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M 50 NiL steel is a variety of tool steel having high toughness. This steel is potential materials for several bearings of aero-engine. This steel is used in carburized condition. However, nitriding has several advantages over carburizing. The main goal of this work is to compare and contrast the microstructural features, mechanical properties and tribolgical performances of this steel nitrided by gas, liquid and plasma nitriding processes. In view of the above, M 50 NiL steel was nitrided by the processes mentioned above. Microstructural characteristics, mechanical properties, sliding wear response in unidirectional and reciprocation mode were evaluated. The results reveal formation of compound layer in gas and plasma nitrided samples. Surface hardness was comparable for all three nitrided layers. Liquid nitrided specimen indicates best wear resistance in unidirectional and reciprocating sliding conditions. The friction coefficient is lower and wear rate is higher under reciprocating sliding than under unidirectional sliding. While delamination cracks are the main features of unidirectional sliding, cracks perpendicular to the surface were noted under reciprocating sliding.

N-doped CoCrFeNiMn high entropy alloy coating (N-HEA) was prepared on Ti-6Al-4V alloy by high-velocity oxygen fuel (HVOF) spraying coupled with double glow plasma nitriding. The results show that the CoCrFeNiMn high entropy alloy coating (HEA) is mainly composed of single CoCrFeNiMn face-centered cube (fcc) phase with a little MnCr 2 O 4 spinel phase, and the thickness is approximately 200 μm. After plasma nitriding, the surface morphology of the coating is reconstructed, changing from the molten and semi-melted coral-like structure to cauliflower-like structure, while the surface roughness and the thickness have no significant change. The phase composition of the N-HEA coating has no obvious change, and the N mainly exists as interstitial atoms in solid solution. The microhardness of the HEA coating is highly significantly higher than Ti-6Al-4V alloy, and it is further increased by 45% after plasma nitriding. The friction coefficient of N-HEA coating is as low as 0.38, and the wear rate is 1.283 × 10 −4  mm −3 N −1  min −1 , which is 53 and 72% lower than those of the HEA coating and the Ti-6Al-4V alloy, respectively. Both the wear mechanism of the N-HEA and HEA samples against GCr15 steel ball are mainly adhesive wear, while more Fe elements are transferred from the GCr15 steel ball onto the surface of the N-HEA sample.

The phenomena of cavitation and cavitation erosion affect hydraulic machines, increasing their maintenance costs. Both these phenomena and also the methods of preventing the destruction of materials are presented. The compressive stress in the surface layer created from the implosion of cavitation bubbles depends on the aggressiveness of the cavitation, which in turn depends on the test device and test conditions, and also affects the erosion rate. Comparing the erosion rates of different materials tested using different tests devices, the correlation with material hardness was confirmed. However, no one simple correlation was obtained but rather several were achieved. This indicates that in addition to hardness, cavitation erosion resistance is also affected by other properties, such as ductility, fatigue strength and fracture toughness. Various methods such as plasma nitriding, shot peening, deep rolling and coating deposition used to increase resistance to cavitation erosion by increasing the hardness of the material surface are presented. It is shown that the improvement depends on the substrate, coating material and test conditions, but even using the same materials and test conditions large differences in the improvement can be sometimes gained. Moreover, sometimes a slight change in the manufacturing conditions of the protective layer or coating component can even contribute to a deterioration in resistance compared with the untreated material. Plasma nitriding can improve resistance by even 20 times, but in most cases, the improvement was about two-fold. Shot peening or friction stir processing can improve erosion resistance up to five times. However, such treatment introduces compressive stresses into the surface layer, which reduces corrosion resistance. Testing in a 3.5% NaCl solution showed a deterioration of resistance. Other effective treatments were laser treatment (an improvement from 1.15 times to about 7 times), the deposition of PVD coatings (an improvement of up to 40 times) and HVOF coatings or HVAF coatings (an improvement of up to 6.5 times). It is shown that the ratio of the coating hardness to the hardness of the substrate is also very important, and for a value greater than the threshold value, the improvement in resistance decreases. A thick, hard and brittle coating or alloyed layer may impair the resistance compared to the untreated substrate material.

