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The properties of metal surface depend on the structure of that which is composite of boundaries, crystal faces and layers. (Afshari and Dehghanian 2009)
electrodeposition is widely used in industry in order to provide a composite coating which is abrasion and corrosion resistance, e.g., Ni/TiO2 and Ni/SiC. (Lajevardi and Shahrabi 2010)
As the electrodeposition technique has many advantages, therefore nano size particles deposition in metallic matrix achieve high attention in many industries and become a new generation of composites (Gyftou, Pavlatou and Spyrellis 2008).
The agglomeration of nano particles in the electrolyte, and provide sufficient co-deposited particles are some of the chalenges of nanoparticles codeposition (Srivastava et al. 2007).
Control of deposition process is concerned with conditions of the solution bath and its composition as well as conditions of product requirements (Gabe 1978).
Application of electroplating
One of the most important mechanical properties of engineering applications is wear resistance, as more than 50% of materials loss occurs because of wear. Reduce of grain size has significant influence to improve the wear resistance of materials with nanostructure (Jeong et al. 2001).
Composite coatings can be used in many industries, e.g. in computers and electronic components which are high-tech industry. They also can be used in automotive industry, food, paper mills and textiles industries which are traditional industries. The main afford of using these kind of coatings in the last decades is to provide a corrosion and wear resistance coating and also produce the system which is self lubricant (Srivastava et al. 2007).
As the Ni-SiC composite coatings improve the corrosion and wear resistance properties, therefore it is used commercially in combustion engines and parts of components which needed to be protect from friction (Yeh and Wan 1994).
(Merk 1995) asserted, there is a limitation in the use of Ni-SiC composite coatings, and they can be used in applications where the temperature is less than 400°C. The limitation is because of nickel silicide formation at above 400°C which enhances the material loss.
Electrodeposits are concerned with improvement of surface properties regarding to the corrosion behaviour and mechanical properties which can be varies by coating thickness and plating conditions. Materials are categorised according to their properties for the purpose of plating. The chromium which is a hard metal is used for plating where wear resistance is a key issue especially in engineering machinery. High internal stress is not a limitation for using materials in electroplating process as separate micro size cracks occur over the coating layers, but these cracks improve lubricant properties of the surface which is suitable for engineering applications. The materials group of platinum is used in electronics applications where frequent switch sparking occurs and hard surface and oxidation resistance is essential. Thickness of coating which used in corrosive environments depends on the nature of environments. The coating thickness of 10-4 to 10-5 mm is used for decorative coatings with the presence of corrosion resistance underlayer, however in some conditions depend on the required properties, the thickness of these coatings increase to 10-1 to 10-2 mm.
electrodeposition is an effective and economically technique to create a nanostructured materials. There are many advantages associated with this technique which are as follows:
Produces a range of nanometals which consist of Cu, Co, Ni and their compositions, their alloy composites as well as nanomatrix composites (Ni-SiC).
Provide products in a range of shapes and sizes such as thick deposited layer, thin films, powders and bulk materials.
Provide a surface without porosity with distribution of narrow grain size which shows high ductility and strength.
The need of low initial capital investment
Low cost and high production rates
Silicon carbide nickel matrix coatings have been
extensively studied and have gained widespread use for
protection of friction inside parts of cylinders in the
The smooth surface of nickel deposit is observed by pulse electroplating which contain fewer pores than rough deposits. However the reduce of porosity was not observed in industrial level, even by well preparation of the surface (Devaraj and Guruviah 1990). Moreover (El-Sherik, Erb and Page 1997) stated, pulse plating reduces porosity of nickel deposits and improves its hardness.
The nickel deposits obtained from amide bath by using pulse reverse current provide ductile and crack free deposits, and improve the brightness of coating (Devaraj and Guruviah 1990).
Silicon Carbide (SIC)
The reinforcement of metal matrix composites by micron and nano size particles improves wear and corrosion resistance and hardness of a coating, therefore it attracts scientific and industrial for using this materials in their applications (Garcia et al. 2003).
The terminology of pulse plating
The results of researches on composite materials electrodeposition, show a development in the process of this method from direct current (DC) electrodeposition to pulse current (PC) electrodeposition which gives higher current density and consequently provides nanocrystalline deposits with finer grains and more compact structure. (Lajevardi and Shahrabi 2010)+refhaye khodesh.
