Wear behaviour and microstructural characterization of worn surfaces and wear debris of a high purity Al and an Al-Si alloy and an Al-Si/SiCp composite sliding against an M2 steel
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Date
1999-05-01T00:00:00Z
Authors
Li, Xianyao
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Abstract
The present dissertation discusses wear behavior and microstructural characterization of the wear surface and wear debris of an Al-Si alloy (A356) and an Al-Si (A356) based composite reinforced with 20 vol.% SiC particulate as well as a high purity (99.99%) Al sliding against an M2 steel under unlubricated conditions using a block-on-ring sliding wear tester. It was observed that the wear resistance of the Al-Si/SiCp composite material was superior to that of the Al-Si alloy by one order of magnitude at a low sliding load. It was also observed that the wear resistance of the Al-Si/SiC composite could slightly be increased by approximately 10% due to the T6 beat treatment of the composite. The increase in the wear resistance, however, was not significant when compared to the increase in the hardness of the matrix due to the beat treatment. This can be partially attributed to the fact that 'in-situ' precipitation occurred in the subsurface during the sliding wear of the as-cast Al composites. Microstructural characterization of the worn surfaces and weer debris was carried out by using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) equipped with Energy Dispersive Spectroscopy (EDS), X-ray mapping and X-ray Diffraction (XRD). In particular, longitudinal cross-section TEM analyses of the worn surfaces and wear debris were successfully conducted in the present work. The results showed that the deformed structures in the worn surfaces of the Al-Si alloy and the composite contained a large number of cells and elongated subgrains with an aspect ratio of 5 to 10. It was also observed that mechanically mixed layers (MMLs) were formed during the sliding wear of the Al-Si/SiC composite and Al-Si alloy as well as the high purity (BP) Al against the steel. Characterization of the wear debris by means of the above mentioned techniques and Mossbauer spectroscopy revealed that the wear debris had features similar to those of the MMLs in the worn surfaces of the Al based materials from which the debris were detached. The present work has provided evidence of the formation of the intermetallic compound FeAl as a result of material transfer and mechanical mixing, as revealed by using XRD, TEM and Mossbauer spectroscopy. Amorphization was observed in the wear debris and worn surfaces of both the Al-Si alloy and the Al-Si/SiC composite sliding against the too) steel under the unidirectional dry sliding wear and a reciprocating dry sliding wear. The amorphous phases were found to coexist with the ultrafine grained aggregates (a few ran to dozens nm depending on the loads) in the wear debris. The correlation of the microstructural characteristics of the worn surface and wear debris to the wear behavior and mechanisms of the Al-Si alloy and the Al based composite was discussed in detail. (Abstract shortened by UMI.)