- Email: [email protected]
Peak-profile analysis of electroless nickel coatings
Journal of Alloys and Compounds 312 (2000) 30–40
L
www.elsevier.com / locate / jallcom
Peak-profile analysis of electroless nickel coatings Nicholas M. Martyak*, Kerry Drake Atochem North America, 900 First Avenue, King of Prussia, PA 19406, USA Received 21 February 2000; accepted 18 May 2000
Abstract Peak-profile analysis (PPA) of X-ray diffraction patterns and differential scanning calorimetry of electroless nickel (EN) coatings were done to determine the changes in the structure with annealing. As-deposited medium phosphorus EN coating exhibit broadened X-ray reflections indicative of a semi-amorphous structure. Heat-treatment decreased the amorphous phase which contributed to broadening of the reflections. A small exotherm was seen in the temperature range where the amorphous phase decreased and the crystalline component increased. No amorphous component was detected above 3008C. Changes in particle size did not occur until the annealing temperature exceeded 3008C. The strain in the EN coatings decreased sharply after 2008C and was negligible after 4008C heat-treatment. 2000 Elsevier Science B.V. All rights reserved. Keywords: Electroless nickel; Peak profile analysis; Particle size; Strain; Heat treatment; X-ray diffraction
1. Introduction Electroless nickel (EN) coatings have gained wide acceptance since their discovery by Brenner and Riddell [1] in the middle part of this century. The coatings are found in many market segments such as automobile, electronics and the food industries [2]. This diverse market penetration is due in part to the unique properties of these coatings. As in many advanced materials, the properties are highly dependent upon the microstructure of the film which in turn is dependent upon the composition of the deposit. EN coatings are not pure nickel deposits but rather alloys of nickel and a metalloid such as phosphorus or boron. Because of the low solubility of metalloids in a nickel matrix, these coatings are supersaturated solutions of either phosphorus or boron in nickel [2,3]. This low solubility of the metalloid drastically alters the microstructure of the metal coating. For example, low concentrations of phosphorus in nickel result in a microcrystalline coating whereas medium levels of phosphorus yield coatings that contain both crystalline and amorphous components. High concentrations of occluded phos-
*Corresponding author. E-mail address: [email protected] (N.M. Martyak).
phorus produce EN deposits that are completely amorphous. The properties of EN films are dependent upon the microstructure of the coatings [4]. Low phosphorus films are required where hard, wear-resistant coatings are specified. The hardness of these films is considerably greater than pure nickel. Non-magnetic nickel coatings are required on magnetic disk drives prior to cobalt sputtering. Therefore, the occluded phosphorus must be high in these deposits. Many EN coatings are subjected to elevated temperatures which results in a concomitant change in the structure and properties of these deposits [2]. X-ray diffraction is widely used to determine the structure and composition of materials. Diffraction patterns contain information showing various phases of a material and residual stresses within a coating. Broadening of XRD lines is associated with small particle size of the coherently diffracting crystallites or strains present within the film, or both [5]. Warren and Averbach [6] showed the microstrain component on line broadening can be separated from the particle size contribution since the latter is independent of the order of the reflection. Hall and Williamson [7] also discussed the broadening of XRD lines from particle size and microstrain contributions. Howard and Snyder [8] and Yau and Howard [9] discussed the use of real space variables rather than Fourier transforms as used in the
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01099-9
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Warren–Averbach method to study XRD profiles. There have been numerous XRD studies investigating the changes in microstructure of EN coatings with annealing [10–12]. Most agree the XRD lines sharpen with lower phosphorus content and after annealing. Sampathkumar and Nair [13] studied XRD profiles of EN deposits containing 5.5% phosphorus and concluded the total XRD profile was composed of crystalline and amorphous components. Annealing from 60 to 3008C changed the ratio of the integrated intensities of the amorphous, Iamp , to the intensity of the crystalline phase, Icry . Sadeghi et al. [14] determined the size of coherently diffracting crystallites in EN coatings from broadened XRD patterns. However, they assumed no strain in their calculations. Mai et al. [15] used pole figure analysis to monitor texture development in low, medium and high phosphorus EN coatings with annealing. This study was done to understand the changes in the XRD patterns of EN deposits with annealing temperature. Peak profile analysis (PPA) of the XRD lines showed changes in the relative amorphous and crystalline phases as well as variations in particle size and strains with annealing temperatures.
