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Alumina toughened zirconia from yttria coated powders
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Journal of the European Ceramic Society 32 (2012) 3911–3918
Alumina toughened zirconia from yttria coated powders Frank Kern ∗ , Rainer Gadow 1 University of Stuttgart, Institut für Fertigungstechnologie keramischer Bauteile (IFKB), Allmandring 7B, D-70569 Stuttgart, Germany Available online 3 April 2012
Abstract Alumina toughened zirconia (ATZ) ceramics are attractive materials for biomedical implants and other engineering applications requiring high strength and abrasion resistance at ambient temperature. As the toughness of conventional ATZ composites is moderate it was tried to improve the mechanical properties by starting from a very tough and transformable 2.5Y-TZP powder derived from powder coating. 2.5Y-TZP and ATZ materials with 20–40 vol% of alumina were produced and tested with respect to mechanical properties, microstructure and phase composition. √ ATZ materials with a bending strength of up to 1900 MPa and a fracture toughness of 4.8–5.5 MPa m were obtained. Transformability of the TZP distinctly declines with rising alumina content. Calculation of cooling and transformation stresses of the materials qualitatively confirms the measured toughness values. Alumina addition initially induces tensile residual cooling stress in the zirconia matrix which facilitates phase transformation. Formation of compressive stresses during transformation limits the transformability and toughness. © 2012 Elsevier Ltd. All rights reserved. Keywords: Hot pressing; ZrO2 ; Al2 O3 ; Microstructure-final; Mechanical properties
1. Introduction Transformation toughened ceramics obtain their high toughness and strength from a martensitic transformation of the meta-stable tetragonal to the stable monoclinic phase which is associated by volume expansion and shear.1 Stabilization of the tetragonal phase by addition of ∼2.5–3 mol% of yttria leads to Y-TZP (yttria stabilized tetragonal zirconia polycrystals), a material with high strength and toughness. Alumina addition to TZP acts in multiple ways. While alumina and zirconia are generally considered immiscible, TEM investigations have shown that alumina is incorporated into the grain boundary of Y-TZP. This alumina accumulation in the first 5–10 monolayers further stabilizes the zirconia and improves the resistance to low temperature degradation (LTD) considerably.2 Vleugels found the maximum contribution to toughness at an alumina fraction of 2.5 vol% added.3 Further increase in alumina fraction > 10 vol% leads to alumina toughened zirconia, a material of outstanding strength.4 LTD resistance rises with alumina addition.5,6 As alumina addition also improves wear resistance, ATZ is an
∗
Corresponding author. Tel.: +49 711 685 68233; fax: +49 711 685 68299. E-mail addresses: [email protected] (F. Kern), [email protected] (R. Gadow). 1 Tel.: +49 711 685 68300; fax: +49 711 685 68299. 0955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2012.03.014
interesting material for biomedical implants. Toughness of YTZP based ATZ is however only moderate, Tsukuma found a high resistance to crack initiation but not to crack propagation.4 One concept to increase the toughness is the shifting to other stabilizer systems. 10Ce-TZP/alumina and 1.5Nd–1.5Y-TZP have much higher toughness but do not reach the extremely high strength of Y-TZP/alumina composites.7,8 Formation of hexaaluminate platelets in situ enhances the toughness of CeTZP/alumina.9 In case of Y-TZP/alumina only a slight increase of toughness but a considerable increase of strength and reliability were observed.10 It was demonstrated that there is no unlimited increase of both strength and toughness in transfor√ mation toughened ceramics.11 Up to a toughness of 6.5 MPa m strength rises with toughness, a behavior typical of brittle solids. Above this √ toughness a narrow strength plateau is reached, from ∼8 MPa m upwards strength declines with rising toughness (R-curve domination). R-curve behavior limits the strength of e.g. Mg-PSZ and Ce-TZP. While these correlations are basically accepted, it seems advisable to be cautious with absolute values, as strength and toughness may differ depending on the protocol of measurement. Most data displayed are measured by 3-pt bending strength and indentation toughness according to Evans or Niihara, methods known to produce “optimistic” values. In the past years more performing TZP materials with improved strength and toughness were developed. Shifting from coprecipitated to coated TZP leads to a substantial increase
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√
of toughness to 6–9 MPa m at identical or higher strength (1100–1600 MPa) for Y-TZP and Yb-TZP.3,12–17 In this study it was therefore attempted to improve the mechanical properties of Y-TZP/alumina composites by replacing coprecipitated 3Y-TZP by tougher and more transformable 2.5Y-TZP produced by powder coating. Concerning the LTD resistance autoclave tests at 134 ◦ C have been made on hot pressed 0.5 mol% alumina-doped 2.5Y-TZP and 2.75Yb-TZP made from pyrogenic nanopowder with almost identical results, the onset of monoclinic formation was found at exposition times > 10 h corresponding to 30–40 years in vivo.16,17 Residual stress can play an important role in the mechanical properties of composite materials. Taya et al. have refined the model of Virkar for calculation of residual stress based toughness increments.18,19 Gregori et al. have calculated the residual stress situation in zirconia toughened alumina composites with and without phase transformation.20
2. Materials and methods The powder coating process was carried out following a procedure described by Yuan.14 The starting powder chosen for the synthesis of 2.5Y-TZP was TZ-0 (Tosoh, Japan), an agglomerated monoclinic zirconia nanopowder with a specific surface area SBET = 14–19 m2 and a primary crystallite size of dc = 25 nm (manufacturer’s data). 286.5 g of TZ-0 were dispersed in 500 ml 2-propanol, 13.5 g of yttria (Aldrich, USA, purity 99.9%) were dissolved in half concentrated boiling nitric acid. The yttrium nitrate solution was added to the zirconia dispersion and the powder was gently ball milled in a polyethylene bottle with 600 g of 3Y-TZP milling balls (d = 5 mm) for 12 h at 30 rpm. The solvent was then evaporated at 125 ◦ C, the dried powder heated to 300 ◦ C, crushed and screened through a 125 m mesh before calcining at 800 ◦ C for 1 h in air. The agglomerated yttria coated powder was then attrition milled for 4 h at 500 rpm with YTZP balls (d = 1 mm) in 2-propanol dried and screened through a 125 m mesh. The 2.5Y-TZP was then ready for processing. The two ATZ blends were produced by attrition milling as described above 85.9 g 2.5Y-TZP and 14.1 g alumina (ATZ20) and 69.5 g 2.5Y-TZP and 30.5 g alumina (ATZ40), respectively. The submicron size alumina used for the blends was APA 0.5 (Ceralox, USA), an ␣-alumina with a mean grain size of d50 = 0.3 m and SBET = 8 m2 /g (manufacturer’s data). Disks of ∼2 mm thickness of TZP, ATZ20 and ATZ40 were hot pressed (KCE, Germany) in a boron nitride clad graphite die. Heating in vacuum was performed with 50 K/min at 2 MPa pre-load. Final temperature was varied between 1400 and 1500 ◦ C at 25 K increments, dwell was 1 h at 50 MPa. As a benchmark material a commercially available ATZ powder hot-pressed at 1400 ◦ C/2 h/30 MPa was investigated (Daiichi ATZ (20), SBET = 10 m2 /g, yttria content 3.34 mol%, 20 mass% = 27.5 vol% alumina content). Sample disks were lapped and polished to a 1 m finish with diamond suspension (Struers, Denmark). Bars of 4 mm width were cut with a diamond wheel (Struers, Denmark). Sides and edges were carefully polished and bevelled to a 15 m finish to eliminate cutting induced defects. Mechanical characterization included measurement of Vickers hardness HV10 on
Fig. 1. Microstructure of benchmark material ATZ (20) sintered at 1400 ◦ C/2 h/30 MPa, thermally etched at 1400 ◦ C/30 min/air.
