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Vickers microhardness and indentation fracture toughness of tantalum sesquinitride, η-Ta2N3
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CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 982–985 www.elsevier.com/locate/ceramint
Short communication
Vickers microhardness and indentation fracture toughness of tantalum sesquinitride, η-Ta2N3 Judith Bourguille, Ovidiu Brinza, Andreas Zerrn Laboratoire des Sciences des Procédés et des Matériaux (LSPM) – Centre National de la Recherche Scientifique (CNRS), Université Paris 13, 99 Avenue J. B. Clément, 93430 Villetaneuse, France Received 29 June 2015; received in revised form 29 July 2015; accepted 20 August 2015 Available online 2 September 2015
Abstract Vickers indentation testing was applied to examine an additive-free monolithic sample of tantalum sesquinitride having orthorhombic U2S3 type structure, η-Ta2N3, synthesised at high pressures and temperatures. Crystallisation of η-Ta2N3 in form of elongated crystals and a high bulk to shear modulus ratio of 2.3–2.6, considered as a qualitative criterion of malleability, suggest elevated fracture toughness for this ceramic material. Here we measured Vickers hardness, HV, indentation size effect, and fracture toughness, KIc-if, as well as their dependences on the surface self-healing (densification) due to mechanical polishing for a sample with the residual porosity of p ¼0.14. HV of η-Ta2N3 was found to exhibit a moderate indentation size effect and approach, for the porous sample, HV(1) ¼16(1) GPa at the maximal load. KIc-if was measured to be 4.6(2) MPa m1/2, comparable with the values reported for hexagonal Si3N4 and SiC. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Hardness; C. Toughness and toughening; D. Nitrides
1. Introduction Tantalum sesquinitride having orthorhombic U2S3 type structure, η-Ta2N3, is the first thermodynamically stable transition metal nitride with the nitrogen to metal ratio (N: M) exceeding 4:3 [1]. This material, discovered recently using high pressure–high temperature (HP–HT) techniques [1,2], exhibits the same structure as binary selenides and tellurides of rare earth metals with atomic numbers between 60 and 66 which are denoted, according to a general systematics covering all sulphides, selenides and tellurides of these metals, as ηphases [3]. The systematics covers also HP–HT nitrides of Zr and Hf (members of the group 4 elements having lower atomic numbers than group 5 elements) crystallising in Th3P4 type structure [4,5] which was also observed for a number of selenides and tellurides of rare earth metals with lower atomic numbers, from 57 to 62 [3].
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Corresponding author. Tel.: þ33 149403493; fax: þ 33 149403938. E-mail address: [email protected] (A. Zerr).
http://dx.doi.org/10.1016/j.ceramint.2015.08.130 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
As any other HP–HT ceramic material, η-Ta2N3 was expected to exhibit enhanced elastic and mechanical properties. Bulk and shear moduli of dense η-Ta2N3 measured to be B¼ 280–320 GPa and G¼ 123 GPa, respectively [2,6], confirmed the expectations and suggested a high Vickers hardness, HV, approaching 30 GPa. This estimation is based on the earlier established nearly linear correlation of hardness and shear modulus [7–9]. Preliminary measurements yielded for a porous sample of η-Ta2N3 HV(0.5)=16 GPa [1] but the initially estimated porosity of 0.1 was significantly underestimated due to the so called self-healing effect taking place by mechanical polishing [6]. This effect manifests itself in closing of pores in the upper sample layer which depth ( 0.3 μm) is comparable with size of the diamond grains used at the last step of polishing (0.25 μm) [6]. The healing mechanism and nature of the mobile phase by mechanical polishing of η-Ta2N3 are still not determined unambiguously. However, a relatively high bulk to shear modulus ratio of B/G¼ 2.3–2.6 suggests, according to the approach developed for metals by Pugh [7] and extended to ceramics by Gandhi and Ashby [10], a malleability comparable to that of such metals as tungsten or molybdenum. Such degree
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of malleability could be sufficient to cause closing of small pores by mechanical polishing of porous η-Ta2N3 and could also enhance its fracture toughness. A high fracture toughness of η-Ta2N3 was also expected due to crystallisation of η-Ta2N3 in form of elongated crystals with a high aspect ratio [1]. Such crystal form is known to enhance fracture toughness of such ceramic materials as β-Si3N4 or β-SiC. In this work the indentation fracture toughness KIc-if of a monlithic sample of η-Ta2N3 having residual porosity p¼ 0.14 (4) was measured for the first time. In order to have a reliable HV value needed for KIc-if calculation, indentation size effect, e.g. a systematic decrease of HV with load [11], was examined.
