Papers by Author: Alejandra Chávez-Valdez

Abstract: Novel materials that can be used as thermal barrier coatings in high temperature applications were obtained by homogenization, mechanical milling and thermal treatment. Samarium oxide was investigated as an alternative to react with the free silica from fly ash and to form new silicate compounds. The main phases found in fly ash-Sm₂O₃ mixtures were mullite and samarium silicate Sm_4.66O(SiO₄)₃. Electrophoretically deposited coatings from these materials were obtained at 50 V and 3 minutes deposition time. The surface microstructure of the coatings was characterized by scanning electron microscopy (SEM-EDXS). The coatings were homogeneous and showed no crack formation. Additionally, thermal conductivity of the bulk samples at room temperature was determined. The thermal conductivity values of the new materials were below 1 W/mK which makes them suitable for thermal and environmental barrier applications.

Abstract: Fly ash cenospheres coatings are a promising material in improving the oxidation and corrosion resistance of SiC. In this work, fly ash cenospheres coatings were deposited on SiC substrates by electrophoretic deposition process at 500 V during 30 seconds, 1, 2 and 3 minutes. These coatings were characterized by SEM to observe the surface morphology. The coatings were sintered at 1000, 1100 and 1200°C during 3 hours by microwave heating. After sintering, the coatings were characterized by XRD. Cross sections of the substrate/coating samples were analyzed by SEM/EDS. The coatings with the best deposition and sintering conditions were evaluated by oxidation and hot corrosion resistance tests. The oxidation test was performed in air at temperatures from 1000 to 1300°C during 100 hours. In addition, fly ash cenospheres coatings on SiC and uncoated SiC substrates were subjected to a hot corrosion test at 1000°C during 100 hours in Na2SO4. The coatings showed good adhesion after sintering; the main phases of the coating were mullite and SiO2. The uncoated substrates were found to gain weight due to the formation of SiO2 at the surface. In contrast, SiC samples coated with fly ash cenospheres showed no weight gain and exhibited no signs of cracking or spallation following the oxidation tests. After the high temperature corrosion test in Na2SO4, the uncoated SiC showed signs of oxidation and corrosion while the coated remained unreacted in the presence of molten Na2SO4, indicating that the electrophoretic deposition fly ash cenospheres acted as a hot corrosion barrier for SiC substrates.

Abstract: Hydroxyapatite, porcelain and wollastonite coatings on stainless steel 316L were produced by electrophoretic deposition (EPD) in ethanol and acetone using a voltage between 200 and 1000 V during 0.5 to 60 s. The particle size distribution of the starting suspension was 0.3 to 4.9 microns with an average size of 1.5 microns. The coatings were analyzed using scanning electron microscopy. The amount of ceramic material on the surface of the metallic samples was evaluated by determining their difference in weight before and after the electrophoretic deposition process. The conductivity and zeta potential of the dispersing media were also evaluated. Dense, homogeneous and crack-free green coatings were obtained. The deposition rate was higher by using acetone as dispersing media. The higher zeta potential and the lower viscosity were attributed to be the cause of the better electrophoretic deposition of the acetone and methanol ceramic suspensions. Submicron particle coatings were then sintered between 800 and 1000 C during 2 h. The sintered coatings presented a very homogeneous polycrystalline structure free of cracks. The results show that the application of high voltage during short periods of time is an effective method to obtain ceramic coatings with good sinterability.

Abstract: Zirconia-alumina composites with additions of a CaO-SiO2 glass are prepared by uniaxial pressing and sintering. In order to promote bioactivity, the composites are biomimetically treated. The effect of immersion time in simulated body fluids (SBF) and that of the presence of a wollastonite powder bed, as a calcium ion provider, on the apatite forming ability are investigated. The influence of replacing the simulated body fluids each 7-day-period for a more concentrated solution is also studied. A bonelike apatite layer is observed after 21 days of immersion when the SBF is renewed, whether the bed of wollastonite powder is present or not. However, a thicker layer is formed by using wollastonite and the agglomerates of the apatite layer are finer on the composites containing CaO-SiO2 glass.