Heterogeneous layered structure in thermal barrier coatings by plasma spray-physical vapor deposition

The unique columnar structure endows thermal barrier coatings(TBCs)prepared by plasma spray-physical vapor deposition(PS-PVD)with high thermal insulation and long lifetime.However,the coating delamination failure resulting from an intra-column fracture(within a column rather than between columns)is a bottleneck in the solid dust particle impact environment for aero-engine.To clarify the intra-column fracture mechanism,a basic layer deposition model is developed to explore a heterogeneous weak-to-strong layered structure formed by a local transient in-situ deposit temperature.During the PS-PVD,an in-situ deposit surface is continuously updated due to constantly being covered by vapor condensation,showing a transient temperature,which means that the in-situ deposit surface temperature rises sharply in short period of 0.2 s of depositing a thin layer during a single pass.Meanwhile,the increasing temperature of the in-situ deposit surface results in an experimentally observed heterogeneous weak-to-strong structure,showing a continuous transition from a porous weak structure at the bottom region to a dense strong structure at the top region.This structure easily makes the intra-column fracture at the porous weak region.The results shed light on improving TBC lifetime by restraining the intra-column fracture.

Progress in ceramic materials and structure design toward advanced thermal barrier coatings

Thermal barrier coatings(TBCs)can effectively protect the alloy substrate of hot components in aeroengines or land-based gas turbines by the thermal insulation and corrosion/erosion resistance of the ceramic top coat.However,the continuous pursuit of a higher operating temperature leads to degradation,delamination,and premature failure of the top coat.Both new ceramic materials and new coating structures must be developed to meet the demand for future advanced TBC systems.In this paper,the latest progress of some new ceramic materials is first reviewed.Then,a comprehensive spalling mechanism of the ceramic top coat is summarized to understand the dependence of lifetime on various factors such as oxidation scale growth,ceramic sintering,erosion,and calcium–magnesium–aluminium–silicate(CMAS)molten salt corrosion.Finally,new structural design methods for high-performance TBCs are discussed from the perspectives of lamellar,columnar,and nanostructure inclusions.The latest developments of ceramic top coat will be presented in terms of material selection,structural design,and failure mechanism,and the comprehensive guidance will be provided for the development of next-generation advanced TBCs with higher temperature resistance,better thermal insulation,and longer lifetime.

Plasma spray-physical vapor deposition toward advanced thermal barrier coatings:a review

Plasma spray–physical vapor deposition(PS–PVD)is a unique technology that enables highly tailorable functional films and coatings with various rare metal elements to be processed.This technology bridges the gap between conventional thermal spray and vapor deposition and provides a variety of coating microstructures composed of vapor,liquid,and solid deposition units.The PS–PVD technique serves a broad range of applications in the fields of thermal barrier coatings(TBCs),environmental barrier coatings(EBCs),oxygen permeable films,and electrode films.It also represents the development direction of high-performance TBC/EBC preparation technologies.With the PS–PVD technique,the composition of the deposition unit determines the microstructure of the coating and its performance.When coating materials are injected into a nozzle and transported into the plasma jet,the deposition unit generated by a coating material is affected by the plasma jet characteristics.However,there is no direct in situ measurement method of material transfer and deposition processes in the PS–PVD plasma jet,because of the extreme conditions of PS–PVD such as a low operating pressure of*100 Pa,temperatures of thousands of degrees,and a thin and high-velocity jet.Despite the difficulties,the transport and transformation behaviors of the deposition units were also researched by optical emission spectroscopy,observation of the coating microstructure and other methods.This paper reviews the progress of PS–PVD technologies considering the preparation of advanced thermal barrier coatings from the perspective of the transport and transformation behaviors of the deposition units.The development prospects of new high-performance TBCs using the PS–PVD technique are also discussed.

Contribution of the Alternative Respiratory Pathway to PSII Photoprotection in C3 and C4 Plants

The mechanism by which the mitochondrial alternative oxidase （AOX） pathway contributes to photosystem II （PSII） photoprotection is in dispute. It was generally thought that the AOX pathway protects photosystems by dissipating excess reducing equivalents exported from chloroplasts through the malate/oxaloacetate （Mal/OAA） shuttle and thus preventing the over-reduction of chloroplasts. In this study, using the aoxla Arabidopsis mutant and nine other C3 and C4 plant species, we revealed an additional action model of the AOX pathway in PSII photoprotection. Although the AOX pathway contributes to PSII photoprotection in C3 leaves treated with high light, this contribution was observed to disappear when photorespiration was suppressed. Disruption or inhibition of the AOX pathway significantly decreased the photorespiration in C3 leaves. Moreover, the AOX pathway did not respond to high light and contributed little to PSII photoprotection in C4 leaves possessing a highly active Mal/OAA shuttle but with little photorespiration. These results demonstrate that the AOX pathway contributes to PSII photoprotection in C3 plants by maintaining photo- respiration to detoxify glycolate and via the indirect export of excess reducing equivalents from chloro-plasts by the MaI/OAA shuttle. This new action model explains why the AOX pathway does not contribute to PSII photoprotection in C4 plants.

