Sokichi Takatsu

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.

Transmission electron microscopy studies of growth and interface structure of chemically vapour deposited TiC and TiN films on WCCo alloy subsrates

Abstract It is known that the interface structures between deposits and substrates and the growth microstructure of deposits are important factors in the wear resistance of coating films. TiN and TiC films prepared by the chemical vapour deposition (CVD) method on WCCo alloy substrates were investigated using transmission electron microscopy (TEM). The thin foil for TEM was prepared using cross-sectional techniques. CVD TiC and TiN seem to be directly bonded with WCCo alloy and η phase without any pores at the interface. Deposits were fine grained at the initial stage of deposition. As the thickness of a film increased, the grains of CVD TiN became elongated in the direction of the growth with a preferred 220 orientation. High resolution TEM revealed that the grains in CVD TiC were connected with each other without any second phases.

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.

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.

Diffusion of cobalt into a TiC layer during chemical vapour deposition and its effects on the cutting performance of TiC/Al2O3-coated cemented carbides

It is assumed that during the chemical vapour deposition (CVD) process onto cemented carbides cobalt from the substrate can diffuse into the coated ceramic layer. It would be very interesting to determine the relation between the CVD conditions and the amount of diffused cobalt and how the cobalt influences the cutting performance of the coated carbides. CVD of TiC was carried out onto cemented carbides. The CVD temperature (1030 and 1100 °C), CH4 concentration (0.9 and 1.81 min -1) and total gas pressure (140 and 760 Torr) were varied. Diffusion of cobalt from the substrate carbides into the TiC layer was measured with an ion microanalyser and an energy-dispersive X-ray analyser. Cobalt diffuses through grain boundaries in the TiC layer. The distribution of the cobalt across the TiC layer is uniform from the interface with the substrate to the top surface of the TiC layer. The mean cobalt content in a TiC layer about 5 μm thick ranges from 0.13 to 0.44 wt.% depending on the CVD conditions. It increases with increases in the temperature, the CH4 concentration and the gas pressure. The cobalt content in the TiC layer is increased by subsequent Al2O3 coating by CVD while essentially no cobalt is detected in the Al2O3 layer. The effects of the diffused cobalt on cutting performance of carbides coated with TiC(5–6 μm)/Al2O3(1–2 μm) were investigated. Two types of carbide, W WC-6 wt.%TiC-9wt.%TaC-5wt.%Co(A) and WC-2wt.%TiC-2wt.%TaC-7wt.%Co (B) were used. The cobalt content in the TiC layer after Al2O3 coating varied from 0.71% and 0.38% for carbide A and from 0.60% and 0.45% for carbide B. In steel turning, flank wear of carbide A with a high cobalt content in the TiC (specimen AH) is less than half that of carbide A with a low cobalt content in the TiC (specimen AL). However, in the case of carbide B, equal flank wear was observed for high and low cobalt contents in the TiC layer (specimens BH and BL respectively). In cast iron turning, flank wear of the high cobalt specimens AH and BH is one-third to one-quarter of that for the low cobalt specimens AL and BL. A chipping test by interrupted turning with a feed-up method demonstrates that the high cobalt specimens AH and BH withstand a feed rate one step higher than the low cobalt specimens AL and BL do. The coating layers of specimens AH and BH show no visual damage after a peeling test by turning, whereas those of specimens AL and BL are peeled from the cutting edge. It is proposed that the reason the cutting performance of coated carbides is improved by diffused cobalt is that the cobalt is effective in increasing the mechanical and adhesive strength of the coating layer, while the negative effects of the cobalt on hardness, wear resistance etc. are negligible for the small amounts of cobalt in this experiment.

The Tool Life of Diamond Coatings in Milling an Al-Si Alloy

Abstract The adhesion of diamond film is improved by surface decarburizing of WC substrate before diamond coating process. Superior turning performance of diamond coating had been confirmed[1]. However, milling is more severe conditions for diamond coated insert compared with turning. In the present study, the excellent cutting performance of diamond coated insert has been confirmed in milling Al-Si alloys and the field test. Especially, the polishing of diamond film is very effective to prolong the tool life and improve the surface finish of work material.

DEPOSITION OF DIAMOND FOR CUTTING TOOL APPLICATIONS

Abstract The poor adhesion of diamond film to substrates is one of the major problems for practical use in a cutting tool (1-4). in this study, sintered tungsten carbide (WC) body without Co metal, not cemented carbide, was used as the substrate (5), and the effects of surface decarburization of the substrate for improvement in the adhesion of diamond films were investigated. The surface decarburization and diamond coating were carried out in a microwave plasma CVD system. From the results of several adhesion tests, including the cutting tests, it is concluded that the good adhesion is obtained by surface decarburization of the substrate before diamond coating. The reasons for improvement in adhesion are considered by observing the interface structure between the film and the substrate. The damage mechanism of diamond coating on cutting an AI-18%Si alloy with increasing cutting speed is also discussed.