Oxidation Behavior Of Nitride-Bonded Silicon Carbide Refractories In Different Atmospheres

As the key facility to transform the municipal Waste into Energy（WtE）,moving grate incinerators are the most widely used waste incinerators.SiC-based refractories are the main lining materials for such facility,which protect the water-cooled metal tube and transfer the heat.Cracks and spalling caused by the high-temperature gas in the incinerators are the main reasons for the failure of SiC-based lining materials.Oxidation is considered as one of the most important factor for the failure of SiC-based lining materials,because there are the complexity of atmosphere and the high content of steam,oxygen and carbon dioxide in the incinerators.This work studied the corrosion of Si₃N₄-SiC,SiAlON-SiC and Si₂N₂O-SiC in the atmosphere of H₂O,H₂O/O₂,O₂ and CO₂,respectively.XRD,and SEM methods were used to analyze the phase composition and microstructure changes of the three materials before and after oxidation.The mercury intrusion instrument was used to detect the pore size distribution of the three materials before and after oxidation.The change of O（1s）before and after oxidation of Si₃N₄-SiC material in water vapor and oxygen atmosphere was detected by XPS,and a comprehensive comparison of the anti-oxidation performance of the three nitride-bonded silicon carbide materials in four different oxidation atmospheres was done.The main conclusions are as follows:（1）In the process of water vapor oxidation,the mass change rate and volume change rate of the three materials increased with the increase of oxidation time;the oxidation weight gain began to slow after 100h;the volume change first slowly increased,after 250h,the internal oxidation of the materials was serious and the volume increased rapidly.The order of the strength of the three materials against high temperature water vapor oxidation was:Si₂N₂O-SiC>SiAlON-SiC>Si₃N₄-SiC.（2）During the H₂O/O₂ oxidation process,the mass change of the three materials increased firstly and then decreased slightly,and the volume change rate increased slowly.As the water vapor diffused inward through the dense and thick oxide layer of the material,it reacted with the SiO₂ and caused mass loss,resulting in mass loss of three materials.The order of oxidation resistance to composite atmosphere was Si₃N₄-SiC>Si₂N₂O-SiC>SiAlON-SiC.（3）In the oxygen atmosphere oxidation,the oxidation reaction rate of the material changed from the interface chemical reaction rate control to the diffusion rate control;the beginning of the mass change of the three materials increased rapidly and then tended to be slow;the oxidation degree was low;the volume expansion was unobvious.The order of the strength of the three materials against high temperature oxygen oxidation was:Si₂N₂O-SiC>Si₃N₄-SiC>SiAlON-SiC.（4）In the process of CO₂ oxidation,the oxidation reaction rate of the material changed from the interface chemical reaction rate control to the diffusion rate control;the inert oxidation of the material was dominant;at the beginning of the mass change increased rapidly and then tended to be slow,and the volume shrinkage of the materiasl might be caused by the active oxidation reaction at interface.For the three materials,the order of resistance to high temperature carbon dioxide oxidation was:Si₂N₂O-SiC>SiAlON-SiC>Si₃N₄-SiC;.（5）According to the O（1s）XPS spectrum analysis result before and after oxidation of the Si₃N₄-SiC sample,it was a siloxane functional group（Si-O-Si）before water vapor oxidation,and a silanol functional group（Si-OH）appears after steam oxidation,before oxygen oxidation and after that were both siloxane functional groups（Si-O-Si）.It was inferred that the volatilization reaction of SiO₂ and water vapor was the main reason for destroying the oxide layer.（6）In this experiment,the order of oxidation strength of four atmosphere to three nitrides combined with silicon carbide material was:H₂O>H₂O/O₂>O₂>CO₂.Water vapor was the most oxidizing atmosphere for nitrites boned silicon carbide materials,and its oxidation detection data had important reference significance for waste incinerator lining selection.