High Temperature Oxidation Mechanism Of MoAlB Modified Glass-ceramic Coatings On Titanium Alloy

The glass coating has become one of the high temperature protective coatings on titanium alloy surface because of its high chemical and high temperature stability,no short circuit diffusion channels such as grain boundary,strong blocking effect on oxygen and other advantages,but the intrinsic brittleness and crack sensitivity of the glass coatings limit their wide application.In this paper,the MAB phase MoAlB with excellent metal and ceramic properties was used to modify the glass coating.Both the glass coatings（PE）and the glass-ceramic composite coatings（P15MAB,P25MAB,P35MAB）modified with different contents（15/25/35 wt.%）of MoAlB particles were prepared on BT25y titanium alloy by air-spraying and high temperature sintering.The mechanical properties at room temperature,oxidation resistance and thermal shock behavior at 800℃of both glass coatings and glass-ceramic composite coatings were systematically studied,and the strengthening mechanism and oxidation resistance mechanism were clarified.The glass-ceramic coatings modified by MoAlB particles showed good film-forming properties when prepared at 1000℃+10 min.With the increase of the MoAlB content（35 wt.%）,the film-forming property of the coating decreased and the thickness of the brittle oxygen-dissoloved layer in the alloy increased.Additionally,the hardness of the glass coating increased from 590 HV～650 HV to 1600 HV～1650 HV,and the cracking load increased from 100 g to200 g.The fracture toughness increased with the increase of the MoAlB content,and the toughening mechanism was microcrack toughening,crack deflection and crack bridging.The crystallization behavior of glass coatings and their oxidation resistance at 800°C can also be influenced by the MoAlB particles.In the sintering and oxidation process,Al atoms in MoAlB diffused outward and reacted with oxygen to formγ-Al₂O₃,Mo Al_1-xB and Mo B,forming a“core-shell structure”in which the MoAlB and Mo B were enveloped by the nanocrystalline Al₂O₃.Al₂O₃ in the shell was beneficial to improve the oxidation resistance of the coating,and MoAlB+Mo B in the core could enhance the mechanical properties of the coating.Mo B was further oxidized to Mo O₃ and B₂O₃.As a result,the liquid Mo O₃ and B₂O₃increased the glass flowability,inhibited the precipitation of Zr O₂ and Zr Si O₄ in the coating,accelerated the interfacial reaction between the coating and the alloy,and finally generated a complex interface layer composed of discontinuousα-Al₂O₃+thicker（Ti,Zr）₅Si₃+thinner（Ti,Zr）₆Si₃,which effectively blocked the diffusion of oxygen to the alloy.Though the higher the content of the MoAlB particles,the greater the oxidation weight gain of the coating,the isothermal oxidation kinetic curves of both PE coatings and P15MAB,P25MAB and P35MAB coatings oxidized at 800°C for 100 h followed the parabolic law,showing excellent high-temperature oxidation resistance.The intrinsic brittleness,crack sensitivity and large thermal mismatch between the glass and the titanium alloy led to the cracking and spalling of PE coating under thermal shock（water quenching）at 800℃,and some cracks even extended into the alloy.The glass-ceramic composite coatings were well combined with the alloy.With the increase of the MoAlB content,the content of Zr Si O₄ with low thermal expansion coefficient decreased,while the content of MoAlB+Al₂O₃ that has the thermal expansion coefficient between the glass coating and the substrate increased.Consequently,the thermal mismatch between the coating and the substrate was significantly improved,which reduced the cracking tendency of the coating and thereby enhanced the fracture toughness of the coating.At the same time,the oxidation products（liquid B₂O₃,Mo O₃）of MoAlB particels increased the glass flowability and crack self-healing ability.As a result,the glass-ceramic composite coatings showed excellent thermal shock resistance,and P25MAB and P35MAB coatings showed higher thermal shock resistance that of P15MAB coatings.In general,P25MAB coatings prepared under the sintering process of 1000°C+10 min showed good film-forming characteristics and exihibitd excellent mechanical properties,oxidation resistance and thermal shock resistance,which can provide effective high temperature protection for BT25y titanium alloy.