The present paper aims at evaluation of the sliding wear behaviour of SiC particles reinforced copper (phosphor-bronze) alloy composite materials studied as a function of sliding speed and applied loads under unlubricated conditions. The content of SiC particles in the composites was varied from 1-5% in steps of 2% by weight. A pin-on-disc wear testing machine was used to evaluate the wear rate, in which EN24 steel disc was used as the counterface. Loads of 20-160 N in steps of 20 N and speeds of 1.25, 1.56, 1.87 m/s were employed. The results indicated that the wear rate of both the composites and the matrix alloy increased with increase in load

and sliding speed. However, the composites exhibited lower wear rate than the alloys.It was found that above a critical applied load, there exists a transition from mild to severe wear both in the unreinforced alloy and in the composites. But the transition loads for the composites were much higher than that of the unreinforced alloy. The

transition loads increase with the increase in weight % of SiC particles, but decrease with the increase in sliding speeds. SEM analysis of the wear surfaces as well as the

sub-surfaces are used to explain the observations made

Girish, B M

Kamath, Rathnakar

Satish, B M

Sharma, S C

English

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72AN EVALUATION OF THE SLIDING WEAR BEHAVIOUR OFSiC PARTICLES REINFORCED COPPER ALLOYCOMPOSITE MATERIALSS.C.SHARMA, B.M.GIRISH, RATHNAKAR KAMATH AND B.M.SATISHResearch and Development, Department of Mechanical Engineering,R. VCollege of Engineering, Bangalore - 560059, Karnataka , INDIA.AbstractThe present paper aims at evaluation of the sliding wear behaviour of SiCparticles reinforced copper (phosphor-bronze) alloy composite materials studied as afunction of sliding speed and applied loads under unlubricated conditions. The con-tent of SiC particles in the composites was varied from 1-5% in steps of 2% byweight. A pin-on-disc wear testing machine was used to evaluate the wear rate, inwhich EN24 steel disc was used as the counterface. Loads of 20-160 N in steps of 20N and speeds of 1.25, 1.56, 1.87 m/s were employed. The results indicated that thewear rate of both the composites and the matrix alloy increased with increase in loadand sliding speed. However, the composites exhibited lower wear rate than the alloys.It was found that above a critical applied load, there exists a transition from mild tosevere wear both in the unreinforced alloy and in the composites. But the transitionloads for the composites were much higher than that of the unreinforced alloy. Thetransition loads increase with the increase in weight % of SiC particles, but decreasewith the increase in sliding speeds. SEM analysis of the wear surfaces as well as thesub-surfaces are used to explain the observations made.1.0 IntroductionMetal matrix composites (MMCs) in the recent years are being increasinglyspecified as new wear resistant materials. The tribological properties of MMCs is ofinterest in several applications like bearing sleeves, piston and cylinder liners, aircraftbrakes, etc.A wide variety of copper (Cu) based alloys are used directly as bearing materi-als and also as backing materials for babbit liners. Their use in the former categorycovers a range of physical properties in terms of hardness and strength, that is widerthan that of any other group of bearing alloys. Cu alloys are traditionally used forsliding applications particularly for bearings and electrical contacts. These alloys whichare also used for various wear resistant applications like bearings are also being con-SLIDING WEAR BEHAVIOUR OF SiC PARTICLES 73sidered for bearing cages in systems where extended life is necessary.The phosphor-bronze alloys are highly potential Cu alloys which exhibit goodwear resistance, good machinability, cold workability, fatigue resistance, corrosionresistance, etc. and can operate in more severe conditions and up to temperaturesaround 450°C. High strength and wear-resistance make them suitable for wear andabrasion resistance applications which involve impact or shock loading. They are widelyused as engineering tools and dies, bushings, guide plates, etc. Incorporation of ce-ramic reinforcement will