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Effect of surfactants on the mechanical properties of electroless (Ni–P) coating
Surface & Coatings Technology 203 (2008) 709–712
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Effect of surfactants on the mechanical properties of electroless (Ni–P) coating R. Elansezhian a,⁎, B. Ramamoorthy a, P. Kesavan Nair b a b
Manufacturing Engineering Section, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai — 600036, India Department of Metallurgical & Materials Engineering, Indian Institute of Technology Madras, Chennai — 600036, India
a r t i c l e
i n f o
Available online 17 August 2008 Keywords: Surfactants Surface roughness Microhardness Microstructure Electroless Ni–P coatings
a b s t r a c t Electroless nickel (EN) coating is a well established surface engineering process that involves deposition of a metal–metalloid alloy coating on various substrates. The effect of surfactants on the surface roughness, microhardness and microstructure of electroless Nickel–Phosphorus (EN) surface protective coating obtained from an alkaline bath is presented in this paper. In this study the influence of surfactants sodium dodecylsulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) on the surface roughness, microhardness and microstructure of coated samples are investigated. EN deposits with addition of surfactant SDS and CTAB at a concentration of 0.6 g/l produce a smooth surface and the average roughness value (Ra) is 1.715 µm for SDS and 1.607 µm for CTAB which is less than the Ra value of EN deposit without surfactant addition (1.885 µm). The mean average roughness value (Ra) with addition of surfactant is 1.796 µm. It was found that without surfactant the micro hardness value of as-coated condition was 450 Hv. The micro hardness value increased to 685 Hv with addition of SDS and 675 Hv with addition of CTAB. The complete experimental details, results obtained and their analysis is presented in this paper. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Good bearing properties in any part are obtained when the surface has large number of irregularities. If the surface is perfectly smooth then seizure would occur due to difficulty of maintaining the lubricating oil film. The rates of wear are proportional to the surface areas in contact and the load per unit area. Thus it is seen that different requirements demand different types of surfaces. Every machining operation leaves characteristic evidence on the machined surface. This evidence in the form of finely spaced micro irregularities left by the cutting tool which are termed as surface irregularity or surface roughness [1]. Roughness is sometimes an undesirable property, as it may cause friction, wear, drag and fatigue, but it is sometimes beneficial, as it allows surfaces to trap lubricants and prevents them from welding together. Hence controlling surface roughness is important in terms of functional and quality aspects. Electroless nickel (EN) coating is a well established surface engineering process that involves deposition of a metal–metalloid alloy coating on various substrates. Although a variety of metals can be plated, electroless Ni–P coating has received widespread acceptance as it provides high hardness and excellent resistance to wear, abrasion and corrosion [2]. Major advantages over the electroless deposition process include the formation of a uniform deposit on irregular surfaces, direct deposition on surface activated non-conductors and the formation of less porous, more corrosion resistant deposits [3]. The effect of adding surfactants to the EN bath is an area which is not explored adequately. In ⁎ Corresponding author. Tel.: +91 44 22574674; fax: +91 44 2257 8578. E-mail addresses: [email protected] (R. Elansezhian), [email protected] (B. Ramamoorthy), [email protected] (P. Kesavan Nair). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.08.021
this investigation surfactants are added to the electrolyte bath and their effects on the surface finish, microhardness, phosphorus content and micro structure are studied. Electroless process for nickel coatings produces deposits having microcrystalline, amorphous or fully crystalline nature over a wide range of compositions [4] and the deposit from the hypophosphite bath with alkaline-citrate stabilizer are crystalline in nature [5] and [6]. Allen et al. (1982) [7] reported that EN coatings are either crystalline, amorphous, or a mixture of both. In general, low phosphorous (1–5% P) EN deposits are microcrystalline, medium phosphorus (6–10% P) coatings have mixed crystalline and amorphous microstructures, whereas high phosphorous (11–13% P) EN coatings have outstanding corrosion resistance, but their hardness and wear resistance are lower than their low phosphorous counterparts [8]. However, limited data are available concerning the effects of anionic surfactant sodium dodecyl sulfate (SDS) and cationic surfactant cetyltrimethyl ammonium bromide (CTAB), on the microhardness of electroless nickel deposits. In this study, the effect of surfactants SDS and CTAB addition was investigated on the surface roughness, microhardness and microstructure of EN coated samples. 2. Experimental 2.1. Preparation of the substrates In this work, mild steel specimens of about 24 mm diameter and 7 mm thickness were prepared from rod stock to be used as substrates. All the specimens were then solutionised at 800°C for 2 h and furnace cooled to ensure uniform initial microstructural conditions. The
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R. Elansezhian et al. / Surface & Coatings Technology 203 (2008) 709–712
Table 1 Compositions of plating bath for electroless Ni–P deposits used in experiments Particulars
NiCl2 H2NaO2P∙H2O Na3C6H5O7 NH4Cl NaC12H25SO4 C19H42BrN Temperature Ph
Quantity (g/l) Bath A
Bath B
30 40 25 50 0–1.5 – 87 °C (± 1 °C) 9–10
30 40 25 50 – 0–1.8 87 °C (± 1 °C) 9–10
samples were then finished by grinding followed by disc polishing. The typical surface finish values measured using a stylus instrument of the finished samples are as follows: average roughness value is the average of peak and valley distances measured along the centerline of one cutoff length (0.762 mm) (Ra) = 0.57 µm. The step-by-step cleaning procedure employed prior to plating consists of cleaning the substrate with acetone, rinsing with distilled water, ultrasonic cleaning in methanol, acid pickling for 1 min [8% H2SO4 by volume], rinsing in distilled water followed by a methanol wash.
