Processing of Zirconia

Abstract

This chapter starts with an introduction, including zircon, zircon structure, zirconia ceramics and their structures as they exhibit a distinct crystal structure at various temperatures while maintaining the same chemical composition. Then the properties of zirconia including mechanical, tribological, and electrical properties and thermal conductivity, were discussed. Zirconia’s properties offer excellent resistance to corrosion and chemicals. When compared to other high-tech ceramic materials, zirconia is very robust at room temperature. The processing of zirconia was explained, starting with the purification of zirconium compounds followed by powder processing of zirconia, colloidal processing of zirconia, additive manufacturing, and zirconia treatments. Zirconia ceramics are processed from their raw ingredients in order to modify and enhance their physical and chemical characteristics and make them more suitable for use in future processes and in the production of finished goods. Various analysis methods of zirconia, including powder processing and sintering, microstructural analysis, phase characterization, mechanical testing, and tribological characterization were discussed. The last section in the chapter discusses the nano zirconia (ZrO2 nanoparticles). In addition to photocatalytic and piezoelectric uses, dental and optical coatings, nano zirconia has additionally been employed as a catalyst in a variety of organic interactions. It appears in the cubic, tetragonal, and monoclinic structural phases.

1. Introduction

1.1 Zircon

Zircon, which is frequently found in highly silicic stones and contains significant quantities of certain trace elements, is regarded as a resistant auxiliary mineral. It is also a helpful instrument for tracking melt compositional alterations throughout magma development [1]. The quantity of Zr in the principal minerals is often negligible in comparison to that housed by zircon, and there is disagreement on the presence of zircon at boundaries of grains vs. the amount hidden as inclusions within different minerals [2]. Zirconium silicate is another name for the mineral zircon (ZrSiO₄). It results from the extraction and processing of historical heavy mineral sand and is most typically found in coastal placers or dune deposits across the world [3]. Zirconium silicates are abundant in essence, along with presence under hydrothermal circumstances (between 300 and 550°C) which seems to have received a lot of interest [4].

The global output of zircon is around 1.1 million t/a, with the majority of it coming from Australia and South Africa. The most important use of zircon is represented by ceramics sector [3]. Zircon may be treated to make zirconia by melting the sand at extremely high temperatures to yield molten zirconium oxide (ZrO₂) [5].

It is debatable how zircons behave in suprasolidus metamorphic environments. Experimental findings suggest zircon growth occurs throughout prograde metamorphism in melt-bearing systems, but computational modeling only anticipates zircon development upon cooling and melt crystallization. Zircon in high-pressure zones and elevated temperatures metamorphic rocks is a probable by-product of inconsistent melting events. However, knowledge of how zirconium, a crucial structural component of zircon, is distributed among the numerous products and reactants that occur during incongruent partial melting is still limited. A fresh source of Zr may become accessible to zircon formation after heating under suprasolidus under certain circumstances, or Zr may redistribute from already-existing zircon [2].

1.2 Zircon structure

Zircon is a mineral that is an orthosilicate accessory. Its chemical formula is ZrSiO₄ (Z = 4) with the space group I4₁/amd. The zircon structure is made up of chains of consecutive, edge-shared SiO₄ and ZrO₈ polyhedra that pass along the c axis. The structure is rather wide; it incorporates “channels” of evidently vacant sixfold sites that run in corresponding direction to the {001} orientation and can accommodate big amount of non-formula components [6].

The edges of the ZrO₈ triangular dodecahedra are coupled in a zigzag manner across the {100} dimensions. The structure shown in Figure 1 accounts for the most prevalent three-dimensional behavior of zircon as well as the uncertain cleavage along {110}. Natural zircon incorporates non-formula elements at all times [7]. On the other hand, large cations (U, Th, Y, lanthanides, Hf) often replace Zr in the eightfold dodecahedral position, however, smaller cations may be placed in the sixfold location [8].

Figure 1.

Two-unit cells shown in the polyhedral depiction of the zircon structure [7].

1.3 Zirconia ceramics

For several reasons, zirconia-based ceramics are the most researched and demanding materials. Zirconia (zirconium dioxide, ZrO₂), popularly known as “ceramic steel”, offers outstanding hardness, strength, and fatigue resistance, as well as good wear characteristics and biocompatibility [9].

Zirconium oxide, silicon nitride, and aluminum oxide are a few frequently employed ceramics that have exceptional chemical and physical qualities including high hardness, high strength, high toughness, and corrosion resistance [1]. Zirconium oxide, on the other hand, outperforms silicon nitride and aluminum oxide in terms of abrasion resistance, toughness, and service temperature [10].

Zirconia is a significantly highly versatile transition metal oxide frequently applied in ceramics, electrolytes for solid-oxide fuel cells and semiconductor technology [11]. The finest qualities come from zirconia stabilized with Y₂O₃. A crystallographic change on a ZrO₂ surface limits fracture formation once is subjected to stress [12].

Baddeleyite, a naturally existing zirconia, is found in igneous rocks such as carbonatite. South Africa was the major producer of baddeleyite until 2002, but these plants have since closed, leaving the Kola Peninsula area of the Russian Federation as the only commercial source of this mineral. The majority of zirconia presently accessible is made using zircon (34,000 t/a), with a relatively minor natural source by the Russian Federation (6000 t/a) [3].

