Improvement in tribological behavior of novel sol-enhanced electroless Ni-P-SiO2 nanocomposite coatings

    Improvement in tribological behavior of novel sol-enhanced electroless Ni-P-SiO2 nanocomposite coatings A. Sadeghzadeh-Attar, G. AyubiKia, M. Ehteshamzadeh PII: DOI: Reference:

S0257-8972(16)31020-9 doi:10.1016/j.surfcoat.2016.10.026 SCT 21671

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

30 April 2016 7 October 2016 9 October 2016

Please cite this article as: A. Sadeghzadeh-Attar, G. AyubiKia, M. Ehteshamzadeh, Improvement in tribological behavior of novel sol-enhanced electroless Ni-P-SiO2 nanocomposite coatings, Surface & Coatings Technology (2016), doi:10.1016/j.surfcoat.2016.10.026

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ACCEPTED MANUSCRIPT Improvement in tribological behavior of novel sol-enhanced electroless Ni-P-SiO2 nanocomposite coatings

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A. Sadeghzadeh-Attara*, G. AyubiKiab, M. Ehteshamzadehb Department of Metallurgy and Materials Engineering, University of Kashan, P.O. Box. 8731753153, Ghotb Ravandi Blvd., Kashan, Iran.

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Department of Metallurgy and Materials Engineering, Shahid Bahonar University of Kerman, P.O. Box 76175-133, Islamic Republic Blvd., Kerman, Iran.

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Abstract

Homogeneous nickel-phosphorus-silica (Ni-P-SiO2) nanocomposite coatings were prepared on low carbon

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steel alloy surfaces by a new process. This method was developed by combining sol-gel and electroless deposition technique in the presence of surfactant to produce a composite coating containing well-dispersed

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SiO2 nanoparticles. The SiO2-sol solution was directly added to the electroless Ni-P solution to produce nanocomposite coating on the St37 steel substrate. The surface morphology and composition analysis were studied by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS), which

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confirmed the presence of silica in the coating matrix. The results showed co-deposited SiO2 nanoparticles

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with a clean surface and no agglomeration. Therefore, this technique can effectively avoid the agglomeration of nanoparticles in the coating matrix. The structural phase analysis indicated that by heat treating, the

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amorphous phase changed to crystalline phases such as Ni, Ni3P and some non-equilibrium phases. The microhardness and wear properties of Ni-P and Ni-P-SiO2 composite coatings were evaluated and compared with each other. The hardness and wear resistance of Ni-P composite coatings increased in the presence of SiO2 nanoparticles, whereas the maximum microhardness of 970 Hv100, minimum coefficient of friction of 0.25, and minimum wear loss of 4.4 mg were achieved in heat treated coating at 400 °C for 1 h. Keywords: Electroless Ni-P-SiO2 nanocomposite coatings, Sol-gel process, High dispersed SiO2 nanoparticles, Hardness, Wear resistance. 1. Introduction

It is well known that electroless deposited Ni-P coatings have been widely used for many industrial applications because of their excellent wear resistance, high hardness, anticorrosive property, paramagnetic property, and electrocatalytic activity. With new prospects to improving properties of composite electro and electroless nickel coatings by incorporating of nanometer size particles, these research fields have lately received increased attention [1, 2]. The presence of nanoparticles in electroless Ni-P coatings is significant *

Corresponding author: Tel.: +98 3155912492; fax: +98 3155912424 E-mail addresses: [email protected] (A. Sadeghzadeh-Attar)

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ACCEPTED MANUSCRIPT for developments of the coating because various types of nanoparticles have a special nature which is much different from that of bulk counterparts [3]. Recently, it has been found that incorporation of finely dispersed nanoparticles such as Al2O3, ZrO2, SiC, SiO2, TiC, TiO2, TiN, and diamond into the Ni-P matrix, can significantly enhance properties of the Ni-P composite coatings, especially in corrosion and wear resistance.

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[4-12]. However, the so-called composite coatings are prone to some problems. Because nanoparticles have a

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high energy surface and activity, these nanoparticles are unstable and easily agglomerated in the electroless nickel plating bath without special surface modification. This issue directly affects homogeneous quality and

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weakens the mechanical properties of coatings [13, 14]. The composite coatings have non-uniformly distributed particulates and numerous defects due to the segregation and agglomeration of the nanoparticles

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and can only improve properties of the coatings to a limited level. To obtain high quality Ni-P composite coating, it is necessary to make the particles evenly distributed within the coating. Several factors influence

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the incorporation of particles in an electroless Ni-P composite coating, including characteristics of the particle in the plating bath (e.g. size, shape, type, concentration, surface charge, and relative density); electrolyte bath composition (electrolyte concentration, temperature, pH, additives, surfactant type, and

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concentration); the methods and degree of agitation (laminar, turbulent, vortex or mixed flow regimes); shape

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and size of electroplating vessel; and the compatibility of the particles with the coating matrix [1, 15]. However, a clear describe of the exact effect of the experimental parameters is difficult to obtain. Agitation

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of the plating solution is one of the key factors in determining particle incorporation and uniform deposition on the coating surface. In order to achieve good dispersion of the particles, various methods of agitation was employed such as circulation by pumping, purging of air, oxygen, nitrogen, ultrasonic vibration and magnetic stirring, air injection, parallel plate electrode, rotating disk electrode (RDE) and rotating cylinder electrode (RCE) techniques [15-18]. In general, if the agitation is too slow (laminar flow), the particles in the bath may not disperse completely, except when their density is low. Besides that, with increasing stirrer speed, the flow may be converted to vortex and agitation is too high [19]. The particles will not have sufficient time to get attached to the surface, and this result in poor particles incorporation. This indicates that the stirring speed is important and should be optimized to be the particles incorporated [15]. It should be noted that the complete dispersion of particles in the bath solution was difficult to achieve, even with the use of mechanical stirring, which effected the dispersion of the material in the coating. The effect of flow can be complex and is often underestimated, despite its overriding importance to particle suspension in the bath and composite deposit quality [20]. Walsh et al. [1, 21, 22] have reported that the flow induced by a magnetic stirrer is poorly defined, difficult to replicate, complex, vortex-prone flow, and is rarely appreciated. They have used RDE and RCE techniques to offer well-defined laminar or turbulent flow together with predictable mass transport to smooth electrode surfaces. Kalantary et al. [23] have also suggested that the laminar-turbulent transition 2

