The Peculiarities of Structure Formation and Properties of Zirconia-Based Nanocomposites with Addition of Al2O3 and NiO
© The Author(s). 2017
Received: 7 December 2016
Accepted: 6 February 2017
Published: 17 February 2017
The present study is devoted to the problem of enhancing fracture toughness of ZrO2 ceramic materials through the formation of composite structure by addition of Al2O3 and NiO particles. In this paper, we analyzed the general and distinguished features of microstructure of both composite materials and its effect on fracture toughness of materials. In this paper, we used the XRD, SEM, and EDS methods for determination of granulometric, phase, and chemical composition of sintered materials. The peculiarities of dependence of fracture toughness values from dopant concentration and changing the Y3+ amount in zirconia grains allow us to assume that at least two mechanisms can affect the fracture toughness of ZrO2 ceramics. Crack bridging/deflection processes with the “transformation toughening” affect the K1C values depending on the dopant concentration. Crack deflection mechanism affects the K1C values when the dopant concentrations are low, and transformation toughening affects the K1C values when the dopant concentrations begin to have an impact on microstructure reorganization–redistribution of Y3+ ions and formation of Y3+-depleted grains with high ability to phase transformation.
KeywordsZirconia composites Fracture toughness Phase transformation
Technical progress every year presents new and more stringent performance requirements for materials and devices. The lifetime and reliability of the devices should be increased, and the wear and fracture should be decreased significantly. This is especially true for durable ceramic products, operating in aggressive environments and at high temperatures, when the fracture of article can be initiated by small damage (pore, scratch, defects). This leads to formation of a new trend in material design—the production of materials tolerant to the defects. So, the structure of material, in our case ceramic material, must withstand external shocks. For example, these are self-healing materials [1, 2]. The corrosion resistance of silicon nitride ceramics can be increased by modifying its secondary phase. The formation of insoluble oxide layers will strongly reduce damage caused by subcritical crack growth. Creation of ceramic matrix composite (CMC) and metal matrix composite (MMC) materials also is the way of formation of the special microstructures in materials with enhanced properties. So, the development of new methods of formation of predetermined structure of ceramic material may improve the reliability of ceramic materials.
As it is known, the damage mechanisms depend on the structure and the type of the materials. Also, it is known that the hard materials tend to be brittle and materials with lower strength tend to be tougher. The damage process is associated with initiation and propagation of cracks in the material. As it was shown by Ritchie “the intrinsic damage processes that operate ahead of the tip of a crack to promote its propagation, and extrinsic crack-tip-shielding mechanisms that act mostly behind the crack tip to inhibit this propagation” . Intrinsic toughening is the source of fracture resistance in ductile materials. This mechanism is effective against the initiation and propagation of cracks. Most metallic materials are toughened by this mechanism. Usually, brittle materials, such as ceramics, cannot be toughened by plastic deformation and have low values of fracture toughness. Typically, fracture toughness values for Al2O3, SiC, and Si3N4 are less than 3–4 MPa*m1/2. The extrinsic mechanisms, which are inherent to ceramic materials, are only effective in resisting crack propagation; they can have no effect on crack initiation. The basic variants of extrinsic toughening mechanisms are crack or fiber bridging and crack deflection.
Ceramic materials on the basis of zirconia are distinguished from other ceramic materials by the highest fracture toughness value because they demonstrate the well-known “transformation toughening effect,” which took part in the field of stresses caused by the propagation of cracks in sintered materials [4, 5]. The martensitic phase transformation (with increasing specific volume of transformed grains), which takes part in zirconia ceramic, is the manifestation of extrinsic toughening mechanism , but it happen ahead of the crack tip. So, zirconia-based materials have three variants of increasing the fracture toughness value—by transformation toughening, crack bridging, and crack deflection processes. Perhaps, the synergetic effects may be realized in the special composite structure.
There are many studies devoted to the formation of composite structures on the basis of zirconia. Zirconia–alumina (ZrO2–Al2O3) composites have been the subject of extensive research because they couple a high toughness with the desirable properties of alumina, i.e., good resistance to wear and chemical stability [7, 8]. Also, it has been reported that the addition of Al2O3 in a ZrO2 matrix can suppress the low-temperature degradation of mechanical properties of zirconia . The addition of Al2O3, besides their low solubility in ZrO2, practically did not affect the phase stabilization, i.e., zirconia and alumina exist as separate phases [7, 10, 11]. But, our investigations  show that method of composite powder preparation has strong effect on fracture toughness value of material. It was shown that the increasing of K1C value of zirconia ceramics with a small amount of alumina, sintered from nanopowders and obtained using co-precipitation technique, can be conditioned through a series of processes for composite structure formation during precipitation, crystallization, and sintering of composite nanopowders.
