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Influence of CaTiO3 impurity in SHS-TiN powder on the mechanical properties of ZrO2–TiN composites
Materials Science and Engineering A 492 (2008) 95–101
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Influence of CaTiO3 impurity in SHS-TiN powder on the mechanical properties of ZrO2 –TiN composites Sedigheh Salehi a , Gerard Bienvenu b , Omer Van der Biest a , Jef Vleugels a,∗ a b
Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium Easyl, ZI 13 rue de Montr´eal, 74100 Ville La Grand, France
a r t i c l e
i n f o
Article history: Received 18 December 2007 Received in revised form 29 February 2008 Accepted 5 March 2008 Keywords: Self-propagating high-temperature synthesis Ceramic composites Mechanical properties Microstructures ZrO2 TiN
a b s t r a c t Yttria-stabilised tetragonal polycrystalline ZrO2 -based composites with 40 vol.% TiN were hot pressed at 1450 ◦ C for 1 h using a jet-milled thermally synthesized and a self-propagating high-temperature synthesis (SHS) TiN powder. The ZrO2 phase of the SHS-TiN powder-based composites was found to be substantially coarser than for the jet-milled TiN powder-based ceramics and prone to spontaneous transformation to m-ZrO2 and microcracking, due to the CaTiO3 impurity in the SHS-TiN starting powder. In order to prove this, a set of experiments was performed to investigate the effect of the addition of CaO and TiO2 on an yttria-stabilised tetragonal ZrO2 polycrystalline (Y-TZP). The addition of 0.2 mol% of CaO to a Y-TZP ceramic was found to destabilise the t-ZrO2 phase, whereas the addition of 1 mol% TiO2 results in significant grain growth and the formation of less transformable t-ZrO2 . The CaTiO3 impurity could be removed from the SHS-TiN powder by hot hydrochloric acid leaching, allowing to obtain a similar microstructure and mechanical properties as with conventional TiN powder. © 2008 Elsevier B.V. All rights reserved.
1. Introduction ZrO2 –TiN (60/40) composites combine excellent mechanical properties with an electrical conductivity that is high enough to allow shaping of the material by electrical discharge machining (EDM) [1–3]. Y2 O3 -stabilised ZrO2 -based TiN composites can be fully densified by hot pressing at 1450 ◦ C and 1.75 mol% Y2 O3 stabilised ZrO2 with 40 vol.% TiN results in a Vickers hardness of 1470 kg/mm2 , an indentation toughness of 5.9 MPa m1/2 and an excellent bending strength of 1674 MPa in combination with an appropriate electrical conductivity of 0.2 × 106 S/m [1]. A possible way to decrease the cost of the starting powder is self-propagating high-temperature synthesis (SHS). This process is based on the exothermal reaction of an ignited powder mixture. The reaction self-propagates via a combustion wave travelling through the starting powder mixture, converting it into the targeted reaction product. Low energy consumption, simplicity of the equipment needed, high combustion temperatures and high synthesis rates are characteristics of SHS production, suggesting the possibility to reduce the production cost of starting powders compared
∗ Corresponding author at: Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44-bus 2450, B-3001 Heverlee, Belgium. Tel.: +32 16 321244; fax: +32 16 321992. E-mail address: [email protected] (J. Vleugels). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.03.013
to that of the conventional high temperature furnace technology [4–7]. This paper reports on the possibility of making ZrO2 -based composites with 40 vol.% SHS-TiN powder. The influence of the impurities in the SHS-TiN powder on the microstructure and mechanical properties is assessed and explained in terms of the influence of the CaO and CaTiO3 contamination. The observed trends were experimentally confirmed by a set of experiments with Y2 O3 + CaO co-doped ZrO2 ceramics. Moreover, a washing step with hot HCl was introduced into the processing route of the SHSTiN powder, allowing removing the CaTiO3 impurity and obtaining identical mechanical properties as for the ZrO2 –TiN composites based on thermally synthesised TiN. 2. Experimental procedure ZrO2 –TiN–Al2 O3 composites with 1.75 and 2.0 mol% (3.2 and 3.7 wt%) Y2 O3 stabiliser, 40 vol.% TiN and 0.8 wt% Al2 O3 were hot pressed for 1 h at 1450 ◦ C. Al2 O3 (grade SM8, 0.6 m, Baikowski, France) was added as ZrO2 grain growth inhibitor and sintering aid [8]. Powder mixtures with an Y2 O3 -content of 1.75 or 2 mol% were obtained by mixing the appropriate amounts of monoclinic (grade TZ-0, BET = 15.9 m2 /g, crystallite size 26 nm, Tosoh, Japan) and 3 mol% Y2 O3 co-precipitated (grade TZ-3Y, BET = 14.5 m2 /g, crystallite size 29 nm, Tosoh, Japan) ZrO2 powder. Two different TiN starting powder sources were used, i.e., SHS-TiN (Easyl,
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Table 1 Impurity content (wt%) of the TiN powder grades, as obtained by XRF analysis TiN powder grade
Ca
Al
O
Fe
C
Ti
N
Jet-milled SHS Acid washed SHS
nd 1.10 <0.005
nd 0.16 0.06
0.91 5.20 1.83
0.10 nd nd
0.11 nd nd
78 76 80
21 22 24
nd, not detected.
France) and jet-milled TiN (Kennametal, USA). The influence of CaO and TiO2 on the microstructure and mechanical properties of ZrO2 and ZrO2 –TiN (60/40) composites was assessed by adding a defined amount of Ca(NO3 )2 ·4H2 O (99.0%, Acros, Belgium) or Ti{OCH(CH3 )2 }4 (99.99%, Aldrich, Germany) to the starting powder mixture. The chemical composition of the TiN powder grades, as obtained by X-ray fluorescence (XRF) spectroscopy (PW 2400, Philips, The Netherlands), is summarized in Table 1. Fifty grams of formulated powder was mixed in ethanol in a polyethylene container of 250 ml on a multidirectional mixer (Type Turbula T2C, WAB, Switzerland) during 24 h at 60 rpm. Two hundred and fifty grams of ZrO2 milling balls (grade TZ-3Y, Tosoh, Japan) with a diameter of 3 mm were added to the container to break up the agglomerates in the starting powder and to enhance powder mixing. The ethanol was removed after mixing, using a rotating evaporator. The dry powder mixture was inserted into a graphite container with an internal diameter of 30 mm, manually coated with boron nitride. After cold compression at 30 MPa, the samples were hot pressed (Model W100/150-2200-50 LAX, FCT Systeme, Germany) under vacuum (∼0.1 Pa) under a mechanical load of 28 MPa for 1 h at 1450 ◦ C, with a heating rate of 50 ◦ C/min and a cooling rate of 20 ◦ C/min. The samples were separated from the furnace atmosphere by the graphite hot-press arrangement. The density of the samples was measured in ethanol, according to the Archimedes method (BP210S balance, Sartorius AG, Germany). The Vickers hardness, HV10 , was measured (Model FV700, Future-Tech Corp., Tokyo, Japan) with an indentation load of 98 N with a dwell time of 10 s. The indentation toughness, KIC , was based on the radial crack pattern produced by Vick-
ers HV10 indentations, and calculated according to the formula of Anstis et al. [9]. The reported values are the mean and standard deviation of five indentations. The elastic modulus, E, of the ceramics was measured using the resonance frequency method [10]. The resonance frequency was measured by the impulse excitation technique (Model Grindo-sonic, Lemmens N.V., Belgium). The flexural strength at room temperature was measured in a 3point bending test (Instron 4467, PA, USA) on rectangular samples (25.0 mm × 5.4 mm × 2.1 mm) with a span width of 20 mm and a crosshead displacement of 0.2 mm/min. The reported values are the mean and standard deviation of five measurements. All surfaces of the bending bars were ground with a diamond-grinding wheel (type D46SW-50-X2, Technodiamant, The Netherlands) on a Jung grinding machine (JF415DS, Jung, Germany). Microstructural investigation was performed by scanning electron microscopy (SEM, XL30-FEG, FEI, The Netherlands), equipped with an energy dispersive X-ray spectrometer with ultra-thin window (EDS, EDAX, The Netherlands) for elemental analysis. In order to reveal the grain boundary and grain size, a selection of ZrO2 ceramics were thermally etched for 30 min at 1300 ◦ C, whereas the selected composites where thermally etched in vacuum (<10−5 Pa) for 25 min at 1300 ◦ C. Cu K␣ (40 kV, 30 mA) X-ray diffraction (XRD, Seifert 3003 T/T, Ahrensburg, Germany) analysis was used for phase identification and calculation of the relative phase content of monoclinic and tetragonal ZrO2 . The average grain size of the TiN powders and ZrO2 phase in the densified composites was measured according to the linear intercept method. The reported values are the average of at least 400 grains measured by means of image analysis on SEM micrographs using Imagepro-plus software (version 4, Media Cybernetics, Silver Spring, MD). 3. Results and discussion 3.1. Microstructure and mechanical properties The morphology of the TiN starting powder grades is compared in Fig. 1, whereas the X-ray diffraction patterns are provided in
Fig. 1. SEM micrographs of the SHS before (a) and after (b) hot acid treatment and jet-milled (c) TiN starting powders.
