Introduction
Recently, crystallographers Paul Paufler and colleagues published a series of articles in Nature and other journals, describing what they saw when closely examining the medieval blade of a Damascus sword, known to have been produced by Islamic craftsmen using a raw material imported from the Indian subcontinent and known as wootz steel. Paufler spotted an arrangement of carbon atoms that they interpreted as carbon nanotubes. Paufler's work was described here at Archaeology at About.com, in these pages, at Damascus Steel - Ancient Technology and Modern Alchemy.
In this rebuttal Madeleine Durand-Charre asserts that although researchers made a difficult investigation resulting in interesting findings, she is doubtful about the formation of carbon nanotubes; contests the interpretation of the microstructure; and offers a different explanation and mechanism for the formation of the microstructure.
Durand-Charre, formerly of the Institut National Polytechnique in Grenoble, is the author of the Microstructure of Steels and Cast Irons, and currently translating a book published in French and entitled "Les aciers damassés. Du fer primitif aux aciers modernes". She may be contacted at mdc.damas@gmail.com.
At Issue
A sample taken from a blade of a genuine wootz steel sword was examined by HRTEM (high-resolution transmission microscopy) by German crystallographers, and reported in several papers [see listed references by Kochmann, Levin, Paufler, and Reibold, among others]. Wootz steel is high carbon steel, reinforced by fine carbide precipitation. The investigation was focused upon fine precipitates which were extracted by completely dissolving the matrix in a 20% solution of HCl over a period of one week. They detected fiber shaped carbide (nanowires), and these fibers appeared enveloped in a layer of carbon.
"We have now also detected carbon nanotubes in a specimen but the nanotubes only become apparent after dissolution of the sample in hydrochloric acid..." (Reibold et al. 2006). And the comment of the corresponding figure 1 in Reibold et al. 2006: "remnants show evidence of incompletely dissolved cementite nanowires, indicating that these wires could have been encapsulated and protected by the carbon nanotubes."
The authors suggested the formation of these nanotubes in steel: "Nanotubes can be formed catalytically, as well as from hydrocarbons inside micropores at reduced temperatures... Thermal cycling and cyclic forging cause catalytic elements to segregate gradually into planar arrays parallel to the forging plane. These elements may give rise to the growth of carbon nanotubes, which in turn initiate formation of cementite nanowires and coarse cementite particles." (Reibold et al. 2006)
An Alternate Explanation
However they do not propose any convincing scientific mechanism explaining the formation of these nanotubes. In addition, I understand that these precipitates that Paufler and colleagues are calling nanotubes are not empty tubes, but full rods.
Revisiting the papers I found a problem with the interpretation of the corresponding micrograph (Fig.1 in Kochmann et al. 2004) which does not exhibit at all a fine grained pearlitic microstructure as mentioned. Instead, my experience suggests that some areas have the characteristic features of lower bainite, and a few others the features of tempered martensite.
Lower bainite forms during the cooling process of austenite at temperatures below 400°C, above the temperature for martensite start. It results in the growth of sheaves of laths rather similar to martensite laths. What enables us to distinguish them from martensite laths is the precipitation inside the laths of tiny elongated precipitates which are oriented with an angle of about 50-60° from the habit plane defined as the interface plane between austenite and bainitic lath as measured on a macroscopic scale. This could explain the observation of rods. But why are they carbon coated?
Possible Explanations for Carbon Coating
The first answer could be an artifact resulting from the dissolution of the matrix, having etched the carbide surface and dissolved selectively iron atoms. Let us assume this has been avoided. The second proposition is the result of the sword having been stored at room temperature for several centuries and having undergone an aging process.
The bainitic constituents are out of equilibrium phases: oversaturated ferrite and metastable FexC coherent carbides (x is about 2.5). The lack of thermodynamic equilibrium induces the gradual approach of the structure to the equilibrium state involving pure ferrite and (carbon) graphite at a temperature that permit a sufficient diffusion rate for the atoms. Consequently the mechanism is governed by carbon diffusion, the only element concerned at room temperature. The transformation nucleates and develops at the interface between ferrite and carbides where a high concentration of vacancies and dislocations exists due to the interface misfit. The rate is very low at room temperature and diffusion operates at a very short range distance, a few atomic rows. The carbon atoms should arrange themselves optimizing the energy of the bonding network. Of course, this assumption of an aging process needs to be discussed and verified.
Lower bainite and tempered martensite develop very good mechanical properties of both good resistance and toughness. In the case of a blade, the famous cutting edge is the result of submicronic i.e. nano scale precipitates. Similar micrographs of fine carbide precipitation corresponding to tempered martensite in another wootz blade are presented in Durand-Charre et al (see in particular Fig.10). In modern steels the cooling rate is well controlled in order to obtain such suitable precipitation. Ancient smiths knew that cooling was the key to achieving good blades. They used many secret, legendary recipes for the cooling process, as for instance hot blades being thrust into the hollow trunk of a banana tree or given to a horse man galloping away.
Read more of this discussion of nanotechnology in Damascene steel swords.
- Damascus Steel, the original article
- Identifying Mechanical Structures in Damascus Sabers, comment by Peter Paufler
References
Durand-Charre M, Roussel-Dherbey F, and Coindeau S. 2010. Les aciers damassés décryptés. Revue de Métallurgie 107(04):131-143
Kochmann W, Reibold M, Goldberg R, Hauffe W, Levin AA, Meyer DC, Stephan T, Müller H, Belger A, and Paufler P. 2004. Nanowires in ancient Damascus steel. Journal of Alloys and Compounds 372(1-2):L15-L19.
Levin AA, Meyer DC, Reibold M, Kochmann W, Pätzke N, and Paufler P. 2005. Microstructure of a genuine Damascus sabre. Crystal Research and Technology 40(9):905-916.
Reibold M, Paufler P, Levin AA, Kochmann W, Pätzke N, and Meyer DC. 2006. Materials: carbon nanotubes in an ancient Damascus sabre. Nature 444(7117):286.
