Perspective Chapter: The Application of Yttria-Stabilized Zirconia (YSZ)

Abstract

Pure zirconia will transform into different phases, which include monoclinic, tetragonal, and cubic, at different high temperature levels. Specific phases can be retained at room temperature by adding stabilizer and yttria is one of the most common stabilizers for zirconia, commonly formed yttria stabilizer zirconia (YSZ). To utilize YSZ in various industry applications, the amount of yttria and sintering temperature played a vital role. Thus far, YSZ has received a warm welcome in the industries of thermal barrier coating (TBC), solid oxide fuel cell (SOFC), and biomaterial. However, the limitations and challenges still occur, and this opens up the room and possibility of enhancing and improving the material properties of YSZ for a better performance in the mentioned area. This chapter explained the working principles of YSZ in the industries respectively and the research been conducted to improve the materials accordingly.

1. Introduction

Zirconia (ZrO₂) possessed attractive material properties (mechanical, thermal, and electrical properties) when additional conditions are included. However, the material will encounter phase transformation at different levels of temperature [1]. As shown in Figure 1, the crystal structure of zirconia will transform from monoclinic to tetragonal and to cubic as the temperature is increasing, and the transformation is reversible.

Figure 1.

The phase transformation of zirconia.

By adding stabilizers to zirconia, it is one of the strategies to enhance the material properties of zirconia through retaining a specific phase, which the ceramics material did not come naturally at room temperature. Yttria (Y₂O₃) is one of the most common stabilizers for zirconia and formed yttria stabilizer zirconia (YSZ). The phase transition of YSZ is complicated but is well documented as the sintering temperature and the yttria concentration vary. The phase diagram, which is illustrated in Figure 2, clearly indicated the phase (or phases) of YSZ will be produced as the mole percentages (mol%) of yttria against the sintering temperature. For example, tetragonal (t) and cubic (c) will be produced with 5 mol% of yttria (5Y) and sintered at temperature 1000°C.

Figure 2.

Phase diagram of YSZ.

The Scanning Electron Microscope (SEM) samples images of different yttria amount is showed in Figure 3. Under the same level of close view observation, the images revealed the higher the amount of yttria, the higher the grain size of the ceramics material be produced. Grain size played a very important role in affecting the material properties which included hardness, fracture toughness, and elastic modulus. By sustaining a particular phase at room temperature, it allows YSZ to function specific role in the industry as specific material properties are induced. In this chapter, thermal barrier coatings (TBC), solid oxide fuel cells (SOFC) and biomaterial, which are the common industry applications, will be explained and discussed. Each application will highlight the advantages as well as the limitations respectively. All these are related with phase retention, which beyond the zirconia natural phase transformation toward the temperature.

Figure 3.

The example of SEM images of 2Y, 3Y, and 8Y sintered at 1500°C [2].

2. Thermal barrier coating (TBC)

2.1 The working principles

Thermal barrier coating (TBC) refers to a thermal insulation layer for the purpose of protecting the original material (base material) at high service operation temperature and hot corrosion environment. The described situation normally happens in heavy duty industry like aircraft, automotive, offshore equipment (power generator). In order to protect and secure the base material from extreme scenario and further prolong the lifespan of the machine and system, TBC is one of the option to be used [3]. The common components will be applied the coating included but not limited, of diesel engine and gas turbine blade [4, 5].

Figure 4 illustrated the concept, construction, and function of thermal barrier coating system. It basically consisted of a surface layer (or been called as “top-coat (TC)”) to form a bond coat (BC) layer. In between these two layers, a thin layer called Thermal Grown Oxide (TGO) is formed due to high-temperature oxidation of the bond coat [6]. The three layers function as the coating to protect the base materials from extreme service environment.

Figure 4.

Illustration of thermal barrier coating.

YSZ is one of the common ceramic materials to form the TC layer and the BC layer is made of different types of metallic material. This is due to YSZ possessing thermal properties like low thermal conductivity and high coefficient of thermal expansion. This main role of BC layer is to generate an adhesion with TC, then to protect the base material from oxidation [7]. Many research showed that 6–8 mol% yttria stabilizer zirconia (6-8YSZ) exhibited excellent thermal properties for example, low thermal conductivity, and high thermal expansion coefficient [8]. The phases of this level normally consist of tetragonal and cubic, or both phases co-exist at the same time.

