Mechanical Properties of Bulk and Nanophase Materials

      Another major effort in the group over the past years has been the investigation of mechanical properties of composite materials. In the past year and a half, however, we have taken this effort to a new level by undertaking a collaboration with the group of Vijay Gupta in the Department of Mechanical and Aerospace Engineering here at UCLA. In this work, we have been examining how nanoscale architecture can control mechanical properties. The work involves applying tension to periodic silica/surfactant composite films to measure Young’s moduli and various elastic limits and failure points. The results are quite dramatic. Producing a nanoscale composite lowers the stiffness of a composite by approximately the expected amount – in our case to about 1/3 the original value for a material that is approximately 1/3 inorganic. The failure strain of the composite, however, is dramatically increased: from 0.08% for the bulk material to over 3% for the nanoscale composite. This remarkable increase in elasticity can be understood by three factors. First, the nanoscale architecture produces a new length scale for deformations that is not present in the bulk material. Second, the nanoscale periodicity prevents crack growth and formation of critical cracks. Finally, the periodic nature of the structure means that unlike disordered porous materials, the composites have no weak points that can cause materials failure. Our current results are an important step toward understanding how nanometer-scale architecture can be used to tune the mechanical properties of materials.

     In collaboration with the Kaner group, we also have a project aimed at establishing a new paradigm for ultra-hard materials. ¹ In this work, materials are synthesized according to a prescription of optimized electron density and maximal covalency. Our initial experiments with OsB₂ resulted in a material with a bulk modulus approaching that of diamond, and a hardness greater than sapphire. We are now working to use physical measurements of elastic moduli, and elastic and plastic deformation limits to refine these synthetic criteria in an effort to produce a new class of boride based ultra-hard materials.