Due to the small grain size of nanostructure materials and volume fraction of grain boundaries, the improvement of physical, mechanical and electrochemical properties is observed in these materials compared with amorphous or polycrystalline materials (Jeong et al. 2001).
There are three types of current which are used for electroplating. They can be direct current (DC), pulse current (PC) and pulsed reverse current (PRC), where Pulse plating can produce higher current density than direct current which provides better properties of coatings. Another advantage of pulse plating is that there is a possibility to change the parameters of pulse plating in order to control the interface between electrolyte and cathode surface. (Ge et al. 1997)
Pulse plating technology provides coatings with brilliant properties, also reduces the effects of hydrogen on cathode surface. Moreover alloy composition coatings can be produced by pulse technique while it is not possible to produce them by DC deposition.
Pulse plating of Ni
Pulse plating of Ni/SiC
The wear resistance of nano sized nickel deposits (10-20 nm) is 100-170 times higher than polycrystalline nickel deposits. He also pointed out, the friction coefficient of them is 45-50% lower than polycrystalline nickel films which have the grain size of 10-100 Î¼m (Jeong et al. 2001).
In regards to Ni/SiC composite coating, researches reveal that there is a difficulty to control the codeposition rate of nano-SiC particles during the deposition process. The reason behind this problem is, due to the size of these particles which is nano, they have high surface energy which causes agglomeration in the electrolyte and metal matrix. (Lajevardi and Shahrabi 2010)
Moreover â€¦â€¦ asserted, use of PC electrodeposition results in increase in particle contents and uniformity of particles in the coatings. (Chang et al. 2006).
In regards to the hardness of coatings, Ni/SiC coatings are 4 times higher than Nickel coating. It should be mentioned that, the effect of decrease in the grain size in the hardness of the coating is significant even higher than effect of SiC particles (zimmerman et al. 2002). Also (Benea et al. 2002) pointed out, the size of nickel grains in the composite coatings is smaller than those in pure nickel coating.
Using the nano-crystals SiC in Nickel matrix, provide a nano-structure material and increase the properties of coating which is higher than those with individual materials. (Benea et al. 2002).
â€¦â€¦ shows that, the grain size of nickel by PC electrodeposition is less than those by DC electrodeposition, where 4 µm is the largest grain size of nickel under PC and 7µm is the largest size under DC conditions.
The comparison between the structure of pure nickel and nano-composite coating under SEM, reveals that nano-composite coating has a nodular structure while the pure nickel coating has a regular surface structure. ???? (Benea et al. 2002)
The nano-crystals SiC particles has a low atomic number and small dimension, therefore they are not visible on the surface. â€¦.. showed that during electroplating, nano-SiC particles disturbed the nickel crystals growth which then nickel matrix showed random orientation rather than preferred orientation and consequently produced nano-size nickel crystals by providing higher nucleation sites. (Benea et al. 2002)
The increase in the amount of SiC particles in the electrolyte, increases the involvement of these particles in the composite layers, and also (Benea et al. 2002) described the importance of sodium dodecylsulfate in this involvement where it reduce the agglomeration of SiC particles and also enhance the amount of them in the coating layers.
As the size of particles decrease, the number of grain boundaries increase and consequently this affect the properties of material, therefore nano-crystalline materials have received significant attention due to their special physical, mechanical and chemical properties. (Afshari and Dehghanian 2009). It is more supported by (Tjong 2007) where he described, the increase in the number of grain boundaries is contributes to higher specific interphase energy than polycrystaline materials. Nano-crystalline particles with high fraction of grain boundaries also improve corrosion resistance of materials.
The waveform has a significant effect on distribution of SiC particles on the composite coatings, where (Heidari, Tavakoli and Mousavi Khoie 2010) asserted, homogenous distribution observed under PC electrodeposition while agglomeration of SiC particles occurred on the surface at DC.
The coatings which contain 30 g/L silicon carbide in solution have higher microhardness than those without these particles which can be related to the mechanism of dispersion hardening (Heidari, Tavakoli and Mousavi Khoie 2010). Moreover (Garcia, Fransaer and Celis 2001) asserted, the strength and hardness of reinforcement particles are high, therefore they reduce the plastic deformation of nickel matrix and consequently the hardness of coating increase. But as mentioned earlier the effect of small grain size in hardness of coating is higher than silicon carbide particles.
