Effect of rare earth (RE) on pack boronising process of titanium alloy

The pack boronising experiments were carried out on the TC21 titanium alloy using monomer boron powder as the boriding agent with and without rare earth oxide CeO2 respectively. The friction and wear behaviour of boride coated TC21 samples were evaluated using a pin on disc friction tester. The boride layers were investigated by means of X-ray diffraction, X-ray photoelectron spectroscopy, SEM and optical microscopy. The results show that the boride layers grown on TC21 consist of continuous top layer TiB2 and sublayer TiB whiskers; with the addition of CeO2 in the boriding agent, the grain size of boride layers is refined, the diffusion efficiency is increased and more TiB changes to TiB2; the microhardness of the top layer is ∼3200 HV(10 g); without CeO2, the friction coefficient of boride coated TC21 sample is ∼0·56, while with the addition of CeO2, it is <0·2. The boride coated TC21 samples show a much better wear resistance than the TC21 substrate.