The Tribological Properties And Applications Of The CVD Diamond Films Under Water-lubricated Conditions

Chemical vapor deposition (CVD) diamond film has many excellentproperties, such as extremely high hardness, wear resistance, chemicalinertness, together with excellent tribological properties with water lubricating.Depositing CVD diamond films on the WC-Co machining tools not onlyprolongs their working life and improves machining quality, but also attributesto the machining technique promotion, resource saving and sustainableeconomy development. The combined application of coatings and waterlubrication technology can not only reduces the environmental pollutionscaused by the oil-based lubricants and excessive consumption of oil resources,but also has important scientific significances and economic values. This thesisconcentrates on (1) deposition CVD films with different morphology on WC-Co substrates,(2) study on the water lubricated tribological properties of CVDdiamond films (3) applications of CVD diamond films on turning inserts anddrawing dies with water as lubricant. Main contents go as follows:1. Deposit conventional micro-crystalline diamond film(MCD), nano-crystalline diamond film(NCD) and the MCD/diamond-like carbon compositefilm(MCD/DLC) on the WC-Co substrates with the HFCVD method. Firstly,the two-step chemical pretreatment method was used to roughen the surfaceand remove the cobalt, then the substrate was placed into the HFCVD chamber to deposit diamond films.Characterized the surface morphology, structure, composition, crystalorientation, surface roughness, grain size, and the contact angle with deionizedwater with energy-dispersive X-ray spectroscopy (EDX), scanning electronmicroscope (SEM), Raman spectroscopy and contact angle measuringinstrument. Residual stress of all CVD diamond films were examined usingboth X-ray diffraction (XRD) analysis and Raman spectroscopy measurements.The calculated residual stresses of MCD, NCD and MCD/DLC compositefilms were about-3.871GPa,-1.07and-1.537GPa, respectively. Silicondoping has no obvious effect on reducing the residual stress of MCD films.2. The tribological behaviors of MCD, NCD and MCD/NCD compositefilms sliding against Si3N4balls were studied by conducting a group of tests onthe ball-on-plate type reciprocating friction tester at several sliding speeds andnormal loads in water-lubricated environment. The worn surfaces on thediamond films were observed by SEM and the wear volumes of diamond filmswere measured by surface profilometer. The tribological influence of films’surface morphology and the friction condition was revealed. The sample withMCD films or NCD films was placed in the flat plate, the counterpart samplewas a6mm diameter silicon nitride ceramic ball. During the test, the ballreciprocated along the surface of CVD diamond film sample with the load inthe range of2~8N, and the sliding speed from300rpm to1200rpm (equivalentto0.126~0.503m/s). From the friction coefficient curves, we could get that theaverage friction coefficient of MCD, NCD and MCD/NCD composite filmssliding against silicon nitride during the stable stage were0.03~0.18,0.02~0.15and0.025~0.12, respectively, in water. The results indicated that the influencesof the normal load on the friction coefficients for both all films were obvious,that was, the value decreased as the load improved. Sliding speed also hadsome impact on the friction coefficient. The friction coefficient of the NCD film decreased as the speed improved. The wear rates of MCD film and NCDfilm were too small to calculate, while the wear rate of MCD/DLC film wasvery low, which was2.179×10-8mm3N-1m-1at the sliding speed of1200rpmand the load of8N.3. Study on the application of CVD diamond coated drawing dies. Firstly,Studied the Friction coefficients of the CVD diamond films in dry sliding,water lubrication sliding and oil lubrication sliding with aluminum alloy,copper, low-carbon steel and stainless steel as counterpart materials, which aretraditional drawing materials. The results showed that the friction coefficientof CVD diamond in water was very close to those in oil, which proved thefeasibility of water-lubricated drawing. Further, we had developed asupporting device for the water-lubricated drawing process, which ensured thatthe working areas of the drawing die and the whole tube/wire were soak in therecycled water so as to be sufficient lubricated and cooled during the drawingprocess. We had conducted the water-lubricated drawing of aluminium plasticscomposite pipe and the results showed that the lifetime of the water-lubricateddrawing die was100times longer than the traditional cemented carbidedrawing die’s.4. Study on the application of CVD diamond coated cutting tools. CVDdiamond films were deposited on the WC-Co turing inserts. Before thedeposition, adopting orthogonal optimization method to optimize thedeposition process parameters: hot filament radius r, spacing of the filament H,longitudinal distance from the filament to the tip of insert D, and filamenttemperature Tf, with the software Fluent. The objective was to ensure thetemperature on the surface of the cutting tool during the depositon was in therange that was suitable for diamond growth, thus prepareing high qualitydiamond film. Deposit CVD diamond films on the WC-Co turing inserts withthe optimize deposition parameters and examined the cutting performance of the coated turning inserts using SiC reinforced aluminum matrix compositesas the workpiece in the condition of water lubricated turning. Adopt orthogonalexperiments to optimize the cutting parameters (cutting speed,feed and depth)of diamond coated tools, then conducted wear test with the optimize cuttingparameters to evalute the working life and quality of machined surfaces. Theresults indicated that the CVD diamond coating can extend the tool life of thecemented carbide insert by3.5times.