The effects of gravity on the combustion characteristics and microstructure of metal-ceramic composites (HfB 2 /Al and Ni 3 Ti/TiB 2 systems) were studied under both normal and low gravity conditions. Under normal gravity conditions, pellets were ignited in three orientations relative to the gravity vector. Low gravity combustion synthesis (SHS) was carried out on a DC-9 aircraft at the NASALewis Research Center. It was found that under normal gravity conditions, both the combustion temperature and wave velocity were highest when the pellet was ignited from the bottom orientation; i.e., the wave propagation direction was directly opposed to the gravitational force. The SHS of 70 vol pct Al (in the Al-HfB 2 system) was changed from unstable, slow, and incomplete when ignited from the top to unstable, faster, and complete combustion when ignited from the bottom. The hydrostatic force (height ϫ density ϫ gravity) in the liquid aluminum was thought to be the cause of formation of aluminum nodules at the surface of the pellet. The aluminum nodules 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 TiB 2 particles and sedimentation of the Ni 3 Ti phase were observed for the Ni 3 Ti/TiB 2 system. The possibility of liquid convective flow at the combustion front was also discussed. Under low gravity conditions, both the combustion temperature and wave velocity were lower than those under normal gravity. The distribution of the ceramic phase, i.e., TiB 2 or HfB 2 , in the intermetallic (Ni 3 Ti) or reactive (Al) matrix was more uniform.

Combustion synthesis of intermetallic−ceramic composite materials usually involves high combustion temperature as well as high temperature gradient. Gravity may play an important role in such systems when the liquid phase is present, and the intermetallic and ceramic phases have different densities. The combustion characteristics and microstructure may both be affected by gravity. Combustion synthesis of (1 − x)Ni3Al + xTiB2 composites, with the weight fraction x varying from 0.05 to 0.8, was carried out under normal gravity conditions. Both the combustion temperature and wave propagation velocity increased, and the propagation mode changed from unstable (x ≤ 0.2) to stable (x ≥ 0.4), as the TiB2 content increased. The combustion temperatures were higher than the melting point of Ni3Al for samples with x ≥ 0.4, resulting in a composite material consisting of ceramic TiB2 particles dispersed in a Ni3Al matrix. Owing to buoyancy of TiB2 particles in the denser molten Ni3Al phase, gravity was found to affect the microstructure of the composite, yielding a nonuniform distribution of phases. The phase separation distance calculated by using Stokes' law compared well with measurements.

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Combustion synthesis (SHS) of Ni 3 Ti-TiB 2 metal matrix composites (MMCs) was selected to investigate the effect of gravity in a reaction system that produced a light, solid ceramic particle (TiB 2 ) synthesized in situ in a large volume (Ͼ50 pct) of the liquid metallic matrix (Ni 3 Ti). The effects of composition, green density of pellets, and nickel particle size on the combustion characteristics are presented. Combustion reaction temperature, wave velocity, and combustion behavior changed drastically with change in reaction parameters. Two types of density effects were observed when different nickel particle sizes were used. The structures of the combustion zones were characterized using temperature profile analysis. The combustion zone can be divided into preflame, reaction, and afterburning zones. The combustion mechanism was studied by quenching the combustion front. It was found that the combustion reactions proceeded in the following sequence: formation of liquid Ni-Ti eutectic at 940 ЊC → Ni 3 Ti ϩ NiTi phases → reduction of NiTi with B → TiB 2 ϩ Ni 3 Ti.

This article discusses the unique material manufacturing process of self-propagating high temperature synthesis (SHS) as applied to the making of porous biomaterials. Porous materials have long been considered as the first step toward in-vivo bone tissue engineering and the creation of patient life-time implants. The authors have approached this challenge by utilizing combustion synthesis, to create novel materials such as NiTi + TiC as well as porous forms of materials that are commonly accepted for biomedical applications such as tricalcium phosphate and hydroxyapatite. In the SHS product, physico-chemical properties are controlled by, but not limited to, reactant stoichiometry; green density; particle size of the reactant mix; use or presence of a gasifying agent; heating rate of the reactants and gravity. By balancing these parameters, the energy of the reaction is controlled to create the desired product stoichiometry, porosity, and mechanical properties. SHS provides a means to rapidly manufacture materials, saving time and production costs as well as enabling the synthesis of custom devices through the use of individual molds. Mold materials can range from graphite to paper or paper machete. Combustion synthesis offers a method for the rapid manufacture of affordable, individual biomedical devices that will reduce patient recovery time.