AngeL L. Ortiz | Universidad de Extremadura (original) (raw)

Papers by AngeL L. Ortiz

Research paper thumbnail of Solid-state spark plasma sintering of super wear resistant B4C-SiC-TiB2 triplex-particulate composites

Ceramics International, 2023

B4C-SiC-TiB2 ceramic composites with equal volume fractions of the three phases (i.e., 1B4C-1SiC-... more B4C-SiC-TiB2 ceramic composites with equal volume fractions of the three phases (i.e., 1B4C-1SiC-1TiB2) were fabricated by solid-state spark-plasma sintering (SPS) from commercially available B4C, SiC, and TiB2 powders, first optimizing their densification temperature and then investigating for the first time the unlubricated sliding wear of the optimally SPS-ed composite. It is shown that SPS is optimal at 1800°C (under 75 MPa pressure and 5 min soaking), which is much lower than the temperatures used so far for both the solid-state hotpressing and SPS of this and other B4C-SiC-TiB2 composites. It is also shown that the optimally SPS-ed 1B4C-1SiC-1TiB2 composite has a triplex-particulate microstructure with evenly distributed carbide and boride grains whose sizes are essentially those of the corresponding starting powders, and that it is ultrahard (i.e., ~35 GPa) and relatively tough (i.e., ~4 MPa•m 1/2). Moreover, it is demonstrated that, due to its ultra-high hardness and proneness to form a coherent oxide tribolayer, it is also very immune to wear, possessing an unprecedented super wear resistance to unlubricated sliding contact (i.e., ~1.6•10 8 (N•m)/mm³), thanks to which it only undergoes very mild abrasion in the form of superficial plastic scratches with hardly any material removal by micro-fracture. Finally, implications of interest for the ceramics and hard-materials communities

Research paper thumbnail of A critical comparison of the tribocorrosive performance in highly-alkaline wet medium of ultrafine-grained WC cemented carbides with Co, Co+Ni, or Co+Ni+Cr binders Aniss-Rabah Boukantar a , Boubekeur Djerdjare a

International Journal of Refractory Metals and Hard Materials, 2021

The tribocorrosion (resistance, mode, and mechanism) of ultrafine-grained cemented carbides of WC... more The tribocorrosion (resistance, mode, and mechanism) of ultrafine-grained cemented carbides of WC-Co, WC-(Co+Ni), and WC-(Co+Ni+Cr) was evaluated and compared critically under the typical conditions of use in highly-alkaline wet medium by means of detailed studies of wet-sliding wear at pH~13.65, complemented by separate detailed studies of both corrosion at pH~13.65 and dry-sliding wear. It is shown that: (i) WC-Co undergoes the most severe tribocorrosion, and separately the most intense corrosion and little mechanical wear; (ii) WC-(Co+Ni) undergoes moderate tribocorrosion, and separately modest corrosion and little mechanical wear; and (iii) WC-(Co+Ni+Cr) undergoes only slight tribocorrosion, and separately almost negligible corrosion and very little mechanical wear. These observations were explicable Manuscript File Click here to view linked References

Research paper thumbnail of Ultra-low temperature spark plasma sintering of super wear-resistant hard B4C composites

Scripta Materialia, 2022

The feasibility was investigated of fabricating super wear-resistant hard B4C composites by spark... more The feasibility was investigated of fabricating super wear-resistant hard B4C composites by spark plasma sintering (SPS) at ultra-low temperature (i.e., 1400°C) using very high proportions of MoSi2 aids (i.e., 35-50 vol.%). It is shown that with 40 vol.% MoSi2 aids already sufficient Si transient liquid phase is formed in situ during SPS (by the reaction B4C+2MoSi2→SiC+2MoB2+3Si) to achieve the ultrafast full densification of B4C at 1400°C, the Si melting point, simply by pore filling, particle rearrangement, and liquid spreading. Importantly, it is also shown that the resulting B4C composites are hard (i.e., ≈23 GPa) and super wear-resistant (i.e., ~10 7 (N•m)/mm 3), attributes both deriving from the composites' quadruplex-particulate (i.e., B4C plus β-SiC, β-MoB2, and MoSi2), fine-grained (i.e., <1 µm), fully-dense microstructure. Thus, this work opens a new avenue for the present and future lowercost fabrication of novel B4C composites for use in contact-mechanical and tribological applications.

Research paper thumbnail of Unlubricated sliding wear of B4C composites spark-plasma sintered with Si aids and of their reference B4C monoliths

Journal of the European Ceramic Society, 2023

Two fully-dense B4C-SiC composites were fabricated by spark-plasma sintering (SPS) from B4C+Si po... more Two fully-dense B4C-SiC composites were fabricated by spark-plasma sintering (SPS) from B4C+Si powders, one superhard (i.e., ~28.7(8) GPa) with abundant SiC by SPS of B4C+20vol.%Si at 1400°C and the other ultrahard (i.e., ~35.1(4) GPa) with little SiC by SPS of B4C+4.28vol.%Si at 1800°C, and their unlubricated sliding wear was investigated and compared with those of the reference B4C monoliths. It was found that the two B4C-SiC composites underwent mild tribo-oxidative wear with preferential removal of the oxide tribolayer, with the one SPS-ed at 1400°C from B4C+20vol.%Si being, despite its lower hardness and greater proneness to form oxide tribolayer, only slightly less wear resistant than the one SPS-ed at 1800°C from B4C+4.28vol.%Si (i.e., ~1.0(5)•10 7 vs 1.37(8)•10 7 (N•m)/mm³). The reference B4C monolith SPSed at 1400°C is comparatively two orders of magnitude less wear resistant (i.e., ~1.70(6)•10 5 (N•m)/mm³), attributable to its undergoing severe purely mechanical wear by microfracturedominated three-body abrasion due to its very poor sintering (i.e., high porosity of ~33.5%), poor grain cohesion, and low hardness (i.e., ~3.1(5) GPa). The reference B4C monolith SPS-ed at Revised Manuscript with Changes Marked Click here to view linked References 2 1800°C, while equally or less hard (i.e., 28.4(9) GPa) and slightly porous (i.e., ~5.3%), is somewhat more wear resistant (~1.8(3)•10 7 (N•m)/mm³) than the B4C-SiC composites, attributable to its undergoing only mild purely mechanical wear by plasticity-dominated two-body abrasion without porosity-induced grain pull-out, but it requires SPS temperatures well above 1400°C. Finally, relevant implications for the ceramics and hard-materials communities with interests in tribological applications are discussed.

