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.

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.

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.

The quest to increase the surface temperatures and resistance to the corrosive environment of thermal barrier coatings topcoats mean that newer coating design strategies are needed. In this study, a performance evaluation of suspension high-velocity oxy-fuel (SHVOF)-sprayed dense vertically cracked (DVC) and suspension-plasma-sprayed (SPS) columnar structure (CS) topcoats is conducted. The calcia magnesia alumino silicate (CMAS) evaluation is conducted at 1300 degrees C for 30 min, whereas the furnace cycling tests (FCT) is conducted at 1135 degrees C for 45 min cycle dwell time. The CMAS infiltrates down to the bond coat layer, but does not induce partial or complete topcoat spallation on all studied topcoat layers. In terms of CMAS infiltration, the CMAS appears to be restricted along the vertical cracks. The FCT of the SPS CS structure indicates a failure largely at the thermally grown oxide (TGO)-topcoat interface, while the DVC topcoat layers indicate a mix-mode failure, i.e., both material-associated cracking and localized spallations at the TGO-topcoat interface. Overall, the SHVOF-sprayed ethanol-based DVC topcoat seems to offer a balanced trade-off, i.e., a majority of the topcoat is still intact after 100 thermal cycles and exceeds the material durability and performance offered by the SPS CS structure. A first-of-its-kind performance evaluation, based on simulated high-temperature furnace cycling and calcia magnesia alumino silicates (CMAS) attack, is carried out on dense vertically cracked (DVC) microstructures, as an alternative to columnar structure (CS) microstructure for thermal barrier applications. The optimized DVC structures restrict CMAS attack and demonstrate promising thermal cycling performance, exceeding material's durability offered by the conventional CS microstructures.image (c) 2023 WILEY-VCH GmbH

The quest to increase the surface temperatures and resistance to the corrosive environment of thermal barrier coatings topcoats mean that newer coating design strategies are needed. In this study, a performance evaluation of suspension high‐velocity oxy‐fuel (SHVOF)‐sprayed dense vertically cracked (DVC) and suspension‐plasma‐sprayed (SPS) columnar structure (CS) topcoats is conducted. The calcia magnesia alumino silicate (CMAS) evaluation is conducted at 1300 °C for 30 min, whereas the furnace cycling tests (FCT) is conducted at 1135 °C for 45 min cycle dwell time. The CMAS infiltrates down to the bond coat layer, but does not induce partial or complete topcoat spallation on all studied topcoat layers. In terms of CMAS infiltration, the CMAS appears to be restricted along the vertical cracks. The FCT of the SPS CS structure indicates a failure largely at the thermally grown oxide (TGO)–topcoat interface, while the DVC topcoat layers indicate a mix‐mode failure, i.e., both material‐associated cracking and localized spallations at the TGO–topcoat interface. Overall, the SHVOF‐sprayed ethanol‐based DVC topcoat seems to offer a balanced trade‐off, i.e., a majority of the topcoat is still intact after 100 thermal cycles and exceeds the material durability and performance offered by the SPS CS structure.

Ice protection techniques have attracted significant interest, notably in aerospace and wind energy applications. However, the current solutions are mostly costly and inconvenient due to energy-intensive and environmental concerns. One of the appealing strategies is the use of passive icephobicity, in the form of coatings, which is induced by means of several material strategies, such as hydrophobicity, surface texturing, surface elasticity, and the physical infusion of ice-depressing liquids, etc. In this review, surface-roughness-related icephobicity is critically discussed to understand the challenges and the role of roughness, especially on superhydrophobic surfaces. Surface roughness as an intrinsic, independent surface property for anti-icing and de-icing performance is also debated, and their interdependence is explained using the related physical mechanisms and thermodynamics of ice nucleation. Furthermore, the role of surface roughness in the case of elastomeric or low-modulus polymeric coatings, which typically instigate an easy release of ice, is examined. In addition to material-centric approaches, the influence of surface roughness in de-icing evaluation is also explored, and a comparative assessment is conducted to understand the testing sensitivity to various surface characteristics. This review exemplifies that surface roughness plays a crucial role in incorporating and maintaining icephobic performance and is intrinsically interlinked with other surface-induced icephobicity strategies, including superhydrophobicity and elastomeric surfaces. Furthermore, the de-icing evaluation methods also appear to be roughness sensitive in a certain range, indicating a dominant role of mechanically interlocked ice.