A small number of studies have been devoted to the influence of Ni and NiO particles on the fracture toughness of monolithic 3Y-TZP ceramic materials [13, 14]. Another widely known system on the basis of zirconia is 8YZrO2-Ni(NiO) system for SOFC anode . The triple junction between zirconia and Ni particles ensues the high level of catalytic properties of porous ZrO2-Ni(NiO) composite material. As engineering material, the composite Al2O3–Ni is more studied. In the studies [16, 17], it was shown that addition of nickel inclusions in alumina and zirconia matrix leads to increase in the fracture toughness of alumina or zirconia ceramic material. In this work, it has found two facts which addition of NiO during sintering promotes to (i) stabilization of a cubic phase of zirconia and (ii) destabilization of tetragonal phase and a formation of monoclinic phase. The formation of monoclinic phase even at small quantities of NiO (0.3–2 wt%) leads to sample destruction. In our previous study , it was found that the phase transformation from tetragonal to monoclinic phase in 3Y-TZP-NiO composite occurs only during sintering in air environment, and during sintering in argon environment, there are no traces of monoclinic phase, but the fracture toughness value increased by 40–50% . The increasing amount of cubic phase of zirconia was also found. Probably in these two cases [12, 19], we can see the synergetic effect—increasing transformability of zirconia T-phase, and crack deflection/bridging caused by appearing of zone of tensile or compressive stresses near the inclusions. In turn, inclusions are realized during creation and decomposition of solid solutions under sintering process, but the physical properties (coefficient of thermal expansion, Young’s modulus, etc.) of chosen dopants (Al2O3 and NiO) are quite different. In this study, we try to separate the influence of residual stresses and transformability of tetragonal phase on fracture toughness of zirconia-based composites. These effects cannot be realized without changes in microstructure and chemical and phase composition of matrix phase.
In this work, we try to analyze and summarize the facts of influence of the Al2O3 and NiO additions on the structure formation of structure of zirconia ceramic materials and linked these structure peculiarities with the fracture mechanisms of zirconia ceramics.
The matrix ZrO2-3 mol% Y2O3 nanopowders (3Y-ZrO2) and composite ZrO2-3 mol% Y2O3-Al2O3 (3Y-ZrO2–Al2O3) nanopowders were synthesized by a co-precipitation technique using ZrOCl2·nH2O and AlCl3·6H2O salts. Based on previous investigations , the amount of Al2O3 was 2 wt%. For understanding the trends of the other composition, the amounts of 0.5 and 1 wt% have been used if needed. All chemicals used were of chemical purity (SiO2 <0.008 wt%, Fe2O3 <0.01 wt%, Na2O <0.01 wt%). At first, the appropriate amounts of Y2O3 were dissolved in nitric acid; then, the zirconium and yttrium salts (in case of matrix material) and zirconium, aluminum, and yttrium salts (in case of composite material) were mixed with a propeller stirrer for 30 min and were subsequently added to an aqueous solution of the precipitant (25% NH4OH) with constant stirring. Sediments were mixed for 1 h at room temperature at a pH of 9. Sediments were then repeatedly washed and filtered with distilled water. Washing was carried out until a negative test for C1− ions is obtained with the use of a silver nitrate solution. After washing and filtration, the hydrogel was dried in a microwave furnace with an output power of 700 W and at a frequency of 2.45 GHz. The calcination of dried zirconium hydroxides and composites was carried out in resistive furnaces at 700 °C with dwelling time 2 h. Because the nickel hydroxide is soluble in ammonia salts, the preparation of the nanocomposite ZrO2-3 mol% Y2O3-NiO (3Y-ZrO2-NiO) powders was conducted by mixing appropriate amounts of zirconia and nickel oxalate powders in distilled water using ultrasound at a frequency of 22 kHz. NiO in the composite nanopowders were obtained by the calcination of powders at 600 °C [fedor]. Based on the previous investigations , the amount of nickel oxide was 10 wt%, but for understanding the trends of the other composition, the amounts from 1 to 7.5 wt% have been used if needed.
Cylindrical (20-mm diameter and 3 mm in height) and rectangular (45 × 4 × 4 mm) specimens were prepared firstly by uniaxial cold pressing, then by isostatic pressing at 200 MPa, and finally by pressureless sintering at 1500 °C for 1 h in air atmosphere in case of ZrO2-3 mol% Y2O3 and ZrO2-3 mol% Y2O3 + Al2O3 and in argon atmosphere in case of ZrO2-3 mol% Y2O3-NiO. The sintering of ZrO2-3 mol% Y2O3-NiO composites was performed in argon atmosphere because the total sample destruction took place in case of sintering in air. The specimens used for mechanical testing were ground with a 180-grit diamond wheel and were subsequently polished with diamond slurries to minimize machining flaws.
