Al 2 O 3 thin films: relation between structural evolution, mechanical properties and stability

In this thesis the relation between structure evolution, mechanical properties, and stability was investigated for Al2O3 thin films. The existence of different polymorphs of this material offers a range of properties for applications, but also meets the process control challenges when the formation of phase pure material is desired. The aim of this work is to affect the phase structure evolution and the stability of vapor deposited alumina thin films, especially for the case of PECVD. This goal is achieved by experimental investigations of the PECVD process which show that the alpha-Al2O3 phase is obtained only at conditions with a low precursor ratio, a long pulse length and a high power density. These allow for more efficient precursor dissociation as suggested from plasma OES data as well as larger ion bombardment. When these conditions are not fulfilled, only gamma-Al2O3 or alpha-gamma-mixture is formed. Due to the use of chloridic precursor, it is found that less Cl content is incorporated in alpha-Al2O3 films with < 1.0 at.% than in gamma-Al2O3 films (~2.0 at.%). This observation is consistent with a high precursor dissociation and significant surface bombardment for the alpha-phase. Smooth and dense alpha-Al2O3 films with negligible Cl incorporation and elastic properties similar to those of the bulk alpha-Al2O3 are deposited at a growth temperature of 560 ± 10°C, applying a novel generator that allows for the operation of a PECVD discharge at approx. 4 times larger power density than those utilized conventionally. Up-scaling of a lab-scale PECVD process to an industrial reactor has been successfully conducted allowing the growth of crystalline alumina on large thixocasting tools at substrate temperatures of 590°C. Steel thixocasting operations on the coated moulds indicate that coatings are intact after thixocasting. No evidence for adhesive or cohesive failure of the coatings was obtained, indicating efficient protecting of the tools during thixocasting. Investigation of thermal stabilities of gamma-Al2O3 films suggest that the gamma-Al2O3 to alpha-Al2O3 transformation in the PECVD films is faster and occurs at lower temperatures in comparison to the FCA films. This could be due to the porous structure of the PECVD grown films which enhance diffusion processes during the phase transformation. The porous structure of PECVD films further enables the oxidation of a TiAlN interlayer and the extrusion of TiO2 towards the film surface due to large volume expansion from TiO2, leading to a complete failure of the coating system. The relative stability of gamma-Al2O3 with respect to alpha-Al2O3 with 2.5 at% of Si, Cr, Ti, Sc, and Y additives is also explored using ab initio calculations, indicating that Si stabilizes gamma-Al2O3, while Cr stabilizes alpha-Al2O3. This is explained by electronic structure changes induced by the alloying element. As Si is added, a bond length increase in alpha-Al2O3 is observed, while strong and short Si-O bonds are formed in gamma-Al2O3, consequently stabilizing this phase. On the other hand, Cr additions induce a smaller bond length increase in alpha-Al2O3 than in gamma-Al2O3, therefore stabilizing the alpha-Al2O3.

Grantor dc:publisher

Shaker

Year dc:date

2011

Author dc:creator

• Jiang, Kaiyun

Contributors dc:contributor

• Schneider, Jochen M.

• info:eu-repo/classification/ddc/550
• Dünne Schicht
• Mechanische Eigenschaft
• Thermodynamische Stabilität
• Geowissenschaften
• Al2O3-Dünnschichten
• Al2O3 Dünnschichten
• strukturelle Entwicklung
• mechanische Eigenschaften
• Phasenstabilität
• Al2O3 thin film
• structural evolution
• mechanical properties
• phase stability

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-124393%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:62907