This paper presents a comparison of two-component ammonia-based inlet atmospheres diluted with either hydrogen (NH3/H2) or nitrogen (NH3/N2). Taking advantage of the features of inlet atmospheres diluted with nitrogen and hydrogen, four two-stage processes were designed and carried out, which were juxtaposed with two single-stage processes carried out only in an NH3 atmosphere. A common parameter of the processes carried out was the same value of nitrogen availability in each process stage. The gas nitriding process was carried out on ASIS 1085 non-alloy steel and ASIS 52100 alloy steel. It was found that the chemical composition of the steels studied, for the adopted nitriding process parameters, did not affect the kinetics of the growth in the mass of nitrided samples as a function of the nitriding time. However, the additions of alloying elements present in the steels studied significantly affected the nitrogen distribution between the resulting iron nitride layer and the diffusion zone in the nitrided substrate. Because of the presence of chromium in AISI 52100 steel, a larger mass of nitrogen accumulated in the nitriding zone in the solution compared with unalloyed AISI 1085 steel. As a result, with the same increase in the mass of nitrided steel, a thicker layer of iron nitrides formed on AISI 1085 steel than on AISI 52100 steel.

The influence of plasma nitriding and gas nitriding processes on the change of surface roughness and dimensional accuracy of 42CrMo4 steel was investigated in this paper. Both processes almost always led to changes in the surface texture. After plasma nitriding, clusters of nitride ions were formed on the surface of steel, while gas nitriding very often led to the new creation of a formation of a “plate-like” surface texture. In both cases of these processes, a compound layer in specific thickness was formed, although the parameters of the processes were chosen with the aim of suppressing it. After the optimizing of nitriding parameters during nitriding processes, it was found that there were no changes in the surface roughness evaluated using the Ra parameter. However, it turned out that when using a multi-parameter evaluation of roughness (the parameters Rz, Rsk and Rku were used), there were presented some changes in roughness due to nitriding processes, which affect the functional behavior of the components. Roughness changes were also detected by evaluating surface roughness profiles, where nitriding led to changes in peak heights and valley depths. Nitriding processes further led to changes in dimensions in the form of an increase of 0.032 mm on average. However, the magnitude of the change has some context on chemical composition of material. A larger increase in dimensions was found with gas nitriding. The change in the degree of IT accuracy is closely related to the change in dimension. For both processes, there was a change of one degree of IT accuracy compared to the ground part (from IT8 to IT9). On the basis of the achieved dimensional accuracy results, a coefficient of change in the degree of accuracy IT was created, which can be used to predict changes in the dimensional accuracy of ground surfaces after nitriding processes in degrees of accuracy IT3–IT10. In this study, a tool for predicting changes in degrees of accuracy of ground parts after nitriding processes is presented.

The gas nitriding of Ti13Nb13Zr alloy was performed in a pure nitrogen atmosphere at 1200 °C with different holding times. The variations in the phase constituents, microstructure, and mechanical properties of the nitriding layer with depth were characterized by SEM, XRD, and nanoindentation. Depending on the phase distributions in depth, the nitriding layer could be sequentially divided into an external nitride layer mainly consisting of TiN, an internal nitride layer consisting of TiN, Ti 2 N and TiN 0.3 , and an N diffusion region consisting of TiN 0.3 . The mechanical properties were closely associated with the phase constituents, where the nanohardness of the internal nitride layer, internal nitride layer N diffusion region and substrate were about 17.126±0.399, 12.120±0.386, 5.627±1.080, and 3.632±0.116 GPa, respectively. In the nitriding process, the N diffusion region containing needle-like TiN 0.3 precipitates formed in the initial nitriding stage, and then the precipitates were gradually converted to the external and internal nitride layers, whose thickness increased with the nitriding time. Furthermore, the influence of the alloying element redistribution on the N diffusion mechanism was discussed.