Zirconia ceramics are traditionally processed using turning, milling, and grinding techniques. These techniques have significant drawbacks, including inadequate productivity and inevitable manufacturing damage, which make it difficult to achieve sufficient quality of the surface. But sandblasting and acid etching have a tendency to pollute the component’s surfaces or alter their physical and chemical characteristics. In order to get an immaculate finish and undo surface and subsurface damage from earlier preparation, polishing can significantly enhance the surface quality [10].

1.4 Zirconia structure

There are three different types of zirconia crystal structures: Cubic, tetragonal, and monoclinic. It is possible to achieve high structural stabilization by combining ZrO₂ and associating to other metallic oxides like ZrO₂—MgO, ZrO₂—CaO, or ZrO₂—Y₂O₃ [13]. However, recent research emerged to be more concentrated on zirconia—yttria ceramics, which can be recognized by their fine-grained in the micro range classified as Tetragonal Zirconia Polycrystals (TZP) [14].

At high temperatures (>2370°C), a cubic zirconia also known as (c-ZrO₂) fluorite crystal configuration with a unit cell of face-centered cubic (fcc) and a (space group Fm-3m) form. A numerous atom of Zr is linked to eight oxygens to create the fcc cube and the oxygen atoms are arranged along the cube’s diagonals [15]. Furthermore, the fluorite structures of the monoclinic and tetragonal are deformed as seen in Figure 2. At intermediate temperatures (1200–2370°C), the structure is transferred to tetragonal (P4₂/nmc), while at low temperatures (<950°C), the structure is monoclinic (P2₁/c) [16]. Throughout thermal cycling, undoped zirconia reveals the following phase transitions [17]:

Figure 2.

The three ZrO₂ polymorphs are shown in a schematic in the following order: (a) cubic, (b) tetragonal, and (c) monoclinic [16].

$$\mathrm{m}‐{\mathrm{ZrO}}_2\underset{950°\mathrm{C}}{\overset{1170°\mathrm{C}}{\rightleftharpoons }}\mathrm{t}‐{\mathrm{ZrO}}_2\stackrel{2370°\mathrm{C}}{\rightleftharpoons }\mathrm{c}‐{\mathrm{ZrO}}_2\stackrel{2650°\mathrm{C}}{\rightleftharpoons }\text{Liquid}$$E1

2. Properties of zirconia

In relation to their usage, this section describes the many varieties and characteristics of zirconia materials. Due to the following qualities, zirconia has recently been the subject of substantial inquiry in the scientific literature:

2.1 Mechanical properties

Recent studies showed that mechanical parameters including hardness, grain size, and fracture toughness, as seen in Table 1 have an important bearing on slide wear resistance of bulk ceramics [18]. Zirconia has mechanical properties that are on par with stainless steels. Its traction resistance might be in the 900–1200 MPa range, and it has a 2000 MPa compression tolerance [19]. 3Y-TZP dentistry zirconia incorporates alumina (Al₂O₃) as a strain-hardening promoter, resulting in great opacity and exceptional mechanical qualities [20].

Property	Minimum value (S.I.)	Maximum value (S.I.)	Units (S.I.)	Minimum value (Imp.)	Maximum value (Imp.)	Units (Imp.)
Atomic volume (average)	0.02	0.021	m^2 kmol^−1	1220.47	1281.5	In^3 kmol^−1
Density	5	6.15	Mg m^−3	312.14	383,932	lb ft^−3
Energy content	200	300	MJ kg^−1	21667.7	32501.6	kcal lb^−1
Bulk modulus	72.3	212	GPa	10,4862	30,748	10^6 psi
Compressive strength	1200	$200	MPa	174,045	754,196	Ksi
Elastic limit	115	711	MPa	16.6793	103.122	Ksi
Endurance limit	107	640	MPa	15,519	92.8241	Ksi
Fracture toughness	1	8	MPa m^1/2	0.910047	7.28037	ksi in^3/2
Hardness	5500	15,750	MPa	797,708	2284.35	Ksi
Loss coefficient	0.0005	0.001		0.0005	0.001	Null
Modulus of rupture	177	1000	MPa	25.6717	145,038	Ksi
Poisson’s ratio	0.22	0.32		0.22	0.32	Null
Shear modulus	53.4	86.4	GPa	7.74501	12.5313	10^6 psi
Tensile strength	115	711	MPa	16.6793	103,122	Ksi
Young’s modulus	100	250	GPa	14.5038	36.2594	10^6 psi
Latent hear of fusion	700	820	kJ kg^−1	300,944	352,535	BTU lb^−1
Maximum service temperature	1248	2522	K	1786.73	4079.93	°F
Melting point	2823	2973	K	4621.73	4891.73	°F
Minimum service temperature	0	0	K	−459.67	−459.67	°F
Specific heat	420	540	J kg^−1 K^−1	0.32502	0.417883	kJ/kg °K
Thermal conductivity	1.7	2.7	W m^−1 K^−1	3.18246	5.05449	W K^−1 m^−1
Thermal expansion	2.3	12.2	10^−6 K^−1	4.14	21.96	10^−6 °F^−1
Breakdown potential	4	6	MV m^−1	101.6	152.4	V per mil
Dielectric constant	10	23	—	10	23	Null
Resistivity	3.16 × 10^13	3.16 × 10^18	10^−8 ohm m	3.16 × 10^13	3.16 × 10^18	10^−8 ohm m

Table 1.

Thermal, electrical, and mechanical properties of zirconia [18].