five indents (Bareiss, Germany), micro-hardness measurement HV0.1 on 12 indents (Fischer, Germany). Indentation modulus EIND was calculated according to the universal hardness method from the loading/unloading curve of the micro-hardness measurements. Bending strength was measured by 3-pt bending with a 15 mm span on 8 samples according to DIN EN 6872 (Hegewald&Peschke, Germany). Fracture toughness was measured by two independent methods. Direct crack length measurement was performed on 5 HV10 indents. KIND was calculated according to Anstis.21 Indentation strength in bending (ISB) according to Chantikul was measured on 3 bars each.22 Bending bars were indented with a HV10 indent with the cracks perpendicular to the sides. The residual strength measured with the indent in the middle of the tensile side in the same 3-pt setup as for bending. Crack length and residual strength were measured immediately after indentation to minimize the influence of subcritical crack growth. The phase composition of polished surface and fracture faces was investigated by XRD (Bruker, Germany, Cu K␣, graphite monochromator). Areas of monoclinic (1¯ 1 1) and (1 1 1) as well as tetragonal (1 0 1) reflexes were determined in the 27–33◦ 2θ-range. Quantitative results of zirconia phase composition were obtained applying the calibration curve of Toraya.23 SEM images were taken from the thermally etched (1400 ◦ C/30 min/air) surfaces (Jeol, Japan). Grain sizes distributions were determined by automatic analysis of SEM images of 20,000× magnification (ImageJ, USA) on 200–300 grains using Mendelson’s correction factor 1.558 for the grain sizes.24 3. Results and discussion 3.1. Benchmark material The Daiichi ATZ (20) benchmark material was characterized by mechanical testing, phase analysis and SEM. Results show that the material is very fine grained (Fig. 1), has a very attractive bending strength but only a moderate toughness (Table 1). The high stabilizer content of 3.34% chosen by the manufacturer restrains transformability and limits fracture toughness.
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Table 1 Mechanical properties of commercially available ATZ benchmark material. Property
Unit
Value ± 2σ
3-pt bending strength (σ 3pt ) Vickers hardness (HV10 ) Vickers hardness (HV0.1 ) Indentation modulus (EIND ) Fracture toughness (KIND , Anstis) Fracture toughness (KISB )
MPa – – GPa √ MPa m √ MPa m
1502 1554 1873 297 4.5 4.0
± ± ± ± ± ±
323 12 22 2 0.1 0.2
Fig. 4. Bending strength σ 3pt according to DIN EN 6872 of 2.5Y-TZP, ATZ20 and ATZ40 vs. sintering temperature.
Fig. 2. Vickers hardness HV10 of 2.5Y-TZP, ATZ20 and ATZ40 vs. sintering temperature.
3.2. TZP and ATZ from coated powder 3.2.1. Mechanical properties Vickers hardness HV10 and indentation modulus are shown in Figs. 2 and 3. Hardness data basically follow the rule of mixture. Addition of 20 vol% of alumina leads to an increase of hardness by ∼1.5 GPa while indentation modulus rises by ∼35 GPa. A slight trend of EIND to increase with sintering temperature can be observed which might be an artefact of the measuring method which only investigates the surface. The penetration depth of a HV0.1 indentation ∼2 m. Moreover the indentation modulus seems systematically too high compared to rule of mixture
Fig. 3. Indentation modulus EIND of 2.5Y-TZP, ATZ20 and ATZ40 vs. sintering temperature.
values based on average data from literature (ETZP = 210–230 GPa, EAlumina = 385–400 GPa). As the indentation modulus is also used for calculation of toughness resulting toughness values depending on calculation model may be too high by ∼5% (Anstis: KIND ∼ E0.5 ) or ∼1% (ISB: KISB ∼ E0.125 ). Bending strength is shown in Fig. 4. TZP and ATZ40 reach a bending strength of 1250–1450 MPa with a trend to declining strength at rising sintering temperature. ATZ20 shows two strength maxima of 1800–1900 MPa at low (1400 ◦ C) and high (1475–1500 ◦ C) sintering temperatures, in between the strength falls back to 1650 MPa. Strength is higher and standard deviations of strength are lower (2σ = 130–250 MPa) than for the coprecipitated reference material. Fracture toughness determined by direct crack length measurement according to Anstis is shown in Fig. 5. The corresponding measurement of toughness by ISB method is shown in Fig. 6. Comparing the two methods Anstis model generally leads to higher absolute values and no clear discrimination between fracture toughness of ATZ20 and ATZ40. The ATZ show a flat toughness vs. sintering temperature curve, the toughness of the TZP tends to rise with sintering temperature. In case of the ISB√model toughness is generally lower √ (TZP: KIC = 7.5–8.5 MPa m, ATZ20: KIC = 5.3–5.5 MPa m,
Fig. 5. Fracture toughness KIND (Anstis model) of 2.5Y-TZP, ATZ20 and ATZ40 vs. sintering temperature.