Research paper thumbnail of Spark plasma sinterability and dry sliding-wear resistance of WC densified with Co, Co+Ni, and Co+Ni+Cr

International Journal of Refractory Metals & Hard Materials, 2020

The spark plasma sinterability and the dry sliding-wear resistance of ultrafine-grained WC densif... more The spark plasma sinterability and the dry sliding-wear resistance of ultrafine-grained WC densified with ~16.7 vol.% of Co, Co+Ni, and Co+Ni+Cr, compositions equivalent to that of the typical WC-10wt.%Co, were investigated and compared critically. Firstly, it was found that the partial substitutions of Co by Ni or by Ni+Cr are detrimental for the pressureless ultrafast sinterability, but much more in the latter than the former case, attributable to the higher eutectic temperatures during liquid-phase sintering. However, it was also observed that, thanks to the auxiliary stress supplementing the sintering stresses, the ultrafast sinterability with pressure is affected by these partial substitutions either not at all (Co by Ni) or very little (Co by Ni+Cr), making it possible in all cases to obtain fully-dense WC cermets with almost no grain growth at the same spark plasma sintering temperature. And secondly, it was found that these cemented carbides are very resistant to dry sliding wear, but that WC-(Co+Ni+Cr) is markedly more so, attributable essentially to its greater hardness. This is due to wear occurring essentially by two-Manuscript File Click here to view linked References 2 body abrasion dominated by plastic deformation (ploughing), plus some oxidative wear with formation of tribo-oxidation layers that, although not entirely coherent, are nonetheless beneficial for the wear resistance.

Research paper thumbnail of Highly sliding-wear resistant B4C composites fabricated by spark-plasma sintering with Ti-Al additives

Scripta Materialia, 2020

The lubricated sliding-wear resistance of a fine-grained B 4 C composite fabricated by transient ... more The lubricated sliding-wear resistance of a fine-grained B 4 C composite fabricated by transient liquid-phase assisted spark-plasma sintering with Ti-Al additives at smooth conditions is critically compared to that of its reference monolithic B 4 C ceramic. It is shown that the former has an excellent sliding-wear resistance that exceeds that of the later by one order of magnitude (far less specific wear rate, worn volume, and wear damage), attributable to its greater hardness and densification. The wear mode is abrasion dominated by plastic deformation, plus localized microfracture in the monolithic ceramic. Implications for fabricating highly wear-resistant tribocomponents based on superhard ceramics are presented.

Research paper thumbnail of Fabricating toughened super-hard B4C composites at lower temperature by transient liquid-phase assisted spark plasma sintering with MoSi 2 additives

Journal of the European Ceramic Society, 2019

Toughened, super-hard B 4 C triplex-particulate composites were densified by spark plasma sinteri... more Toughened, super-hard B 4 C triplex-particulate composites were densified by spark plasma sintering with MoSi 2 additives (5, 10, and 15 vol.%) at temperatures in the range 1750-1850 °C at which the reference monolithic B 4 C ceramics are porous. It is proved that MoSi 2 is a reactive sintering additive that promotes densification by transient liquid-phase sintering, thus yielding fully-dense B 4 C-MoB 2-SiC composites at relatively lower temperatures. Specifically, the MoSi 2 first reacts at moderate temperatures (<1150 °C) with part of B 4 C to form MoB 2 , SiC, and Si. This last is a transient component that eventually melts (at ~1400 °C), contributing to densification by liquid-phase sintering, and then (at 1500-1700 °C) reacts with free C present in the B 4 C starting powders to form more SiC, after which densification continues by solid-state sintering. It is found that these B 4 C-MoB 2-SiC composites are super-hard (~30 GPa), tough (~3-4 MPa•m 1/2), and fine-grained, a combination that renders them very appealing for structural applications. Finally, research opportunities are discussed for the future microstructural design of a novel family of toughened, ultra-hard/super-hard multi-particulate composites based on B 4 C plus refractory borides and carbides. *Manuscript Click here to view linked References

Research paper thumbnail of Transient liquid-phase assisted spark-plasma sintering and dry sliding wear of B4C ceramics fabricated from B4C nanopowders

Journal of the European Ceramic Society, 2021

With the motivation of developing B 4 C composites with superior wear resistance for tribological... more With the motivation of developing B 4 C composites with superior wear resistance for tribological applications, an ultrafine-grained (~200− 300 nm) B 4 C composite was fabricated, characterized microstructurally, and tested mechanically and tribologically. First, a well-dispersed powder mixture of B 4 C nanopowders (~40 nm) with coarse Ti-Al powders (~38 μm) as transient liquid-phase sintering additives was environmentally-friendly prepared by aqueous colloidal processing, optimized by measurements of the zeta potential of dilute suspensions and rheological studies of concentrated suspensions. Second, the powder mixture obtained by freeze-drying was densified by spark-plasma sintering (SPS), identifying the optimal SPS temperature (1850 • C) by measurements of density, hardness, and toughness. Third, the dry sliding-wear behaviour of the optimal superhard B 4 C composite (~31.5 GPa) was investigated by pin-on-disk tests and observations of the worn surface, determining its specific wear rate (~4.4⋅10 − 8 mm 3 /(N⋅m)) as well as wear mode (two-body abrasion) and mechanism (plastic deformation). And lastly, the wear behaviour of the ultrafine-grained B 4 C composite was compared with that of a reference fine-grained (~0.7− 0.9 μm) B 4 C composite, finding that both have the same mode and mechanism of wear but with the former being more resistant than the latter (~2.3⋅10 7 vs 1.9⋅10 7 (N⋅m)/mm 3). Implications for the fabrication of B 4 C tribocomponents with greater superior wear resistance are discussed.

Research paper thumbnail of Effect of type of solvent alcohol and its molar proportion on the drying critical thickness of ZrO2–3mol% Y2O3 films prepared by the sol–gel method

Surface and Coatings Technology, 2011

The effect of the type of the solvent alcohol and its molar proportion on the drying critical thi... more The effect of the type of the solvent alcohol and its molar proportion on the drying critical thickness of 3 mol% Y 2 O 3-stabilized ZrO 2 (YSZ) sol-gel films was examined experimentally and modeled analytically. To this end, zirconium n-propoxide was dissolved using different solvent alcohols (ethanol, methanol, and propanol) and different molar proportions of propanol (alcohol-to-alkoxide molar ratios of~6.5, 11.5, or 16.5), and the resulting sol-gel solutions were dip-deposited at different substrate withdrawal rates until cracking occurred after drying at 100°C. It was found that the drying critical thickness, i.e., the maximum thickness that can be reached without film cracking, was greatest when the solvent alcohol was propanol, despite the tensile strength of the gelled network being the lowest. The use of ethanol and methanol as solvent alcohols results in sol-gel films with greater strength but thinner drying critical thicknesses. The analysis of the results using a drying stress model showed the drying critical thickness to be conditioned by the balance between the evaporation rate and viscosity of the sol-gel solutions, and the experimental data indicated that the predominant factor to take into account when choosing the solvent alcohol is the evaporation rate of the solution. It was also found that the critical thickness increases with increasing molar proportion of solvent alcohol, again despite the decreasing tensile strength of the gelled network. The analytical modeling of the drying stresses indicated that, once the solvent alcohol has been chosen, the determining factor in the drying critical thickness is the viscosity of the sol-gel solutions. Implications for obtaining thicker, crack-free, sol-gel films of YSZ or other oxide ceramics during drying that have greater tensile strength are discussed.

