G. Antou, R. Belon, M. Georges, N. Pradeilles, A. Maître Spark plasma sintering of boron carbide ceramics: densification mechanisms and thermomechanical properties  Thank you Mr Chairman. This talk deals with « the study of the densification mechanisms and thermomechanical properties of spark plasma sintered boron carbide ceramics ». This work has been conducted at the IRCER Institute of the University of Limoges in France. G. Antou, R. Belon, M. Georges, N. Pradeilles, A. Maître University of Limoges, IRCER - UMR CNRS 7315, France

Outline 1. Boron carbide: properties and applications - UHTCs 2. Spark Plasma Sintering ability of B4C-based ceramics a. Commercial raw powder b. Sintering map c. Densification mechanism 3. Thermomechanical properties of B4C-based ceramics a. Effect of the impurity content at RT b. Mechanical behavior up to 1600°C 4. Conclusions and perspectives This talk is divided in four parts : At first, I would like to briefly introduce the main physical properties and main applications of B4C, which belongs to the Ultra High Temperature Ceramics’ class; Then, the “sintering ability” of B4C ceramics is studied using the SPS process. The applied sintering conditions have been optimized to obtain fully-dense specimens. The involved “densification mechanisms” have been investigated. In the third part, “the thermomechanical properties of the sintered B4C ceramics” have been characterized at RT and at HT (up to 1600°C); Finally, “some conclusions and perspectives” of this work will be given.

1. Boron carbide: properties and applications -UHTCs Physico-chemical properties B4C Melting point (°C) 2445 ± 100 Density (g/cm3) 2.45 – 2.52 Vickers hardness (GPa) 32 - 37 Mechanical strength (MPa) 350 - 450 Civil application: nuclear control rods (mechanical resistance at HT, neutron absorption) Military application : armor Shock resistance and low density B4C exhibits good “physico-chemical properties” like: high meting point, high hardness, good mechanical strength. Regarding these properties, B4C can be used in various “applications”: for instance, as “control rods in the next generation of nuclear reactors, thanks to its good neutron absorption and mechanical resistance at HT”; B4C is also applied in “armor systems, thanks to its high shock resistance and low density”.

2. Spark Plasma Sintering ability of B4C-based ceramics a. Commercial raw powder Supplier H.C. Starck Reference HD 20 grade Particle size (vol) D[50] : 0.4 µm Specific surface 24.4 m2/g sub-micron elementary particles ( 0.2-0.4 µm) + agglomerates of a few μm BxC graphite HD20 presence of Cfree under the graphite form + H3BO3 in surface In this work, the sintering behavior of « a commercial raw powder supplied by HC Stark » is studied using the « SPS process ». This is a submicronic powder exhibiting « sub-micron elementary particles with size ranged from 0.2 to 0.4 µm, and exhibiting also some agglomerates of a few μm”. XRD and TEM analyses reveal the presence of impurities, that is to say “the presence of Cfree under the graphite form and boric acid at the surface of the carbide particle”.

2. Spark Plasma Sintering ability of B4C-based ceramics a. Commercial raw powder Determination of the stoichiometry of the BxC phase from the abacus of the literature [1] Chemical dosing of Cfree by standard addition method [2] (applicable method if C under the graphite form with C wt.% < 4%) HD: B4.48C 0.56 wt.% Cfree In order to refine “the chemical composition of this powder”: “the stoichiometry of the BxC phase” has been identified by determining the cell parameters of the BxC phase (through the method of Le Bail), and using “the abacus established in the literature by Aselage”. It leads to a Boron over Carbon ratio around 4.5. The amount of Cfree has been also identified by “chemical dosing through the standard addition method”. The amount of Cfree is around 0.56 wt. %. Chemical composition B4.48C + 0.56 wt.% Cfree + H3BO3 [1] T. Aselage et al., J. Am. Ceram. Soc. 75 [8] (1992) 2207-2212. [2] M. Beauvy et al., J. Common Met. 80 (1981) 227–233.