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Fig. 6. Fracture toughness KISB (residual strength) of 2.5Y-TZP, ATZ20 and ATZ40 vs. sintering temperature.
√ ATZ40: KIC = 4.5–4.8 MPa m). Toughness of ATZs is, considering the standard deviation, independent on sintering temperature. The toughness of TZP shows two maxima at 1400 ◦ C and 1475 ◦ C. Considering the uncertainties of the determination of EIND and the recent harsh criticism of indentation methods, the ISB method appears more conservative and credible especially at high toughness values.25 Compared to the commercially available ATZ with an alumina content of 27 vol% a significantly higher toughness value is observed for both tested ATZ compositions. Probably the lower yttria-content at least partly accounts this increment.
Fig. 8. Monoclinic fractions in polished surface (Vm,polished ) and fracture face (Vm,fractured ) and transformability (Vm,fractured − Vm,polished ) of ATZ20.
3.2.2. Phase analysis Phase analysis of the three materials (Figs. 7–9) shows that the polished surfaces of TZP always contain a certain amount of monoclinic phase. The monoclinic fraction rises from low to high sintering temperature, reaches its maximum of ∼12% at 1475 ◦ C and falls back to the starting value of ∼2% at 1500 ◦ C. Polished ATZ materials are entirely free of monoclinic phase with the exception of ATZ20 which contains 2% of monoclinic at a sintering temperature of 1400 ◦ C. Ohnishi found monoclinic phase in coated and pressureless sintered 2.5–3.5 mol% Y-TZP even at the highest yttria content. Thus the presence of
monoclinic phase in the polished surfaces is most probably not an indicator for incomplete stabilization.13 In studies of LTD resistance of coated 2.5Y-TZP and 2.75Yb-TZP by the authors no correlation between monoclinic fraction in polished surfaces and aging resistance was found.16,17 The fractured faces of TZP contain ∼60% of monoclinic while the transformed fraction in ATZ20 amounts to 15–30% for ATZ20 and 9–15% for ATZ40. For the ATZ materials a trend to higher transformability at higher sintering temperature is clearly visible while the TZP shows a transformability minimum at 1425 ◦ C. A closer look at the XRD results of fracture faces shows an interesting detail which was previously reported for alumina doped Y-TZP, Yb-TZP and yttria free ZTA.15,16,26 At some sintering temperatures (actually in more than half of the tested parameters) there appear two additional reflexes between the monoclinic (1¯ 1 1) and the tetragonal (1 0 1) reflex. These reflexes are very pronounced especially for the case of ATZ40 sintered at 1425–1450 ◦ C (Fig. 10). As no systematic relation between boundary conditions and appearance of the peaks is found and it is yet unclear which structures are associated with these reflexes, the area of these peaks was not included in the transformability calculation. They may – if involved in an additional toughening mechanism such as e.g. ferroelastic
Fig. 7. Monoclinic fractions in polished surface (Vm,polished ) and fracture face (Vm,fractured ) and transformability (Vm,fractured − Vm,polished ) of 2.5Y-TZP.
Fig. 9. Monoclinic fractions in polished surface (Vm,polished ) and fracture face (Vm,fractured ) and transformability (Vm,fractured − Vm,polished ) of ATZ40.
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Fig. 10. XRD of the 26.5–31.5 2θ-range for fracture faces of ATZ40 sintered at 1425 ◦ C, two additional reflexes.
toughening – hint at a significant further contribution to toughness. The intensity ratio of the peaks is similar to ␦-phase Y4 Zr3 O12 . As such reflexes were also found in yttria free ZTA the unknown phase does not necessarily contain yttria.26 The average transformability of the three materials correlates very well with the measured average toughness. If we √ subtract a basic toughness of ∼3 MPa m from the measured values, the remaining toughness increment supposedly delivered by transformation toughening KTT divided by transformability T is almost constant KTT /T = 10 √ = 5/0.5 (TZP) = 2.5/0.25 (ATZ20) = 1.5/0.15 (ATZ40) [MPa m]. In detail TZP shows a very strong correlation but especially for the ATZs some deviations especially at lower sintering temperatures can be observed. 3.2.3. Microstructure SEM images of the microstructures of TZP, ATZ20 and ATZ40 sintered at 1400 ◦ C are shown in Fig. 11. Despite the low sintering temperatures the structures obtained are not extremely fine grained. Both zirconia and alumina grains are around 500 nm in size. With rising sintering temperatures the zirconia
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grains grow to ∼570 nm at 1500 ◦ C. Grain size of Alumina in ATZ20 due to its volume content close to the percolation limit showed no pronounced growth, in case of ATZ40 a coalescence of adjacent grains can be observed (Fig. 12). In order to quantify the grain growth process SEM images were analyzed by automatic image analysis. Fig. 13 shows the evolution of grain sizes with sintering temperature. Evidently at the lowest sintering temperature a very narrow size distribution of grains can be observed. At 1425–1450 ◦ C a broadening of the grain sizes begins which seems to be completed at 1475 ◦ C. Vleugels has attributed the higher transformability and toughness of coated TZP to a size broadening of the TZP grains, in fact such a broadening is clearly visible.3 The transformability at least of the ATZ materials and the TZP (after the minimum at 1425 ◦ C) seems to rise with broadening grain sizes. However we observe that the toughness KISB stays almost constant or shows a first maximum at 1400 ◦ C. A second explanation model for the high toughness of coated Y-TZP is based on an yttria gradient from the surface to the bulk of the grains. Grains, according to this model, have a transformable core and a stable shell.13 Thus as the transformation is believed to start at the grain boundary they can bear high loads before starting to transform but once phase transformation proceeds into the bulk they reach high transformability. Diffusion will equilibrate this yttria gradient with rising sintering temperature. While previous investigations on TZP sintered at temperatures as low as 1200 ◦ C derived from ultra fine powders are in favor of the gradient model, here the gradient model seems only applicable at the lowest sintering temperature.16,17 3.2.4. Calculation of residual stress Assuming that the sample is stress-free at sintering temperature, residual stresses arise during the cooling of a composite consisting of materials of different CTE. This effect influences the toughness. An inclusion with low CTE in a matrix of high CTE will favor crack opening and reduce toughness. According to Taya the absolute values of the residual stress based toughness increment depend on the residual cooling stress qC , the
Fig. 11. SEM images of thermally etched 2.5Y-TZP, ATZ20 and ATZ40 (1400 ◦ C, 30 min, air) sintered at 1400 ◦ C/1 h/50 MPa.
