Microstructure Control And High-temperature Oxidation Resistance Of Ir Coating Prepared By Electrodeposition In Molten Salt

The preparation of Ir coating is the key technology of the third-generation Ir/Re-C/C combustion chambers used in the liquid-propellant rocket engines.Due to the high deposition rate and good throwing power,electrodeposition in molten salt is considered as an ideal method of preparing the Ir coating.At present,the compact and intact Ir coating with columnar crystal has been successfully prepared by using this method.However,so far there have been few investigations on the effects of electrodeposition process on the coating structure and the effects of coating structure on the high temperature oxidation resistance.Therefore,in this dissertation,the effects of deposition process on the microstructure and properties of Ir coating were studied.Then the effects of processing parameters of the pulse electrodeposition on the deposition rate and the covering power of the Ir coating were investigated.Finally the effect of the microstructure of the Ir coating on its high temperature oxidation resistance was studied to reveal the relationship between the microstructure and the oxidation resistance,providing theoretical and technical support for preparaing Ir coating by electrodeposition in molten salt.The influence of the current waveform,molten salt system and surface treatment on the microstructure of Ir coating was studied.The parameters of the pulse electrodeposition have a significant effect on the microstructure of Ir coating.With the increase of toff,the grain size and the microscopic compactness of the Ir coating decrease with the order of the preferred orientation changing from <111> to <200> to <311>.With the increase of Jp,the grain size of the coating decreased with the preferred orientation changing in the order of <311>-<111>-<220> and a tendency of the grain structrue transformation from columnar crystal to isometric crystal was observed.The grain size increases with the increase of the ton,and the order of preferred orientation of the coating changs from <200> to <111>.With the increase of the content of F-in the quaternary molten salt of NaCl-KCl-CsCl-NaF,the size of Ir coating was significantly reduced(< 1 ?m),while the surface roughness increases.The as-deposited Ir coating in this quaternary molten salt exhibited an equiaxed grain structure,After electroplating-mechanical polishing-electroplating process,the Ir coating exhibited a multilayered structure,and the grain size of outer layer is finer than the inner layer.The influence of the processing parameters of the pulse electrodeposition on the deposition rate and the covering power of the Ir coating was discussed.Compared with the direct current plating,the pulse electrodeposition technology has increased the deposition rate of the Ir coating by 50% due to its high current efficiency,and meanwhile the allowable upper limit of current density for the pulse current plating improves from 20mA/cm2 to 50mA/cm2 without compromise of the Ir coating quality.With increasing depth-to-width ratio of the grooves on the substrate surface,the covering power of the direct current deposition dropped significantly.The thickness of Ir coating on the bottom of the grooves dropped below 30% of that on the substrate surface.Due to the high instantaneous current density,the pulse electrodeposition technique can effectively increase the current density within the groove.As a result,the thickness of Ir coating on the bottom of the grooves only reduced to 50% of that on the substrate surface,which means that the pulse electrodeposition technology has a better covering power than the traditional direct current electrodeposition.The high-temperature thermal stability of the microstructure of Ir coating was investigated under Ar atmosphere between 1350~1450?.For the columnar crystal structure,,the final stable microstructure of the Ir coating exhibited the bulky columnar crystal grains with length-width ratio of 1:1 under all the heat treatment conditions irrespective of the original grain size.For the multi-layered columnar Ir coating,the growth of the Ir grains is restricted within each layer,while the multilayered structure is reserved.For the equiaxed Ir coating,with the coarsening of the Ir grains,the equiaxed structure is unchanged after the heat treatmentcu.The Ir/Re/Ir specimens were oxidized at 1950? in stagnant air.Comparative investigation on the oxidation of Ir coatings with different micro-structure was conducted and its failure mechanism was discussed.It was found that the lifetimes of the multi-layer columnar Ir coating and equiaxed Ir coating at 1950? were about 170 min and 210 min,respectively.The respective oxidation rates were about 1.39 mg·cm-2·min-1 and 1.18 mg·cm-2·min-1.The oxidation failure of Ir coating on Re substrates was mainly caused by uniform oxidation reduction of Ir grains and preferential oxidation of Ir grain boundaries,which was mainly caused by the boundary diffusion of Re atoms at high temperature in the Ir coating.Since the diffusion distance of equiaxed Ir coatings was much longer than that of multilayer columnar Ir coatings,the lifetime of equiaxed grains Ir coating was improved accordingly.