Research On Fabrication And Machining Performance Of Boron-Doped Diamond Coated Tools

The application of diamond coated film to tools, dies and wear-resistant apparatus material have been attracting great interest for the outstanding mechanical characteristics of diamond. However, the adhesive strength of diamond coating and substrate is very poor, which is primarily due to the large mismatch of thermal expansion coefficient and the cobalt-catalyzed formation of non-diamond carbon at the diamond-carbide interface, has restrained the successful development and commercialization of diamond-coated tools. The research of p-type diamond has indicated the incorporation of boron into the diamond coating in the process of diamond deposition is a dynamic boron doping technique, which can decrease the grains sizes, reduce internal stress and improve quality of diamond thin film. It is of great significance for the application and development of boron-doped diamond coating.In the present paper, boron-doped diamond coatings were deposited on silicon wafers, cemented carbide tools and cemented carbide drawing dies by bias-enhanced hot filament chemical vapor deposition. The trimethyl borate (B(OCH3)3) dissolved in acetone solution was used as the boron resource, the gas mixture of hydrogen and acetone were used as reaction gases. The deposited mechanism, process and influencing factors of boron-doped diamond coatings were studied. The surface morphology and quality of boron doped diamond coatings were studied and analyzed carefully by Scanning electron microscopy (SEM), Raman spectra (Raman) and X-ray diffraction (XRD), etc.. Furthermore, the cutting performance of the boron-doped diamond coated inserts and drills were studied by cutting tests of composite materials, the machining effect of boron-doped diamond coated drawing dies were tested by practical drawing brass tubes.The research results indicated:①The deposited mechanism and growth law of boron-doped diamond coatings are similar with conventional diamond coatings, which can be interpreted by the non-equilibrium thermodynamics model.②The diamond thin film deposited on round silicon wafers existed internal stress, the grains size and resistance of diamond films decreased as the boron concentration increased.③Diamond films doped with different boron concentrations grown on WC-Co cemented carbide inserts (SCMT120408-HM) showed the two-step pretreatment method could improve mechanical interlocking effect and adhesion between diamond coating and WC-Co substrate effectively. Diamond nucleation density increased as the boron doping concentration was moderate. Meanwhile, the stable cobalt boride compounds were produced on the substrate surface in the process of nucleation, which suppressed outward diffusion of cobalt, reduced the catalytic effect of cobalt. Surface morphology analysis showed a great decrease of grain size as a function of boron concentration increase. Raman spectra of diamond coatings showed that the diamond peaks decreased and downshifted to lower energy as the boron concentration increased. Turning tests displayed that the cutting performance of boron-doped diamond coated inserts have been greatly improved when the boron doping concentration was 3000ppm.④Diamond coatings deposited on conventional cemented carbides YG8(Co 8%) drills showed the grains were faceted with uniform texture. The boron doping decreased the grains size and improved the quality of diamond film, so the machining performance of boron-doped diamond coated drill was better than that of conventional diamond coated drill.⑤The diamond coatings deposited on the inner holes shape of the drawing dies had small surface roughness, uniform thickness and excellent adhesive strength. The films surface was even and the average grain size was 2-5μm. The practical drawing brass tubes test demonstrated the boron-doped diamond coated drawing dies had excellent machining performance, which can prolonged the dies lifetime, saved resource materials, improved the quality of drawing brass tubes and increased production efficiency.