Silicon Carbide (SiC) coatings offer exceptional wear resistance and excellent tribological characteristics; however, it is a challenging material to be thermally sprayed due to a lack of melting point. In this study, a hybrid, single-step suspension and solution precursor feedstock design is proposed, consisting of a SiC suspension modified with Yttrium Aluminium Garnet (YAG) precursors, for thermal spraying of SiC/YAG coatings. The decomposition of SiC was restricted in all spray campaigns. The solid loading of SiC (from 10 wt% to 20 wt%) and YAG phase (from 20 wt% and up to 50 wt%) were varied in an attempt to improve wear performance, enhance coating cohesion, and minimise porosity of the studied coatings. Among all studied coatings, 60 wt% SiC/40 wt% YAG and 50 wt% SiC/50 wt% YAG coatings at a 10 wt% solid loading were the best-performing coatings, demonstrating a promising wear resistance up to a sliding distance up of 1000 m, a dense coating structure with porosity at 0.4 ± 0.2%. The feedstock design opens up a new method to process materials which are difficult, if not impossible, to process using a conventional thermal route.

Carbides are interesting materials for many wear resistant and high temperature applications, however, the production of coatings with these materials represents a significant challenge as they tend to oxidise or decompose into gaseous phases when they are exposed to extreme thermal spray conditions. An innovative method merging suspension and solution precursors was developed to allow the production of carbide composite coatings. Suspensions of carbides and borides were modified with the addition of oxide precursors to obtain composite coatings by high-velocity oxy-fuel (HVOF) thermal spray. The transformation of these oxides precursors and their subsequent melting during spraying contribute to protect the carbides from oxidising conditions, avoid their degradation during the spray process and support the development of dense coatings, as it was demonstrated by dispersive X-ray spectroscopy and X-ray diffraction analysis. The relationships between processing and microstructure were studied in terms of porosity phase distribution and mechanical properties, proving that this novel approach could be applied to obtain coatings of materials that are prone to decompose during thermal spraying.

Liquid-infused polymeric surfaces have demonstrated promising icephobicity. However, the capability to maintain the icephobic performance after material damage has been a challenge, both in terms of conserving a smoother surface and the replenishment of the infused liquid. Cetacean skin possesses a microscopically smooth texture in the form of cells lubricated with lipid proteins and consists of structural fibers that ensure durability. Concerning the structure of cetacean skin, glycerol-infused fiber-reinforced polyurethane (GIFRP) coatings were proposed. Instead of hosting the lipid proteins, the coatings were infused with glycerol, a known cryoprotectant to induce the supercooling of water, a strategy inspired by wood frogs and red flat dark beetles to prevent freezing. The inclusion of glycerol delayed water droplet freezing duration by 659%, while negligible frost accumulated on the fabricated coatings during anti-icing tests. The reinforcement of fibers was effective and the surface damage was reduced by a factor of 4, compared to the pure polyurethane coatings after erosion impact. The incorporation of fibers has proven to be beneficial for infused-liquid replenishment and the slow-releasing capabilities of GIFRP coatings. Minimized surface deterioration and the continued presence of glycerol on GIFRP coatings demonstrated a small increase in ice adhesion from 0.22 to 0.77 kPa after the erosion tests, one of the lowest values reported in the literature after substantial surface damage. The concept inspired by cetacean skin and the cryoprotective features of chionophiles was instrumental in keeping the ice adhesion under 1 kPa after erosion impact.

A comparative study of de-icing evaluation methods was conducted in this work, and their variations in response to surface characteristics were investigated. The mechanical de-icing measurements include centrifugal, push, and tensile methods. The centrifugal and the horizontal push (shear) methods suggested a linear relationship of ice adhesion strength with surface roughness, whereas the tensile (normal) method indicated an inverse curvilinear relationship with contact angle hysteresis. A partial correlation of contact angle hysteresis on the shear-based methods was also indicated over a specified range of surface roughness. Further attempts were also made on 1H,1H,2H,2H-perfluorooctyltriethoxysilane-coated surfaces, and the ice adhesion indicated a clear reduction in the normal de-icing method, whereas the shear-based methods did not show a considerable change in ice adhesion, highlighting their mechanical forces-centric response. Lastly, a further evaluation using a hybrid de-icing method was conducted, to verify the influence of surface characteristics on ice removal involving heating, which demonstrated a partial correlation of energy consumption with the ice adhesion strength over a specified range of surface roughness. The results obtained in this study provide crucial information on the influence of surface characteristics on ice adhesion and offer material-dependent correlations of the popular de-icing evaluation methods. The conclusions could be applied to define an appropriate testing method for the evaluation of icephobic surfaces and coatings. Graphical abstract

