between the CMAS and YSZ coating, the YSZ coating was partially dissolved in the CMAS, inducing the phase transformation from tetragonal phase to monoclinic phase. Also, the porosity of the coating was reduced from ∼25% to 5%. As a result, the thermal diffusivity at 1200 °C increased from 0.3 mm 2/s to 0.7 mm 2/s, suggesting a significant degradation in the thermal barrier effect. Also, the coating showed a ∼40% increase in the microhardness. The degradation mechanism of TBC induced by CMAS was discussed.

Laser surface modification of plasma sprayed yttria stabilised zirconia coatings on type 316L stainless steel

Plasma sprayed yttria stabilised zirconia/NiCrAlY thermal barrier coatings proposed for molten LiCl–KCl salt environments contain pores and microcracks that may cause corrosion of the substrate on prolonged exposure. In the present work the as sprayed samples were laser remelted with various scan speeds of 1, 2·5 and 5 mm s−1 to reduce the segmented cracks. Microstructural inhomogeneities like pores and voids have been eliminated on laser remelting, however, segmented cracks were formed irrespective of scan speed. Distinct interface separating fine and coarse grains were observed at all scan speeds. The microhardness of the glazed surface improved and surface roughness was reduced. The beneficial non-transformable tetragonal phase was formed after laser remelting while the as sprayed coating consisted of insignificant monoclinic phase. An attempt was made to seal the pores of the zirconia coating by applying ZrO2 and ZrO2 + SiO2 powder.

Feasibility of ceramic coating to guard stainless steels against intergranular corrosion

The aim of this study is to examine the corrosion behaviour of yttria stabilised zirconia (YSZ) coated 304LN stainless steel, with or without bond coat, in order to assess the potential of the coatings to protect stainless steels against intergranular corrosion (IGC). The coatings have been prepared by plasma spraying technique, and their effectiveness in providing corrosion resistance has been examined by carrying out double loop electrochemical potentiokinetic reactivation test, anodic polarisation studies and boiling nitric acid tests. Characterisations of the steel, the coating and the interfaces, supplement the primary corrosion tests. The effectiveness of the coatings has been mainly assessed using the nature of spalling of the coated material subjected to boiling nitric acid tests, and the results have been discussed using the porosity in the coatings and their nature of the interfaces. It can be inferred from the obtained results that the developed plasma sprayed YSZ coatings, with or without bond coat, are unable to act as protective environmental barrier to prevent IGC of the stainless steel substrate primarily due to inappropriate plasma spray processing which leads to considerable amount of interconnected porosity. However, these results provide some interesting guidelines to develop corrosion resistant ceramic coatings.

A stochastic coating model to predict the microstructure of plasma sprayed zirconia coatings

A stochastic model has been developed to simulate the formation of thermal sprayed yttria-stabilized zirconia coatings. Porosity in the ceramic coating was assumed to be due to incomplete filling of cavities on the rough surface of the coating. Values of thermal spray particle size, velocity, temperature and trajectories were generated assuming they followed known distributions, with means and standard deviations measured experimentally. Predictions from the model of porosity and roughness of zirconia coatings agreed with experimental results, accurately predicting a reduction in porosity with particle size. The model also predicted that porosity, thickness and roughness would decrease with increasing particle velocity. Increasing spray-gun standoff distance had little effect on porosity (keeping particle velocity constant), while it decreased coating thickness and roughness.

Thermo-physical properties of plasma sprayed Yttria Stabilized Zirconia Coatings

This article investigates the thermo-physical properties of plasma sprayed zirconia coatings produced by agglomerates of submicron size particles as the feedstock. The microstructure of these deposits is consisted of splats and non-molten particles. This bimodal structure generally shows a better performance than conventional coatings. The agglomerated feedstock with internal submicron size porosity may significantly affect porosity related properties, such as the thermal diffusivity. In this study different process parameters were used to deposit yttria stabilized zirconia coatings with conventional and bimodal structures. Results showed a good correlation between the shape and distribution of pores and thermal diffusivity of the coatings.

Improvement in wear resistance of plasma sprayed yttria stabilized zirconia coating using nanostructured powder

Plasma sprayed yttria stabilized zirconia coatings were prepared using nanostructured and conventional powders with optimized process parameters for the highest deposition efficiency, the smallest porosity and the highest microhardness. The tribological properties of these coatings against 100C6 steel were then tested with a ball-on-disc arrangement. Results showed that although the friction coefficients of the coatings sprayed using the nanostructured powder were slightly different from those of the coatings sprayed using the conventional powder, the former coatings were more wear resistant than the latter coatings. The wear mechanisms of all the coatings were explained in terms of adhesion-induced spallation and micro-fracturing of lamellae. The improvement in wear resistance of the coatings sprayed using the nanostructured powder could be mainly ascribed to the decrease of micrometer-sized defects such as pores and interlamellar and intralamellar cracks in the coatings.