VTT TIEDOTTEITA ¨C MEDDELANDEN ¨C RESEARCH NOTES 1792
Mechanical and physical properties of engineering alumina ceramics
Pertti AuerkariVTT Manufacturing Technology
ISBN 951-38-4987-2ISSN 1235-0605
Copyright Valtion teknillinen tutkimuskeskus (VTT) 1996
Chemical and thermal stability, relatively good strength, thermal and electrical insulation characteristics combined with availability in abundance have made aluminium oxide Al2O3, or alumina, attractive for engineering applications. Much of its traditional use is in classical refractory service. However, the present work is limited to grades of alumina that qualify for structural engineering, ie. Topoly crystalline grades with at least 80% (mostly at least 90%) Al2O3 and no open porosity. In practice impermeability at room temperature requires that the total porosity is less than about 6% (Ryshkewitch 1960, Richerson 1982).
Alumina has several allotropic forms, but only the usual type or α-alumina is considered here. It has an internal crystal structure where the oxygen ions are packed in a close-packed hexagonal (cph) arrangement with aluminum (and other metal) ions in two-thirds of the octahedral sites. Alumina does not deviate much from stoichiometry but even small levels of impurities can influence high-temperature diffusion rates greatly. Alumina-based high-strength ceramic alloys are also available but not considered here. Alumina has a melting temperature of about 2040℃, but impurities and alloying elements form secondary phases that can melt at considerably lower temperatures.
Engineering grade polycrystalline alumina products are usually made by sintering alumina powder at high temperature (>1300℃). The manufacturing route limits the component and section size that can be produced in reasonably full density. The manufacturing process is also a major source of the initial defects that through fracture toughness will limit the strength of alumina components in service. As a consequence, strength of alumina is not a strict material property but dependent on stressed volume (or stressed surface, if surface defects dominate). The size dependence of strength makes larger components relatively weaker, and this is further amplified by the difficulty of sintering large pieces to an equivalent end density with small ones, and by generally higher maximum residual stresses in larger components.
While strength via the inherent brittleness is an important design-limiting factor, most engineering alumina is used primarily for its other functional qualities. This frequently creates an optimization task in design, when functionality needs to be maximized without compromising mechanical integrity. This requires also consideration of the manufacturing, because the high-temperature manufacturing processes including joining and coating operations control both the material properties and residual stresses in the final product.
Alumina is here graded into two main groups, the first of high-alumina grades with at least 99% Al2O3 (Table 1) and the second of alumina grades between 80% and 99% Al2O3 (Table 2). These main groups can be further divided into subclasses according to type, purity and intended service (Morrell 1987).
The difference between the grades is mainly in the amount of impurities and some deliberate alloying agents such as sintering aids. Alloying with other oxides does not necessarily impair the mechanical properties, but on average the best mechanical and other properties are seen in high purity grades of alumina. The first group of Table 1 is generally characterized by high density (> 3.75 g/cm3), high sintering temperatures in manufacturing (1500 – 1900 ℃) and relatively good mechanical performance. The lower grade alumina of Table 2 are cheaper to produce and therefore attractive for purposes where the properties are sufficient.
Table 1. High-alumina engineering ceramics (grades A1 – A4, at least 99% Al2O3) and their characteristics (Morrell 1987).
Table 2. Engineering alumina grades A6 – A9 (80% ≤ Al2O3 ≤ 99% as requirement) and their characteristics (Morrell 1987).
This work aims to describe the physical and mechanical properties of engineering alumina for the purpose of materials characterization under wide-ranging stresses and temperatures. Particular applications are anticipated in evaluating the complex deformation and stress histories as well as the residual stresses of dissimilar joints and coatings involving alumina as one component. Therefore, emphasis lies in the temperature dependence of the relevant properties that determine the performance after manufacturing processes and later in service.