Iranian Journal of Materials Science and Engineering

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Abstract: Formation of a hybrid coating by the use of plasma nitriding and hard chromium electroplating on the surface of H11 hot work tool steel was investigated. Firstly, specimens were plasma nitrided at a temperature of 550 °C for 5 hours in an atmosphere of 25 vol. % H2: 75 vol. % N2. Secondly, electroplating was carried out in a solution containing 250 g/L chromic acid and 2.5 g/L sulphuric acid for 1 hour at 60 °C temperature and 60 A/dm2 current density. Thirdly, specimens were plasma nitrided at a temperature of 550 °C for 5 and 10 hours in an atmosphere of 25 vol. % H2: 75 vol. % N2. The obtained coatings have been compared in terms of composition and hardness. The compositions of the coatings have been studied by X-ray diffraction analysis. The surface morphology and elemental analysis was examined by using scanning electron microscopy. The improvement in hardness distribution after third step is discussed in considering the forward and backward diffusion of nitrogen in the chromium interlayer. Also, the formed phases in the hybrid coating were determined to be CrN+Cr2N+Cr+Fe2-3N+Fe4N.

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Abstract: In this research plasma nitriding of pure aluminium and effect of iron elemental alloy on the formation and growth of aluminium nitride was investigated. Also corrosion properties of formed AlN were investigated. After preparation, the samples were plasma nitrided at 550^oC, for 6, 9 and 12 h and a gas mixture of 25%H₂-75%N₂. The microstructure and phases analysis were investigated using scanning electron microscopy and X-ray diffraction analysis. Moreover corrosion resistance of samples was investigated using polarization techniques. The results showed that only a compound layer was formed on the surface of samples and no diffusion zone was detected. Dominant phase in compound layer was AlN. Scanning electron microscopy results showed that nitride layer has particulate structure. These nitrided particles have grown columnar and perpendicular to the surface. It was also observed that the existence of iron in the samples increases the nitrogen diffusion, thus growth rate of iron containing nitrides are higher than the others. Corrosion tests results showed that formation of an aluminium nitride layer on the surface of aluminium decreases the corrosion resistance of aluminium significantly. This is due to elimination of surface oxide layer and propagation of cracks in the formed nitride layer

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This study investigates the synthesis of Al/MoS₂ nanocomposite coating by the electro spark deposition (ESD) method for its lubricating properties. ESD method was selected because it is a very easy, rapid, and cost-saving method and the resulting coating has a strong bonding with the substrate. As a substrate, a Ti-6Al-4V alloy sheet containing 6.12 % Al, 4.06 % V, 0.19% Fe, and 0.05 % Ni was used. For coating, an aluminum-molybdenum disulfide composite electrode in the form of a cylindrical rod was employed. Three frequencies of 5, 8, and 11 kHz, three current limits of 15, 25, and 35 amps, and three duty cycles of 50, 60, and 70% were used in the coating operation. AFM analysis was used to study the topography, morphology, and calculate roughness. The samples were then subjected to hardness tests. To determine the wear resistance of the samples, pin on disk tests were performed. XRD analysis was performed to identify the phases on the surface of the coated samples. SEM was used to examine the microstructure of the coating before and after wear testing, in order to determine the wear mechanism. The results indicated that the Al/MoS2 nanocomposite coating was synthesized on the substrate surface. The hardness of the reference sample is 353 Vickers, and that of the coated samples is about 200 Vickers. For the reference sample, the roughness was measured at 15.7 nm, and for the coated sample at 268.1 nm. As spark energy increased, the coefficient of friction increased by approximately 0.09. As spark energy increased, the wear rate increased by 27%. A significant increase in the Lancaster coefficient occurred around 5 joules of energy. According to the wear rate results, the sample with the lowest thickness wears 4% less than the sample with the highest thickness. The wear rate of sample 351170 is 78% lower than that of sample 150550.

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The Mo-30 wt% W solid solution is a well-established molybdenum-based alloy. A recent approach to alloying Mo and W involves the simultaneous reduction of their oxides by hydrogen. Several studies have reported the production of Mo-W alloys via the reduction of mixed oxides. But the alloying mechanism and the effects of key reduction parameters on alloy formation remain uninvestigated. In the present study, the simultaneous hydrogen reduction of MoO₃–WO₃ mixed oxides was investigated as a potential route for producing pre-alloyed Mo–W powders. In this study pre-alloyed Mo-30 wt% W is. The effects of reduction temperature and holding time on phase evolution, morphology, and alloy formation were systematically examined. The chosen temperature range is 600-1050°C. The influence of time on phase formation and morphology during the hydrogen reduction of molybdenum and tungsten oxides is examined over a 30-360 minute period. Other parameters such as heating rate (20℃min ), bed height (10mm), H₂ flow rate (500mlmin ), and material weight (14.3g) were kept constant. The reduced samples are analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of temperature and time on the reduction processes are closely linked. Results indicate the formation of MoO₂ and WO₂ phases at 600°C after 60 minutes of hydrogen reduction. No Oxide phases were detected in XRD analysis of the sample that was reduced at 800°C for 180 minutes. Therefore, tungsten and molybdenum coexist in all particles of the samples. Mass transfer between Mo and W takes place during the reduction of oxides to the metallic phase. But isothermal reduction does not yield a homogeneous alloy. In these studies, for T > 800°C, increasing time has minimal effect on alloying these elements. At T=800°C, a significant change in morphology occurs at t>180 min. At T=800°C, a notable morphological change occurs after t>180 min. These changes are caused by the CVT mechanism. The optimal conditions for achieving chemical homogeneity in the alloy powder particles after reduction were identified as: 600°C for 60 min, 600 to 1050 for 240 min, and 180 min at 1050°C. This cycle leads to the formation of homogeneous alloy powder particles. The achieved homogeneity is attributed to providing sufficient time for mass transport between molybdenum and tungsten oxide particles through gaseous chemical species formed within this temperature range.