ACCEPTED MANUSCRIPT region is the most effective agitation condition for maximizing incorporation of SiC particles in electroless Ni-P composite coatings. Anyway, a suitable flow regime could effectively assist the dispersion of the particles in the Ni-P bath solution. On the other hand, some researchers showed that adding surfactant in the bath has potential benefits in maintaining good particle dispersion, which improves the deposit morphology

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and its properties [24, 25]. The surfactants are often utilized in colloidal systems, where they are responsible for uniformity and stability, as well an adsorption capacity. Surfactants are surface active agents with the

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molecules composed of a polar hydrophilic group, the ‘‘head”, attached to a non-polar hydrophobic group,

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the ‘‘tail’’. The hydrophilic part of the molecule prefers to interact with water while the hydrophobic part is repelled from water. Surface active molecules absorb at the air/water interface, decreasing surface tension or

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surface free energy of the interfaces. As the interface becomes saturated, the surfactant molecules start to form aggregates or micelles in the bulk of the solution, with the surface tension remaining constant [26].

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Surfactants may be classified in four categories depending on the nature of their head groups: anionic, cationic, non-ionic (steric stabilized), or zwitter-ionic (electrostatic interaction). A non-ionic surfactant has no charge groups in its head and an ionic surfactant carries a net charge. The cationic surfactant contains

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positive charge in hydrophobic position and the anionic surfactant contains negative charge in hydrophobic

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position [27-30]. Many attempts have been made to find out the effect of surfactants on the morphology and properties of electroless composite coatings. Elansezhian et al. [31] performed studies on Ni-P coatings

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influenced by adding sodium dodecyl sulfate (SDS) during deposition. They reported that the surfactant utilized in the process modified the surface morphology and dispersion of nickel particles of the coating. Similarly, a very brief conclusion was derived by Hagiwara et al. [32] as well, who studied effect of three different surfactants added in the electroless Ni-P bath on the morphology of the resulting particles. Therefore, excellent comprehensive properties of the electroless Ni-P based composite coatings are highly dependent on the preparation techniques, specifically stable dispersion of the nanoparticles [33, 34]. Zhou et al. [35] developed a new process combining sol-gel and electroless plating technique to form Ni-P-TiO2 nanocomposite coatings with highly dispersed TiO2 nanoparticles. This technique utilizes transparent sol containing desirable components to form TiO2 nanoparticles in the electroless plating solution, which can be co-deposited with Ni-P coating matrix to produce Ni-P-TiO2 dispersive nanocomposite coatings. Up to now, this technique which could obtain interesting results has not been utilized for SiO2 nanoparticles. This method can be used to examine the possibility of other oxide nanoparticles to taking advantage of its benefits. This work aimed to study the possibility of a novel technique, sol-gel enhanced electroless plating, to synthesize highly dispersive SiO2 nanoparticles. Furthermore, the formation mechanism of SiO2 nanoparticle was discussed and the influence of these nanoparticles on the structure, hardness and wear resistance of the coatings was also investigated. 3

ACCEPTED MANUSCRIPT 2. Experimental procedure

2.1. Sol preparation For the preparation of transparent SiO2 sol, from tetraethyl orthosilicate (TEOS, 99.9%, Merck), absolute

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ethanol (EtOH; 99.9%, Merck), acetic acid (99.8%, Merck) and distilled water were used. The chemicals were employed without any further purification. The molar ratio of precursor materials for preparation of sol

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solutions are given in Table 1. The solution was stirred for 1 h under magnetic stirring for complete mixing.

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First, ethanol, distilled water and acetic acid were mixed. Then, after 10 min of stirring, TEOS was added into water/acetic acid/ethanol solution and the mixture was stirred for 1 h.

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2.2. Deposition of electroless Ni-P/nano-SiO 2 composite coating Commercial St37 steel alloy samples (10×10×5 mm3) were used as the substrate with composition (wt.%) of

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C: 0.21, P: 0.065, S: 0.04, Si: 0.17, Mn: 0.35 and Fe: base. The specimens were mechanically polished using 1200 grit SiC paper. Prior to coating deposition, the substrates were cleaned in acetone, followed by cleaning in NaOH for 5 min and finally, they were given a pickling treatment with immersion in 50 vol.% sulphuric

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acid solution for deoxidization for 30 s. After each step, the samples must be washed with distilled water for

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1 min to remove the residual liquid of the last process in order to avoid affecting the next process. After cleaning, the specimens were immediately transferred to the plating solution. A rigid stand was used to hold

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and support the substrate and temperature sensor. A 250 mL glass beaker containing the electroless solution was held by a tight-fitting metal ring on the magnetic stirrer hotplate for coating deposition. A polygonal PTFE-coated cylindrical bar with length of 25 mm and 6 mm in diameter was applied as magnetic stirrer material at 100 rpm to generate a consistent bath agitation. The important function of magnetic stirrer is to maintain the suspension of particles in the electroless bath and to avoid agglomeration of SiO2 particles. When the magnetic stirrer begins to rotate, the electroless bath column starts moving and after some time a statistically stationary flow sets in. Inherent fluctuations around the mean arise due to the periodic motion of the stirrer bar, the lack of a fixed axis of rotation, and it seems that the flow is mainly turbulence. The bath composition and operating conditions of the electroless composite coatings are given in Table 2. pH value of the baths was maintained in the range of 5.0±0.1 by adding the required quantity of ammonia solution. The SiO2-sol solution was added to the bath in three steps: first before transferring the specimen to plating solution; next, 15 min after coating and finally 30 min after coating. At the same time, the solution was kept homogeneous by magnetic stirring at speed of 100 rpm. After the deposition on the substrate, the samples were dried and isothermally heat-treated at temperatures 300, 400 and 500 °C for 1 h, subsequently cooled

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ACCEPTED MANUSCRIPT down to room temperature to minimize the internal stresses. The heat treatment was carried out in an electric resistance furnace to improve the hardness and wear resistance of composite coatings. 2.3. Adsorption isotherm