2. Material and methods The monolithic sample of η-Ta2N3 examined in this work was synthesised at P ¼ 11 GPa and T ¼ 1500 1C using a multianvil HP–HT apparatus [1]. Powder of Ta3N5, compacted in a platinum capsule, served as the starting material. After recovery to the ambient conditions, the product composition was accurately determined to be Ta1.94(N0.95O0.05)3. Moreover, the sample was found to be chemically homogeneous, free of any other amorphous or crystalline intergranular phase. A detailed description of the sample synthesis, crystallographic and chemical characterisation is given elsewhere [1]. The recovered cylindrical sample of η-Ta2N3 in the Pt capsule of about 1.2 mm in diameter and 1.5 mm in height was mounted into a molybdenum support for further Vickers indentation tests and for surface examination using a scanning electron microscope (SEM) Zeiss Supra 40VP. The SEM was operated in high vacuum mode providing the lateral resolution of 2 nm. From the SEM images we derived porosity of the sample material, sizes of the impressions produced by the Vickers indenter as well as lengths of the cracks emanating from or close to the impression corners. Indentations were performed with loads, F, between 0.098 N (10 g) and 9.8 N (1 kg) using the microhardness testers Duramin-X (Struers, Denmark) and FM-700 (Future-Tech Corp., Japan) with the aim to examine the indentation size effect. In all indentation tests the loading time was fixed at 10 s. The sample was first mechanically polished with diamond abrasives which grain size reduced to 0.25 μm at the final step. SEM images of the obtained surface, collected with magnifications between 10k and 40k, showed the apparent porosity of p ¼ 0.03(2) (Fig. 1a). Then the mechanically modified surface layer was removed by a treatment with an Ar þ -iron beam (GATAN Precision Etching Coating System) for 15 min. A good surface quality was obtained when the rotated sample was tilted by 70–801 with respect to the Ar þ -ion beam of 400 mA accelerated by 4–5 kV. The treated surface showed a significantly higher porosity of p ¼ 0.14(4) due to mesoscopic pores with dimensions down to 10 nm (Fig. 1b) which were not visible after the mechanical polishing (Fig. 1a). The main source of the mesoscopic pores with sizes below 100 nm appears to be the nitrogen fluid forming upon HP–HT
Fig. 1. SEM images of the η-Ta2N3 sample surfaces: (a) after mechanical polishing, (b) after Ar þ -ion etching, (c) untreated cleavage surface. The images (a) and (b) were collected with the magnification of 40k.
synthesis when the starting Ta3N5 with a higher nitrogen content decomposes to η-Ta2N3 with a lower nitrogen content. SEM images of the cleaved sample surface (Fig. 1c) showed that such pores mostly appear inside η-Ta2N3 grains having sizes of 0.5–1 μm. The mesoscopic pores are spherical/elliptical and separated from each other thus forming closed porosity which cannot be accessed by intrusion porosimetry or similar techniques. Volume shrinkage due to the transition and a high hardness of η-Ta2N3 also contributed to pores formation because the uniform compression geometry of the multi-anvil apparatus does not favour plastic deformation of products at HP–HT conditions even when heated to 1500 1C. 3. Results and discussion Vickers indentations were performed on both mechanically polished- and Ar þ -ion etched sample surfaces (Fig. 2) and only a week indentation size effect was recognised (Fig. 3). At loads o 4.9 N the HV-data scatter but at higher loads the
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HV (GPa)
20
18
16
14
0
2
4
6
8
10
Load (N)
Fig. 2. SEM image of the Vickers impression produced with the load of 4.9 N on the η-Ta2N3 sample polished mechanically. Lengths of the cracks emanating from or close to the imprint corners were used to determine KIc-if.
scattering vanishes and the values become almost identical and equal to 16(1) GPa. This value was used below for determination of KIc-if of our η-Ta2N3 sample. It is well known that hardness decreases rapidly with porosity and an exponential law was suggested to describe the dependence: H¼ H0 exp(bp) where b is a material constant, p porosity, H and H0 are hardness of a porous and dense specimen, respectively [11,12]. Analysis of a wide range of hardness data for ceramic materials showed that b varies between 3 and 7 [12– 15]. Assuming for our sample (with p=0.14(4)) the average value of b=5 we predict for the dense η-Ta2N3 HV(1)E32 GPa. This estimation is supported by the available data for δ-TaN, the hardest member of the family of transition metal mononitrides: for a sample having porosity of p=0.05 HV(1)=20.9 GPa was reported [16]. Taking this HV value and applying the same material constant b¼ 5 we calculated for the dense δ-TaN HV ¼ 27 GPa, in good agreement with the experimental values of HV ¼ 25–30 GPa [17]. The estimated Vickers hardness for dense η-Ta2N3 of HV(1)¼ 32(7) GPa is similar to that of γSi3N4 measured to be 33–37 GPa [18]. Thus, η-Ta2N3 belongs to the family of novel high-pressure nitrides with a nitrogen-tometal ratio exceeding unity which exhibit microhardness HV above 30 GPa and compete for the rank of the third hardest material after diamond and cubic BN [18–20]. Vickers indentations obtained with the loads F Z 4.9 N, where the HV values become constant and independent of the load, were used to determine KIc-if from length of cracks emanating from or close to the impression corners (Fig. 2) using the procedure described elsewhere [21,22]. At the first stage the crack under-surface geometry was evaluated. The most typical are radial/median cracks (mostly having the halfpenny geometry) or radial (Palmqvist) cracks [23] which require different analytical expressions to calculate KIc-if. In order to assess the cracks geometry we analysed dependences of the distances from the indentation centre to the crack end (c) on F. For Palmqvist cracks the dependence c/F on F 1/2 should be linear [24] while for half-penny cracks c/F should depend linearly on F 1/3 [22]. In some series of our indentation tests, considered separately, we could recognise a linear
Fig. 3. Vickers microhardness (diamonds) of our η-Ta2N3 sample as a function of the indentation load. Triangles represent results of our nanoindentation tests [31]. Solid symbols indicate data for the mechanically polished- and open symbols for the Ar þ -ion etched surface.
