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oai:riubu.ubu.es:10259/58182024-07-25T07:32:20Zcom_10259_9476com_10259.4_106com_10259_2604col_10259_9477

2021-06-28T11:37:21Z urn:hdl:10259/5818 Ultra high temperature high-entropy borides: effect of graphite addition on oxides removal and densification behaviour Barbarossa, Simone Orrù, Roberto Garroni, Sebastiano Licheri, Roberta Cao, Giacomo High-entropy metal borides Oxide impurities Spark plasma sintering Self-propagating high-temperature synthesis X-ray diffraction The introduction of 0.5–1.0 wt.% graphite to the powders prepared by Self-propagating High-temperature Synthesis (SHS) is found to be highly beneficial for the removal of oxide impurities (from 2.7-8.8 wt.% to 0.2–0.5 wt.%) during spark plasma sintering (1950°C/20 min, 20 MPa) of (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 and (Hf0.2Mo0.2Ta0.2Zr0.2Ti0.2)B2 ceramics. Concurrently, the consolidation level achieved is enhanced from about 92.5% and 88%, respectively, to values exceeding 97%. While a further increase of graphite slightly improves samples densification, final products become progressively richer of the unreacted carbon. It is assumed that graphite plays a double role during SPS, e.g. not only as a reactant during the carbothermal reduction of oxides contaminant, but also as lubricating agent for the powder particles. The latter phenomenon is likely the main responsible for the densification improvement when 3 wt.% or larger amounts of additive are used. Another positive effect is the crystallite size refinement of the high-entropy phases with the progressive abatement of oxides, to confirm that their presence promotes grain coarsening during the sintering process. 2021-06-28T11:37:21Z 2021-06-28T11:37:21Z 2021-03 info:eu-repo/semantics/article 0272-8842 http://hdl.handle.net/10259/5818 10.1016/j.ceramint.2020.10.200 eng Ceramics International. 2021, V. 47, n. 5, p. 6220-6231 https://doi.org/10.1016/j.ceramint.2020.10.200 http://creativecommons.org/licenses/by-nc-nd/4.0/ info:eu-repo/semantics/openAccess Attribution-NonCommercial-NoDerivatives 4.0 Internacional Elsevier