Electroless Coatings Research Articles

Improved multilayer coatings by combined use of electrochemical and ultra-short pulsed laser deposition techniques

Abstract Electrochemical and ultra-short pulsed laser deposition (USPLD) methods were combined to produce multilayer coatings on various base materials that are challenging to be coated by using traditional electrochemical techniques. A thin and well-adherent metallic intermediate layer was coated on each base material by USPLD method before final nickel or nickel-trivalent chrome coating by electrochemical methods. In this way, the pretreatment steps for electrochemical deposition were simplified compared with traditional methods and hazardous chemicals were not used during the pretreatment process. The morphology, coating thickness and the elemental composition of the multilayer coatings were analyzed using scanning electron microscope (SEM) coupled with Energy Dispersive X-ray spectroscopy (EDS), and the adhesion strengths of each coating were tested by scratch and pull-off tests. The results show good adherence for all studied coatings on aluminum alloy (6082) and stainless acid-proof steel (SS316L), whereas obtaining a sufficient adhesion of coatings on glass (borosilicate) and polycarbonate (PC) requires a correct choice of coating materials and process steps. The critical loads in scratch test are approximately 20 N for coatings on aluminum and glass, at least 50 N for coatings on SS316L and approximately 5 N for coatings on PC. In pull-off tests, maximum adhesion strengths for coatings on aluminum and SS316L are approximately 25 MPa for electroless coatings and 10 MPa for electroplated coatings. Coatings on glass and PC reach adhesion strengths of approximately 4–9 MPa, which is sufficient for many applications.

Corrosion and wear resistance behaviors of electroless Ni–Cu–P–TiN composite coating

Composite coating of Ni–Cu–P alloys containing TiN particles was prepared by electroless technique based on the excellent wear resistance of TiN and better anti-corrosion property of electroless Ni–Cu–P alloys on carbon steel surfaces. Electrochemical method which uses Tafel polarization curves was carried out to study the corrosion performance of the coating. The results indicate that the anti-corrosion ability of the Ni–Cu–P–TiN composite coating (7.92 μA) is almost doubled compared with that of the as-coated Ni–P (13.60 μA). Furthermore, heat treatment results in first increase and then decrease in anti-corrosion ability. And the Ni–Cu–P–TiN composite coatings heat-treated for 40 min have maximum hardness of HV 960 and a self-corrosion current of 28.20 μA. The friction coefficient of electroless composite coatings was measured by end-facing tribometer. It is found that the friction coefficient of the Ni–Cu–P–TiN composite coating decreases apparently compared with those of Ni–P and Ni–Cu–P electroless coatings.

Novel Investigation on Nanostructured Multilayer and Functionally Graded Ni-P Electroless Coatings on Stainless Steel

In this study, step-wise multilayer and functionally graded Ni-P coatings were deposited with electroless in which the content of phosphorus and nickel would be changed gradually and step-wise through the thickness of the coatings, respectively. To compare the properties of these coatings with Ni-P single-layer coatings, three types of coatings with different phosphorus contents were deposited. Heat treatment of coatings was performed at 400 °C for 1 h. The microstructure and phase transformation of coatings were characterized by SEM/EDS, TEM, and XRD. The mechanical properties of coatings were studied by nanoindentation test. According to the results of the single-layer coatings, low P coating had the maximum hardness and also the ratio of hardness (H) to elasticity modulus (E) for the mentioned coating was maximum. In addition, low and medium P coatings had crystalline and semi-crystalline structure, respectively. The mentioned coatings had 〈111〉 texture and after heat treatment their texture didn’t change. While high P coating had amorphous structure, after heat treatment it changed to crystalline structure with 〈100〉 texture for nickel grains. Furthermore, the results showed that functionally graded and step-wise multilayer coatings were deposited successfully by using the same initial bath and changing the temperature and pH during deposition. Nanoindentation test results showed that the hardness of the mentioned coatings changed from 670 Hv near the substrate to 860 Hv near the top surface of coatings. For functionally graded coating the hardness profile had gradual changes, while step-wise multilayer coating had step-wise hardness profile. After heat treatment trend of hardness profiles was changed, so that near the substrate, hardness was measured 1400 Hv and changed to 1090 Hv at the top coat.

