Structure and Phase Transformation Behaviour Of

Structure and phase transformation behaviour of

electroless Ni–P composite coatings

J.N. Balaraju a, T.S.N. Sankara Narayanan b,*, S.K. Seshadri c,*a Surface Engineering Division, National Aerospace Laboratory, Bangalore 560017, India

b National Metallurgical Laboratory, Madras Centre, CSIR Complex Taramani, Chennai 600113, Indiac Department of Metallurgical Engineering, Indian Institute of Technology Madras, Chennai 600036, India

Received 3 May 2005; received in revised form 21 August 2005; accepted 30 September 2005

Available online 21 October 2005

Abstract

This paper addresses the structural characteristics and phase transformation behaviour of plain electroless Ni–P coating and

electroless Ni–P–Si3N4, Ni–P–CeO2 and Ni–P–TiO2 composite coatings. The X-ray diffraction patterns of electroless Ni–P–Si3N4,

Ni–P–CeO2 and Ni–P–TiO2 composite coatings are very similar to that of plain electroless Ni–P coating, both in as plated and heat-

treated conditions. Selected area electron diffraction (SAED) patterns obtained on the Ni–P matrix of Ni–P–Si3N4, Ni–P–CeO2 and

Ni–P–TiO2 composite coatings exhibit diffuse ring patterns resembling the one obtained for plain electroless Ni–P coating. Phase

transformation behaviour studied by differential scanning calorimetry (DSC) indicates that the variation in crystallization

temperature and the energy evolved during crystallization of plain electroless Ni–P coating and electroless Ni–P–Si3N4, Ni–

P–CeO2 and Ni–P–TiO2 composite coatings is not significant. The study concludes that incorporation of Si3N4, CeO2 and TiO2

particles in the Ni–P matrix does not have any influence on the structure and phase transformation behaviour of electroless Ni–P

coatings.

# 2005 Elsevier Ltd. All rights reserved.

Keywords: A. Amorphous materials; B. Chemical synthesis; C. Differential scanning calorimetry (DSC); D. Crystal structure

1. Introduction

Electroless plating is a well-established surface engineering process that involves the deposition of a metal–

metalloid alloy on various substrates. During the past five decades electroless plating has gained immense popularity

due to its ability to produce coatings that possess excellent corrosion, wear and abrasion resistance. Among the various

types of electroless plating, hypophosphite reduced electroless nickel plating has received commercial success

because of its low cost, ease of control, and ability to offer good corrosion and wear resistance surface [1,2]. The ability

to codeposit fine particulate matter with electroless Ni–P matrix to prepare composite coatings is a key development in

this area of research as they extend the potential of electroless nickel coatings for innumerable applications. Numerous

studies are available on the development of electroless Ni–P composite coatings with the incorporation of various hard

and soft particles, emphasizing the mechanism of particle incorporation, factors influencing particle incorporation and

the effect of particle incorporation on hardness, friction, wear resistance, abrasion resistance, corrosion resistance, etc.

www.elsevier.com/locate/matresbu

Materials Research Bulletin 41 (2006) 847–860

* Corresponding authors.

E-mail addresses: [email protected] (J.N. Balaraju), [email protected] (T.S.N. Sankara Narayanan), [email protected] (S.K. Seshadri).

0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2005.09.024

Feldstein [3] has presented a detailed account of electroless nickel composite coatings. Balaraju et al. [4] has recently

reviewed the various aspects of electroless nickel composite coatings.

Crystallization and phase transformation behaviour of electroless Ni–P coating during thermal processing has been

the subject of various investigations [5–10]. Keong et al. [11–13] studied the crystallization kinetics and phase

transformation behaviour of electroless Ni–P coatings with different phosphorus contents, namely low phosphorus (3–

5 wt.% P) [11], medium phosphorus (5–8 wt.% P [11] and 6–9 wt.% P [12]) and high phosphorus (10–14 wt.% P) [13].

The crystallization of Ni–P alloys has been investigated by many different methods [14–21]; differential scanning

calorimetry (DSC), X-ray and electron diffraction, small angle X-ray scattering, transmission electron microscopy,

magnetothermal methods and by the change of alloy properties such as microhardness and magnetic properties during

crystallization. Though the crystallization process of electroless Ni–P coatings essentially involves the formation of

crystalline nickel, nickel phosphide (Ni3P) and/or other intermediate phases, the phase transformation is not a single

stage process and occurs through many stages involving one or more intermediate crystalline phases [11–13,22–24].

