The Structural and Morphological Characterization of Electrodeposited Cobalt Tungsten Alloys

Traditional hexavalent hard chrome coatings produced from chromium trioxide are providing reasonable corrosion and wear resistance, which recognised as a carcinogen and its use controlled by reach legislation. Many researchers have investigated as the ability of Mo and W to codeposit with iron group metals (Ni, Fe, Co) to form crystalline and amorphous compounds and more recently, work has shown that nano-crystalline coatings may be produced with high hardness.

In this thesis, Co-W electrodeposited coatings are investigated structurally, compositionally, and morphologically. Series of Co-W alloy coatings with variable structures and composition were obtained using a gluconate bath and a soluble Co anode at 80 °C temperature and 2.7 Adm-2 current density with air agitation.

Some of these samples at the same compositions illustrate at different preferred growth directions of (100), (101), (002) and (110) and these differences occur due to variations in agitation. SEM/EDX and XRD and microhardness were employed to characterise the coatings and how they differ across the samples due to edge effects. Variation in W content of 2 – 3 at.% changes in W content of the coatings affect the hardness significantly up to around 50%.

Coatings were selected for heat treatment process based on composition and preferred orientation. The effect on the structure and hardness of the coatings after heat treatment at 600 °C for the duration up to 160 hours was explored using XRD and SEM. Two of 20 at.% W electrodeposited coatings at (100) and (101) preferred growth direction examined on TEM as electrodeposited and after 2 hours heat treatment by sample preparations of ion beam vertically and horizontally to the coating in the cross section using nanotechnological developments. The wear behaviour some of these samples examined under dry sliding conditions.