SYNTHESIS AND PROCESSING
2.02 Processing of Alumina and Corresponding Composites
2.02.1 Introduction
Alumina materials are a family of ceramics whose principal constituent is aluminium oxide (Al2O3), known as alumina in the ceramic sector. On a weight basis, these materials have the largest share of the ceramics’world market.
Alumina is the ceramic material most extensively studied, both from a theoretical and practical stand points.
There exist several reviews about alumina materialsdproperties, production, and uses (Briggs, 2007; Doremus, 2008; Dörre & Hübner, 1984; Gitzen, 1970; Hart, 1990; Kingery, 1984; McColm, 1990; Riley, 2009)don which this chapter relies to summarize the basic aspects of traditional alumina materials. This chapter tries to add to the previous publications by dealing with new developments and trends that were just proposed as possibilities in some of these reviews.
InTable 1, the main commercial applications of alumina materials and the properties required for the envisaged application are summarized.
As a single crystal (sapphire), alumina is prized as gemstone (sapphire or ruby, depending on the impurities) and for special applications such as watch bearings. Moreover, some high-grade glass substitutes are made of single-crystal alumina (artificial sapphire). Thefirst commercial application as ceramic material is described in a German patent from 1907 (Riley, 2009). In coarse, polycrystalline form, alumina is the major constituent of high-grade shaped and unshaped refractory materials. Porous sintered aluminas are used as catalyst supports for chemical processes. In the form of powder and grids, alumina is used as a grinding and abrasive media. In the form of whiskers andfibers, alumina is used for low–thermal mass furnace insulation and metal reinforcement.
Last but not the least, a wide range of wear-resistant and electrically insulating components are constituted by alumina-based materials with micrometer grain size.
Structural applications of alumina at room temperature are based on the combination of hardness, wear resistance, and corrosion resistance that alumina ceramics provide. Room temperature applications include wear parts in medical engineering (total prostheses for hip joints), in process plants (pump components and valve faces, lining of pipework), and in mechanical engineering (bearings and valves). Alumina pieces of different shapes and sizes are currently used as textile guides. The main disadvantage of alumina in room temperature applications is its brittleness that leads to a lack of reliability of the pieces in use. In fact, even though fine-grained and dense aluminas with high strength are available, they still present relatively low Weibull modulus; thus, a number of monolithic and layered alumina-based composites have been developed and are still under development, seeking reliability.
Alumina also presents high refractoriness, that is, high melting point (2050C; McColm, 1990) and retention of structural integrity at a high temperature. In particular, it experiences practically no deformation
Table 1 Commercial applications of alumina materials and the related properties
Applications Primary property Other properties
Single
crystal Gemstones Aesthetics Mechanical stability at room temperature
Glass substitutes Translucency/transparency Hardness and stiffness up to 1000C Special applications
(e.g., watch bearings) Hardness and stiffness
Grains Abrasives Hardness
Aggregates for unshaped
refractories Chemical inertness at
high temperatures Hardness and stiffness at high temperature
Shaped:
polycrystalline Shaped refractories
Medical engineering (e.g., total
prostheses for hip joints) Biocompatibility Wear resistance at room temperature Mechanical engineering
(e.g., bearings and valves) Wear resistance at room
temperature Chemical inertness at room temperature
Process plants (e.g., pump components and valve faces, lining of pipework)
Cutting tools Wear resistance up
to 1200C Hardness and stiffness at high temperature chemical inertness 32 Processing of Alumina and Corresponding Composites
under compressive loads at temperatures up to 1200C, above which sapphire can deform by dislocation motion (Kronenberg, 1957; Lagerlof, Heuer, Castaing, Rivière, & Mitchell, 1994; Scott & Orr, 1983; Snow &
Heuer, 1973). Only under extremely high hydrostatic stresses, which prevent crack propagation, plasticity may be extended at low temperatures such as 200C; deformation is further enhanced in hydrogen atmosphere (Korinek & Castaing, 2003). Due to its high refractoriness, one of the most successful applications of alumina is in cutting tools for high-speed machining of metals. Applications of alumina bodies as components for engines are limited by its low thermal shock resistance.