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Fig. 2. X-ray diffraction patterns of the jet-milled, SHS and acid-treated SHS-TiN starting powders.
Fig. 2. The jet-milled TiN powder has an angular shape with a wider grain size distribution and an average particle size of 0.8 m, whereas the SHS-TiN powder is rounded with a narrow particle size distribution and an average particle size of 1.5 m. The XRD patterns of the TiN starting powders reveal the presence of a small amount of CaTiO3 in the SHS powder (Fig. 2), whereas the jet-milled material is pure TiN. 1.1 wt% Ca is measured by XRF analysis, as summarized in Table 1. In the SHS process, TiO2 , Ca and Ca3 N2 are used as starting powders [4], which are converted into TiN and CaO by a self-propagating exothermal reaction according to: 2TiO2 + Ca + Ca3 N2 → 2TiN + 4CaO, TiO2 + CaO → CaTiO3 , 2CaTiO3 + Ca + Ca3 N2 → 2TiN + 6CaO.
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The major amount of CaO is subsequently removed from the reaction product by means of HCl leaching at room temperature. Unfortunately, the CaTiO3 is too stable to be dissolved by HCl at room temperature and atmospheric pressure and remains as a minor impurity. Four grades of 1.75 or 2 mol% Y2 O3 -stabilised ZrO2 –TiN–Al2 O3 (60/40-0.8) composites were hot pressed using SHS and jet-milled TiN starting powders. An overview and details of the polished microstructures of a selection of ceramic composites is given in Fig. 3. Three phases can be distinguished in the hot pressed microstructures, i.e., ZrO2 (grey), TiN (dark) and Al2 O3 (black). Full densification of the composites was obtained since no residual porosity could be identified by means of SEM investigation of polished cross-sections. The TiN phase is homogeneously distributed in both grades, but the grain morphology of the jet-milled TiN powder is more angular and smaller than that of the SHS powder, which is more rounded, reflecting the particle morphology of the starting powders (Fig. 1). The ZrO2 grains are substantially larger in the SHS-TiN-based composite (Fig. 3c) than in the jet-milled TiN powder-based ceramic (Fig. 3d). Small black particles are observed in-between the ZrO2 grains and even inside the TiN grains in the SHS-based ZrO2 –TiN composites. EDS compositional point analysis of these particles revealed the presence of Al and O, indicating Al2 O3 . Moreover, EDS point analysis qualitatively revealed the presence of Ca and Ti in the large ZrO2 grains of the SHS-TiN powder-based grade. Microcracks were frequently observed in the SHS-TiN-based composites (Fig. 3a and c), whereas no microcracks were present in the jet-milled TiN powder-based composites. The CaTiO3 impurity seems to dissolve into the ZrO2 phase during hot pressing, since both Ca and Ti are found to be present inside the large ZrO2 grains. Tetragonal, as well as monoclinic ZrO2 were detected on polished SHS-TiN-based composites, as shown by the X-ray diffraction patterns in Fig. 4. In the polished jet-milled TiN-based composites however, no monoclinic phase was measured. This reveals that not all t-ZrO2 is stable in the SHS-TiN-based composites and par-
Fig. 3. Backscattered electron micrographs of polished cross-sectioned 1.75 mol% Y2 O3 -stabilised SHS (a and c) and 2 mol% Y2 O3 -stabilised jet-milled (b and d) TiN powderbased ZrO2 –TiN–Al2 O3 (60/40-0.8) composites.
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Fig. 4. X-ray diffraction patterns of polished 1.75 or 2.00 mol% yttria-stabilised ZrO2 –TiN (60/40) composites made from jet-milled and SHS-TiN powder.
tially transforms from tetragonal to monoclinic during cooling. The 1.1 vol.% alumina added is below the XRD detection limit. The mZrO2 volume fraction in the 1.75 and 2 mol% yttria-stabilised SHS powder-based ZrO2 –TiN composites was calculated to be 51 and 40 vol.%, respectively. The calculations were performed according to the formula of Toraya et al. [11], applied to the XRD patterns. The mechanical properties of the composites are summarised in Table 2. Although the jet-milled TiN powder-based composites were found to be fully dense and microcracks were observed in the SHS-TiN powder-based composites, the measured density of the jet-milled powder-based composites is slightly lower. This has to be attributed to the CaTiO3 impurity in the SHS-TiN powder. The Vickers hardness of the jet-milled TiN-based composites is higher than for the SHS powder-based material due to the absence of microcracks. The indentation toughness however is slightly higher, most probably due to the crack arresting potential of the microcracks. Crack deflection by the TiN particles was found to be an active toughening mechanism, beside ZrO2 transformation toughening which is the major toughening mechanism [1].
Table 2 Mechanical properties of the ZrO2 –TiN–Al2 O3 composites Y2 O3 Stabiliser content (mol%)
TiN grade
(g/cm3 )
E (GPa)
HV10 (kg/mm2 )
1.75 2.00 1.75 2.00 1.75
Jet-milled Jet-milled SHS SHS Acid treated SHS
5.81 5.83 5.87 5.88 5.80
290 263 300 300 278
1329 1312 1143 1201 1366
± ± ± ± ±
12 13 19 21 14
KIC (MPa m1/2 ) 5.1 4.8 6.0 6.0 5.2
± ± ± ± ±
0.1 0.1 0.3 0.3 0.1
Fig. 5. Backscattered electron micrographs of thermally etched polished 0 (a), 0.5 (b), 1 (c), 1.8 (d), 4 (e) mol% CaO and 1 mol% TiO2 and (f) co-stabilised 2 mol% Y2 O3 -stabilised ZrO2 with 2 wt% Al2 O3 addition, sintered for 1 h at 1450 ◦ C.
