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R-curve behavior and flexural strength of zirconia-toughened alumina and partially stabilized zirconia composite laminates
Author’s Accepted Manuscript R-CURVE BEHAVIOR AND FLEXURAL STRENGTH OF ZIRCONIA-TOUGHENED ALUMINA AND PARTIALLY STABILIZED ZIRCONIA COMPOSITE LAMINATES Diego Blaese, Tobias Benitez, Marcelo Barros, Hans Jelitto, Nahum Travitzky, Dachamir Hotza, Rolf Janssen
PII: DOI: Reference:
www.elsevier.com/locate/ceri
S0272-8842(18)30977-5 https://doi.org/10.1016/j.ceramint.2018.04.107 CERI18024
To appear in: Ceramics International Received date: 1 March 2018 Accepted date: 12 April 2018 Cite this article as: Diego Blaese, Tobias Benitez, Marcelo Barros, Hans Jelitto, Nahum Travitzky, Dachamir Hotza and Rolf Janssen, R-CURVE BEHAVIOR AND FLEXURAL STRENGTH OF ZIRCONIA-TOUGHENED ALUMINA AND PARTIALLY STABILIZED ZIRCONIA COMPOSITE LAMINATES, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.04.107 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
R-CURVE BEHAVIOR AND FLEXURAL STRENGTH OF ZIRCONIATOUGHENED ALUMINA AND PARTIALLY STABILIZED ZIRCONIA COMPOSITE LAMINATES Diego Blaesea, Tobias Benitezb,c, Marcelo Barrosb, Hans Jelittoa, Nahum Travitzkyc, Dachamir Hotzab, Rolf Janssena* a
Institute of Advanced Ceramics, Hamburg University of Technology, Hamburg, 21073, Germany b Department of Chemical Engineering (EQA), Federal University of Santa Catarina (UFSC), 88040-900 Florianópolis, SC, Brazil c Institute of Glass and Ceramics, Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Martensstr. 5, 91058 Erlangen, Germany
*
Corresponding author: [email protected]
Abstract
A composite-laminate formed by thick layers (~320µm) of zirconia-toughened alumina (ZTA) with thin (~50µm) interlayers of zirconia partially stabilized (Y-PSZ) has been fabricated by tape casting and pressureless sintering. Fracture behaviour and strength has been investigated and compared to a “monolithic” reference, e.g. a stack of zirconiatoughened alumina (ZTA) without interlayers. The fracture behaviour has been analysed using stable crack growth in V-notched specimens loaded in 3-point bending. The ZTA+YPSZ composite laminate presented a rising crack resistance with maximum values between 6 and 14 MPa.m1/2. In contrast, the “monolithic” ZTA laminate shows a plateau R-curve behaviour at 2.7 MPa.m1/2. Several toughening mechanisms were identified in the ZTA+YPSZ composite laminate, such as, crack arrest/slow down, micro cracking and bifurcation. These toughening mechanisms are most likely caused by high tensile residual stresses that were estimated theoretically.
Keywords: R-curve, reliability, toughening mechanisms, optimizing laminate design.
1.
Introduction
Laminar ceramics offer a versatile route to modify the crack propagation path during loading thereby enhancing the fracture resistance [1,2,3,4,5]. Particularly, compositelaminates of zirconia-toughened alumina (ZTA) with yttrium-stabilized zirconia (Y-PSZ) appear as an attractive route, as those can be designed with a customized residual-stress profile to deflect and slow down or even arrest cracks, enabling a toughening effect [4,6,7]. ZTA+Y-PSZ composite-laminates may also keep similar mechanical resistance than reference “monolithic” ZTA laminates [5]. Additionally, altering the crack path may enhance contact, wear and erosion performance if related cracks are similar arrested [4,5]. Residual stresses and toughness mechanisms in ZTA+Y-PSZ composite laminates such as: crack arrest, bifurcation and interface delamination have been studied extensively [6,8,9,10]. Strategies for reliable optimization configuration of composite laminates have been as well proposed [3,4,5]. Indeed, numerical designed composite laminates with customized residual stress profile have reported offrering high reliability with standard deviation of the mean flexural strength of 3.6% compared with 8-12% of monolithic materials [4,5]. However, in some of those works, fracture toughness was determined by conventional indentation fracture method [11,12], which is a technique not appropriate for the measurement of the fracture toughness of composites [13]. The mechanical behavior of ceramics materials has been always motivated to produce components of “invariant” final strength, i.e independent of the initial flaw size [14]. Fracture toughness as a function of crack extension, e.g. the fracture resistance curve or Rcurve, can be determined by two different bending test methods [4,13]. The first relies on the determination of fracture toughness of several pre-cracked specimens (SEPB [15], CNB [16] or SCF [17]) each with a different initial crack length. This method uses unstable crack growth and each specimen gives a value of fracture toughness for a respective crack length. The second method relies on the measurement of one single pre-cracked specimen as used in advanced set up by Jelitto and coworker[18]. In this case the specimen is tested in a controlled manner which enables stable crack growth. This was achieved by partially unloading when there is crack propagation and subsequent reloading until the crack further
propagates. This method allows the acquisition of several values of fracture toughness for different crack lengths on one single specimen [18]. In this work, the crack propagation of ZTA+Y-PSZ composite-laminates and reference “monolithic” ZTA laminates prepared by tape casting and sintering have been examined. In addition, characteristic strength and Weibull moduli have been tested with a conventional loading frame in 3-point bending. The toughening mechanisms and residual stresses induced by Y-PSZ interlayers on the composite laminate mechanical behavior are identified and assessed.