The diffusion of the component elements in the Ni–P alloy is essential for the simultaneous formation of various

crystalline phases (Ni and Ni3P) and any hindrance in diffusion could either control or inhibit the crystallization

process of the Ni–P alloy. Deposition of Ni–P amorphous alloy on high surface area SiO2 carriers or the introduction of

metals such as Pd to the Ni–P alloy confirms such an occurrence [25]. Chen et al. [26] have shown that codeposition of

Cr and W in the Ni–P alloy film retards the formation of Ni3P phase and increases the crystallization temperature from

380 to 420 8C. The crystallization temperature of electroless and sputtered ternary Ni–Cu–P alloy deposit is

approximately 400 8C [10] and 370 8C [27], respectively. Yu et al. [28] have compared the crystallization behaviour of

electroless Ni–P binary and Ni–Cu–P ternary alloy deposits and proved that the codeposition of copper in the Ni–P

alloy has a definite influence on the crystallization behaviour of Ni–P alloy deposit. Since codeposition of a third

element in Ni–P alloy and deposition of Ni–P alloy deposit over SiO2 influence the crystallization behaviour, studies

on the influence of particle incorporation on the structure and phase transformation behaviour of electroless Ni–P

matrix assumes significance. Hansen and Moller [29] have studied the effect of TiC particle incorporation on the

structure of the electroless Ni–P matrix and showed that the incorporated TiC particles did not affect the structure of

the Ni–P matrix. However, McCormack et al. [30] have shown that addition of yttria particles to a Watt’s nickel plating

bath significantly change the surface morphology and preferred crystallographic orientation of the Ni–yttria composite

coatings. In this context, the present work aims to study the effect of incorporation of hard ceramic particles, namely

Si3N4, CeO2 and TiO2, on the structure and phase transformation behaviour of electroless Ni–P matrix.

2. Experimental details

The electroless Ni–P coatings of the present study were prepared using an acidic hypophosphite reduced electroless

nickel plating bath. Nickel sulphate hexahydrate was the source of nickel while sodium hypophosphite served as the

reducing agent and the source of phosphorus. Besides these, the bath also contains suitable amounts of complexing

agents and stabilisers. During plating, the temperature of the bath was maintained at 88 � 1 8C using a constant

temperature oil bath and the pH was maintained between 4.8 and 5.0 with frequent addition of small quantities of

NaOH. The plating solution was mechanically agitated (600 rpm) using a magnetic stirrer and Teflon coated magnetic

paddle. The plating rate was in the range of 10–12 mm/h. The composition of the plating bath and the operating

conditions employed for plating are shown in Table 1.

The Si3N4, CeO2 and TiO2, particles were used as received without any further treatment. The particles size of these

particles is as follows: among the three, TiO2 particles possess a very narrow range of 2.4–3 mm, followed by Si3N4

(20% 8–10 mm; 40% 6–8 mm; 10% 3–4 mm; 20% 2–3 mm; remaining 10% 16–28 mm) and CeO2 (40% 8–10 mm;

30% 6–8 mm; 20% 4–6 mm; remaining 10% 13–17 mm). In the deposition of electroless Ni–P composite coatings, a

slurry was prepared by thoroughly mixing the required amount of second phase particles (Si3N4/CeO2/TiO2) and a

small portion of plating solution in a mortar to ensure prevention of agglomeration and to enable uniform distribution

of these particles in the plating bath. Thin foils of electroless Ni–P composite coatings used for structural and thermal

characterization were deposited using stainless steel (AISI 304 grade) substrate and subsequently peeled off.

A nickel strike using Woods nickel bath makes the stainless steel substrate catalytically active for electroless plating.

The nickel content of the deposits was determined gravimetrically after precipitating the nickel as nickel–dimethyl

glyoxime complex, whereas the phosphorus content was determined by Inductively Coupled Plasma Atomic Emission

Spectrometry (ICP-AES). The amount of incorporation of second phase particles in their respective deposits was

J.N. Balaraju et al. / Materials Research Bulletin 41 (2006) 847–860848

determined by dissolving a known weight of the deposits in nitric acid and then filtering the particles through a

weighed 0.1 mm membrane. From the weight of the second phase particle and weight of the electroless Ni–P

composite coatings taken for analysis, the percentage weight of second phase particles incorporated in the deposit was

calculated.

