Abstract: High temperature oxidation resistance of graphite can be effectively improved by a gradient TiC coating, which is in-situ reacted on the graphite surface. The in-situ reaction mechanism, using Ti, TiO2 and NH4Cl as pack cementation agent to prepare TiC gradient coating on the surface of graphite, was investigated by the combination of experimental and chemical thermodynamic calculations in this paper. Also, based on the analyses of coating formation kinetics, the influence of temperature and pressure on the structure of the coating was studied. The results show that in the process of pack cementation on graphite substrate, TiO2 does not act as an inert filler, but participates in the reaction as a reactant and promotes the formation of TiC. This is different from the formation mechanism of TiC coating on the carbon steel surface using the same cementing agents. Without changing the composition of the cementing agent, increasing the partial pressure of the TiCl3 gas is beneficial to the increase of the gas diffusion rate, thereby increasing the thickness of the gradient coating. On the other hand, though the diffusion rate of TiCl3 gas is increased with the increase of the pack cementation temperature, the permeability of the TiCl3 gas is decreased as the rate of reaction between TiCl3 gas and graphite increases with temperature. Therefore, the structure of the gradient coating can be modulated by the controlling of pack cementation temperature. Under the existing experimental conditions, the thickness of the gradient TiC in the graphite is continuously reduced and the thickness of the surface TiC coating on the graphite is continuously increased when the pack cementation temperature is increased from 1 000 ℃ to 1 500 ℃, resulting in the coating structure is changed from the TiC gradient coating to the TiC surface coating.
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2019, Vol. 33 