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Fig. 12. SEM images of thermally etched 2.5Y-TZP, ATZ20 and ATZ40 (1400 ◦ C, 30 min, air) sintered at 1500 ◦ C/1 h/50 MPa.
particle diameter of the inclusion d and the particle distance of the inclusions λ (Eq. (1)). 2(λ − d) KR = qC · (1) π The mean inter-particle distance λ is related to the particle diameter √ and the volume fraction of the dispersion f, λ = 1.085 d/ f. The residual stress qC can be either measured (e.g. by XRD or piezo-spectroscopy) or calculated (analytically or by FEM simulations). In this study the model by Gregori was applied to calculate the hydrostatic stress qC (Eq. (2)). In the present case the results are such that the residual stress based toughness reduction by the addition √ of alumina is almost negligible at KR ∼ −0.1 √ to 0.15 MPa m for ATZ20 and KR ∼ −0.15 to 0.2 MPa m for ATZ40, these increments are in the range of standard deviations of measured KIC values. In the present case we assume a fine grained zirconia matrix which can dissipate cooling stress by grain boundary sliding and creep, the effective temperature T will be lower than the difference of sintering and ambient temperature. We may
tentatively assume a effective temperature difference T of ∼750 K, Poisson’s ratio νZ for zirconia is 0.23. Values αA and αZ for CTE and KA and KZ for bulk moduli of alumina (index A) and zirconia (index Z) as well as the value for transformation strain εT were taken from the paper by Gregori. The effective modulus of the composite K* and the Poisson’s value coefficient n are calculated as shown by Gregori (Eqs. (3) and (4)). qC = K∗ · (αZ − αA ) · T K∗ = n=
3nKA KZ fA KA fZ + nKZ fZ + nKA fA + KA fA
2(1 − 2νZ ) 1 + νZ
(2) (3) (4)
The calculation leads to value of qC = 81 MPa for ATZ20 and qC = 156 MPa for ATZ40. Zirconia in the composites is under tension and should be initially more transformable the more alumina is added. If we assume that plain untransformed TZP is stress free, we can calculate the total stress in the zirconia matrix as the sum of cooling stress and transformation related stress. Transformation stress qT is the product of the elastic modulus
Fig. 13. Grain size evolution from image analysis data of 2.5Y-TZP, ATZ20 and ATZ40 sintered at 1400–1500 ◦ C.
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composition and toughness. The applied analytical models show that initial tensile hydrostatic stress in the zirconia matrix rises with alumina addition. During the process of transformation the slope d(qT /Xt-m ) describing the rise of compressive stress with proceeding phase transformation is proportional to the alumina content. Thus in ATZ rising compressive stresses counteract the transformation process much more than in case of TZP. With respect to developing tougher ATZ materials than the ones presented in this study, a TZP matrix of even higher transformability and of lower Young’s modulus will be beneficial. We may however expect that these tougher materials may suffer from a certain trade-off in strength and LTD resistance.
Fig. 14. Estimation of effective hydrostatic stress in the zirconia matrix of TZP, ATZ20 and ATZ40 vs. transformed fraction of zirconia.
E of the composite, the transformation strain εT (0.016) and the fraction transformed Xt-m (Eq. (5)): qTOT = qT + qC = E · εT · Xt-m + qC
(5)
The result of this estimation is shown in Fig. 14. With proceeding transformation compressive stress is generated. The slope of the curves is determined by the elastic modulus E. As E rises with alumina addition, qT of ATZ materials rises faster than for TZP. At a transformation Xt-m of ∼15% and a residual compressive stress of ∼600 MPa identical conditions are reached for all materials. Beyond this point compressive stresses rise faster in ATZ as they scale with alumina addition. As a consequence we can expect that these compressive stresses counteract transformation. The estimation leads to a result at least qualitatively in good accord with experimental values. Other effects like a change in grain boundary chemistry by incorporation of alumina or formation of microcracks may also be considered. 4. Conclusions Based on a monoclinic zirconia 2.5Y-TZP powder was produced by a coating process via the nitrate route. 2.5Y-TZP and ATZ with 20% and 40% of alumina in 2.5Y-TZP matrix were produced by hot pressing of yttria coated powders. Compared to a benchmark material derived from coprecipitated Y-TZP the composites reach the same or higher strength and higher toughness. The high toughness of the 2.5Y-TZP cannot be transferred entirely into the ATZ composites. Materials with an initially very narrow grain size distribution run through a grain coarsening process at rising sintering temperatures. The characteristics of this coarsening process is a moderate growth of average grain sizes (from 500 nm to 570 nm) while a considerable broadening of the size distribution can be observed which takes place predominantly between 1425 ◦ C and 1475 ◦ C. The broadening of grain size scales with transformability but not with toughness. At low sintering temperature a second toughness maximum can be observed which hints at the presence of an yttria gradient which is eliminated at higher sintering temperatures. By estimation of the cooling and transformation stresses it was attempted to find a semi-quantitative explanation for the relations between
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Journal of the European Ceramic Society 32 (2012) 3911–3918
Alumina toughened zirconia from yttria coated powders Frank Kern ∗ , Rainer Gadow 1 University of Stuttgart, Institut für Fertigungstechnologie keramischer Bauteile (IFKB), Allmandring 7B, D-70569 Stuttgart, Germany Available online 3 April 2012
Abstract Alumina toughened zirconia (ATZ) ceramics are attractive materials for biomedical implants and other engineering applications requiring high strength and abrasion resistance at ambient temperature. As the toughness of conventional ATZ composites is moderate it was tried to improve the mechanical properties by starting from a very tough and transformable 2.5Y-TZP powder derived from powder coating. 2.5Y-TZP and ATZ materials with 20–40 vol% of alumina were produced and tested with respect to mechanical properties, microstructure and phase composition. √ ATZ materials with a bending strength of up to 1900 MPa and a fracture toughness of 4.8–5.5 MPa m were obtained. Transformability of the TZP distinctly declines with rising alumina content. Calculation of cooling and transformation stresses of the materials qualitatively confirms the measured toughness values. Alumina addition initially induces tensile residual cooling stress in the zirconia matrix which facilitates phase transformation. Formation of compressive stresses during transformation limits the transformability and toughness. © 2012 Elsevier Ltd. All rights reserved. Keywords: Hot pressing; ZrO2 ; Al2 O3 ; Microstructure-final; Mechanical properties