Fig. 2. Microhardness of EN deposits vs. surfactant concentrations: (a) SDS (b) CTAB.
2.2. Plating bath and operating conditions The composition of the plating bath for electroless Ni–P deposition had: Nickel chloride as the source of nickel, sodium hypophosphite as the reducing agent, sodium citrate as the stabilizer and ammonium chloride as the complexing agent. The specific bath compositions and plating conditions used are presented in Table 1. The surfactant sodium dodecyl sulfate (SDS) was added into solution before electroless nickel deposition with various concentrations ranging from 0.15 g/l to 1.5 g/l and cetyltrimethyl ammonium bromide (CTAB) was added with concentrations ranging from 0.15 g/l to 1.8 g/l. Temperature of the plating bath was maintained at 87 °C (±1 °C). The pH of the bath was maintained between 9 and 10 by addition of sufficient quantity of ammonia solution as and when required. The electrolyte was heated indirectly by an electrically heated oil bath whose temperature was regulated by a Proportional Integral Derivative (PID) controller. The temperature of the oil medium was controlled and the corresponding temperature of
Fig. 1. Average surface roughness of EN deposits vs. concentration of surfactant: (a) SDS (b) CTAB.
Fig. 3. Diffractograms of EN coated samples at various surfactant concentration: (a) SDS (b) CTAB.
R. Elansezhian et al. / Surface & Coatings Technology 203 (2008) 709–712
711
scanning electron microscope, Netherlands. The amount of phosphorous and nickel contents on the EN deposits were analyzed using FEI Quanta 200 OXFORD EDAX attachment, Netherlands. X-ray diffraction results were obtained from experiments using a computer controlled Shimadzu diffractometer using iron radiation (Fe-Kα). 3. Results and discussions 3.1. Surface roughness of electroless Ni–P deposits
Fig. 4. Phosphorus content of EN deposits vs. concentration of surfactants: (a) SDS (b) CTAB.