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For measurements of the adsorption isotherm, SDS with concentrations from 0 to 2 g/L were added to a set of 100 ml SiO2-sol solution and the mixture were stirred at 100 rpm speed for 24 h at 87 oC to ensure

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equilibration. The amount of SDS adsorbed on the surface of the SiO2 particles, Γ (mg·g-1), was determined

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by measuring different concentrations in the solution before and after adsorption by colorimetric method using the following mass balance formula [36, 37]:

(1)

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Γ= (C0-Ce)V/m

where, C0 and Ce are the initial and equilibrium concentrations of SDS (mg·g-1) respectively, V is the volume

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of the solution (L), and m is the weight of the SiO2 particles (g). The values of surfactant adsorbed per gram of solid were then plotted against their corresponding Ce values at constant temperature and pH to construct the adsorption isotherm. The concentration of sodium dodecyl sulfate can be calculated using Beer's law,

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which holds for many dilute solutions, states that absorbance is linearly related to the concentration was used

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[38]:

(2)

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A= ε.b.c

where, A is absorbance, ε is the molar absorptivity, b is length of light path through the sample (cm), and c is sample concentration. There is a UV adsorption peak of sodium dodecyl sulfate at 210 nm, which was selected as the detection wavelength for the maximal UV adsorption occurs there. 2.4. FTIR analysis

Transmission FTIR analysis was carried out on SiO2 sol and dried powder using Bruker (Tensor 27 model) spectrometer in the wavenumber region between 400 and 4000 cm-1. 2.5. Morphology and structure of the coatings The morphology and microstructure of the coatings were analyzed using scanning electron microscope (SEM; VEGAII TESCAN, Czech Republic) equipped with an attachment for energy dispersive spectrometry (EDS) analysis. The images were taken using a secondary electron (SE) detector. X-ray diffraction (XRD; Panalytical X’Pert PRO, CuKα radiation) analyzer with a scanning rate of 0.2 o/min equipped with HighScore software was used for identification of phases formed in the coatings. A Hitachi H-800 5

ACCEPTED MANUSCRIPT transmission electron microscope (TEM) operated at 200 kV was employed to study the detailed microstructure and distribution of the SiO2 particles in the Ni-P composite coating. The structure of the coating was characterized by the selected area electron diffraction (SAED) technique. The peeled deposits

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were cut to 4 mm diameter and thinned to 20 μm by mechanical polishing, and finally thinned to a suitable

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thickness for TEM analysis by ion-beam technique.

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2.6. Bonding strength of the coatings

The bonding strength between the coatings and the substrate was evaluated using a scanning scratch tester

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(SHIMADZU, SST-101), and the critical load at which the coatings start to be parted from the substrate was measured as the bonding strength of the coatings. A diamond indenter of conical shape with a tip radius of

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0.2 mm, conical angle of 120o, scratch rate of 0.002 mm/s, and scratch length of 5 mm was selected to perform the test. Five readings of such critical load values were averaged for each sample, and the standard deviation of these data was taken as the error.

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2.7. Hardness and wear resistance measurements The Vickers microhardness of nanocomposite coatings was measured by a micro-hardness tester with load of

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100 g for 15 s. During data processing the highest and lowest values were removed and hardness value as an average of seven measurements was reported. Pin-on-disc tests were performed to determine the wear resistance of the coatings at a load of 30 N and a sliding speed of 0.1 m/s. The wear tests were carried out against a SAE 52100 bearing steel pin with hardness of 55 HRC under 1000 m sliding distance. Ambient temperature was kept at 25 oC and relative humidity at 35% in all experiments. No lubrication was used during the test. 3. Results and discussion

3.1. Formation mechanism of SiO 2 nanoparticles Formation of SiO2 nanoparticles during the preparation and deposition process can be intricate. The sol-gel process produces SiO2-sols directly based on the hydrolysis-condensation reactions of alkoxides (tetraethyl orthosilicate; Si(OC2H5)4) as precursor with water and in the presence of acetic acid as catalyst. In the preparation of SiO2-sol utilizing neutral ethanol as solvent, condensation process of organic metal macromolecule ions started before completion of hydrolysis, and the formation of ordered structure was hindered, hence the as-synthesized SiO2 nanoparticles were found to be amorphous. Hydrolysis occurred by the nucleophic attack of the oxygen contained in water on the Si atom as evidenced by the reaction of 6

ACCEPTED MANUSCRIPT isotopically labeled water with TEOS that produces unlabeled alcohol in both acid- and base-catalyzed systems [39]. The hydrolysis reaction replaced alkoxide groups (OC2H5) with hydroxyl groups (OH). Hydrolysis was facilitated in the presence of alcohol as homogenizing agent that was especially beneficial in promoting hydrolysis of silanes containing bulky organic or alkoxy ligands. The condensation/

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polymerization between the silanol groups or between silanol groups and ethoxy groups created siloxane

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bridges (Si-O-Si) that form entire silica structure. In summary, hydrolyzed TEOS monomer was polymerized to form a large microgel cluster which reaches the size and degree of cross-linking where they became

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insoluble and collapse. After, or during, the collapse, the microgel underwent additional condensation to form high density, colloidally stable particles. Once the particles were formed, they were prevented from

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aggregating with other stable particles due to double layer effects. However, monomers were still being hydrolyzed in the solution .They reacted until formation of polymers which reach a large enough size to

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collapse or encountering a particle and attaching to its surface. When polymers reacted with a surface, they could collapse onto the surface and enable the particles to maintain a spherical shape. This was the postulated formation mechanism of the colloidal silica particles from alkoxides [40]. Hydrolysis had the

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highest speed and completeness when sol was added to Ni-P electroless bath solution. When the SiO2-sol

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solution is added into the plating solution, the hydrate Ni ions in the solution will lead to the polymerization reaction of sol, forming well dispersed SiO2 nanoparticles. The nanoparticles can be absorbed into the

