Effects Of Deposition Conditions On Microstructures Of Low Temperature Isotropic Pyrocarbon

Low temperature isotropic pyrocarbon(LTIC) possess low anisotropy degree, dense structure,small grain size and uniform mechanical properties, so it not only has common advantages ofgeneral carbon materials, such as high temperature resistance, wear resistance and so on, but alsohas high strength, excellent sealing performance and biocompatibility. At present, LTIC is widelyused as coating materials in clinical medicine. But different deposition conditions lead to LTIC withdifferent structures, while the coatings performance is determined by structures. Therefore, themicrostructure of LTIC is the bridge to contact deposition parameters and material properties, andalso the key to coatings quality and performance.In this paper, the quasi-steady-state fluidized bed chemical vapor deposition (FBCVD) wasused to prepare low temperature isotropic pyrocarbon at deposition temperature from1250to1450℃with propane at different concentrations. Microstructural information and structure unit of lowtemperature isotropic pyrocarbon coatings obtained were described and analyzed by using X-raydiffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM)and polarized light microscopy (PLM). On the basis of characterizations mentioned above, theeffect of deposition temperature and propane concentration on the density, fracture morphology andtexture of the LTIC has been investigated. According to the results analyzed above, pyrocarbonformation mechanism research status were summarized and referred, the chemical vapor deposition(CVD) process was investigated, on base of which LTIC formation mechanism was discussed andLTIC droplet deposition model was established. What’s more, the effect of deposition temperatureand propane concentration was explored. The main contents and conclusions of this paper can bedivided into the following sections:(1) The microstructure of LTIC coatings has been analyzed and characterized. The resultsshow that the LTIC obtained is principally stacked and coalesced by spherical particles that are lessthan2μm in diameter. These spherical particles only composed of two kinds’ lattice fringes, namelyβ-SiC in regular arrangement and turbostratic structure pyrocarbon. There exists a growth conestructure transition layer between LTIC and graphite substrate, which caused by substrate surface"memory effect". Fractured spherical particle revealed an inner onion-like structure with graphitecrystalline nuclear-laminated shell structure.(2) The effect of deposition temperature and propane concentration on the density of LTIC hasbeen studied. The results show that the density of LTIC obtained arranged from1.65g/cm3to2.03g/cm3.Reducing deposition temperature or propane concentration could improve the density ofLTIC, and the influence of deposition temperature on LTIC density is more significant than propane concentration.(3) The effect of deposition temperature and propane concentration on the fracturemorphology of LTIC has been studied. The results show that the influence of propane concentrationon LTIC fracture morphology is more significant than deposition temperature. As the depositiontemperature and propane concentrations were raised, the spherical particles fusion degree and rangeis reduced, the number of globular-like particles is increased, and the stacking way is changed fromdense and uniform wrinkled laminar type structure to incompact “grape cluster” type structure withmore pores and less density. And the globular-like fracture structure of LTIC was more and moreobvious with the deposition process mode changes from growth mode to nucleation mode andnucleation rate is increased.(4) The effect of deposition temperature and propane concentration on the texture of LTIC hasbeen studied. The results show that as the deposition temperature and propane concentrations wereraised, the length and tortuosity of lattice fringes inner globular-like particles is increased and theorientation turn to disorder gradually with the texture decreased, which are correlated to variationsin the tortuosity of the aromatic carbon layers.(5) In terms of deposition temperature and propane concentration, the selection and control ofdeposition conditions of LTIC has been studied. The results show that the higher the density, thedenser of LTIC coatings, and the higher the nucleation rate, the more isotropic of LTIC. Ifdeposition temperature or propane concentration is too low, it’s hard to increase the nucleation rate,whereas if deposition temperature or propane concentration is too high t, it’s hard to increase thedensity.(6) LTIC formation mechanism has been explored, LTIC droplet deposition model has beenestablished and the effect of deposition temperature and propane concentration on themicrostructure of LTIC has been explained. The results show that the change of the morphology ofLTIC can be determined by the change of deposition mode of LTIC with deposition conditions,which were correlated to variations in the proportion of linear molecules and small aromatichydrocarbons to polycyclic aromatic hydrocarbons (PAHS), besides the viscosity of the droplets.