CO addition in low-pressure chemical vapour deposition of medium-temperature TiCxN1-x based hard coatings

TiCxN1-x base layers with Al2O3 top layers, both grown by chemical vapour deposition (CVD), are state-of-the-art in metal cutting with cemented carbide inserts. In order to influence the microstructure and properties of the base layer, five different TiCxN1-x coatings were grown by medium-temperature CVD with increasing CO fractions in the TiCl4–CH3CN–H2–N2–CO feed gas using an industrial-scale low-pressure CVD system. With the CO fraction in the feed gas rising up to 2.2 vol.%, oxygen contents of 2.2. at.% could be detected, which are distributed homogeneously over the coating thickness. The originally equiaxed TiCxN1-x grains with preferred {110} orientation obtained without CO addition change to randomly distributed plate-like grains, while the grain size decreases to the sub-micron range. Also the surface roughness decreases with rising CO addition. The incorporation of oxygen leads to a homogenous distribution of dislocations within the TiCxN1-x grains and to an increasing density of twin boundaries. The residual tensile stress shows a minimum of 200 ± 47 MPa, while the hardness increases to 29.1 ± 1.3 GPa and the elastic modulus reaches a maximum 569 ± 29 GPa for CO fractions of 1.5 vol.% in the feed gas.