TFTR Limiter Study: coating adhesion testing and graphite surface preparation

A pull-rod adhesion test was used to evaluate the bonding of chemical vapor deposited coatings of TiC and TiB2 on POCO AXF5Q graphite. The samples were from two different coating suppliers. They were tested in the as-deposited, 2000C annealed and electron beam thermally cycled conditions. All of the substrates were ultrasonically cleaned before coating. All samples (except the tests where the pull-rod adhesive failed) exhibited fracture in the graphite substrate, while no coating/substrate separations were observed. Since proper substrate preparation appears to assure good bonding, a detailed study of an ultrasonic cleaning sequence was made, with characterization of the surface after each step. The quality of the preparation is clearly revealed by visual appearance, an adhesive tape stripping test, and by SEM micrographs of the surface.