Influence of zirconia sol-gel coatings on the fracture strength of brittle materials
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dro Miranda and Antonio Díaz-Parralejo Departamento de Electrónica e Ingeniería Electromecánica, Escuela de Ingenierías Industriales, Universidad de Extremadura, 06071 Badajoz, Spain
Antonia Pajares Departamento de Física, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain
Fernando Guiberteaua) Departamento de Electrónica e Ingeniería Electromecánica, Escuela de Ingenierías Industriales, Universidad de Extremadura, 06071 Badajoz, Spain (Received 18 November 2004; accepted 18 March 2005)
In this work, the effect of a sol-gel ZrO2–3 mol% Y2O3 thin film on the fracture properties of a variety of brittle substrates was investigated. The results suggest that the film does not have any appreciable influence on the fracture behavior of crystalline substrates but dramatically affects the fracture properties of amorphous layers. In particular, a significant reduction of average fracture strength and a major increase of the Weibull modulus were observed on coated glassy slides. The origin of such variations is attributed to the generation of a homogeneous flaw population in the vitreous substrates, and the possible mechanisms for the production of flaws are analyzed. Implications of these results for the practical use of coated glassy layers are discussed.
I. INTRODUCTION
Zirconia (ZrO2) ceramics have an interesting combination of mechanical, chemical, and physical properties. In particular, yttria (Y2O3)-doped zirconia can exhibit excellent mechanical properties, combining high wear resistance with moderate toughness. Also, zirconia has a thermal expansion coefficient (10 × 10−6 °C−1) of the same order as those of metals, while its thermal conductivity (0.05 cal cm−1 °C−1 s−1) is one order of magnitude lower, enabling its use as an effective thermal barrier coating.1 In addition, zirconia has a high refractive index (2.21 at ⳱ 630 nm), low absorption, and a large optical band gap (3.8–3.2 eV), and therefore, new zirconiabased coatings have long been sought for applications in optics.2,3 Among zirconia coatings, sol-gel thin films have aroused great interest for optical applications. Indeed, sol-gel zirconia films can be used to provide glass with specific optical properties (anti-reflection, selective reflection, photochromism, etc.).4 The sol-gel route5,6 has
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0191 1544
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 6, Jun 2005 Downloaded: 15 Mar 2015
such advantages as simplicity and low cost, ability to coat large surface areas, low processing temperatures, and a very high optical quality of the films. Consequently, the literature concerning the control and optimization of the processing route and the optical and mechanical characterization of zirconia sol-gel thin films is vast.7–9 However, until recently, no studies have been devoted to analyzing how these zirconia coatings affect the fracture behavior of the glass substrate. We addressed this issue in a previous study for the particular
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