dependence of c/F on F 1/2 but the data from different sets scattered. Other criteria considered in the literature supported formation of Palmqvist cracks in our experiments: Miyazaki and co-authors proposed that if the ratio c/a 42 (a represents the indentation flank length) then the cracks should have the half-penny geometry [25]. A similar conclusion was made by Lube [26] but for c/a 42.2. According to the latter author, Palmqvist cracks should form when c/a o 1.8 while for the values 1.8o c/a o 2.2 cracks geometry cannot be determined unambiguously. For our η-Ta2N3 sample we always found that c/a o 2 and in most cases closer to 1.5. Obviously, the criterion of Niihara et al. [27] for Palmqvist cracks that l/ ao 2.5 (l ¼ c a) was also fulfilled. Accordingly, KIc-if values were calculated here using an expression developed for Palmqvist cracks [24]: KIc-if ¼ β (HVF / 4l) 1/2, where β ¼ 1 / [3π1/2(1 ν2)(21/2 π tanψ)1/3] is a geometry factor calculated to be β¼ 0.092 using ψ¼ 681, for the Vickers pyramid, and Poisson's ratio ν ¼ 0.29 for our porous η-Ta2N3 [6]. An advantage of this expression is that only Poisson's ratio of the tested material is needed to derive KIc-if. The equation of Niihara et al. [27] requires also Young's modulus which was found to be much stronger biased by porosity than ν [6]. KIc-if of our η-Ta2N3 sample was found to be 4.6(2) MPa m1/2 independent of the surface state. This value approaches KIc-if of the densified/sintered monoliths of β-Si3N4 and β-SiC as well as of their composites considered as promising tough ceramics: hot-pressed Si3N4 sintered with rear-earth oxides RE2O3 (RE ¼ La, Nd, Y, Yb, and Lu) exhibits KIc-if ¼ 5.2– 7.1 MPa m1/2 and addition of 5vol% of SiC resulted in KIc-if ¼ 4.3–5.4 MPa m1/2 [28]. A similar value of KIc1/2 was reported for hot-pressed Si3N4 sintered if ¼ 5.2 MPa m with 5 wt% of Y2O3 while addition of 8 wt% of SiC leads to KIc-if ¼ 5.4–5.8 MPa m1/2 [29]. Hot-pressed β-SiC sintered with 9 wt% of Y2O3 and Al2O3 was found to exhibit KIc-if between 2.9 and 4.5 MPa m1/2 [30]. We expect that KIc-if of pores-free monoliths of η-Ta2N3 could surpass the value reported here because our nanoindentation tests on the mechanically polished samples, where a contribution of the densified layer to the total sample response is significant, have shown 10–20%
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higher KIc-if than for the sample without the densified layer [31]. 4. Conclusion In this work we measured the Vickers hardness and indentation fracture toughness of a binder-free monolith of η-Ta2N3 having residual porosity p¼ 0.14. The indentation size effect was found to be moderate, and at loads F Z 0.5 kg HV approaches 16(1) GPa. For the fully dense material we expect HV 430 GPa. We have measured for the sample a high fracture toughness of KIc-if ¼ 4.6(2) MPa m1/2 approaching that of densified β-Si3N4, β-SiC, and their composites. Thus, ηTa2N3 is a promising ceramic material exhibiting self-healing behaviour by mechanical polishing combined with a high hardness and fracture toughness. Acknowledgements Technical support of A. Hocini and J.-Ph. Couzinie is acknowledged. References [1] A. Zerr, G. Miehe, J.W. Li, D.A. Dzivenko, V.K. Bulatov, H. Hofer, N. Bolfan-Casanova, M. Fialin, G. Brey, T. Watanabe, M. Yoshimura, High-pressure synthesis of tantalum nitride having orthorhombic U2S3 structure, Adv. Funct. Mater. 19 (2009) 2282–2288. [2] A. Friedrich, B. Winkler, L. Bayarjargal, E.A.J. Arellano, W. Morgenroth, J. Biehler, F. Schroder, J.Y. Yan, S.M. Clark, In situ observation of the reaction of tantalum with nitrogen in a laser heated diamond anvil cell, J. Alloy. Compd. 