Microstructural stability and phase transformations in electrodeposited cobalt-phosphorus coatings

Electrodeposited Co-P coatings are a potential replacement for electrolytic hard chrome coatings in critical engineering applications. The superior performance of the Co-P coatings is derived from as-deposited microstructures consisting of metastable crystalline and amorphous phases. These microstructures depend upon the deposition conditions and, in particular, on the P content. In this study, Co-P coatings with P contents of 6.4–28.4 at.% were produced by DC electrodeposition onto low alloy steel coupons. The coatings were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The initial deposit microstructures are either crystalline (<10.0 at.% P), hybrid crystalline/amorphous (10.0–12.1 at.% P) or amorphous (>12.1 at.% P). The thermal signatures within each microstructural class were determined by DSC to identify the temperatures at which transformations occur in these metastable microstructures. The transformation products were then identified by XRD and TEM on samples of the deposits annealed isothermally in the DSC. These data show that: the crystalline deposits transform to an equilibrium mixture of HCP-Co and Co₂P by precipitation of the phosphide from the initially supersaturated HCP-Co; the amorphous deposits crystallize to form a mixture of metastable FCC-Co and Co₂P; and the hybrid deposits transform to a mixture of the products observed for the crystalline and amorphous deposits. The activation energies of the processes involved were obtained by Kissinger analysis on the scan-rate dependence of the peak temperatures in the DSC curves. The transformation mechanisms were then deduced by comparing these values with published values for such systems in the literature. - Highlights: • DC electrodeposited coatings of Co-P grown with up to up to 28.4 at.% P. • Deposits are crystalline, hybrid or amorphous depending upon P content. • Crystalline deposits transform to HCP-Co with Co₂P precipitates on heating. • Amorphous deposits crystallize to microstructures with metastable FCC-Co + Co₂P. • Hybrid deposits show both processes giving 3-phase HCP-Co + FCC-Co + Co₂P.