Cellular Ceramics / p2

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structure against shrinkage and to remove nonfibrous inclusions, which would cause density variations and devitrification of the fibers.

These additional processing steps are essential since crystalline silica fibers would exhibit polymorphic phase transformations during thermal cycling that would compromise the performance of the heat shield. Due to the slight variability in glass melt composition and purity and the resulting effect on reproducibility of insulation properties, a number of fiber lots are blended together prior to use. The fibers are also heat-treated to eliminate any porosity that may have formed during the purification process (acid leaching). The blended, heat-treated bulk fiber is then cleaned by using a hydrocyclone to remove glass shot and dried [18].

The colloidal silica binder must also be of high purity so as to adequately bond the silica fibers together to form rigid insulation without contaminating them. Of particular concern are devitrifying agents such as sodium. Contamination sources include processing silica sols from sodium silicate or using NaOH to prevent gelation. The Na+ ions are typically removed by a series of column deionization cycles involving prolonged storage of the sols and/or heat treatments between cycles to facilitate the migration of Na+ to sol particle surfaces for removal. These cycles are repeated until the Na2O content reaches an acceptably low level, as determined by heat treatment and X-ray diffraction to determine the amount of crystalline phase formed (typically less than 300 ppm) [19].

After the constituents are chemically refined, the tile insulation blocks are formed by using a slurry casting method. Casting generally results in preferential orientation of fibers perpendicular to the pressing direction. This, in turn, produces anisotropy in the thermal and mechanical properties of the insulation billet. Hence, tile properties are often reported for both the through-thickness (parallel to the pressing direction) and in-plane (perpendicular to the pressing direction) orientations. When bonded to the vehicle, the in-plane orientation is parallel to the surface.

To form the slurry, predetermined weights of fibers and deionized water are combined in a low-shear mixer. After the fibers are dispersed, the mixture is transferred to a casting tower and de-aired. The water is removed by a combination of pressing from the top surface and applying a vacuum to the bottom surface of the mold. Water is removed until the billet reaches a predetermined thickness and the water makes up approximately 80 % of the billet weight. The colloidal silica binder is then pumped into the top of the mold while vacuum is again applied to the bottom so that the binder displaces the water. The binder is then gelled to ensure a uniform distribution after drying [18].

The billets are dried in a convection oven or microwave drier or both and have a density of approximately 0.11 g cm–3. Once dry, the billets are sintered at 1290 C until they reach the desired density of 0.15 g cm–3. They are then machined to shape, heat-treated to remove organic contaminants, and coated. Although most tiles are nominally 6 0 6-inch squares (except for close-out tiles), no two tiles on an orbiter are exactly alike. The service heat load determines the thickness of each tile and the curvature of each tile’s underside is matched to the contour of the Shuttle’s skin at the exact point the tile is to be bonded. This custom machining adds considerable expense to the system.

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The inner coating is generally the same for each of the tile systems discussed and is required since tiles cannot withstand airframe load deformation. Therefore, strain isolation is necessary between the tiles and the orbiter structure. This isolation is provided by a compliant pad. These are thermal insulators made of Nomex felt material and are bonded to the tiles with a high-temperature silicon adhesive. The same adhesive is then used to bond the assembly to the vehicle surface [14].

The bond assembly introduces stress concentrations in the tile. This results in localized failure of the tile just above the bond line [20]. To solve this problem, the inner surface of the tile is densified to distribute the load more uniformly. The densification material comprises a Ludox ammonia-stabilized silica binder mixed with silica slip particles and a silicon tetraboride colorant added to facilitate visual inspection of the penetration depth in the tile. Several coats of the pigmented slurry are brushed onto the tile surface that is to be bonded to the strain isolator pad. The tile is then air-dried for 24 h before the coating is sintered. The densification coating penetrates the tile to a depth of approximately 3 mm, and the strength and stiffness of the tile and bonding system are increased sufficiently for reliable attachment to the vehicle [21].