502 (2010) 5–12. [3] J. Flahaut, L. Domange, M. Guittard, M.P. Pardo, Étude cristallographique des séléniures et des tellurures des terres rares orthorhombiques, de type U2S3, Bull. Soc. Chim. Fr. (1) (1965) 326–327. [4] A. Zerr, G. Miehe, R. Riedel, Synthesis of cubic zirconium and hafnium nitride having Th3P4 structure, Nat. Mater. 2 (2003) 185–189. [5] D.A. Dzivenko, A. Zerr, V.K. Bulatov, G. Miehe, J.W. Li, B. Thybusch, J. Brötz, H. Fuess, G. Brey, R. Riedel, High-pressure multi-anvil synthesis and structure refinement of oxygen-bearing cubic zirconium (IV) nitride, Adv. Mater. 19 (2007) 1869–1873. [6] A. Zerr, N. Chigarev, O. Brinza, S.M. Nikitin, A.M. Lomonosov, V. Gusev, Elastic moduli of η-Ta2N3, a tough self-healing material, via laser ultrasonics, Phys. Status Solidi-Rapid Res. Lett. 6 (2012) 484–486. [7] S.F. Pugh, Relations between the elastic moduli and the plastic properties of polycrystalline pure metals, Philos. Mag. Ser. 45 (1954) 823–843. [8] D.M. Teter, Computational alchemy: the search for new superhard materials, MRS Bull. 23 (1998) 22–27. [9] A. Zerr, R. Riedel, Introduction: novel ultrahard materials, in: R. Riedel (Ed.), Handbook of Ceramic Hard Materials, Wiley-VCH, Weinheim, 2000, pp. XLV–LXXVII. [10] C. Gandhi, M.F. Ashby, Fracture-mechanism maps for materials which cleave: fcc, bcc and hcp metals and ceramics, Acta Metall. 27 (1979) 1565–1602.
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[11] I.J. McColm, Ceramic Hardness, Plenum Press, New York, 1990. [12] R.W. Rice, Porosity of Ceramics, Marcel Dekker, New York, 1998. [13] J. Adachi, K. Kurosaki, M. Uno, S. Yamanaka, Porosity influence on the mechanical properties of polycrystalline zirconium nitride ceramics, J. Nucl. Mater. 358 (2006) 106–110. [14] S.A. Cho, I.B. de Arenas, F.J. Arenas, J. Ochoa, J.L. Ochoa, Porosity; microhardness correlation of sintered (Al1 yCry)2O3 solid solutions, J. Alloy. Compd. 288 (1999) 211–216. [15] J. Luo, R. Stevens, Porosity-dependence of elastic moduli and hardness of 3Y-TZP ceramics, Ceram. Int. 25 (1999) 281–286. [16] H. Yusa, F. Kawamura, T. Taniguchi, N. Hirao, Y. Ohishi, T. Kikegawa, High-pressure synthesis and compressive behavior of tantalum nitrides, J. Appl. Phys. 115 (2014) 103520. [17] C. Politis, G. Rejman, Zum Aufbau der Systeme Nb-N und Ta-N, in: H. Holleck (Ed.), Aufbau, Herstellung und Eigenschaften hochschmelzender Verbindungen und Systeme (Hartstoffe und hartmetalle), Kernforschungszentrum Karlsruhe, Karlsruhe, 1978, pp. 102–123 – Jahresbericht 1977 – (KfK-Ext. 6/78–1). [18] J.Z. Jiang, F. Kragh, D.J. Frost, K. Stahl, H. Lindelov, Hardness and thermal stability of cubic silicon nitride, J. Phys.: Condens. Matter 13 (2001) L515–L520. [19] A. Zerr, M. Kempf, M. Schwarz, E. Kroke, M. Göken, R. Riedel, Elastic moduli and hardness of cubic silicon nitride, J. Am. Ceram. Soc. 85 (2002) 86–90. [20] A. Zerr, N. Chigarev, R. Brenner, D.A. Dzivenko, V. Gusev, Elastic moduli of hard c-Zr3N4 from laser ultrasonic measurements, Phys, Status Solidi-Rapid Res. Lett. 4 (2010) 353–355. [21] A.G. Evans, E.A. Charles, Fracture toughness determinations by indentation, J. Am. Ceram. Soc. 59 (1976) 371–372. [22] M. Sakai, R.C. Bradt, Fracture-toughness testing of brittle materials, Int. Mater. Rev. 38 (1993) 53–78. [23] R.F. Cook, G.M. Pharr, Direct observation and analysis of indentation cracking in glasses and ceramics, J. Am. Ceram. Soc. 73 (1990) 787–817. [24] D.K. Shetty, I.G. Wright, P.N. Mincer, A.H. Clauer, Indentation fracture of WC–Co cermets, J. Mater. Sci. 20 (1985) 1873–1882. [25] H. Miyazaki, H. Hyuga, Y. Yoshizawa, K. Hirao, T. Ohji, Crack profiles under a Vickers indent in silicon nitride ceramics with various microstructures, Ceram. Int. 