Cellular Ceramics / p2

102Part 2 Manufacturing

2.4.2

Oxide Fibers

Several types of fibers have been used to produce insulation for applications ranging from furnace linings to thermal protection systems for space vehicles. Glass fibers were among the earliest available synthetic fibers and are generally processed from the melt [1–4]. Melts can be formed directly from mineral ores prior to spinning or may be homogenized through a prior melting cycle to form marbles. Molten glass is passed through a series of heated orifices to form individual filaments that may be subjected to one or more drawing methods to modify the fiber diameter and quench the amorphous filament. Melt spinning or blowing processes can be used to form fibers of various diameters (< 1 mm to 10 mm) and compositions. These compositions are, however, limited to glass formers that have the appropriate rheological properties to form continuous strands when melted [5]. The primary constituent is typically silica, but other chemical components are necessary to decrease the melting temperature and melt viscosity to allow homogenization and fining. Processes that can be used to draw the molten glass into filaments include rotary air attenuation (similar to the method used to make cotton candy), steam or air attenuation, and flame attenuation. Bundles of strands or filaments can be collected and wound together to produce strands suitable for textile methods such as weaving [6].

Of available glass fibers, high-purity, small-diameter silica fibers have been the primary constituent of all types of rigid tile insulation and have been used in a flexible mat form in thermal protection blankets. These require specialized processing to form small-diameter filaments and to purify the fibers. Their processing is described in more detail in Section 2.4.2.1.

The limitations imposed by melt spinning arise from the restricted set of chemical compositions that melt at temperatures low enough for practical processing and have the proper viscosity for fiber production. Other refractory oxide fibers of Al2O3, ZrO2, mullite, and yttrium aluminum garnet (YAG) were developed by using alternative approaches [5]. These are typically produced from a liquid precursor solution that can be made viscous through the use of additives and/or by increasing their concentration. The fibers are converted to their final crystalline form by pyrolysis and sintering. These fibers, too, can be produced with a wide range of diameters

depending on the drawing method. Both small- (~ 3 mm) and large-diameter (~ 11 mm) fibers produced from precursor solutions have been used to produce heatshield materials. The two most prevalent compositions used are based on alumina– silica and alumina–borosilicate compositions. The drawing methods used for smalland large-diameter fibers are quite different, and both will be described.

2.4.2.1

Melt-Blown Silica Fibers

Of the various glass fibers available, only those with the highest thermal stability are used for aerospace insulation applications. These are typically high-purity silica fibers (e.g., Q-fiber, Johns Manville Corporation) formed by using a flame attenua-

2.4 Connected Fibers: Fiber Felts and Mats 103

tion process suited to producing small-diameter filaments. They require composition refinement to prevent crystallization during use, which can compromise both the thermal and mechanical performance of the heat-shield insulation.

The fibers are blown from high-silica sand which is melted once to form marbles that are subsequently remelted and drawn through furnace bushings consisting of a sievelike arrangement of orifices to produce coarse fibers. These are again melted in a second attenuation step to form several smaller diameter fibers. This second step uses a high-temperature gas flame, typically impinging at right angles to the primary fibers (Fig. 1a) [7]. The resulting amorphous fibers exhibit a range of diameters (Fig. 1b) with an average diameter of 0.75–1.6 mm [8]. They are propelled by high-ve- locity gas through a forming tube and collected as an entangled mass on a conveyor belt. In addition to the fibers, inclusions or “shot” which have a different morphology and can be of different chemical composition are also collected in the mat [9].

Glass

Marbles

Glass

Wool

Flame

Fig. 1 a) Schematic of the flame-attenuation fiber-blowing process. b) Scanning electron micrograph of high-purity silica fibers (Q-fiber, Johns Manville).

Alternatively, blown fibers can be collected on a perforated rotating drum and aligned as they are removed as an agglomerated mass to pass through an orifice and be spooled onto a second drum rotating faster than the first. This process is called drafting, and the tension produced by the differential drum speeds aligns the fibers in a well-defined bundle [10]. Drafting cycles can be repeated until the desired degree of fiber alignment is achieved. The drafted fiber bundles can then undergo additional processes such as twisting to form continuous strands suitable for textile processing such as weaving.

