Material properties of diamond

Diamond is the allotrope of carbon in which the carbon atoms are arranged in the specific type of cubic lattice called diamond cubic. It is a crystal that is transparent to opaque and which is generally isotropic (no or very weak birefringence). Diamond is the hardest naturally occurring material known. Yet, due to important structural brittleness, bulk diamond's toughness is only fair to good. The precise tensile strength of bulk diamond is little known; however, compressive strength up to 60GPa has been observed, and it could be as high as 90GPa in the form of micro/nanometer-sized wires or needles (~100nm in diameter, micrometers long), with a corresponding maximum tensile elastic strain in excess of 9%. The anisotropy of diamond hardness is carefully considered during diamond cutting. Diamond has a high refractive index (2.417) and moderate dispersion (0.044) properties that give cut diamonds their brilliance. Scientists classify diamonds into four main types according to the nature of crystallographic defects present. Trace impurities substitutionally replacing carbon atoms in a diamond's crystal structure, and in some cases structural defects, are responsible for the wide range of colors seen in diamond. Most diamonds are electrical insulators and extremely efficient thermal conductors. Unlike many other minerals, the specific gravity of diamond crystals (3.52) has rather small variation from diamond to diamond. Known to the ancient Greeks as ἀδάμας (adámas, 'proper, unalterable, unbreakable') and sometimes called adamant, diamond is the hardest known naturally occurring material, and serves as the definition of 10 on the Mohs scale of mineral hardness. Diamond is extremely strong owing to its crystal structure, known as diamond cubic, in which each carbon atom has four neighbors covalently bonded to it. Bulk cubic boron nitride (c-BN) is nearly as hard as diamond. Diamond reacts with some materials, such as steel, and c-BN wears less when cutting or abrading such material.

Micro-mechanics of 3D architectured metals synthesized by electrodeposition

Maxime Alain Baptiste Mieszala

Cellular metallic materials have emerged as a new promising class of materials due to their lightweight porous structures and advanced multi-functional properties. Originally limited to random metallic foams, modern lithography techniques have enabled the design of hierarchical cellular structures with tailored mechanical properties. In addition, the mechanical properties of micro-cellular metals can be further improved by taking advantage of the properties of metals, which are size-sensitive at this length scale. It is therefore expected that the combination of architectured cellular designs with size effects in metals would result in materials with unprecedented mechanical properties. The present thesis aims at investigating and understanding the influence of size effects on the mechanical properties of three-dimensional micro-architectured metal structures synthesized using advanced lithography techniques and electrochemical deposition of metals. Electrodeposited copper films with engineered microstructures and nickel-based hierarchical micro-cellular structures were manufactured and tested under uniaxial loading conditions. The results of the tests were used to evaluate the contribution of the size effects and/or architecture to the change in mechanical properties and deformation behavior. The influence of the intrinsic size effect was first demonstrated in electrodeposited pure copper films. Using nanoscale twins to engineer the grain boundaries, it was possible to produce copper films with high strength and significantly tune the mechanical properties by adjusting the twin orientation. The combination of size effects was then investigated for two types of micro-architecture. The first type of metal micro-architecture was obtained by plating nanocrystalline Ni inside a self-assembled colloidal crystal of microspheres in order to produce a regular inverse opal. The second type was fabricated by metallizing a 3D polymer template produced by 3D laser lithography with an amorphous NiB coating. Results show that there is a complex interaction between the characteristic external dimension and cellular structure. For regular Ni inverse opal, there is a complex interaction between the ligament size and the grain size which defines the overall strength of cellular metals. In the case of hybrid NiB/polymer structures, the brittle-to-ductile transition in the amorphous NiB coating and the architecture effect are coupled. By varying the NiB thickness and the architecture geometry, it is possible to control the deformation mechanism from global buckling to brittle failure and to tune the energy absorption characteristics of the micro-architectures. Through these case studies, it was demonstrated that the combination of grain boundary engineering, sub-micron geometrical features and overall architecture can yield 3D metallic micro-architectures, which are both light and strong and have tailored mechanical properties. The mechanical properties of some of these materials exceed the properties of either of the parent materials. Therefore, the constitutive mechanical laws for porous metals should be modified to account for these three effects all together. The results confirmed the potential of our experimental approach and provide a practical way to extend the material property-space.

EPFL2018

Micro-mechanics of 3D architectured metals synthesized by electrodeposition

Maxime Alain Baptiste Mieszala

EPFL2018