Comportamiento tribológico de materiales cerámicos de nitruro de silicio texturados

Se han fabricado materiales densos de nitruro de silicio (Si3N4) texturados incorporando semillas de β- Si3N4, favoreciendo su alineación mediante extrusión de las piezas en verde y prensado en caliente. Estos materiales presentan elevada anisotropía microestructural que conduce a un comportamiento anisótropo de sus propiedades, tanto mecánicas como tribológicas. Se han realizado ensayos de desgaste en seco mediante movimiento recíproco lineal de pares homólogos con geometría esfera-placa. El material texturado presenta mayor resistencia al desgaste que el de referencia, sin texturar, identificándose como principal mecanismo de desgaste la abrasión debida a microfractura y arranque de granos de Si3N4. En este material, además, se desarrolla un tercer cuerpo que protege la superficie del material

Superior Performance of Ablative Glass Coatings Containing Graphene Nanosheets

Glass compositions in the Y₂O₃-Al₂O₃-SiO₂ (YAS) system are envisaged as promising coatings for high‐temperature protection, in particular for the thermal protection systems (TPS) looked for aerospace applications. Recently, thermally sprayed YAS hybrid coatings containing a small amount of graphene nanoplateletes (GNPs) showed enhanced performance as compared to the blank YAS coating, demonstrated by the occurrence of unusual electrical conductivity for these glasses and the development of better mechanical compliance, both phenomena associated with the presence of GNPs. Nevertheless, a crucial issue is to demonstrate if these kinds of coatings would also have superior behavior under ablation conditions, particularly regarding the mentioned TPS applications. This work goes far beyond, exploring the ablative behavior of new YAS/GNPs coatings flame sprayed over SiC substrates. These essential tests were carried out under laboratory conditions, reaching limit temperatures of 1350°C while blowing gas. Results evidence that hybrid coatings having just 1.05 vol% GNPs show enhanced ablation resistance, actually withstanding up to 30 thermal cycles (between 200°C and 1350°C) without apparent damage. This satisfactory performance is linked to the benefits of the GNP additions, and fundamentally to the higher emissivity and the directional thermal conduction characteristics of the hybrid coatings—produced by the formation of a GNP network with a preferential surface parallel arrangement—that preclude the creation of hot spots and also hinder heat propagation toward the substrate; accordingly, coating degradation is constrained to the uppermost layer of these coatings

Development of HVOF-sprayed nanostructured TiO2 coatings for high temperature applications

Peer reviewed: YesNRC publication: Ye

Improved crack resistance and thermal conductivity of cubic zirconia containing graphene nanoplatelets

Composites of 8 mol.% yttria-stabilized zirconia (8YSZ) with graphene nanoplatelets (GNP) have been pointed as alternative interconnectors in SOFC due to their mixed ionic-electronic conduction. Here we show that GNP addition provides rising crack-resistance behavior, with long crack toughness up to 78% higher than that of 8YSZ, also improving its thermal conductivity (up to 6 times for the in-plane direction). Toughness versus crack length is measured for 7 and 11 vol.% of GNP using single edge V-notched beam technique and ultrashort pulsed laser notching; and thermal behavior is analyzed by the laser flash method. Materials also have highly anisotropic coefficient of thermal expansion. These properties contribute to enhance their performance under the harsh operating conditions of SOFC, as thermal residual stresses could be reduced while significantly improving the system mechanical stability. Moreover, the heat transfer may be enhanced especially along the interface direction which would increase the system efficiency.This work was supported by Spanish project RTI2018-095052-B-I00, Ministerio de Ciencia, Innovacion y Universidades, Spain (MCIU/AEI/FEDER, UE). AG and PM acknowledge support from Ministerio de Economía, Industria y Competitividad, Spanish government (Project FIS2017- 87970-R) and Junta de Castilla y León, Spain (Project SA287P18). C. R. thanks the financial support by MCIU under contract IJCI-2017-34724 of “Juan de la Cierva” Program

The effect of graphene nanoplatelets on the thermal and electrical properties of aluminum nitride ceramics

The thermal conductivity (κ) of AlN (2.9 wt.% of Y_{2}O_{3}) is studied as a function of the addition of multilayer graphene (from 0 to 10 vol.%). The κ values of these composites, fabricated by spark plasma sintering (SPS), are independently analyzed for the two characteristic directions defined by the GNPs orientation within the ceramic matrix; that is to say, perpendicular and parallel to the SPS pressing axis. Conversely to other ceramic/graphene systems, AlN composites experience a reduction of κ with the graphene addition for both orientations; actually the decrease of κ for the in-plane graphene orientation results rather unusual. This behavior is conveniently reproduced when an interface thermal resistance is introduced in effective media thermal conductivity models. Also remarkable is the change in the electrical properties of AlN becoming an electrical conductor (200 S m⁻¹) for graphene contents above 5 vol.%