the electrolyte was monitored using a thermometer. The coating was done for a period of 2 h with total initial volume of the plating bath restricted to 150 ml (unless otherwise mentioned). 2.3. Analysis of deposits Micro hardness of the EN deposits was estimated using a FutureTech microhardness tester Japan, with a diamond pyramid as an indenter, 200 g load, 15 seconds loading time and five trials per sample. Surface roughness of EN deposits were measured using a stylus instrument and five trials per sample. The microstructures of the electroless nickel deposits were observed using a FEI Quanta 200 high resolution
The variation of average roughness value with respect to surfactant concentration is presented in Fig. 1 (a) and (b). At lower concentration of surfactants, the average surface roughness value is higher and when concentration reaches to 0.6 g/l and above, the roughness value gets stabilized and it ranges from 1.579 µm to 1.884 µm for SDS and the mean average value is 1.796 µm which is less than the average roughness value of ENi–P deposit without surfactant addition. The average roughness value of ENi–P deposit without surfactant is 1.885 µm. In the presence of surfactants the Ni–P deposits changes the surface topography from a smooth state to non-smooth state at lower concentrations and at higher concentrations of SDS the surface is smoother than that of a conventional deposit and the reason is the amount of nickel particles deposited on the coating surface is increased. This is due to the fact that at higher concentration of SDS the contact angle is reduced and this leads to the better wettability of Ni–P deposit. It is apparent that SDS addition during Ni–P deposit can improve the roughness. Researchers Lin and Duh (2006) [9] reported that adding SDS during electrodeposition of Ni–P modified the surface roughness and surface morphology of deposit for under bump metallization (UBM) in microelectronic industry. Wheeler et al. (2004) [10] reported that the surface of electroplated Ni–P deposited with SDS addition was smoother at higher concentrations and an exactly similar trend was observed in our experimental work as well. When surfactant CTAB is added in the electrolyte bath, at lower concentration the average surface roughness value is higher and when concentration reaches to 0.6 g/l and above, the roughness value gets stabilized and it ranges from 1.569 µm to 2.557 µm and the mean average value is 2.238 µm. In the presence of CTAB the Ni–P deposits change the surface topography from a smooth state to non-smooth state. Coalescence of nickel particles deposited on the coating surface at lower concentration of CTAB. Addition of CTAB as a surfactant promotes the fusion of nickel particles into a blend at lower concentration. As the CTAB concentration is increased, the surface is smoother as in the case of a conventional deposit. These results follow a similar trend as reported by earlier researchers Kajihara et al. [11] who studied the influence of CTAB surfactant on the roughness of titania sol–gel films. 3.2. Microhardness of electroless Ni–P deposits The microhardness of ENi–P deposits increase with increase in SDS concentration up to its critical micelle concentration (CMC) and then
Fig. 5. SEM micrographs (1000×) of EN deposition: (a) without surfactant (b) 1.2 g/l SDS (c) 1.5 CTAB.
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R. Elansezhian et al. / Surface & Coatings Technology 203 (2008) 709–712
decreased. Fig. 2 (a) shows the variation of microhardness of ENi–P deposit with respect to SDS surfactant concentrations. In the as plated condition, hardness is 450 VHN200. In the presence of SDS the hardness increased up to 685 VHN200 (52% increase). The reason is due to the change in crystalline structure. In the absence of surfactant, the EN deposits are purely crystalline and with addition of SDS it is changed to a mixture of nano crystalline and amorphous structure. The X-ray diffractogram of EN deposits in the as plated condition and with addition of SDS at various concentration is shown in Fig. 3 (a). When SDS is added into the electrolyte bath the phosphorus content is increased from 6–9%. At higher concentrations above the CMC value of SDS the phosphorus content is high (9–10%) and their hardness value is reduced. Fig. 4 (a) shows the variation of phosphorus content with respect to SDS concentrations. Mukherjee et al. (1992) [8] have reported that high phosphorus amorphous EN coatings have outstanding corrosion resistance. In the present study, a similar trend has been observed as well. Fig. 2 (b) shows the variation of microhardness of ENi–P deposit with respect to CTAB surfactant concentrations. In the presence of CTAB the hardness increased up to 675 VHN200 (50% increase). The reason is due to the change in crystalline structure. The EN deposits are changed to a mixture of nano crystalline and amorphous structure and the corresponding X-ray diffractograms of EN deposits with addition of CTAB is presented in Fig. 3 (b). Fig. 4 (b) shows the variation of phosphorus content with respect to CTAB concentrations. At lower CTAB concentration the P content is 7.5% and is increased with increase in CTAB concentration to a maximum value of 12%. 3.3. Morphology of electroless Ni–P deposits The morphology of the deposits was analyzed by employing scanning electron microscope. The SEM micrographs of ENi–P deposits without addition of surfactant, with addition of 1.2 g/l concentrations of SDS and 1.5 g/l concentrations of CTAB are presented in Fig. 5 (a), (b) and (c) respectively. In the case of ENi–P deposit without surfactant, the dispersion of nickel particles seem to be less when compared to deposit with surfactant resulting in uniform surface finish. 4. Conclusions Based on the experimental results and analysis, the following conclusions have been drawn which clearly indicate that there is a pos-
