Increasing the Sintering Rate and Strength of ZrO₂–Al₂O₃ Ceramic Materials by Iron Oxide Additions

Abstract—

We have prepared powders and ceramic composite materials in the ZrO₂–Al₂O₃ system containing 10 and 20 wt % Al₂O₃ and examined the effect of ferric oxide additions on the linear shrinkage, phase composition, porosity, microstructure, and mechanical properties of the ZrO₂–Al₂O₃ ceramic materials. The results demonstrate that the addition of ferric oxide leads to a considerable increase in linear shrinkage and ensures porosity as low as under 1% even at a sintering temperature of 1450°C in both composite materials. Moreover, small amounts of the additive stabilize the tetragonal phase of ZrO₂, whereas increasing the Fe content to 3 mol % leads to an increase in the amount of the monoclinic phase. We have obtained densely sintered ZrO₂–10% Al₂O₃ and ZrO₂–20% Al₂O₃ ceramic materials at 1450°C, with a bending strength of up to 760 and 475 MPa, respectively.

INTRODUCTION

Ceramic materials based on the yttria-stabilized zirconia–alumina (YSZ–Al₂O₃) system offer unique properties: high bending strength and fracture toughness of YSZ in combination with the chemical inertness and hardness of Al₂O₃ make them widely demanded as structural and functional ceramics [1]. Such materials are used in various areas of engineering: in vital parts of machinery, the aerospace industry, medical tools, and implants [2–4]. The technology of such composite ceramics has a number of drawbacks: high sintering temperature (1600–1750°C), resulting in grain growth [5]; necessity of prolonged high-temperature heat treatment [6]; or the use of expensive processes, such as spark plasma sintering [7] or hot pressing [8].

One topical issue is the development of sintering aids based on transition metal oxides, which will make it possible to lower the sintering temperature and simplify the fabrication of YSZ–Al₂O₃ based ceramic materials. The increase in sintering rate on the addition of such sintering aids is due to the increase in the density of structural defects when heterovalent cations or cations larger than Zr⁴⁺ are incorporated into the YSZ lattice [9]. Of particular interest among sintering aids is ferric oxide [10, 11]. The addition of Fe₂O₃ to YSZ containing 3 and 8 mol % yttria made it possible to obtain a dense structure at a temperature of 1150°C owing to the formation of substitutional and interstitial solid solutions in the YSZ lattice, but no data were reported on the mechanical properties of the materials thus prepared. Smirnov et al. [12] demonstrated the possibility of lowering the sintering temperature of ceramics containing 5 wt % Al₂O₃ by adding ferric oxide, but grain growth prevented them from reaching good mechanical properties of the resultant materials. Raising the Al₂O₃ content helps improve the mechanical properties of composite ceramics [13]. The effect of ferric oxide on the sintering behavior of YSZ-based ceramic materials containing more than 5 wt % Al₂O₃ has not been studied previously.

The purpose of this work was to study how the concentration of ferric oxide additions influences the phase composition, sintering behavior, and mechanical properties of YSZ–Al₂O₃ ceramics containing 10 and 20 wt % Al₂O₃.

EXPERIMENTAL

ZrO₂–Al₂O₃ composite powders containing 10 and 20 wt % alumina were prepared via chemical precipitation from salt solutions. We used aqueous solutions of the ZrOCl₂ · 8H₂O and АlCl₃ · 6H₂O chlorides, which were added to a 9% aqueous ammonia. To obtain tetragonal zirconia (t-ZrO₂), yttrium chloride (YCl₃ · 6H₂O) was added to the starting reagents to give 3 mol % yttria relative to zirconia. After the synthesis, the resultant precipitate was filtered off in a Buchner funnel, dried, and then calcined at a temperature of 650°C. The powders were mechanically activated under ethanol in a planetary mill. Ferric oxide was added in the form of the soluble salt (NH₄)Fe(SO₄) · 12H₂O. The amount of the additive was evaluated from Fe³⁺ content: 0, 0.33, 1.0, and 3.0 mol % relative to the ZrO₂–10% Al₂O₃ and ZrO₂–20% Al₂O₃ composites. For this purpose, the as-synthesized ceramic powder was mixed with an aqueous solution containing a dissolved additive. The resultant powders were dried and passed through a 100 μm nylon sieve.