As-drawn fibers contain impurities such as sodium that are beneficial in the melting and spinning processes but detrimental in service since they promote fiber crystallization. To eliminate such impurities, the fibers are subjected to an acid leaching process. The process starts with glass fiber and then any extraneous monoor diva-

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lent ions are leached from the glass to leave an open pore structure. Depending on the purity of the starting raw materials the resulting silica content after leaching can be quite high (99.5+ %) [8]. All subsequent processing steps are performed with the goal of minimizing contamination.

2.4.2.2

Blown Alumina–Silica Fibers

Fibers with a high volume fraction of alumina (e.g., Saffil, J&J Dyson) are also used as components of both flexible and rigid thermal protection systems [8]. Saffil fibers are made by using a solution-precursor spinning process especially developed to manufacture small-diameter fibers. The fibers are made from an aqueous solution comprising aluminum oxide chloride, a silica sol (added to control the crystallization behavior), and a nonionic, soluble, high molecular weight organic polymer (added to control viscosity). The viscosity of the composition is tailored to the fibrizing method (blowing) and is typically less than 10 poises [11].

During blowing, the solution is extruded through an aperture into a high-velocity gas stream. The extruded liquid stream is drawn down by the action of the gas stream to reduce its diameter. A diameter reduction factor of about 20 is usual for this type of blowing process. For very fine fibers the viscosity of the extruded composition must be carefully maintained during this step. This means that loss of solvent from and/or gelling of the composition must be controlled by manipulation of velocity, temperature, and particularly the drawing atmosphere. To minimize the loss of solvent from the composition, air with a high relative humidity is typically used. This fiber-forming method typically results in a product with a lower shot content than the melt-spinning process described above.

The fiber can be dried further after attenuation in the gas stream and is then exposed to ammonia vapor or a basic amine atmosphere to cause gelling of the precursor and to preserve the fiber shape. Fibers are then subjected to a hydrothermal treatment after being collected in the form of a loose mat [11]. It is during low-tem- perature hydrothermal heat treatment that the aluminum salt is decomposed to an aluminum hydroxide. The precise mechanism whereby ammonia or a basic amine assists the process is not fully understood, but it is believed that the release of acid anions is assisted by the formation of a soluble substance that is more easily removed in hydrothermal treatment. In general, the use of ammonia or a basic amine in conjunction with hydrothermal treatment gives fibers with a higher BET surface area and smaller grain size. The fibers are subjected to a final high-tempera- ture heat treatment step to fully crystallize the fiber and remove residual porosity. This heat treatment is accompanied by a number of phase changes involving transition aluminas with a progression from g-Al2O3 to c-Al2O3 to d-Al2O3 to h-Al2O3 and finally the stable form a-Al2O3 [5]. Careful control of the heat-treatment parameters is required to control the fiber microstructure. A fine, uniform grain size and high density are desired for good mechanical performance but this is difficult to achieve in practice in pure alumina fibers. Therefore, silica is added as a second phase to stabilize the transition alumina forms to allow pore removal and inhibit

2.4 Connected Fibers: Fiber Felts and Mats 105

Fig. 2 Scanning electron micrograph of alumina–silica fibers (Saffil, J&J Dyson).

crystal growth. The resulting fibers (Fig. 2) comprise approximately 95 wt % Al2O3 and 5 wt % SiO2 and are relatively fine (~ 3 mm) and have short staple lengths (2–4 cm) [5].

2.4.2.3

Drawn Alumina–Borosilicate Fibers

Large diameter continuous oxide fibers are typically produced by extruding precursor solutions of appropriate viscosity through an orifice and drawing them to the desired diameter [12]. Commercially available fibers are produced for the most part from aqueous solution precursors based on alumina. Viscosity control is the key to the process. The solution must not shear-thin or change viscosity during extrusion. Furthermore, to produce continuous fibers the drawing and gravitational forces must balance the surface tension, viscosity, and inertial drag of the fiber as it is accelerated during attenuation [13]. This requirement establishes an appropriate range of viscosity so that fibers are not subject to necking (viscosity too low) or capillary fracture (viscosity too high) during drawing. Once formed, the fibers are subjected to heat treatments to pyrolyze the precursor to remove organics and water and heat-treated to higher temperatures to obtain the desired microstructure and crystalline phase. The removal of fugitive components without forming defects becomes increasingly difficult as the fiber diameter increases and ultimately limits the maximum diameter of fibers that can be processed from solution precursors or sols (typically ~ 20 mm).