Although the sintering of ZrO₂ stabilized with Y₂O₃ is substantially more complex, this is the primary type of zirconia recognized for present medical usage. Surface treatments have the potential to alter the physical characteristics of zirconia. Furthermore, prolonged wetness contact may have a negative impact on its assets. The term for this is zirconia aging. Surface grinding can also diminish toughness. The mechanical characteristics of zirconia might deteriorate as it ages. Mechanical forces and moisture exposure are essential for speeding up this process [20].

Due to transition toughening, ZrO₂ has the ability to achieve extremely high toughness. It is regarded as a strong contender for numerous cutting tool applications as well as a biomedical material. Nevertheless, the intrinsic fragility of these substances remains the primary bottleneck. This issue has been addressed by developing suitable composites. Novel materials, including carbon nanotubes and fiber-like structures, have been produced in recent years. The fibrous character of the toughening components may result in increased fracture toughness. Furthermore, compared to the other two non-metallic additions, carbon’s electrical conductivity allows for additional electrical qualities to be helpful [21].

Using Eq. (2), it is possible to conduct a three-point bending test and a biaxial flexure test to assess flexural capacity and can be examined using a universal testing device (Autograph AG-X).

$$\sigma =\frac{\mathit{-}\mathbf{0.2387}\mathit{P}\left(\mathit{X}\mathit{-}\mathit{Y}\right)}{{\mathit{b}}^{\mathbf{2}}}$$E2

where the center’s highest tensile stress is σ (in MPa), the total load producing the rupture is P (in N), and the cross-section of the test piece at the fracture point is b (in mm) [22].

Meanwhile, fracture toughness (K_IC) test can be performed using the single-edge pre-cracked beam technique and can be measured by applying Eq. (3). Where B represents the material length (in m), W represents the sample width (in m), and a represents the pre-crack length; P represents the breaking force (N), and S represents the support span (m) [22].

$$K_{\mathit{IC}}=\left(\frac{P\times S}{B\times W^{3/2}}\right)\times \left\{\frac{3}{2}{\left(\frac{a}{W}\right)}^{\frac{1}{2}}\times Y\left(\frac{a}{W}\right)\right\}$$E3

2.2 Tribological properties

Zirconia ceramics have been extensively researched as a wear resistant material for engineering fields throughout the last few years. The kind of Hall-Petch law governs the connection between wear resistance and grain size in tetragonal zirconia ceramics. As a result, it is reasonable to predict that nano-structured zirconia coatings will have more wear resistance than standard zirconia. Friction and wear tests may sometimes be performed on an MM-200 wear tester’s block-on-ring configuration. As seen in Figure 3, the stationary zirconia-coated blocks are pressed up against a moving stainless steel. The tester provides the friction coefficient immediately. Additionally, wear frequencies can be calculated by dividing the wear mass loss by the force implemented, sliding distance, and material density [24].

Figure 3.

The block on ring test [23].

Throughout wear testing, the surface morphology, particularly big pores, may result in excessive stress concentration and fracture development, resulting in poor resistance. Additionally, it is shown that, as evidenced by Eq. (4), the wear rate of polycrystalline ceramics matched an exponential function. In which 1/K_0w represents the TZP substance wear resistance with no pores (in N.m/mm³) and P represents porosity.

$$\frac{1}{k_w}=\frac{1}{k_{0w}}\mathit{exp}\left(-23P\right)$$E4

Eq. (4) above illustrates that decreasing the porosity of zirconia ceramics resulted in enhanced wear resistance. The existence of gaps reduces the probability of grain boundary sliding, which delays or prevents the occurrence of plastic deflection [24].

2.3 Electrical properties and thermal conductivity

Zirconia additionally has a high degree of flexibility, a superior conductivity, a poor thermal conductivity, and a strong corrosion resistance as seen in Table 1 [18]. Because of that, zirconia is utilized in many applications at various high temperatures, such as fuel cells and thermal barrier coatings [18]. Monoclinic zirconia was demonstrated at 1000°C to be an amphoteric semiconductor in a prior work. When oxygen pressures are high (10⁻⁶ to a value of 1 atm), the most common defects are fully ionized zirconium holes, which produced exceptionally well, implying charge transfer via a thermally induced hopping mechanism [25].

On the other hand, at ambient temperature, zirconia in its monoclinic phase has a thermal conductivity of 7.2 Wm⁻¹ K⁻¹. However, at high temperature (1100°C) zirconia begins to have very poor heat conductivity (1.2–2.6 Wm⁻¹ K⁻¹), making it an ideal candidate for thermal barrier (TBC) coatings [26]. However, the volume variation (5%) caused by phase transformation at elevated heat as well as during cooling the apparatus limits the use of zirconia in aggressive environments. The crumbling of the zirconia-based components is caused by phase transformation and volume change [27].

Yet, when the temperature rises, the phase transition (Figure 4) occurs, resulting in structural failure and fractures in the coat. This would be prevented by enriching the zirconia matrix with yttria, which stabilizes the zirconia at high temperatures. Some Zr⁴⁺ cations are substituted by Y³⁺ during this stabilizing procedure [29]. To preserve charge balance, one oxygen gap is formed for every two replacing Y³⁺ cations. Because of the presence of voids, YSZ is valuable not only for TBCs but also as an electrolyte in oxygen sensors and solid-oxide fuel cells (SOFC) [30].

Figure 4.

Thermal conductivity of ZrO₂ [28].

