Synthesis And Mechanical Properties Of Periodic Nitrogen-doped Single Crystal Diamond By MPCVD

Diamond is the hardest substance in nature,which makes it have important position and application in the industrial field,such as high-hard cutting tools,fine grinding materials,various drills and wire drawing mold.However,the toughness of diamond is poor.In order to give full play to the excellent performance of diamond and expand its application range,for example,on long-life tools and high-pressure diamond anvils,diamonds are required to have high hardness and high toughness.Therefore,it is necessary to study the toughening method of CVD single crystal diamond.In order to improve the toughness of single crystal diamond,this paper uses the method of microwave plasma chemical vapor deposition(MPCVD)to grow periodic nitrogen-doped nanolayers on the surface of single crystal diamond on the basis of systematically studing the growth law and influencing factors of CVD single crystal diamond.A smooth and flat single-crystal surface was obtained by optimizing the size relationship between the seed substrate and the square hole of the shielding ring.The polarization characteristics of two kinds of CVD layers were compared between flat and uneven surfaces.Abnormally high-order birefringence occurs in both morphologies,which is related to the dislocations density in the CVD layers.The higher dislocation density of the sample,the more obvious high-order anomalous birefringence.When the diamond surface demonstrates an edge-to-center morphology,the dislocations in the seed substrate,interface-induced dislocations,and dislocations formed in the growth layer begin to converge toward the central region of the CVD layer,which results in a smaller area of low stress in the center of the CVD layer.High dislocation density causes the crystal to have a mosaic structure and orientation deviation.Fluorescence characteristics showed that the sectors of the High Pressure High Temperature(HPHT)seed substrates were obviously partitioned.Raman and absorption spectra showed that the CVD seed substrate produced higher-quality crystals with fewer nitrogen impurities.X-ray topography showed that the HPHT seed substrate had obvious growth sector boundaries,inclusions,dislocations,and stacking faults.The polarization characteristics of HPHT seed substrate were obvious,and the stress distribution was not uniform.When etching HPHT and CVD seed substrates using the same parameters,the etching morphology and extent of different growth sectors of the two substrates differed.The dislocation density of a 1 mm-thick CVD layer grown on a CVD seed substrate was only half that of a 1 mm-thick CVD layer grown on an HPHT seed substrate.It was found that low nitrogen concentrations in the gas phase are enough to dramatically increase the growth rate.When the gas phase nitrogen concentration is high,large conical and pyramidal features are formed on the surface of the sample.Raman mapping and photoluminescence imaging of the polished cross sectional slice shows a broadband emission,with a characteristic zero phonon line(ZPL)at 575 nm in the doped layers,and strong compressive stress was formed in the nitrogen-doped layers.X-ray topography shows that the defects at the interface can induce dislocation.The pyramid feature is formed at the defect,and more nitrogen-related defects are doped into the pyramid region.Thin nitrogen-doped multilayers were successfully prepared,and the thickness of the nitrogen-doped and buffer layers was about 650 nm each.The indentation measurements reveal that the thin nitrogen-doped multilayers are 13.7 MPa m1/2,which is about 61%higher than Ib-type HPHT seed substrate(8.5 MPa m1/2).The influence of the size of nitrogen-doped layer is considered.Periodic nitrogen-doped nanolayers were grown on the surface of singley crystal diamond.A periodic nitrogen-doped CVD single crystal diamond with single nitrogen-doped layer thickness of 96 nm was grown.The fracture toughness of periodic nitrogen-doped CVD layer(18.2 MPa m1/2)is about 2.1 times that of the HPHT seed substrate(8.5 MPa m1/2).This value is increased by about 32%compared to the fracture toughness of the CVD layer doped with the same nitrogen content but grown with non-periodic nitrogen doping(13.8 MPa m1/2).And this value is increased by about 64%compared to the fracture toughness of non-nitrogen-doped CVD layer(11.1 MPa m1/2).The periodic compressive stress is formed at the periodic nitrogen-doped layer and non-nitrogen-doped layer,which significantly improved the fracture toughness of diamond.