Development Of Nanostructured ZrO₂/Y₂O₃ Powder And Thermal Barrier Coating

Zirconia has low thermal conductivity and high thermal expansion coefficient close to nickel-based superalloy substrate. It is an ideal material used as thermal barrier coatings applicable to high temperatures. Plasma spraying is the most often used method for depositing thermal barrier coatings. Conventional ceramic coatings, consisted of micro-lamellae, are brittle. However, nanostructured ceramic materials have low brittleness and improved toughness. ZrO2/Y2O3 nanocrystalline powder, prepared by the method of solution precipitation, can not be directly used as thermal spray feed stock, because of its low mass of individual nanocrystalline particle.This study was to prepare ZrO2-8%Y2O3 agglomerated nanocrystalline particles and to form nanostructured thermal barrier coatings by plasma spraying under varied conditions. Spray drying and heat treatment were used to reprocess the nanocrystalline ZrO2-8%Y2O3 powder purchased from market. Optical microscopy and scanning electron microscopy were used to observe the morphology and particle size of the starting nanocrystalline powder and the agglomerated powder as well as to examine the microstructure of the thermal barrier coatings. X-ray diffraction was used to analyze the crystal structure of the powders and the coatings. Thermal gravimetric analysis and diffraction thermal analysis were also conducted. In addition, free holding density, tap density, and flow ability of the powders were measured. Experimental results show that the large agglomerated particles are spheric, have the size of 10-100um, and remain original nanostructure. Such powders with good flow ability and high tap density are suitable for thermal spray used in the deposition of nanostructured thermal barrier coatings. Experimental results of the thermal barrier coatings deposited from the agglomerated nanocrystalline powders with different particle size distributions (<45um and 45~75um) demonstrate that the thickness of the as-sprayed coatings is uniform, and the lamellae always found in conventional microstructured coatings are not obvious in these nanostructured coatings. The porosity of the best coating sample is approximately 7%. TheHImonoclinic zirconia phase existed in the starting powder has been transformed into tetragonal phase after the plasma spraying. The nanostructured coatings are composed of tetragonal and cubic phases. It is revealed that these coatings consist of a large fraction of nanostructure transformed from unmelted agglomerates and a mall fraction of microstructure formed from melted agglomerates in plasma.