certainly enhance the wear and friction properties of thesewear resistant alloys.Though very less, some extent of research has been carried out on Cu alloysbased composite materials. Chen et al., [ 1 ] who have worked on tribological behaviourof Cu based composites using a pin-on-disc wear tester, have evaluated the coefficientof friction, wear rate and bulk temperature of the composites in terms of effect ofnormal pressure and sliding speed. They have reported that the wear rate increasedwith increasing normal pressure and decreased with increasing sliding speed in thestudied range of normal pressure and sliding speed. Liu et al., [2] also have worked ontribological behaviour of Cu-20% Nb in-situ composites. It was found that Nb fila-ments underwent re-orientation, shearing and refinement. The SEM analysis used tostudy the micro-mechanisms of wear showed that surface deformation, filament re-orientation and refinement, and oxidation seem to play important roles iii the frictionand wear processes.Even though Saka and Karalekas [3] have reported that the Cu based MMCsexhibit wear behaviour which is similar to that of Al based MMCs, the tribologicalproperties of Cu alloys based MMCs have not yet been studied extensively. In addi-tion, there is still an incomplete understanding of the wear mechanism of Cu basedMMCs, due to which there is a great need for research on these highly potential alloybased MMCs for wear and frictional resistance applications. Moreover, Cu matrixcomposites have the potential for use as wear-resistance and heat-resistant materials,electronic materials, brush and torch nozzle materials. Hence studies on Cu alloy basedcomposites with regard to their wear behaviour under dry sliding condition are ofsignificance.2. Experimental ProcedureSiC particles of size 50-100 p.m were used as the reinforcement. Phosphor-bronze alloy with the chemical composition in weight %: Sn-18 to 20%, Pb-0.25%,Fe-0.25% and P-1.0%, Cu-remainder, was used as the base matrix alloy. Liquid met-allurgy technique was used to fabricate the composites, in which the preheated SiCparticles were introduced into the vortex created in the molten alloy through an alumina74coated steel impeller rotated at around 500 rpm. The coating of alumina is necessary inorder to prevent contamination of the melt. The melt was thoroughly stirred, degassedsubsequently by passing nitrogen at the rate of 2-3 litres/minute and poured intopreheated split-type permanent moulds.2.1 Wear test equipmentThe dry tests of the wear specimens were conducted in accordance with ASTMG99 standards using a pin-on-disc sliding wear testing machine. EN24 steel disc ofhardness. HRc 57 and diameter of 200 nun was used as the counterface on which thetest specimens slide. Arrangements were made to hold a specimen and also for applica-tion of the load on the specimen. The test specimen was clamped in a vertical sampleholder and held against the rotating steel disc. More detailed description of the appara-tus is available. elsewhere [4]. In the present investigation, loads of 20-160 N in stepsof 20 N were used. The rotational speeds employed were 200, 250 and 300 rpm, whichat an average distance of 80 mm from the centre gave corresponding linear speeds of1.25, 1.56, 1.87 m/s respectively.2.2 Testing of SpecimensThe specimens of the material under investigation were 6mm in diameter and15mm in length. The disc was cleaned with acetone to remove any possible traces ofgrease and other surface contaminants. The specimens which were cleaned-with etha-nol were weighed before and after the tests using a balance accurate to ± 0.OOIg. Thewear results were computed from weight loss measurements. The duration of each testwas exactly 60 minutes. The wear volume was calculated from the ratio of weight lossto density and wear rate was calculated using sliding distance and wear volume. Usageof such relations to calculate the wear parameters is not uncommon and has been usedby Rohatgi et al [5]. The data for the wear tests was taken from the average of threemeasurements. The standard deviation was about 5 %. The surfaces of the worn speci-mens were cleaned thoroughly to remove the loose wear debris and then observedusing a scanning electron microscope.3. RESULTS3.1 Effect of