36 (2010) 173–179. [26] T. Lube, Indentation crack profiles in silicon nitride, J. Eur. Ceram. Soc. 21 (2001) 211–218. [27] K. Niihara, R. Morena, D.P.H. Hasselman, Evaluation of Klc of brittle solids by the indentation method with low crack-to-indent ratios, J. Mater. Sci. Lett. 1 (1982) 13–16. [28] P. Tatarko, M. Kasiarova, J. Dusza, P. Sajgalik, Effect of various grain boundary phases and SiC addition on the fracture characteristics of Si3N4 based ceramics, Chem. Listy 105 (2011) S850–S851. [29] M. Balog, J. Keckes, T. Schoberl, D. Galusek, F. Hofer, J. Krest'an, Z. Lences, J.L. Huang, P. Sajgalik, Nano/macro-hardness and fracture resistance of Si3N4/SiC composites with up to 13 wt% of SiC nanoparticles, J. Eur. Ceram. Soc. 27 (2007) 2145–2152. [30] A. Kovalcikova, J. Dusza, P. Sajgalik, Thermal shock resistance and fracture toughness of liquid-phase-sintered SiC-based ceramics, J. Eur. Ceram. Soc. 29 (2009) 2387–2394. [31] J. Bourguille, O. Brinza, F. Tétard, S. Nikitin, A. Zerr, Elastic moduli and hardness of η-Ta2N3 from nanoindentation measurements, EPL 111 (2015) 18006.
CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 982–985 www.elsevier.com/locate/ceramint
Short communication
Vickers microhardness and indentation fracture toughness of tantalum sesquinitride, η-Ta2N3 Judith Bourguille, Ovidiu Brinza, Andreas Zerrn Laboratoire des Sciences des Procédés et des Matériaux (LSPM) – Centre National de la Recherche Scientifique (CNRS), Université Paris 13, 99 Avenue J. B. Clément, 93430 Villetaneuse, France Received 29 June 2015; received in revised form 29 July 2015; accepted 20 August 2015 Available online 2 September 2015
Abstract Vickers indentation testing was applied to examine an additive-free monolithic sample of tantalum sesquinitride having orthorhombic U2S3 type structure, η-Ta2N3, synthesised at high pressures and temperatures. Crystallisation of η-Ta2N3 in form of elongated crystals and a high bulk to shear modulus ratio of 2.3–2.6, considered as a qualitative criterion of malleability, suggest elevated fracture toughness for this ceramic material. Here we measured Vickers hardness, HV, indentation size effect, and fracture toughness, KIc-if, as well as their dependences on the surface self-healing (densification) due to mechanical polishing for a sample with the residual porosity of p ¼0.14. HV of η-Ta2N3 was found to exhibit a moderate indentation size effect and approach, for the porous sample, HV(1) ¼16(1) GPa at the maximal load. KIc-if was measured to be 4.6(2) MPa m1/2, comparable with the values reported for hexagonal Si3N4 and SiC. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Hardness; C. Toughness and toughening; D. Nitrides
1. Introduction Tantalum sesquinitride having orthorhombic U2S3 type structure, η-Ta2N3, is the first thermodynamically stable transition metal nitride with the nitrogen to metal ratio (N: M) exceeding 4:3 [1]. This material, discovered recently using high pressure–high temperature (HP–HT) techniques [1,2], exhibits the same structure as binary selenides and tellurides of rare earth metals with atomic numbers between 60 and 66 which are denoted, according to a general systematics covering all sulphides, selenides and tellurides of these metals, as ηphases [3]. The systematics covers also HP–HT nitrides of Zr and Hf (members of the group 4 elements having lower atomic numbers than group 5 elements) crystallising in Th3P4 type structure [4,5] which was also observed for a number of selenides and tellurides of rare earth metals with lower atomic numbers, from 57 to 62 [3].
n
Corresponding author. Tel.: þ33 149403493; fax: þ 33 149403938. E-mail address: [email protected] (A. Zerr).