The powders and sintered specimens were characterized by XRD (Dron-3) with Cu-Kα radiation for crystallite sizes and quantitative phase analyses by a proven method . For identifying of the monoclinic (M), tetragonal (T), and cubic (C) phases of zirconia, as well as Ni, NiO, and Al2O3, the angular regions of 25°–45° and 71°–77° were used. Particle sizes of different calcined powders were estimated by transmission electron microscopy (TEM) (JEM 200, Jeol, Japan). Reliable data were obtained by analyzing data from 30 TEM fields.
The flexural strength was measured using a four-point bending test on polished samples with a cross-head speed of 0.5 mm/min (Tinius Olsen H50kT, USA). The inner and outer spans were 20 and 40 mm, respectively. The hardness and fracture toughness of the materials was measured at room temperature by the Vickers indentation technique (Vickers tester TP-7p-1) on mirror-polished surfaces with a 98- and 196-N load, respectively. At 196-N loads, the Palmquist type cracks were propagated in 3Y-TZP and composited with alumina. The fracture toughness values were calculated by Niihara equation for Palmquist type cracks . The density was measured using the Archimedes method. The microstructures of the ceramics were studied by scanning electron microscopy (JSM 6490LV Jeol) of thermally etched surfaces at 1450 °C polished surfaces as well as fractured surfaces.
Results and Discussion
In the case of ZrO2-3 mol% Y2O3-Al2O3 nanopowder, incorporation of Al3+ cations into the ZrO2 particles limited its crystallization  and consequently decreased the particle size of zirconia-alumina composite powders during calcination. According to ultrasonic mixing technology, the NiO NPs in zirconia matrix nanopowder can be distinguished by TEM (Fig. 1c) but not Al2O3 NPs (Fig. 1b).
Characterization of Structure of Sintered Ceramic Materials
After sintering at 1500 °C in argon atmosphere, the phase composition of zirconia in 3Y-ZrO2 matrix material and 3Y-ZrO2-NiO composites changes according to XRD results. The amount of cubic phase in 3Y-ZrO2-NiO composite increased up to 20% in comparison with 3Y-ZrO2 (11%). SEM analysis of thermally etched surfaces also shows the macroscopic difference in the structure of zirconia grains. The amount of big grains, which traditionally corresponds to cubic phase of zirconia, and their size increased (Fig. 3c) in comparison with matrix 3Y-ZrO2. The average grain sizes with tetragonal and cubic phases in 3Y-ZrO2-NiO material by SEM data were 0.2–0.4 and 2–4 μm, respectively. The EDS analysis shows the increasing amount of Y3+ ions in the big grains up to 9–11 wt% (5–6 mol%). This value is approaching the concentration of Y3+ ions, which corresponds to chemical composition of cubic phase of ZrO2 (7–8 mol%). The concentration of Y3+ ions in the matrix small grains of tetragonal phase decreased to 1.6–2.5 mol% instead of 3 mol%, which corresponds to chemical composition of tetragonal phase of ZrO2. So, it was found that addition of NiO to 3Y-ZrO2 ceramics leads to depletion of ZrO2 grains of tetragonal phase by Y3+ ions, even more than addition of Al2O3.
Mechanical Properties of Sintered Ceramic Materials
All samples were sintered to greater than 99% of theoretical density. The four-point bending strength values for 3Y-ZrO2-Al2O3 and 3Y-ZrO2-NiO composites decreased by less than 10% in comparison with 3Y-ZrO2 matrix material (from 850 ± 60 to 760 ± 70 and 820 ± 78 MPa, respectively). Hardness values for 3Y-ZrO2-Al2O3 composite increased slightly from 12.0 ± 0.2 to 12.45 ± 0.3 GPa and for 3Y-ZrO2-NiO composite from 12.0 ± 0.2 to 12.1 ± 0.3 GPa. We know that the absolute fracture toughness values obtained by the indentation method could be overestimated, but this technique has been approved by many authors to provide the estimation of the fracture toughness values for samples with high-density levels, where the porosity cannot have effect on crack propagation [11, 24–28].
