Effect of Cr/CrNx transition layer on mechanical properties of CrN coatings deposited on plasma nitrided austenitic stainless steel

doi:10.1016/j.surfcoat.2019.03.068

Surface and Coatings Technology

Volume 367, 15 June 2019, Pages 100-107

https://doi.org/10.1016/j.surfcoat.2019.03.068 Get rights and content

Highlights

•
Plasma nitriding + transition layer + CrN coating can be in-situ performed in a plasma-enhanced magnetron sputtering system.
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In case of 4-hour nitriding samples, transition layers suppress nitride precipitation, and hence improve the adhesion force.
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4-hour nitriding samples with CrN_x transition layer show best wear resistance.
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1-hour nitriding samples without transition layer exhibit highest micro-hardness and adhesion force.

Abstract

In this work, two types of transition layers (pure Cr and gradient CrN_x) were fabricated between plasma-assisted nitrided (PAN) stainless steel and magnetron sputtering CrN coating to investigate the effect of transition layer on mechanical properties of CrN coating on PAN stainless steel substrates. For comparison, samples without transition layer were also synthesized. The phase structure, morphology, abrasive resistance and adhesion of the CrN coatings, as well as the nano-hardness and Young's modulus were investigated by XRD, SEM, ball-on-disk tribometer, scratch tester and nano-indentation tester, respectively. The results showed the transition layers of pure Cr and gradient CrN_x absorbed the excess nitrogen diffusing outwards from the nitrided layer, and thus suppressed partly the precipitation. As a result, the adhesion force was improved to 22.5 and 25.7 N, respectively. The nano-hardness and Young's modulus of modified layers were improved to the range of 22.8–25.8 GPa and 331–347.3 GPa, corresponding to the substrate of 7.2 GPa and 230.5 GPa. The modified layer with no transition layer had the best wear resistance for nitriding of 1 h; while the layer with gradient CrN_x had the best wear resistance for nitriding of 4 h.

Introduction

Austenitic stainless steels exhibit faced-centered cubic (fcc) crystal lattice containing a minimum of 16 wt% chromium and sufficient nickel and/or manganese to retain the austenitic structure at all temperatures [1]. They are attractive materials for various industrial applications where corrosion resistance (especially in wet environment) and toughness are primary requirements [2,3]. However, they show poor surface properties, such as low hardness, low resistance to wear and pitting, which greatly limits their applications [[4], [5], [6]]. The surface properties can be improved by reliable surface engineering techniques such as plasma nitriding, and physical vapor deposition (PVD).

Chromium nitride coating shows good toughness, excellent abrasion and corrosion resistance [[7], [8], [9]], though its hardness is lower than that of titanium nitride. Unfortunately, the adhesion of CrN coating directly deposition on the austenite substrate remains unsatisfactory due to their excessive physical differences. Variable strategies have been proposed to eliminate this large hardness difference between ceramic films and substrate, for instance, by inserting an intermediate layer. Pure Cr and gradient CrN_x coatings were developed by some researchers to improve adhesion property and wear resistance [10,11]. Further, it is expected that the poor adhesion of CrN coating on the austenite can be enhanced by adding an intermediate diffusion layer using plasma-assisted nitriding [3,12].

Plasma-assisted nitriding (PAN) is a flexible and multifunctional thermo-chemical diffusion treatment for surface modifications to improve hardness, fatigue and wear resistance of ferrous and/or non-ferrous materials at the industrial scale [[13], [14], [15]]. After nitriding below 450 °C, a metastable supersaturated solid solution phase with high nitrogen content (from 10 to 40 at.%), usually called expanded austenite. This phase is hard, wear resistant and a paramagnetic to ferromagnetic transition takes place for nitrogen content higher than 14 at.% [16]. Another particular feature of the expanded austenite is nitrogen depth profiles in the nitrided layer [17], resulting in a hardness gradient which provides a good intermediate transition to enhance the adhesion of hard coatings on soft substrates. It was reported that duplex treatments in this way exhibited low wear characteristics of the ceramic coating, combined with the high load-bearing capacity and high fatigue strength characteristics of nitrided layer [18]. However, the detailed mechanism of the enhanced adhesion and wear resistance of the duplex layers has been less understood clearly, and the interface effects between the nitriding layer and CrN coatings need to be further investigated.

In this work, pure Cr and gradient CrN_x transition layers were deposited in-situ on plasma-assisted nitrided 316 L stainless steels in a hot wire plasma-enhanced magnetron sputtering (MS) system. Successively, CrN coatings were deposited as the outmost layer using MS technique in the same chamber. One aim of this study is to investigate the effect of transition layer on phase structure and mechanical properties of CrN coating-nitrided layer systems. The other aim is to investigate the effect of nitrogen outward-diffusion with variable nitrogen content on interfacial bonding.

Section snippets

Experimental

Samples used in this work were AISI 316 L stainless steel cylinders (17.2 wt% Cr, 10.0 wt% Ni, 2.0 wt% Mo, 1.3 wt% Mn, 0.6 wt% Si, 0.08 wt% C, and Fe as balance) with a size of Φ30 mm × 5 mm. The samples were mechanically polished to mirror-like, followed by a 20-min ultrasonic clean in ethanol. Then they were dried with nitrogen and placed into the vacuum chamber which was 900 mm in diameter and 1000 mm in height. Four balanced magnets were installed around the chamber wall, each with a 99.9%

Phase and microstructures

The XRD patterns of all the samples and the substrate are shown in Fig. 1. The substrate showed an austenite phase having an fcc structure with γ (111), (200) and (220) diffraction peaks. After nitriding, these diffraction peaks of austenite broadened and shifted to lower angles, the CrN and Fe₄N phases occurred as well for nitriding of 4 h, as shown in Fig. 1(b). Peak shift and broadening are generally attributed to the lattice expansion associated with stress and faulting [19]. All the CrN

Conclusions

The duplex treatments of PAN and successive MS CrN coatings were in-situ performed on 316 L stainless steel in a plasma-enhanced magnetron sputtering system. The pure Cr and gradient CrN_x transition layer were added between PAN layer and CrN coating to investigate the effect of transition layer on mechanical property. After nitriding for 1 h, the expanded austenite was formed due to nitrogen diffusion into the lattices. As the nitriding time increased to 4 h, the iron nitride phase (γ’-Fe₄N)

Acknowledgments

This research work was supported by National Natural Science Foundation of China (No. 51502126, 51101080, 51672109 and 11805089), Natural Science Foundation of Liaoning Province (20180550802), Provincial Key Laboratory Open Project of USTL (USTLKFSY201705) and School Science Foundation for Youth Scholars of USTL (2018QN12).

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Effect of Cr/CrNx transition layer on mechanical properties of CrN coatings deposited on plasma nitrided austenitic stainless steel

Highlights

Abstract

Introduction

Section snippets

Experimental

Phase and microstructures

Conclusions

Acknowledgments

Appl. Surf. Sci.

Mater. Sci. Eng. A

Surf. Coat. Technol.

Appl. Sci. Surf.

Surf. Coat. Technol.

Appl. Surf. Sci.

Thin Solid Film

J. Alloys Compd.

Corr. Sci.

Surf. Coat. Technol.

Surf. Coat. Technol.

Thin Solid Films

Surf. Coat. Technol.

Surf. Coat. Technol.

Effect of Cr/CrN_x transition layer on mechanical properties of CrN coatings deposited on plasma nitrided austenitic stainless steel