WEAR BEHAVIOUR OF SQUEEZE CAST AI-A1203-MgO PARTICULATE MMCs UNDER DRY SLIDING CONDITIONS

Abstract

An attempt has been made in the present work to study the wear behaviour of squeeze cast Al-Al,,03-MgO particulate MMCs in comparison with gravity chill cast pure aluminium, under dry sliding conditions, using a pin-on-disc apparatus. Gravity chill cost pure Al, 160 and 240 MFa squeezed composite are respectively termed as specimen No. 1, 2 and 3, in the present work. Cylindrical specimens of 10 mm dia and 30 mm length prepared out of above materials were slid against a mild steel disc of 200 mm dia (HV ... ), under bearing loads of 50, 100, 200, 300 and 400 gins and sliding speeds of 124, 198.54 and 273 in min-1. Both, the cylindrical specirne.ns and steel disc were polished to 4/0 grade emery paper prior to start of any new run. The wear data was generated in terms of weight loss under different sets of experimental conditions. From this data., the wear rate was calculated in individual cases. This data was then related with bearing load and sliding speed employed in different sets of wear experiments. The weight loss data was also related with sliding distance for different materials under investigation. The worn surface of different specimens and the debris material generated during the process of wear were examined optically and also by the SEM. The measurable depl~*, upto which individual materials undergo work hardening was also measured with the help of microhardness measurements. The results of present study show that wear rate in all the cases studied (specimen No. 1, 2 and 3) progressively increases, at all sliding speeds and track lengths, as the bearing load is systematically increased in steps. Also, composite squeezed to highest pressure (240 MPa - specimen No. 3) displays highest wear resistance compared to specimen No. 2 and I under ail experimental conditions of bearing load, sliding speeds and track lengths. Superior wear resistance of specimen No. 3 (240 MPa) becomes more and more distinct as bearing loa.d is progressively increased. it was also observed that, in general, the wear rate of materials, progressively decreases as sliding speed is increased in stages. The relationship between weight loss and sliding distance, at different sliding speeds and bearing loads also shows that the wear resistance of specimen No. 3 is distinctly superior than the rest two specimens. The optical and SEll examination of worn surface of different specimens revealed typical features which are indicative of the mechanism of wear that has taken place in all these specimens. The examination of debris material, generated during process of wear, was also of great help in this regard. It was found that a mixed mode of adhesive cum abrasive wear has occured in most of the specimens depending upon the amount of bearing load applied, to general, the mechanism of wear has been; severe work hardening of the worn surface leading to the generation of cracks which propogate deeper into base matrix upto a. measurable depth. The worn surface was found to undergo, the process of delamination and further fragmentation through the generation of big cracks. Also this effect of severe work hardening at the worn surface, progresses deeper into the base matrix upto a measurable depth. This process leads to subsurface deformation upto varying depth in different specimens. The present studies therefore reveal that it Is not only the actual surface which is rubbing a.t the steel disc, that is involved in the process of wear, but also a definite volume of material, below the worn surface which is also directly involved in the overall process of wear.