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freshly deposited surface with the help of Van der Waals forces and mechanical movement driven by continuous agitation. If the particles are not deposited uniformly on coated surface causes numerous defects in the coating and the resulting are not helpful to the improvement of the properties of the composite coatings. The main reason for non-uniform depositing of the particles may be agglomeration and segregation of the particles in electroless bath. Therefore, maintain good dispersion of particles in electroless Ni-P bath is an important factor in the formation of a suitable coating. Also, the added ammonium to bath solution in term of pH regulation seriously affected the hydrolysis and condensation rate of sol. Resolved mineral acids and ammonia in bath solution acted as catalysts and forwarded hydrolysis and condensation reactions. Therefore, progress in the addition of ammonia to electroless bath led to degradation in sol stability. Finally, the dispersed SiO2 nanoparticles were co-deposited into the Ni-P matrix to produce the Ni-P-SiO2 nanocomposite coatings. 3.2. Surface modification of SiO 2 nanoparticles Due to the high surface free energy and abundant hydroxyl groups on the surface of ultrafine SiO2 particles, these nanoparticles tend to agglomerate and make bigger structures, which result in a poor performance of the composite material in applications. Therefore, for practical applications, surface modification and suitable 7

ACCEPTED MANUSCRIPT surfactant choice is important and necessary to improve the dispersion stability and compatibility of the particles in the electroless plating bath [41, 42]. Adsorption of surfactants on the particle surface serves to aid the achievement and maintenance of a good dispersion in the bath which facilitates a reliable and predictable dispersion of particles within the growing metal deposit [1, 36, 43]. Sodium dodecyl sulfate (SDS) is by far

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the most important surfactant that often utilized in electroless Ni deposition. The presence of SDS in the

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electroless bath can increase the stability of the system and increases the suspension of particles to avoid the decomposition of electroless bath [44]. It reduces the agglomeration of composite SiO2 particles by

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increasing the repulsion force between particle charges and reducing the electrostatic adsorption of suspended particles on the substrate, which is accountable for smooth surface of composite coating [45]. At

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low surfactant concentration the SDS molecules are adsorbed individually and at higher concentration the monolayer and bilayer structures are formed. These structures act as micelles and the concentration at which

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surfactants begin to form micelles is known as the critical micelle concentration or CMC [31]. CMC is a key parameter for the optimization of surfactants, which directly affects the performance of a surfactant, is unique to every surfactant and is affected by many factors in the electroless deposition. However, micelles are in

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dynamic equilibrium with surfactant monomers in the bulk, which are frequently being exchanged with the

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surfactant molecule in the micelles. When micelles formation takes place, the head group repulsions are balanced by hydrophobic attractions and for ionic micelles, also by attractions between head groups and

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counter ions. Hydrogen bonds can also be formed between adjacent head groups [26]. The adsorption isotherm was drawn to predict the amount of adsorption at a certain concentration of the SDS on the surface of SiO2 particles (Fig. 1). Adsorption isotherm shows how the amount of surfactant adsorbed onto the surface of SiO2 particles increased with the concentration of the surfactant in the bulk solution, provides a plausible explanation for the occurrence. From this isotherm study the maximum adsorption capacity was found to be 1.25 mg/g and it occurred when the concentration of SDS was about 1500 mg/L. The CMC condition was established through the concentration variation of a particular surfactant so as to achieve the maximum dispersion of SiO2 nanoparticles. Therefore, the concentration of SDS should be maintained at CMC levels (about 1000 mg/L) as discussed. 3.3. FTIR studies Fig. 2 shows FTIR spectra of the resulting SiO2-sol solution (Fig. 2a) and as-prepared powder dried at room temperature (Fig. 2b) at the same molar ratios of the components. The band around 3441 cm-1 was attributed to Si-OH stretching vibration, hydrogen bonded. Intensity of C-H absorbance group and broad absorption band in the O-H stretching 3300-3600 cm-1 region (-H bonded H2O, -H bonded OH vibrations of alcohol, hydroxyl terminals, and -H bonded Si-OH in chain) decreased or disappeared when the silica-sol solution 8

ACCEPTED MANUSCRIPT was dried [46]. After drying, the adsorbed water peaks almost were disappeared while vibration of hydroxyl groups could be still observed. Infrared spectrum of the samples indicated that there were two bands around 2980 and 2930 cm-1 which could be corresponds to CH3 and CH groups. Also, absorption band around 1716 cm-1 corresponded to C=H stretching. The band around 1090 cm-1 is related to asymmetric stretching

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vibration of Si-O-Si band, in which the bridging oxygen atom moved parallel to the Si-Si lines in the

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opposite direction to their Si neighbors. The band around 800 cm-1 could be related to Si-O bending vibration, in which the oxygen moved at the right angle to the Si-Si lines in the Si-O-Si plane. The band at

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~463 cm-1 is corresponded to Si-O rocking vibration, in which oxygen atom moved perpendicular to the SiO-Si plane [47]. However, the results showed that the colloidal silica particles were formed in both sol

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solution and as-prepared powders.

3.4. Surface and Cross-sectional morphologies of the composite coatings

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Fig. 3 shows SEM images of the surface morphology and elemental composition of the conventional Ni-P and the novel Ni-P-SiO2 composite coatings, respectively. The coatings indicated typical spherical nodular

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structure with shallow boundaries. It is evident from the micrographs shown in Figs. 3a and c that surface of

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electroless Ni-P composite coating has more pores with slightly larger grain size compared to that of the novel Ni-P-SiO2 composite coating with as smaller nodular morphology. For Ni-P composite coating (Fig.

