Microstructure and properties evolution of plasma sprayed Al_(2)O_(3)-Y_(2)O_(3) composite coatings during high temperature and thermal shock treatment

Al_(2)O_(3)-Y_(2)O_(3) composite powder with TiO_(2) additive was plasma sprayed to prepare Al_(2)O_(3)-Y_(2)O_(3) composite coatings.The micro structure and properties evolution of the Al_(2)O_(3)-Y_(2)O_(3) coatings during high temperature and thermal shock resistance were investigated.The results show that the micro structure of the Al_(2)O_(3)-Y_(2)O_(3)-TiO_(2) coating is more uniform than that of the Al_(2)O_(3)-Y_(2)O_(3) coating.Meanwhile,amorphous phase is formed in the two coatings.The Al_(2)O_(3)-Y_(2)O_(3)(-TiO_(2)) coatings were heat treated for 2 h at temperatures of 800,1000 and 1200℃,respectively.It is found that the microstructure and properties of the two coatings have no obvious change at 800℃.Some of the amorphous phase is crystallized at1000℃,and meanwhile Y_(2)O_(3) and Al_(2)O_(3) react to form YAG phase and YAM phase.At 1200℃,all of the amorphous phases are crystallized.After heat treatment,the micro hardness of the two coatings is increased.The thermal shock resistance of the Al_(2)O_(3)-Y_(2)O_(3) system coatings can be improved by using TC4 titanium alloy as substrate and with NiCrAlY bonding layer.Moreover,the Al_(2)O_(3)-Y_(2)O_(3)-TiO_(2) coating exhibits better thermal shock resistance due to the addition of TiO_(2).

In-situ synthesis,microstructure,and properties of NbB_(2)-NbC-Al_(2)O_(3)composite coatings by plasma spraying

The high melting point and strong chemical bonding of NbB_(2)pose a great challenge to the preparation of high-density nanostructured NbB_(2)composite coating.Herein,we report a novel,simple,and efficient method to fabricate in-situ NbB_(2)–NbC–Al_(2)O_(3)composite coating by plasma spraying Nb_(2)O_(5)–B_(4)C–Al composite powder,aiming at realizing the higher densification and ultra-fine microstructure of NbB_(2)composite coating.The microstructure and properties of in-situ NbB_(2)–NbC–Al_(2)O_(3)composite coating were studied comparatively with ex-situ NbB_(2)–NbC–Al_(2)O_(3)composite coating(plasma spraying NbB_(2)–NbC–Al_(2)O_(3)composite powder).The reaction mechanism of Nb_(2)O_(5)–B_(4)C–Al composite powder in plasma jet was analyzed in detail.The results showed that the in-situ nanostructured NbB_(2)–NbC–Al_(2)O_(3)composite coating presented a lower porosity and superior performance including higher microhardness,toughness and wear resistance compared to the plasma sprayed ex-situ NbB_(2)–NbC–Al_(2)O_(3)coating and other boride composite coatings.Densification of the in-situ NbB_(2)–NbC–Al_(2)O_(3)coating was attributed to the low melting point of Nb_(2)O_(5)–B_(4)C–Al composite powder and the exothermic effect of in-situ reaction.The superior performance was ascribed to the density improvement and the strengthening and toughening effect of the nanosized phases.The in-situ reaction path could be expressed as:Nb_(2)O_(5)+Al®Nb+Al_(2)O_(3),and Nb+B_(4)C®NbB_(2)+NbC.