TBC can be constructed by two common methods—thermal spraying and electron beam-physical vapor deposited (EB-PVD). Thermal spraying methods included atmospheric-plasma spray (APS), plasma spray-physical vapor deposition (PS-PVD) and high-velocity oxy-fuel (HVOF) spraying [9]. The basic mechanism of thermal spraying is to melt the coating material and propel it into the substrate material with high and strong velocity. Plasma spraying basically created the scenario through plasma jet and HVOF is to use high-velocity jet instead.

It is more common to adopt thermal spraying method for made up of TBC for a few reasons. First, thermal spraying was widely accepted by the industry with the technique compatible with various materials, which included ceramics, metals, and alloys. Second, low operational cost and third, low equipment specification requirement. All these caused the mentioned technique received high welcoming small and medium-sized factory. However, many previous works had reported EB-PVD method did produce a higher quality result compared with plasma spray methods [10, 11]. Even though in recent years, new and advanced plasma spray techniques are emerging, the full exploration of the method is yet to be fully accepted by the industry. For example, new techniques like high-velocity air fuel (HVAF) and liquid feedstock thermal spraying, had claimed it able to produce a high quality of surface by increasing the adhesive strength of the coating [12]. The technique of HVAF spraying environment is showed in Figure 5.

Figure 5.

High-velocity air fuel (HVAF) technique [12].

2.2 Limitation and solutions

Even though YSZ is a proven material to form TC, degradation is still becoming one of the biggest challenges, and the major failure mechanisms included residual stress, hot corrosion, oxidation, and phase transformation [13]. All these became the factors of shortened the life span of TBC and hinder it continuing to perform in an optimum condition. The durability of TBC refers to the high quality of adhesive strength and low thermal conductivity (insulation) at high temperature [14]. Once these two capabilities been destroyed, TBC basically will fail under the tough operation and service environment, and the mentioned failure mechanisms are playing the role to degrade TBC.

Phase transformation can be related to degrading TBC through the tetragonal transformation back to monoclinic when thermal gradient occurs during the operation. The happened due to the oxygen diffused to bond coating layer and allow chemical reaction with YSZ. This had greatly decreased the amount of oxygen of YSZ. Since oxygen ion is one of the factors to stabilize tetragonal phase by forming chemical bonding with zirconium ion, decreasing of the amount of the ion directly destabilized tetragonal phase and drove the phase transformation happen. Besides that, the cooling of TBC also contributed to the phase transformation, as that is the natural characteristic of zirconia. However, this mechanism does not occur immediately. Various research has been conducted to overcome the challenges of TBC to prolong the service life, and one of the common methods is by using dopant. Co-doping Yb₂O₃-Gd₂O₃-Y₂O₃ co-doped with ZrO₂ (YGYZ), the outcome showed the tetragonal phase able to be retained 40% compared to the undoped YSZ [15].

The X-ray diffraction (XRD) result revealed that the doped sample consisted of stable tetragonal phase greater than the undoped YSZ, which is showed in Figure 6. Take note that YVO₄ is the corrosion product, due to the experimental testing by using Na₂SO₄ + V₂O₅ molten salts to simulate the real hot corrosion environment. In another word, the doping technique also improved the hot corrosion resistance of YSZ. This research outcomes in Figure 7 showed a similar trend of creating double layer of topcoats by using La₂Zr₂O₇ under the similar experimental condition [16].

Figure 6.

XRD analysis after hot corrosion at 1100°C [15].

Figure 7.

SEM image of double layer topcoat [16].

Slow cooling rate is another finding to avoid phase transformation, as well as to prolong the life span of TBC as showed in Figure 8. The research reported by reducing the cooling rate to 10 K/s, compared to 100 K/S, which is the common practice, also increased the operational temperature from 1200°C to 1500°C [17]. All the research revealed that doping (or co-doping) is an effective solution to minimize the phase transformation, which will avoid the mentioned failure mechanisms.

Figure 8.

Number of cycles to failure for YSZ as TBC [17].

Besides avoiding the failure mechanism, the advantages of doping technique also bring benefits and improvement to YSZ as TBC. By doping TiO₂ to YSZ, as one of the recent finding, showed the dopant able to increase the operational temperature of TBC from 1200°C to 1600°C [18]. In Figure 9, it also resulted the doped samples (TZ) had a lower thermal conductivity compared with the undoped sample (YSZ). The research claimed that low thermal conductivity was attributed by the lattice disorder, which restrict the movement of phono and lower the thermal conductivity.

Figure 9.

Thermal conductivity of doped and undoped YSZ [18].

