Abstract
The wear behavior of extruded nano-SiC particulates reinforced AZ61 magnesium matrix composites fabricated by ultrasonic method was investigated with wearing test machine, electronic balance, and scanning electron microscopy (SEM). The results show that, at the sliding speed of 0.5 m/s or 1 m/s, the extruded composites exhibit superior wear resistance over the matrix with the rise of wear load. At the high load, the softening and melting of composites are delayed to higher loads and speed because of their higher thermal stability.
1. Introduction
Magnesium matrix composites, due to their high specific strength and stiffness, low coefficient of thermal expansion, and well wear resistance, are attractive light metal matrix composites besides aluminum matrix composites [1]. Magnesium matrix composite materials are not only widely used in electrics, automotive, aerospace, and other fields, but also require a higher physical and mechanical properties level. Wear is a serious problem in many engineering applications such as bearing, moving parts, and engine parts [2]. Various parts of magnesium matrix composites will cause in wear when, contact with other materials; also they will be abraded in molding, machining, and assembly processes. Wear can reduce fine tolerances and destroy a surface finish necessitating early replacement of components [3]. Therefore, wear properties of materials are extremely important for mechanical properties.
Sharma et al. [2] studied the wear behavior of magnesium matrix composites reinforced with 1%, 3%, and 5% (mass fraction) feldspars particles, respectively. The results showed that the wear rates decreased with increasing the feldspar content. The impact of sliding speed and load on SiCp-reinforced magnesium matrix composites was studied by Lim et al. [4]. They reported that the wear rates of the composite materials were lower than that of the matrix alloy, approximately 15–30%. Abachi et al. [5] investigated the wear behavior of SiCp/QE22 magnesium alloy matrix composites; they found that the wear rates of the composites reinforced with the sharp shape particles were higher than that of the composites with the particles of the other two kinds of shapes (blocky and round). The authors believed that formation of more cluster zones, especially in the case of sharp shape particles, porosities, and cracks at matrix/SiC reinforcement interfaces and delamination process, can be attributed to more easy pull out and machining away of reinforcing particles from the composites.
Most of the previous works focused on tribological characteristics of magnesium matrix composites reinforced with micron-sized particles [2, 4, 5]. The studies of the wear behavior of magnesium matrix composites with nanosized particles are rare, especially for the extruded magnesium matrix composites. So we investigate the wear behavior of 1 vol.% extruded nano-SiCp reinforced AZ61 magnesium matrix composites by the experiments, hoping to provide a reliable theoretical basis for the further use of magnesium matrix composites and the improvement of magnesium matrix composites' wear properties.
2. Experimental
2.1. Preparation of Experimental Materials
The matrix alloy of the composites was AZ61 magnesium alloy; its chemical compositions were listed in Table 1. Figure 1 shows the reinforcement phase (SiC particles) with the size range of 100 nm. The cylindrical billets (Φ = 42 mm) of 1 wt.% nano-SiCp/AZ61 magnesium matrix composites were fabricated by ultrasonic method [6]. The microstructure of AZ61 magnesium alloy is shown in Figure 2. Finally, the billets were extruded at YG32-200T horizontal extrusion machine; the extrusion ratio was 6.25, the extrusion temperature was 400°C, and the outlet speed was 1 m/min.
Chemical compositions of AZ61 magnesium alloy.
The TEM image of nano-SiC particles.
The SEM of the composite's microstructure.
2.2. Wear Experimental Methods
The sliding wear tests were conducted using M-2000 wear testing machine at room temperature. 1 wt.% nano-SiCp/AZ61 composites and the matrix were machined into cuboid specimens (7 mm × 7 mm × 30 mm). Before the tests, the specimens were cleaned using ultrasonic cleaning instrument with ethanol. The tests were carried out at velocities of 0.5, 1 m/s and loads of 10, 30, 50, 100, and 120 N; the wear time was 15 min. TPX-JA210 electronic balance scale was used to calculate the wear mass loss by weighing specimens before/after wear tests.
