Wear 259 (2005) 609–612

Short communication

Numerical analysis of pin on disc tests on Al–Li/SiC composites

C. Gonzaleza, A. Martına, J. Llorcaa, M.A. Garridob, M.T. Gomezb, A. Ricob, J. Rodrıguezb,∗a Department of Materials Science, Polytechnic University of Madrid, Ciudad Universitaria s/n 28040, Madrid, Spain

b Departamento de Ciencia e Ingenier´ıa de Materiales, Universidad Rey Juan Carlos, Tulip´an s/n E28933, M´ostoles, Madrid, Spain

Received 16 September 2004; received in revised form 4 February 2005; accepted 4 February 2005Available online 10 May 2005

Abstract

It is well known that metal matrix composites exhibit marked improvements in wear resistance when compared to unreinforced alloys. Inoperating conditions, components made of Al-based composites are usually subjected to elevated temperatures and high contact loads. Inthis work, an experimental programme on a pin on disk test on Al–Li/SiC metal matrix composite and the corresponding unreinforced alloywas carried out at different loads and temperatures. A finite element model to simulate wear tests was also developed. The sliding distance ofthe pin was discretized in several steps according to the input velocity. Wear is taken into account updating the geometry of the pin in everys . The nodeso tomaticallyu viour of thep every stepw©

K

1

fyrpeoavUcsdo

atrix

pinSiCaqusionsvali-

l–Lisili-thisorethe

0d

tep. The material worn out in each step is computed using the Archard law and the normal pressure acting on the contact surfacef the contact surface were displaced in the normal direction and the geometry and the finite element mesh of the pin were aupdated. The model includes some other important features such as Coulombic friction, temperature-dependent plasticity behain, heat generation at the contact surface by plastic deformation of the pin. Thermo-mechanical coupled equations resulting inere integrated using Abaqus Standard code.2005 Elsevier B.V. All rights reserved.

eywords:Al–Li composites; Finite element model; Wear

. Introduction

The wear behaviour of aluminium matrix composites hasocused the interest of many researchers during the last 15ears. From an experimental point of view, the tribologicalesponse of these materials has been widely studied. Severalapers can be consulted to remind the established knowl-dge about the effects of load, sliding speed or temperaturen the wear rates and the predominant mechanisms associ-ted with each condition[1,2]. Wear maps have been de-eloped showing transitions between different wear regimes.nfortunately, this extensive experimental work has not beenompleted with a similar modelling effort. In that sense, theupport of the finite element method may be valuable to un-erstand the thermo-mechanical state under the conditionsf a wear test[3]. This may be a valuable point of a com-

∗ Corresponding author. Tel.: +34 1488 7159; fax: +34 1488 8150.E-mail address:j.rodriguez@escet.urjc.es (J. Rodrıguez).

plete understanding of the wear behaviour of metal mcomposites.

In this work, a numerical analysis of the experimentalon disc tests of an aluminium alloy 8090 reinforced withparticles is performed using the finite element code Ab[4]. Experiments were also carried out at different conditof pressure and environmental temperature, in order todate the goodness of the numerical model.

2. Experiments

2.1. Materials

The investigation was performed on a commercial Aalloy 8090 and the composite reinforced with 15% ofcon carbide particles. Due to the presence of lithium,alloy exhibits elastic modulus significantly higher than mconventional aluminium alloys. The elastic modulus ofcomposite Al–Li 8090 + 15% of SiC is over 100 GPa.

043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved.

oi:10.1016/j.wear.2005.02.107

610 C. Gonzalez et al. / Wear 259 (2005) 609–612

The material was supplied by Cospray (Banbury, UK) inthe form of an extruded bar produced by spray co-depositionof the matrix and particles onto a substrate. The size ofthe reinforcements was 7.5± 2.4�m with an aspect ratio of2.4± 1.2. Matrix grain size is on average 12�m in the longi-tudinal direction and 6�m in the long and short transversalones. Details about microstructure can be consulted in thereference[5].

