Microstructure evolution and mechanical performance of tetrahedral amorphous carbon coatings with dense droplets during annealing

Abstract

Tetrahedral amorphous carbon (ta-C) coatings with approximately 50% sp3C were deposited onto cemented carbide alloy YG6 and Si wafers via direct vacuum cathode arc (DVCA) evaporation at different substrates temperatures. Coated specimens were annealed in a 5 Pa argon atmosphere. The micromorphology and chemical bonding structure of these coatings with different treatments were characterized using field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The mechanical performance was tested using a nanoindenter and film stress instrument. The obtained ta-C coatings contained dense droplets of various sizes (“abnormal particles” (An-Ps)). The densities of the An-Ps were controlled by the graphite cathode target arc rather than the substrate temperature, and their ID/IG ratios were close to their counterpart coating matrix. Increasing the substrate temperature was beneficial for increasing the sp2C fraction and the number of aromatic ring sp2C sites. Growth, coalescence, and leveling-diffusion of aggregates in the coating matrix and An-Ps occurred during annealing, which was related to the existing state of the sp2C sites, including the olefinic chain and aromatic ring. The residual stress variation with annealing temperature also depended on the sp2C state. The residual stress in the coating increased during annealing when the number of aromatic ring sp2C sites exceeded a certain critical value.