Thermo-Physical Properties and Thermal Shock Resistance of Segmented La2Ce2O7/YSZ Thermal Barrier Coatings

La₂Ce₂O₇ (LCO)/yttria-stabilized zirconia (YSZ) thermal barrier coating (TBC) with segmentation crack structure was produced by atmospheric plasma spraying. Thermo-physical properties, such as thermal diffusivities and thermal conductivities, and thermal cycling performance of the segmented LCO/YSZ TBC were investigated. The thermal conductivity of the segmented coating was measured to be around 1.02 W/mK at 1200 °C, relatively lower than that of the non-segmented coating, respectively. The segmented LCO/YSZ TBC exhibited a thermal cycling lifetime of around 2100 cycles, improving the durability by nearly 50% as compared to the non-segmented TBC. The failure of the segmented coating occurred by chipping spallation and delamination cracking within the coating.     

Isothermal oxidation of physical vapor deposited partially stabilized zirconia thermal barrier coatings

Thermal barrier coatings (TBCs), consisting of physical vapor deposited (PVD) partially stabilized zirconia (PSZ, 8 wt.%Y₂O₃) and a diffusion aluminide bond coat, were characterized as a function of time after oxidative isothermal heat treatment at 1373 K in air. The experimental characterizations was conducted by X-ray diffraction analysis and scanning electron microscopy (SEM) with energy-dispersive spectroscopy. During cooling to room temperature, spallation of the PSZ ceramic coatings occurred after 200 and 350 h of isothermal heat treatment. This failure was always sudden and violent, with the TBC popping from the substrate. The monoclinic phase of zirconia was first observed on the bottom surface of the PVD PSZ after 200 h of isothermal heat treatment. The failure of TBCs occurred either in the bond coat oxidation products of αAl₂O₃ and rutile TiO₂ or at the interface between the oxidation products and the diffusion aluminide bond coat or the PSZ coating.     

HVOF thermal spray deposited Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-stabilized ZrO<Subscript>2</Subscript> coatings for thermal barrier applications

High velocity oxy-fuel (HVOF) thermal spray has been successfully used to deposit yttria-stabilized zirconia (YSZ) for thermal barrier coating (TBC) applications. Adherent coatings were obtained within a limited range of spray conditions using hydrogen as fuel gas. Spray parameters such as hydrogen-to-oxygen ratio, spray distance, and substrate cooling were investigated. Spray distance was found to have a pronounced effect on coating quality; adherent coatings were obtained for spray distances between 75 and 125 mm from the gun exit for the hydrogen-to-oxygen ratios explored. Compared to air plasma spray (APS) deposited YSZ coatings, the HVOF deposited coatings were more fully stabilized in the tetragonal phase, and of similar density, surface roughness, and cross-sectional microhardness. Notably, fracture surfaces of the HVOF coatings revealed a more homogeneous structure. Many theoretical models predict that it should not be possible to melt YSZ in an HVOF flame, and therefore it should not be possible to deposit viable YSZ coatings by this process. The experimental results in the present work clearly contradict those expectations. The present results can be explained by taking into account the effect of partial melting and sintering on particle cohesion, as follows. Combustion chamber pressures (P _o) of ∼3.9 bar (58.8 psi) realized during HVOF gun operation allows adiabatic flame temperature values that are above the zirconia melting temperature. Under these conditions, the Ranz-Marshall heat transfer model predicts HVOF sprayed particle surface temperatures T _p that are high enough for partial melting of small (∼10 μm) zirconia particles, T _p=(1.10−0.95)T _m. Further analysis shows that for larger particles (38 μm), adherent coatings are produced when the particle temperature, T _p=0.59−0.60 T _m, suggesting that sintering may have a role in zirconia particle deposition during HVOF spray. These results suggest two different bonding mechanisms for powders having a broad particle size distribution.     

