Microstructure and Failure Behavior Controlling of NiCoCrAlYTa Based Plasma Coatings

Author:Tao Li

Supervisor:wang lei


Degree Year:2017





Surface coating technology on the metals is one of the most effective methods to prolong the life of industrial equipment servicing at high temperature,such as aerospace engine,steam turbine and so on.It is of great significant to develop the coatings with excellent mechanical properties corrosion resistance and oxidation resistance avoiding from the failure of oxidation and corrosion at high temperature.Therefore,a new type of Al2O3 and Cr2O3 composite oxide reinforced ceramic coating is researched in the present study.The effects of microstructure of oxide ceramic coating on the mechanical properties,high temperature oxidation resistance and corrosion resistance are examined,through the controlling of the compositions and microstructure,and mechanism is also discussed,in order to provide a fundamental for the safety application in the engineering.One alloy powder and two commercial oxide ceramic powders were selected as the raw materials in the present study.The NiCoCrAlYTa-Al2O3/Cr2O3 coatings with different ratios of Al2O3 and Cr2O3 were prepared by atmospheric plasma spraying.The microstructures of the coatings were studied by an optical microscope,a scanning electron microscope and a transmission electron microscope,respectively.The results show the phase of Cr2O3,Ni3Al and Al2O3 crystals and some amorphous or nanocrystalline were obtained in NiCoCrAlYTa-Al2O3/Cr2O3 coating by adjusting the ratios of Al2O3 and Cr2O3.The formation of amorphous or nanocrystalline phases is ascribed to the rapid cooling of coatings,and these phases are beneficial to improve the mechanical properties of coatings.The structure the coating is layered,parallelling to the interface of coating and substrate,which includes melted or semi melted particles,micro cracks and black substances.It is also found that there is a negative correlation between the porosity and microhardness.When m(Al2O3):m(Cr2O3)was 3:2,the porosity of the coatings is the lowest,while the microhardness of the coatings is the highest.This kind of negative correlation is related to the solubility,particle size and shape of the two kinds of oxide ceramics.The thermal shock properties of the coatings at 700℃,900℃ and 1100℃,respectively were investigated.The results show that,the cycles of coatings thermal shock decrease as the rise of the temperature.That indicates the temperature has a decisive influence on the cycles of coatings thermal shock.Furthermore,the thermal shock performance of the coating only with Cr2O3 exhibits better than that of other coatings,which proves that Cr2O3 in coatings is helpful to improve the performance of coating thermal shock.There are two reasons.On one hand,the thermal expansion coefficient of Cr2O3 is closer to that of NiCoCrAlYTa and austenitic stainless steel.On the other hand,the powder particles of Cr2O3 and NiCoCrAlYTa are both spherical.And the pores in NiCoCrAlYTa-Cr2O3 coating are also spherical and small which are uniform distributed.Thus these pores could alleviate the thermal stress during thermal shock.By studying the high temperature friction properties of the coatings,it is found that the friction resistance of the NiCoCrAlYTa-Al2O3 coating is lower than that of other coatings,which is due to lower microhardness and density.At the initial stage of high temperature friction,the coating shows a good friction resistance.The main friction mechanism of this stage is the slight plastic deformation.At the middle stage,irregular pits appear on the coating surface,so the friction mechanism of this stage is micro fracture and fatigue wear.At the last stage,many pits on the coating surface are filled the wear debris.And there are not any obvious cracks.Fe had been transferred from friction pair to the coating surface.The friction mechanism of this stage is adhesive wear.The oxidation behaviors of the coatings at 700℃,900℃ and 1100℃,respectively have been studied.It has been found that the oxidation resistance of NiCoCrAlYTa-Cr2O3 coating is better than that of other coatings at 700℃.But at 900℃,the oxidation resistance of NiCoCrAlYTa-Cr2O3 coating is worse than that of other coatings,and the oxidation resistances of the other coatings is close to that of each other.While at 1100℃,the oxidant resistances of the different coatings have no significant difference.The oxidation process is accompanied with the formation of Cr2P3 and Al2O3 and the transformation from 0-Al2O3 to a-Al2O3.The corrosion behaviors of the coatings at 700℃,900℃ and 1100℃,respectively have been studied.The corrosion kinetic curves of the coatings show a rapid rise within 15 hours at 700℃.After 20 hours,the corrosion kinetic curves of the coatings rise slowly and steadily.While at 900℃,the corrosion resistance of the NiCoCrAlYTa-Cr2O3 coating was the worst.At 1100℃,the coatings were seriously damaged within 25 hours,because this temperature is close to the upper limit of the bearing capacity of the coating.The performance of Al2O3 protective film is better than that of Cr2O3 film at high temperature.That is ascribed to several orders of magnitude lower diffusion coefficient of metal ions and oxygen ions in Al2O3 than that of Cr2O3.But on the other side,the atmospheric plasma sprayed coatings has low density,containing a large number of holes and cracks,so the corrosion medium could enter the inner coating through defects in coatings,accelerating the failure of coating structure and consuming a large amount of Al.Along with the corrosion process going the Al content in the coating could not sustain the growth of protective Al2O3 film,so the protective Cr2O3 film could be replaced the Al2O3 film on the coating surface.