A study on the oxidation and carbon diffusion of TiC in alumina–titanium carbide ceramics using XPS and Raman spectroscopy

Abstract

Diamond-like-carbon (DLC) coated and non-coated alumina–titanium carbide ceramics used in magnetic recording heads were annealed in air and nitrogen atmospheres from 200°C to 800°C. Micro-Raman spectroscopy found that carbon of TiC began to diffuse at 350–400°C in air, but above 400°C in N 2. The lower onset temperature of carbon diffusion in air was due to titanium oxidation. The diffused carbon was amorphous at 350–600°C with mixed sp 2 and sp 3 bonds, and nano-crystalline graphite above 600°C. With increase in annealing temperature, the structure became more graphitic, and the degree of crystallization and crystal size increased. The structure changes with annealing temperature were revealed in Raman measurements by increased ratio of D-band to G-band intensities, shifting of G-band to higher frequencies and reduced relative width of the bands at half-height maximum. In a nitrogen environment, carbon still diffused from TiC to leave elemental Ti. Oxidation of titanium was also observed in air. At temperatures less than 600°C, titanium was oxidised to several states, however, at higher temperatures, only polycrystalline TiO 2 was observed. At 800°C, the products consisted of sub-micron needle-like TiO 2. Al 2O 3–TiC ceramic composites with a 60 Å of DLC coating on the surface effectively protected them from oxidation and carbon diffusion at temperatures below 500°C. DLC coating delayed the onset of carbon diffusion and TiC oxidation.