X-ray diffraction (XRD) measurements were made with a Shimadzu (Model: XD-D1) diffractometer using Cu Ka

(l = 1.54 A) radiation. A Philips scanning transmission electron microscope (Model CM 12) attached with an EDAX

to facilitate elemental distribution analysis was used for the analysis of deposits. The foils were electrolytically

thinned using a twinjet polishing using perchloric acid-methanol mixture at very low temperatures (about 5 8C) and

carefully placed inside a copper grid for analysis. The bright field/dark field TEM micrographs and selected area

electron diffraction (SAED) patterns were obtained to gain a better insight into the structure of the electroless Ni–P

coatings. The EDAX analysis were performed both on electroless Ni–P matrix region as well as in the region of

incorporated second phase particles to analyse the constituents present. The phase transformation behaviour of

electroless Ni–P and Ni–P composite coatings were studied using differential scanning calorimetry (DSC)

(NETZSCH-Geratebau GmbH) at non-isothermal (linear heating) conditions, which involves the measurement of heat

flow corresponding to the nucleation and growth processes. The activation energy of the crystallization process was

calculated using the method proposed by Duswalt [31], based on the major exothermal DSC peaks obtained at

different heating rates.

3. Results and discussion

The optical micrograph of the cross section of plain electroless Ni–P coating and electroless Ni–P–Si3N4,

Ni–P–CeO2 and Ni–P–TiO2 composite coatings is depicted in Fig. 1a–d, respectively. It is evident from the

micrographs that the Si3N4, CeO2 and TiO2 particles are uniformly distributed throughout the thickness of the coating.

The X-ray diffraction pattern of plain electroless Ni–P coating in its as deposited condition exhibits only a single broad

peak around 458 2u (Fig. 2). Theoretically, a disorder in arrangement of atoms manifests itself as a broad peak in X-ray

diffractograms [32]. The mechanism of formation of electroless Ni–P coatings suggests that during electroless nickel

deposition, random capturing of phosphorus on nickel atoms occurs and the variation in the rate of segregation of

nickel and phosphorus atoms determines the crystallinity of the resultant coating. Between these two atoms, the rate

of diffusion of phosphorus is relatively small compared to that of nickel [33]. Hence, electroless Ni–P coating having

a higher amount of phosphorus involves the movement of a larger number of phosphorus atoms from a given area per

unit time during deposition to achieve segregation of nickel and phosphorus [34]. Chemical analysis of the plain

electroless Ni–P coating shows that it contains 89 wt.% Ni and 11 wt.% P. Since the required phosphorus segregation

is very large, it prevents the nucleation of face centered cubic nickel phase and this results in an amorphous structure in

as deposited condition. Other investigators [13,28] also confirm the amorphous nature of electroless Ni–P coatings

with high phosphorus content (>7 wt.% P) in its as deposited condition.

Fig. 3 shows the X-ray diffraction pattern of the electroless Ni–P coating heat-treated at 400 8C for 1 h. During heat

treatment, transformation of electroless Ni–P coating from a disordered structure to an ordered arrangement of face

centered cubic Ni and body centered tetragonal Ni3P occurs and, this results in the formation of several sharp peaks.

No metastable phases are observed in the X-ray diffraction pattern as the composition of the Ni–P alloy (89 wt.% Ni

J.N. Balaraju et al. / Materials Research Bulletin 41 (2006) 847–860 849

Table 1

Chemical composition of the electroless nickel plating bath and its operating conditions

Chemical composition

Nickel sulphate (NiSO4�6H2O) 23 g/l

Sodium hypophosphite (NaH2PO2�H2O) 21 g/l

Complexing agents and stabilizers Proprietary

Particles in suspension (Si3N4/CeO2/TiO2) 0–25 g/l

Operating conditions

pH 4.8–5.0

Temperature 88 � 1 8CPlating duration 2 h

Plating rate 10–12 mm/h

and 11 wt.% P) is very close to eutectic. The X-ray diffraction patterns obtained for electroless Ni–P–Si3N4, Ni–P–

CeO2 and Ni–P–TiO2 composite coatings, both in as-plated and heat-treated conditions, are very similar to those

obtained for plain electroless Ni–P coating both in as plated and heat-treated conditions. This suggests that the

incorporated second phase particles have no influence on the structure of electroless Ni–P matrix. However, X-ray

diffraction measurements alone are not sufficient to reach such a conclusion and hence warrant further characterization

using transmission electron microscopy (TEM).