2. Experimental EN deposits were plated from a medium phosphorus EN solutions operated at 908C. The pH of the EN solution was 4.8. Mild air agitation was used during nickel plating. Low carbon steel panels were cleaned in a sodium hydroxide:sodium gluconate solution at 4.0 V for 30 s at 608C. This was followed by water rinses then immersion in 10% HCl for 5 s. The panels were plated for 90 min to deposit an EN coating between 25 and 30 mm. After plating, the samples were doubly rinsed in distilled water and dried. Samples were annealed from 100 to 5008C for 60 min. X-ray diffraction patterns were acquired on the as-deposited and annealed samples using a Philips APD 3720 instrument with variable slit optics. Scans were done from 30 to 1008 using a 0.01 o step scan. The Kb was removed using a nickel filter. The diffraction patterns were analyzed using computer programs [16]. A convoluted algorithm combining an instrumental broadening and Lorentzian function was used in the curve fitting analysis. The patterns were corrected for instrumental broadening and shifts using a well crystallized LaB 6 sample, NIST SRM 660. Peak-profile analysis for particle size and microstrain was calculated from:
b cos(u ) 5 57.296( l /t 1 4e 57.296 sin(u )) where b is the integral breadth of the profile, t is the crystallite size, 2u is the diffraction angle, e is the microstrain component and l is the wavelength of the
31
incident radiation. A plot of b cos(u ) vs. sin u yields a straight line. The microstrain is calculated from the slope of this line and the particle size from the ordinate-intercept. The background was subtracted from each pattern. Differential scanning calorimetry was done on the EN sample using a TA 910 DSC cell. The EN deposit was first plated on a copper substrate so the thickness of the coating was about 20 mm. The copper substrate was removed by floating the foil on a solution containing 500 g / l chromic acid and 50 ml / l sulfuric acid. Once the copper dissolved, the EN foil was triply rinsed in distilled water and dried. The DSC scans were done under nitrogen at a scan rate of 58C / min.
3. Results and discussion X-ray diffraction of as-deposited EN containing 5–6 w / w% phosphorus is shown in Fig. 1a. Well defined (111) and (200) reflections are seen. On the low angle side of the (200) appears some skewing which may be indicative of stacking faults in this coating [17]. PPA of the pattern required an amorphous component to be added to the crystalline pattern. This amorphous line was located between the (111) and (200) reflections, centered at about 49.58. Small (220), (311) and (222) reflections are seen at higher 2u values. The five reflections were fitted using the instrumental broadening and Lorentzian functions to separate the peak broadening due to inherent instrumental factors (optics, X-ray source) and those due to sample characteristics. This peak broadening observed in these samples appears to be dependent upon both the small particle size and the strain in the coating as seen in Fig. 1b. The size of the coherently diffracting EN crystallites and the strain is seen in Table 1. Annealing to 1008C sharpened the (111) and (200) reflections slightly as seen in Fig. 2a. The (200) appears to grow at the expense of the (111) manifested by the changes in the relative intensities of these two reflections compared to those in Fig. 1a. The amorphous component
Table 1 Effect of annealing on crystallite size, strain and amorphous phase EN coating
Size of EN crystallites (nm)
Strain310 23
Amorphous component %
As-deposited Annealed 1008C Annealed 2008C Annealed 3008C Annealed 4008C
4.8 5.0 4.1 4.4 24.6 1129 a 61.9 1715 a
8.07 7.75 0.06 21.9 0.12 3.2 a 0.07 1.78 a
22.4 19.6 14.9 9.7 0
Annealed 5008C a
Size and strain for Ni 3 P reflections.
0
32
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Fig. 1. (a) XRD pattern of unannealed EN coating. (b) Size and strain analysis for as-deposited EN sample.
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Fig. 2. (a) XRD scan for EN deposit heat-treated at 1008C. (b) Size and strain fit for 1008C annealed sample.
33
34
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Fig. 3. (a) XRD pattern of 2008C annealed sample. (b) Size and strain fit for 2008C annealed sample showing particle size controlling broadening. (c) DSC trace showing exotherms during annealing.