S. Salehi et al. / Materials Science and Engineering A 492 (2008) 95–101
Fig. 6. Average grain size and toughness of 0–4 mol% CaO co-stabilised 2 mol% Y2 O3 stabilised ZrO2 with 2 wt% Al2 O3 addition, sintered for 1 h at 1450 ◦ C.
The addition of TiO2 to pure ZrO2 [12], ZTA (ZrO2 -toughened Al2 O3 ) [13] and 3–6 mol% Y2 O3 -stabilised ZrO2 [14] is reported to increase the ZrO2 grain size and to decrease the ZrO2 martensitic transformation temperature (Ms ). A reduced ZrO2 transformability due to TiO2 dissolution from TiN and TiCN phases in ZrO2 -based composites has also been described [15]. The addition of 1–2 mol% CaO to yttria–ytterbia-stabilised ZrO2 increases the cubic ZrO2 grain size [8]. The dissolution of TiO2 from the CaTiO3 can explain the large ZrO2 grains in the SHS powder-based material as well as the ZrO2 transformation from tetragonal to monoclinic. The influence of small amounts of CaO on the microstructure and properties of yttria-stabilised ZrO2 however is not clear from the literature. Because of the lack of literature data on the influence of small amounts of CaO on the microstructure and mechanical properties of yttria-stabilised tetragonal zirconia polycrystals (Y-TZP) and YTZP-based ceramics, a set of experiments was conducted in order to elucidate the influence of CaO addition, as discussed below. 3.2. Influence of CaO and TiO2 addition on the microstructure and mechanical properties of Y-TZP ceramics and composites 0.2, 0.5, 1.0, 1.8 and 4 mol% of CaO was added to a powder mixture of TZ-0 and TZ-3Y with an overall Y2 O3 content of 2 mol% with the addition of 2 wt% Al2 O3 and hot pressed for 1 h at 1450 ◦ C. In order to investigate the influence of TiO2 , a 2 mol% Y2 O3 -stabilised ZrO2 ceramic was doped with 1 mol% TiO2 and hot pressed under the same conditions. The microstructures of the polished and thermally etched crosssectioned ceramics, revealing the grain boundaries and grain size distribution, are shown in Fig. 5. All ceramic grades are fully densified, except the 0.2 mol% CaO-doped material that spontaneously transformed. The average ZrO2 grain size and toughness of the CaO doped ceramic grades are shown in Fig. 6 and are summarised in
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Fig. 7. XRD patterns of 0–4 mol% CaO and 1 mol% TiO2 co-stabilised 2 mol% Y2 O3 stabilised ZrO2 with 2 wt% Al2 O3 addition, sintered for 1 h at 1450 ◦ C.
Table 3. The XRD patterns are shown in Fig. 7. The 2 wt% (3.1 vol.%) alumina added is below the XRD detection limit. The addition of 0.2 mol% CaO to a 2 mol% Y2 O3 -stabilised ZrO2 ceramic clearly results in a destabilisation of the t-ZrO2 phase, due to spontaneous transformation and concomitant material degradation. Co-stabilising with 0.5 mol% CaO results in tetragonal ZrO2 with high transformability (66%) and concomitantly high fracture toughness. The ZrO2 grain size however is substantially larger than for the 2Y-TZP ceramic. Increasing the CaO to 1.8 mol% reduces the tetragonal ZrO2 grain size, fracture toughness and ZrO2 transformability. Further increasing the CaO content up to 4 mol% (=1.8 wt%) results in a bimodal grain size distribution, as shown in Fig. 5e. The XRD pattern of the 4 mol% CaO grade, shown in Fig. 7, reveals the presence of m-, t- as well as c-ZrO2 . The cubic ZrO2 phase has a larger grain size than t-ZrO2 [16], increasing the overall average grain size. The average grain size of the CaO co-stabilised Y-TZP ceramics, shown in Fig. 6, is evolving in a similar way as that of Y2 O3 -stabilised ZrO2 , i.e., going through a minimum at intermediate stabiliser contents where a fully tetragonal material is obtained [17]. At low CaO contents, m-ZrO2 is present and the grain size decreases with increasing CaO addition until a (Ca,Y)-TZP ceramic is obtained. The smallest grain size is obtained for (1.0–1.8 Ca,2Y)-TZP that has an acceptable t-ZrO2 transformability and fracture toughness. Further increasing the CaO content results in the formation of binary systems composed of smaller tetragonal and larger cubic ZrO2 grains, the average grain size increases with increasing CaO content and the transformability decreases with increasing c-ZrO2 content. In the current investigation, also m-ZrO2 was observed in the (4Ca,2Y)ZrO2 ceramic, what should be attributed to the partial spontaneous transformation of the t-ZrO2 phase. The addition of 0.2 mol% CaO to a 2 mol% Y2 O3 -stabilised starting powder however results in a sintered ZrO2 ceramic that sponta-
Table 3 Mechanical properties of 2Y-TZP, co-stabilised with different CaO contents CaO content (mol%)
(g/cm3 )
HV10 (kg/mm2 )
KIC (MPa m1/2 )
Transformability (%)
Average grain size (m)
0.0 0.2 0.5 1.0 1.8 4.0 1 mol% TiO2
6.09 6.30 6.30 6.28 6.00 5.81 6.01
1224 ± 11 Cracked 1206 ± 012 1279 ± 13 1266 ± 09 1231 ± 15 1283 ± 7
8.0 ± 0.1 Cracked 8.4 ± 0.1 6.7 ± 0.2 6.3 ± 0.2 2.8 ± 0.3 3.9 ± 0.2
63 – 66 59 51 0 28
0.34 ± 0.14 Cracked 0.74 ± 0.30 0.24 ± 0.08 0.26 ± 0.08 0.38 ± 0.30 0.44 ± 0.16
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Fig. 10. Microstructure of polished 1.75 mol% Y2 O3 -stabilised ZrO2 –TiN–Al2 O3 (60/40-0.8) composites obtained with hot acid treated SHS.
Fig. 8. X-ray diffraction patterns of SHS-TiN powder-based 2 mol% yttria-stabilised ZrO2 –TiN (60/40) composites with 1–6 mol% CaO addition.
neously transforms, whereas the 2 mol% Y2 O3 -stabilised ceramic is fully tetragonal, indicating a competitive dissolution of the CaO and Y2 O3 stabiliser. The addition of 0.5 mol% CaO results in a (Ca,Y)-TZP with a substantially larger grain size and slightly higher transformability as well as fracture toughness than the 2Y-TZP grade. The addition of 1 mol% TiO2 to 2 mol% Y2 O3 -stabilised ZrO2 causes a more pronounced t-ZrO2 grain growth than the addition of 1 mol% CaO, as shown in Fig. 5. On the other hand, the t-ZrO2 transformability decreases drastically with TiO2 addition (Table 3), due to the formation of non-transformable tetragonal zirconia, as reported in the literature [12–15]. Based on the above, it can be assumed that the TiO2 from the dissolving CaTiO3 will result in t-ZrO2 grain growth and a reduced transformability, whereas the small amount of CaO will destabilise the t-ZrO2 matrix and cause spontaneous transformation. This is in excellent agreement with the observed t-ZrO2 grain growth and
spontaneous transformation observed in the hot pressed SHS-TiN containing composites (see Figs. 3a, b and 4). In order to try to establish a TZP matrix in the SHS-TiN powderbased composites, 1–6 mol% CaO was added to a 1.75 mol% yttriastabilised ZrO2 –TiN (60/40-SHS) composite. The XRD patterns of the composites hot pressed for 1 h at 1450 ◦ C, presented in Fig. 8, reveal that m-ZrO2 can indeed be avoided upon adding ≥3 mol% CaO. The overall ZrO2 grain size in the composites increases with increasing CaO addition, as shown in Fig. 9, due to an increasing volume fraction of larger grained c-ZrO2 . The composite with 3 mol% CaO has the highest strength (∼1 GPa) which is however substantially lower than for the jet-milled TiN powder-based 1.75 mol% Y2 O3 -stabilised ZrO2 composite, having a strength of 1.7 GPa. The toughness and bending strength decrease dramatically with increasing CaO addition above 3 mol%, whereas the hardness only slightly decreased, as summarized in Table 4. 3.3. Hot acid treatment of SHS-TiN starting powder The literature reports however claim the possibility to completely dissolve SrTiO3 and BaTiO3 perovskites within 4 h in HCl at
Fig. 9. Backscattered electron micrographs of thermally etched polished 1.75 mol% yttria-stabilised powder-based ZrO2 –TiN–Al2 O3 (60/40-0.8) composites with jet milled (a) and SHS (b–d) TiN with 0 (a), 1 (b), 4 (c) and 6(d) mol% CaO addition.
