Materials and Design 31 (2010) 1526–1532

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Materials and Design

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Technical Report

Dry sliding wear behaviour of AA 6351-ZrB2 in situ composite at room temperature

G. Naveen Kumar a, R. Narayanasamy a,*, S. Natarajan b, S.P. Kumaresh Babu b,K. Sivaprasad b, S. Sivasankaran a

a Department of Production Engineering, National Institute of Technology, Thiruchirappalli 620 015, Indiab Department of Metallurgical and Materials Engineering, National Institute of Technology, Thiruchirappalli 620 015, India

a r t i c l e i n f o

Article history:Received 19 May 2009Accepted 9 September 2009Available online 12 September 2009

0261-3069/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.matdes.2009.09.017

* Corresponding author. Tel.: +91 431 250 1801; faE-mail address: narayan@nitt.edu (R. Narayanasam

a b s t r a c t

In the present work, AA 6351-xZrB2 [x = 0, 3, 6 and 9 weight percentage (wt.%)] in situ composites havebeen prepared by the reaction of mixture of K2ZrF6 and KBF4 with molten aluminium alloy at a reactiontemperature of 850 �C. The in situ prepared composites were characterized by using scanning electronmicroscope (SEM), X-ray diffractometer (XRD), and microhardness analysis. The sliding wear propertiesof the prepared composite at room temperature were estimated by a pin-on-disc wear testing equipmentusing the composite material; the pins were machined according to standard sizes, and the tests wereconducted as per the standards recommended by the ASTM G99-95a designation of different weighingpercentage at room temperature. The wear characteristics of the composite in the as-cast, the solution-ized and the solutionized-aged conditions were studied by conducting sliding wear test at the load of9.81 N. The results indicated that the wear rate was decreased with an increase in the weight percentageof ZrB2 and the wear resistance was increased with an increase in the fraction of ZrB2 particulates in com-posite before and after heat treatment.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Aluminium-based particulate reinforced metal matrix compos-ites have emerged as an important class of high performance mate-rials for use in aerospace, automobile, chemical and transportationindustries because of their improved strength, high elastic modu-lus and increased wear resistance over conventional base alloys[1]. Aluminium metal matrix composite (MMC) is potentially animportant material for tribological applications because of its lowdensity and high thermal conductivity. However, MMC by itselfexhibits poor tribological properties. Therefore, the study of the tri-bological behaviour of aluminium based composites is becomingincreasingly important [2].

Most of the Al-based ex situ composites have low reinforcementmatrix bonding strength [3]. To overcome this problem, in situ syn-thesis techniques were employed. In situ techniques lead to betteradhesion at the interface and hence better mechanical properties[4]. Eventhough the in situ composites have significant advantages,some synthesis routes may lead to composites with inhomoge-neous microstructure with various unstable and/or undesirablephases. These undesirable phases might drastically reduce themechanical properties [5–11].

Zhang et al. [12] prepared in situ composites reinforced by Al3Zrand zirconium boride (ZrB2) particles made via magneto chemistry

ll rights reserved.

x: +91 431 250 0133.y).

in situ reaction of K2ZrF6–KBF4 chemicals in the aluminium melt at1173 K and the dry sliding wear properties of the composites wereinvestigated. The results of various composites showed that wearresistance increased with an increase in volume fraction of rein-forcements [13–20].

ZrB2 has the unique combination of mechanical and physicalproperties, including high melting points (>3000 �C), high thermaland electrical conductivities, chemical inertness against moltenmetals, and great thermal shock resistance [21]. ZrB2 ceramiccomposites are difficult to deform due to its inherent brittleproperty [22,23]. The research of ZrB2 has become one of the mosthotspots in composites. An in situ composite with ZrB2 as rein-forcing particle and aluminium as base posed to exhibit high per-formance characteristics at high temperatures [24,25]. Not muchwork has been done on the study of wear behaviour and hightemperature performance characteristics. The AA 6351 wasselected as the base line material as it possesses good formability,weldability, machinability, and corrosion resistance, with mediumstrength compared to the other grades of aluminium alloys. Itsnominal chemical composition is shown in Table 1. Also ZrB2 isone of the most promising candidates for ultra-high temperatureapplications such as leading edges and thermal protection systemsfor reusable atmosphere re-entry vehicles, and rocket propulsionsystems [26,27]. In addition, owing to its good chemical stabilityand wear resistance, ZrB2 can be used as high temperatureelectrodes and crucibles for molten metal contact as well aswear-resistant components [28,29].