Two top surface coatings are used for the LI-900 tiles to give the tiles different heat-rejection capabilities. Tiles on the lower or windward vehicle surface have a black coating for high emittance at high temperature and the tiles on the upper surface (now mostly replaced by blankets; see Section 2.4.4.2) have a white coating to limit in-orbit vehicle temperature. The black tiles are coated on the top and sides with a mixture of powdered silicon tetraboride and borosilicate glass frit in an alcohol carrier with a methylcellulose prebinder [22]. The viscosity and component particle sizes are measured prior to application. This coating slurry is sprayed onto the top and side surfaces of tiles. The slurry is applied to achieve a targeted weight based on coated area that will result in a dense coating approximately 300 mm thick after heating to a temperature of 1230 C [18]. The white coating contains no silicon tetraboride but is applied in the same manner and sintered to a slightly lower temperature. The black tiles of this type are used to protect vehicle surfaces that reach temperatures of less than 1200 C and the white tiles for areas that reach temperatures of less than 650 C. After the ceramic coatings are sintered, the tiles are waterproofed with a silane vapor. Dimethylethoxysilane is injected into each tile through an existing hole in the surface coating with a needleless gun.

For some areas of the vehicle, the strength of the low-density LI-900 is inadequate. A higher density tile system, LI-2200, with a slightly different processing scheme was developed by NASA, Ames, for those areas. Although these insulation billets are also produced by slurry casting, they do not contain colloidal silica as a binder. They do, however, contain silicon carbide powder as an emittance aid [18].

Several other details of the insulation billet-processing scheme were changed to reach higher densities. The silica fibers were not heat-treated prior to casting, and a more aggressive slurry mixing process was employed. The fibers were mixed with water, silicon carbide powder, and ammonium hydroxide (all in preweighed amounts) in a V-blender with an intensifier bar. The size of the fiber agglomerate was modified during this step to allow a higher packing density during casting. The

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casting process itself was unchanged, although the degree of fiber compaction was increased. After drying, the green density of the billet was approximately 0.21 g cm–3, and after firing to 1300 C the final density was approximately 0.35 g cm–3. The tiles were machined and the surface coatings applied as previous described.

Following the development of all-silica tiles, other compositions were investigated. These blended-fiber tiles generally consisted of insulation blocks formed by blending two or more types of fibers to impart additional specific strength. This benefit, however, was often achieved at the expense of other desirable properties such as low thermal conductivity or dimensional stability at elevated temperatures. In most cases parametric studies were performed to characterize the effects of varying the composition and processing parameters on the microstructure and physical properties of the insulation. The results of a few of the published studies of this type are summarized herein. Several groups, however, continue to conduct proprietary research on these systems using fibers and binders other than those discussed in this summary, and future improvements may be expected.

The first of the blended-fiber systems, developed at NASA, Ames, was termed fibrous refractory composite insulation (FRCI) [18, 23, 24]. In these insulation materials, high-purity silica fibers were combined with aluminoborosilicate fibers. Both fiber types are heat-treated prior to blending. The silica fibers were preconditioned by dispersing them in a mixture of deionized water and hydrochloric acid (pH » 3) while at the same time bubbling nitrogen gas through the slurry to sediment out shot and other contaminants. The fibers were treated in this manner for a couple of hours and then rinsed with deionized water. Afterward, the fibers were thermally treated as described above for LI-900 tiles.

The relatively larger diameter (~ 11 mm) aluminoborosilicate fibers are available as rovings that are cut to lengths of approximately 3 mm prior to use. They are then heated to at least 1090 C to promote crystallization. The stiffer fully crystalline fibers have been observed to disperse more uniformly during fiber blending than amorphous forms [25]. The resulting mullite content is determined prior to use. A number of tile compositions have been investigated with the aluminoborosilicate fibers making up 20–80 wt % of the billet. Of these, the composition with 20 % aluminoborosilicate fibers and a density ranging from 0.12 to 0.22 g cm–3 has been produced in a production capacity (FRCI-20-12).