Preparation and Tribological Behavior of Ni-P-B<sub>4</sub>C Electroless Coatings

In this research, micro-hardness and wear resistance of two types of electroless coatings were investigated including Ni-P and Ni-P-B4C composite coatings. Dispersible B4C particles and electroless Ni-P alloy were codeposited on carbon steel by electroless plating and then heat treated at 200, 400 and 600 °C for 1 h, respectively. The cross-section morphology and microstructure of the composite coatings were characterized. Meanwhile, the micro-hardness and tribological behavior of composite coatings were evaluated. The results showed that the Ni-P-B4C composite coating presents better wear resistance in comparison with that of Ni-P coating. The Ni-P-B4C composite coating with heat treated at 400 °C exhibits high micro-hardness and good wear resistance, which the highest hardness is 1200 HV, the minimum wear weight loss is 0.12 mg and the lowest friction coefficient is 0.2054.

Effect of sodium sulfate on the characteristics and corrosion behavior of high phosphorus Ni-P electroless coatings

In this study, the effect of sodium sulfate on the microstructure and corrosion resistance of high phosphorus Ni–P electroless coatings has been investigated. Ni–P electroless coating was deposited on AISI 1045 steel substrate with using sodium sulfate as an additive in the plating bath (with concentration of 0–1.5 g/L). Electrochemical behavior and corrosion resistance of the coatings were investigated by evaluation of open circuit potential (OCP) and potentiodynamic polarization curves. The results showed that at an optimum concentration of sodium sulfate (∼1 g/L), deposition rate was maximum and the size and number of microporosities in the coating decreased to the minimum values. In addition, the best corrosion resistance and the most extensive passive region in potentiodynamic polarization curve was obtained at the optimum concentration of additive. Also its OCP was the same as pure nickel.

Chemical synthesis of TiO2 nanoparticles and their inclusion in Ni–P electroless coatings

The work addresses the preparation of Ni3P3TiO2 nanocomposite coatings on mild steel substrate by the electroless technique. Nanosized TiO2 particles were first synthesized by the precipitation method and then were codeposited (4 g/l) into the Ni3P matrix using alkaline hypophosphite reduced EL bath. The surface morphology, particle size, elemental composition and phase analysis of as-synthesized TiO2 nanoparticles and the coatings were characterized by field emission scanning electron microscopy (FESEM), energy-dispersive analysis of X-ray (EDAX) and X-ray diffraction (XRD). Coatings with 20 µm thickness were heat treated at 400 °C for 1 h in argon atmosphere. The morphology, microhardness, wear resistance and friction coefficient characteristics (ball on disc) of electroless Ni3P3TiO2 nanocomposite coatings were determined and compared with Ni3P coatings. The results show that as-synthesized TiO2 nanoparticles are spherical in shape with a size of about12 nm. After heat treatment, the microhardness and wear resistance of the coatings are improved significantly. Superior microhardness and wear resistance are observed for Ni3P3TiO2 nanocomposite coatings over Ni3P coatings.

