X-ray diffraction technique was used to follow Ni-P and Ni-P-Re coatings structure and its relation with the chemical composition. Diffractograms of the as plated coatings are shown in Fig.1, presenting two types of relation: first-how the coating structure changes with increasing phosphorus level(Fig.1a) and second-is there any difference in structure, when some amount of rhenium is included in the layer(compare Fig.1a and 1b). It should be noted, that the peaks of copper are associated with the signal coming from the substrate underneath the coating. The reason of application of this particular substrate was dictated by the potential application of the coatings, which was mentioned above. The copper substrate, which has been initially prepared by rolling at ambient temperature is characterized by strong(200) crystallographic texture. This is the reason that the diffraction pattern has an intense reflection at 2theta of about 59. Thus, the Ni-P-Re layer is eventually deposited on a highly anisotropic material from the microstructural point of view. XRD results collected in Fig.1a allow compare the structure changes of the coatings, when the phosphorous level rises. The largest difference can be observed with the phosphorus content change from 5.2 wt% to 7.7 wt%. These results clearly indicate a change from crystalline microstructure (clearly observed for composition of 3.2 wt% of P) into partly amorphous one, which stays in agree-ment with the structure of Ni-P layers reported by Keong et al. It is, however, difficult to explain such observations, when comparing these XRD data to the equilibrium phase diagram.
The Ni-P equilibrium phase diagram given by Okamoto shows only two phases for concentra-tions of P below 15 wt%, which is the case of electroless Ni-P plating like those studied here. So,a mixture of these two crystalline phases: the solid solution(x), consisting in less than 0.17 wt% of P, and NisP which contains 15 wt% of P can be expected. However, electroless nickel coatings in as-deposited state are not in their equilibrium state. There-fore, the equilibrium phase diagram fails in their case, when from particular level of phosphorus, the amorphous state is observed. The nonequilibrium phase diagram was first only theoretically predicted by Moyseyev , and later Duncan, based on the experimental data, showed that when phosphorus content changes from 1 to 13 of weight percentage, the structure varies from pure crystalline p phase (a crystalline solid solution of phosphorus in nickel containing up to 4.5 wt% of P) to totally amorphous y phase(11-15 wt% of P). Therefore, between4.5 and 11 wt% of P the mixture of p and y phases is observed. Based on the presented diffractograms(Fig.1a) it can be confirmed that coating of 5.1 wt% of P are crystalline, while for the layers of higher amount of P(7.7 wt% P), it is difficult to unambiguously state they are the mixture of amorphous and crystalline phases.
An important and new observation is that the same tendency can be evidenced for Ni-P-Re layers(Fig.1b), where the crystalline character of the coating with the composition of Ni-4.8P-7.6Re changes into amor-phous when the content reaches 7.3 wt% of P and 4.4 wt% of Re.
Fig.1. XRD spectra of freshly deposited Ni-P(a) and Ni-P-Re(b) coatings.A change of color from red to blue defines an arbitray "boundary"between the coatings in amorphous and crystalline states, respectively.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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