Rare-earth phosphates (REPO4) can be used in combination with gadolinium zirconate (Gd2Zr2O7 or GZO) to increase CMAS resistance. In this study, coarse gadolinium zirconate powder was mixed with fine lanthanum phosphate powder (LaPO4) using resonant acoustic mixing (RAM) to produce a composite feedstock powder with a core-shell microstructure. Composite GZO-LPO coatings with low porosity (∼5%) were deposited using spray parameters chosen from trials using single-phase gadolinium zirconate. Alongside pure Gd2Zr2O7, 4 distinct phases containing La and P were observed. The La:P ratios varied in the range of ∼1.1-2.6 at.% which was a result of the loss 62 mol.% of P2O5 during the spray process. The morphologies of the La-P phases were ascribed to variable mixing during RAM. After CMAS exposure at 1300 °C for 0.5 h, the composite GZO-LPO coating showed significantly reduced relative penetration depth of ∼40 μm compared to ∼130 μm of the single-phase Gd2Zr2O7 coating due to lower porosity and higher concentration of rare-earth elements.

Suspension plasma spraying (SPS) enables the fabrication of environmental barrier coatings (EBCs) with complex multilayer architectures; however, degradation in such systems often initiates locally at buried interfaces, making it difficult to resolve using conventional laboratory-scale characterization techniques. In this work, the applicability of synchrotron-based micro-x-ray diffraction (µXRD), combined with micro-x-ray fluorescence (µXRF), is evaluated for the characterization of SPS-deposited ytterbium disilicate (YbDS) EBCs. An as-sprayed YbDS coating was investigated as a baseline case to examine differences between conventional XRD and spatially resolved µXRD, while an annealed and CMAS-exposed YbDS coating was studied as a service-relevant case to probe localized phase evolution. The samples were selected from previously optimized SPS process conditions and are not intended for direct comparison. Laboratory-scale XRD provided global phase information, whereas µXRD enabled layer-specific phase identification and resolved localized interfacial features. In the as-sprayed condition, µXRD confirmed phase-pure YbDS, resolved the crystallinity of individual coating layers, and verified the absence of unintended interfacial reaction phases that are not accessible by conventional XRD. In the annealed + CMAS-exposed coating, µXRD and µXRF revealed the formation of a calcium–ytterbium–silicate oxyapatite phase confined to the YbDS/Si interface, highlighting the localized nature of CMAS-induced degradation. These results demonstrate that synchrotron microanalysis provides valuable complementary insight for probing localized phase evolution in thermally sprayed EBC systems.

Suspension plasma sprayed (SPS) yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) deposited from ethanol-based suspensions typically exhibit columnar microstructures that provide excellent strain tolerance and low thermal conductivity. However, the flammability of ethanol presents significant safety and scalability challenges, motivating the search for alternative non-flammable suspension media capable of producing comparable microstructural features and performance. For the first time, in this study, a non-flammable solvent based YSZ suspension was employed for the first time to deposit columnar SPS YSZ coatings, and their suitability as TBCs was studied through furnace cyclic testing (FCT), erosion, and calcium–magnesium–alumino–silicate (CMAS) infiltration experiments. The resulting coating microstructure exhibited a well-developed columnar morphology with discontinuous intercolumnar porosity, demonstrating a 97.4% longer thermal cyclic lifetime than previously reported water-based YSZ coating microstructure deposited by suspension high-velocity oxy-fuel spraying. In the non-flammable SPS coatings, failure during thermal cycling occurred predominantly by delamination at the thermally grown oxide (TGO)/top-coat interface, driven by TGO thickening and thermal expansion mismatch stresses. The erosion rate for the same coating was approximately seven times higher than that of previously reported ethanol-based SPS YSZ coatings because of its lower fracture toughness, with material removal dominated by crack propagation along interpass porosity bands. Compared with a water-based SPS YSZ coating, the non-flammable SPS coating microstructure exhibited a 45.3% reduction in CMAS infiltration, owing to its discontinuous intercolumnar pore network that restricted continuous penetration pathways. These findings demonstrate the potential of non-flammable suspensions as a novel and viable route for suspension-derived YSZ TBCs.