This kind of stabilization not only assists in preventing phase change but also reduces the thermal conductivity of YSZ, for example, from 1.42 W/m·K (at ambient temperature) to 1.35 Wm⁻¹ K⁻¹at 1200°C since c-phase zirconia exhibits less thermal conductivity than m-zirconia [27].

Zirconia is also a metal oxide which is composed of a weak base and a weak acid and is one of the ceramic semiconductors. Because of its crystalline structure, it can be an insulator utilized as an n-type semiconductor or a high-resistance ceramic. The forms of the nanocrystals determine the potential uses for them. Oxygen sensors, fuel cell electrolytes, and gate dielectrics have all been employed with spherical ZrO₂ [31].

3. Processing of zirconia

This section discusses the purification process of zircon, followed by stating various methods for zirconia processing (thermal dissociation, chlorination, additive manufacturing, and colloidal processing) depending on the characteristics asked for and the purpose or the industry that asks for a zirconia supply.

3.1 Purification of zirconium compounds

For usage in the electronics sector and the creation of partly stabilized zirconia, highly pure zirconia is commonly used today. A large portion of the world’s zirconia supply found naturally as ZrSiO₄ [32].

By caustic fritting of zircon sand, the zirconium nitrate solution is obtained. Caustic fritting was accomplished with the aid of magnesium hydroxide carbonate and magnesium oxide. These additives reduce the quantity of soluble silica in nitric acid by less than 1000 ppm. With no additional ingredients, caustic fritting produces some nitric acid-soluble silica, which is eliminated by dehydrating with sulfuric acid [32].

Other procedures currently are utilized to extract zirconia from zircon generally by including heat or chemical disintegration of zircon to provide zirconia and silica. But even so, the resulting zirconia needs to be purified through additional chemical processing. A financially appealing method of producing zirconia is the thermal dissociation of zircon, which results in a combination of zirconia and silica. The dissociated zircon result can be leached either with a strong base, like sodium hydroxide (NaOH), to breakdown the silica and end up leaving zirconia or with a strong acid, like sulfuric acid (H₂SO₄), to solubilize the zirconia and produce a zirconium salt, leaving non-soluble silica. Zirconia may be created using the caustic-leach technique with a purity of up to 99.5%. Still, it must undergo subsequent chemical treatment, usually including dissolving the zirconia in acid to attain greater purity zirconia [32].

In reality, however, acid-leaching precipitates are gel-like, challenging to filter, and enclose numerous contaminants. Because of this, the basic sulfate precipitation using zirconium sulfate solution is favored, even though it is challenging to regulate the circumstances. To precipitate the basic sulfate, sulfuric acid or a sulfate salt is frequently added after the zirconium sulfate is converted to zirconium oxychloride. It has been discovered that creating zirconium compounds with good quality, including zirconia, is feasible by precipitating zirconium from zirconium sulfate solution at a low pH [32].

It is desired to utilize a zirconium sulfate solution with a zirconium level of more than 75 g/L and a sulfate content of more than 180 g/L. The ammonia source is introduced to the aqueous zirconium sulfate solution throughout the procedure of the present invention till the solution’s pH value is between 0.1 and 2.5. Whenever the pH is in the interval of 1.0 to 2.0, it is preferable to stop adding the ammonia source. The zirconium compositions created using the procedure of the present invention are white, compressible, unrestricted, finely dispersed substances after washing and drying [32].

3.2 Chlorination

Chlorinating impure zirconia in the presence of carbon is among the best ways to create highly pure zirconia. To purge contaminants from the ZrCl₄, partial solidification and sublimation procedures are performed in the range of temperature between 623 and 673 K. Additionally, highly pure ZrO₂ is created by the interaction of pure solid ZrCl₄ with water steam. Another crucial stage in the industrial scale of titanium and zirconium is chlorination in the presence of carbon. In the temperature range of 1000 K–1100 K, chlorine and ZrO₂ do not really thermodynamically interact, and a reducing agent like carbon is necessary to complete the process. Consequently, the process is known as carbochlorination [33].

By determining the variation in the free energy of the different reactions, it is possible to know why a reducing agent is required. By creating a low oxygen potential environment, the existence of carbon atoms lowers the inclination of the production of oxides and stimulates the creation of chlorides. It has been discovered that between 1075 K and 1275 K, the following reaction (CO₂ + C = 2CO) moves more slowly than during the chlorination process of ZrO₂ with a ΔG of −4.6 kJ mol⁻¹ at 1000 K to a −22.0 kJ mol⁻¹ at 1100 K [34]. On the other hand, thermodynamically speaking, reaction 1 cannot take place at 1100K but other reactions may. It’s possible that reaction (ZrO₂ + C + 2Cl₂ = ZrCl₄ + CO₂) with a ΔG of −248.3 kJ mol⁻¹ and reaction (CO₂ + C = 2CO) will produce reaction (ZrO₂ + 2C + 2Cl₂ = ZrCl₄ + 2CO) with a ΔG of −270.4 kJ mol⁻¹.