The comparison between two different sizes of silicon carbide in Ni-SiC composite coating is reported by (Garcia, Fransaer and Celis 2001). They showed that the wear resistance of this coating which contained submicron SiC (0.3-0.7 Î¼m) is higher than those with micron SiC (5 Î¼m).
In regads to the tribological study of nickel composite coatings under both DC and PC conditions, (Giftou et al. 2005) reported that the coatings achieved from sulphate bath which contained macron SiC (1 Î¼m) had higher coefficient of friction values than coatings which contained nano (20 nm) particles. They also observed that wear resistance of nickel composite coating increased as the size particles increase from nano to micron size which is in contrast with other results from other experiments.
The change of shape and grain size is observed in nickel crystals by using SiC particles in nickel matrix where they reduce the grain size of nickel and change their orientation from preferred orientation to random orientation (Ebrahimi et al. 1999). However it is in contrast with the results of (Gyftou, Pavlatou and Spyrellis 2008) which showed that random orientation did not occurred in deposited layer and also the lowest value of crystallographic texture observed which they pointed out, it could be related to the formation of oriented micron-sized [2 1 1] crystallites and weak formation of small sized [1 0 0] crystallites.
â€¦â€¦. pulse electroplating is an effective technique for perturbing of adsorption and desorption process at cathode and plating solution interface and make it an economical process in order to provide a nanostructure material (Li et al. 2007).
Mechanism of general electrodeposition
The deposition process occurred by diffusion of metal ions which are in the plating solution on the cathode surface in the course of the potential gradient. Then the metal ions become adatoms by discharging process at Helmholtz double layer and absorbed on the surface of cathode and produce a crystal (Watanabe 2004).
(Watanabe 2004)described the model of nucleation, where the impurity atoms adsorbed at active sites on the surface of cathode and by continuing this process, normal crystal growth occurred. This growth is continued when two dimensional nuclei expand through the cathode surface.
Dendrites are formed in deposited layers on low current density or high current density. Each dendrite is a single crystal however it can be a series of fine grains. Even in deposition process which produces dendrites, the deposited films provide a uniform layer and dendrites start to form on this layer. This is more supported by the fact that the distribution of metal ions which are near the cathode surface is uniform over the surface. After formation of deposited layer, MIDL is produced over the surface and its thickness changes on the surface. The metal ions discharge over MIDL region and dendrites start to form on that region and growing of them increase through the tips which have high current density. Moreover, there is a increase in the temperature of tips areas which is because of increase in the discharge of metal ions on that regions. Figure x shows dendrites which formed over the surface of gold films. (page 37 watanaba)
The researches reveal that, a rough microstructure start to form in deposited film at the first step of layer growth, which produce amorphous structure. The study on nickel deposited films by TEM has been shown a fine grained structure at the early stage of layer growth where the substrate material affected the microstructure of deposited films (Toth-Kadar et al. 1997).
Mechanism of pulse deposition
â€¦â€¦.. pointed out, in theory increase in deposition rate occurred at high overpotential which is contributes to increase the concentration of ions around the cathode surface and increase nucleation rate. High nucleation rate produce fine grains which then provide a smooth surface.
Nickel plating solutions
Types of bath and their composition
In nickel deposition process four types of bath are used which are as follows:
Nickel deposited films obtained at high current density from chloride bath tend to  texture and those from sulphate bath tend to  texture. Moreover the deposited film from watts solution which is the mixture of chloride and sulphate bath yielded the  texture. as the watts bath is the mixture of sulphate bath with higher ratio than chloride bath therefore it provides the texture which is similar to those from sulphate bath. The texture of nickel films from sulfamat bath is  which is different from other types of bath. This shows that the type of anions in plating solution affect the texture of nickel films. Temperature of plating solution and current density also change the texture of deposited films (Watanabe 2004).
Chloride bath is used to produce fine grains and harder deposited layers while sulphate bath is used where insoluble conductive anode is needed. Sulfamat bath is used for specific applications where fast plating is essential and gives more advantages (Gabe 1978).
