This work investigated the influence of pre-formed yttria alumina garnet (YAG) and Al 2 O 3 mixed oxide layer induced by laser surface melting (LSM) on the oxidation behaviour of a CoNiCrAlY bond coat alloy. As-cast CoNiCrAlY alloy was vacuum heat treated at 1100 °C for 2 h and LSM was conducted at a laser power of 400 W and scanning speeds from 200-800 mm/min in air. Both heat-treated (HT) and LSM CoNiCrAlY alloys were subsequently oxidised at 1100 °C for 100 h. It was found that the β phases were refined and redistributed along the γ phase boundaries after LSM. In addition, an initial mixed oxide layer composed of YAG and Al 2 O 3 was developed on the surface during LSM, which was due to the high oxygen affinity of Y and Al. It was further demonstrated that the pre-formed YAG/Al 2 O 3 mixed oxide layer at scanning speeds of 200-600 mm/min effectively suppressed oxide growth and inhibited β depletion during oxidation. This was attributed to the fact that the stable YAG acted as grain boundary barriers for oxygen inward diffusion. However, a newly formed Al 2 O 3 layer, associated with a corresponding β depletion zone, appeared underneath the mixed oxide layer at the highest scanning speed of 800 mm/min. This was due to that the spallation of the mixed oxide layer and refined γ phases allowed the oxide growth at the alloy surface and subsequently corresponding β depletion occurred. • A pre-formed YAG/Al 2 O 3 oxide layer was induced during laser surface melting due to the high oxygen affinity of Y and Al. • High dislocation density was observed in the YAG/Al 2 O 3 oxide layer after LSM. • No β depletion was observed after long-term oxidation for LSM CoNiCrAlY at 200-600 mm/min. • A β depletion developed at 800 mm/min due to oxide microcracking and subsequent growth of Al 2 O 3 .

Environmental barrier coatings (EBC) are essential to protect silicon nitride (Si 3 N 4 ) from hot gas corrosion, enabling the utilization of Si 3 N 4 in gas turbines. Otherwise the initially formed silica (SiO 2 ) layer degrades through reactions with the water vapor from fuel combustion. Therefore, hafnia (HfO 2 ) as doping additive in combination with ytterbium silicates was used to reduce the growth rate of silica and to enhance adhesion of EBCs without a bond coat. Finally, a uniform and gradient coating was generated in one stage, applied via spraying and sedimentation of a respective slurry. The in situ formed composite gradient coatings composed of HfO 2 -Yb 2 SiO 5 /Yb 2 Si 2 O 7 -Si 3 N 4 without Si bond coat, exhibited excellent adhesion (42.6 ± 6.4 MPa) and high microhardness (up to 4.3 ± 0.4 GPa). The durability of these coatings was confirmed through more than 20 thermal cycling tests in the temperature range of 1200-20 °C, in which self-healing ability of cracks was proven.

Interfacial stress and failure behavior of EB-PVD YSZ thermal barrier coatings with different bond coats

Abstract Although inner-diameter (ID) thermal barrier coating (TBC) processes are well established for large aero/land-based turbine liners, the relationships between the processes, microstructures, and properties of coatings applied to smaller, highly confined passages (ID < 200 mm), such as combustor liners, exhaust manifolds, and pipes that face comparable thermal loads, remain largely undocumented. This study examines 8 wt.% YSZ top coats, which are deposited using an ID-atmospheric plasma-spray (APS) torch, as well as bond coats, which are deposited using an ID-high-velocity oxyfuel torch, inside 200 mm diameter, 8 mm wall tubes. The results are then compared with those of flat substrates sprayed under otherwise identical conditions. Single splats of YSZ demonstrate that oblique impact angles imposed by the tube create splashed lamellae, resulting in slightly higher local porosity than on flat substrates. Pull-off tests reveal the adhesion strengths of ID-TBCs when sprayed using the two ID spraying systems. Two types of top coat microstructures were developed using a lower-power ID-APS torch: porous variants with different porosities ranging from 9% to 17 vol.% and a dense, vertically cracked (DVC) variant. The microstructures were subjected to (1) burner-rig thermal gradient cycling and (2) 1100 °C furnace cycling. In the burner-rig tests, it was found that lifetime scaled with bond coat thickness and less distinctly with through-thickness temperature gradient. The best porous ID coating endured 246 cycles, while the specimen with a thin bond coat failed after 101 cycles. Furnace tests impose uniform oxidation; all porous coatings spalled between 60 and 100 cycles, and the DVC cracked after 40 cycles. The failure modes indicate, respectively, oxidation-driven delamination of the ceramic from the bond coat and cracking through the ceramic top coat. In conclusion, disk-type splat formation and adequate bond and top coat thicknesses (>130 and > 300 µm, respectively) are microstructural prerequisites for durable ID-TBCs.