2.2. Experimental setup

Pin on disk tests were performed to evaluate the wear be-haviour of the materials. Wear tests were carried out in aWazau machine TRM 1000, able to perform a standardizedpin on disc test[6]. Nevertheless, several modifications wereintroduced, mainly regarding the pin shape. Prismatic pinswere made of the material under study with rectangular sec-tion of 2.5 mm× 6.3 mm. With this geometry, the nominalcontact area was maintained constant during the tests in spiteof the wear process. The disk, made of carbon steel (SAE1045), rotates horizontally at sliding speed of 0.1 m/s. A deadweight loading system was used to perform three sets of testscarried out at normal loads of 100, 150 and 250 N, corre-sponding to nominal pressures of 6.3, 12.5 and 16.5 MPa, re-spectively. The bulk test temperature was modified from roomtemperature to 250◦C by a furnace installed in the machine.P ce tor ot lu-b tiont mea-s andw enta-t

3

t arem 4 for

Fig. 2. Temperature effect on wear behaviour.

all conditions tested. On the contrary, wear rates show cleardifferences depending on the load and, specially, the tem-perature of the test.Fig. 2 shows the results obtained whena nominal pressure of 16.5 MPa is applied. As it can be ap-preciated, the wear rates of the unreinforced alloy and thecomposites are quite moderate up to a transition tempera-ture (mild wear), above which the wear rate experiments ahuge increase (severe wear). In fact, the presence of the re-inforcement does not represent an improvement of the wearresistance under the experimental conditions. The beneficialeffect of the reinforcement is limited to increase the transi-tion temperature from the mild to the severe wear regime. Theeffect of normal pressure on the wear behaviour of the com-posite is included inFig. 3. An approximately linear responseis observed for the three temperatures analysed. Room tem-perature results are apparently anomalous because wear ratesare lower at elevated temperatures. This effect is confirmedby different tests and it shows pressure dependency. Temper-ature may alter the wear mechanism, assisting the formation

reliminary tests show that 500 m was sufficient distaneach a steady-state wear regime. Specimens were nricated and the debris formed was not eliminated. Fric

orque and the linear wear amount were continuouslyured during the test, providing data of friction coefficientear rate. Curves registered by the tribometer instrum

ion are presented inFig. 1.

. Results

Experimental measurements of the friction coefficienaintained in values approximately constant around 0.

Fig. 1. Linear wear amount and coefficient of friction curves.

Fig. 3. Pressure effect on wear behaviour of Al–SiC composite.

C. Gonzalez et al. / Wear 259 (2005) 609–612 611

of tribolayers, but not any experimental evidence has beencurrently found.

4. Numerical simulations

We have developed a model to simulate wear during thepin on disk tests. According to the well-established Archardlaw [7], the wear rate (or pin height loss per unit time) isproportional to the product of the applied pressurep and thesliding velocity following:

w = K

H(T )pv (1)

whereH(T) is the hardness of the material – which depends ontemperature – andK is the wear coefficient. Under steady-state assumptions, the differential equation can be numeri-cally integrated to obtain the material worn as a function ofthe sliding distance of the pin:

�h = K

H(T )pv�t (2)

wherev�t is the distance travelled by the pin and�h is itslocal height variation. The 3D geometry of the problem wassimplified to a 2D plane stress thermo-mechanical problem.The initial geometry of the pin was assumed to be rectan-g andt hess alueso desb hor-i intca con-t hem y oft ng af

ation( singt d us-i ss as

sumption. Wear usually involves strongly coupled problemssuch as mechanical, thermal and contact. The heat producedby friction must be redistributed towards the two bodies incontact. Thus, this heat source decreases the hardness of thematerial and increases the wear rate. The contact problemwas solved using the contact algorithm provided by the finiteelement package. The Coulomb friction model together withthe friction coefficient obtained from the tests was used to thisend. The total heat flux produced by friction,q = µpv, mustbe distributed between the two bodies in contact accordingto the following Equation[9]:

α = 1

1 + √ρdcdκd/ρpcpκp

(3)

whereρ, c andκ are the density, specific heat and thermalconductivity of the disk and pin, respectively andα is theheat fraction transmitted to the pin. These physical propertieswere taken from well-established values obtained from theliterature for Al alloys and SiC particles.

