| Fusion energy is a promising energy because of its cleanliness and high efficiency.Hydrogen isotope deuterium and tritium are used as fuel in the fusion reactor,however,hydrogen and its isotopes have strong penetrating power,which results in the hydrogen embrittlement of structural materials and loss of nuclear fuel.Therefore,preventing the diffusion of hydrogen and its isotopes in structural materials is an important issue.Since the diffusion coefficient of hydrogen in ceramic materials is several orders of magnitude lower than that in stainless steel,a coating that impedes hydrogen permeation is often prepared on the surface of the material.Based on the difference of thermal expansion coefficient between oxide layer and stainless steel substrate,the iron-aluminum alloy bond coat was introduced to reduce the thermal stress inside the coating and a new composite coating preparation process was proposed:low-temperature pack cementation aluminizing process is adopted to prepare the iron-aluminum alloy bond coat on the substrate,then ZrO2 film is formed on the surface of the bond coat by sol-gel method.Electrodeposition is further used to densify the ZrO2 coating since the sol-gel method is prone to produce pores.Finally,a phosphate protective layer is prepared.In addition,the effects of preparation process parameters on the microstructure and properties of the coating were investigated.This study provides a new idea for the engineering application of hydrogen barrier coating.The main conclusions are as follows:(1)The iron-aluminum alloy bond coat was prepared on the 316L stainless steel substrate by low-temperature pack cementation.The effects of different substrate surface roughness and aluminizing agent composition on the coating surface roughness,phase structure and the thickness of each phase in the aluminide coating were studied.The surface of the coating became smoother with smaller substrate surface roughness and higher AICl3 content.With the increase of AICl3 content,the thickness of Fe3Al,FeAl and FeAl3 phases in the aluminide coating decreases,while the Fe2Al5 phase remains unchanged.The phase structure of the aluminized coating changed after heat treatment.After 30h of heat treatment,the initial phase structure of Fe3Al,FeAl,Fe2Al5 and FeAl3 completely converted into the structure of Fe3Al and FeAl,and the brittle phases of Fe2Al5 and FeAl3 completely disappeared.The average bonding strength of the aluminized coating to the substrate was 67.60 MPa and the coating has excellent thermal shock resistance.At 600?,the aluminide coating is compatible with CO2 and the CO2 gas has a repairing effect on the cracking phenomenon of the coating.(2)The formation mechanism of the aluminized coating by low-temperature pack cementation was studied from the perspective of thermodynamics and kinetics.The aluminum reacts with the activator AlCl3(g)to form active aluminum atoms.A Fe2Al5 phase layer is formed on the surface of the substrate with the rapid infiltration of aluminum atoms.Some active aluminum atoms react with the surface Fe2A15 layer to form FeAl3 phase.At the same time,the Fe2Al5 phase in contact with the substrate reacts with the iron atoms to form FeAl phase.The Fe3Al phase is formed during the cooling process.The growth activation energy and diffusion constant of the aluminized coating were 164.78 kJ/mol and 3.86×10-4 m2/s,respectively.The relationship between the growth rate of the aluminized coating and the temperature is D=(3.86×10-4)exp(-16478/RT)The lower activation energy value is determined by the phase structure of the aluminized coating.(3)The zirconia coating was prepared on the iron-aluminum alloy bond coat by a composite process of sol-gel and electrodeposition method.The effects of pulling speeds,coating times and heat treatment temperatures in sol-gel and deposition current,time and heat treatment temperatures in electrodeposition were investigated.The ZrO2 film appears as a partial laminate structure and porous structure after sol-gel.After coating 6 times at 120 mm/min,the film had a porosity of 6.26%?8.96%and pore diameter of 1.67 ?m?1.86 ?m.After 4 mins of electrodeposition,the pores on the surface of the zirconia film were filled,but pores were still present inside the coating.After annealing at 600?,the ZrO2 coating is a mixed phase of tetragonal and monoclinic.(4)The average bonding strength of the ZrO2 coating to the aluminized coating is about 37.30 MPa.After 40 thermal shocks,the coating did not peel off and the thermal shock resistance was excellent.The ZrO2 coating has good compatibility with CO2 gas.The deuterium permeation test was conducted at 350?,450?,550?,600? and 650?,and the PRF values were 1626.5,171.5,114.8,30.2 and 16.7,respectively.(5)A phosphate protective coating was prepared by sol-gel method.The effects of pulling speed and phosphate concentration on phosphate coating were studied.The phosphate coating can almost completely cover the zirconia coating with high concentration phosphate solution and a pulling speed greater than 90 mm/min.The bonding strength between the phosphate coating and the zirconia coating was 13.33 MPa.The phosphate coating has good thermal shock resistance and good compatibility with CO2 gas. |
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