http://dx.doi.org/10.1016/j.ceramint.2015.08.130 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
As any other HP–HT ceramic material, η-Ta2N3 was expected to exhibit enhanced elastic and mechanical properties. Bulk and shear moduli of dense η-Ta2N3 measured to be B¼ 280–320 GPa and G¼ 123 GPa, respectively [2,6], confirmed the expectations and suggested a high Vickers hardness, HV, approaching 30 GPa. This estimation is based on the earlier established nearly linear correlation of hardness and shear modulus [7–9]. Preliminary measurements yielded for a porous sample of η-Ta2N3 HV(0.5)=16 GPa [1] but the initially estimated porosity of 0.1 was significantly underestimated due to the so called self-healing effect taking place by mechanical polishing [6]. This effect manifests itself in closing of pores in the upper sample layer which depth ( 0.3 μm) is comparable with size of the diamond grains used at the last step of polishing (0.25 μm) [6]. The healing mechanism and nature of the mobile phase by mechanical polishing of η-Ta2N3 are still not determined unambiguously. However, a relatively high bulk to shear modulus ratio of B/G¼ 2.3–2.6 suggests, according to the approach developed for metals by Pugh [7] and extended to ceramics by Gandhi and Ashby [10], a malleability comparable to that of such metals as tungsten or molybdenum. Such degree
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of malleability could be sufficient to cause closing of small pores by mechanical polishing of porous η-Ta2N3 and could also enhance its fracture toughness. A high fracture toughness of η-Ta2N3 was also expected due to crystallisation of η-Ta2N3 in form of elongated crystals with a high aspect ratio [1]. Such crystal form is known to enhance fracture toughness of such ceramic materials as β-Si3N4 or β-SiC. In this work the indentation fracture toughness KIc-if of a monlithic sample of η-Ta2N3 having residual porosity p¼ 0.14 (4) was measured for the first time. In order to have a reliable HV value needed for KIc-if calculation, indentation size effect, e.g. a systematic decrease of HV with load [11], was examined.
2. Material and methods The monolithic sample of η-Ta2N3 examined in this work was synthesised at P ¼ 11 GPa and T ¼ 1500 1C using a multianvil HP–HT apparatus [1]. Powder of Ta3N5, compacted in a platinum capsule, served as the starting material. After recovery to the ambient conditions, the product composition was accurately determined to be Ta1.94(N0.95O0.05)3. Moreover, the sample was found to be chemically homogeneous, free of any other amorphous or crystalline intergranular phase. A detailed description of the sample synthesis, crystallographic and chemical characterisation is given elsewhere [1]. The recovered cylindrical sample of η-Ta2N3 in the Pt capsule of about 1.2 mm in diameter and 1.5 mm in height was mounted into a molybdenum support for further Vickers indentation tests and for surface examination using a scanning electron microscope (SEM) Zeiss Supra 40VP. The SEM was operated in high vacuum mode providing the lateral resolution of 2 nm. From the SEM images we derived porosity of the sample material, sizes of the impressions produced by the Vickers indenter as well as lengths of the cracks emanating from or close to the impression corners. Indentations were performed with loads, F, between 0.098 N (10 g) and 9.8 N (1 kg) using the microhardness testers Duramin-X (Struers, Denmark) and FM-700 (Future-Tech Corp., Japan) with the aim to examine the indentation size effect. In all indentation tests the loading time was fixed at 10 s. The sample was first mechanically polished with diamond abrasives which grain size reduced to 0.25 μm at the final step. SEM images of the obtained surface, collected with magnifications between 10k and 40k, showed the apparent porosity of p ¼ 0.03(2) (Fig. 1a). Then the mechanically modified surface layer was removed by a treatment with an Ar þ -iron beam (GATAN Precision Etching Coating System) for 15 min. A good surface quality was obtained when the rotated sample was tilted by 70–801 with respect to the Ar þ -ion beam of 400 mA accelerated by 4–5 kV. The treated surface showed a significantly higher porosity of p ¼ 0.14(4) due to mesoscopic pores with dimensions down to 10 nm (Fig. 1b) which were not visible after the mechanical polishing (Fig. 1a). The main source of the mesoscopic pores with sizes below 100 nm appears to be the nitrogen fluid forming upon HP–HT
Fig. 1. SEM images of the η-Ta2N3 sample surfaces: (a) after mechanical polishing, (b) after Ar þ -ion etching, (c) untreated cleavage surface. The images (a) and (b) were collected with the magnification of 40k.