Icephobic performance of low-ice adhesion polymeric coatings has been studied intensively for passive ice protection. However, limited efforts were conducted to identify strategies for enhancing the durability of the coatings to maintain low ice adhesion after erosion impact. In this work, we developed and investigated several polyurethane-based nanocomposite and fibre-reinforced coatings to understand the deteriorating behaviour of the coatings under rigorous impinging erosion tests and the subsequent impact on ice adhesion. The inclusion of fillers resulted in up to 38 points increase in Shore hardness relative to the pristine PU coatings. The ice adhesion strengths on 3 wt% nanoparticle-reinforced coatings after the erosion tests were nearly halved, whereas, a 5-fold reduction was observed on 3 wt% fibre-reinforced coatings compared to that of the pure PU coatings. Our results indicated that the incorporation of fillers was effective in reducing the ice anchoring points, and that, after the impingement, the icephobic performance was retained by either lowering surface roughness or by minimizing surface deterioration. Fibres took a more effective role in limiting crack initiation and resisting crack propagation. The ice adhesion strength of the coatings increased from 5.6 kPa to 8.4 kPa with 20 wt% carbon fibres incorporated PU coatings, essentially keeping the adhesion below 10 kPa even after rigorous impinging tests and a similar to 10-fold reduction in ice adhesion strength as compared to the pure PU coatings. The incorporation of the fibres led to enhanced durability and retention of excellent icephobic performance via a mechanism that is adaptable to other polymeric coatings.

The roles of surface roughness on icephobicity including ice adhesion strength have been long debated in icephobicity studies. However, the direct/systematic influence of surface roughness on ice adhesion strength while keeping other surface characteristics such as surface wettability and interfacial cavitation unchanged are seldom reported. In this paper, systematic reduction of ice adhesion strength with the decrease in surface roughness regardless of the surface wettability was demonstrated across all the studied material types, i.e. metallic surfaces and polymeric coatings with different surface wettability. In-situ icing observation studies indicated that the ice did not anchor on smooth metallic surfaces and polymeric coatings but anchored on rough surfaces including superhydrophobic coatings. Effect of surface wettability was argued against the ice adhesion strength based on our results and similar ice adhesion strength was found on materials having different wettability (i.e. hydrophilic and hydrophobic coatings, and surfaces having different contact angle hysteresis). On the contrary, the introduction of low surface energy chemicals (via deposition and/or functionalization) on the surface having similar surface roughness showed a direct reduction of ice adhesion strength. These results indicated the surface roughness is vital in achieving icephobic performance, however, the ultra-low ice adhesion strength could be achieved by the synergetic effect of low surface roughness and low interfacial cavitation (in line with the interfacial correlation factor).

Icephobic surfaces have attracted increasing attention due to their wide ranging application areas from wind and solar energy systems to aviation. Slippery liquid-infused porous surfaces (SLIPS) are being explored for passive ice protection due to their lower ice adhesion strength. In this study, we present a cost-effective and scalable electrospinning technique to produce freestanding nanofibrous polymeric surfaces for the fabrication of transparent icephobic SLIPS. The diameter of the electrospun fibres produced varied from 200 to 400 nm and the membranes had a theoretical porosity of 71.6 +/- 4.1%. Furthermore, three different lubricants polychlorotri-fluoroethylene oil (PcTFE), silicone oil and liquid paraffin, were used and it was observed that when silicone oil and PcTFE were used as lubricants for SLIPS, they provided high optical transparency (> 90%) in the visible light spectrum compared to PVDF-co-HFP itself. All SLIPS were subjected to centrifugal ice adhesion testing which revealed their ice adhesion strengths lower than 1 KPa with significant delay in droplet icing compared to aluminum reference, from 5 up to 41 s. The results indicated that enhanced icephobic properties of electrospun membranes have been clearly demonstrated.

Icephobicity is intrinsically affected by rough asperities and the surface voids provide anchoring points for the ice. The anchor of ice is likely to form on the surface under high humidity conditions. In-situ water condensation and icing observation were conducted to understand water condensation and ice retracting patterns in controlled humidity, pressure and temperature conditions. It was observed that water micro-condensation and icing occurred on rougher surfaces and the water droplets condensed along the surface cracks of the superhydrophobic polydimethylsiloxane (PDMS) based nanocomposite coatings. Further analysis revealed that ice anchoring was present on both aluminum and superhydrophobic coating surface, but it was more severe and intensified on the as-received aluminum substrates. No water condensation or subsequent icing was found on smooth PDMS hydrophobic surfaces due to the incapacity of the smooth surfaces to anchor water drops. It is the first time to validate ice anchoring over retracting ice on different wettability surfaces from in-situ icing observation. Ice adhesion strengths were also measured on the studied surfaces and the results indicated a strong linkage between centrifugal shearing of ice and anchoring mechanism due to surface rough voids, and there was no clear relevancy between ice adhesion strength and the surface wettability or hydrophobicity.

