Diffusion barrier coatings for graphite, C/C and C/SiC racks in vacuum heat treatment orhigh temperature brazing processes

摘要

In order to overcome the disadvantage of local carburizing of steel components in contact with light-weight graphiteor carbon fiber reinforced ceramic racks alumina based thermal spray coatings are produced as diffusion barrierswith improved life time compared to rapidly degrading alumina or boron nitride pastes. The powder flame sprayedcoatings are also capable to prevent damage by excess filler material in high temperature brazing processeseffectively. Besides graphite also C/C racks are coated with pure alumina, Al2O3-TiO2 and Al2O3-Cr2O3.Conventional powder flame spraying is applied in order to provide a low-cost solution for realization of diffusionbarriers. Coatings are characterized by means of optical microscopy and SEM with regard to the interface to thesubstrates and their porosity. Coated racks are used in field tests for case hardening of steel components. The lifetime of thermal spray coatings is compared to alumina and boron nitride based pastes. Comparative liquid metalcorrosion tests are carried out with NiCr7Si4.5B3.1Fe3 filler at 1,050 °C.