Preparation, Structure And Performance Of Al₂O₃-Y₂O₃ System Amorphous/eutectic Wear Resistant Composite Coating

The friction and wear of a pair under high load conditions(namely,high PV=contact pressure P?friction velocity V,generally means PV?15 MPa×m×s^-1)often accompanied with high temperature,oxygen enrichment and thermal shock in a wide temperature range.Such severe wear condition that plays a decisive role in determining the reliability and lifetime of some mechanical systems such as aerospace engines,space vehicles and some pumps or valves.The thermal conductivity of a ceramic coating is crucial in such condition.Therefore,Al₂O₃ coating has been chosen as a candidate by virtue of its better thermal conduction in comparison with other oxide ceramics?Cr₂O₃,ZrO₂,TiO₂?.However,previous studies can hardly achieve high thermal and mechanical properties simultaneously.The addition of Cr₂O₃ into Al₂O₃coating investgated by our team has improved the content ofa-Al₂O₃,mechanical and thermal conduction properties,and the related wear resistance via heterogeneous nucleation and part of the solid solution effects.The use of nano-sized feedstock can strengthen the solid solution effect,which may further improve its performance,but it is hard to guarantee the high thermal conduction and structure stability of a coating.Try to improve the content ofa-Al₂O₃,decrease the number of grain boundaries and keep the stability of the coating microstructure under high temperature,thus the mechanical,thermal conduction and the related wear resistance of the coating may be guaranteed.Directionally solidified Al₂O₃-based eutectic ceramic composite materials have become one of the hot topic of researches in the scope of high temperature structure materials by virtue of their outstanding high temperature microstructure stability and thermal-mechanical properties.Bulk Al₂O₃-based eutectic ceramic composites have no grain boundary of the same phases,no amorphous transition zone,and the phase boundary is closely bonded.Further,the high temperature microstructure stability and mechanical properties of Al₂O₃-based eutectic ceramic composites are favorable,among them,Al₂O₃/Y₃Al₅O₁₂?YAG?eutectic is a typical composition.The crucial preparation factors of these eutectic composites are high thermal enthalpy,large temperature gradient and fast cooling rate,which coincide with plasma spraying.Therefore,in this study,atmospheric plasma spray?APS?was conducted to fabricate Al₂O₃/YAG coating.The Al₂O₃/YAG eutectic composite coating can be obtained by crystallization treatment of as-sprayed amorphous Al₂O₃-YAG coating.This paper investigated micro-/nano-powders on the effect of amorphous phase formation and structure in the coating,also,the evolvement rules of structure,mechanic and thermophysical properties during the states of amorphous?crystalline are studied.Based on the previous study,the wear resistant assessment of the coatings were conducted in order to realize the coating with high thermal conduction,stable microstructure and thermal-mechanical performance.Thus,a scientific and technical guidance can be provided for the coating applied in severe wear conditions.The main conclusions drawn from the study are as follows:?1?Achieved a kind of nano-granulated Al₂O₃/Y₂O₃ powders that suitable for thermalspray.The powders were fabricated by spray drying technique and followed by1000?@2h calcination process.Moreover,the powders display nearly sphericalshape and Al₂O₃&Y₂O₃ nano-particles were uniformly mixed in a singlegranulated particle.?2?With nano-granulated powders as a feedstock,the amorphous Al₂O₃-YAG coatingcan be fabricated by plasma spraying,and a small amount of Al₂O₃ and YAGphases homogeneously distributed in the coating.The formation mechanism ofamorphous phases were attributed to the large cooling rate of melted splats duringthe plasma spraying process,thus resulted in low solid-liquid interface temperatureduring solidification which easily contributed to solute trapping,followed with thestopping grow of eutectic phases and then amorphous phases formed.Usingmicron-sized coarse grain mixed powders as a feedstock,the Al₂O₃ and Y₂O₃crystalline phases were strip-like arrangements in the as-received coating and lowcontent of amorphous phases existed.?3?Achieved the crystallization behavior of Al₂O₃-YAG amorphous coating.Thecrystallization of the coating belongs to a two-stage crystallization process,whichYAG precipited first and followed with the precipitation ofa-Al₂O₃.Based on theevolvement rules of crystallized volume fraction under the effect of temperatures,the crystallization mechanism for YAG anda-Al₂O₃ were bulk nucleation withthree-dimensional growth and two-dimensional growth in the initial,and graduallychanged to surface nucleation,respectively.The calculated values of activationenergy for crystallization showed that the nucleation of YAG grains were easierthana-Al₂O₃ grains.Besides,the high values of T_g,T_c,E_c,and E_c/RT_g denote anoutstanding high temperature thermal stability of the Al₂O₃-YAG amorphouscoating.?4?According to the crystallization behavior of Al₂O₃-YAG amorphous coating,itsmechanic and thermophysical properties were further studied before and aftercrystallization.Compared with Al₂O₃ and Al₂O₃-Cr₂O₃ coating,Al₂O₃-YAGcoating possesses superior thermal diffusivity and toughness,which conducive toreduce the cumulated thermal stress as well as brittle peel off of coatings duringthe friction process.The hardness of as-sprayed Al₂O₃-YAG coating was lowerthan Al₂O₃ and Al₂O₃-Cr₂O₃ coating.With crystallization proceeding,the hardnessof Al₂O₃-YAG coating gradually increased.The hardness of Al₂O₃-YAG eutecticcoating surpassed 15 GPa,which was higher than that of Al₂O₃ coating?9.4 GPa?and Al₂O₃-Cr₂O₃ coating?12.1 GPa?.?5?Contrast experiments were carried out to evaluate the wear resistance of coatings.The results showed that,only Al₂O₃-YAG coatings maintained its surface integrityafter wear tests,indicating it has the best wear resistance.However,certain crackswere both existed in Al₂O₃?Al₂O₃-Cr₂O₃?Al₂O₃-Y₂O₃ coatings after wear tests.Moreover,the Al₂O₃-YAG coating also has favorable stability of frictioncoefficient and low worn surface temperature.?6?Based on the morphologies of worn surfaces,the worn mechanisms of coatingswere elucidated.The worn mechanism of Al₂O₃-YAG coating was plasticdeformation in rich Al amorphous region and form strip-like spreading layersunder thermal-mechanical coupling condition,thus greatly alleviated the stressconcentration.Then the plastic deformated layers were gradually peeled off andform hard fine debris during the wear process,which intensified the abrasive wearand resulted in the continuously losing of the coating surface.The wornmechanisms of Al₂O₃?Al₂O₃-Cr₂O₃ and Al₂O₃-Y₂O₃ were similar.Under thefunction of thermal-mechanical coupling,the cracks initiated from the surfaces ofcoatings,then cracks extended,delaminated and then abrasive wear formed whichintensified the wear of the coating.