3. Results and Discussion
3.1. The Wear Mass Loss versus Load Curve of the Matrix and Composites
Figure 3 is the wear mass loss versus load curve of extruded matrix and composite materials at various loads and sliding velocities. As we can see from Figure 3, the wear resistance of extruded SiCp/AZ61 magnesium matrix composites is higher than that of the extruded matrix alloy in the same test conditions. When the load was 10 N, the wear resistance of composites was 28.7% higher than that of the matrix. When the load was 50–100 N, the wear resistance of composites was 17.4%–19.6% higher than that of the matrix. When the load was 120 N, the wear resistance of composites was 21.8%–25% higher than that of the matrix. For extruded SiCp/AZ61 composites, the wear behavior is a certain rule, which is that the wear mass loss of composites increases with the increasing of the load and velocity. This indicates that the wear resistance of composites decreases with the increasing of load and velocity. As for extruded AZ61 alloy, the rule is similar to that of the composites, but its wear mass loss increases dramatically compared to the composites under the load of 120 N.
The wear mass loss versus load curve of extruded matrix and composite materials at various loads and sliding velocities.
3.2. The Discussion of Wear Mechanisms
3.2.1. Abrasion
Figure 4 shows the worn surface of specimens in the initial period of wear under low loads (10 N–50 N). Numerous grooves and scratch marks are evident on the worn surface, which are the characteristics of abrasion. Figure 4(a) is the worn surface of the matrix; grooves and scratch marks are evident on the worn surface; Figure 4(b) is the worn surface of the composites; compared with the matrix alloy, it has shallow grooves and less scratch marks. This is mainly because the hardness of the composites is higher than that of the matrix alloy.
The worn surface morphology of the matrix (a) and composites (b).
Abrasion is caused by hard asperities on the steel counterface or hard particles in between the contacting surfaces, which can be a result of plowing or cutting but mostly by a combination of both. Plowing creates grooves in the worn surface; materials are displaced on either side of the abrasion groove without being removed. While cutting creates cut marks in the worn surface, the dislodged debris is strip or ribbon [7]. We analyzed debris of the composites and matrix under the low load by SEM and found that the wear debris was strip or ribbon, as shown in Figure 5. However, the occurrence of such ribbons in the wear debris is rare. This suggests that abrasion takes place primarily via plowing. Studying the reasons of grooving and scratching, there are the following two reasons. (1) The steel counterface causes hard asperities, when rubbing with specimens; because the hardness of specimens is lower than that of the steel, asperities of the steel counterface would plow or cut into the surface of specimens. (2) As wear progressed under the low load, the worn surface creates powdery debris due to oxidation; some of these debris exist in the contacting surfaces of the steel and specimens; these powdery debris plow or cut into the surface of specimens during the wear, so the surface of the specimens showed grooves and scratch marks.
The ribbon debris of the matrix alloy and composites.
3.2.2. Oxidation
The wear mechanism of the composites and matrix changes with the load increases. For the matrix alloy, when the load was 30 N, the worn surface appeared dark, the wear debris was the powder black MgO (Figure 6), and the wear mass loss decreased. That means that the matrix has caused oxidative wear. For the composites, the worn surface's color became dark and the wear mass loss decreased when the load was 50 N. That means that the composites has caused oxidative wear under this load. Compared with the matrix, the worn surface's color of the composites under the load of 50 N was darker than that of the matrix under the load of 30 N, and the wear mass loss was less than the matrix. This indicates that the oxide film of the composites is more durable than that of the matrix, and it can better prevent the surface of the composites from directly contacting with the steel counterface, which leads to the wear mass loss of the composites less than that of the matrix.
The oxide debris of the composite and matrix generated in oxidative wear.
