A STUDY OF WEAR BEHAVIOUR OF SQUEEZED AI—A1203—MgO PARTICULATE MMCs AT LOW BEARING LOADS f100-400gms) UNDER DRY SLIDINGC ONDITIONS

Abstract

An attempt has been made in the present wcrk to study the wear behaviour of squeeze cast A1-A1203-MgO particulate MMCs in comparison with gravity chill cast Al-A1203-MgO particulate MMC, under dry sliding conditions, using a pin-ondisc apparatus. Gravity chill cast composite,80 and 140 MPa 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 100, 200, 300 and 400 gms and sliding speeds of 173.73,E 287.895 and 372.28 m min ~1. Both, the cylindrical specimens 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 were examined optically and also by the SEM. The debris material generated during the process of wear was examined by the SEM. The measurable depth 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 (140 MPa - specimen no. 3) displays highest wear resistance compared to specimen no. 2 and 4 under all experimental conditions of bearing load, sliding speeds and track lengths. Superior wear resistance of specimen no. 3 (140 MPa) becomes more and more distinct as bearing load i s progressively increased. It was also observed that, in general, the wear rate of materials, progressively decreases 'as sliding speed is increased in stages. The reationship 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 SEM 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 ragard. It was,ythat a mixed mode of adhesive cum abrasive wear has occured in most of the specimens depending upon the amount of bearing load applied. In 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 delami'nation and further fragmentation through the generation of big cracks. Also the effect of severe work hardening at the worn surface, progressive 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 at 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.