Research paper thumbnail of Effect of hexagonal-BN additions on the sliding-wear resistance of fine-grained α-SiC densified with Y3Al5O12 liquid phase by spark-plasma sintering

Journal of the European Ceramic Society, 2014

The effect of flaky hexagonal (h) BN additions (1, 5, and 10 vol.%) on the lubricated sliding-wea... more The effect of flaky hexagonal (h) BN additions (1, 5, and 10 vol.%) on the lubricated sliding-wear behavior of fine-grained, liquid-phase-sintered SiC ceramics fabricated by spark-plasma sintering was investigated. It was found that resistance to the initial mild, deformation-controlled wear decreases with increasing h-BN content in the composite, which progressively exhibits a greater wear rate and a sooner transition to severe wear. This is because the softer h-BN particles reduce the hardness and do not act as internal lubricant, while promoting poorer grain cohesion due to their morphologically-favored segregation at grain boundaries. By contrast, their addition is increasingly beneficial in terms of resistance to the subsequent severe, fracture-controlled wear upon prolonged sliding contact, with a lower wear rate. This is because the flaky h-BN particles increase the fracture toughness, and also act efficiently as external lubricant when pulled-out from the microstructure. Finally, implications for the design of advanced triboceramics are discussed.

Research paper thumbnail of Carbon nanotubes prevent the coagulation at high shear rates of aqueous suspensions of equiaxed ceramic nanoparticles

Journal of the European Ceramic Society, 2014

Equiaxed ceramic nanoparticles and their mixtures are expected to exhibit shear-thinning behaviou... more Equiaxed ceramic nanoparticles and their mixtures are expected to exhibit shear-thinning behaviour when dispersed colloidally in aqueous media, whereas shear-thickening is the expectation for large aspect ratio phases such as, for example, carbon nanotubes (CNTs). Here, contrary experimental evidence is presented demonstrating the occurrence of severe coagulation at high shear rates in colloidally stable, semi-concentrated, aqueous suspensions of equiaxed SiC nanoparticles (major phase) mixed with equiaxed Y 3 Al 5 O 12 nanoparticles (liquid-phase sintering additive), and how CNT addition prevents this coagulation if sufficient sonication is applied. It is also shown that although shear-thinning is the natural behaviour of the ceramic suspension up to moderate shear rates, coagulation is eventually a phenomenon inherent to the aqueous colloidal processing of these suspensions, with the critical shear rate for coagulation increasing and the rheopexy decreasing the better is the initial dispersion state achieved with the sonication. It is also shown that the critical shear rate for coagulation depends on the exact condition of shear rate increase, and that the re-sheared suspensions coagulate more significantly and at lower shear rates than the fresh suspensions. The mechanisms by which this coagulation occurs and is impeded by the CNTs are discussed, together with broader implications of these phenomena for the environmentally friendly processing of nanostructured ceramics and ceramic composites.

Research paper thumbnail of Effect of liquid-phase content on the contact-mechanical properties of liquid-phase-sintered α-SiC

Journal of the European Ceramic Society, 2007

... 2.3. Mechanical properties characterization. 2.3.1. Hertz indentation. ... 18. The use of bot... more ... 2.3. Mechanical properties characterization. 2.3.1. Hertz indentation. ... 18. The use of both blunt (Hertz) and sharp (Vickers and Berkovich) indentation techniques allows studies of the mechanical response of the materials from the elastic regime to the plastic regime. 20 Fig. ...

Research paper thumbnail of Complex impedance spectroscopy study of a liquid-phase-sintered α-SiC ceramic

Journal of the European Ceramic Society, 2007

The electrical response of a liquid-phase-sintered (LPS) ␣-SiC with 10 wt.% Y 3 Al 5 O 12 (YAG) a... more The electrical response of a liquid-phase-sintered (LPS) ␣-SiC with 10 wt.% Y 3 Al 5 O 12 (YAG) additives was studied from near-ambient temperature up to 800 • C by complex impedance spectroscopy. The electrical conductivity of this LPS SiC ceramic was found to increase with increasing temperature, which was attributed to the semiconductor nature of the SiC grains. It was concluded that the contribution of the SiC grains to the electrical conductivity of the LPS SiC ceramic at moderate temperatures (<450 • C) is a somewhat greater than that of the YAG phase. In contrast, at higher temperatures the SiC grains control the electrical conductivity of the LPS SiC ceramic. It was also found that there are two activation energies for the electrical conduction process of the ␣-SiC grains. These are 0.19 eV at temperatures lower than ∼400 • C and 2.96 eV at temperatures higher than ∼500 • C. The existence of two temperature-dependence conduction regimes reflects the core-shell substructure that develops within the SiC grains during the liquid-phase sintering, where the core is pure SiC (intrinsic semiconductor) and the shell is mainly Al-doped SiC (extrinsic semiconductor).

Research paper thumbnail of Aqueous colloidal processing of submicrometric SiC plus Y3Al5O12 with diamond nanoparticles

Journal of the European Ceramic Society, 2013

Aqueous colloidal processing was used for the environmentally friendly preparation of well-disper... more Aqueous colloidal processing was used for the environmentally friendly preparation of well-dispersed concentrated suspensions and powder mixtures of submicrometric SiC powders with submicrometric Y 3 Al 5 O 12 as sintering additive plus diamond nanoparticles as reinforcing phase. It is shown that the addition of nano-diamond markedly increases the viscosity and thixotropy of the SiC + Y 3 Al 5 O 12 suspensions, and also that, by adjusting the pH, deflocculant content, and sonication time it is possible to co-disperse these three rheologically different ceramic phases (i.e., non-oxide, oxide, and hydrophobic compounds) in aqueous media, thereby avoiding the otherwise irremediable severe hetero-aggregation. Moreover, the microstructural characterization of the powder mixtures obtained by freeze-drying the suspensions confirmed the homogeneous dispersion of the diamond nanoparticles among the submicrometric SiC and Y 3 Al 5 O 12 particles in the form of isolated or adhered nanoclusters and nanodeposits. Implications for engineering the microstructure of non-oxide ceramics with diamond nanodispersoids are discussed.