Recent year, a method calls sol-gel method to fabricate YSZ aerogel had been developed. The research outcome reported the new coating successfully lower down the thermal conductivity through a finer porosity in the aerogel structure [19]. The conceptual illustration of the method on gas turbine is illustrated in Figure 10.

Figure 10.

Conceptual illustration of the YSZ aerogel TBC coating on the gas turbine blade [19].

This earlier work revealed that applying dopant to YSZ and using different sintering method are one of those effective strategies to overcome the limitation which been mentioned in TBC application. Some research work even showed that doping and sintering methods can affect the average grain size which further affected the mechanical properties of the materials. However, the effect also depends very much on the substrates to be used.

3. Solid-oxide fuel cells (SOFC)

3.1 The working principles

Green technology and energy are becoming one of the major trends globally, and the Department of Economic and Social Affairs, United Nations has established 17 sustainable development goals (SDGs). Without any surprise, clean energy and a green environment are one of the major aspects.

Solid Oxide Fuels Cell (SOFC) received great attention for supply of environmental-friendly energy. One of the main advantages of SOFC is to generate power from hydrogen, natural gas, and other renewable fuels. Based on Figure 11, the SOFC reaction can be explained by the following chemical equation:

Figure 11.

Illustration diagram of SOFC [20].

Anode:

$${\mathrm{H}}_2+{\mathrm{O}}^{2-}➔{\mathrm{H}}_2\mathrm{O}+\text{2e}-$$E1

$$\mathrm{CO}+{\mathrm{O}}^{2-}➔{\mathrm{CO}}_2+\text{2e}-$$E2

Cathode:

$${\mathrm{O}}_2+\text{2e}-➔{\text{2O}}^{2-}$$E3

By burning the fuel (hydrogen or hydrocarbon) as the input on anode, the concentration of oxygen, from the surrounding environment (air), will be “consumed,” as illustrated in Eqs. (1) and (2). The electrons will be transported through external circuits for industry applications. Meanwhile, oxygen on cathode side will react with the electron been transported and produced oxygen ion, as shown in Eq. (3). The porous character of electrolyte will allow the oxygen ions to diffuse to anode and continue the cycle. The whole operation needs to be happened at high operating temperature in between 800°C and 1000°C [21]. This operation of conversion greatly reduced the gas emission (heat), which is bringing air pollution to the environment, and the other product from the reaction will be water (H₂O) [22].

YSZ is one of the promising materials to be utilized as solid electrolyte material for SOFC. A qualify material as the medium and play the role of solid electrolyte required characteristic of conducting ions effectively. 8YSZ possess characteristics of high concentration of oxygen ion vacancies, which allow an effectiveness and efficiency performance of SOFC, due to 8 mol% yttria able to retain cubic phase (c-YSZ) of zirconia, which allow highest oxygen ion vacancies concentration compare to the other phase of zirconia [23].

3.2 Limitation and solution

However, utilizing 8YSZ as the electrolyte of SOFC comes with restrictions and limitations which require the high operating temperature to “activate” SOFC to reach high ion conductivity. In such an operational environment, the components of SOFC may encounter thermal expansion and contraction and further consume the durability of the cell. To overcome this, it drives the scientist and engineers to study and develop different types of materials to widen the application of SOFC. At the same time, it also opened the possibility of continuously enhancing and improving 8YSZ without compromising the conductivity yet continues to lower the operational temperature as well as the mechanical properties [24]. 8YZP basically exhibited low mechanical properties, so several research and investigation had been conducted to reach a breakthrough of these limitations.

Instead of using conventional sintering method, flash sintering method had be proven as the sintering method not only increase the ionic conductivity of 8YZP as electrolyte in SOFC, at the same time, produced 8YSZ to reach full densification at a lower sintering temperature [25].

In Figure 12, the Nyquist plot, which generated by Electrochemical Impedance Spectroscopy (EIS), indicated flash sintering produce higher conductivity, smaller grain size and porosity. In the plot, flash sintering showed the semicircle with smaller diameter, which referred to a smaller grain size. Smaller grain size offered higher surface area for more ion oxygen to move or transport. This factor attributed a higher conductivity of the ceramic material [26].

Figure 12.

Nyquist plot of 8YSZ with (a) conventional sintering (b) flash sintering method at 215°C [26].

Besides sintering method, utilizing dopant also become another strategy to increase the ionic conductivity of 8YSZ. Dopant included iron(III) oxide (Fe₂O₃) reported an satisfactory result by increasing the ion conductivity [27].

