Wear Analysis of Al7068 Metal Matrix Composite Reinforced with Silicon Nitride

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Wear Analysis of Al7068 Metal Matrix Composite Reinforced with Silicon Nitride

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Abstract- In present scenario pure material is unable to provide the varieties of properties required for the variations in applications hence acquainted with the composite material are quiet relevant. Thus for these point of consideration, Metal matrix composites (MMCs) possess significantly improved properties including high strength ratio; specific modulus, damping capacity and high wear resistance compared to unreinforced ,means original basic alloys. There has been an increasing interest in composites containing light weight and low cost of component .Also These material shows required properties and it will have very less impact on environment they are easily recycled and it is very helpful for solid waste management, which is in a great concern of our country and society. In this paper Al7068 alluminium alloy reinforced with silicon nitride with varying percentage is used for fabrication of composite material. casting is done by liquid state fabrication process that is stir casting process. After casting process for insuring homogeneity of material SEM is done. And after various testing of strength, wear analysis is done by well known solid state Pin on disk ,wear analysis process. As it is great point of consideration that failure of parts due to friction is happens and about one third energy losses is due to it. But as we know silicon nitride has excellent hardness which enhance the properties of base alloy. Thus we can aspect to reduce the failure tendency of component in future. These material will have very vast application in automobile industries, aerospace as well as marine where strength and durability is main point of consideration.

Keywords- Metal matrix composites (MMCs), Al7068 alluminium alloy, SEM, Wear analysis, silicon nitride

1. Introduction

Theengineeringcommunityhasalwaysbeeninthesearch for materials which would suitable for all kinds of conditions during service. It originates from the requirements to make improved discoveries made by

scientists,ordable. This affordability factor has persuaded various researchers to make such materials which satisfy manyhithertounexploredconditions.Inthecurrent world most materials are used for various uses and their limitations have been seen. In current world almost all materials have been used for various applications and their limitations have been met. But in today’s world materials are required to face harsher environments. This situation requires that new materials should be created from various combinations of other compatible materials. It should be analysed that this method is not new, it has been considered by humans since ages. In each and every division of the world, different materials have been combined to obtain some desired properties, although each case has variation from the others, i.e. one can generate new materials with specified properties, which are user desirement and are vary from their base ingredients. This idea holds good for a type of materials called Composite materials where, various variety of matrices are combined with reinforcements that contribute to the enhancement of the properties. Neither the matrices nor the reinforcements when taken individually can fulfill the desired requirement, but the compositematerialscan do. Thischange inpropertiescan be controlled by many different ways, by controlling the matrixandreinforcementquality,theircompositionorthe fabrication tricks and technology. This flexibility in manufacturing of material allows to develop composites with different properties in a precisely restrict manner. It is the special mechanical as well as thermal properties of compositematerialswhich hastriggeredtheirindentation into all fields of manufacturing. Metal Matrix Composites (MMCs) have came as a new branch of materials suitable for structural, electronic, aerospace, vehicle, thermal and damage applications due to their advance character over theconventional materials. They are far superiorin terms of modulus ratio, specific strength, wear resistance, chemical inactiveness, high temperature sustainability, resistant co efficient of thermal expansion, and so on. But

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on the other hand they have low toughness and high cost of manufacturing in comparison with Polymer Matrix Composites (PMCs). But MMC’s have higher transverse strength and stiffness, shear strength and high temperature capabilities when compared to Polymer Matrix Composites (PMCs) . The desirable physical propertyarehighelectrical andthermalconductivities, no moisture absorption, non-flammability and resistance to most radiations. Compositionally, Metal Matrix Composite (MMCs)hasatleasttwocomponents,i.ethematrixandthe reinforcement. The matrix is usually a metal, but often a pure one. In some cases, it is generally an alloy. The most widely used alloys are based on Aluminum and Titanium. Both of the materials have low density and are easily available in a wide range of alloy compositions. Other alloys are also used in some distinct cases, because they have their own advantages and disadvantages. For Ex Beryllium is the lightest of all manufacturing related materials and has a tensile strength greater than that of steel, but it is highly brittle, making it unsuitable for general purpose use. Magnesium is light, but has high reactivity to Oxygen. Nickel and Cobalt based super alloys are also used in some cases, but some of the alloying elements present in them have been found to have undesirable result (promoting oxidation) on the reinforcingfibersatveryhightemperatures.

