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Hard And Tough Materials At The Nanoscale

Max Planck scientists prove that the extreme strength of biomaterials is due to an as yet unknown flaw tolerance threshold in the nanometer range

The nanoscale size of mineral particles in bone, tooth, and other biological materials may have evolved to ensure optimum strength and maximize tolerance of flaws, according to new research conducted at the Max Planck Institute for Metals Research and Austrian Academy of Sciences.

While it is quite clear that the composite character of biological materials plays an important role, little is known about the role of the nanometer scale of mineral particles. The new study provides further insight into the mystery how nature produces hard and tough materials out of protein as soft as the human skin and mineral as fragile as the classroom chalk.

The key finding of this research is that there exists a critical nanometer size at which the particles found in biocomposites become insensitive to flaws: They maintain strength equivalent to a perfect crystal despite inherent defects. This phenomenon also suggests that the engineering concept of stress concentration at flaws is no longer valid for nanoscale design. (Gao et al., PNAS, 2003, Vol. 100, No. 10, pp. 5597-5600.)

The science and technology of materials have set milestones of human civilization. We have mastered the art of making a wide variety of materials with many interesting mechanical properties. Some materials like ceramics, glasses and mineral crystals are hard and fragile. Others like rubbers and collagen are soft and tough.

During the Bronze and Iron ages, man learned to make hard and tough materials in the form of various metals and metallic alloys. With the maturity of the field of metallurgy during the 20th century, we now understand that the hardness and toughness of metals are largely attributed to their crystalline structure which allows an important class of material defects called dislocations to move around and relieve stress concentration at crack-like flaws.

One of the next big challenges for humanity is to develop hard a