Ceramic nanocomposites obtained by sol–gel coating of submicron powders

MgAl₂O₄–ZrO₂ nanocomposites were fabricated by conventional sintering of composite powders obtained by sol–gel coating of a submicron spinel powder. In the composite powder the zirconia grains remain narrow sized and completely tetragonal even after being heat treated at temperatures where a free xerogel is completely monoclinic. The sintered material exhibits a dense, fine and highly homogeneous microstructure. The zirconia nanoparticles are located at both inter- and intragranular positions and exhibit heteroepitaxial relationships with the surrounding crystals. Tetragonal zirconia seems to be stabilised by an interface effect. Both the scale of the microstructure and the fraction of intragranular grains were controlled by adjusting the mean grain size of spinel grains before coating and sintering conditions.     

Effect of increased water vapor levels on TBC lifetime with Pt-containing bond coatings

To investigate the effect of increased water vapor levels on thermal barrier coating (TBC) lifetime, furnace cycle tests were performed at 1150 °C in air with 10 vol.% water vapor (similar to natural gas combustion) and 90 vol.%. Either Pt diffusion or Pt-modified aluminide bond coatings were applied to specimens from the same batch of a commercial second-generation single-crystal superalloy and commercial vapor-deposited yttria-stabilized zirconia (YSZ) top coats were applied. Three coatings of each type were furnace cycled to failure to compare the average lifetimes obtained in dry O₂, using the same superalloy batch and coating types. Average lifetimes with Pt diffusion coatings were unaffected by the addition of water vapor. In contrast, the average lifetime of Pt-modified aluminide coatings was reduced by more than 50% with 10% water vapor but only slightly reduced by 90% water vapor. Based on roughness measurements from similar specimens without a YSZ coating, the addition of 10% water vapor increased the rate of coating roughening more than 90% water vapor. Qualitatively, the amount of β-phase depletion in the coatings exposed in 10% water vapor did not appear to be accelerated.     

Development of a Pre-Oxidation Treatment to Improve the Adhesion between Thermal Barrier Coatings and NiCoCrAlY Bond Coatings

High-temperature coating systems, consisting of a René N5 superalloy, a Ni–23Co–23Cr–19Al–0.2Y (at.%) bond coating (BC), and a yttria (7 wt%)-stabilized zirconia (YSZ) thermal barrier coating (TBC), were thermally cycled to failure for seven different controlled pre-oxidation treatments and one commonly employed industrial pre-oxidation treatment to establish the preferred microstructures of the thermally-grown oxide (TGO) on a NiCoCrAlY bond coating after pre-oxidation. It was found that the failure of the coating system occurred along the TGO/BC interface when the TGO attained a critical thickness, except if a NiAl₂O₄ spinel layer developed contiguous to the TBC/TGO interface. Then, the coating system failed at a smaller TGO thickness along the NiAl₂O₄/α-Al₂O₃ interface. The value for the TGO thickness at failure increased for a larger area fraction of Y-rich oxide pegs at the TGO/BC interface after pre-oxidation. A desired slow-growing oxide layer on the BC surface was promoted when the presence of the oxides NiAl₂O₄, θ-Al₂O₃, Y₃Al₅O₁₂ at the TGO surface after pre-oxidation was avoided. The α-Al₂O₃ layer, which developed adjacent to the BC upon thermal cycling, grew at a low rate if the initial oxide at the onset of oxidation consisted of θ-Al₂O₃ instead of α-Al₂O₃. Based on these results a pre-oxidation treatment is proposed for which the lifetime of the entire coating system during service is enhanced.     

Post-deposition treatment of zirconia thermal barrier coatings using Sol-Gel alumina

