Investigation of Tensile behaviour, Seizure Conditions and Frictional Characteristics of Al-Zn-Cu-Mg Alloy based Composites

In a current scenario, light weighted composites are the prime need of manufacturers in aerospace, automotive, marine and other applications due to their excellent properties compared to traditional materials. The current study includes the fabrication of Al Alloy 7068-based composites reinforced with Si3N4 nanoparticles by 0.5 to 2.0 wt. % by maintaining a difference of 0.5 wt. % through stir casting approach advanced with ultrasonic treatment. The effect of reinforcement on the tensile behaviour of casted samples was analyzed in terms of ultimate tensile strength, and ductility. The wear performance of casted samples was analysed in terms of volumetric weight loss and COF under the effect of applied pressure, sliding velocity and sliding distance. On the investigation of tensile strength, 1.5 wt. % nanocomposite is the optimum one beyond which agglomeration starts which affects the strength. Wear loss increased with applied stress, sliding velocity and sliding distance. COF also increased with sliding distance but decreased with applied stress. There is a significant increment in seizure pressure noticed with reinforcement addition. Adhesion is the dominant wear mechanism observed during sliding, whereas mixed (adhesion and abrasion) type of wear is noticed for 2 wt. % at higher applied pressure and distance.


Introduction
Aluminum is the most useful material for aerospace and automotive industries due to its lightweight and ultrahigh strengthto-weight ratio, their properties were tremendously enhanced after the incorporation of alloying elements in the Aluminum.It has been noticed that the appropriate quantity and specific combination of Zn, Cu and Mg significantly improved the structural, as well as wear characteristics along with thermal and electrical conductivity.The structural and wear properties of Al alloy significantly improved with the addition of ceramic nano particulates such as SiC [1], B 4 C [2], Al 2 O 3 [3], TiC [4], TiB 2 [5], Si 3 N 4 [6] and Gr [7] etc. Ghassemieh et al. [8] stated in their research that the efficiency of automobiles is directly affected by their weight, a weight reduction of approx.10% improves fuel efficiency by 7%.So, the nanocomposites containing a matrix of aluminium and copper alloys are a suitable replacement for steel to improve fuel efficiency and very trending in manufacturing industries.
Mistry & Gohil et al. [9] investigated the mechanical properties and microstructure of as-cast and T-6 treated Si 3 N 4 reinforced AA7075 composites, where reinforcement was done in 4 to 12 wt.% in a step of 4 wt.%, they noticed a significant improvement in strength and hardness with the incorporation of reinforcement and performing heat treatment and these properties were maximized at 12 wt.%.Mohan &Pugazhenthi et al. [10] prepared composites of Si 3 N 4 and Al alloy in the proportions of 3 to 9 wt.% in the step of 3 wt.%, they observed an increase in wear resistance with their weight % as well as mechanical properties also improved.This improvement is credited to strengthening due to grain refinement, interfacial bonding between matrix and particles, thermal coefficient mismatch and other microstructural changes in composites.Modi et al. [11] noticed seizure pressure significantly minimizes with velocity, this reduction continues more effectively in composites compared to pure alloy.They also observed higher seizure pressure in the case of composites comparatively.The increase in wear loss was reported with applied pressure and sliding speed.Veeresh et al. [12] reported that the incorporation of ceramic particles in matrix alloy improves their resistance to seizure significantly, in addition to that, tensile properties and hardness also enhance the expense of ductility.Singh et al. [13] investigated the wear performance and friction behaviour of sillimanite-reinforced Al composites and reported that the seizure pressure of pure alloy and composites degraded with increasing sliding velocity, in addition to that, the rise in temperature near the matrix interface was almost uniform for alloy and composites with increasing normal pressure but in case of composites higher seizure temperature was investigated compared to alloy.Also, noticed that COF decreased with normal pressure and for composites its valves obtained lesser compared to alloy.Rao et al. [14] developed (Al-Zn-Mg-Cu)/SiC composites through liquid processing and noticed a significant rise in seizure pressure by 37.5% with the incorporation of 10 wt.% SiC compared to unreinforced alloy, and the wear mechanism is influenced and dedicated by the generation of the mechanically mixed layer (MML), oxide layer, subsurface cracking and deformation.In addition to that, they observed that, at the lower pressure the formation of MML is thinner and more stable, for intermediate pressure, thicker and stable MML with richer content of iron was formed, whereas these formed MML starts destroying and became unstable above the critical load and these layers completely destroyed at seizure pressure.Kumar et al. [15] stated that the COF significantly reduced with the addition of graphite in Al/ Si3N4 composites due to the additional self-lubricating properties of graphite.
The current study includes the development of Al Alloybased nanocomposite through the latest stir casting with ultrasonication technique.Metallographic characterization was done by using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and Field emission Scanning Electron Microscopy (FESEM).Tensile, wear and friction behaviour of matrix alloy and nanocomposites were examined along with worn surface characterization.

