The dependence of structural and mechanical properties on film thickness in sol-gel zirconia films
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The dependence of structural and mechanical properties on film thickness in sol-gel zirconia films Melissa J. Paterson School of Mathematics, Physics, Computing and Electronics, Macquarie University, Sydney, New South Wales 2109, Australia
Peter J. K. Paterson Department of Applied Physics, Royal Melbourne Institute of Technology, GPO Box 2476V, Melbourne, Victoria 3001, Australia
Besim Ben-Nissan Department of Materials Science, University of Technology, Sydney, P.O. Box 123, Broadway, New South Wales 2007, Australia (Received 18 November 1996; accepted 16 April 1997)
The structure, morphology, and mechanical properties of sol-gel zirconia films have been examined using XRD, AES depth profiling, AFM, and ultramicro indentation. There is a systematic variation in the structure and morphology of the zirconia films with increasing thickness. These changes include increases in the amount of monoclinic phase, substrate oxides, and a decrease in grain size. Ultramicro indentation measurements indicate measured hardness increases with film thickness. The highest hardness value was 6.12 GPa for a 900 nm thick film. However, these values may be influenced by the substrate oxide layer at the film/substrate interface which increases with film thickness. The modulus of the films appears to be thickness independent. As the films are made up of a number of separately fired layers, it appears that the property changes observed are also related to the number of thermal cycles experienced by the sample.
I. INTRODUCTION
Zirconia has promising physical and chemical properties such as high hardness and corrosion resistance1 which make it potentially useful as a wear coating in harsh environments. Zirconia films have been produced using various techniques including plasma and thermal spraying, physical vapor deposition, and solgel deposition.1–4 Of these the sol-gel method has the advantage of being able to form homogeneous films at low temperatures and allows deposition of thin ceramic films onto complex shapes.5 The term sol-gel is derived from the words solutiongelation and describes the transition of a liquid (or sol) into a gel phase. In the case of the alkoxide (metalorganic) based sols, gelation occurs through hydrolysis in the presence of water. The hydrolysis reaction is a multistage process which can be described by the simplified chemical equations: ZrsORd4 1 XOH ! ZrsORd3 OX 1 ROH ZrsORd3 OX 1 aH2 O ! ZrsORd32a sOXd sOHda 1 aROH Subsequent condensation reactions result in the formation of a gel. Heat treatment of the gel removes alcohol, water, and residual organic groups and results in the formation of the ZrO2 ceramic.6 388
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J. Mater. Res., Vol. 13, No. 2, Feb 1998
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The structure and thermal barrier properties of solgel derived zirconia films have been widely examined1,2 ; however, there has been little examination of their surface morphology and mechanical properties.5,7 The current investigation utilizes atomic for
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