2.
Materials and Characterization ZTA slurries containing 25 vol% of solids were prepared using 70 vol% of alumina
(CT 3000 SG, Almatis, d50 = 0.40 μm, Ss 7.60 m2/g) and 30 vol% of zirconia partially stabilized with 3% mol Y2O3 (TZ-3YB-E, Tosoh, Ss 16 m2/g) [7]. 3Y-TZP slurries with 18 vol% of solids were also prepared using the same partially stabilized zirconia powder (TZ3YB-E from Tosoh). First, the ceramic powders were deagglomerated in deionized water with addition of dispersant (Darvan C-N, Vanderbilt) - 1.5 wt% referred to solids content using ball milling with zirconia grinding balls (diameters of 5 and 10 mm) for 24h. Afterwards, binder (Mowilith LDM 6138, Clariant), surfactant (coconut diethanolamide, Stepan) and antifoamer (Antifoam Y-30, Sigma-Aldrich) - 25.0 wt%, 1.5 wt% and 0.5 wt% respectively, referred to solids content - were added and mixed by ball milling for 30 min. The slurries were tape casted using a commericial tape casting machine (CC-1200, Mistler). Tapes were casted onto a carrier film (Mylar G10JRM, Mistler) with casting speed of 6 cm/min and allowed to dry in air at room temperature for 24 h. Composite laminate tiles (30×30×3.2 mm³) were fabricated by stacking interchangeably nine ZTA and eighth 3Y-TZP tapes and warm pressing at 20 MPa and 60 ºC for 5 minutes. The thickness of the ZTA and Y-PSZ layers were 322 ± 16 µm and 50 ± 4 µm, respectively. The reference “monolithic” ZTA laminates were fabricated using the same procedure. Debinding of green laminates was performed by heating up to 600 ºC with a rate of 1 ºC/min and holding time of 1 hour. The laminates were sintered at 1550 °C for 2 hours with heating rate of 5 ºC/min. Cooling was not controlled.
For the flexural test, 11 prismatic bars (3×3×30 mm³) were cut from the tiles. For the R-Curve tests, two smaller prismatic bars (3×1.5×30 mm³) were cut from the tiles as well. To facilitate control of crack growth, one side of these specimens were polished down to 1 µm diamond suspension. The specimens were then notched to 1/3 of the height (1 mm) using a diamond disc saw. Subsequently, the notch was sharpened using an automated razor blade device [19,20]. The final notch radius of all samples was between 5 and 7 µm. A representative image of the R-curve specimen can be seen in Figure 1. Due to the advanced set up, one repetition was sufficient for conclusive results. As previously mentioned, R-curves of brittle materials can be obtained in two different ways: unstable fracture and stable crack growing. The last one represents a more realistic condition; therefore, it was selected for this work. Thus, SEVNB stable crack growth is achieved by partial unloading and reloading cycles (unloading cycles must be fast to avoid additional crack growth) The crack tip is continuously observed with an optical microscope and as soon as the crack startst to propagate the sample is partially unloaded by an automatic, computer-controlled process, based on the continuous evaluation of the sample compliance [18]. Maximum force and crack length are recorded to determine fracture toughness and R-curve. Linear elastic behavior is assumed and the fracture toughness, KIc, calculated following the Griffith-Irwin theory [21]. For a 3-point-bending experiments, we use: KI
3F L a M ( ) b h2
(1)
Here, F is the force on the specimen, L half of the support distance (L = 10 mm), b and h the width and height of the specimen, respectively, a the crack length, and = a/h the relative crack length. The geometric factor M () is:
M ( )
(1 ) 3 / 2
4 0 , 3738 ( 1 ) A h / 2 L , 0
with the 25 coefficients A listed in the reference [22]. Fracture occurs for KI KIc, meaning that F is the maximum force at each loading cycle. The level of residual stresses was considered using Equation 3 and 4 from Chartier and coworkers [23].
