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Zoli L.,CNR Institute of Science and Technology for Ceramics | Sciti D.,CNR Institute of Science and Technology for Ceramics
Materials and Design | Year: 2017

A series of high density ceramic composites with carbon fibre content between 40 and 65% and ultra-refractory ceramic matrix was produced by slurry infiltration and hot pressing. The matrix consisted of ZrB2–10 vol% SiC or ZrB2–40 vol% SiC ceramic mixtures. Water–based and polymer–based routes were tested to analyse the effects on microstructure, mechanical properties and oxidation resistance at 1650 °C in air. Changing the process and/or the processing parameters was found to affect the final composition, the amount of residual porosity, the matrix/fibre adhesion. Composites with nearly fully dense matrix and optimized infiltration of fibre preforms were found to possess the highest strength (240 MPa) and oxidation resistance. Composites with weak interface and higher porosity in the matrix showed higher toughness (up to 12 MPa m0.5) but were more prone to oxidation and erosion. © 2016 Elsevier Ltd


Silvestroni L.,CNR Institute of Science and Technology for Ceramics | Meriggi G.,CNR Institute of Science and Technology for Ceramics | Sciti D.,CNR Institute of Science and Technology for Ceramics
Corrosion Science | Year: 2014

This paper deals with the oxidation behavior of ZrB2-based composites sintered with different additives, namely ZrSi2, MoSi2, TaSi2 and WSi2. The oxidation mechanisms were investigated between 1200 and 1800°C for 15min in a bottom loading furnace. The scope of this study is to draw a classification of goodness for the 4 composites depending on the temperature range and understand how each cation influences the oxidation behavior of ZrB2 by acting either on glass or on ZrO2 modification. MoSi2 was the best additive for improving the oxidation resistance of ZrB2, even up to 1800°C. © 2014 Elsevier Ltd.


Medri V.,CNR Institute of Science and Technology for Ceramics | Ruffini A.,CNR Institute of Science and Technology for Ceramics
Ceramics International | Year: 2012

Silicon carbide (SiC) foams were developed with a low temperature process by using an inorganic alkali aluminosilicates binder, also known as geopolymer. The foaming agent was the metallic silicon present as impurity in the SiC powder. Si 0 in the alkaline solution led to gas evolution that induced the foaming of the slurries. The binder was a geopolymeric resin with atomic ratio Si/Al = 2 and potassium as alkaline cation, classified as (K)poly(silalate-siloxo). The geopolymeric resin was prepared using metakaolin as aluminosilicatic raw powder, while the alkali aqueous solution was KOH/K 2SiO 3. Metakaolin in alkaline conditions dissolved and re-precipitated to form geopolymeric nano-particulates that acted as a glue to stick together SiC particles (90 wt.%). Process parameters such as water addition, mixing time and curing temperature were correlated to the foam structure. The formation of prolate pores induced anisotropy in the compressive strength. The foams were studied by dilatometric analysis in inert and oxidative atmospheres up to 1200 °C. © 2011 Elsevier Ltd and Techna Group S.r.l.


Silvestroni L.,CNR Institute of Science and Technology for Ceramics | Sciti D.,CNR Institute of Science and Technology for Ceramics
Journal of the European Ceramic Society | Year: 2013

The purpose of this work was to produce a dense ZrB 2-SiC ceramic and to identify a suitable annealing cycle to crystallize the glassy phase and promote SiC growth along the c axis. The concept behind this work exploits the irreversible SiC β→α transformation occurring at temperature above 1900°C, in such a way to have SiC platelets able to trigger effective toughening mechanisms. La 2O 3 and MgO were used as sintering additives. The as sintered and annealed materials were examined through X-ray diffraction (XRD), to identify the crystalline phases, scanning electron microscope (SEM), to study the distribution of the secondary phases, and transmission electron microscope (TEM), to analyze the microstructure at nanoscale level, with particular attention to new crystalline phases and to the interfaces in high resolution mode. A model for the microstructure evolution during densification and upon annealing is presented. © 2012 Elsevier Ltd.


Silvestroni L.,CNR Institute of Science and Technology for Ceramics | Sciti D.,CNR Institute of Science and Technology for Ceramics
Journal of the American Ceramic Society | Year: 2011

The microstructure of fully dense hot-pressed ultra-high-temperature ceramics (UHTCs), namely ZrB2 and HfB2 containing 15 vol% of TaSi2, was characterized by X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). ZrB2 and HfB2 grains displayed a core-shell structure: the core was constituted by the original MB2 grain and the shell by a (M, Ta)B2 solid solution, which grew epitaxially on the core. The compositional misfit between core-shell was accommodated by low-angle grain boundaries and dislocations pile-up, especially pronounced in the ZrB 2-based composite, where a higher amount of Ta entered the boride lattice. Ta5Si3, Ta4.8Si3C 0.3, and Ta5SiB2, with Zr or Hf impurities, were detected at the triple points and wetting of the grain boundaries by a Ta-Si-B-C-O amorphous phase was observed. Based on the new microstructural features detected by TEM, thermodynamic calculations and the available phase diagrams, a densification mechanism for ZrB2 and HfB2 with addition of TaSi2 is proposed. The microstructures of the UHTC composites presented here are compared with composites sintered with the addition of MoSi2 in the same amount. © 2011 The American Ceramic Society.


