Thermal,corrosion and wear analysis of copper based metal matrix composites reinforced with alumina and graphite

Venkatesh R,Vaddi Seshagiri Rao

aDepartment of Mechanical Engineering,St.Joseph's College of Engineering,Chennai,600119,Tamilnadu,India

bSt.Joseph's College of Engineering,Chennai,Tamilnadu,India

Keywords:TGA FTIR Corrosion Pin-on-disc Wear rate

A B S T R A C T The wear and corrosion resistances are important in marine applications,especially when it comes to structural support components like bearings,bushes and blocks.The copper hybrid metal matrix components are the new avenues explored in this front.A novel combination of alumina and graphite were considered as the reinforcements in a copper base for the development of a metal matrix composite.Power metallurgical techniques were used for the development of the MMC.The Vickers's hardness value of 64.9Hv has been observed by increasing the volume of alumina.Thermogravimetric analyses were carried out on material samples to estimate the exact sintering temperature and identified that 450-700°C would be conducive.The TGA curves shows two step decomposition exists between 4300C-460°C.FT-IR analysis was done to confirm the peak values of the materials.FTIR exposed the peak value of 1600 cm-1for alumina where as for Copper and graphite peak values have been 2840 cm-1and 17260 cm-1respectively.The potentio dynamic analysis was done to estimate the rate of corrosion on the samples.The sample with nano and micro reinforcements offered intensive resistance to corrosion.The presence of graphite minimized the weight loss of the samples during the corrosion test.Finally the wear rates of the samples were estimated using the Pin On Disc experimental setup.The samples with nano material reinforcement and with a maximum proportion of graphite exhibited a better wear rate of 1.52×10-12m2/kg under maximum load conditions.

1.Introduction

Fretting wear is a greater phenomenon happens in the mechanical joints right from household applications to automobiles.A low amplitude oscillatory sliding between the mating surfaces without lubrication causes this wear in applications ranging from aircrafts to human body implants,describes Roy et al.[1].There are certain heavy loading conditions where metals slide against each other,without the benefits of lubrication,as mating components,as analyzed by Kovalchenko et al.[2].Hybrid composite materials with copper as the base and other suitable reinforcements like graphite and alumina could prevent scuffing and catastrophic damages.The composites developed with Copper as matrix and with the dispersoids capable of providing lubrication like graphite alongside a sizable proportion of a ceramic either SiC or Alumina as hard reinforcement have experimentally proved the increase in tensile strength,lower co-efficient of friction and an appreciable hardness.These reinforcements have greater influence on the machinability of copper and its inherent conductivity as well,reveals Ramesh et al.[3].

Mechanical properties,wear and corrosion behavior of composite materials are greatly influenced with the reinforcements of steel machining chips.These copper matrix composites are being extensively used,in the design of radiators for its ability to resist corrosion.Kenneth et al.[4],comprehends that copper based heterogeneous combinations are used in digital devices for their better conductivity.Also used in the casing of jet engines where combined effects of corrosion,wear and thermal conductivity are predominant.Such components are widely used in the intricate components of automotive industry.The wear resistances keep on increasing with the addition of alumina in the nano regime as reinforcement in copper matrix and it also helps to maintain stability in the thermo dynamical aspects.The compressive strength also increases with the addition of nano alumina particles[5,6].Two different materials are used as the matrix materials in an investigation,copper as the matrix and chromium as the matrixalloying element with copper coated carbon nano tubes as reinforcements.These combinations not only enhance the mechanical properties but also find solution for the interfacial problem of non wetting with other constituent element in the composite,as explained by Ke chu et al.[7].

Rajmohan et al.[8],justifies the inclusion of two different ceramic reinforcements one in microns and the other in nano scale to form the hybrid composites.The investigations justify the hybrid nature of the composites upon the inclusion of Mica or graphite along with a metal and a ceramic in appropriate proportions.The powder metallurgical techniques have been used to develop the composite materials[8,9].Wear rate exhibited by copper coated graphite are better than the uncoated ones similar to the differences between coated and uncoated silicon carbides,as investigated by Sapate et al.[10].The increase in volume fraction of ceramics,be it silicon carbide or alumina,keeps the rate of wear at the low level.Alumina and graphite have been the predominant reinforcements in any metal matrix composites to improve wear resistances,as hinted by Gheorghe Iacob et al.[11].

The presence of graphite,with the size in the range of 10-9,shows excellent wear resistances in the copper matrix composites as ascertained by Rajkumar[12].Kannan et al.[13],says that stir casting and squeeze casting techniques have also been widely used in the fabrication of hybrid nano composites and comprehensive analysis have been made to study their mechanical and microstructural changes.The load carrying capacity and material loss due to friction have drastically come down with the reinforcements of carbon nanotubes and NbSe2in the copper matrix,as reported by Beibei Chen et al.[14].N.Ch.Kaushik et al.[15],observes a higher wear resistance and a lower surface roughness for hybrid metal matrix compared to that of metal matrix composites.