(2)
(
)
(
(3)
)
(4)
Here, Δε is the difference in thermal strain between adjacent layers, Ei is the Young’s modulus and νi the Poisson’s ratio, n represents the number of layers and t the thickness. Δε was calculated as upper bound using ∆α*∆T with the sintering temperature as freezing temperature”, e.g. stress relaxation during cooling was neglected. The data used are summarized in Table 1.
Table 1: Number, thickness, coefficient of thermal expansion, Young modulus, and Poisson coefficients of Y-PZS and ZTA layers used for residual stress calculation. ZTA Y-PSZ
n 9 8
t (μm) 322 50
α (10¯⁶K¯¹) 9.39 10.50
E (Gpa) 329 210
υ 0.275 0.310
3.
Results
3.1
Mean flexural strength, characteristic strength and Weibull modulus The ZTA+Y-PSZ composite-laminate and ZTA laminate 3-point mean flexural
strength were 425 ± 53 MPa and 471 ± 65 MPa, respectively. The Weibull modulus of the ZTA+Y-PSZ and ZTA are 7.6 and 6.6 (Figure 2). The mean characteristic strength of the composite-laminate is 450 MPa and for reference “monolithic” ZTA laminate was ~500 MPa.
3.2
R-curve A representative load-displacement curve of the ZTA+Y-PSZ composite-laminate is
presented in Figure 3. It can be seen that the first loading cycle presents the expected linear elastic behavior. After reaching 80% of the maximum force, the crack starts propagating
horizontally, being then slowed down close to the first Y-PSZ layer. By further loading, the crack propagates until the maximum force is reached, followed by growth through the adjacent ZTA layer towards the second Y-PSZ. This growth is accompanied by a sudden load drop. After each load drop, the machine is completely unloaded and reloaded. As the load again increases, the compressive stresses offer the capability to slow down or even arrest the crack growth until the new threshold load is achieved. This phenomenon also takes places for the next Y-PSZ layers. The load-displacement diagram presents the expected step-wise behavior of composite-laminate bending tests [12]. Due to stable crack growth throughout the test, a R-curve load-displacement is achieved as expected [18].
Crack propagation in the specimen can be identified by the first load drop. A visible crack growth, however, can just be identified by the optical microscope after a certain number of loading-unloading cycles. This is due to uneven crack growth throughout the specimen, e.g. the crack front is curved with less progress at the surface. The load-displacement curve for the reference “monolithic” ZTA laminate is presented in Figure 4. The point after which visible crack growth becomes detectable is pointed out in Figure 4 and 5. The subsequent loading-unloading cycles allow the stable crack growth in the specimen and are represented by the short lines in Figure 4. The crack path of a ZTA+Y-PSZ specimen is presented in Figure 5a. As mentioned earlier, the crack was slowed down and deflected close to each Y-PSZ layer and extended afterwards horizontally. After a threshold load, the crack grew through the ZTA layer. A non-linear crack path can be seen. For “monolithic” ZTA specimens (Figure 5b and c), the crack grew linearly throughout the sample, not being arrested or deflected. The R-curve for both ZTA+Y-PSZ and ZTA laminates are presented in Figure 6. Results for two specimens of each material are presented in the graph. The ZTA+Y-PSZ composite-laminates present a rising R-curve starting at about 6 MPa.m1/2. During growth the crack passes five layers of Y-PSZ, resulting in an increment to 14 and 10,5 MPa.m1/2 for sample 1 and 2, respectively. Thereby, the reproducibility between both samples is excellent up to the third 3Y-PSZ layer, i.e. within the crack length area of only moderate toughness increase. From layer four to five, the toughening is more advanced in sample one even though sample two shows also an increased slope.
In contrast, the “monolithic” ZTA laminate exhibits a plateau-like behavior, e.g. nocurve effect. Since the crack propagated in stable manner throughout the sample, values for fracture toughness were determined along the whole specimen thickness. The mean fracture toughness value is 2.7 MPa.m1/2. Several toughening mechanisms in the ZTA+Y-PSZ composite-laminate can be identified. Figure 7a and b indicates crack slow-down phenomena [14,24], which took place almost close to all Y-PSZ/ZTA interfaces. Figure 7c and d indicate microcracking toughening mechanism [14]. After crack slow-down, several microcracks were created along the Y-PSZ layers. These microcracks extended themselves horizontally for more than 500 µm before the main crack propagated transversally towards the next Y-PSZ layer. In addition, crack bifurcation occurs in the YSZ layer. Thereby, impinging cracks bifurcate and propagate inside the Y-PSZ layer before growing inside the ZTA layer (see Figure 7e and f) [25]. 4.
