Investigation on dry sliding wear behavior of Mg/BN nanocomposites

R.Vr Prsd Kviti,D.Jeysimmn,∗,Gururj Prnde,Mnoj Gupt,R.Nrynsmy

a Department of Mechanical Engineering,Periyar Maniammai Institute of Science&Technology,Thanjavur 613403,Tamil Nadu,India

b Department of Mechanical Engineering,National Institute of Singapore,9 Engineering drive,Singapore 117576,Singapore

c Department of Production Engineering,National Institute of Technology,Tiruchirappalli 620015,Tamil Nadu,India

Abstract The present research objective is to investigate the effect of boron nitride nanoparticles reinforcement on dry sliding wear behavior of pure Magnesium and magnesium nanocomposites.The fabricated nanocomposites contains varied percentages of boron nitride such as 0%(pure Mg),0.5%,1.5%and 2.5%were synthesized by using powder metallurgy technique and followed by a hot working process called hot extrusion.The pin on disk equipment was used for conducting the wear tests for traditional loads of 5 N,7 N and 10 N at different sliding speeds of 0.6,0.9 and 1.2 m/s against the steel disk at room temperature.For all traditional loads and sliding speeds,the changes in wear rate and friction co-efficien(μ)with respect to sliding distances were observed and analyzed.The wear characteristics are observed with the help of scanning electron microscopy under given test conditions.To investigate dominant wear mechanisms for various test conditions,the morphologies of all worn composites surfaces were analyzed.Final results show that,for all nanocomposites the wear level raises with respect to the sliding speeds and loads.Magnesium reinforced with 0.5%boron nitride shows lower wear rates and low friction coefficien values compare with magnesium reinforced with 1.5%boron nitride and 2.5%boron nitride nanocomposites.

Keywords:Metal matrix composites;Wear;Friction coefficient Wear mechanisms.

1.Introduction

Magnesium is the lightest metal,making it very useful for automobile,aerospace and transportation sectors due to its potential to dramatically reduce the weight of components that would otherwise be made from aluminium,which is 65%denser than magnesium[1,2].The addition of reinforcement to magnesium and its alloys improves its strength,and stiffness.These materials have very low flxibility compared with other materials and hence limit its broad applications[3–6].

Magnesium can take care of the vast majority of the issues looked by enterprises in which the strength to weight proportion is vital,for example,the automobile,space and telecommunication industries.The available literature shows that the usage of magnesium is constantly increasing and can be expected to continue to increase in future[7,8].Metal matrix composites(MMCs)produced by adding ceramic materials for reinforcement exhibit improved mechanical properties,including structural,wear and creep properties,among others,and thereby fin many applications.The properties of MMCs mainly depend up on matrix material,particle size and materials used for reinforcement and manufacture technique of the composite[9].The main drawbacks of magnesium and its alloys are wear and consumption protections.From all the problems,wear is most predominant problem in mechanical segments,prompting a lessened life time for magnesium-based parts and making magnesium unsuitable for use in bearings,gears,pistons and cylinders[10–12].

Fig.1.Wear rate v/s sliding distance and load curves for sliding speed of 0.6 m/s.

The reinforcement stage in MMC’s could be a particle,continuous and small fibe.Among these stages,particle reinforced MMC’s are isotropic in nature and easier to fabricate;because of its high damping capacity and stiffness magnesium has been fortifie with an assortment of ceramic particulates,for example,Al2O3,zinc oxide,TiC,SiC,B4C,TiB2.Among these ceramic particulates,Al2O3and TiC have emerged as exceptional artistic fortification due to their prominent mechanical,optical and electrical properties and extensive variety of uses[13–21].

Magnesium is a better metal than Al and Ti in terms of its physical properties,including processing,machining and recycling properties,which can tremendously reduce recurring costs[22].Even though friction and rate of wear depends on many factors,like applied load,sliding speed,specimen geometry,material type and surface roughness,it is observed that sliding speed and load had a particularly solid influenc on the wear rate[23–33].In light of this unique situation,the present research work aims to analyze the influenc of the sliding velocity and load on the wear mechanism and friction co-efficien(μ)of a pure Mg,0.5 wt%BN,1.5 wt%BN,and 2.5 wt%BN reinforced Mg metal matrix nanocomposites.

2.Experimental details

2.1.Nanocomposites fabrication

Pure magnesium(Mg)powder of 98.5%purity with a size range of 60–300μm supplied by Merck(Darmstat,Germany)was used as major metal matrix.Boron Nitride(BN)with a size range of～50 nm from Nabond,Hong Kong,China was used as reinforcement.The Mg/BN nanocomposites were prepared by mechanical alloying(MA)and then uniaxially compacted at a pressure of 50 tons to obtain billets.Hybrid microwave assisted two directional sintering was used to sinter the cold compacted billets and thus explained elsewhere[34,35].After sintering,the sintered billets were extruded to obtain rods of 8 mm in diameter.After extrusion the required nanocomposites of diameter 8 mm were used for wear study.

