Comment on ?effect of phosphorus content on the relative proportions of crystalline and amorphous phases in electroless NiP deposits?

Abstract

In a recent paper, Kumar and Nair [1] presented a quantitative analysis of electroless Ni-P deposits with phosphorus concentrations between 4.35 and 9.10 wt %. They performed X-ray diffraction (XRD) experiments on their deposits and analysed the XRD profiles on the basis of a mixture of an amorphous Ni-P phase and fcc nickel. It is the aim of this paper to show that their analysis based only on these two phases is incomplete and incorrect. To show the presence of amorphous Ni-P and fcc nickel Kumar and Nair presented selected area electron diffraction (SAED) patterns of a crystalline phase and an amorphous phase. While the diffuse halo ring pattern shown in one of their micrographs is typical for an amorphous alloy, the spot pattern shown in the other micrograph was reported to indicate a [1 1 1] orientation of fcc nickel. There is, however, strong evidence .that the diffraction spot pattern shown by Kumar and Nair [1] does not show the [1 1 1] orientation of nickel but the [0 0 1] orientation of a well-known metastable hexagonal Ni3P phase. The diffraction pattern shown by Kumar and Nair [1] consists of strong diffraction spots and two weak Debye-Scherrer rings. The ratio of the spacings of these rings is identical to the ratio of the lattice spacings of the {1 11} and {200} lattice planes of an fcc lattice. If we assign these diffraction rings to nickel and use them to calibrate the micrograph, the d-values of the spot pattern can be determined. The d-values of some spots determined in this way and the corresponding values of the a-axis of a hexagonal crystal are listed in Table I. Within the uncertainty of our measurements, all d-values result in the same lattice parameter. Hexagonal Ni-P phases with nearly the same lattice parameter were found for chemically and electrolytically deposited amorphous Ni-P alloys after heat treatment between 270 and 400°C (a =0.672nm) [2], in crystallized thin (20-50nm) films of amorphous NislPx9 (a = 0.656 nm) [3], in the thin ( < 5 0 n m ) regions of melt-spun NislP19 , and in thin regions (< 50 nm) of a crystallized Ni69Cr14P17 alloy (a = 0.690 nm) [4]. Further evidence that the diffraction pattern shown by Kumar and Nair represents a hexagonal phase is given by a comparison with our own measurements on Ni69Cr14P17 , an alloy of similar composition. Selected area electron diffraction patterns of this material in the amorphous state and in thinned regions after heat treatment to 305 °C are shown in Fig. 1 and Fig. 2, respectively. Both patterns were taken with the same camera length printed at identical magnifications. By tilting of the specimen, the lattice parameters of this crystalline phase were determined to be a = 0.690 nm and c = 1.04 nm [4]. The similarity of these two micrographs with those published by Kumar and Nair for Ni-P alloys is evident and supports our assertion that the electron diffraction pattern in [1] represents that of a hexagonal Ni-P phase together with the