Discussion The “monolithic” ZTA laminates present a higher mean flexural strength than
ZTA+Y-PSZ composite-laminates. However, t tests shown that these values are not significantly different for a 95% confidence interval. The ZTA+Y-PSZ compositelaminates retain around 90% of the ZTA laminate mean flexural strength – in spite of the inner residual stresses. As expected, the Weibull modulus was higher for ZTA+Y-PSZ. As consequence, the design stress level can be higher for the composite-laminates in case classical layout schemes for structural parts are used. The ZTA+Y-PSZ composite-laminates present a rising R-curve behavior. The initial fracture toughness is ~150% higher than the mean fracture toughness of the “monolithic” ZTA laminates (6.9 to 2.7 MPa.m1/2). After 1.5 mm of crack extension, the fracture toughness was app four times higher than the toughness of the reference, e.g. 11,6 to 2.7 MPa.m1/2. The ZTA+Y-PSZ composite-laminate presented a step-wise fracture. The crack was slowed down and accelerated again by the residual stresses which can result already in a low to moderate toughening [26]. More important, the residual stress favor a modification of the crack path as well as bifurcation/branching and microcracking, the latter most likely in a zone around the advancing main crack, resulting in increase of the crack resistance
during crack growth. Thereby, the R-curve effect was particularly apparent with advancement from the fourth to the fifth layer of Y-TZP. It is postulated that the toughening is essentially caused by residual stresses created by alternating thin Y-PSZ and thick ZTA layers. Calculation of the stress level in these layers by equation 3 and 4 gives approximately 390 MPa for the Y-PSZ layers (Y-PSZ) and -50 MPa, for ZTA layers (ZTA). It is important to note that this analytical model does not take into account relaxation of transient stresses during cooling, specifically at higher temperatures. This means that the effective residual stresses are most likely lower than those calculated. Detailed experimental quantification would be needed in order to access these stresses precisely, However, this afford is not adequate here as the general behavior described above will be not altered. The ZTA laminates didn´t exhibit any macroscopic toughening mechanisms resulting in a so called “R-curve effect”, e.g. a straight and constant crack growth resistance during crack advancement is observed.
5.
Conclusions The flexural strength and crack resistance curve (R-curve) of ZTA+Y-PSZ
composite-laminates have been assessed with “monolithic” ZTA laminates as reference. The R-curves have been measured in 3-point bending using a stiff frame with computercontrolled loading regime which even enables precise R-curve detection of bars with residual stresses. ZTA+Y-PSZ composite-laminate presented significantly higher fracture toughness and a rising R-curve whereas ZTA laminates presented a plateau R-curve at much lower toughness level. It was postulated that the rising R-curve is due to toughening mechanisms caused by the tensile residual stresses in the Y-PSZ layers. The main identified toughening mechanisms addressed are crack-velocity variations, microcracking and crack bifurcation. Both laminates presented a comparable flexural with no statistically significant difference between the ZTA+Y-PSZ and ZTA laminates. This approach of constructing multi-laminates with thin layers under high tensile and thick layers of low compressive residual stresses shows promising first and offer potential for further optimization.
Acknowledgement: The work was sponsored by a bilateral funding program of DAAD (Germany) and CNPq (Brazil). D.B. and T.B. thank both agencies for supporting their exchange visits.
6.
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Figure 1. a) Side view of R-curve ZTA+Y-PSZ specimen. Notch is 1/3 of the height and notch radius 6.28 µm b) Detail on notch tip. Notch radius is 6.28 µm. f (MPa) 330
403
493
602
735
898 100
2
80
0
60
Pf (%)
ln ln (1/(1-Pf)
1
ZTA ZTA + Y-PSZ Sglavo et al., (2006) AZ40 Sglavo et al., (2006) AMZ
-1 40 -2
-3
-4
m (slope) Characteristic Strength R-Square Nunmer of specimens
5.8
ZTA
ZTA+Y-PSZ AZ40
6.6 500 92.2 11
7.6 450 96.5 11
6.0
6.2
20
AMZ 12 856 98.5 15
41 790 96.9 16
6.4
0 6.6
6.8
ln f (ln MPa)
Figure 2. Weibull plot of ZTA+Y-PSZ composite-laminate and ZTA laminate. Additionally, a reference optimized laminate of alumina/zirconia and alumina/mullite is plotted for comparison.
Figure 3. Force-displacement graph of R-curve measurement for ZTA+Y-PSZ compositelaminate
Figure 4. Force-displacement graph of R-curve measurement for “monolithic” ZTA specimen. Red dots represent the points used for calculation of the R-curve.
Figure 5. Crack path. a) ZTA+Y-PSZ, crack was arrested and deflected in each Y-PSZ layer. Crack bifurcation can be seen. (compound image). b) and c) ZTA (reference), crack grew linearly throughout the specimen.
