Materials Science Forum Vol. 802

Abstract: Excellent tribological properties of hard materials surface are desirable in several sectors of industry. Diamond-like carbon (DLC) coatings are well known for their low friction, excellent wear resistance, and high hardness. In this work, DLC films were deposited on AISI M2 steel using a modified PECVD pulsed-DC discharge. Multilayer of carbon and silicon were grown, alternately. Samples were produced with different layer thickness for carbon and silicon, and the same parameters for each material layer, in order to investigate friction coefficient in each layer, evaluate rate deposition variation and the gradient behavior of different layers. Raman spectroscopy was used to verify the structural arrangement of carbon atoms. The films were also characterized by scanning electron microscopy and EDX. Tribological tests were performed to observe adhesion between layers and substrate, friction, and wear. The results showed the variation of friction coefficient and that deposition rate declines when increasing number of layers.

Abstract: In this work was investigated the microstructural evolution process of the duplex stainless steel SAF 2205 as-cast after solution annealing treatment. The aim was to detect the effects on the material microstructure by the cooling rate variation. The studied material were submitted to solution anneal at 1100 °C for 240 min, followed by cooling in water, air and furnace. The results evaluation was based on micrographic analysis, energy dispersive spectrometry measurements (EDS), X-ray diffraction and hardness tests. The ferrite volume fraction obtained in the microstructure increased with the cooling rate, because it causes diffusion inhibition of the steel constituent elements, promoting retention of the ferrite. The volume fraction of austenite phase increased with a lower cooling rate. The cooling rate is an important factor in defining the steel microstructure, particularly about intermetallic phases precipitation, which occurred by the slow cooling rate. Additionally, it was observed the precipitation of sigma phase.

Abstract: The main aim of this work is to show porosity evolution during application of various processing conditions to a high-purity (99.7 wt.%) iron powder, including compacting, sintering and equal channel angular pressing (ECAP). Iron powder bars with dimensions of 8x8x30 mm and 8x8x10 mm were axially pressed with pressures ranging from 100MPa to 250MPa, followed by sintering at 1100oC during 30 minutes under argon atmosphere. Sintered bars were processed by ECAP at room temperature in a single pass, using a SAE 1045 steel die with an internal angle of 120o. Microstructural characterization was performed by light optical microscopy (LOM) and quantitative stereology. ECAP processing resulted in a substantial reduction in the porosity levels for specimens pressed at 100 MPa and 150 MPa. The sample compacted with 150MPa and processed by ECAP with back-pressure showed the lowest volume fraction of porosity. Higher compacting pressures caused an increase in porosity levels. This result is explained by presence of cracks prior to ECAP and the concurrent action of severe stress-strain states during extrusion.

Abstract: As aircraft and thermoelectric turbine blades work in aggressive environments (high temperatures and pressures), they are exposed to oxidation reactions. Ceramic coatings are employed to increase the turbine work temperature (improving its performance) and a bond coat (BC), base of particulate material of Ni-Cr-Al powders, which assure a good adhesion, gradual decrease in thermal expansion coefficient between the metallic substrate and the ceramic top coat, avoiding the oxidation effect in the metallic substrate. This research aims the study and comparison of two different deposition process routes of particulate materials of BC (MCrAlY) on AISI 316 stainless steel substrate. In the first case, the BC powder was pre-deposited by segregation method and irradiated by a CO₂ laser beam. In the second case, laser surface texturing was done on the stainless steel surface by a Yb: fiber laser beam, the BC was deposited by the same method, and further, irradiated by a CO₂ laser beam. The main focus of this work was to evaluate the resulting interface for both mentioned cases. For this propose, characterizations were made using the techniques of optical microscopy and roughness measurements. In the first case, homogenous layers of bond coat were obtained. Optical microscopy suggest the formation of a metallurgic bonding between the substrate and the MCrAlY. For the laser surface texturing, the surface roughness can be adjusted by the laser beam parameters.

