טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentRibak Eliezer
SubjectCoating of Graphite and Carbon-Carbon Composite via
Reaction with Cr Powder
DepartmentDepartment of Materials Science and Engineering
Supervisors Dr. Irena Gotman
Professor Elazar Gutmanas


Abstract

Carbon-based materials and especially C/C composites are leading candidates for high temperature airspace applications due to their extraordinary high temperature strength, low density and thermal stability in inert environments. Unfortunately, these materials suffer from inadequate oxidation resistance and require protection when used in air and other oxidizing atmospheres. The main goal of the present research, therefore, was to transform the carbon surface into the much more oxidation resistant chromium carbide via an original Powder Immersion Reaction Assisted Coating (PIRAC) procedure and to characterize the coatings obtained in terms of their microstructure and oxidation behavior.

Towards this goal, graphite and carbon-carbon composite plates were immersed into a Cr powder and annealed at 800-1200oC in sealed stainless steel containers that provide a very low partial oxygen pressure. As a result of the interaction between carbon and Cr atoms, continuous coatings were formed on the graphite and carbon-carbon surfaces. In some cases, a crystalline halogen (iodine) was admixed to the metal powder in order to accelerate the transfer of metal atoms onto the ceramic surface.

Phase identification and microstructure characterizations of the modified surfaces were performed employing x-ray diffraction (XRD), optical microscopy and scanning electron microscopy with chemical analysis (SEM/EDS). The surface of PIRAC treated graphite and C/C composites were found to contain different Cr carbides (Cr23C6, and Cr7C3) and, occasionally, Cr nitride, Cr2N. The coatings were characterized by excellent conformity and strong adhesion to the substrate. The addition of iodine allowed the coatings to infiltrate into the substrate pores and to 'seal' the graphite and C/C surfaces from in service oxygen penetration.

Depending on the processing temperature and duration, layers of different thickness up to several tens of microns were formed on the carbon surfaces. Kinetic analysis revealed that the coating grew according to the parabolic law typical of a diffusion controlled process. Oxidation tests conducted in open air at 800-1200oC revealed a much better oxidation resistance of PIRAC coated graphite and C/C composites compared to their uncoated counterparts.

The results of the research suggest that the developed simple PIRAC coating procedure could be considered for protecting graphite and C/C composites against oxidation at elevated temperatures. PIRAC coatings grown can also serve as diffusion barrier layers preventing the interaction of carbon fibers with metallic matrices.