Understanding the age-hardening mechanism of CrWN coating

doi:10.1016/j.tsf.2020.138298

Thin Solid Films

Volume 711, 1 October 2020, 138298

https://doi.org/10.1016/j.tsf.2020.138298 Get rights and content

Highlights

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Structure evolution and age-hardening mechanism of CrWN coating were studied.
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The as-deposited coating showed un-uniformly distributed coherent solid solution.
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Thermal activation induced spinodal decomposition of supersaturated solid solution.
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Thermal activation driven diffusion and homogenization of solid solution matrix.
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Large area coherent diffusion solid solution resulted in a profound age-hardening.

Abstract

A microstructure and age-hardening investigation was performed on a chromium nitride and tungsten nitride (CrWN) transition metal nitride coating for optical glass molds using electron microscopy and nano-indentation. The as-deposited coating had a non-uniformly distributed coherent solid solution matrix causing limited strengthening and unsatisfactory mechanical performance. Annealing of the coating induced spinodal decomposition of supersaturated solid solution and promoted diffusion within and homogenization of the solid solution matrix. Large areas of homogenized solid solution greatly strengthened the coherent matrix resulting in significant age-hardening of the annealed coating. These findings provide supporting data for understanding the age-hardening mechanism of transition metal nitride coatings.

Introduction

Ternary transition metal nitride coatings have received widespread attention for optical glass molding due to their excellent anti-wear and anti-sticking performance [1], [2], [3]. Most studies focus on the anti-sticking and anti-oxidation properties of these coatings [4], [5], but thermal stability is a critical factor in determining their useful life. It has been found that transition metal nitride coatings show outstanding thermal stability and age-hardening. Chen et al. [6], [7] attributes the hardness enhancement to microstructure evolution. Supersaturated cubic TiAlN is one such coating, but it suffers from spinodal decomposition to form cubic (c) TiN and c-AlN domains. The formation of nano-sized cubic TiN and AlN within the remaining cubic matrix are reported to cause prominent age-hardening, confirming previous results reported by Mayrhofer et al. [8], [9] and Hörling et al. [10]. Thus, spinodal decomposition is considered a key mechanism leading to the age-hardening of transition metal nitride coatings, but relatively little direct observation has been presented. Herein, a detailed electron microscopy and nano-indentation study of CrWN coating microstructures is reported; including a description of the age-hardening mechanism of the transition metal nitride coating.

Section snippets

Materials and methods

A CrWN coating was synthesized using plasma-enhanced magnetron sputtering (PEMS). PEMS has independent plasma generation control that can be optimized to improve the adhesive strength between the coating and substrate [11], [12]. Therefore, it has been widely used to synthesize transition metal nitride or carbide films with outstanding mechanical and tribological properties. Pure W (99.9 %) and Cr (99.6 %) were used as the sputtering targets. Silicon wafers and cemented carbides (WC – 8 wt.%

Results and discussion

Fig. 1 shows the SEM images (cross-section as insert) with corresponding EDS spectra of the as-deposited and annealed coating. As shown in Fig. 1a, the as-deposited coating exhibits a uniform surface morphology having many nano-sized grains tightly packed in large clusters; a columnar grain texture with a thickness of 1.41 µm is also observed. The chromium, tungsten, nitrogen, and oxygen contents in this coating were 26.4 ± 1.4 at.%, 31.9 ± 6.1 at.%, 40.3 ± 4.5 at.%, and 1.4 ± 0.5 at.%,

Conclusions

The detailed TEM characterization clearly confirmed that the varying energy and random trajectory of the sputtered W and Cr particles applied to a CrWN coating formed a non-uniformly distributed supersaturated solid solution matrix. This provided limited strengthening effects resulting in the poor mechanical properties of the as-deposited coating. A vacuum annealing process induced significant spinodal decomposition of the supersaturated solid solution. Strain fields originating from the

CRediT authorship contribution statement

F. Guo: Investigation. X.F. Huang: Investigation. Z.W. Xie: Conceptualization. K.S. Li: Methodology. F. Gong: Conceptualization. Y.J. Chen: Methodology. Q. Chen: Supervision.

Declaration of Competing Interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgement

This work was supported by National Natural Science Project of China (51771087), Liaoning Innovative Talents Support Plan (LR2017052), University of Science and Technology Liaoning Talent Project Grants (601011507-07), Innovation Team of Liaoning University of Science and Technology (2017TD04). We sincerely thank the Institute of Electron Microscope Center of Shenzhen University for their TEM technical supports.

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¹: The authors contributed equally to this work.

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Understanding the age-hardening mechanism of CrWN coating

Highlights

Abstract

Introduction

Section snippets

Materials and methods

Results and discussion

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgement

Surf. Coat. Technol.

Surf. Coat. Technol.

Thin Solid Films

Appl. Surf. Sci.

Thin Solid Films

Surf. Coat. Technol.

Scr. Mater.