2. Spark Plasma Sintering ability of B4C-based ceramics b. Sintering map Top water cooled electrode Applied stress (MPa) Temperature (°C) Time (min) Total enclosure Punches in graphite B4C powder grahite felt Bottom water cooled electrode device: Dr. Sinter SPS 825 (Syntex) SPS process = high densification rates, inhibiting grain growth effect  high heating rate, and application of an uniaxial load The sintering behavior of this commercial powder is studied using « a Dr Sinter-type SPS device, supplied by Syntex ». « The SPS process » promotes « high densification rates and inhibites grain growth effect », thanks to « the application of high heating rate and application of an uniaxial load ». « The applied sintering conditions » are the following: dwell temperature ranged from … soaking time between… applied stress ranged from… Applied sintering conditions: - die temperature (1450-1750ºC) - soaking time (5-20 min) - applied uniaxial stress (50-100 MPa)

2. Spark Plasma Sintering ability of B4C-based ceramics b. Sintering map Grain size (µm) “A sintering map” has been established thanks to the conducted SPS experiments. It gives the evolution of the mean grain size as a function of the relative density. It shows that, when densifying this powder by SPS, “grain growth is limited to the ultimate stage of sintering, that is to say to relative density > 97%”. Relative density (%) Grain growth during the ultimate stage of sintering ( > 97%).

2. Spark Plasma Sintering ability of B4C-based ceramics c. Identification of densification mechanism Micromechanical models of nonlinear-viscous flow of porous solids (reduced equation) 𝜃 =−𝐹(𝜃) 𝜎 𝑧 𝑛 𝑑 𝑝 𝑘𝑇 exp⁡ −𝑄 𝑅𝑇 (in case of pressing in a rigid die) n: stress exponent p: grain size exponent Q: apparent activation energy viscoplastic parameters related to densification mechanism : porosity level F(): stress intensity factor z: uniaxial applied macroscopic stress T: temperature d: grain size Application to boron carbide Used of an expression of F() suggested by Wei et al. [1] based on the rheological model of Olevsky [2] 𝜃 =− 3𝜃 2 𝑛+1 2 1−𝜃 1−3𝑛 2 𝜎 𝑧 𝑛 𝐴 0 𝑇 exp⁡ −𝑄 𝑅𝑇 The “densification mechanism” involved during the SPS treatment of this B4C powder has been investigated. Whatever the considered “micromechanical models of nonlinear-viscous flow of porous solids”, it reduces to the following form “in the case of pressing in a rigid die”. It gives the densification kinetic as a function of: “viscoplastic parameters related to densification mechanism” (that is to say “n, p and Q”) and “F, a stress intensity factor” that describes the evolution of the mechanical response of the porous body with the porosity level, In this study, we used “an expression of F suggested by Wei and based on the rheological model of sintering of Olevsky”. So “the applied methodology” consists in: “determining Q by … “determining n by … Applied methodology: determination of Q by measuring 𝜃 for 3 dwell temperatures determination of n by considering the previous equation (using least squares fitting) [1] X. Wei et al., Materials. 8 (2015) 6043–6061. [2] E.A. Olevsky, Mater. Sci. Eng. R Rep. 23 (1998) 41–100.

2. Spark Plasma Sintering ability of B4C-based ceramics c. Identification of densification mechanism Identification of Q Under isobar conditions: analysis of shrinkage rates at fixed porosity level for different dwell temperatures: 𝑙𝑛 𝑇. 𝜃 =− 𝑸 𝑅𝑇 +𝐾  = 82.5 %  = 82.5 % Relative density (%) 𝒍𝒏 𝑻. 𝜽 Q has been identified “under isobar conditions”, by “analyzing shrinkage rates at fixed porosity level for different dwell temperatures ranged from 1465 to 1525°C”. “An activation energy around 100 kJ/mol has been determined”. Q = 112 ± 20 kJ.mol-1 Dwell time (s) -1/RT

2. Spark Plasma Sintering ability of B4C-based ceramics c. Identification of densification mechanism Identification of n Under isothermal condition: least squares fitting with 𝜃 =− 3𝜃 2 𝒏+1 2 1−𝜃 1−3𝒏 2 𝜎 𝑧 𝒏 𝐴 0 𝑇 exp⁡ −𝑄 𝑅𝑇 n  3.6 Shrinkage rate, 𝜽 (s-1) The stress exponent has been determined “under isothermal condition”, by “fitting” the evolution of the measured “shrinkage rate” with the analytical expression suggested by Wei. It leads to a stress exponent “around 3.6”, which shows “the strong dependence of the densification rate to the applied stress” for this powder during the intermediate stage of sintering. (T=1650°C, z = 75 MPa) Porosity level,  (-) Strong dependence of the shrinkage rate to the applied stress.

