ABSTRACT
This study presents research results concerning the vacuum carburizing of four steel grades, specifically conforming to European standards 1.7243, 1.6587, 1.5920, and 1.3532. The experimental specimens exhibited variations primarily in nickel content, ranging from 0 to approximately 3.8 wt. %. As a comparative reference, gas carburizing was also conducted on the 1.3532 grade, which had the highest nickel content. Comprehensive structural analysis was carried out on the resultant carburized layers using a variety of techniques, such as optical and electron scanning, transmission microscopy, and X-ray diffraction. Additionally, mechanical properties such as hardness and fatigue strength were assessed. Fatigue strength evaluation was performed on un-notched samples having a circular cross-section with a diameter of 12 mm. Testing was executed via a three-point bending setup subjected to sinusoidally varying stresses ranging from 0 to maximum stress levels. The carburized layers produced had effective thicknesses from approximately 0.8 to 1.4 mm, surface hardness levels in the range of 600 to 700 HV, and estimated retained austenite contents from 10 to 20 vol%. The observed fatigue strength values for the layers varied within the range from 1000 to 1350 MPa. It was found that changing the processing method from gas carburizing, which induced internal oxidation phenomena, to vacuum carburizing improved the fatigue properties to a greater extent than increasing the nickel content of the steel.
ABSTRACT
This article presents the results of a study on reducing deformations resulting from high-temperature vacuum carburizing and post-carburizing heat treatment. The idea was to increase the strength of steel at elevated temperatures by pre-carburizing at heat-up to the process temperature (SC-stage carburizing). It has been shown that the use of carburizing in stages from a lower temperature to the target temperature, compared to traditional vacuum carburizing at a constant temperature (Constant-Temperature Carburizing-CTC), has a significant impact on the chemical and phase composition of the technological layer, surface after the process and, consequently, on its mechanical properties. It was shown that the retained austenite content after stage carburizing was reduced by approximately 45%, as was the thickness of the gear teeth measured at the pitch diameter. Additionally, uniform stress distribution was demonstrated for the SC process. Carbon saturation of austenite increases the yield strength, and therefore the dimensional stability of steel heat-treated at elevated temperatures also improves, which effectively permits high-temperature treatment of critical steel parts such as, for example, gear wheels, for which high dimensional accuracy is required.
ABSTRACT
Graphene transfer onto ceramics, like Si/SiO2, is well-developed and described in the literature. However, it is problematic for other ceramic materials (e.g., Al2O3 and ZrO2), especially porous ones. In this case, it is mainly due to poor adhesion to the substrate, resulting in strong degradation of the graphene. For these reasons, the research topic of this study was undertaken. This article presents research on the development of the methodology of graphene transfer onto ceramic Al2O3 surfaces. Polycrystalline graphene chemical vapour deposition (CVD) monolayer and quasimonocrystalline high-strength metallurgical graphene (HSMG®) synthesised on liquid copper were used. When developing the transfer methodology, the focus was on solving the problem of graphene adhesion to the surface of this type of ceramic, and thus reducing the degree of graphene deterioration at the stage of producing a ceramic-graphene composite, which stands in the way of its practical use. Plasma and chemical ceramic surface modification were applied to change its hydrophobicity, and thus to improve the adhesion between the graphene and ceramic. The modification included the use of dielectric barrier discharge (DBD) plasma, oxygen plasma (RF PACVD method - Radio Frequency Plasma Assisted Chemical Vapour Deposition), and hydrofluoric acid treatment. Changes in surface properties caused by the modifications were determined by measuring the contact angle and (in the case of chemical modification) measuring the degree of surface development. The effectiveness of the applied surface preparation methodology was evaluated based on the damage degree of CVD and HSMG® graphene layer transferred onto modified Al2O3 using optical microscopy and Raman spectroscopy. The best average ID/IG ratio for the transferred HSMG® graphene was obtained after oxygen plasma modification (0.63 ± 0.18) and for CVD, graphene DBD plasma was the most appropriate method (0.17 ± 0.09). The total area of graphene defects after transfer to Al2O3 was the smallest for HSMG® graphene after modification with O2 plasma (0.251 mm2/cm2), and for CVD graphene after surface modification with DBD plasma (0.083 mm2/cm2).
ABSTRACT
Graphene has been considered as a material that may overcome the limitations of polymer semi-permeable membranes in water treatment technology. However, monolayer graphene still suffers from defects that cause leakage. Here, we report a method of sealing defects in graphene transferred onto porous polymer substrate via reduced graphene oxide (rGO). The influence of various reducing agents (e.g., vitamin C, hydrazine) on the properties of rGO was investigated by SEM, Raman, FTIR, and XRD. Subsequently, membranes based on graphene/reduced graphene oxide were tested in a forward osmosis system using sodium chloride (NaCl). The effect of the effectiveness of the reduction of graphene oxide, the type and number of attached groups, the change in the distance between the rGO flakes, and the structure of this material were examined in terms of filtration efficiency. As a result, semi-permeable centimetre-scale membranes with ion blocking efficiency of up to 90% and water flux of 20 mL h-1 m-2 bar-1 were proposed.
ABSTRACT
To access the properties of theoretical graphene, it is crucial to manufacture layers with a defect-free structure. The imperfections of the structure are the cause of deterioration in both electrical and mechanical properties. Among the most commonly occurring crystalline defects, there are grain boundaries and overlapping zones. Hence, perfect graphene shall be monocrystalline, which is difficult and expensive to obtain. An alternative to monocrystalline structure is a quasi-monocrystalline graphene with low angle-type boundaries without the local overlapping of neighboring flakes. The purpose of this work was to identify factors that directly affect the structure of graphene grown on a surface of a liquid metal. In the article the growth of graphene on a liquid copper is presented. Nucleating graphene flakes are able to move with three degrees of freedom creating low-angle type boundaries when they attach to one another. The structure of graphene grown with the use of this method is almost free of overlapping zones. In addition, the article presents the influence of impurities on the amount of crystallization nuclei formed, and thus the possibility to order the structure, creating a quasi-monocrystalline layer.