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
35
Fig. 3. (continued)
was again necessary to obtain an adequate fit to the data. The crystallite size and strain were about the same as that in the as-deposited sample. The size and strain fit curve seen in Fig. 2b appears to indicate the crystallite size may be influencing peak broadening more than the strain in the coating. Continued annealing to 2008C shows a further strengthening of the (200) reflection and a smaller (111), Fig. 3a. The amorphous component was also smaller than that seen in Figs. 1a and 2a. Refinement of the pattern showed the size and not the strain component controlled the broadening of the reflections as seen in Fig. 3b and Table 1. There appears to be no strain in the EN deposit after annealing to 2008C. DSC of the EN coating shows a small exotherm about 1728C, Fig. 3c. It is likely the reduction in strain is associated with microstructural changes occurring in the EN coating between 100 and 2008C. The amorphous phase in the EN film annealed at 2008C is almost one-half of that seen in the as-deposited coating.
After annealing at 3008C, the (200) is considerably greater than the (111) and the amorphous phase contributes only about 10% to the total broadening, Fig. 4a. Unlike the work of Sampathkumar and Nair [13] who showed the amorphous phase was eliminated after a 2008C anneal, there is a noticeable amorphous phase present in these coatings after the 3008C anneal. The crystallite size again dominates the broadening effect as seen in Fig. 4b and the strain is slightly negative. There are studies which show various metastable nickel structures in EN coatings during low temperature anneals. Cziraki et al. [18] examining the crystallization of EN coatings found a Ni 5 P2 metastable phase during annealing which subsequently transformed to Ni 3 P with further annealing. Pittermann and Ripper [19] using high energy electron diffraction confirmed the existence of a metastable Ni 5 P2 or Ni 2.55 P structure prior to the formation of the stable Ni 3 P. The DSC trace seen in Fig. 3c shows a small exotherm at 338.48C which may be due to the formation of these metastable phases. The formation of such intermediates after annealing to 3008C in
36
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Fig. 4. (a) XRD pattern of 300QDC annealed sample showing sharpening of reflections. (b) Size and strain fit for 3008C annealed sample–size control broadening.
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
37
Fig. 5. (a) XRD scan for EN deposit heat-treated at 4008C showing onset of nickel phosphide precipitation. (b) Size and strain fit for 4008C annealed sample–nickel reflections only. (c) Size and strain fit for 4008C annealed sample–nickel phosphide reflections only.
38
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Fig. 5. (continued)
these coatings may relieve the strain prior to the onset of Ni 3 P precipitation. Annealing at 4008C resulted in the precipitation of nickel phosphide, Ni 3 P, as seen in Fig. 5a. No amorphous phase was needed to accurately curve fit the reflections. PPA of the nickel phase showed any broadening was a result of the crystallite size and not any residual strain effects, Fig. 5b. PPA of the nickel phosphide phase also showed the crystallite size of Ni 3 P was considerably greater than that of metallic nickel, Fig. 5a and c and Table 1. There was also a strain component to the nickel phosphide crystallites. This strain may be due in part to the precipitation hardening mechanism whereby the nickel and nickel phosphide phases are coherent until precipitation occurs, similar to that observed in the Al–Cu system. During precipitation of the phosphide phase, coherency between the nickel and the nickel phosphide crystallites is lost. The 5008C anneal intensified the nickel phosphide reflections slightly as seen in Fig. 6a. The strongest reflections were those of metallic nickel. PPA of both the nickel and nickel phosphide reflections showed a slight increase in crystallite size for both phases, Ni8 and Ni 3 P. There was no strain associated with the metallic nickel
phase and the strain observed at 4008C in the nickel phosphide phase was almost negligible after the 5008C heat-treatment, Fig. 6b and c. The changes in crystallite size and strain with annealing temperature is seen in Table 1. The size of the nickel crystallites does not increase to any appreciable extent up to 3008C. However, there is a sharp decrease in the strain component with temperature and the strain becomes compressive at 3008C. Thus, the sharpening of the (111) and (200) reflections seen in Figs. 1a–4a is due to the elimination of strain within the coating and not an increase in the crystallite size.
4. Conclusions The amorphous and crystalline components of the EN coatings changed with heat treatment. There is a decrease in the amorphous phase with an increase in heat treatment temperature up to 3008C. Beyond 3008C, no amorphous component was detected in the EN diffraction patterns. Broadening of the XRD reflections is due mainly to the crystallite size component and not the strain. Annealing
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
39
Fig. 6. (a) XRD scan for EN deposit heat-treated at 5008C showing growth of nickel and nickel phosphide crystallites. (b) Size and strain fit for 5008C annealed sample–nickel reflections only. (c) Size and strain fit for 5008C annealed sample–nickel phosphide reflections only.