Table 2The amounts of chemicals added for various composites.

AA 6351-0 wt.% ZrB2

AA 6351-3 wt.% ZrB2

AA 6351-6 wt.% ZrB2

AA 6351-9 wt.% ZrB2

K2ZrF6 (g) 0 36 71 106KBF4 (g) 0 38 76 114

Dead Weight

Balancing Weight

Disk

Pin

Load cell for measurement of force

Fig. 1. Photograph of pin-on-disc wear testing equipment.

Table 1The nominal chemical composition of AA 6351.

Element Si Mg Fe Mn Cu Zn Ti Aluminium

Wt.% 0.7–1.3 0.4–0.8 0.5 0.4–0.8 0.1 0.2 0.2 Balance

G. Naveen Kumar et al. / Materials and Design 31 (2010) 1526–1532 1527

In the present work, attempts to prepare an in situ compositecomprising AA 6351 alloy as base matrix and ZrB2 as reinforcing

10 15 20 25 30 35 40

Rel

ativ

e In

tens

ity (

a. u

.)

Diffraction Ang

Al (

1 1

1)

ZrB

2 (

0 0

1)

ZrB 2

(1

0 0)

ZrB 2

(1 0

1)

x = 9

x = 6

x = 3

x = 0

Fig. 2. XRD results for the pre

particle have been made. The prepared composites have been ana-lyzed for dry sliding wear behaviour at room temperature with thevarying weight percentage of reinforcement system and also fordifferent heat treated conditions such as solutionizing and solu-tionizing followed by aging.

2. Experimental details

2.1. Fabrication and method of characterization of in situ composites

AA 6351-xZrB2 (x = 0, 3, 6 and 9 wt.%), were prepared by theaddition of salts, namely, K2ZrF6 and KBF4. The salts were stirredin the above aluminium alloy melt at a temperature of 850 �C inthe furnace for a period of 30 min. The chemical reaction betweenthe two required salts is as shown in Table 2. The chemical reactionbetween the salts produces in situ ZrB2 particulates in Al–Mg–Simatrix. Both alloy and composites were cast into rods ofØ25 � 100 mm. One set of the samples were retained without heattreatment to test in as-cast condition. The second set was solution-ized at 505 �C for 1 h followed by quenching in water. The third setof composites was solutionized as above followed by aging at170 �C for 6 h to get peak hardness.

The composites have been characterized by X-ray diffraction(XRD) using Cu Ka radiation (1.54056 A�), optical microscopy(OM), scanning electron microscopy (SEM) and energy dispersivespectroscopy (EDS). X-ray diffraction test was carried out to con-firm the formation of ZrB2 phase. XRD phase evaluation was per-formed on D/MAX ULTIMA III XRD machine supplied by RigakuCorporation, Japan. The specimens were polished using standardmetallographic procedure and the polished samples were etchedwith Keller’s reagent. The optical microscope (supplied by M/s.Chennai Met. Company, Chennai, India) was used for the examina-tion of microstructure of the samples in as-cast, solutionized andsolutionized-aged heat treated conditions. The composite materi-als were examined under a SEM to find out the formed ZrB2 parti-cles and their surface morphology; the surface morphology on theworn samples was also studied to analysis the wear behaviour. Thehardness of the composites was measured using Zwick Vicker’shardness tester at a load of 0.5 kg. The experimental error in

45 50 55 60 65 70 75 80

le (2θ)

Al (

2 0

0)

Al (

2 2

0)

Al (

3 1

1)

AA 6351-xZrB2

pared in situ composites.

1528 G. Naveen Kumar et al. / Materials and Design 31 (2010) 1526–1532

measuring hardness is less than 9% and is represented as error barsin the corresponding table.

