Significantly reduced process duration
‘With the new method we can now maintain the stability of the peripheral zone and even slightly increase it’, adds Mee. Since he implements every step of the process with a microwave plasma, the process chain need not be interrupted. This eliminates extra work steps and thus valuable time. Another important factor: tools produced by this method can be recycled once they become worn by removing the old coating and applying the process again. This reduces the amount of materials required, which is particularly advantageous in regard to the tungsten used – this is extracted primarily in China and cannot be reliably obtained on the world markets.
Multiple processes combined into one
Apart from the wet-chemical procedure currently used almost exclusively for ensuring the adhesion of diamond to cemented carbide, other methods of surface treatment are possible – and Dr. Mee has bundled these into an integrated process for the functionalization of the tool material. ‘By combining different process approaches I have been able to make use of the advantages of each of the factors influencing adhesion while also being able to compensate for their individual disadvantages’, he explains. First the carbon is removed from the surface of the carbide at high temperature, which results in the formation of what is known as the eta phase. Re-enrichment with carbon then leads to a large proportion of the unwanted cobalt in the peripheral zone being vaporized. At the same time, however, the surface structure, hardness and fracture toughness are positively influenced. The process can be controlled in such a way that the eta phase previously created remains in the grain boundaries beneath the surface, and can then be used in the following treatment step to cause a conversion to cobalt tungstate. This has proven itself capable of stabilizing the cobalt and inhibiting its subsequent diffusion. This process step does not, however, prevent a wetting of the surface by a very thin cobalt film. Mee has therefore added a further step to the procedure in which a silicon-based interlayer is added that finally keeps the cobalt away from the diamond layer.