synthesis when the starting Ta3N5 with a higher nitrogen content decomposes to η-Ta2N3 with a lower nitrogen content. SEM images of the cleaved sample surface (Fig. 1c) showed that such pores mostly appear inside η-Ta2N3 grains having sizes of 0.5–1 μm. The mesoscopic pores are spherical/elliptical and separated from each other thus forming closed porosity which cannot be accessed by intrusion porosimetry or similar techniques. Volume shrinkage due to the transition and a high hardness of η-Ta2N3 also contributed to pores formation because the uniform compression geometry of the multi-anvil apparatus does not favour plastic deformation of products at HP–HT conditions even when heated to 1500 1C. 3. Results and discussion Vickers indentations were performed on both mechanically polished- and Ar þ -ion etched sample surfaces (Fig. 2) and only a week indentation size effect was recognised (Fig. 3). At loads o 4.9 N the HV-data scatter but at higher loads the
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HV (GPa)
20
18
16
14
0
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4
6
8
10
Load (N)
Fig. 2. SEM image of the Vickers impression produced with the load of 4.9 N on the η-Ta2N3 sample polished mechanically. Lengths of the cracks emanating from or close to the imprint corners were used to determine KIc-if.
scattering vanishes and the values become almost identical and equal to 16(1) GPa. This value was used below for determination of KIc-if of our η-Ta2N3 sample. It is well known that hardness decreases rapidly with porosity and an exponential law was suggested to describe the dependence: H¼ H0 exp(bp) where b is a material constant, p porosity, H and H0 are hardness of a porous and dense specimen, respectively [11,12]. Analysis of a wide range of hardness data for ceramic materials showed that b varies between 3 and 7 [12– 15]. Assuming for our sample (with p=0.14(4)) the average value of b=5 we predict for the dense η-Ta2N3 HV(1)E32 GPa. This estimation is supported by the available data for δ-TaN, the hardest member of the family of transition metal mononitrides: for a sample having porosity of p=0.05 HV(1)=20.9 GPa was reported [16]. Taking this HV value and applying the same material constant b¼ 5 we calculated for the dense δ-TaN HV ¼ 27 GPa, in good agreement with the experimental values of HV ¼ 25–30 GPa [17]. The estimated Vickers hardness for dense η-Ta2N3 of HV(1)¼ 32(7) GPa is similar to that of γSi3N4 measured to be 33–37 GPa [18]. Thus, η-Ta2N3 belongs to the family of novel high-pressure nitrides with a nitrogen-tometal ratio exceeding unity which exhibit microhardness HV above 30 GPa and compete for the rank of the third hardest material after diamond and cubic BN [18–20]. Vickers indentations obtained with the loads F Z 4.9 N, where the HV values become constant and independent of the load, were used to determine KIc-if from length of cracks emanating from or close to the impression corners (Fig. 2) using the procedure described elsewhere [21,22]. At the first stage the crack under-surface geometry was evaluated. The most typical are radial/median cracks (mostly having the halfpenny geometry) or radial (Palmqvist) cracks [23] which require different analytical expressions to calculate KIc-if. In order to assess the cracks geometry we analysed dependences of the distances from the indentation centre to the crack end (c) on F. For Palmqvist cracks the dependence c/F on F 1/2 should be linear [24] while for half-penny cracks c/F should depend linearly on F 1/3 [22]. In some series of our indentation tests, considered separately, we could recognise a linear
Fig. 3. Vickers microhardness (diamonds) of our η-Ta2N3 sample as a function of the indentation load. Triangles represent results of our nanoindentation tests [31]. Solid symbols indicate data for the mechanically polished- and open symbols for the Ar þ -ion etched surface.
