Defect and Diffusion Forum Vol. 405

Abstract: In this study three-layered materials composed of one zinc layer between two magnesium layers were prepared. Diffusion at the Mg-Zn boundary leads to the formation of thermodynamically more stable, yet mechanically very brittle intermetallic phase. Homogenous distribution of the fine-grained MgZn₂ intermetallic phase in magnesium or zinc alloys has a positive effect on strength of these alloys. In a form of continuous thick layer stretching throughout the whole material, the phase may leads to deterioration of mechanical properties. However, the mechanism of fracture has not yet been sufficiently described. The Mg based materials with one layer of Zn were investigated in terms of chemical composition and mechanical properties and fractographic evaluation. The materials with 0.25 mm, 0.5 mm, 1 mm and 2 mm thick layer of Zn were processed via bidirectional hot pressing method at 300 °C and 500 MPa. The phase and chemical composition of prepared materials was characterized by XRD and SEM-EDS methods. The mechanical properties were evaluated based on the results of three-point bend test and fractographic analysis of fracture surface. The results showed formation of MgZn₂ intermetallic phase on the interface of Mg and Zn layers and solid solution of Zn in Mg. The results showed that the presence of Zn layer leads to improvement of mechanical properties when compared to pure Mg prepared at the same condition. The strengthening effect of solid solution and intermetallic phase may be the reason of the increase of flexural strength.

Abstract: Dispersion strengthened Cu composites are studied over recent years to find an optimum processing route to obtain a high strength, thermal-stable copper alloy designed for modern applications in electrical engineering. The experimental Cu–4Al₂O₃–1MgO material was prepared by in situ thermo-chemical technique and mechanical milling followed by spark plasma sintering (SPS). The study analyses the influence of the Al₂O₃ and MgO secondary phases on strengthening the copper matrix. Microstructure of the composite was studied by X-ray diffraction analysis, scanning and transmission electron microscopy. The sintered microstructure shows a grain size distribution characterized by ultrafine grains/twins embedded inside the matrix of nanocrystalline grains. The microstructure is thermal stable up to 900 °C due to the dispersed alumina nano-particles that effectively strengthen crystallite/grain boundaries during the SPS process and annealing of the sintered compact at elevated temperatures. On the other hand, the coarsened MgO particles are responsible for ultrafine grains/twins formation. The obtained microstructure is important for practical utilization of the material because this structure is characterized by a good combination of strength and ductility.

Abstract: In the present study, bulk 10 % cerium stabilized zirconia was prepared by spark plasma sintering technique. Various time temperatures regimes were used and prepared sample were subjected to microstructure observation by electron microscopy and X-ray diffraction and mechanical testing by nanoindentations. It was shown, that conditions of spark plasma sintering process can strongly influence properties of resultant sample, mainly grain size which warried from some tens or hundreds of nanometre to approximately 100 micrometres. Also some structure changes in the sintering process were observed resulted to phase changes and decomposition.

Abstract: The economic, environmental and healthcare aspects are pushing cemented carbide industry to reduce or even avoid the usage of conventional binder metals – nickel and cobalt. Commonly, austenitic Fe-Ni alloys have been preferred choice for substituting Co. Similar to Ni, manganese acts as austenite stabilizer and studies have shown that Fe-Mn alloys offer alternative binder metal to Co and Ni in cemented tungsten carbides. In addition, Fe-Mn as a binder potentially offers improved wear resistance due to the well-known wear properties of Fe-Mn-C steels. Addition of chromium to the binder composition increases corrosion performance of composite. Cemented carbides bonded with austenitic FeCrNi binder have demonstrated promising performance. In present work the possibility of achieving austenitic binder phase through substitution of nickel by manganese as an austenite stabilizer is investigated. Structure formation, phase composition and mechanical performance of WC-FeMn and WC-FeCrMn cemented carbides are discussed.

Abstract: Granulated iron oxide particles were incompletely reduced to structured particles comprised metallic iron and residual iron oxides. Structured particles were pressed into prismatic compacts and sintered. Some of sintered specimens were subsequently phosphatized and calcined. Specimens with an iron phosphate coating were found stiffer than specimens without coating. In Hanks' solution, a galvanic corrosion was induced by more noble iron oxides coupled to a less noble metallic iron. This could explain higher corrosion potentials and higher rates of iron dissolution in comparison with a pure iron. The coating of specimens with iron phosphates shifted corrosion potentials towards more negative values and slowed down the dissolution of iron. This was most likely caused by a reduction in oxygen flow through the coating to iron-oxide cathodes, which has enhanced the influence of diffusion control on the kinetics of reduction reaction.

