Study Of Deposition And Application Of Amorphous Ceramic Interlayered Diamond Coatings On Cemented Carbides

Chemical vapor deposition diamond coatings have many excellent properties,such as extreme hardness,high elasticity modulus,high thermal conductivity,good self-lubrication and chemical stability.Due to these excellent properties,the fabrication of CVD diamond coated cemented carbide tools have exhibited wide application prospect in machining difficult-to-machine materials.Nevertheless,the failure of CVD diamond coatings caused by inadequate adhesion between diamond coatings and cemented carbides has always restrained the industrialization of CVD diamond coated tools.The amorphous ceramics have typical covalent chemical bond,on which the deposition of CVD diamond can achieve a high nucleation density.And there's no effect of metallic cobalt that leads to the graphitization of CVD diamond.Therefore,the application of amorphous ceramic intermediate films is expected to improve the adhesive strength between diamond coatings and cemented carbides.The purpose of this dissertation is to solve the key problems inhibiting the adhesive strength of CVD diamond coatings and promote the industrialization of CVD diamond-coated cemented carbide tools,especially for the high cobalt cemented carbide substrates.We investigate the fabrication and characterization of amorphous ceramics and amorphous ceramic interlayered diamond coatings on cemented carbide substrates.Subsequently,the tribological properties of amorphous ceramic interlayered diamond coatings are studied.Finally,the effect of amorphous ceramic interlayers on the cutting performance of CVD diamond coated cutting tools is investigated.The chief contents of this dissertation are concluded as follows:1.Study on the interface structure in the film/substrate system with amorphous ceramic interlayers.The structure analyses of cemented carbide/a-SiO₂,cemented carbide/a-SiC,a-SiO₂/diamond,a-SiC/diamond and cemented carbide/diamond interfaces are given using the quantum mechanics method.The preferred orientations of tungsten carbide and diamond are detected by experimental XRD data.Therefore,the interfacial modelling process is simplified.The interfacial bonding and binding energy are analyzed by the investigation of total energy,nuclear population,total and local density of state,and density distribution of electrons after the geometry optimization and relaxation of interfaces.The results show that the bonding energy of cemented carbide/a-SiO₂,cemented carbide/a-SiC,a-SiO₂/diamond and a-SiC/diamond interfaces are-1.629,-4.896,-14.107 and-1.752 J?m^-2,respectively.Strong Si-C bonds and C-C bonds form between these interfaces,which exhibit attractive behavior.However,in the case of cemented carbide/diamond interface,the interface bonding energy is1.165 J?m^-2,and there are few hybird bonds,which make the cemented carbide/diamond interface exhibits repulsive behavior.The amorphous ceramic?a-SiO₂ and a-SiC?can share electrons with diamond and tungsten carbide and form covalent bonds at the interfaces.Consequently,the bonding strength between diamond coatings and cemented carbide substrates are increased.2.Study on the fabrication of amorphous ceramic interlayered diamond coatings.Amorphous SiO₂?a-SiO₂?and amorphous SiC?a-SiC?intermediate films are deposited on cemented carbide substrates with 6 and 10 wt.%Co content,respectively.The amorphous ceramic interlayers are fabricated by precursor pyrolysis using tetraethoxysilane?TEOS?and dimethyldiethoxysilane?DMDEOS?as reactant gas,which aims at improving the flaw of chemical pretreatment on cemented carbide substrates.The influence of amorphous ceramic interlayers on the growth characterization of CVD diamond coatings is studied.And Rockwell indentation tests are performed to investigate the effect of interlayer thickness on the adhesion of diamond coatings.The results show that the amorphous ceramic interlayers have little influence on the growth characterization?morphology,growth velocity,quality and crystalline orientation?of diamond coatings.However,with the increase of interlayer thickness,the adhesion of diamond coatings increases and afterwards decreases.For both types of cemented carbide substrate,the optimized thickness of a-SiO₂ and a-SiC interlayers are 400 nm and 1?m respectively,in which case the adhesion of diamond coatings can be significantly improved.The amorphous ceramics can react with the cobalt?either remained after pretreatment or diffused from the bulk?and harmless silicon-cobalt compounds come into being.As a result,the graphitization caused by metallic cobalt can be greatly reduced.Moreover,the amorphous ceramic interlayer can improve the morphology of cemented carbide substrates.The SEM images show that the amorphous materials cover the asperities and edges of tungsten carbide particles and fill up the gully area between tungsten carbide particles.After the deposition of interlayers the substrate morphology exhibits a ball-like appearance,which can improve the residual stress distribution of diamond coatings near the interfaces.3.Study on the tribological properties of amorphous ceramic interlayered diamond coatings.The amorphous ceramic interlayered diamond coatings are deposited on cemented carbide plane substrates with 6 and 10 wt.