1. Introduction Transformation toughened ceramics obtain their high toughness and strength from a martensitic transformation of the meta-stable tetragonal to the stable monoclinic phase which is associated by volume expansion and shear.1 Stabilization of the tetragonal phase by addition of ∼2.5–3 mol% of yttria leads to Y-TZP (yttria stabilized tetragonal zirconia polycrystals), a material with high strength and toughness. Alumina addition to TZP acts in multiple ways. While alumina and zirconia are generally considered immiscible, TEM investigations have shown that alumina is incorporated into the grain boundary of Y-TZP. This alumina accumulation in the first 5–10 monolayers further stabilizes the zirconia and improves the resistance to low temperature degradation (LTD) considerably.2 Vleugels found the maximum contribution to toughness at an alumina fraction of 2.5 vol% added.3 Further increase in alumina fraction > 10 vol% leads to alumina toughened zirconia, a material of outstanding strength.4 LTD resistance rises with alumina addition.5,6 As alumina addition also improves wear resistance, ATZ is an
∗
Corresponding author. Tel.: +49 711 685 68233; fax: +49 711 685 68299. E-mail addresses: [email protected] (F. Kern), [email protected] (R. Gadow). 1 Tel.: +49 711 685 68300; fax: +49 711 685 68299. 0955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2012.03.014
interesting material for biomedical implants. Toughness of YTZP based ATZ is however only moderate, Tsukuma found a high resistance to crack initiation but not to crack propagation.4 One concept to increase the toughness is the shifting to other stabilizer systems. 10Ce-TZP/alumina and 1.5Nd–1.5Y-TZP have much higher toughness but do not reach the extremely high strength of Y-TZP/alumina composites.7,8 Formation of hexaaluminate platelets in situ enhances the toughness of CeTZP/alumina.9 In case of Y-TZP/alumina only a slight increase of toughness but a considerable increase of strength and reliability were observed.10 It was demonstrated that there is no unlimited increase of both strength and toughness in transfor√ mation toughened ceramics.11 Up to a toughness of 6.5 MPa m strength rises with toughness, a behavior typical of brittle solids. Above this √ toughness a narrow strength plateau is reached, from ∼8 MPa m upwards strength declines with rising toughness (R-curve domination). R-curve behavior limits the strength of e.g. Mg-PSZ and Ce-TZP. While these correlations are basically accepted, it seems advisable to be cautious with absolute values, as strength and toughness may differ depending on the protocol of measurement. Most data displayed are measured by 3-pt bending strength and indentation toughness according to Evans or Niihara, methods known to produce “optimistic” values. In the past years more performing TZP materials with improved strength and toughness were developed. Shifting from coprecipitated to coated TZP leads to a substantial increase
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√
of toughness to 6–9 MPa m at identical or higher strength (1100–1600 MPa) for Y-TZP and Yb-TZP.3,12–17 In this study it was therefore attempted to improve the mechanical properties of Y-TZP/alumina composites by replacing coprecipitated 3Y-TZP by tougher and more transformable 2.5Y-TZP produced by powder coating. Concerning the LTD resistance autoclave tests at 134 ◦ C have been made on hot pressed 0.5 mol% alumina-doped 2.5Y-TZP and 2.75Yb-TZP made from pyrogenic nanopowder with almost identical results, the onset of monoclinic formation was found at exposition times > 10 h corresponding to 30–40 years in vivo.16,17 Residual stress can play an important role in the mechanical properties of composite materials. Taya et al. have refined the model of Virkar for calculation of residual stress based toughness increments.18,19 Gregori et al. have calculated the residual stress situation in zirconia toughened alumina composites with and without phase transformation.20
2. Materials and methods The powder coating process was carried out following a procedure described by Yuan.14 The starting powder chosen for the synthesis of 2.5Y-TZP was TZ-0 (Tosoh, Japan), an agglomerated monoclinic zirconia nanopowder with a specific surface area SBET = 14–19 m2 and a primary crystallite size of dc = 25 nm (manufacturer’s data). 286.5 g of TZ-0 were dispersed in 500 ml 2-propanol, 13.5 g of yttria (Aldrich, USA, purity 99.9%) were dissolved in half concentrated boiling nitric acid. The yttrium nitrate solution was added to the zirconia dispersion and the powder was gently ball milled in a polyethylene bottle with 600 g of 3Y-TZP milling balls (d = 5 mm) for 12 h at 30 rpm. The solvent was then evaporated at 125 ◦ C, the dried powder heated to 300 ◦ C, crushed and screened through a 125 m mesh before calcining at 800 ◦ C for 1 h in air. The agglomerated yttria coated powder was then attrition milled for 4 h at 500 rpm with YTZP balls (d = 1 mm) in 2-propanol dried and screened through a 125 m mesh. The 2.5Y-TZP was then ready for processing. The two ATZ blends were produced by attrition milling as described above 85.9 g 2.5Y-TZP and 14.1 g alumina (ATZ20) and 69.5 g 2.5Y-TZP and 30.5 g alumina (ATZ40), respectively. The submicron size alumina used for the blends was APA 0.5 (Ceralox, USA), an ␣-alumina with a mean grain size of d50 = 0.3 m and SBET = 8 m2 /g (manufacturer’s data). Disks of ∼2 mm thickness of TZP, ATZ20 and ATZ40 were hot pressed (KCE, Germany) in a boron nitride clad graphite die. Heating in vacuum was performed with 50 K/min at 2 MPa pre-load. Final temperature was varied between 1400 and 1500 ◦ C at 25 K increments, dwell was 1 h at 50 MPa. As a benchmark material a commercially available ATZ powder hot-pressed at 1400 ◦ C/2 h/30 MPa was investigated (Daiichi ATZ (20), SBET = 10 m2 /g, yttria content 3.34 mol%, 20 mass% = 27.5 vol% alumina content). Sample disks were lapped and polished to a 1 m finish with diamond suspension (Struers, Denmark). Bars of 4 mm width were cut with a diamond wheel (Struers, Denmark). Sides and edges were carefully polished and bevelled to a 15 m finish to eliminate cutting induced defects. Mechanical characterization included measurement of Vickers hardness HV10 on
Fig. 1. Microstructure of benchmark material ATZ (20) sintered at 1400 ◦ C/2 h/30 MPa, thermally etched at 1400 ◦ C/30 min/air.