3.3 Colloidal processing of zirconia

During lower sintering temperatures, high-density zirconia structures with decently crystal frames and improved mechanical characteristics may be created using fine nanocrystalline 3Y-TZP powders. However, throughout most of the production of the 3Y-TZP nanocrystalline initial crystals, significant agglomeration takes place [35]. The outer surface of the colloidal matter is decreased by agglomerates, which consist of networks of particles linked by van der Waals forces [36]. Ultimately, accumulation could lead to the development of undesirable variabilities, including defects, fractures, and porous surfaces with big granules, which can impair the mechanical capabilities of zirconia structures. As a result, attempts must be considered to prevent agglomeration due to consolidation. Powders are merged in greenish structures with the necessary shapes using various consolidation techniques, such as solid free-form manufacturing, wet slip casting, dry isostatic and uniaxial pressing. A slip casting is a common colloidal procedure that creates structures with a significant density and few harmful variabilities. By adjusting the intensity of the attracting and repelling interparticle interactions between zirconia particles, steady solutions with high particle dispersion may be created, which are necessary for reducing the production of agglomerates [37]. An electric double surface is produced by generating comparable charges on the interfaces of suspended particles of sufficient size [36].

CAD/CAM, which denotes computer-aided engineering and production, is one way to create zirconia copings and frames. An automatic manufacturing process using a software power tool follows the editing of a 3D model on a screen in the CAD/CAM manufacture of zirconia. There are two methods for creating CAD/CAM zirconia foundations: pre-sintered blanks can be “soft-machined” or completely sintered blanks can be “hard-machined”. These foundations have undergone high-quality procedures to produce homogeneous material structures with little to no cavities, defects, or fractures. A binder is blended with partly stabilized zircon powder (silicone oxide 0.02%, yttrium oxide >4%, hafnium oxide >1%, zirconium dioxide 96%, aluminum oxide 1%) to create zirconia [38].

Well-dispersed zirconia solution has just been subjected to direct coagulation casting (DCC) and gel casting are two innovative methods for treating colloidal materials to produce greenish particles [39]. DCC focuses on creating ceramic components closer to their final form from suspensions of ceramic particles in concentration solutions. It combines the DLVO concept with colloidal ceramic formation. The saturated suspension is disrupted by the decomposition generating base, electrolyte, and acid, whether to boost the ionic strength or to let the pH reach the ceramic powder isoelectric point framework [35, 36].

3.4 Powder processing of zirconia

Because of their acceptable mechanical qualities, minimal neutron absorption cross-section, and superior corrosion tolerance, zirconium that concentrates on alloys has been employed extensively in industrial power stations.

Nevertheless, because zirconium has a great melting point (>1800°C) and a significant attraction for H, O, and other contaminating atoms, it is challenging to produce zirconium workpieces using standard methods. AM will make it easier to produce complex parts that are employed in a variety of industrial applications. The quality of the finished products, however, is heavily dependent on the raw powders. When used in the AM process, powders with desirable morphological properties display improved rheological properties and more optimum particle packing. Research on producing sphere powder from commercial products like Ti64 has advanced significantly. Spherical metal components have been created using various techniques, such as Electrode Induction Melting Gas Atomization, Plasma Atomization, Plasma Rotating Electrode Process, Plasma Spheroidization (most flexible approach), and Vacuum Induction Melting Inert Gas Atomization [40].

3.5 Additive manufacturing (AM)

3D printing technology commonly referred to as additive manufacturing techniques, opens up new possibilities for material shaping by increasing design flexibility, accelerating production, and minimizing waste and expense. The revolutionary possibilities of using these technologies to produce technical ceramics have been widely acknowledged, despite the ceramic industry’s slower adoption of 3D printing technology than the polymer and metal industries. Stereolithography (SLA), one of the several 3D printing technologies currently accessible, offers advantages in the areas of printing resolution and precision, which—along with the exceptional efficiency of exterior coating—make it the ideal suitable AM technology for constructing ceramics [41]. The greatest range for achieving uniform mixing, according to an investigation by Jang et al. on the synthesis of zirconia employing DLP additive manufacturing, was 58 vol% of zirconia by volume. The 3-point bending durability improves as the volume fraction rises. The research did note that the viscosity rose quickly to 56 vol% [42]. SLA technique is built on the layer-by-layer polymerization of a liquid photocurable monomer incorporating ceramic particles; more specifically, it uses a beam of UV laser that shifts from one location to another while tracking the photo-polymerized pattern [43].

A near fabrication path to the manufacture of progressed ceramics with extremely complicated geometries is provided by additive manufacturing (AM). It does not involve the original building of a mold, making it a quick-response solution for engineers and designers to create new items [44].

A possible method for creating intricately shaped ceramic components with high accuracy is digital light processing (DLP) [43]. A different type of SLA utilizes a UV projector as the light source for treating an entire resin layer by layer, improving the speed of the process. Due to the variety of utilizations for which this material can be applied, including restorative dentistry and frameworks, fuel cells for monolithic support, tooling and blades, and other precision parts for various applications, including mechanical and thermal ones, 3D-printed zirconia is becoming more and more popular among technical ceramics [44].

DLP, which has excellent printing resolution, a high interfacial polish, and a rapid building process, is becoming increasingly popular for manufacturing ceramic components. However, compared to traditional ceramic processing, DLP is still in its developing phase [44].

Making completely dense, defect-free ceramics with characteristics equal to those of materials treated using traditional technologies is considered a difficult task. This requires the supervision of numerous manufacturing steps, including printing optimization techniques, the slurry preparation, and the sintering heating cycles. Nevertheless, even though DLP and SLA are established for zirconia 3D printing technologies, most articles briefly discuss the mechanical characteristics of the resulting materials [43] within the beginning, zirconia green entities are created for the specimen preparation using a heavily zirconia-loaded slurry on a DLP 3D printer utilizing a UV light with a wavelength of 405 nm. In Figure 5, a diagram of the DLP printing principles is displayed. For all builds, the depth can be adjusted at 25 μm. For further characterization and assessment, a variety of zirconia green bars, disks, and parts are acquired.