Drop of tensile stress is observed in nickel deposits obtained from chloride bath by using periodic alternate current which then consequently change the grain size from 100 to 5 grain/pm (Devaraj and Guruviah 1990).
Watts nickel plating solution
The traditional type of bath which used for nickel plating is Watts solution and industries still use this kind of plating solution for the engineering purpose. The composition of this bath consists of nickel chloride which used for anode dissolution improvement and recovers the limitation of current density by film distruption, nickel sulphate which is the source of nickel ions, boric acid in order to control PH of the electrolyte and some additional agents for brightening and reduce stress (Gabe 1978).
Pulse electrodeposition parameters and surface morphology
The adsorption rate of anions affects the crystal growth, grain size and consequently surface morphology. As mentioned earlier there is a difference between experimental results and theory where (Fischer 1973) asserted it can be related to incorporation of impurities in the deposited layer and also difficulties to control the thickness and condition of electrolyte around the cathode surface.
(Watanabe 2004) classified the microstructure of deposited film in to seven categories which is shown in figure x. It should be mentioned that there is a possibility to control each microstructure individually and independent from others, however the overlap can be occurred among them. (page 4, watanabe). In general the surface morphology is changed by plating conditions.
The microstructure and properties of deposited films are affected by parameters of pulse plating (Toth-Kadar et al. 1997).
Pulse plating parameters play an important role to control the composition of coating. Some of important variables in pulse plating that should be considered in to account are as follows:
Types of anions (Bath composition)
DC AND PDC:
The most important factor in electrodeposition process is the rate of metal ions transportation to the surface of cathode where these ions discharge, in order to produce the structure or coating. The deposition rate in PC technique can be controlled by its parameters.
In DC electrodeposition process, the metal ion concentration reduces by discharging cations at the cathode and diffusion layer start to form as the process of deposition continues. By forming this layer, concentrations of metal ions decrease to less than concentration of metal in the electrolyte. An equilibrium is appeared in the process by continuing deposition where metal ions with the same rate, go through the diffusion layer. The rate of electrodeposition, can be determined by looking at the diffusion layer thickness, and one of the ways to eliminate the thickness of this layer is agitation.
There are three independent variables in pulse plating that can affect the properties of coating, which are on-time, off-time and current density. The concentration of metal ions around cathode surface reduce during the on-time and as mentioned earlier, diffusion layer start to form which its thickness is depends on current density. The concentration of metal ions starts to recover again during the off-time from the bulk electrolyte which needs enough time to do it.
(Heidari, Tavakoli and Mousavi Khoie 2010) stated that the microhardness of coating which produced under pulse plating is higher than those with direct current plating. It is more supported by (zimmerman et al. 2002) where described, in composite materials distribution of reinforcement particles and grain size of matrix define the hardness of composite, and therefore higher hardness of coating under PC plating can be related to smaller grain size of coating that can be produced under PC conditions.
In regards to corrosion properties of coating (Garcia et al. 2003) pointed out, as grain boundaries are the preferred place for corrosion and pulse plating produce smaller grain size by increasing the rate of nucleation, therefore longer rout of corrosion can be achieved by PC deposition than DC which consequently increase the corrosion resistance of coating produced by pulse current deposition.
The grains with columnar shape are observed in conventional electrodeposites where the grain size increases by increasing the deposition time which consequently results to increase the thickness of deposited layer. The crystallographic texture of some deposits finded strong as the crystals growth occurred in preferred orientation and the size of grains are more than nano-regime. This is occurred under DC deposition conditions and pure nickel deposits shows the highest value of texture strength where the crystals growth in micron size and [1 0 0] orientation (Srivastava et al. 2007).
The main important factor to produce the small size and also nano-size crystals, is to increase the nucleation rate of crystals on the surface (Gyftou, Pavlatou and Spyrellis 2008). According to (Amblard et al. 1979)There are three ways to increase the nucleation rate which are as follows:
Using reinforced particles
Using grain refining agents in electrolyte
Using pulse current for deposition
The percentage of corporation in composite coatings which produced with PC is higher than those prepared with DC. Moreover decreasing the duty cycle is a key factor in order to increase the proportion of embedded SiC particles (Gyftou, Pavlatou and Spyrellis 2008)
â€¦.. shows that surface morphology of Ni-SiC coating is affected by current density, where increase in current density is contribute to decrease in nickel grain size (Hu and Chan 2005). Moreover â€¦â€¦ asserted, microhardness value of Ni-SiC coating by pulse electrodeposition is about 2 times higher than those with polycrystalline composites (zimmerman et al. 2002).
