Fanghong Sun

Improvement of adhesive strength and surface roughness of diamond films on Co-cemented tungsten carbide tools

Abstract Diamond-coated tools were fabricated using Co-cemented carbide inserts as substrates by the bias-enhanced hot filament chemical vapor deposition. The surface of the WC–Co substrate was decarburized by microwave plasma with Ar–H2 gas. Effect of the new substrate pretreatment on the adhesion of diamond films was investigated. A boron-doped solution was brushed on the tool surface to diffuse boron into the substrates during diamond deposition. A new process was used to lower the surface roughness of diamond thin films by appropriately controlling acetone concentration and reactive gases pressure. It consists of a two-step chemical vapor deposition procedure that includes, first, the deposition of the rough polycrystalline diamond and then the fine-grained diamond. The research results show that the pretreatment using Ar–H2 etching decarburization by microwave plasma is an effective method to enhance adhesive strength. An adequate amount of boron dopant solution can effectively suppress the cobalt diffusion to the surface and avoid the catalytic effect of Co at the high temperature. Smooth diamond films with low roughness can be deposited by the two-step CVD process. It is of great significance for improvement of the cutting performance of diamond-coated tools using the above new technology to deposit diamond coatings with the low surface roughness and high adhesive strength on WC–Co substrates.

Fabrication and application of chemical vapor deposition diamond-coated drawing dies

Abstract A diamond coating has been fabricated by straight hot filament chemical vapor deposition (CVD) passing through the interior hole of the drawing die using a mixture of hydrogen and acetone as source gases. The substrates are drawing dies made by cemented carbides with large apertures (φ>2 mm), and are pre-treated by various methods including leaching Co by acid solution, scratching the substrate by diamond powder and decarburizing the WC substrate by microwave plasma. The homogeneity of coatings is estimated by scanning electron microscopy and Raman spectroscopy. The preliminary applied tests show that the adhesion strength of diamond coatings can meet the need of the practical drawing wires. As compared with the cemented carbide drawing die, the working lifetime of the diamond-coated drawing die can be increased by a factor of five to 10.

Fabrication and application of high quality diamond-coated tools

Abstract Diamond-coated tools were fabricated using Co cemented carbide inserts as substrates by electronically aided hot filament chemical vapor deposition (EACVD). An amount of additive in an acid solution was used to promote the Co etching of the substrate surface. The surface of the WC–Co substrate was decarburized by microwave plasma with Ar–H 2 gas. The effect of the new substrate pre-treatment on the adhesion of the diamond films was investigated. A boron-doped solution was brushed on to the tool surface to diffuse the boron into the substrate during diamond deposition. A new process was used to reduce the surface roughness of the diamond thin films by appropriately controlling the deposition parameters, which consists of a composite diamond film chemical vapor deposition procedure including first the deposition of rough polycrystalline diamond and then fine-grained diamond. The research results show that the pre-treatment, including both Co etching in acid solution and Ar–H 2 etching decarburization by microwave plasma, is an effective method to enhance the adhesive strength. An adequate amount of boron dopant solution can effectively suppress cobalt diffusion to the surface and avoid the catalytic effect of Co at high temperature. The composite film CVD process can deposit smooth diamond films with low surface roughness. It is of great significance for the improvement of the cutting performance of diamond-coated tools to use the above new technology to deposit diamond coatings with low surface roughness and high adhesive strength on WC–Co substrates.

