Types, Properties, Production and Application of Ceramic Fiber

What is Ceramic Fiber?
Ceramic fibers have a very large aspect ratio and a very small cross-sectional area. Commercially available ceramic fiber, for example, have a fiber diameter of approximately 10 mm. Due to their geometry, the properties of the fibers differ significantly from those of bulk forms of the same material. In particular, fibers have much higher strength, higher strain to failure but also greater chemical vulnerability. Fibers can be manufactured as fiber fabrics, flows, and short or long fibers. Fine fibers are generally bundled into so-called ‘rowings’, which consist of 500–10,000 single filaments. Bundling keeps the fibers flexible, makes them considerably easier to process and handle, and makes it possible for them to be manufactured into various fibrous forms.

ceramic fiber products
Fig: Ceramic fiber products

Ceramic fibers were originally called Refractory Ceramic fibers when produced in the 1940s, and the material is now called high temperature insulation wool. In the past half-century, there has been significant development in the processing and use of high performance ceramic fibers. Generally, it is made from equal parts alumina and silica, and it ranges from white to cream in color. The need for reinforcements for structural ceramic matrix composites (CMC) to be used in air at temperatures above 1000°C, as well as for the reinforcement for metals (MMCs), has encouraged great changes in small-diameter ceramic fibers since their initial development as refractory insulation. Ideally, ceramic fiber should show sufficient flexibility so that preforms can be made by weaving and subsequently infiltrated by the matrix material. Applications envisaged are in gas turbines, both aeronautical and ground-based, heat exchangers, first containment walls for fusion reactors, as well as uses for which no matrix is necessary such as candle filters for high temperature gas filtration.

Ceramic fiber is found in two forms, continuous (long length) and discontinuous (short length). Alumina- and silicate-based continuous oxide fibers are made by sol-gel process but short oxide fibers by melt-spinning route. On the other hand, silicon- and boron-based nonoxide ceramic fibers are currently being developed and produced by thermal conversion of polymer precursor process.

Types of Ceramic Fibers:
Mainly ceramic fibers are two types,

  1. Ceramic oxide fibers
  2. Ceramic nonoxide fibers

1. Ceramic oxide fibers:
Ceramic oxide fibers that are in the form of long length and short length have been commercially available since the 1970s. These fibers that mostly consist of alumina (Al2O3) and alumina-silica (Al2O3-SiO2) mixtures due to their high melting points are generally used for high-temperature applications.

Ceramic oxide fibers are used both as insulation and as reinforcement material. The mostly known examples for oxide ceramic fibers are composed of oxides such as silica (SiO2), mullite (3Al2O3.2SiO2), alumina (Al2O3), and zirconia (ZrO2) having different characteristic properties.

Recently, different methods such as slurry spinning, sol-gel spinning, and single-crystal growth have been developed for manufacturing of oxide-based ceramic fibers.

Alumina (Al2O3)-based fibers are the most common of oxide fibers. These fibers exhibit excellent thermal, mechanical, and electric properties such as high-temperature strength, high thermal-shock and creep resistance, high dimensional stability, low thermal expansion coefficient, and good dielectric properties. Alumina-based fibers are often used as structural reinforcements in a variety of metal, ceramic, and polymer composites, making them stiffer and stronger. They are suitable for load-bearing applications; resulting composites withstand higher temperatures than metals.

Nextel ceramic fibers are spun, dried, and then sintered at high temperatures. Nextel fibers are manufactured from continuous oxide composite grade fibers designed for load-bearing structural applications in metal, ceramic, and polymer matrices.

Nextel 610 and 720 fibers have crystal structures based on α-alumina and α-alumina/ mullite, respectively. Nextel 610 fibers have higher strength at room temperature than Nextel 720 fibers. Another commercial alumina-based fiber that is Nextel 650 was developed as composite reinforcement for high-temperature applications. Nextel 650 oxide fibers contain α-alumina and cubic zirconia phases.

