Research On Microstructure And Properties Of Low Temperature Isotropic Pyrolytic Carbon

Low temperature isotropic pyrolytic carbon is considered as the best coating material for making artificial heart valves due to its low elastic modulus and high flexural strength,good abrasion resistance,chemical inertness,anti-coagulation,biocompatibility and so on.Usually take the method of chemical vapor deposition(CVD)to prepare Low temperature isotropic pyrocarbon,the process included the pyrolysis of hydrocarbons in gaseous environment,a series of complex chemical reactions,final deposition on hot substrate.Due to the complexity of deposition process and deposition mechanism,the low temperature isotropic pyrocarbon coating prepared through CVD probably accompanied by some defects,which restricted the progress of the production and research of artificial mechanical heart valve.Therefore,the study of microstructure and properties of low temperature isotropic pyrocarbon has a great significance for solving the defects of artificial mechanical heart valve.In this paper,low temperature isotropic pyrocarbon samples were obtained by steady state fluidized bed chemical vapor deposition at the deposition temperature of 1 250 ?~1450 ? and the volume concentration of propane 25%~60%.Characterized and analyzed the microstructure of low-temperature isotropic pyrolytic carbon by X-ray diffraction(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM),discussed the deposition mechanism of low temperature isotropic pyrolytic carbon,and using the three point bending method to test samples' bending strength and fracture toughness which prepared at 1350 ? deposition temperature and 40% propane volume,measured the microhardness of pyrolytic carbon which prepared at different deposition conditions.At last,measured the thermal expansion coefficient of pyrolytic carbon coating which prepared at 1350? deposition temperature,40% propane and graphite,combined with finite element software ANSYS analyzed the distribution of residual thermal stress in coating.The main contents and results are as follows:(1)In X-ray diffraction,as for low temperature isotropic pyrolytic carbon under different deposition conditions,all the diffraction peaks of 002 carbon layer are stronger than other layers and with a certain extent wide,their 2? are about 26o,combined Prague formula calculate their crystallite size,the result shows that the layer distance of(d002)plane all are among 3.455 ? and 3.495 ?,which is higher than the d002 value(3.354 ?)of graphite crystal,in accordance with the characteristics of turbostratic graphite structure.(2)By SEM and TEM characterization,the result show that the low temperature isotropic pyrocarbon is composed of lamellar carbon and spherical particles,and the pyrolytic carbon spherical particles are not truly sphere,the core is formed by a small amount of tobacco particles gathered together,the surface of spherical particles is clad by clutter carbon layers;The crystal lattice stripe of samples arranged in disorder,bending and frequent interlacing,small microcrystalline size and disorder in long-range,in accordance with the structural characteristics of isotropic pyrocarbon.With the increase of deposition temperature and volume concentration of propane gas,in fracture morphology of pyrocarbon the spherical particles become thinner,increase in quantity,lamellar structure reduce or even completely disappear,the lattice fringes are becoming more and more disorder,orientation decreases,isotropic degree increases.(3)At deposition temperature is 1350 ?,volume concentration of propane gas is 40%,measured the bending strength of low temperature isotropic pyrolytic carbon,the value is about 390~450 MPa,and the fracture toughness of low temperature isotropic pyrolytic carbon coated on graphite substrate is about 2.97 MPa (?).Measured the microhardness of low temperature isotropic pyrolytic carbon sample at different deposition conditions,the value is among 1383 MPa and 3 044 MPa,its hardness decrease with the increase of the deposition temperature and volume concentration of propane gas,it is related to the microstructure and pore inside.(4)By thermal expansion instrument measured the thermal expansion coefficient of low temperature isotropic pyrolytic carbon coating,which prepared at 1350? and 40% propane concentration,the value is among 2.97×10-6/?~9.98×10-6/?,and also measured the thermal expansion coefficient of graphite substrate,the value is among 5.23×10-6/?~10.99×10-6/?.And combined building a reliable ANSYS finite element model,the results show that in the coating mainly exists radial stress,in the area intersected by the interface and the outer edge found stress concentration;With the increase of coating thickness,the first principal stress in coating is mainly concentrated near the upper surface of the coating and the outer edge of the interface,the maximum principal stress of the coating increases with the increase of the thickness of the coating,and the increment becomes stable after the coating thickness reaches 0.5 mm.