Kinetics of the Oxidation of TiC-Coated Cemented Carbide

Abstract

The oxidation of TiC in the temperature range 450 – 1000°C has previously been proposed to be parabolic according to several investigations. However, a linear relationship for the oxidation of TiC-layers on steel has also been found.

Thermogravimetric investigation of the oxidation of cemented carbide both as uncoated and as coated with a titanium carbide layer has shown:

• i. The oxidation rate of uncoated cemented carbide samples proceeds according to a linear law and with a rate which is 20 – 50 times larger than for TiC-coated samples.

• ii. The oxidation kinetics for TiC-coated samples;s gradu- ally changing from a parabolic rate law at 600 – 700°C to approximately a linear rate law at about 900°C.

• iii. Only TiO2 (rutile) has been found as the oxidation product during oxidation in air or oxygen. However, formation of whiskers has been shown at low oxygenpartial pressure in the electron microscope, but not directly on TiC. These whiskers could possibly consist of Ti2O3, but not of TiO2.

• iv. A dark, narrow zone between the TiO2 and TiC layers forms during the oxidation at 600 – 700°C. This zone has a good bonding to the TiC layer, which presumably can be explained by an epitaxial growth of the oxide.

• v. No borgerline zone can be detected on samples oxidized at 800 – 900°C. Instead the TiO2 forms as a rather porous layer. Cracks and blisters easily develop in the boundary.