Figure 6 R-Curves of ZTA+Y-PSZ and ZTA laminates.
Figure 7. Evolution of toughening mechanisms caused by the Y-PSZ layers.
PII: DOI: Reference:
www.elsevier.com/locate/ceri
S0272-8842(18)30977-5 https://doi.org/10.1016/j.ceramint.2018.04.107 CERI18024
To appear in: Ceramics International Received date: 1 March 2018 Accepted date: 12 April 2018 Cite this article as: Diego Blaese, Tobias Benitez, Marcelo Barros, Hans Jelitto, Nahum Travitzky, Dachamir Hotza and Rolf Janssen, R-CURVE BEHAVIOR AND FLEXURAL STRENGTH OF ZIRCONIA-TOUGHENED ALUMINA AND PARTIALLY STABILIZED ZIRCONIA COMPOSITE LAMINATES, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.04.107 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
R-CURVE BEHAVIOR AND FLEXURAL STRENGTH OF ZIRCONIATOUGHENED ALUMINA AND PARTIALLY STABILIZED ZIRCONIA COMPOSITE LAMINATES Diego Blaesea, Tobias Benitezb,c, Marcelo Barrosb, Hans Jelittoa, Nahum Travitzkyc, Dachamir Hotzab, Rolf Janssena* a
Institute of Advanced Ceramics, Hamburg University of Technology, Hamburg, 21073, Germany b Department of Chemical Engineering (EQA), Federal University of Santa Catarina (UFSC), 88040-900 Florianópolis, SC, Brazil c Institute of Glass and Ceramics, Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Martensstr. 5, 91058 Erlangen, Germany
*
Corresponding author: [email protected]
Abstract
A composite-laminate formed by thick layers (~320µm) of zirconia-toughened alumina (ZTA) with thin (~50µm) interlayers of zirconia partially stabilized (Y-PSZ) has been fabricated by tape casting and pressureless sintering. Fracture behaviour and strength has been investigated and compared to a “monolithic” reference, e.g. a stack of zirconiatoughened alumina (ZTA) without interlayers. The fracture behaviour has been analysed using stable crack growth in V-notched specimens loaded in 3-point bending. The ZTA+YPSZ composite laminate presented a rising crack resistance with maximum values between 6 and 14 MPa.m1/2. In contrast, the “monolithic” ZTA laminate shows a plateau R-curve behaviour at 2.7 MPa.m1/2. Several toughening mechanisms were identified in the ZTA+YPSZ composite laminate, such as, crack arrest/slow down, micro cracking and bifurcation. These toughening mechanisms are most likely caused by high tensile residual stresses that were estimated theoretically.
Keywords: R-curve, reliability, toughening mechanisms, optimizing laminate design.
1.
Introduction
Laminar ceramics offer a versatile route to modify the crack propagation path during loading thereby enhancing the fracture resistance [1,2,3,4,5]. Particularly, compositelaminates of zirconia-toughened alumina (ZTA) with yttrium-stabilized zirconia (Y-PSZ) appear as an attractive route, as those can be designed with a customized residual-stress profile to deflect and slow down or even arrest cracks, enabling a toughening effect [4,6,7]. ZTA+Y-PSZ composite-laminates may also keep similar mechanical resistance than reference “monolithic” ZTA laminates [5]. Additionally, altering the crack path may enhance contact, wear and erosion performance if related cracks are similar arrested [4,5]. Residual stresses and toughness mechanisms in ZTA+Y-PSZ composite laminates such as: crack arrest, bifurcation and interface delamination have been studied extensively [6,8,9,10]. Strategies for reliable optimization configuration of composite laminates have been as well proposed [3,4,5]. Indeed, numerical designed composite laminates with customized residual stress profile have reported offrering high reliability with standard deviation of the mean flexural strength of 3.6% compared with 8-12% of monolithic materials [4,5]. However, in some of those works, fracture toughness was determined by conventional indentation fracture method [11,12], which is a technique not appropriate for the measurement of the fracture toughness of composites [13]. The mechanical behavior of ceramics materials has been always motivated to produce components of “invariant” final strength, i.e independent of the initial flaw size [14]. Fracture toughness as a function of crack extension, e.g. the fracture resistance curve or Rcurve, can be determined by two different bending test methods [4,13]. The first relies on the determination of fracture toughness of several pre-cracked specimens (SEPB [15], CNB [16] or SCF [17]) each with a different initial crack length. This method uses unstable crack growth and each specimen gives a value of fracture toughness for a respective crack length. The second method relies on the measurement of one single pre-cracked specimen as used in advanced set up by Jelitto and coworker[18]. In this case the specimen is tested in a controlled manner which enables stable crack growth. This was achieved by partially unloading when there is crack propagation and subsequent reloading until the crack further
propagates. This method allows the acquisition of several values of fracture toughness for different crack lengths on one single specimen [18]. In this work, the crack propagation of ZTA+Y-PSZ composite-laminates and reference “monolithic” ZTA laminates prepared by tape casting and sintering have been examined. In addition, characteristic strength and Weibull moduli have been tested with a conventional loading frame in 3-point bending. The toughening mechanisms and residual stresses induced by Y-PSZ interlayers on the composite laminate mechanical behavior are identified and assessed.