In this chapter, fine-grained (30mm), wear-resistant aluminas and alumina-matrix composites are addressed. Such materials also dominate the engineering ceramics market. As a term of comparison, alumina shipments (372106V) constituted more than half from the total shipments of structural ceramics in the Japanese market in 2003 (715106V) (Okada, 2008). Commercial aluminas for high-responsibility appli- cations such as hip parts for arthroplasty or cutting tools have typical grain sizes under 5mm. First, the pro- cessing methods to optimize the microstructure of single-phase and fine-grained materials are addressed.
Second, the development of special microstructures in single-phase as well as in composite systems is described.
Table 2summarizes the advanced structural aluminas with special microstructures and alumina-based com- posites that are described in this chapter together with their applications.
The optimization of the microstructure of single-phase materials and the development of alumina-based composites have been the research subjects since the general use of aluminas in the 1930s. The example of three of the most performance-demanding applications allows summarizing the main processing developments of alumina materials.
Alumina-based cutting tools have experienced a huge development since their commercial introduction in the early 1950s. Initially, major applications of alumina tools were the high-speed machining of cast iron. Later, processing improvement as well as the development of composites allowed new uses as machining of relatively harder and stronger steels quite effectively and economically and also intermittent cutting at reasonably high speeds, feeds, and depths of cut. Main developments have been reduced grain sizes and porosity and the development of composites, specially alumina–zirconia (ZrO2), alumina–TiC, and alumina–SiCw silicon carbide whiskers.
Alumina implant technology has been improving since the 1970s, focusing on the grain size refining, density increase, and significant reduction in the level of inclusions. Also, alumina-matrix composites, in particular, alumina–zirconia (ZrO2), have been proposed.
Evolution from the translucent aluminas used in high-pressure sodium vapor lamp tubes to the transparent ones with promising applications as windows, armor, and bulbs of high-pressure metal halide lamps is being done by reducing the grain size to the submicrometric scale together with further density increase.
Table 2 Advanced structural aluminas with special microstructures and alumina-based composites and their applications
Microstructure size Status Secondary phase
Application/envisaged properties
Textured aluminas Under development Remaining glass in
some cases Flaw tolerance Monolithic
composites Micrometric Commercial ZrO2 Implants
TiC Cutting tools
SiCw
Under development SiCw Electrical
Al2TiO5 Thermal shock
and wear Nanocomposites Micro-nanometric
Nano-nanometric Under development ZrO2 Implants
SiC Wear
FeAl2O4
Al2SiO5
Laminates Mili-micro–nanometric Under development CNTsZrO2 Strengthening and/or
flaw tolerance
Al2SiO5 Flaw tolerance
CaAl12O19
Processing of Alumina and Corresponding Composites 33
In summary, for single-phase aluminas, in general, microstructures with submicrometer grain sizes for obtaining components with improved hardness (Krell, 1995), wear resistance (Goh, Lim, Rahman, & Lim, 1997; Krell, 1996), strength (Krell & Blank, 1996), or optical performance (Apetz & van Bruggen, 2003) are required. When dealing with composites, processing procedures that allow the homogeneous mixing of phases with different compositions, sizes, and even shapes together with the sintering of the mixture to full density are major issues.
2.02.1.1 Single-Crystal Aluminium Oxide 2.02.1.1.1 Crystalline Structure
The pure form of aluminium oxide exists only in one crystalline form,a-Al2O3, throughout the whole tem- perature range up to the melting point, even though the confusing nomenclaturebandg-Al2O3would suggest the existence of different polymorphs. The crystalline structure of a-Al2O3 is described in several sources (e.g.,Brook, 1991): Hexagonal structure, D3a6 space group, and two Al2O3 units per unit cell. Slip occurs on {0001}<11–20> systems. The mean aluminium–oxygen distance is 192 pm. The single-crystal density is 3990 kg m3(Powder X-ray diffractionfile ASTM42-1468). Aluminium oxide is ionic, constituted by Al3þand O2; however, its bonds have some covalent character (Sousa, Illas, & Pacchioni, 1993).b-Al2O3is a ternary oxide with general composition Na20.11 Al2O3, andg-Al2O3describes a group of phases with the cubic form of a defect spinel that are produced by dehydration of gelatinous Al(OH)3, giving Al2O3with impurity protons.
