Plasma-sprayed Zirconia Thermal Barrier Research Articles

The effect of vermiculite on the degradation and spallation of plasma sprayed thermal barrier coatings

A study has been made of the effect of vermiculite (VM) on the sintering-induced spallation of plasma sprayed zirconia thermal barrier coatings (TBCs). The glass transition (~1040°C) and melting (~1360°C) temperatures of VM are representative of a broad class of so-called CMAS (calcia–magnesia–alumina–silica) particulates likely to be ingested into gas turbines. Selected loadings of VM powder were introduced onto the surface of free-standing coatings, followed by heating (1500°C) for periods of up to 80h. The presence of VM induces various microstructural changes in the coatings and also accelerates the rise in their (in-plane) Young's modulus. Finally, results are presented concerning the effect of VM on spallation resistance, using coatings sprayed onto dense alumina substrates. Spallation lifetimes can be substantially reduced by VM, even at relatively low levels (~1wt.%). This is related to acceleration of the sintering-induced increases in coating stiffness.

Thermal cyclic oxidation performance of plasma sprayed zirconia thermal barrier coatings with modified high velocity oxygen fuel sprayed bond coatings

In this study, the Hastelloy-X superalloy samples were overlaid by a CoNiCrAlY bond coating using a high pressure high velocity oxygen fuel (HVOF) spray process. A thin platinum film around 7.5μm thick was further applied to the surface of CoNiCrAlY coating by the magnetron sputtering deposition. Then the superalloy coupons with bare bond coat and with Pt deposited bond coat were pack aluminized at 850°C for 4h to produce (Ni,Co)Al and PtAl2 phases on surfaces, respectively. After that, all samples were overlaid with the yttria-stabilized zirconia (YSZ) top coats by air plasma spraying (APS). Then specimens were subjected to a thermal cycling test at 1100°C. Thermal cycling tests were 1h at 1100°C followed by 10min of forced-air cooling to ambient temperature outside the furnace. The weights of all the specimens were measured every 2cycles. Then effects of aluminizing and Pt-aluminizing on the cyclic oxidation performance and microstructure evolutions of the coatings were evaluated. Scanning electron microscopy (SEM), X-ray diffractometry (XRD) and electron probe microanalyzer (EPMA) were used to identify crystalline phases and microstructures of each coating. The results proved that the roughness of the CoNiCrAlY coating was not changed after the aluminizing or the Pt-aluminizing process. The specific weight gain of the thermal barrier coatings (TBCs) with aluminizing or with Pt-aluminizing bond coating was lower than that of TBCs with only HVOF sprayed bond coat. The cyclic oxidation life of sprayed MCrAlY/ZrO2–Y2O3 thermal barrier coating can be improved effectively by either aluminizing or Pt-aluminizing treatment.

Structure–property differences between supersonic and conventional atmospheric plasma sprayed zirconia thermal barrier coatings

Yttria-stabilized zirconia (YSZ) based thermal barrier coatings (TBCs) were deposited by high efficiency supersonic atmospheric plasma spraying (SAPS) system. The microstructure and thermal shock resistance of the SAPS-TBCs were investigated. As compared to conventional atmospheric plasma sprayed TBCs (APS-TBCs) with the same composition, the microstructure of SAPS-TBCs was much finer. It was found that the thickness of lamellar structure consisted of columnar crystals in the SAPS- and APS-coatings was in the range of 1–4μm and 2–8μm, respectively. Besides, the statistical results revealed that the average thickness of the lamellar structure in SAPS-coating was 2.5±0.6μm, while that of APS-coating was 5.3±0.9μm. The desirable structure was attributed to higher impact velocity of in-flight particles during SAPS process, which resulted in the improvement of flattening degree of molten particles after impinging on the target. The well-adhered fine lamellar structures, fine micro-cracks and lower growth rate of thermally grown oxide (TGO) appeared to be responsible for greatly improved thermal cycling lives of SAPS-TBCs as compared to their conventional plasma sprayed counterparts. The results of water-quenching test from 1100°C into room temperature showed that SAPS-TBCs presented high thermal shock resistance, only 10% coating area spalled after 265 thermal cycles, about 90% higher than that of APS-TBCs. The SAPS method, which offered some unique advantages over the conventional plasma spraying process, is expected to be potentially used to deposit high-performance TBCs at lower cost.