The bulk material (composite and the unreinforced al-loy) was assumed to behave as isotropic thermo-elastoplasticsolids that follow the incremental J2 theory of plasticity. Theelastic modulus and yield stress as a function of tempera-ture can be found in[10]. Hardness was assumed to de-pend on the yield stress of the material through the well-k et dur-i icalb d itsb rmalm

100,1 r-mA f thew le. Att attaint o notv

ula-t desw ture.

iCP pin fo

ular in shape where the initial height was set to 2 mmhe length in the sliding direction 6 mm, respectively. Tliding distance is discretized in steps ofv�t of length. Eachtep starts mapping the nodal and element variables vbtained in the former step onto the new one. All the noelonging to the upper edge were forced to remain in a

zontal straight line during the simulations using multipoonstraint equations. Then, the pin slides the distancev�t

nd the temperature and pressure distribution along theact is obtained. Equation(2) is applied to each node of todel resulting in a modification of the actual geometr

he pin. Finally, the mesh is updated automatically usiree mesher package[8].

The contact pressure and temperature needed in Equ2) were computed through the finite element method uhe code Abaqus. The geometry of the pin was discretizeng three nodes isoparametric triangles under plane stre

Fig. 4. Temperature distribution in the Al/S

-

nown fully plastic relationH= 3σy. No heat generation duo transformation of plastic work was assumed to occurng the simulations. For simplicity, the thermo-mechanehaviour of the disk was not taken in to account anehaviour was assumed to respond to a rigid isotheaterial.Simulations were carried out at room temperature,

50, 200, 250 and 300◦C, respectively, with an applied noal pressure of 16 MPa and a sliding velocityv = 0.1 m/s.ll the simulations were stopped when the difference oear rate between two consecutive steps was negligib

his point, the heat boundary problem was assumed tohe steady-state regime and the mechanical properties dary significantly.

Fig. 4shows the nodal temperature for one of the simions carried out at 20◦C. The temperature of the upper noas kept constant and equal to the simulation tempera

r one of the simulations carried out at 20◦C.

612 C. Gonzalez et al. / Wear 259 (2005) 609–612

Fig. 5. Normal stress distribution in the Al/SiCP pin for one of the simulations carried out at 20◦C.

The maximum increase of temperature,≈9◦C, is attainedin the right corner. No attempt was made to obtain the flashtemperature resulting from the real roughness of the slidingsurfaces. Another interesting result is presented inFig. 5,where the normal stress in the vertical loading direction isplotted through out the pin. The stress distribution is fairlyconstant and equal to the applied nominal normal pressure,16 MPa. However, the stress distribution differs from the ho-mogeneous one at the right and left contacts as a result ofthe boundary conditions. The nodes belonging to the uppersurface were enforced to remain horizontal during the simu-lations introducing a bending moment that was proportionalto the friction force and the pin height. This fact modifies thestress distribution around these points increasing the com-pression force at the right edge and decreasing at the left one.Thus, the real contact area does not correspond to the nominalone.

Finally, Fig. 6shows the results of the adimensional wearrate (wear rate related to the 20◦C wear rate) as a function of

F ratureo e-i

temperature for the Al/SiCp composite and the unreinforcedalloy. In all the cases, the wear rate increases continuouslywith temperature as a result of the softening effect of hardnessof the Archard wear law.

5. Conclusions

Experiments performed have confirmed that, under theconditions analysed, the temperature effect on the wear rateis more critical than that of pressure. As it was observed inprevious investigations with other aluminium matrix compos-ites[11], the role of the reinforcement particles is to increasethe transition temperature from the mild to the severe wearregimes. These tendencies can also been observed from thenumerical model, although quantitative agreement with theexperiments is not achieved.

Acknowledgement

The authors are indebted to Comunidad de Madrid for thefinancial support through grant.

References

l. 57

03.cket,

pin

h/

ut.

[ 4)

[ 3)

ig. 6. Adimensional wear rate (wear rate related to the room tempene) as function of temperature for the Al–Li/SiCp composite and the unr

nforced alloy.

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