2.
Materials and Characterization ZTA slurries containing 25 vol% of solids were prepared using 70 vol% of alumina
(CT 3000 SG, Almatis, d50 = 0.40 μm, Ss 7.60 m2/g) and 30 vol% of zirconia partially stabilized with 3% mol Y2O3 (TZ-3YB-E, Tosoh, Ss 16 m2/g) [7]. 3Y-TZP slurries with 18 vol% of solids were also prepared using the same partially stabilized zirconia powder (TZ3YB-E from Tosoh). First, the ceramic powders were deagglomerated in deionized water with addition of dispersant (Darvan C-N, Vanderbilt) - 1.5 wt% referred to solids content using ball milling with zirconia grinding balls (diameters of 5 and 10 mm) for 24h. Afterwards, binder (Mowilith LDM 6138, Clariant), surfactant (coconut diethanolamide, Stepan) and antifoamer (Antifoam Y-30, Sigma-Aldrich) - 25.0 wt%, 1.5 wt% and 0.5 wt% respectively, referred to solids content - were added and mixed by ball milling for 30 min. The slurries were tape casted using a commericial tape casting machine (CC-1200, Mistler). Tapes were casted onto a carrier film (Mylar G10JRM, Mistler) with casting speed of 6 cm/min and allowed to dry in air at room temperature for 24 h. Composite laminate tiles (30×30×3.2 mm³) were fabricated by stacking interchangeably nine ZTA and eighth 3Y-TZP tapes and warm pressing at 20 MPa and 60 ºC for 5 minutes. The thickness of the ZTA and Y-PSZ layers were 322 ± 16 µm and 50 ± 4 µm, respectively. The reference “monolithic” ZTA laminates were fabricated using the same procedure. Debinding of green laminates was performed by heating up to 600 ºC with a rate of 1 ºC/min and holding time of 1 hour. The laminates were sintered at 1550 °C for 2 hours with heating rate of 5 ºC/min. Cooling was not controlled.
For the flexural test, 11 prismatic bars (3×3×30 mm³) were cut from the tiles. For the R-Curve tests, two smaller prismatic bars (3×1.5×30 mm³) were cut from the tiles as well. To facilitate control of crack growth, one side of these specimens were polished down to 1 µm diamond suspension. The specimens were then notched to 1/3 of the height (1 mm) using a diamond disc saw. Subsequently, the notch was sharpened using an automated razor blade device [19,20]. The final notch radius of all samples was between 5 and 7 µm. A representative image of the R-curve specimen can be seen in Figure 1. Due to the advanced set up, one repetition was sufficient for conclusive results. As previously mentioned, R-curves of brittle materials can be obtained in two different ways: unstable fracture and stable crack growing. The last one represents a more realistic condition; therefore, it was selected for this work. Thus, SEVNB stable crack growth is achieved by partial unloading and reloading cycles (unloading cycles must be fast to avoid additional crack growth) The crack tip is continuously observed with an optical microscope and as soon as the crack startst to propagate the sample is partially unloaded by an automatic, computer-controlled process, based on the continuous evaluation of the sample compliance [18]. Maximum force and crack length are recorded to determine fracture toughness and R-curve. Linear elastic behavior is assumed and the fracture toughness, KIc, calculated following the Griffith-Irwin theory [21]. For a 3-point-bending experiments, we use: KI
3F L a M ( ) b h2
(1)
Here, F is the force on the specimen, L half of the support distance (L = 10 mm), b and h the width and height of the specimen, respectively, a the crack length, and = a/h the relative crack length. The geometric factor M () is:
M ( )
(1 ) 3 / 2
4 0 , 3738 ( 1 ) A h / 2 L , 0
with the 25 coefficients A listed in the reference [22]. Fracture occurs for KI KIc, meaning that F is the maximum force at each loading cycle. The level of residual stresses was considered using Equation 3 and 4 from Chartier and coworkers [23].
(2)
(
)
(
(3)
)
(4)
Here, Δε is the difference in thermal strain between adjacent layers, Ei is the Young’s modulus and νi the Poisson’s ratio, n represents the number of layers and t the thickness. Δε was calculated as upper bound using ∆α*∆T with the sintering temperature as freezing temperature”, e.g. stress relaxation during cooling was neglected. The data used are summarized in Table 1.
