镍基单晶合金MCrAlY涂层防护性能研究

【作者】 罗顺；

【导师】 刘立斌； 陈和兴；

【作者基本信息】 中南大学 ， 材料学， 2009， 硕士

【摘要】 单晶高温合金由于其优秀的高温机械性能,目前已经成为航空发动机叶片的首选材料。然而,单晶高温合金叶片的抗高温氧化和抗熔盐的热腐蚀性能较差,需要进行表面防护处理。本文采用低压等离子喷涂技术（LPPS）在单晶高温合金上制备了NiCoCrAlYTa六元合金涂层。并对涂层进行了真空热扩散处理。采用X射线衍射（XRD）、带能谱的扫描电镜（SEM/EDS）、电子探针和显微硬度仪等分析手段对喷涂态和热处理态涂层的相组成、显微结构、结合强度和韧性等进行了评价,重点对单晶高温合金和有涂层的单晶高温合金的高温氧化和两种熔盐（Na₂SO₄熔盐和75%（wt%）Na₂SO₄+25%（wt%）熔盐）的热腐蚀行为进行了研究,探讨高温氧化和热腐蚀失效机理,主要的研究结果如下:1)喷涂态涂层结构致密、元素分布均匀、孔隙率低。主要相由γ-Ni相、β-NiAl相和少量γ’-Ni₃Al组成。热处理后涂层与基体出现互扩散。涂层与基体结合良好。2) 900℃条件下单晶合金的抗高温氧化性能远远不及涂层的抗高温氧化性能,单晶经过100h氧化后出现剥落失重。在覆盖有涂层后,试样表现出优异的抗氧化性能,在900℃、1000℃、1100℃氧化200h后其仍具有完整的氧化膜,增重曲线分别符合对数规律、抛物线规律、立方规律。增重量都在0.003mg/cm²以下。在不同温度下形成的氧化膜具有不同的晶型。3)单晶合金在Na₂SO₄熔盐和75%（wt%）Na₂SO₄+25%（wt%）熔盐中都未能形成有效的保护膜,试样在早期就出现破裂或者剥落,最终都出现了严重的内氧化和内硫化。单晶合金在两种盐中形成不同形貌氧化膜。NiCoCrAlYTa涂层在在Na₂SO₄熔盐和75%（wt%）Na₂SO₄+25%（wt%）熔盐中都具备较好的抗热腐蚀性能,主要原因是在试样表面都能形成以Al₂O₃为主的致密连续的氧化膜。在混合熔盐中由于Cl^-的加入,涂层氧化膜出现了开裂,涂层出现了轻微的内硫化和内氧化。更多还原

【Abstract】 Becauses of the excellent high temperature mechanical performance, the single crystal superalloy have been chosen as the primary aeroengine materials. Howerver, a protective coating must be performed due to its poor high temperature oxidation resistance and hot corrosion resistance to molten salt.In this paper, the NiCoCrAlYTa coating was sprayed on the substrate of the single crystal superalloy by low pressure plasma spray technique, and then the sprayed coating was vacuum heat treated. The phase composition, microstructure, adhesive strength and toughness of sprayed coating and heat treated coating were characterized by using the XRD, SEM, EDS, EMPA and micro-hardness tester analysis equipment. The high temperature oxidation resistance and the hot corrosion resistance in the Na₂SO₄ melted salt and 75%（wt%）Na₂SO₄+25%（wt%） melted salt were investigated. The results show that:The sprayed coat possesses dense and even micro-structure with very low porosity. The sprayed coating is mostly composed byγ-Ni andβ-NiAl, also with a few content ofγ’ -Ni₃Al. After heat treat, mutural elemental diffusions between the coating and single crystal superalloys take place, which revealed the good adhesive strength for the treated coating.The oxidation resistance for coated single crystal superalloy at 900℃is much better than that of single crystals without coating. The single crystals without coating present great weight lost after 100h at 900℃, while the coated single crystal exhibited excellent oxidation resistant, because a dense protective film was formed after 200h at 900℃, 1000℃and 1100℃. The oxidation kinetics curve tends to be logarithm shape at 900℃, a near-parabolic law at 1000℃, and cube curve at 1100℃. The mass gain was below 0.003mg/cm . The oxidation films present as different crystal structure at various temperature.Effectively protective films did not form in the single crystals at the Na₂SO₄ molten salt and the 75%（wt%）Na₂SO₄+25%（wt%） molten salt, which lead to occurance of internal oxidation and sulfidation,and finally the breakdown and exfoliation of superalloy in a very short time. NiCoCrAlYTa coated superalloy present good corrosion resistance at the Na₂SO₄ molten salt and the 75%（wt%）Na₂SO₄+25%（wt%） molten salt., due to the formation of continuous and dense Al₂O₃ films. But the latter reveal that crack, slight internal oxidation and internal sulfidation take place in mixed molten salt corrosion because of the strong penetration and corrosion of Cl^-.更多还原