An elaborate study has been made by Michael oluwatosin Bodunrin et al.[16],in the philosophies behind the development of MMCs,the characterization techniques involved and a significant attention has been given to the tribological characterizations as well.N.Nemati et al.[17],enumerates on the worn out surface observations,it has been revealed that the dominant mechanism of wear for pure Al specimen without the reinforcements has been delaminating while the worn out surfaces of the nano-particle reinforced composites are smoother and the deformations due to depth of cut are smaller, fine grooves are seen in the unreinforced matrix specimens.

Scanning electron microscope,XRD,and TEM are the techniques being widely used for the morphological analysis.To determine the mechanical characteristics Hardness,compression test,tensile tests,bending strength tests and fractography analysis were studied.Pin-on disk analysis has been adopted to evaluate the wear rate of the developed composites[18-21].

Wear has been experimentally and numerically exemplified by the researchers and reported[22,23].The nuances of development of composites,their characterization and fatigue analyses have been meticulously shown and comparative analyses between micro and nano reinforcement have also been made and reported by Senthilkumar et al.[24].The mechanism of tribological wear given in the scientific article by Kovalchenko et al.[25],inspires to proceed further in the research.Material characterizations have been detailed by Saravanakumar et al.[26]and Suryanarayana et al.[27]in their investigations on hybrid nano composites.Extensive works have been done by researchers[28-30]on copper based nanocomposites and the work put forth by Preetkanwal et al.[31]stands as a guideline to the researchers to work on hybrid nano composites.

All the investigations show the elevation of material behavior resulting in the resistances to any adverse effects;they call for further refinement in finding a better combination of materials at appropriate proportions.To address the issue of minimizing the wear rate,a novel combination of copper,alumina and graphite has been tried in this investigation.The thermal effects have also been studied alongside the corrosion resistances in addition to the mechanical and metallurgical evaluations.Moreover,the works related to the combinations of copper,alumina and graphite are scanty.

2.Experimental procedures

Copper was selected as the base material.Since the objective is to develop hybrid reinforcement,it has been decided to choose the metal copper as the base and a proportionate combination of preferablyalumina a ceramic along with graphitean allotropic form of carbon as the particulate reinforcements.This combination justifies the term hybrid reinforcements.These constituents were taken at different proportions as shown in Table 1.

The selected materials were subjected to mechanical alloying technique using a planetary ball milling machine.The constituent elements were subjection to milling at 550 rpm under atmospheric ambience for 3 h.The materials were kept in the air tight tungsten carbide coated vessel carrying 10 carbide coated balls so as to maintain the powder(in grams)to ball(in numbers)ratio of 10:1[24],In order to avoid micro aggregation an intermediate cooling was carried out to facilitate the heat dissipation from the carbide coated vial.

Green pellets were made using pneumatic compaction techniques.Dies were fabricated to make circular pellets of 60 mm in diameter and 10mm in thickness.Zinc Stearate was applied as the binder to make pellets.A compaction load of 23 tons was applied to fabricate the pellets.The compacted pellets were sintered in an induction furnace to the temperature of 700°C and for a soaking period of 180 min.In this investigation,proper care was taken to avoid excessive oxidation.Many a researchers say that,the powder metallurgical processes are usually being carried out in inert gas ambience preferably argon[26,27,29,30].The work piece was allowed to cool within the furnace[32,33].Subsequent to sintering,the pellets were tested for their mechanical and metallurgical properties[34,35].Pin-on disk apparatus was used to conduct the wear analysis.

3.Results and discussion

3.1.Scanning electron microscopy

The hybrid powders,a homogeneous blend of copper,alumina and graphite particles were characterized by SEM(JOEL)analysis after milling for 3h.The morphologies of the observed particles lie in the nano range.The Figs.1-3(a,b)SEM images reveal the presence of nano alumina particles and also the clear visibility of interface between reinforcement particles and the matrix.The above image shows that nano alumina particles in the copper matrix were dispersed fairly well.Agglomerations of nano alumina particles were also observed in the composites but the absence of micro cracks is an indicator of good interfacial strength between the matrix and particles.It is also seen that dispersed phases of nano precipitate components are in the matrix phase.EDAX image shown in Fig.4 confirms the presence of all the constituents.