Dissolution of anode in the electrolyte increases at high current density, which it contributes to increase in concentration of nickel ions around the surface of cathode. Moreover the result of this concentration is the increase of repulsive forces between SiC particles on the surface of cathode and nickel ions around the cathode. Therefore, dispersion of SiC particles on the composite coating and agglomeration occurrence of them increases (Heidari, Tavakoli and Mousavi Khoie 2010).
Roughness of deposited layer is affected by overpotential, where high overpotential decrease surface roughness and low overpotential increase it.
Figure x illustrates how the irregularity of plated surface is changed by different current density. As can be seen in figure x(a) at low current density small number of metal ions discharge in the surface. Moreover, by continuing this process at low current density discharging of metal ions occurred at Protrusion where the surface irregularities increase by growing of these protrusions. While the discharge of metal ions on the surface of cathode increases at high current density.
According to the literatures, PC electrodeposition provides higher current density than DC where (Watanabe 2004)asserted high current density provides high density of metal ions around the cathode surface which then distribute on the surface along with the repulsive forces of metal ions.
Furthermore, redistribution of metal ions over the cathode surface provides a chance for distribution of discharge sites to become more uniform and produce a smooth surface. Figure x(b) and x(c) illustrate these process. Therefore, current density distribution over the surface of cathode is one of the main important reasons for surface roughness in deposited layers.
It is proved by researches that overpotential can control the grain size of deposited films where the smooth surface can be obtained by high overpotential. Moreover, deposition rate increases by rising the overpotential and consequently reduces the rate of film growth. High deposition rate provides high nucleation rate which then results to produce fine and smaller grain size which forms the smooth surface.
...... "High overpotentials should be used to obtain electrodeposites with smooth surfaces, however it is not correct to say such surface smoothing resulted from the formation of fine grains" (Watanabe 2004). (figure hamintori baraye size page 40 watanabe)
Because of the electrolyte resistance and irregularities on the surface of cathode, there are different current densities over its surface. Moreover, the current density decreases by increasing the space between anode and cathode in solution. The geometric arrangement in the solution space can be conquer this problem to some extent, but it is not affect the ability and efficiency of the bath for plating process (Gabe 1978).
The effect of current density also is shown in figureâ€¦ which illustrates nickel electrodeposits surface morphology achieved from 6 and 8 Adm2. As can be seen from the figure, high current density provides small grain size where there are no microscopic defects on their surface with low roughness (Li et al. 2007)
Fig. 2. Surface morphology of nickel electrodeposits obtained at average current
densities Jm of (a) 6 Admâˆ’2 and (b) 8 Admâˆ’2. (bedune phraseeeeeeeeee) (Li et al. 2007)
The current density effects on the deposited layers are more supported by (Li et al. 2007) where they provided a graph to show changes in grain size, microhardness and tensile strength by applying different current densities. As can be seen, there is a rapid decrease in the grain size from 180 nm to 10 nm by increasing the current density from 1 Adm2 to 10 Adm2. Moreover, it reveals that the combination of high current density and inhibitors can provide refine grains on the surface. The inhibitors are needed in order to decrease surface diffusion of ions on the surface of cathode which improve grain size refining process (Li et al. 2007).
According to electrochemical theory, in order to produce a nanostructure film on the surface higher nucleation rate is required rather than grain growth which is done by controlling the deposition parameters. High overpotential, high concentration of metal ions and low mobility of ions on the cathode surface are essential to increase the nucleation rate and consequently reduce the grain growth (El-Sherik and Erb 1995).