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3a), the average grain size is on the order of ≤20 μm, while incorporation of SiO2 particles in the coating are believed to inhibit growth of the Ni-P nodules thus reducing their size to approximately 4 μm, as estimated from the inset of the Fig. 3c. The grain refinement of Ni-P-SiO2 composite coating is due to this fact that the SiO2 particles impede the growth of the nodules and can be acted as more nucleation sites. In order to prove the presence of SiO2 particles in the nanocomposite coating, EDS analysis of area scanning was carried out and the corresponding spectra are shown in Figs. 3b and d. The EDS measurement presented in Fig. 3d clearly indicates the presence of Si and O peaks that were observed together with Ni and P peaks. This confirmed that there are second phase SiO2 nanoparticles in the Ni-P matrix. Several conditions, such as method and sol addition procedure, change in pH, temperature and bath composition were examined for this purpose. The aim was achieved only by adding of SDS as surfactant to electroless solution before the beginning of deposition. Chemical composition of Ni-P and Ni-P-SiO2 composite coatings is given in Table 3. EDS analysis showed that Electroless Ni-P and novel Ni-P-SiO2 coatings contained about 6.21 wt.% P, 93.79 wt.% Ni, and 8.37 wt.% P, 87.92 wt.% Ni, respectively. A considerable variation in phosphorus content could be seen after co-deposition of nano-SiO2 particles in Ni-P matrix. It is evident that this higher phosphorus content of novel coating prevented the nucleation of f.c.c nickel phase and has resulted in an amorphous structure of Ni-P coatings. Thus, chemical composition of the Ni-P matrix was influenced by 9

ACCEPTED MANUSCRIPT adding silica-sol solution, which was due to the change in electroless bath composition and thereupon the change in deposition process as a result of sol addition. In this work, coating composition of 2.5 wt.% incorporation of SiO2 nanoparticles was obtained with using a surfactant but only 1 g/L of SiO2-sol solution

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in Ni-P bath. Low and high magnification SEM surface morphology of the electroless of Ni-P-SiO2 composite coating

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annealed at 400 oC for 1 h at low and high magnification are illustrated in Fig. 4. As can be clearly observed the coating surface had nodular structure and the SiO2 particles uniformly were distributed in the surface of

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the Ni-P matrix. High magnification view (Fig. 4b) revealed the absence of any agglomerates from the microstructure of coating confirms the uniform dispersion of SiO2 nanoparticles in the Ni-P matrix. It can be

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also seen from Figs. 3c and 4b that after annealing at 400 °C, the average nodular size of the Ni-P-SiO2 coating increased to ≥5 μm compared to that of the as-plated Ni-P-SiO2 coating.

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Fig. 5 shows cross-sectional SEM images with line scan analysis of the electroless Ni-P and novel Ni-P-SiO2 composite coatings. The cross-sectional images of composite coatings (Figs. 5a and c) indicated that the

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coatings were dense, crack-free, uniform, and homogeneous with an average thickness of 14 μm for the Ni-P

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and 17 μm for the novel composite coatings. There was a smooth interface between steel substrate and coatings, which was due to the good pre-treatment. For the Ni-P-SiO2 coating, a clean and homogeneous

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surface could be seen without any visible SiO2 particles probably due to their extremely small size and relatively low content of 2 ± 0.5 wt%. The EDS line scans show relatively homogeneous distribution of Ni and P along the composite coatings, which is shown in Figs. 5b and d. The corresponding EDS line scanning of Fig. 5c shows that the nickel, phosphorus and silicon distributed uniformly and longitudinally from the surface to the bottom of the composite coating. The EDS analysis represents a low quantity but uniform distribution of Si in novel Ni-P-SiO2 composite coating, corresponding to the good dispersion of SiO2 small particles in the novel Ni-P-SiO2 composite coating. Typical TEM bright field image and electron diffraction patterns of the Ni-P-SiO2 composite coating annealed at 400 oC for 1 h are demonstrated in Fig. 6. The Ni-P-SiO2 composite coating shows a homogeneous contrast with the highly dispersive SiO2 particles with the size less than 150 nm were uniformly distributed in the composite coatings as pointed by the arrows. As illustrated in the diffraction pattern (inset of Fig. 6), the pattern consisted of continuous rings that can be assigned as the crystalline structure of coating. 3.5. Phase analysis of the composite coatings

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ACCEPTED MANUSCRIPT XRD patterns of the electroless Ni-P and Ni-P-SiO2 composite coatings in as-plated and after different heat treatment conditions are shown in Figs. 7a and b, respectively. The effects of annealing temperature on the structure of the Ni-P and Ni-P-SiO2 coatings were compared. There was a single broad diffraction peak at 44.6o for both as-plated composite coatings that is corresponded to the (111) plane of Ni (JCPDS: 01-1260).

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It can be seen that both coatings have a typical amorphous structure. Since the well-known, formation of

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amorphous structure in the electroless Ni-P coating is attributed to the distortion of the crystal lattice of Ni by the P atoms [48]. Basically, a disorder in arrangement of atoms manifests itself as a broad peak in XRD

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patterns. Since phosphorous has lower solubility in nickel, it causes lattice disorder in the crystalline nickel. The higher phosphorous in the coating, the higher disorder and the structure becomes amorphous. Therefore,

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the system remains in the amorphous state because of kinetic constraints and the low solubility of the phosphorus [49]. It is evident that the XRD pattern of the Ni-P-SiO2 composite coating is similar to that of

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nanoparticle free coating. Also, diffraction peak corresponding to silica could not be traced in the as-plated Ni-P-SiO2 nanocomposite coating even though presence of SiO2 particles was evident from the EDS analysis, again probably due to the low quantity, low crystallinity, and highly dispersive distribution of the nanosized

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SiO2 particles or high intensity of Ni diffraction peak. Crystallization of electroless Ni-P deposits begins

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when the thermal energy provided by thermal processing exceeds the activation energy for crystallization from the amorphous state. In general, the overall crystallization process has two stages: thermal activation

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leading to local atom movements and incipient crystallization into metastable crystalline structures, followed by long range thermal migration of atoms and decomposition to equilibrium phases [50]. According to the XRD patterns shown in Fig. 7, the broad peak corresponding to Ni is eliminated and Ni diffraction peaks were heightened after heat treatment at 300 oC for 1 h. The present phases at the transformation temperature identified by XRD patterns of Ni-P-SiO2 coating are metastable Ni2P, Ni5P2 and Ni12P5 phases before the transformation to the final stable Ni3P phase, same as obtained for the Ni-P matrix [49]. The full width of the peaks at their half maximum (FWHM) also decreased with increase in temperature. It can be observed that there was no visible peak belonging to SiO2 phase at 300 oC. Thus, we can conclude that the phase transformation of amorphous to crystalline SiO2 has not started yet. After annealing at 400 oC for 1 h, XRD pattern of the samples indicated the existence of stable Ni and Ni3P phases. Annealing at high temperature causes phosphorous segregation in the grain boundaries of the Ni-P matrix and formation of phosphorous rich zones. Nickel atoms from the matrix react with phosphorous atoms once the phosphorous content in the area exceeds a certain amount, contributing to precipitation of Ni3P with body centered tetragonal (BCT) crystal structure (JCPDS: 01-074-1384) [51]. These changes exhibited that annealing at 400 oC improves the crystallite orientation along the (111) and (200) planes of Ni, which is reflected in the sharpened peaks in the Ni-P coating matrix. Thus, at this temperature, the complete transformation can be occurred for the 11