2. Literature Review

S.Madhavarao et al. [1] studied the frictional and wear behaviour of Aluminium 7075 reinforced with 10% and 15% weight % of SiC particles. The aluminium matrix composites were manufactured by stir casting process. Wear and frictional properties of the aluminium matrix composite in dry sliding conditions were studied by using pinondiskmachine.Thewearexperimentwereconducted by using Taguchi orthogonal array DOE approach and result were analyzed by using ANOVA Method. The result showsthattheslidingdistancehasmoreinfluenceonwear rate than other parameters for AL7075+10%SiC while applied load has more influence on wear rate for Al 7075+15%SiCcomposite.

Bhoopati Mahendra et.al [2] calculated the mechanical properties and the micro structure of Al 2024 reinforced with Silicon Carbide and Fly Ash mixture metal matrix composite. A total of 9 specimen were prepared with varying weight percentage of SiC and Fly Ash in Al alloy 2024.ThespecimenwerepreparedbyStirCastingMethod alsoknown asliquidstate method. The micro structureof the composites were studied under optical micrograph. The test shows non-uniform distribution of the reinforcement in the case of Al/(5% SiC),Al/(10%

SiC),Al/(5% fly ash) and Al/(10% fly ash).While uniform distribution of the reinforcement is seen in Aluminum mixed with SiC-Fly Ash mixture at various composition. Mechanical Properties such as Tensile Strength, Yield Strength, Hardness and Elongation were evaluated for all the specimen. The test results shows that both tensile strength and yield strength increases compared to unreinforced Al alloy. Tensile strength of unreinforced Al is 235 N/mm2 and this number increases to 267 N/mm2 forAl/(10%SiC),263N/mm2forAl/(10%flyash)and293 N/mm2 for Al/(10%SiC+10%fly ash)composite, which is about 57% improvement to that of the unreinforced matrixwhencompared.Similarresultsofincreasingtrend hadbeenobservedbothinhardnessandyieldstrength.

Mir IrfanUl Haq et.al [3] studied the effect of sliding speedonfrictionanddamagepropertiesofAA7075-Si3N4 composites has been studies. The tribological testing was held on a unidirectional pin on disc tribometer at three variable sliding speeds (1 m/s, 3 m/s, and 7 m/s) at a constant load of 32 N. The wear loss increased with an increaseinspeedandcoefficientoffrictiondecreasedwith an increase in speed. The morphologies of the worn samples were investigated by SEM and it was observed thatincaseofunreinforcedalloyandlowerconcentrations (2 wt% and 4 wt%) delamination was the dominant wear mechanism and, in case of higher Si 3N4content (8 wt%) abrasive wear was the dominant wear mechanism. Heavy plasticdeformation and deepergrooveswereseen incase of higher speeds. Mechanically Mixed Layer (MML) formationwasalsoprovedbyEDS.Thedevelopedmaterial couldservevarioushighspeedslidingwearapplicationsin automotive sector. Graphite and silicon carbide MetalMatrix Composite by Tagauchi and ANOVA method. Aluminum alloysofgrade356wereusedwithadditionof reinforcementindifferent weightpercentageconstituents such as SiC 2%,Alumina 2%,Al356 and SiC 2%,Alumina 4%,Al356. The material was fabricated by stir casting method. The wear nature of the material wasinvestigated by pin on disk wear testing equipment. The experimental result showed that the value of coeffcient of friction µ of thematerialdecreasesasthereinforcementincreasesand increasingtheslidingtimeandincreasesasloadincreases.

