**1. Introduction**

Ceramic materials have played an important role for many years. Their main use has been in bricks and in pottery. Because of its inertness and corrosion resistance, table wear is a quite popular, centuries-old application of ceramics. All goods named are made out of natural raw materials. About 90 years ago, the first publications showed alumina based ceramic materials based on synthetic alumina (Al2O3) powders [1].

In the early 50s of the last century, alumina ceramics became popular for some wear applications. Especially in the textile industry, it is still used as the most wear resistant material against yarns. Additional developments related to several applications, like cutting tools, lead to further improvements of the materials [2].

Due to its reversible phase transformation, zirconia (ZrO2) didn't play a significant role for 80 years, because of the need of stabilizing oxides in order to keep a stable cubic crystal structure. In refractories, such zirconia became popular, because of their low thermal conductivity. However, mechanical strength properties of such kinds are quite limited. In mid 70s of the last century, it has been found that by a partial stabilization of the zirconia, special properties by microstructural design have been achieved [3]. Finally, in the early 80s it has been found that by stabilization of zirconia by yttria (Y2O3), the tetragonal modification can be stabilized. Such materials show a very high mechanical strength, and since that time, many new developments have been made.

Mixtures of alumina and zirconia were introduced in 1977 for ceramic cutting tools [4]. By incorporation of very fine grained zirconia particles into an alumina matrix, mechanical

**Citation:** Burger, W.; Kiefer, G. Alumina, Zirconia and Their Composite Ceramics with Properties Tailored for Medical Applications. *J. Compos. Sci.* **2021**, *5*, 306. https:// doi.org/10.3390/jcs5110306

Academic Editor: Francesco Tornabene

Received: 4 October 2021 Accepted: 12 November 2021 Published: 22 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

strength properties and fracture toughness can be enhanced. Finally, in 1991 it was published that reinforcement of alumina with zirconia and hexagonal platelets led to a further improvement of the materials.

One of the major uses of yttria stabilized zirconia started in 1986 when ball-heads for total hip prosthesis have been introduced into the market [5]. Today ball-heads on the bases of yttria stabilized zirconia do not play a role in this field. They have been substituted by composite ceramics. However, in the dental industry yttria stabilized zirconia ceramics are quite popular in prosthetics and as implants [6].

In the following, alumina, zirconia and its composites will be described more in detail.

#### **2. High Purity Alumina**

The most popular processing technology for making alumina powder is a disintegration of bauxite with caustic soda. By this process, called the Bayer-process, the alumina is dissolved at temperatures of about 120–140 ◦C and a pressure of about 2–3 bar. The Al(OH)4 −-ions are then precipitated by the addition of seeds. These Al(OH)3 (aluminium hydroxide) precipitates normally contain impurities like magnesium, calcium, silica, iron and sodium ions. After a thermal treatment, the hydroxide changes to several intermediate oxides, before finally the thermodynamically stable α-phase of alumina is reached. Usually, this powder then is used for making synthetic ceramic materials [7].

One of the first applications for an alumina ceramic material has been the isolating component of spark plugs. Since densification during the sintering process of such alumina powders is very difficult, additional components are added. Therefore, most of the alumina ceramic materials contain silica (SiO2), magnesia (MgO) and/or calcium oxide (CaO) [8,9]. By addition of these components, glassy phases are formed and such glassy phases support the densification behaviour significantly. Besides spark plugs, which are usually based on 97 wt-% of alumina and additional silica and calcium oxide or magnesia, seal-discs and substrates for electronic applications are based on similar formulations. Unfortunately, these glassy-phase containing alumina ceramics tend to corrode under long term treatment in humid atmospheres.

Alumina ceramics with a purity of 99.7% based on Bayer-alumina still contain a certain small amount of calcium and silica impurities and do not have a very high sintering activity. Therefore, in order to achieve a density of at least 97.5% it is mandatory to apply sintering temperatures up to 1700 ◦C. By application of such high sintering temperatures a significant grain-growth occurs. Furthermore, grain-growth control is very difficult. Even the addition of magnesium oxide, which is well known as grain-growth inhibitor, does not help any more to control the growth of the grains at these high temperatures [10]. As a result, discontinuous grain growth occurs. Grain-size of such a kind varies significantly, and single grains with a size of 20 μm or more are quite often found in the microstructures. The typical mechanical strength of these materials is about 250–300 MPa. Figure 1 shows the microstructure of such a ceramic.

Looking back to the 1960s, only alumina powders based on the Bayer process have been available on the market. In parallel, already electro corundum has been applied for grinding applications. Electro corundum are alumina single crystals with a size of about 15–300 μm. Due to the melting process, which occurs in order to achieve the single crystals, the purity of these grains is much higher compared to alumina powders derived from the Bayer process. The only remaining impurities are small amounts of silica (SiO2) and sodium oxide (Na2O).

Erhard Dörre, a pioneer in the development of high purity alumina ceramics, recognized the advantage of the higher purity of electro corundum, milled the single crystals down to a size of about 0.6 μm, cleaned the milled powder with hydrofluoric acid in order to get rid of the silica impurities, followed by a second cleaning step with hydrochloric acid in order to get rid of sodium oxide. By this approach, he realized a high purity powder for making high purity ceramics. In addition, this ceramic material could be sintered at temperatures of less than 1600 ◦C. By addition of magnesia as grain-growth inhibitor

a very uniform microstructure with a mean grain-size of about 5 μm has been realized (Figure 2) [11]. Furthermore, the density achieved has been at 99%. As a result of this, the mechanical strength could be increased to 420 MPa.

