氮化鉻/氮化鋁奈米多層硬質薄膜氧化行為、微觀結構與熱穩定性

Oxidation Behavior, Microstructure Characterization and Thermal Stability in CrN/AlN Multilayer Hard Coatings

田世綱(Shih-Kang Tien)

指導教授 ： 杜正恭

國立清華大學/工學院/材料科學工程學系/博士(2007年)

摘要

表面工程技術是藉由表面改質或是表面鍍膜來保護工件、延長使用壽命、降低生產成本以及擴展應用領域。而硬質鍍膜由於具有高強度、抗氧化、抗腐蝕、耐磨耗等機械特性以及其他光、電、熱性質，可廣泛使用在各種工業領域，例如半導體中的擴散屏障層與絕緣層、切削刀具保護層、裝飾鍍層以及汽車工業中抗高溫腐蝕與磨耗的引擎等。本研究採以兩種具高性能之氮化鉻與氮化鋁材料，復以反應性磁控濺鍍系統沈積形成不同週期厚度的奈米多層氮化鉻/氮化鋁硬質薄膜。由實驗的結果發現，結合氮化鋁以及氮化鉻形成奈米多層膜結構的硬度高達30 GPa，比單層氮化鉻(20 GPa)與氮化鋁(12 GPa)的平均硬度提升將近一倍。硬度的提升主要是來自於奈米多層膜內由氮化鋁轉變成介穩態所形成的契合界面所致。此外，在850oC高溫真空熱處理環境之下，氮化鋁/氮化鉻奈米多層薄膜依然能維持如初鍍狀態般的高強度。在900oC高溫氧化實驗中，藉由微結構、表面粗糙度與機械強度測試結果發現，奈米氮化鉻/氮化鋁多層硬質薄膜展現了優越的抗氧化能力，甚至較單相的氮化鋁薄膜要好，主要是因為生成互溶且具非晶質(AlxCr1-x)2O3氧化物，在熱力學上較單獨的Al2O3穩定。再者，相較於具有較大週期之奈米氮化鉻/氮化鋁多層薄膜在經過950oC、1小時高溫或是800oC、16小時長時間熱處理條件下開始形成局部結晶性的富鉻相的氧化物，具有較小週期之奈米氮化鉻/氮化鋁多層薄膜表面生成的氧化層依然能維持非晶質的(AlxCr1-x)2O3氧化物。如此證明了多層薄膜中許多的契合界面也具有阻止氧化擴散的發生。另外，本研究也提出了氮化鉻/氮化鋁與目前常被研究氮化鈦/氮化鋁奈米多層薄膜性質上的比較。整體言之，具有較小週期之奈米氮化鉻/氮化鋁多層硬質薄膜在機械性質、高溫抗氧化與熱穩定性上都有卓越的提升效果。

關鍵字

氮化鉻/氮化鋁 ； 奈米多層薄膜 ； 抗氧化行為 ； 硬度強化

並列摘要

Surface modification engineering, including coating deposition and surface treatment techniques, is the technology to deposit a foreign material onto the surface of interest to improve specific desired properties. In this study, nanostructured CrN/AlN multilayer coatings with different modulation periods were fabricated by RF magnetron sputtering technique. The hardness of as-deposited CrN/AlN coating with of 4 nm was 28.2 GPa, which was 60% higher than that estimated by rule of mixture. The hardness enhancement was caused by the specific coherent interfaces between cubic CrN and metastable cubic AlN. The enhanced hardness for CrN/AlN multilayer coatings annealed at 850oC in vacuum could prevail, similar to the as-deposited state, and the nano-layered structure still existed. The hardness degradation ratio of CrN/AlN coating with modulation period of 4 nm was only 8.1% at 700oC, which was superior to that of CrN coating. Furthermore, the microstructure of CrN/AlN coatings exhibited a dense columnar structure and the surface roughness of multilayer coating retained below 5 nm after annealing at elevated temperatures.
After heat treatment at 800oC for 1hr, only one oxide layer smaller than 50 nm in thickness was found in the annealed CrN/AlN coating with 4 nm. This amorphous oxide layer identified by EDS was a metal deficient oxide, in which Al2O3 and Cr2O3 were mixed to form solid solution. It is worthy to note that this Al2O3-Cr2O3 solid solution was still existed even after heat treatment at 950oC for 1 hr. In comparison, a thick oxide layer around 260 nm was formed on the surface of TiN/AlN coating with 4 nm. The oxide layer formed on the coating was composed of three distinct regimes, including Al-riched oxide with excess oxygen on the top surface, a crystalline Al-depleted TiO2 layer, 30-80 nm thick above the nitride coating and in between, was mixed with nano-crystalline Al2O3 and TiO2 films. After heat treatment at 950oC, the bilayer structure of TiN/AlN coating disappeared instead of the thick oxide layer with cracks found on the surface. As a result, the CrN/AlN coating exhibited superior stability compared to the TiN/AlN coating at elevated temperatures.
In addition, for CrN/AlN multilayer coating with 12.3 nm modulation followed by heat treatment at 800oC for 1hr, three kinds of oxide layer around 60 nm formed on the surface was observed, including the Al-rich layer covered at the topmost surface, the mixed nano-crystalline Al2O3 and Cr2O3 film and the spherical Cr-rich grains embedded in between. After heat treatment at 900oC and 950oC for 1 hr, a large crystalline grains were formed on the surface due to the grain growths of both Al-rich and Cr-rich oxides, which was much different in the CrN/AlN coating with 4 nm. This implied that the interface in the multilayer coating played an important role in oxidation resistance at elevated temperatures.
The oxidation behaviors and mechanisms of CrN/AlN with different modulation periods and TiN/AlN multilayer coatings were discussed and proposed. It was concluded that mechanical properties and thermal stability of CrN/AlN multilayer coating with 4 nm were much superior to that of CrN, AlN, CrN/AlN with 12.3 nm and TiN/AlN coatings. A promising nanostructured hard coating candidate was then developed.

並列關鍵字

CrN/AlN ； multilayer coating ； anit-oxidation behavior ； hardness strrengthening

參考文獻

2. K.G. Budinski, Surface engineering for wear resistance, 1988, Prentice Hall, New Jersey, USA

4. C. M. Lieber, “One-dimensional nanostructures: chemistry, physics & applications,” Solid State Commum. 107, 607 (1998).

6. A. P. Alivisatos, “Semiconductor clusters, nanocrystals, and quantum dots,” Science 271, 933 (1996).

7. C. B. Murray, C. R. Kagan, and M. G. Bawendi, “Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies,” Annu. Rev. Mater. Sci. 30, 545 (2000).

8. M. Narihiro, G. Yusa, Y. Nakamura, T. Noda, and H. Sakaki, “Resonant tunneling of electrons via 20 nm scale InAs quantum dot and magnetotunneling spectroscopy of its electronic states,” Appl. Phys. Lett. 70, 105 (1996).