Fig. 4 shows the TEM micrograph (bright field image) of plain electroless Ni–P coating in as plated condition. The

micrograph exhibits a dark network due to the inhomogeneous distribution of phosphorus in the electroless Ni–P

matrix. In general, inhomogeneity in phosphorus content is prevalent in electroless Ni–P coatings having high

phosphorus content. The energy dispersive X-ray analysis (EDAX) (Fig. 5) performed on the Ni–P matrix indicates a

phosphorus content of about 11 wt.%. The selected area diffraction (SAED) pattern of the plain electroless

Ni–P coating (Fig. 6) exhibits a diffuse ring pattern, which is typical of an amorphous structure [15,35–37].


Fig. 1. Optical micrograph of the cross section of the plain electroless Ni–P coating and electroless Ni–P–Si3N4, Ni–P–CeO2 and Ni–P–TiO2

composite coatings (a) Ni–P; (b) Ni–P–Si3N4; (c) Ni–P–CeO2; (d) Ni–P–TiO2 [R, resin; C, Cu over layer; EN, electroless Ni–P coating; ENSN,

electroless Ni–P–Si3N4 coating; ENCE, electroless Ni–P–CeO2 coating; ENTI, electroless Ni–P–TiO2 coating].

These observations confirm that the plain electroless Ni–P coating is amorphous in as deposited condition and strongly

support the observations made from X-ray diffraction measurements.

Fig. 7a–c show the TEM micrographs (bright field images) of electroless Ni–P–Si3N4, Ni–P–CeO2 and Ni–P–TiO2

composite coatings, respectively. The contrast from the matrix is uniform. The TEM micrographs (Fig. 7a–c) clearly

reveal the presence of individual second phase particles having an angular/sub-angular shape as well as agglomerated

particles in some regions. Fig. 8 shows the EDAX pattern obtained in the matrix region of electroless Ni–P–Si3N4

composite coating. Besides nickel and phosphorus, the presence of silicon is also apparent from the EDAX pattern,


Fig. 2. X-ray diffraction pattern of electroless Ni–P deposit in its as deposited condition.

Fig. 3. X-ray diffraction pattern of the electroless Ni–P deposit heat-treated at 400 8C for 1 h.

making evident of the incorporation of Si3N4 particles in the electroless Ni–P matrix. Similarly, electroless Ni–P–

CeO2 and Ni–P–TiO2 composite coatings shows the presence of cerium and titanium, confirming the incorporation of

CeO2 and TiO2 particles in the Ni–P matrix of the respective coatings. SAED patterns obtained on the Ni–P matrix

of electroless Ni–P–Si3N4, Ni–P–CeO2 and Ni–P–TiO2 composite coatings exhibit diffuse ring patterns resembling

the one obtained for plain electroless Ni–P coatings. However, the EDAX pattern taken on the particle region of the

matrix reveals the presence of the silicon, cerium and titanium in the electroless Ni–P–Si3N4, Ni–P–CeO2 and Ni–P–

TiO2 composite coatings, respectively. Fig. 9a–c, respectively, show the SAED patterns taken on the region of the

Si3N4, CeO2 and TiO2 particles and confirm the crystalline nature of these particles. Hansen and Moller [29] also made

similar observations for incorporation of TiO2 particles in electroless Ni–P matrix. The observations of the present

study and that of Hansen and Moller [29] postulate that the incorporated second phase particles did not alter the

structure of the electroless Ni–P matrix.