Thermal barrier coatings (TBCs) are an effective means of providing thermal insulation and protecting the hot-section components of gas turbine engines. Their quality and performance characteristics largely depend on the microstructural features and the bond strength between the bonding layer and the substrate. The present study aims to determine the optimal plasma spraying parameters that ensure the formation of NiCrAlY coatings with superior microstructural integrity and adhesion strength. The objective of the study is a thermally sprayed nickel–chromium–aluminum–yttrium (NiCrAlY) bond coat deposited onto an Inconel 718 nickel-based superalloy, which is widely used in aircraft gas turbine engines due to its high strength and excellent oxidation resistance at elevated temperatures. It was found that the coating produced under the optimized conditions exhibited a significantly higher adhesion strength compared with the samples obtained under other spraying regimes. The results confirm that a precise adjustment of the atmospheric plasma spraying (APS) process parameters, taking into account the equipment configuration, allows for a substantial improvement in coating quality and performance.

Machine-learned accelerated discovery of oxidation-resistant NiCoCrAl high-entropy alloys

Development of an internal diameter spraying system for CoNiCrAlY/YSZ thermal barrier coatings: microstructure, properties, and engineering verification

High-temperature oxidation and β-phase depletion kinetics of CoNiCrAl bond coat alloys: effects of Al content

The ongoing demand for high-thrust turbine engines necessitates the advance of next-generation structural materials capable of withstanding higher temperatures. Commercial MCrAlY alloy, used as bond coats crucial for thermal barrier coating (TBC) systems, face a fundamental temperature ceiling of ∼1100 °C due to accelerated oxidation and spallation. Here, we design a novel Y and Hf co-doped NiCoCrAl-type multi-principal element alloy (MPEA) that achieves exceptional 1200 °C oxidation resistance primarily through lattice distortion-induced diffusion suppression. Compared with typical NiCoCrAlY alloy, the MPEA exhibits 59% lower in thermally grown oxide (TGO) growth rate, as well as negligible TGO spallation after 500 h at 1200°C. This performance stems from a significantly refined eutectic structure enabling rapid formation of a protective Al2O3 scale during initial oxidation, coupled with lattice distortion that elevates vacancy formation energy and Al migration barriers within the Al-depletion zone (ADZ), drastically reducing sustained diffusion rates. This co-design strategy, integrating tailored microstructure and lattice distortion, establishes a new paradigm for ultra-stable performance in extreme environments.

In this study, La₂Ce₂O₇ (LC) powders synthesized via a molten-salt process were deposited as a thermal barrier coating on Hastelloy-X substrates by atmospheric plasma spraying (APS), with a NiCrAlY bond coat as an interlayer. The APS-deposited LC coating exhibited phase stability up to 1600 °C for 100 h while retaining its fluorite structure. The thermal conductivity of the coating remained low, ranging from 0.64 to 0.82 W·m⁻¹·K⁻¹ over the temperature range of 30–1000 °C. The coating was further subjected to thermal cycling tests to evaluate its thermal cycling lifetime and failure behavior, during which chipping-type spallation developed within the ceramic topcoat near the bond-coat interface due to thermally induced stresses. Hot-corrosion tests were performed in Na₂SO₄, V₂O₅, and a eutectic mixture consisting of 32 wt.% Na₂SO₄–68 wt.% V₂O₅ at 900 °C for 30 h. The coating exhibits high stability in pure Na₂SO₄ and moderate degradation in V₂O₅. In contrast, severe degradation is observed in the Na₂SO₄–V₂O₅ eutectic mixture, where LaVO₄ forms as the dominant corrosion product, accompanied by CeVO₄, LaCeVO₄, and CeO₂. This study presents a coating-level evaluation of the high-temperature phase stability of APS-deposited La₂Ce₂O₇ coatings, together with a systematic assessment of their hot-corrosion behavior in sulfate- and vanadate-containing service environments.

Effect of vibratory finishing of APS NiCrAlYSi bond coat on high-temperature oxidation performance of EB-PVD YSZ thermal barrier coatings

Oxidation behavior of RuAl, PdAl, and ruthenium-palladium aluminides for bond coat applications

ABSTRACT A systematic investigation was conducted on the oxidation behavior of silicon‐bond coats within environmental barrier coating (EBC) systems applied to Si‐carbide (SiC) substrates, aiming to understand how different underlying SiC substrates influence the bond coat's thermally grown oxide (TGO) and its properties. The study examined (Y/Yb) 2 Si 2 O 7 /Si coatings on three cost‐effective surrogate SiC substrates (chemical vapor deposition [CVD]‐grown β‐SiC, sintered α‐SiC, and reaction‐bonded [RB] SiC) for SiC fiber /SiC matrix ceramic matrix composites (CMCs). Discrepancies in TGO growth were observed, with noticeably higher growth rates reported for the coated CMC specimens than for the four monolithic SiC specimens. The CMC samples produce an amorphous TGO, whereas the other monolithic substrates formed a crystalline TGO, which lowered the oxygen permeability through the SiO 2 scale. The vitrification of the TGO in the (Y/Yb) 2 Si 2 O 7 /Si/CMC system resulted from the migration of boron species from the CMC substrate to the SiO 2 scale, leading to network modification via boron doping.

