Microstructural, phase, hardness, and oxidation resistance studies of AlN/h-BN-reinforced Ti6Al4V matrix composites synthesized by spark plasma sintering

One of the identified shortcomings militating against the widespread application of Ti6Al4V alloy in the aerospace sector of the economy includes its unsatisfactory hardness and proneness to high-temperature oxidation which in turn adversely impact its abrasion resistance properties in certain service conditions. The work reported in this paper forms an important aspect of the effort put together in addressing these composite issues. Precisely, the effects of the nano-reinforcements of aluminum nitride (AlN) and hexagonal boron nitride (h-BN) on the microstructure, phase contents, hardness, and oxidation resistance of Ti6Al4V-based composites were considered. The prepared powders were sintered by spark plasma sintering (SPS), an innovative sintering method which enables the implementation of fast heating and cooling rates. Thereafter, scanning electron microscopy, optical microscopy, and X-ray diffractometry were employed to observe and study the microstructure and phase contents, Archimedes’ principle was applied to evaluate the densification, Vickers’ microhardness test was used to measure the hardness values, and thermogravimetric analysis technique was used to study the oxidation resistance of the sintered composites. A homogeneous blend of the nano-reinforcements of 1.5 wt% each of AlN and h-BN gave the highest relative densification value of 99.77% while the hardness value (656.01 HV) was less than that of Ti6Al4V-3h-BN (716.80 HV) which gave more credence to the phase contents in the microstructure as the most influencing factor determining the hardness property of the composites than ordinary relative densification. Meanwhile, Ti6Al4V-1.5AlN-1.5h-BN with the minimum positive weight change (0.36 mg/cm2) exhibited the greatest thermal oxidation resistance in air.