The friction and wear properties of nano-SiO 2 and -TiO 2 particle-reinforced PMMA composites

Materials and specimens

Nano-SiO₂ with an average diameter of 40 nm was applied as additional reinforcement. Commercially available PMMA plates (thickness, ds = 2.0 mm; Shinkolite A, Mitsubishi Rayon, Tokyo, Japan) were used as matrix.

PMMA plates and nanofillers with different content were hot-pressed at 452 K for 2 h under applied pressure of 1.5 MPa. Hot pressing was done in ambient air. After hot-press, the composite was allowed to furnace cool. Parallel surfaces of the hot-pressed specimen were mechanically ground and polished to 0.05 μm finish to obtain optically flat surfaces. For the purpose of comparison, a pure PMMA specimen was fabricated by the same hot-press process and the surfaces were polished.

Evaluation of mechanical and tribological properties

The tribological tests were carried out in a ball-on-disc tribometer of HT-500 at room temperature (25°C). The upper specimen was a GCr15 ball with a diameter of 4 mm, whose microhardness was 800 HV. The down specimen was a disc of the composite specimen. The GCr15 ball slid against the down specimen under dry friction condition at the loads of 100 N, 150 N, 200 N and 250 N (speed was 0.28 m/s) for a distance of 500 m. The friction coefficients of the specimens were obtained by the software attached to HT-500 and the wear losses of which were measured by the analytical balance of TG328B with precision of 0.1 mg.

Effects of loads on friction and wear properties of the TiO₂/PMMA composite

The variation curves on friction coefficients and wear losses of the composite at different loads are shown in Figure 1. It can be seen that the friction coefficients of all PMMA composites take on downward trend with the increase in loads (Figure 1(a)). At the small load of 50 N, the friction coefficient of PMMA is 0.59, and gradually decreases to 0.48 as the load increasing. The friction coefficient of the TiO₂/PMMA composite is reduced and presents a decreasing trend with the increasing TiO₂ content. At the same sliding condition, the 7-vol% TiO₂-filled composite shows lower friction coefficient. This can be attributed to fact that at low and medium loads, the tribochemical reaction layers formed at the interface controls the tribological behavior of the composite.

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Figure 1.

The variation curves on friction coefficients and wear losses of the TiO₂/PMMA composite at different loads.

Figure 1(b) exhibits the wear losses of both the TiO₂/PMMA composite and PMMA increase with the increase in loads. The wear decreased significantly with increasing TiO₂ content, which indicated that TiO₂ had good friction-reducing and antiwear abilities.

When the TiO₂ content is about 7 vol%, the wear of the TiO₂/PMMA composite is the lowest, which is only 56% of that of the pure PMMA. A minimum point of friction coefficient is formed at the TiO₂ content of about 7 vol% in Figure 1 and the minimum friction coefficient is 87% that of pure PMMA for the TiO₂/PMMA composite. However, at higher loads, mechanical stability and hardness of such oxide layers decreases and may partially be removed resulting in actual surface contacts, leading to higher wear.

Effects of loads on friction and wear properties of the SiO₂/TiO₂/PMMA composite

The coefficient of friction decreased with increasing SiO₂ content up to 5 vol% and then assumed little change with further increase in SiO₂ content from 1 vol% to 5 vol%. Moreover, the coefficient of friction decreased with increasing SiO₂ content up to 7 vol% and then showed a very large increase at SiO₂ content above 15% (Figure 2(a)). In addition, the friction modifier SiO₂ was beneficial to increasing wear resistance of the PMMA-based composites within a volume fraction of 1%–5% but harmful to the wear resistance of the composites above a volume fraction of 7 vol%.

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Figure 2.

The variation curves on friction coefficients and wear losses of the SiO₂/TiO₂/PMMA composite at different loads.

It can be seen that the wear losses are also found to increase in ascending order with loads. The wear of all PMMA composites decreased with increasing SiO₂ content. This improvement in the wear resistance correlates with an increase in the hardness with increasing SiO₂ content. The addition of 5 vol% SiO₂ to the PMMA matrix caused about 50% reduction in the wear.

It is not as obvious for mixing SiO₂/TiO₂/PMMA as for TiO₂/PMMA, as the friction coefficient changes with the variation in filler content. It can be seen from Figure 2(b) that the wear is much lower for the SiO₂/TiO₂/PMMA than for the mixing TiO₂/PMMA with the same components. Choosing the appropriate content of SiO₂ can obviously reduce the wear of PMMA.

The typical worn surface morphologies of SiO₂/TiO₂/PMMA composite with different content of SiO₂ are shown in Figure 3. When the content of SiO₂ is null (Figure 3(a)), the worn surface is flat with some shallow and narrow scratches along the sliding direction, which were created by microcutting of asperities on the surface of GCr15 counterpart. So the composite experienced slight microcutting wear. The friction coefficient and wear loss of the composite are minimum. With the increase in the content of SiO₂ (Figure 3(b) and (c)), the worn surface becomes rough accompanied with some furrows and cracks as well as some wear debris; moreover, obvious ridges and delamination caused by plastic deformation are observed, which indicates that multiplastic deformation wear, adhesive wear and abrasive wear occur in the friction process. This is because the frictional surface suffered higher contact stress and was severely ploughed and extruded by counterpart with the increase in loads, and adhesion between the contact surfaces were exacerbated. Meanwhile, the repeating plastic deformation leading to occlusion between the worn surfaces and the material of composite coating was desquamated and changed into debris under relatively big contact stress, which also acted as abrasives and resulted in the increase in friction coefficient and wear loss. While due to the grain boundary strengthening of SiO₂ particles in the composite coating, the surface hardness and deformation resistance were improved.

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Figure 3.

The worn surface morphologies of SiO₂/TiO₂/PMMA composite.

For the SiO₂/TiO₂/PMMA composite filled with low content of SiO₂, the worn surface is relatively flat with slight scrapes caused by abrasion of rough surfaces (Figure 3(b)). Because of the low content of SiO₂, the asperities on contact surfaces were able to separate friction pairs and reduce adhesion, so the composite shows lower friction coefficient and wear loss. The shear force on the surface increased and more asperities were ruptured and worn off to act as abrasives. So deep furrows on the worn surface are obvious, which indicate the existence of abrasive wear. More obvious cracks are found on the worn surface. So the microcracks in subsurface gradually propagated to surface forming macrocracks, which illuminated that the wear mechanism was fatigue wear. Therefore, the friction coefficient and wear loss also increased.

The wear grooves and the material fragments indicate that the solely incorporated SiO₂ particles, in particular at high content, result in a severe wear, although the hard particles also help to develop the toughness and stiffness of the matrix to some extent. This effect is ascribed to that the particles act as an abrasive third body and meanwhile increase the discontinuities in matrix. The combination of the two kinds of fillers, however, promotes a pronounced change in wear process. It can be seen in Figure 3(c) that a considerable amount of compacted wear debris are collected around the fibers, acting temporarily as a distance holder between the composite pin and the steel counterpart, and thus protect the fibers.

The TiO₂ particles, when being used in combination with the SiO₂, are not easily ploughed out of the matrix by counterpart asperities. Instead, they are gradually released and likely start to act as a rolling body between the two mating surfaces.