ACCEPTED MANUSCRIPT metastable nickel phosphides to stable Ni3P of the electroless Ni-P coatings [49]. Besides, peaks were also observed at the diffraction angles of 20.8 o, 26.7 o, and 50.1o attributed to crystalline SiO2 nanoparticles. It is being noticed that the peaks related to Ni and Ni3P phases remain without change, which confirms the undistorted structure of the phases on reinforcement of SiO2 nanoparticles and the co-deposited particles

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have not influenced the phase transformation. With the increasing of temperature to 500 oC, coarsening of

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Ni3P phase occurred, which in turn reduced the P content of the remaining material to form Ni and Ni 3P phases. Heat treatment at higher temperatures encouraged both the formation and growth of the Ni, Ni3P, and

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other phases such as NiO [48, 52]. However, XRD analysis showed that there were no changes in the

became larger. 3.6. Mechanical properties of the composite coatings

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position of SiO2 diffraction peaks during crystallization although the peaks became sharper as crystallites

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The bonding strength between the coating and the substrate is an important indicator for the tribological properties of electroless plating Ni-P coatings, and is directly related to the reliability and durability of the

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material using. The bonding strength is force required peel unit area of the coating from the substrate. The

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bonding strength of the electroless Ni-P and Ni-P-SiO2 composite coatings in as-plated and after various heat treatment temperatures was determined by using scratch test method, and the results are shown in Fig. 8.

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Critical load (Fc) is the powerful criterion in the scratch method, which can qualitative that the bonding between the coating and the substrate. As can be seen, the bonding strength of the Ni-P coatings is all close to the Ni-P-SiO2 coatings. It is found that adding SiO2 nanoparticles had no remarkable effect on the bonding strength of the Ni-P coatings. It can also be observed that bonding strength of coatings increased with the increasing treatment temperature. The critical load reaches the maximum value when the heat treatment temperature increases 500 oC, whereas the microhardness of the coatings is not the highest. Therefore, it can be deduced that the heat treatment at higher temperatures may result in stronger diffusion between the coating and the substrate, consequently increasing the bonding strength of the coatings [53]. Hardness of coating is defined as resistance to deformation caused by the indentation, which is an important indicator of coating performance, and to a certain extent, reflects the wear resistance of coatings. The Vickers microhardness of substrate, electroless Ni-P and Ni-P-SiO2 composite coatings are shown in Fig. 9. The microhardness of the as-plated Ni-P coating was measured 380±10 Hv and the hardness increased to 540±10 Hv by applying sol-enhancing technology. Obviously, the Ni-P-SiO2 samples have higher microhardness than the conventional Ni-P coatings both in as-plated and heat-treated conditions. It is due to the dispersion strengthening of hard nanoparticles and precipitation strengthening in Ni-P matrix [54, 55]. During the deposition of coatings, SiO2 nanoparticles are adsorbed onto the surface and serve as the nuclei of grains. 12

ACCEPTED MANUSCRIPT These particles as a second phase are capable to reinforcing the Ni-P coating and impede fast propagation of dislocations in ductile Ni-P matrix [56]. It is well known that the refinement of grain size causes improvement of strength and microhardness of coating, in accordance with the Hall-Petch relationship [57]. Also, they could act as a barrier against plastic deformation of Ni-P matrix and hence increase the

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microhardness [58]. The heat treatment further greatly enhances the microhardness of all samples as compared to the untreated samples. Heat treatment can improve the microhardness whereas Ni-P and Ni-P-

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SiO2 coatings heat-treated at 400oC have a maximum hardness of about 785 and 970 Hv, respectively. It can

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be due to the complete transformation of amorphous to crystalline Ni-P coatings and formation of the hard intermetallic precipitates of Ni3P during heat treating at 400 oC, which prevent the grain growth in coatings

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[48, 59]. Besides, the existence of high internal stress could be the reason for the increase in microhardness. This internal stresses could be the result of the distortion caused by the non-equilibrium locations of the

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nickel and phosphorus atoms [60]. During the crystallization of Ni-P-SiO2 composite coating, SiO2 nanoparticles embedded in Ni-P matrix prevent from rapid growth of grains. Precipitation of hard particles acts as barriers for dislocation movement thereby increasing the hardness further. This unique property of

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electroless alloys makes them suitable for applications requiring wear resistance [54, 61, 62]. After

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increasing the heat treatment temperature up to 500 oC, the maximum microhardness of Ni-P-SiO2 coating reduced to 885±10 Hv, which could be attributed to the coarsening of grain size. It has been also reported that

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semi-coherent Ni3P precipitates can be transformed to non-coherent precipitates at higher temperatures, contributing to lower coating microhardness [63]. Generally, it is found that microhardness of heat treated composite coatings depends on three factors including level of incorporation of particles, heat treatment temperature and uniform distribution with less agglomeration of particles [64]. The tribological behavior of solid bodies results in the two major phenomena of friction and wear. The friction coefficient and weight loss of the electroless Ni-P and Ni-P-SiO2 composite coatings are shown in Figs. 10 and 11, respectively. From Fig. 10a, it is clear that average friction coefficient of the conventional Ni-P coatings (the ratio of tangential friction force to normal force) during the steady-state period is ranging from 0.65 for as-plated sample to 0.43 for annealed sample at 500°C. The values of the friction coefficient vary slightly, mainly due to the crystalline nickel phosphides produced by the heat treatment. The Ni-P coating heat treated at 400 oC for 1 h exhibited the lowest value of the coefficient of friction equal to 0.39, which could be associated with the formation of hard Ni and Ni3P phases. Variations of friction coefficient with sliding distance for novel Ni-P-SiO2 composite coatings were also shown in Fig. 10b that is vary from 0.47 for as-plated coating to 0.34 for annealed coating at 500 oC. The friction coefficient is found to be higher in as-plated coating when compared to the heat-treated coatings. Composite coating with SiO2 nanoparticles 13