2.2. Sliding wear test

To study the dry sliding wear behaviour of the in situ compos-ite at room temperature, a pin-on-disc wear testing machine wasused (M/s. Wear and friction Tech; Chennai, India) as shown inFig. 1. In this method three different sets (as-cast, solutionizedand solutionized-aged) of pins were fabricated from compositesviz., AA 6351-xZrB2 (x = 0, 3, 6 and 9 wt.%). The pin dimensionwas 10 mm in diameter and 25 mm in length. The counterpartdisc with 70 mm in outside diameter and 10 mm in thicknesswas fabricated using high carbon high chromium steel. The eachtest pin was loaded against the disc with a dead weight. The testswere carried out at the room temperature with the fixed slidingwear parameters, namely, the load as 9.8 N, the sliding speed as1.0 m/s for 20 min. A digital balance (Model: Sartorius CP423S)with a precision of 1 mg was used to measure the weight of thepin before and after each test. The sliding speed was kept con-stant by fixing the track diameter and the angular speed of discat 50 mm and 400 rpm respectively. Each wear test was carriedout for a sliding distance of 1.2573 km approximately. Duringthe rotation of disc, the frictional torque was applied on thepin. The movable arm holding the pin against the frame on whichload cell being mounted, was used in the apparatus. The wearrate was calculated from the weight-loss measurements. The fric-tional torque was recorded during the experiment and the coeffi-cient of friction was determined using the standard procedure asdescribed elsewhere [1].

Fig. 3. Optical microscopic images of AA 6351-ZrB2 in situ composite in solutionized cond(d) AA 6351-9 wt.% ZrB2.

3. Results and discussion

3.1. Characterization of the composites

3.1.1. X-ray diffraction analysisX-ray diffraction patterns of the matrix and of the specimens

reinforced with different amounts of ZrB2 are shown in Fig. 2.The increase in relative intensity of the ZrB2 peaks with an in-creased amount of ZrB2 in composite was also evident from theXRD. It is evident that the reaction has been taken between moltenaluminium alloy matrix and the salts namely, K2ZrF6 and KBF4 andformed ZrB2 particles on the matrix. The gradual and the marginalshift in the Al peaks to higher angles is an evidence for increase inthe weight fraction of the reinforcement. This indicates decrease inthe lattice parameter of aluminium. The volume fractions of ZrB2

particles in the corresponding composites were determined as2.88%, 5.52% and 8.17% against the targeted values. These reducedfractions against the targeted values were due to losses observedduring the melt operation.

3.1.2. Microstructural examinationThe optical micrographs of the composites AA 6351-xZrB2

(x = 0, 3, 6 and 9 wt.%), in the solutionized condition are shown inFig. 3. Equiaxed grains with an average grain size of 90 lm were ob-served in the solutionized AA 6351 alloy, as shown in Fig. 3a. The ob-served volume fraction of ZrB2 was increased against the targetedvalues. Moreover, the average size of ZrB2 particles was also in-creased with an increased volume fraction. The ZrB2 particle sizeand its distributions are shown in Fig. 4a. The volume fraction of big-ger size particles increases with an increased volume of ZrB2. As

ition: (a) AA 6351-0 wt.% ZrB2, (b) AA 6351-3 wt.% ZrB2, (c) AA 6351-6 wt.% ZrB2 and

Fig. 4. (a) Particle size distribution of ZrB2 and (b) mean particle size of ZrB2 in AA6351-x wt.% ZrB2 (x = 3%, 6% and 9%) in situ composite.

Fig. 5. (a) and (b) SEM images of AA 6351-9% ZrB2 and

G. Naveen Kumar et al. / Materials and Design 31 (2010) 1526–1532 1529

shown in Fig. 4b, the mean particle size of ZrB2 also increased with anincreased volume fraction of ZrB2 in composite. This is attributed tothe higher reaction kinetics close to ZrB2 nuclei during the chemicalreaction between the above mentioned salts in aluminium alloy melt.

Fig. 5a and b shows the SEM micrographs of the composites,namely, AA 6351 alloy with ZrB2 (9 wt.%) particles. It is observedthat the ZrB2 particles were more spherical in shape. The agglomer-ation of ZrB2 particles was observed in the matrix. Fig. 5c shows theEDAX analysis of the above spherical particles with the evidence ofthe formation of ZrB2 as fine dispersion. The above particles wereobserved to be uniformly distributed throughout the matrix. Theaverage size of ZrB2 particles is in the order of 5–13 lm in diameter.

3.1.3. Microhardness analysisThe Vickers microhardness values of AA 6351 with and without

ZrB2 particles are shown in Table 3. The hardness of both alloy andcomposites varied from 46.603 Hv0.5 to 69.036 Hv0.5 for alloy,54.327 Hv0.5 to 82.010 Hv0.5 for 3% ZrB2, 60.427 Hv0.5 to98.976 Hv0.5 for 6% ZrB2, 67.406 Hv0.5 to 115.020 Hv0.5 for 9% ZrB2

after heat treatment. The results revealed that with an increase inthe weight percentage of ZrB2, the hardness values also increased.The in situ composites with solutionized-aged condition exhibitedthe highest hardness value compared to composites with solution-ized as well as as-cast condition. This higher hardness of the alloyand composites can be attributed to the controlled precipitationof b0-Mg2Si particles after solutionizing and finer particles of ZrB2.In the solutionized and aged condition, optimum precipitation ofb0 precipitates occur and this leads to highest hardness in that con-ditioned. The hardness of alloy and composites in solutionized-agedcondition is 1.4–2.5 times higher than as-cast conditions which im-proves the mechanical properties i.e strength and stiffness, andwear resistance. The experimental error and standard deviationon each hardness value is also mentioned in Table 3.