The result in Figure 13 was generated through molecular dynamics (MD) simulation, and it showed that the optimum result of the conductivity was 4 wt% of Fe₂O₃ dopant. The simulation result agreed with another research outcome by using the same dopant [28]. The research claimed that doping can stabilize cubic phase of 8YSZ, which is high of ion conductivity, through creating oxygen vacancies.

Figure 13.

8YSZ doped with Fe₂O₃ of: (a) MD simulation diagram and (b) oxygen ion conductivity of [27].

The result of XRD in Figure 14 validated the claim, which showed the doped sample consist high volume of cubic phase regardless the sintering temperature. By using the cold sintering process (CSP), the study also showed the enhancement of ion conductivity compared with conventional sintering method.

Figure 14.

XRD pattern of 4 wt% Fe₂O₃-doped 8YSZ sintered by using cold sintering process [28].

Figure 15 illustrated the how doping created oxygen vacancies and allow ion to migrate and increased the ion conductivity characteristic. Dopant will introduce lattice defects and distortions, which produced oxygen vacancies in the ceramic material. The vacancies allowed migration of oxygen ion, thus increasing the ionic conductivity of the material [29].

Figure 15.

The illustration diagram of ion migration mechanism [29].

When entering millennium year, the doping method on 8YSZ started to focus on improving mechanical properties, which is important to enhance the durability of the solid electrolyte in the cell. A study had been conducted by using 8 mol% Lu₂O₃ doped 8YSZ and successfully improved the flexural strength of the materials without compromising the ion conductivity [30]. Other studies on the dopant like CuO, TiO₂ and Bi₂O₃, also reported the sintering aids for 8YSZ effectively aided the ceramics material to reach full densification at a lower sintering temperature without compromising ion conductivity [31, 32, 33]. Lower sintering temperature for full densification, which is highly related to the mechanical properties like hardness and fracture toughness, means less energy required during processing.

Transition metal oxide doping strategy showed significantly of improving the mechanical properties of 8YSZ. However, an interesting study and investigation had been conducted. Instead of using transition metal oxide as the dopant, 3YZP (10, 25 and 35 wt.%) had been doped to 8YSZ and both mechanical and electrical properties had been evaluated. The study reported the doped material had an improved version in terms of Vickers’ hardness and fracture toughness. At the same time, ionic conductivity also increased, but with only happen under the service temperature below 550°C [34].

Sintering methods and sintering aids (dopant) had been the main factor to enhance the ionic conductivity of the materials and elevate the performance of 8YSZ as the solid electrolyte in SOFC. While the mentioned investigation also revealed doping strategies able to improve the mechanical properties of the material by controlling the amount of the dopant.

4. Biomaterials

4.1 The working principles

The phase diagram (Figure 2) showed the amount of yttria stabilizer determine the phase of retention of zirconia (monoclinic, tetragonal, or cubic), and 3 mol% yttria tetragonal zirconia polycrystal (3Y-TZP) is one of the ceramic materials received great welcoming as biomaterial. Among all the three phases of zirconia, tetragonal phase exhibited high hardness strength and fracture toughness [35]. Besides that, zirconia also showed a friendly biocompatible characteristic, which is greatly benefit to bone and tissue implantation [36]. The high fracture toughness is attributed by a phenomenon called stress-induced transformation toughening. This mechanism happens when cracking happens on the surface of 3Y-TZP, metastable tetragonal will transform back to monoclinic, and this transformation will increase the toughness of the material, as illustrated in Figure 16.

Figure 16.

Illustration of transformation toughening of 3Y-TZP.

Due to these attractive mechanical properties, ceramic material is usually utilized in biomaterial industry to enhance the mechanical structure of the product. Besides the mentioned material properties, 3Y-TZP also showed itself with the criteria as the bio-ceramic, which include high corrosion and wear resistance, and esthetics [37].

4.2 Limitation and solution

However, 3Y-TZP does come with limitations and the most common one is called low temperature degradation (LTD) or hydrothermal aging. It is a phenomenon when 3Y-TZP exposed to moisture or humidity environment (water or water vapor presence) at temperature between 65°C and 400°C, which commonly for biomaterial application [38]. Such condition will allow phase transformation, from tetragonal (t) to monoclinic (m), on the surface of the material then produced intergranular cracking [39]. This is because the tetragonal phase of 3Y-TZP is a metastable state at room temperature and transformed back to monoclinic phase, which is the most stable phase of zirconia at room temperature, will easily take place under such environment. Once monoclinic phase saturated and dominated the microstructure of the material, the mechanical properties will also degrade. It received a great welcome from the biomedical industry to study and investigate the factors to improve and overcome the mentioned challenge. However, the effect of LTD did not happen immediately but the degradation will take long period of time. In order to study LTD effectively, scientists and researchers simulated the condition under the environment of laboratory. The condition commonly by using conventional water-vapor autoclave operating at low temperature (98, 121 and 132°C) respectively and under adiabatic pressure for different exposure time [40]. In Figure 17 showed the example of the evaluation of the phase transformation (t - > m) when exposed the simulated degradation environment [41]. The method has been proven as an effective way to assess any modified and improved 3Y-TZP of the characteristic toward the resistance of LTD.