Medium entropy rare earth (RE) silicates represent a promising frontier for environmental barrier coatings, yet the interplay between chemical complexity, phase formation, and calcium-magnesium-aluminosilicate (CMAS) resistance remains poorly understood. This study elucidates how medium entropy substitution at the RE site modulates [REOn] coordination polyhedra to dictate the phase formation and corrosion behavior of mono-silicates (RE2SiO5), disilicates (RE2Si2O7), and oxyapatite silicates (RE9.33(SiO4)6O2). We find that X2-RE2SiO5 formation is highly sensitive to local [REOn] configurations, where increased cation size mismatch and larger average radii destabilize the framework, promoting the formation of secondary disilicate and vacancy rich oxyapatite phases. In contrast, the RE2Si2O7 system exhibits superior structural adaptability, accommodating broad multi cation substitutions across monoclinic and orthorhombic polymorphs. CMAS corrosion tests at 1300 degrees C reveal that X2 dominated compositions facilitate rapid formation of thin, dense Ca apatite barriers that effectively arrest melt infiltration. Conversely, formulations rich in RE9.33(SiO4)6O2 provide poor protection, where the vacancy-rich oxyapatite framework is associated with enhanced Ca-mediated reaction and deep melt penetration. To unify these findings, this study identifies ionic field strength (IFS), derived from the local chemistry of [REOn] coordination polyhedra, as a key descriptor linking medium entropy substitution, phase formation, and CMAS corrosion behavior. Higher IFS, associated with smaller RE ions and lower coordination numbers, is found to promote increased lattice rigidity and self-protecting corrosion kinetics. By establishing IFS as a structure-based metric for rational rare earth selection, this work provides a mechanistic foundation for the accelerated design of multi component silicates for high temperature environmental barrier coatings.

The volatilization of Nb2O5 during suspension plasma spraying (SPS) poses a significant challenge to maintaining the stoichiometry and phase stability of gadolinium niobate (GdNbO4, GNO) in advanced thermal barrier coating applications. This study systematically investigates the influence of feedstock particle size, SPS processing parameters, and post-deposition heat treatment on Nb2O5 volatilization and phase evolution in SPS-deposited GNO coatings. Lower plasma power significantly reduced Nb2O5 loss, while coatings deposited from 2 μm feedstock exhibited improved compositional retention compared with those produced from 0.5 μm powders, demonstrating the critical role of particle size in mitigating volatilization during SPS. Post-deposition heat treatment facilitated oxygen vacancy replenishment and promoted phase transformation from the as-sprayed cubic phase to a mixed-phase microstructure comprising monoclinic, tetragonal, and residual cubic GNO. Raman spectroscopy corroborated the structural evolution, revealing the progressive dominance of the monoclinic phase with increasing thermal exposure. These findings establish the interplay between SPS processing conditions, feedstock characteristics, and post-deposition thermal treatment in governing compositional stability and phase evolution. The results provide practical guidelines for minimizing Nb2O5 volatilization and tailoring the phase constitution of SPS-derived GNO coatings, thereby advancing the development of high-performance rare-earth niobate thermal barrier coatings for next-generation gas turbine applications.

This work investigates the fabrication and microstructural evolution of tungsten-copper (W-Cu) functionally graded material (FGM) coatings produced using a shrouded axial-injection atmospheric plasma spray (APS) process. The central novelty of this study lies in the use of an inert-gas shrouded axial-injection APS system, which markedly suppresses in-flight oxidation and enables deposition of W-rich layers, overcoming a major limitation of conventional APS for refractory metals and allowing the formation of a continuous W-Cu compositional gradient. The resulting architecture exhibits a controlled transition from a Cu-rich region at the substrate interface to a W-rich layer at the coating surface. Microstructural characterisation by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and quantitative XRD revealed phase-pure W and Cu without evidence of intermetallic formation, together with a well-defined spatial gradient in elemental distribution. Through systematic adjustment of stand-off distance, gas composition, and plasma net power, it was observed that elevated net power and reduced stand-off distance improved melting efficiency and splat deformation, leading to significantly enhanced bonding and reduced porosity, particularly in W-rich sections where deposition is most sensitive to thermal input. Porosity analysis demonstrated that coatings produced under higher plasma power exhibited enhanced consolidation, with porosity decreasing from similar to 2 % to similar to 0.2 % in W-rich regions of the FGM after sub-solidus heat treatment at 650 degrees C in an argon atmosphere. Heat treatment further promoted diffusion-assisted healing of crack defects. Microhardness values reflected the compositional gradient, ranging from similar to 125 HV in Cu-rich zones to similar to 310 HV in W-rich regions. These findings establish shrouded APS as a viable, oxidation-controlled method for producing graded W-Cu coatings and provide the processing foundation for future mechanical, thermal, and functional assessment in high-temperature and plasma-facing applications.