This article addresses the problem of gas permeability of thermal sprayed yttria-stabilized zirconia thermal barrier coatings (TBC)s. The objective of this study was to decrease the open porosity of TBCs through deposition of dense alumina ceramic on the surface of the pores. A simple infiltration technique was used, beginning with aluminum isopropoxide as sol precursor, subsequently hydrated to aluminum hydroxide sol, which decomposed at relatively low temperatures to extra-fine, readily sinterable aluminum oxide. In some experiments, the sol-gel (SG) precursor was combined with fine grains of calcined alumina, constituting high solid-yield composite sol-gel (CSG) deposits within the pores of TBCs. Sinterability in the model systems, including aluminum hydroxide sol-calcined alumina and aluminum hydroxide sol-calcined alumina-zirconia, has been studied. A number of TBC specimens were impregnated with suspensions of alumina sols and CSG. It is shown that these ceramics effectively penetrated into the pores and cracks of TBCs and reduced the coating permeability to gases. The overall reduction of porosity was however small (from ∼12 to ∼11%), preserving the strain and thermal shock tolerance of the coatings. Burner rig tests showed an increase in sealed coating lifetime under thermomechanical fatigue conditions.     

Improved oxidation life of segmented plasma sprayed 8YSZ thermal barrier coatings

Unconventional plasma sprayed thermal barrier coating (TBC) systems were produced and evaluated by interrupted or cyclic furnace oxidation life testing. First, approximately 250 μm thick 8YSZ coatings were directly sprayed onto grit blasted surfaces of PWA 1484, without a bond coat, to take advantage of the excellent oxidation resistance of this superalloy. For nominal sulfur (S) contents of 1 ppmw, total coating separation took place at relatively short times (200 h at 1100°C). Reductions in the S content, by melt desulfurization commercially (0.3 ppmw) or by hydrogen (H₂) annealing in the laboratory (0.01 ppmw), improved scale adhesion and extended life appreciably, by factors of 5–10. However, edge-initiated failure persisted, producing massive delamination as one sheet of coating. Secondly, surfaces of melt desulfurized PWA 1484 were machined with a grid of grooves or ribs (∼250 μm wide and high), resulting in a segmented TBC surface macrostructure, for the purpose of subverting this failure mechanism. In this case, failure occurred only as independent, single-segment events. For grooved samples, 1100 °C segment life was extended to ∼1000h for 5 mm wide segments, with no failure observed out to 2000 h for segments ≤2.5 mm wide. Ribbed samples were even more durable, and segments ≤6 mm remained intact for 2000 h. Larger segments failed by buckling at times inversely related to the segment width and decreased by oxidation effects at higher temperatures. This critical buckling size was consistent with that predicted for elastic buckling of a TBC plate subject to thermal expansion mismatch stresses. Thus, low S substrates demonstrate appreciable coating lives without a bond coat, while rib segmenting extends life considerably.     

Characterizations of cold-sprayed Nickel-Alumina composite coating with relatively large Nickel-coated Alumina powder

Previous studies have shown that the fabrication of metal matrix composites (MMCs) by cold spraying is effective and promising. When light materials, such as SiC and Al₂O₃, were used as reinforcements, it was diffcuclt to obtain a high volume fraction of hard phase in the composite just through the simple powder mixture. Therefore, in this study, a Ni-coated Al₂O₃ powder, which was produced through hydrothermal hydrogen reduction method, was employed aiming at increasing the volume fraction of ceramic particles in the deposited composite coating. It was found that a dense Ni-Al₂O₃ composite coating could be deposited with the Ni-coated Al₂O₃ powder under the present spray conditions. X-ray diffraction analysis indicated that the composite coating had the same phase structures as the feedstock. The volume fraction of Al₂O₃ in the composite was about 29 ± 6 vol.%, which is less than that in the feedstock (nominal: 40-45 vol.%) due to the rebound of some Al₂O₃ particulates upon kinetic impacting. The microhardness of the composite coating was about 173 ± 33Hv_0.2.     

Effect of annealing on mechanical properties of silicon carbide sintered with aluminum nitride and scandium oxide

Beta-SiC powder samples containing 1 wt.% α-SiC as a seed and 10 vol.% AlN-Sc₂O₃ as a sintering additive were hot-pressed at 1900 °C for 1 h and subsequently annealed at 2000 °C for 1 h, 3 h and 6 h. When the annealing time was increased, the microstructure changed from equiaxed to elongated grains, which resulted in a self-reinforced microstructure consisting of elongated grains (hexagonal platelet grains in 3-dimensions). The development of a self-reinforced microstructure resulted in significant improvement in toughness. However, the improved toughness was offset by a reduction in strength. The typical fracture toughness and strength of the ceramics annealed for 6 h were 7.3 MPa·m^1/2 and ∼500 MPa, respectively.     

