1

Introduction

Zirconia toughened ceramics (ZTC) are attractive materials for several medical and engineering applications as they exhibit high strength and toughness compared to other oxide ceramics. These superior properties result from the stress induced tetragonal to monoclinic phase transformation occurring in these materials. During crack propagation, the volume increase (∼4%) accompanying the transformation creates compressive stresses that shield the crack tip from the applied stress and thus, enhances the fracture toughness . The properties of ZTC depend on their microstructure and composition, especially the amount of transformable tetragonal phase. The major difficulty for the development of transformation-toughened zirconia composites is to combine high strength and high toughness, as the strength is limited by the stress-activated transformation in these materials . For example, ceria-doped zirconia (Ce-TZP) can exhibit high toughness (more than 15 MPa m ^1/2 ) but moderate strength (300–500 MPa) as compared to yttria-doped zirconia (Y-TZP) with high strength (800–1500 MPa) but low toughness (∼6 MPa m ^1/2 ). The moderate strength of Ce-TZP ceramics is due to large grains and a high propensity to stress induced phase transformation, which can be autocatalytic and may occur at low stresses.

During the past two decades, a new generation of zirconia Ce-TZP based composites was developed with different stabilizers and compositions to obtain tough and strong materials, through microstructure refinement. A Ce-TZP/Al ₂ O ₃ nanocomposite in which nanometer Al ₂ O ₃ and 10Ce-TZP particles were trapped within submicron 10Ce-TZP and Al ₂ O ₃ grains, respectively, showed high toughness (9.8 MPa m ^1/2 ) and a strength of 950 MPa, equivalent to that of conventional 3Y-TZP. A Ce-TZP/MgAl ₂ O ₄ nanocomposite with inter and intragranular dispersion of nano-scaled magnesia spinel in a Ce-TZP matrix, developed by Apel et al. showed a biaxial flexural strength of more than 900 MPa. Another approach consists of developing “in situ” platelets reinforced composites with Ce-TZP zirconia reinforced with alumina and strontium or lanthanum hexaaluminate . High fracture resistance (up to 12 MPa m ^1/2 ) and attractive strength (∼800 MPa) were also reported for these materials .

However, comparison between reported properties is difficult, as the characteristics are generally not obtained with the same methods, while it is evident from prior art that the testing method plays a large role on measured mechanical values. High toughness values (>14 MPa m ^1/2 ) are for example observed when evaluated by the indentation fracture method or with long propagated cracks in double cantilever beam or double torsion methods, while they are generally lower when measured by single-edge-V-notched beam (SEVNB) . This is due to the variation of transformation morphology and the stress distribution around the crack path, as evidenced recently by Nawa in the case of a Ce-TZP/alumina nanocomposite . Different methods are also used for strength measurement. Parallel to the popular flexural in bending tests, biaxial flexural tests are increasingly used, especially for bioceramics due to the simple preparation of required samples. Different configurations were used as piston-on-three balls , ball-on-ring and ball-on-three balls . Again, direct comparison between strength values derived from different testing methods is not always valid, and this is generally analyzed through Weibull statistics and the consideration of volume under tensile stresses . In the case of transformation-toughened ceramics, the dependence of tetragonal to monoclinic (t–m) transformation to the testing geometry may also account, but has been little investigated.

In this study, the fracture behavior of a 10Ce-TZP/Al ₂ O ₃ /La ₂ AlO ₃ composite exhibiting a high propensity to transformation is investigated. The fracture toughness and flexural strength were evaluated with a focus on the influence of the testing configuration on the flexural strength. Uniaxial and biaxial flexural configurations were performed using four-point bending and piston-on-three balls methods respectively, as model tests often used to the certification of bio-ceramics (see ISO 6872 , dedicated to ceramic materials in dentistry).

2

Material and methods

A10Ce-TZP/Al ₂ O ₃ /La ₂ AlO ₃ composite, processed by a German laboratory (DOCERAM) in the European framework of Longlife project, currently developed as a biomaterial for dental application was tested. It is composed of 80 vol% 10Ce-TZP with 10 mol% ceria, 10 vol% Al ₂ O ₃ and 10 vol% of (La ₂ AlO ₃ ). Two types of samples were provided: rectangular bars with polished edges (4 mm × 3 mm × 40 mm) for bending tests and disks (diameter of 15 mm and thickness of 1.2 mm) for biaxial flexural tests. Poisson’s ratio and elastic modulus were determined by the resonance vibration method. The density was measured by Archimedes method using distilled water, and the hardness with a Vickers indentation. The microstructure of the composite was observed by scanning electron microscopy (SEM), and X-ray diffraction (XRD) analysis was applied to determine the monoclinic zirconia content .

