DEFINICIÓN DE TÉRMINOS BÁSICOS 1. Accesibilidad

Commercially available oxide reinforcements are typically alumina-based continuous, polycrystalline fibres produced by the sol-gel process. Whilst this method is more expensive than traditional melt-blow techniques, fibres with higher alumina content can be produced and greater microstructural control is possible. High alumina content provides improved chemical stability, an elevated melting point and superior strength up to 1200°C. The sol-gel process involves spinning chemically-derived precursors followed by pyrolysis and ceramisation.

Precursors of commercially available fibres include aluminium chlorohydrate (Almax™

fibres), aluminium acetate (Nextel™ series of fibres) and aluminoxane polymers (Sumitomo Altex). The sol is pumped through a multiple orifice spinneret to form 400-1000 filaments simultaneously under controlled temperature, humidity and air flow. Varying the pump rate relative to the speed of the draw wheels enables control of fibre diameter. After spinning, the green fibres are heat treated, or pyrolysed, forming ceramic fibres. Gentle decomposition of the green fibre limits defects and flaws, which reduce fibre strength. To achieve high fibre strength at room temperature, a small grain size (<0.5µm) is desired. A low firing temperature will give a smaller grain size but generally leads to an unacceptable level of residual porosity caused by grain boundary migration. Porosity can be eliminated with higher processing temperatures but leads to excessive grain growth. To overcome this dilemma, it is common to introduce a second phase that prevents grain growth at high sintering temperatures whilst simultaneously eliminating porosity [12], [14], [20].

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The first pure (99.9%) α-alumina continuous fibre was developed by Du Pont in the late 1970s. α-alumina is the most thermally stable and crystalline form of alumina. The FP fibre had a polycrystalline microstructure with a grain size of 0.5µm and good room temperature strength. However, the fibre exhibited a rapid degradation of mechanical properties above 1000°C owing to its fine microstructure, which led to grain sliding and high creep strain rates.

In addition, its large diameter (18µm) and lack of ductility rendered it unsuitable for weaving and thus the fibre was not produced commercially [21], [22]. Du Pont developed a second polycrystalline continuous fibre, PRD-166, which consisted of 80wt% α-alumina with 20wt%

yttria-stabilised zirconia particles. The presence of zirconia improved room temperature strength by phase transformation toughening and inhibited grain growth at high temperature.

Nevertheless, these improvements were not sufficient to allow commercial development [23].

Nextel™ 610 (3M) and Almax™ (Mitsui Mining) fibres, developed in the early 1990s, are both 99% α-alumina with a diameter of 10µm. This reduction in diameter compared with FP and PRD-166 fibres has the immediate advantage of increasing the flexibility of the fibres, allowing weaving and thus the production of more complex shapes [22]. The Almax fibre has a grain size of 0.5µm; however, the fibre exhibits a large amount of intergranular porosity (8%) caused by a lack of control over grain growth during manufacturing. Consequently, mechanical properties are severely degraded above 1000°C. The Nextel 610 fibre has high strength and thermochemical stability, and maintains excellent high temperature strength and creep resistance below 1000°C [24]. A fine grained (0.1µm) α-Al2O3 fibre is obtained by seeding α-Al2O3 with a very fine hydrous colloidal iron oxide (Fe2O3), which acts to improve the nucleation rate of α-Al2O₃. In addition, Nextel 610 contains 0.35% SiO₂ which acts to retard grain growth above 1000°C during manufacture. It is believed that silica does not form

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a second phase at grain boundaries [22]. Nextel 610 fibres suffer from poor creep performance above 1000°C as a result of their small grain size [12].

Pure α-alumina fibres have superior chemical and thermal stability compared with silica-containing fibres and are therefore less reactive with potential oxide matrices during manufacture and are more stable in corrosive service environments. Nevertheless, difficulties remain in controlling porosity and grain growth during processing of pure α-alumina fibres, and fibres generally have a higher density and higher elastic modulus which lowers strain-to-failure and increases brittleness [14]. The introduction of silica allows the transformation to α-alumina to be limited since it reacts with α-alumina to form mullite (3Al₂O₃:2SiO₂). The presence of mullite at grain boundaries controls the grain growth of residual α-alumina.

Silica-containing fibres are generally produced at lower cost than pure α-alumina fibres, and are stable up to 1200°C. The Altex fibre, produced by Sumitomo Chemicals Co., comprises 85% alumina and 15% silica to produce nano-scale γ-alumina grains intimately dispersed in amorphous silica. Silica inhibits the transformation to α-alumina below 1127°C in both oxidising and inert atmospheres [25]. The presence of silica in the Altex fibre does not reduce fibre strength at lower temperatures compared with pure alumina fibres; however lower activation energy is required for the creep of the fibre [22]. Nextel 720 fibres (3M) are also composed of 85% α-Al₂O₃ and 15% SiO₂ by weight. In contrast to the Altex fibre, heat treatment forms a two-phase mixture of α-Al2O3 and mullite with approximately 4% porosity.

The fibres contain 0.3% Fe2O3 as a nucleating agent for the formation of α-alumina, as described for Nextel 610 fibres. Nextel 720 fibres exhibit superior creep performance compared with other oxide fibres owing to the high content of mullite, a highly creep-resistant compound, and due to the unique crystalline structure which reduces deformation by grain boundary sliding at high temperature. In addition, the larger grain size of Nextel 720 fibres

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reduces creep, since creep is inversely proportional to grain size in fine-grained oxides. The high content of mullite lowers density by 13% and thermal expansion by 30% compared to pure α-alumina fibres (Nextel 610). Nextel 720 fibres have a circular cross section and a diameter of 12µm, woven in an 8-harness satin weave with approximately 400 filaments in each tow. The fibres have a single filament tensile strength and modulus of 2100MPa and 260GPa, respectively. The Nextel 720 fibre displays an in-service temperature capability of 1200°C, currently the highest of any commercially available oxide fibre. In addition, the fibres retain 90% room temperature multi-filament tensile strength at 1300°C (Figure 2.4) [24], [26], [27].

Figure 2.4. Tensile strength retention of multi-filament strands of Nextel fibres at elevated temperature [24].

The Nextel 650 fibre was developed for high temperature applications where the presence of mullite and other silicon-containing phases is not desirable. The fibre comprises 89% α-alumina, 10% cubic zirconia and 1% yttria (Y2O3) and is produced in continuous form with a diameter of 11.2µm. Zirconia with a grain size of 5-30nm is present on both grain boundaries and within alumina grains and limits α-alumina grain growth at high temperature. Nextel 650

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fibres have an alumina grain size of 0.1µm, yet display 100 times lower creep than Nextel 610 fibres due to the presence of yttria. The single-filament tensile strength of Nextel 650 fibres is reported to be 2.75GPa, with only 73% strength retained at 800°C and just 30% strength retained at 1200°C (Figure 2.4). As such, the fibre has a maximum use temperature of 1080°C [24], [28].

The Saphikon fibre (Saint-Gobain Saphikon), a single crystal α-alumina fibre, was developed to allow higher temperature capability and better creep resistance than polycrystalline alumina fibres such as Nextel 610 or 720. Saphikon fibres are produced by the ‘edge-defined film-fed growth’ method whereby a single crystal is grown from a molten film of alumina. During growth, the melt wicks into a molybdenum die and the fibre growth, initiated by a seed crystal, occurs at the top of the die. The shape of the crystal is defined by the external shape of the die. Continuous growth lengths of more than 300m have been demonstrated. Whilst the Saphikon fibre displays thermochemical stability at 1400°C, its large diameter (125µm) prevents weaving and the production method is very expensive [8], [29], [30].