five indents (Bareiss, Germany), micro-hardness measurement HV0.1 on 12 indents (Fischer, Germany). Indentation modulus EIND was calculated according to the universal hardness method from the loading/unloading curve of the micro-hardness measurements. Bending strength was measured by 3-pt bending with a 15 mm span on 8 samples according to DIN EN 6872 (Hegewald&Peschke, Germany). Fracture toughness was measured by two independent methods. Direct crack length measurement was performed on 5 HV10 indents. KIND was calculated according to Anstis.21 Indentation strength in bending (ISB) according to Chantikul was measured on 3 bars each.22 Bending bars were indented with a HV10 indent with the cracks perpendicular to the sides. The residual strength measured with the indent in the middle of the tensile side in the same 3-pt setup as for bending. Crack length and residual strength were measured immediately after indentation to minimize the influence of subcritical crack growth. The phase composition of polished surface and fracture faces was investigated by XRD (Bruker, Germany, Cu K␣, graphite monochromator). Areas of monoclinic (1¯ 1 1) and (1 1 1) as well as tetragonal (1 0 1) reflexes were determined in the 27–33◦ 2θ-range. Quantitative results of zirconia phase composition were obtained applying the calibration curve of Toraya.23 SEM images were taken from the thermally etched (1400 ◦ C/30 min/air) surfaces (Jeol, Japan). Grain sizes distributions were determined by automatic analysis of SEM images of 20,000× magnification (ImageJ, USA) on 200–300 grains using Mendelson’s correction factor 1.558 for the grain sizes.24 3. Results and discussion 3.1. Benchmark material The Daiichi ATZ (20) benchmark material was characterized by mechanical testing, phase analysis and SEM. Results show that the material is very fine grained (Fig. 1), has a very attractive bending strength but only a moderate toughness (Table 1). The high stabilizer content of 3.34% chosen by the manufacturer restrains transformability and limits fracture toughness.
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Table 1 Mechanical properties of commercially available ATZ benchmark material. Property
Unit
Value ± 2σ
3-pt bending strength (σ 3pt ) Vickers hardness (HV10 ) Vickers hardness (HV0.1 ) Indentation modulus (EIND ) Fracture toughness (KIND , Anstis) Fracture toughness (KISB )
MPa – – GPa √ MPa m √ MPa m
1502 1554 1873 297 4.5 4.0
± ± ± ± ± ±
323 12 22 2 0.1 0.2
Fig. 4. Bending strength σ 3pt according to DIN EN 6872 of 2.5Y-TZP, ATZ20 and ATZ40 vs. sintering temperature.
Fig. 2. Vickers hardness HV10 of 2.5Y-TZP, ATZ20 and ATZ40 vs. sintering temperature.
3.2. TZP and ATZ from coated powder 3.2.1. Mechanical properties Vickers hardness HV10 and indentation modulus are shown in Figs. 2 and 3. Hardness data basically follow the rule of mixture. Addition of 20 vol% of alumina leads to an increase of hardness by ∼1.5 GPa while indentation modulus rises by ∼35 GPa. A slight trend of EIND to increase with sintering temperature can be observed which might be an artefact of the measuring method which only investigates the surface. The penetration depth of a HV0.1 indentation ∼2 m. Moreover the indentation modulus seems systematically too high compared to rule of mixture
Fig. 3. Indentation modulus EIND of 2.5Y-TZP, ATZ20 and ATZ40 vs. sintering temperature.
values based on average data from literature (ETZP = 210–230 GPa, EAlumina = 385–400 GPa). As the indentation modulus is also used for calculation of toughness resulting toughness values depending on calculation model may be too high by ∼5% (Anstis: KIND ∼ E0.5 ) or ∼1% (ISB: KISB ∼ E0.125 ). Bending strength is shown in Fig. 4. TZP and ATZ40 reach a bending strength of 1250–1450 MPa with a trend to declining strength at rising sintering temperature. ATZ20 shows two strength maxima of 1800–1900 MPa at low (1400 ◦ C) and high (1475–1500 ◦ C) sintering temperatures, in between the strength falls back to 1650 MPa. Strength is higher and standard deviations of strength are lower (2σ = 130–250 MPa) than for the coprecipitated reference material. Fracture toughness determined by direct crack length measurement according to Anstis is shown in Fig. 5. The corresponding measurement of toughness by ISB method is shown in Fig. 6. Comparing the two methods Anstis model generally leads to higher absolute values and no clear discrimination between fracture toughness of ATZ20 and ATZ40. The ATZ show a flat toughness vs. sintering temperature curve, the toughness of the TZP tends to rise with sintering temperature. In case of the ISB√model toughness is generally lower √ (TZP: KIC = 7.5–8.5 MPa m, ATZ20: KIC = 5.3–5.5 MPa m,
Fig. 5. Fracture toughness KIND (Anstis model) of 2.5Y-TZP, ATZ20 and ATZ40 vs. sintering temperature.
