Nanostructured Zirconia Coatings Research Articles

Development of ZrO2 and YSZ coatings deposited by PE-CVD below 800 °C for the protection of Ni alloys

One possibility to increase the safety of structural Ni alloys is the use of yttria stabilized zirconia coatings in order to avoid deleterious thermal effects on their mechanical properties and corrosion resistance above 800 °C. However, the deposition of YSZ with a fully cubic structure below 800 °C cannot be achieved by traditional processing routes. In this work, we show the deposition of ZrO2 and Y2O3-stabilized ZrO2 coatings by Plasma Enhanced Chemical Vapor Deposition (PE-CVD). To this purpose, the effect of deposition temperature (500–800 °C), plasma power (50–250 W) and precursor concentration (0.5–2 g) on the microstructure, composition and crystalline phase of ZrO2 and YSZ coatings was studied. Contrary to previous reports, we show that it is feasible to use zirconium (IV) acetylacetonate [Zr (acac)4] to produce nanostructured zirconia coatings with low carbon content (1.8 at.%) and metastable tetragonal phase. A fully cubic YSZ structure was achieved with the use of Y-acetylacetonate with concentrations above 12 mol% at 700 °C and obtain a columnar microstructure.

Understanding of degradation-resistant behavior of nanostructured thermal barrier coatings with bimodal structure

Nanostructured thermal barrier coatings (TBCs) often provide high degradation resistance, as well as extended lifetime. However, the underlying mechanism has not been fully understood. In this study, the sintering characteristics of nanostructured yttria-stabilized zirconia (YSZ) coatings were investigated, and compared with those of the conventional YSZ coatings. Multiscale characterizations of the changes in microstructures and properties were performed. Results showed that the enhanced high-performance durability was mainly attributed to different sintering mechanisms of lamellar zones and nanozones. Sintering characteristics of the lamellar zones were similar to those of the conventional coatings. Stage-sensitive healing of two-dimensional (2D) pores dominated the sintering behavior of the lamellar zones. However, the differential densification rates between nanozones and lamellar zones of the nanostructured TBCs led to the formation of coarse voids. This counteractive effect, against healing of 2D pores, was the main factor contributing to the retardation of the performance degradation of bimodal TBCs during thermal exposure. Based on the understanding of the performance-degradation resistance, an outlook towards TBCs with higher performances was presented.

Structural evolution of plasma-sprayed nanoscale 3 mol% and 5 mol% yttria-stabilized zirconia coatings during sintering

The microstructure of plasma-sprayed nanostructured yttria-stabilized zirconia (YSZ) coatings may change during high-temperature exposure, which would influence the coating performance and service lifetime. In this study, the phase structure and the microstructural evolution of 3YSZ (zirconia-3mol% yttria) and 5YSZ (zirconia-5mol% yttria) nanostructured coatings were investigated by means of sintering at 1400°C for 50–100h. The microhardness, elastic moduli, and thermal shock cycles of the 3YSZ and 5YSZ nanostructured coatings were also investigated. The results showed that the redistribution of yttrium ions at 1400°C caused the continuous increase of monoclinic-phase zirconia, but no obvious inter-splat cracking formed at the cross-sections, even after 100h. Large voids appeared around the nanoporous zone because of the sintering of nanoscale granules upon high-temperature exposure. The microhardness and elastic moduli of the nanostructured coatings first increased and then decreased with increasing sintering times. The growth rate of the nanograins in the 3YSZ coating was lower than that in 5YSZ, which slowed the changes in 3YSZ coating porosity during sintering. Although the 3YSZ coating was prone to monoclinic phase transition, the experimental results showed that the thermal shock resistance of the 3YSZ coating was better than that of the 5YSZ coating.