2. Spark Plasma Sintering ability of B4C-based ceramics c. Identification of densification mechanism Correlation with structural observations by TEM twins High density of dislocations Several twins within B4C crystals  possible role of twin-dislocation interaction?  low twin spacing (walls at the nanoscale)  effect on dislocation mobility? The identified densification parameters were “correlated with structural observation conducted by TEM”. It reveals: “a high density of dislocations” And “several twins within B4C crystals”, with a “possible twin-dislocation interaction”. Indeed, “the low twin spacing”, which forms walls at the nanoscale” within the crystals, could alter “dislocation mobility”. In conclusion, based on the identified densification parameters and TEM observations, it appears that, “during the intermediate stage of sintering, the deformation mechanism corresponds to “a PLC regime controlled by dislocation mobility (as suggested by the stress exponent value and TEM observations), with a potential twinning effect at HT”. During the intermediate stage of sintering (0.75 <  < 0.9): PLC regime controlled by dislocation mobility (n 3), with a potential twinning effect at HT.

3. Thermomechanical properties of B4C-based ceramics a. Effect of the impurity content at RT [R. Belon et al., Ceram. Int. 43 (2017) 6631–6635] Pre-heat treatment of the raw powder to reduce impurity content (Cfree, oxide phases) (1350°C – 5h – mixture of Ar + 2.9 mol.% H2) raw HT Same microstructure (  98 % - d50  0.6 µm) raw-B4C HT-B4C In addition to the study of sintering mechanism, « the thermomechanical properties of the sintered B4C ceramics » have been investigated. It leads to « a published paper in 2017 ». « The effect of the impurity content at RT » has been studied by applying « a pre-heat treatment to the commercial raw powder in order to reduce the impurity content (that is to say the amounf of Cfree and oxide phases) ». This treatment consists in a dwell « at 1350°C during 5 min », under a controlled atmosphere corresponding to a « mixture of Ar and dihydrogen ». The sintered specimen from both powders (raw- and HT- ones) exhibit « similar microstructure (i.e. high relative density and a d50 around 0.6 µm) », but the heat treatment allows reducing the amount of Cfree by about 50% (from 0.6 to 0.3wt%). Chemical composition (from XRD analyses) raw-B4C B4.48C + 0.56 wt.% Cfree HT-B4C B4.48C + 0.31 wt.% Cfree

3. Thermomechanical properties of B4C-based ceramics [R. Belon et al., Ceram. Int. 43 (2017) 6631–6635] a. Effect of the impurity content at RT raw-B4C (B4.48C + 0.56 wt.% Cfree) HT-B4C (B4.48C + 0.31 wt.% Cfree) HV [GPa] 42.6 ± 3.9 39.6 ± 3.1 KIc [MPa.m1/2] 2.6 ± 0.2 2.3 ± 0.2 (d50 = 0.6 µm -  = 98%) Properties similar to those of Moshtaghioun et al. [1]: (B?C + 2.9 wt.% WB2, d50 = 0.45 µm -  = 98.8%)  HV = 37 GPa (slightly lower)  KIc = 2.9 MPa.m1/2 raw-B4C vs. HT-B4C: heat treatment ( oxide phases and Cfree)  slight reduction of HV ( -7%) and KIc ( -12%) « At RT, the measured hardness and toughness are similar to those of Moshtaghioun for B4C exhibiting similar microstructure (i.e. mean grain size around 0.5 µm and fully-dense specimens) ». If we compare now the raw- and the HTed samples, it appears that « the heat treatment, that induced a reduction the amount of oxide phases and Cfree, finally leads to a slight reduction of hardness by about 7% and toughness by about 12% ». [1] B.M. Moshtaghioun et al., J. Eur. Ceram. Soc. 34 (2014) 841–848.

3. Thermomechanical properties of B4C-based ceramics [R. Belon et al., Ceram. Int. 43 (2017) 6631–6635] b. Mechanical behavior up to 1600 C Three-point bending tests (strain rate = 1.5.10-4 s-1, span = 20 mm) In addition, three-point bending tests have been conducted up to 1600°C. At RT, it reveals that « heat treatment allows a slight reduction of E by 14% and a small increase of r by 8% ». It could be related to the « reduction of free graphite content », which exhibit an « onion-like structure at GB » and could be an area of weakness. However, at HT, both materials exhibit similar elasticity and mechanical strength (around 400 MPa). “onion-like” structure at GB (area of weakness) at RT: heat treatment  slight  E (-14%)  r (+8%)  reduction of Cfree at high temperature: no more difference