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Fig. 6. Fracture toughness KISB (residual strength) of 2.5Y-TZP, ATZ20 and ATZ40 vs. sintering temperature.
√ ATZ40: KIC = 4.5–4.8 MPa m). Toughness of ATZs is, considering the standard deviation, independent on sintering temperature. The toughness of TZP shows two maxima at 1400 ◦ C and 1475 ◦ C. Considering the uncertainties of the determination of EIND and the recent harsh criticism of indentation methods, the ISB method appears more conservative and credible especially at high toughness values.25 Compared to the commercially available ATZ with an alumina content of 27 vol% a significantly higher toughness value is observed for both tested ATZ compositions. Probably the lower yttria-content at least partly accounts this increment.
Fig. 8. Monoclinic fractions in polished surface (Vm,polished ) and fracture face (Vm,fractured ) and transformability (Vm,fractured − Vm,polished ) of ATZ20.
3.2.2. Phase analysis Phase analysis of the three materials (Figs. 7–9) shows that the polished surfaces of TZP always contain a certain amount of monoclinic phase. The monoclinic fraction rises from low to high sintering temperature, reaches its maximum of ∼12% at 1475 ◦ C and falls back to the starting value of ∼2% at 1500 ◦ C. Polished ATZ materials are entirely free of monoclinic phase with the exception of ATZ20 which contains 2% of monoclinic at a sintering temperature of 1400 ◦ C. Ohnishi found monoclinic phase in coated and pressureless sintered 2.5–3.5 mol% Y-TZP even at the highest yttria content. Thus the presence of
monoclinic phase in the polished surfaces is most probably not an indicator for incomplete stabilization.13 In studies of LTD resistance of coated 2.5Y-TZP and 2.75Yb-TZP by the authors no correlation between monoclinic fraction in polished surfaces and aging resistance was found.16,17 The fractured faces of TZP contain ∼60% of monoclinic while the transformed fraction in ATZ20 amounts to 15–30% for ATZ20 and 9–15% for ATZ40. For the ATZ materials a trend to higher transformability at higher sintering temperature is clearly visible while the TZP shows a transformability minimum at 1425 ◦ C. A closer look at the XRD results of fracture faces shows an interesting detail which was previously reported for alumina doped Y-TZP, Yb-TZP and yttria free ZTA.15,16,26 At some sintering temperatures (actually in more than half of the tested parameters) there appear two additional reflexes between the monoclinic (1¯ 1 1) and the tetragonal (1 0 1) reflex. These reflexes are very pronounced especially for the case of ATZ40 sintered at 1425–1450 ◦ C (Fig. 10). As no systematic relation between boundary conditions and appearance of the peaks is found and it is yet unclear which structures are associated with these reflexes, the area of these peaks was not included in the transformability calculation. They may – if involved in an additional toughening mechanism such as e.g. ferroelastic
Fig. 7. Monoclinic fractions in polished surface (Vm,polished ) and fracture face (Vm,fractured ) and transformability (Vm,fractured − Vm,polished ) of 2.5Y-TZP.
Fig. 9. Monoclinic fractions in polished surface (Vm,polished ) and fracture face (Vm,fractured ) and transformability (Vm,fractured − Vm,polished ) of ATZ40.
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Fig. 10. XRD of the 26.5–31.5 2θ-range for fracture faces of ATZ40 sintered at 1425 ◦ C, two additional reflexes.
toughening – hint at a significant further contribution to toughness. The intensity ratio of the peaks is similar to ␦-phase Y4 Zr3 O12 . As such reflexes were also found in yttria free ZTA the unknown phase does not necessarily contain yttria.26 The average transformability of the three materials correlates very well with the measured average toughness. If we √ subtract a basic toughness of ∼3 MPa m from the measured values, the remaining toughness increment supposedly delivered by transformation toughening KTT divided by transformability T is almost constant KTT /T = 10 √ = 5/0.5 (TZP) = 2.5/0.25 (ATZ20) = 1.5/0.15 (ATZ40) [MPa m]. In detail TZP shows a very strong correlation but especially for the ATZs some deviations especially at lower sintering temperatures can be observed. 3.2.3. Microstructure SEM images of the microstructures of TZP, ATZ20 and ATZ40 sintered at 1400 ◦ C are shown in Fig. 11. Despite the low sintering temperatures the structures obtained are not extremely fine grained. Both zirconia and alumina grains are around 500 nm in size. With rising sintering temperatures the zirconia
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grains grow to ∼570 nm at 1500 ◦ C. Grain size of Alumina in ATZ20 due to its volume content close to the percolation limit showed no pronounced growth, in case of ATZ40 a coalescence of adjacent grains can be observed (Fig. 12). In order to quantify the grain growth process SEM images were analyzed by automatic image analysis. Fig. 13 shows the evolution of grain sizes with sintering temperature. Evidently at the lowest sintering temperature a very narrow size distribution of grains can be observed. At 1425–1450 ◦ C a broadening of the grain sizes begins which seems to be completed at 1475 ◦ C. Vleugels has attributed the higher transformability and toughness of coated TZP to a size broadening of the TZP grains, in fact such a broadening is clearly visible.3 The transformability at least of the ATZ materials and the TZP (after the minimum at 1425 ◦ C) seems to rise with broadening grain sizes. However we observe that the toughness KISB stays almost constant or shows a first maximum at 1400 ◦ C. A second explanation model for the high toughness of coated Y-TZP is based on an yttria gradient from the surface to the bulk of the grains. Grains, according to this model, have a transformable core and a stable shell.13 Thus as the transformation is believed to start at the grain boundary they can bear high loads before starting to transform but once phase transformation proceeds into the bulk they reach high transformability. Diffusion will equilibrate this yttria gradient with rising sintering temperature. While previous investigations on TZP sintered at temperatures as low as 1200 ◦ C derived from ultra fine powders are in favor of the gradient model, here the gradient model seems only applicable at the lowest sintering temperature.16,17 3.2.4. Calculation of residual stress Assuming that the sample is stress-free at sintering temperature, residual stresses arise during the cooling of a composite consisting of materials of different CTE. This effect influences the toughness. An inclusion with low CTE in a matrix of high CTE will favor crack opening and reduce toughness. According to Taya the absolute values of the residual stress based toughness increment depend on the residual cooling stress qC , the
Fig. 11. SEM images of thermally etched 2.5Y-TZP, ATZ20 and ATZ40 (1400 ◦ C, 30 min, air) sintered at 1400 ◦ C/1 h/50 MPa.