SN Prashant et al. [4] studied the mechanical and wear properties of Aluminum 6061 reinforced with SiC and Graphite form 6% to 12% increasing in the steps of 3%. The specimen were prepared by stir casting method and were stirred automatically for 10mins to ensure the uniform distribution of reinforcement. Mechanical Test such as Hardness Test and Ultimate Tensile Strength and wear testwere performed onthehybridcomposite.It can beconcludedfromthetestthatwhenSiCandGraphiteare

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mixed with Aluminum gives opposite result to each other. Hardness of SiC reinforced composite increases with the compositionwhilethatofgraphitedecreases.Ittestresult concludes that the ultimate tensile strength of hybrid compositeisgreaterthantheunreinforcedAluminum.The wear test was performed at different loads(10N, 30N and 50N)bykeepingtheslidingspeedconstantat1.6m/s.Itis foundthatthewearforSiCreinforcedcompositedecreases at all compositions and nominal load while wear rate for graphite reinforced composite decreases uptil 6% after whichstartsdecreasing.

Mohammed Amair et al.[5]says that the utility and advantage of metal matrix composites are increasing day todaybecauseofhighstrengthtoweightratio.Thehybrid aluminum composites can be taken into mind as an outstanding and uncomparable material where high strength and wear-resistant components are of bigger importance, predominantlyintheaerospace andcraftand automotive like cars of various variety in engineering sectors. In the present work , Al7068 reinforced with that of the tur husk ash(THA) and Al2O3 hybrid metal matrix composite is formed from sintering of mechanically alloyed powder (ball milling) in powder metallurgy process. Different combinations of compositions (Al7068 reinforced with 0%,4%,8%,12%0F THAandAl2O3) were taken with total 16 combinations. Hardness was found to beincreasingwithincreasingpercentageofAl2O3butwas decreasingwithincreasingpercentageofTHA.

3. Work & Methodology

Working Steps:

The material is selected to reduce the cost and also improvethemechanicalpropertiessuchasstrength ofthe materials, hardness, toughness and wear resistance of the components.

 After doing literature survey we select an aluminiumalloyAL7068asmetalmatrixandSi3N4 as reinforcement. Since Aluminium alloys gives good strength to weight ratio. Aluminium 7068 alloy is a heat operablewrought alloy with well anodizing results,high thermal conductivity & good fatigue strength. It is manufactured as a higher strength alternative to other aluminium alloyssuchasAl7075forordnanceapplications.It also gives the highest mechanical strength of all aluminium alloys. Silicon nitride is the most thermodynamically stable as a reinforcement and alsohaveveryhighstrength.



Stir casting is used for casting of the composite material as it is relatively less costly than other methods of fabrication and also gives comparatively good results. Since we know Powder metallurgy is one of the best processes butitistoocostlyandcomplicatedprocess 

InstircastingweuseSiliconNitridemicropowder of20-40µmwhichstirredwithAl7068alloy.



After stir casting various mechanical testing such as tensile strength (by UTM),hardness will be analysed such that we can observe the improved propertiesofthecomposite.



Finally wear analysis will be done by the Pin on disc methodsothattherate ofwearofcomposite can be calculated and also compared with the otherreinforcingelement.

4. Materials and Methods

4.1

Instruments used and materials required

This chapter describes the experimental process adopted in the present project work. The instruments used for the several experiments in this work are taken into paper are below.Adetailedreportisalsogivenontherawmaterials utilised for fabrication of the test work piece and the characterization of the original material used for fabrication process. Details of each working steps accepted for the fabrication of the testing work piece, the processofmechanicaltestingcarriedout,thegenerationof the micrograph through Scanning Electron Microscopy(SEM) has been furnished. The instrument / equipments used for the manufacturing of Aluminium

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Material used

4.2.1

Al7068 Alloy:

The material used is Al7063 with SiN3 which is a composite made by using Stir-casting process. Its advantages are: Cost reduction can be achieved by changing the microstructure of the connecting rod and strengthtoweightratioishigh.Thismaterialexhibitshigh wearresistanceincomparetotheconventionalconnecting rodandcanbesplittedintothe2pieces(thebigbodyand the cap) by fracturing it with an instant impact loads. There is always a demand for a light-weighting material withimprovedfuelefficiencyinaerospaceandautomobile industries. This lead to increase the use of materials like aluminum over traditional metals as steel for increasing bothrangeandefficiency.Powdermetallurgyisoneofthe best technology for mass production of the parts in these industries.Aluminumishavingexcellentspecificstrength, thermalpropertiesandstiffness,itisalsocosteffectivefor the mass production. However, it suffers from poor wear resistance so its usefulness lowers in moving parts applications. The inclusion of ceramic reinforcements to give the metal matrix composites (MMCs) which can increase both specific mechanical properties as well as frictional and wear properties of aluminium. The silicon nitride is a ceramic material having high strength, low coefficient of thermal expansion and stiffness. It also have high toughness for a ceramic material hence when combines to give it the extremely high thermal shock resistance.[22] Aluminum Alloy 7068 is selected as a matrix material. Aluminum Alloy 7068(AA7068) and SiliconNitrideispurchasedParshwamanimetals,mumbai, India. AA7068 alloy consists of mainly other elements as Zn,Mg,Cu,Fe,Si,Cr,MnandNi.Thechemicalconstituents areshowninbelowtable. Table 1

