石墨表面TiC梯度涂层的制备及结构调制

Preparation and Structure Modulation of TiC Gradient Coating on Graphite Surface

在石墨表面原位反应形成TiC梯度涂层可以有效地提高石墨的高温耐氧化性能。本实验采用实验与化学热力学计算相结合的方法,探究了以Ti、TiO2和NH4Cl为固渗剂在石墨表面原位反应制备TiC梯度涂层的机理,并在涂层形成动力学过程分析的基础上,研究了温度和压力对涂层结构的影响规律。研究结果表明,在石墨基体固渗Ti的过程中,TiO2不是起惰性填料的作用,而是作为反应物参与反应,并促进TiC的原位反应形成,这与该固渗剂在碳钢表面形成TiC的机理不同。在不改变固渗剂成分的条件下,增大反应气体TiCl3(g)分压有利于气体扩散速率的提高,从而增大梯度涂层的厚度;提高固渗温度一方面提高TiCl3(g)扩散速率而增加梯度涂层的厚度,另一方面又提高TiCl3(g)与石墨原位反应形成TiC的速度,从而降低反应气体的渗透率。因此,通过调整固渗温度可使涂层的结构发生改变。在现有的实验条件下,将固渗温度从1000℃提高到1500℃,梯度涂层的厚度不断减小,表面涂层的厚度不断增大,涂层由TiC梯度涂层转变为TiC表面涂层结构。

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,TiO 2 and NH 4Cl 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,TiO 2 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 TiCl 3 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 TiCl 3 gas is increased with the increase of the pack cementation temperature,the permeability of the TiCl 3 gas is decreased as the rate of reaction between TiCl 3 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.