Decarbonizing aviation aims to reduce greenhouse gas emissions from aircraft operations, paving the way for sustainable air travel. This endeavor requires adopting advanced technologies, alternative fuels, high-performance coatings and efficient engineering solutions that minimize environmental impact while maintaining performance and safety standards. Two of the most prevalent and cost-effective methods for applying protective and functional coatings to aerospace components are thermal spray and physical vapor deposition. These techniques enhance the durability and efficiency of several components, resulting in fuel savings and an extended service life across the aviation industry. This supports the broader aim of transitioning the aviation industry to net-zero emissions and sustainable growth. This roadmap explores how these two coating techniques can promote sustainable aviation and identifies the challenges and opportunities in the aerospace sector for researchers and manufacturers of thermal spray and physical vapor deposition (PVD) coatings. It also proposes research directions to address these challenges and discusses the role of AI, which is crucial for breakthrough technologies in process optimization and integration, new coating development and coating design optimization. The roadmap is organized into 20 concise subsections, each focusing on a specific topic. Renowned specialists in each area were invited to summarize the current status of their field, discuss the challenges it faces, and offer recommendations for necessary research and development to overcome these issues. Together, these contributions vividly highlight the essential elements of the field and the challenges that lie ahead. The innovative ideas and concepts outlined in the roadmap reveal that the future path is both expansive and far-reaching. A decade after the JTST released its roadmap on thermal spray, which emphasized the processes, coatings, and applications of thermal spray, the current roadmap shifts its focus to spray processes and vapor deposition methods aimed at decarbonizing aviation. Academic and industry experts are collaborating to share insights on how thermal spray and vapor deposition techniques and coatings can advance sustainable aviation and the necessary research to overcome associated challenges. This roadmap can serve as a valuable reference point for researchers aiming to understand the field’s trajectory and identify critical gaps to address.

This work presents the first steps in using Eu3+ as a structural probe using photoluminescence (PL) emission spectra. The sol-gel synthesis of four Eu3+-doped powders encompassing seven phases of the yttrium silicate phase system is detailed. Following heat treatment at 1100 and 1400 degrees C, the powders were then interrogated using x-ray diffraction (XRD), Rietveld refinement, electron dispersive spectroscopy, and thermal analysis techniques. The following Eu3+-doped phases were identified and assigned in the PL-emission spectra: cubic Y2O3; X1-Y2SiO5; X2-Y2SiO5; Y4.67(SiO4)3O and the alpha, beta, and y polymorphs of Y2Si2O7. A validation of the assigned peak database is achieved through the prediction of the major and minor phases present in an Eu3+-doped yttrium disilicate powder synthesized through coprecipitation synthesis from its PL-emission spectra alone, with outputs corroborated through XRD analysis.

