ICMCTF2018 Session F4-1: Functional Oxide and Oxynitride Coatings

Thermal barrier coatings (TBCs) are applied on aircraft and industrial gas turbine engines protecting the operating components from demanding high temperature environments. However, the thermal mismatch leads to generation of stresses in the TBCs resulting in micro-cracks that grow and coalesce, leading to ultimate failure of the coating. A new, unique self-healing thermal barrier coating for turbines and other thermally loaded structures has been developed in orderto realize a significant extension of the lifetime of critical high-temperature components..

The concept is based on novel Al₂O₃ coated MoSi₂ particles embedded in the TBC layer, typically consisting of yttria partially stabilized zirconia (YPSZ) [1]. Upon high-temperature exposure in an oxidising environment the embedded MoSi₂ particles react to form a viscous silica (SiO₂), which fills the cracks and re-establishes adherence in the TBC. Subsequently the SiO₂ reacts with the matrix forming zircon (ZrSiO₄), which is a load bearing and solid crystalline ceramic phase. This new concept involves the creation of an inert, oxygen impenetrable, shell of alumina (Al₂O₃) around the actual healing agent, which prevents premature triggering of the healing reaction. With this approach, the healing mechanism will become active only when a crack penetrates the alumina shell.

The MoSi₂ particles are alloyed with boron to promote the kinetics of the healing reaction and filling of crack gap with amorphous silica [2]. For manufacturing the self-healing TBC by atmospheric plasma spraying, encapsulation by selective oxidation of aluminium, added to the healing particles, proved to be successful. The core of the encapsulated and embedded healing particles remained intact when exposed to high temperatures in air for long times.

In furnace cycle tests, mimicking TBCs in applications, the crack damage evolution due to mismatch in thermal expansion is determined. The lifetime of the self-healing TBC in the furnace cycle tests compared to a similar TBC but without healing particles, was prolonged and the scatter in the lifetime data reduced making the self-healing TBC more reliable.

[1] W.G. Sloof, S.R. Turteltaub, A.L. Carabat, Z. Derelioglu, S.A. Ponnusami and G.M. Song. Crack healing in yttria stabilized zirconia thermal barrier coatings. In: Self-healing materials - pioneering research in the Netherlands, S. van der Zwaag & E. Brinkman Eds., IOS Press, Amsterdam, 2015, pp. 217-225.

[2] Z. Derelioglu, A.L. Carabat, G.M. Song, S.V.D. Zwaag, W.G. Sloof. On the use of B-alloyed MoSi2 particles as crack healing agents in yttria stabilized zirconia thermal barrier coatings, J Eur Ceram Soc 35 (2015) 4507-4511.

Photocatalysis has been widely studied for the removal of contaminants of emerging concern from water. The use of the catalyst in a powdered form results in high surface area, but hinders the catalyst recovery. An alternative approach is to deposit the photocatalyst onto a flexible substrate material that can conform to the shape of a reactor vessel. Titania (TiO₂) in the anatase phase is the most widely used photocatalyst, but when deposited by conventional magnetron sputtering, the coating usually requires elevated temperatures or post-deposition annealing in order to form the desired crystalline structure. This precludes the use of thermally sensitive substrates. However, deposition in HiPIMS (high power impulse magnetron sputtering) mode allows the deposition of anatase titania in a single stage process directly onto polymeric substrates. This paper, therefore, presents data on the performance of photocatalytic thin films of titania deposited onto polyethylene terephthalate (PET) supports via HiPIMS. Photocatalytic activity of the coated film was assessed by the degradation of the photostable fungicide carbendazim (CBZ) in aqueous solution, in the presence of a photosensitizing agent, reaching 35% of CBZ removal under UV-A and visible radiation. The reusability of the coatings was implied by negligible drop in activity after 5 cycles. The titania coatings have been characterized by SEM, XRD and UV-vis spectroscopy.

Development of novel multicomponent ceramic oxide systems is the promising way how to extend application potential of binary oxides. Zirconia is one of the most studied oxide ceramic materials because of its excellent chemical inertness and good mechanical, electrical, optical and thermal properties. Tantalum pentoxide used as thin-film material exhibits interesting electrical and optical properties. The limit for an application of these oxides is the stability of their structure and properties at elevated temperatures.

The present study focuses on investigation of the thermal stability of the structure and optical and mechanical properties of Zr-Ta-O films with a low and high Ta content. Two ternary Zr-Ta-O films (Zr₂₅Ta₅O₇₀ and Zr₅Ta₂₅O₇₀) and two binary films (ZrO₂ and Ta₂O₅) were prepared by reactive high-power impulse magnetron sputtering of a single Zr-Ta target (with a varying Ta fraction in the target erosion area) in argon-oxygen gas mixtures using a pulsed reactive gas flow control. The films were deposited either without any external substrate heating or at 400°C onto Si substrates at a floating potential. In the as-deposited state, the structure, microstructure, mechanical and optical properties of the films were analyzed and their thermal stability in air in a temperature range of 700°C – 1300°C investigated.

Oxide ceramic coatings produced by Plasma Electrolytic Oxidation (PEO) in electrolytes with nanoparticle additions have been gaining increasing attention. Many electrical and electronic applications such as capacitors, resistors and integrated circuits would benefit from dielectric surface layers produced by green and facile PEO technology. The PEO coatings produced with incorporation of nanoparticles directly from electrolyte are known to be denser, however the influence of nanoparticle additions on electrophysical properties of such coatings requires further investigation.

The PEO coatings have been produced on AA6082 alloy samples using a pulsed bipolar current mode with 1-3 kHz frequency. The composition of the dilute alkaline electrolyte and the concentration of alpha alumina nanoparticles additions were varied from 10 to 30 g/l. Since incorporation of nanoparticles allowed coating porosity to be reduced, thin yet dense PEO coatings with thickness ranging from 10 to 30 micron have been produced in relatively short treatment times, varied from 5 to 15 min.

A Mott-Schottky analysis was performed on the coatings to determine the concentration charge carriers and the flat band potential. For that, electrochemical impedance spectroscopy (EIS) was conducted at increasing potentials from 0 to 1.3 V vs. OCP with step size of 100 mV. The capacitance of the coatings was calculated by fitting the experimental data to an equivalent circuit and a linear relationship between the inversed squared capacitance and applied voltage was found. The dielectric strength of the studied materials was evaluated in the metal-oxide metal configuration by applying an increasing DC voltage until the coating breakdown is achieved. Morphology of the coatings was studied by scanning electron microscopy (SEM) and phase composition of the coatings was analysed by X-ray diffraction. Correlations were sought between characteristics of surface morphology, phase composition and electrophysical properties of the studied coatings.

The results indicate that the electrophysical properties of PEO coatings are comparable and in some cases even better compared to epoxy-based materials used, up to now, for insulated metal substrates. Additionally, these coatings present higher thermal stability and lifetime which makes them potential candidates for electronic applications.