Lightweight epoxy-based abradable seal coating with high bonding strength

To gain high efficiency and low fuel consumption, aluminum-based abradable seal coatings had been widely used in the compressor casing of aero engines or gas turbines owing to the low elastic modulus. However, the adhesive transfer phenomenon frequently occurs when the radial rubbing generates between titanium alloy blade tips and aluminum-based coating. It tends to increase scratch force and results in blades vibration and even engine jam. To eliminate this problem, a new lightweight epoxy-based abradable seal coating with high bonding strength was developed by an effective and porosity controllable mixing process that distributes spherical pores uniformly in the continuous matrix. A lightweight coating of 63% porosity with a hardness of 33.1(HR15 Y) can be reached when the content of hollow microspheres is 31 wt.%. The coating density is 0.5 g/cm3 and the bonding strength is as high as 18 MPa.The performance of the epoxy-based coating is comprehensively better than aluminum-based coatings in five essential indices. This study is expected to provide a new technical path for obtaining high-quality abradable seal coatings to guarantee the efficient and safe operation of compressors.

Improving WC-Co coating adhesive strength on rough substrate:Finite element modeling and experiment

The adhesive strength in a coating-substrate system is of primary importance for the coating lifetime in service.However,the underlying mechanism is not fully understood due to the complex internal structure of composite coatings.In this study,the effect of substrate roughness on the adhesive strength of WCCo coatings was investigated by experiment and simulation.Results show that the adhesive strength is significantly affected by the roughness.In the case of the Ra<2μm,the adhesive strength is approximately 35–46 MPa.When the Ra is 4μm,the adhesive strength increases to nearly 60 MPa.A finite element model was developed to correlate the roughness with adhesive strength.It is found that the predicted values are well consistent with the experimental data.In addition,with the increase of the roughness,the residual stress would be changed from concentrated state to widespread state,which decreases the critical stress to result in crack propagation.That’s why a larger roughness can cause a higher adhesive strength.This study gives understanding on the mechanism of adhesive strength affected by roughness,which contributes to the parameter optimization with better performance.

Condensation behavior of gaseous phase during transported in the near-substrate boundary layer of plasma spray-physical vapor deposition

Plasma spray-physical vapor deposition(PS-PVD)has exhibited the potential ability to prepare columnar structures for advanced thermal barrier coatings(TBCs).The coating structure is nominally affected by operating parameters,but it is controlled by the type of deposition unit actually and essentially.In order to realize the columnar structure deposited by gaseous phase units,the transition behavior of gaseous phase units to clusters must be fundamentally understood.This work investigated the transport process of gaseous phase units in the PS-PVD near-substrate boundary layer along with the condensation behavior.The Monte Carlo method was used to examine the transport process and condensation behavior of gaseous phase units under different scale boundary layers.Simulation results show that it is easier to form more numerous larger clusters at the edge of the plasma jet than at the center.Based on the understanding of the changes in deposition unit caused by the condensation of gaseous phase in the near-substrate boundary layer of PS-PVD,an outlook towards TBCs with different structures is presented.And it is in good agreement with the experimental data.

Pressure infiltration of molten aluminum for densification of environmental barrier coatings

Environmental barrier coatings(EBCs)effectively protect the ceramic matrix composites(CMCs)from harsh engine environments,especially steam and molten salts.However,open pores inevitably formed during the deposition process provide the transport channels for oxidants and corrosives,and lead to premature failure of EBCs.This research work proposed a method of pressure infiltration densification which blocked these open pores in the coatings.These results showed that it was difficult for aluminum to infiltrate spontaneously,but with the increase of external gas pressure and internal vacuum simultaneously,the molten aluminum obviously moved forward,and finally stopped infiltrating at a depth of a specific geometry.Based on the wrinkled zigzag pore model,a mathematical relationship between the critical pressure with the infiltration depth and the pore intrinsic geometry was established.The infiltration results confirmed this relationship,indicating that for a given coating,a dense thick film can be obtained by adjusting the internal and external gas pressures to drive a melt infiltration.