dependence of c/F on F 1/2 but the data from different sets scattered. Other criteria considered in the literature supported formation of Palmqvist cracks in our experiments: Miyazaki and co-authors proposed that if the ratio c/a 42 (a represents the indentation flank length) then the cracks should have the half-penny geometry [25]. A similar conclusion was made by Lube [26] but for c/a 42.2. According to the latter author, Palmqvist cracks should form when c/a o 1.8 while for the values 1.8o c/a o 2.2 cracks geometry cannot be determined unambiguously. For our η-Ta2N3 sample we always found that c/a o 2 and in most cases closer to 1.5. Obviously, the criterion of Niihara et al. [27] for Palmqvist cracks that l/ ao 2.5 (l ¼ c a) was also fulfilled. Accordingly, KIc-if values were calculated here using an expression developed for Palmqvist cracks [24]: KIc-if ¼ β (HVF / 4l) 1/2, where β ¼ 1 / [3π1/2(1 ν2)(21/2 π tanψ)1/3] is a geometry factor calculated to be β¼ 0.092 using ψ¼ 681, for the Vickers pyramid, and Poisson's ratio ν ¼ 0.29 for our porous η-Ta2N3 [6]. An advantage of this expression is that only Poisson's ratio of the tested material is needed to derive KIc-if. The equation of Niihara et al. [27] requires also Young's modulus which was found to be much stronger biased by porosity than ν [6]. KIc-if of our η-Ta2N3 sample was found to be 4.6(2) MPa m1/2 independent of the surface state. This value approaches KIc-if of the densified/sintered monoliths of β-Si3N4 and β-SiC as well as of their composites considered as promising tough ceramics: hot-pressed Si3N4 sintered with rear-earth oxides RE2O3 (RE ¼ La, Nd, Y, Yb, and Lu) exhibits KIc-if ¼ 5.2– 7.1 MPa m1/2 and addition of 5vol% of SiC resulted in KIc-if ¼ 4.3–5.4 MPa m1/2 [28]. A similar value of KIc1/2 was reported for hot-pressed Si3N4 sintered if ¼ 5.2 MPa m with 5 wt% of Y2O3 while addition of 8 wt% of SiC leads to KIc-if ¼ 5.4–5.8 MPa m1/2 [29]. Hot-pressed β-SiC sintered with 9 wt% of Y2O3 and Al2O3 was found to exhibit KIc-if between 2.9 and 4.5 MPa m1/2 [30]. We expect that KIc-if of pores-free monoliths of η-Ta2N3 could surpass the value reported here because our nanoindentation tests on the mechanically polished samples, where a contribution of the densified layer to the total sample response is significant, have shown 10–20%
J. Bourguille et al. / Ceramics International 42 (2016) 982–985
higher KIc-if than for the sample without the densified layer [31]. 4. Conclusion In this work we measured the Vickers hardness and indentation fracture toughness of a binder-free monolith of η-Ta2N3 having residual porosity p¼ 0.14. The indentation size effect was found to be moderate, and at loads F Z 0.5 kg HV approaches 16(1) GPa. For the fully dense material we expect HV 430 GPa. We have measured for the sample a high fracture toughness of KIc-if ¼ 4.6(2) MPa m1/2 approaching that of densified β-Si3N4, β-SiC, and their composites. Thus, ηTa2N3 is a promising ceramic material exhibiting self-healing behaviour by mechanical polishing combined with a high hardness and fracture toughness. Acknowledgements Technical support of A. Hocini and J.-Ph. Couzinie is acknowledged. References [1] A. Zerr, G. Miehe, J.W. Li, D.A. Dzivenko, V.K. Bulatov, H. Hofer, N. Bolfan-Casanova, M. Fialin, G. Brey, T. Watanabe, M. Yoshimura, High-pressure synthesis of tantalum nitride having orthorhombic U2S3 structure, Adv. Funct. Mater. 19 (2009) 2282–2288. [2] A. Friedrich, B. Winkler, L. Bayarjargal, E.A.J. Arellano, W. Morgenroth, J. Biehler, F. Schroder, J.Y. Yan, S.M. Clark, In situ observation of the reaction of tantalum with nitrogen in a laser heated diamond anvil cell, J. Alloy. Compd. 502 (2010) 5–12. [3] J. Flahaut, L. Domange, M. Guittard, M.P. Pardo, Étude cristallographique des séléniures et des tellurures des terres rares orthorhombiques, de type U2S3, Bull. Soc. Chim. Fr. (1) (1965) 326–327. [4] A. Zerr, G. Miehe, R. Riedel, Synthesis of cubic zirconium and hafnium nitride having Th3P4 structure, Nat. Mater. 2 (2003) 185–189. [5] D.A. Dzivenko, A. Zerr, V.K. Bulatov, G. Miehe, J.W. Li, B. Thybusch, J. Brötz, H. Fuess, G. Brey, R. Riedel, High-pressure multi-anvil synthesis and structure refinement of oxygen-bearing cubic zirconium (IV) nitride, Adv. Mater. 19 (2007) 1869–1873. [6] A. Zerr, N. Chigarev, O. Brinza, S.M. Nikitin, A.M. Lomonosov, V. Gusev, Elastic moduli of η-Ta2N3, a tough self-healing material, via laser ultrasonics, Phys. Status Solidi-Rapid Res. Lett. 6 (2012) 484–486. [7] S.F. Pugh, Relations between the elastic moduli and the plastic properties of polycrystalline pure metals, Philos. Mag. Ser. 45 (1954) 823–843. [8] D.M. Teter, Computational alchemy: the search for new superhard materials, MRS Bull. 23 (1998) 22–27. [9] A. Zerr, R. Riedel, Introduction: novel ultrahard materials, in: R. Riedel (Ed.), Handbook of Ceramic Hard Materials, Wiley-VCH, Weinheim, 2000, pp. XLV–LXXVII. [10] C. Gandhi, M.F. Ashby, Fracture-mechanism maps for materials which cleave: fcc, bcc and hcp metals and ceramics, Acta Metall. 27 (1979) 1565–1602.
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