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Table 4 Mechanical properties of 1.75 mol% yttria-stabilised SHS-TiN powder-based ZrO2 –TiN–Al2 O3 (60/40-0.8) composites with CaO addition CaO content (mol%) a
0 1 2 3 4 6
a
(g/cm3 )
E (GPa)
HV10 (kg/mm2 )
5.81 5.68 5.66 5.66 5.64 5.65
290 301 299 297 296 305
1329 1244 1155 1186 1365 1206
± ± ± ± ± ±
12 28 28 37 17 48
KIC (MPa m1/2 ) 5.1 4.2 4.9 3.1 3.1 2.9
± ± ± ± ± ±
0.1 0.3 0.8 0.1 0.2 0.3
Bending strength (MPa) 1674 613 943 1070 778 559
± ± ± ± ± ±
314 276 105 308 113 202
Prepared from jet-milled TiN powder.
240 ◦ C and elevated pressure using continues agitation [18]. In order to test the possibility of removing the CaTiO3 impurity, the SHS-TiN powder was heat treated in HCl at 100 ◦ C for 4 h in a pressure vessel and subsequently washed in demineralised water, dried in vacuum and deagglomerated and sieved. X-ray fluorescence analysis clearly revealed a strongly reduced Ca content (see Table 1). Moreover, XRD analysis indicates complete removal of the CaTiO3 phase, as shown in Fig. 2. The average grain size of the original SHS powder (1.5 m) slightly decreased to 1.2 m after hot acid treatment (Fig. 1). The hot acid treated SHS-TiN powder was used to make yttriastabilised ZrO2 –TiN–Al2 O3 (60/40-0.8) composites by hot pressing at 1450 ◦ C for 1 h. The obtained microstructure, shown in Fig. 10, is comparable to that of the jet-milled TiN powder-based composite, shown in Fig. 3b. Moreover, the fracture toughness and hardness of the jet-milled and hot acid treated SHS-TiN powderbased ZrO2 –TiN composites are comparable, as summarised in Table 2. 4. Conclusions The CaTiO3 impurity in SHS-TiN starting powder resulted in an increased t-ZrO2 grain size, partial transformation to m-ZrO2 and microcracking in hot pressed yttria-stabilised ZrO2 –TiN (60/40) composites. The addition of 0.2 mol% CaO to a 2 mol% Y2 O3 -stabilised ZrO2 destabilises the t-ZrO2 phase resulting in a spontaneously transforming material. Adding 0.5 mol% CaO allows to obtain (Ca,Y)-TZP material with a high transformability of 66% and fracture toughness of 8.4 MPa m1/2 . 1.0–1.8 mol% CaO addition results in (Ca,Y)-TZP ceramics with minimum grain size, whereas a bimodal t- + c-ZrO2 material is obtained when adding 4 mol% CaO. The grain size of the CaO + 2 mol% Y2 O3 co-stabilised ceramics clearly shows a minimum at 1.0–1.8 mol% CaO. The addition of 1 mol% TiO2 to a 2 mol% Y2 O3 -stabilised ZrO2 causes a more pronounced t-ZrO2 grain growth than the addition of 1 mol% CaO and a strongly reduced t-ZrO2 transformability. The t-ZrO2 destabilising effect of the CaTiO3 impurity in 40 vol.% SHS-TiN powder-based 2 mol% Y2 O3 -stabilised composites could
be avoided by the addition of 3 mol% CaO, but the strength and toughness are inferior to those of pure TiN powder base composites. The CaTiO3 impurity could be removed by hot hydrochloric acid leaching of the SHS-TiN starting powder, allowing to obtain a similar microstructure and mechanical properties as with conventional thermally synthesized TiN powder. Acknowledgements S. Salehi thanks the Research Council of K. U. Leuven for a doctoral scholarship (DB/07/012). This work was supported by the Commission of the European Communities within the Framework 6 Program under project No. STRP 505541-1. The authors also acknowledge the support of the Belgian Federal Science Policy Office (BELSPO) through IUAP project INANOMAT (contract P6/17). References [1] S. Salehi, O. Van der Biest, J. Vleugels, J. Eur. Ceram. Soc. 26 (2006) 3173–3179. [2] A. Pitman, J. Huddleston, Br. Ceram. Trans. 99 (2000) 77–84. [3] K. Vanmeensel, A. Laptev, O. Van der Biest, J. Vleugels, Acta Mater. 55 (2007) 1801–2181. [4] J. Russias, S. Cardinal, C. Esnouf, G. Fantozzi, K. Bienvenu, J. Eur. Ceram. Soc. 27 (2007) 327–335. [5] P. Mossino, Ceram. Int. 30 (2004) 311–332. [6] G. Merzhanov, Ceram. Int. 21 (1995) 371–379. [7] Z.A. Munir, U. Anselmi-Tamburini, Mater. Sci. Rep. 3 (1989) 277–336. [8] Z. Lu, R. Guo, P. Yao, F. Dai, Mater. Des. 28 (2007) 1399–1403. [9] G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, J. Am. Ceram. Soc. 64 (1981) 533–538. [10] ASTM C 1259-98. Standard test method for dynamic Young’s modulus, shear modulus, and Poisson’s ratio for advanced ceramics by impulse excitation of vibration. Annual Book of Standards, vol. 15.01; ASTM, West Conshohocken, PA, 1998. [11] H. Toraya, M. Yoshimura, S. Somiya, J. Am. Ceram. Soc. 67 (1984) 119–121. [12] V.C. Pandolfelli, J.A. Rodrigues, R. Stevens, J. Mater. Sci. 26 (1991) 5327–5334. [13] C.-S. Hwang, Y.-J. Chang, J. Mater. Res. 11 (1996) 1545–1551. [14] C.L. Lin, D. Gan, P. Shen, J. Mater. Sci. Eng. A 129 (1990) 147–155. [15] J. Vleugels, O. Van der Biest, J. Am. Ceram. Soc. 82 (1999) 2117–2120. [16] J.L. Shi, M.L. Ruan, T.S. Yen, Ceram. Int. 22 (1996) 137–142. [17] F.F. Lange, J. Am. Ceram. Soc. 69 (1986) 240–242. [18] J. Dolezal, J. Lenz, Z. Suleck, Anal. Chim. Acta 47 (1969) 517.
