Sadao Takeuchi

Chemical vapour deposition of a diamond coating onto a tungsten carbide tool using ethanol

Abstract Diamond coating of a tungsten carbide alloy using the hot filament chemical vapour deposition process was investigated for application to tool manufacture. Previously, diamond deposition onto a cobalt-rich tungsten carbide alloy has been considered to be impossible using this process. However, it was found that using ethanol as the carbon source a good diamond coating layer could be obtained. Moreover, when a diamond-coated tool was applied to the shearing of an aluminium plate 2.5 mm thick, no abrasion of the diamond coating and no adhesion of chips of the sheared material to the tip of the tool were found even after 5 × 10 4 punchings, demonstrating the excellent hard coating effect as well as reasonably good adhesion of the coated diamond layer.

Evaluation of tribological properties of DLC films used in sheet forming of aluminum sheet

Diamond-like carbon (DLC) films, i.e. hard carbon films having amorphous structure, are known to have excellent tribological properties against various materials. However, because of its low adhesion strength to a substrate and/or the difficulty of preparing the required thick film, thin films have not been as extensively applied to tools as expected. In this paper, the basic tribological properties of DLC films against aluminum alloys are reported. More specifically, films with different compositions prepared by various techniques and coated onto the forming tools were subjected to tribology tests using aluminum alloys as the counter face material. We constructed a heavy-load friction tester that enables the application of load similar to that in actual metal forming of the material. It was found that among various hydrogenated DLC films with or without inclusion of a silicon element, non-silicon containing films show the best tribological properties.

Mechanical applications of thin and thick diamond films

Abstract Several applications of thin and thick diamond films have been developed in our laboratory which enable us to exploit the well-known property of diamond, i.e. the highest hardness on earth. These applications include cutting inserts of either a coated (thin film) type or a brazed (thick film) type whose performance has been evaluated and compared with both conventional sintered and single-crystal diamond insert (tool) as well as tungsten carbide insert in cutting very-hard-to-turn materials such as a 18 wt.% Si aluminium alloy or a dispersion-strenthened aluminium alloy including 20 vol.% SiC whiskers. The performance test is extended to a precision turning operation of pure aluminium which requires a very good surface finish. Other applications of diamond film here have been found in antiabrasive tools; one is a dresser used for forming grinding wheels and another is a shearing tool for cutting very abrasive materials such as a polishing tape with a top SiC coating layer. Finally, the development of an indenter mounting a thick diamond film flake shaped into a scratch tester tip has been explained as one example of diamond films for use as various mechanical parts or elements.

Performance of a rotating gear pair coated with an amorphous carbon film under a loss-of-lubrication condition

The ability of gear boxes to complete their missions following loss of lubrication is an important parameter in design criteria for various machines. This paper shows the results of the actual gear performance test conducted using a gear testing machine that was subjected to a loss-of-lubrication condition immediately after a short operating period under fully lubricated conditions. The result showed that a gear pair subjected to an initial shot peening process and a subsequent PVD process to deposit an amorphous carbon film (WC/C film) could endure the conditions for a very long time before it failed or seized even under a very severe loading condition, thus demonstrating the utility of gears coated with an amorphous carbon film.

Synthesis of multilayer diamond film and evaluation of its mechanical properties

When diamond is used as a wear-resistant material for tools, its unique wear resistance cannot be fully exploited due to the possibility of brittle fracture. In general, diamond films synthesized by the vapor phase method are polycrystalline exhibiting columnar crystal growth. In the polycrystalline structure, once cracks occur on the surface of the film, they tend to propagate through the columnar particles, leading to a decrease in toughness. In this study, using the hot-filament chemical vapor deposition (CVD) method, diamond secondary nuclei are grown on a substrate by applying bias current to the substrate repeatedly and intermittently; multilayer diamond films, in which the continuity of grain growth is suppressed, are synthesized. The interfaces formed by the multilayer structure are expected to prevent crack propagation. To confirm this effect, mechanical characteristics, such as the bending strength of the multilayer diamond film are evaluated. The results indicate that the bending strength of the multilayer diamond film is approximately 30% higher than that of a conventionally-produced diamond film.

