Friederike Deuerler

Pretreatment of substrate surface for improved adhesion of diamond films on hard metal cutting tools

Abstract The d.c. plasma jet CVD process is one of the most promising coating processes used for the production of polycrystalline diamond films. In comparison with other CVD processes, its obtainable linear growth rates, in the range of 100 μm/h, are much higher than growth rates of microwave or hot filament CVD (1–10 μm/h). One interesting application is the diamond coating of cutting tools. The main problem here is the poor adhesion of the films. Therefore, a mechanical or chemical pretreatment or intermediate layers are used to improve the adhesion. In these investigations the influence of mechanical pretreatment by grinding and polishing with diamond powder of different grain sizes as well as chemical etching are examined on WC-Co hardmetals. Sputtered metallic interlayers of different thicknesses and arc-ion plated amorphous carbon films are deposited on these substrates, and diamond films were synthesized on these pretreated cutting tools by d.c. plasma jet CVD. Adhesion and wear resistance of the diamond films have been examined by dry turning tests on very abrasive metal-matrix composites. Distinct improvement in adhesion of diamond coatings on hard metal substrates was achieved by two methods of substrate surface pretreatment: etching with Murakamis solution and following ultrasonically seeding with diamond particles or using an amorphous carbon film as intermediate layer.

Enhanced nucleation of diamond films on pretreated substrates

Abstract One interesting application of polycrystalline diamond films is the coating of cutting tools. The main problem herewith—the poor adhesion of the films—has been successfully overcome using mechanical or chemical pretreatments or intermediate layers. Besides adhesion, a significant aspect of film quality and performance is connected to size and orientation of the diamond grains, which depends on nucleation. In these investigations, different mechanical and chemical pretreatments as well as intermediate layers are produced to examine their influence on nucleation density, orientation of nuclei, etc. The effects of pretreatment on the nucleation process have been studied by EDX and scanning electron microscopy. It has been shown, for example, that the nucleation density can be significantly enhanced if combined methods of pretreatment are used.

Influence of chemical pretreatment of hardmetal substrates for diamond deposition

Abstract Diamond coated cutting tools are of increasing importance in the fields of high speed cutting, dry machining or machining of special materials such as metal-matrix-composites. A well-known problem is the poor adhesion of diamond films on hard metals due to the Co- or Ni-binder that catalyzes the formation of graphite. Several methods––such as the application of intermediate layers or mechanical or chemical pretreatment of the hardmetal substrate––have been developed to overcome this effect. Usually chemical pretreatment is used in order to reduce the concentration of binder phase on the surface that is to be coated. Surprisingly pretreatment with agents such as Murakamis solution result in improved adhesion and nucleation of diamond films while the concentration of the binder phase on the surface is enhanced. This “contradiction” can be explained by proving that the surface is converted into a very thin oxide/hydroxide film.

Process control during diamond coating of tools

The plasma jet CVD process is especially suited for the low pressure synthesis of diamond films from the vapour phase due to its high deposition rate. The process parameters as e.g. gas composition and pressure in the reaction chamber, temperature of the substrate and of the gas phase, have to be adjusted very carefully due to the intimate correlations between them. If set correctly this technique allows the high quality diamond coating of tools with complex shape, even with big coating areas.