Aluminium 7068 Composition

was made as a higher ability alternative to the Al7075 for theordnanceusefulness.Itistheveryhigh attractivewith a overall of the mixture mechanical properties (which allowsittoretainattheelevatedtemperaturesbetterthan Al7075)andtheotherimportantcharacteristicsofAl7068 havealsoresultedinawidespreadspecificationsofalloyto markedly reducing the weight/cross section and widely increase in the strength of the critical components in the diversemarketsectors.

Table 2 Typical properties AL7068

Tensilestrength 586MPa Ultimatestrength 642MPa

Young’smodulus 73GPa Hardness(Brinell) 188 NominalDensity(68ºF /20ºC) Specific heat(212 ºF / 100ºC) 1050J/kg ºK

2850kg/m3

Al7068alloyhasoneofthemaximummechanicalstrength ofallaluminiumalloysandmuchmorethanthatofcertain steels. This is the outstanding alloy combining yield strengthofupto710MPa(whichisuptoover31%more than that of the Al7075 alloy) and the good ductility with thecorrosionresistancewhichissimilartoAl7075andthe other features is high working component/equipment designers.Itisdevelopedinmiddleof1990‟s,Al7068alloy

MeltingRange 476ºC-635ºC

4.2.2

Silicon nitride :

Silicon nitride is a preferred ceramic for the high mechanical working engineering applications. This ceramic can also be considered great candidate for the biomedical applications because of the same properties which makes it so important for the engineering application like high wear resistance, chemical stability, low friction coefficient and it is mainly because of mechanical behaviour under the articulation which is generally considered better than that for the alumina ceramics. Silicon nitride (Si3N4) is strong and the heat resistant material which can manufactured in many differentformswithspecificbenefitsofdifferenttypeslike surgical implants. Silicon nitride was first produced in year 1955 for thermocouple tubes and molten metal crucible and also used in rocket nozzles. Such materials were formed by nitriding silicon powder compacts. Considerable changes in internal arrangement took place to form silicon nitride crystal which grows with the pre existing pores of silicon powder compact as this is heated in nitrogen at 1200 degree celcius. After that they observed its original dimensions of component remain same by nitriding then they heated to provide strength. The final material contains 20-30% microporosity that limits its strength (<200MPa) for certain applications. So,

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finally sintered to produce the dense and high strength material.

Silicon nitride exhibits : 

Highfractureresistance.  Highflexuralstrength.  Goodcreepresistance .  Highhardnessnumber  Verygoodresistancetoerrosion.

Table 3 properties of Silicon Nitride

DENSITY 3020kg/m3

HARDNESS 20.5GPa ELASTICMODULUS 280GPa COMPRESSIVE STRENGTH 4900MPa TENSILESTRENGTH 420MPa THERMALEXPANSION 2.5x10-6/oC

4.3 fabrication of metal matrix composite ( casting method):

4.3.1 Stir casting method:

Among the above mentioned fabrication methods in the introduction part, stir castingmethod comes outto be the mostpromisingandeconomicalwayoftheproduction.

