Kunio Shibuki

Adhesion strength of diamond films on cemented carbide substrates

Diamond-coated cemented carbides are expected to be superior cutting tools for Al-Si alloys. For practical use in cutting, the adhesion strength of the diamond film is a major problem which could be improved. The adhesion strength of diamond particles, deposited by hot filament chemical vapour deposition (CVD) of CH4, was qualitatively measured in relation to the conditions of surface preparation of the carbide substrate and CVD. The adhesion strength is improved by deposition of diamond in pores formed by cobalt removal from near the surface of the substrate. Good adhesion is obtained when the size of the pore and the deposited particle is nearly equal. Better adhesion is obtained at a substrate temperature of 1000 °C compared with 800 and 900 °C. A diamond-coated carbide cutting insert was produced using the above results by microwave plasma CVD. The coating layer is mainly composed of diamond with a small amount of non-diamond carbon. The insert demonstrates a very small steady wear without flaking of the diamond film in cutting Al-10%Si alloy.

The improvement of the adhesion strength of diamond films

Abstract One of the expected applications of diamond coatings is in cutting tools for non-ferrous metals and alloys such as Al-Si and non-metals such as hard carbons etc. However, the poor adhesion strength of diamond films must be improved for practical use in cutting. In this study, sintered tungsten carbide (WC) without cobalt metal was used as the substrate and the effect of surface decarburization of the substrate to improve the adhesion strength of diamond films was investigated. Surface decarburization and diamond coating were carried out using a microwave plasma chemical vapour deposition apparatus. Good adhesion was obtained by surface decarburization of the substrate before diamond coating. From the results of observations by scanning electron microscopy and transmission electron microscopy, the improvement in the adhesion strength can be considered to be due to an increase in the contact area between the film and the substrate by generation of fine WC grains on the surface of the substrate, with the film embedded in the shape of a wedge at the fine WC grain boundaries. The coated insert demonstrates a very small steady wear without film flaking when milling a hard carbon compared with the large amount of wear exhibited by uncoated cemented carbides.

Improvements in adhesive strength and cutting performance of diamond-coated tools

Abstract The surface of WC substrates was decarburized before coating, to improve the adhesive strength of diamond film. The adhesive strength and cutting performance of diamond-coated tools on decarburized WC substrates were examined. The diamond film showed high adhesive strength because of the rugged substrate surface formed during diamond coating. The diamond-coated tool demonstrated excellent cutting performance when milling and turning high silicon content aluminum alloys. The cutting performance was improved further by lapping the rough diamond film surface.

Growth of oriented diamond on single crystal of silicon carbide (0001)

Diamond was deposited on a (0001) plane of an α‐silicon carbide single crystal by the microwave method. The substrate surface was cleaned by pretreatment with hydrogen gas at 1200 °C. Cubo‐octahedral diamond crystals with (111)D∥(0001)SiC were obtained.

Friction and wear properties of hard carbon films formed on cemented carbides by D.C. plasma chemical vapour deposition

Abstract Hard carbon film was formed on a cemented carbide substrate from CH4 gas using d.c. plasma chemical vapour deposition (CVD). The effects of added argon and hydrogen gases and electrode power on the deposition rate and the hardness of the film were investigated. The friction and wear properties of the coated carbides were measured using a block-on-rotating-ring sliding test with a FALEX I type tester. A film with a flat and very smooth surface was obtained at a deposition rate of between 0.2 and 1.4 μm h-1 according to the CVD conditions. From the Raman spectra the film was estimated to be mainly composed of graphite-like carbon; however, no crystalline structure was observed in the film using scanning electron microscopy. The coefficients of friction of the films were in the range 0.13 – 0.25 throughout dry sliding with hardened steel. Film with a high critical load in the scratch test generally showed better resistance to film flaking and wear. A film with a very stable low coefficient of friction and superior wear resistance without film flaking was obtained. The specific wear of this coated carbide was 1 23 of that of the uncoated carbides, and many antiwear applications can be expected.

ORIENTED DIAMOND ON GRAPHITE FLAKES

Diamond was deposited by the microwave plasma chemical vapor deposition method on a Si(100) substrate on which graphite flakes had been spread with their basal planes parallel to the substrate before deposition. Before diamond deposition, the substrate was preheated at 1200 °C under hydrogen at 60 Torr to clean the surface of graphite flakes. Scanning electron micrographs showed that most of diamond particles were cubo‐octahedral in morphology. The {111} planes of some diamond particles, which were judged by their triangular shape, were often parallel to the (0001) plane of graphite. Furthermore, some 〈111〉‐oriented diamond particles were clearly nucleated at the edge of graphite. The possibility of heteroepitaxy of diamond on graphite was discussed based on crystallographic considerations.

Diamond coating on WC-Co and WC for cutting tools

Abstract Cemented carbide (WC-Co) and hot-pressed tungsten carbide (WC) were diamond coated at 850–950°C by the microwave plasma chemical vapour deposition method for use as cutting tools. First, WC inserts without Co were diamond coated utilizing the decarburization method to roughen the surface before diamond deposition. Accordingly, the absence of Co improved the adhesion strength with diamond films, which enabled the machining of Al-Si alloys with a sufficient lifetime. Similarly, the decarburization method was applied to WC-6 wt.%Co inserts and their cutting performance was compared with that of diamond-coated WC. The diamond-coated WC-6%Co inserts showed quite high adhesion strength, enough to cut Al-Si alloys for practical purposes, although slightly inferior to that of diamond-coated WC.

Grain boundary in cemented carbide

Abstract σ = 2 grain boundaries in cemented carbide (WC-Co) were investigated by high-resolution electron microscopy on the atomic scale. The following crystallographic relationships were often obtained at the grain boundaries, these originating in the way that the interplanar spacings of the basal plane (0002) and the prism plane (1110) are quite close with a 2% misfit, since a/c ∼ 1: The role of Co in grain-boundary structures during sintering is discussed briefly.

Microstructure of diamond films near the interface with WC substrate

Abstract A diamond-coated cutting tool has been developed by plasma chemical vapour deposition with H 2 CH 4 . The adhesion strength of the diamond film is remarkably improved by predecarburizing the surface of the WC substrate to withstand severe cutting. To investigate the effects of decarburization on the adhesion strength, the microstructure of the diamond film near the interface with the substrate was observed by scanning and transmission electron microscopy. A rugged surface with a large amount of fine roughness is generated on the decarburized substrate after diamond deposition, while the non-decarburized surface remains flat. A thin graphite-like non-diamond carbon layer is observed near the tip of the diamond film contacted to the substrate. No significant difference in the amount of non-diamond carbon is found between substrates with and without decarburization. Therefore the main effect of predecarburization is considered to be mechanical reinforcement of the adhesion strength caused by an increase in contact area between the diamond film and the substrate.

Growth of an oriented graphitic layer on a TiC nanocube

Cubic‐shaped titanium carbide (TiC), 30–60 nm in size, was sprayed onto aluminum oxide powder and hot pressed at 1800 °C. The preferential evaporation of Ti atoms out of the surface resulted in the formation of graphitic layers on the {100} planes of TiC with the following crystallographic relationships: (010)TiC∥(1120) graphite; 〈001〉TiC∥〈0001〉graphite. The oriented graphite layers form a continuous nanocube of carbon, closely analogous to carbon nanotubes.