A Novel Structural Ceramic with Enhanced Properties

In an exciting development for the ceramic market, Innovnano has developed an alternative to current 3 mol % yttria-stabilized zirconia (3YSZ) powder suitable for physically demanding structural ceramics applications.

Their new powder, 2 mol % YSZ (2YSZ), is produced through a unique synthesis method that offers beneficial properties to the powder. Significantly, this includes a balance between two highly required features of a structural ceramic - resistance to ageing and outstanding fracture toughness.

Consequently, pressed ceramic parts formed from Innovnano 2YSZ have short-term benefits due to the enhanced wear-resistance and toughness. Additionally, the parts show good results in hydrothermal/thermal ageing and cyclic fatigue tests, which have long-term benefits to the user.

Structural Zirconia Ceramics

Many interesting chemical and physical properties make pure zirconia a constructive material for a number of applications. This versatility is achieved as a result of its ability to exist in three different phases, each with unique properties to go with a specific use and requirement.

Zirconia’s tetragonal phase transition takes place at temperatures above 1175 °C. This high temperature exposure is often reached during sintering. Following this, a stabilizing dopant (in this case yttria) ensures the stability of this phase in the long term and at lower temperatures.

The tetragonal phase of stabilized zirconia is the phase that is achieved through the process of sintering at high temperatures and doping that offers many of the highly sought after properties desired of a structural ceramic. Exceptional durability, flexural strength, high component and excellent wear resistance are some of the desired properties.

During the process of doping zirconia with yttria to produce yttria-stabilized zirconia (YSZ), some of the Zr4+ ions in the crystal lattice are substituted for the slightly larger Y3+ ions. It allows YSZ to reveal all of the desirable properties of the zirconia tetragonal phase even after cooling, making it ideal for engineering and industrial applications under normal operating conditions.

Choosing a Structural Ceramic Material

Zirconia can be stabilized in its tetragonal form using different amounts of yttria. The amount of yttria used depends on the specific requirements of the final pressed ceramic part.

Generally, an increase in fracture toughness is correlated with a lower amount of yttria. It would thus be advantageous to choose an YSZ that contains less yttria for structural ceramic applications. However, this improved fracture toughness brings with it a trade-off in the form of decreased ageing resistance and reduced mechanical strength.

However, for Innovnano 2YSZ, this relationship between the amount of yttria used for stabilization and these two important structural ceramic properties does not stand true. Tests have shown that this 2YSZ maintains ageing resistance values and mechanical strength in-line with 3YSZ due to its unique synthesis process. Thus, without compromising on ageing resistance or mechanical strength, the user can benefit from the improved fracture toughness inherent to a lower-yttria-content zirconia.

An Ultra-Tough Alternative to 3YSZ

Tests have been carried out in an independent testing facility as well as in-house at Innovnano to confirm the powder properties and ensure compliance with ISO material standards.

While fracture toughness is considerably increased from 5 to up to 14 MPa.m0.5 (indentation method) when compared to benchmark 3YSZ, these tests showed that the mechanical strength of Innovnano 2YSZ remains above 1,000 MPa (biaxial flexure). A fracture toughness of this value corresponds to an ultra-tough material that can exceed the performance of many structural ceramic powders on the market.

For structural ceramic applications, the wear-resistance behavior of a pressed ceramic part is another vital property. This feature strongly relies on the hardness and surface finishing of the ceramic part. In accordance with the standard test method for abrasion (ASTM G65), wear-resistance was tested. The test showed that Innovnano 2YSZ has a very similar result to benchmark 3YSZ.

Table 1. Properties of 2YSZ from Innovnano compared to benchmark 3YSZ.

Performance Benchmark 3YSZ Innovnano 2YSZ
Fracture toughness (MPa.m0.5) 5 14
Flexural strength (MPa) 1200 1200
Hardness (HV10) 1250 1250
Wear-resistance (%ΔP/P) 0.020 0.028
Cyclic fatigue resistance 50% of static resistance 87% of static resistance

 

Combining Improved Mechanical Properties with Stability

Innovnano has performed many ageing tests to allay any concerns about using this material because of its lower yttria content. Due to the lower concentration of yttria, the material will be less stable and more prone to severe ageing effects in spite of the outstanding fracture toughness values seen in 2YZ testing. However, tests demonstrate that Innovnano 2YSZ powder can challenge the present structural ceramic powder of choice, 3YSZ, in terms of stability.

Based on the ISO 13356 methodology, cyclic fatigue testing was conducted to make sure that the mechanical properties of 2YSZ were ideal for applications that involve periodic load demands. For this test, 2YSZ ceramic parts were prepared by uniaxial pressing followed by cold isostatic pressing and usual sintering. Only a 13% loss in flexural strength was measured after 106 cycles with a load of 320 MPa (maximum) and 110 MPa (minimum) at 20 Hz frequency.

Standard cyclic fatigue testing of Innovnano 2YSZ showed only a 13% loss in flexural strength.

Figure 1. Standard cyclic fatigue testing of Innovnano 2YSZ showed only a 13% loss in flexural strength.

Using a hot pressed 2YSZ ceramic part, further cyclic fatigue testing was also performed. For this extreme testing, 1100 MPa was used for loading, which is nearly three times the amount specified by ISO 13356.

The exceptional cyclic fatigue behavior of Innovnano 2YSZ was shown by the sample resisting the load cycles, even using this much higher maximum pressure. It must be noted that this stress loading corresponds to almost double the values obtained with benchmark 3YSZ.

The hydrothermal ageing of Innovnano 2YSZ was also investigated, in addition to cyclic fatigue behavior, according to the ISO 13356:2015 methodology. 2YSZ pellets produced by sintering and uniaxial pressing were used. Samples must retain 80% of their flexural strength following five hours at 134 °C and 0.2 MPa in order to fit the standard. By retaining 85% of its flexural strength, Innovnano 2YSZ achieved and exceeded this standard.

Unique Synthesis Method

Emulsion Detonation Synthesis (EDS) is a synthesis approach through which 2YSZ obtains its improved properties. Being a proprietary method to Innovnano, EDS is accredited with providing the ideal ageing resistance that is possible even though the yttria content is low.

EDS is a defined cycle of rapid quenching, pressures and high temperatures in a fully automated system, based on the detonation of two water-in-oil emulsions in a single-step reaction. Zirconia is stabilized owing to the energetic nature of EDS. This is a process that has been tested to a great extent. A nanostructure (with increased specific surface area due to smaller grain sizes) is observed in the resultant powders to which the improved structural properties of resistance to thermal shock, flexural strength, fracture toughness and hardness are attributed.

Schematic representation of Emulsion Detonation Synthesis (EDS).

Figure 2. Schematic representation of Emulsion Detonation Synthesis (EDS).

EDS controls the morphology, chemical structure, purity and final properties of the synthesized ceramic powders. This improves consistency and quality, as well as production capacity, to produce improved powders in a cost-effective and successful manner.

Summary

Innovnano 2YSZ combines two major structural ceramics requirements with its unique synthesis method. It provides a material that can benefit from both the phase stability of a high yttria-containing YSZ and fracture toughness of a low yttria-containing YSZ.

Thus, Innovnano 2YSZ proves to be an exciting alternative to the present structural ceramic powder of choice because of this combination of properties. In addition, it also offers superior fracture toughness with no loss in ageing resistance.

Innovnano logo

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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Comments

  1. Neil Farbstein Neil Farbstein United States says:

    Ultra High Strength Nanomaterials and methods of Manufacture divulges methods of manufacturing ultra-tough ceramics made form common earth elements- silicon carbon and boron, not rare earth elements.

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of AZoM.com.

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