Dense graphene nanoplatelet/yttria tetragonal zirconia composites: Processing, hardness and electrical conductivity

Abstract

Yttria tetragonal zirconia ceramic composites with 1, 2.5, 5 and 10 vol% nominal contents of graphene nanoplatelets (GNPs) were fabricated and characterized. First, the GNP dispersion in isopropanol was optimized to de-agglomerate the GNPs without damaging their structure. Then, submicrometric fully dense composites were obtained via spark plasma sintering (SPS) at 1250ºC with a 5 min holding time. The processing routine produced a nearly homogeneous GNP dispersion in the ceramic matrix, and the GNPs preferential orientation was perpendicular to the sintering compression axis. The Ceramic grain refinement due to the GNPs was also detected. The Vickers hardness measured on the plane perpendicular to the sintering compression axis (basal plane) was lower than on the cross sections. This anisotropy increased with the increasing GNP content, while the average hardness decreased. The electrical conductivity was also highly anisotropic, up to seven times higher for the basal planes. The electrical percolation threshold for these composites was estimated to be between 2.2 and 4.4 vol% of the GNP measured content.

Spark Plasma Sintered Zirconia Ceramic Composites with Graphene-Based Nanostructures

“…The highest electrical conductivity values are measured in the perpendicular direction to the SPS compression axis (σ ⊥ ) where the GNPs lie in an "in-plane" configuration. The hardness values, on the contrary, are lower in the "in-plane" surfaces and higher in cross-sections [8]. This difference is again related to the microstructural anisotropy due to the GNP's geometry and preferential orientation, which favor delamination of the GNPs when the indenter hits the "in-plane" surface [14].…”

“…As for the Vickers hardness, although a slight increase has been reported for 3YTZP composites with low GNP content, this trend is reverted for higher GNP content (2.2-9.5 vol%) [8]. Both the electrical and mechanical anisotropy seem to increase with the GNP content.…”

“…The SPS approach, which is a fast sintering technique that greatly reduces the sintering time, is more convenient for preserving the microstructural integrity of the GBNs, minimizing their degradation. For instance, sintering conditions of a 5 min holding time at 1250 • C produce fully dense GNP/3YTZP composites with sub-micrometric equiaxed ceramic grains and refined microstructures due to the GNP distribution at the ceramic grain boundaries [8]. This technique, due to the uniaxial pressure exerted on the powders during the sintering process, can make the two-dimensional GBNs adopt a preferential orientation with their major ab axis lying perpendicular to the sintering compression axis.…”

“…For that reason, techniques that apply pressure during sintering are usually preferred. Though conventional hot pressing has been successfully used to sinter these ceramic/GBN composites [11], spark plasma sintering (SPS) is up to now the dominant method [7,8,12]. The SPS approach, which is a fast sintering technique that greatly reduces the sintering time, is more convenient for preserving the microstructural integrity of the GBNs, minimizing their degradation.…”

“…Concerning the starting materials, different advanced ceramics have been used as matrices, such as Si 3 N 4 [1], which has the highest fracture toughness reported for 1.5 vol% GBN filler content [2], SiC [3,4], hydroxyapatite [5], alumina [6,7], and, more recently, zirconia [8,9].…”