Stress Distributions in Plasma-Sprayed Thermal Barrier Coatings Under Thermal Cycling in a Temperature Gradient

Abstract The residual stress distribution in plasma-sprayed zirconia thermal barrier coatings subjected to cyclic thermal gradient testing was evaluated using Raman piezospectroscopy and finite element computation. The thermal gradient testing (approximately 440°C/mm at temperature), consisted of repeated front-side heating with a flame and constant cooling of the back-side of the substrate either with front-side radiative cooling only or with additional forced air cooling between the heating cycles. The coatings exhibited characteristic “mud-cracking” with the average crack spacing dependent on the cooling treatment. This is consistent with finite element calculations and Raman spectroscopy measurements in which the sudden drop in coating surface temperature on initial cooling leads to a large biaxial tension at the surface. The key to proper interpretation of the Raman shifts is that the stress-free Raman peaks need to be corrected for shifts associated with the evolution of the metastable tetragonal phase with aging.

A sintering model for plasma-sprayed zirconia thermal barrier coatings. Part II: Coatings bonded to a rigid substrate

The sintering model described in Part I, which relates to free-standing plasma-sprayed thermal barrier coatings, is extended here to the case of a coating attached to a rigid substrate. Through-thickness shrinkage measurements have been carried out for coatings attached to zirconia substrates, and these experimental data are compared with model predictions. The model is then used to explore the influence of the substrate material (zirconia vs. a nickel superalloy), and of the in-plane coating stiffness. Both differential thermal expansion stresses and tensile stresses arising from the constraint imposed on in-plane shrinkage can be relaxed via two diffusional mechanisms: Coble creep and microcrack opening. This relaxation allows progression towards densification, although the process is somewhat inhibited, compared with the case of a free-standing coating. Comparison of the stored elastic strain energy with the critical strain energy release rate for interfacial cracking allows estimates to be made of whether debonding is energetically favoured. 1 A compiled version of the sintering model can be downloaded from www.msm.cam.ac.uk/mmc/publications/software.html. 1

Improving the erosion resistance of plasma-sprayed zirconia thermal barrier coatings by laser glazing

In this study under review, substrates of 100 mm × 25 mm × 2 mm SUS 420 stainless steel coupons were first sprayed with a Ni–22Cr–10Al–1Y bond coat and then with a 19.5 wt.% yttria-stabilized zirconia (YSZ) top coat using an air-plasma spray system. After that, the plasma-sprayed yttria-stabilized zirconia thermal barrier coatings (TBCs) were glazed using a pulsed CO 2 laser. The subsequent effects of laser glazing on the microstructure and erosion behavior of applied coatings were then evaluated. Several erosion tests were conducted at room temperature using 50 μm silica erodent particles with impact velocity of 50 m s − 1. The microstructures of both the as-processed and the tested TBCs were investigated by scanning electron microscopy (SEM). The phase of the coatings was measured by X-ray diffractometry (XRD). The results of this investigation showed that the laser-glazing process increased the microhardness from about 550 Hv for the as-sprayed layer to about 1550 Hv for the as-glazed layer. The erosion rate increased as the impingement angle increased for both plasma-sprayed and laser-glazed TBCs. Laser glazing enhanced the erosion resistance of plasma-sprayed TBCs by about 1.5 to 3 times with the impingement angle ranging between 30° and 75°, while the erosion resistance did not significantly improve when the impingement angle reached 90°. Erosion morphology analysis clearly indicates that the erosion of the plasma-sprayed TBCs was deemed to be the erosion of the protrusions and the sprayed splats. The erosion of the laser-glazed TBCs was proven to be the spallation of the glazed layer. Spallation occurred in the laser-glazed layer/plasma-sprayed splats interface.

