ICMCTF2016 Session A2-2: Thermal and Environmental Barrier Coatings

A thermal detector (camera) can detect a temperature difference between a person and his/her surroundings. However, a thermal detector does not capture the temperature of the person; it detects the thermal radiation from an object. The intensity of the radiation depends on the temperature and natural emissivity of the material's surface. Avoiding heat transfer between the body and the garment or coating low emissivity materials onto the fabric surface are two effective ways to provide a person with thermal camouflage.

Coating silver on textiles using various coating processes for antibacterial effects has been intensively investigated [1-3]. Here we develop an effective route to apply silver nanoparticles on a fabric surface using chemical deposition. An effective layer-by-layer (LBL) silver plating process for applying silver nanoparticles on fabric surface has been investigated. The LBL coating based on a spraying process produces a smooth and dense silver coating layer with small amounts of silver nanoparticles. The treated fabric has a reasonably low emissivity property and good bonding strength. Two catalysts, SnCl2 and L-Cystine, were used to improve the silver loading on fabrics. Chitosan/DMDHEU was proven to be the best resin for overlay protection of the silver coating. It has a very small influence on the emissivity properties of the fabric, but gives excellent washing fastness for the silver coating film.

This technique can also be used to develop other silver related products. Silver is a wide spectrum and high efficiency antibacterial agent, and the controlled release of the silver atom/ion to the fabric surface can provide a long-lasting antibacterial function.

Reference

1. Klueh, U., Wagner, V., Kelly, S., Johnson, & A, Bryers, JD. Efficacy of silver-coated fabric to prevent bacterial colonization and subsequent device-based biofilm formation. J Biomed Mater Res. 53, 621-31(2000).

2. Chen, Y.H., Hsu, C. C.,& He, J. L. Antibacterial silver coating on poly(ethylene terephthalate) fabric by using high power impulse magnetron sputtering. 232, 868-875 (2013).

3. El-Rafie, M.H., Mohamed, A.A., Th.I. Shaheen., & A. Hebeish, A. Antimicrobial effect of silver nanoparticles produced by fungal process on cotton fabrics. Carbohydrate Polymers. 80, 779–782 (2010).

The conversion of electric power from renewable energy sources into liquid fuels (power-to-liquid) is currently an upcoming issue of special interest. For achieving a high efficiency, the steam electrolysis used to split water to hydrogen and oxygen is run under pressure up to 30 bar at temperatures around 850°C. The impact of such severe conditions on the behavior of the interconnector coatings designed to retain evaporating chromium is a crucial issue and needs detailed understanding to ensure a reliable operation.

Two coatings, one based on Mn-Co-Fe oxide (MCF) and the other on La-Sr-Mn oxide (LSM) were deposited on the interconnector material Crofer 22 APU by thermal spraying and by roll coating for cost savings. 100% water vapor and pure dry oxygen, both representing the opposite extremes of the possible process atmospheres, were selected for the study. Laboratory test autoclaves from Alloy 602 were prepared to conduct the experiments under pressure. The samples were exposed at 850°C and 30 bar to 100% water vapor for up to 1000 h and to pure oxygen for up to 3000 h.

In water vapor the MCF coating decomposes after 300 h to metallic Co and manganese oxide. The higher redox potential of Co compared to that of Mn is considered as an explanation of the selective reduction of Co by hydrogen from the water vapor. No Cr was detected in the coating after exposure to pure water vapor indicating that the evaporated gaseous Cr species does not react with no component of the coating. The LSM coating shows in water vapor a grain coarsening after 1000 h as well as a separation of grains with higher La content towards the surface and grains containing Mn towards the interface. Cr is detected very weakly in the areas with high La contents.

Yttria-stabilised zirconia (YSZ) is a good thermal insulator that provides thermal protection of working components in high temperature applications, such as gas turbines and hot sections of aircraft engine turbine blades. However, the thermal mismatch in the thermal barrier coating (TBC), leads to generation of cracks, delamination of the coating and, ultimately, to failure of the TBC system.

To prolong the lifetime of TBCs a new method of crack healing has been developed, which relies on embedding molybdenum disilicide (MoSi₂) healing particles within the TBC matrix. Upon high temperature oxidation, MoSi₂ decomposes and amorphous silica (SiO₂) is formed as a reaction product. SiO₂ fills and seals the cracks created during thermal cycling thus providing a self-repairing mechanism to prolong the lifetime of the TBC.

MoSi₂ has relatively high thermal conductivity (30 W/mK at 1400 ^oC). Therefore, embedding a good thermal conductor into a thermally insulating matrix (thermal conductivity of dense YSZ is about 2.3 W/mK at 1000 ^oC) could have an unfavourable impact on thermal conductivity of the TBCs.

High-temperature high-velocity water vapor exposures of Y₂Si₂O₇ and Y₂SiO₅ were conducted to determine the potential of these oxides as Environmental Barrier Coatings (EBCs) and/or matrix constituents for SiC-based composites. Liquid water was pumped at a controlled rate into a fused quartz capillary held inside a tube furnace at temperatures between 1200°C and 1400°C. The vaporization of water within the capillary resulted in a large volume expansion and acceleration of the steam out of the capillary, simulating the high-temperature high-velocity steam conditions of a turbine engine. The high-velocity steam impinged on the specimen under study leading to surface silica depletion via the formation of volatile silicon hydroxide species. Specimens were characterized by Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, Electron Backscatter Diffraction and X-ray Diffraction before and after exposure. The effects of sample density, exposure time and temperature on silica depletion were explored. Steam-jet exposures of Y₂Si₂O₇ resulted in the formation of porous Y₂SiO₅. SiO₂ depletion from Y₂Si₂O₇ increased with decreasing density due to additional paths for ingress of steam to the specimen interior. SiO₂ depletion rates from dense Y₂Si₂O₇ slowed with time due to increasing path lengths for steam ingress through the growing porous layer of Y₂SiO₅. Surprisingly, SiO₂ depletion from dense Y₂Si₂O₇ decreased with increasing temperature due to sintering of the porous conversion layer. Y₂Si₂O₇ has four polymorphs, however, no effects of phase on SiO₂ depletion were observed. Surface faceting was found to correspond to the disilicate crystallographic orientation. No SiO₂ depletion from Y₂SiO₅ was observed for steam-jet exposures at 1200 – 1300°C, exposure times of 250 – 500 hours, and steam velocities of 150 – 200 m/s.

Thermal barrier coatings (TBCs) for aeronautical applications are mostly based on yttria stabilized zirconia (YSZ) deposited by either thermal spray or electron-beam physical vapour deposition (EB-PVD) in the combustion chamber and the high pressure turbine components. The increase of the turbine inlet temperatures demands greater oxidation and thermal insulation to the turbine parts that are not currently TBcoated. This work explores the mechanisms involved in the synthesis of full TBC systems in a single step process from slurries containing Al microspheres. The microspheres simultaneously deform and oxidise allowing Al to be supplied to the Ni-based superalloy substrate. The combustion synthesis mechanisms allow then fast aluminisation of the superalloy. Once Al is consumed by diffusion to the substrate, a bisque of hollow alumina microspheres is left on top that provides very low thermal conductivity (k) as measured by a laser flash method. The thermal conductivity values with respect other conventional –industrial- TBCs will be established. While the erosion resistance is to be improved, this coating system appears to provide resistance to both molten salt in Na2SO4-V2O5 and to oxidation in wet and dry air.