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of CaTiO3 impurity in SHS-TiN powder on the mechanical properties of ZrO2 –TiN composites Sedigheh Salehi a , Gerard Bienvenu b , Omer Van der Biest a , Jef Vleugels a,∗ a b
Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium Easyl, ZI 13 rue de Montr´eal, 74100 Ville La Grand, France
a r t i c l e
i n f o
Article history: Received 18 December 2007 Received in revised form 29 February 2008 Accepted 5 March 2008 Keywords: Self-propagating high-temperature synthesis Ceramic composites Mechanical properties Microstructures ZrO2 TiN
a b s t r a c t Yttria-stabilised tetragonal polycrystalline ZrO2 -based composites with 40 vol.% TiN were hot pressed at 1450 ◦ C for 1 h using a jet-milled thermally synthesized and a self-propagating high-temperature synthesis (SHS) TiN powder. The ZrO2 phase of the SHS-TiN powder-based composites was found to be substantially coarser than for the jet-milled TiN powder-based ceramics and prone to spontaneous transformation to m-ZrO2 and microcracking, due to the CaTiO3 impurity in the SHS-TiN starting powder. In order to prove this, a set of experiments was performed to investigate the effect of the addition of CaO and TiO2 on an yttria-stabilised tetragonal ZrO2 polycrystalline (Y-TZP). The addition of 0.2 mol% of CaO to a Y-TZP ceramic was found to destabilise the t-ZrO2 phase, whereas the addition of 1 mol% TiO2 results in significant grain growth and the formation of less transformable t-ZrO2 . The CaTiO3 impurity could be removed from the SHS-TiN powder by hot hydrochloric acid leaching, allowing to obtain a similar microstructure and mechanical properties as with conventional TiN powder. © 2008 Elsevier B.V. All rights reserved.
1. Introduction ZrO2 –TiN (60/40) composites combine excellent mechanical properties with an electrical conductivity that is high enough to allow shaping of the material by electrical discharge machining (EDM) [1–3]. Y2 O3 -stabilised ZrO2 -based TiN composites can be fully densified by hot pressing at 1450 ◦ C and 1.75 mol% Y2 O3 stabilised ZrO2 with 40 vol.% TiN results in a Vickers hardness of 1470 kg/mm2 , an indentation toughness of 5.9 MPa m1/2 and an excellent bending strength of 1674 MPa in combination with an appropriate electrical conductivity of 0.2 × 106 S/m [1]. A possible way to decrease the cost of the starting powder is self-propagating high-temperature synthesis (SHS). This process is based on the exothermal reaction of an ignited powder mixture. The reaction self-propagates via a combustion wave travelling through the starting powder mixture, converting it into the targeted reaction product. Low energy consumption, simplicity of the equipment needed, high combustion temperatures and high synthesis rates are characteristics of SHS production, suggesting the possibility to reduce the production cost of starting powders compared
∗ Corresponding author at: Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44-bus 2450, B-3001 Heverlee, Belgium. Tel.: +32 16 321244; fax: +32 16 321992. E-mail address: [email protected] (J. Vleugels). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.03.013
to that of the conventional high temperature furnace technology [4–7]. This paper reports on the possibility of making ZrO2 -based composites with 40 vol.% SHS-TiN powder. The influence of the impurities in the SHS-TiN powder on the microstructure and mechanical properties is assessed and explained in terms of the influence of the CaO and CaTiO3 contamination. The observed trends were experimentally confirmed by a set of experiments with Y2 O3 + CaO co-doped ZrO2 ceramics. Moreover, a washing step with hot HCl was introduced into the processing route of the SHSTiN powder, allowing removing the CaTiO3 impurity and obtaining identical mechanical properties as for the ZrO2 –TiN composites based on thermally synthesised TiN. 2. Experimental procedure ZrO2 –TiN–Al2 O3 composites with 1.75 and 2.0 mol% (3.2 and 3.7 wt%) Y2 O3 stabiliser, 40 vol.% TiN and 0.8 wt% Al2 O3 were hot pressed for 1 h at 1450 ◦ C. Al2 O3 (grade SM8, 0.6 m, Baikowski, France) was added as ZrO2 grain growth inhibitor and sintering aid [8]. Powder mixtures with an Y2 O3 -content of 1.75 or 2 mol% were obtained by mixing the appropriate amounts of monoclinic (grade TZ-0, BET = 15.9 m2 /g, crystallite size 26 nm, Tosoh, Japan) and 3 mol% Y2 O3 co-precipitated (grade TZ-3Y, BET = 14.5 m2 /g, crystallite size 29 nm, Tosoh, Japan) ZrO2 powder. Two different TiN starting powder sources were used, i.e., SHS-TiN (Easyl,
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Table 1 Impurity content (wt%) of the TiN powder grades, as obtained by XRF analysis TiN powder grade
Ca
Al
O
Fe
C
Ti
N
Jet-milled SHS Acid washed SHS
nd 1.10 <0.005
nd 0.16 0.06
0.91 5.20 1.83
0.10 nd nd
0.11 nd nd
78 76 80
21 22 24
nd, not detected.
France) and jet-milled TiN (Kennametal, USA). The influence of CaO and TiO2 on the microstructure and mechanical properties of ZrO2 and ZrO2 –TiN (60/40) composites was assessed by adding a defined amount of Ca(NO3 )2 ·4H2 O (99.0%, Acros, Belgium) or Ti{OCH(CH3 )2 }4 (99.99%, Aldrich, Germany) to the starting powder mixture. The chemical composition of the TiN powder grades, as obtained by X-ray fluorescence (XRF) spectroscopy (PW 2400, Philips, The Netherlands), is summarized in Table 1. Fifty grams of formulated powder was mixed in ethanol in a polyethylene container of 250 ml on a multidirectional mixer (Type Turbula T2C, WAB, Switzerland) during 24 h at 60 rpm. Two hundred and fifty grams of ZrO2 milling balls (grade TZ-3Y, Tosoh, Japan) with a diameter of 3 mm were added to the container to break up the agglomerates in the starting powder and to enhance powder mixing. The ethanol was removed after mixing, using a rotating evaporator. The dry powder mixture was inserted into a graphite container with an internal diameter of 30 mm, manually coated with boron nitride. After cold compression at 30 MPa, the samples were hot pressed (Model W100/150-2200-50 LAX, FCT Systeme, Germany) under vacuum (∼0.1 Pa) under a mechanical load of 28 MPa for 1 h at 1450 ◦ C, with a heating rate of 50 ◦ C/min and a cooling rate of 20 ◦ C/min. The samples were separated from the furnace atmosphere by the graphite hot-press arrangement. The density of the samples was measured in ethanol, according to the Archimedes method (BP210S balance, Sartorius AG, Germany). The Vickers hardness, HV10 , was measured (Model FV700, Future-Tech Corp., Tokyo, Japan) with an indentation load of 98 N with a dwell time of 10 s. The indentation toughness, KIC , was based on the radial crack pattern produced by Vick-
ers HV10 indentations, and calculated according to the formula of Anstis et al. [9]. The reported values are the mean and standard deviation of five indentations. The elastic modulus, E, of the ceramics was measured using the resonance frequency method [10]. The resonance frequency was measured by the impulse excitation technique (Model Grindo-sonic, Lemmens N.V., Belgium). The flexural strength at room temperature was measured in a 3point bending test (Instron 4467, PA, USA) on rectangular samples (25.0 mm × 5.4 mm × 2.1 mm) with a span width of 20 mm and a crosshead displacement of 0.2 mm/min. The reported values are the mean and standard deviation of five measurements. All surfaces of the bending bars were ground with a diamond-grinding wheel (type D46SW-50-X2, Technodiamant, The Netherlands) on a Jung grinding machine (JF415DS, Jung, Germany). Microstructural investigation was performed by scanning electron microscopy (SEM, XL30-FEG, FEI, The Netherlands), equipped with an energy dispersive X-ray spectrometer with ultra-thin window (EDS, EDAX, The Netherlands) for elemental analysis. In order to reveal the grain boundary and grain size, a selection of ZrO2 ceramics were thermally etched for 30 min at 1300 ◦ C, whereas the selected composites where thermally etched in vacuum (<10−5 Pa) for 25 min at 1300 ◦ C. Cu K␣ (40 kV, 30 mA) X-ray diffraction (XRD, Seifert 3003 T/T, Ahrensburg, Germany) analysis was used for phase identification and calculation of the relative phase content of monoclinic and tetragonal ZrO2 . The average grain size of the TiN powders and ZrO2 phase in the densified composites was measured according to the linear intercept method. The reported values are the average of at least 400 grains measured by means of image analysis on SEM micrographs using Imagepro-plus software (version 4, Media Cybernetics, Silver Spring, MD). 3. Results and discussion 3.1. Microstructure and mechanical properties The morphology of the TiN starting powder grades is compared in Fig. 1, whereas the X-ray diffraction patterns are provided in
Fig. 1. SEM micrographs of the SHS before (a) and after (b) hot acid treatment and jet-milled (c) TiN starting powders.