Development of diamond-coated drills and their cutting performance

Abstract Compared with sintered polycrystalline diamond, deposited thin film diamond has great advantage in the fabrication of cutting tools with complex geometries such as drills. Because of their low costs for fabrication equipment and their high performance in the high speed machining of non-ferrous metals and alloys, metal-compound materials, and hard brittle non-metals, diamond-coated drills find great potentialities for commercial application. However, the poor adhesion of the diamond film to the substrate becomes the main technical barrier for the successful development and commercialization of diamond-coated tools. In this paper, diamond thin films were deposited on to WC–Co-based drills by electron-aided hot filament chemical vapor deposition (EACVD). A new multiple chemical pretreatment technology including microwave oxidation, reaction in alkaline solution and cleaning by ultrasonic treatment in acid solution was developed and its affects on diamond adhesion strength, nucleation and surface morphology were investigated. At the same time, a special set-up to fabricate diamond-coated drills was also developed. A novel CVD process based on the varied parameters controlling model is presented, whereby a uniform quality and homogenized thickness of the diamond coating can be obtained. From the practical utilization viewpoint, the cutting performance of the diamond-coated drill was studied by drilling SiC particle reinforced aluminum matrix composite. Satisfactory wear resistance and decrease of torque and thrust led to the long life of this kind of drill. This paper presents meaningful work for the development of cutting tools with complex geometries, to further extend the application of diamond material.

Study on fabrication and cutting performance of high quality diamond coated PCB milling tools with complicated geometries

Abstract The diamond film was deposited on a complex shaped WC–Co printed circuit board (PCB) milling tool in a revised hot filament chemical vapour deposition apparatus, in which the vertical spiral hot filament arrangement was adopted. The SEM investigation on the surface of the fabricated diamond coated PCB milling tool exhibited a continuous layer of fine grained diamond films, with grain size of about 2–3 μm, was uniformly deposited. To evaluate the cutting performance of the fabricated diamond coated tool, comparative milling tests were conducted for both diamond coated and uncoated WC–Co PCB milling tools, with glass fibre reinforced composite materials as the workpiece. The milling test results showed that the flank wear on circle and end cutting edge of the fabricated diamond coated PCB milling tool were only 0·08 and 0·18 mm respectively after 40 min milling, about one-fifth and a quarter of that for uncoated WC–Co PCB milling tool, which could attribute to both the good wear resistance of the deposited diamond coating and the strong adhesion between them and the tool surface. Additionally, the thrust forces acted on the milling tools during the milling process were measured using a Kistler dynamometer measuring system. The results presented that all measured axial, radial and tangential thrust forces were smaller for diamond coated PCB milling tool, which was due to the low friction coefficient of the diamond film that could facilitate the chip evacuation.

Pre-treatment for diamond coatings on free-shape WC–Co tools

Abstract A new multiple chemical pre-treatment including microwave oxidation, reaction in alkaline solution and cleaning by ultrasonic treatment in acid solution has been performed for free shape cemented WC–Co tools in order to increase the diamond nucleation and to enhance the coating adhesion. High quality diamond films were deposited on such pre-treated substrates by a hot filament chemical vapor deposition (CVD) method using a mixture of acetone and hydrogen gases. After pre-treatment, the surface of the WC–Co substrate becomes slightly rough, but its composition or structure shows no changes identified by X-ray diffraction (XRD). Scanning electron microscopy (SEM) indicates a distribution of uniform micro-roughness WC grains on substrate surface. The results show that the multiple chemical pre-treatment effectively increases the diamond nucleation as well as greatly enhancing the coating adhesion. Especially, it is suitable for free-shape substrates, which may open the way to the use of diamond coatings for coated tool applications.

Si-doped diamond films prepared by chemical vapour deposition

Abstract The effects of Si doping on morphology, components and structure characteristics of CVD diamond films were studied. Si-doped CVD diamond films were deposited on Si substrate by adding tetraethoxysilane (TEOS) into acetone as source of reactant gas. The morphology and microstructure of diamond films were characterized by scanning electron microcopy (SEM). The crystalline quality of diamond films was studied by Raman spectroscopy and X-ray diffractometry (XRD). The surface roughness of the films was evaluated with surface profilometer. The results suggest that Si doping tends to reduce the crystallite size, enhance grain refinement and inhibit the appearance of (111) facets. Raman spectra indicate that Si doping can enhance the formation of sp 2 phase in diamond films. Moreover, Raman signal of SiC was detected, which suggests the existence of Si in the diamond films. Smooth fine-grained diamond (SFGD) film was synthesized at Si to C ratio of 1%.