Stir Casting:

In this casting process,the process constitutes electric powered furnace,a preheater for preheating the reinforcement particle so that working temperature difference can be minimised, mechanical stirrer & electric motor.Thetemperatureoftheofthefurnaceismaintained at700-780℃.Thediesofparticularshapeisplacedinside the furnace before the alloy so that it can be preheated about250-300℃.Itistoensureforpropersolidificationto obtain a desirable grain growth as if boundaries cools faster strength will vary. Graphite crucibles are used to hold and keep aluminium alloy bars for placing inside the furnace. The matrix material Al 7068 is being heated in a crucibleabout750°Candpreheatedsiliconnitrideceramic particlesforreinforcingintothemetalfluidaremixedinto the crucible at a not changingrate. The ceramics were preheated in an electric furnace up to350- 400 °C. The stirrerconnectedtomachinepoweredbyanelectricmotor

was rotated at an RPM of 250-300.The stirring done for around 20-30minutes so that proper mixing can be ensuredAlso Mg tablet mixed to improve wetability.Four differentworkpieceofAA7068-Si3N4arepreparedbystir casting . The ceramic reinforcement was varied in four differentlevelsbyweight(1,3,5and7).Theliquidformof composite was added into the permanent mould, which was thereafter allowed to cool in ambient conditions. The cast samples were machined to form cylindrical pin of 8 mm diameter and 8 mm height. The presence of silicon nitride ceramic was confirmed by EDS method,before the hardnesstesting, the samples were rubbed by different grades of emery papers.and final polished component is sent for different testings so that desirable properties can beconfirm.

4.4 Wear Theory:

When two surfaces of relative motion interact with each other,wearingofthesurfacesoccurs.Wearcanbedefined as the gradual loss of material from the contact surface during relative motion. Scientists have generate various wear theories, which take into account the physical and mechanical properties and physical conditions of the material (such as the resistance of the friction body and the stress state of the contact area). In 1940, Holm calculated the volume of the worn material on the unit slidingpathfromtheatomicmechanismofwear.Kragelski put forward the theory of wear fatigue. Due to the irregularities in the solid, their in teraction on the slide is discrete, and the various locations where they touch together occur, forming the actual contactarea.Under the action of normal force, these irregularities penetrate or flatteneachother,andthecorrespondingstressandstrain riseintheactualcontactarea.Duringtheslidingprocess,a fixed volume of matter is subjected to many more repetitions, which weakens the material andeventually causescrackingandfracture.

5. RESULTS & CONCLUSION

5.1 Evaluation of Mechanical Properties

This chapter contains the results obtained from the tests performed on the composite materials. Mechanical Test such as Tensile Strength Test and Brinell Hardness Test and Wear Test were conducted on the composite materials.Thetestspecimensforalltheexperimentswere conducted according to ASTM standards. To study the micro structure of the materials SEM micrographs have alsobeenprovided.

5.1.1

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Tensile Strength Test

To determine the Ultimate Tensile Strength and Yield Strength of the hybrid composite materials, Tensile Strength Test was conducted by using Universal Testing Machine (UTM). The test was conducted in the workshp Thetestspecimenlengthis100mm. σuts=F/a where FutsistheUltimateTensileLoad(kN) FyieldistheYieldLoad(kN) Aoistheinitialcrosssectionalofthespecimen(mm3)

Table 4 UTS AND YS of the Material

S.No Material composition UTS(MPa) Yield Strength(MPa) 1 0% Si3N4 600 630 2 3% 622 615 3 5% 739 725 4 7% 748 733

5.1.2

Brinell Hardness Test

TodeterminetheHardnessNo.ofthecompositematerials, HardnessTestwasconductedby usingBrinellHardnessMachine.Thetestwasconductedin the workshop. The test specimen had a length of 30mm anddiameterof30mm.

Table 5 Hardness comparison

5.1.3 Charpy Impact Test

Tocalculatetheimpactenergymeansthetoughnessvalue ,charpy test was conducted in the workshop. the test specimenwasofdimention10*10mm.

Table 6 Impact strength Comparison

S.No Material composition Impactvalue(j) 1 0% 23 2 3% 18 3 5% 14 4 7% 10

5.1.4

Wear calculation and analysis

tocalculatewearratethereductioninweightiscalculated after pin on disk test. The test was also conducted in the workshop. the weight deduction is variable for different composition.

