Thermally sprayed diffusion barrier coatings on C/C light-weight charging racks for furnace applications

Charging racks are used for the positioning and transport of workpieces in heat treatment and high-temperature brazing processes. Conventionally, they are made of steel or cast iron, but in recent years, more and more companies have changed over to charging racks made of light-weight materials such as graphite or carbon fiber-reinforced carbon (C/C). However, the main problem of these carbon-based workpiece carriers is the undesired carburization of components, which are in contact with the rack, by means of diffusion processes. For that reason, within the scope of the presented work, thermally sprayed coatings were applied on C/C and extensively tested for their suitability as high-temperature diffusion barrier. The ceramic powders Al2O3, Al2O3/Cr2O3, Al2O3/TiO2 and ZrO2/Y2O3 were chosen as coating materials and processed by powder flame spraying (PFS) and atmospheric plasma spraying (APS). Molybdenum and silicon carbide served as adhesion-promoting intermediate layers. Coating characterization included materialographic analyses, tensile strength and tribometric tests as well as long-term high-temperature tests, both laboratory-scaled and in industrial practice. Especially Al2O3 and Al2O3/Cr2O3 coatings on siliconized C/C exhibited excellent results, proving the wide application potential of thermally sprayed ceramic coatings on carbon-based light-weight materials.