Materials
Al Alloy 7068 was taken as a continuous matter i.e., matrix materials procured from paraswamani metals Mumbai, and Si 3 N 4 nanoparticles were chosen as a reinforcement to strengthen matrix alloy procured from nano research elements, U.S.A. Fig. 1 (a, and b) represents the SEM of Al alloy 7068 and their EDS spectra, it can be noticed that the major alloying element is Zn (6.19 wt.%) followed by Mg (1.69 wt.%), and Cu (1.42 wt.%). Figure 1 (c, d and e) represents the FESEM, particles size frequency and EDS of Si 3 N 4 nanoparticles, from the obtained results, the size of Si 3 N 4 particles ranges from 40 to 95 nm, whereas size frequency maximized in a small range of 55 to 60 nm.The EDS shows the weight % of Si and N i.e., 33.09 and 66.91 wt.% respectively.

Fabrication of Nanocomposites
Nanocomposites were fabricated using an ultrasonically assisted agitation casting process, initially raw Al alloy was heated up to 780 °C until became in the molten state.Preheat treatment of reinforcement and dies was performed at 350 °C to minimize temperature gradient and avoid moisture content.Reinforcement was wrapped in Al foil and mixed into the molten alloy in two-three steps with the continuation of the stirring process at 450 rpm for 10-15 min.Ultrasonic treatment was given to the mixture with the help of a niobium probe for 5 min at the frequency of 20 Hz to avoid reinforcement clustering in order to get uniform distribution of reinforcing particles.Finally, the prepared composite mixture was poured into the preheated dies and left for solidification.By considering the above-mentioned process nanocomposites were cast consisting of 0.5 to 2.0 wt.% Si 3 N 4 separately.

Density
Figure 2 represents the Actual and theoretical densities of alloy and their composites samples.A gradual increment trend in density is noticed after adding Si 3 N 4 reinforcement when calculated theoretically by using the rule of mixture.The increment after reinforcement is credited to the higher density of Si 3 N 4 particles (3.52 g/cc) in comparison to Al alloy (2.85 g/cc) because the rule of mixture simply deals with the theoretical densities and their weight percentage.There is a decrement in density is noticed when observed practically through Archimedes' principle due to the casting defects such as voids or porosity mainly introduced during the stirring process.The void content increases with the addition of nano reinforcement, attributed to the lower wettability and higher surface area of Si 3 N 4 .As the surface area increases particles come closer due to surface energy and the entrapment of surrounding gas facilitates.The void content or porosity increases with the content of Si 3 N 4 (Fig. 2).

Tensile behaviour
Figure 3, depicts the stress vs percentage strain relationship after the tensile test performed on a computerized tensile machine (UTM).It is noticed from the results that the strength of pure alloy significantly enhanced after the insertion of Si 3 N 4 nanoparticles from 0.5 to 2.0 wt.%, interestingly the strength decreased for 2.0 wt.% in comparison to 1.5 wt.% but still higher than unreinforced alloy.The enhancement in tensile strength is attributed to different strengthening mechanisms named Hall Petch, Orowan, load bearing and thermal miss-match strengthening.According to Hall Petch, tensile strength will be increased after the incorporation of nanoparticles due to the grain refinement, Orowon strengthening applies when the particles size is less than 1 µm, and the transfer of load from matrix to reinforcement enhances the tensile strength of composite as per the load bearing strengthening.There is a mismatch in the thermal coefficient of Al alloy and Si 3 N 4 reinforcement increases the number of dislocations in composites and facilitates significant resistance to the plastic flow of material during loading conditions resulting strengthening of material.On the other hand, the modulus of the composite decreases at 2 wt.% mainly credited to improper mixing of Si 3 N 4 particles, as these particles are nanosized and have a large surface area as well as higher surface energy so they cluttered very soon after mixing, stirring and ultrasonication break clusters up to an extent but for a higher volume of reinforcement some clusters are left in the mixture facilitates air entrapment in between, resulting early failure of material during loading.Some of the studies also used nanoparticles as reinforcement and faced similar problems for higher weight % due to nonhomogeneous mixture or uneven distribution of Si 3 N 4 particles [16][17][18].Refereeing to Fig. 3, it is noticed that the ductility of the alloy decreases with the incorporation of nano Si 3 N 4 and this decrement is continuing up to 2 wt.%.The very first 22.34% reduction in % elongation is noticed for 0.5 wt.%, whereas this increases up to 54.52% for 2 wt.% compared with the unreinforced alloy.The reduction of ductility with the addition of reinforcement is credited to the fact that the brittleness of the composite enhanced with an increase in weight percentage due to the brittle nature of ceramic Si 3 N 4, the increased brittleness facilitates faster crack propagation and leads to ductility decrement.Reduction in % elongation (ductility) with ceramic reinforcement is a suitable match with prior reported research by [19,20].