High-Entropy/multicomponent rare-earth oxides (HECs and MCCs) show promise as alternative materials for thermal barrier coatings (TBC) with the ability to tailor properties based on the combination of rare-earth elements present. By enabling the substitution of scarce or supply-risk rare-earths with more readily available alternatives while maintaining comparable material performance, HECs and MCCs offer a valuable path towards alternative TBC material design. However, navigating this search space of compositionally complex materials is both time and resource intensive. In this study, an active learning (AL) framework was employed to identify HEC/MCC materials with a pyrochlore structure, with acceptable thermal conductivity (TC) for TBC applications. The AL framework was applied through a Bayesian optimisation (BO) strategy, coupled with a random forest surrogate model. TC was selected as the optimisation criterion as that is the most fundamental requirement of TBC materials. Over two iterations of the AL cycle, four compositions were generated and synthesised in the lab for experimental evaluation. The first iteration yielded two single-phase pyrochlores, (La0.29Nd0.36Gd0.36)2Zr2O7 and (La0.333Nd0.26Gd0.15Ho0.15Yb0.111)2Zr2O7, with measured thermal conductivities of 2.03 and 1.90 W/mK, respectively. The surrogate model predicted a TC of 2.009 W/mK for both compositions, demonstrating its accuracy for completely new compositions. The second iteration compositions showed dual-phase when synthesised, highlighting the need to take into account phase formation in the AL framework.

Emerging technological advancements are aimed at improving the operating temperatures of gas turbine engines to improve their overall efficiency. This has triggered the development of yttria-stabilized zirconia-based thermal barrier coatings (YSZ TBC) with microstructures designed for enhanced performance achieved by optimizing process parameters and deposition techniques. The present study aims to evaluate the performance of YSZ columnar structure deposited using suspension plasma spraying (SPS) on Pt-Al bond-coated CMSX-4 single-crystal nickel-based superalloy substrate, reported for the first time in the literature. The thermal cycling performance and calcium-magnesium-alumino-silicate (CMAS) corrosion behavior of SPS YSZ coatings were compared with YSZ deposited using electron beam physical vapor deposition (EBPVD) and air plasma spraying (APS) techniques. It was observed that SPS YSZ coatings showed the highest thermal cycling resilience when compared to EBPVD and APS. In addition to this, the CMAS penetration was also substantially lower in SPS YSZ coatings compared with EBPVD and APS coatings. Unlike SPS and EBPVD coatings, APS coatings also showed signs of tetragonal-to-monoclinic phase transitions in some regions away from the top surface. These observations lay the baseline for the potential development of optimized microstructures using the SPS deposition technique with improved performance relevant to the TBCs.

Cold spray technique employs the kinetic energy of the feedstock to generate coatings while maintaining the original microstructures of the particles during deposition. Inconel 718 is well-known for its high tensile strength and corrosion resistance at elevated temperatures thanks to a complex alloying chemistry. Cold spraying Inconel 718 often demonstrates porosity and delamination due to limited plastic deformation. Hence, we looked into improving and understanding the deformability of such particles by modifying feedstock microstructures through solution treatment at two temperatures. At 950 degrees C, the gamma(y) phase transforms into a stable delta precipitate, while at 1050 degrees C, a near-complete solution transition is observed through X-ray diffraction (XRD), electron backscatter diffraction (EBSD) mapping, and electron channelling contrast imaging (ECCI). Cold-sprayed, solution-treated Inconel 718 at 950 degrees C exhibits porosity reduction by approximately 60 % compared to the as-sprayed feedstock. ECCI micrographs suggest enhanced particle-substrate bonding and jet formation at the interface. Conversely, depositing Inconel 718 solution-treated at 1050 degrees C decreases coating hardness by around 23 % compared to that at 950 degrees C. Coatings' microstructures denote differences in dislocation accumulation mechanisms, suggesting potential dislocation formation scenarios.

This study focuses on the deposition and post-processing behavior of commercially pure copper produced using cold spray additive manufacturing (CSAM) with compressed air. By evaluating the microstructural evolution and mechanical performance of as-deposited and heat-treated copper samples, this work aims to provide insights into optimizing CSAM processes for industrial applications.

