Wear resistance investigation of titanium nitride-based coatings

High-entropy ceramics with five or more cations have recently attracted significant attention due to their superior properties for various structural and functional applications. Although the multi-component ceramics have been of interest for several decades, the concept of high-entropy ceramics was defined in 2004 by producing the first high-entropy nitride films. Following the introduction of the entropy stabilization concept, significant efforts were started to increase the entropy, minimize the Gibbs free energy and achieve stable single-phase high-entropy ceramics. High-entropy oxides, nitrides, carbides, borides and hydrides are currently the most popular high-entropy ceramics due to their potential for various applications, while the study of other ceramics, such as silicides, sulfides, fluorides, phosphides, phosphates, oxynitrides, carbonitrides and borocarbonitrides, is also growing fast. In this paper, the progress regarding high-entropy ceramics is reviewed from both experimental and theoretical points of view. Different aspects including the history, principles, compositions, crystal structure, theoretical/empirical design (via density functional theory, molecular dynamics simulation, machine learning, CALPHAD and descriptors), production methods and properties are thoroughly reviewed. The paper specifically attempts to answer how these materials with remarkable structures and properties can be used in future applications.

High-entropy ceramics: Review of principles, production and applications

Recent progress of residual stress measurement methods: A review

Validity of “sputtering and re-condensation” model in active screen cage plasma nitriding process

Recently there has been considerable interest in active screen plasma nitriding (ASPN), through-cage plasma nitriding, and cathodic-cage plasma nitriding. In these processes, the edge effect is completely eliminated because plasma is produced on the cage and not directly on the samples. However, little information has been reported on the effect of the distance between the screen and the sample on the ASPN properties. The objective of this study is to investigate this effect by nitriding low-carbon steel S15C and austenitic stainless steel SUS 304 by ASPN using a 304 steel screen. The sample was treated for 18 ks at 723 K under 600 Pa in 25% N2 + 75% H2. The distance between the screen and the sample was changed to 10, 20, 30, 40, and 50 mm. The nitrided samples were characterized by observing their appearance and surface roughness by optical microscopy, scanning electron microscopy, x-ray diffraction, and microhardness testing. After nitriding, polygonal particles with a normal distribution were observed at the entire of all the ASPN-treated sample surfaces. The particles on the sample surface became coarser with a decrease in the distance between the screen and the sample. Furthermore, the thickness of the surface-hardened layer increased with a decrease in the distance between the screen and the sample.

Effect of the distance between screen and sample on active screen plasma nitriding properties

Nitriding steel sample SACM 645 was nitrided by active screen plasma nitriding (ASPN) using a titanium screen to form simultaneously TiN coating/nitrogen-diffusion layer on the sample surface. ASPN experiments were carried out using a DC plasma-nitriding unit. The sample was placed on the sample stage in a floating potential and a cathodic potential. A titanium double screen was mounted on the cathodic stage around the sample stage. ASPN treatments at 0%-bias and 100%-bias were performed in a nitrogen–hydrogen atmosphere with 75% N2 + 25% H2 for 0–54 ks at 873 K under 100 Pa. After nitriding, the nitrided microstructure was examined with a scanning electron microscope, glow discharge optical emission spectroscopy and X-ray diffraction studies. In addition, the hardness of the surface and the cross‐sections of the nitrided sample were measured using a Vickers microhardness tester under a 0.1-N load. The thickness of the TiN layer grew linearly with increasing nitriding time. In this case, the deposition rate of the TiN layer was 0.22 μm/h. The nitrided layer formed by ASPN at 100%-bias consisted of a TiN compound layer followed by a nitrogen-diffusion layer.

Simultaneous duplex process of TiN coating and nitriding by active screen plasma nitriding

A feasibility study of active screen plasma nitriding (ASPN) of Ti6Al4V titanium alloy was conducted using a CP-Ti cage as the active screen under various process conditions. The process variables included active screen setup parameters (two different potentials of 0%-bias and 100%-bias), treatment temperature (750 and 850°C), gas mixture (10/90, 25/75, 50/50 and 100/0 of N2/H2) and treatment time (10 h) under 1·5 mbar pressure. The samples were characterised by X-ray diffraction, optical microscopy, scanning electron microscopy, microhardness and surface roughness tests. The experimental results have demonstrated that Ti–6Al–4V alloy can be treated by ASPN using a Ti screen. A Ti nitride compound layer consisting of TiN followed by Ti2N was formed on top of the surface and an Al rich layer was formed between the Ti nitride layer and the α-Ti case. The surface hardness of the samples nitrided at 750 and 850°C was 800 and 1400 HV respectively.