ACCEPTED MANUSCRIPT heat treated at 400 oC showed the lowest coefficient of friction (0.25). Comparison of the friction coefficient values between Ni-P and Ni-P-SiO2 composite coatings indicated that the co-deposition of SiO2 particles in Ni-P matrix has decreased the friction coefficient of Ni-P matrix. Lower values of friction coefficient may be attributed to the incorporation of the hard SiO2 particles in the Ni-P-SiO2 composite coatings. Results of the

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wear resistance tests of electroless Ni-P and Ni-P-SiO2 composite coatings are presented in Figs. 11a and b,

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respectively. The weight loss during the wear test for as-plated Ni-P coating is highest under the applied load (Fig. 11a). Heat treatment of the samples could improve the wear behavior and mechanical properties of Ni-P

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matrix. Heat treatment process transforms amorphous phase to crystalline Ni and nickel phosphides such as Ni2P and Ni3P that leads to changes in the microstructure [48]. These phases have low reciprocal solubility

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with iron resulting in an incompatible surface with the steel pin, hence, effectively increase the hardness and wear resistance of the Ni-P matrix [65]. Minimum weight loss during tests is attributed to the samlple

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annealed at 400 oC for 1 h, that shows better wear resistance campared to other Ni-P coatings. The improvement of wear resistance could be mainly attributed to the formation of hard and stable phases of Ni and Ni3P and the improved hardness [66]. Annealing at 500 oC, leads to the gradual decrease of the wear

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resistance. This is caused by the coarsening of the Ni grains and of the phosphide precipitates. Negative

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influence of the coarsening is partially compensated by the formation of hard intermetallic phases on the interface of substrate/coating. This effect is most significant in case of the coating annealed at high

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temperatures [67]. In case of wear behavior of Ni-P-SiO2 composite coatings, shown in Fig 11b, the ascoated sample had the highest weight loss, while the sample annealed at 400 oC had the lowest weight loss, which is in good agreement with microhardness of the coatings. After co-depositing SiO2 nanoparticles into the electroless Ni-P matrix by using sol-enhancing technique, the wear resistance was decreased. The SiO2 nanoparticles not only increases the nucleation rate but also drastically prevents grain growth, which makes for much finer grains formation of numerous dislocations on the strengthened coatings [2, 54]. The comparison between Ni-P and Ni-P-SiO2 composite coatings exhibits that the highly dispersed hard SiO2 nanoparticles and trapping of these particles in the Ni-P matrix leads to an increase in the hardness and wear resistance. However, among all the coating samples, the Ni-P-SiO2 composite coating annealed at 400 oC for 1 h had the highest microhardness, lowest friction coefficient and best wear resistance. The possible reasons for the perfect wear resistance of these coatings are attributed to the reinforcing function of SiO2 nanoparticles and changes in the microstructure by formation of hard crystalline precipitates such as Ni3P in the coating after annealing at 400 oC. To understand the mechanism of wear in the coatings, the wear track patterns were studied using SEM. The SEM images of typical wear scar on the surface of the Ni-P and Ni-PSiO2 composite coatings annealed at 400 oC are shown in Figs. 12a and b, respectively. It can be observed that worn surface for the Ni-P-SiO2 coating is narrower than the Ni-P coating in the same sliding wear 14

ACCEPTED MANUSCRIPT conditions. The wear surface image reveals that abrasive grooves and scuffing of the Ni-P-SiO2 coating with absence of prominent plastic deformation. The formation of grooves can be associated with the presence of co-deposited SiO2 particles between steel pin and the coating surface. During the wear test process the hard SiO2 particles have reduced the direct contact of steel pin with coating and acts as lubricating layer which

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increases the wear resistance of the coating. The wear resistance of the strengthened coating is correlated to

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the content and distribution of the hard particles and microstructure of the coatings [64]. Therefore, the above results suggest an abrasive wear mechanism in studied coatings that is the most important wear mechanism in

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the similar electroless Ni-P composite coatings containing hard particles [68, 69]. The presence of hard SiO2 nanoparticles and fine intermetallic nickel phosphide phases constitute a favorable combination that resists

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abrasion. 4. Conclusion

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SiO2 nanoparticles were successfully applied to electroless Ni-P composite coating via addition of transparent sol to the conventional electroless solution. This novel method of introducing nanoparticles by

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adding SiO2-sol solution could effectively avoid particle agglomeration. The mechanism of SiO2 nanoparticle

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formation was also studied in detail in terms of the hydrolysis and condensation. The SEM images showed the presence of SiO2 nanoparticles, in which the absence of any agglomerates from the microstructure of

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coating confirmed the uniform dispersion of SiO2 nanoparticles throughout the surface of the Ni-P matrix. The hardness and tribological properties of coatings are improved in the presence of SiO2 nanoparticles and hard crystalline precipitates of Ni and Ni3P, which prevent grain growth and mobility of matrix dislocations. A very good correlation between the surface morphology, chemical composition, microhardness, and tribological behavior of coatings was found in this investigation. The wear mechanism was also studied and found the dominant mechanism for wear of coatings was abrasive wear. References: [1] F.C. Walsh, C.P. de León, A review of the electrodeposition of metal matrix composite coatings by inclusion of particles in a metal layer: an established and diversifying coatings technology, Transactions of the Institute of Materials Finishing 92 (2014) 83-98. [2] M. Islam, M.R. Azhar, N. Fredj, T.D. Burleigh, O.R. Oloyede, A. Almajid, S.I. Shah, Influence of SiO2 nanoparticles on hardness and corrosion resistance of electroless Ni-P coatings, Surface & Coatings Technology 261 (2015) 141-148. [3] J.N. Balaraju, T.S.N. Narayanan, S.K. Seshadri, Structure and phase transformation behaviour of electroless Ni-P composite coatings, Materials Research Bulletin 41 (2006) 847-860.

