Ceramics and Composite Materials

Biography

Biography: Ghatu Subhash

Abstract

Ultrahard ceramics mainly stem from two structural forms: diamond-like and icosahedral. While diamond-like structures (e.g., naturally occurring diamond and c-BN) hold the microhardness record at over 100 GPa, icosahedral boron-rich solids have attracted considerable attention in recent years due to their strong thermal/chemical stability and an excellent combination of high hardness (~40 GPA) and low mass density (~2.5 g/cm3). Boron carbide (B4C) and boron suboxide (B6O) are two popular ceramics in this class of materials and are preferred candidates for impact and wear resistance applications. While their structure mainly consists of a 12-atom icosahedron and few atoms that bond to the icosahedron, the arrangement of these atoms and their chemical nature controls their deformation mechanisms. For e.g., B4C has a 3-atom chain attached to the equatorial atoms of the icosahedron, B6O does not have a chain but one oxygen atom bonded on each side of the icosahedron. Due to the closeness in atomic radii of boron and carbon, B4C exhibits polymorphisms where a carbon can substitute for boron and potentially yield more than 200 polymorphs. On the other hand, B6O has no polymorphs. Thus B6O is structurally more homogeneous than B4C. These structural differences influence their properties and deformation mechanisms. Both materials have high hardness (>30 GPa), low density (2.52 g/cm3 for B4C and 2.6 g/cm3 for B6O), high compressive strength (up to 5 GPa), moderate fracture toughness (3.4 MPa·m1/2 for B4C and 4.2 MPa·m1/2 for B6O) and exhibit amorphization (localized crystal structure collapse) under high pressure loads. But, amorphization in B4C can be detected in Raman spectroscopy (through appearance peaks beyond 1200 cm-1), B6O does not show any new peaks due to amorphization. While the 3-atom chain bending has been proposed as the main mechanism for amorphization in B4C, the lack of chain structure in B6O raises new questions as the root-cause of amorphization in this material. Similarly, B4C has been found to occasionally undergo deformation twinning but B6O has been shown to undergo ‘nanotwinning’ (twin spacing of nm scale) even in virgin state and has been theorized to provide extremely high hardness if the entire specimen undergoes nanotwinning at critical twin spacing of two atomic planes. In this research, a coordinated experimental, spectroscopic, microscopic, and quantum mechanical investigations are performed to provide fundamental insight into the above issues. Finally, implications of these structural, behavioral and bonding differences during high pressure dynamic deformation will be discussed. The long-term goal of this research is to identify novel avenues for designing of ultrahard materials with tailored properties.