load and speed on the wear rateThe results were averaged to obtain the final wear rate which are presentedgraphically in Figures 1-3. The wear rate of the unreinforced alloy is also plotted inorder to enable comparison with the composites. It is found from the graphs that thewear rate of both the unreinforced alloy and the composite specimens increased withthe applied load. The wear rate of each phosphor-bronze alloy/SiC reinforced compos-SLIDING WEAR BEHAVIOUR OF SiC PARTICLES 75ite specimens reduced with the increase in SiC content.It follows from the graphs shown in Fig 1-3 that the wear rate of the compositesis lower than that of the alloy . The wear rate also decreases with the increase in SiCcontent . It is evident from the graphs that there exists a certain applied load, i.e. atransition phenomena at which there is a sudden increase in the wear rate of bothreinforced as well as unreinforced materials . However, the transition loads for thecomposites were much higher than that observed for the unreinforced alloy and alsothe transition load increased with the increase in SiC particle content.The unreinforced matrix alloy showed a transition from mild to severe wear at aload of 60 N, while the 1 and 3% fibre reinforced composites showed a transition at140 N, and the same was observed at a load of 160 N in case of 5% reinforced com-posite . This observation indicates that the presence of SiC reinforcement delays thetransition from mild to severe wear , and increases the transition load of the 5% rein-forced composite by almost 2 . 5 times with respect to the unreinforced alloy.The results show that at lower loads , comparatively low wear rates exist, indi-cating the regime of mild wear . In this regime of mild wear, the composites demon-strate more significant wear resistance than the alloy counterpart. At higher loads, thematerials exhibit rapid increase in wear rate . At loads greater than the transition load,severe wear occurs leading to seizure of the materials . The severe wear manifesteditself by a rapid rate of material removal in the form of generation of coarse metallicdebris , and also by massive surface deformation and material transfer to the counterface.It follows from the graphs shown in the Fig. 1-3 , that the unreinforced -alloyshows atransition at 60 N when tested at 200 and 250 rpm , while the same is observedat 40N in case of 300 rpm test . Similarly, the composite with 1 and 3% SiC shows atransition at 140N when tested at 200 and 250 rpm , while the same is observed at120N in case of 300 rpm test . The composite with 5% SiC shows a transition at 160Nwhen tested at 200 and 250 rpm , while the same is observed at 140N in case of 300rpm test . The above observation clearly indicated that the sliding speeds employedhave a significant effect on the wear rate transition of the materials . The transition inwear rate decreases with the increase in speed in all the materials. The results obtainedare on par with the one obtained by Lee et al ., [6] who have reported that the wearmechanisms are strongly dependent upon the sliding speeds.The mild wear of the alloy is oxidation dominated wear at low sliding speedsand loads . In the case of composites , due to the existence of SiC particles , the oxidefilm of the metal is not continuous and tenacious , it is removed by sliding frictionforces resulting in oxidation assisted mild abrasion wear . Hence , it can be consideredthat the dominating wear mechanism is the removal and reproduction of the oxide film.76This kind of wear is maiptained until higher loads are employed. at which the wearmechanism transforms from mild wear to severe wear. The morphologies of wearsurfaces of 5% SiC reinforced composites are shown in Fig.4. The wear tracks inFig.4 (a) show typical abrasive wear for the composites tested at low loads of 40N.Hence it can be concluded that the dominating wear mechanism is abrasive wear atlow loads.At higher loads at which transition occurs from mild to severe wear, the wearrate quickly increases sharply. Due to the high loads employed, the friction and wearincreased obviously. In this condition. the removal and formation of oxide films arefaster than that of mild oxidation. thereby resulting in relatively higher wear rates.Severe