Al2O3-nanodiamond composite coatings with high durability and hydrophobicity prepared by aerosol deposition

We attempted the room-temperature fabrication of Al₂O₃-based nanodiamond (ND) composite coating films on glass substrates by an aerosol deposition (AD) process to improve the anti-scratch and anti-smudge properties of the films. Submicron Al₂O₃ powder capable of fabricating transparent hard coating films was used as a base material for the starting powders, and ND treated by 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) was added to the Al₂O₃ to increase the hydrophobicity and anti-wear properties. The ND powder treated by PFOTES was mixed with the Al₂O₃ powder by ball milling to ratios of 0.01 wt.%, 0.03 wt.%, and 0.05 wt.% ND. The water contact angle (CA) of the Al₂O₃-ND composite coating films was increased as the ND ratio increased, and the maximum water CA among all the films was 110°. In contrast to the water CA, the Al₂O₃-ND composite coating films showed low transmittance values of below 50% at a wavelength of 550 nm due to the strong agglomeration of ND. To prevent the agglomeration of ND, the starting powders were mixed by attrition milling. As a result, Al₂O₃-ND composite coating films were produced that showed high transmittance values of close to 80%, even though the starting powder included 1.0 wt.% ND. In addition, the Al₂O₃-ND composite coating films had a high water CA of 109° and superior anti-wear properties compared to those of glass substrates.     

Yttria stabilized zirconia corrosion destabilization followed by Raman mapping

The incidence of V₂O₅ corrosion on yttria stabilized zirconia (YSZ) thermal barrier coatings has been studied as a function of application methods and powder initial granulometry. Commercial fused-and-crushed 8 wt.% yttria-stabilized zirconia was sprayed by Atmospheric Plasma Spraying (APS) and High Frequency Pulse Detonation. Hollow Spherical Powder (HOSP™) with the same composition was sprayed by APS. The coatings where covered with V₂O₅ powder and treated at 1000 °C for different times. The extent of corrosion was followed by X-ray diffraction, scanning electron microscopy and Raman micro-spectrometry. A relationship between coating porosity and corrosion resistance is explored: the HOSP coating presented deeper penetration of corrosion than the other coatings. The authors present the extended capabilities of Raman semi-quantitative phase analysis to describe the depth and density of yttria leaching by vanadia leading to YSZ destabilization.     

Preparation and oxidation protection of CVD SiC/a-BC/SiC coatings for 3D C/SiC composites

An amorphous boron carbide (a-BC) coating was prepared by LPCVD process from BCl₃-CH₄-H₂-Ar system. XPS result showed that the boron concentration was 15.0 at.%, and carbon was 82.0 at.%. One third of boron was distributed to a bonding with carbon and 37.0 at.% was dissolved in graphite lattice. A multiple-layered structure of CVD SiC/a-BC/SiC was coated on 3D C/SiC composites. Oxidation tests were conducted at 700, 1000, and 1200 °C in 14 vol.% H₂O/8 vol.% O₂/78 vol.% Ar atmosphere up to 100 h. The 3D C/SiC composites with the modified coating system had a good oxidation resistance. This resulted in the high strength retained ratio of the composites even after the oxidation.     

In situ deposition behavior of SiO2 on YSZ-TBC-coated IN738LC during a burner rig test

This work was planned as a preliminary test to obtain an optimal condition for in situ deposition of SiO₂ on zirconia-based thermal barrier coating (TBC)-coated IN738LC specimens using a burner rig. The effect of the in situ deposited SiO₂ on the long-term reliability of TBCs upon cyclic burner rig operations will be tested in the next study. Tetraethylorthosilicate (TEOS), the precursor for the deposition, was fed admixed with methanol into the combustion chamber of the burner rig at an exhaust flame temperature of 1530 °C. All five coating processes were selected by varying the concentration of TEOS in the mixture and applied to hollow type pin specimens infixed to the rotating carousel to face the downstream exhaust gas vertically. A total of 1500 cc of the mixture at a given vol.% TEOS was fed evenly to the burner rig for 54 min during the coating process.     