Thermal Shock Failure Mechanism of Nanostructured Zirconia Coating by Atmospheric Plasma Spraying

Nanostructured zirconia coatings has been prepared based on reasonable spraying technical parameters and the corresponding thermal shock property of the as-sprayed coating was examined at 1100 °C. The structure and the surface/interface morphology of the coatings have been analyzed using X-ray diffraction (XRD), metallographic microscope, scanning electron microscopy (SEM), energy dispersive spectrum (EDS) and transmission electron microscopy (TEM). Based on the detailed analysis of the structure and phase composition, a rational mechanism has been proposed for thermal shock failure of the coating. Compared with other nanoparticles, those particles close to pores and pre-existing microcracks would preferentially grow up during the thermal shock process due to a better growth space. The growth of these nanoparticles is conducive to the formation of new microcracks which would lead to the growth of the other nanoparticles. With the growth of most or all of the nanoparticles, the nanostructured zirconia coating correspondingly changes into the quasi-microstructured coating. The thermal shock failure mode of the as-sprayed coating is similar to that of traditional zirconia thermal barrier coatings (TBCs).

Computational Development of a Novel Aerosol Synthesis Technique for Production of Dense and Nanostructured Zirconia Coating

The feasibility of a new processing method solution precursor high-velocity oxygen fuel spray (SP-HVOFS) is presented for the production of dense ZrO2-based nanostructured coatings, in which organometallic chemical precursor droplets are injected into the HVOF spray system. With the help of developed computational fluid dynamics (CFD) solver (Fluent), the evolution of particle volume, area, and number concentration is simulated considering nucleation, coagulation, and sintering. The aerosol model is validated with the experimental data available in the literature. When the oxygen-fuel gas flow rate (GFR) is increased, the (i) velocity and (ii) enthalpy of the HVOF flame is increased. The former reduces the particle residence time in the HVOF flame while the latter favors the sintering. Overall the results show that, by controlling the GFR, single scale nanometre particles (∼1–5 nm) can be fabricated without any agglomeration.

Microstructure and properties of nanostructured zirconia coatings deposited by atmosphere plasma spraying process

Nanostructured yttria zirconia coatings were deposited by air plasma spraying process with agglomerated yttria partially stabilised zirconia (ZrO2-6.8%Y2O3) powder. Field emission scanning electron Microscope, scanning electronic microscopy, transmission electron microscopy and X-ray diffraction were applied to analyse the phase composition and microstructure of the nanostructured powders and coating. The results showed that the agglomerated powders and as-sprayed nanostructured coating were both consisted of tetragonal phase; No monoclinic phase were observed in the coating .The average grain size of the powders and coating were 23 and 45 nm, respectively. The coating presented three kinds of microstructures, columnar grains, equiaxed grains and non-molten particles.

Abrasive Wear Analysis of Plasma-Sprayed LaCeYSZ Nanocomposite Coatings Using Experimental Design and ANN

The abrasive wear characteristics of plasma-sprayed nanostructured yttria-stabilized zirconia (YSZ) coatings on Inconel 718 substrates was evaluated using AFS 50/70-grade silica sand as abrasives. This article depicts the dependence of abrasive wear characteristics of plasma-sprayed nanocomposite LaCeYSZ coatings on abrading distance, keeping the applied load constant. The influence of four operating parameters—that is, load, wheel speed, time, and temperature with four different levels each—on the performance output (i.e., abrasion wear rate) is studied using Taguchi's L16 orthogonal array design and analysis of variance (ANOVA). Out of the four parameters, load has been found to be most significant factor followed, by speed of the abrasive wheel and temperature influencing abrasion. The morphology of the worn-out surface also showed microcutting and small crater formation in the binder matrix caused by the repetitive impacts of abrasive particles. It was observed that coating with nano-LaCeYSZ grains exhibited higher wear resistance compared to conventional YSZ coating and the reason may be attributed to embedded crack-arresting nanozones, which toughen the coating. An artificial neural network (ANN) approach is then implemented taking into account training and test procedures to predict the triboperformance under different operating conditions. This technique helps in saving time and resources for a large number of experimental trials and successfully predicts the wear rate of the coatings both within and beyond the experimental domain.

Tribo-performance of plasma-sprayed nanostructured yttria-stabilized zirconia coatings using Taguchi’s experimental design

Plasma-sprayed thermal barrier coatings are frequently subjected to impacts by solid particles, which induce surface erosion. This article depicts the solid particle erosion response of plasma sprayed nanostructured yttria-stabilized zirconia (YSZ) coatings on Inconel 718 substrates. The influence of five operating parameters i.e. impact velocity, erodent size, erodent temperature, impingement angle, and stand-off distance with four different level each, on performance output (i.e. erosion rate) are studied using Taguchi’s L16 orthogonal array design. Out of the five parameters, impact velocity has been found to be most significant factor followed by erodent temperature and impingement angle influencing erosion. Maximum erosion takes place at an impingement angle of ∼60° showing the semi-ductile response of the coating to solid particle erosion. The morphology of the eroded surface also showed micro-cutting and small crater formation in the binder matrix caused by the repetitive impacts of erodent particles. It was observed that coating with nano-YSZ grains exhibited higher erosion resistance compared to conventional YSZ coating.