Studies on Electroless Coatings at IIT Roorkee - A Brief Review

Since the introduction of Electroless (EL) coating in 1946 by Brenner and Riddle, the process has been the subject of steady growth. It is one of the most elegant methods available for the production of alloy coatings on surface. The technique involves the autocatalytic reduction, at the substrate/solution interface, of cations by EL bath released from suitable chemical reducing agents. EL coating technique is simple one, as can be manifested just by controlling pH and temperature of the coating bath. Such coatings are reported to provide excellent physical and mechanical properties. The electroless coatings are being studied at Indian Institute of Technology Roorkee since 1985. The structural and morphological behavior of Ni-P coatings for different phosphorous contents has been extensively studied. Sub-micron size coating islands and their transformations have been deduced. The metallography studies using hot stage within TEM to follow the phase transformations occurring at various temperatures have been studied for Ni-B EL coatings. The realization of mechanical bonding along with chemical bonding between EL coating and the substrate has been explained by coated copper on ceramic powder. As a forward step towards composite coatings, Ni-P-C, Ni-P-Al2O3, Ni-P-ZrO2has been developed by EL co-deposition technique. Ag-graphite coatings produced by EL technique exhibits nearly five times higher wear resistance and nearly two times better corrosion resistance apart from a good electrical conductor. The tribological behavior of electroless Ni-P-X and Ni-P coatings on steel and aluminium substrates in different conditions i.e., as coated, heat treated at various temperatures at different extents of time with different normal loads, have been studied in terms of dry sliding friction and wear against counter face of case hardened steel. In Ni-P-X nanocoating (X= ZrO2-Al2O3-Al3Zr), X has been producedin-situand are of nanosize particles. Such coating could be done on carbon fibre of 7µ diameter uniformly. Ni-P-ferrite nanocoatings with thickness less than nearly 1mm thick, is exhibiting good absorption of microwave in the range of 12-18 GHz which can be exploited for radar applications. Micro-thickness coatings are paying ways to nanocoatings. Nanocoatings are the coatings in which either the thickness of the coating is in nanolevel or second phase that dispersed in the coat matrix is of nanosize. To further explore the field of EL nanocomposite coatings, now days, a work on EL Ni-P-ZnO, TiO2, Al2O3, ZrO2and Ni-B-ZrO2for its mechanical properties has been carried out.

A Study on Ni-P and Ni-P-ZnO Composite Coatings Developed by Electroless Technique

A new processing concept has been developed to produce nano structured metal matrix composite coatings. Electroless coatings of Ni-P and composite Ni-P-ZnO were developed by co-deposition process on mild steel substrates. Ni-P-ZnO coatings were prepared using conventional route i.e., ZnO particles of ~ 20 μm with average diameter were added to the bath (900C, pH ~9.0) and kept in suspension by stirring. Uniform and grayish bright coatings were observed. The coatings were heat treated at 400°C for 1h in argon atmosphere to attain crystalline nature of coating. The as-synthesized and heat treated Ni-P and composite Ni-P-ZnO coatings were characterized by XRD, FESEM and AFM.

Application of Electrochemical Techniques in the Porosity Assessment of Electroless Coatings

Chemical and electrochemical techniques were applied to evaluate the porosity of various electroless coatings (single-layer coating, multilayer coating and ternary coating). The electrochemical techniques include linear polarization resistance and Tafel extrapolation method. The effects of these techniques were compared. It’s evident that electrochemical methods performed better than chemical method when they were applied to assess the porosities of coatings with very fine through pores.

Direct Electroless Deposition of Low Phosphorous Ni-P Films on AZ91D Mg Alloy

Magnesium metal and its alloys are known to have excellent physical and mechanical properties suitable for a number of industrial and technological applications. Yet due to their highly reactive nature, magnesium alloys have continued to see little industrial use. This paper demonstrates an extension of the recently introduced, novel method established to deposit electroless Cu films on Mg alloys to the next step of direct electroless deposition of nickel phosphorous [Ni-P] and nickel-zinc-phosphorous [Ni-Zn-P] alloy films on polished AZ91D magnesium alloys. These reasonably adhered, quick depositing, continuous, low phosphorous, electroless coatings contain phosphorous, atomically, at, or below, 10% due to the use of an alkaline deposition bath.

The performance of surfactant on the surface characteristics of electroless nickel coating on magnesium alloy

Magnesium and its alloys corrode rapidly in the electrolyte bath. Surfactants while used extensively as surface active agents in the electrolyte bath, have been little studied on magnesium surfaces. The influence of surfactants cetyltrimethyl ammonium bromide (CTAB) and sodium lauryl sulfate (SLS) on the surface properties such as roughness, morphology and topography of electroless Ni–P deposits on magnesium alloy was researched. The research reveals that the surfactant solutions has significant influence on the composition of coating, surface roughness and surface morphology. In addition, it has marginal effect on the microhardness. Electroless coatings with addition of surfactants produce a smooth surface and average roughness value of 1.412μm for CTAB and 1.789μm for SLS, which are less than the value (2.98μm) without surfactant addition. There was a significant improvement in the rate of deposition. However, the surfactants influence reached maximum at critical micelle concentration and above this value it gets stabilized. The initial structure appears to be dependent upon the percent occluded surfactants. The surface microstructures are discussed in line with the experimental observations.