The phase composition of the powders was determined by X-ray diffraction (Difray 401 diffractometer, CrK_α radiation) using JCPDS Powder Diffraction File data and PCPDFWIN. The morphology of the synthesized powders was examined by transmission electron microscopy (TEM) on a JEOL JEM 2100 (accelerating voltage of 200 kV). Their specific surface area was determined by BET measurements (Micromeritics TriStar).

The sinterability of the materials was analyzed using dilatometry during heating to 1500°C at a rate of 10°C/min in air (Netzsch DIL 402 C). To study the properties of ceramics, the powders were pressed into beams 30 × 4 × 4 mm in dimensions. Compaction was carried out in a metallic press die set at a pressure of 100 MPa by uniaxial pressing. The resultant specimens were fired in a furnace with silit rod heaters in air.

The porosity of the ceramics was determined in accordance with the Russian Federation State Standard GOST 2409-2014. The materials were tested in three-point bending on an Instron 3382 universal testing machine. The microstructure of the ceramic materials was examined scanning electron microscopy (SEM) on a Tescan Vega II.

RESULTS AND DISCUSSION

X-ray diffraction characterization showed that, after synthesis and heat treatment at 600°C, the powders consisted of pseudocubic zirconia, c-ZrO₂ (JCPDS card no. 49-1642), with a low degree of crystallinity (Fig. 1). The X-ray diffraction patterns of the powders contained no peaks of alumina, which was due to the ability of zirconia to suppress grain growth of alumina, making it X-ray amorphous. Characteristically, the ZrO₂–20% Al₂O₃ materials had a lower intensity of reflections from c-ZrO₂ than did the ZrO₂–10% Al₂O₃ materials, which was due to the larger amount of Al₂O₃. Similar behavior of ZrO₂–Al₂O₃ composite materials heat-treated at 1000°C was reported by Balagopal et al. [1].

Fig. 1.

X-ray diffraction patterns of the ZrO₂–10% Al₂O₃ and ZrO₂–20% Al₂O₃ powders.

TEM examination of the morphology of the ZrO₂–10% Al₂O₃ powder showed that it consisted of rounded particles less than 10 nm in size (Fig. 2a). Raising the alumina content to 20% led to the formation of powder consisting of not only small particles, less than 10 nm in size, but also larger, faceted particles, up to 25 nm in size (Fig. 2b).

Fig. 2.

TEM micrographs of the (a) ZrO₂–10% Al₂O₃ and (b) ZrO₂–20% Al₂O₃ powders.

Analysis of the specific surface area (S) data showed that the addition of the lowest ferric oxide concentration to the synthesized powders caused no significant changes in their surface area. The Fe-free ZrO₂–10% Al₂O₃ composites had S = 47 m²/g. The addition of ferric oxide led to changes in specific surface by no more than 5%, independent of its concentration. The ZrO₂–20% Al₂O₃ materials had a larger specific surface area, 54 m²/g, which was probably due to the lower degree of aggregation of the powders. The ferric oxide-containing powders also had S in the range 50–55 m²/g.

According to the dilatometry data, the addition of even the lowest ferric oxide concentration (0.33 mol % Fe³⁺) to the composite materials of the ZrO₂–Al₂O₃ system significantly improved their sinterability (Fig. 3). In the case of the ZrO₂–10% Al₂O₃ materials, a well-defined effect was observed on the addition of 0.33 mol % Fe, which increased the shrinkage of the samples from 13.8 (with no additive) to 24.1%. The first stage of sintering, at temperatures in the range 900–950°C, was shown to be associated with the phase transition from the monoclinic phase of ZrO₂ (m-ZrO₂) to the tetragonal phase (t-ZrO₂). The onset of an active sintering process, associated with the heat-treatment-induced densification of the material, was observed at a temperature of 1010°C. Raising the ferric oxide content of the ZrO₂–10% Al₂O₃ samples led to an increase in linear shrinkage to 26.2%. The materials underwent considerable densification in the temperature range 1320–1350°C.

Fig. 3.

Shrinkage curves of the (a) ZrO₂–10% Al₂O₃ and (b) ZrO₂–20% Al₂O₃ materials from dilatometry data.