Abradable environmental barrier coatings (EBCs) are essential for improving gas turbine efficiency with ceramic matrix composites (CMCs). This study examined ytterbium disilicate (Yb2Si2O7 or YbDS) abradable EBCs with 8 %, 15 %, and 22 % porosity, deposited via atmospheric plasma spraying. Coatings were exposed to steam, CMAS, and combined steam-CMAS at 1350 degrees C for 100 hr. Results showed that porosity had minimal impact on corrosion resistance. Steam exposure formed a thin ytterbium monosilicate (Yb2SiO5 or YbMS) layer, while CMAS caused dissolution-precipitation, forming reprecipitated Yb2Si2O7 and Yb-apatite (Ca2Yb8(SiO4)6O2). Combined exposure increased CMAS penetration depth by 25 %, enhanced Yb-apatite formation, and introduced ytterbium aluminium garnet (Yb3Al5O12, YbAG). These findings demonstrate that the interaction of CMAS and steam accelerates coating degradation, highlighting the need for optimized phase composition and microstructure to enhance abradable EBC durability under turbine-relevant conditions.

This research aims to enhance the deformability of Inconel 718 by modifying the feedstock microstructures prior to deposition through solution treatment process at 950 °C and 1050 °C. It was observed that the gamma phase has transformed into a stable delta precipitate at 950 °C. Higher treatment temperature at 1050 °C shows a complete solution transition, proven by EBSD phase map, and electron channelling contrast imaging (ECCI).

Quantitative structure-activity relationship (QSAR) modelling is widely employed in materials sci- ence to predict properties of interest and extract useful descriptors for measured properties. In thermal barrier coatings (TBC), QSAR can significantly shorten the experimental discovery cycle, which can take years. Although machine learning methods are commonly employed for QSAR, their performance depends on the data quality and how instances are represented. Traditional, hand-crafted descriptors based on known material properties are limited to represent materials that share the same basic crystal structure, limited the size of the dataset. By contrast, graph neural networks offer a more expressive representation, encoding atomic positions and bonds in the crystal lattice. In this study, we compare Random Forest (RF) and Gaussian Process (GP) models trained on hand-crafted descriptors from the literature with graph-based representations for high-entropy, rare-earth pyrochlore oxides using the Crystal Graph Convolutional Neural Network (CGCNN). Two different types of augmentation methods are also explored to account for the limited data size, one of which is only applicable to graph-based representations. Our findings show that the CGCNN model substantially outperforms the RF and GP models, underscoring the potential of graph-based representations for enhanced QSAR modelling in TBC research.

Cold spray has been extensively applied to deposit a range of materials in many industries. In the recent times, such a method has also shown its potential to deposit nickel-based superalloys, which currently are in demand due to their high tensile strength and corrosion resistance (especially at elevated temperatures); however, cold sprayed nickel super alloy coatings have poor mechanical properties due to the materials’ limited ability to undergo plastic deformation. Regarding this, numerous cold spray process modifications have been experimented, including preheating substrate and feedstock powder, applying laser irradiation, heat treating coatings post deposition, and heat treating feedstock powder, to promote plastic deformation, eliminate porosity and enhance inter particle bonding. Specifically, the important influence of external heat input on the underlying substrate and/or the incoming particles during cold spray deposition was highlighted in multiple studies. These studies indicated that the addition of external heat during cold spray increased the adhesion strength of the coatings due to an increase in the thermal softening effect of the deposited particles. In general, an attempt is made here to systematically review the influence of cold spray process modifications on the microstructure, mechanical properties and residual stresses of nickel super alloy coatings.

The first-of-its-kind use of the active learning (AL) framework in thermal spray is adapted to enhance the prediction accuracy of the in-flight particle characteristics. The successful AL framework implementation via Bayesian Optimisation is beneficial in, first, reducing the maximum uncertainty, which greatly improves the prediction accuracy and informativeness of the existing database. Second, it reduces local uncertainty around a contrived test point that offers the capability to find improvement in a limited search area, allowing an accurate prediction around a desired test point. The dataset for Machine Learning (ML) training consists of 26 atmospheric plasma spray (APS) parameters of silicon and a further six AL-guided spray runs carried out to reduce maximum uncertainty in the initial database. On average, a 52.9% improvement (error reduction) of RMSE and an R2 increase of 8.5% were reported on the predicted in-flight particle velocities and temperatures after the AL-driven optimisation. Furthermore, the contrived test point optimisation to predict the best possible characteristics in a limited search space resulted in a three-fold increase in prediction accuracy compared to the non-optimised prediction. The AL-driven optimisation proved to be greatly beneficial for resource-intensive thermal spraying, as the framework not only allowed an accurate prediction of the in-flight particle characteristics but also found expected improvement around a desired in-flight characteristic. Furthermore, the framework uses the Gaussian Process (GP) ML model as a surrogate that generalises a global solution without necessarily involving physical and underlying mechanisms, thus extending the framework to other thermal spraying methods.