Toughness and strength measurement were conducted on a universal hydraulic INSTRON 8500 testing machine at room temperature (∼0 °C) and humidity (RH ∼0%). The fracture toughness was determined by the single edge V notched beam (SEVNB) method . Rectangular bars were polished with diamond pastes down to 1 μm to observe the crack extension behavior and notched to a relative depth of 0.4, using a diamond blade with a thickness of 0.2 mm. The notches were then sharpened with a fine razor blade and diamond paste of 1 μm. The samples were annealed at 1200 °C for 20 min to eliminate the machining residual stresses and loaded in a 4-point-bending device (10–35 mm) at a cross-head speed of 5 mm/min. The crack growth resistance curve ( R -curve) was determined using annealed SENVB samples loaded in three-point bending, with a span of 35 mm and a cross-head speed of 0.005 mm/min. The crack growth resistance was determined, from the recorded load–displacement curve, in terms of stress intensity factor, K _R , plotted versus the crack extension, Δ a , optically measured.

Strength measurements were conducted by four-point bending (4PB) tests with roller spacings of 35 and 10 mm and biaxial bending tests (piston-on-three balls, POB) according to ISO 6872, dedicated to ceramic materials in dentistry. The only difference with the ISO recommendation was that the surface of the majority of the samples was polished down to 1 μm with a diamond paste, to be able to observe the t–m transformation with an optical microscope in Normarski contrast. In that case, the strength measurements were conducted on polished and annealed samples (1200 °C for 20 min) in order to suppress any t–m transformation and residual stresses that may be present after the preparation steps. Strength tests were performed at a displacement rate of 5 mm/min, using different testing devices to achieve two stress states: uniaxial tension in four-point bending with rectangular bars and roller spacing of 35 and 10 mm, and Biaxial bending, with disks in piston-on-three balls tests ( Fig. 1 ). To investigate the effect of residual stresses on the strength, a set of “as received” machined disks were also tested in the biaxial configuration. The biaxial flexure strength, σ _B was calculated as follows :

where P is the fracture load, t is the specimen thickness, X and Y are respectively given by:

Schematic illustration of the piston-on-three balls biaxial flexural test.

where υ is the poison’s ratio, a = 6 mm is the radius of the supporting circle ( Fig. 1 ), c = 0.795 mm is the radius of contact area with the piston and R is the radius of specimen.

The stress field in the disk specimen was analyzed using the finite element (FE) software COMSOL, with Boundary conditions and loading modeled so as to reproduce the conditions of the piston-on-three balls tests: (i) the loading piston is allowed to move only vertically; (ii) the applied load was considered uniformly distributed on the contact area between the piston and the disk; (iii) the support balls positioned at equal distance from each other are fixed in position.

2

Material and methods

A10Ce-TZP/Al ₂ O ₃ /La ₂ AlO ₃ composite, processed by a German laboratory (DOCERAM) in the European framework of Longlife project, currently developed as a biomaterial for dental application was tested. It is composed of 80 vol% 10Ce-TZP with 10 mol% ceria, 10 vol% Al ₂ O ₃ and 10 vol% of (La ₂ AlO ₃ ). Two types of samples were provided: rectangular bars with polished edges (4 mm × 3 mm × 40 mm) for bending tests and disks (diameter of 15 mm and thickness of 1.2 mm) for biaxial flexural tests. Poisson’s ratio and elastic modulus were determined by the resonance vibration method. The density was measured by Archimedes method using distilled water, and the hardness with a Vickers indentation. The microstructure of the composite was observed by scanning electron microscopy (SEM), and X-ray diffraction (XRD) analysis was applied to determine the monoclinic zirconia content .

Toughness and strength measurement were conducted on a universal hydraulic INSTRON 8500 testing machine at room temperature (∼0 °C) and humidity (RH ∼0%). The fracture toughness was determined by the single edge V notched beam (SEVNB) method . Rectangular bars were polished with diamond pastes down to 1 μm to observe the crack extension behavior and notched to a relative depth of 0.4, using a diamond blade with a thickness of 0.2 mm. The notches were then sharpened with a fine razor blade and diamond paste of 1 μm. The samples were annealed at 1200 °C for 20 min to eliminate the machining residual stresses and loaded in a 4-point-bending device (10–35 mm) at a cross-head speed of 5 mm/min. The crack growth resistance curve ( R -curve) was determined using annealed SENVB samples loaded in three-point bending, with a span of 35 mm and a cross-head speed of 0.005 mm/min. The crack growth resistance was determined, from the recorded load–displacement curve, in terms of stress intensity factor, K _R , plotted versus the crack extension, Δ a , optically measured.

Strength measurements were conducted by four-point bending (4PB) tests with roller spacings of 35 and 10 mm and biaxial bending tests (piston-on-three balls, POB) according to ISO 6872, dedicated to ceramic materials in dentistry. The only difference with the ISO recommendation was that the surface of the majority of the samples was polished down to 1 μm with a diamond paste, to be able to observe the t–m transformation with an optical microscope in Normarski contrast. In that case, the strength measurements were conducted on polished and annealed samples (1200 °C for 20 min) in order to suppress any t–m transformation and residual stresses that may be present after the preparation steps. Strength tests were performed at a displacement rate of 5 mm/min, using different testing devices to achieve two stress states: uniaxial tension in four-point bending with rectangular bars and roller spacing of 35 and 10 mm, and Biaxial bending, with disks in piston-on-three balls tests ( Fig. 1 ). To investigate the effect of residual stresses on the strength, a set of “as received” machined disks were also tested in the biaxial configuration. The biaxial flexure strength, σ _B was calculated as follows :

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