Increasing the alumina content of the materials had a negative effect on their sinterability. In particular, the Fe-free ZrO₂–20% Al₂O₃ samples showed a linear shrinkage of 7.5%. The addition of 0.33 mol % Fe³⁺ increased the shrinkage to 10.0%. Raising the Fe³⁺ content to 3 mol % lead to further increase in shrinkage, up to 13.3%.

According to X-ray diffraction data, the Fe-free ZrO₂–10% Al₂O₃ materials sintered at 1450°C contained t-ZrO₂ (JCPDS card no. 42-1164) and a small amount of the m-ZrO₂ phase (JCPDS card no. 37-1484), within 5 wt %. The amount of this phase increased as the sintering temperature was raised to 1500°C (Figs. 4a, 4b). Al₂O₃ crystallized in the form of corundum (JCPDS card no. 10-0173).

Fig. 4.

X-ray diffraction patterns of the (a, b) ZrO₂–10% Al₂O₃ and (c, d) ZrO₂–20% Al₂O₃ ceramics prepared by sintering at (a, c) 1450 and (b, d) 1500°C.

The addition of 0.33 or 1.0 mol % Fe to the ZrO₂–10% Al₂O₃ composite materials led to an increase in the amount of t-ZrO₂, whereas m-ZrO₂ almost completely disappeared (Fig. 4). Increasing the amount of the additive to 3 mol % led to the formation of up to 20 wt % m-ZrO₂, whereas the intensity of the reflections from Al₂O₃ remained essentially unchanged. Raising the sintering temperature to 1500°C led to an increase in m-ZrO₂ content to 40% as the amount of the additive was increased to 3 mol % Fe³⁺. Note that the addition of 3 mol % Fe³⁺ eliminated the splitting of the peaks at 2θ = 49.46° and 50.06° to give a single peak at 50.22°, characteristic of c-ZrO₂.

The oxide-additive-free ZrO₂–20% Al₂O₃ samples are characterized by the formation of up to 10 and 20 wt % m-ZrO₂ at 1450 and 1500°C, respectively (Figs. 4c, 4d). The addition of 0.33 or 1.0 mol % Fe³⁺ stabilizes t-ZrO₂, whereas the addition of 3 mol % Fe³⁺ leads to an increase in the amount of m-ZrO₂ and eliminates the doublet in the range 49.8°–51.2° to give a single peak characteristic of c-ZrO₂, at both 1450 and 1500°C.

The measured porosity data for our samples are presented in Table 1. All of the ZrO₂–10% Al₂O₃ samples containing additives were shown to be densely sintered, with a porosity under 1% even at 1450°C. The additive-free materials were characterized by a 7.7% porosity at a sintering temperature of 1450°C. The additive-free ZrO₂–20% Al₂O₃ materials sintered at 1450°C had higher porosity, up to 18.55%. The addition of 3 mol % Fe³⁺ to ZrO₂–20% Al₂O₃ allowed a densely sintered state to be reached. Both the ZrO₂–10% Al₂O₃ and ZrO₂–20% Al₂O₃ materials heat-treated at 1500°C were in a densely sintered state.

Table 1.   Influence of the composition and sintering temperature of the materials on their porosity

Figure 5 illustrates the microstructure of the ZrO₂–10% Al₂O₃ ceramics containing 0, 0.33, 1.0, and 3.0 mol % Fe and sintered at 1450°C. The ceramics have a dense microstructure. Characteristically, the additive-free materials have a nonuniform microstructure made up of fine grains, 0.1–0.3 μm in size, and larger grains, 1.0–2.0 μm in size. Alumina in the form of dark grains is uniformly distributed over the material in the form of agglomerates up to 0.5 μm in size. The addition of 0.33 mol % Fe³⁺ leads to an increase in the size of the fine grains, which reaches 0.5–0.7 μm, and an increase in the number of large crystals, up to 50% of the field of view. In the structure of the ceramic containing 1 mol % Fe³⁺, we observe further recrystallization, resulting in the formation of large, irregularly shaped grains ranging in size from 2.0 to 3.0 μm. At 3 mol % Fe³⁺, the large grains account for up to 70% of the field of view and are a predominant phase. The microstructure of the ZrO₂–10% Al₂O₃ ceramic containing 3 mol % Fe is made up of sintered large grains, ranging in size from 1.5 to 3 μm; intermediate grains (0.5 to 1 μm in size), and fine grains (0.3 μm).

