The world was shocked by the disintegration upon reentry of NASA’s Columbia space shuttle on February 1, 2003. Until the Columbia Accident Investigation Board (CAIB, www.caib.us) has completed its report, both experts and laymen will continue to wonder exactly what happened to cause Columbia to break up attempting to complete its 28th space flight. Indeed, journalists and politicians are also adding their nontechnical opinions and laying the blame.
Perhaps it is inherent in human nature to be fascinated by disasters. James L. Robinson, the editor of the Journal of Metals, reports that the article with the highest on-line Science Citation Index in 2002 was “Why Did the World Trade Center Collapse” by T. W. Eager and C. Musso. In 1998, it was an article about the Titanic. I predict that interpretation of the Columbia disaster will be a future favorite.
A few years ago, my student Brian Cockeram and I were engaged in research to develop diffusion coatings to protect Ti and Ti alloys. If successful, we hoped to permit an increase in temperature for Ti blades in a gas turbine compressor. We demonstrated that silicided alloys, especially Ti-base substrates coated with Si-B and Si-Ge diffusion coatings, were very resistive to isothermal and cyclic oxidation. But the leading manufacturer of aircraft gas turbines would not adopt our protective coating process because “a man-rated vehicle may not rely for primary oxidation protection on a brittle protective coating”. This ‘rule’ makes good sense for commercial carriers because foreign objects, such as birds and tire parts, can enter the compressor and lead to blade and, ultimately, turbine failure.
For the nose cones and wing leading edges of the space shuttle, which experience the highest temperatures upon reentry, protection of the highly oxidizable carbon-carbon (C/C) composite structure is provided by a siliconized diffusion coating of SiC 500–1000 μm thick. The failure of Columbia seems to be a violation of the no brittle protective coating rule. On the other hand, without taking this risk, the NASA missions could not have happened at all.
The fact that the Columbia survived 27 ‘uneventful missions’ points to either an in-flight impact with some upstream spalled object — space junk or a meteor — or a degradative aging process, or a combination of these. Indeed, after ten shuttle flights, NASA engineers discovered pinholes (380–510 μm in diameter and 380–710 μm deep) in the protective SiC coating on the wing leading edges. The frequency and size of these pinholes were tracked and apparently some repair procedure was selectively applied.
With Nate Jacobson of NASA-Glenn, I helped to interpret the cause of the pinholes [NASA Tech. Mem. 106793 (1995)] and later predicted the consequences of a pinhole penetrating the SiC coating and reaching the C/C substrate [Carbon (1999) 37, 411]. We suggested that the pinholes arise because chloride sea salt spray contaminates tiny flaws on the wing surfaces while on the launch pad. Upon reentry, when the wings are subjected to an oxidizing plasma of 1700°C for 7–8 minutes, a progressive deepening of the pinholes could be supported by a vapor cycle whereby volatile SiCl2 species are formed at the base of the pit, releasing C to the substrate. As the SiCl2 reaches the mouth of the pit, oxidation would deposit a pile of amorphous SiO2 at the surface, while the released Cl (atoms) could return to the base of the pit for another cycle. Mathematical modeling based on diffusion control was consistent with the observed pit depths and SiO2 piles. Furthermore, after a pit reaches the C/C interior, oxidation could occur to form hemispherical voids at the base of the capillary-shaped pit. Thus, oxidation of the C/C matrix would not be catastrophic, but rather diffusion-controlled by the arrival of oxidant at the substrate and escape of CO2 to the surface. One possible cause of the shuttle disaster, among many, could have been a chance impact of some foreign object at an existing pinhole site, corresponding to a subsurface void. Such an impact might have led to a much larger entry hole for oxygen to reach the C/C matrix. An ensuing catastrophic oxidation would have resulted in destruction of the whole structure.
So, who or what is at fault in the Columbia failure? Perhaps just the passion of the human race to advance our knowledge by undertaking risky and heroic endeavors.