Direct nitridation of aluminium and fabrication of aluminium-aluminium nitride composite using powder metallurgy route

Abstract

The enviable attributes of aluminium nitride (AlN) such as; high melting point, good wear and corrosion resistance, wide band gap similar to that of silicon, high resistivity and low coefficient of thermal expansion have continued to attract research attention in the last few decades. Direct nitridation of aluminium is one of the industrial methods of synthesizing this ceramic but difficulty in achieving 100% conversion of aluminium to aluminium nitride (AlN) has been the major setback. In order to find solution to this problem, aluminium powder was heated at the rate of 20oC/min in a stainless steel tube furnace at temperatures ranging from 650 to 9500C in flowing nitrogen at the rate of 1.0L/min for 4-6 hours. 5.0 and 10.0wt% magnesium were added to facilitate the process while hydrogen gas flowing into the furnace at the rate of 0.2L/min was used to maintain low oxygen pressure. SEM and XRD were used to analyze the nitrided powders. Problems of swelling and liquid exudation during production of Al-AlN composite using powder metallurgy route have also been reported. In order to find the causes and remedies to these problems, Al-AlN composite consisting of 0 – 16.25%AlN reinforcement and 0 - 2.0%Mg were compacted using load of 3.8 - 5.0 tons. The green compacts were sintered using normal and two-step sintering techniques in nitrogen and hydrogen atmospheres at temperatures between 600 and 650oC for 30 – 90 minutes. The Al-AlN compacts produced were used for various tests. The electrical resistivity/conductivity was measured using four point probes; while coefficient of thermal expansion (CTE) was measured using dilatometer at temperatures from 30 to 450oC at heating rate of 10oC/min in nitrogen atmosphere. Corrosion rate of Al-AlN compact was determined by immersion of the compacts in 0.05M NaOH for 24 hours. XRD results show that aluminium powder containing 10%Mg gave 100% conversion from Al to AlN at temperature of 950oC after 5 hours nitridation. The main cause of swelling in the compacts was due to presence of volatiles which were eliminated by two-step sintering. The factors that promote liquid exudation during sintering were identified as: presence of magnesium and low temperature eutectic liquid, high sintering temperature, long sintering time and use of hydrogen (inactive) atmosphere during sintering. The values of electrical conductivities of the compacts increase with weight fractions of AlN and magnesium added. The highest value of electrical conductivity obtained was 131.326(Ωm)-1 in compact containing Al-16.25wt%AlN-1.0wt%Mg. CTE results show that Al-6.5wt%AlN-0.5wt%Mg gave the lowest (best) values equivalent to 10.60x10-6K-1 at temperature of 50oC and 20.91x10-6K-1 at 450oC. Corroded samples show that the main corrosion product on the compacts was aluminium hydroxide, Al(OH)3. Unreinforced sintered aluminium corroded more than compacts reinforced with AlN. Compact containing Al-16.25wt%AlN-1.0wt%Mg offered the best corrosion resistance with lowest value of corrosion rate equivalent to 22.74 mm/y.