Sciti D.,CNR Institute of Science and Technology for Ceramics | Silvestroni L.,CNR Institute of Science and Technology for Ceramics
Journal of the European Ceramic Society | Year: 2012

Borides and carbides of early transition metals are considered a class of promising materials for several applications, the most appealing ones being in the aerospace and energy sectors. The present work is mostly focused on toughening of UHTCs through incorporation of SiC chopped fibers. Mechanical properties of reinforced composites are compared to those of un-reinforced, whisker- and particle-reinforced materials and the effect of different kinds of sintering aids is studied. Addition of fibers allows toughness to be increased from 3-4MPam 1/2 (for un-reinforced materials) to 5.0-6.2MPam 1/2. The high temperature behavior is also investigated both in air furnace and in arc jet facility. Eventually, a paragraph is dedicated to potential of UHTCs as sunlight absorbers for future solar concentrating systems operating in the high temperature regime. © 2011 Elsevier Ltd.


Medri V.,CNR Institute of Science and Technology for Ceramics | Ruffini A.,CNR Institute of Science and Technology for Ceramics
Journal of the European Ceramic Society | Year: 2012

Silicon carbide (SiC) foams were developed by using a low temperature process such as chemical consolidation that is suitable to replace the sintering step. An alkali aluminosilicates binder, also known as geopolymer, was used. It was prepared from metakaolin, as aluminosilicatic raw powder, and KOH/K 2SiO 3 aqueous solution. The foaming agent was the metallic silicon present as impurity in SiC powders. Different grades of SiC were used as the main component (90wt%) of the foams and the micro and macrostructures varied with the morphologies of the SiC raw powders. The surface of SiC grains participates to the geopolymeric process because of the dissolution of the silica layer into the alkaline solution. SiC foams were tested and characterized under oxidative atmospheres up to 1200°C. © 2011 Elsevier Ltd.


Sciti D.,CNR Institute of Science and Technology for Ceramics | Guicciardi S.,CNR Institute of Science and Technology for Ceramics | Silvestroni L.,CNR Institute of Science and Technology for Ceramics
Scripta Materialia | Year: 2011

ZrB2 composites containing SiC chopped fibers were hot pressed at 1600-1900 °C using different sintering aids. The change of sintering aid strongly affected the densification temperature and the mechanical properties of the composites. Addition of MoSi2 was detrimental to the final microstructure and properties, due to the high temperature needed to reach a full densification. Addition of ZrSi2 decreased the sintering temperature to 1650 °C, and the resulting composite containing 20 vol.% fibers reached a toughness of 6.2 MPa m0.5. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.


Silvestroni L.,CNR Institute of Science and Technology for Ceramics | Sciti D.,CNR Institute of Science and Technology for Ceramics
Journal of Alloys and Compounds | Year: 2014

The microstructure of a dense ultra-high-temperature ceramic, namely ZrB2 containing 15 vol% of WSi2, was characterized by X-ray diffraction, scanning and transmission electron microscopy. ZrB 2 displayed a core-shell structure: the core was constituted by the original MB2 grain and the shell by a (Zr,W)B2 solid solution which grew epitaxially on the core. According to the final microstructure with mixed Zr,W-silicides, -borides and -carbides at the triple points and clean grain boundaries, a densification mechanism for ZrB2 in presence of WSi2 is proposed. This composite displayed excellent thermo-mechanical properties, like strength retention of 84% at 1500 °C in air and moderate oxidation up to 1650 °C. Correlation between microstructure and properties are here presented in relationship to other ultra-refractory ceramics available in the literature. © 2014 Elsevier B.V. All rights reserved.


Sciti D.,CNR Institute of Science and Technology for Ceramics | Guicciardi S.,CNR Institute of Science and Technology for Ceramics | Silvestroni L.,CNR Institute of Science and Technology for Ceramics
Materials and Design | Year: 2014

This paper deals with the effect of the addition of Hi-Nicalon SiC fibers to Zr- and Hf-borides. The main scope is to understand the fiber/matrix chemical interaction and correlate it to the fracture toughness. Transmission electron microscopy (TEM) was used as key investigation tool to disclose the microstructural features at nanoscale level. Several sintering additives were used to enable densification in the temperature range 1600-1850. °C. It was observed that the fiber strongly reacts with the matrix at the same temperature at which the sintering additive starts to be effective. At this point, the fibers themselves locally behave as sintering aid promoting a strong fiber/matrix bonding which prevents any possibility of fiber pullout. Fiber modification was correlated with the fracture toughness and it was at last deduced that these fibers exert a toughening action only when the sintering temperature is kept below 1700. °C. Above this temperature fibers start to significantly degrade and can be considered just as a secondary phase. © 2013 Elsevier Ltd.

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