sibility of a significant improvement in the average surface roughness and microhardness of ENi–P deposited layers. 1. The surface finish of the coated layer significantly improved when the concentration of the surfactant exceeds 0.6 g/l. But at the lower levels of concentration the surface finish was poor. 2. There was a considerable improvement in the hardness of the coated layers. With the addition of SDS the improvement was 52% (685 VHN200) and with CTAB it was 50% (675 VHN200). 3. By adding the above two surfactants during deposition of ENi–P, the phosphorus content had gone up resulting in improved quality of the deposits. Particularly it improves the corrosion resistance. 4. With SDS, fine nickel particles have dispersed uniformly on the substrate surface resulting in smoother surface finish of the deposited layers. 5. In the presence of CTAB, at lower concentrations, coalescence of nickel particles have been deposited on the substrate surface and at the higher concentration uniformly improved surface finish of the deposited layer is resulted. References [1] W. Wolf, in: E.A. Avallone, T. Baumeister (Eds.), Marks' Standard Handbook for Mechanical Engineers, Section 13.5 “Surface Texture Designation, Production, and Control”, McGraw-Hill, 1996. [2] G.O. Mallory, J.B. Hajdu, Electroless Plating, American Electroplaters and Surface Finishing Society, Orlando, 1991, p. 203. [3] Riedel Wolf, Electroless Plating, ASM International, Ohio, 1991, p. 287. [4] D.W. Baudrand, Electroless Nickel Plating, Surface Engineering, ASM Hand book, vol. 5, American Society for Materials, Material Park, Ohio, 1994, p. 290. [5] P. Sampath Kumar, P. Kesavan Nair, Plating Surf. Finish. 71 (5) (1994) 96. [6] J.T. Winowlin Jappes, B. Ramamoorthy, P. Kesavan Nair, J. Mater. Process. Technol. 169 (2005) 308. [7] M. Allen, J.B. VanderSande, Scr. Metall. 16 (1982) 1161. [8] D. Mukherjee, D. Rajagopal, Met. Finish. 90 (1) (1992) 15. [9] Yun-Chi Lin, Jenq-Gong Duh, J. Alloys Compd. 439 (1–2) (2006) 74. [10] D. Wheeler, T.P. Moffat, G.B. McFadden, S. Coriell, D. Josell, J. Electrochem. Soc. 151 (2004) C538. [11] K. Kajihara, K. Nakanishi, K. Tanaka, K. Hirao, N. Soga, J. Amer. Chem. Soc. 81 (10) (1998) 2670.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Effect of surfactants on the mechanical properties of electroless (Ni–P) coating R. Elansezhian a,⁎, B. Ramamoorthy a, P. Kesavan Nair b a b
Manufacturing Engineering Section, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai — 600036, India Department of Metallurgical & Materials Engineering, Indian Institute of Technology Madras, Chennai — 600036, India
a r t i c l e
i n f o
Available online 17 August 2008 Keywords: Surfactants Surface roughness Microhardness Microstructure Electroless Ni–P coatings
a b s t r a c t Electroless nickel (EN) coating is a well established surface engineering process that involves deposition of a metal–metalloid alloy coating on various substrates. The effect of surfactants on the surface roughness, microhardness and microstructure of electroless Nickel–Phosphorus (EN) surface protective coating obtained from an alkaline bath is presented in this paper. In this study the influence of surfactants sodium dodecylsulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) on the surface roughness, microhardness and microstructure of coated samples are investigated. EN deposits with addition of surfactant SDS and CTAB at a concentration of 0.6 g/l produce a smooth surface and the average roughness value (Ra) is 1.715 µm for SDS and 1.607 µm for CTAB which is less than the Ra value of EN deposit without surfactant addition (1.885 µm). The mean average roughness value (Ra) with addition of surfactant is 1.796 µm. It was found that without surfactant the micro hardness value of as-coated condition was 450 Hv. The micro hardness value increased to 685 Hv with addition of SDS and 675 Hv with addition of CTAB. The complete experimental details, results obtained and their analysis is presented in this paper. