Hard yet tough nc-CrAlN/a-SiNx nanocomposite coatings

In the past decades, hard or even super-hard ceramic coatings have been vigorously developed to satisfy the various requirements of practical applications in the machining industry. It is increasingly recognized that the enhanced hardness must be complemented by a proper toughness to avoid the catas...

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Main Author: Wang, Yuxi
Other Authors: Lew Wen Siang
Format: Theses and Dissertations
Language:English
Published: 2013
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Online Access:https://hdl.handle.net/10356/54697
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Institution: Nanyang Technological University
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spelling sg-ntu-dr.10356-546972023-03-11T17:52:00Z Hard yet tough nc-CrAlN/a-SiNx nanocomposite coatings Wang, Yuxi Lew Wen Siang Lee Jyh‐Wei Sam Zhang Shanyong School of Mechanical and Aerospace Engineering DRNTU::Engineering::Materials In the past decades, hard or even super-hard ceramic coatings have been vigorously developed to satisfy the various requirements of practical applications in the machining industry. It is increasingly recognized that the enhanced hardness must be complemented by a proper toughness to avoid the catastrophic fracture when the coatings are subjected to an intensive contact force as in harsh conditions. However, results of the recent researches indicate that the increasing toughness is usually accompanied with a large sacrifice of the hardness. In this project, the state-of-the-art nitride hard coatings (i.e. CrAlN) and their nanocomposite variants (i.e. nc-CrAlN/a-SiNx, nc-: nanocrystalline, a-: X-ray amorphous) are fabricated using magnetron sputtering. The effect of the negative bias voltage on the CrAlN and the role of the Si content on the nc-CrAlN/a-SiNx are studied. The obtained results show that the bias voltage and the Si content are essential factors in determining the coating’s microstructure, and thus the mechanical properties. A proper bias voltage is beneficial for achieving a simultaneous improvement of the hardness and toughness since a dense microstructure and higher compressive stress are ready to be obtained. However, the increasing Si content leads only to a drop of toughness although a significant increase of hardness. To overcome the brittleness of the nc-CrAlN/a-SiNx nanocomposite, the distribution of the doped Si is varied and the influence of the distribution behavior is investigated. With the homogeneous and heterogeneous grading of Si, the toughness of brittle nc-CrAlN/a-SiNx is restored but still at an expense of hardness to different extents. Heterogeneous grading is more effective due to the laminated structure. It is believed that the inner layer with less Si provides the good toughness and adhesion while the outer layer with more Si offers the great resistance to the plastic deformation. On the other hand, a conventional toughening method by adding the ductile phase Ni is also evaluated. The obtained results indicate that with proper concentration of Ni (4.3 at.%), the toughness of the nc-CrAlN/a-SiNx nanocomposite is improved by 200% at an expense of 18% hardness drop. Based on the analyses of the above monolayers, a new routine in achieving hard-yet-tough ceramic coatings is proposed. A multilayers consisting of soft-tough CrAlN and hard-brittle nc-CrAlN/a-SiNx is fabricated. The dependence of the microstructure and mechanical properties on the bilayer thickness is revealed. Detailed studies on the microstructure indicate that the mixture of nanograins with different crystallographic orientations, the complex atomic configuration at the layer interface and the reduced grain size governed by the bilayer thickness are beneficial to achieve the enhanced toughness and high hardness. At bilayer thickness of 20 nm, the optimum toughness of the multilayers is improved by 5 times as compared to the nc-CrAlN/a-SiNx monolayer. Moreover, the hardness of multilayers also increases, and thus the achievable hard-yet-tough ceramic coatings. DOCTOR OF PHILOSOPHY (MAE) 2013-07-23T08:27:31Z 2013-07-23T08:27:31Z 2013 2013 Thesis Wang, Y. (2013). Hard yet tough nc-CrAlN/a-SiNx nanocomposite coatings. Doctoral thesis, Nanyang Technological University, Singapore. https://hdl.handle.net/10356/54697 10.32657/10356/54697 en 210 p. application/pdf