Studying the effects of the addition of TiN nanoparticles to Ni–P electroless coatings

The effects of the addition of nano TiN on the surface morphology, deposition rate, hardness and corrosion properties of Ni–P electroless coatings were studied. Heat treatment was conducted to compare the corrosion and hardness behavior of the coatings before and after heat treatment. It was observed that the incorporation of TiN particles into the coating has an adverse effect on the corrosion properties of the specimens. The hardness of the specimens increased dramatically by adding TiN. Furthermore, the hardness of the specimens increased after conducting the heat treatment. The corrosion and hardness behavior of the Ni–P system after heat treatment largely depended on the temperature of heat treatment. The heat treatment temperatures at which the desired corrosion and hardness properties were expected were determined.

Electroless copper-phosphorus coatings with the addition of silicon carbide (SiC) particles

Cu-P-silicon carbide (SiC) composite coatings were deposited by means of electroless plating. The effects of pH values, temperature, and different concentrations of sodium hypophosphite (NaH2PO2·H2O), nickel sulfate (NiSO4·6H2O), sodium citrate (C6H5Na3O7·2H2O) and SiC on the deposition rate and coating compositions were evaluated, and the bath formulation for Cu-P-SiC composite coatings was optimised. The coating compositions were determined using energy-dispersive X-ray analysis (EDX). The corresponding optimal operating parameters for depositing Cu-P-SiC are as follows: pH 9; temperature, 90°C; NaH2PO2·H2O concentration, 125 g/L; NiSO4·6H2O concentration, 3.125 g/L; SiC concentration, 5 g/L; and C6H5Na3O7·2H2O concentration, 50 g/L. The surface morphology of the coatings analysed by scanning electron microscopy (SEM) shows that Cu particles are uniformly distributed. The hardness and wear resistance of Cu-P composite coatings are improved with the addition of SiC particles and increase with the increase of SiC content.

Preparation and properties of Ni–Co–P/nano‐sized Si3N4 electroless composite coatings

Ni–Co–P/nano‐sized Si3N4 composite coating was successfully fabricated on aluminum alloys by electroless plating in this work. The surface and cross‐sectional morphologies, composition, microstructure, microhardness, friction and wear behavior of deposits were investigated with SEM, EDS, XRD, Vickers hardness and high‐speed reciprocating friction, respectively. It was found that a Ni–Co–P/nano‐sized Si3N4 composite coating on aluminum alloy substrate is uniform and compact. The existence of nano‐sized Si3N4 particles in the Ni–Co–P alloy matrix causes a rougher surface with a granular appearance, and increases the microhardness but decreases the friction coefficients and wear rate of electroless coatings. Meanwhile, the effects of heat treatment at 200, 300, 400 and 500 °C for 1 h on the hardness and tribological properties were researched. It is revealed that both of the microhardness and tribological properties of Ni–Co–P coatings and Ni–Co–P/Si3N4 composite coatings increase with the increase of heating temperature in the range of 200–400 °C, but show different behavior for the two coatings after annealing at 500 °C. Copyright © 2011 John Wiley & Sons, Ltd.

Increased Corrosion Resistance of Closed-Cell Aluminum Foams by Electroless Ni-P Coatings

The Ni-P coatings were deposited on closed-cell aluminum foams substrate by electroless process. Electroless coatings cover the entire surface of the foams (thickness ∼30 μm) with a high corrosion resistance (corrosion current density 15.5 μA/cm2, as compared to the 308.8 μA/cm2 of as-received aluminum foams).

Electroless Ni-based coatings for biodiesel containers

Biodiesel commonly experiences oxidative and hydrolytic degradations, leading to problems of low storage stability and corrosion of fuel containers. The present study investigates the fabrication and use of electroless-deposited nickel alloys, containing phosphorus and tungsten, as potential coating materials that effectively protect steel-based biodiesel containers from corrosion. Through long-term static immersion and high-temperature oxidation tests, coupled with surface analyses of the coatings and assessments of the biodiesel’s acidity and storage stability, it is determined that a nickel coating with 15 wt% of phosphorus is favorably compatible with biodiesel, both in terms of corrosion protection and fuel stability.