3.2.X-ray diffraction techniques(XRD)

A Sample was taken for XRD study to estimate the crystal lite sizeof the powder,phase constituents,lattice strain.The crystallite size and lattice strain was calculated using formula [26,27],Bcosθ=0.9λ/d+ε sin θ.Where λis the wavelength of Cu Kα radiation in nm,dis the crystallite size in nm,θis the Bragg angle,ε is the lattice strain andBis the Full width half maximum of the ray in nm.The XRD tests were conducted individually to all the elements to ascertain their entry into the nano regime.The XRD pattern for Al2O3,Cu and graphite were shown in Fig.5.The output results obtained from the X-Ray Diffraction studies as shown in Table 2,with the help of the data base provided by Joint Committee on Powder diffraction studies(JCPDS)and supported by Material report Analyzer software,the peak values of all the constituent elements were compared with the values available in the database and confirmed the presence of all the constituent elements.With the same results morphology and the lattice crystal structure of the constituent elements at the nano scale were studied.Substituting the values for the parameters in the Scherrer's formula the particle size reaching the nano regime has been confirmed.The phase(Al2O3)is identified with the patterns with high peak values of 2 theta=(35,44,53,58)at the planes[104],[113][24],and [116]

respectively.The 2θvalue for copper at 44.95°gives the maximum peak value in plane[111]similarly graphite gains the maximum value at 2θ=26.52°with the plane[002].Similar to the studies of other investigators[26],graphs reveals the diffraction of graphite with peak of 2(θ)25°the same peak were obtained in the graphite powder.

Table 1 Percentage of composition of copper and other reinforcement.

Table 2 Crystalline size and strain of the constituent elements in the hybrid.

3.3.Hardness

Vicker's hardness tester was used to measure the micro hardness of the samples at a load of 70gm applied for the duration of 20s shown in Table 3.The presence of alumina in nano scale with higher proportion increases the hardness of the composition.Micro hardness has always been a measure of reinforcement in the heavy conducting copper matrix,that the increase of Vicker's hardness value is a function of the increase in the dispersive of Al2O3in the copper matrix[5].The physical parameter hardness indicates the capability to pose hindrance to the local plastic deformation.Nano sized Al2O3copper matrix resists the movement of dislocations preventing the occurrences of plastic deformation.A slight increase in the content of graphite also increases the hardness of the composition,from the analysis it is obvious that mere 4%increase in graphite increases the hardness equal to the composition containing nano alumina.A slight variation has been observed in the micro hardness between the composites.The results match with the findings of researchers worked with copper and alumina combinations[11].

3.4.Thermal analysis

3.4.1.Thermogravimetric analysis

Thermal gravimetric analysis(TGA)is a technique of thermal analysis in which the mass of a test sample is measured overtime as the temperature changes.This measurement affords about the physical information such as phase transitions,as well as chemical phenomena thermal decomposition.TGA can be used to calculate the thermal stability to a substantial level.In a preferred temperature range,if samples are thermally stable,there will be no experiential mass change.Negligible mass loss relates to little or no slope in the TGA trace[36,37].This technique also provides the upper use temperature of a material and above this temperature the material will begin to degrade[38].

Hence to identify our samples working temperature as lubricant thermogravimetric analysis was performed.The formed TGA curves are given in Fig.6.The thermo grams of three samples showed two steps of decomposition.The decomposition step of micro alumina reinforced sample-1 exists between 430°C-460°C.Sample-2 and sample-3 shown exposed the same decomposition due to the nano alumina and graphite with copper base metal.This TGA,DTA and DTG thermograms showed the withstanding ability of the samples for further mechanical studies.However,there is a class of thermally stable materials that are capable to withstand working temperatures without structural changes or strength loss.The reinforced materials are characterized by thermogravimetric analysis for the stability and identification of operating conditions.From the results,all the materials are shown ±40°C degradation temperature.Hence to identify the operating stability and to selectthe best composition this research tested the corrosion nature of the reinforced materials.

Table 3 Vickers hardness of the developed composite pellet materials.

3.4.2.FT-IR analyses

Reinforced copper metal samples were analyzed by FTIR for the existence of the alumina and graphite in pellets.Sometimes copper alloys surfaces showed blue,green,dark brown and white corrosion products and have been identified such as sodium copper carbonate actetate,copper(II)hydroxide,copper and tin oxide,and sodium acetate trihydrate,respectively.Hence the FT-IR images of the reported base metal and reinforced materials shown in Fig.7 were compared with the sample FTIR.The FT-IR result showed the peaks relevant to the constituents of the prepared copper base metals and graphite structure shown in Table 4.Similar results were obtained by several researchers[38-40].

Sample-1 FT-IR exposed the peaks relevant to copper and graphite.But alumina peak is absent in the FT-IR due to the 2%of micro alumina.Sample-2 and sample-3 FTIR showed the peaks for alumina between 1500 and 1600 cm-1due to nano size of the alumina.This shows the nano material's vibration property.Copper,graphite and alumina showed peaks at 2840 cm-1,1760 cm-1,1600cm-1respectively.Almost all the samples also showed the peaks at the same range and also observed that there are no surface products on sample surface.This clearly showed the chemically inert nature improved on copper base metal due to reinforced graphite and alumina.