8 Icephobic surfaces have attracted increasing attention due to their wide ranging application 9 areas from wind and solar energy systems to aviation. Slippery liquid-infused porous surfaces 10 (SLIPS) are being explored for passive ice protection due to their lower ice adhesion strength. 11 In this study, we present a cost-effective and scalable electrospinning technique to produce 12 freestanding nanofibrous polymeric surfaces for the fabrication of transparent icephobic 13 SLIPS. The diameter of the electrospun fibres produced varied from 200 to 400 nm and the 14 membranes had a theoretical porosity of 71.6 ±4.1%. Furthermore, three different lubricants 15 polychlorotrifluoroethylene oil (PcTFE), silicone oil and liquid paraffin, were used and it was 16 observed that when silicone oil and PcTFE were used as lubricants for SLIPS, they provided 17 high optical transparency (>90%) in the visible light spectrum compared to PVDF-co-HFP 18 itself. All SLIPS were subjected to centrifugal ice adhesion testing which revealed their ice 19 adhesion strengths lower than 1 KPa with significant delay in droplet icing compared to 20 aluminium reference, from 5 up to 41 sec. The results indicated that enhanced icephobic 21 properties of electrospun membranes have been clearly demonstrated. 22

Icephobicity of materials has received intensive attention in recent years due to the increasing requirement of ice protection in aerospace, wind energy and power lines. However, the influencing factors of material icephobicity have not been well identified. In this work, the effect of surface gaseous adsorption on icing behaviour of materials was investigated for the first time. Ni-Cu-P coatings with different surface morphologies were fabricated and used as the objects of the study. Environmental scanning electron microscopy (ESEM) was utilized to observe the water condensation and ice formation on the coatings. X-ray photoelectron spectroscopy (XPS) was employed to analyse the variations of surface adsorption. Droplets icing time and ice adhesion strength of the coatings were also studied. The results showed that the icing time of water droplets on the Ni-Cu-P coatings increased significantly, and the ice adhesion strength decreased sharply with the spontaneous surface adsorption of gaseous species (mainly hydrocarbon groups) in air. The adsorbed hydrocarbon species would promote the formation of air pockets between the ice-coating interface, which could effectively reduce the interfacial contact of the formed ice with the coating. When the adsorbed hydrocarbon species were removed by plasma cleaning, water droplets tended to have more direct contacts with the coatings prior to icing, leading to the formation of interlocked ice and significantly increased the ice adhesion on the surface. The variation of surface icephobicity can also be attributed to the changes of surface energy due to the surface adsorption. The results indicated that the surface gaseous adsorption in air played an important role in determining the surface icing behaviour and the icephobicity of the materials.

Hydrophobic/icephobic coatings have been fabricated using a combination of thermal sprayed metallic MCrAlY (M = Ni, Co) coatings with a subsequent deposition process using 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS). The MCrAlY coatings provide the desirable surface roughness feature for hydrophobicity, and water contact angle of 135 degrees was directly obtained after aged in the atmosphere for 1 week. However, it was found that the hydrophobicity of MCrAlY was not stable under water impinging due to unstable hydrocarbon absorption. Better hydrophobicity with water contact angle of 154 degrees and improved durability have been achieved by further modification using POTS vapour on the rough MCrAlY coatings. X-ray photoelectron spectroscopy results revealed that replacement of absorption of hydrocarbon by functional C-F groups played important role in the improvement of hydrophobicity and durability. The ice adhesion test confirmed that lower ice adhesion strength of MCrAlY based coatings have been obtained compared with the threshold for icephobicity which is desirable to be applied as icephobic coatings for aircraft. The electo-thermal heating de-icing test showed an energy saving of 28.6% for de-icing with the two-step MCrAlY based coatings. The combination of strong metallic MCrAlY rough layers and the subsequent functionalization enables a new approach for the fabrication of durable hydrophobic/icephobic coatings.

Formation and accretion of ice on the leading edge surface of aircrafts wings may lead to disasters. The current de-icing system for aircraft will build up weight, increase energy consumption and add complexity to the aircraft systems. Development of icephobic coatings is a potential solution to prevent ice formation and/or reduce accretion on the critical surface of aircraft. Icephobic coatings based on poly(dimethylsiloxane) (PDMS) with modification by fluorosilane and incorporation of silica nanoparticles have been fabricated. The hydrophobicity of the coatings has been measured in normal conditions with atmospheric pressure and room temperature, showing improvement of hydrophobicity by the fluorination of PDMS and incorporation of silica nanoparticles. The water droplet icing behaviour shows better anti-icing performance for fluorinated PDMS (F-PDMS)/silica coatings with a rough surface. The ice adhesion strength test results show that F-PDMS coatings without silica nanoparticles have lower ice adhesion strength implying better de-icing performance. The wettability of the coatings was also measured at reduced pressure and temperature, to study the mechanism of higher ice adhesion strength of F-PDMS/silica coatings comparing with F-PDMS based coatings. For the design and fabrication of icephobic coatings, compromise on the roughness induced hydrophobicity may become a critical requirement to avoid mechanical interlock between the ice and the rough surface.