Study On The Fabrication Of High-performance Diamond Coatings And Their Application In CFRP Drilling

Carbon fiber reinforced plastics (CFRP) have been widely used in the field ofaeronautics and astronautics technology, owing to their excellent properties such as lowweight, high specific strength, low expansion coefficient, excellent anti-thermal shock abilityand good vibration absorption. In the application of CFRP, lots of mechanical machining areconducted, especially the drilling. However, the material anisotropic and low strengthbetween layers caused by the difference of the physical properties of resin matrix and carbonfibers, which lead to the defects including delamination, spalling, burrs and fiber pullout.What’s more, rapid tool wear and low-efficiency are encountered during the cutting processresulted by the high strength, high hardness and low thermal conductivity of CFRP.Chemical vapor deposition (CVD) diamond films have many excellent properties, such ashigh hardness, good thermal conductivity, high wear resistant together with low friction,which make them as the ideal materials for tool coatings. CVD diamond coated tools havegreat potential in solving the problems of drilling CFRP. This dissertation aims at studying onthe fabricating of high-performance diamond coatings and their application in CFRP drilling.The different types of diamond films are deposited on Co-cemented tungsten carbide (WC-Co) drills, in order to fabricate diamond coated drills with high quality. Experiments indrilling CFRP using diamond coated drills are conducted to investigate the drillingperformance of the coated tools and obtain the appropriate cutting parameters for drillingCFRP. The main accomplishments in this dissertation can be concluded as follows:1. Study on the simulation and experiment of physical field in HFCVD diamond filmsgrowth on WC-Co tools with complex geometries. The finite volume method (FVM) isadopted to simulate the temperature and gas density distribution during the HFCVD deposition process. The simulation models with tools arranged in single-row, multi-row andlarge-quantity are established to study the effects of arrangements of filaments and gas inletson substrate temperature and gas density distribution. The optimum parameters are obtainedfor the three cases. The grid structure with different distances between filaments is proper toimprove the uniformity of the substrate temperature. The results of temperature-measuringexperiments indicate that the simulation is reasonable and reliable. The deposition results inthe three cases show that film flaking occurs with the higher substrate temperature, caused bythe accumulation of residual stress; while the lower substrate temperature will lead to thediscontinuous films. Deposition experiments with the optimum parameters from simulationare beneficial for fabricating high-quality diamond coated tools, which provides guide fordiamond coated tools in industrial application.2. Study on the optimum of processing parameters for diamond depositing on WC-Cosubstrates. The orthogonal experiments are introduced to study the comprehensive effects ofparameters including substrate temperature, total pressure, carbon concentration, filamentpower and the distance between filament and substrate, on the quality and growth rate ofdiamond films deposited on WC-Co substrates. The results show that the lower total pressure(10Torr) is favorable for nano-crystalline diamond films without other gas addition, highergrowth of3.03μm/h is obtained. There are more non-diamond carbon phases in the filmswhen deposited at lower substrate temperature. As to the substrates with complex geometry,higher power and lower pressure are the suitable parameters for the growth of nano-crystalline diamond films. A new method for silicon doped diamond films is explored. Thesilicon incorporation can improve the adhesive strength between the diamond films and WC-Co substrates. Furthermore, the grain size of diamond films reaches to nanometer scale whenthe Si/C ratio reaches50000ppm. The silicon doped can prepare the diamond films withsmooth surfaces and excellent adhesion on WC-Co substrates.3. Study on the formation mechanisms of CFRP drilling and the drilling experiments ofdiamond coated drills. The drilling mechanism of CFRP drilling and access of hole qualityare analyzed, associating with the tool wear. The combined effects of forces caused by themain cutting and chisel edges are the main reason of the delamination. The burrs and fiberpullout in the hole exit are mainly caused by the squeezing of the chisel edge. The results of drilling experiments show that the hole quality is improved by the drill with large point angle,and the diamond coated drills exhibit good drilling performance due to their high wearresistant and good thermal conductivity.4. Study on the characterization of tribological properties between CVD diamond filmsand CFRP. The frictional properties of the micro-crystalline diamond films (MCD), nano-crystalline diamond films (NCD), micro-and nano-cryatalline composite diamond films(MCD/NCD), boron doped diamond films (B-doped) and silicon doped diamond films (Si-doped) are comparably examined on a ball-on-plate friction tester, using the CFRP laminatesas counterpart materials. The friction performance between WC-Co inserts and CFRP is alsoexamined for comparison. The friction coefficients of WC-Co and five types of diamondfilms are0.250.310.240.130.35and0.23. The experimental results show that thefilm morphology has great effect on the friction properties. The MCD/NCD films show thesmallest coefficient due to their smooth surfaces and the anti-friction effect of compositefilms. The specific wear rates of CFRP are4.77×10-5mm3/(N·m)95.47×10-5mm3/(N·m)44.65×10-5mm3/(N·m)85.63×10-5mm3/(N·m)66.49×10-5mm3/(N·m) and61.81×10-5mm3/(N·m), respectively. The material removal rate of the five diamond films to CFRP are20,9,18,14and13times than that of WC-Co. Due to the sp2non-diamond structure in theNCD, the material removal rate of NCD to CFRP is lower than that of MCD. For theMCD/NCD, it displays the higher material removal rate for the interlayer of MCD betweensubstrate and NCD.5. Study on the application of CVD diamond coated drills in drilling CFRP and theadaptation of drilling parameters for the coated tools. The drilling performance of MCD,NCD, MCD/NCD, B-doped and Si-doped diamond coated drills in drilling CFRP are carriedout. The orthogonal experiments are adopted to investigate the effect of film structure anddrilling parameters on machining CFRP. The tool wear and hole quality are considered toevaluate the drilling performance. The optimum drilling parameters for diamond coated toolsin drilling CFRP are obtained.