3.2. Sliding wear test at room temperature

The purpose of solutionizing treatment is to obtain the highestpractical amount of hardening solutes b0-Mg2Si in the aluminium

its (c) sum spectrum produced by EDAX analysis.

Table 3Hardness values with standard deviation of the in situ composite as different conditions.

Wt.% of ZrB2 As-cast condition, Hv0.5 (STDEV) Solutionized condition, Hv0.5 (STDEV) Solutionized-aged condition, Hv0.5 (STDEV)

0 46.603 ± 3.757 (2.991) 59.036 ± 4.50 (3.8854) 69.036 ± 4.0 (3.295)3 54.327 ± 4.567 (4.281) 64.010 ± 5.050 (4.745) 82.010 ± 4.025 (3.389)6 60.427 ± 3.792 (3.293) 72.976 ± 5.940 (4.632) 98.976 ± 5.020 (3.774)9 67.406 ± 4.915 (4.091) 85.02 ± 5.430 (4.592) 115.020 ± 7.401 (6.393)

STDEV = standard deviation.

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matrix [28]. The solutionizing temperature must be limited to asafe level below the highest temperature (correspond to maximumsolid solubility) to avoid the consequences of overheating and par-tial melting [30]. Song et al. [27] showed that an increase in agingtemperature enhanced the hardness and the wear resistance. Theimproved micro hardness of the alloy during aging can be attrib-uted to the fact that aging accelerates the kinetics of precipitatehardening.

Fig. 6a shows the variation of wear rate as a function of theamount of reinforcement in various conditions of AA 6351 alloyreinforced with ZrB2. The lower wear rates in composites withthe higher amount of ZrB2 particles can be attributed to the highpeak hardness and the good interfacial bonding between thein situ reinforcement and the matrix alloy. For all reinforcementlevels, the solutionized and aged samples showed a better perfor-mance compared with the as-cast and the solutionized conditions.The lower wear rates can be attributed to reinforcement of ZrB2

Fig. 6. (a) Wear rate, (b) wear resistance, (c) coefficient of friction of AA 6351-x wt.% ZrB2

for as-cast and heat treated conditions.

particles and b0 precipitates. According to earlier work the spheri-cal ZrB2 could also be leads to higher wear resistance. Sphericalmorphology could discourage crack initiation [31] and growth oftheir matrix interface because it causes low stress concentrationat the interface. The decrease in wear rate with an increase inthe amount of ZrB2 was not linear due to the complex wear mech-anism processes occurred during the wear of composites. A studyof the wear surfaces and the wear debris was necessary to under-stand the non-linearity observed in the wear rates of composites.

The wear resistance as a function of weight percentage of ZrB2

reinforcement is shown in Fig. 6b. It is observed to have an in-creased wear resistance with an increase in reinforcements in thecomposite. The solution treatment helps in homogenizing the caststructure and minimize the segregation of alloying elements incasting. The difference in coefficient of thermal expansion betweenaluminium matrix and ZrB2 reinfocement causes development ofstrain field at the interface between the reinforcement particle

(x = 0%, 3%, 6% and 9%) in situ composites as a function of amount of reinforcement

G. Naveen Kumar et al. / Materials and Design 31 (2010) 1526–1532 1531

on the matrix which ultimately leads to an increase in dislocationdensity. This location increases hardness of the alloys and ulti-mately wear resistance of the composites.