Of the family of larger diameter continuous oxide fibers (Nextel, 3M Co.), the most widely used in thermal protection systems is Nextel 312 which contains 14 wt % boria. These fibers are produced from a mixture of aluminum carboxylates and colloidal silica mixed with organic and boria-containing additives. The precursor is

106 Part 2 Manufacturing

Fig. 3 a) Schematic illustration of the dry spinning process. b) Scanning electron micrograph of continuous aliminoborosilicate fibers (Nextel 312, 3M Co.).

formed as a dilute solution that is filtered prior to being concentrated by solvent evaporation to form a viscous spin dope suitable for fiber drawing. A dry-spinning process is used in which the spin dope is pumped via a metering pump through a spinneret. The fiber diameter is controlled by varying the pumping rate and drawing speed during spinning. The fibers are then heat-treated to pyrolyze, crystallize, and sinter the fibers. The dry-spinning process is shown schematically in Fig. 3a [12].

The composition of the formed fibers is 62 % Al2O3, 24 % SiO2, and 14 % B2O3, and the predominant crystalline species after heat treatment are aluminum borate and mullite. The fibers are approximately 11 mm in diameter (Fig. 3b) and each roving or tow comprises 390 individual filaments. The continuous fibers can be woven to produce fabrics or chopped to shorter lengths and combined with low-conductiv- ity oxide fibers to form insulation tiles.

2.4.3

Fiber Product Forms

Fibers produced by the methods described above can be used to produce a variety of three-dimensional porous bodies. Several examples are given although a number of additional processing approaches exist. These are summarized in Fig. 4 [5].

2.4 Connected Fibers: Fiber Felts and Mats 107

Fig. 4 Fiber forms.

2.4.3.1

Continuous Monofilaments

Monofilaments of continuous blown or drawn fibers can be used to produce composite reinforcements or fiber preforms by weaving, knitting, or braiding. The fibers are typically collected as aligned fiber bundles or roving. Continuous fiber bundles can also be chopped to short lengths of several millimeters and mixed with discontinuous fibers to form mats or molded felts or blocks.

2.4.3.2

Fiber Mat

Mats are formed by collecting staple fibers that are randomly oriented in the length and breadth directions on a moving belt. The structure is built up in the thickness direction as layers of such deposits are formed atop one another. This results in the physical properties differing in the in-plane and through-thickness directions. It

108 Part 2 Manufacturing

Fig. 5 Commercially available oxide fiber forms include a) mat, b) board, c) bulk fibers, d) paper, and e) felt.

also produces a loose flexible mat with a striated appearance and two-dimensional layers that can be easily separated from one another (Fig. 5a). This allows the weight of the mat per unit area to be easily adjusted. Mats can be used in their as-deposited form or processed into rigid board by using organic or inorganic binders (Fig. 5b). They can also be further processed into bulk or milled fiber.

Mat properties can be modified by several techniques. Needled mats are produced by incorporating organic nonwoven fiber scrim cloths between layers of the mat and pushing an array of needles through the assembly in the through-thickness direction. The needles force the organic fibers and some of the inorganic fibers to follow their trajectory. This results in a fraction of the fibers interpenetrating several of the two-dimen- sional mat layers. The process can be repeated to change the area fraction of interpenetrating fibers. The organic fibers can then be removed by heat treatment if desired.

Mats can also be reconfigured as “stacked blocks” for use as insulation in applications where delamination of the two-dimensional fiber layers is especially problematic. For this configuration, strips of insulation are cut, turned 90 to their original orientation, and reassembled. While useful for furnace insulation, this product has not been incorporated in thermal protection systems.

2.4 Connected Fibers: Fiber Felts and Mats 109

2.4.3.3

Bulk Fiber

Mats are shaped into bulk form in a shredding or chopping machine that produces three-dimensional fiber agglomerates (Fig. 5c). Chopping can produce a range of fiber lengths. Further reduction in the fiber length, if required, can be achieved by high-energy milling processes such as hammer milling. Bulk fibers are typically processed into insulation components by dispersing the fiber agglomerates in a liquid and then removing the liquid (usually water) by filtration to leave a shaped body. Depending on the method used for consolidation, products can range from paper to felt to board.