Figure 5.

Diagram illustrating the DLP printing principles [39].

The extra slurry is then washed off the artificial green parts in an ultrasonic bath. The debinding and sintering then performed in a tubular furnace with an air flow [44].

3.6 Thermal dissociation/calcination

Calcination is the practice of heating a substance at 400, 600 and 800°C without allowing it to fuse to affect changes in its physical or chemical composition [45]. Electrospinning, a newly improved combination of the ceramic precursor zirconium acetylacetonate and the bond polymer polyacrylonitrile, may be utilized to create zirconia incorporating nanofibers. It is discovered that the steps that follow the ceramic precursor conversion occur simultaneously with the transformation of electrospun zirconium acetylacetonate/polyacrylonitrile fibers into zirconia nanofibers:

$$\begin{array}{ll}\mathrm{Zr}{\left({\mathrm{C}}_5{\mathrm{H}}_7{\mathrm{O}}_2\right)}_4\to & \mathrm{Zr}\left(\mathrm{OH}\right){\left({\mathrm{CH}}_3\mathrm{COO}\right)}_3\to \mathrm{ZrO}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\to \mathrm{t}-{\mathrm{ZrO}}_2\to \mathrm{t}\\ & -{\mathrm{ZrO}}_2+\mathrm{m}-{\mathrm{ZrO}}_2\to \mathrm{m}-{\mathrm{ZrO}}_2.\end{array}$$E5

Fibers are heated in two stages; first, they are heated to a temperature of 500°C with a heat rate of 1°C/min, and subsequently to a specified temperature with a heat rate of 5°C/min, calcining them at various temperatures between 500 and 1300°C for 1 hour. A low-temperature profile was adopted for the first annealing stage to ensure the sensitive removal of the ceramic precursor’s breakdown products and binding polymer to minimize fiber breaking [46].

Additionally, furnaces for manufacturing glassware have often been built using and repaired with zircon refractories. Its breakdown tendency is a crucial zircon property that might affect furnace and glass performance. A decrease in the proportion of aluminum, titanium, iron, and alkali is necessary to lessen the rate of zircon deterioration [47].

Generally, the below process may be used to describe zircon dissociation:

$${\text{ZrSiO}}_4\to 3{\mathrm{ZrO}}_2+{\mathrm{SiO}}_2$$E6

where zircon is transformed into zirconia and silica, often by thermal methods at a temperature of around 1676°C. Zirconia acquires a tetragonal shape at 1173°C, whereas underneath this temperature, it exists as a monoclinic phase [47].

Zirconia precipitates are calcined at 800°C for 2 hours (h) in the air, which changes the monoclinic crystal structure and improves the crystallinity as well as the dimensions of the precipitate. Additionally, it was noted that zirconia with a cubic and tetragonal structure undergoes a partial transformation from its monoclinic state. In additional research, acidic and ammoniacal zirconium chloride solutions in different concentrations were used to create nanoscale zirconia precipitates at temps between 110 and 150°C for times ranging from 1 to 4 hours [48].

3.7 Zirconia treatments

Surface treatment to form a strong chemical connection with such composite resin is one method for successfully mending chemically inert zirconia foundations. Consequently, the effectiveness of dental restorations is determined by the adhesiveness of zirconia and composite resin.

It is extremely hard to chemically or physically change the zirconia ceramic surfaces because of its superior stiffness and chemical inertness to avoid corrosion [49]. In addition, unlike other types of ceramics, such as glass, zirconia is not etchable, making it difficult to carry out the adhesion processes [50]. According to earlier research, sandblasting significantly enhanced the ceramic surface’s wettability, surface energy, and roughness. Because of this, the resin cement is retained by micro-interlocking due to the bonding area’s enhancement [49].

However, implant technology has been working hard to change zirconia in terms of morphological and bioactive features essential for suitable cell interaction and differentiation throughout most of the neighboring bone healing. Numerous physicochemical techniques have been utilized to improve zirconia properties, such as PVD, grit blasting, laser treatment, PVD, micro-machining, and CVD [51].

Although it has been asserted that zirconia pretreatment techniques like tribochemical silica coating, sol-gel processes, silicon nitride hydrolysis, and vapor-phase deposition technique increase resin adherence to zirconia, the long-term bond performance of tribochemical silica coating is in doubt. Condensed silanol layers produced by the sol-gel technique and hydrolyzed from tetraethyl orthosilicate are prone to cracking when heated, and coatings produced by silicon nitride hydrolysis or vapor-phase deposition require complex equipment and take a long time, making them unsuitable for use in clinical settings. With 5% HF inscription for 90–120 seconds at a pressure of 0.3 Mpa, a new glass ceramic spray deposition (GCSD) approach has recently been described to increase the zirconia bond strength. It is possible to create an extremely thin layer that does not alter the zirconia’s physical characteristics by spraying glass-ceramic powders over zirconia surfaces which are subsequently sintered [52].

3.8 Powder sintering

During sintering, powder components are condensed and go under consolidation at a high temperature but still below their melting temperature. A particle substance is transformed into a rigid, compact design. Somewhere at macro scales, changes in geometry, dimension changes (shrinkage), and density variation may be seen, as well as changes in mechanical qualities like mechanical strength (as a result of shrinkage) [53].