Tribological Characterization
Dry sliding tribological behaviour and the seizure condition of alloy and reinforced composites were analyzed using Pin on Disc tribometer.Volumetric weight loss is taken as  output which is calculated through the weight loss method.Samples were properly cleaned using acetone before and after the wear test and weight is calculated using a digital precision balance having 0.01 gm least count.The effect of wear parameters such as applied stress (applied load per unit area), and sliding velocity on volumetric weight loss and coefficient of friction has been observed.

Effect of Applied Stress
Figure 4 represents the volumetric wear loss of reinforced composites and pure alloy; all samples were tested at a constant 3.14 m/s velocity for a 4500 running distance and various applied stress (from 0.19 to 2.67 MPa) considered for wear and seizure investigation.Significant improvement was observed in wear resistance with the addition of Si 3 N 4 in Al alloy, attributed to the increased dislocation density, strength, and hardness.As per the Archard law, wear loss is inversely proportional to the hardness of the material, harder material will be associated with lower wear loss.Increasing trends of volumetric wear loss with applied stress were also noticed from results mainly attributed to increased asperityto-asperity contact, change in surface chemistry, material transfer between contacting surfaces and material softening due to the temperature rise of softer surfaces.Referring to Fig. 4, the condition of seizure for pure alloy observed at lower stress compared to its composites, the incorporation of Si 3 N 4 makes it more resistant to plastic deformation during the wear test.An enhancement of 40.52% was noticed in seizure pressure for 1.5 wt.% compared to unreinforced alloy.During a wear test initially, asperities of the sample (softer surface) come under contact with the asperities of the harder counter disc, asperities at softer surface raptured and deformed by hard asperities and the rate of deformation increased with applied stress.Hard asperities ploughed the softer surface and make micro grooves, with an increase in applied pressure and distance more debris or small fragments were formed between contacting surfaces [21].In this stage, a mechanically mixed layer or oxide layer start forming due to an increase in temperature and followed by subsurface deformation, and wear debris starts aligned in the sliding direction.later, subsurface melting also started due to the generation of more amount of heat followed by material transfer from the sample surface.Further sliding results in higher plastic deformation, degradation of oxide layers, melting of subsurface over the worn surface and finally sticking of surface or seizure come in the picture [22].
In the case of nanocomposites, asperities of counter face raptured, deformed and fragmented the softer asperities as well as protruded the reinforcing particles from the matrix [23].These fragmentations further rubbed and formed wear debris that compressed and aligned on the composite surface followed by the generation of oxide or mechanically mixed layers.These layers are generally in a discontinuous manner and combination of equiaxed, oxidized and elongated wear debris of counterpart, nanoparticles and matrix [14].Further increase in applied stress and distance more heat generated between the surfaces and cracking of oxide layers and wear fragments happens along the compacted interface.The surface also started melting below the oxide layers due to the heat transfer and material softening by the action of prolonged friction and higher pressure, leading to an upward flow of melted material towards the worn surface through microcracks and seizure of condition [11], Composites are observed seized at higher pressure credited to the Si 3 N 4 reinforcement which restricts the flow of partially melted sub-layers.On the other hand, seizure pressure decreases for 2 wt.% compared to 1.5 wt.% mainly credited to the uneven distribution and agglomeration of reinforcing particles.At the higher applied stress and distance the protrusion of Si 3 N 4 particles increases also the reinforcement clusters brakes and deboned from the matrix and come in between the contacting surface during sliding action leading to a higher rate of three-body abrasion, heat generation and acceleration of seizure condition [24].