Nanoflex stainless steel is a promising material for medical applications. However, improvement of its mechanical properties without compromising its corrosion resistance is still a challenge. In order to investigate the effect of the nitriding process on the corrosion and wear resistance of Sandvik NanoflexTM steel, a number of processes were carried out in a gas atmosphere with differing ammonia contents in the temperature range of 425–475 °C for 4 h. The mechanical properties and wear resistance of the layers were tested using the nanoindentation and pin-on-disc methods, respectively. In order to assess corrosion resistance, potentiodynamic tests were carried out in Ringer’s artificial body fluid and in a 3% aqueous solution of sodium chloride. The results are discussed herein with respect to the microstructural characteristics of the layers studied using light and scanning electron microscopy, X-ray diffraction phase analysis and wavelength dispersive X-ray microanalysis. The structure of nitrided layers included three zones: the subsurface zone composed of nitrides and the zones composed of metastable phases, i.e., the S phase (γN) and expanded martensite (αN) with possible precipitates of nitrides. The third zone adjacent to the steel core was enriched with carbon. The nitrided samples showed significant improvement in the wear rate while maintaining good corrosion resistance in comparison to the non-treated steel. We concluded that nitriding should be carried out at a temperature below 450 °C and in an atmosphere containing no more than approximately 50% ammonia in order to avoid nitrides precipitation.

A kind of gas nitriding method catalyzed by rare earth for 40CrNiMoA alloy steel was researched in this article. Effect of temperature on surface hardness of gas nitriding method catalyzed by rare earth, change law of layer depth with time at 500 °C were carried out and compared with normal gas nitriding. Based on these researches, gas nitriding method catalyzed by rare earth was optimized. The results show that gas nitriding catalyzed by rare earth can not only increase the nitriding speed, but also enhance the surface hardness of the nitriding layer. Using three - stage gas nitriding method catalyzed by rare earth and after 40 hours, the samples can meet the need of nitrided layer depth no less than 0.5mm, surface vickers hardness no less than 600.

Surface texturing-plasma nitriding duplex treatment was conducted on AISI 316 stainless steel to improve its tribological performance. Tribological behaviors of ground 316 substrates, plasma-nitrided 316 (PN-316), surface-textured 316 (ST-316), and duplex-treated 316 (DT-316) in air and under grease lubrication were investigated using a pin-on-disc rotary tribometer against counterparts of high carbon chromium bearing steel GCr15 and silicon nitride Si3N4 balls. The variations in friction coefficient, mass loss, and worn trace morphology of the tested samples were systemically investigated and analyzed. The results showed that a textured surface was formed on 316 after electrochemical processing in a 15 wt % NaCl solution. Grooves and dimples were found on the textured surface. As plasma nitriding was conducted on a 316 substrate and ST-316, continuous and uniform nitriding layers were successfully fabricated on the surfaces of the 316 substrate and ST-316. Both of the obtained nitriding layers presented thickness values of more than 30 μm. The nitriding layers were composed of iron nitrides and chromium nitride. The 316 substrate and ST-316 received improved surface hardness after plasma nitriding. When the tribological tests were carried out under dry sliding and grease lubrication conditions, the tested samples showed different tribological behaviors. As expected, the DT-316 samples revealed the most promising tribological properties, reflected by the lowest mass loss and worn morphologies. The DT-316 received the slightest damage, and its excellent tribological performance was attributed to the following aspects: firstly, the nitriding layer had high surface hardness; secondly, the surface texture was able to capture wear debris, store up grease, and then provide continuous lubrication.

TC4 titanium alloy was treated by ion nitriding. The structure of nitriding layer was analyzed by scanning electron microscopy. The depth and microhardness of nitriding layer were measured. The frictional properties of titanium alloy before and after nitriding were compared by friction test. The results show that the ion nitriding technology can form a stable nitriding layer with a depth of up to 20μm and a surface hardness of 560 HV0.2. At the same time, after nitriding, the wear resistance of the titanium alloy surface is improved. And the coefficient of friction between the friction pair is reduced.