Long‐Term Behavior and Application Limits of Plasma‐Sprayed Zirconia Thermal Barrier Coatings

Investigations of changes in phase composition, mechanical properties, and microstructure of ZrO2‐based plasma‐sprayed thermal barrier coatings (TBCs) with 8 mol% CeO2, 19.5 mol% CeO2/1.5 mol% Y2O3, 35 mol% CeO2, and 4.5 mol% Y2O3 after long‐term heat treatments at typical operation temperatures (1000°–1400°C) are presented. Experimental studies include X‐ray diffractometry, mechanical testing, and scanning electron microscopy. Thermal cycling experiments also have been performed. TBCs with 8 mol% CeO2 contain mainly the tetragonal equilibrium phase and, therefore, show rapid failure because of the high amount of tetragonal → monoclinic phase transformation, even after relatively short heat treatments (1250°C/1 h). In the case of the other systems that consist mainly of the tetragonal or cubic nonequilibrium phases, TBCs with 19.5 mol% CeO2/1.5 mol% Y2O3 or 35 mol% CeO2 reveal a smaller amount of monoclinic phase after long‐term heat treatments (1250°C/1000 h) compared with TBCs containing 4.5 mol% Y2O3. TBCs containing 35 mol% CeO2 show a higher degree of sintering than the TBCs with 19.5 mol% CeO2/1.5 mol% Y2O3 and, therefore, a greater increase of the elastic modulus. Among the systems investigated, TBCs containing 4.5 mol% Y2O3 exhibit the highest resistance to failure in thermal‐cycling experiments.

Phase transformation and bond coat oxidation behavior of plasma-sprayed zirconia thermal barrier coating

ZrO 2–CeO 2–Y 2O 3 and ZrO 2–Y 2O 3 thermal barrier coatings were prepared using the air plasma spray process. Phase transformation in the ceramic top coating, bond coat oxidation and thermal barrier properties were investigated to compare ZrO 2–CeO 2–Y 2O 3 with ZrO 2–Y 2O 3 at 1300°C under high temperature thermal cycles. In the as-sprayed condition, both coatings showed a 7∼11% porosity fraction and typical lamellar structures formed by continuous wetting by liquid droplets. In the ZrO 2–CeO 2–Y 2O 3 coating, the phase ratio of tetragonal to cubic phase was 75:25 and ZrO 2–Y 2O 3 coating had a 100% non-transformable tetragonal phase. There was no monoclinic phase in either coating. However, the phase ratio of the coatings was changed after 1300°C thermal cycles. In the ZrO 2–CeO 2–Y 2O 3 coating, the ratio of tetragonal to cubic was changed to 88:12 and a monoclinic phase was still not detected, but a 10∼19% monoclinic phase had formed in the ZrO 2–Y 2O 3 coating. The life of the coatings was found to be strongly dependent on the temperature which the bond coat experienced during exposure to a peak temperature of 1300°C. When the bond coat experienced a temperature higher than 1100°C, the useful life of both thermal barrier coatings was decreased drastically and this was related to the oxidation behavior of the bond coat. Al 2O 3 formed preferentially along the bond and top coat interface and the other oxides such as NiO and Ni(Cr,Al) 2O 4 spinel, which were believed to decrease the coating life by oxide growth stress, formed rapidly at the top coat side of the interface at temperature higher than 1100°C. The thermally shocked ZrO 2–Y 2O 3 coating exhibited a non-linear thermal expansion curve which was probably due to a reversible tetragonal–monoclinic transformation. The ZrO 2–CeO 2–Y 2O 3 coating was found to have better thermal cycling behavior than the ZrO 2–Y 2O 3 coating. The reason for this could be that there was no phase transformation from tetragonal to monoclinic phase, which induces volume expansion. The thermal expansion mismatch is smaller and the effect of oxide growth stress is relatively small because of better thermal insulation.