Figure 17.

Monoclinic vs degradation time of 3Y-TZP under degradation exposure time (134°C, 4 bar) [41].

Several studies showed doped 3Y-TZP exhibited good LTD resistance by delaying the phase transformation then further minimized the degradation of mechanical properties. Co-doping CaO and CeO₂ to 3Y-TZP exposed to the simulated LTD environment, the result in Figure 18(a), reported that the no monoclinic phase exists in the microstructure of the samples regardless the amount of both the dopants [42].

Figure 18.

XRD pattern of the samples after degradation exposure time of (a) 30 h [42] and (b) 120 h [43].

Manganese dioxide (MnO₂) (0.5 wt% and 1.0 wt% respectively) achieved a similar result under the same LTD laboratory condition but exposed to a longer holding time, 120 h, which showed in Figure 18(b). The dopant effectively increase the LTD resistance of 3Y-TZP [43]. Both XRD diagrams also revealed a stable tetragonal phase in the microstructures, which enhanced the mechanical properties like hardness and fracture toughness of the doped 3Y-TZP as well.

Besides the mentioned dopant, other transition metal oxide like Copper Oxide (CuO), graphene oxide (GO), Flaysh also had been studied and investigated and achieved a similar outcome [44, 45, 46]. Some research even brings further insight by relating the LTD resistance of 3Y-TZP with grain size. It reported grain size less than 3 μm will improve the LTD resistance and beyond the mentioned size, the effect will not take place. And doping Ceria and Alumina to 3Y-TZP able to control the grain size below the mentioned level effectively [47].

Sintering condition, which included sintering temperature and sintering holding time, also played a vital role for improving the LTD resistance of 3-YZP. The sintering temperature, 1450–1650°C and holding time, 1, 2 and 4 h, became the study scope. The outcome revealed that the higher both the mentioned parameters, the higher sensitivity of LTD of 3Y-TZP [48]. In year 2018, a research showed that the level of LTD resistance as well as the microstructure of 3Y-TZP had been improved through different sintering cycle and conditions [49]. The effect of pressureless sintering with a range of temperature, 1400–1600°C, and to produce an optimum result between the resistance of LTD, hardness and fracture toughness also had been studied. The outcomes of the study showed that the optimum sintering temperature, 1500°C, is the best temperature to balance the three (3) mentioned material properties of 3Y-TZP [50].

There is a huge potential to continue to improve 3Y-TZP application for biomedical related industries. Both sintering and doping methods have shown promise in enhancing the aging resistance and mechanical properties of 3Y-TZP, making it more suitable for use in biomedical applications. Ongoing research in these areas is expected to further enhance 3Y-TZP’s potential for biomedical use and lead to new innovations. Therefore, both methods are valuable tools in unlocking the potential of 3Y-TZP for biomedical applications.

5. Conclusion

YSZ is a versatile material that can be applied in various industries, which include TBC, SOFC, and biomaterials.

In the context of TBC, YSZ has become the promising material due to its material characteristic to withstand high-temperature environments and protect underlying components from thermal damage. This makes it ideal for use in gas turbine engines and other high-temperature applications.

In the field of SOFC, YSZ is commonly used as an electrolyte due to its high ionic conductivity and stability under operating conditions. The use of YSZ electrolytes has contributed to the development of high-performance SOFCs, which have the potential to play a significant role in the future of clean energy production.

YSZ’s biocompatibility and high mechanical properties like hardness and fracture toughness make it a suitable material especially for implants, and other biomedical structural-related applications.

The use of different sintering methods and various dopants played an important role in to overcome the different challenges and able to enhance the material properties of YSZ, such as its crystal structure (phase) stability, ionic conductivity, and mechanical properties. Even though it seems that thermal properties, electrical properties, and mechanical properties are emphasized in TBC, SOFC and biomaterials respectively, it can be challenging to achieve significant improvements in all properties simultaneously. Therefore, it is important to prioritize specific properties based on the requirements of each application.