S.No Material composition Load (N) SLIDING SPEED (m/s)

Wear rate (mm3/ min)

COF

1 0% 40 .25 .442 0.127 2 3% 40 .25 .135 0.084 3 5% 40 .25 .0682 0.061 4 7% 40 .25 0.0125 0.043

6. Conclusions & Future Scope

Based on the study of various research paper, model was developed byaddingceramic asa reinforcingagent Si3N4 in the Al7068 aluminium alloy, the following conclusion canbeobtained:

* There are an more value in the hardness value with an increase in Si3N4 content. With an increase in Si3N4 content from 0 wt% to 7 wt%, the hardness of the componentisseen to increasefrom61.2to 123(BHN). It means that the increment in the hardness of the 7 wt% Si3N4mixedAluminiumcomposites.

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* The tensile strength increased from 98 MPa to 175MPa from neat Aluminium to the 7wt% of the Al-Si3N4 composites. It shows that with an introducing Si3N4 into Aluminium alloy improves material properties because of good compatibility and the interfacial adhesion between theSi3N4andtheAluminiummatrix.

* It is observed that the occurrence of the hard and the brittle Si3N4 particles in soft and ductile Al alloy matrix reduces the ductility content of the fabricated metal compositeduetothesmallductilecontentofmatrixmetal in composite, which considerably enhances hardness of thefabricatedAMCs.

* The brittleness nature of Si3N4 particles shows significant role in compromising its ductility; because, Si3N4 as reinforcement is a brittle and it improves brittlenessinthenewlyformedcomposites,whichin-turns reduced ductility content of the composites. Further, increment in wt. % of the Si3N4 particles in composites reject the flow-ability of aluminium matrix and decreases ductile matrix content, which results in decrease of percentage elongation of the composites. Identical consequences of the reduce in percentage elongation with theincreasingvalueofreinforcementinaluminiummatrix on comparision of fabrication process of the composites wereachieved.

Thuswecanexpectthatthiscompositematerialdefinitely reduce the cost or reduce the weight of component withoutcompromisingthequalityrequirement.

References

[1] S. Madhavarao, C. R. Raju, J. Madhukiran, N. S. Varma, and P. R. Varma, “A study of tribological behaviour of aluminum-7075/sic metal matrix composite,” Materials Today:Proceedings,vol.5,no.9,pp.20013–20022,2018

[2] M. M. Boopathi, K. Arulshri, and N. Iyandurai, “Evaluation of mechanical properties of aluminium alloy 2024 reinforced with silicon carbide and fly ash hybrid metal matrix composites,” American journal of applied sciences,vol.10,no.3,p.219,2013

[3]S.Prashant,M.Nagaral,andV.Auradi,“Preparationand evaluation of mechanical and wear prop- erties of al6061 reinforced with graphite and sic particulate metal matrix composites,” Internationaljournal of mechanical engineering and robotics research, vol. 1, no. 3, pp. 106–112,2012.

[4]Panwar N, Chauhan A. Fabrication methods of particulatereinforcedAluminiummetalmatrixcompositeA review. Materials Today: Proceedings. 2018 Jan 1;5(2):5933-9.

[5]Azimi A, Shokuhfar A, Nejadseyfi O, Fallahdoost H, SalehiS.OptimizingconsolidationbehaviorofAl7068–TiC nanocomposites using Taguchi statistical analysis. Transactions of Nonferrous Metals Society of China. 2015 Aug1;25(8):2499-508.

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[7]Synthesis and characterization of graphene nanoplatelets reinforced AA7068 matrixnanocomposites produced by liquid metallurgy route ;Mohammad Alipour, RezaEslami-Farsani:material science&engineering A706(2017)71-82.

[8].Corrosion resistance of aluminum alloy AA7022 wire fabricated by friction stir extrusion ;Kamin TAHMASBI1, Masoud MAHMOODI1, Hossein TAVAKOLI2;Trans. NonferrousMet.Soc.China29(2019)1601−160.

[9]Investigation of mechanical properties of Al7075-SiCMoS2 hybride composite;K.maruthiVarun,R.Raman Goud(2019);Department of mechanical engg.GRIET,Hyderabad500092,india.

[10]Wear behaviour of Al2618 alloy reinforced with Si3N4,AlN,ZrB2 insitu composite atelevatedtemp.;N.Mathan umar,S.Senthil,L.A.Kumaraswamidash(2015);Alexandria EngineeringJournal(2016)55,19-36.