Using Zirconia to Improve the Damage Tolerance for Longer Lasting Materials

Zirconia is an adaptable material with many interesting chemical and physical properties. When it is stabilized using yttria, zirconia is useful for a wide array of purposes, especially in those industrial applications that demand wear- and fracture-resistance, as well as high strength. 3 mol % yttria-stabilised zirconia (3YSZ) has been utilized in structural ceramic applications for several years on account of its impressive mechanical properties, which include, when compared with other technical ceramics, strong fracture toughness and high flexural strength.

Now, another material - Innovnano 2 mol % yttria-stabilised zirconia (2YSZ) –comes with all the positive characteristics of 3YSZ but with significantly higher outstanding values of fracture toughness. In addition to this, it boasts resistance to aging and damage tolerance that are better for any application that demands strong and long-lasting materials.

Zirconia Structural Ceramics

Zirconia exists in its monoclinic phase at room temperature, but transitions to a tetragonal phase at temperatures of greater than 1175 °C. This transformation affects the properties of the zirconia, allowing for exceptional wear resistance, high component and flexural strength, and impressive durability. These very sought-after characteristics make the tetragonal phase of zirconia an optimal structural ceramic material for any physically challenging applications.

It is therefore key to be able to properly maintain zirconia in its tetragonal state at a variety of suitable temperatures. Doping, a process that involves the addition of oxides to the crystalline structure of the zirconia allows this to be feasible. Despite the fact that different oxides can be utilized to stabilize the higher temperature tetragonal phase (e.g. CaO, MgO and CeO2), yttria (Y2O3) is the most often used as it displays a high level of solubility in the zirconia lattice1.

Some of the Zr4+ ions are substituted during the doping process, with the Zr4+ ions in the crystal lattice being removed in favor of the marginally larger Y3+ ions, which forms yttria-stabilized zirconia (YSZ)2,3. YSZ is suitable for use under the usual operating conditions, as it displays all of the desirable properties of the zirconia tetragonal phase at room temperature.

Minimizing yttria to Maximize Fracture Toughness

Depending on the desired properties of the ceramic end-product, the quantity of yttria dopant used to stabilize the zirconia can be changed to produce alternative crystalline structures. Decreasing the amount of stabilizing yttria leads to a clear increase in the fracture toughness. This is highly sought-after to improve performance of the component, though it is usually the case that zirconia stabilized with low quantities of yttria is not as resistant to aging and has decreased flexural strength. In contrast to the usual state of affairs, 2YSZ manufactured using Emulsion Detonation Synthesis (EDS) preserves all of the attractive mechanical characteristics of YSZs with higher yttria content.

The unique synthesis method used by EDS involves a well-established cycle of high temperatures and pressures, together with fast quenching in a completely automated system, dependent upon the detonation of two water-in-oil emulsions in a single step reaction (Figure 1). The high-energy nature of EDS makes the zirconia more likely to be stable, and the resultant powders exhibit a nanostructure - with an increased specific surface area - which in turn contributes to the improved structural properties.

Among these properties are fracture toughness, hardness, flexural strength, ability to sinter at lower temperatures, and an improved resistance to thermal shock. EDS creates 2YSZ ceramics that combine impressive fracture toughness with sturdy stability and excellent aging resistance, all the while maintaining a high level of flexural strength (Table 1).

2YSZ provides a viable alternative to the standard 3YSZ, when synthesized using EDS. It boasts a flexural strength of at least 1000 MPa, with corresponding fracture toughness substantially increased from 5 to higher than 14 MPa.m0.5 when compared against the benchmark 3YSZ. Whether it is being used as a ready-to-press powder or instead as the zirconia component in zirconia-toughened alumina/alumina-toughened zirconia (ZTA/ATZ) and cermets, 2YSZ therefore is an outstanding alternative for applications in the domain of structural ceramics.

Emulsion Detonation Synthesis (EDS) technology from Innovnano.

Figure 1. Emulsion Detonation Synthesis (EDS) technology from Innovnano.