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Good bearing properties in any part are obtained when the surface has large number of irregularities. If the surface is perfectly smooth then seizure would occur due to difficulty of maintaining the lubricating oil film. The rates of wear are proportional to the surface areas in contact and the load per unit area. Thus it is seen that different requirements demand different types of surfaces. Every machining operation leaves characteristic evidence on the machined surface. This evidence in the form of finely spaced micro irregularities left by the cutting tool which are termed as surface irregularity or surface roughness [1]. Roughness is sometimes an undesirable property, as it may cause friction, wear, drag and fatigue, but it is sometimes beneficial, as it allows surfaces to trap lubricants and prevents them from welding together. Hence controlling surface roughness is important in terms of functional and quality aspects. Electroless nickel (EN) coating is a well established surface engineering process that involves deposition of a metal–metalloid alloy coating on various substrates. Although a variety of metals can be plated, electroless Ni–P coating has received widespread acceptance as it provides high hardness and excellent resistance to wear, abrasion and corrosion [2]. Major advantages over the electroless deposition process include the formation of a uniform deposit on irregular surfaces, direct deposition on surface activated non-conductors and the formation of less porous, more corrosion resistant deposits [3]. The effect of adding surfactants to the EN bath is an area which is not explored adequately. In ⁎ Corresponding author. Tel.: +91 44 22574674; fax: +91 44 2257 8578. E-mail addresses: [email protected] (R. Elansezhian), [email protected] (B. Ramamoorthy), [email protected] (P. Kesavan Nair). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.08.021
this investigation surfactants are added to the electrolyte bath and their effects on the surface finish, microhardness, phosphorus content and micro structure are studied. Electroless process for nickel coatings produces deposits having microcrystalline, amorphous or fully crystalline nature over a wide range of compositions [4] and the deposit from the hypophosphite bath with alkaline-citrate stabilizer are crystalline in nature [5] and [6]. Allen et al. (1982) [7] reported that EN coatings are either crystalline, amorphous, or a mixture of both. In general, low phosphorous (1–5% P) EN deposits are microcrystalline, medium phosphorus (6–10% P) coatings have mixed crystalline and amorphous microstructures, whereas high phosphorous (11–13% P) EN coatings have outstanding corrosion resistance, but their hardness and wear resistance are lower than their low phosphorous counterparts [8]. However, limited data are available concerning the effects of anionic surfactant sodium dodecyl sulfate (SDS) and cationic surfactant cetyltrimethyl ammonium bromide (CTAB), on the microhardness of electroless nickel deposits. In this study, the effect of surfactants SDS and CTAB addition was investigated on the surface roughness, microhardness and microstructure of EN coated samples. 2. Experimental 2.1. Preparation of the substrates In this work, mild steel specimens of about 24 mm diameter and 7 mm thickness were prepared from rod stock to be used as substrates. All the specimens were then solutionised at 800°C for 2 h and furnace cooled to ensure uniform initial microstructural conditions. The
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R. Elansezhian et al. / Surface & Coatings Technology 203 (2008) 709–712
Table 1 Compositions of plating bath for electroless Ni–P deposits used in experiments Particulars
NiCl2 H2NaO2P∙H2O Na3C6H5O7 NH4Cl NaC12H25SO4 C19H42BrN Temperature Ph
Quantity (g/l) Bath A
Bath B
30 40 25 50 0–1.5 – 87 °C (± 1 °C) 9–10
30 40 25 50 – 0–1.8 87 °C (± 1 °C) 9–10
samples were then finished by grinding followed by disc polishing. The typical surface finish values measured using a stylus instrument of the finished samples are as follows: average roughness value is the average of peak and valley distances measured along the centerline of one cutoff length (0.762 mm) (Ra) = 0.57 µm. The step-by-step cleaning procedure employed prior to plating consists of cleaning the substrate with acetone, rinsing with distilled water, ultrasonic cleaning in methanol, acid pickling for 1 min [8% H2SO4 by volume], rinsing in distilled water followed by a methanol wash.
Fig. 2. Microhardness of EN deposits vs. surfactant concentrations: (a) SDS (b) CTAB.
2.2. Plating bath and operating conditions The composition of the plating bath for electroless Ni–P deposition had: Nickel chloride as the source of nickel, sodium hypophosphite as the reducing agent, sodium citrate as the stabilizer and ammonium chloride as the complexing agent. The specific bath compositions and plating conditions used are presented in Table 1. The surfactant sodium dodecyl sulfate (SDS) was added into solution before electroless nickel deposition with various concentrations ranging from 0.15 g/l to 1.5 g/l and cetyltrimethyl ammonium bromide (CTAB) was added with concentrations ranging from 0.15 g/l to 1.8 g/l. Temperature of the plating bath was maintained at 87 °C (±1 °C). The pH of the bath was maintained between 9 and 10 by addition of sufficient quantity of ammonia solution as and when required. The electrolyte was heated indirectly by an electrically heated oil bath whose temperature was regulated by a Proportional Integral Derivative (PID) controller. The temperature of the oil medium was controlled and the corresponding temperature of
Fig. 1. Average surface roughness of EN deposits vs. concentration of surfactant: (a) SDS (b) CTAB.
