AlCr(Si)N和CrAl(Si)N涂层对TiAl合金900℃循环氧化性能的影响

Influence of AlCr(Si)N and CrAl(Si)N Coatings on the Cyclic Oxidation of TiAl Alloys at 900℃

TiAl合金作为新型高温结构材料在航空航天和汽车工业领域已获应用,然而在850℃及以上温度服役时抗氧化性不足,施加氮化物涂层在提高TiAl合金抗氧化性和耐磨性等综合性能方面独具优势,目前关于氮化物涂层对TiAl基合金抗氧化性影响的研究有限。采用多弧离子镀方法在TiAl合金表面分别制备AlCrN涂层、AlCrSiN涂层、CrAlN涂层和CrAlSiN涂层,研究涂层对TiAl合金900℃循环氧化行为的影响。XRD结果表明,AlCrN和AlCrSiN涂层主要呈现AlN结构,而CrAlN和CrAlSiN涂层为CrN结构。在AlCrSiN和CrAlSiN涂层中,Si可能固溶于晶格中形成(Al,Cr,Si)N和(Cr,Al,Si)N固溶体。在900℃经过300个周期的循环氧化,AlCrN和AlCrSiN涂层表面氧化膜主要由Al2O3组成,热循环过程中涂层中形成大量裂纹。CrAlN和CrAlSiN涂层表面氧化膜主要由Cr_(2)O_(3)组成,连续致密,无开裂剥落。其中CrAlN涂层表面Cr_(2)O_(3)膜下面形成了Al的内氧化物,其氧化增重高于CrAlSiN涂层。此外,CrAlN和CrAlSiN涂层均与TiAl合金发生较严重的互扩散,在CrAlN/TiAl界面和CrAlSiN/TiAl界面处形成较厚的互扩散层。可见,CrAlN和CrAlSiN涂层显著提高了TiAl合金的抗高温氧化性能,而涂层中Si的添加使得涂层抗氧化性得到进一步的提升。研究结果可为TiAl合金施加氮化物涂层高温防护提供潜在可能。

TiAl-based alloys are used in the aerospace and automotive industries because of their low density,reliable strength,and good oxidation resistance.However,the Al content of the TiAl-based alloys is approximately 50 at.%;thus,it cannot form a protective alumina scale when oxidized over 850℃,resulting in poor high-temperature oxidation resistance.Many coatings such as metallic,ceramic,aluminide or silicide diffusion,and glass coatings have been investigated to improve the oxidation resistance of TiAl alloys.Furthermore,the effects of alloying or halogens on the oxidation of TiAl alloys have been studied.Nitride coatings have unique advantages for improving the comprehensive properties of TiAl alloys owing to their wear and oxidation resistance.However,studies on the effect of nitride coatings on the oxidation resistance of TiAl-based alloys are limited.In this study,a multi-arc ion plating system was used to deposit AlCrN,AlCrSiN,CrAlN,and CrAlSiN coatings on a TiAl alloy.The influence of AlCr(Si)N and CrAl(Si)N coatings on the cyclic oxidation of the TiAl alloy was investigated at 900℃.Furthermore,the influence of the Al content on the coating structure and thermal cycle resistance was investigated.The as-deposited AlCr(Si)N and CrAl(Si)N coatings were homogeneous,compact,and well combined with the matrix alloy.Compared with the CrAl(Si)N coatings,the AlCr(Si)N coatings exhibited a deeper contrast in scanning electron micro scopes back scattered electron pattern,which might be related to the higher Al content.X-ray diffraction patterns showed that the AlCrN and AlCrSiN coatings exhibited an AlN structure,whereas the CrAlN and CrAlSiN coatings exhibited CrN structures.Si may have dissolved into the crystal lattice of the AlCrSiN and CrAlSiN coatings to form(Al,Cr,Si)N and(Cr,Al,Si)N solid solutions.During oxidation for 300 cycles at 900℃,oxide scales primarily composed of Al2O3 formed at the surface of AlCrN and AlCrSiN coatings,and numerous cracks formed in the coatings.Aluminum and titanium reacted with oxygen through cracks to form many ridged oxides on the surface of the coatings.However,the matrix was severely oxidized.Therefore,the AlCr(Si)N coatings exhibited poor thermal cycle resistance,which could not improve the cycle oxidation resistance of TiAl alloys.Therefore,research on the development of coatings should consider not only the improvement of oxidation resistance,but also other properties to adapt to complex environmental conditions in the actual service process.After oxidation,continuous and dense oxide scales without cracking or peeling formed on the surface of the CrAlN and CrAlSiN coatings primarily composed of Cr_(2)O_(3).The weight gain of the CrAlN coating was higher than that of the CrAlSiN coating,which was attributed to the formation of an internal oxidation zone under the outer Cr_(2)O_(3)layer.Therefore,the addition of Si efficiently inhibited the formation of an inner oxidation zone in the CrAlSiN coating.In addition,the CrAl(Si)N coating decomposed into CrN,Cr2N,Cr,and h-AlN after annealing.After oxidation,thick interdiffusion zones formed at both the CrAlN/TiAl and CrAlSiN/TiAl interfaces owing to the significant interdiffusion between the CrAlN or CrAlSiN coatings and the TiAl matrix.TiN and Ti2AlN beneath the CrAlSiN coating in the interdiffusion zone(IDZ)at the CrAlSiN/TiAl interfaces were primarily formed by the diffusion of N from CrAlSiN to TiAl.Moreover,the released Al diffused from the TiN and Ti2AlN layers to form TiAl2.In the discontinuous Laves phase,Ti5Si3 and Al3Nb were formed between the TiAl2 and nitride layers,and the diffusion barrier of Si addition for N to the TiAl alloy was not apparent,which might be due to the lack of formation of a continuous Ti5Si3 layer in the IDZ.It could be concluded that both the CrAlN and CrAlSiN coatings significantly improved the high-temperature oxidation resistance of the TiAl alloy,and the addition of Si to the coating further improved the oxidation resistance.These results provide prospects for the application of nitride coatings for high-temperature oxidation protection of TiAl-based alloys.