Table 1. Properties of 2YSZ synthesized using EDS and benchmark 3YSZ.

Performance Benchmark 3YSZ Innovnano 2YSZ
Fracture toughness (MPa.m0.5) 5 14
Flexural strength (MPa) 1200 1200
Hardness (HV10) 1250 1250
Cyclic fatigue resistance 50% of static resistance 85% of static resistance
Shock absorption capability Innovnano 2YSZ shock absorption capability is 2 times higher than 3YSZ (for the same part thickness)

 

Testing the Theory of Better Ageing Resistance

In order to guarantee the lower yttria content of this 2YSZ does not negatively impact important structural ceramic properties, independent stability and aging tests have been carried out

Cyclic stress-strain aging tests in saline solution were carried out using EDS-synthesized 2YSZ bars that have undergone both conventional sintering and cold isostatic pressing (CIP). All pieces passed the ISO 13356 standard methodology (106 cycles, 320 MPa, 20 Hz frequency) without any issues. The four-point bending strength was also derived prior to and after the experiment, and the results demonstrated a decrease in flexural strength of just 13%. Subsequent cyclic stress-strain aging experiments were successfully carried out (20 Hz, 106 cycles) with 1100 MPa being used as the maximum pressure, drawing attention to the impressive resistance to mechanical aging. Experimented using 3YSZ in the literature show that among 13 specimens tested at a maximum pressure of 650 MPa, none were able to attain 106 cycles4.

Hydrothermal aging investigations under ISO 13356:2015 (5 h, 134 °C, 0.2 MPa) of 2YSZ pellets were produced using uniaxial pressing and sintering. Results showed that EDS-synthesized 2YSZ exceeds the standard to retain 85% of its flexural strength. In addition to this, these 2YSZ pellets were aged subject to even more challenging conditions for a period of 96 hours, and were still able to retain 80% of the mechanical properties than it started with.

Improved Damage Tolerance

It is important for structural ceramic components to be able to endure tough conditions when in service, and to therefore have decent damage tolerance. Both flexural strength and the inherent critical defect sizes have been investigated both theoretically and practically.

The flexural strength of a ceramic part is, according to the Griffith criteria5, both positively impacted by the material fracture toughness and negatively impacted by the inherent critical defect size. On this basis of this theory, two materials with identical flexural strength but different fracture toughness values can behave differently in the presence of defects.

In the case of 2YSZ and 3YSZ, investigations have shown that both parts have identical flexural strength but differ in fracture toughness values,  with the parts exhibiting fracture toughness values of 14 and 5 MPa.m0.5, respectively (Table 1).

Using the equation describing the Griffith criteria, this corresponds to a critical flaw size of 1.76 µm for 3YSZ and 13.83 µm for 2YSZ. Consequently, the EDS-synthesized 2YSZ material can tolerate defects of increased size with improved mechanical properties, when compared to 3YSZ.

To evaluate these theoretical predictions, a practical evaluation of the flaw tolerance behavior of EDS-synthesized 2YSZ was carried out to compare with the benchmark 3YSZ. Uniaxial pressed disk samples were formulated at 100 MPa in a 20 mm die and sintered at 1450 ºC (3YSZ) and 1350 ºC (2YSZ) for two hours. Samples were mirror polished after sintering and defects of different sizes were created on purpose by way of a micro-indentation system with a Vickers tip (Figure 2A) and the application of different loads for 15 seconds. The biaxial flexural strength was determined prior to and following the insertion of defects, utilizing a piston-on-three ball system in accordance with ISO 6872. In order to guarantee that the pre-crack effect is taken into consideration in the mechanical test, the introduced defect is placed face down (Figure 2B).

Biaxial flexural strength was assessed before and after defect insertion using piston-on-three ball system according to ISO 6872. (A) Schematic representation of the indented defect and corresponding crack length. (B) The introduced defect is placed face down to ensure the pre-crack effect is taken into consideration.

