氧化对含氟化物共晶耐磨自润滑涂层机械和摩擦学性能影响

Effect of Oxidation on the Mechanical and Tribological Properties of Eutectic Fluorides Containing Wear Resistant Self-lubricating Coating

目的考察NiCr/Cr_(3)C_(2)−BaF_(2)/CaF_(2)涂层在高温氧化环境下成分与结构变化,着重研究氧化对涂层机械和摩擦学性能影响。方法采用超音速火焰喷涂(HVOF)制备出NiCr/Cr_(3)C_(2)−BaF_(2)/CaF_(2)涂层,对涂层进行700、800、850℃氧化处理,借助扫描电镜、X射线衍射仪、显微硬度计、力学性能试验机和高温摩擦磨损试验机等设备,比较了喷涂态涂层与氧化处理后涂层的微观结构、物相成分及机械和摩擦学性能。结果涂层在700℃以上氧化环境中,会发生氧化诱导下以氟化物润滑相表面迁移和表面Cr选择性氧化等行为构成的铬酸盐反应。该反应在850℃时会使涂层性能形成“嬗变”,在该温度下氧化处理后涂层的结合强度由喷涂态的75 MPa急剧下降至20 MPa,涂层近表显微硬度由735HV下降至190HV;此外,涂层在300℃时体积磨损率由喷涂态的2.19×105 mm^(3)/N·m剧增至16.3×105 mm^(3)/N·m。结论高温氧化诱导下的铬酸盐反应,不仅会破坏涂层中粘结相、耐磨相和润滑相的分布均匀性,而且会使涂层孔洞、裂纹等缺陷显著增加,由此导致涂层的机械和摩擦磨损性能大幅下降。对于工作时摩擦闪温超过800℃的含氟化物刷式封严涂层而言,涂层在经历闪点后所发生的性能“嬗变”是其短期失效的重要因素。提高涂层的抗氧化性能、降低高温下铬酸盐反应烈度将会是改善涂层失效的有效方法。

The work investigates the compositional and structural changes of NiCr/Cr_(3)C_(2)-BaF_(2)/CaF_(2) coatings under high temperature oxidation,with emphasis on the effect of oxidation on the mechanical and tribological properties of the coatings which were prepared by HVOF and oxidized at 700,800 and 850℃.With the aid of scanning electron microscopy,X-ray diffractometer,microhardness tester,tensile tester and high temperature tribometer,the microstructure,composition and mechanical and tribological properties of the as-sprayed coating were compared with those of the oxidized coatings.One specific,frictional wear test was carried out on a high temperature ball-on-disk tribotester(UMT-Tribolab).The results of previous studies have shown that the coating wears most severely when tested near 300℃.This temperature point was chosen to investigate the frictional wear performance of the coating before and after the oxidation treatment,based on the most demanding frictional wear environment.The results show that the coatings undergo oxidation-induced chromate reactions consisting of surface migration of fluoride lubricant and selective oxidation of surface Cr in an oxidizing environment above 700℃.At oxidation temperatures between 700 and 800℃,the reaction is dominated by the preferential participation of BaF_(2) in the formation of the BaCrO_(4) phase and the subsequent participation of CaF_(2) in the formation of the Ca_(2)Cr_(2)O_(5) metastable phase with a lignite type structure.As the oxidation temperature increases to 850℃,the Ca_(2)Cr_(2)O_(5) phase undergoes further oxidation and is completely transformed into the CaCrO_(4) phase.At the same time,the formation of Cr_(2)O_(3) on the surface of the coating intensifies as the Cr in the coating diffuses outwards along fast diffusion channels such as grain boundaries and lamellae interfaces,while the fluoride phase continues to migrate to the surface and the chromate reaction process proceeds rapidly,resulting in the formation of a large amount of BaCrO_(4) on the surface of the coating.The rapid chromate reaction results in the"transmutation"of the coating properties.In particular,the bond strength decreases sharply from 75 MPa in the sprayed state to 20 MPa after oxidation and the near-surface microhardness of the coating decreases from 735HV to 190HV after the oxidation treatment at this temperature.In addition,the volume wear rate of the coating after oxidation at 850℃ increases dramatically from 2.19×10^(5) mm^(3)/N·m in the sprayed state to 16.3×10^(5) mm^(3)/N·m when tested at 300℃.To summarize,the high temperature oxidation-induced chromate reaction not only destroys the uniformity of the distribution of the bonded,wear-resistant and lubricant phases,but also causes a significant increase in defects such as holes and cracks in the coating,resulting in a significant decrease in the mechanical and frictional wear properties.For fluorides containing brush seal coatings with frictional flash temperature exceeding 800℃ during operation,the performance"transmutation"that occurs after the flash point is an important factor in its short-term failure.Improving the oxidation resistance of the coating and reducing the intensity of chromate reaction at high temperature will be an effective way to improve the coating failure.