Results and Characterization of Electroless Nickel Phosphorus Alloy. The micro-structure of freshly deposited EN films seems to be independent of the

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1 4.5 Structure of Electroless Nickel Deposits The micro-structure of freshly deposited EN films seems to be independent of the nature of the substrate and thickness of the EN deposit (Goldenstein et al., 1957). The structure of the as-deposited (freshly deposited) electroless nickel coatings have been a controversial subject since the discovery of EN process (Mai et al., 1998). Recently, much attention has been given to the corrosion resistance of Ni-P coatings and their characteristics including the content and distribution of phosphorus as well as surface morphology and structure (Salvago and Fumagalli, 1987; Salvago et al., 1981, Ratzker et al., 1986). Ni-P coatings are easier to passive (the formation of a thin, transparent oxide film which blocks the acid penetrating into the bulk alloy and thus deter the corrosion rate) than pure nickel in acidic environments owing to the presence of phosphorus alloyed with nickel. The phosphorus content of coatings seemed to play an important role in the passivity. However some researchers indicated that the structure of a coating (e.g., crystallinity) was another important factor affecting the properties of the coating (Duncan, 1996, 1982). Recently, the characterization of microstructure of surface coatings has been well documented (Schesinger et al., and Lin et al., 1991). Many researchers have reported that as-deposited Ni-P is crystalline provided the phosphorus is within a composition range of 1-3%, whereas amorphous structure is displayed for phosphorus content higher than 10 wt% and semiamorpoorus or amorphous with microcrystallines is given for 4-8wt%P. However, as we known, seldom research work takes part in the surfactant s influence on the EN film microstructure. In the present study, three kinds of surfactants, nonionic (TGT 15- S-12 at 5ppm level), anionic surfactant (SDS at 5 ppm level) and cationic surfactant (CTAB at 5ppm level) are used in the EN plating bath in order to investigate the effect of Surfactants on structure of electroless nickel phosphorus deposit. To investigate 68

2 their structural changes induced by heating, EN deposits are isothermally heat treated for one hour at different temperature of 200, 300 and 400 C. Figure 4.9 shows the typical diffraction diagram of EN plates in which various surfactants used in the plating bath before undergoing heat treatment. Peak intensity is plotted versus diffraction angle. For the sake of clarity the XRD profiles were shifted arbitrarily on the vertical scale. Despite the hindrance, due to the large sharp substrate peaks, to definite characterization of the Ni-P layers, the results clearly reveal a considerable broadness of diffraction line in the angle range 2θ = corresponding to Ni(1 1 1) plane which is the characteristic of amorphous nickel for all Ni-P deposits. Broadening of XRD lines is associated with small particle size of the coherently diffracting crystallites or strains present within the film, or both. It is agreed with the studies on high phosphorus deposits from other studies (Martyak, 1994; Ma et al., 1988, Vafaei-Makhsoos et al., 1978; Guo et al., 2003). The two weak auxiliary peaks which are the reflection peak from the brass substrate. The XRD patterns of Ni- P deposits and that of brass substrate are compared in Figure 4.9. The spectra of all the as-deposits Ni-P are same. However, the XRD spectrum of Ni-P deposits shows more prominent effect due to the less amorphous phase in the deposits. Figure 4.10 indicates that XRD spectra of the plates after heat treatment for 1 hr at 200 C. It is obviously seen that the micro structures of deposits in which surfactants are added in the plating bath are remain the same but the broaden peak of deposit without using surfactant become stronger than before undergoing heat treatment. Similarly, the same microstructure can be seen after annealing at 300 C for one hour as shown in Figure The deposit without using surfactant increase in intensity of 69

