FUNDAMENTAL-STUDIES OF THE INFLUENCE OF BORON ON THE GRAPHITE-OXYGEN REACTION USING IN-SITU ELECTRON-MICROSCOPY TECHNIQUES

Controlled atmosphere electron microscopy coupled with in situ electro n diffraction has been used to follow the manner by which the addition of boron oxide influences the graphite-oxygen reaction. Continuous ob servations of the process show that at about 450-degrees-C the boron o xide undergoes a strong interaction with both the graphite edge and th e basal plane regions, and this results in a spreading of the oxide to form a uniform thin adherent film over the entire substrate. The coat ed graphite specimens appear to be impervious to attack by oxygen at t emperatures below 815-degrees-C. Above this temperature, however, ther e is a weakening of the additive-graphite interaction with the ''armch air'' {1120BAR} faces, and these regions then become vulnerable to att ack by oxygen. At the same time very shallow pits are observed to deve lop in the basal plane, and this action coincides with the appearance of boron carbide species in the electron diffraction pattern. In a com plementary series of experiments, it is found that boron carbide is an extremely active catalyst for the graphite-oxygen reaction even at te mperatures as low as 100-degrees-C. The impact of these low pressure s tudies on the behavior of carbon structures used in aerospace applicat ions is discussed.