Mechanical Properties of LaTi<Subscript>2</Subscript>Al<Subscript>9</Subscript>O<Subscript>19</Subscript> and Thermal Cycling Behaviors of Plasma-Sprayed LaTi<Subscript>2</Subscript>Al<Subscript>9</Subscript>O<Subscript>19</Subscript>/YSZ Thermal Barrier Coatings

Recently, extensive efforts have been made to develop new thermal barrier coating (TBC) materials which can operate at temperatures above 1523 K over a long term. In this article, LaTi₂Al₉O₁₉ (LTA) was synthesized by solid-state reaction at 1773 K, and the mechanical properties of the LTA bulk were evaluated. The microhardness is about 14 GPa, comparable to that of YSZ bulk, whereas the Young’s modulus is about 44 GPa, lower than the value of YSZ. However, the fracture toughness of 0.8-1 MPa m^1/2 is much lower than that of bulk YSZ. A double-ceramic-layer LTA/YSZ TBC structure was proposed and the TBC sprayed by plasma spraying. Thermal cycling tests of the TBC specimens were performed at 1373 K with a dwell time of 10 min. The LTA remained good stability with ZrO₂ and Al₂O₃. However, the single layer LTA TBC was cracked at the LTA/bond coat interface after about 300 cycles, due to its poor thermal shock resistance, while the YSZ TBC yielded a lifetime of about 1000 cycles. The LTA/YSZ TBC remained intact even after 3000 cycles, exhibiting a promising potential as new TBC materials.     

Failure of physical vapor deposition/plasma-sprayed thermal barrier coatings during thermal cycling

ZrO₂-7 wt.% Y₂O₃ plasma-sprayed (PS) coatings were applied on high-temperature Ni-based alloys precoated by physical vapor deposition with a thin, dense, stabilized zirconia coating (PVD bond coat). The PS coatings were applied by atmospheric plasma spraying (APS) and inert gas plasma spraying (IPS) at 2 bar for different substrate temperatures. The thermal barrier coatings (TBCs) were tested by furnace isothermal cycling and flame thermal cycling at maximum temperatures between 1000 and 1150 °C. The temperature gradients within the duplex PVD/PS thermal barrier coatings during the thermal cycling process were modeled using an unsteady heat transfer program. This modeling enables calculation of the transient thermal strains and stresses, which contributes to a better understanding of the failure mechanisms of the TBC during thermal cycling. The adherence and failure modes of these coating systems were experimentally studied during the high-temperature testing. The TBC failure mechanism during thermal cycling is discussed in light of coating transient stresses and substrate oxidation.     

Oxidation Behaviour of TBC Systems on γ-TiAl Based Alloy Ti–45Al–8Nb

The lifetime of thermal barrier coating (TBC) systems on gamma titanium aluminides was determined in the temperature range between 850 °C and 950 °C under cyclic oxidation conditions in air. Coupons of the alloy Ti–45Al–8Nb (at.%) were coated by pack aluminizing. A subset of samples was subsequently annealed at 910 °C for 312 h in argon. During this heat treatment, the two-phase (Nb,Ti)Al₃ plus TiAl₂ microstructure of the coating transformed into single phase γ-TiAl. On pre-oxidised aluminized, annealed and bare samples, TBCs of yttria partially stabilized zirconia were deposited using electron-beam physical vapour deposition (EB-PVD). No spallation of the TBCs was observed in cyclic oxidation tests at 850 °C for up to 3,000 cycles of 1 h dwell time at high temperature. The two-phase aluminide coating provided effective oxidation protection due to the formation of a continuous alumina scale. The lifetime of this TBC system exceeded 1,400 cycles at 950 °C, whereas an aluminized and annealed sample failed after approximately 500 cycles. The TBC on bare substrate failed when thermally cycled at 900 °C. In contrast, no spallation occurred with an aluminized and annealed specimen at this temperature during the maximum exposure length of 1,000 cycles, probably related to an increased aluminium concentration in the subsurface region. EB-PVD zirconia top coats were well adherent to the alumina scale and the thermally grown mixed oxides. Failure of the TBC systems observed with bare and annealed samples was associated with spalled oxide scales formed on γ-TiAl.     

