Applied Mechanics and Materials Vols. 719-720

Abstract: Emissivity is the ratio between the radiant hemispherical power emitted by a real body, at an absolute temperature, and the radiant hemispherical power emitted by a black body at the same temperature. The energy emitted is proportional to the fourth power of the object ́s temperature. Emissivity may vary from 0 (reflected by a mirror) to 1.0 (black body theory). Studies are being carried out at the University of Brasilia to investigate the microstructural behavior of materials used in the repairing of hydroelectric turbines, after several thermal cycles of welding. These studies use thermographic techniques for monitoring the temperature and require that the correct emissivity value for specific materials and surface conditions are used in order to guaranty that the temperatures reported by the radiometric sensors are consistent with the actual temperatures. This study aims to validate an experimental methodology for evaluating the emissivity of the steel ASTM A 743 CA6NM and the AWS 410 NiMo as deposited by a GMAW process at temperatures ranging from 100oC to 1000oC. The experiment consists of heating a small sample of the material with an oxyacetylene torch while a thermocouple, welded on surface of the sample, an infrared sensor and a thermographic camera monitor the surface temperature. During the heating and the cooling process, the sample surface is protected from the air by an argon gas flow directed towards the visualized area. Results consistent with the reported in the literature for similar materials were attained and curves of the emissivity “versus” temperature for the tested materials were produced, providing a basis for proper thermographic temperature monitoring.

Abstract: Photocatalytic TiO₂–Copper oxides composite coatings were prepared by the mechanical coating technique (MCT) and subsequent heat oxidation. The results showed that Cu particles were mainly adhered to the surfaces of the Ti–Cu composite coatings. After heat oxidation,the Ti coatings were partly oxidized to rutile TiO₂ and a Ti–TiO₂–Cu₂O–CuO composite microstructure was formed. These composite coatings had relative high photocatalytic activity because of the formation of the TiO₂–Ti composite microstructure and TiO₂–Cu₂O–CuO p–n heterojunctions.

Abstract: Titanium (Ti) coatings on alumina (Al₂O₃) balls fabricated by mechanical coating technique (MCT) were oxidized at relatively low temperatures. Crystal structure and microstructure of the coatings were investigated. Photocatalytic activity of the coatings was evaluated and discussed. The results showed that TiO₂ films on Ti or Ti/TiO₂ coatings were fabricated by the MCT and subsequent heat oxidation. The TiO₂ is anatase phase if the heat oxidation temperature is under 773 K. The TiO₂ is the mixed phases of anatase and rutile if the heat oxidation temperature is in the range of 673–973 K. Besides, rutile TiO₂ is formed if the heat oxidation temperature is beyond 1073 K. Photocatalytic activity of anatase TiO₂ is higher than rutile TiO₂.

Abstract: Photosensitizers immobilized in polymers can serve as antibacterial surfaces or coatings and can be applied for disinfection of water or medical instruments. The antibacterial activity of the immobilized photosensitizers is based on their excitation by visible light followed by energy transfer from the photosensitizers to oxygen dissolved in an aqueous phase which produces reactive oxygen species that cause irreversible damage to bacterial cells. The photosensitizer Rose Bengal immobilized in polystyrene, polycarbonate and poly (methyl methacrylate) was shown to eradicate Gram-positive Staphylococcus aureus bacteria under moderate illumination.

Abstract: In this study, a developed new boriding method called as “Cathodic Reduction and Thermal Diffusion based Boriding” (CRTD-Bor) was applied to increase the surface hardness of 400 series steels. The cross-sectional examination of borided steel revealed that the boride layers consisted of single phase Fe₂B. A dense and continuously 25μm thick Fe₂B layer could be formed after 20 minutes of CRTD-Bor. The grown boride layer exhibited 1500±200 HV on top, and gradually decreased to the matrix (325 ± 25 HV).

Abstract: Aluminium alloys are the predominant materials in modern industries. Increased knowledge about the surface characteristics of bare aluminium can enhance the understanding about how to optimize the working conditions for the equipment involving aluminium parts. This work focusses on the properties of native surface of aluminium alloy 8011, which is the main construction material for the production of air-to-air heat exchanger fins. In this study, we address its water wettability, surface roughness and frost formation in different psychometric parameters. The contact angle measurements revealed that this aluminium alloy exhibits a relatively high contact angle of about 78 degree, i.e. is not wetted completely. AFM measurements revealed significant surface roughness of typical heat exchanger fins. The thickness of formed frost was studied in relation to the wettability, humidity and the cold surface temperature.

Abstract: This research presents a developed simple analytical model to estimate the residual stress state and its magnitude in plasma sprayed coating based on the Vickers instrumented indentation results. By means of energy method during a Vickers indentation cycle, it is convenient to compute the stress state and its magnitude without any comparison of load-displacement curves of stress-free reference material. Computed results show that the residual stress in plasma sprayed Al-Si coating mainly ranges between 23-32 MPa in tensile state, which is consistent with the measurement of residual stress in as-sprayed NiCrAlY coating using neutron diffraction.

Abstract: The plasma cutting technology has been emerged as a developing technology which finds tremendous potential in fabrication and metal cutting industries. Thus for the cutting operation, the electrode inside the plasma torch plays a vital role for the plasma arc generation. The temperature of the arc is very high and at the electrode is around 3500°C. The cutting torch requires proper cooling system in order to prevent the electrode from quick wear due to the existence of high thermal gradient. The presented work aimed to study the impact of three coolants propylene glycol, ethylene glycol and de-ionized water flow over the electrode life. The experimental setups were arranged to study the heat transfer capabilities of the three coolants for different flow values and aimed to achieve the optimal flow rates for the efficient heat removal. The electrode life test trials were conducted to measure the electrode life for the flow values of three coolants in the temperature rise test. The optimal flow rates arrived from temperature rise test and the electrode life measured from life test are compared for the three coolant cases considered.

Abstract: Nowadays, polymeric composites reinforced with natural fibers are being considered in the civil engineering area. The use of polymeric composites to reinforce degraded timber structures can improve its behavior. Fibers with larger structural applications are glass and carbon but the use of natural fibers is an economical alternative and posses many advantages such as biodegradability, low cost and is derived from natural and renewable sources. Epoxy composite reinforced with sisal fabric was processed by resin transfer molding (RTM) at room temperature and this work studies thermal behavior and its respective mechanism of thermal decomposition. Samples of sisal fiber, epoxy resin and sisal/epoxy composite were characterized by Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). Thermogravimetric curves showed that sisal fibers can be used in manufacturing process where the processing temperature does not exceed 177°C and shown that the epoxy resin has the greatest stability material followed by sisal/epoxy composite.