All non-cubic materials present anisotropy in properties at the crystalline level, such as thermal expansion that is increased in ionic oxides due to partial polarization. On average, simple image of the structures of ionic oxides is a series of close-packed layers of O2ions generating cubic or hexagonal symmetry, with the cations localized at octahedral or tetrahedral interstices in the close-packed layers. The real structure is not so sym- metrical as polarization is produced when the small cations with large charges occupy the interstices. Then, a series of unequal Mnþ–O2distances are generated. In the case of aluminium oxide, three O2are closer to each Al3þthan the other three and thefinal structure is formed by highly distorted AlO6octahedra. These distorted octahedra and the overall hexagonal symmetry lead to anisotropy in the properties at the crystalline level.
2.02.1.1.2 Mechanical and Elastic Properties
The single crystals of aluminium oxide might present extremely high strengths due to the strong aluminium– oxygen bonds. Room temperature bending strengths of 400–700 MPa and tensile strengths of 500 MPa, depending on orientation and the surface perfection, are common (Brook, 1991; Riley, 2009). Moreover, values up to 7 GPa have been reported forflame-polished artificial sapphire rods (Brook, 1991; Watchman &
Maxwell, 1959), which are increased up to 11 GPa for thinfilaments. Recent data obtained using cantilever beams (10–30mm long, width and depth 2–5mm) machined by Focused Ion Beam (FIB) and tested in a nanoindenter range from 10 to 13 GPa for monocrystals. Strengths of polycrystals tested in the same way are about 5 GPa, failure is intergranular and usually initiates at the grain boundary. Carbon has been demon- strated to strengthen the grain boundaries of alumina, changing the fracture mode to transgranular. Then, strengths of polycrystals doped with 0.01% C are almost double than those obtained for pure alumina (about 8 GPa) (Yahya & Todd, in press).
Such strength values are of the order of theoretical strength for aluminium oxide (31 GPa;Riley, 2009).
Aluminium oxide also presents high Young’s modulus (z520 GPa; Brook, 1991) and hardness. Sapphire presents a hardness of 9 in the Mohs scale (nonlinear, 1: talc, 10: diamond) and Vickers hardness up to 30 GPa, depending on the orientation (Ryshkewitch & Richerson, 1985). However, the fracture energy and fracture toughness of alumina single crystals in the rhombohedral plane, which is the preferred cleavage plane, are relatively low, as reviewed byIwasa and Bradt (1984). A value ofgfy6 J m2for alumina monocrystals at room temperature and a fracture toughness of about 2.4 MPa m1/2 have been reported (Wiederhorn, 1969;
Wiederhorn, Hockey, & Roberts, 1973). Most of the work on the development of alumina materials has been devoted to take advantage of the desired properties of the single crystals while increasing toughness. For such purpose, another property of the single crystal, the thermal expansion anisotropy, has been exploited, as described below.
2.02.1.1.3 Thermal Expansion Anisotropy
a-Al2O3presents a relatively low crystalline average thermal expansion coefficient (az8.7106K1between 25 and 1000C;Taylor, 1984a). However, there is a perceptible larger thermal expansion coefficient parallel to 34 Processing of Alumina and Corresponding Composites
the c-axis (az9.2106K1and 8.4106K1between 25 and 1000C, in the parallel and perpendicular directions, respectively;Taylor, 1984a). This thermal expansion anisotropy is responsible for the development of stresses in alumina materials when cooling from the sintering temperature, as described by different authors (e.g.,Blendell & Coble, 1982).