Research paper thumbnail of Sliding-wear resistance of liquid-phase-sintered SiC containing graphite nanodispersoids

Journal of the European Ceramic Society, 2014

The influence was investigated of a graphite nanodispersoid addition on the lubricated sliding-we... more The influence was investigated of a graphite nanodispersoid addition on the lubricated sliding-wear behaviour of liquid-phase-sintered (LPS) SiC ceramics fabricated by spark-plasma sintering (SPS). The graphite nanodispersoids, introduced into the microstructure of the LPS SiC ceramic to act as self-lubricating phase, were obtained by graphitization of diamond nanoparticles during the SPS. It was found that the graphite nanodispersoid addition results in a lower resistance to mild wear, attributable to the lower hardness of graphite and the null lubrication it provides. Moreover, the graphite nanodispersoids, which mostly locate at grain boundaries/faces, worsen the cohesion of SiC grains, contributing together with the higher mild-wear rate to an early transition to the severe-wear stage. On the contrary, the graphite nanodispersoids were effective at improving the resistance to severe wear because they increase the fracture toughness while providing some external lubrication. Relevant implications for the microstructural design of advanced triboceramics are discussed.

Research paper thumbnail of Microstructural effects on the sliding-wear resistance of pressureless liquid-phase-sintered SiC under diesel fuel

Journal of the European Ceramic Society, 2013

The sliding wear of pressureless liquid-phase-sintered (PLPS) SiC ceramics under diesel fuel lubr... more The sliding wear of pressureless liquid-phase-sintered (PLPS) SiC ceramics under diesel fuel lubrication was investigated. It was found that the sliding-wear behaviour under diesel fuel is similar to what is observed under lubrication with the chemically similar paraffin oil: initial mild, plasticity-controlled wear followed by severe, fracture-controlled wear, with a well-defined wear transition. Compared to paraffin oil, however, diesel fuel lubrication decreases the extent of the mild-wear regime and results in damage that is more severe due to its lower viscosity. Also, it was found that the wear resistance under diesel fuel decreases with increasing contact load and sliding speed. Subsequent investigation of the effects of the intergranular phase content, grain size, and grain shape demonstrates nevertheless that the wear performance of PLPS SiC under diesel lubrication can be improved via microstructural control by (i) decreasing the intergranular phase content, and (ii) decreasing the SiC grain size or increasing its aspect ratio. The results of this work are likely to have relevant implications for the design of wear resistant PLPS SiC ceramic components for diesel engines.

Research paper thumbnail of Effect of Microstructure on Sliding-Wear Properties of Liquid-Phase-Sintered α-SiC

Journal of the American Ceramic Society, 2005

We have studied, for the first time, the effect of the content of intergranular phase and grain s... more We have studied, for the first time, the effect of the content of intergranular phase and grain size on the sliding-wear resistance of pressureless, liquid-phase-sintered (LPS) a-SiC (silicon carbide) ceramics. It was found that the sliding-wear behavior of these ceramics is similar to what is observed in other polycrystalline ceramics: initial mild, plasticity-controlled wear followed by severe, fracture-controlled wear, with well-defined wear transition. We have found that the increase in the content of intergranular phase and the grain coarsening leads to the degradation of the sliding-wear resistance in LPS a-SiC ceramics. A mechanistic model is used to rationalize these wear results, and to provide guidelines for the design and fabrication of low cost, highly wearresistant SiC-based ceramics. This is likely to have important implications because SiC-based materials are being used increasingly in contact-mechanical and tribological applications.

Research paper thumbnail of Microstructural Evolution in Liquid‐Phase‐Sintered SiC: Part I, Effect of Starting Powder

Journal of the American Ceramic Society, 2004

The effect of starting SiC powder (β‐SiC or α‐SiC), with simultaneous additions of Al2O3 and Y2O3... more The effect of starting SiC powder (β‐SiC or α‐SiC), with simultaneous additions of Al2O3 and Y2O3, on the microstructural evolution of liquid‐phase‐sintered (LPS) SiC has been studied. When using α‐SiC starting powder, the resulting microstructures contain hexagonal platelike α‐SiC grains with an average aspect ratio of 1.4. This anisotropic coarsening is consistent with interface energy anisotropy in α‐SiC. When using β‐SiC starting powder, the β→α phase transformation induces additional anisotropy in the coarsening of platelike SiC grains. A strong correlation between the extent of β→α phase transformation, as determined using quantitative XRD analysis, and the average grain aspect ratio is observed, with the maximum average aspect ratio reaching 3.8. Based on these observations and additional SEM and TEM characterizations of the microstructures, a model for the growth of these high‐aspect‐ratio SiC grains is proposed.

Research paper thumbnail of High‐Energy Ball Milling of ZrB2 in the Presence of Graphite

Journal of the American Ceramic Society, 2010

The effect of graphite addition on the high‐energy ball‐milling behavior of zirconium diboride (Z... more The effect of graphite addition on the high‐energy ball‐milling behavior of zirconium diboride (ZrB2) powder is investigated. It is shown that, regardless of presence or absence of graphite during ball milling, comminution occurs by repeated brittle fracture followed by cold‐welding, thereby resulting in the formation of agglomerates comprising primary particles of 10 nm in average diameter. However, addition of 2 wt% graphite leads to the formation of nanometer agglomerates (∼50 nm) rather than the submicrometer counterparts (∼120 nm) formed in the absence of graphite. The simultaneous attainment of reduced agglomerate size and homogeneous graphite dispersion at the nanometer scale can have important implications for the ultra‐high‐temperature ceramics community because of the expected twofold enhancement in the sintering kinetics of the compacts made from these nanoscale ZrB2 powders and the simplification of the processing routine.

Research paper thumbnail of Creep and Microstructural Evolution at High Temperature of Liquid-Phase-Sintered Silicon Carbide

Journal of the American Ceramic Society, 2007

The compressive creep characteristics at 16251C of liquidphase-sintered silicon carbide ceramics ... more The compressive creep characteristics at 16251C of liquidphase-sintered silicon carbide ceramics containing 5 and 15 wt% of crystalline Y 3 Al 5 O 12 (YAG) as the secondary phase were studied. In the two cases, strains between 10% and 15% were reached without failure. The creep behavior was characterized by a stress exponent n% 2, and the proportion of secondary phase was related to the creep resistance of the materials. The microstructural evolution during creep consisted firstly in the redistribution of the secondary phase, probably as a consequence of its viscous flow at the creep conditions, and secondly an extensive nucleation and growth of cavities, which was more important for the highest YAG content. The latter reflects the carbothermal reduction that the secondary phase undergoes during creep.

