Preparation Of NiCrBSi-ZrB₂ Composite Coating And Its Corrosion-Wear Resistance Mechanism In High Temperature Cl Medium

The global ecological and environmental problems caused by large-scale CO₂ emissions worldwide are increasingly serious.With the implementation of the Paris Agreement,the carbon emission reduction was promoted and the goal of carbon peak-carbon neutrality was put forward actively by our country.As the main power supply mode in China,coal-fired power generation and its CO₂ emissions account for more than half of the total carbon emissions.Reducing the proportion of coal-fired power supply will be an important way to achieve the dual-carbon target.As a CO₂ neutral fuel,the amount of CO₂ released by the combustion process is equal to the amount of CO₂ absorbed by the growth process.Therefore,the biomass direct combustion power generation technology has become the preferred method to replace coal-fired power generation and reduce CO₂ emissions.Biomass boiler is the main energy conversion place for biomass direct combustion power generation.However,the ash produced by biomass combustion process causes serious high temperature Cl corrosion and high temperature wear on the heating surface of the boiler four tubes,which has a huge impact on the power generation efficiency and economic benefits of power plants,and also restricts the promotion of biomass power generation technology.In this paper,NiCrBSi alloy was used as metal matrix and ZrB₂ as ceramic phase.NiCrBSi-ZrB₂ cermet powder was prepared by spray drying,followed by NiCrBSi-ZrB₂ cermet protective coating was prepared on the surface of 12Cr MoV boiler steel using high-velocity oxygen fuel spraying（HVOF）.The performance effect of NiCrBSi-ZrB₂ powders and coatings at different sintering temperatures and different ZrB₂ contents were systematically investigated.The metal/ceramic liquid phase sintering process was optimized by regulating the powder sintering temperature and ZrB₂ content,which improved the ZrB₂ ceramic phase sintering performance,and revealed the influence of liquid phase reaction process on the microstructure and mechanical properties of composite coatings.The high temperature Cl corrosion model and wear mechanism of composite coatings were established through hot corrosion experiment and high temperature wear test,respectively.The main research results are listed as follows:（1）NiCrAlY and NiCrBSi coatings under different oxygen-fuel ratio were prepared by HVOF technology.The results showed that with the increase of oxygen-fuel ratio,the powder melting degree in the NiCrAlY coating increased,the unmelted particle boundary in the coating decreased,the coating porosity decreased,and the binding strength increased.Under different oxygen-fuel ratio parameters,the NiCrBSi powder melting state is good and can reach the droplet spreading state.The coating structure is dense,with low porosity and high binding strength.During hot corrosion,the corrosion resistance of NiCrAlY coating increased with the increase of oxygen-fuel ratio.The lowest hot corrosion kinetic constant（K_p）value of NiCrAlY coating with oxygen-fuel ratio of 5.39 can reach 1.55 mg²/（cm⁴·h）,the corrosion resistance of NiCrBSi coating is overall higher than that of NiCrAlY coating,and the lowest K_p value of NiCrBSi coating with oxygen-fuel ratio of 4.62 is 0.87mg²/（cm⁴·h）.（2）The NiCrBSi-30ZrB₂ metal ceramic powders were prepared using different powder sintering temperatures,and the corresponding composite coatings were prepared by HVOF technique.The results showed that,with the increase of the powder sintering temperature,the liquid phase sintering area between NiCrBSi and ZrB₂ powder expanded,the CrB phase formed in the powder,the structure of powder became denser,and the sphericity,fluidity and apparent density of the powder increased.Accordingly,the microhardness,bonding strength and fracture toughness of composite coatings also gradually increased with the increase of the powder sintering temperature.The coating prepared with 1000℃sintered powder has the lowest hot corrosion mass change and and hot corrosion kinetic constant with a value of 12.97mg/cm² and 1.48 mg²/（cm⁴·h）,respectively.The coating prepared with 1100℃sintered powder has the lowest wear volume and wear rate of 0.73×10^-9 m³ and 0.93×10^-13 m³/（N·m）,respectively.Compared with the NiCrBSi coating,its wear resistance is improved by about 65%.In the process of high temperature wear,the NiCrBSi coating is mainly characterized by abrasive wear and adhesive wear,and the metal matrix is removed by shear deformation under the force of friction.The ZrB₂ and CrB ceramic phases in the composite coating provide hard skeleton support for the coating,which improves the resistance to contact deformation of the coating.At the same time,the superior oxidation resistance and the higher content of ZrO₂ on the worn surface reduce the degree of adhesive wear of the coating,which greatly improves the wear resistance of the coating.（3）The NiCrBSi-ZrB₂ cermet powders were prepared by adding different contents of ZrB₂ ceramic phase,and the corresponding composite coatings were prepared by HVOF technique.The results implied that with the increase of ZrB₂ content,liquid phase and CrB phase were formed during powder sintering,the powder sphericity,density and fluidity increased,while apparent density decreased.The microhardness,bonding strength of the composite coatings increased first and then decreased,while the fracture toughness gradually increased with the increase of ZrB₂ content.The NiCrBSi-40ZrB₂ coating has the lowest hot corrosion weight and kinetic constants of 8.14 mg/cm² and 0.61 mg²/（cm⁴·h）,respectively.Compared with the NiCrBSi coating,its corrosion resistance is improved by about 30%.It also has the lowest wear volume and wear rate of 0.91×10^-9 m³ and 1.16×10^-13 m³/（N·m）,respectively.In the process of hot corrosion,oxide layers mainly composed of SiO₂ and ZrO₂ are formed at the interface of the composite coating,so that the pressure of O₂ decreases in a gradient trend from the atmosphere/oxide layer interface to the coating/oxide layer interface and then to the corrosion diffusion front of Cl,effectively blocking the O-diffusion process and improving the hot corrosion resistance of the coating.