40
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Fig. 6. (continued)
temperatures greater than 3008C result in the precipitation of nickel phosphide.
References [1] A. Brenner, G. Riddell, J. Res. Nat. Bur. Std. 37 (1946) 31. [2] G.O. Mallory, J.B. Hajdu, Electroless Plating, AESF, Orlando, FL, 1990. [3] N.M. Martyak, S. Wetter, L. Harrison, M. McNeil, R. Heu, A.A. Neiderer, Plating and Surface Finish (1993) 60. [4] R. Weil, J.H. Lee, K. Parker, Plating and Surface Finish (1989) 62. [5] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publ., Reading, MA, 1978. [6] B.E. Warren, B.L. Averbach, J. Appl. Phys. 21 (1950) 595. [7] W.H. Hall, G.K. Williamson, Acta Met. 1 (1953) 22.
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
S.A. Howard, R.L. Snyder, J. Appl. Cryst. 22 (1989) 238. J.K. Yau, S.A. Howard, J. Appl. Cryst. 22 (1989) 244. K. Parker, Plating and Surface Finish (1981) 71. M. Lambert, D. Duquette, Thin Solid Films 177 (1989) 207. S.H. Park, D.N. Lee, J. Mat. Sci. 23 (1988) 1643. P. Sampathkumar, P.K. Nair, Prog. Thermal Treat. Mat. (1996) 169. M. Sadeghi, P.D. Longfield, C.F. Beer, Tran. Inst. Metal Finish 61 (1983) 141. Q.X. Mai, R.D. Daniels, H.B. Harpalani, Thin Solid Films 166 (1988) 235. Materials Data Inc., Livermore, CA, 1994. A.H. Graham, R.W. Lindsay, H.J. Read, J. Electrochem. Soc. 112 (4) (1965) 401. A. Cziraki, B. Fogarassy, I. Bakonyi, K. Tompa, T. Bagi, Z. Hegedus, J. Phys. Col. 41 (C8) (1980) 141. U. Pittermann, S. Ripper, Z. Metallkde. Bd. 74 (1983) 783.
L
www.elsevier.com / locate / jallcom
Peak-profile analysis of electroless nickel coatings Nicholas M. Martyak*, Kerry Drake Atochem North America, 900 First Avenue, King of Prussia, PA 19406, USA Received 21 February 2000; accepted 18 May 2000
Abstract Peak-profile analysis (PPA) of X-ray diffraction patterns and differential scanning calorimetry of electroless nickel (EN) coatings were done to determine the changes in the structure with annealing. As-deposited medium phosphorus EN coating exhibit broadened X-ray reflections indicative of a semi-amorphous structure. Heat-treatment decreased the amorphous phase which contributed to broadening of the reflections. A small exotherm was seen in the temperature range where the amorphous phase decreased and the crystalline component increased. No amorphous component was detected above 3008C. Changes in particle size did not occur until the annealing temperature exceeded 3008C. The strain in the EN coatings decreased sharply after 2008C and was negligible after 4008C heat-treatment. 2000 Elsevier Science B.V. All rights reserved. Keywords: Electroless nickel; Peak profile analysis; Particle size; Strain; Heat treatment; X-ray diffraction
1. Introduction Electroless nickel (EN) coatings have gained wide acceptance since their discovery by Brenner and Riddell [1] in the middle part of this century. The coatings are found in many market segments such as automobile, electronics and the food industries [2]. This diverse market penetration is due in part to the unique properties of these coatings. As in many advanced materials, the properties are highly dependent upon the microstructure of the film which in turn is dependent upon the composition of the deposit. EN coatings are not pure nickel deposits but rather alloys of nickel and a metalloid such as phosphorus or boron. Because of the low solubility of metalloids in a nickel matrix, these coatings are supersaturated solutions of either phosphorus or boron in nickel [2,3]. This low solubility of the metalloid drastically alters the microstructure of the metal coating. For example, low concentrations of phosphorus in nickel result in a microcrystalline coating whereas medium levels of phosphorus yield coatings that contain both crystalline and amorphous components. High concentrations of occluded phos-
*Corresponding author. E-mail address: [email protected] (N.M. Martyak).
