In this research, in order to increase the oxidation resistance of graphite, kaolin and alumina powder with different ratios (26A-74S, 49A-51S, 72A-28S) and slurry method were used to create an aluminosilicate coating on the graphite substrate. In order to reduce the difference in the coefficients of thermal expansion of graphite with aluminosilicate coating, aluminum metaphosphate coating as an interlayer was prepared on the surface of graphite by cathodic electrochemical treatment. The isothermal oxidation test of the samples was carried out in air at a temperature of 1250 °C for 1, 3 and 5 h. The microstructure, chemical composition, and phase components of the coating were, respectively, analyzed by scanning electron microscope equipped with an energy-dispersive spectrometer and X-ray diffraction. The results indicated that, by increasing the withdrawal speed of the samples in slurry method, the amount of changes in the weight of the samples has increased and therefore had a direct effect on oxidation. In addition, it was approved that, at high-temperature oxidation, AlPO4 glass phase forms on aluminum metaphosphate interlayer which retards graphite oxidation. Along with aluminum metaphosphate, aluminosilicate coating also produces a glass phase which fills and seals the voids on the surface which prevents the oxygen to reach the surface of graphite. The created double-layer coating including an interlayer of aluminum metaphosphate + slurry coating prepared with the ratio of 26A-74S as the optimal coating in this research was able to increase the oxidation resistance of graphite by 73% at a temperature of 1250 °C.

Improvement of oxidation resistance of graphite by aluminosilicate coating with aluminum metaphosphate interlayer
    Abstract
    1 Introduction
    2 Experimental procedure
    3 Results and discussion
        3.1 Characterization of aluminum electrophosphate coating
            3.1.1 Mechanism of aluminum metaphosphate coating formation
        3.2 Characterization of aluminosilicate coating
            3.2.1 Weight change of the samples after coating with the slurry
            3.2.2 Microstructural analysis of the aluminosilicate coating
        3.3 Investigation of the oxidation behavior
            3.3.1 Phase characterization and microstructure of the oxide layer
            3.3.2 Microstructure and chemical composition of the coating
    4 Conclusion
    References

• M. Fekri(Faculty of Materials and Manufacturing Technologies, Malek Ashtar University of Technology, Tehran, Iran)
• K. Jafarzadeh(Faculty of Materials and Manufacturing Technologies, Malek Ashtar University of Technology, Tehran, Iran)
• S. A. Khalife Soltani(Faculty of Materials and Manufacturing Technologies, Malek Ashtar University of Technology, Tehran, Iran)
• Z. Valefi(Faculty of Materials and Manufacturing Technologies, Malek Ashtar University of Technology, Tehran, Iran)
• H. Mazhari Abbasi(Faculty of Materials and Manufacturing Technologies, Malek Ashtar University of Technology, Tehran, Iran)