The Decisive Role Played by Graphene Nanoplatelets on Improving the Tribological Performance of Y2o3-al2o3-sio2 Glass Coatings

Graphene nanoplatelets (GNPs) have proved to be effective fillers for enhancing the mechanical and tribological properties of bulk ceramics and glasses, also with the added benefit of developing electrical and thermal functionalities. Similarly, enhanced transport performance has recently been shown for glass-ceramics coatings of the Y2O3-Al2O3-SiO2 (YAS) system containing a small amount of GNP fillers, intended for applications in the aerospace industry. In the present work, the wear and friction behaviour of GNP/YAS coatings –containing 0, 1.2 and 2.3 wt.% GNPs- on silicon carbide substrates is evaluated. The flame spraying process used for coating fabrication induces a structure of splats oriented parallel to the substrate with GNPs located at the inter-splat boundaries forming a connected network of platelets mainly oriented parallel to the surface as well. Unlubricated ball–on-plate reciprocating wear tests show that both the friction coefficient and the wear rate decreased by 35% and 65%, respectively, for 2.3 wt.% of GNPs. A wear mechanism for GNP/YAS coatings based on both the progressive exfoliation of the graphene sheets and the effect of the GNPs on preventing crack propagation within the coating is proposed

Análisis de la adhesión de recubrimientos del sistema Y2O3-Al2O3-SiO2 sobre sustratos de interés para la industria aeroespacial

En la industria aeroespacial se necesitan materiales ligeros que tengan unas altas prestaciones mecánicas combinadas con una baja densidad. El carburo de silicio, el carbono reforzado con fibra de carbono y el carburo de silicio reforzado con fibra de carbono son materiales que cumplen con estos requisitos, pero a altas temperaturas presentan problemas de oxidación. Una de las formas más efectivas de prevenir este fenómeno es la utilización de recubrimientos cerámicos, cuya correcta adhesión sobre los distintos sustratos es fundamental para garantizar su funcionamiento. En el caso del presente trabajo, se analiza la adhesión de recubrimientos vítreos del sistema Y2O3-Al2O3-SiO2 obtenidos mediante proyección térmica por llama oxiacetilénica. Para ello, se realizan ensayos de rayado a carga creciente analizando el tipo y la carga de fallo y su relación con las propiedades elásticas y mecánicas de los recubrimientos. Los resultados indican que la adhesión sobre los sustratos carburo de silicio y carburo de silicio reforzado con fibra de carbono es buena, mientras que el carbono reforzado con fibra de carbono no es un material adecuado para recubrir

INFLUENCE OF PROCESSING METHOD ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MULLITE/ZrO2 COMPOSITES

Des matériaux composites Mullite/ZrO2 ont été préparés par deux méthodes différentes : frittage et frittage réactif. Les propriétés mécaniques et les mécanismes de ténacité des deux composites ont été analysés sur la base de considérations microstructurales.Mullite/ZrO2 composites have been prepared by two different methods : sintering and reaction sintering. The mechanical properties and toughening mechanisms of both composites are analized on the basis of microstructural considerations

Quemadores de gas cerámicos

The extended use of natural gas for domestic applications is spurring the continuous improvement in the gas boiler technology. Thus, compact boilers that are able to work in a wider power range, in order to satisfy the exact energy requirements of the users, are a common goal. This requires the development of gas burners capable of working in radiant mode for increasing the combustion efficiency besides reducing the emissions of hazardous gases. The ceramic materials are strong candidates for this application. They have the properties required for working in the severe conditions associated with the combustion of gases, i.e. high temperatures and the presence of corrosive chemical species. These work conditions produce a slow but continuous degradation of their properties and the ultimate burner failure. This paper discusses the different types of ceramic gas burners available, their working requirements, modes of operation and the possible degradation mechanisms.<br><br>El creciente uso del gas natural, como fuente de energía térmica en aplicaciones domésticas, está produciendo una continua evolución en la tecnología de este tipo de calderas. Estas son cada vez más compactas y capaces de trabajar en un mayor rango de potencias caloríficas, para satisfacer las demandas de energía requeridas, en cada momento, por los usuarios. Para ello, deben estar provistas con quemadores que puedan funcionar intermitentemente en modo radiante, lo que permite un aumento en la eficiencia del proceso de combustión y, además, una reducción en la emisión de gases nocivos. Los materiales cerámicos son unos buenos candidatos para fabricar estas placas radiantes de los nuevos quemadores de gas. En general, estos materiales poseen las propiedades adecuadas para funcionar en las severas condiciones que se producen en la combustión, caracterizadas por las altas temperaturas y la presencia de especies químicas corrosivas. Estas condiciones de trabajo producen una degradación lenta pero progresiva de sus propiedades y, con el tiempo, dan lugar al fallo del quemador. En este trabajo se presenta una revisión sobre los tipos de quemadores de gas cerámicos existentes, requisitos que deben cumplir, su modo de operación y los modos en que se degradan