CVD Diamond Coated Carbide Cutting Tool Pretreatment—Acid Etching

As a result of several technical matters of CVD coated cemented carbide, the researchers and scholars has developed some pretreatment for tungsten carbide cutting tool, which can remarkably improve the binding force or adhesion between CVD diamond coating and tungsten carbide matrix. It mainly includes acid etching removes cobalt (Co), plasma etching removes cobalt (Co), applying various transition layer and mechanical or chemical heat treatment and so on. This paper is focused on introducing acid etching.

Theoretically, acid etching usually uses some inorganic acid (such as hydrochloric acid (HCl), nitrate ACID (HNO₃), sulphuric acid (H₂SO₄), hydrofluoric acid (HF), and phosphoric acid (H₃PO₄)) to remove the metallic oxide of the surface, which is the most widely used in the surface of tungsten carbide Co removing. The cemented carbide containing corrosive acid into the etching is performed so that the surface layer of the cobalt and the acid react to form a stable compound, thereby removing the surface layer contained cobalt (Co). Since after the removal of molybdenum, tungsten carbide WC substrate surface defects left, which will also reduce the surface of the barrier, so that the carbon atoms in the defect aggregate to form carbon radicals, radicals increases as the particle size, wherein the diamond growing phase, so the final formation of the diamond core, diamond between the film and the cemented carbide is improved.

Acid etching includes one-step process and two-step process. Although one-step etching method, the deposited diamond film and the substrate have good adhesion, but because tungsten can not easily etched by acid, thus impeding the acid erosion of deep cobalt. The two-step etching method of the deposited diamond crystal is better, and the combined strength of the matrix is higher, but it remains some defects, one is loose structure controlling (Nucleation loose structure is due to cobalt-depleted cobalt zone at high temperature and pressure to the cemented carbide substrate surface diffusion occurs, the surface of the cobalt-depleted zone will be gradually reduced until it disappears, strong graphite suppression of Co bonded diamond grains meet and growth, and the surface of the substrate nucleation and growth of a large number of non-diamond phase, which ultimately affect the quality of diamond film deposition.); The other one is two-step method is not suitable for the tools machining with small size (it has an bad influence on breaking strength).

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