In our present work, TiN suspension was sprayed onto the 316 stainless steel substrate using the suspension high velocity oxy fuel thermal spray. An in-situ reaction between TiN and O2 2 occurred during the spray process to form TiO2 2 and TiN/TiO2 2 composite coatings were fabricated. The phase evolution, microstructures and tribological properties of TiN/TiO2 2 composite coatings were studied. Results showed that the interface between the coating and the substrate was well bonded, which was mainly owing to the in-situ reaction heat and the sufficient deformation of particles. The structure of molten particles, small grain size and low porosity could contribute to improving the mechanical and tribological properties of the in-situ coating. A schematic exfoliation process of TiN/TiO2 2 composite coating was proposed to analyze the failure of coatings. The antiwear mechanisms of the TiN/TiO2 2 composite coatings were discussed. The good cohesive bonding strength and high hardness of the TiN/ TiO2 2 composite coating, the pinning role of TiN in crack propagation, and the newly formed TiO2 2 in the friction layer during the sliding test contributed to the improvement of tribological properties for the TiN/TiO2 2 composite coating. This work could provide a reference for the application of TiN/TiO2 2 composite coating in the field of friction and wear.

The inherent phase instability of the state-of-the-art 7-8 mol% partially stabilised Yttria-Stabilised Zirconia (YO1.5, 8 YSZ, tetragonal) under a molten CMAS attack lacks the technological readiness needed to increase the gas inlet temperatures for more thermally efficient gas turbine engines. In this study, the concentration of Yttria in YSZ was systemically increased to impart CMAS resistance and their applicability via emerging Suspension Plasma Sprayed (SPS) coatings and Spark Plasma Sintered (SpPS) pellets under simulated conditions was evaluated. The fully stabilised higher Yttria YSZ compositions (21.4 mol% and 50.2 mol%, cubic) severely restricted the CMAS infiltration and interaction areas, with the later composition forming an apatite layer and the transgranular cracking and chipping of YSZ into layers reduced with an increase in Yttria content. However, their application into water-based SPS coatings resulted in a complete coating failure following the high-temperature CMAS tests. During Furnace Cycling Test (FCT) tests, the 8 YSZ coating survived 34 thermal cycles compared to just one on the 50.2 mol% YSZ coatings. The premature delamination of the higher Yttria coating seems to have been caused by the spallation associated with horizontal cracks within the coating. The YSZ composition could be modified into CMAS-resistant chemistry; however, their applicability as a functioning TBC with inherent microstructural features, such as the Dense Vertically Cracked (DVC) coating strategy examined in this study, requires new coating design strategies that impart sustainable thermal cycling performance.

Gas turbines rely on thermal barrier coating (TBC) to thermally insulate the nickel-based superalloys underneath during operation; however, current TBCs, yttria stabilised zirconia (YSZ), limit the operating temperature and hence efficiency. At an operating temperature above 1200 degrees C, YSZ is susceptible to failure due to phase instability and CMAS (Calcia-Magnesia-Alumina-Silica) attack. Gadolinium zirconates (GZ) could overcome the drawback of YSZ, complementing each other with the multi-layer approach. This study introduces a novel approach utilising axial suspension plasma spray (ASPS) and axial solution precursor plasma spray (ASPPS) to produce a double-layer and a triple-layer TBCs with improved CMAS resistance. The former comprised suspension plasma sprayed GZ and YSZ layers while the latter had an additional dense layer deposited through a solution precursor to minimise the columnar gaps that pre-existed in the SPS GZ layer, thus resisting CMAS infiltration. Both coatings performed similarly in furnace cycling test (FCT) and burner rig testing (BRT). In the CMAS test, the triple-layer coating exhibited better CMAS reactivity, as evidenced by the limited CMAS infiltration observed on the surface.