Graphitization Behavior Of CVD Free-standing Diamond Films At High Temperature

CVD free-standing diamond film is a potential window material for infrared transmission.The diamond window,however,can be heated to an ultra-high temperature instantaneously under extreme environment,typically in hypersonic flight conditions.Thermal induced the transformation of diamond to graphite results in attenuation or even failure in optical properties of the diamond material.Therefore,it is necessary to study the graphitization behavior of CVD free-standing diamond films at high temperatures.In this paper,the free-standing diamond films prepared with DC arc plasma jet chemical vapor deposition method were heat treated by rapid heating process throuth argon-hydrogen plasma arc.The graphitization behavior of diamond film in transient high temperature environment was systematically studied.The results show that,(1)Under argon-hydrogen plasma conditions,the initial temperatures of surface graphitization and macroscopic graphitization of CVD free-standing diamond films are 1450 ? and 1800 ?,respectively.And the graphitization rate is faster on the nucleation side which has more grain boundaries than the growth side.(2)The higher the heat treatment temperature,the greater the decrease of infrared transmittance.Compared with graphitization on surfaces,graphitization at grain boundaries is the main reason for the degradation of optical properties.The full band absorption of graphite on the infrared spectrum causes the decrease in infrared transmittance,while the scattering of light by graphite crystallites is a secondary cause.(3)The binding energy(EB)of sp3 carbon centered at 285.6 eV in the C1s photoelectron spectra,and the difference of the binding energy bentween sp3-and sp2-bonded components is 1.35 eV.An additional peak appears at the shift of 0.8 eV lower than the peak position of sp3.This component is caused by the reconstruction at diamond surface and disordered atomic arrangement at grain boundary.Through curve fits,we obtained the ratio between sp3-and sp2-bonded carbon after heat treatment at different temperatures.(4)The surface graphitization activation energy of diamond film is 366 kJ·mol-1 and 887 kJ·mol-1 in the temperature range of 1773-1973 K and 2073-2173 K,respectively.It indicates that the Debye temperature,(?)D 2021 K,can be taken as the boundary between the two graphitization mechanism.At or above(?)D,the graphitization is promoted by simultaneously breaking multiple C-C bonds,while below(?)D,the graphitization is activated by generating vacancies or breaking single carbon bond.(5)The difference in activation energy has effect on the graphitization rate for different crystallographic planes.At the treating temperature lower than 1850 0C,the graphitzation rate of the diamond(110)face is higher than that of(111)face.As the temperature increases,the graphitization rate of the(111)face gradually increases.At 1900 ?,the graphitization rate is comparable between diamond(110)and(111)faces.(6)The relationship between temperature and lattice constant of diamond/graphite was established.It is demonstrated that the compressive stress of diamond has inhibition effect on the formation of graphite nuclei.From the perspective of crack expansion resuting from graphite nucleation,the critical nuclei radius inside the diamond grain is calculated to be 0.7 nm.(7)The mechanism of micro-circulation phase transition on the surface of diamond film is proposed.The process can be divided into three steps:(a)Firstly,the C-C bonds between diamond(111)sufaces are destroyed by thermal activation,resulting in flattening of the buckled hexagon structrures at some regions to form graphite sheets.(b)Then,partial carbon atoms are sparated from the solid sites into the gas by the formation of hydrocarbon radicals.(c)Finally,the spontaneous deposition of sp2 clusters is accomplished step by step through the process of carbon atoms bonding into graphite lattice from the gas phase.(8)A graphitzation mechanism of diamond grain boundaries with trans-polyacetylene as an intermediate product is proposed.The formation of trans-polyacetylene,which is a precursor of sp2-bonded carbon with conjugated structure alternating C-C single bonds and C=C double bonds,is the result of residual hydrogen at diamond grain boundaries.Since it is not necessary to directly break the a bond of diamond,the grain boundary graphitization activation energy Ea=247 kJ·mol-1 is lower than the C-C bond energy.(9)The mechanism of the transformation from three(111)diamond planes to two(0002)graphite planes is verified.This mechanism still plays an important role in the position of stack fault inside the diamond grains.The cubic diamond with ABC stack can be directly converted into hexagonal graphite structure with AB stack under this mechanism.