Comparative study on effect of oxide thickness on stress distribution of traditional and nanostructured zirconia coating systems

Numerical based assessment of traditional and nanostructured yttria stabilized zirconia (YSZ) thermal barrier coating systems (TBCs) has been carried out with varying thickness of thermally grown oxide (TGO). Radial, axial and shear stresses are determined for both coatings and are presented in comparison with few novel and interesting results. Elastic strain energy for TGO failure assessment is determined from calculated stress within TGO for varying thickness. Radial stresses at TGO/bond coat interface and maximum axial stresses in nanostructured zirconia coatings are found to be lower than in traditional YSZ up to a critical TGO thickness of 6 –7μm, after which stresses in nanostructured zirconia coatings increase considerably. However, radial compressive stresses in nanostructured TBCs are lower in all TGO thickness cases and shear stresses are slightly higher with relatively more prominent difference at high oxide thickness.

HOT CORROSION RESISTANCE AND MECHANICAL BEHAVIOR OF ATMOSPHERIC PLASMA SPRAYED CONVENTIONAL AND NANOSTRUCTURED ZIRCONIA COATINGS

Conventional and nanostructured zirconia coatings were deposited on In-738 Ni super alloy by atmospheric plasma spray technique. The hot corrosion resistance of the coatings was measured at 1050°C using an atmospheric electrical furnace and a fused mixture of vanadium pent oxide and sodium sulfate respectively. According to the experimental results nanostructured coatings showed a better hot corrosion resistance than conventional ones. The improved hot corrosion resistance could be explained by the change of structure to a dense and more packed structure in the nanocoating. The evaluation of mechanical properties by nano indentation method showed the hardness (H) and elastic modulus (E) of the YSZ coating increased substantially after hot corrosion.

Microstructure, mechanical properties and thermal shock resistance of plasma sprayed nanostructured zirconia coatings

Nanostructured yttria stabilized zirconia (YSZ) coatings were deposited by Atmospheric Plasma Spraying (APS). X-ray diffraction (XRD) was used to investigate their phase composition, while scanning electron microscopy (SEM) was employed to examine their microstructure. The coatings showed a unique and complex microstructure composed of well-melted splats with columnar crystal structure, partially melted areas, which resembled the morphology of the powder feedstock, and equiaxed grains. Vickers microhardness of nanostructured zirconia coatings was similar to that of the conventional ones and strongly depended on the indentation load. Otherwise, a higher thermal shock resistance was found. This effect was addressed to the retention of nanostructured areas in coating microstructure and to the corresponding high porosity.

Thermal properties of plasma-sprayed thermal barrier coating with bimodal structure

Nanostructured zirconia coatings have been prepared by atmospherical plasma spraying (APS) on NiCrAlY-coated superalloy substrates. The isothermal oxidation test results indicate that the oxidation kinetics of nanostructured TBC follows a parabolic law and the oxidation resistance of the nanostructured TBC is comparable to that of the conventional TBC. The nanostructured thermal barrier coatings exhibit excellent thermal cyclic resistance and low thermal diffusivity. The failure of the nanostructured TBC occurs within the top coat and close to the YSZ/thermal growth oxide interface. The thermal diffusivity of the coating is 90% of that of conventional thermal barrier coatings, and it increases after heat treatment at 1050°C for 34h. The increase in the thermal diffusivity of the coating is ascribed to grain growth, the crack healing as well as sintering neck formation.