Application of ultra-smooth composite diamond film coated WC–Co drawing dies under water-lubricating conditions

Abstract A specific revised HFCVD apparatus and a novel process combining HFCVD and polishing technique were presented to deposit the micro- and nano-crystalline multilayered ultra-smooth diamond (USCD) film on the interior-hole surface of WC–Co drawing dies with aperture ranging from d 1.0 mm to 60 mm. Characterization results indicate that the surface roughness values ( R a ) in the entry zone, drawing zone and bearing zone of as-fabricated USCD coated drawing die were measured as low as 25.7, 23.3 and 25.5 nm, respectively. Furthermore, the friction properties of USCD films were examined in both dry sliding and water-lubricating conditions, and the results show that the USCD film presents much superior friction properties. Its friction coefficients against ball-bearing steel, copper and silicon nitride balls ( d 4 mm), is always lower than that of microcrystalline diamond (MCD) or WC–Co sample, regardless of the lubricating condition. Meanwhile, it still presents competitive wear resistance with the MCD films. Finally, the working lifetime and performance of as-fabricated USCD coated drawing dies were examined under producing low-carbon steel pipes in dry-sliding and water-lubricating conditions. Under the water-lubricating drawing condition, its production significantly increases by about 20 times compared with the conventional WC–Co drawing dies.

Effect of Boron-Doped Diamond Interlayer on Cutting Performance of Diamond Coated Micro Drills for Graphite Machining

Thin boron doped diamond (BDD) film is deposited from trimethyl borate/acetone/hydrogen mixture on Co-cemented tungsten carbide (WC-Co) micro drills by using the hot filament chemical vapor deposition (HFCVD) technique. The boron peak on Raman spectrum confirms the boron incorporation in diamond film. This film is used as an interlayer for subsequent CVD of micro-crystalline diamond (MCD) film. The Rockwell indentation test shows that boron doping could effectively improve the adhesive strength on substrate of as deposited thin diamond films. Dry drilling of graphite is chosen to check the multilayer (BDD + MCD) film performance. For the sake of comparison, machining tests are also carried out under identical conditions using BDD and MCD coated micro drills with no interlayer. The wear mechanism of the tools has been identified and correlated with the criterion used to evaluate the tool life. The results show that the multilayer (BDD + MCD) coated micro drill exhibits the longest tool life. Therefore, thin BDD interlayer is proved to be a new viable alternative and a suitable option for adherent diamond coatings on micro cutting tools.

Friction and wear performance of boron doped, undoped microcrystalline and fine grained composite diamond films

Chemical vapor deposition (CVD) diamond films have attracted more attentions due to their excellent mechanical properties. Whereas as-fabricated traditional diamond films in the previous studies don’t have enough adhesion or surface smoothness, which seriously impact their friction and wear performance, and thus limit their applications under extremely harsh conditions. A boron doped, undoped microcrystalline and fine grained composite diamond (BD-UM-FGCD) film is fabricated by a three-step method adopting hot filament CVD (HFCVD) method in the present study, presenting outstanding comprehensive performance, including the good adhesion between the substrate and the underlying boron doped diamond (BDD) layer, the extremely high hardness of the middle undoped microcrystalline diamond (UMCD) layer, as well as the low surface roughness and favorable polished convenience of the surface fine grained diamond (FGD) layer. The friction and wear behavior of this composite film sliding against low-carbon steel and silicon nitride balls are studied on a ball-on-plate rotational friction tester. Besides, its wear rate is further evaluated under a severer condition using an inner-hole polishing apparatus, with low-carbon steel wire as the counterpart. The test results show that the BD-UM-FGCD film performs very small friction coefficient and great friction behavior owing to its high surface smoothness, and meanwhile it also has excellent wear resistance because of the relatively high hardness of the surface FGD film and the extremely high hardness of the middle UMCD film. Moreover, under the industrial conditions for producing low-carbon steel wires, this composite film can sufficiently prolong the working lifetime of the drawing dies and improve their application effects. This research develops a novel composite diamond films owning great comprehensive properties, which have great potentials as protecting coatings on working surfaces of the wear-resistant and anti-frictional components.