As the next step, the Ni-P and Ni-P-Re coatings crystallization temperatures were determined using differential scanning calorimetry technique. The values of crystallization temperature for investigated coatings were collected in Table 1 and also presented in Fig.1(in this Figure-for the sake of statistics-more data points is shown). It is evidenced that the increase of temperature of crystallization for Ni-P and Ni-P-Re layers is associated with modifications of the chemical composition. Due to the fact that the experiment included production of Ni-P coatings in the same plating conditions as Ni-P-Re (except of presence of rhenium source), the results obtained for Ni-P can be treated as a reference. From the report of Sohn et al.it was found that the changes of the phosphorus level contribute to the large changes in crystallization temperature.
The series of obtained compositions of Ni-P coatings obeys three groups with respect to the phosphorus content: low (less than 5 wt% of P), medium(6-9 wt% of P) and high (above 10 wt% of P). In the case of the lowest phosphorus containing coating (3.2 wt%), the main peak was observed at 423℃. XRD spectra obtained for this coating after DSC experiment revealed that this effect is associated with the crystallization of the NigP stable phase from the Ni solid solution. As for the samples with medium (5.1 and 7.2 wt%) and high (11.7 wt%) phosphorus content the main exothermic peak occurred in the temperature range of411-345℃, and its position depends on the content of phosphorus reaching the highest values for low content of P. This peak is correlated to the transformation of the Ni-P matrix to the Ni and NisP crystallites. Similar observations are also presented in the literature. Taheri has been studied low phosphorus Ni-P coating with 4.5 wt% content of P and in his examination the peak of the NisP phase crystallization was observed to appear at 403℃.
It should be noted that the heating rate in was of 20℃/min, while here for 3.2 wt% P the crystallization temperature was of 423℃, revealed with the heating rate of 25℃/min. Additionally, Sha et al.investigated a medium(5,6 and 8 wt% P)and high (12% wt.P) phosphorus containing Ni-P electroless coatings.The crystallization peak related to the creation of Ni5P phase and the Ni crystallites was in the range of 388-406℃ for the medium P containing layers(in this study for 5.1 wt% P the temperature was of 411 ℃,332-362℃(in this study for 7.2 wt% P-at 375℃). But for the coating with 12% wt. of P the temperature was 350℃, while in this study for 11.7 wt% of Pit was 345℃.Obtained results are in agreement with those described by Martyak and Drake for medium-phosphorus (5-6 wt% P) Ni-P coatings, where the crystallization peak was observed at 397 ℃(5 ℃/min), comparing to the one obtained in this study located at 411℃.
The shifts between the acquired crystallization temperatures and presented in the literature are caused mainly by two factors: some discrepancies in the coatings chemical composition and differences in the used heating rates in DSC. The impact of these factors was described in detail in previous studies .
Table 1 Chemical compositions of the coatings Ni-P and Ni-P-Re produced in electroless plating process on copper versus their thermal stability.
Fig.1. Crystallization temperature versus phosphorus content for Ni-P(blue)and Ni-P-Re(red) coatings. Rhenium content in Ni-P-Re alloys varies in the range of 2.5-7.0 wt%.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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