Research paper thumbnail of Solid-state spark plasma sintering of super wear resistant B4C-SiC-TiB2 triplex-particulate composites

Ceramics International, 2023

B4C-SiC-TiB2 ceramic composites with equal volume fractions of the three phases (i.e., 1B4C-1SiC-... more B4C-SiC-TiB2 ceramic composites with equal volume fractions of the three phases (i.e., 1B4C-1SiC-1TiB2) were fabricated by solid-state spark-plasma sintering (SPS) from commercially available B4C, SiC, and TiB2 powders, first optimizing their densification temperature and then investigating for the first time the unlubricated sliding wear of the optimally SPS-ed composite. It is shown that SPS is optimal at 1800°C (under 75 MPa pressure and 5 min soaking), which is much lower than the temperatures used so far for both the solid-state hotpressing and SPS of this and other B4C-SiC-TiB2 composites. It is also shown that the optimally SPS-ed 1B4C-1SiC-1TiB2 composite has a triplex-particulate microstructure with evenly distributed carbide and boride grains whose sizes are essentially those of the corresponding starting powders, and that it is ultrahard (i.e., ~35 GPa) and relatively tough (i.e., ~4 MPa•m 1/2). Moreover, it is demonstrated that, due to its ultra-high hardness and proneness to form a coherent oxide tribolayer, it is also very immune to wear, possessing an unprecedented super wear resistance to unlubricated sliding contact (i.e., ~1.6•10 8 (N•m)/mm³), thanks to which it only undergoes very mild abrasion in the form of superficial plastic scratches with hardly any material removal by micro-fracture. Finally, implications of interest for the ceramics and hard-materials communities

Research paper thumbnail of A critical comparison of the tribocorrosive performance in highly-alkaline wet medium of ultrafine-grained WC cemented carbides with Co, Co+Ni, or Co+Ni+Cr binders Aniss-Rabah Boukantar a , Boubekeur Djerdjare a

International Journal of Refractory Metals and Hard Materials, 2021

The tribocorrosion (resistance, mode, and mechanism) of ultrafine-grained cemented carbides of WC... more The tribocorrosion (resistance, mode, and mechanism) of ultrafine-grained cemented carbides of WC-Co, WC-(Co+Ni), and WC-(Co+Ni+Cr) was evaluated and compared critically under the typical conditions of use in highly-alkaline wet medium by means of detailed studies of wet-sliding wear at pH~13.65, complemented by separate detailed studies of both corrosion at pH~13.65 and dry-sliding wear. It is shown that: (i) WC-Co undergoes the most severe tribocorrosion, and separately the most intense corrosion and little mechanical wear; (ii) WC-(Co+Ni) undergoes moderate tribocorrosion, and separately modest corrosion and little mechanical wear; and (iii) WC-(Co+Ni+Cr) undergoes only slight tribocorrosion, and separately almost negligible corrosion and very little mechanical wear. These observations were explicable Manuscript File Click here to view linked References

Research paper thumbnail of Ultra-low temperature spark plasma sintering of super wear-resistant hard B4C composites

Scripta Materialia, 2022

The feasibility was investigated of fabricating super wear-resistant hard B4C composites by spark... more The feasibility was investigated of fabricating super wear-resistant hard B4C composites by spark plasma sintering (SPS) at ultra-low temperature (i.e., 1400°C) using very high proportions of MoSi2 aids (i.e., 35-50 vol.%). It is shown that with 40 vol.% MoSi2 aids already sufficient Si transient liquid phase is formed in situ during SPS (by the reaction B4C+2MoSi2→SiC+2MoB2+3Si) to achieve the ultrafast full densification of B4C at 1400°C, the Si melting point, simply by pore filling, particle rearrangement, and liquid spreading. Importantly, it is also shown that the resulting B4C composites are hard (i.e., ≈23 GPa) and super wear-resistant (i.e., ~10 7 (N•m)/mm 3), attributes both deriving from the composites' quadruplex-particulate (i.e., B4C plus β-SiC, β-MoB2, and MoSi2), fine-grained (i.e., <1 µm), fully-dense microstructure. Thus, this work opens a new avenue for the present and future lowercost fabrication of novel B4C composites for use in contact-mechanical and tribological applications.

Research paper thumbnail of Unlubricated sliding wear of B4C composites spark-plasma sintered with Si aids and of their reference B4C monoliths

Journal of the European Ceramic Society, 2023

Two fully-dense B4C-SiC composites were fabricated by spark-plasma sintering (SPS) from B4C+Si po... more Two fully-dense B4C-SiC composites were fabricated by spark-plasma sintering (SPS) from B4C+Si powders, one superhard (i.e., ~28.7(8) GPa) with abundant SiC by SPS of B4C+20vol.%Si at 1400°C and the other ultrahard (i.e., ~35.1(4) GPa) with little SiC by SPS of B4C+4.28vol.%Si at 1800°C, and their unlubricated sliding wear was investigated and compared with those of the reference B4C monoliths. It was found that the two B4C-SiC composites underwent mild tribo-oxidative wear with preferential removal of the oxide tribolayer, with the one SPS-ed at 1400°C from B4C+20vol.%Si being, despite its lower hardness and greater proneness to form oxide tribolayer, only slightly less wear resistant than the one SPS-ed at 1800°C from B4C+4.28vol.%Si (i.e., ~1.0(5)•10 7 vs 1.37(8)•10 7 (N•m)/mm³). The reference B4C monolith SPSed at 1400°C is comparatively two orders of magnitude less wear resistant (i.e., ~1.70(6)•10 5 (N•m)/mm³), attributable to its undergoing severe purely mechanical wear by microfracturedominated three-body abrasion due to its very poor sintering (i.e., high porosity of ~33.5%), poor grain cohesion, and low hardness (i.e., ~3.1(5) GPa). The reference B4C monolith SPS-ed at Revised Manuscript with Changes Marked Click here to view linked References 2 1800°C, while equally or less hard (i.e., 28.4(9) GPa) and slightly porous (i.e., ~5.3%), is somewhat more wear resistant (~1.8(3)•10 7 (N•m)/mm³) than the B4C-SiC composites, attributable to its undergoing only mild purely mechanical wear by plasticity-dominated two-body abrasion without porosity-induced grain pull-out, but it requires SPS temperatures well above 1400°C. Finally, relevant implications for the ceramics and hard-materials communities with interests in tribological applications are discussed.