Influence of dopant on the thermal properties of two plasma-sprayed zirconia coatings Part I: Relationship between powder characteristics and coating properties

Plasma- sprayed coatings produced with two zirconia powders (− 90 + 10 μm, spray dried and partially sintered) that were stabilized (9 wt %) with dysprosia (DSZ) and ytterbia (YbSZ) were compared to coat-ings sprayed with a yttria (7 wt %) stabilized zirconia (YSZ) powder (45 + 22 μm, fused and crushed). The YSZ particles in the coating were almost fully molten (less than 0.2 % monoclinic m- phase), with excellent contact between the layered splats (adhesion of 54 MPa). The DSZ particles were only partially melted (3.1 % m- phase), with coating adhesion greater than 34 MPa; the YbSZ particles were less melted (6.1 % m- phase), with coating adhesion of 27 MPa. The thermal properties (diffusivity, a; specific heat, c_p; and thermal conductivity, κ) of the coatings were about the same. Under thermal cycling (1 h heating at 1100 °C in a furnace followed by fast cooling for approximately 3 min by air jets) of the coatings sprayed on FeCrAl alloy manufactured by powder metallurgy, the behavior of the DSZ coating was simi-lar to that of the YSZ, whereas the YbSZ coating was partially detached. However, in all cases the percent-age of the monoclinic phase decreased and the ratio of the hexagonal structure increased to 1.013 of the nontransformable tetragonal phase t′.     

Dependence of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> coating thickness and annealing conditions on microstructural and electrochemical properties of LiCoO<Subscript>2</Subscript> film

We studied the dependence of Al₂O₃ coating thickness and annealing conditions on the surface morphology and electrochemical properties of Al₂O₃ coated LiCoO₂ films. The optimum coating thickness allowing for the highest capacity retention was about 24 nm. A sample consisting of Al₂O₃ coated on annealed LiCoO₂ film with additional annealing at 400 °C had a uniform coating layer between the coating materials and cathode films. This sample showed the best capacity retention of ∼91 % with a charge-cut off of 4.5 V after 30 cycles, while the bare cathode film showed a capacity retention of ∼32 % under the same conditions. The formation of second phases such as Co-Al-O was observed in the coating films by X-ray photoelectron spectroscopy (XPS). The Co-Al-O containing samples showed a higher initial capacity because of their smaller grain size, but less capacity retention than the Al₂O₃ containing samples.     

Mechanistic Study on the Degradation of Thermal Barrier Coatings Induced by Volcanic Ash Deposition

Thermal stress generated on thermal barrier coatings (TBCs) by volcanic ash (VA) deposition was assessed measuring the tip deflection of a multilayered beam structure as a function of temperature. The TBC in this study was deposited onto the surface of a blade utilized in a land-based gas turbine which is composed of 8 wt.%Y₂O₃-ZrO₂/CoNiCrAlY on a Ni-based superalloy. The VA-deposited TBC sample was heated at 1453 K, and the effect of VA deposition on TBC delamination was examined in comparison with a TBC sample without VA deposition as a reference. On the basis of the VA attack damage mechanism which was investigated via the tip deflection measurement and a comprehensive microstructure examination, a damage-coupled constitutive model was proposed. The proposed model was based on the infiltration of the molten VA inside pores and phase transformations of yttria -tabilized zirconia in the TBC system. The numerical analysis results, which were simulated utilizing the finite element code installing the developed constitutive model, showed us that VA attack on the TBC sample induced near-interfacial cracks because of a significant increasing in the coating stress.