Table 1: Number, thickness, coefficient of thermal expansion, Young modulus, and Poisson coefficients of Y-PZS and ZTA layers used for residual stress calculation. ZTA Y-PSZ
n 9 8
t (μm) 322 50
α (10¯⁶K¯¹) 9.39 10.50
E (Gpa) 329 210
υ 0.275 0.310
3.
Results
3.1
Mean flexural strength, characteristic strength and Weibull modulus The ZTA+Y-PSZ composite-laminate and ZTA laminate 3-point mean flexural
strength were 425 ± 53 MPa and 471 ± 65 MPa, respectively. The Weibull modulus of the ZTA+Y-PSZ and ZTA are 7.6 and 6.6 (Figure 2). The mean characteristic strength of the composite-laminate is 450 MPa and for reference “monolithic” ZTA laminate was ~500 MPa.
3.2
R-curve A representative load-displacement curve of the ZTA+Y-PSZ composite-laminate is
presented in Figure 3. It can be seen that the first loading cycle presents the expected linear elastic behavior. After reaching 80% of the maximum force, the crack starts propagating
horizontally, being then slowed down close to the first Y-PSZ layer. By further loading, the crack propagates until the maximum force is reached, followed by growth through the adjacent ZTA layer towards the second Y-PSZ. This growth is accompanied by a sudden load drop. After each load drop, the machine is completely unloaded and reloaded. As the load again increases, the compressive stresses offer the capability to slow down or even arrest the crack growth until the new threshold load is achieved. This phenomenon also takes places for the next Y-PSZ layers. The load-displacement diagram presents the expected step-wise behavior of composite-laminate bending tests [12]. Due to stable crack growth throughout the test, a R-curve load-displacement is achieved as expected [18].
Crack propagation in the specimen can be identified by the first load drop. A visible crack growth, however, can just be identified by the optical microscope after a certain number of loading-unloading cycles. This is due to uneven crack growth throughout the specimen, e.g. the crack front is curved with less progress at the surface. The load-displacement curve for the reference “monolithic” ZTA laminate is presented in Figure 4. The point after which visible crack growth becomes detectable is pointed out in Figure 4 and 5. The subsequent loading-unloading cycles allow the stable crack growth in the specimen and are represented by the short lines in Figure 4. The crack path of a ZTA+Y-PSZ specimen is presented in Figure 5a. As mentioned earlier, the crack was slowed down and deflected close to each Y-PSZ layer and extended afterwards horizontally. After a threshold load, the crack grew through the ZTA layer. A non-linear crack path can be seen. For “monolithic” ZTA specimens (Figure 5b and c), the crack grew linearly throughout the sample, not being arrested or deflected. The R-curve for both ZTA+Y-PSZ and ZTA laminates are presented in Figure 6. Results for two specimens of each material are presented in the graph. The ZTA+Y-PSZ composite-laminates present a rising R-curve starting at about 6 MPa.m1/2. During growth the crack passes five layers of Y-PSZ, resulting in an increment to 14 and 10,5 MPa.m1/2 for sample 1 and 2, respectively. Thereby, the reproducibility between both samples is excellent up to the third 3Y-PSZ layer, i.e. within the crack length area of only moderate toughness increase. From layer four to five, the toughening is more advanced in sample one even though sample two shows also an increased slope.
In contrast, the “monolithic” ZTA laminate exhibits a plateau-like behavior, e.g. nocurve effect. Since the crack propagated in stable manner throughout the sample, values for fracture toughness were determined along the whole specimen thickness. The mean fracture toughness value is 2.7 MPa.m1/2. Several toughening mechanisms in the ZTA+Y-PSZ composite-laminate can be identified. Figure 7a and b indicates crack slow-down phenomena [14,24], which took place almost close to all Y-PSZ/ZTA interfaces. Figure 7c and d indicate microcracking toughening mechanism [14]. After crack slow-down, several microcracks were created along the Y-PSZ layers. These microcracks extended themselves horizontally for more than 500 µm before the main crack propagated transversally towards the next Y-PSZ layer. In addition, crack bifurcation occurs in the YSZ layer. Thereby, impinging cracks bifurcate and propagate inside the Y-PSZ layer before growing inside the ZTA layer (see Figure 7e and f) [25]. 4.