Ceramic paper is formed by blending short bulk inorganic fibers with binders and a dispersing medium (Fig. 5d). Several types of fiber can be included depending on the final product requirements and the necessary fiber bonding characteristics. Binders can be organic or inorganic and are added to achieve a specific concentration relative to the fiber content. The binders must be uniformly distributed to produce a suitable strength, modulus, and other mechanical properties. The organic binders impart strength in the green state to allow ease of handling. After forming, the paper can be densified by compressing or calendering.

Felts are produced in much the same way as paper excepting the final densification steps (Fig. 5e). They, too, may contain organic fibers or binders to improve their handling strength but are often available in flexible binderless forms. Their density and homogeneity depend on the length of the fibers, their degree of agglomeration, and the process used to mix the fibers in the dispersing medium. Rigid boards can also be produced with bulk fiber bonded together with second-phase fibers or binding agents. These can be made rigid at low temperatures by using organic binders and can be compressed to form insulation with a range of densities. Inorganic binding agents can be incorporated to promote fiber bonding during thermal excursion such that the boards retain their shape and rigidity in service at elevated temperatures.

2.4.4

High-Performance Insulation for Space Vehicles

Space Shuttle thermal protection systems are a special class of high-performance insulation. They must withstand high temperatures, severe thermal shock and gradients, high acoustic loads during launch, and structural deflection due to aerodynamic loads, and they must remain dimensionally stable and be reusable. Furthermore, they must satisfy these requirements while being as light as possible and inexpensive to produce, install, and maintain. When the Shuttle program began, the primary heat-shield candidates included replaceable ablator panels, re-radiative panels produced from carbon–carbon composites, metals, and oxide insulation [8]. Oxide insulation was selected for all areas except control surfaces that experience the highest temperatures.

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Originally, most surfaces of the Space Shuttle orbiters were protected by insulation tiles [14]. These rigid tiles were primarily comprised of silica fibers and a silica bonding agent, and their success was dependent on developing purification methods for both constituents and modifying their surface characteristics to control the optical properties and impart additional durability. Tile compositions and processing approaches have changed somewhat since the first systems were developed. These changes were made to improve the temperature resistance and mechanical performance. Even with such improvements, however, they remain inherently expensive to produce and maintain, and this has led to the development of alternative systems.

In an effort to decrease the overall cost of space-vehicle heat shields, additional concepts including flexible insulation systems and/or thermal protection blankets have been implemented [15]. Initially, these were also comprised primarily of silica and were used on vehicle surfaces that experience lower peak temperatures during vehicle reentry. Their temperature capability has, however, been improved through the use of new constituents and processing methods, and they are now being considered for higher temperature areas [16].

2.4.4.1

Rigid Space Shuttle Tiles

The two major components of preliminary Space Shuttle heat shields were carbon–- carbon hot structure used to protect vehicle leading edges and oxide fibrous insulation tiles used to protect the remaining vehicle surfaces [14]. The tile system was selected over competing approaches such as ablative or metallic reradiative panels because it was reusable and of a relatively simpler design. Furthermore, the insulation thickness could be tailored in a straightforward manner to meet local thermal performance requirements [8].

All of the rigid insulation heat shield materials are formed by a slurry casting method and incorporate bulk fiber. The earliest tiles comprised silica fibers and a silica binder [17]. The first system was of lower density and thermal capability than the all-silica systems developed subsequently. By incorporating additional fiber types and bonding agents, improvements in the tile strength and thermal capacity were achieved. The evolution of these systems and their processing methods will be described.

Although the primary focus is the fibrous insulation block used to produce thermal protection tiles, two additional key components will also be discussed. These are an outer (exposed) surface coating that provides the necessary optical properties and enhances the surface durability, and an inner coating that provides adequate mechanical properties for bonding the tiles to the underlying vehicle structure. Without these components and the processing approaches developed to co-process them with the insulation system, the tiles would not have been adequate for service.

Lockheed developed the first Space Shuttle heat shield tiles in the early 1960s. These all-silica tiles are designated LI-900, they have a low density of 0.15 g cm–3 (compared to 2.2 g cm–3 for fused quartz) and are comprised wholly of silica. The primary constituent is the high-purity amorphous fiber described above. Before the fiber is used, however, it undergoes additional processing steps to stabilize the micro-