Fig. 3. Diffractograms of EN coated samples at various surfactant concentration: (a) SDS (b) CTAB.
R. Elansezhian et al. / Surface & Coatings Technology 203 (2008) 709–712
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scanning electron microscope, Netherlands. The amount of phosphorous and nickel contents on the EN deposits were analyzed using FEI Quanta 200 OXFORD EDAX attachment, Netherlands. X-ray diffraction results were obtained from experiments using a computer controlled Shimadzu diffractometer using iron radiation (Fe-Kα). 3. Results and discussions 3.1. Surface roughness of electroless Ni–P deposits
Fig. 4. Phosphorus content of EN deposits vs. concentration of surfactants: (a) SDS (b) CTAB.
the electrolyte was monitored using a thermometer. The coating was done for a period of 2 h with total initial volume of the plating bath restricted to 150 ml (unless otherwise mentioned). 2.3. Analysis of deposits Micro hardness of the EN deposits was estimated using a FutureTech microhardness tester Japan, with a diamond pyramid as an indenter, 200 g load, 15 seconds loading time and five trials per sample. Surface roughness of EN deposits were measured using a stylus instrument and five trials per sample. The microstructures of the electroless nickel deposits were observed using a FEI Quanta 200 high resolution
The variation of average roughness value with respect to surfactant concentration is presented in Fig. 1 (a) and (b). At lower concentration of surfactants, the average surface roughness value is higher and when concentration reaches to 0.6 g/l and above, the roughness value gets stabilized and it ranges from 1.579 µm to 1.884 µm for SDS and the mean average value is 1.796 µm which is less than the average roughness value of ENi–P deposit without surfactant addition. The average roughness value of ENi–P deposit without surfactant is 1.885 µm. In the presence of surfactants the Ni–P deposits changes the surface topography from a smooth state to non-smooth state at lower concentrations and at higher concentrations of SDS the surface is smoother than that of a conventional deposit and the reason is the amount of nickel particles deposited on the coating surface is increased. This is due to the fact that at higher concentration of SDS the contact angle is reduced and this leads to the better wettability of Ni–P deposit. It is apparent that SDS addition during Ni–P deposit can improve the roughness. Researchers Lin and Duh (2006) [9] reported that adding SDS during electrodeposition of Ni–P modified the surface roughness and surface morphology of deposit for under bump metallization (UBM) in microelectronic industry. Wheeler et al. (2004) [10] reported that the surface of electroplated Ni–P deposited with SDS addition was smoother at higher concentrations and an exactly similar trend was observed in our experimental work as well. When surfactant CTAB is added in the electrolyte bath, at lower concentration the average surface roughness value is higher and when concentration reaches to 0.6 g/l and above, the roughness value gets stabilized and it ranges from 1.569 µm to 2.557 µm and the mean average value is 2.238 µm. In the presence of CTAB the Ni–P deposits change the surface topography from a smooth state to non-smooth state. Coalescence of nickel particles deposited on the coating surface at lower concentration of CTAB. Addition of CTAB as a surfactant promotes the fusion of nickel particles into a blend at lower concentration. As the CTAB concentration is increased, the surface is smoother as in the case of a conventional deposit. These results follow a similar trend as reported by earlier researchers Kajihara et al. [11] who studied the influence of CTAB surfactant on the roughness of titania sol–gel films. 3.2. Microhardness of electroless Ni–P deposits The microhardness of ENi–P deposits increase with increase in SDS concentration up to its critical micelle concentration (CMC) and then
Fig. 5. SEM micrographs (1000×) of EN deposition: (a) without surfactant (b) 1.2 g/l SDS (c) 1.5 CTAB.