Discussion The “monolithic” ZTA laminates present a higher mean flexural strength than
ZTA+Y-PSZ composite-laminates. However, t tests shown that these values are not significantly different for a 95% confidence interval. The ZTA+Y-PSZ compositelaminates retain around 90% of the ZTA laminate mean flexural strength – in spite of the inner residual stresses. As expected, the Weibull modulus was higher for ZTA+Y-PSZ. As consequence, the design stress level can be higher for the composite-laminates in case classical layout schemes for structural parts are used. The ZTA+Y-PSZ composite-laminates present a rising R-curve behavior. The initial fracture toughness is ~150% higher than the mean fracture toughness of the “monolithic” ZTA laminates (6.9 to 2.7 MPa.m1/2). After 1.5 mm of crack extension, the fracture toughness was app four times higher than the toughness of the reference, e.g. 11,6 to 2.7 MPa.m1/2. The ZTA+Y-PSZ composite-laminate presented a step-wise fracture. The crack was slowed down and accelerated again by the residual stresses which can result already in a low to moderate toughening [26]. More important, the residual stress favor a modification of the crack path as well as bifurcation/branching and microcracking, the latter most likely in a zone around the advancing main crack, resulting in increase of the crack resistance
during crack growth. Thereby, the R-curve effect was particularly apparent with advancement from the fourth to the fifth layer of Y-TZP. It is postulated that the toughening is essentially caused by residual stresses created by alternating thin Y-PSZ and thick ZTA layers. Calculation of the stress level in these layers by equation 3 and 4 gives approximately 390 MPa for the Y-PSZ layers (Y-PSZ) and -50 MPa, for ZTA layers (ZTA). It is important to note that this analytical model does not take into account relaxation of transient stresses during cooling, specifically at higher temperatures. This means that the effective residual stresses are most likely lower than those calculated. Detailed experimental quantification would be needed in order to access these stresses precisely, However, this afford is not adequate here as the general behavior described above will be not altered. The ZTA laminates didn´t exhibit any macroscopic toughening mechanisms resulting in a so called “R-curve effect”, e.g. a straight and constant crack growth resistance during crack advancement is observed.
5.
Conclusions The flexural strength and crack resistance curve (R-curve) of ZTA+Y-PSZ
composite-laminates have been assessed with “monolithic” ZTA laminates as reference. The R-curves have been measured in 3-point bending using a stiff frame with computercontrolled loading regime which even enables precise R-curve detection of bars with residual stresses. ZTA+Y-PSZ composite-laminate presented significantly higher fracture toughness and a rising R-curve whereas ZTA laminates presented a plateau R-curve at much lower toughness level. It was postulated that the rising R-curve is due to toughening mechanisms caused by the tensile residual stresses in the Y-PSZ layers. The main identified toughening mechanisms addressed are crack-velocity variations, microcracking and crack bifurcation. Both laminates presented a comparable flexural with no statistically significant difference between the ZTA+Y-PSZ and ZTA laminates. This approach of constructing multi-laminates with thin layers under high tensile and thick layers of low compressive residual stresses shows promising first and offer potential for further optimization.
Acknowledgement: The work was sponsored by a bilateral funding program of DAAD (Germany) and CNPq (Brazil). D.B. and T.B. thank both agencies for supporting their exchange visits.
6.
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Figure 1. a) Side view of R-curve ZTA+Y-PSZ specimen. Notch is 1/3 of the height and notch radius 6.28 µm b) Detail on notch tip. Notch radius is 6.28 µm. f (MPa) 330
403
493
602
735
898 100
2
80
0
60
Pf (%)
ln ln (1/(1-Pf)
1
ZTA ZTA + Y-PSZ Sglavo et al., (2006) AZ40 Sglavo et al., (2006) AMZ
-1 40 -2
-3
-4
m (slope) Characteristic Strength R-Square Nunmer of specimens
5.8
ZTA
ZTA+Y-PSZ AZ40
6.6 500 92.2 11
7.6 450 96.5 11
6.0
6.2
20
AMZ 12 856 98.5 15
41 790 96.9 16
6.4
0 6.6
6.8
ln f (ln MPa)
Figure 2. Weibull plot of ZTA+Y-PSZ composite-laminate and ZTA laminate. Additionally, a reference optimized laminate of alumina/zirconia and alumina/mullite is plotted for comparison.
Figure 3. Force-displacement graph of R-curve measurement for ZTA+Y-PSZ compositelaminate
Figure 4. Force-displacement graph of R-curve measurement for “monolithic” ZTA specimen. Red dots represent the points used for calculation of the R-curve.
Figure 5. Crack path. a) ZTA+Y-PSZ, crack was arrested and deflected in each Y-PSZ layer. Crack bifurcation can be seen. (compound image). b) and c) ZTA (reference), crack grew linearly throughout the specimen.
Figure 6 R-Curves of ZTA+Y-PSZ and ZTA laminates.
Figure 7. Evolution of toughening mechanisms caused by the Y-PSZ layers.