抄録
Vapor-deposited coatings have become increasingly important as heat- and corrosion-resistant materials. However, the columnar growth morphology in vapor-deposited films frequently causes a pitting-type corrosion, making vapor deposition less attractive in its application to the production of corrosion-resistant coatings. The present study aimed at clarifying the relationship between coating microstructure and corrosion resistance. Molybdenum films of 5-20μm thickness were ion-plated onto iron substrates. A change in the microstructure of the films with the temperature of the substrate could be explained by the Movchan-Demchishin model. The films became dense and porous at around 800°C, and changed to equi-axed structures at temperatures of about 900°C or higher, in which range the number of pores was found to decrease markedly. Corrosion resistance was evaluated both by a modified CASS test and by weight loss measurements of the coated specimens. The results indicated that the corrosion resistance of the coating varied markedly with the number of pores in the film. The high-temperature deposits (800-1000°C) exhibited excellent corrosion resistance due to the formation of an almost pore-free layer. The influence on corrosion resistance of other deposition parameters such as the substrate bias voltage, coating thickness and angle of vapor incidence was also discussed. A preliminary experiment on the alternate evaporation of Mo and Al revealed that a lamellar film structure resulted in reduced columnar growth morphology.
