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Bocanegra-Bernal M.H.,CIMAV | Dominguez-Rios C.,CIMAV | Echeberria J.,University of Navarra | Reyes-Rojas A.,CIMAV | And 2 more authors.
Ceramics International | Year: 2016

Multi-walled carbon nanotube (MWCNT), single-double walled carbon nanotube (SDWCNT) and single-walled carbon nanotube (SWCNT) were dispersed for the first time into alumina matrix using conventional mixing/sonication for dispersion of pristine CNTs followed by spark plasma sintering (SPS) process. The resultant composites displayed an increase of the grain size, a decrease in fracture toughness from 8% to 40% over monolithic Al2O3 and the hardness values did not change significantly with the addition of CNTs. In the light of the observed fracture surfaces in different CNT reinforced alumina composites, poor dispersion of these into ceramic matrix will lead to a decrease in fracture toughness. On the other hand, the lower ratio aspects, large surface areas of CNTs, as well as large grain size could be unfavorable to achieve high fracture toughness and the results are worse than for pure alumina questioning whether worth the effort to reinforce alumina with CNTs to obtain marginal improvement under the present conditions, although the literature has reported significant improvement of mechanical properties with alumina and other ceramics systems reinforced with CNTs, resulting from different testing techniques adopted. © 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Source

Bocanegra-Bernal M.H.,CIMAV | Echeberria J.,University of Navarra | Ollo J.,University of Navarra | Garcia-Reyes A.,Interceramic | And 3 more authors.
Carbon | Year: 2011

The use of multi-wall carbon nanotubes (MWCNTs) or single-wall carbon nanotubes (SWCNTs) as filler in ceramic matrices could create composites with exceptional mechanical properties. We have prepared dense monolithic alumina (Al2O3) and zirconia-toughened alumina (ZTA) composites with additions of 0.01 wt% of MWCNTs or 0.01 wt% of SWCNTs by conventional sintering and have demonstrated that the mechanical properties depend on (a) the distribution of CNTs in the matrix and (b) the interaction between the ceramic phases and CNTs. The fracture toughness of Al2O3 ceramics reinforced with SWCNTs was significantly better than those reinforced with MWCNTs. However, fracture toughness in MWCNT-reinforced ZTA increased 41% over ZTA free of the toughening agent and 44% over ZTA reinforced with SWCNTs. A well dispersed and small amount of MWCNTs was enough to produce an increase of fracture toughness in ZTA composites. © 2010 Elsevier Ltd. All rights reserved. Source

Estili M.,Japan National Institute of Materials Science | Echeberria J.,University of Navarra | Vleugels J.,Catholic University of Leuven | Vanmeensel K.,Catholic University of Leuven | And 8 more authors.
Ceramics International | Year: 2015

Zirconia-based ceramics have been introduced in biomedical applications, for example, in hip implants. Certain zirconia composites are prone to spontaneously transform from the tetragonal phase to the monoclinic phase during long-term storage in the presence of moisture at low temperatures. This phenomenon is time-dependent and can be accelerated by water or water vapour. Herein, we report strong experimental evidence of a delayed t→m ZrO2 phase transformation in alumina-toughened zirconia (ATZ) and ATZ/multi-walled carbon nanotube (MWCNT) composites when pressureless sintered in air in a graphite powder bed. The m-ZrO2 phase in ATZ and ATZ/MWCNT composites sintered in a graphite powder bed after hydrothermal ageing for 20 h at 134 °C decreased by 81% and 87%, respectively, compared to an ATZ sample sintered in an alumina powder bed. The enhanced hydrothermal stability could be attributed to the formation of a thin continuous alumina protective layer covering the surface of the ceramic composites. © 2014 Elsevier Ltd and Techna Group S.r.l. Source

Echeberria J.,University of Navarra | Ollo J.,University of Navarra | Bocanegra-Bernal M.H.,CIMAV | Garcia-Reyes A.,Interceramic | And 3 more authors.
International Journal of Refractory Metals and Hard Materials | Year: 2010

This work describes the microstructure and fracture toughness of zirconia toughened alumina (ZTA) nanocomposite in which multi-wall carbon nanotubes (MWCNTs) and nanosized ZrO2 particles were used as reinforcement. The ZTA nanocomposites with additions of 0, 0.005, and 0.01 wt.% MWCNTs and 2 wt.% nanosized ZrO2 particles were pressureless sintered in an anti-oxidant sagger with graphite powder bed at 1520 °C during 1 h in air and then HIPed at 1475 °C in argon atmosphere 1 h at a pressure of 150 MPa. Relative densities ranging 94-98% were reached. In HIPed composites the hardness and fracture toughness values were increased up to ∼17% and ∼37%, respectively, compared to the "as sintered" composites free of carbon nanotubes. A combined fracture mode, crack deflection, pull-outs of a small amount of carbon nanotubes, and bridging effect were the mechanisms leading to the improvement in fracture toughness. © 2009 Elsevier Ltd. All rights reserved. Source

Echeberria J.,University of Navarra | Rodriguez N.,University of Navarra | Vleugels J.,Catholic University of Leuven | Vanmeensel K.,Catholic University of Leuven | And 5 more authors.
Carbon | Year: 2012

It is demonstrated that 0.1 wt% of multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs) added to zirconia toughened alumina (ZTA) composites is enough to obtain high hardness and fracture toughness at indentation loads of 1, 5, and 10 kg. ZTA composites with 0.01 and 0.1 wt% of MWCNTs or SWCNTs were densified by spark plasma sintering (SPS) at 1520 °C resulting in a higher hardness and comparable fracture toughness to the ZTA matrix material. The observed toughening mechanisms include crack deflection, pullout of CNTs as well as bridged cracks leading to improved fracture toughness without evidence of transformation toughening of the ZrO2 phase. Scanning electron microscopy showed that MWCNTs rupture by a sword-in-sheath mechanism in the tensile direction contributing to an additional increase in fracture toughness. © 2011 Elsevier Ltd. All rights reserved. Source

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