To achieve a greater compactness and performance, more machined and sintering operations are required [44]. However, these existing techniques methods have a variety of drawbacks, including the inability to produce highly complex geometry because of the common usage of molds, their increased price, and their prolonged processing times. Furthermore, machining ceramic materials is challenging due to their extraordinary hardness and brittleness. Additionally, flaws and unwanted shrinkages may also be produced in the ceramic parts [54].

DLP-produced sintered ceramic components may have shrinkage problems. An effective way to address them is to scale the CAD/CAM model appropriately for each axis before printing to account for shrinks [44]. Through DLP, a single layer of material is first made, and by repeating this process, a body with three dimensions is developed. The three-dimensional cross-linked polymer that makes up the green body has ceramic fragments caught inside it. The components are then cleaned, debonded, and sintered to create the final thick ceramic part. This technique makes it possible to create intricate three-dimensional ceramic structures with excellent precision and creative freedom [55].

The microstructure changes throughout sintering, and this development is characterized by a rise in grain compaction and rearrangement and cohesive bonds among granules develop in the early stages. Because of the mass transmission, the bonds between particles expand as the sintering process continues. Surface and grain boundary diffusion are often the primary mass transport modes in sintering. Surface tension and bonds’ stresses cause particle attraction, which causes the system to contract. The decrease and occasionally the actual eradication of material porosity is a result of neck development and shrinking [53].

Owing to the possibility of the ceramic material’s cracking or breaking, thin restorations for minimally invasive dentistry might be challenging to create employing the subtractive method. In addition, subtractive manufacturing usually results in significant cutting tool usage and significant volumes of generated waste (zirconia powder) after milling. Additionally, zirconia’s resilience and lifetime are improved by surface treatment and adhesive bonding with primers [55].

4. Analysis methods

4.1 Microstructural analysis

For microstructural characterization, Raman microanalysis can be demonstrated. On the other hand, for studying the chemical composition, high vacuum X-ray energy dispersive microanalysis, X-ray photoelectron spectroscopy (XPS) are usually used. XPS is accelerated under a voltage of 14 kV, emission of current of o.2 A, pressure of 10⁻⁹ mbar, energy resolution of 0.30 eV, maximum power of 2.8 kW, sampling area of 6 × 0.5 mm sampling area, and around 90° take-off angle. Finally, to charge compensation, an electron flood gun works at 4 eV. All spectra can be shown and recorded on the binding energy with an excellent resolution of 150 eV pass energy [56].

In order to recognize and define the pattern of the tetragonal (which can be demonstrated by 145 and 262 cm⁻¹ Raman shift) and monoclinic (by 180 and 190 cm⁻¹ Raman shift) ZrO₂ phases just at the surface area, then the cervical collar that is prepared for XRD investigation is examined. The following requirements are used to conduct a Raman microscope: 100 m slit, Ar laser (532 nm), 10 mW at the sample, a grating of 1800 grit/mm, a confocal hole of 1000 μm, and a sampling duration of 10s.

The microscope’s optical system independently finds and examines three locations at every implant surface site (collar/root). 20 × 35 m regions are chosen and examined at 5 m increments with a 5 s scan time in order to map the monoclinic ZrO₂ phase [56].

4.2 Phase characterization

For morphological analysis, LV-SEM is used. And for the roughness, optical profilometry is used. In X-ray diffraction (XRD), a sample from each implant is embedded in epoxy resin and sliced into longitudinal sections using a microtome while being continually cooled by water to test for the presence of tetragonal and monoclinic ZrO₂ phases across the whole implantation. The materials are smoothed using SiC paper and polished using 3 M diamond paste, and then for 3 minutes, ultrasonic cleaning in distilled water is applied [56]. The zirconia X-ray diffraction spectra is displayed in Figure 6. Their X-ray diffraction patterns may identify zirconia’s monoclinic and metastable tetragonal phases [57].

Figure 6.

Diagrammatic representation of the parameters (L, W, and H) change along the zirconia DLP pathway [44].

The threaded portions of the implants’ 3D surface roughness characteristics are evaluated using optical profilometry. At 10–100× magnification, an optical profiler may be utilized to look at the gap between two subsequent implant threads. The roughness metrics of a 3D surface, including arithmetic mean deviation (Sa), root average square deviation variance (Sq), and max peak to valley altitude (Rt) are then measured using Veeco Vision software [56].

4.3 Mechanical testing

Using the techniques specified in Table 2 at ambient temperature, the mechanical characteristics, including fracture toughness, Vickers hardness, and flexural strength, are thoroughly studied. A Weibull assessment of the tensile and flexural data is thus carried out to determine the possibility that zirconia would rupture. By employing the single-edge V-notched beam (SEVNB) technique and indentation, correspondingly, the fracture toughness value (K_IC) is assessed (Table 3) [44].

Reaction	Temperature (K)	∆G (kJ mol^−1)
ZrO2 + 2Cl2 = ZrCl4 + O2	1000	154.2
	1100	147.1
ZrO2 + C + 2Cl2 = ZrCl4 + CO2	1000	−241.2
	1100	−248.3
ZrO2 + 2C + 2Cl2 = ZrCl4 + 2CO	1000	−245.7
	1100	−270.4
ZrO2 + 2CO + 2Cl2 = ZrCl4 + 2CO2	1000	−236.6
	1100	−226.6
CO2 + C = 2CO	1000	−4.6
	1100	−22.0

Table 2.