The fiber slurry is prepared in several steps. The aluminoborosilicate fibers are first mixed with a portion of the silica fibers in a high-shear mixer. This mixture is then combined with the remaining silica fibers and silicon carbide powder in a V- blender with an intensifier bar. The slurry pH is adjusted with ammonium hydroxide to aid dispersion. After mixing, the slurries are transferred to a casting tower and de-aired.

Rapid removal of the water during casting is required to prevent sedimentation, which results in composition and density gradients in the final billet. During pressing, the fibers become aligned perpendicular to the pressing direction.

After drying, the tile billets are fired to 1315 C to promote fiber-to-fiber bonding. The bonding in this system is effected by the formation of borosilicate glass at fiber junctions with the boron fluxing agent provided by the aluminoborosilicate

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fibers. The difference in fiber bonding between silica tiles and blended fiber tiles is shown in Fig. 6. During sintering, the heating rate is chosen to permit relatively uniform temperatures to be achieved throughout the tile. A faster heating rate causes nonuniform sintering and may result in cracking. Longer or higher temperature firing results in more initial shrinkage during sintering but less subsequent shrinkage in service. Increased firing temperatures may lead to crystallization of silica fibers.

Fig. 6 After sintering, the fibers bonded together in a) LI-900 silica tiles and b) alumina-enhanced thermal barrier blendedfiber tiles appear different.

In general, blended-fiber tiles can be produced with higher through-thickness strength, lower density, and lower thermal conductivity than LI-2200. FRCI-20-12 (20 % nominal aluminoborosilicate fibers with a nominal density of 12 lb ft–3) tiles have, in fact, been successfully used in place of LI-2200 on Shuttle orbiters.

Further modifications to the blended-fiber tile systems include the incorporation of small-diameter alumina fibers to increase the temperature to which the tiles can be exposed without dimensional instability or slumping. These alumina-enhanced thermal barrier (AETB) systems are of nominal composition 20 % small-diameter alumina fibers (Saffil), 12 % large diameter aluminoborosilicate fiber (Nextel 312), and 68 % silica fiber (Q-fiber) [25]. They are processed similarly to FRCI insulation and in two nominal densities, 0.13 and 0.19 g cm–3. The lower density version has successfully been flown on the Shuttle base heat shield, replacing LI-900 tiles, and the higher density version has been flown as an experimental material.

The improved dimensional stability of AETB over FRCI tiles has been attributed to the presence of the more refractory alumina fiber as well as a reduction in the concentration of aluminoborosilicate fiber resulting in a lower boria content and less softening of the silica fibers. The alumina fibers, however, also increase the thermal expansion coefficient of the tile, which, in turn, necessitates modification of the surface coating composition. The effect of various additives was investigated, as was the effect of changing the coating morphology from a thin glass shell to thicker tile-infiltrating glaze. These combined modifications have been quite successful at preventing coating spallation and undesirable tile shrinkage during sintering. These

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coatings (toughened uniform fibrous insulation or TUFI) penetrate several millimeters into the tile surface and also improve impact resistance [26].

To produce the tile-infiltrating coating slurry, the glass frit, fluxing agent, and emittance agent are separately milled to reduce their particle size before they are mixed together. The glass frit comprises a small amount of boron oxide added to a commercially available, relatively pure, acid-leached borosilicate glass available under the name Vycor. The coating contains a silicon tetraboride fluxing agent that is oxidized exothermically to produce a high boron oxide borosilicate glass flux. The resultant multicomponent heterogeneous glass encapsulates the molybdenum disilicide emittance agent. To adjust the sintering compatibility to that of the AETB insulation, approximately 20 wt % molybdenum disilicide is incorporated. All of the components are milled in ethanol and then mixed together in a Kendall mixer or equivalent to blend the individual slurries together. No methycellulose is added to increase the viscosity of this coating composition, and this in conjunction with milling (which increases the surface area of the particles and decreases the viscosity of the fluxed glass composition during sintering) facilitates penetration of the coating into the substrate.