Research paper thumbnail of Spark plasma sinterability and dry sliding-wear resistance of WC densified with Co, Co+Ni, and Co+Ni+Cr

International Journal of Refractory Metals & Hard Materials, 2020

The spark plasma sinterability and the dry sliding-wear resistance of ultrafine-grained WC densif... more The spark plasma sinterability and the dry sliding-wear resistance of ultrafine-grained WC densified with ~16.7 vol.% of Co, Co+Ni, and Co+Ni+Cr, compositions equivalent to that of the typical WC-10wt.%Co, were investigated and compared critically. Firstly, it was found that the partial substitutions of Co by Ni or by Ni+Cr are detrimental for the pressureless ultrafast sinterability, but much more in the latter than the former case, attributable to the higher eutectic temperatures during liquid-phase sintering. However, it was also observed that, thanks to the auxiliary stress supplementing the sintering stresses, the ultrafast sinterability with pressure is affected by these partial substitutions either not at all (Co by Ni) or very little (Co by Ni+Cr), making it possible in all cases to obtain fully-dense WC cermets with almost no grain growth at the same spark plasma sintering temperature. And secondly, it was found that these cemented carbides are very resistant to dry sliding wear, but that WC-(Co+Ni+Cr) is markedly more so, attributable essentially to its greater hardness. This is due to wear occurring essentially by two-Manuscript File Click here to view linked References 2 body abrasion dominated by plastic deformation (ploughing), plus some oxidative wear with formation of tribo-oxidation layers that, although not entirely coherent, are nonetheless beneficial for the wear resistance.

Research paper thumbnail of Highly sliding-wear resistant B4C composites fabricated by spark-plasma sintering with Ti-Al additives

Scripta Materialia, 2020

The lubricated sliding-wear resistance of a fine-grained B 4 C composite fabricated by transient ... more The lubricated sliding-wear resistance of a fine-grained B 4 C composite fabricated by transient liquid-phase assisted spark-plasma sintering with Ti-Al additives at smooth conditions is critically compared to that of its reference monolithic B 4 C ceramic. It is shown that the former has an excellent sliding-wear resistance that exceeds that of the later by one order of magnitude (far less specific wear rate, worn volume, and wear damage), attributable to its greater hardness and densification. The wear mode is abrasion dominated by plastic deformation, plus localized microfracture in the monolithic ceramic. Implications for fabricating highly wear-resistant tribocomponents based on superhard ceramics are presented.

Research paper thumbnail of Fabricating toughened super-hard B4C composites at lower temperature by transient liquid-phase assisted spark plasma sintering with MoSi 2 additives

Journal of the European Ceramic Society, 2019

Toughened, super-hard B 4 C triplex-particulate composites were densified by spark plasma sinteri... more Toughened, super-hard B 4 C triplex-particulate composites were densified by spark plasma sintering with MoSi 2 additives (5, 10, and 15 vol.%) at temperatures in the range 1750-1850 °C at which the reference monolithic B 4 C ceramics are porous. It is proved that MoSi 2 is a reactive sintering additive that promotes densification by transient liquid-phase sintering, thus yielding fully-dense B 4 C-MoB 2-SiC composites at relatively lower temperatures. Specifically, the MoSi 2 first reacts at moderate temperatures (<1150 °C) with part of B 4 C to form MoB 2 , SiC, and Si. This last is a transient component that eventually melts (at ~1400 °C), contributing to densification by liquid-phase sintering, and then (at 1500-1700 °C) reacts with free C present in the B 4 C starting powders to form more SiC, after which densification continues by solid-state sintering. It is found that these B 4 C-MoB 2-SiC composites are super-hard (~30 GPa), tough (~3-4 MPa•m 1/2), and fine-grained, a combination that renders them very appealing for structural applications. Finally, research opportunities are discussed for the future microstructural design of a novel family of toughened, ultra-hard/super-hard multi-particulate composites based on B 4 C plus refractory borides and carbides. *Manuscript Click here to view linked References

Research paper thumbnail of Transient liquid-phase assisted spark-plasma sintering and dry sliding wear of B4C ceramics fabricated from B4C nanopowders

Journal of the European Ceramic Society, 2021

With the motivation of developing B 4 C composites with superior wear resistance for tribological... more With the motivation of developing B 4 C composites with superior wear resistance for tribological applications, an ultrafine-grained (~200− 300 nm) B 4 C composite was fabricated, characterized microstructurally, and tested mechanically and tribologically. First, a well-dispersed powder mixture of B 4 C nanopowders (~40 nm) with coarse Ti-Al powders (~38 μm) as transient liquid-phase sintering additives was environmentally-friendly prepared by aqueous colloidal processing, optimized by measurements of the zeta potential of dilute suspensions and rheological studies of concentrated suspensions. Second, the powder mixture obtained by freeze-drying was densified by spark-plasma sintering (SPS), identifying the optimal SPS temperature (1850 • C) by measurements of density, hardness, and toughness. Third, the dry sliding-wear behaviour of the optimal superhard B 4 C composite (~31.5 GPa) was investigated by pin-on-disk tests and observations of the worn surface, determining its specific wear rate (~4.4⋅10 − 8 mm 3 /(N⋅m)) as well as wear mode (two-body abrasion) and mechanism (plastic deformation). And lastly, the wear behaviour of the ultrafine-grained B 4 C composite was compared with that of a reference fine-grained (~0.7− 0.9 μm) B 4 C composite, finding that both have the same mode and mechanism of wear but with the former being more resistant than the latter (~2.3⋅10 7 vs 1.9⋅10 7 (N⋅m)/mm 3). Implications for the fabrication of B 4 C tribocomponents with greater superior wear resistance are discussed.

Research paper thumbnail of Effect of type of solvent alcohol and its molar proportion on the drying critical thickness of ZrO2–3mol% Y2O3 films prepared by the sol–gel method

Surface and Coatings Technology, 2011

The effect of the type of the solvent alcohol and its molar proportion on the drying critical thi... more The effect of the type of the solvent alcohol and its molar proportion on the drying critical thickness of 3 mol% Y 2 O 3-stabilized ZrO 2 (YSZ) sol-gel films was examined experimentally and modeled analytically. To this end, zirconium n-propoxide was dissolved using different solvent alcohols (ethanol, methanol, and propanol) and different molar proportions of propanol (alcohol-to-alkoxide molar ratios of~6.5, 11.5, or 16.5), and the resulting sol-gel solutions were dip-deposited at different substrate withdrawal rates until cracking occurred after drying at 100°C. It was found that the drying critical thickness, i.e., the maximum thickness that can be reached without film cracking, was greatest when the solvent alcohol was propanol, despite the tensile strength of the gelled network being the lowest. The use of ethanol and methanol as solvent alcohols results in sol-gel films with greater strength but thinner drying critical thicknesses. The analysis of the results using a drying stress model showed the drying critical thickness to be conditioned by the balance between the evaporation rate and viscosity of the sol-gel solutions, and the experimental data indicated that the predominant factor to take into account when choosing the solvent alcohol is the evaporation rate of the solution. It was also found that the critical thickness increases with increasing molar proportion of solvent alcohol, again despite the decreasing tensile strength of the gelled network. The analytical modeling of the drying stresses indicated that, once the solvent alcohol has been chosen, the determining factor in the drying critical thickness is the viscosity of the sol-gel solutions. Implications for obtaining thicker, crack-free, sol-gel films of YSZ or other oxide ceramics during drying that have greater tensile strength are discussed.