Zirconia’s carbochlorination reactions and the change in Gibbs free energy [34].

Table 3.

Investigation techniques for mechanical properties [44].

It is crucial to pick a reference based on the actual indentation fracture morphology since the selection of the equations also depends upon the type of crack. A femtosecond laser is used to construct an incredibly sharp V-notch (0.5 m, indicated by a red dashed line as seen in Figure 7) to measure fracture toughness accurately [49, 58, 59].

Figure 7.

XRD spectra of zirconia [57].

Figure 7 shows how U-grooves are created at particular thicknesses using a reduced-speed blade and diamond rims with a 200 m thickness under irrigation water. A 2.9 W femtosecond laser is now being used to create a crisp V-notch just at the bottom of the U-grooves. With a repeating rate of 100 kHz, the femtosecond lasers produced 290 fs linearly polarized pulses at 515 nm [50, 58].

5. Future perspectives of zirconia

5.1 Nano zirconia

Nano solids, which are tiny structures of ZrO₂, have been extensively proposed. ZrO₂ on a nanoscale, which exhibits better mechanical properties and superior biocompatibility, is frequently included into many technologies utilized in tissue engineering and dental applications [51]. Even though sophisticated powder synthesis processes can generate nano-sized particles [60, 61, 62]. Nano zirconia also showed a promising future for fuel cell applications. Cubic ZrO₂ nanoparticle production is challenging because of the range of phases that the reactions create. In a previous study, a simple precipitation method was established to adjust the form and crystallinity of cubic (Arkelite) and monoclinic (baddeleyite) zirconia nanoparticles. The manufactured cubic and monoclinic zirconia can be put into the Nafion® membrane to increase the fuel cell’s efficiency. The polytetrafluoroethylene backbone and perfluorinated ether sidechain are characteristics of the proton conductor electrolyte Nafion®. Additionally, due to the transpiration of water that leads the membrane to buckle, Nafion® loses conductivity at a higher temperature of 100 C [31].

Toward the latter situation, the rise in clinical defects in the dental sector has been noticed owing to veneer chipping due to pressure accumulation throughout fabrication. Additionally, the use of ceramics’ nanocrystalline structure has gained the interest of engineers since they strengthen their resistance to low-temperature deterioration (LTD) [63].

Current efforts involve synthesizing nanoparticle agglomerates within 30 mm aggregates. The high-velocity oxy-fuel (HVOF) method can be subjected to spray nano-zirconia powders made in radio frequency (RF) plasma. This method employs relatively low flame temperatures (<3000°C) to ensure that the particles are only a little heated. The nano-zirconia particle-reinforced coatings are examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effects of the post-spray treatment on the coatings are investigated [62].

Wet-chemical synthesis techniques, such as co-precipitation, hydrothermal synthesis, and sol–gel preparation, are primarily used to manufacture nano-ZrO₂ powders. The physical approach cannot achieve nanoscale results, and the gas-chemical process is too expensive to be repeated in a real-world setting [64].

The nanoparticles’ surfaces are treated with various surfactants that have different attaching head groups and carbon chain lengths. Because of their distinct nature and properties, quantum dots (QDs) fall within the special category of nanomaterials. A QD is defined as an individual semiconducting nanocrystal that ranges in dimension from 2 to 10 nm. Several techniques, including soot vapor deposition, microwave synthesis, laser ablation or chemical oxidation of graphite, and graphite oxidation heat-induced oxidation of a molecular precursor, can be used to create QDs. In recent research, scientists devised a simple approach for producing a highly effective photocatalytic composite by adsorbing carbon quantum dots (CQDs) onto the surface of ZrO₂ NPs as demonstrated in Figure 8. The developed photocatalyst could be useful for the rapid fading of textile colors. The composite’s production method, which entailed calcination of an ammonium citrate solution, is used to make QDs. Then, using a conventional solvent-based chemical method, produced QDs are adsorbed onto the surface of ZrO₂ NPs with ultrasound assistance Figure 9 [65].

Figure 8.

(A) Diagrammatic representation of SEVNB samples. (B) Equipment for creating a razor-sharp V-notch using a blade [49, 58].

Figure 9.

TEM pictures of ZrO₂ nanoparticles with CQD embellishments [65].

Other studies show another development regarding a nano-structured glass-zirconia in order to increase the resilience of the interface between dental zirconia substrate and veneered porcelain. A new SiO₂–Li–Al–O₃ material to create a glass-zirconia nanostructure, prepped (SLA) glass was infiltrated through the outermost layer of fully sintered dental zirconia [66].

6. Conclusions

Zirconia has been indicated and used in various industries including, dentistry, glassware, and electronics. This comes from different reasons especially, its mechanical, electrical, thermal conductivity, and tribological properties. Due to the high demand, zirconia supply had to be produced after purifying zirconium compounds by chemical processing using acids or bases. Different processes might be performed to produce pure zirconia, such as chlorination under a high-temperature range between 623 K and 673 K, colloidal processing of zirconia, and additive manufacturing which uses a 3D printing technology with high precision and resolution. Zirconia’s microstructure is analyzed by XPS and X-ray energy, phase characterization by XRD, and Mechanical properties are tested analyze the fracture toughness and flexural strength. The uses of zirconia have been constantly increasing over the last years, and the most discussed structures are nano zirconia integrated with other systems such as CQDs to enhance its characteristics.

Conflict of interest

The authors declare no conflict of interest.