Pankaj Kumar Mehrotra

Computational investigations of the bonding layer in CVD-coated WC+Co cutting tools

The effect of processing parameters on the bonding layer structure of the chemical vapor deposition (CVD) coated cemented carbide inserts was investigated through thermodynamics calculations. It was found that, under equilibrium conditions, the existing processing parameters would deposit graphite. To eliminate the thermodynamic driving force for the formation of graphite, the amount of the methane participating in equilibrium calculations must be significantly reduced. Under such circumstances, various titanium carbides formed instead, in agreement with recent experimental investigations. Thus the methane may have not fully cracked during the CVD coating process.

1.07 – Powder Processing and Green Shaping

Hardmetals are composites based on WC-Co, with minor additions of carbides of Ti, Ta, Nb, Zr, and Hf. Carbides of V and Cr can be added for grain growth inhibition. Composition selection is based on the intended application and chosen process route. WC is most commonly made by stepwise reduction of W-bearing precursor to W oxides, W metal, and finally to WC. Powder selection is made with considerations of final microstructure and composition and minor impurities, especially those of oxygen, to ensure that the target carbon balance is achieved. Fugitive binders and additives are used to impart green strength to powder compacts and facilitate shaping processes. Milling in a wet medium is conducted for mixing and particle size reduction using ball or attritor mills. This is followed by spray or vacuum drying to obtain granulated powder. Uniaxial pressing, cold isostatic pressing, extrusion, injection molding, and green machining are common methods for green forming of the hardmetal powder.