The DSC trace of plain electroless Ni–P coating (at a heating rate of 10 K/min) exhibit a well-defined exothermic

peak at 337.4 8C and a shoulder around 344 8C in the temperature range of 310–370 8C (Fig. 10). The crystallization

occurs over a narrow temperature range of 320–360 8C and the energy evolved during the exothermic transition,

calculated from the area of the peak is 68.14 J/g. The peak temperature and the energy evolved during the phase

transition calculated for electroless Ni–P coating, 337.4 8C and 68.14 J/g, respectively, is comparable to the values

obtained for similar coatings reported earlier [38].

Figs. 11–13, respectively, depict the DSC traces of electroless Ni–P–Si3N4, Ni–P–CeO2 and Ni–P–TiO2 composite

coatings obtained under similar conditions. Similar to plain electroless Ni–P coating, the electroless Ni–P–Si3N4,

Ni–P–CeO2 and Ni–P–TiO2 composite coatings also exhibit a well-defined exothermic peak in the temperature range


Fig. 4. Transmission electron micrograph (bright field image) of electroless Ni–P deposit in its as plated condition.

Fig. 5. EDAX pattern of electroless Ni–P deposit in its as plated condition.

of 320–360 8C. However, the DSC traces obtained for the composite coatings differ from one another and from that

obtained for plain electroless Ni–P coating. Electroless Ni–P–Si3N4 coating exhibit only a single sharp peak at

339.3 8C. Electroless Ni–P–CeO2 coating, besides the sharp exothermic peak at 339.1 8C, also exhibit a shoulder

around 345 8C. Electroless Ni–P–TiO2 coating, besides the sharp peak at 338.7 8C, exhibit an additional peak at

347.5 8C. The energy evolved during the exothermic transition, calculated from the area of the peaks of Ni–P–Si3N4,

Ni–P–CeO2 and Ni–P–TiO2 composite coatings are 70.37, 70.45 and 80.22 J/g, respectively. Table 2 shows the peak

temperatures and DH values calculated for all the coatings studied.

The peak temperature and the energy evolved during phase transformation in electroless Ni–P coatings are very

sensitive to the phosphorus content and the rate of heating of the coatings [15,33,37–40]. Since a constant heating rate

(10 K/min) is adopted in the DSC studies of all these coatings, this variable will not have any influence on the peak

temperature and the energy evolved during phase transformation. However, it is interesting to recall the results of

chemical analysis of these coatings. The electroless Ni–P coating is found to have 11 wt.% phosphorus and 89 wt.%

nickel, whereas with the incorporation of Si3N4, CeO2 and TiO2 particles in the Ni–P coating, the nickel and

phosphorus content vary slightly. The nickel and phosphorus contents of the composite coatings are as follows:

10.1 wt.% phosphorus and 81.79 wt.% nickel for electroless Ni–P–Si3N4 coating; 10.18 wt.% phosphorus and

82.38 wt.% nickel for electroless Ni–P–CeO2 coating; 10.40 wt.% phosphorus and 84.18 wt.% nickel for Ni–P–TiO2

coating. Many process variables could cause variation in phosphorus content of the electroless Ni–P coating and

among them; the pH of the electroless nickel bath is the most influencing parameter. One possible reason for variation

of the pH of electroless nickel plating bath could be the adsorption of H+ ions over the second phase particles added to

the bath. However, Yeh and Wan [41] and Szczygiel [42–44] suggest that such a presumption is not valid in plating

baths that contain very high concentration of Ni2+ ions (1 mol/dm3), where the adsorption of Ni2+ ions will be

preferred over the H+ ions. This appears to be true as very little change in phosphorus content of the Ni–P coatings of

the present study. Hence, it is evident that some other mechanism is responsible for the decrease in the phosphorus

content of the electroless Ni–P–Si3N4, Ni–P–CeO2 and Ni–P–TiO2 composite coatings.

During deposition of electroless Ni–P composite coatings, adsorption of the second phase particles on the electrode

surface is a prime factor that decides their incorporation in the coating. This adsorption process continues during the


Fig. 6. SAED pattern of electroless Ni–P deposit in its as plated condition (aperture size: 155 mm).


