Processing And Characterisation Of Bulk Al2 O3 p /AIN-Al Composites By Pressureless Infiltration
Al-Mg alloys were infiltrated into porous alumina preforms at temperatures greater than 950°C where significant amount of nitride forms in the matrix. The present work aims to obtain a process window for growing A1N rich composites over uniform thicknesses so that bulk fabrication of these composites could be carried out. Initial experiments were carried out in a thermo-gravimetric analyser (TGA) to establish suitable conditions for growing useful thicknesses. Al- 2wt% Mg alloy, alumina preforms of particle size 53-63μm and N2 - 2% H2 (5ppm O2) were used for the present study based on previous work carried out in the fabrication of MMCs at low temperatures. Experiments carried out in the TGA indicate that oxygen in the system has to be gettered for the growth of nitride rich composites. Infiltration heights of about 8mm were obtained using an external getter (Al - 5wt%Mg) alloy in addition to the base alloy used for infiltration. The above process conditions were subsequently employed in a tube furnace to fabricate bulk composites and to study the effect of temperature on the volume fraction of aluminium nitride in the matrix. The volume fraction of nitride in the composite varied between 30 and 95 vol % with increase in process temperature from 950°C to 1075°C. Microstructures of these composites indicate that A1N starts to form on the particle surface and tends to grow outwards. The metal supplied through channels adjacent to the particle surface nitride until a point is reached when the composite growing from the adjacent particles meet each other and isolate the melt underneath from nitrogen thereby leading to a metal rich region underneath. Increase in temperature results in an increased nitridation rate resulting in reduced metal pocket size. Composites fabricated at 975°C had a minor leak at the O-rings, which seal the tube. This led to infiltration under conditions of varying oxygen partial pressure leading to different nitride fractions in the composite. The above fact was confirmed by conducting an experiment with commercial purity nitrogen, which has an oxygen content of about 5000ppm. The composite had an A1N content of about 30% whereas the composite fabricated with N2 -2%H2 (5ppm oxygen) showed a nitride content of 64%. This suggests that one can vary the nitride content in the composite by varying the oxygen content in the system at a particular process temperature. The hardness of the matrix increases with increase in process temperature from 3.5 ± 0.7 GPa at 975°C to about 9.8 ± 0.9 GPa at 1075°C. Porosity was observed in the composite processed at 1075°C. This increased porosity leads to decreased hardness though the nitride content in the composite has increased by 11%. The scatter in the data is attributed to variations in the microstructure as well as due to interference from underlying metal pockets or particles as well as due to porosity introduced in the composite at high processing temperatures.