3.5.Corrosion study

Copper is a cathodic metal which is easily undergoes oxidation by accepting electrons.When compare to iron bearings reinforced copper bearings lifetime will be higher due to its corrosion resistivity.Hence this study selected the copper as base metal for the bearing preparation purpose after the reinforcement and its corrosion nature was studied under two different conditions.Due to the alumina reinforcement this research tested corrosion behavior in sodium hydroxide and sodium chloride.1%Sodium hydroxide solution was prepared initially and serially diluted like 0.5%,0.25%respectively.The prepared solutions were taken in 250mL beaker and the pallets were dipped in the solutions[39-41].The reinforced pellets were dipped in the solutions for 24h and potentio dynamic study carried for the samples.Similarly the samples tested in sea water after 24h and weight loss was calculated.

Table 4 FT-IR frequencies of different composites.

After 24h potentiodynamic study graph was plotted for an each samples and shown in Fig.8.From the graph 8,sample-3 nano and micro alumina reinforced copper pellet showed higher resistivity and more cathodic nature.Highly cathodic metals are having more corrosion resistivity than anodic metals[42-44].The same dimension pallets are used for the corrosion using sea water for sodium chloride impact on material surface.Weight loss was calculated and shown in Tables 5 and 6.From Table 5,sample-3 showed good corrosion resistivity than other samples similar to the studies published in the research articles[45,46].

3.6.Wear analysis

Wear is a universal phenomenon that even the hardest material diamond has not been left untouched by this factor so far.The research on wear mechanisms and their influences on the life of cutting tools and mating surfaces have attracted the research enthusiasts.The deformation due to the behavior of materials after the transition from elastic state to plastic state,nature of cracks and their propagation influences on tribology are the vital ingredients which called in a greater response towards the research.In many research works to date,the authors have investigated the effects of contact abrasion,erosion and impact effects on uncoated components,mainly as separate problems.Pin on disc has been extensively used to measure the wear rate.

Table 5 Corrosion study on the samples using sodium hydroxide solution.

Table 6 Corrosion study on the samples using sea water.

Weights of the pin made of the material EN8,with the dimension of 60 mm length and 6mm diameter were measured initially.The disc made of the developed composites with the dimensions of 60 mm diameter and 10 mm thick were also measured initially with the weighing machine of a very high precision.The dimensions of the Pin and Disc follow the G99 ASTM standards.Appropriate clamping was done for both the pin and the disc in the apparatus shown in Fig.9.Then the predetermined load is applied against the pin supported beam.The track diameter varies between 20 mm,45 mm and 55 mm with the corresponding sliding velocities of 0.72 m/sec,0.57 m/sec and 0.55 m/sec respectively.Frictional force vs time curve shown in Fig.10 and wear rate vs load characteristics is shown in Fig.11.

A computer based controller has been interfaced with the pin on disc apparatus to control the process parameters.The parameters required are speed in rpm,and load in Kg.In each disc 3 different tracks were made by varying the load where as the speed was kept constant.During the process,the computing system generates the values of coefficient of friction,friction force generated,for the given time interval as given in Table 7.Loads of 0.5,0.75 and 1kg were varied between Track1,Track2 and Track3 respectively.The total sliding distances were calculated,they are 45 m,35m and 15 m for the respective tracks.The following equations(2)-(5)proposed by G99 standards were used to calculate the wear rate.

Table 7 The observations in wear rate analysis on Pin-on-Disc.

The presence of nano materials in the samples could not reduce the wear rate considerably.The increase in application of load has a telling effect on the wear rate.Wear rates of the samples goes down with the cohesiveness of the bond between the ingredients.The samples with micro and nano alumina reinforcements give an unpredictable characteristic curves,whereas the pure nano reinforcement with sizable proportion of graphite gives a meticulous outcome of the frictional force variation with time.There are few uneven variations with respect to sliding distance and loading.This is due to the unevenness happened at the time of compaction or the uneven distribution of reinforcements in the composite material developed.The formation of oxides in any of the composition while sintering made the material prone to wear as seen in the investigations carried out by Selvam et al.[47].

4.Conclusions

With the assistance of Thermogravimetric analysis the material stability of the composite mixtures were ascertained.The sample peak values were read with the help of FTIR analysis.The corrosion resistance of the composite with the maximum alumina reinforcement in nano scale exhibited better wear resistance.The presence of graphite certainly provides lubricating effect.There are only subtle changes in the wear resistance,since the material compaction required more compaction pressure.Had the distribution of reinforcements been even,the net wear characteristics would have been meticulous.The uneven surface occupation of the reinforcements could not yield a structural pattern in the outcome.The compacting pressure,sintering temperature and the mixing of the composite powders using a ball mill have a definite implication in enhancing the wear resistance.