Microstructure and thermal cycling behavior of nanostructured yttria partially stabilized zirconia (YSZ) thermal barrier coatings

Nanostructured yttria partially stabilized zirconia (YSZ) coatings were prepared by atmospheric plasma spraying (APS) using the conglomeration made by zirconia nanoparticle as the raw materials. The measurement methods, which consisted of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermal cycling behavior, were used to character the morphology, composition and thermal oxidation behavior of the powder and the coatings. From the results, it was shown that the YSZ coating was the laminar structure, and the elements distribution in the bond and top coat were well-proportioned. The YSZ coatings were composed of fine grains with size ranging from 30 to 110 nm. The laminar layers with columnar grains were surrounded with unmelted parts of the nanostructured powder and some equiaxed grains. In the as-sprayed nanostructured zirconia coatings, there existed pores that were less than 1 μm. The cracks were observed on some of the crystal border. The cyclic oxidation experiment showed that the nanostructured coating had longer thermal cycling lifetime to exhibit the promising thermal cyclic oxidation resistance. The failure of the nanostructured TBC was similar to the failure of conventional APS TBC.

Microstructure analyses and thermophysical properties of nanostructured thermal barrier coatings

Nanostructured 8 wt% yttria partially stabilized zirconia coatings were deposited by air plasma spraying. Transmission electron microscopy, scanning electron microscopy, and X-ray diffraction were carried out to analyze the as-sprayed coatings and powders. Mercury intrusion porosimetry was applied to analyze the pore size distribution. Laser flash technique and differential scanning calorimetry were used to examine the thermophysical properties of the nanostructured coatings. The results demonstrate that the as-sprayed nanostructured zirconia coatings consist of the nonequilibrium tetragonal phase. The microstructure of the nanostructured coatings includes the initial nanostructure of powder and columnar grains. Moreover, micron-sized equiaxed grains were also exhibited in the nanostructured coatings. Their evolution mechanisms are discussed. The as-sprayed nanostructured zirconia coating shows a bimodal pore size distribution, and has a lower value of thermal conductivity than the conventional coating.

Nanostructured zirconia coatings processed by PROSOL deposition

The development of nanostructured coatings is a new field of interest in the world of thermal spraying. Novel techniques, such as HVOF or plasma spraying have been developed with respect to thermally activated processes, in order to reach the nanometric scale (10–20 nm). Frequently, the evolution consists in using agglomerated nanoparticles or suspension feedings. The CEA has patented a process called PROSOL [K. Wittmann-Ténèze, K. Vallé, L. Bianchi, F. Blein, P. Belleville, CEA Le Ripault, France, “Revêtement nanostructuré et procédé de revêtement”, Fr patent n°04 52390, (2004).], which is based on the injection of a sol–gel colloidal solution in a plasma source. Studies on the resulting coatings have principally shown that a nanostructured material could be prepared all the while preserving the intrinsic properties of the nanoparticle sol, the nanoparticle size and the crystalline phase distribution. The present investigation illustrates the PROSOL process through an example of spraying of a zirconia (ZrO 2) sol, prepared under hydrothermal conditions, in an atmospheric plasma jet. A time-stabilized sol of zirconia nanoparticles (10 nm) crystallized in both monoclinic and tetragonal phases was obtained. The injection of such a sol in the plasma plume was performed through a liquid injector that dispensed a sol jet under pressure without any pulverization gas. Because of the high temperature and the high velocity medium, the sol was fragmented into droplets and the liquid phase was vaporized. The resulting nanoparticle agglomerates embedded in droplets were accelerated and collected on a substrate without melting. By adjusting the plasma spraying parameters, more or less dense coatings could be obtained. Various characterization techniques were performed on the as-sprayed layers, including SEM, XRD and TEM. Results showed that the coatings were constituted of 10 nm-sized grains in the monoclinic and tetragonal phases. The flexibility of the process was also highlighted, i.e., the possibility to obtain coatings with thicknesses between one and several microns, produced without any physical change at a given deposition rate, on a variety of geometries and on a vast range of substrates.

Mechanical Properties of Nanostructured Zirconia Coatings Fabricated by Air Plasma Spraying

Nanostructured coatings are experiencing a rapid development in recent years due to their potential applications in a wide variety of technological areas. In order to improve the properties of thermal barrier coatings, nanostructured 8 wt% yttria partial stabilized zirconia coatings were deposited by air plasma spraying with reconstituted nanoparticles on titanium alloy substrates. For comparison, the conventional counterparts were also fabricated. Their microstructures were investigated by using transmission electron micrograph and scanning electron micrograph. Their mechanical behaviors were evaluated via Vickers microhardness test and tensile test. The results indicate that the nanostructured zirconia coatings with original nanostructure and columnar grains have higher microhardness and adhesive strength than the conventional counterparts. Both the nanostructured coating and the conventional coating exhibit a bimodal distribution of microhardness values, which are analyzed via Weibull statistics. This phenomenon is attributed to the presence of molten and non-molten parts in the coatings.