Effect of Sliding Velocity
Figure 5(A) and (B) represent the volumetric loss of pure alloy and its composites at 0.19 MPa and 0.57 MPa applied stress respectively, there were five different velocities from 1.047 m/s to 3.67 m/s considered for the investigation of their effect on wear loss, the running distance kept constant during the wear test as 3000 m.From Fig. 5(A), a noticeable rise in the volumetric loss was obtained with sliding velocity, for example at 0.19 MPa, wear loss of pure alloy was noted as 3.54 mm 3 at 1.47 m/s and it tremendously increases up to 5.9 mm 3 when velocity increased to 3.67 m/s.Whereas, the incorporation of Si 3 N 4 reinforcement significantly enhances the wear resistance and the volumetric loss minimizes at 2 wt.% at lower pressure (0.19 MPa), for example, the wear for 2 wt.% noticed as 1.9 mm 3 and it increased up to 4.9 mm 3 .The increment in wear loss with sliding distance is credited to material softening due to prolong friction and lowering of interfacial bonding between reinforcement and matrix.The formation of oxide layers also affects the wear loss after 2.094 m/s but the fragmentation initiated of these layers when distance further increased due to the temperature rise by the effect of friction [25].Figure 5(B), depicts the relation between sliding velocity and applied pressure at a higher load of 0.57 MPa, similar trends of volumetric wear loss were observed for alloy and its composites (up to 1.5 wt.%) with a higher volume.The contact pressure between the asperities of both surfaces increases when the load increases, facilitating intense rubbing action and higher plastic deformation due to the temperature increment, material softening and higher adhesion [26].The increment in volumetric wear loss with velocity was also noticed at higher load, for example, at 1.047 m/s wear loss of unreinforced alloy was noted as 5.6 mm 3 and it increases up to 8.6 mm 3 at 3.67 m/s sliding velocity.Whereas, for 2 wt.% wear loss suddenly increased at higher velocity attributed to the uneven distribution and three-body abrasion.Since the reinforcing particles are agglomerated in this composition, interfacial bonding also affects due to the presence of void contents in between, these clusters are breaks when normal load increases and these hard reinforcing particles set and rubbed in between the contacting surfaces leading to severe plastic deformation, ploughing and higher wear loss.

Coefficient of Friction (COF)
The properties of reinforcement and matrix, reinforcement weight percentage, wear test parameters such as applied pressure, sliding velocity, rubbing distance, and wear test condition severely influenced the frictional co-efficient.The values of COF have been obtained from the Ducom software at different levels during wear tests and represented their relationship with affecting parameters.

Effect of Si3N4 on COF
The relationship between the frictional coefficient and sliding distance at 0.19 MPa and 0.57 MPa is shown in Fig. 6 (A) and (B) respectively, at 1.57 m/s sliding velocity for a running distance of 3000 m.From the observations, it is noticed that COF maximized for unreinforced alloy and their values come down after adding Si 3 N 4 reinforcement from 0.5 to 2.0 wt.%.The reduction of the frictional coefficient is credited to the enhanced harness, load bearing and anti-wear properties of composites.The incorporation of Si 3 N 4 introduced the lubricating properties in composites, Si 3 N 4 became reactive with atmospheric humidity under high temperature, resulting formed SiO 2 and other oxide layers between the surfaces that acts like a solid lubricant, resulting in minimizes the direct contact of the sample to the counter surface and reducing the COF [9].For example, the maximum value of COF as 0.515 is noted for unreinforced alloy and it minimized up to 0.47 when Si 3 N 4 was added in alloy by 2 wt.%, after a run of 500 m at 0.19 MPa applied pressure.From the figures it can be noticed that the trends of COF in a zigzag manner, are credited to strain hardening, generation of mechanically mixed layers or oxide layers, breaking of junctions, and abrasion of reinforcing particles (in the case of composites).Figure 6(B), represents the variation of the frictional coefficient of pure and reinforced composites considering different running distances varied from 500 to 3000 m at constant 0.57 MPa applied pressure.Similar trends were obtained for pure alloy and composites as for 0.19 MPa but with a lower value of COF.This reduction in COF values is credited to mechanically mixed layers formation during sliding due to the temperature rise by the effect of intense friction between surfaces under high normal pressure.These layers minimize the direct contact of contacting surfaces but these layers also vanished during continuous sliding or friction after a distance.for example, COF was noted for unreinforced alloy as 0.46 and it decreases up to 0.43 after the addition of 2 wt.% Si 3 N 4 at 0.57 MPa normal pressure after a run of 500 m, whereas this variation is noted from 0.479 to 0.452 after a run of 3000 m.For 2 wt.%, higher fluctuation in COF values was noticed, credited to uneven dispersion and clustering of Si 3 N 4 particles, presence of porosity facilitates adhesion and abrasion type of wear due to the formation of wear debris under high pressure sliding and breaking of clusters [27].