The stress level depends on the particular relative orientation of the grain boundaries. For grain sizes above a critical one, these stresses can lead to fracture. However, for the small grain sized (<30mm) structural aluminas, the developed stresses remain as residual stresses in the sintered materials and are partially responsible for the dependence of properties such as hardness, fracture toughness, and strength on grain size because they add to the externally applied stresses (Bueno & Baudín, 2006a; Bueno, Berger, Moreno, & Baudín, 2008; Mussler, Swain, & Claussen, 1982; Rice, Freiman, & Becher, 1981). Impurity phases, often glassy at grain boundaries, add further to these variations due to the thermal expansion mismatch between these phases and alumina, which also depend on the particular orientation of the grain boundary.
The residual stresses created in single-phase polycrystalline aluminium oxide and in composites as a result of its constrained anisotropic thermal contraction can be measured with the technique of piezospectroscopy using thefluorescence from trace Cr3þimpurities (Ma & Clarke, 1994). Over the range of grain sizes from 2 to 16mm, the residual stresses exhibit a dependence on grain size consistent with the prediction of the Evans–Clarke model of thermal stress relaxation by grain boundary diffusion.
The hardness of Al2O3increases from grain sizes of 5mm down to 0.5mm because of an increasing limitation microplastic deformation by movement of dislocations and twins (Krell, 1995, 1996). In addition, large-grain materials tend to present grain boundary microcracking under localized loads with associated hardness lowering; a decrease from 20 to 17 GPa for an increase in average grain size fromz1 toz5.5mm has been reported (Bueno & Baudin, 2006a). On the contrary, the adequate manipulation of the crystalline thermal expansion anisotropy can lead to increased toughness due to crack deflection, crack branching, and micro- cracking during the fracture process, acting as toughening mechanisms. A number of alumina-matrix com- posites have been developed on the basis of thermal expansion mismatch of the second phase with alumina to increase toughness.
2.02.1.2 Thermodynamic Stability of Aluminium Oxide
As stated byBrewer (1953)in his exhaustive revision, from a thermodynamic stand point,a-Al2O3is one of the most stable of the metal oxides. This fact is clearly demonstrated by the large negative Gibbs energy of formation from the metal,DGT, according to:
2Al þ 1:5O2ðgÞ/Al2O3 (1)
Whose values range from1590 to945 kJ mol1fromT¼273 to 2273 K (calculations using the program Outokumpu, 1993). This high thermodynamic stability is responsible for the absence of natural deposits of free metal.
Dissociation of Al2O3to suboxides, Al2O and AlO, has been reported to take place at high temperature and low oxygen partial pressures (Brewer, 1953; Brewer & Searcy, 1951) according to the following.
Al2O3ðcÞ / 2AlOðgÞ þ 2O2ðgÞ (2)
Al2O3ðcÞ / Al2OðgÞ þ O2ðgÞ (3)
However, both reactions present a negative Gibbs energy and the corresponding equilibrium constants are extremely low (e.g., at 1773 K, K¼2.761030 and 4.561024 for Reactions (2) and (3), respectively;
calculations using the program Outokumpu, 1993). Therefore, alumina experiences extremely low weight losses in vacuum, even at high temperatures (e.g., z106 to 105kg/m$s in the temperature interval of 1973–2273 K;Harper, 2001).
Even though the fusion temperature of alumina is very high, liquids can be formed at lower temperatures due to impurities that originate at low invariant points. Some examples of liquid-forming temperatures (Levin, Robbins, & McMurde, 1964) are those corresponding to the systems Al2O3–SiO2 (1590C:
mullite–tridymite), Al2O3–SiO2–CaO (1170C: pseudowollastonite–tridymite–anorthite), Al2O3–MgO–SiO2
(1355C: protoensteatite–cordierite–tridymite), Al2O3–SiO2–K2O (689C: quartz-potash feldpart-K2Si4O9, 990C: leucite–mullite–cristobalite), and Al2O3–SiO2–Na2O (720C: quartz-albite-anorthite). Some of these impurities can be used as sintering aids to increase thefinal density and/or reduce the sintering temperature.
Processing of Alumina and Corresponding Composites 35
However, they remain at the grain boundaries in the sintered material, often in glass form, and are detrimental for the high temperature strength and creep resistance of the material.