Black glasses as a solution to the problem of high-temperature oxidation

Always too hot...

The problem of high-temperature oxidation affects many branches of the economy because higher temperatures very often translate to an increase in process efficiency; for example, a higher temperature of fuel combustion in aircraft engines means higher effectiveness of this process, which, unfortunately, brings about the creation of a hot, gaseous environment affecting all metallic elements. The problem also impacts the energy industry, as well as a very interesting branch of low-emission SOFC (Solid Oxide Fuel Cell) technology, which requires high operational temperatures (currently about 800 °C). This requirement results from the need to provide an appropriate level of ion conductivity of the electrolyte made of YSZ (Yttrium Stabilised Zirconia) – zirconium dioxide ZrO₂ stabilised with yttrium oxide Y2O₃. Unfortunately, such high temperatures mean the acceleration of degradation of individual elements of the fuel cell, whereby high-temperature oxidation of the so-called interconnects, the ‘backbone’ of the cell that facilitates the connection of individual cells in larger stacks, remains one of the most burning issues of SOFC technology.

Black glasses to the rescue

A way to counteract the aforementioned problems is to use protective coatings. As far as interconnects in SOFCs are concerned, the task is even more difficult because of the strict material requirements for potential interconnect materials (therefore also for coatings) that encompass not only resistance to oxidation at high temperatures, but also a high level of electric conductivity. Interestingly, this novel idea of using the potential hidden within the titular black glasses to solve the research problem at hand originated in Professor Sitarz’s research team. Black glasses are materials with the structure of amorphous silica, which belong to the so-called polymer-derived ceramics (PDC). The key to their success is the possibility of ‘target-design’ their properties for specific applications. This is a result of the fact that the process of manufacturing black glasses can be controlled at each stage, starting with the choice of precursors, through the selection of form (of the coating, for example), to determining the parameters of thermal treatment.

What’s with the carbon...

The key element that determines the interesting properties of black glasses is carbon, which can be found in the material in two variants. The first one constitutes carbon atoms bonded with silicon atoms by strong, covalent Si-C bonds, which contribute to a high thermomechanical resistence of the material.  The other variant is expressed by means of the so-called free carbon phase, which occurs when a certain threshold of carbon content in the glassy matrix is exceeded. This phase is responsible for the electrical conductivity and black colour of the material. Only after nine years of research work conducted by Dr Maciej Bik as part of his degree theses and numerous research projects, as well as this university grant (implemented during the final stage of his doctoral dissertation process), and due to an enormous input of labour, was it possible to discover the optimal ratio of both these phases and develop a process of obtaining tight and homogeneous black glass-based coatings from ‘pure’ silicon oxycarbide (SiOC) and aluminium cations silicon oxycarbide (SiAlOC). During high-temperature tests (800 °C) of steel samples intended for interconnects and modified with the use of black glass-based coatings, promising results were obtained as uncontrolled diffusion processes were impeded, which translated into a significant reduction in material degradation rate with the simultaneous maintenance of satisfactory levels of electrical conductivity. It is worth noting that this was the first such attempt in the world where silicon oxycarbide-based coatings were used as interconnects, which are characterised by a working mechanism that is completely different from the ones described in the literature for the most commonly used layers based on spinel- or perovskite-structured compounds.

More than interconnects

One of the greatest advantages of this coating material is the wide scope of possible applications, which include not only metallic substrates (ferritic steels, intermetallic alloys), but also work in various atmospheres (oxidation, reducing) with equally positive effect of blocking uncontrolled diffusion processes. During the implementation of the university grant and as part of a scientific cooperation with the DECHEMA-Forschungsinstitut in Frankfurt, the first joint paper has already been published (https://doi.org/10.1016/j.apsusc.2021.151826), in which the scientists describe the huge success of using a black glass-based coating on ferritic steel intended for interconnects (800 °C; air); and two more papers are almost complete, in which the scientists describe an equally effective use of the coating on the substrate of TiAl alloy, intended for low-pressure turbine vanes for aircraft engines (750 °C; air and air with steam), as well as pure chromium (950 and 1050 °C; air).

The project was funded by a university grant within the framework of the “Excellence Initiative – Research University” project.