The slurry is sprayed onto the insulation block with an airbrush or spray gun in several applications. A preweighed amount of slurry is applied and this effectively impregnates the outer surface of the tile and creates a porous graded composite. After spraying, the substrate is dried overnight at room temperature or for several hours at temperatures up to about 70 C. After drying, the coating is sintered in a preheated furnace for about 90 min at about 1200–1260 C. The fired coating is porous and black in color.

Tile systems that have been used to date on Space Shuttle orbiters have been described in the preceding paragraphs. Advanced concepts for next-generation vehicles are discussed at the end of this chapter. The tile compositions and processes used to make them have evolved since the earliest systems were developed. Improved strength and dimensional stability have been realized through the incorporation of additional types of fibers. As these additions were made, however, more aggressive mixing approaches were needed to ensure adequate homogeneity. In the filter pressing process, sedimentation due to mass differences between different fiber types and fiber agglomerate sizes is detrimental and leads to nonuniform through-thickness density and tile warping. Mixing the slurry until a rapid liquidremoval process is employed minimizes this problem. As a final check to ensure that each tile is uniform and homogeneous, witness specimens are also usually taken from various locations on the billets and tested for strength, density, and other properties.

General properties of rigid tile insulation systems are summarized in Table 1.

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atop the surface of the fabric. The density of the assembled AFRSI blanket is approximately 0.1 g cm–3and it varies in thickness from 1.1 to 2.4 cm. The appropriate thickness is determined by the heat load the blanket encounters during vehicle reentry, and thicker blankets are used to protect hotter areas. The blankets are cut to the required shape and bonded directly to the orbiter by RTV silicone adhesive 0.5 cm thick. The thin glue line reduces weight and minimizes thermal expansion during temperature changes.

A blanket waterproofing agent is typically applied before each flight. Compared to tiles, the AFRSI blankets have a much rougher surface and therefore an increased propensity to force a laminar boundary layer into turbulence. This increased roughness can also produce markedly amplified local heating where boundary layers are relatively thin, such as the windward side of most reentry vehicles. Improving the surface smoothness of these materials is key to their use on additional vehicle surfaces.

The original AFRSI constructions have been modified to increase their thermal capabilities. These improvements involved substituting a woven Nextel 312 fabric and sewing thread for the Astroquartz materials used in the first construct, and incorporating alumina mat insulation in place of quartz felt. These higher temperature blankets are used for the dome heat shields that surround the base of the main engine nozzles [16, 28]. Additional improvements, not yet incorporated on vehicles, have also been explored and are discussed in the Section 2.4.4.3.

2.4.4.3

Innovations in Thermal Protection Systems

Several potential improvements to the performance of thermal protection tiles and blankets may result from recent or on-going research aimed at reducing the thermal conductivity of the insulation or improving the durability of the overall system. The first category of improvements can be achieved by incorporating additives that modify heat conduction processes or by combining more than one type of insulation in a single component. The second category of improvements typically involves integrating new materials and surface treatments.

Heat transfer through fibrous insulation occurs by a combination of conduction, convection, and radiation. Reducing contributions from any of these mechanisms may decrease the thermal conductivity of tile or blanket insulation and enhance its performance. At high temperatures, the radiative component becomes increasingly important. A number of attempts have been made to modify radiative heat transfer in fibrous insulation by incorporation of reflective particles [29] or sheets embedded in the insulation, or by coating individual fibers with a reflective material by using sol–gel and other processes [30]. These have met with varying degrees of success. The most widely used example is commercially available multilayer insulation (MLI) blankets, in which highly reflective metal foils are sandwiched in a predetermined stacking sequence throughout a flexible fibrous ceramic mat. Although this construction is effective in decreasing the thermal conductivity of the insulation, it also reduces the maximum use temperature.

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Another approach for decreasing the thermal conductivity of tile and blanket insulation materials is to reduce gas convection through the insulation by incorporation of aerogels [31, 32]. The fine pore structure of these materials, typically on the order of 50 nm or so, is very effective at reducing convective heat flow. In addition, the weight penalty is very small. Processing techniques based on supercritical drying have been successfully used to incorporate aerogels of various compositions in both rigid and flexible fibrous insulations. The primary limitation of the approach lies in the poor high temperature stability of the aerogels themselves. They sinter and densify at temperatures well below the peak surface temperatures of thermal protection components. For this reason, they are likely to be of use only when embedded in insulation near the cooler face.

