The effect of electrodeposition conditions and post-annealing on nanostructure of Co–W coatings

Nanocrystalline Co–W alloys were electrodeposited at 60 °C from citrate–borate electrolyte at pH 6.7 and 8.0. The effect of electrodeposition conditions in the direct (DC) and pulse (PC) current modes on tungsten content in the alloys and their structure was investigated. The tungsten content in the alloys was varied between 13 and 36 at.%, and it defines the grain size of the Co–W coatings irrespectively on the deposition mode or current density allowing us to obtain this particular content of alloy. A grain size of 27–40 nm was obtained for a lower content of tungsten, and 3–6 nm is characteristic for as-deposited coatings containing more than 23 at.% W. The formation of W solid solution in the Co and Co3W phases is discussed. The transition from nanocrystalline to polycrystalline structure of the Co–W coatings with an as-deposited grain size of 3–6 nm was obtained by annealing. In coatings containing 24.5 at.% W, a well developed polycrystalline structure of Co3W appears at 400 °C with {200}, {201} and {220} textures which remain stable up to 1000 °C. The grain size of the coatings containing 24.5 at.% W increases from 3–5 nm up to 26–30 nm after annealing at 400 °C. Whereas, Co–W coatings with a tungsten content of 34.4 at.% maintain their nanocrystallinity at 400 °C, and transforms to the polycrystalline structure at 600 °C with an increase of grain size up to 45 nm.