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R. Elansezhian et al. / Surface & Coatings Technology 203 (2008) 709–712
decreased. Fig. 2 (a) shows the variation of microhardness of ENi–P deposit with respect to SDS surfactant concentrations. In the as plated condition, hardness is 450 VHN200. In the presence of SDS the hardness increased up to 685 VHN200 (52% increase). The reason is due to the change in crystalline structure. In the absence of surfactant, the EN deposits are purely crystalline and with addition of SDS it is changed to a mixture of nano crystalline and amorphous structure. The X-ray diffractogram of EN deposits in the as plated condition and with addition of SDS at various concentration is shown in Fig. 3 (a). When SDS is added into the electrolyte bath the phosphorus content is increased from 6–9%. At higher concentrations above the CMC value of SDS the phosphorus content is high (9–10%) and their hardness value is reduced. Fig. 4 (a) shows the variation of phosphorus content with respect to SDS concentrations. Mukherjee et al. (1992) [8] have reported that high phosphorus amorphous EN coatings have outstanding corrosion resistance. In the present study, a similar trend has been observed as well. Fig. 2 (b) shows the variation of microhardness of ENi–P deposit with respect to CTAB surfactant concentrations. In the presence of CTAB the hardness increased up to 675 VHN200 (50% increase). The reason is due to the change in crystalline structure. The EN deposits are changed to a mixture of nano crystalline and amorphous structure and the corresponding X-ray diffractograms of EN deposits with addition of CTAB is presented in Fig. 3 (b). Fig. 4 (b) shows the variation of phosphorus content with respect to CTAB concentrations. At lower CTAB concentration the P content is 7.5% and is increased with increase in CTAB concentration to a maximum value of 12%. 3.3. Morphology of electroless Ni–P deposits The morphology of the deposits was analyzed by employing scanning electron microscope. The SEM micrographs of ENi–P deposits without addition of surfactant, with addition of 1.2 g/l concentrations of SDS and 1.5 g/l concentrations of CTAB are presented in Fig. 5 (a), (b) and (c) respectively. In the case of ENi–P deposit without surfactant, the dispersion of nickel particles seem to be less when compared to deposit with surfactant resulting in uniform surface finish. 4. Conclusions Based on the experimental results and analysis, the following conclusions have been drawn which clearly indicate that there is a pos-
sibility of a significant improvement in the average surface roughness and microhardness of ENi–P deposited layers. 1. The surface finish of the coated layer significantly improved when the concentration of the surfactant exceeds 0.6 g/l. But at the lower levels of concentration the surface finish was poor. 2. There was a considerable improvement in the hardness of the coated layers. With the addition of SDS the improvement was 52% (685 VHN200) and with CTAB it was 50% (675 VHN200). 3. By adding the above two surfactants during deposition of ENi–P, the phosphorus content had gone up resulting in improved quality of the deposits. Particularly it improves the corrosion resistance. 4. With SDS, fine nickel particles have dispersed uniformly on the substrate surface resulting in smoother surface finish of the deposited layers. 5. In the presence of CTAB, at lower concentrations, coalescence of nickel particles have been deposited on the substrate surface and at the higher concentration uniformly improved surface finish of the deposited layer is resulted. References [1] W. Wolf, in: E.A. Avallone, T. Baumeister (Eds.), Marks' Standard Handbook for Mechanical Engineers, Section 13.5 “Surface Texture Designation, Production, and Control”, McGraw-Hill, 1996. [2] G.O. Mallory, J.B. Hajdu, Electroless Plating, American Electroplaters and Surface Finishing Society, Orlando, 1991, p. 203. [3] Riedel Wolf, Electroless Plating, ASM International, Ohio, 1991, p. 287. [4] D.W. Baudrand, Electroless Nickel Plating, Surface Engineering, ASM Hand book, vol. 5, American Society for Materials, Material Park, Ohio, 1994, p. 290. [5] P. Sampath Kumar, P. Kesavan Nair, Plating Surf. Finish. 71 (5) (1994) 96. [6] J.T. Winowlin Jappes, B. Ramamoorthy, P. Kesavan Nair, J. Mater. Process. Technol. 169 (2005) 308. [7] M. Allen, J.B. VanderSande, Scr. Metall. 16 (1982) 1161. [8] D. Mukherjee, D. Rajagopal, Met. Finish. 90 (1) (1992) 15. [9] Yun-Chi Lin, Jenq-Gong Duh, J. Alloys Compd. 439 (1–2) (2006) 74. [10] D. Wheeler, T.P. Moffat, G.B. McFadden, S. Coriell, D. Josell, J. Electrochem. Soc. 151 (2004) C538. [11] K. Kajihara, K. Nakanishi, K. Tanaka, K. Hirao, N. Soga, J. Amer. Chem. Soc. 81 (10) (1998) 2670.