Thus, it was found that increasing in fracture toughness value of zirconia-based ceramic composites by 40–50% in comparison with 3Y-ZrO2 is caused by addition of different types of dopants—Al2O3 and NiO. These materials strongly differ by crystal lattice (Al2O3—trigonal, NiO—cubic), density (Al2O3—3.96 g/cm3, NiO—7.45 g/cm3), CTE (Al2O3—8.86 × 10−6K−1, NiO—12.8 × 10−6K−1), and Young’s modulus (Al2O3—400 GPa, NiO—95 GPa). The analogical parameters for ZrO2 are 5.95–6.1 g/cm3 (density), 10.8–11.5 × 10−6 K−1 (CTE), and 195–205 GPa (Young’s modulus). So, besides the direct influence on crack propagation as inclusions of alien material, these inclusions affect the structure and phase composition of matrix zirconia, the distribution of residual stresses, and other physical and chemical properties. Let us consider the impact of these dopants on the fracture toughness of zirconia ceramics and try to find the general and distinguish features.
where β and A are the composition from the Young and Poisson modules of ZrO2 and Al2O3 or NiO .
The chemical composition on the polished and thermally etched surfaces of the grains of 3Y-ZrO2 matrix, 3Y-ZrO2-2 wt%Al2O3, and 3Y-ZrO2-10 wt%NiO composite materials. The numbers of spectrum are coinciding with the points marked on Fig. 3
C-phase, Fig. 3a
C-phase, Fig. 3a
T-phase, Fig. 3a
T-phase, Fig. 3a
T-phase, Fig. 3b
T-phase, Fig. 3b
C-phase, Fig. 3b
C-phase, Fig. 3b
C-phase, Fig. 3c
C-phase, Fig. 3c
T-phase, Fig. 3c
T-phase, Fig. 3c
As it is known , this process took part in zirconia ceramic during sintering–cooling process. When the sintering temperature increases, the amount of segregated Y3+ ions on grain boundaries also increases because the diffusion process and ion segregation are enhanced, and a part of the tetragonal phase in Y-TZP transform into the cubic phase which is thermodynamically stable. During cooling, the grains, which were depleted by Y3+, become thermodynamically unstable and can be easily transformed into monoclinic phase in the stress field of propagated crack and stopped it. This is the transformation toughening effect . In this case, the conception of critical grain size d c is introduced. The grains with size smaller than d c are stable, and grains with size greater than d c are unstable and easily transformed into monoclinic phase. The d c depends from dopant type and concentration and varied in a wide range from 150 to 1000 nm [34–36]. The most common value for tetragonal 3Y-ZrO2 is 300–400 nm. Increasing the dopant concentration leads to increase the critical grain size and, respectively, decreasing the dopant concentration leads to decreasing d c . The average grain size, which formed during sintering, depends on sintering conditions, initial powder characteristics, etc. For standard sintering conditions (1500 °C), which were used in this study, the average grain size is near 400 nm. So, decreasing the Y3+ ion concentration in zirconia grains in this study leads to decreasing the d c to the value less then experimentally observed values of average grain size of ZrO2 in T-phase.
The key condition is that the dopant should enhance the diffusion of Y3+ ions in zirconia lattice and should not form the unwanted chemical compounds. By changing the type of dopant and its concentration, the formation of a ceramic material with enhanced level of fracture toughness and predetermined strength value can be created. So, the combination of structure peculiarities–multilevel inclusion structure and phase metastability can enhance the toughening mechanisms in zirconia-based composites.
By SEM data, it was shown that Al2O3 and NiO additions lead to acceleration of bimodal grain structure formation, when the Y3+ ion enrichment and depleted zirconia grains are formed.
Analysis of influence of Al2O3 and NiO additions on indentation fracture toughness values of 3Y-ZrO2 ceramics shows the increasing of fracture toughness values with increasing dopant concentration and its stabilization on certain value.
The combination of such behavior of K1C dependences and structure peculiarities allows us to assume that at least two mechanisms can influence on fracture ZrO2 ceramics: (i) first mechanism is the crack bridging and deflection process, which affects in a region when K1C grows with increasing dopant concentration; (ii) second mechanism is the transformation toughening effect, which exerts one’s influence with increasing dopant concentration to the highest level.
The combination of multilevel inclusion structure and phase metastability can enhance the toughening mechanisms in zirconia-based composites.
- 3Y-ZrO2 :
3 mol% yttria-stabilized tetragonal zirconia polycrystal
- K1C :
Critical coefficient of stress intension (K1)
Scanning electron microscopy
Transmission electron microscopy
This work was supported by the grant AZ 90355 of the VW foundation program “Trilateral Partnerships.” Authors are grateful to the KIPT (Kharkov) for SEM investigation.
ID produced the main idea for writing this manuscript, analyzed the data, designed the experiments, and wrote the manuscript. GL performed the calculation of residual stresses in composites. IB performed the synthesis of nanopowders. VB performed the analysis of composite structure by SEM and EDS. LA performed the sample compaction, sintering, and preparation for mechanical testing. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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