SYNTHESIS AND PROCESSING
2.02 Processing of Alumina and Corresponding Composites
2.02.7 Fabrication of Alumina Nanocomposites .1 Alumina–SiC
Since the seminal work byNiihara (1991), the recognition of the appealing mechanical, physical, and tribo- logical properties of bulk nanocomposites, defined by the grain size of at least one of the constituent micro- structural phases being smaller than 100 nm or finer, has generated considerable recent research activity.
(a) (b) (c)
(d) (e) (f)
Figure 10 Capability of direct shaping of aqueous slurries methods to fabricate ceramic laminates. Examples of alumina–zirconia laminates. Different layer thicknesses can be obtained as a function of the method. Layers containing the maximum amounts of alumina appear darker. (a) and (b) Stacking pre-sintered cast tapes of two different compositions (Al2O3þ5 vol% YTZP and Al2O3þ40 vol% YTZP) at room temperature with low pressures. (a) Dip coating of pre-sintered cast tapes (Al2O3þ5 vol% YTZP) in binder-containing slurries (pure ZrO2), stacking at room temperature with low pressures and sintering (Nicolaidis et al., 2008).
(d), (e), and (f) Sequential slip-casting.Bermejo, Baudín, et al. (2007), Bermejo, Sánchez-Herencia, et al. (2007).
Processing of Alumina and Corresponding Composites 59
A major challenge in this researchfield is concerned with processing, in particular, dispersion of thefine second phase and restriction of grain growth during sintering. In this regard, novel processing techniques have been developed with the aim of fabricating bulk ceramic nanomaterials. Once more, most of the work has been done on alumina-matrix composites. Reviews on processing and properties of alumina–SiC composites can be found elsewhere (Mukhopadhyay & Basu, 2007; Sternitzke, 1997).
Most work on alumina-based nanocomposites has been done in the system alumina–SiC. Figure 11 shows a typical microstructure of such kind of materials, with alumina grain sizes in the range of micro- meters and nanometric SiC grains located inside the alumina grains and at grain boundaries (Winn & Todd, 1999).
Original works by Niihara’s group give little processing detail while reporting the use of ethanol, toluene, and acetone. A later detailed study on processing with nonaqueous media found methanol to be a suitable dispersing medium (Stearns, Zhao, & Harmer, 1992; Zhao et al., 1993). Other groups have used methanol (O’Sullivan et al., 1995, 1996) or ethanol (Jeong, Nakahira, & Niihara, 1999) as the medium. Nonpolar organic molecules, hexane and toluene, are not a suitable dispersing medium for alumina–SiC mixtures (Stearns et al., 1992; Timms & Ponton, 2002).
Aqueous processing has been widely used to fabricate these composites. Different additives and ultrasonic dispersion (Borsa, Jones, Brook, & Todd, 1997; Walker, Borsa, Todd, Davidge, & Brook, 1994), attrition milling (Cock, Shapiro, Todd, & Roberts, 2005; Kumar, Baron, & Hampshire, 1999; Limpichaipanit & Todd, 2009), or ball milling (Timms & Ponton, 2002) have been used in the mixing stage in which polyethyleneglycol was added as a binder. The main drawback associated with aqueous processing is the occurrence of hard agglom- erates and segregation on slow drying of the slurry, so different rapid drying methods have been investigated.
Freeze-drying was found to avoid the formation of hard agglomerates (Walker et al., 1994) and has been successfully used later (Cock et al., 2005; Limpichaipanit & Todd, 2009). It has also been found that pH adjustment can be used to induceflocculation of the slurry, preventing segregation and agglomeration of the silicon carbide particles (Borsa et al., 1997; Walker et al., 1994). Pressurefiltration has also been proposed as a means to maintain the same quality of dispersion as that of the slurry in the mixed dried powder (Timms, Ponton, & Strangwood, 2002).
As discussed before for Al2O3–SiCw composites (Section 2.02.4.2.2), the incorporation of the high bond energy second phases such as the SiC nanoparticles severely inhibits sintering (Mukhopadhya & Basu, 2007;
Mukhopadhyay & Todd, 2011; Sternitzke, 1997).
More than 99% of theoretical density materials with 5 vol% SiC have been reported by Timms et al. (Timms et al., 2002) after pressureless sintering at 1900C. Anya and Roberts (1997) claimed still higher density (>99.6% of theoretical) after sintering inflowing nitrogen at 1700–1775C for amounts of SiC up to 15 vol%.
However, in general, pressureless sintering is only possible with low volume fractions of SiC and impure Figure 11 SEM micrograph showing the characteristic microstructure of an alumina–SiC nanocomposite (Winn & Todd, 1999).
Nanometric and submicrometric grains of SiC appear clearer than the micrometric alumina matrix.
60 Processing of Alumina and Corresponding Composites
alumina powders (Baron, Kumar, Le Gonidec, & Hampshire, 2002) or through the use of sintering aids (Cock et al., 2005; Kumar et al., 1999).
The most used solution is to use pressure-assisted sintering techniques, such as HP and SPS (Borsa et al., 1997; Limpichaipanit & Todd, 2009; Mukhopadhyay & Basu, 2007; Sternitzke, 1997; Zhao et al., 1993).
Jeong et al. (Jeong et al., 1999) proposed the use of sintering additives and pressureless sintering followed by HIP to circumvent the limitationsdshape, size, anisotropydof HP. However, such expensive two-step process has not been generally accepted.