Fig. 7. Bright field images of electroless Ni–P composite coatings in their as deposited condition (a) Ni–P–Si3N4; (b) Ni–P–CeO2; (c) Ni–P–TiO2.

entire duration of the deposition. Such a process is most likely to cause a decrease in the active surface area available

for reduction of nickel and phosphorus. Electroless deposition of nickel, being an autocatalytic process, continuously

makes available surface sites that are catalytically active for further reduction of nickel and phosphorus. The

adsorption of second phase particles over this catalytically active area affects the possibility of reduction of nickel and


Fig. 9. SAED patterns obtained from the particle region (a) Ni–P–Si3N4 (aperture size: 110 mm); (b) Ni–P–CeO2 (aperture size: 530 mm); (c) Ni–

P–TiO2 (aperture size: 530 mm).

Fig. 8. EDAX pattern of electroless Ni–P–Si3N4 composite coating in its as plated condition.

Fig. 10. DSC trace of electroless Ni–P coating.

phosphorus at those sites and this result in coatings with decreased nickel and phosphorus content. Dennis et al.

[45] also observed a similar phenomenon with the incorporation of Cr2C3 particles in the Ni–P coating.

The other interesting feature of the composite coatings of the present study is that in spite of the decrease in the nickel

and phosphorus content with the incorporation of the second phase particles, the ratio of nickel to phosphorus remains the


Table 2

Peak temperatures and DH values obtained for electroless Ni–P and Ni–P composite coatings

Type of coating Peak temperature (8C) DH (J/g)

I II

Ni–P 337.4 – �68.14

Ni–P–8.10 wt.% Si3N4 339.3 – �70.37

Ni–P–7.44 wt.% CeO2 339.1 – �70.45

Ni–P–5.42 wt.% TiO2 338.7 347.5 �80.22

Fig. 11. DSC trace of electroless Ni–P–Si3N4 coating.

Fig. 12. DSC trace of electroless Ni–P–CeO2 coating.

Fig. 13. DSC trace of electroless Ni–P–TiO2 coating.

same as that of the plain Ni–P coating. As a result, the peak temperature for all the coatings remains around 338� 1 8C.

Similarly, the DH value of all the coatings of the present study is also of the same order. The higher DH value obtained for

electroless Ni–P–TiO2 coating is due to the contribution from the additional higher temperature peak at 347.5 8C.

DSC traces were recorded at different heating rates to get a better insight on the effect of these second phase

particles on the phase transformation behaviour of electroless Ni–P coatings. Figs. 14 and 15, respectively, show the

DSC traces obtained for electroless Ni–P and Ni–P–Si3N4 coatings at varying heating rates, viz., 3, 7, 12 and 15 K/

min. Using the onset temperatures obtained at different heating rates (Table 3) it is possible to calculate the activation

energy (Qc) for crystallization of the electroless nickel coatings using the relationship developed by Duswalt [31]:

Qc ¼ �Rdðln vÞdð1=TÞ


Fig. 14. DSC traces of electroless Ni–P coating at varying heating rates, viz., 3, 7, 12 and 15 K/min.

Fig. 15. DSC traces of electroless Ni–P–Si3N4 coating at varying heating rates, viz., 3, 7, 12 and 15 K/min.

Table 3

Variation of peak temperature for electroless Ni–P and Ni–P–Si3N4 coatings with heating rate

Type of coating Heating rate (K/min) Onset temperature (K) 1/T (�103 K) ln v (K/min)

Ni–P 3 591.4 1.690 1.098

7 602.0 1.661 1.945

12 609.1 1.641 2.484

15 612.2 1.633 2.708

Ni–P–8.10 wt.% Si3N4 3 592.7 1.687 1.098

7 603.3 1.657 1.945

12 610.5 1.638 2.484

15 613.2 1.631 2.708

where R is the gas constant (8.314 J/(K mol)), v the heating rate (K/min) and T is the onset of crystallization

temperature (in K). A plot of ln v versus 1/T is made using the variation in peak temperatures with heating rate

(Fig. 16a and b). The activation energy calculated using the slope values of the above plot are 234.39 and 237.75 kJ/

mol, respectively, for electroless Ni–P and Ni–P–Si3N4 coatings. Mahoney and Dynes [38] also performed a similar

analysis for electroless Ni–P coatings. According to them, the activation energy for crystallization of electroless Ni–P

(11 wt.% P) coating is 227.6 kJ/mol. For pure nickel, the activation energy for self-diffusion is approximately 289 kJ/

mol.