Nanostructured yttria stabilized zirconia coatings deposited by air plasma spraying

Nanostructured yttria partially stabilized zirconia coatings were deposited by air plasma spraying with reconstituted nanosized powder. The microstructures and phase compositions of the powder and the as-sprayed nanostructured coatings were characterized by transmission electron microscopy(TEM), scanning electron microscopy(SEM) and X-ray diffraction(XRD). The results demonstrate that the microstructure of as-sprayed nanostructured zirconia coating exhibits a unique tri-modal distribution including the initial nanostructure of the powder, equiaxed grains and columnar grains. Air plasma sprayed nanostructured zirconia coatings consist of only the nontransformable tetragonal phase, though the reconstituted nanostructured powder shows the presence of the monoclinic, the tetragonal and the cubic phases. The mean grain size of the coating is about 42 nm.

Microstructural feature, thermal shock resistance and isothermal oxidation resistance of nanostructured zirconia coating

Nanostructured and conventional zirconia coatings on superalloys have been deposited to show thermal shock resistance and oxidation resistance by atmospheric plasma spraying. The results showed that the nanostructured zirconia coating mainly contained two kinds of microstructures, nanosized zirconia particles embedded in the matrix and micrometer-sized columnar grain structures of zirconia similar to those of conventional zirconia coating. Compared with the conventional zirconia coating, the nanostructured coating is denser and has finer and less porous structure and fewer microcracks with higher thermal shock resistance and isothermal oxidation resistance. With increasing zirconia coating thickness from 100 to 500 μm, the thermal shock resistance of both nanostructured and conventional coatings decreased. The increased properties of nanostructured zirconia coating are related to improved toughness and decreased porosity and microcrack of the coating.

Wear Characteristics of Plasma-Sprayed Nanostructured Yttria Partially Stabilized Zirconia Coatings

Reconstituted nanostructured and conventional yttria partially stabilized zirconia coatings were deposited by atmospheric plasma spray. The tribologic properties of the coatings against 100C6 steel were evaluated with a ball-on-disc configuration under dry friction conditions at room temperature. Microstructure and the phase composition of the powders and the coatings were examined using a scanning electron microscope, optical microscope, and x-ray diffraction. Microhardness and the Young’s modulus of coatings were measured by indentation testing. Results showed that the wear resistance of the coatings produced using the nanostructured powder is improved compared with the coating produced using the conventional powder. The wear rates of nanostructured zirconia coatings are about four-fifths of those of conventional counterparts under a load of 5 N. The wear mechanism is also discussed.

Evaluation of unlubricated wear properties of plasma-sprayed nanostructured and conventional zirconia coatings by SRV tester

Atmospheric plasma spraying method was used to deposit nanostructured and conventional zirconia coatings using spray-dried nanostructured zirconia powder and conventional zirconia powder as feedstock, respectively. Their wear properties were evaluated comparatively by a sliding, reciprocating and vibrating (SRV) tester under dry conditions. The obtained results show that the wear properties of the plasma sprayed zirconia coatings deposited from spray-dried nanostructured zirconia powder were greatly improved compared with those of plasma sprayed zirconia coatings produced from conventional powder. The wear rates of nanostructured zirconia coatings are approximately half of those of conventional zirconia coatings. Under dry conditions, the wear mechanism for the plasma-sprayed nanostructured zirconia coatings is abrasive wear. Whilst in the case of plasma sprayed conventional zirconia coatings, it is a combination of abrasive wear and brittle fracture, the former is dominant wear mechanism. Their wear properties were explained in terms of their microstructure as well as mechanical properties and compared with the wear properties obtained under distilled-water lubricated conditions. Based on the experimental results, it is concluded that the finer debris is a critical factor for the improvement of wear properties of plasma-sprayed nanostructured zirconia coating under dry conditions. The wear properties of plasma sprayed zirconia coatings can be increased by the presence of water during the SRV testing.