TEMPERATURE-DEPENDENT MECHANICAL PROPERTIES OF ALUMINA

ABSTRACT

In the present paper, the basic mechanical properties of alumina were thoroughly analyzed as understanding its mechanical properties is crucial for optimizing its performance and ensuring its safe and reliable use. The temperature-dependent behavior of main mechanical properties was also analyzed for alumina with purity higher than 99.5%. All mechanical properties of alumina demonstrated rapid decrease at elevated temperatures except flexural strength which was stable till 1000 °C.

АННОТАЦИЯ

В настоящей статье основные механические свойства оксида алюминия были тщательно проанализированы, поскольку понимание его механических свойств имеет решающее значение для оптимизации его характеристик и обеспечения его безопасного и надежного использования. Температурно-зависимое поведение основных механических свойств было проанализировано также для оксида алюминия чистотой более 99,5%. Все механические свойства оксида алюминия быстро снижались при повышенных температурах, за исключением прочности на изгиб, которая оставалась стабильной до 1000 °С.

Keywords: alumina, purity, mechanical properties, elevated temperature.

Ключевые слова: оксид алюминия, чистота, механические свойства, повышенная температура.

Introduction. In the world of modern materials, ceramics play a prominent role due to the wide range of their mechanical, physical, and chemical properties. High chemical and thermal stability, relatively good strength, and thermal and electrical insulation characteristics have made aluminum oxide Al₂O₃, or alumina, attractive for engineering applications [1]. Furthermore, other additional factors made aluminum oxide a preferable material in various industry fields. The first is the wide distribution of raw materials: aluminum oxide and its derivatives are obtained from natural minerals bauxite, nepheline, and kaolin, which provides many advantages over oxides of zirconium, hafnium, thorium, etc., the source minerals for the production of which are not readily available. The second is the high hardness of corundum (α-modification of aluminum oxide) - 9 on the Mohs scale - in contrast, for example, to CaO and MgO with a hardness of 5-6 on the Mohs scale [2]. The high chemical and thermal stability of aluminum oxide is due to its structure. Aluminum oxide crystallizes in several modifications (α, χ, η, δ, κ, θ, γ, ρ), which are obtained as a result of the decomposition of aluminum trihydroxides under different conditions under the influence of temperature, the most stable of which is α-Al2O3 (corundum) [3]. Aluminum oxide can be divided into classes according to type, purity, and application. Alumina can be further classified into two basic groups of high and low-purity alumina in terms of grade as most properties of aluminum oxide are dependent on the amount of impurities in it. The high-purity alumina grades with at least 99% Al₂O₃ demonstrate the best mechanical and other properties which make them suitable for application of relevant fields while the low-purity one is vice versa [4].

Mechanical properties of alumina at ambient. Ideally, before we can use alumina materials in a load-bearing application, the mechanical properties of them should be carefully investigated. The mechanical properties of alumina are determined using a wide range of tests [1]. Main mechanical properties of alumina depend on purity degree and they increase as the amount of impurities in the composition of alumina decrease (Table 1). And values of mechanical properties for alumina can be different according to testing methods.

Table 1.

Mechanical properties of various alumina grades [4]

Flexural strength. This type of strength is frequently ascertained by the bend test to circumvent the high cost and challenges of conducting tensile tests on alumina. Apart from being less expensive, the bend test's primary benefit is that it uses straightforward sample geometries. The specimens' geometries are either cylindrical or rectangular. Because there is a long area between the inner rollers with a constant bending moment, the four-point bend test is recommended. Another name for the bend test is the flexure test [1].

Figure 1. The four-point bend test

After the test, the bending strength is calculated using the formula:

Where, M = Pl/4 - maximum bending moment;

M = bh²/6 - a moment of resistance of the sample;

P - breaking load;

b and h are the width and height of the sample;

l - distance between supports [13].

Fracture toughness. One important characteristic of alumina is its fracture toughness, which is a measure of brittleness that is highly dependent on the specimen's microstructure. One material that seems to demonstrate R-curve behavior—that is a toughness increase with increasing crack length—is alumina [5-6]. The fracture toughness, K_I, was evaluated as,

Where the geometric factor Y has been determined to be 1.02, σ_f is the fracture stress, and δ is the crack length.  The units of K_I are MPa·m^1/2 [1].

Some approaches have been put forth in the past to determine K_I. A large number of the current methods are based on samples that have indentation cracks or controlled sharp cracks. However, there remains some discrepancy between the outcomes of various experimental methodologies. Because the methods used to measure fracture toughness values for alumina may vary [7, 10].

Hardness. Measuring the hardness of an alumina is important, and it is usually done using an indentation test. Hardness is intended to be a measure of the resistance to plastic deformation, which may include effects such as material displacement and fracture. The hardness is then determined by dividing the applied force, F, by this area. The hardness obtained by the Vickers method is 

HV = 1.8544 P/d²

Where P is the applied load and d is the mean diagonal length of the irreversible impression produced by the indentor. For most ceramics, the value of HV varies with the load, at least for lower loads [9].

Mechanical properties of alumina at elevated temperatures. We know that alumina-based materials are especially employed in fields like cutting tools, structural material for mechanic seals, and also heat engine parts as pure alumina is chemically inert and possesses high stability, excellent compressive strength, and creep resistance. Usually, all these properties are measured for quality control purposes at room temperature, while in practical applications the material has to withstand elevated temperatures. Such temperatures can affect the mechanical properties of the material, compromising its performance during service. Accordingly, the interest in analyzing the mechanical properties of alumina above room temperature is evident [8, 11]. The flexural and compressive strength of alumina with a purity of 99.5% demonstrated different changes during the heating up to 1500 °C. A dramatic decrease can be seen in compressive strength at elevated temperatures while there is a minor change in flexural strength up to 1000 °C after that it decreases rapidly (Figure 2).

Figure 2. Flexural and compressive strength of alumina at elevated temperatures

Both fracture toughness and hardness are very sensitive to high temperatures (Figure 3). It can be seen that with prolonged exposure of alumina to high temperatures (more than 1300 °C) and under constant load, irreversible creep occurs, which directly depends on its density and porosity, the amount and type of additives. It also depends on temperature, stress, and crystal size [12].

Figure 3. Vickers Hardness and Fracture toughness of alumina at elevated temperatures

Therefore, pure alumina is not preferable in the fields where both high temperatures and load are applied.  Therefore, the mechanical properties of alumina should be enhanced by using additives, optimizing sintering parameters, and so on.

Conclusion. It is worth noting that the value of hardness and fracture toughness of aluminum oxide may differ according to testing methods. Moreover, the purity degree of aluminum oxide is important in the determination of mechanical properties as the mechanical properties of alumina decrease with increasing content of impurities. Investigating the mechanical properties of various alumina grades from A1-A9, it was found that Al₂O₃ with 99% purity has better flexural strength, hardness, and fracture toughness. Furthermore, temperature-dependent behavior of alumina with 99,5% purity was studied and it was noted that fracture toughness, hardness, and compressive strength drastically decreased at elevated temperatures while the flexural strength depicted stable values until 1000 °C.

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