S. Salehi et al. / Materials Science and Engineering A 492 (2008) 95–101
Fig. 2. X-ray diffraction patterns of the jet-milled, SHS and acid-treated SHS-TiN starting powders.
Fig. 2. The jet-milled TiN powder has an angular shape with a wider grain size distribution and an average particle size of 0.8 m, whereas the SHS-TiN powder is rounded with a narrow particle size distribution and an average particle size of 1.5 m. The XRD patterns of the TiN starting powders reveal the presence of a small amount of CaTiO3 in the SHS powder (Fig. 2), whereas the jet-milled material is pure TiN. 1.1 wt% Ca is measured by XRF analysis, as summarized in Table 1. In the SHS process, TiO2 , Ca and Ca3 N2 are used as starting powders [4], which are converted into TiN and CaO by a self-propagating exothermal reaction according to: 2TiO2 + Ca + Ca3 N2 → 2TiN + 4CaO, TiO2 + CaO → CaTiO3 , 2CaTiO3 + Ca + Ca3 N2 → 2TiN + 6CaO.
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The major amount of CaO is subsequently removed from the reaction product by means of HCl leaching at room temperature. Unfortunately, the CaTiO3 is too stable to be dissolved by HCl at room temperature and atmospheric pressure and remains as a minor impurity. Four grades of 1.75 or 2 mol% Y2 O3 -stabilised ZrO2 –TiN–Al2 O3 (60/40-0.8) composites were hot pressed using SHS and jet-milled TiN starting powders. An overview and details of the polished microstructures of a selection of ceramic composites is given in Fig. 3. Three phases can be distinguished in the hot pressed microstructures, i.e., ZrO2 (grey), TiN (dark) and Al2 O3 (black). Full densification of the composites was obtained since no residual porosity could be identified by means of SEM investigation of polished cross-sections. The TiN phase is homogeneously distributed in both grades, but the grain morphology of the jet-milled TiN powder is more angular and smaller than that of the SHS powder, which is more rounded, reflecting the particle morphology of the starting powders (Fig. 1). The ZrO2 grains are substantially larger in the SHS-TiN-based composite (Fig. 3c) than in the jet-milled TiN powder-based ceramic (Fig. 3d). Small black particles are observed in-between the ZrO2 grains and even inside the TiN grains in the SHS-based ZrO2 –TiN composites. EDS compositional point analysis of these particles revealed the presence of Al and O, indicating Al2 O3 . Moreover, EDS point analysis qualitatively revealed the presence of Ca and Ti in the large ZrO2 grains of the SHS-TiN powder-based grade. Microcracks were frequently observed in the SHS-TiN-based composites (Fig. 3a and c), whereas no microcracks were present in the jet-milled TiN powder-based composites. The CaTiO3 impurity seems to dissolve into the ZrO2 phase during hot pressing, since both Ca and Ti are found to be present inside the large ZrO2 grains. Tetragonal, as well as monoclinic ZrO2 were detected on polished SHS-TiN-based composites, as shown by the X-ray diffraction patterns in Fig. 4. In the polished jet-milled TiN-based composites however, no monoclinic phase was measured. This reveals that not all t-ZrO2 is stable in the SHS-TiN-based composites and par-
Fig. 3. Backscattered electron micrographs of polished cross-sectioned 1.75 mol% Y2 O3 -stabilised SHS (a and c) and 2 mol% Y2 O3 -stabilised jet-milled (b and d) TiN powderbased ZrO2 –TiN–Al2 O3 (60/40-0.8) composites.
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Fig. 4. X-ray diffraction patterns of polished 1.75 or 2.00 mol% yttria-stabilised ZrO2 –TiN (60/40) composites made from jet-milled and SHS-TiN powder.
tially transforms from tetragonal to monoclinic during cooling. The 1.1 vol.% alumina added is below the XRD detection limit. The mZrO2 volume fraction in the 1.75 and 2 mol% yttria-stabilised SHS powder-based ZrO2 –TiN composites was calculated to be 51 and 40 vol.%, respectively. The calculations were performed according to the formula of Toraya et al. [11], applied to the XRD patterns. The mechanical properties of the composites are summarised in Table 2. Although the jet-milled TiN powder-based composites were found to be fully dense and microcracks were observed in the SHS-TiN powder-based composites, the measured density of the jet-milled powder-based composites is slightly lower. This has to be attributed to the CaTiO3 impurity in the SHS-TiN powder. The Vickers hardness of the jet-milled TiN-based composites is higher than for the SHS powder-based material due to the absence of microcracks. The indentation toughness however is slightly higher, most probably due to the crack arresting potential of the microcracks. Crack deflection by the TiN particles was found to be an active toughening mechanism, beside ZrO2 transformation toughening which is the major toughening mechanism [1].
Table 2 Mechanical properties of the ZrO2 –TiN–Al2 O3 composites Y2 O3 Stabiliser content (mol%)
TiN grade
(g/cm3 )
E (GPa)
HV10 (kg/mm2 )
1.75 2.00 1.75 2.00 1.75
Jet-milled Jet-milled SHS SHS Acid treated SHS
5.81 5.83 5.87 5.88 5.80
290 263 300 300 278
1329 1312 1143 1201 1366
± ± ± ± ±
12 13 19 21 14
KIC (MPa m1/2 ) 5.1 4.8 6.0 6.0 5.2
± ± ± ± ±
0.1 0.1 0.3 0.3 0.1
Fig. 5. Backscattered electron micrographs of thermally etched polished 0 (a), 0.5 (b), 1 (c), 1.8 (d), 4 (e) mol% CaO and 1 mol% TiO2 and (f) co-stabilised 2 mol% Y2 O3 -stabilised ZrO2 with 2 wt% Al2 O3 addition, sintered for 1 h at 1450 ◦ C.