3. Thermomechanical properties of B4C-based ceramics [R. Belon et al., Ceram. Int. 43 (2017) 6631–6635] b. Mechanical behavior up to 1600 C Three-point bending tests (strain rate = 1.5.10-4 s-1, span = 20 mm) 1600 °C RT for raw and HT-B4C Now, if we take a look at the stress-strain curve measured during the bending tests, it appears that the mechanical behavior remains purely brittle up to 1600°C. It is « associated with transgranular fracture surfaces”. It means that: “the temperature of brittle-to-plastic transition is higher than 1600 °C” for these fine-grained and fully-dense B4C samples. So “the BPTT is higher than the one determined by Abzianidze around 1500°C for a porous B4C ceramic with a relative density around 94%” elaborated by hot pressing. This lower BPTT of Abzianidze could be related to the porosity level of their samples, “leading to a non-structural response”. Brittle behavior (associated with transgranular fracture surfaces)  temperature of brittle-to-plastic transition > 1600 °C  higher than the BPTT  1500°C determined by Abzianidze et al. [1] for a porous B4C ceramic (=94%, HP), leading to a non-structural response of B4C [1] T.G. Abzianidze et al., J. Solid State Chem. 154 (2000) 191–193.

Conclusions Perspectives 1) Manufacturing of fine-grained and dense B4C ceramics by SPS without sintering additives 2) Identification of the involved densification mechanism at the intermediate stage:  PLC regime controlled by dislocation mobility 3) No strong effect of the Cfree content on the mechanical prop. at RT and HT (for Cfree<1 wt.%)  slight  of E, HV, KIc   of r 4) As shown by the stress-strain curves measured by bending tests at high temperature  no BPT up to 1600°C for these fully-dense SPSed B4C ceramics Perspectives 1) To perform mechanical tests at higher temperatures (> 1600°C) to observe the BPT 2) To analyze the effect of stoichiometry (B/C ratio) on mechanical properties and sintering mechanisms (in progress) So, in conclusion: In this work, « fine-grained and dense B4C ceramics have been manufactured by SPS without sintering additives » « The densification mechanism involved during the intermediate stage of sintering have been determined, and corresponds to « a PLC regime controlled … » As concerns the thermomechanical properties of B4C, it is shown that « there is no significant effect of the Cfree content on the mechanical properties at RT and HT for the considered amount lower than 1 wt.%, with a slight decrease of … but a small increase of…” Moreover, “as shown by the stress-strain curves…”, “no BTP transition is revealed up to 1600°C” for this submicronic and fully-dense B4C ceramics elaborated by SPS. “The perspectives to this work” will consist in: “performing mechanical tests at….” “also analyzing the effect of …”. These experiments are “in progress”.

Thanks for your attention Financial support through PF NEEDS “Matériaux” Thanks for your attention  G. Antou *, R. Belon, M. Georges, N. Pradeilles, A. Maître IRCER Institute (UMR CNRS 7315), Univ. of Limoges, France * guy.antou@unilim.fr

2. Spark Plasma Sintering ability of B4C-based ceramics c. Identification of densification mechanism Discussion about the identified densification parameters Compressive creep (dense state) SPS (porous state) Viscoplastic parameters Moshtaghioun et al. [1] (at 1600-1800°C, 250-500 MPa) Abzianidze et al.[2] (at 1650-1800°C, 20-110 MPa) This work (at 1465-1525°C, 75 MPa) n (-) 3 3.6 Q (kJ.mol-1) 632 385 112 n  3-4  viscoplastic flow controlled by dislocation mobility Different values of Q : porous state vs. dense state Moshtaghioun[1] : presence of a secondary phase WB2 (2.9 wt.%) at GB (diffusion mechanism) Abzianidze[2] : no information on P02 during creep (oxidation  stoichiometry?) In the literature, this work is the first one dealing with the SPS densification mechanism of B4C ceramic. The densification parameters identified here can be compared with “compressive creep tests performed on dense specimens”. As concerns “n”, similar values around 3 have been also determined during “compressive creep experiments at similar temperatures”. It suggests “a viscoplastic flow controlled by dislocation mobility”. As concerns “Q”, “very different values of Q were identified in the literature”. This difference could be related to: The fact that the B4C materials are not under the same state: “dense in the case of creep” and “porous in the case of sintering experiments”, which could alter the involved deformation mechanism (the atomic flux field and the path length for diffusion can be affected by the porous state) Moreover, the values determined by “Moshtaghioun can be altered by the presence of a secondary phase” at the grain boundary, coming from a milling step of the powder and which could modify the diffusion mechanism In the case of “Abzianidze, no information on the oxygen partial pressure during creep” is given. Oxygen traces could induce “oxidation phenomenon” and modify the “stoichiometry of carbide”. [1] B.M. Moshtaghioun et al., J. Eur. Ceram. Soc. 35 (2015) 1423–1429. [2] T.G. Abzianidze et al., J. Solid State Chem. 154 (2000) 191–193.