Abstract The durability of thermal barrier coating (TBC) systems is strongly influenced by the interaction between oxidation of the metallic bond coat and its mechanical behavior. While the response of bond coats under isothermal monotonic loading has been widely studied, the effect of thermal cycling remains poorly understood, even though cyclic loading naturally arises from the mismatch in coefficients of thermal expansion between the ceramic top coat, thermally grown oxide, metallic bond coat, and the superalloy substrate. In this work, high-energy X-ray diffraction was used to investigate the strain and stress evolution in the $$\beta $$ β -(Ni, Pt)Al bond coat of a standard TBC deposited on a nickel-based single-crystal superalloy during thermal cycling. Before in situ cycling, some of the studied specimens were aged through long furnace cycles. Strains and stresses in the $$\beta $$ β phase were quantified in situ using the $${\hbox {sin}}^{2}\psi $$ sin 2 ψ method combined with micromechanical modeling. The results reveal that plastic deformation in $$\beta $$ β is strongly controlled by evolving interfacial effects and by the cyclic $$\beta \rightleftharpoons \gamma '$$ β ⇌ γ ′ phase transformation during thermal cycling. These mechanisms govern the accumulation of plastic strain in $$\beta $$ β and may promote rumpling, spallation, and ultimately TBC degradation. This study provides new mechanistic insight into bond-coat plasticity under thermal cycling.

Abstract In high-temperature, adverse environments, the premature failure of environmental barrier coatings (EBCs) is a critical phenomenon that can significantly impact their applications in both aircraft engines and land-based gas turbines. The delamination failure of EBCs typically occurs at the topcoat/TGO or TGO/bond–coat interfaces, primarily due to thermal and shrinkage strains, as well as water vapor corrosion, resulting in crack propagation and coating spallation. This paper presents a systematic study of the degradation of bi-layer disilicate Yb 2 Si 2 O 7 (YbDS)/Si EBCs using COMSOL Multiphysics methodologies. Thermal-cycle-induced temperature fields were implemented into the EBC model, aiming to simulate the system’s in-service operation. The high-temperature creep models of topcoat YbDS, monosilicate Yb 2 SiO 5 (YbMS), silica TGO, Si bond coat, and SiC substrate are included for the built-up undulated coating geometries. Based on the local stress evolution and distribution in the EBC system during thermal cycles in water vapor environments, and on the transformation from YbDS to YbMS and TGO growth kinetics observed at elevated temperatures, the experimentally observed EBC degradation mode was explained in terms of the simulated results. The J-integral, virtual crack extension, and phase-field damage model were implemented to investigate crack nucleation and propagation under thermal cycles, considering the cristobalite TGO, which undergoes a displaced β-α phase transformation between 220 °C and 270 °C with an associated large volume shrinkage upon cooling. The significant phase-field value of TGO, due to its high tensile stress, leads to the automatic nucleation of cracks in TGO and their subsequent bifurcation and propagation along both YbDS/TGO and Si/TGO interfaces. The linking of these bifurcation-induced cracks during cooling cycles could be the primary mechanism leading to the EBC’s spallation and failure. The simulated TGO crack growth pattern was compared with that experimentally observed in the literature.

DFT-assisting experimental insights into the thermally grown oxide of thermal barrier coatings with Cr2AlC as the bond coat

Abstract Environmental barrier coatings (EBCs) are required for the use of silicon carbide‐based ceramic matrix composites in gas turbine engines. Current‐generation EBCs consist of a silicon bond coat and a rare earth (RE) silicate topcoat. The RE silicate topcoat is exposed to high‐velocity steam during engine operation, and SiO 2 within the coating can preferentially volatilize to form Si(OH) 4 gas. Therefore, the volatility of the RE silicates in a combustion environment is of interest. The volatility of the RE silicates in steam is related to SiO 2 activity (), which was measured in this work for the gadolinium silicates via Knudsen Effusion Mass Spectrometry (KEMS). The Gd 2 O 3 –SiO 2 system is of interest because gadolinium has been considered as a component in solid solution RE disilicate topcoats. Silica activities measured within the biphasic fields Gd 2 O 3 ‐Gd 2 SiO 5 () and Gd 9.33 (SiO 4 ) 6 O 2 ‐Gd 2 Si 2 O 7 () were compared to those reported for RE 2 O 3 –RE 2 SiO 5 and RE 2 SiO 5 –RE 2 Si 2 O 7 with RE = Y, Yb, Lu. The SiO 2 activity of Gd apatite (Gd 9.33 (SiO 4 ) 6 O 2 ) was determined for the first time (), and apatite phase formation was assessed in air between 1100°C and 1600°C. Experimental silica activities were also compared to those modeled by Thermo‐Calc. Implications for the Gd silicates as coating materials are discussed in the context of thermal expansion, water vapor exposure, and CaO‐MgO‐Al 2 O 3 ‐SiO 2 (CMAS) exposure.

Structural and compositional design of a double-layer NiCoCrAlY bond coat: HfSi co-doping in the top layer for enhanced oxidation resistance in water-vapor/oxygen atmospheres

Improving the thermal cycling lifetime of thermal barrier coatings with an atmospheric plasma-sprayed carbon-alloyed MCrAlY bond coat

Combined Effects of Hot and CMAS Corrosion and Initial Heat Treatment on HVOF-Applied CoNiCrAlY Bond Coats