Figure 2. Biaxial flexural strength was assessed before and after defect insertion using piston-on-three ball system according to ISO 6872. (A) Schematic representation of the indented defect and corresponding crack length. (B) The introduced defect is placed face down to ensure the pre-crack effect is taken into consideration.

The biaxial flexural strengths of benchmark 3YSZ and EDS-synthesized 2YSZ products before and following the creation of defects are compared in Figure 3A. The non-damaged parts of both 3YSZ and 2YSZ have flexural strengths close in value which are comparable with the properties shown in Table 1. However, when a defect is introduced in the ceramic component, and using an identical applied load, the parts prepared using 2YSZ product exhibit significantly higher flexural strength values in comparison to those of benchmark 3YSZ products.

On account of the significantly higher fracture toughness of EDS-synthesized 2YSZ (14 MPa.m0.5), the formation of the defect also results in reduced crack length for equivalent applied loads compared to 3YSZ (Figure 3C). This is consistent with theoretical predictions that materials with a high value of fracture toughness have improved energy absorption abilities and consequently a reduced crack propagation rate.

Therefore, a comparison can be made between the biaxial flexural strengths of EDS-synthesized 2YSZ and benchmark 3YSZ products, as a function of defect crack length (Figure 3B), to provide further evidence of the greater damage tolerance capabilities of this 2YSZ. For identical crack lengths, EDS-synthesized 2YSZ products have a biaxial flexural strength that is 1.5-1.8 times larger than for 3YSZ.

(A) Biaxial flexural strength of benchmark 3YSZ and Innovnano 2YSZ tested against applied load. (B) Biaxial flexural strength of benchmark 3YSZ and Innovnano 2YSZ tested against crack length. (C) SEM of the indented 2YSZ sample at 300 N, shown at 700x magnification. (D) SEM of the indented 3YSZ sample at 300 N, shown at 490x magnification.

Figure 3. (A) Biaxial flexural strength of benchmark 3YSZ and Innovnano 2YSZ tested against applied load. (B) Biaxial flexural strength of benchmark 3YSZ and Innovnano 2YSZ tested against crack length. (C) SEM of the indented 2YSZ sample at 300 N, shown at 700x magnification. (D) SEM of the indented 3YSZ sample at 300 N, shown at 490x magnification.

Summary

Using EDS manufacturing technology, it is feasible to create a product with unparalleled potential for structural ceramics applications. 2YSZ synthesized in this way uniquely combines the benefits of desirableaging and damage tolerance capabilities with higher fracture toughness.

Compared to benchmark 3YSZ, mechanical tests have shown that EDS-synthesized 2YSZ products have:

  • Impressive resistance to both hydrothermal and cyclic stress-strain aging.
  • 1.5-1.8 times higher biaxial flexural strength for equivalent flaw size, when compared with 3YSZ products.
  • Reduced flaw size for the equivalent applied load, when compared with 3YSZ products.
  • The capability to support defects of increased size with improved mechanical properties over 3YSZ products.

References

[*] Proprietary to Innovnano

  1. J. J. Swable, “Role of Oxide Additives in Stabilizing Zirconia for Coating Applications,” US, 2001.
  2. B. Basu, “Toughening of yttria-stabilised tetragonal zirconia ceramics,” International Materials Reviews, vol. 50, no. 4, Kanpur, India, pp. 239–256, 2005.
  3. R. M. Nunes Soares, “Phd Thesis - Development of Zirconia based phospors for application in lighting and as luminescent bioprobes,” University of Aveiro, 2013.
  4. Renato Chaves Souza, et al. Performance of 3Y-TZP Bioceramics under Cyclic Fatigue Loading. Materials Research, Vol. 11, No. 1, 89-92, 2008
  5. C.B. Carter, M.G. Norton, “Ceramic Materials – Science and Engineering”, Springer; 2007 edition

This information has been sourced, reviewed and adapted from materials provided by Innovnano.

For more information on this source, please visit Innovnano.

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