Primary atomic structure of pyrocarbons is close to atomic arrangement of natural graphite (stacks of hexagonal atomic planes). Further microstructure formation critically depends on pyrolysis conditions. Low-temperature (LT) pyrolysis gives different types of microstructure but only isotropic pyrocarbon is of special interest. The exceptional mechanical properties in combination with chemical and biological inertness have determined the main field of isotropic pyrocarbon application: the material is employed for manufacturing cordial prosthetic devices. Cardiovascular applications impose hard demands on pyrocarbon materials. Reliability of prosthetic devices depends on microstructure of pyrocarbons and presence of microdefects, on local mechanical properties and their distribution over the specimen body. We have developed high frequency focus ultrasound methods for evaluating local mechanical properties and microstructure, for visualizing defect distribution inside the material body and non-destructive inspection of material to be employed for prosthesis production.
Microstructure characterization has been performed by acoustic imaging of structural elements inside the material body. The layered structure resulted from a large-scale microscopic ordering has been observed. Acoustic microscopy technique has been employed for revealing inclusions, cracks and grain structure. Layer-by layer imaging provides 3D reconstruction of matter structure.
The elastic characteristics of pyrocarbons have been measured by microacoustical technique. Despite the substances being considered as isotropic materials, the ultrasonic measurements revealed elastic anisotropy. It seems the anisotropy is correlated with occurrence of large-scale ordering in pyrocarbons. A part of specimens reveals an elastic orthotropy (transversal isotropy), while others demonstrate a more complicated anisotropy. A complete set of elastic moduli has been measured for the orthotropic material.