S. Salehi et al. / Materials Science and Engineering A 492 (2008) 95–101
Fig. 6. Average grain size and toughness of 0–4 mol% CaO co-stabilised 2 mol% Y2 O3 stabilised ZrO2 with 2 wt% Al2 O3 addition, sintered for 1 h at 1450 ◦ C.
The addition of TiO2 to pure ZrO2 [12], ZTA (ZrO2 -toughened Al2 O3 ) [13] and 3–6 mol% Y2 O3 -stabilised ZrO2 [14] is reported to increase the ZrO2 grain size and to decrease the ZrO2 martensitic transformation temperature (Ms ). A reduced ZrO2 transformability due to TiO2 dissolution from TiN and TiCN phases in ZrO2 -based composites has also been described [15]. The addition of 1–2 mol% CaO to yttria–ytterbia-stabilised ZrO2 increases the cubic ZrO2 grain size [8]. The dissolution of TiO2 from the CaTiO3 can explain the large ZrO2 grains in the SHS powder-based material as well as the ZrO2 transformation from tetragonal to monoclinic. The influence of small amounts of CaO on the microstructure and properties of yttria-stabilised ZrO2 however is not clear from the literature. Because of the lack of literature data on the influence of small amounts of CaO on the microstructure and mechanical properties of yttria-stabilised tetragonal zirconia polycrystals (Y-TZP) and YTZP-based ceramics, a set of experiments was conducted in order to elucidate the influence of CaO addition, as discussed below. 3.2. Influence of CaO and TiO2 addition on the microstructure and mechanical properties of Y-TZP ceramics and composites 0.2, 0.5, 1.0, 1.8 and 4 mol% of CaO was added to a powder mixture of TZ-0 and TZ-3Y with an overall Y2 O3 content of 2 mol% with the addition of 2 wt% Al2 O3 and hot pressed for 1 h at 1450 ◦ C. In order to investigate the influence of TiO2 , a 2 mol% Y2 O3 -stabilised ZrO2 ceramic was doped with 1 mol% TiO2 and hot pressed under the same conditions. The microstructures of the polished and thermally etched crosssectioned ceramics, revealing the grain boundaries and grain size distribution, are shown in Fig. 5. All ceramic grades are fully densified, except the 0.2 mol% CaO-doped material that spontaneously transformed. The average ZrO2 grain size and toughness of the CaO doped ceramic grades are shown in Fig. 6 and are summarised in
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Fig. 7. XRD patterns of 0–4 mol% CaO and 1 mol% TiO2 co-stabilised 2 mol% Y2 O3 stabilised ZrO2 with 2 wt% Al2 O3 addition, sintered for 1 h at 1450 ◦ C.
Table 3. The XRD patterns are shown in Fig. 7. The 2 wt% (3.1 vol.%) alumina added is below the XRD detection limit. The addition of 0.2 mol% CaO to a 2 mol% Y2 O3 -stabilised ZrO2 ceramic clearly results in a destabilisation of the t-ZrO2 phase, due to spontaneous transformation and concomitant material degradation. Co-stabilising with 0.5 mol% CaO results in tetragonal ZrO2 with high transformability (66%) and concomitantly high fracture toughness. The ZrO2 grain size however is substantially larger than for the 2Y-TZP ceramic. Increasing the CaO to 1.8 mol% reduces the tetragonal ZrO2 grain size, fracture toughness and ZrO2 transformability. Further increasing the CaO content up to 4 mol% (=1.8 wt%) results in a bimodal grain size distribution, as shown in Fig. 5e. The XRD pattern of the 4 mol% CaO grade, shown in Fig. 7, reveals the presence of m-, t- as well as c-ZrO2 . The cubic ZrO2 phase has a larger grain size than t-ZrO2 [16], increasing the overall average grain size. The average grain size of the CaO co-stabilised Y-TZP ceramics, shown in Fig. 6, is evolving in a similar way as that of Y2 O3 -stabilised ZrO2 , i.e., going through a minimum at intermediate stabiliser contents where a fully tetragonal material is obtained [17]. At low CaO contents, m-ZrO2 is present and the grain size decreases with increasing CaO addition until a (Ca,Y)-TZP ceramic is obtained. The smallest grain size is obtained for (1.0–1.8 Ca,2Y)-TZP that has an acceptable t-ZrO2 transformability and fracture toughness. Further increasing the CaO content results in the formation of binary systems composed of smaller tetragonal and larger cubic ZrO2 grains, the average grain size increases with increasing CaO content and the transformability decreases with increasing c-ZrO2 content. In the current investigation, also m-ZrO2 was observed in the (4Ca,2Y)ZrO2 ceramic, what should be attributed to the partial spontaneous transformation of the t-ZrO2 phase. The addition of 0.2 mol% CaO to a 2 mol% Y2 O3 -stabilised starting powder however results in a sintered ZrO2 ceramic that sponta-
Table 3 Mechanical properties of 2Y-TZP, co-stabilised with different CaO contents CaO content (mol%)
(g/cm3 )
HV10 (kg/mm2 )
KIC (MPa m1/2 )
Transformability (%)
Average grain size (m)
0.0 0.2 0.5 1.0 1.8 4.0 1 mol% TiO2
6.09 6.30 6.30 6.28 6.00 5.81 6.01
1224 ± 11 Cracked 1206 ± 012 1279 ± 13 1266 ± 09 1231 ± 15 1283 ± 7
8.0 ± 0.1 Cracked 8.4 ± 0.1 6.7 ± 0.2 6.3 ± 0.2 2.8 ± 0.3 3.9 ± 0.2
63 – 66 59 51 0 28
0.34 ± 0.14 Cracked 0.74 ± 0.30 0.24 ± 0.08 0.26 ± 0.08 0.38 ± 0.30 0.44 ± 0.16
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Fig. 10. Microstructure of polished 1.75 mol% Y2 O3 -stabilised ZrO2 –TiN–Al2 O3 (60/40-0.8) composites obtained with hot acid treated SHS.
Fig. 8. X-ray diffraction patterns of SHS-TiN powder-based 2 mol% yttria-stabilised ZrO2 –TiN (60/40) composites with 1–6 mol% CaO addition.
neously transforms, whereas the 2 mol% Y2 O3 -stabilised ceramic is fully tetragonal, indicating a competitive dissolution of the CaO and Y2 O3 stabiliser. The addition of 0.5 mol% CaO results in a (Ca,Y)-TZP with a substantially larger grain size and slightly higher transformability as well as fracture toughness than the 2Y-TZP grade. The addition of 1 mol% TiO2 to 2 mol% Y2 O3 -stabilised ZrO2 causes a more pronounced t-ZrO2 grain growth than the addition of 1 mol% CaO, as shown in Fig. 5. On the other hand, the t-ZrO2 transformability decreases drastically with TiO2 addition (Table 3), due to the formation of non-transformable tetragonal zirconia, as reported in the literature [12–15]. Based on the above, it can be assumed that the TiO2 from the dissolving CaTiO3 will result in t-ZrO2 grain growth and a reduced transformability, whereas the small amount of CaO will destabilise the t-ZrO2 matrix and cause spontaneous transformation. This is in excellent agreement with the observed t-ZrO2 grain growth and
spontaneous transformation observed in the hot pressed SHS-TiN containing composites (see Figs. 3a, b and 4). In order to try to establish a TZP matrix in the SHS-TiN powderbased composites, 1–6 mol% CaO was added to a 1.75 mol% yttriastabilised ZrO2 –TiN (60/40-SHS) composite. The XRD patterns of the composites hot pressed for 1 h at 1450 ◦ C, presented in Fig. 8, reveal that m-ZrO2 can indeed be avoided upon adding ≥3 mol% CaO. The overall ZrO2 grain size in the composites increases with increasing CaO addition, as shown in Fig. 9, due to an increasing volume fraction of larger grained c-ZrO2 . The composite with 3 mol% CaO has the highest strength (∼1 GPa) which is however substantially lower than for the jet-milled TiN powder-based 1.75 mol% Y2 O3 -stabilised ZrO2 composite, having a strength of 1.7 GPa. The toughness and bending strength decrease dramatically with increasing CaO addition above 3 mol%, whereas the hardness only slightly decreased, as summarized in Table 4. 3.3. Hot acid treatment of SHS-TiN starting powder The literature reports however claim the possibility to completely dissolve SrTiO3 and BaTiO3 perovskites within 4 h in HCl at
Fig. 9. Backscattered electron micrographs of thermally etched polished 1.75 mol% yttria-stabilised powder-based ZrO2 –TiN–Al2 O3 (60/40-0.8) composites with jet milled (a) and SHS (b–d) TiN with 0 (a), 1 (b), 4 (c) and 6(d) mol% CaO addition.
