Using Glass Coatings for Heat Shielding

The bonding capacity with different metals and alloys, from stainless steel to titanium superalloys, has made glass coatings effective heat shields against corrosion and oxidation at high temperatures.¹ This has made glass coatings particularly valuable in the aerospace industry due to equipment exposure to highly oxidizing conditions and high temperatures.

Image Credit: iStock

For instance, titanium alloys, recognized for their lightweight and robust properties, exhibit reduced resistance to oxidation and corrosion under such extreme conditions.² By applying glass coatings to these alloys, their vulnerability to cracks and failure can be mitigated.

Combining composites of glass and ceramics to form a coating has also demonstrated similar benefits as glass coatings. However, combining glass with ceramic materials allows the maintenance of glass’ processability to work with the enhanced mechanical properties derived from the crystalline nature of ceramics.

The design of specific coatings with different glass and ceramic components allows for the development of coatings with a range of thermo-mechanical properties, thus ensuring compatibility with the substrate.

An important aspect of their utility lies in the ability to adjust their thermal expansion characteristics to match that of the substrate, making them especially well-suited for high-temperature applications.

This article will discuss the range of applications and advantages of glass coatings in heat shielding applications, providing insight into their potential to transform aerospace engineering and other fields reliant on thermal protection.

Glass-Ceramic Coatings

Numerous studies have been dedicated to exploring the potential of glass–ceramic coatings for titanium alloys and titanium-aluminum intermetallic compounds.² These coatings offer protection against high-temperature oxidation and corrosion and exhibit excellent chemical stability and durability.

Glass-ceramic coatings are typically made through a multi-step melt-quenched method where raw materials are mixed and heated to form a molten glass, usually at temperatures exceeding 1500 °C. Water is then used to quench the molten glass to produce ‘frit,’ which is then milled into glass powders.

The resulting powder can then be sprayed as a slurry onto the substrate before the component, now coated, is itself heat-treated at elevated temperatures. By incorporating ceramic inclusions into the glass matrix and carefully controlling their species and quantity, the thermal and mechanical properties of the glass–ceramic coatings can be customized to meet specific requirements.

Moreover, glass composite coatings have proven to be effective in safeguarding nickel-based superalloys, displaying exceptional resilience against hot corrosion even at temperatures of 1000 °C.⁴

These superalloys are particularly important in advanced aircraft, accounting for a significant portion of their overall weight. They are extensively utilized in crucial components such as rotating turbine parts, engine casings, links, and specific engine mounts.

Additionally, these alloys find wide-ranging applications in diverse sectors, including the manufacturing of turbocharger discs for large diesel engines and high-performance racing car engines. Recent research has also shed light on the potential of using glass-ceramics in coatings to prolong the durability of nickel-based superalloys.⁵

In conventional practices, alloys are coated with a metallic bond coat, followed by a ceramic topcoat.

However, when exposed to high temperatures, oxidation occurs at the interface between the bond coat and topcoat, resulting in the gradual accumulation of oxide and the generation of internal stresses within the material, ultimately leading to cracks and failure. As a result, this oxidation process often determines the lifespan of thermal barrier coatings.

Emerging studies have suggested that employing glass-ceramics as the bond coat between the ceramic topcoat and the nickel-based superalloy substrate can prevent this specific type of oxidation and subsequent degradation. By incorporating glass-ceramics into the coating design, the risk of material failure or degradation can be mitigated, thus extending the lifespan of the thermal barrier coating.

Applications of Glass-Ceramic Coatings in Space

Glass-based coatings are also proving attractive in one of the harshest environments a vehicle might encounter: space.

NASA researchers have utilized porous silica to create reaction-cured glass and glass coatings, which have the potential to serve as protective layers for heat shields. Additionally, these coatings can be applied to reaction vessels requiring durability against extreme heat and abrupt temperature changes.⁶

The importance of glass-ceramic coatings is exemplified by the NASA Space Shuttle Orbiter, the first spacecraft designed for reuse. The success of its missions hinged in part on its heat shield, which had to withstand extreme temperatures, significant temperature variations, and intense thermal shocks during re-entry into the Earth’s atmosphere.

When the orbiter re-enters the Earth’s atmosphere, the chemical reactions between high-temperature gases and its surface can lead to the vaporization of certain compounds. Hence, any heat shield must endure multiple re-entries into the Earth’s atmosphere at temperatures reaching up to 1400 °C.⁶

To address the thermal challenges of its journey in and out of Earth’s atmosphere, NASA’s team developed glassy–ceramic metal composites combining glasses made of porous high silica borosilicate glass and boron oxide. The mixture was treated with additives to enhance the glass’s stability, including silicon hexaboride, silicon tetraboride, boron, and boron silicide.

The composite then underwent several treatments once applied to the substrate, including glazing in a furnace. The resulting coating allowed for a stable glass coating exhibiting strong mechanical properties even when ambient temperatures exceeded 1092 °C.⁶

The coatings made of porous silica composites developed by NASA operated effectively in temperatures ranging from -100 °C to 1482 °C.⁶

The coatings can even be used at temperatures above their glazing temperature, demonstrating their reliability and versatility for space and aeronautical applications in the most demanding environments.

References

1. Chen M. et al. Glass coatings on stainless steels for high-temperature oxidation protection: Mechanisms. Corrosion Science, Volume 82, May 2014, Pages 316-327
2. Li W. et al. SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy: Oxidation and interfacial reaction behavior. Corrosion Science Volume 74, September 2013, Pages 367-378
3. Donald I.W et al. Recent developments in the preparation, characterization and applications of glass- and glass–ceramic-to-metal seals and coatings. Journal of Materials Science volume 46, pages1975–2000 (2011)
4. Chen M. et al. Preparation and thermal shock behavior at 1000 °C of a glass-alumina-NiCrAlY tri-composite coating on K38G superalloySurface and Coatings Technology Volume 206, Issues 8–9, 15 January 2012, Pages 2566-2571
5. Das S. et al. Glass–ceramics as oxidation resistant bond coat in thermal barrier coating system. Ceramics International Volume 35, Issue 4, May 2009, Pages 1403-1406
6. Fletcher J.C et al. US Patent. Reaction cured glass and glass coatings. June 6, 1978. Reaction cured glass and glass coatings – NASA Technical Reports Server (NTRS)
7. Mo-Sci Corporation. Applications of Thin and Thick Glass Films. 2022. Applications of Thin and Thick Glass Films Mo-Sci Corporation

This information has been sourced, reviewed and adapted from materials provided by Mo-Sci.

For more information on this source, please visit Mo-Sci.