Decorative boride coatings based on LaB 6

Abstract

Decorative coatings of lanthanum boride were deposited onto steel and molybdenum substrates employing d.c. magnetron sputter deposition using LaB 6 targets. The characterization of the coatings was achieved by means of scanning electron microscopy, electron probe microanalysis and X-ray diffraction. The results were discussed in connection with application-related properties, such as film hardness and colour determined by Vickers microhardness and CIE- L* a* b* colorimeter measurements. The aim of this work was the investigation of the interrelationships between the chemical composition, microstructure, hardness and colour of the coatings. Sputtering of LaB 6 targets results in the formation of extremely fine columnar films with predominantly (100)-oriented LaB 6 crystals. With increasing process gas pressure, an increase in the (100) texture and a decrease in the lattice distortion were observed. Likewise, the boron-to-lanthanum atomic ratio decreased from 6.8 to 6.5. With increasing argon pressure, a distinct decrease in the brilliance value L* was observed, resulting in a shift in colour from dark violet to almost black. The maximum Vickers microhardness of the coatings was approximately 2900 HV 0.01. The results of the investigations establish the correlations between the stoichiometry, microstructure, hardness and film coloration for decorative coatings based on LaB 6.