Influence of countersamples on wear process of friction couples with aluminium matrix composites
Dionizy Biało Politechnika Warszawska, Wydział Mechatroniki, Instytut Inżynierii Precyzyjnej i Biomedycznej, ul. św. A. Boboli 8, 02-525 Warszawa
Quarterly No. 1, 2006 pages 15-19
DOI:
keywords: aluminium matrix composites, wear, countersample hardness, friction coefficient
abstract The paper shows the results of investigation on influence of countersample material on wear of both partners of friction pair i.e. composite samples and countersamples. This problem is usually over-looked in scientific publications. Results of research work presented by different authors are controversial, especially with regard to the effect of countersample material, its state and processing, hardness and surface roughness. A very important problem in wear process of composite is effect of the type and hardness of the countersample on that process. This paper is going to focus on this subject. Composite material manufactured by powder metallurgy route contains Al-20%Si-3%Cu-1%Mg matrix and reinforcing phase - Al2O3 particles with 10% vol. fraction. Figure 1 shows the microstructure of the composite, with the visible dark particles Al2O3 and the grey silicon crystals in the matrix. Countersamples made of grey cast iron (210 HB) and steel with hardness of 28, 40 and 58 HRC were used. It had shape of disc with surface roughness Ra ≈ 0.32 μm. Tribological experiments were carried- out on the pin-on-disk apparatus at dry conditions at the following parameters: contact pressure of 0.5÷3.0 MPa, friction velocity of 1 m/s, friction distance of 6000÷12000 m. It was noticed that material and hardness of countersamples play significant role in wear process of composites. Figures 2 and 3 show the wear rate of composite samples versus sliding distance. The wear steadily increases with increasing sliding distance and is higher at higher contact pressure between the sample and countersample. With decreasing in the hardness of countersample the wear of composite is growing (Fig. 4). The highest wear rate was observed for grey cast iron countersample. On the soft countersample friction surface, local adhesive accretions areas of material transferred from the sample matrix were found (Fig. 5). More over the surface of the softest countersamples undergo to „reinforcing” with Al2O3 particles transferred from the composite during friction and act as abrasives to composite leading to few times higher wear rate (Fig. 6). For the hardest countersample build-ups on the friction surface are hardly found and only when the highest contact pressure is applied some material transfer is observed. There is not shielding of the surface by the hard particles. This results in less wear. The role of abrasion is less important and the sample surface is smoother. As a result the delamination mechanism of wear occurred. We may confirm that changing in countersamples hardness involves of several wear mechanisms: abrasive, adhesive and delamination. In Figures 6 and 7 SEM of composite worn surface undergone abrasive and delamination mechanisms of wear are shown. During friction with composite the highest wear of countersamples is occurring for countersamples made of cast iron and steel with lowest hardness (Fig. 8). For these countersamples lowest value of friction coefficient was noticed (Fig. 9).