institution Nanyang Technological University
building NTU Library
continent Asia
country Singapore
Singapore
content_provider NTU Library
collection DR-NTU
language English
topic DRNTU::Engineering::Materials
spellingShingle DRNTU::Engineering::Materials
Wang, Yuxi
Hard yet tough nc-CrAlN/a-SiNx nanocomposite coatings
description In the past decades, hard or even super-hard ceramic coatings have been vigorously developed to satisfy the various requirements of practical applications in the machining industry. It is increasingly recognized that the enhanced hardness must be complemented by a proper toughness to avoid the catastrophic fracture when the coatings are subjected to an intensive contact force as in harsh conditions. However, results of the recent researches indicate that the increasing toughness is usually accompanied with a large sacrifice of the hardness. In this project, the state-of-the-art nitride hard coatings (i.e. CrAlN) and their nanocomposite variants (i.e. nc-CrAlN/a-SiNx, nc-: nanocrystalline, a-: X-ray amorphous) are fabricated using magnetron sputtering. The effect of the negative bias voltage on the CrAlN and the role of the Si content on the nc-CrAlN/a-SiNx are studied. The obtained results show that the bias voltage and the Si content are essential factors in determining the coating’s microstructure, and thus the mechanical properties. A proper bias voltage is beneficial for achieving a simultaneous improvement of the hardness and toughness since a dense microstructure and higher compressive stress are ready to be obtained. However, the increasing Si content leads only to a drop of toughness although a significant increase of hardness. To overcome the brittleness of the nc-CrAlN/a-SiNx nanocomposite, the distribution of the doped Si is varied and the influence of the distribution behavior is investigated. With the homogeneous and heterogeneous grading of Si, the toughness of brittle nc-CrAlN/a-SiNx is restored but still at an expense of hardness to different extents. Heterogeneous grading is more effective due to the laminated structure. It is believed that the inner layer with less Si provides the good toughness and adhesion while the outer layer with more Si offers the great resistance to the plastic deformation. On the other hand, a conventional toughening method by adding the ductile phase Ni is also evaluated. The obtained results indicate that with proper concentration of Ni (4.3 at.%), the toughness of the nc-CrAlN/a-SiNx nanocomposite is improved by 200% at an expense of 18% hardness drop. Based on the analyses of the above monolayers, a new routine in achieving hard-yet-tough ceramic coatings is proposed. A multilayers consisting of soft-tough CrAlN and hard-brittle nc-CrAlN/a-SiNx is fabricated. The dependence of the microstructure and mechanical properties on the bilayer thickness is revealed. Detailed studies on the microstructure indicate that the mixture of nanograins with different crystallographic orientations, the complex atomic configuration at the layer interface and the reduced grain size governed by the bilayer thickness are beneficial to achieve the enhanced toughness and high hardness. At bilayer thickness of 20 nm, the optimum toughness of the multilayers is improved by 5 times as compared to the nc-CrAlN/a-SiNx monolayer. Moreover, the hardness of multilayers also increases, and thus the achievable hard-yet-tough ceramic coatings.
author2 Lew Wen Siang
author_facet Lew Wen Siang
Wang, Yuxi
format Theses and Dissertations
author Wang, Yuxi
author_sort Wang, Yuxi
title Hard yet tough nc-CrAlN/a-SiNx nanocomposite coatings
title_short Hard yet tough nc-CrAlN/a-SiNx nanocomposite coatings
title_full Hard yet tough nc-CrAlN/a-SiNx nanocomposite coatings
title_fullStr Hard yet tough nc-CrAlN/a-SiNx nanocomposite coatings
title_full_unstemmed Hard yet tough nc-CrAlN/a-SiNx nanocomposite coatings
title_sort hard yet tough nc-craln/a-sinx nanocomposite coatings
publishDate 2013
url https://hdl.handle.net/10356/54697
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