فصلنامه علوم و مهندسی سطح ایران
پیاپی 40 (تابستان 1398)

In this study, high-velocity oxygen-fuel (HVOF) process was applied to produce Fe3TiO4 coating on the low carbon steel by adding NiCr. For this purpose, NiCr powder with the same ratio of 25% was mixed with Fe3TiO4. The structural and mechanical properties of the coatings were evaluated using scanning electron microscopy, X-ray diffractometry, micro hardness and roughness test. The results showed that the microstructure of the coatings is homogeneous and uniform, consisting of a nickel and iron solid metal solution with nickel chromium in titanium oxide. The XRD results indicated that the coatings included Fe 2O3, FeNi3, TiO2 and Fe2TiO4 phases. It was also found that the hardness was increased, roughness and porosity was decreased by addition of NiCr

In this paper, the effects of anodizing, solution heat-treatment and hot-rolling on microstructure and corrosion behavior of a biocompatible magnesium alloy containing Zn, Ca, and rare earth elements (REEs) were investigated. The anodizing process using HAE method in 3 various current densities and 2 different times was performed on each of the specimens. The corrosion resistance of samples was evaluated by immersion test in PBS solution at 37 ° C. The resulting oxide film were studied by scanning electron microscopy and the weight loss of the specimens was measured after eliminating the oxide residues. The results showed that a uniform structure with increased grain size is made by solution heat-treatment. This treatment had a favorable effect on the corrosion resistance of the alloy and reduced the corrosion rate by 20%. The hotrolling process likewise increased grain size and reduced the corrosion rate by 9%. Also, the anodizing process generally had a favorable effect on the corrosion properties of the specimens and in the best condition, caused 80% reduction in the corrosion rate. The effect of time and applied current density of the anodizing process was also studied. Results showed different effects for each of samples.

Synthesis of a hard layer on the surface of mild steel is a proper way to rise its performance. Titanium carbide is among the preferred wear-resistant materials. In this research, the carbothermal synthesis of TiC via pulsed laser is studied. Ilmenite is used, as an economic raw material, for in-situ synthesis of TiC. However, the conversion of FeTiO3 to TiC in the short period of laser processing is challenging. Therefore two solutions were accompanied: the applied carbon content was more than the stoichiometry of reduction, while the blended ilmenite and graphite powders were mechanically activated. The increase of activation time to 200 hours was investigated via X-ray diffraction analysis. Afterward, a layer of the powder was preplaced on the substrate and the laser cladding was conducted. The effect of mechanical activation on the formation of the dispersed particles is studied, especially considering the transformation of oxides to TiC. SEM with EDS analyzer and XRD analysis were applied for this aim. The morphology of TiC particles and the microstructure of the matrix were also deliberated. The characteristic microstructure of directional solidification with equiaxed, cellular, dendritic and eutectic regions was observed. Otherwise, the Vickers microhardness measurements represented a gradual increase from the substrate toward the surface, which has reached to 1600 HV. It is confirmed that the synthesis of a composite layer with gradual dispersion of TiC particles via laser reduction of ilmenite is a feasible process.

Improvement of copper surface properties, such as wear resistance and corrosion resistance, increases the application of this metal in various industries .For this purpose, Inconnell 625 alloy was coated with Gas Tangsten Arc Welding (GTAW) on copper. Phase analysis and solidification behavior of coating were performed using X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results showed that the coating structure contains the γ phase, with the secondary products of niobiumrich. Due to the full solubility of nickel and copper in solid state, it was observed that the region created between the coating and the substrate has a chemical composition of Cu-24.26 Ni-9.31 Cr-5.77 Mo and is free from metallurgical defects. The microhardness of sunstrate and coating was measured 50 and 338 vickers, respectively. In order to investigate the wear behavior of the coating and the substrate, the pin on disk test was used at ambient temperature. The wear surfaces produced at ambient tempreture characterised at micro-scale level using SEM and EDS. The presence of coating increased wear resistance of about 85%, which can be attributed to the change in the wear mechanism from delamination wear (related to copper) to oxidation wear (related to inconell 625). The oxide layer formed from the wear pair, in addition to reducing the wear rate, reduced the friction range from about 0.2 - 1.5 (related to copper) to 0.3 - 0.8 (for inconell 625). The study corrosion property of coating and substrate was also done by polarization test. The results showed a significant increase corrosion resistance of copper in the presence of this coating and corrosion current decreased from 239 nA/cm2 to 0.27 nA/cm2 .