2. Nonoxide ceramic fibers:
Production of nonoxide fibers is difficult due to their high melting points and resistance to densification. Oxidation resistance tends to be their main deficiency. For these fibers, extensive research has been conducted and reported in terms of processing, microstructure, mechanical, and thermal stability. These ceramic fibers are either fine or thick diameter.

Silicon carbide based fibers:
Silicon carbide (SiC) fibers have an excellent combination of high strength, modulus, and thermal stability, including good oxidation resistance and mechanical properties (compressive-tensile strength) at high temperatures. Silicon carbide based fibers are generally applied as continuous fiber in ceramic matrix. This type of CMCs is used in hot section of engines for power, etc.

Boron-based fibers:
Boron fibers are commercially produced by CVD techniques; boron is generally coated on a fine tungsten wire (∼12 μm diameter) substrate. But a carbon substrate can also be used. Boron fiber with 100 μm diameter has a density of 2.6 g/cm3. It has high melting point of 2040°C. The average tensile strength of boron fiber is 3–4 GPa, while its Young’s modulus is between 380 and 400 GPa. Due to their superior mechanical properties and low density, they are used in military air crafts, space shuttle, and sports equipment such as golf shafts, tennis rackets, and bicycle frames.

Other fibers:
There are lots of different nonoxide fiber types, in addition to above mentioned ones. Another type of synthetic ceramic fiber is silicon oxynitride (SiNO). SiCN based ceramic fibers and Si-Al-O-N ceramic fibers are also important types of nonoxide fiber.

Basic Compositions of Ceramic Fiber:

  1. SiO2 ——————->50-60 %
  2. Al2O3 ——————>30-50 %
  3. Na2O-H2O ————->0.1 %
  4. Fe2O3 ——————>0.04 %
  5. Leachable Chlorides —>10 ppm

Properties of Ceramic Fiber Textile Products:

  1. Resistant to oil, solvents and chemicals
  2. Good resistance to oil
  3. Good resistance to chemical
  4. Good resistance to solvents
  5. Flame retardant
  6. Low thermal conductivity
  7. Heat-Resistant—it can withstand temperatures up to 1,260°C
  8. Abrasion resistant.
  9. High temperature stability
  10. Excellent thermal shock resistance
  11. Good dimensional stability
  12. Low density
  13. Incombustible
  14. Good flexibility
  15. Continuous use limit 1260°C
  16. Melting Point 1790°C

Production Techniques of Ceramic Fibers:
Ceramic fibers may be produced in various forms like blankets, felts, bulk fibers, vacuum-formed or cast shapes, paper, and textiles depending on the application area.

Flow chart for making polymer-derived ceramic fibers via the melt-spinning process
Fig: Flow chart for making polymer-derived ceramic fibers via the melt-spinning process

There are several methods / techniques for manufacturing of ceramic fiber. Few important techniques are described below.

CVD technique:
CVD is a very common primary technique to produce ceramic fibers. Many nonoxide fibers employed in CMCs, MMCs, and intermetallic composites (IMC) have been manufactured via CVD. This process is based on the deposition of a material’s vapor phase on a core substrate that is in the form of a monofilament.

Melt spinning technique:
In the traditional melt-spinning route, the precursor materials (e.g., oxides like Al2O3) are melted and subjected to spinning process. During spinning, the oxides in the molten state are forced throughout the nozzles at relatively high pressure and solidified by cooling. The produced fibers generally exhibit amorphous structure due to the high cooling rates that lead to the instability. The rapid viscosity change owing to the tremendous temperature variation causes the lack of diameter control. In addition to that, a number of parameters such as spinning speed, draw ratio, temperature, and environmental circumstances considerably affect the structures of fabricated fibers.

Slurry spinning:
This technique has been developed to avoid the handicaps of melt-spinning method. Both oxide and nonoxide fibers can be manufactured via extrusion (spinning) of ceramic slurries. In this process, the slurry is composed of three basic components: alumina particulates (either in solid or in aqueous form), suspension of alumina precursor (e.g., aluminum chlorohydroxide), and organic polymer. The precursor used in this process promotes the densification during sintering.

