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Damascus Steel: A Premier in Material Science and Nanoengineering

 

By Allison Kubo Hutchison
Reproduced Wootz Damascus blade showing both a ladder and rose pattern made by Alfred Pendray. Photo by JD Verhoeven, AH Pendray and WE Dauksh.

Material science and nanoengineering are emerging fields promising to revolutionize the industry, medicine, and energy technologies. But our understanding of both is rooted in ancient knowledge. Would it surprise you to know that we had knowledge of and used nanoparticles as far back as 400 CE? Roman technologists were using gold-silver nanoparticles to make colored glass and the Mayans used indigo dyes stored in clay nanopores to make brilliant long-lasting pigments. One of the most impactful of these technologies was Damascus steel developed in 300 CE in the Middle East. 

 Although you may have seen the beautiful wavy patterns in kitchen knives, this is not ancient Damascus. The modern “Damascus steel” is formed by welding different pieces of steel together then folding it together. The technique to make true damascene blades was lost around 1750 although one possible method was rederived in 1998. However, the blades exist today and the legends of their quality and durability drove metallurgic study in Europe in an attempt to match them. Modern metallurgy produces higher quality steels but Damascus blades were the best possible blades for over a thousand years. Damascus steel produced in the third century was known for its incredible resilience to bending while maintaining a wickedly sharp edge.

Carbon nanotubes are the source of these valued properties. The carbon nanotubes, the same ones that cost $1500 per gram today and are the subject of intense nanotechnology research, allow for incredible tensile strength while maintaining an edge. High carbon content in the sword can be traced back to the ingots the swords were made from called Wootz. Wootz was imported from Southern India (modern-day Sri Lanka and Tamil Nadu). Wootz was produced by heating ore in a sealed clay crucible with plant fiber inside as the source of carbon. Just adding fallen leaves to iron isn’t a recipe for good quality steel, it must be in the correct portions and also forged at the correct temperatures to produce optimal results. Iron and carbon when mixed form a solid solution: mixing at the atomic level forming complex crystal structures. The temperature and rate affect the crystal structure and the structure controls the tensile properties of the steel.

Steel shown under reflected light microscopy with cementite (dark) embedded parallel in ferrite (white) crystals. Photo by Professor TW Clyne, University of Cambridge.
 
It is desirable to have multiple types of steel structures such as austenite and ferrite in a blade. Different crystal structures which are stable at different temperatures and compositions can be achieved through thermal cycling, multiple rounds of heating and cooling. This process is difficult to achieve since it requires precise temperature control. Wootz steel was also found to have trace impurities (as low as 40 parts per million by weight (ppmw)) of rare-earth elements such as vanadium and molybdenum. The impurities cause certain crystal structures to form preferentially in-band rather than randomly. These impurities and the presence of carbon nanotubes facilitated the banding of different steel crystals particularly cementite which is very hard but brittle. However, in the damascene blades, the cementite formed nanowires, less than a nanometer-thin, that span the blade adding hardness but allowing for flexibility. The microstructure which was interwoven in the blade-like rebar in cement strengthened the steel. This was also the source of the mesmerizing pattern made by the most skilled bladesmiths such as Assad Ullah although other smiths could achieve high metallurgic quality without achieving the sought-after patterns. The patterns were emphasized and made to stand out using acid etching to cause the cementite to appear black on the white steel matrix.

While modern Damascus is made by welding together metals of different colors, the wavy patterns of ancient Damascus were formed by crystalline structures although both are beautiful. The exact way of making Damascus blades was lost around the middle 18th century. It could have been that they lost access to the special ores which contained the trace impurities necessary for the formation of the nanowires or that the difficult and specific heat cycling process was not passed down. The attempt to recreate the Damascus steel produced throughout India and the Middle East was important in driving the development of metallurgy as science and still provides insight today.

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