Synthesis of thick DLC film for micromachine components

Abstract A diamond-like carbon (DLC) film generally exhibits low adhesion strength, i.e. easiness of peeling off from the surface of a substrate due to the internal compressive stress, as high as 2 GPa, generated in the film, thus resulting in the limitation of the film thickness to as small as 1–3 μm. The purpose of this study is to establish a technology, which can synthesize DLC film thicker than 30 μm, for micromachine components, using the RF plasma chemical vapor deposition (CVD) method. The results showed that the introduction of silicon can reduce the internal stress to approximately 0.8 GPa, and the formation of a multilayered structure in addition to silicon can reduce it further to 0.68 GPa. Thus, these measures succeeded in reducing the stress by as much as 40% compared with that in the film synthesized by conventional techniques. Furthermore, the reduction of the stress allowed the formation of a freestanding thick DLC film that can be used as the material for micromachine components. Microcutting and grooving performance of the film, by the usage of YAG laser and focused ion beam (FIB) techniques, are shown. Finally, we fabricated a DLC rod of 30-μm length and 5-μm diameter.

An experiment in large area diamond coating using a combustion flame torch in its traversing mode

Abstract Synthesis of diamond films by conventional chemical vapour deposition methods using a vacuum chamber is difficult when coating a large area substrate because of the limited space in the chamber. This paper deals with an experiment in which a large area substrate is diamond coated using a combustion method which does not have this problem. A tungsten substrate (50 mm × 50 mm × 5 mm) was used, over which an oxyacetylene combustion flame was passed at a speed of 2 mm min -1 to obtain a diamond coating. By establishing an optimum combination of coating parameters, including gas flow rate and substrate cooling rate, the substrate could be completely coated with a good diamond film over an area of approximately 40 mm × 40 mm.

Tribological behavior of amorphous hard carbon films against zinc-plated steel sheets

Zinc-plated steel sheets extensively used in the automotive industry, particularly on body parts which are usually produced by stamping processes, such as deep drawing and blanking, are known to often cause flakes and powdering and consequent adhesion onto the tool when used without heavy lubricants. With heavy lubricants, strong degreasing agents must be used, which is detrimental to the environment. Accordingly, in an effort to achieve deep drawing of zinc-plated steel sheets either with a very light lubricant or, better yet, with no lubricant at all, various deep drawing dies, including those coated with amorphous hard carbon coatings such as DLC (diamond-like carbon) and WC/C (hard amorphous hydrogenated carbon), were subjected to a tribological test using a ball-on-disc tribometer, as well as to a field test. These amorphous hard carbon coatings showed excellent performance in terms of basic tribological properties. Furthermore, the results of the field test, in which the work material was deep drawn with no lubrication for 5000 operations using the WC/C coating, showed that even with no lubrication the coating can effectively reduce the chance of powdering that can cause surface damage or wrinkles over the cup product, thereby prolonging the die life.

CVD diamond, DLC, and c‐BN coatings for solid film lubrication

The main criteria for judging coating performance were coefficient of friction and wear rate, which had to be less than 0.1 and 10-6 mm3/(N.m), respectively. Carbon‐ and nitrogen‐ion‐implanted, fine‐grain, chemical‐vapor‐deposited (CVD) diamond and diamondlike carbon (DLC) ion beam deposited on fine‐grain CVD diamond met the criteria regardless of environment (vacuum, nitrogen, and air).

Quantitative adhesion strength measurement of diamond coatings

Abstract Various adhesion measurement methods for hard coatings have been studied. However, not one has yet been established for use with a superhard coating such as diamond. This study proposes an adhesion tester which can determine quantitatively the adhesion strength of diamond films coated, e.g. on WCCo alloy substrates using the thermal CVD method. The tester comprises a scraping blade of tungsten carbide which is moved along the interface between the coating and the substrate; the scraping force is measured so as to provide the adhesion strength. Finally, the quantitative adhesion strength values thus determined were compared with the adhesion strength qualitatively determined with the shearing test in which the diamond-coated tool tip was mounted in the shearing die set. The results show that the adhesion strength values obtained either qualitatively or quantitatively relate well with each other, thus providing the usefulness of the present adhesion testing method for diamond films.