The embedded-aerogel approach to optimizing the performance of heat-shield components is similar to other methods explored to combine more than one type of insulation material in a single component. Multilayer tile material has been produced from layers of AETB tiles of two different densities [33]. The insulation layers were bonded together by a high-strength, high-temperature alumina or silica binder having a coefficient of thermal expansion similar to that of the insulation layers. In this way, the surface exposed to the highest peak temperature can comprise higher density tile that is dimensionally stable, while the embedded portion of the insulation, which reaches lower temperatures, can comprise lower density tile that is lighter and of lower thermal conductivity. This approach provides a means of optimizing several performance parameters if successful.

In addition to improving heat shields by decreasing the conductivity of the insulation, they can be improved by using new construction methods or more effective protective surface treatments. For example, changing the form of the insulation used in thermal protection blankets from a flexible mat to a rigid board significantly improves the resulting surface smoothness, as needed for better aerodynamic performance [34]. The rigid insulation board allows more uniform tensioning of threads during the sewing process and better control of the fabric tension along blanket edges. The boards can be processed with organic binders that are removed by heat treatment to restore the requisite flexibility of the component subsequent to fabrication.

Flexible reusable insulation requires a coating on the outer woven sheet that infiltrates and stiffens the fabric to provide an aerodynamic surface. The coating must act as a “high-temperature starch” without embrittling the fabric. Since the coated fabric layer is essentially a thin ceramic matrix composite in which the infiltrated coating is the matrix, the requirements for blanket durability are the same as those for damage tolerance in structural ceramic composites: a weak bond is needed between the matrix and the fibers to prevent embrittlement. The upper surface of the orbiter is protected by blankets that consist of silica-based fabric, insulation, and coating. At temperatures above about 700–800 C, the silica-based coating bonds strongly to the fibers, embrittling the outer fabric and limiting its lifetime. Development of more refractory blanket fabrics and compatible coatings with temperature capability up to the range of 1000–1200 C would allow use of blankets on additional vehicle surfaces. This concept has been explored, and several improved blanket systems have been developed [35–38]. In general, varying the fabric/coating combina-

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Fig. 7 A family of improved thermal protection blankets has recently been demonstrated.

tion affects the thermal capability and durability of the system. Suites of new flexible insulation systems with tailored properties now exist, and examples are shown in Fig. 7. In some cases, these materials can be used up to temperatures of 1200 C with no loss of durability.

An alternative approach that has been taken to produce durable blanket surfaces is to incorporate a woven hybrid aluminoborosilicate/Inconel wire/braze alloy wire outer fabric [39]. A thin Inconel foil is brazed onto the surface of the blanket after the sewing operation. No waterproofing agent is applied to the durable advanced flexible reusable surface insulation (DurAFRSI), since the superalloy outer surface is waterproof. Compared to tile-type TPS, the installed cost of DurAFRSI is relatively low, but it has a more highly catalytic surface and a lower emissivity, both of which can lead to amplified surface heating. Additionally, its maximum use temperature is limited to below 1000 C.

Improvements in surface treatment for tiles include combining the two types of glass coatings described in Section 2.4.4.1 such that the first coating penetrates into the tile and the second thin dense glass shell covers and seals the surface. These double-coated tiles have been shown to be more impact resistant than when either coating is used separately [38]. Tiles have also been protected with ceramic composite surfaces [38]. These are typically produced by using matrices derived from preceramic polymer precursors. The composite is laminated directly atop the tile by conventional polymer composite processing methods. The precursor is then converted to an oxide ceramic matrix in a heat treatment step that also bonds it to the insulation block. The surface durability and maximum use capability depend on the composite system used and its construction details (fabric weave, number of fabric layers, etc.). In general, these can be produced with a higher specific impact resistance than any of the glass-coated systems. An example of such a tile undergoing impact testing is shown in Fig. 8.