3 broaden peak. It may be further loss of amorphous phase in the deposits. It can be observed that surfactant have some effect on the microstructure of Ni-P deposits. When NiP deposits are heat-treated for 1 hr at 400 C, their structures undergo modification. Many research have been investigated that heat treatment of the asdeposited coatings causes a transformation from a supersaturated solid solution of phosphorus in nickel to a nickel matrix plus Ni 3 P (Lambert and Duquette, Lin and Lai, 1989). It has been reported that different heating conditions also have shown significant influences on both the microstructural properties and crystallization behaviors of the EN deposits (Keong et al., 2001). As a result of solid state diffusion, the structures will revert to the thermodynamically most stable state. The amorphous deposits undergo a crystal growth process, and such heat treatment results in a mixture of relatively coarse-grained metallic nickel together with intermetallic phase.a marked increase in grain size and the formation of Ni 3 P compound was also observed. The peak intensities of spectra are changed by the use of different surfactants as shown in the Figure 4.12 (a), (b), (c), and (d). A large number of reflection peak emerged indicating the presence of another phase. According to the international Centre for Diffraction Data (ICDD) card file, two peaks are indexed as those of the nickel phase [(Ni(1 1 1) and (Ni (2 0 0).The spectra for the as-deposited coating also consists of Ni 3 P [{2 3 1}, {3 3 0}, {1 1 2 }, {2 4 0}, {1 4 1 }, {2 2 2}, {1 3 2 }, etc.] Some study indicated that the electroless nickel coating form Ni 7 P and Ni 5 P 2 as intermediate phase during heat treatment, and then these phase eventually transform to the Ni 3 P phase (Cziraki et al., 1980). The Ni(1 1 1) peak was relatively narrow and more intense than other Ni and Ni 3 P peaks for the as-deposited Ni-P except with the use of SDS in the plating bath,revealing a very strong preferred orientation in that direction. Figure

4 (a) shows XRD spectrum of the Ni-P deposit without using any surfactant in the plating bath. From this figure, it can be seen that Ni {1 1 1} peak is strongest and there are a small amount of a second phase Ni 3 P, is detected. The formation of Ni 3 P phase in this sample is weaker than that of Ni phase. It may be influence of initial concentration of phosporus in the deposit. Initially, the reference deposit may have lower phosphorus concentration. Therefore, there may be little chance to form Ni 3 P when heat treated and it shows strong formation of Ni {1 1 1} crystal. Figure 4.12(b) shows the XRD spectrum of Ni-P deposits using 5 ppm CTAB in the plating bath. It can be seen that Ni 3 P {2 3 1} and Ni {3 3 0} peaks become sharper and strengthen than reference sample. The change in peak intensity of Ni and Ni 3 P are also observed for the deposit using TGT 15-S-12-5 ppm in the plating bath as shown in Figure 4.12(b). Ni {1 1 1} peak for the deposit using TGT14-S-12 is stronger than that of using CTAB..The preferred orientation of nickel matrix is reduced for using CTAB and TGT 15-S-12 compared with reference sample. However, a strong {1 1 1} orientation was still observed. Figure 4.12(d) demonstrates that the diffraction peak of Ni {1 1 1} decrease and reduce grain size while Ni 3 P phase increases for the deposit using SDS 5 ppm. The change in relative intensity of this peak indicates the development of a preferred orientation in EN coating. It can be observed that the addition of surfactants have influenced on the structure of EN coating according to the structure of surfactant and the preferred orientations of the deposits were also affected by heating process. 71

5 SDS (5ppm) TGT 15 -S-12 (5ppm) CTAB (5ppm) Reference Brass Substrate substrate Ni (1 1 1) Figure 4.9 XRD Spectra of Ni-P Deposits and Brass Substrate before Annealing 72

6 SDS (5ppm) relative intensity(counts) TGT15-S-12 (5ppm) CTAB (5ppm) Reference substrate Ni( 1 1 1) Figure 4.10 XRD Spectra of Ni-P Deposits after Annealing at 200 C for 1hr 73

7 SDS (5 ppm) relative intensity(counts) TGT15-S-12 (5ppm) CTAB (5ppm) Reference substrate Ni (1 1 1) Figure 4.11 XRD Spectra of Ni-P Deposits after Annealing at 300 C for 1 hr 74

8 Ni 3 P (2 3 1) (3 3 0) (2 4 0 ) (2 0 2) (2 2 2) ( 1 3 2) Ni Substrate (a) Reference Ni 3 P (2 3 1) (3 3 0) (2 0 2) (2 4 0 ) (1 4 1 ) (2 2 2) ( 1 3 2) Ni Substrate (b) TGT 15-S-12 (5 ppm) 75

9 (2 3 1) Ni 3 P (3 3 0) (2 4 0 ) (2 0 2) (1 4 1 ) (2 2 2) ( 1 3 2) Ni Substrate (c) SDS (5 ppm) Ni 3 P Ni (2 3 1) (3 3 0) (1 41) (2 0 2) (2 2 2) ( 1 3 2) Substrate (d) CTAB (5 ppm) Figure 4.12 XRD Spectra of Ni-P Deposit after Annealing at 400. C for 1 hr 76