Zirconium Dioxide, “Zirconia” (ZrO2)

Formula: ZrO2

Melting point: 2715°C

Density: 5.68 g/cc

Solubility: Virtually inert. Attacked only by the acids HF, HCl, H2SO4, and hot alkalis.

The history of zirconium dioxide (ZrO₂) , commonly known as zirconia, is intimately linked to the development of modern technical ceramics and the evolution of materials science in the 20th century. Although the element zirconium was identified by Martin Heinrich Klaproth in 1789 from the mineral zircon, it was not until much later that its oxide began to be systematically synthesized and used in industrial applications. Zirconia emerged as a strategic material when its ability to withstand extreme temperatures and aggressive chemical environments was discovered, making it an ideal candidate to replace or complement other oxides such as alumina (Al₂O₃).

During the second half of the 20th century, research in advanced ceramics revealed that ZrO₂ could be stabilized in its tetragonal phase by the addition of oxides such as yttrium oxide (Y₂O₃), leading to what is known as partially stabilized zirconia. This finding was revolutionary, as it allowed leveraging its phase transformation to improve fracture toughness, a property uncommon in ceramic materials. From then on, zirconia ceased to be a simple refractory oxide and became a multifunctional material, used in dental prostheses, engine components, oxygen sensors, and membranes for solid oxide fuel cells.

The history of zirconia is also the story of how an apparently simple oxide was transformed into a pillar of precision engineering, thanks to its structural versatility and its ability to adapt to the demands of contemporary technology. From its origins as a byproduct of zircon to its consolidation as a high-performance material, ZrO₂ has traveled a path that reflects the progress of materials science in its search for durable, resistant, and efficient solutions.

Before addressing its physical characteristics, it is essential to understand that zirconia is not thermodynamically stable in its cubic phase at ambient temperature and pressure. This statement may seem contradictory, given its widespread use in high-demand applications, but it responds to basic principles of thermodynamics. Just as diamond, over time, tends to transform into graphite because the latter is more stable under normal conditions, zirconia also presents crystalline phases with different levels of stability. In simple terms, the form that requires less energy to maintain will be favored over a more energetic one, depending on the external conditions of the system. This phenomenon can be illustrated with the behavior of water: ice predominates in the Arctic, while it is unthinkable to find it in the Sahara; in contrast, water vapor is abundant in tropical climates. Water adapts to temperature and pressure conditions, favoring its liquid form between 0°C and 100°C at atmospheric pressure, while below 0°C it solidifies and above 100°C it vaporizes. This type of transition also occurs in elements like carbon or iron, where the most stable crystalline structure depends on the physical environment.

In the case of ZrO₂, the cubic phase, which is the most desired due to its superior mechanical properties, is not stable at room temperature. In the absence of a stabilizing agent, this phase tends to slowly transform into other more stable ones, such as the monoclinic phase, leading to a significant loss of toughness and strength. To prevent this transformation, yttrium oxide (Y₂O₃) is introduced, which acts as a stabilizer and allows the cubic phase of zirconium dioxide to be fixed. Although the chemical formula remains unchanged, the atomic arrangement is modified, resulting in a material with improved properties and greater durability.

Once this structural aspect is understood, the physical description of the oxide can be addressed. Zirconia is an inorganic compound belonging to the group of technical ceramics, along with silicon carbide and alumina, being one of the most relevant due to its versatility. In its pure state, ZrO₂ is crystalline, transparent, and possesses high hardness, reaching 8.5 on the Mohs scale. Its chemical resistance is remarkable, making it an ideal material for corrosive environments. This gem quality of the oxide has been exploited in jewelry as a diamond substitute, not only for its appearance but also for its higher light dispersion index, known as “fire,” which even surpasses that of the purest diamond. Although it is an imitation, it is not an inexpensive material, and its use in gemology responds to both aesthetic and technical reasons.

In the industrial field, zirconia is obtained as a white powder by the calcination of zirconium minerals, such as zircon (ZrSiO₄), which also has applications in jewelry and industrial processes. It is important not to confuse zircon with zirconia, as the former is a silicate and the latter the pure oxide of the metal. In its calcined form, zirconia presents a bright white color, retaining its hardness and chemical resistance. Although it could be used in bulk form, this is uncommon due to the existence of more economical alternatives for applications such as abrasives. Its most widespread use is based on its toughness, which approaches that of silicon nitride (Si₃N₄), a more expensive and exotic material. Industrial zirconia is processed by sintering, and its use without yttria stabilization is not recommended, as it would compromise its mechanical properties.

Extreme heat resistance is significantly improved by the presence of Y₂O₃, and its behavior against oxidation at high temperatures —a phenomenon known as crepitation— is excellent. Although considered chemically resistant, it is not completely inert: hydrofluoric acid can attack it even when cold, albeit slowly, while when hot it reacts with hydrochloric acid and non-concentrated sulfuric acid with relative ease. Finally, it is worth noting that zirconia has a high density for a ceramic material, which contributes to its robust mechanical behavior and its effectiveness in demanding structural applications.

Zirconia has consolidated itself as one of the most recognized high-performance ceramics in modern industry, and not by chance: its versatility is exceptional. Its resistance to extreme heat, abrasion, and fracture —even under high thermal conditions— makes it an effective alternative to more costly materials such as silicon nitride (Si₃N₄), especially in applications where toughness is crucial but the budget is limited. This ability to substitute more exotic compounds without sacrificing performance has led to ZrO₂ being used in a wide range of technical components, from crucibles and laboratory parts to precision bearings, pipes, and ducts subjected to considerable mechanical and thermal stresses.

One of the most widespread and everyday uses of zirconia is in dentistry, where it is employed as a material for dental prostheses. In this context, industrial-grade zirconia is used, although subjected to specific treatments that improve its aesthetics and biocompatibility. Its original marbled white color can be chemically modified in the laboratory to mimic the shade of the patient's other teeth, allowing the replacement part to blend in visually. This customization capability is essential from an aesthetic point of view, as the ZrO₂ prosthesis is more resistant and chemically inert than the natural tooth. This means that, even after cosmetic procedures like tooth whitening, zirconia maintains its color without alterations, ensuring perfect visual integration with the rest of the dentition. However, the cost of these prostheses is usually high, regardless of the country or healthcare system in which they are acquired, which limits their accessibility to certain segments of the population.

Another widely known use of zirconia is in jewelry, where it is used as a diamond substitute. Despite being an imitation, it surpasses diamond in terms of light dispersion index, which translates into a more intense and brilliant “fire”. This optical property has led to zirconia being used not only as a diamond replica but also as a base for the creation of synthetic gems of various colors. Through doping and heat treatment techniques, ZrO₂ can acquire shades that closely imitate precious stones such as sapphire (blue), ruby (red), or amethyst (violet), among others. This chromatic versatility, coupled with its hardness and brilliance, has made zirconia an undisputed protagonist in the synthetic gem market.