Abstract: The development of dry self-lubricating materials is directly linked to the rising requirements of performance. Iron and steel are the most used metals around the world and molybdenum disulfide (MoS₂) one of the most used solid lubricant. Therefore it is expected that one might try to develop self-lubricating steels containing MoS₂, however MoS₂ reacts with steel matrices during sintering. This work has focused on the study of these reactions; temperatures at which this occurs; the influence of particle sizes and MoS₂ content and also MoS₂ influence during processing and in the parts properties. The results showed that the reaction occurs at lower temperatures than the predicted and the products are iron-molybdenum and iron sulfides. The sintering studies revealed that MoS₂ enhances sintering and the particle size and MoS₂ content directly influenced the morphology and chemical composition of the resulting phases.

Abstract: The effect of the substitution of La with Pr on the microstructure and some electrochemical properties of La_0.7-xPr_xMg_0.3Al_0.3Mn_0.4Co_0.5Ni_3.8 (x=0.0, 0.1, 0.3, 0.5, and 0.7) ingot alloys as powder electrode was studied in this paper. XDR and SEM (+EDX) analyses revealed that the as-cast alloys consist mainly of similar LaNi₅ and (La,Pr)Mg₂Ni₉ phases. With the increase in Pr content, both the relative abundance of the phases and microstructure changed. The electrochemical studies showed that the maximum discharge capacity decreased with the increase in Pr content. For the alloy without Pr, the self-discharge studies revealed a major stability of the hydride until the first 984 hours (41 days) of the charge/discharge cycles. The high-rate dischargeability of this alloy electrode showed the best result HRD=88% at a discharge current density of I_d=250 mA/g.

Abstract: The microstructure, chemical composition and self-discharge of some retail available supercapacitors have been investigated. Standard capacities of 1.0, 0.47 and 0.1 F at a maximum potential of 5.5 V were employed in this study. Self-discharge were carried out at room temperature and close to the nominal maximum working temperature of the supercapacitors (specified for 343 K). Internal resistance of the supercapacitors were calculated using the discharge curve for room temperature and 333 K. The microstructures of the electrode powder material have been investigated using scanning electron microscopy (SEM) and chemical microanalyses employing energy dispersive X-ray analysis (EDX). A compositional and morphological evaluation of these commercial supercapacitors materials has been carried out.

Abstract: In Petrobrás SA refineries, there are thousands of heat exchangers in which cooling water is treated for recycling, often by outsourced companies. In all of these refineries there happens shutdown for corrective maintenance of the exchangers by problems related to corrosion caused by the cooling water, requiring about four days to repair a unit. The shutdown of a production line results in loss of revenue between U$ 200,000.00 and U$ 300,000.00 per day. To control the cooling water quality, it is used coupons for corrosion analysis. Tests are limited to visual inspection and by mass loss analysis. In this work, inhibitors such as sodium nitrite, sodium dichromate and benzoic acid are used to protect metals immersed in water in order to avoid the corrosion and consequently the shutdown of the exchanger for corrective maintenance.

Abstract: Titanium and stainless steel are examples of biomaterials widely used in dental and orthopedic implants owing to their properties of good corrosion resistance and excellent biocompatibility. This paper reports on a study of the biomimetic method applied to titanium (cp-Ti) and 316L stainless steel. The method consists in immersing the metal substrate in a synthetic solution of SBF (simulated body fluid) whose composition, pH and temperature resemble those of human blood plasma. The coating on the two metals was effective for obtaining hydroxyapatite, which was confirmed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR).

Abstract: Pit morphology on sensitized 310S stainless steel has been studied using an image processing method based on reflected light microscopy (profile and surface). Salt Spray test has been used to induce the pitting corrosion. Morphological pits character do not depend on sensitization heat treatments here applied. Nucleation rates and growth may be associated with quantity and distribution of chromium carbides. This being so, condition I (heating up to 1065^o C during 1 h and air cooling) and condition II (heating up to 1065^o C during 1 h and air cooling followed by reheating up to 670°C during 5 h and again air cooling) are the most susceptible to pitting, in particular the first one. In these two conditions, pits are nucleated in grains and in grain boundaries, while in condition III (heating up to 1065^o C during 1 h and air cooling followed by reheating up to 620^o C during 24 h and again air cooling), pits are preferentially nucleated in boundaries of small grains. Thence, pits usually grow more rapidly in depth than in width, being able to occur partial or total grains separation. Pits are mainly hemispherical, near-hemispherical, near-conical and near-cylindrical without significant geometric transition associated with an increasing exposure period.