Fig. 6c shows the variation of coefficient of friction as a functionof weight percentage of ZrB2 particles for the different condition ofthe alloy. These plots showed that the friction coefficient of thecomposite was the highest for the as-cast condition and the lowestfor the solutionized-aged condition. The alloy without ZrB2 rein-forcements showed the highest coefficient of friction. This is dueto the presence of ZrB2 particles in the composites. If the particlesremained well bonded with the matrix, the aluminium matrix sur-rounding the particles would be worn away during the wear testand essentially all the contact would be provided between thereinforcing particles and the steel counter face. Since the particlede-cohesion was very easy, a contact would occur between the alu-minium matrix and the steel counter face, and the loosened hardparticles start third body abrasion. This could have resulted in sur-face roughness between the contacting surfaces and subsequentlylead to increases the friction coefficient. The wear rate was largelycontrolled by the rate at which particulates delaminated from thealuminium alloy matrix. Therefore, the frictional coefficients werevery low for the reinforced metal matrix composite, comparedwith the pure alloy. Also, because of the highest hardness for thesolutionized-aged condition, the frictional coefficient was the low-est among the samples tested.

3.3. Analysis of worn surfaces using SEM

The SEM micrographs of the worn surface of AA 6351 and thecomposite specimens slid at room temperature for solutionizedcondition are shown in Fig. 7a–d. It can be observed that the wear

Fig. 7. SEM micrograph of the worn surface of AA 6351 as a function of reinforcemen

track was covered with compacted wear debris; the cleaning ofspecimens in an ultrasonic bath with acetone could remove someof the loose debris. This suggested that the primary wear mecha-nisms under these conditions are abrasive wear. Surface projec-tions or asperities present in the contact surface plasticallydeformed and eventually welded together by high local pressureduring the relative motion between the contact surfaces.

In Fig. 7a, the SEM image of the base metal showed deep growsand numerous pits and cracks, where in the composite samples,the grooves were shallow in nature and contained fewer pits(Fig. 7b–d). The general wear pattern seems to be ploughing outof the material from the surface.

The wear debris in the composite seems to be loose in nature,the bigger in size and non-adherent because of the hard ZrB2 par-ticles. Also the wear debris particle seems to round in shape, whichwill not get embedded further in the matrix and will carry awayeasily.

Fig. 7(a–d) shows that numerous grooves on the worn surface ofthe monolithic matrix and composites were existed. The grooveson the worn surface of the monolithic matrix were coarse andthe plastic deformation at the edge of grooves was heavy. How-ever, the worn surfaces of the other three composites were seemsto be smooth. The grooves were fine and the plastic deformation atthe edge of grooves was less.

The SEM micrographs (Fig. 7(a–d)) of the worn surface of AA6351-xZrB2 (x = 0, 3, 6 and 9 wt.%), composite showed that thesegrooves were distributed evenly throughout the worn surfaceand eventually broken off to become debris. Interestingly, as theZrB2 content increased, it became difficult to examine the wearsurface in SEM without coating. The surface charging phenomenawas enhanced with an increase in the amount of ZrB2 particles.

t in solutionized condition: (a) 0% ZrB2, (b) 3% ZrB2, (c) 6% ZrB2 and (d) 9% ZrB2.

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This suggested that the worn out surface was oxidized easily caus-ing surface charging. Also the frictional heating during the wearcontributed to the oxidation of the finer debris more easily thanthe coarser ones. The oxide layer prevented metal to metal contact.This might be the reason why the alloy as well as the compositeshowed a different wear rate depending upon the level of oxidelayer formation during the test. The micro grooves of the materialwere one of the interesting scenarios observed at roomtemperature.

4. Conclusions

In the present work, the feasibility of obtaining ZrB2 reinforce-ment particles in the AA 6351 matrix via in situ reactive sinteringis investigated using XRD, OM, SEM and EDS. The effects of ZrB2

reinforcement with different weight ratios on the microstructures,hardness and the sliding wear behaviour at room temperature ofthe resultant composites are studied. From the results of this study,the following conclusions can be drawn:

� The XRD analysis and micro hardness test results confirmed thepresence of ZrB2 particles in the AA6351 matrix and hence thefeasibility of the composite for industrial applications.

� The hardness value of the composite increases with the increasein percentage of ZrB2 reinforcement and maximum hardnesswas observed for solutionized and aged sample with highestZrB2 reinforcement (9%).

� Lowest wear rate and highest wear resistance were observed forsolutionized and aged samples.

� The coefficient of friction seems to be higher for as-cast samples.� The primary wear mechanism at room temperature appears to

be adhesive wear.

Acknowledgements

The authors wish to express their gratitude for the provision ofexcellent experimental facilities of Surface Engineering Laboratory,established under (TEQIP) Technical Education Quality Improve-ment Programme in the Department of Metallurgical and MaterialsEngineering, National Institute of Technology, Tiruchirappalli, aWorld Bank funded MHRD Scheme, Government of India.

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