THE EFFECT OF GRAVITY ON THE COMBUSTION SYNTHESIS OF METAL-CERAMIC COMPOSITES

The effects of gravity on the combustion characteristics and microstru cture of metal-ceramic composites (HfB2/Al and Ni3Ti/TiB2 systems) wer e studied under both normal and low gravity conditions. Under normal g ravity conditions, pellets were ignited in three orientations relative to the gravity vector. Low gravity combustion synthesis (SHS) was car ried out on a DC-9 aircraft at the NASA-Lewis Research Center. It was found that under normal gravity conditions, both the combustion temper ature and wave velocity were highest when the pellet was ignited from the bottom orientation; i.e., the wave propagation direction was direc tly opposed to the gravitational force. The SHS of 70 vol pCt Al (in t he Al-HfB2 system) was changed from unstable, slow, and incomplete whe n ignited from the top to unstable, faster, and complete combustion wh en ignited from the bottom. The hydrostatic force (height x density x gravity) in the liquid aluminum was thought to be the cause of formati on of aluminum nodules at the surface of the pellet. The aluminum nodu les that were observed on the surface of the pellet when reacted under normal gravity were totally absent for reactions conducted under low gravity. Buoyancy of the TIE, particles and sedimentation of the Ni,Ti phase were observed for the Ni3Ti/TiB2 system. The possibility of liq uid convective flow at the combustion front was also discussed. Under low gravity conditions, both the combustion temperature and wave veloc ity were lower than those under normal gravity. The distribution of th e ceramic phase, i.e., TiB2 or HfB2, in the intermetallic (Ni3Ti) or r eactive (Al) matrix was more uniform.