Effect of Sliding Distance on COF
The relationship between COF and sliding/running distances (500 to 3000 m) of pure and reinforced composites at 0.19 MPa and 0.57 MPa shown in Fig. 6(A), and (B) respectively, during this observation sliding velocity, kept constant at 1.57 m/s.It is noticed that the COF of pure alloy and composites gradually increased with running distance, and maximum COF is obtained for the pure alloy.The incorporation of Si 3 N 4 in alloy minimizes its COF significantly.The rise in COF with distance is attributed to the breaking of asperities junctions and changes in surface chemistry, material transfer between contacting surfaces and increased surface roughness.The material removed from the sample starts due to the sliding of surfaces, as the distance increases temperature starts to rise at the contact area followed by weakening of intermetallic bonds and higher material removal.Furthermore, the amount of wear debris increases and also the formation of new asperities junctions under high pressure and temperature, resulting in higher frictional force required to break these junctions and material removal [9,28,29].In the case of composites, lower values of COF were noted compared to pure alloy and these values were further reduced when the weight % of Si 3 N 4 increased, this type of behaviour obtained credited to the reactive properties of Si 3 N 4 with atmospheric humidity, rich oxide layers of SiO 2 formed at the contact area which minimize the direct contact, resulting in decreases the coefficient of friction.

Effect of Applied Stress on COF
The variation of COF of pure alloy and their composites is represented in Fig. 6(A), and (B) at the applied stress of 0.19 and 0.57 MPa respectively.From the figures, it is noticed that the values of COF decreased when the load increases from 0.19 to 0.57 MPa.The higher frictional coefficient at a lower load is credited to the fact that the formation of the oxide layer/ mechanically mixed layer totally depends on the temperature at the contact region and at this load lesser amount of oxide layer is generated compared to a higher load, leading to adhesion and abrasive type of wear and higher frictional coefficient.On the contrary part, more amount of mechanically mixed layers are generated at the contact region at higher normal pressure which reduces the direct contact of surfaces, resulting in frictional coefficient reductions, [30].In addition to that, with the increase in contact pressure, more amount of Fig. 6 COF plotted against sliding distance of pure alloy and composites (A) at 0. 19  wear debris is formed and trapped between the contacting surfaces, which facilitates the generation of mechanically mixed layers or oxide layers which minimizes the contact between the surfaces, leading to a decrease in frictional coefficient [15,31].On the other hand, the frictional coefficient is inversely proportional to normal pressure, so as the normal load/pressure increases the counter surface asperities more profoundly penetrate the softer surface which facilitates a higher degree of plastic deformation, temperature rise and material transfer leading towards softening of the sample surface and increased slipping behaviour, resulting in COF decrement [14].