Research paper thumbnail of Effect of hexagonal-BN additions on the sliding-wear resistance of fine-grained α-SiC densified with Y3Al5O12 liquid phase by spark-plasma sintering

Journal of the European Ceramic Society, 2014

The effect of flaky hexagonal (h) BN additions (1, 5, and 10 vol.%) on the lubricated sliding-wea... more The effect of flaky hexagonal (h) BN additions (1, 5, and 10 vol.%) on the lubricated sliding-wear behavior of fine-grained, liquid-phase-sintered SiC ceramics fabricated by spark-plasma sintering was investigated. It was found that resistance to the initial mild, deformation-controlled wear decreases with increasing h-BN content in the composite, which progressively exhibits a greater wear rate and a sooner transition to severe wear. This is because the softer h-BN particles reduce the hardness and do not act as internal lubricant, while promoting poorer grain cohesion due to their morphologically-favored segregation at grain boundaries. By contrast, their addition is increasingly beneficial in terms of resistance to the subsequent severe, fracture-controlled wear upon prolonged sliding contact, with a lower wear rate. This is because the flaky h-BN particles increase the fracture toughness, and also act efficiently as external lubricant when pulled-out from the microstructure. Finally, implications for the design of advanced triboceramics are discussed.

Research paper thumbnail of Carbon nanotubes prevent the coagulation at high shear rates of aqueous suspensions of equiaxed ceramic nanoparticles

Journal of the European Ceramic Society, 2014

Equiaxed ceramic nanoparticles and their mixtures are expected to exhibit shear-thinning behaviou... more Equiaxed ceramic nanoparticles and their mixtures are expected to exhibit shear-thinning behaviour when dispersed colloidally in aqueous media, whereas shear-thickening is the expectation for large aspect ratio phases such as, for example, carbon nanotubes (CNTs). Here, contrary experimental evidence is presented demonstrating the occurrence of severe coagulation at high shear rates in colloidally stable, semi-concentrated, aqueous suspensions of equiaxed SiC nanoparticles (major phase) mixed with equiaxed Y 3 Al 5 O 12 nanoparticles (liquid-phase sintering additive), and how CNT addition prevents this coagulation if sufficient sonication is applied. It is also shown that although shear-thinning is the natural behaviour of the ceramic suspension up to moderate shear rates, coagulation is eventually a phenomenon inherent to the aqueous colloidal processing of these suspensions, with the critical shear rate for coagulation increasing and the rheopexy decreasing the better is the initial dispersion state achieved with the sonication. It is also shown that the critical shear rate for coagulation depends on the exact condition of shear rate increase, and that the re-sheared suspensions coagulate more significantly and at lower shear rates than the fresh suspensions. The mechanisms by which this coagulation occurs and is impeded by the CNTs are discussed, together with broader implications of these phenomena for the environmentally friendly processing of nanostructured ceramics and ceramic composites.

Research paper thumbnail of Effect of liquid-phase content on the contact-mechanical properties of liquid-phase-sintered α-SiC

Journal of the European Ceramic Society, 2007

... 2.3. Mechanical properties characterization. 2.3.1. Hertz indentation. ... 18. The use of bot... more ... 2.3. Mechanical properties characterization. 2.3.1. Hertz indentation. ... 18. The use of both blunt (Hertz) and sharp (Vickers and Berkovich) indentation techniques allows studies of the mechanical response of the materials from the elastic regime to the plastic regime. 20 Fig. ...

Research paper thumbnail of Complex impedance spectroscopy study of a liquid-phase-sintered α-SiC ceramic

Journal of the European Ceramic Society, 2007

The electrical response of a liquid-phase-sintered (LPS) ␣-SiC with 10 wt.% Y 3 Al 5 O 12 (YAG) a... more The electrical response of a liquid-phase-sintered (LPS) ␣-SiC with 10 wt.% Y 3 Al 5 O 12 (YAG) additives was studied from near-ambient temperature up to 800 • C by complex impedance spectroscopy. The electrical conductivity of this LPS SiC ceramic was found to increase with increasing temperature, which was attributed to the semiconductor nature of the SiC grains. It was concluded that the contribution of the SiC grains to the electrical conductivity of the LPS SiC ceramic at moderate temperatures (<450 • C) is a somewhat greater than that of the YAG phase. In contrast, at higher temperatures the SiC grains control the electrical conductivity of the LPS SiC ceramic. It was also found that there are two activation energies for the electrical conduction process of the ␣-SiC grains. These are 0.19 eV at temperatures lower than ∼400 • C and 2.96 eV at temperatures higher than ∼500 • C. The existence of two temperature-dependence conduction regimes reflects the core-shell substructure that develops within the SiC grains during the liquid-phase sintering, where the core is pure SiC (intrinsic semiconductor) and the shell is mainly Al-doped SiC (extrinsic semiconductor).

Research paper thumbnail of Aqueous colloidal processing of submicrometric SiC plus Y3Al5O12 with diamond nanoparticles

Journal of the European Ceramic Society, 2013

Aqueous colloidal processing was used for the environmentally friendly preparation of well-disper... more Aqueous colloidal processing was used for the environmentally friendly preparation of well-dispersed concentrated suspensions and powder mixtures of submicrometric SiC powders with submicrometric Y 3 Al 5 O 12 as sintering additive plus diamond nanoparticles as reinforcing phase. It is shown that the addition of nano-diamond markedly increases the viscosity and thixotropy of the SiC + Y 3 Al 5 O 12 suspensions, and also that, by adjusting the pH, deflocculant content, and sonication time it is possible to co-disperse these three rheologically different ceramic phases (i.e., non-oxide, oxide, and hydrophobic compounds) in aqueous media, thereby avoiding the otherwise irremediable severe hetero-aggregation. Moreover, the microstructural characterization of the powder mixtures obtained by freeze-drying the suspensions confirmed the homogeneous dispersion of the diamond nanoparticles among the submicrometric SiC and Y 3 Al 5 O 12 particles in the form of isolated or adhered nanoclusters and nanodeposits. Implications for engineering the microstructure of non-oxide ceramics with diamond nanodispersoids are discussed.