2.2.Pin and disk preparation for the test

The specimens of diameter 8 mm and height 15 mm were utilized for testing the wear rate.The end surfaces of

Fig.2.Wear rate v/s sliding distance and load curves for sliding speed of 0.9 m/s.

specimens were finishe in order to eliminate conceivable harmed sharp edges on the surface of the disk while sliding against it.The specimen’s surfaces were flattene utilizing six hundred coarse grit Silicon carbide emery paper and then washed by a cleaning agent(acetone).After checking the flatnes and perpendicularity a proper surface contact is maintained between the samples and disk.The surface fin ish of the disk was done by six hundred coarse grit silicon carbide emery papers to evacuate collected particles on disk and finall washed by a cleaning agent(acetone).After grinding,fin cleaning operation was conducted with the help of cleaning equipment according to ASTM:E3 utilizing precious stone glue having molecule size of 1μm.

2.3.Method of testing

Nanocomposites samples were tested by sliding the pins on an OHNS steel disk.The 100%surface contact is maintained between the sample specimens and disk.The samples weight was measured by an electronic balancing instrument with a least count of 0.0001 g.Tests were led beneath sliding speeds of 0.6,0.9 and 1.2 m/s at ordinary weights of 5,7 and 10 N for a sliding separation of 500,1000 and 1600 m[16,20,23].The weight misfortunes were ascertained at each interim of sliding separation for various ordinary burdens concerning different sliding speeds.

WhereFis the force due to friction andPis the normal load acting on sample.

The Volume loss(Vloss)is determined with the help of a Weight loss(Wloss)as per the formula given below[20]

Where

Vloss=Volume loss,

Wloss=Weight loss andρ=Density.

The following formula was used to determine the wear rate

Fig.3.Wear rate v/s sliding distance and load curves for sliding speed of 1.2 m/s.

3.Results and discussions

3.1.Wear rate v/s normal load and sliding distance

The variations in wear levels as a function of sliding distances for all magnesium reinforced with Boron nitride nanocomposites were plotted in Figs.1–3.From the Figs.1–3,the wear rate slope occurs at a normal load of 10 N is higher compared to the typical loads of 5 N&7 N for sliding speeds up to 1.2 m/s.Fig.1 shows that wear rate increments straightly with sliding distance;At a sliding speed of 0.6 m/s oxidation did not occur while sliding samples against the disk.As appeared in Fig.2,the wear level isn’t direct;adjustments in wear level increment from 500 m to 1000 m sliding separations and marginally diminish past 1000 m.Oxidation occurred at a 0.9 m/s sliding speed from 500 m to 1000 m.After 1000 m,the oxidation layer happened in view of sliding activity of sample against disk surface.Fig.3 demonstrates that,above 1000 m sliding separation,wear rate diminished independent of ordinary load at sliding speed of 1.2 m/s.

3.2.Wear rate v/s normal load and sliding speeds

Fig.4 demonstrates for all sliding speeds the wear level increments with respect to the connected load.For all working speeds same wear level was obtained for Mg-0.5 wt%BN nanocomposite,at low load(5 N)and more wear level was acquired for 7 N&10 N normal loads.From Fig.4 it was clearly observed that as the load and sliding speed raises wear rate also raises for Mg-BN(0,1.5&2.5%)nanocomposites and for Mg-0.5 wt%nanocomposite the deviation in wear rate was very less in comparison with 0%,1.5%BN and 2.5%BN reinforced Mg composites.

3.3.Volume loss v/s normal load and sliding distance

From Fig.5 it is observed thatVlossdirectly expanded with expanding distances and loads for all readied nanocomposites and very lowVlossis observed for Mg-0.5 wt%BN nanocomposite compared with all other fabricated nanocomposites(Mg-1.5 wt%BN and Mg-2.5 wt%BN)and unreinforced pure magnesium.

Fig.4.Curves for wear rate v/s load for various sliding speeds.

Table 1 Average friction coefficien values for all given sliding speeds.

3.4.Volume loss v/s normal load and sliding speeds

For all fabricated nanocomposites the changes in friction coefficien (μ)for all working speeds and distances were shown in Figs.6–8.The average friction coefficien values for all fabricated nanocomposites were tabulated in Table 1.From Table 1 it is observed that friction coefficien is more at a sliding speed of 0.9 m/s and gradually increments for 0.9 m/s sliding speed and thereby decreases with the sliding speed increases.Among all Mg/BN nanocomposites 0.5 percentage of BN had low friction coefficient For a given sliding distance the value of μ increases with increasing applied load for all fabricated samples.