plastic deformation in the subsurface layers may be caused at high loads lead-ing to subsurface cracking. In view of the subsurface crack seen in Fig.. it can beconcluded that it is the adhesion and delamination wear that is dominant at high loads.Seizure may occur with the further increase in the applied load.3.2 Effect of SiC particles on wear rateSiC which is a ceramic. due to its directional covalent bonds and relativelylower concentrations and mobility of crystalline defects., is not prone to seizure underrubbing conditions. This property along with high strength, wear resistance. corrosionresistance, etc., have shown that this material is a possible candidate for use in wearresistant applications.It is also to be observed that the transition load increases with the increase inSiC content and also the wear rate of the composite was lower than that of the basealloy without SIC reinforcement. This is obviously due to the release of SiC by thecomposite specimens onto the mating surface during sliding which provide resistanceto wear. Alpas and Zhang [71 who have studied the sliding wear behaviour of Al-Sialloys reinforced with SiC particles have indicated that under the conditions whereparticles support the load. SIC reinforcement leads to significant improvement in thewear resistance. Ames and Alpas [8] have reported that the addition of SiC particlesdelay the transition to the severe wear and increase the transition load over 2.5 timeswith respect to the unreinforced A356 alloy.During the sliding wear, the SIC particles get sheared and then the shearedlayers adhere to the metal surface with the major axis parallel to the sliding directionforming a thin film between the mating surfaces. Moreover, the hard SIC h,1-, verylimited ductility and has the ability to withstand stress without plastic deformation orfracture under low load conditions. It is well established by investigators 191 that thewear rate and surface damage can be minimised if the plastic deformation of the mate-rial at the contact interface is prevented. The hard SiC withstands high stresses with-SLIDING WEAR BEHAVIOUR OF SiC PARTICLES 77out plastic deformation or fracture and is very effective in reducing the wear rate in thecase of composites. Hence, it can be concluded that the ability of the sheared reinforce-ment lavers to adhere to the sliding surface decides the effectiveness of the SiC parti-cles in reducing the wear rate of the composite materials.3.3 SEM analysisFor brevity and convenience , the SEM micrographs of only 5% SiC compositeswear tested at speed of 300 rpm (1.87 mis) have been presented . However. the expla-nation holds good even for the composites with I and 3% reinforcement . The SEMmicrographs of typical worn surfaces of the 5% SiC reinforced composites are pre-sented in Fig.4 which show the wear track morphology of the specimens tested atvarious loads.It can be seen that a lot of parallel, continuous and deep ploughing grooves existon the wear surface of the composite and abrasion phenomenon is observed at lowloads. The parallel grooves suggest abrasive wear as characterised by the penetrationof the hard SIC particles into a softer surface, which is an important contributor to thewear behaviour of ZA-27/SiC composites. The worn surfaces in some places revealpatches from where the material was removed from the surface of the material duringthe course of wear and smeared on to the sliding surface.This abrasive wear is quite possible at low loads where the reinforcing SiCparticles remain intact without fracture during wear and thus act as load-bearing ele-ments. Also the macroscopic observation reveal the presence of the microgrooves onthe surface of the steel disc which provides sufficient evidence for the abrasive effectof the carbide particles. Hence, it can be stated that a prerequisite for this type of wearis that the SiC particles should remain unfractured during wear so that they can sup-port the applied load and act as effective abrasive elements [ 10]. This observation wasmade at low loads where the SiC particles could resist the fracture. But at higher loads.the induced stresses exceed the fracture strength of the carbide particles causing parti-cle fracture. When the particles fracture, they lose their effectiveness as load-bearingcomponents and the damage-accumulation processes including subsurface void nu-cleation and crack propagation may become operative. Further, the shear strains in-duced in the process get transmitted to the matrix alloy and the wear mechanism pro-ceeds by a subsurface crack propagation induced delamination wear.Clear evidence of subsurface cracks in the composite material, the rate of p ropa-gation of which controls the wear rate , can be seen from the SEM micrograph shownin Fig . 