Chemical conversion:
The chemical conversion/processing approach is based on the conversion of ceramic/inorganic precursor fibers into a different composition via chemical reactions by using an external compound. In this technique, the precursor fiber is subjected to the (atomic) deposition of the external compound that diffuses through the fiber surface.

After the successful reaction of components, the final fiber exhibits the composition of desired chemical content. The most common example of this process is the conversion of carbon precursor into the SiC fiber by utilizing Si or SiO agents (carbide-forming material) in the vapor phase.

Sol-gel process:
In the traditional processing techniques, the manufacturing of high-purity fibers and various compositions of mixed ceramic oxide filaments is relatively difficult. Particularly, the liquid immiscibility at the melting temperature and phase separation/ crystallization during cooling are the main problems in those techniques. The sol-gel method is generally used to produce oxide-based ceramic fibers. Theoretically, the “sol” is composed of either colloidal particles, which may be crystalline or amorphous, or polymers in a solvent. The “gel” includes a three-dimensional (3-D) continuous network that covers a liquid phase, and in a colloidal gel, the stacking of particles leads to the formation of this network. In a typical sol-gel route, low-molecular-weight metal alkoxides, which dissolve in a liquid, are used as the starting materials.

The resultant properties of fibers (e.g., physical, mechanical, electric, and optical) can be modified with the addition of different oxides and metal components. The final diameter of the continuous filaments produced by sol-gel method is generally in the range of 10–20 μm where the elastic modulus of the products changes from 150 to 373 GPa.

Uses / Application of Ceramic Fiber:
The use of ceramic fibers in the composite applications is taking attraction/attention since the last decades. In particular, continuous ceramic fibers/filaments are generally employed in high-temperature applications instead of metals due to their high thermal tolerance and corrosion resistance.

From the point of view of industrial implementation, ceramic fiber reinforced composites are utilized in many different commercial products such as aircraft engine components (turbine combustors, compressors, and exhaust nozzles), automotive and gas turbine elements, aerospace missiles, heat exchangers, hot gas filters, rocket nozzles, gasket, and wrapping insulations. Ceramic fiber is used in a ballistic vest, bulletproof jacket or bullet- resistant vest. Metal or ceramic plates can be used with a soft vest. High-tech ceramic is used in watch making for producing watch cases.

High-temperature door seals and linings for furnaces, mainly. It is also used for safety curtains in theatres, cable and pole protection, expansion joint seals and high-temperature filtration systems. In the automotive industry, it is used to insulate catalytic converters, brake pads, airbags, clutch facings and seat belt controls. In the aerospace industry, ceramic fibers are used for the tiles on space shuttles and heat shields on other spacecraft and aeroplanes. Ceramic threads are coated with Teflon so that sewing threads can be used on ceramic fabrics. In the home, ceramic fibers insulate toasters, deep fat fryers, self-cleaning ovens and boilers. Ceramic fiber products can be used for high temperature electrical insulation such as fire doors, fire curtain, fire blanket, spark pad and insulation cover.

Oxide and nonoxide ceramic fibers are being used as reinforcement materials for composites due to their unique properties of high elastic modulus and high-temperature durability. Their properties make them valuable to use in automotive, aerospace, and heat-resistant structural applications.

References:

  1. Fiber Technology for Fiber-Reinforced Composites Edited by Ozgur Seydibeyoglu, Amar Mohanty, Manjusri Misra
  2. High-Performance fibres by J W S Hearle
  3. Fibres to Fabrics by Bev Ashford
  4. Handbook of Textile fibre Structure | Volume 2: Natural, Regenerated, Inorganic and Specialist fibers Edited by S.J. Eichhorn, J.W.S. Hearle, M. Jaffe and T. Kikutani
  5. Ceramic Fibers and Their Applications By Toshihiro Ishikawa
  6. https://www.textileblog.com/ceramic-fiber-properties-production-and-applications/

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