phorus produce EN deposits that are completely amorphous. The properties of EN films are dependent upon the microstructure of the coatings [4]. Low phosphorus films are required where hard, wear-resistant coatings are specified. The hardness of these films is considerably greater than pure nickel. Non-magnetic nickel coatings are required on magnetic disk drives prior to cobalt sputtering. Therefore, the occluded phosphorus must be high in these deposits. Many EN coatings are subjected to elevated temperatures which results in a concomitant change in the structure and properties of these deposits [2]. X-ray diffraction is widely used to determine the structure and composition of materials. Diffraction patterns contain information showing various phases of a material and residual stresses within a coating. Broadening of XRD lines is associated with small particle size of the coherently diffracting crystallites or strains present within the film, or both [5]. Warren and Averbach [6] showed the microstrain component on line broadening can be separated from the particle size contribution since the latter is independent of the order of the reflection. Hall and Williamson [7] also discussed the broadening of XRD lines from particle size and microstrain contributions. Howard and Snyder [8] and Yau and Howard [9] discussed the use of real space variables rather than Fourier transforms as used in the
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01099-9
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Warren–Averbach method to study XRD profiles. There have been numerous XRD studies investigating the changes in microstructure of EN coatings with annealing [10–12]. Most agree the XRD lines sharpen with lower phosphorus content and after annealing. Sampathkumar and Nair [13] studied XRD profiles of EN deposits containing 5.5% phosphorus and concluded the total XRD profile was composed of crystalline and amorphous components. Annealing from 60 to 3008C changed the ratio of the integrated intensities of the amorphous, Iamp , to the intensity of the crystalline phase, Icry . Sadeghi et al. [14] determined the size of coherently diffracting crystallites in EN coatings from broadened XRD patterns. However, they assumed no strain in their calculations. Mai et al. [15] used pole figure analysis to monitor texture development in low, medium and high phosphorus EN coatings with annealing. This study was done to understand the changes in the XRD patterns of EN deposits with annealing temperature. Peak profile analysis (PPA) of the XRD lines showed changes in the relative amorphous and crystalline phases as well as variations in particle size and strains with annealing temperatures.
2. Experimental EN deposits were plated from a medium phosphorus EN solutions operated at 908C. The pH of the EN solution was 4.8. Mild air agitation was used during nickel plating. Low carbon steel panels were cleaned in a sodium hydroxide:sodium gluconate solution at 4.0 V for 30 s at 608C. This was followed by water rinses then immersion in 10% HCl for 5 s. The panels were plated for 90 min to deposit an EN coating between 25 and 30 mm. After plating, the samples were doubly rinsed in distilled water and dried. Samples were annealed from 100 to 5008C for 60 min. X-ray diffraction patterns were acquired on the as-deposited and annealed samples using a Philips APD 3720 instrument with variable slit optics. Scans were done from 30 to 1008 using a 0.01 o step scan. The Kb was removed using a nickel filter. The diffraction patterns were analyzed using computer programs [16]. A convoluted algorithm combining an instrumental broadening and Lorentzian function was used in the curve fitting analysis. The patterns were corrected for instrumental broadening and shifts using a well crystallized LaB 6 sample, NIST SRM 660. Peak-profile analysis for particle size and microstrain was calculated from:
b cos(u ) 5 57.296( l /t 1 4e 57.296 sin(u )) where b is the integral breadth of the profile, t is the crystallite size, 2u is the diffraction angle, e is the microstrain component and l is the wavelength of the
31
incident radiation. A plot of b cos(u ) vs. sin u yields a straight line. The microstrain is calculated from the slope of this line and the particle size from the ordinate-intercept. The background was subtracted from each pattern. Differential scanning calorimetry was done on the EN sample using a TA 910 DSC cell. The EN deposit was first plated on a copper substrate so the thickness of the coating was about 20 mm. The copper substrate was removed by floating the foil on a solution containing 500 g / l chromic acid and 50 ml / l sulfuric acid. Once the copper dissolved, the EN foil was triply rinsed in distilled water and dried. The DSC scans were done under nitrogen at a scan rate of 58C / min.