Strain gradients in plasma-sprayed zirconia thermal barrier coatings

Abstract Neutron diffraction was used to measure the residual strain field in plasma-sprayed zirconia thermal barrier coatings (TBCs). Data were collected at the British neutron spallation source of ISIS (Didcot), on ENGIN, a recently installed TOF (time-of-flight) instrument designed for residual strain depth profiling. Its particular geometry permitted a direct measurement of the interplanar distances of crystallographic planes lying parallel to the component surface as well as the measurement of zero-strain reference samples. The latter were annealed samples of the three present phases: zirconia (top coat), NiCoCrAlY (bondcoat), and copper (substrate). In this way ϵ 33 , the strain component perpendicular to the sample surface, was determined at several positions inside the component, for all the present phases. The results of this analysis, consisting of a strain profile throughout the entire cross-section of the coated component, were integrated by those obtained by a destructive testing, performed after TOF data collection, consisting in the measurement of curvature change of the ceramic after substrate removal by chemical attack.

Microstructural characterization of plasma-sprayed zirconia thermal barrier coatings by X-ray diffraction full pattern analysis

Thick two-layered thermal barrier coatings made of Ni-Co-Cr-Al-Y and yttria-partially stabilized zirconia were plasma sprayed at controlled deposition temperature onto stainless steel substrates. During the spraying of the top coat, the deposition temperature was kept as constant as possible using a front air or liquid-argon-cooling system. The correlation between process parameters and microstructure was investigated by scanning electron microscopy and X-ray diffraction (XRD) analysis. The application of a new procedure for XRD data processing, based on the Rietveld method, made it possible to obtain good accuracy in the phase determination and quantitative evaluation of zirconia polymorphs. Moreover, the crystallite size and microstrain were refined simultaneously to phase percentages. Also the preferred orientation in the coating could be taken into account, adding useful information and increasing the reliability of all the other results. The influence on the coating microstructure of changing the deposition temperature and environment is also discussed.

A comparison of techniques for the metallographic preparation of thermal sprayed samples

Metallographic preparation of thermal spray coated samples is often difficult because hard and soft materials, which normally require different polishing techniques, are commonly present in a single spraycoated sample. In addition, the microstructures of many spray- deposited materials make them prone to pull-out damage during cutting, grinding, and polishing operations. This study compares alternative metallographic techniques to prepare three common types of thermal sprayed coatings: (1) a plasma sprayed alumina-titania wear coating, (2) a plasma sprayed zirconia thermal barrier coating, and (3) a high-velocity oxy-fuel (HVOF) sprayed tungsten- carbide/cobalt (WC/Co) hard coating. Each coating was deposited onto a steel substrate and was prepared with metallographic protocols based on silicon carbide (SiC) papers, bonded diamond platens, and diamond slurries. Polishing with SiC papers generally produced edge rounding and significant pull- out, which increased the apparent porosity of the coatings. Polishing with bonded diamond platens produced less edge rounding, but some pull- out was still observed. Preparation by diamond slurry lapping consistently produced the best overall results. Porosity artifacts produced by polishing with SiC papers and bonded diamond platens also resulted in spuriously low hardness values for the WC/Co samples; however, hardness results for the two ceramic coatings were not affected by the polishing method.

Phase composition and properties of plasma-sprayed zirconia thermal barrier coatings

The use of X-ray diffraction combined with TEM analysis has been used to study the crystalline structure and change in phase composition of zirconia coatings containing 6–12 wt% Y2O3. The optimum composition for maximum durability, observed for coatings within this composition range, is believed to be related to the microstructure developed on rapid cooling and to the volume fractions of t′, c and m phases formed during the evolution of the coating. The amount of these phases present in commercial thermal barrier coatings has been determined using X-ray diffraction and the mechanisms of toughening deduced from TEM examination of the sections of the coatings. The results obtained are discussed in relation to the degree of toughness and hence the thermal shock resistance which is a major factor in determining service life.

Phase composition and properties of plasma-sprayed zirconia thermal barrier coatings

Phase composition and properties of plasma-sprayed zirconia thermal barrier coatings