Fig. 5.

Microstructure of the ZrO₂–10% Al₂O₃ ceramics containing (a) 0, (b) 0.33, (c) 1.0, and (d) 3.0 mol % Fe and sintered at 1450°C.

The ZrO₂–20% Al₂O₃ ceramics containing different amounts of ferric oxide and sintered at 1450°C have a similar microstructure (Fig. 6). The ZrO₂–20% Al₂O₃ ceramics containing 0.33 mol % Fe³⁺ have a uniform microcrystalline structure ranging in grain size from 0.1 to 0.5 μm. Increasing the ferric oxide content leads to the formation of large, sintered grains 2 μm in size, while the grains less than 0.5 μm in size also persist. We observe accumulations of alumina grains (dark grains) up to 1 μm in size. The alumina grain agglomerates are uniformly distributed over the entire field of view.

Fig. 6.

Microstructure of the ZrO₂–20% Al₂O₃ ceramics containing (a) 0, (b) 0.33, (c) 1.0, and (d) 3.0 mol % Fe and sintered at 1450°C.

The addition of Fe³⁺ led to a significant increase in the strength of the sintered ZrO₂–10% Al₂O₃ ceramic materials (Fig. 7). The additive-free materials sintered at 1450°C were characterized by an average strength of 530 MPa. Reaching a densely sintered state at 1450°C and stabilizing t-ZrO₂ in the materials containing 0.33 mol % Fe³⁺ allowed the strength in three-point bend tests to reach 760 ± 25 MPa. Further raising Fe₂O₃ content to 1.0 and 3.0 mol % led to a reduction in strength to 710 ± 20 and 600 ± 20 MPa, respectively, which was due to the formation of a large-grained microstructure and the phase transition of t-ZrO₂ to m-ZrO₂. Raising the sintering temperature to 1500°C led on average to a decrease in strength in three-point bend tests because of the formation of monoclinic phase in the additive-free materials and the increase in its content in the Fe-containing materials. Moreover, the increase in sintering temperature led to ceramic grain growth, thereby impairing the mechanical properties of the ceramics [5].

Fig. 7.

Effects of composition and sintering temperature on the strength of the ZrO₂–10% Al₂O₃ ceramics.

The ZrO₂–20% Al₂O₃ sample sintered at 1450°C had lower strength than did the ZrO₂–10% Al₂O₃ materials because they retained porosity (Fig. 8). In particular, the strength of the additive-free materials was 320 ± 15 MPa, whereas the addition of 1.0 mol % Fe³⁺ led to an increase in strength to 475 ± 20 MPa. The strength of the materials containing 3.0 mol % Fe³⁺ did not exceed 350 MPa, even though their porosity was under 1%. X-ray diffraction results suggested that this was due to the considerable concentration of m-ZrO₂, a low-strength phase. Raising the sintering temperature to 1500°C made it possible to reach a densely sintered state in all of the samples and an average strength near 700 MPa for the sample containing 1.0 mol % Fe. This sample was characterized by the largest amount of t-ZrO₂, a high-strength phase. The average strength of the additive-free material was 630 ± 15 MPa.

Fig. 8.

Effects of composition and sintering temperature on the strength of the ZrO₂–20% Al₂O₃ ceramics.

CONCLUSIONS

The addition of ferric oxide has been shown to cause a considerable increase in the linear shrinkage of the materials containing 10 and 20 wt % Al₂O₃. The addition of 0.33–1.0 mol % Fe³⁺ helps stabilize the tetragonal phase of zirconia, whereas the addition of 3 mol % Fe³⁺ stabilizes m-ZrO₂ and leads to appreciable ceramic grain growth. We have obtained densely sintered ZrO₂–10% Al₂O₃ ceramic materials at 1450°C and Fe³⁺-containing ZrO₂–20% Al₂O₃ ceramics at 1500°C. The addition of ferric oxide during sintering at 1450°C for 2 h has allowed the strength of the materials in three-point bend tests to be raised by 45–50%: to 760 MPa for the ZrO₂–10% Al₂O₃ composite materials and to 475 MPa for the ZrO₂–20% Al₂O₃ composites.