| The excellent thermal shock resistance and slag erosion resistance of aluminum–carbon（Al2O3–C）refractories are lost because of oxygen sensitivity,so their application is great limited.Glass is a very popular anti-oxidation coating material due to its low melting point,low oxygen permeability coefficient of molten glass phase and low production cost.However,the instability of the glass coating after sintering for a long time,the low viscosity and fluidity at high temperature mean that the glass phase cannot exist stably on the surface of the substrate to protect the substrate material during the service process.However,the instability,low and high fluidity of the glass coating at high temperature for a long time mean that the glass phase cannot exist stably on the surface of substrate to protect it from oxidation during service.SiO2 and B2O3 as additives can improve the stability of glass,and then improve the oxidation resistance of the coating.Slurry brushing method has the advantages of simple operation,low cost,controllable coating thickness and structure.However,the coating prepared by the brush method has more holes and cracks,which will provide diffusion channels for oxygen before the formation of a continuous glass phase,so that the matrix is oxidized Therefore,it is very important for the oxidation resistance of coating to make its structure compact and smooth by heat treatment before application.In addition,the bonding strength between coating and substrate is low,so it is necessary to form a"mosaic structure"at the interface between coating and substrate by preoxidizing the matrix,so as to improve the bonding strength of interface.In this paper,Al2O3-C refractory material is used as the substrate,SiO2,B4C and glass powder are used as raw materials,sodium silicate is used as the binder,and carboxymethyl cellulose（CMC）is used as the suspension agent,the SiO2-B4C-glass anti-oxidation coating was prepared by slurry brushing method.Firstly,the effects of SiO2,B4C and sodium silicate additions on the morphology,phase composition and oxidation resistance of the coating were studied by controlling process parameters,and the oxidation protection mechanism and failure reasons of the coating were explored.After heat treatment,the influence of heat treatment temperature and time on the morphology of SiO2-B4C-glass coating was explored,and the change of the weight of the coating sample was observed.Finally,the matrix material was preoxidized,and effect of the pre-oxidation process on the bonding strength between coating and matrix and the oxidation resistance of coating sample was analyzed.The main research contents are as follows:（1）With the increase of SiO2 content,the number of holes and cracks in the coating decreased first and then increased,and the mass loss of the sample decreased first and then increased.With the increase of B4C content,the holes in the coating were less and less.However,when the B4C content was too high,the"boron anomaly"occurred,which reversed the structure and properties of the glass phase in the coating and made the mass loss of the sample decreased first and then increased.With the increase of binder content,the compactness of the coating gradually increased.However,the volume shrinkage caused by excessive water glass during curing was large,which leaded to the increase of cracks and holes in the coating,and the mass loss of sample decreased first and then increased.The optimal slurry ratio was as follows:55.0 wt%SiO2,15.0 wt%B4C,30.0wt%glass powder,60.0 wt%sodium silicate,1.0 wt%CMC.The coating prepared with this ratio had the best oxidation resistance and could protect the substrate from oxidation for at least 100 h.（2）Oxidation resistance of the coating mainly relied on the oxygen consumption of B4C oxidation,B2O3 glass phase and molten glass powder on the substrate and the repair of coating defects,and the stabilizing effect of SiO2 and B2O3 on the glass phase.The main reasons for coating failure were:a.with the increase of oxidation time,the glass phase was gradually lost,resulting in the thickness of the coating is gradually reduced,and it was difficult to prevent the diffusion of oxygen;b.fatal defects that could not be repaired appear in the coating.（3）With the increase of heat treatment temperature,the glass viscosity decreased and the fluidity increased.With the increase of heat treatment time,the glass phase content formed in the coating increased.However,when the fluidity was too large,part of glass phase would be lost from the surface of the substrate due to instability,which reduced the thickness of the coating and could not effectively prevent the oxidation of the substrate.After heat treatment at 900℃for 0.5 h,the surface of coating was smooth and compact,the pores and cracks were the least,the oxidation resistance of the coating was the best,and the mass loss ratio of the coating was-1.041%after 80 h of oxidation at800℃.（4）With the increase of pre-oxidation temperature and time,the mass loss ratio and apparent porosity increasing ratio of Al2O3-C refractory matrix gradually increased.After the substrate was preoxidized at 900℃for 10 min,the bonding strength between coating and substrate increased from 0.930 MPa to 1.316 MPa,an increase of 41.51%.The"mosaic structure"formed between coating and substrate after pre-oxidation could alleviate the thermal stress in the coating and reduce the formation of cracks in the coating.The mass loss ratio was-1.308%after oxidation at 800℃for 98 h,which improved the oxidation protection ability of coating... |
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