Research Of Microstructure And Properties Of Low Temperature Isotropic Pyrocarbon Coating

Low temperature isotropic pyrocarbon(LTIC)has a good biocompatibility and excellent mechanical properties,especially silicon deposited together with pyrolytic carbon,which improve its strength and abrasion resistance,so LTIC is considered to be the best material for manufacture of artificial mechanical heart valves.At present,the domestic LTIC coating often appears defects of crack,hole and stratification,which the development process of the domestic high quality total carbon artificial mechanical heart valves.The process of preparing LTIC is greatly influenced by the process parameters,and under different deposition conditions,the obtained LTIC structure is multiple,and the properties of different structures is also different.Only by understanding the relationship between the deposition condition and the structure and properties,the needed LTIC can be deposited by adjusting the deposition parameters.Therefore,studying the structure and properties of LTIC prepared under different deposition conditions will help clarify the reason of the defects,which is the key to improve the quality and properties stability of the LTIC.In this paper,the LTIC coating is prepared by the method of quasi-steady-state chemical vapor deposition in the deposition temperature of 1250? and propane concentration between 25%~60%.The microstructure of the LTIC was investigated using multi-scale characterization,such as X-ray diffraction,scanning electron microscope,transmission electron microscope,and polarizing microscope,and the microstructure of the LTIC prepared under different deposition conditions was observed and analyzed by high resolution scanning electron microscope and transmission electron microscope,meanwhile the effect of propane concentration on the microstructure of LTIC was also studied.Then,the properties of thermal expansion coefficient,microhardness and silicon content were tested by dilatometer,nanoindentation and energy dispersive spectroscopy,and the effect of propane concentration on the microhardness and silicon content of LTIC was also studied.Finally,the model of LTIC coating was simulated by ANSYS software.The main contents and conclusions of this paper can be divided into the following sections:(1)The microstructure of the LTIC was investigated using multi-scale characterization.Result shows that: the prepared LTIC coating is composed of globular features around 0.5~1?m,and the globular features are connected by lamellar structure carbon.The globular feature is composed by two kinds lattice fringes,namely ?-SiC and turbostratic structure pyrocarbon,and the globular feature has a fine core-shell structure.(2)The effect of propane concentration on the microstructure of LTIC was studied.Result shows that: with the increase of propane concentration,the five-membered rings in grapheme plane gradually increase,which makes the texture of globular-like structure reduce.In addition,the size of globular-like structure is influenced by the concentration of the pyrolysis product and the supersaturation vapour of polycyclic aromatic hydrocarbons,and the supersaturation vapour of polycyclic aromatic hydrocarbons also has an effect on the number of globular-like structure.(3)The properties of LTIC coating were tested,and the effect of propane concentration on the properties was also studied.Result shows that: the thermal expansion coefficient(CTE)of LTIC coating is smaller than the graphite matrix,and the CTE of coating/matrix is the result of the interaction between the coating and the matrix.The microhardness value of the prepared LTIC is in around 2.396 GPa ~ 3.044 GPa,and silicon content of LTIC is around 10.7 wt% ~ 22.1 wt%.Meanwhile,the microhardness and silicon content decrease gradually with the increase of propane concentration.(4)The properties and distribution of residual stress in the coating were analyzed.Result shows that: the model is reasonable and can be used to simulate and analyze the distribution of the residual stress in the coating.The surface of the coating is affected by tensile stress,and stress concentration in the edge region of the coating interface.With the increase of the thickness of coating,the maximum value of the radial stress and shear stress at the interface of the coating decreases first,then increases.while the axial stress at the interface is not affected by the thickness of the coating.The maximum principal stress in the coating mainly occurs in the intersection region of the coating and the substrate side,the upper surface of the coating,and the outer edge region of the coating,and the maximum principal stress value increases with the increase of the coating thickness.When the corner of the matrix is circular arc,it can effectively improve the stress concentration in the corner.