Effect of graphite particle size on wear property of graphite and

Al₂O₃ reinforced AZ91D-0.8%Ce composites

ZHANG Mei-juan(张美娟), LIU Yong-bing(刘勇兵), YANG Xiao-hong(杨晓红),

AN Jian(安 健), LUO Ke-shuai(罗克帅)

Key Laboratory of Automobile Material of Ministry of Education, Jilin University, Changchun 130025, China

Received 12 June 2008; accepted 5 September 2008

Abstract: The graphite particles and Al₂O₃ short fibers reinforced AZ91D-0.8%Ce composites were fabricated by squeeze-infiltration technique. The researches, about the effects of different graphite particle sizes on the microstructure and wear property of the composites, were performed under the condition of constant contents of graphite particles and Al₂O₃ short fibers. The results reveal that the grain size of the composites changes less when the graphite particle size descends. Moreover, Ce enriches around the graphite particle and Al₂O₃ short fibers and forms Al₃Ce phase with Al element. The graphite which that works as lubricant decreases the wear loss. The wear resistance of the composites increases as the graphite particle size increases. At low load the composites have similar wear loss; at high load the composite with the largest graphite particle size has the best wear resistance. The wear mechanism of all the composites at low load is abrasive wear and oxidation wear; at high load, except the composites with the particle size of 240 μm whose wear mechanism is still abrasive wear and oxidation wear, the wear mechanism of others changes to dellamination wear.

Key words: magnesium matrix composite; particle size; graphite; wear property

1 Introduction

Magnesium alloy has high potential as light structure material in the development of automobile and aerospace industries. Its high specific strength and mechanical properties makes it valuable in many applications[1-2]. However, its corrosion and wear resistance limit it to be used as widely as Al alloy[3-4]. One possible solution to improve the wear property of magnesium alloy is to use particles, fibers or rare earth elements as additives to magnesium matrix[5-8]. But most of the researches focus on hard particles (like SiC and TiC particles) reinforced magnesium matrix composites[9-11]. Investigation on the graphite particle that has lubricious effect during sliding process is absent. Recently, I et al.[12-13] have studied the effect of rare earth on graphite reinforced Mg alloy and find better wear property by adding rare earth. WUet al.[14] have investigated the effects of graphite on the microstructures and properties of Mg composite. In the present work, for the combining use of lubricating graphite particles and hard Al₂O₃ short fibers, the AZ91D-0.8%Ce composites were fabricated and the wear property of the composites was investigated along with the wear mechanism, especially investigating the effect of graphite particle size on the microstructure and wear property.

2 Experimental

AZ91D-0.8%Ce alloy was chosen as matrix. The short fibers with a diameter of 8-12 μm and a length of 300-700 μm contained 98.9% Al₂O₃. The contents of Al₂O₃ short fiber and graphite were kept constant with volume fractions of 8% and 15%, respectively. The average graphite particles size was 240, 125, 83 and 55 μm, respectively.

The process of fabricating the composites was carried out by two steps. First, the graphite particle and Al₂O₃ short fibers were prepared into a prreform. Second, the squeeze-infiltration technique was used to add molten AZ91D-0.8%Ce alloy into the preform. The pouring temperature was 680 ℃, applied pressure was 55 MPa and maintaining time was 60 s. Dry sliding wear tests were operated using a pin-on-disc type wear apparatus MM2000. In this system, the specimen size was φd6 mm×12 mm, the friction disc kept rotating at a constant speed of 0.785 m/s, and slid a distance of 376.8 m. The worn surface was analyzed by JSM-6700F scanning electron microscope(SEM) with an energy-dispersive X-ray spectrometer(EDS). The density of sample was measured using the standard Archimedes method with distillated water. Mass changes before and after wear tests were used to calculate the volume loss.

3 Results and discussion

3.1 Microstructures of the composites

The optical microstructure photographs of the composites with different graphite particle sizes are shown in Fig.1. It reveals that the graphite particle and Al₂O₃ short fibers disperse uniformly in the composites and no agglomeration is observed evidently. The graphite takes the form of flake and Al₂O₃ short fibers appear in round and needle shapes as pointed in Fig.1(b). Some rod-like phases are found around the boundaries of graphite and Al₂O₃ short fibers which can be seen in the backscattering image of the composite. By comparing the electronegativity of Mg with Al, Ce prefers to form rod-like Al₃Ce phase with Al, which is confirmed by XRD analysis. As graphite particle size descends, the grain size changes less. This indicates that during solidification the graphite particle size affects the process of nucleation and growth less, so the grain size of the composites has no evident changes.

3.2 Influence of graphite particle size on wear loss of composites

The variations in wear loss as a function of load of the composites with different graphite particle sizes are shown in Fig.2. It reveals that the wear loss increases with increasing the load. At 20 N, all the composites show low wear loss due to the presence of Al₂O₃ short fibers that can keep intact and efficiently bear load. The composites with 55, 83 and 125 μm particles have similar wear loss under 140 N, but the composite with the 240 μm particle shows difference that keeps a low wear loss. Although the wear loss of all the composites increases quickly from 140 to 180 N, the composite with the 240 μm particle still keeps the lowest wear loss. Compared with the composite with the 55 μm particle, the wear loss of the composite with the 240 μm particle descends by 77% and 46% at 20 N and 180 N, respectively. This implies that although the fine grains strengthen the matrix, the effect of graphite particle is primary and works effectively under high load.

