Surface characterization and influence of pyrolysis temperature on microstructure, phase and oxidation kinetics of CVD pyrolytic graphite coatings

The pyrolytic graphite (PyG(1)) coatings have been synthesized on high-density graphite (HDG(2)) substrates by using chemical vapor deposition (CVD3) at 1800, 2000 and 2200 degrees C with propane as hydrocarbon source at constant gas flow rates and chamber pressure. Surface characterization and the influences of pyrolysis temperature on the density, phase, chemical purity, and structural aspects have been studied by using sink and flotation technique, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, polarized light optical microscopy and scanning electron microscopic techniques. The results reveal that the change in density, lattice parameters, disorderness, graphitization, and microstructures of PyG coatings were strongly controlled and dependent on the pyrolysis temperature. Higher pyrolysis temperature results in an increase in the density, degree of graphitization and finer columnar grown microstructural features with relatively small cone angles. The kinetics of nucleation, growth mechanism and its temperature dependence is explained by using the Volmer-Weber 3D island growth model. The thermal stability of PyG coatings studied by linear oxidation up to 1000 degrees C using thermogravimetric analysis under a simulated open atmosphere revealed the higher onset of oxidation (830 degrees C) and lower weight loss (36%) for PyG synthesized at 2200 degrees C.