The above observations suggest that the incorporation of the Si3N4, CeO2 and TiO2 particles do not seem to have

any influence on the phase transformation behaviour of electroless Ni–P coating. This is not surprising as the

temperature range involved in the phase transformation of these coatings is of the order of 300–350 8C, the second

phase particles will not undergo any modification to cause a major change in the phase transformation behaviour of the

coating. It seems possible that even the observed variations in the DSC traces of the electroless Ni–P composite

coatings, such as, the formation of the second peak in the case of electroless Ni–P–TiO2 coating, is not due to influence

of the incorporated second phase particles but due to the Ni–P matrix itself.


Fig. 16. Plot of ln v vs. 1/T made using the variation in peak temperatures with heating rate.

Literature reports on electroless Ni–P coatings present different mechanisms based on which the phase

transformation occurs [15,16,19,24,34–40,46]. These studies classify the electroless Ni–P coatings in three categories

with respect to their phosphorus content. Accordingly, hypoeutectic alloys (<20 at.% P) exhibit at least two peaks; one

corresponds to the primary crystallization of Ni which occurs at lower temperature, whereas the other peak due to the

phase transformation with Ni and Ni3P as final products occurs at higher temperatures. Eutectic Ni–P alloys (20 at.%

P), in general, shows only a single sharp peak in their DSC traces. Hypereutectic Ni–P alloys (>20 at.% P) also exhibit

at least two peaks. Ni, Ni3P and Ni5P2 phases form at low temperature, whereas at higher temperature, the metastable

Ni5P2 phase transforms into stable Ni3P. All these reactions exhibit distinct peaks in the DSC thermograms.

The phosphorus content of plain electroless Ni–P and electroless Ni–P–Si3N4, Ni–P–CeO2 and Ni–P–TiO2

composite coatings lies close to 20 at.% P and hence their DSC traces are expected to be similar to that of an eutectic

Ni–P alloy coating. However, Figs. 10–13, besides the sharp exothermic peak, also exhibit other features such as the

appearance of a shoulder (Figs. 10 and 12) and a second peak (Fig. 13). The appearance of the shoulder (Figs. 10 and

12) may be due to the relaxation of lattice strain energy during the phase separation. Park and Lee [36] also observed a

similar shoulder pattern for electroless Ni–P (10.07 wt.% P) coating. Keong et al. [13] suggest that the formation of

shoulder is due to the first stage of transformation involving short-range atomic movements and incipient

crystallization of metastable crystallization structures [8,36]. The additional high temperature peak observed for

electroless Ni–P–TiO2 coating cannot be accounted for the recrystallization and growth of Ni3P phase, as its

occurrence is expected only around 400 8C [34,35]. The small difference in the peak temperature between the first

(338.7 8C) and second peak (347.5 8C) suggest a possible splitting. Hypoeutectic Ni–P alloys also exhibit a split in the

higher temperature peak [40] due to the differences in activation energy for crystallization from partially crystallized

matrix and amorphous matrix. Though such occurrence is not prevalent in eutectic Ni–P alloys, Rajam et al.

[40] suggest that these peaks too may show splitting.

4. Conclusion

The structural characteristics and phase transformation behaviour of plain electroless Ni–P coating and electroless

Ni–P–Si3N4, Ni–P–CeO2 and Ni–P–TiO2 composite coatings are studied. X-ray diffraction pattern, transmission

electron micrograph, selected area electron diffraction pattern and energy dispersive X-ray analysis reveal that the

structure of electroless Ni–P matrix is not changed with the incorporation of Si3N4, CeO2 and TiO2 particles. Phase

transformation behaviour studied by differential scanning calorimetry indicates that the variation in crystallization

temperature and the energy evolved during crystallization of plain electroless Ni–P coating and electroless Ni–P–

Si3N4, Ni–P–CeO2 and Ni–P–TiO2 composite coatings, is not significant. Hence, the study concludes that

incorporation of Si3N4, CeO2 and TiO2 particles in the Ni–P matrix does not have any influence on the structure and

phase transformation behaviour of electroless Ni–P coatings.

Acknowledgement

The authors wish to express their sincere thanks to Prof. T.S. Sampath Kumar, Department of Metallurgical and

Materials Engineering, Indian Institute of Technology Madras, Chennai, for his help and suggestions in DSC studies.

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Structure and Phase Transformation Behaviour Of

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