2.02.7.2 In situ-Formed Nanocomposites
As occurs with alumina–zirconia composites, when less refractory, oxide nanoparticles are incorporated into the powder mixture, pressureless sintering is feasible but the particles tend to coarsen beyond the nanosized regime and adhere to the grain boundaries. In order to avoid both the inhibition of sintering by refractory non-oxide second phases and the coarsening of oxide phases, the fabrication of ceramic nanocomposites in situ by solid- state reaction has been proposed as a means to overcome the sintering and coarsening limitations to make nanocomposites. Solid-state reaction involves not only reaction between incompatible phases during sintering but also solid-state precipitation. In general, it is necessary to start from a nanosized second phase that would react with the matrix particles during sintering process. Al2O3–FeAl2O4nanocomposites, with improved wear resistance as compared to that of single-phase alumina with similar grain sizes, have been produced by solid- state precipitation during the aging of Al2O3–10 wt% Fe2O3 solid solutions in reducing atmosphere (Mukhopadhyay & Todd, 2010a; Mukhopadhyay & Todd, 2011). The microstructure of the materials was further improved by doping with 250 ppm Y2O3(Mukhopadhyay & Todd, 2010b, 2011). Reaction sintering method has been successfully applied to fabricate alumina–mullite nanocomposites (Burgos-Montes, Moreno,
& Baudín, 2010) from mixtures of microsized alumina and colloidal silica.
2.02.7.3 Alumina–Carbon Nanotubes
Since their discovery by Iijima (1991), carbon nanotubes (CNTs) have been considered as potential re- inforcements of ceramics due to their excellent mechanical characteristics such as high Young’s modulus and tensile strength (Padture, 2009; Popov, 2004; Ruoff & Lorents, 1995). There are two main challenges in the processing of CNTs asfillers: the homogeneous dispersion of CNTs in the matrix and the interfacial bonding between matrix and CNTs.Figure 12shows the microstructure of a series of alumina–CNTs compared to that of single-phase alumina processed in a similar way (Zhang, Fahrenholtz, Hilmas, & Yadlowsky, 2010). Well- dispersed, independent CNTs as well as CNTs clusters are located at the grain boundaries of the alumina matrix. The presence of CNTs limits matrix grain growth. The presence of clusters is due to the fact that nanotubes are difficult to process as they have unfavorable geometrical characteristics for colloidal processing.
Consequently, they tend to agglomerate due to their strong Van der Waals interactions. Mixing with the alumina matrix is difficult and the agglomerates of CNTs act as critical defects in the composites (Lee, Mo, Park, & Ho, 2011). Moreover, sintering presents the same kind of problems as alumina–SiCwcomposites but still enhanced difficulty. First works on CNT-reinforced ceramics dealt with alumina matrix composites (Laurent, Peigney, Dumortier, & Rousset, 1998; Peigney, Laurent, Dumortier, & Rousset, 1998) and since then effort is being dedicated to the fabrication of alumina–CNT composites by different green-forming processes before the sintering step, usually by HP or SPS. Physical mixing has been conducted under wet conditions to disperse CNTs into alumina powder, followed by ultrasonication and ball-milling (Ahmad, Pan, & Shi, 2006; Fan, Zhao, Wu, Xu, & Song, 2006; Zhan, Kuntz, Wan, & Mukherjee, 2003; Zhang et al., 2010). Chemical treatments for surface modification have been performed on CNTs to obtain better uniformity of dispersion during colloidal pro- cessing (Estili et al., 2008; Poorteman, Traianidis, Bister, & Cambier, 2009; Sun, Gao, & Jin, 2005; Sun, Gao, &
Li, 2002; Unwin et al., 2010). The in situ growth of CNTs using chemical vapor decomposition onto alumina particles has been demonstrated to be adequate to avoid CNTs agglomerates in the composites (An, You, & Lim, 2003; Zhang et al., 2009).
In order to improve the poor interfacial bonding between the CNTs and alumina matrix after densification, some methods have been proposed to reach intimate contacts between them. The sol–gel method has been used to improve the poor interfacial bonding; however, the mechanical properties were not increased to the expected level (Mo, Cha, Kim, Lee, & Hong, 2005). The optimization of the interfacial interactions of the alumina matrix and the CNTs in the green state, referred to as molecular-level mixing, has allowed to reach a significant Processing of Alumina and Corresponding Composites 61
5 µm 2 µm
1 µm 5 µm
5 µm 2 µm
2 µm 5 µm
(a) (b)
(c) (d)
(f) (e)
(g) (h)
Figure 12 Microstructures of nominally pure Al2O3and composites containing 1, 3, and 5 vol% CNTs sintered at 1500C for 2 h.
Arrows in images (c) and (d) indicate individual CNTs, while arrows in images (f) and (h) indicate CNT clusters. (a) and (b) Pure Al2O3. (c) and (d) 1 vol% CNT–Al2O3. (e) and (f) 3 vol% CNT–Al2O3. (g) and (h) 5 vol% CNT–Al2O3.Zhang et al. (2010).
62 Processing of Alumina and Corresponding Composites
improvement in the alumina matrix CNTs bonding in the sintered state (Cha, Kim, Arshad, Mo, & Hong, 2005;
Cha, Kim, Lee, Mo, & Hong, 2005; Lee et al., 2011).
Some of these methods have had some successful results for relatively small specimens; however, scaling is still difficult. Therefore, till date, there are no reports of defect free high-density pieces to be commercialized. In the same way as CNTs, boron nitride nanotubes/alumina composites are being investigated. Fabrication of dried mixtures by HP has been reported (Wang et al., 2011).