3.5.Components of wear

Three different components of wear(delamination,abrasion&oxidation)were seen while conducting the test[16].Figs.9–12 demonstrates examining pictures of unadulterated Mg,various proportions of Mg/BN nanocomposites under various connected loads at different sliding speeds.In order to decide the impacts of different components of wear Mg/BN nanocomposites were inspected with the help of Scanning Electron Microscopy images.

3.5.1.Oxidation

Oxidation occurs at a mass of 10 N for all Mg/BN nanocomposites.All Mg/BN nanocomposites by and large show better wear protection on account of their better load bearing limit and capacity than keep up a steady layer of oxide[36].

3.5.2.Abrasion

Fig.5.Volume loss v/s sliding distance and load curves for sliding speed of 1.2 m/s.

At 5 N normal loads,it is predominant component of wear because various scratches and sections are obvious similar to the action of sliding.Figs.9–12 of a,d&g show that all fabricated nanocomposites were distinguished by debris and notches for all working speeds at 5 N load.The very much characterized profound depressions were recognized through the unadulterated magnesium nanocomposites.

3.5.3.Delamination

For 5 N load,insignifican delamination was observable for all fabricated nanocomposites.When force builds up,indications of deformation start to show up,prompting extreme delamination,as appeared in Figs.9–12.The wear path of a steel plate ended up noticeably secured with an obvious fil of exchanged material.The deformation prompts shear twisting close to the subsurface locale of a softer material,along these lines making miniaturized scale breaks in this district.Parts at that point withdraw from the stick surface as wear trash.This conduct is normally known as delamination wear[37].The formation of wear debris was seen at low sliding velocities in view of delamination hypothesis.Fig.10(c)demonstrates that the crack formed in a direction perpendicular to sliding was significantl clearer for serious delamination.

From Figs.9–12 it is observed that with increasing sliding velocity and load the debris particles size increases.This results in plough formation with deep groove for higher sliding velocities and loads.The deep grooves were formed due to excess stress caused on the pin at higher loads.

As shown in Figs.9–12 for all prepared nanocomposites the scratches and grooves were formed due to wear,this results in increasing the surface roughness and decreases the specimen’s surface quality.The results of SEM images showed the compacted delamination and oxidation occurs at a load of 10 N.

Fig.6.Friction coefficien v/s sliding distance for various normal loads for sliding speed of 0.6 m/s.

The Mg-0.5 wt%of Boron nitride nanocomposite samples formed a compacted oxide layer on the surface for all applied loads and sliding velocities as shown in Fig.9.This oxide layer will protect the surface of the specimen from excessive wear and improves the wear resistance.

4.Conclusion

In this research work,the effect of Boron nitride(0,0.5,1.5 and 2.5 wt%of Boron Nitride)reinforced with magnesium composites was investigated by conducting the wear analysis experiment.From the obtained results the following interpretations were done:For all prepared nanocomposites under working load,the increase in sliding speed and load increases the wear rate and loss of volume.For Mg-0.5 wt%BN nanocomposite the raise of rate of wear and loss of volume were very less compared with other compositions of Mg/BN nanocomposites.The raise of sliding speed distance results in low friction coefficien(μ).Higher friction coefficien value occurred at a normal load of 7 N compared with other working loads.From morphologies of all worn surfaces three mechanisms of wear were acknowledged for all synthesized nanocomposites.The addition of 0.5 wt%of boron nitride as reinforcement to magnesium improves wear resistance of the material compared with other proportions of Mg/BN nanocomposites.

Fig.7.Friction coefficien v/s sliding distance for various normal loads for sliding speed of 0.9 m/s.

Fig.8.Friction coefficien v/s sliding distance for various normal loads for sliding speed of 1.2 m/s.

Fig.9.SEM images of pure Mg nanocomposites under sliding speeds of(A)0.6 m/s;(B)0.9 m/s;(C)1.2 m/s;and normal loads of 5 N(a,d&g);7 N(b,e&h);10 N(c,f&i).

Fig.10.SEM images of pure Mg-0.5 wt%BN nano-composites under sliding speeds of(A)0.6 m/s;(B)0.9 m/s;(C)1.2 m/s;and normal loads of 5 N(a,d&g);7 N(b,e&h);10 N(c,f&i).

Fig.11.SEM images of pure Mg-1.5 wt%BN nano-composites under sliding speeds of(A)0.6 m/s;(B)0.9 m/s;(C)1.2 m/s;and normal loads of 5 N(a,d&g);7 N(b,e&h);10 N(c,f&i).

Fig.12.Worn surface morphologies of Mg-2.5 wt%BN nanocomposites(A)at a velocity of 0.6 m/s;(B)at a velocity of 0.9 m/s;(C)at a velocity of 1.2 m/s;(a)5 N;(b)7 N;(c)10 N;(d)5 N;(e)7 N;(f)10 N;(g)5 N;(h)7 N;(i)10 N.