3. Results and discussion X-ray diffraction of as-deposited EN containing 5–6 w / w% phosphorus is shown in Fig. 1a. Well defined (111) and (200) reflections are seen. On the low angle side of the (200) appears some skewing which may be indicative of stacking faults in this coating [17]. PPA of the pattern required an amorphous component to be added to the crystalline pattern. This amorphous line was located between the (111) and (200) reflections, centered at about 49.58. Small (220), (311) and (222) reflections are seen at higher 2u values. The five reflections were fitted using the instrumental broadening and Lorentzian functions to separate the peak broadening due to inherent instrumental factors (optics, X-ray source) and those due to sample characteristics. This peak broadening observed in these samples appears to be dependent upon both the small particle size and the strain in the coating as seen in Fig. 1b. The size of the coherently diffracting EN crystallites and the strain is seen in Table 1. Annealing to 1008C sharpened the (111) and (200) reflections slightly as seen in Fig. 2a. The (200) appears to grow at the expense of the (111) manifested by the changes in the relative intensities of these two reflections compared to those in Fig. 1a. The amorphous component
Table 1 Effect of annealing on crystallite size, strain and amorphous phase EN coating
Size of EN crystallites (nm)
Strain310 23
Amorphous component %
As-deposited Annealed 1008C Annealed 2008C Annealed 3008C Annealed 4008C
4.8 5.0 4.1 4.4 24.6 1129 a 61.9 1715 a
8.07 7.75 0.06 21.9 0.12 3.2 a 0.07 1.78 a
22.4 19.6 14.9 9.7 0
Annealed 5008C a
Size and strain for Ni 3 P reflections.
0
32
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Fig. 1. (a) XRD pattern of unannealed EN coating. (b) Size and strain analysis for as-deposited EN sample.
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Fig. 2. (a) XRD scan for EN deposit heat-treated at 1008C. (b) Size and strain fit for 1008C annealed sample.
33
34
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Fig. 3. (a) XRD pattern of 2008C annealed sample. (b) Size and strain fit for 2008C annealed sample showing particle size controlling broadening. (c) DSC trace showing exotherms during annealing.
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
35
Fig. 3. (continued)
was again necessary to obtain an adequate fit to the data. The crystallite size and strain were about the same as that in the as-deposited sample. The size and strain fit curve seen in Fig. 2b appears to indicate the crystallite size may be influencing peak broadening more than the strain in the coating. Continued annealing to 2008C shows a further strengthening of the (200) reflection and a smaller (111), Fig. 3a. The amorphous component was also smaller than that seen in Figs. 1a and 2a. Refinement of the pattern showed the size and not the strain component controlled the broadening of the reflections as seen in Fig. 3b and Table 1. There appears to be no strain in the EN deposit after annealing to 2008C. DSC of the EN coating shows a small exotherm about 1728C, Fig. 3c. It is likely the reduction in strain is associated with microstructural changes occurring in the EN coating between 100 and 2008C. The amorphous phase in the EN film annealed at 2008C is almost one-half of that seen in the as-deposited coating.
After annealing at 3008C, the (200) is considerably greater than the (111) and the amorphous phase contributes only about 10% to the total broadening, Fig. 4a. Unlike the work of Sampathkumar and Nair [13] who showed the amorphous phase was eliminated after a 2008C anneal, there is a noticeable amorphous phase present in these coatings after the 3008C anneal. The crystallite size again dominates the broadening effect as seen in Fig. 4b and the strain is slightly negative. There are studies which show various metastable nickel structures in EN coatings during low temperature anneals. Cziraki et al. [18] examining the crystallization of EN coatings found a Ni 5 P2 metastable phase during annealing which subsequently transformed to Ni 3 P with further annealing. Pittermann and Ripper [19] using high energy electron diffraction confirmed the existence of a metastable Ni 5 P2 or Ni 2.55 P structure prior to the formation of the stable Ni 3 P. The DSC trace seen in Fig. 3c shows a small exotherm at 338.48C which may be due to the formation of these metastable phases. The formation of such intermediates after annealing to 3008C in
36
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Fig. 4. (a) XRD pattern of 300QDC annealed sample showing sharpening of reflections. (b) Size and strain fit for 3008C annealed sample–size control broadening.
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
37
Fig. 5. (a) XRD scan for EN deposit heat-treated at 4008C showing onset of nickel phosphide precipitation. (b) Size and strain fit for 4008C annealed sample–nickel reflections only. (c) Size and strain fit for 4008C annealed sample–nickel phosphide reflections only.