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Table 4 Mechanical properties of 1.75 mol% yttria-stabilised SHS-TiN powder-based ZrO2 –TiN–Al2 O3 (60/40-0.8) composites with CaO addition CaO content (mol%) a
0 1 2 3 4 6
a
(g/cm3 )
E (GPa)
HV10 (kg/mm2 )
5.81 5.68 5.66 5.66 5.64 5.65
290 301 299 297 296 305
1329 1244 1155 1186 1365 1206
± ± ± ± ± ±
12 28 28 37 17 48
KIC (MPa m1/2 ) 5.1 4.2 4.9 3.1 3.1 2.9
± ± ± ± ± ±
0.1 0.3 0.8 0.1 0.2 0.3
Bending strength (MPa) 1674 613 943 1070 778 559
± ± ± ± ± ±
314 276 105 308 113 202
Prepared from jet-milled TiN powder.
240 ◦ C and elevated pressure using continues agitation [18]. In order to test the possibility of removing the CaTiO3 impurity, the SHS-TiN powder was heat treated in HCl at 100 ◦ C for 4 h in a pressure vessel and subsequently washed in demineralised water, dried in vacuum and deagglomerated and sieved. X-ray fluorescence analysis clearly revealed a strongly reduced Ca content (see Table 1). Moreover, XRD analysis indicates complete removal of the CaTiO3 phase, as shown in Fig. 2. The average grain size of the original SHS powder (1.5 m) slightly decreased to 1.2 m after hot acid treatment (Fig. 1). The hot acid treated SHS-TiN powder was used to make yttriastabilised ZrO2 –TiN–Al2 O3 (60/40-0.8) composites by hot pressing at 1450 ◦ C for 1 h. The obtained microstructure, shown in Fig. 10, is comparable to that of the jet-milled TiN powder-based composite, shown in Fig. 3b. Moreover, the fracture toughness and hardness of the jet-milled and hot acid treated SHS-TiN powderbased ZrO2 –TiN composites are comparable, as summarised in Table 2. 4. Conclusions The CaTiO3 impurity in SHS-TiN starting powder resulted in an increased t-ZrO2 grain size, partial transformation to m-ZrO2 and microcracking in hot pressed yttria-stabilised ZrO2 –TiN (60/40) composites. The addition of 0.2 mol% CaO to a 2 mol% Y2 O3 -stabilised ZrO2 destabilises the t-ZrO2 phase resulting in a spontaneously transforming material. Adding 0.5 mol% CaO allows to obtain (Ca,Y)-TZP material with a high transformability of 66% and fracture toughness of 8.4 MPa m1/2 . 1.0–1.8 mol% CaO addition results in (Ca,Y)-TZP ceramics with minimum grain size, whereas a bimodal t- + c-ZrO2 material is obtained when adding 4 mol% CaO. The grain size of the CaO + 2 mol% Y2 O3 co-stabilised ceramics clearly shows a minimum at 1.0–1.8 mol% CaO. The addition of 1 mol% TiO2 to a 2 mol% Y2 O3 -stabilised ZrO2 causes a more pronounced t-ZrO2 grain growth than the addition of 1 mol% CaO and a strongly reduced t-ZrO2 transformability. The t-ZrO2 destabilising effect of the CaTiO3 impurity in 40 vol.% SHS-TiN powder-based 2 mol% Y2 O3 -stabilised composites could
be avoided by the addition of 3 mol% CaO, but the strength and toughness are inferior to those of pure TiN powder base composites. The CaTiO3 impurity could be removed by hot hydrochloric acid leaching of the SHS-TiN starting powder, allowing to obtain a similar microstructure and mechanical properties as with conventional thermally synthesized TiN powder. Acknowledgements S. Salehi thanks the Research Council of K. U. Leuven for a doctoral scholarship (DB/07/012). This work was supported by the Commission of the European Communities within the Framework 6 Program under project No. STRP 505541-1. The authors also acknowledge the support of the Belgian Federal Science Policy Office (BELSPO) through IUAP project INANOMAT (contract P6/17). References [1] S. Salehi, O. Van der Biest, J. Vleugels, J. Eur. Ceram. Soc. 26 (2006) 3173–3179. [2] A. Pitman, J. Huddleston, Br. Ceram. Trans. 99 (2000) 77–84. [3] K. Vanmeensel, A. Laptev, O. Van der Biest, J. Vleugels, Acta Mater. 55 (2007) 1801–2181. [4] J. Russias, S. Cardinal, C. Esnouf, G. Fantozzi, K. Bienvenu, J. Eur. Ceram. Soc. 27 (2007) 327–335. [5] P. Mossino, Ceram. Int. 30 (2004) 311–332. [6] G. Merzhanov, Ceram. Int. 21 (1995) 371–379. [7] Z.A. Munir, U. Anselmi-Tamburini, Mater. Sci. Rep. 3 (1989) 277–336. [8] Z. Lu, R. Guo, P. Yao, F. Dai, Mater. Des. 28 (2007) 1399–1403. [9] G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, J. Am. Ceram. Soc. 64 (1981) 533–538. [10] ASTM C 1259-98. Standard test method for dynamic Young’s modulus, shear modulus, and Poisson’s ratio for advanced ceramics by impulse excitation of vibration. Annual Book of Standards, vol. 15.01; ASTM, West Conshohocken, PA, 1998. [11] H. Toraya, M. Yoshimura, S. Somiya, J. Am. Ceram. Soc. 67 (1984) 119–121. [12] V.C. Pandolfelli, J.A. Rodrigues, R. Stevens, J. Mater. Sci. 26 (1991) 5327–5334. [13] C.-S. Hwang, Y.-J. Chang, J. Mater. Res. 11 (1996) 1545–1551. [14] C.L. Lin, D. Gan, P. Shen, J. Mater. Sci. Eng. A 129 (1990) 147–155. [15] J. Vleugels, O. Van der Biest, J. Am. Ceram. Soc. 82 (1999) 2117–2120. [16] J.L. Shi, M.L. Ruan, T.S. Yen, Ceram. Int. 22 (1996) 137–142. [17] F.F. Lange, J. Am. Ceram. Soc. 69 (1986) 240–242. [18] J. Dolezal, J. Lenz, Z. Suleck, Anal. Chim. Acta 47 (1969) 517.