Combined Diffusion Barrier and Wear Resistant Thermal Spray Coatings on Light-Weight Charging Racks in Furnace Applications

Conventional charging racks of industrial furnaces for high-temperature applications are commonly made of temperature resistant metal alloys. Due to materials creeping at elevated temperatures these racks often show a short service life. Therefore, more and more creep and temperature resistant carbon and carbon based materials as well as ceramic matrix composites are used in charging rack applications. The hence resulting size accuracy in combination with the lower weight of these materials allows new possibilities in the mechanical charging of furnaces. However, especially the carbon based racks may cause severe carburisation of component within their area of direct contact. An appropriate solution for this problem is the application of thermally sprayed diffusion barrier and protection coatings, which also reduce mechanical wear and so additionally increase the service life of the charging racks.

The present contribution shows updated results in this matter. Investigation subjects are C/C substrates coated with alumina or molybdenum. The component material to be set onto the rack surface is steel. In the application of coated racks, chemical reactions and diffusion occur between substrate and coating, coating and component as well as substrate and component. To evaluate visible material and property changes in these zones (lowered thermal resistance, crack formation and delamination, material spill off), software aided calculations of the chemical equilibrium of the materials in contact for temperatures between room temperature and 2500 K are done. Other results answer to questions concerning residual stresses within the coatings and contact zones according to chemical and geometrical parameters. The contribution end with recommendations for the design and application of light-weight carbon based charging racks for high-temperature furnace processes.