Key Engineering Materials Vol. 694

Abstract: Low density silica aerogel beads were synthesized from rice husk ash via sodium silicate route. The gel beads were prepared by the ball dropping method and dried at ambient pressure after surface modification (silylation) with trimethylchlorosilane (TMCS) solution. The characteristics of porous structure of silica aerogel beads were measured by Brunauer-Emmett-Teller (BET) nitrogen adsorption and desorption method. The silica aerogel beads showed a specific surface area as high as 773 m²/g. They also displayed water-repellant effect (hydrophobic) due to silylation as revealed by Fourier transform infrared spectroscopy (FTIR).

Abstract: The effect of thermal ageing on the properties and microstructure of Al-Mg-Si alloy was investigated. In this work, an extruded Al-6063 alloy samples were used as the main materials. In order to study the effect of thermal ageing, the alloy samples were solution treated at 530 °C and then quenched into water before artificially aged at elevated temperatures between 120 and 250 °C. The ageing response and mechanical properties was monitored by Vickers hardness and tensile tests, respectively. The analysis of surface fracture and microstructure of peak aged alloy were carried out by means of scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. Result shows that the highest hardness value and tensile properties is gained by the alloy that aged at 120 °C. It is found that increasing in hardness and strength values of the alloy are due to precipitates formation during thermal ageing. Fracture analysis on peak-aged condition indicates that the alloy having more ductility after thermal ageing. The result shows that the higher ageing temperature will lead to the higher ductility of the Al-6063 alloy, as a results the alloy’s strength is reduced.

Abstract: A copper alloy consists of 2.6% Fe, 0.15% P and 0.2% Zn with modified grain size of 500 and 750 nm were studied on their rate of diffusion at different oxidation temperature using electron microscopic imaging technique. Different oxidation temperature contributed to the variation of copper oxide particle size, surface porosity level, particle agglomeration and particle nucleation. High oxidation temperature resulted in large oxide particles formation as well as high surface porosity. The magnitude of the copper oxide growth depended on the oxidation temperature. The increase in the oxidation rate at high oxidation temperature was likely a result of faster transport of the reactants through the bulk copper due to a significant contribution from grain-boundary diffusion.

Abstract: (K, Na)NbO₃ (KNN) thin films were prepared by sol-gel technique. Spin coating deposition and rapid thermal annealing (RTP) process were applied to produce the KNN thin films. The films obtained demonstrated that highly crystallographic orientation was produced at five layer deposition with increase (preferred orientation) peak at (1 1 1). The thickness of five layers thin films observed by field emission scanning electron microscopy (FE-SEM) was determined to be ~200nm. However, the inhomogeneous distribution of KNN particles was detected in KNN thin films. The distribution of KNN elements was confirmed by energy-dispersive X-ray (EDX) spectra. Improvement was observed in resistivity (2.71-7.81x10⁶ Ω.cm) and dielectric loss (0.35%-0.21%) following the increasing number of layers.

Abstract: Barium strontium cobalt ferrite (BSCF) and lanthanum strontium carbonate ferrite (LSCF)–samarium-doped ceria carbonate (SDCc) composite cathodes were developed based on various molar ratio of binary carbonate. The percentage of molar ratio for (Li/Na)₂ binary carbonate in the composite cathodes were 67:33, 62:38, and 53:47. Influence of (Li/Na)₂ binary carbonate addition on BSCF-SDCc and LSCF-SDCc were studied in terms of chemical, thermal, and physical properties. The composite-cathode powders were prepared using high-energy ball milling (HEBM) and followed by calcination at 750 °C for 2h. Characterizations of the composite cathode were performed through Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and dilatometry. The FTIR result verified the existence of carbonates in all the composite cathodes. The increment in the Na₂CO₃ molar ratio has contributed to the growth of the BSCF-SDCc particles as observed from the FESEM micrographs and particle size. The LSCF-SDCc composite cathodes revealed a lower (1.38-6.69%) thermal coefficient difference with SDCc electrolyte. The BSCF-SDCc and LSCF-SDCc composites with 53:47 mol.% of (Li/Na)₂ binary carbonate exhibit the applicable properties as SOFC cathode material.