3.2.3. Adhesion
When the load was 100 N, naked-eye inspection revealed that the metallic fragments existed in the steel counterface; these fragments were gradually pulled out and became the wear debris. We examined the surface of the composites and matrix by SEM. Figure 7 shows the surface morphology of the composites. Figure 8 shows the surface morphology of the matrix. Combining the worn surface and wear debris (Figure 9), we can know that the composites and matrix have caused adhesive wear.
The surface morphology of the composites after adhesive wear.
The surface morphology of the matrix after adhesion.
The sheet debris of the composites after adhesive wear.
Analyzing wear debris of the composites and matrix loaded of 100 N, we found that wear debris of the matrix were more than those of the composites. And combining the surface morphology of both, we can obtain that the adhesive wear of the composites is less severe than that of the matrix. This conclusion is consistent with the results of Cao et al. and Wang and Rack [8, 9], who studied the wear properties of SiC-whisker and SiC-particulate reinforced aluminum composites. They found that the adhesive wear of composites was less severe than that of the matrix alloy. This is mainly because the hardness of extruded SiCp/AZ61 composites is higher than that of the extruded matrix, improving load-bearing capacities of the composites.
3.2.4. Thermal Softening and Melting
When the sliding speed was 0.5 m/s or 1 m/s, the load was 120 N and the wear mass loss of the matrix increased dramatically. A large number of fragments existed in the steel counterface and the specimens' surface had caused severe plastic deformation. Layers of material were seen protruding slightly at the trailing edge. The surface morphology of the matrix in these conditions is shown in Figure 10 and the wear debris is shown in Figure 11. It suggests that the matrix alloy causes severe softening or melting. As for the composites, there is no similar phenomenon observed in these conditions. That means the composites didn't cause softening or melting in these conditions.
The surface morphology of softening or melting of the matrix.
The blocky debris of softening or melting of the matrix.
The surface of the matrix causes softening or melting; this is mainly related to the temperature of specimens' surface. When the sliding velocity and load of the matrix alloy reach a certain critical threshold, with the increase of the wear time, the temperature of specimens' surface increases sharply. When the temperature of the specimens' surface is higher than the melting point of the matrix alloy, the surface of the matrix causes severe plastic deformation, the surface's metal protrudes slightly at the trailing edge, and the softening metal adheres the surface of the steel counterface and becomes the blocky debris under a certain pressure. Observing the wear of composites under these conditions, there is no softening or melting of the materials. This is mainly because the thermal stability and hardness of SiCp/AZ61 composites are higher than those of the matrix. When the composites were tested at the sliding velocity of the 0.5 m/s or 1 m/s and the load of 120 N, the frictional temperature of the specimens' surface could not reach the softening or melting temperature of specimens' surface, so we could not observe softening or melting similar to the matrix. This conclusion is consistent with the results of Zhang and Alpas and Wang and Rack [9, 10]. This is because the addition of SiC particulates improves the thermal stability of magnesium matrix composites, delaying softening or melting of the composites to a higher velocity and load.
4. Conclusions
At the sliding speed of 0.5 m/s or 1 m/s, the wear rates of extruded SiCp/AZ61 composites are lower than those of the matrix alloy with the rise of wear load. That means that the wear resistance of the composites is higher than that of the matrix. When the composites and matrix cause abrasive wear under the low load (10 N–50 N), the wear mass loss of the composites is less than that of the matrix. The wear debris is strip or ribbon, but the occurrence of such ribbons in the wear debris is rare. When the matrix alloy causes softening or melting at the high load (120 N), the composites do not soften or melt in the same conditions. This is mainly because the addition of SiC particulates improves the thermal stability of composites, delaying the softening or melting of the composites to a higher velocity and load.
Footnotes
Acknowledgments
This research is supported by the National Natural Science Foundation of China (51364035, 51165032), Innovative Group of Science and Technology of College of Jiangxi Province (00008713), Jiangxi Province Education Commission Foundation (GJJ13203), and Production and Teaching and Research Cooperation Plan of Nanchang Non-Party Experts and Doctor (2012-CYH-DW-XCL-002).