Improvement of Vickers hardness measurement on SWNT/Al2O3 composites consolidated by spark plasma sintering

Abstract

Dense alumina composites with different carbon nanotube content were prepared by colloidal processing and consolidated by Spark Plasma Sintering (SPS). Single-wall carbon nanotubes (SWNTs) were distributed at grain boundaries and also into agglomerates homogeneously dispersed. Carrying out Vickers hardness tests on the cross-section surfaces instead of top (or bottom) surfaces has shown a noticeable increase in the reliability of the hardness measurements. This improvement has been mainly attributed to the different morphology of carbon nanotube agglomerates, which however does not seem to affect the Vickers hardness value. Composites with lower SWNT content maintain the Vickers hardness of monolithic alumina, whereas it significantly decreases for the rest of compositions. The decreasing trend with increasing SWNT content has been explained by the presence of higher SWNT 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Heterogeneous dispersion and distribution of CNTs in the ceramic matrix, poor chemical compatibility between CNTs and alumina hindering effective transfer load and the large differences in the scales of the matrix microstructure and the carbon nanotubes have been stated as main obstacles to transfer the desirable CNT mechanical properties to the brittle ceramic matrix [5,7,11,14,15]. Adequate dispersion of CNTs is very difficult owing to their tendency to form bundles in order to minimize their surface area.Although aqueous colloidal processing has been assessed as an efficient technique producing adequate dispersion of CNTs throughout ceramic matrix grain boundaries after sintering [18][19][20], the presence of agglomerates seems to be unavoidable.Recently, Poorteman et al [21] fabricated MWNT/alumina composites with low MWNT content (0.6 and 1.4 vol. %) by a colloidal processing route to optimize electrostatic

Enhanced carbon nanotube dispersion in 3YTZP/SWNTs composites and its effect on room temperature mechanical and electrical properties

“…The highest value is 0.5%, found in C3, similar to C1 (0.3%) which both used low energy ultrasonic agitation and no electrical insulation during SPS. This value is also similar to the same composition with alumina matrix processed with the same conditions [40]. However, taking into account that the total SWNT content was 1 vol%, and using the Delesse principle to extrapolate volumetric to surface concentrations, these would mean that half the total SWNT of the composite C3 is in agglomerates while the other half is distributed in the grain boundaries.…”

“…C4 and C5 show SWNTs (mainly bundles) in most ceramic grain boundaries, while in C1, some SWNT are in agglomerates and less in grain boundaries. Most SWNT agglomerates appear elongated in the perpendicular direction to the sintering compression axis, as it has been previously reported elsewhere [40], which makes them easier to identify. The aspect ratio has been estimated in~5 (see Table 2).…”

“…The aspect ratio has been estimated in~5 (see Table 2). This elongation of SWNT agglomerates can influence hardness, producing anisotropy in its values when measured in the top surfaces or in cross sections of the sintered composites [40].…”

Recent advances in understanding the reinforcing ability and mechanism of carbon nanotubes in ceramic matrix composites

“…Similar performances have been observed in the highly contact-damage tolerant, mica-based glass-ceramics, which are indeed brittle in nature, with the toughness ∼1.5 MPa m 0.5 [81]. Indeed, the strengthening and toughening abilities of SWCNTs in a ceramic matrix are poor in our opinion, as they cannot be dispersed individually with direct interfaces with the matrix and exist mainly in bundled form [9, 18, 20, 21, 28, 31 and 32]. We should note that the possibility of energy dissipation by the individual SWCNTs of a SWCNT bundle during the crack bridging process seems very low simply because they are covered by neighboring easy-to-slide SWCNTs; thus, they cannot be tightly fixed by the matrix to allow their loading and contribution to the energy dissipation during the crack bridging process.…”

Mechanical Properties and Thermal Conductivity of Ytterbium-Silicate-Mullite Composites

“…In general, the hardness of ceramics is highly influenced by the phase constituents, the grain size and the porosity of the materials. The newly formed Al2O3 phase in the YbAM composite shows much higher Vickers hardness (Hv: 18-21 GPa [39,40]) than that of the mullite phase (Hv: 6.9 ± 0.2 GPa). In the current case, the fine Al2O3 grains are distributed between the grain boundaries and can hinder the local deformation, thus improving the hardness.…”