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Fig. 12. SEM images of thermally etched 2.5Y-TZP, ATZ20 and ATZ40 (1400 ◦ C, 30 min, air) sintered at 1500 ◦ C/1 h/50 MPa.
particle diameter of the inclusion d and the particle distance of the inclusions λ (Eq. (1)). 2(λ − d) KR = qC · (1) π The mean inter-particle distance λ is related to the particle diameter √ and the volume fraction of the dispersion f, λ = 1.085 d/ f. The residual stress qC can be either measured (e.g. by XRD or piezo-spectroscopy) or calculated (analytically or by FEM simulations). In this study the model by Gregori was applied to calculate the hydrostatic stress qC (Eq. (2)). In the present case the results are such that the residual stress based toughness reduction by the addition √ of alumina is almost negligible at KR ∼ −0.1 √ to 0.15 MPa m for ATZ20 and KR ∼ −0.15 to 0.2 MPa m for ATZ40, these increments are in the range of standard deviations of measured KIC values. In the present case we assume a fine grained zirconia matrix which can dissipate cooling stress by grain boundary sliding and creep, the effective temperature T will be lower than the difference of sintering and ambient temperature. We may
tentatively assume a effective temperature difference T of ∼750 K, Poisson’s ratio νZ for zirconia is 0.23. Values αA and αZ for CTE and KA and KZ for bulk moduli of alumina (index A) and zirconia (index Z) as well as the value for transformation strain εT were taken from the paper by Gregori. The effective modulus of the composite K* and the Poisson’s value coefficient n are calculated as shown by Gregori (Eqs. (3) and (4)). qC = K∗ · (αZ − αA ) · T K∗ = n=
3nKA KZ fA KA fZ + nKZ fZ + nKA fA + KA fA
2(1 − 2νZ ) 1 + νZ
(2) (3) (4)
The calculation leads to value of qC = 81 MPa for ATZ20 and qC = 156 MPa for ATZ40. Zirconia in the composites is under tension and should be initially more transformable the more alumina is added. If we assume that plain untransformed TZP is stress free, we can calculate the total stress in the zirconia matrix as the sum of cooling stress and transformation related stress. Transformation stress qT is the product of the elastic modulus
Fig. 13. Grain size evolution from image analysis data of 2.5Y-TZP, ATZ20 and ATZ40 sintered at 1400–1500 ◦ C.
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composition and toughness. The applied analytical models show that initial tensile hydrostatic stress in the zirconia matrix rises with alumina addition. During the process of transformation the slope d(qT /Xt-m ) describing the rise of compressive stress with proceeding phase transformation is proportional to the alumina content. Thus in ATZ rising compressive stresses counteract the transformation process much more than in case of TZP. With respect to developing tougher ATZ materials than the ones presented in this study, a TZP matrix of even higher transformability and of lower Young’s modulus will be beneficial. We may however expect that these tougher materials may suffer from a certain trade-off in strength and LTD resistance.
Fig. 14. Estimation of effective hydrostatic stress in the zirconia matrix of TZP, ATZ20 and ATZ40 vs. transformed fraction of zirconia.
E of the composite, the transformation strain εT (0.016) and the fraction transformed Xt-m (Eq. (5)): qTOT = qT + qC = E · εT · Xt-m + qC
(5)
The result of this estimation is shown in Fig. 14. With proceeding transformation compressive stress is generated. The slope of the curves is determined by the elastic modulus E. As E rises with alumina addition, qT of ATZ materials rises faster than for TZP. At a transformation Xt-m of ∼15% and a residual compressive stress of ∼600 MPa identical conditions are reached for all materials. Beyond this point compressive stresses rise faster in ATZ as they scale with alumina addition. As a consequence we can expect that these compressive stresses counteract transformation. The estimation leads to a result at least qualitatively in good accord with experimental values. Other effects like a change in grain boundary chemistry by incorporation of alumina or formation of microcracks may also be considered. 4. Conclusions Based on a monoclinic zirconia 2.5Y-TZP powder was produced by a coating process via the nitrate route. 2.5Y-TZP and ATZ with 20% and 40% of alumina in 2.5Y-TZP matrix were produced by hot pressing of yttria coated powders. Compared to a benchmark material derived from coprecipitated Y-TZP the composites reach the same or higher strength and higher toughness. The high toughness of the 2.5Y-TZP cannot be transferred entirely into the ATZ composites. Materials with an initially very narrow grain size distribution run through a grain coarsening process at rising sintering temperatures. The characteristics of this coarsening process is a moderate growth of average grain sizes (from 500 nm to 570 nm) while a considerable broadening of the size distribution can be observed which takes place predominantly between 1425 ◦ C and 1475 ◦ C. The broadening of grain size scales with transformability but not with toughness. At low sintering temperature a second toughness maximum can be observed which hints at the presence of an yttria gradient which is eliminated at higher sintering temperatures. By estimation of the cooling and transformation stresses it was attempted to find a semi-quantitative explanation for the relations between
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