**Figure 1.** Microstructure of alumina with a purity of 99.7% sintered at 1650 ◦C.

**Figure 2.** Microstructure of 99.7% alumina based on purified and milled electro corundum.

The pioneering work of Dörre has been underestimated for a long period. Furthermore, the costs of Dörre's material have been significantly higher. Therefore, managing people fought against this material, because they only have seen the higher costs and didn't want to see the unique performance of this material.

Dörre's approach shows the importance of the raw material in relation to the mechanical properties. Furthermore, by his approach he could avoid impurities, which are sensitive to corrosion. While in the Bayer alumina raw materials always small amounts of calcium oxide and silica are present, the high purity alumina ceramics made by Dörre no longer had any impurities. As a consequence of this, besides the improved mechanical properties, this material also shows a significant higher corrosion resistance.

Finally, Dörre achieved the break-through with this material in a publicly funded project, which has been related to bioceramic ball-heads. Mechanical strength and corrosion resistance are mandatory for a long-term stability. Since Dörre's material fulfilled these requirements, finally his material has been qualified for bioceramic applications as ballheads in total HIP replacement systems, while approaches on the basis of Bayer alumina only had limited success.

In 1970s, new powder processing routes for high purity alumina have been developed. All of these processes start from defined chemicals, which are isolated and afterwards transferred to high purity alumina. Typical precursor salts are Ammoniumaluminiumsulfate (Alaun) (NH4)Al(SO4)2·12H2O, Aluminiumchloride [Al(H2O)6]Cl3, Ammoniumaluminiumcarbonate (NH4)Al(CO3)2 or Aluminiumalkoxide Al(OR)3. These salts are easily dissolved and precipitation can be controlled, which means that the primary crystallites formed are influenced by the precipitation method. Especially for the Alkoxides by precipitation either hexagonal or ball-like precipitates can be tailored.

At this point it has to be stated that chemically derived alumina powders are significantly higher in costs compared to Bayer alumina. However, these powders can be sintered already below 1500 ◦C to a final density of 99.7% of the theoretical density with a very homogeneous fine grained microstructure of about 2.5–3 μm in oxidizing atmosphere (Figure 3, left) or between 1–2 μm after HIPing (Figure 3, right) in mean grain-size. In case these ceramics are only pre-fired to a density of about 97–98% and afterwards are hot isostatic pressed, the theorectical density of 3.98 g/cm3 is achieved. By this approach the homogeneous microstructure with a mean grain-size of about 1.5 μm or even less can be realized, and mechanical properties can be enhanced to about 620–650 MPa [12].

**Figure 3.** Microstructure of alumina ceramics; **left**: starting powder with a mean grain-size of 0.33 μm; **right**: starting powder with a mean grain-size of 0.22 μm.

Because of the higher mechanical strength properties of high purity alumina ceramics, chemically derived powders have substituted Dörre's alumina in 1987, followed by introduction of the HIP process in 1994. However, all of these materials with improved mechanical strength properties are extremely brittle and very stiff. It is well known that a brittle material with failures on the surface has a catastrophic breakage. This means that a failed part, i.e., a fractured ball-head, generates many fine particles. These have to be removed before a new ball-head can be replaced in the hip.

Taking into account that within a period of 20 years the mechanical strength properties have been increased by about 50% compared to the original material, it can be concluded that a continuous process improvement including new alumina powders, has been quite successful in order to enhance the mechanical strength properties and therefore the safety of ball-heads made out of alumina ceramics [13].

#### **3. Zirconia**

Zirconia did not play a significant role in engineering ceramic applications for a long period because it has a reversible phase transformation. While at room temperature the monoclinic phase is stable, it transfers at 1174 ◦C diffusion-less into a tetragonal modification. By cooling down, re-transformation into the monoclinic phase takes place. Figure 4 shows the hysteresis, which occurs during phase transformation in pure zirconia in comparison to doped zirconia with a different amount of stabilizing calcium oxide (CaO) [14]. Only after the addition of 19.5 Mol-% of CaO the expansion behaviour becomes reversible, while at lower concentrations, i.e., 5 Mol-%, does the hysteresis effect still occur. The phase transformation from monoclinic to tetragonal is combined with a re-orientation of the ions within the lattice. Due to the higher symmetry of the tetragonal modification, the density of it is 6.1 g/cm3, while in the monoclinic phase it is only 5.85 g/cm3. This means that for a non-stabilized sample, sintering takes place in the tetragonal modification and by cooling down, it re-transforms to monoclinic combined with a volume increase of about 4%. As a consequence of this, cracks are induced within the ceramic body and it is not stable. Figure 5 shows the symmetries in both modifications of the [111]-direction of the cationic lattice.

**Figure 4.** Hysteresis curve of the reversible phase transformation in zirconia.

**Figure 5.** Orientation of the Zr-ions in the monoclinic and tetragonal lattice.