The Study of Zink-Nickel Composite Coatings after Annealing

The Zinc composite coatings containing Ni powder were obtained by the electrodeposition and electroless methods. Electrodeposited Zn+Ni coatings were plated with the current density jk = 150 mA/cm2 from the zinc chloride bath containing the suspension of nickel powder. Electroless (Zn-Ni)+Ni coatings were obtained by chemical reduction of Ni2+ and Zn2+ ions from the sulphate bath containing sodium hypophosphite as a reducing agent and mechanically dispersed Ni powder suspension. The thickness of (Zn-Ni)+Ni layer was ~8 m. In order to enhance the Zn content the obtained coatings were covered with the electrolytic Zn layers of different thickness (5 m, 8 m and 14 m) – (Zn-Ni)+Ni/Zn. The thermal treatment of the obtained composites was carried out at a temperature of 320oC, during 2h in argon atmosphere. The electrodeposited coatings show the presence of Zn, Ni(Zn) and ZnO phases. The electroless coatings show the presence of Zn, Ni(Zn) and ZnO phases. The additional electrodeposition of Zn leads to the creation of dilayer coatings (Zn-Ni)+Ni/Zn. The annealing of such obtained coatings leads to the creation of Ni2Zn11 intermetalic phase. The average Ni(Zn) and Ni crystallite size before annealing is in a range of 200 Å and after annealing the size is increasing to values of 600-800 Å.

Influence of cubic nanostructure additions on the properties of electroless coatings

In this research, the influence of co-deposited within nickel-phosphorus matrix cubic nano- and micro-sized particles (nanodiamond and micron cubic boron nitride) on the properties of the composite coatings was investigated. Nanometer diamond was synthesised by using the detonation method. It is a kind of materials with properties of diamond and nanometer particle. High phosphorus electroless nickel bath was used to prepare electroless nickel composite coatings from a suspension. In order to characterise the deposits, their structure, morphology and hardness were analysed. The incorporation of particles has a marginal influence on the composition. The influence of thermal processing of the samples on the coating properties was investigated. The highest microhardness, the superior corrosion and wear resistance of the nickel-phosphorus-nanodiamond composite coating was obtained by heat-treated samples.

Studying the influence of nano-Al2O3 particles on morphology and corrosion improvement of Ni–9%P electroless coatings

A possibility to increase materials' performance for different applications is to protect them by coatings. Electroless nickel deposition represents an alternative method to obtain coatings on various substrates. Advancement in electroless Ni–P deposition is the codeposition of solid particles within coatings. By this method, composite layers with very good characteristics for specific applications can be produced. Ni–9%P/nano-Al2O3 composite coatings were deposited by electroless plating method, while their morphology and properties were examined via scanning electron microscopy, energy dispersive X-ray spectroscopy and microhardness tests. Electrochemical impedance spectroscopy and polarisation method was used to analyse the corrosion resistance of the coatings. The results showed that the addition of nano-Al2O3 particles improved the corrosion resistance and mechanical properties of electroless Ni–P coatings.

Influence of pH value on microstructure and thermal stability of Ni–P electroless coating prepared in acidic condition

Three kinds of Ni–P electroless coatings were prepared in nickel sulphate solution at different pH values of 4.5, 5.5 and 6.5 with the purpose of ascertaining the influence of pH value on microstructure, internal stress statue and thermal stability of the coatings. Laser curvature (LC) method was used to measure the residual stress level in the coatings. Scanning electronic microscopy (SEM) and transmission electronic microscopy with energy dispersive spectrum (TEM/EDS) were used to examine the surface morphology and internal phase structure of the coatings, respectively. Differential scanning calorimeter (DSC) was used to analyze the phase transformation and thermal stability of the coatings at high temperature. Results showed the Ni–P coating prepared at pH 5.5 with nano-crystal mixed in amorphous structure had the worst thermal stability. The relatively higher stability of Ni–P coatings prepared at pH 4.5 and 6.5 was ascribed to the lower tensile stress level and much finer grain size, respectively. Besides, inverse Hall–Petch effect of annealing strengthening might also contribute to the integrity of Ni–P coating prepared at pH 6.5.