Fig.1 Optical microstructure photographs of composites with different graphite particle sizes of: (a) 240 μm; (b) 125 μm; (c) 83 μm;   (d) 55 μm

3.3 Wear mechanism of composites

The SEM images of worn surfaces of the composites

Fig.2 Variations of wear loss with load of composites

at 20, 100 and 180 N are displayed in Fig.3. The composite with the 240 μm particle has evidently bare graphite particle on the worn surface, as pointed in Fig.3(a). EDS analysis shows that the peak of oxygen element is high, so oxidation phenomenon takes place in the matrix. As the graphite particle size decreases to 55 μm the worn surface shows many parallel ploughs and powder debris that are the characters of abrasive wear and oxidation wear.

At 100 N the oxidation phenomenon of the composite with the 55 μm particle deteriorates, but the worn surface of the composite with 240 μm particle changes less. And the wear mechanism of both is still abrasive wear and oxidation wear. This reveals that no matter how the graphite particle size is larger or small, the presence of graphite can always keep the composites with high wear resistance.

When the tested load increases up to 180 N the

Fig.3 SEM morphologies of worn surface of composites with particle size of 240 μm at 20 N (a), 100 N (c), 180 N (e) and with particle size of 55 μm at 20 N (b), 100 N (d) and 180 N (f)

composite with 55 μm particle has large flakes peeled along the sliding direction combining with the appearance of cracks. The graphite particle cannot keep

intact and it is peeled off with flakes. The wear loss increases sharply, which accords with the curve in Fig.2. So its wear mechanism transforms to delamination wear. But the worn surface of the composite with 240 μm particle looks the same as that under 100 N, except that partial area of the matrix is peeled off. This is because the composite with 240 μm particle has evident bare graphite particle on the worn surface. At low load the graphite particle decreases the actual contact area between composites matrix and friction disk; at high load it is extruded and smeared on the worn surface and benefits to form graphite film[15-16]. The graphite film has lubricious effect and descends the extent of the friction. Finally, the presence of graphite film decreases the wear loss of the composite. So the wear mechanism of the composite with the 240 μm particle is still abrasive wear and oxidation wear.

As shown in the force analysis of Fig.4-(a), the specimen has positive pressure (N) and friction force (f) on it. Both of them form the resultant force (F) with which the angle of axis of horizontal plane is θ. N is made up of the disk gravity (G_disk) and the tested load (N_load). So it goes up when the tested load increases, as shown in Eqn.(1). When the friction coefficient (μ) changes from 1 to 0, based on Eqn.(3), θ changes from -45? to 90? as the tested load increasinges. So the cracksFig.4 Force analysis of composite (a) and optical photograph of lengthways profile of composite with particle size of 55 μm (b) are easy to form and extend when θ changes from -45? to -90?. As a result, the cracks decrease the strength of the matrix and increase the wear loss of the composites.

N = G_disk + N_load                                (1)

f = μN                                       (2)

tanθ=?N/f=?N/μN=?1/μ                       (3)

The optical photograph of lengthways profile of the composite with 55 μm particle is shown in Fig.4-(b). The plastic deformation occurs on the surface layer, as pointed in area A. The grains are elongated along the contrary side of the sliding direction and the grains next to the surface have serious deformation due to the friction force. But inside specimen, as pointed in area B, , the deformation of grains changes less because the influence of friction force is not strong here. Compared with the composite with 55 μm particle, the composite with 240 μm particle has larger graphite particle on the worn surface. While sliding it smears on the worn surface and acts as lubricant that decreases the effect of friction force. At the same time, the temperature of the contact surface decreases as frictional heat becomes less, which gives stability to the microstructure of the matrix. So the presence of graphite film delays the changes from mild wear to severe wear and works effectively at high load, which results in better wear resistance. Therefore, at low load the wear mechanism of all the composites is abrasive wear and oxidation wear; at high load except the composite with the 240 μm particle which is still dominated by abrasive wear and oxidation wear, the wear mechanism of others changes to delamination wear.

Fig.4    Force analysis of composite (a) and optical photograph of lengthways profile of composite with particle size of 55 μm (b)

4 Conclusions

1) Graphite particles and Al₂O₃ short fibers reinforced AZ91D-0.8%Ce composites can be fabricated by squeeze-infiltration technique. The grain size of the composites changes less when the graphite particle size descends.

2) The wear resistance of the composites increases as graphite particle size increases. At the same load, the composite with the particle size of 240 μm keeps the best wear resistance.

3) The wear mechanism of all the composites at low load is abrasive wear and oxidation wear; at high load, except the composites with the particle size of 240 μm which is still dominated by abrasive wear and oxidation wear, the wear mechanism of others changes to delamination wear.

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(Edited by CHEN Wei-ping)

Foundation item: Project(2006BAE04B04-1) supported by the National Science and Technology Program; Project(20060308) supported by the Development of Science and Technology of Jilin Province Program; Project supported by “985 Project” of Jilin University, China

Corresponding author: LIU Yong-bing; Tel: +86-431-85095874; E-mail: ybingliu@jlu.edu.cn