38
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Fig. 5. (continued)
these coatings may relieve the strain prior to the onset of Ni 3 P precipitation. Annealing at 4008C resulted in the precipitation of nickel phosphide, Ni 3 P, as seen in Fig. 5a. No amorphous phase was needed to accurately curve fit the reflections. PPA of the nickel phase showed any broadening was a result of the crystallite size and not any residual strain effects, Fig. 5b. PPA of the nickel phosphide phase also showed the crystallite size of Ni 3 P was considerably greater than that of metallic nickel, Fig. 5a and c and Table 1. There was also a strain component to the nickel phosphide crystallites. This strain may be due in part to the precipitation hardening mechanism whereby the nickel and nickel phosphide phases are coherent until precipitation occurs, similar to that observed in the Al–Cu system. During precipitation of the phosphide phase, coherency between the nickel and the nickel phosphide crystallites is lost. The 5008C anneal intensified the nickel phosphide reflections slightly as seen in Fig. 6a. The strongest reflections were those of metallic nickel. PPA of both the nickel and nickel phosphide reflections showed a slight increase in crystallite size for both phases, Ni8 and Ni 3 P. There was no strain associated with the metallic nickel
phase and the strain observed at 4008C in the nickel phosphide phase was almost negligible after the 5008C heat-treatment, Fig. 6b and c. The changes in crystallite size and strain with annealing temperature is seen in Table 1. The size of the nickel crystallites does not increase to any appreciable extent up to 3008C. However, there is a sharp decrease in the strain component with temperature and the strain becomes compressive at 3008C. Thus, the sharpening of the (111) and (200) reflections seen in Figs. 1a–4a is due to the elimination of strain within the coating and not an increase in the crystallite size.
4. Conclusions The amorphous and crystalline components of the EN coatings changed with heat treatment. There is a decrease in the amorphous phase with an increase in heat treatment temperature up to 3008C. Beyond 3008C, no amorphous component was detected in the EN diffraction patterns. Broadening of the XRD reflections is due mainly to the crystallite size component and not the strain. Annealing
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
39
Fig. 6. (a) XRD scan for EN deposit heat-treated at 5008C showing growth of nickel and nickel phosphide crystallites. (b) Size and strain fit for 5008C annealed sample–nickel reflections only. (c) Size and strain fit for 5008C annealed sample–nickel phosphide reflections only.
40
N.M. Martyak, K. Drake / Journal of Alloys and Compounds 312 (2000) 30 – 40
Fig. 6. (continued)
temperatures greater than 3008C result in the precipitation of nickel phosphide.
References [1] A. Brenner, G. Riddell, J. Res. Nat. Bur. Std. 37 (1946) 31. [2] G.O. Mallory, J.B. Hajdu, Electroless Plating, AESF, Orlando, FL, 1990. [3] N.M. Martyak, S. Wetter, L. Harrison, M. McNeil, R. Heu, A.A. Neiderer, Plating and Surface Finish (1993) 60. [4] R. Weil, J.H. Lee, K. Parker, Plating and Surface Finish (1989) 62. [5] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publ., Reading, MA, 1978. [6] B.E. Warren, B.L. Averbach, J. Appl. Phys. 21 (1950) 595. [7] W.H. Hall, G.K. Williamson, Acta Met. 1 (1953) 22.
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
S.A. Howard, R.L. Snyder, J. Appl. Cryst. 22 (1989) 238. J.K. Yau, S.A. Howard, J. Appl. Cryst. 22 (1989) 244. K. Parker, Plating and Surface Finish (1981) 71. M. Lambert, D. Duquette, Thin Solid Films 177 (1989) 207. S.H. Park, D.N. Lee, J. Mat. Sci. 23 (1988) 1643. P. Sampathkumar, P.K. Nair, Prog. Thermal Treat. Mat. (1996) 169. M. Sadeghi, P.D. Longfield, C.F. Beer, Tran. Inst. Metal Finish 61 (1983) 141. Q.X. Mai, R.D. Daniels, H.B. Harpalani, Thin Solid Films 166 (1988) 235. Materials Data Inc., Livermore, CA, 1994. A.H. Graham, R.W. Lindsay, H.J. Read, J. Electrochem. Soc. 112 (4) (1965) 401. A. Cziraki, B. Fogarassy, I. Bakonyi, K. Tompa, T. Bagi, Z. Hegedus, J. Phys. Col. 41 (C8) (1980) 141. U. Pittermann, S. Ripper, Z. Metallkde. Bd. 74 (1983) 783.