Study On Construction Mechanism Of Ceramic-metal Coating System By Plasma Spraying

Plasma spraying can construct coatings with special properties on the surface of the substrate to strengthen or improve the surface properties of the parts in a targeted manner,enabling some parts to overcome oxidation,high temperatures,corrosion,and other harsh service environments.Based on years of research,ceramic-metal coating systems combining a ceramic top-coat and a metal bond-coat are gradually getting widely used.Previous research on coating construction has mainly focused on single-layer coatings,and there is still to be improved in its validity and reference function when exploring the construction of ceramic-metal coating systems.The present study constructed three ceramic-metal coating systems（Y₂O₃-MCrAlY,8YSZ-MCrAlY,and Cr₂O₃-Ni20Cr）on nickel-base alloy（Inconel 718 alloy）substrates by plasma spraying and investigated the main construction mechanisms by using both experiments and simulations,which can provide an effective guide for the construction of high-quality ceramic-metal coating systems in industry.The main work and its results are as follows:（1）The morphological structure characterizations reveal that:Y₂O₃,8YSZ,and MCrAlY coatings have large thickness changes,no obvious stacking texture,and are constructed with a high softening degree;Cr₂O₃ and Ni20Cr coatings have stable thickness changes,obvious stacking texture,and are constructed with a low softening degree;Y₂O₃ droplets spread in a pie shape,8YSZ droplets spread in a crater and pie shape,and Cr₂O₃ droplets spread in a hill shape.The elemental diffusion analyses reveal that the diffusion occurs weakly between the top-coat and the bond-coat,while being relatively obvious between the substrate and the bond-coat.（2）Performance tests have shown that each coating has good basic properties:the porosities of Cr₂O₃,8YSZ,Y₂O₃,Ni20Cr,and MCrAlY coatings are 12%-17%,10%-16%,1%-3%,3%-7%,and 1%-2%,respectively;the average microhardnesses are1007.15 HV_0.5,730.27 HV_0.5,478.69 HV_0.5,320.77 HV_0.5,and 280.54 HV_0.5,respectively.The average bond strengths of Y₂O₃-MCrAlY,8YSZ-MCrAlY,and Cr₂O₃-Ni20Cr are 61.5 MPa,47.8 MPa,and 31.2 MPa,respectively.（3）In the simulation of the coating construction,the 8YSZ particles are constructed in a crater shape with a high softening degree（high degree of strain）,the Y₂O₃ particles are constructed in a pie shape with a moderate softening degree（moderate degree of strain）,and the Cr₂O₃ particles are constructed in a hill shape with a minimum softening degree（minimum degree of strain）.The comparisons show that the actual droplet spreading is consistent with the simulation results,which show the reasonableness of this simulation model.（4）The particle impact simulations indicate that the temperatures of MCrAlY and Ni20Cr particles are 1400-1800 K,and the corresponding equivalent stresses are about75 MPa and 100 MPa,respectively.The temperatures of 8YSZ,Y₂O₃,and Cr₂O₃particles are about 1800 K,1900 K,and 2150 K,respectively,and the corresponding equivalent stresses are about 25 MPa,50 MPa,and 75 MPa,respectively.The temperature of the overall coating mainly results from the top-coat particles,while the equivalent stress mainly results from the bond-coat particles.In addition,the difference in particle temperature is considered to be mainly due to the difference in their thermal conductivity,while the difference in particle equivalent stress is mainly due to the difference in their softening degree.（5）The substrate impact analyses indicate that the Inconel 718 substrates produce multiple circular high-temperature zones and circular stress zones at 600 ns.At 1400 ns,more circular high-temperature zones and circular stress zones are generated,all high-temperature areas expand,and the overall temperature increases slightly.At 2400 ns and4800 ns,all high-temperature areas continue to expand while the overall temperature decreases slightly,and the overall stress is similar to that at 1400 ns.Therefore,considering that the temperature and equivalent stress of the impacted surface of the substrate are mainly related to the impact of the metal bond-coat particles,the metal bond-coat constructed first can protect the substrate and reduce the effect of the later-constructed ceramic top-coat on the substrate.