Feasibility study of active screen plasma nitriding of titanium alloy

The duplex stainless steel has high strength and excellent corrosion resistance. Since this steel has low hardness and wear resistance, the surface hardening treatment should be carried out under the wear environment. In this study, plasma nitriding has been applied for the improvement of wear resistance. However, traditional plasma nitriding at temperatures ranging from 723 to 823 K decreases the corrosion resistance of stainless steel. This is because the chromium in the steel reacts with nitrogen to form chromium nitride, and this decreases the concentration of chromium in the matrix, which is required for the formation of stable passive layers. Plasma nitriding carried out below 723 K forms a nitrided layer that is called the S phase. This phase improves the surface hardness without decreasing the corrosion resistance. Therefore, a low temperature plasma nitriding technique was used for treating stainless steel. Active screen plasma nitriding (ASPN) was carried out as the plasma nitriding process in addition to the conventional direct plasma nitriding (DCPN). ASPN has several advantages over the conventional DCPN, such as eliminating the edge effect, arcing, and carrying out hollow cathode discharge. The sample used was SUS 329J4L (X2CrNiMoCuN25-6-3). Nitriding was carried out at 673 and 723 K for 18 ks by DCPN and ASPN. The nitrided samples were then analyzed by various tests such as appearance observation, X-ray diffraction, SEM-EDX analysis, hardness testing, wear testing, and corrosion testing by measuring the pitting potential. After nitriding, the ASPN process improved the wear resistance and eliminated the edge effect when compared with the DCPN process. The S phase was observed on all the treated samples, and this phase was uniformly formed on both austenitic and ferritic phases of ASPN samples. Moreover, the corrosion tests showed that the pitting potential did not decrease with plasma nitriding.

Surface hardening of duplex stainless steel by low temperature active screen plasma nitriding

Plasma nitriding is used for materials that are challenging to nitride by conventional techniques. Because of the sputtering effect of positive ions during glow discharge, protective oxide films on the surfaces of materials including stainless steels, aluminum alloys, and titanium alloys, can be effectively removed by ion and energetic particle bombardment; thus, nitrogen mass transfer from the plasma into the component subsurface can be achieved effectively. Plasma nitriding therefore has potential applicability to treat high-entropy alloys (HEAs) containing large amounts of strong oxide-forming elements such as chromium. In this study, the effects of plasma nitriding were investigated on the microstructural, mechanical, and corrosion properties of a CoCrFeMnNi HEA with an fcc structure that is soft and ductile. The HEA was produced by melting pure metals and casting. Direct-current plasma nitriding was then performed in a gas mixture of 25% N2 and 75% H2 for 54 ks at 673–823 K under 200 Pa with an auxiliary cathodic screen. After nitriding, the nitrided samples were examined using scanning electron microscopy, X-ray diffraction (XRD), Vickers microhardness testing, electron probe microanalysis, and glow discharge optical emission spectroscopy (GD-OES). After nitriding, GD-OES revealed that the thickness of the nitriding layer tended to increase with increasing nitriding temperature. XRD analysis revealed that an fcc supersaturated solid solution phase was formed on surfaces nitrided below 723 K, whereas a CrN phase was formed on those nitrided above 723 K. The Vickers microhardness of the nitrided sample surfaces reached approximately 1300 HV. Ball-on-disk wear tests revealed that the wear loss of nitrided samples was considerably lower than that for untreated samples. Finally, the pitting corrosion resistance of samples nitrided at 673 K was higher than that of untreated samples.

Akio Nishimoto

Papers

Effect of the distance between screen and sample on active screen plasma nitriding properties

Simultaneous duplex process of TiN coating and nitriding by active screen plasma nitriding

Feasibility study of active screen plasma nitriding of titanium alloy

Surface hardening of duplex stainless steel by low temperature active screen plasma nitriding

Microstructural, mechanical, and corrosion properties of plasma-nitrided CoCrFeMnNi high-entropy alloys