5 . Das et al ., ( l l ] are of the opinion that in delamination wear. the subsurfacecracks which may either exist earlier or get nucleated due to the stresses , propagateduring the course of wear and when such subsurface cracks join the wear surface,78delamination is the dominant wear mechanism. The surface material gets removed andthe cracks get nearer to the surface and the shear strain is increased thus causing theremoval of the surface layers by delamination.4. ConclusionUnder unlubricated sliding wear, with the increase in applied load and speed,the wear behaviour of the Cu/SiC composites transforms from mild wear to severewear and then to seizure. The mild wear is characterised by oxidation assisted abra-sion, while the severe wear is dominated by subsurface cracks assisted delamination.The transition in the wear rate which occurs above the critical load, where a suddenincrease in the wear rate occurs, restricts the application of the alloys in some impor-tant tribocomponents such as cylinder liners, high-speed bearings or high-load bear-ings.. It follows from the results obtained in the present study, that the presence of hardSiC particles hinder the mild-to-severe wear transition. The wear behaviour furtherimproves with the increase in SiC content, thereby improving the wear resistance ofthe alloys even at higher loads.References(1) Chen.Z, Liu.P, Verhoeven.J.D, and Gibson.E.D, Tribological behaviour ofCu-20% Nb and Cu-15% Cr in-situ composites under dry sliding conditions,Wear, 199 (1996) 74-81.(2) Liu.P, Bahadur.S, and Verhoeven.J.D, Further investigation on the tribologicalbehaviour of Cu-20% Nb in-situ composites, Wear, 162 (1993) 211-219.(3) Saka.N and Karalekas. D. P, Friction and wear of particle reinforced metal ma-trix composites, in Wear of Materials, ASME, New York, 1995, pp. 784-793.(4) Poonawala N.S., Chakrabarti A.K. and Chattopadhyay.A.B., Wear characteristics of nitrogenated chromium cast irons, Wear, 162 (1993) 580.(5) Rohatgi.P.K, Ray.S and Lin.Y, Tribological Properties of metal matrix graph-ite particle composites, Int.Mater.Rev., 37 (3) (1992) 129.(6) Lee.C.S, Kim.Y.H, Han'.K.S and Lim.T, Wear behaviour of aluminium matrixcomposite materials, J.Mater. Sci., 27 (1992) 793-800.(7) Alpas.A.T and Zhang.J, Effect of SIC particulate reinforcement on the drysliding wear of aluminium-silicon alloy (A356), Wear, 155 (1992) 83.SLIDING WEAR BEHAVIOUR OF SiC PARTICLES 79(8) Ames.W and Alpas.A.T, Wear mechanism in hybrid composites of graphite-20pct SiC in A356 Aluminium alloy (A1-7pct Si-0.3 pct Mg), Metall. Mater.Trans., 26 A (1995) 85-98.(9) Komvopoulos.K, Saka.N and Suh.N.P. Role of hard layers in lubricated anddry sliding, J. Tribology, 109 (2) (1987) p 223.(10) ZumGahr.K.H, Microstructure and wear of materials, Elsevier,Amsterdam, 1987.(11) S.Das, S.V.Prasad and T.R.Ramachandran, Microstructure and wear of(Al-Si alloy) - graphite composites, Wear, 133 (1989) 187.87 +0% SiCt1% SICt 3% Sic5% Sic020 40 60 80 100 120 140 160Loads (N)Fig. 1 - Graph of wear rate vls loads at a speed of 1.25 m/s.8080 100 120 140 ' S'JLoads (N)Fig. 2 - Graph of wear rate vs loads at a speed of 1 .56 m/s.Fig. 3 - Graph of wear rate vs loads at a speed of 1.87 m/s.SLIDING WEAR BEHAVIOUR OF SiC PARTICLES 81Fig.-4. SEM showing wear track morphology of 5% SiC composite at 1.87 m/s andloads of (a) 40 N, and (b) 100 N.82Fig.-5. SEM showing the subsurface crack in 5% SiC composite.-01

An Evaluation of the sliding Wear Behaviour of SiC Particles Reinforced Copper Alloy Composite Materials

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An Evaluation of the sliding Wear Behaviour of SiC Particles Reinforced Copper Alloy Composite Materials

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