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ACCEPTED MANUSCRIPT Table Captions: Table 1. Optimized molar ratio for the preparation of SiO2 sol.

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Table 2. Chemical composition and operating conditions for electroless plating bath. Table 3. Chemical composition of as-plated electroless Ni-P and Ni-P/nano-SiO2 coatings determined by

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EDS analysis.

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ACCEPTED MANUSCRIPT Table 1. Optimized molar ratio for the preparation of SiO2 sol.

Ethanol

H2O

CH3COOH

1.4

7.7

2

1.2

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TEOS

operating conditions

Nickel sulphate

21 g/L

Sodium hypophosphite

24 g/L

Lactic acid

25 g/L

Propionic acid

3 g/L

SDS

1 g/L 80 ml/L

Temperature

Quantity 87±2˚C

pH value

5.0±0.1

Stirrer speed

100 rpm

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SiO2 sol

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Quantity

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Bath composition

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Table 2. Chemical composition and operating conditions for electroless plating bath.

Ni-P

Nickel (w%)

Phosphorous (w%)

Silicon (w%)

Oxygen (w%)

93.79

6.21

-

-

87.92

8.37

1.56

2.15

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Type of deposited coating

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Table 3. Chemical composition of as-plated electroless Ni-P and Ni-P/nano-SiO2 coatings determined by EDS analysis.

Ni-P-SiO2

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ACCEPTED MANUSCRIPT Figure Captions: Fig. 1. Adsorption isotherm of surfactant (SDS) onto the surface of SiO2 particles as a function of the equilibrium SDS concentration at 78 oC and pH=5.

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molar ratio of 1.4:7.7:2:1.2. (a) sol solution, (b) as-prepared powder.

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Fig. 2. FTIR spectra of the samples obtained from reaction mixture TEOS:Ethanol:H2O:CH3COOH with

Fig. 3. (a, c) SEM images of the surface morphology and (b, d) EDS spectra of the electroless Ni-P and Ni-P-

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SiO2 composite coatings.

Fig. 4. Surface microstructure of the Ni-P-SiO2 composite coating annealed at 400 oC for 1 h with different

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(×3500), and (b) high magnification view (×10000).

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magnification; showing surface morphology and SiO2 particle dispersion; (a) low magnification view

Fig. 5. (a, c) SEM images of the cross-section and (b, d) EDS line analysis of the electroless Ni-P and Ni-PSiO2 composite coatings.

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Fig. 6. Typical TEM bright field image and the corresponding SAED pattern showing the microstructures of

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the Ni-P-SiO2 composite coating annealed at 400 oC for 1 h. The arrows indicate the SiO2 particles.

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Fig. 7. XRD patterns of electroless coatings: a) conventional Ni-P and b) novel Ni-P-SiO2 composite coatings before and after heat treatment at different temperatures for 1 h. Fig. 8. The bond strength of the electroless Ni-P and Ni-P-SiO2 composite coatings without and with heat treatment in various temperatures.

Fig. 9. Variation in microhardness for bare alloy, as-prepared electroless Ni-P, and Ni-P-SiO2 composite coatings and those annealed at various temperatures. Fig. 10. Friction coefficient as function of the sliding distance for a) electroless conventional Ni-P and b) novel Ni-P-SiO2 composite coatings before and after heat treatment at different temperatures. Fig. 11. Weight loss as function of the sliding distance for a) electroless conventional Ni-P and b) novel NiP-SiO2 composite coatings before and after heat treatment at different temperatures. Fig. 12. SEM images of worn surface for a) electroless conventional Ni-P and b) novel Ni-P-SiO2 composite coatings after heat treatment at 400 oC.

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Fig. 1. Adsorption isotherm of surfactant (SDS) onto the surface of SiO 2 particles as a function of the equilibrium SDS concentration at 78 oC and pH=5.

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Fig. 2. FTIR spectra of the samples obtained from reaction mixture TEOS:Ethanol:H2O:CH3COOH with molar ratio of 1.4:7.7:2:1.2. (a) sol solution, (b) as-prepared powder.

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Fig. 3. (a, c) SEM images of the surface morphology and (b, d) EDS spectra of the electroless Ni-P and Ni-P-SiO2 composite coatings.

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Fig. 4. Surface microstructure of the Ni-P-SiO2 composite coating annealed at 400 oC for 1 h with different magnification; showing

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Fig. 5. (a, c) SEM images of the cross-section and (b, d) EDS line analysis of the electroless Ni-P and Ni-P-SiO2 composite coatings.

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Fig. 6. Typical TEM bright field image and the corresponding SAED pattern showing the microstructures of the Ni-P-SiO2 composite coating annealed at 400 oC for 1 h. The arrows indicate the SiO2 particles.

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Fig. 7. XRD patterns of electroless coatings: a) conventional Ni-P and b) novel Ni-P-SiO2 composite coatings before and after heat

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Fig. 8. The bond strength of the electroless Ni-P and Ni-P-SiO2 composite coatings without and with heat treatment in various temperatures.

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Fig. 9. Variation in microhardness for bare alloy, as-prepared electroless Ni-P, and Ni-P-SiO2 composite coatings and those

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annealed at various temperatures.

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Fig. 10. Friction coefficient as function of the sliding distance for a) electroless conventional Ni-P and b) novel Ni-P-SiO2

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Fig. 11. Weight loss as function of the sliding distance for a) electroless conventional Ni-P and b) novel Ni-P-SiO2 composite

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coatings before and after heat treatment at different temperatures.

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Fig. 12. SEM images of worn surface for a) electroless conventional Ni-P and b) novel Ni-P-SiO2 composite coatings after heat treatment at 400 oC.

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Electroless Ni-P and Ni-P-SiO2 composite coatings were prepared on St37 steel.

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Transparent SiO2-sol was added to a conventional electroless nickel deposition solution.

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The SiO2 nanoparticle formation mechanism was studied in detail.

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Hardness and wear resistance of Ni-P composite coatings is enhanced in the presence of SiO2

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nanoparticles.

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