Abstract: Nowadays commonly used thermal barrier coatings (TBC) are based on yttria stabilized zirconia (YSZ). Addition of mullite phase into the YSZ coating can improve resulting high temperature properties. The contribution focuses on high temperature cyclic oxidation behaviour of two TBC systems with different top coats (TC) deposited by the means of atmospheric plasma spraying. The initial mullite-YSZ powder mixture consisted of 29 vol. % of mullite and 71 vol. % of YSZ. The conventional TBC system consisted of ~ 150 µm thick NiCoCrAlYHfSi bond coat (BC) and ~ 300 µm thick YSZ top coat. The experimental mullite-YSZ (MYSZ) TBC system consisted of ~ 150 µm thick NiCoCrAlYHfSi bond coat, ~ 100 µm thick YSZ interlayer and ~ 200 µm thick mullite-YSZ top coat. The experimental TBC proved higher lifetime, durability and phase stability and also lower grow rate of thermally grown oxide (TGO) compared to conventional TBC. Lifetime, phase stability and changes in the microstructure of TBCs after the furnace cyclic oxidation test were evaluated by the means of scanning electron microscopy equipped with EDX analyzer and X-ray diffraction techniques. Oxidation kinetics of TGO was calculated based on thickness determined utilizing digital image analysis.

Abstract: According to the value of water contact angle (WCA), the surfaces can be roughly defined as hydrophilic (with WCA less than 90°) or as hydrophobic (with WCA higher than 90°). Water wetting behavior plays important role and surfaces with special wettability (hydrophobic-superhydrophobic; hydrophilic-superhydrophilic) can be used both in the daily life (solar cells, smartphones, car windows, etc.) and in the industry (corrosion resistance, self-cleaning, anti-icing properties, etc.). Nowadays, the development of hydrophobic surface treatment that may be applied in the industry is very interesting topic. Therefore, it was decided to estimate the influence of radiofrequency (RF) plasma jet in atmosphere on wetting behavior of ceramic plasma sprayed coatings. As the initial material for surface treatment, yttria stabilized zirconia suspension plasma sprayed coatings were used. The influence of RF plasma jet on suspension plasma sprayed coatings was estimated on both hydrophilic and hydrophobic surfaces, and resulted water contact angle and free surface energy of modified samples were measured by sessile droplet method. Microstructure, phase composition and topography investigation were carried out by means of light microscopy, X-ray diffraction techniques and non-contact profilometry.

Abstract: This paper presents the results of the investigation of NiCrRe coating deposited by plasma spray process at the atmospheric pressure on boiler steel substrate. These coatings were characterized by means of a scanning electron microscopy, and Energy-dispersive X-ray spectroscopy. The wear resistance of plasma sprayed NiCrRe coatings has been investigated under dry sliding conditions at applied load of 10 N in air. The continuous stiffness measurement (CSM) method was used for the investigation of nanohardness using Agilent G200 Nano-indenter in order to determine the mechanical properties of the coatings. Microstructural observations pointed out that the NiCr layer with white isles of rhenium possessed porosity, oxidized, un-melted and semi-melted particles, and inclusions. According to the results the thickness of the layer is 450 µm, the indentation modulus 158 ± 24.4 GPa, hardness 3.74 ± 0.76 GPa and the coefficient of friction is 0.45.

Abstract: The high-velocity oxy-fuel technique (HVOF) was used to produce dense NiCrRe coating on boiler steel substrate with a minimal amount of oxide impurities and low porosity. Microstructure analysis, tribology and nano-hardness measurement have been realized with the aim to characterize the systems. The microstructure was studied using scanning electron microscopy and Energy-dispersive X-ray spectroscopy. Tribological characteristics have been studied under the dry sliding condition at applied loads of 5, 10 and 20 N using the ball-on-flat technique with SiC ball at room temperature. Nano-hardness was investigated in continuous stiffness measurement (CSM) mode, the indentation depth limit was 1500 nm. Microstructure analyses proved that the HVOF sprayed layer, with a thickness approximately 870 µm, contains a relatively low volume fraction of porosity with a chemical composition based on Nickel, Chromium, with white areas of Rhenium. The wear rate of the coating is significantly lower than the wear rate of 16Mo3 steel. The average values of indentation modulus and hardness were E_IT = 237.6 GPa and H_IT = 6.3 GPa, respectively.

Abstract: Anodizing is an electrochemical process to produced anodic coatings for improving magnesium (Mg) properties such as corrosion-resistant. In this study, anodizing of pure magnesium in 1 M NaOH electrolyte for 1800 s and at 21 °C and different constant current or voltage was investigated. The effect of voltage and current on morphology and thickness of the resulting anodic layers was evaluated by scanning electron microscopy (SEM) equipped with EDX analyser. The thickness of the produced layers was determined to utilize digital image analysis. The results showed that using lower current of 0.08 A non-compact anodic layer was produced. When a higher current of 0.2 and 0.5 A was used compact and thicker anodic layers were produced compare to lower current of 0.08 A. The anodic layer produced at a constant voltage of 20 V was rougher, thicker and contained microcracks compare to anodic layers formed at constant voltage of 12 V and at constant current (0.2 and 0.5 A).