Production and properties of Silicon Carbide coatings by thermal spray

Gas turbines are one of the more used combustion systems for power generation. It is fundamentally a simple design with only one moving component. Consequently, very well suited for transportation means since the power to weight ratio is in the order of thirty times higher than for normal piston engines for automobile transport. The common feature with all combustion processes is the importance of temperature during the combustion. For rocket engines the exothermic effect of combusting the fuel is proportional to the generated thrust. The desire to push performance and efficiency has for long been a driver for increasing maximum operating temperature. Material development giving higher tolerance of substrate operating temperature, active cooling within and on top of turbine airfoils, and insulating thermal barrier coatings reducing the exposure to the hot environment. This PhD work has focused on the development of a new type of thermal barrier coatings (TBCs) to increase the operating temperature of gas turbines to make them more efficient. This new material is Yttrium Aluminium Granite (YAG). Seram coatings uses this type of oxide as a matrix for its first commercial product, ThermaSiC. Therefore this PhD work has focused on understanding this material in detail and at the same time explore the possibilities for exploiting it as a new product for turbine applications (gas or aircraft). This project has shown the potential YAG powders have as a thermal barrier coating. Different approaches for obtaining a successful deposition and a good coating have been explored. Small-sized industrial-supplied powder and larger in-house-made powder have been compared, emphasising the importance of energy used for deposition and crystallinity in the final coating. Highly crystalline material has successfully been produced with F4 atmospheric plasma spray system without post-treatment or substrate heating.

This project can have large effects in the next generation of thermal barrier coatings (TBCs). For long time has Yttria Stabilized Zirconia (YSZ) been an established material for TBCs. The well-known system consisting of an YSZ insulating top coating and a NiCrAlY metallic bond coat developed by the NASA Lewis Research Centre in Cleveland by Stecura and Leibert back in the 1970s is still today of the most used TBC systems with an operational temperature up to 1100 C. However, as demands for higher turbine efficiency keep increasing, new TBC materials with higher operating temperatures and better performance are required. One of the most discussed and severe issues related to the degradation of TBCs is the exposure to calcium–magnesium–alumina–silicates (CMAS) at elevated temperatures. The CMAS are often introduced to the system from the intake air in the form of dust, sand, volcanic ashes or similar. YSZ is proven to be prone to CMAS attacks, and research has been carried out to overcome this. New materials like YAG appear as a promising alternative to YSZ both from a performance perspective in CMAS and similar degradation mechanisms, but also to increase the operational temperatures of turbines. The successful development of YAG as alternative material or new material for turbines will not only bring new knowledge to the field in terms of materials development for such demanding applications, but it opens a commercial opportunity for a company like Seram Coatings to enter this market with a new and disruptive product.

Seram Coatings AS er en oppstartsbedrift med utspring fra NTNU som nå er i ferd med å kommersialisere NTNU-oppfinnelsen ThermaSiC. Kort fortalt er Seram Coatings først i verden med å kunne tilby slitasjebelegg av silisiumkarbid (SiC) som kan termisk sprøytes. Selskapet er imidlertid avhengig av betydelig forskning og utvikling for å komme frem til produkter som kan brukes til flest mulig applikasjoner og som kan påføres med flest mulig metoder innen termisk sprøyting. Dette krever forskning. Spesielt viktig er dette for oss ettersom vi ønsker å ha et produkt som er mest mulig likt SiC (som er 80% av vekten i vårt pulver). De resterende 20 % (foreløpig en oksid) må gi en effekt ved både påføring og i egenskaper ved endelig slitasjebelegg som vi må lære mer om. Det er i tillegg slik at et perfekt pulver fra oss (som jo er vårt produkt) likevel kan gi en håpløs coating om man bruker ikke-optimale sprøytepistoler eller parametre for disse. Derfor forskning. Fokuset er da på målene nevnt tidligere i søknaden, kort oppsummert: Hvordan får man best mulige coatinger/slitasjebelegg basert på SiC ved såkalt termisk sprøyting? Dette med ulike metoder som plasmasprøyting og HVOF. Og i tillegg: Hvilke binders (den delen av pulveret som ikke er silisiumkarbid) gir hvilken effekt på coatingen? Potensialet er skyhøyt. SiC kan erstatte veldig mange ulike slitasjebelegg som er på markedet i dag (markedet inkludert utstyr er på mer enn 7 milliarder USD) og for Seram Coatings som eneste tilbyder av dette p.t. vil prosjektet være avgjørende for å garantere god kvalitet.