OXIDATION-KINETICS OF CU-30VOL.PERCENT-CR COATING

The oxidation kinetics of a Cu-30vol.%Cr coating were studied in air o ver the temperature range from 500 to 700 degrees C. The Cu-Cr coating was deposited on a Cu-15vol.%Nb substrate by co-evaporation of Cu and Cr from a single Cu-Cr composite target in a vacuum chamber. The coat ing microstructure was characterized by scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The coating m icrostructure consists of an intimate mixture of Cr particles in a Cu- rich matrix. The size of the Cr phase was less than one micrometer. Th ermogravimetric analyses show the oxidation kinetics to be parabolic a t all investigated temperatures. The activation energy of oxidation wa s calculated to be 24.8 kcal mole(-1). While the uncoated Cu-15vol.%Nb oxidized rapidly to form external Cu-oxide scales, the Cu-Cr coating surface formed continuous Cr2O3 scales. With the coating, the Cu-Nb su bstrate was protected from oxidation for an extended time.