%Co content,respectively.Friction tests of as-fabricated diamond coatings dry sliding against Si₃N₄ in ambient air at room temperature are performed on a“ball on plate”type rotating tribometer.The results of friction tests show that the diamond coatings with/without the amorphous ceramic interlayers exhibit similar evolution of friction curves and wear morphology.Nevertheless,after employing the amorphous ceramic interlayers,the COF and wear rate of diamond coatings are greatly reduced.For WC-6 wt.%Co substrates,the COF of non-interlayered,a-SiO₂ interlayered and a-SiC interlayered diamond coatings is 0.274,0.177 and 0.168 respectively.For WC-10wt.%Co substrates,the COF of non-interlayered,a-SiO₂ interlayered and a-SiC interlayered diamond coatings is 0.216,0.173 and 0.164 respectively.The surface roughness measurement of non-interlayered,a-SiO₂ interlayered,a-SiC interlayered WC-Co substrates and as-deposited diamond coatings is performed on the surface profiler.For WC-6 wt.%Co substrates,after chemical pre-treatment,the substrate surface roughness is 231.61 nm,and the surface roughness of a-SiO₂ interlayer and a-SiC interlayer is 194.65 nm and 145.79 nm,respectively.Correspongdingly,the surface roughness of non-interlayered,a-SiO₂interlayered and a-SiC interlayered diamond coatings is 259.95 nm,233.23 nm and 200.21nm,respectively.For WC-10 wt.%Co substrates,after chemical pre-treatment,the substrate surface roughness is 243.44 nm,and the surface roughness of a-SiO₂ interlayer and a-SiC interlayer is 194.84 nm and 154.55 nm,respectively.Correspondingly,the surface roughness of non-interlayered,a-SiO₂ interlayered and a-SiC interlayered diamond coatings is 257.06nm,233.97 nm and 203.84 nm,respectively.The results show that the surface roughness of WC-Co substrates?6 and 10 wt.%Co content?is significantly reduced after employing the amorphous ceramic interlayers.Consequently,the surface roughness of amorphous ceramic interlayered diamond coatings is reduced,which is of great importance to the improvement of friction coefficient and friction performance.4.Study on the cutting performance of CVD diamond coated cutting tools with amorphous ceramic interlayers.Amorphous ceramic interlayered diamond coatings are deposited on cemented carbide inserts with 6 wt.%Co content.Their cutting performance is evaluated by machining glass fiber reinforced plastic?GFRP?and Al-Si alloy.In the cutting tests against GFRP,the a-SiO₂ interlayered diamond coating exhibits better adhesion and wear endurance,and the lifetime of insert is doubled.In the process of machining Al-Si alloy,the conventional diamond coatings begins to peel off after the turning length reaches 2050 m.When a-SiC interlayer is applied,however,no delamination of diamond coatings occurs.The diamond coated tool with a-SiC interlayer shows much less flank wear than the diamond coated tool with no interlayer,which is only a quarter of the latter's.The amorphous ceramic interlayer can improve the adhesive strength and wear enduration of diamond coatings on cemented carbide tools with low Co content.Afterwards,this work focuses on cemented carbide substrates with higher Co content?10 wt.%?and fabricate diamond coated PCB milling tools and ball-end milling tools with amorphous ceramic interlayers.The influence of amorphous ceramic interlayers on the cutting performance of as-fabricated milling tools is studied using printed circuit board?PCB?and zirconia as work piece respectively.During machining of PCB,the flank wear of conventional diamond-coated PCB tool surpasses 0.18mm when the cutting length reaches 25 m.Nevertheless,the diamond-coated PCB tools with amorphous ceramic interlayers exhibit less flank wear,which is only 0.15 mm for a cutting length of 30 m.In the cutting tests against zirconia,the conventional diamond-coated ball-end milling tool exhibits peeling off after machining for 30 min.For the samples with amorphous ceramic interlayers,however,no peeling-off can be observed,and the flank wear is only 0.045 mm after machining for 50 min.The results illustrate that the amorphous ceramic interlayer can increase the adhesion of diamond coatings,reduce coating delamination,improve the wear rate of diamond coatings and enhance the tool cutting ceramic interlayer can increase the adhesion of diamond coatings,reduce coating delamination,improve the wear rate of diamond coatings and enhance the tool cutting performance,which shows great potential application in the fabrication of diamond coated tools with high Co content.