| Graphite materials have a variety of excellent performance, but under conditions of high temperature and oxygen-containing they are easily oxidized which greatly limits their scope of application. Therefore, to improve its high-temperature performance, this article prepared anti-oxidation coating on graphite by pack cementation method.On basis of sintered principle, Twyman temperature (sintering starting temperature) and pratical conditions, the sintering temperature and the heating rate is determined. Generally, Twyman temperature of metals is about (0.3-0.4) TM, Twyman temperature of silicate is about (0.8-0.9) TM. As coating densification need fast sinter at high temperature, the sintering temperature was set at a temperature higher than Twyman, unified set at1600℃. Limited by laboratory equipment, we choose heating rate as follow:below800℃, set as4℃/min, higher than800℃, set as2℃/min, holding2h at1600℃.First, pure anti-oxidation coating was prepaired through pack cementation method, then the samples were subjected to oxidation test. Oxidation test was conducted under static air, the process was as follow:from room temperature heating to1400℃was2℃/min, held that temperature for2h, then cooled to room temperature by2.5℃/min. Using field emission scanning electron microscope (SEM), X-ray diffraction (XRD), the morphology and structure of the samples (before and after oxidatrion test) was researched, their protection mechanism were summed up its.Prepaired pure SiC coating on the specimen got some antioxidant effects and self-healing ability. But because the vapor pressure of SiO2coating at high temperature was large, the coating would be excessive consumptive in the course of anti-oxidation. This made pure SiC coating not suitable for long period of antioxidant protection. Further, since the the coating was not continuous or dense enough but with holes and microcracks, these provided channels for oxygen penetrating the coating and the substrate interface or within the matrix, causing failure of the antioxidant protection. Pure Si powder and C powder to embed the samples to get anti-oxidation coating had a certain antioxidant activity. Theoretically, oxidation resistance of graphite material would be enhanced with the coating thickness increase, but with Si content in the coating increasing, brittleness of the coating also increased.Then, considering the good wettability between graphite material and metal material, by reacting with the graphite substrate or physical doping, mixed high melting point metal powder to introduce them into the coating, got a metal-silicide coating. Then oxidation test was taken, calculate the mass loss, morphology and structure of the sample before and after the oxidation test was observed.After oxidized at1400℃for2h, the weight loss of samples with W-modified coatings was0.95%; MoSi2modified ones were increased; Cr modified ones were0.05%; Mo-modified ones were0.97%. After modification, antioxidant property had been enhanced. And MoSi2modified ones were dense, smooth, without penetrating pores or micro-cracks. MoSi2distributed in the SiC coating, they could adjust the stress of the coating. MoSi2and SiC interface buffered the tip of the crack tip stress, the micro cracks would not further extended. so that penetrating cracks would be reduced, effectively protective the coating.Mixing different additives improved the anti-oxidation coating of varying differently. Since the melting point of the metal was different, their anti-oxidant temperature was ranged. Therefore, in order to further enhance oxidation resistance of the coating, different additives were embedded to preparir anti-oxidation coating. The uniform design method was to study mixing various admixtures to anti-oxidation coating on the graphite substrate, and optimal dosage was determined. |
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