Abstract: Sol-gel dip-coating technique was utilized to prepare Ag-TiO₂ coating with additive (Degussa P25) on unglazed ceramic tiles. Three different numbers of coating layers (5, 7 and 10 layers) were deposited on unglazed ceramic tiles made of waste glass. The coatings were sintered at 500 °C with the heating rate of 2 °C/min. The crystalline phase and cross sectional areas of the coatings were evaluated using GAXRD and SEM analysis, respectively. Results showed that the average thickness of the coating increased from 7 µm to 9 µm and to 12 µm for 5, 7 and 10 coating layers, correspondingly. Coating of 10 layers exhibited a high crystallinity of anatase phase, a good surface morphology without significant cracks, and a homogeneous coating compared to the 5 and 7 layers of coating, which suggested a better performance in Ag-TiO₂ coating.

Abstract: Acidic TiO₂ solutions were prepared via sol-gel using hydrochloric acid (HCl) as catalyst for TiO₂ coating deposition. The correlation between the HCl content and ageing time with the stability of the solutions was determined by controlling the content of HCl used (0.2, 0.5, 1.0 and 1.7 ml). The stability was evaluated by observing the pH and physical changes that occurred during 30 days of ageing time. The influence of HCl content on the morphology of TiO₂ thin film is further studied by depositing the films on commercialized unglazed ceramic tiles via dip-coating technique. Results of the ageing experiment showed that the PH of the solutions was stable throughout the ageing period. However, significant physical changes occurred only during the first seven days of the ageing period. Energy dispersive X-ray (EDX) analysis disclosed that the TiO₂ coatings were distributed unevenly on the substrate surface creating multilayers of coating. Scanning electron microscope (SEM) micrographs suggest that the TiO₂ solution prepared using 0.5ml of HCl (pH 1.0 – 1.5) produces better TiO₂ coating performance due to less amount of cracks and more homogeneous coating than other HCl content solutions.

Abstract: The time-transition curve of open circuit potential (Eoc) during the single zincating process of aluminium alloy 7075 (AA7075) was recorded. Then, a suitable single zincating duration was selected from the Eoc slope, in order to get the optimum zincating duration. The evolution of surface morphology and composition of AA7075 substrate at various single zincating durations were characterized by scanning electron microscopy (SEM) and energy dispersive analysis of x- rays (EDX). The coating adhesions of single zincated samples produced at various durations were investigated using a scratch tester. SEM results showed that the substrates produced at a longer zincating duration were fully covered by a dense layer of zinc particles. The adhesion of the electrodeposited nickel on single zincated samples produced at longer zincating duration was higher than the conventional single zincating process. This morphology has contributed to the improvement of coating adhesion.

Abstract: The use of electroless deposition method to deposit nickel alloy attracts attention due to its uniformity, corrosion resistance in neutral media and low friction. Quaternary nickel alloy deposit can be achieved by adding metal ion additive into the plating bath. Furthermore, the use of alkaline bath can accelerate the deposition rate, and provide sufficient thickness for corrosion protection. In this study, an electroless quaternary nickel alloy is deposited on iron coupons by adding cobalt and copper ions in hypophosphite based Ni-P alkaline bath. The nickel alloy deposit surface morphology is studied using scanning electron microscope (SEM) and x-ray fluorescence (XRF). Corrosion behavior of the nickel alloy is investigated using polarization curve measurement in 3.5wt% NaCl aqueous solution. From the results, the elecroless Ni-Co-Cu-P alloy coating produced at higher plating bah pH is harder than the lower bath pH. Higher Co, Cu and P content in the Ni alloy exhibit broader passive area in the polarization curve measurement results.

Abstract: Thermally grown oxide (TGO) plays important roles in thermal barrier coating system (TBC) for high temperature application such as in aircraft gas turbine engine blades (GTE). The TGO formed between the bond coat and topcoat interface can increase oxidation resistance to creep of GTE blades by minimizing oxygen diffusion into the metal substrate. In this research a NiCoCrAlYTa metallic bond coat was deposited on Inconel 625 substrate using two methods of deposition namely; high velocity oxy-fuel (HVOF) and atmospheric plasma spray (APS). After coating process, both types of samples underwent pre-oxidation in argon furnace for 12-24 hours at 1000 °C. Results showed that the TGO formation for samples in which the bond coat deposited via HVOF method produced much thinner and continuous TGO formation compared to APS deposition. This TGO characteristic is very useful to lengthen the lifetime of the metals substrate.