Research paper thumbnail of Sliding-wear resistance of liquid-phase-sintered SiC containing graphite nanodispersoids

Journal of the European Ceramic Society, 2014

The influence was investigated of a graphite nanodispersoid addition on the lubricated sliding-we... more The influence was investigated of a graphite nanodispersoid addition on the lubricated sliding-wear behaviour of liquid-phase-sintered (LPS) SiC ceramics fabricated by spark-plasma sintering (SPS). The graphite nanodispersoids, introduced into the microstructure of the LPS SiC ceramic to act as self-lubricating phase, were obtained by graphitization of diamond nanoparticles during the SPS. It was found that the graphite nanodispersoid addition results in a lower resistance to mild wear, attributable to the lower hardness of graphite and the null lubrication it provides. Moreover, the graphite nanodispersoids, which mostly locate at grain boundaries/faces, worsen the cohesion of SiC grains, contributing together with the higher mild-wear rate to an early transition to the severe-wear stage. On the contrary, the graphite nanodispersoids were effective at improving the resistance to severe wear because they increase the fracture toughness while providing some external lubrication. Relevant implications for the microstructural design of advanced triboceramics are discussed.

Research paper thumbnail of Microstructural effects on the sliding-wear resistance of pressureless liquid-phase-sintered SiC under diesel fuel

Journal of the European Ceramic Society, 2013

The sliding wear of pressureless liquid-phase-sintered (PLPS) SiC ceramics under diesel fuel lubr... more The sliding wear of pressureless liquid-phase-sintered (PLPS) SiC ceramics under diesel fuel lubrication was investigated. It was found that the sliding-wear behaviour under diesel fuel is similar to what is observed under lubrication with the chemically similar paraffin oil: initial mild, plasticity-controlled wear followed by severe, fracture-controlled wear, with a well-defined wear transition. Compared to paraffin oil, however, diesel fuel lubrication decreases the extent of the mild-wear regime and results in damage that is more severe due to its lower viscosity. Also, it was found that the wear resistance under diesel fuel decreases with increasing contact load and sliding speed. Subsequent investigation of the effects of the intergranular phase content, grain size, and grain shape demonstrates nevertheless that the wear performance of PLPS SiC under diesel lubrication can be improved via microstructural control by (i) decreasing the intergranular phase content, and (ii) decreasing the SiC grain size or increasing its aspect ratio. The results of this work are likely to have relevant implications for the design of wear resistant PLPS SiC ceramic components for diesel engines.

Research paper thumbnail of Effect of Microstructure on Sliding-Wear Properties of Liquid-Phase-Sintered α-SiC

Journal of the American Ceramic Society, 2005

We have studied, for the first time, the effect of the content of intergranular phase and grain s... more We have studied, for the first time, the effect of the content of intergranular phase and grain size on the sliding-wear resistance of pressureless, liquid-phase-sintered (LPS) a-SiC (silicon carbide) ceramics. It was found that the sliding-wear behavior of these ceramics is similar to what is observed in other polycrystalline ceramics: initial mild, plasticity-controlled wear followed by severe, fracture-controlled wear, with well-defined wear transition. We have found that the increase in the content of intergranular phase and the grain coarsening leads to the degradation of the sliding-wear resistance in LPS a-SiC ceramics. A mechanistic model is used to rationalize these wear results, and to provide guidelines for the design and fabrication of low cost, highly wearresistant SiC-based ceramics. This is likely to have important implications because SiC-based materials are being used increasingly in contact-mechanical and tribological applications.

Research paper thumbnail of Microstructural Evolution in Liquid‐Phase‐Sintered SiC: Part I, Effect of Starting Powder

Journal of the American Ceramic Society, 2004

The effect of starting SiC powder (β‐SiC or α‐SiC), with simultaneous additions of Al2O3 and Y2O3... more The effect of starting SiC powder (β‐SiC or α‐SiC), with simultaneous additions of Al2O3 and Y2O3, on the microstructural evolution of liquid‐phase‐sintered (LPS) SiC has been studied. When using α‐SiC starting powder, the resulting microstructures contain hexagonal platelike α‐SiC grains with an average aspect ratio of 1.4. This anisotropic coarsening is consistent with interface energy anisotropy in α‐SiC. When using β‐SiC starting powder, the β→α phase transformation induces additional anisotropy in the coarsening of platelike SiC grains. A strong correlation between the extent of β→α phase transformation, as determined using quantitative XRD analysis, and the average grain aspect ratio is observed, with the maximum average aspect ratio reaching 3.8. Based on these observations and additional SEM and TEM characterizations of the microstructures, a model for the growth of these high‐aspect‐ratio SiC grains is proposed.

Research paper thumbnail of High‐Energy Ball Milling of ZrB2 in the Presence of Graphite

Journal of the American Ceramic Society, 2010

The effect of graphite addition on the high‐energy ball‐milling behavior of zirconium diboride (Z... more The effect of graphite addition on the high‐energy ball‐milling behavior of zirconium diboride (ZrB2) powder is investigated. It is shown that, regardless of presence or absence of graphite during ball milling, comminution occurs by repeated brittle fracture followed by cold‐welding, thereby resulting in the formation of agglomerates comprising primary particles of 10 nm in average diameter. However, addition of 2 wt% graphite leads to the formation of nanometer agglomerates (∼50 nm) rather than the submicrometer counterparts (∼120 nm) formed in the absence of graphite. The simultaneous attainment of reduced agglomerate size and homogeneous graphite dispersion at the nanometer scale can have important implications for the ultra‐high‐temperature ceramics community because of the expected twofold enhancement in the sintering kinetics of the compacts made from these nanoscale ZrB2 powders and the simplification of the processing routine.

Research paper thumbnail of Creep and Microstructural Evolution at High Temperature of Liquid-Phase-Sintered Silicon Carbide

Journal of the American Ceramic Society, 2007

The compressive creep characteristics at 16251C of liquidphase-sintered silicon carbide ceramics ... more The compressive creep characteristics at 16251C of liquidphase-sintered silicon carbide ceramics containing 5 and 15 wt% of crystalline Y 3 Al 5 O 12 (YAG) as the secondary phase were studied. In the two cases, strains between 10% and 15% were reached without failure. The creep behavior was characterized by a stress exponent n% 2, and the proportion of secondary phase was related to the creep resistance of the materials. The microstructural evolution during creep consisted firstly in the redistribution of the secondary phase, probably as a consequence of its viscous flow at the creep conditions, and secondly an extensive nucleation and growth of cavities, which was more important for the highest YAG content. The latter reflects the carbothermal reduction that the secondary phase undergoes during creep.