Worn Surface Analysis
SEM images of worn-out surfaces of pure alloy and reinforced composites are represented in Fig. 7 to investigate the various wear mechanisms and surface morphology.Figure 7(a) represents the worn surface of pure alloy which was slid against its counterpart at 1.0 MPa applied for 4500 m. it shows adhesion patched at the major portion along with a high amount of plastic deformation, and delamination of the surface.At this high normal pressure, hard asperities of the counter surface deformed and raptured the softer asperities, and intermediate temperature also increases leading to softening of material and plastic deformation [12].Figure 7(b), shows the worn surface of the pure alloy at a higher pressure of 1.57 MPa, severe adhesive, the sticking and ploughed region can be identified, which witnesses a high amount of volumetric wear loss.As the pressure increases, a large amount of wear debris (containing matrix as well as reinforcement) was collected at the contacting region which compressed under higher pressure along the sliding direction leading to partial melting of the subsurface region.This melted material undergoes an upthrust towards the sliding surface through microvoids and partial cracks presented near the surface, leading to the sticking of both surfaces resulting in seizure conditions [13]. Figure 7(c), and (d) represent the worn surface of 1.5 wt.% composites at 1.0 MPa and 2.67 MPa respectively and the sample was slid for 4500 m at constant 3.14 m/s sliding velocity.Referring to Fig. 7(a), at similar wear conditions, a smoother worn surface was obtained compared to pure alloy witnessing the increased wear resistance with the incorporation of Si 3 N 4 particles.However, at a higher normal pressure surface material is plastically deformed intensely, hence the adhesion patches, thicker grooves and delamination observed on the surface.The surface damage Fig. 7 (continued) became severe when the load increases to 2.67 MPa (as shown in Fig. 7(b)), and severe and bigger sticking/adhesion region was observed with thicker grooves and delamination on the worn surface.at this load seizure condition of the sample was investigated credited to the melting of the subsurface region at the application of high normal pressure and prolonged friction.at higher pressure, wear debris of both surfaces collected in between the contacting surface, was compressed and deformed along the sliding direction [12].In the continuation of sliding, formation and degradation of the mechanically mixed layer repeatedly, parallelly the interfacial temperature rises severely by the action of prolonged friction, leading to melting of the subsurface region and this melted material starts moving towards contacting region through the cracks (generated during sliding), and microvoids presented in the composite, resulting in sticking of the sample with the counterpart and stop sliding motion [32].In addition to that the seizure pressure obtained for 1.5 wt.% is higher compared to pure alloy and other composites, credited to increased wear resistance, strength at elevated temperatures and load-bearing properties with the incorporation of Si 3 N 4 particles.The embedded reinforcement acts as a secondary phase in the primary matrix and increased surface hardness and increased the possibility of tribo-chemical layer generation which prevents the direct exposure of the sample to the counterpart and makes the material plastically constrained [33].
Whereas for 2 wt.%, obtained seizure pressure is lower than 1.5 wt.% attributed to the uneven distribution and formed cluster of Si 3 N 4 particles.deeper grooves and pits were observed in the worn surface of 2 wt.% (as shown in Fig. 7(f)), which witnessed higher volumetric loss, material spallation, agglomeration and casting defects.This agglomerated reinforcement is responsible for increased wear resistance at the lower and medium pressure, velocity and rubbing distance due to the formation of mechanically mixed layers and increased hardness which improves the wear resistance [33,34].Whereas, at higher normal pressure the tendency of material spallation and three-body abrasion increases, as particles are deboned from clusters and sets between the contacting surface [23].Hence, for 2 wt.%, the volumetric loss increases and seizure pressure decreases due to the temperature rise at the early stage by the effect of severe abrasion compared to 1.5 wt.% composites.

Conclusions
Nanocomposite compositions of Al alloy 7068 and Si 3 N 4 (0.5 to 2.0 wt.%) were successfully fabricated using a stir casting technique advanced with ultrasonic treatment.Following conclusions are withdrawn from this study such as: 1. Tensile strength was significantly enhanced after the addition of Si 3 N 4 for all compositions except 2.0 wt.% due to the agglomeration of particles.A reduction in ductility was noticed after the incorporation of Si 3 N 4 nanoparticles due to the increased brittleness when Si 3 N 4 particles were incorporated.2. Wear resistance was significantly enhanced with Si 3 N 4 addition for all compositions, irrespective of wear parameters, but for 2 wt.% it reduces at higher pressure due to the increase in abrasion and excessive heating at the contacting interface.3. Volumetric wear loss of pure alloy and nanocomposites increased with applied stress, sliding distance and sliding velocity.The coefficient of friction is enhanced with sliding distance whereas it reduces with applied stress.4. The seizure pressure significantly improved with the addition of nano Si 3 N 4 in the matrix, and it maximized at 1.5 wt.%.It reduced for 2 wt.% compared to 1.5 wt.% due to the porosity, uneven dispersion and clustering of particles.5.At higher values of applied stress, distance and velocity, abrasive wear is also present along with the adhesive wear.

Fig. 4
Fig.4 Variation of volumetric loss with applied stress/pressure of pure alloy and composites