Fig. 3. Changes with average current densities Jm in: (a) grain size (bottom); (b)
microhardness (center); (c) tensile strength (top). (bedooooneeeeeeeee phraseeeeee) (Li et al. 2007)
According to (Jeong et al. 2003), there is a linear relationship between wear resistance of nano-sized nickel coating and its hardness. Moreover, figure x reveals that the hardness of deposited layer rises from 300 Kgmm2 to 600 Kgmm2 by increasing the current density from 1 Adm2 to 6 Adm2, then the hardness reaches to a constant value till 12 Adm2 and then start to decrease after this value. (Li et al. 2007) described that, the mechanical properties of deposited layers is highly depend on the grain size of them which also affect the hardness of deposited films. Therefore, by decreasing the grain size of films, it hardness increases and also keep the constant value where the grain size of films has the constant value in the range of current density. The decrease of hardness occur by rising the current density to more than 12 Admm2 which is because of formation of curl and tear deposits at higher current densities due to the rising of internal stress.
In regards to the tensile strength of deposited layers figure ..... also shows that it increased rapidly and reached to the highest value at 8 Admm2, and then decreased dramatically at higher current densities which is due to the lower ductility of the deposited layers at high current density. The low ductility itself is because of the presence of defects and impurities in the deposited layers.
(Kim and Weil 1989) and also showed that, increase of current density results to rise the internal stress and porosity of nickel deposits which consequently affect the hardness of films. This could be a reason for decreasing the hardness of nickel deposits by raising the current density over 12 Admm2.
The definition of duty cycle is the ratio of the on-time to the sum of the on-time and off-time of deposition which shown in the formulaâ€¦â€¦..
Duty Cycle (Î¦) = Ã- 100%
As there is no off time for DC plating, therefore the duty cycle for this technique is 100%.
off-time in PC electrodeposition is contribute to decrease in polarization rate and concentration of hydrogen molecules on the surface of cathode, and concequently concentration of nickel ions increases around cathode surface which results to increase in nucleation rate. As the smaller grain size can be achieved by higher nucleation rate, therefore nickel grain size under PC is smaller than DC electrodeposition (El-Sherik, Erb and Page 1997).
During the on-time in pulse plating the chance for deposition of nickel ions is higher than SiC particles, whereas this chance is same for both of them during the off-time of plating. Therefore, the deposition rate of SiC particles in pulse plating process is higher than DC plating (Chen et al. 2006).
According to the literature on pulse electrodeposition parameters, different influence of these parameters has been found on the incorporation rate of particles in the coating. For example (Bahrololoom and Sani 2005)tried to find out the influence of frequency and duty cycle on the incorporation rate of alumina particles in nickel matrix. The results of experiment showed that, incorporation rate of Al2o3 particles in coating decreased by raising the pulse frequency while (Thiemig, Lange and Bund 2007)found different results in this case. However, both of them found the same result in regards to the duty cycle and showed that the rate of particles incorporation increase by reducing the duty cycle.
Pulse electroplating is an effective technique to control the microstructure of nickel electrodeposits and produces fine grains on the surface. The most important factor to achieve these properties is controlling the on and off time of deposition where, on time of current should be less than off time which affect electrocrystallization process during plating (Qu et al. 2003).
Deposition time and thickness of co-deposited layer
Types of anions (Bath composition)
The effect of agitation rate on the surface morphology of deposited nickel layers in chloride bath is shown in figure x below where the agitation was controlled by magnetic stirrer speed. High surface irregularities is observed over nickel films which obtained from non-agitated bath (see figure x(a)). The rougher film surface is formed by increasing the agitation rate and also it caused formation of secondary irregularities over the surface which is on the side of primary irregularities which is illustrated in figure x(b)-(d). The reason for formation of secondary irregularities is, agitation of solution provides side face of primary irregularities with metal ions where they can discharge on those sites. (Page 35 watanabe)
General description of the equipments
Double beam oscilloscope
Preparation of substrate
According to (Gabe 1978) there are four general processes of surface preparation in the case of electrodeposition which are as follows:
Using abrasive which used for failed components after recoating process or castings.
Cleaning of the surface is essential in order to remove dirt or remaining metal compounds after polishing or buffing.
Atmospheric oxidation and dark films are removed from the surface by picking which is essential to provide good coating conditions and metal adhesion. The most common techniques are acid dip with electrolytic treatment and agitation which is followed by washing by water and electroplating process.
The last stage or finishing stage is done by etching or brightening.