Ceramic matrix composites
Ceramics are inorganic non-metallic materials with a heterogeneous structure. They consist of glass, pores and minerals of different compositions. Material based ceramics have high strength at elevated temperatures and are resistant to oxidation. Their main disadvantage is their brittleness.
- Advantages of ceramic composites (compared with metals)
- low density
- low coefficient of thermal expansion
- higher hardness
- higher temperature and corrosion resistance and even wear resistance
- higher melting point and the stability of mechanical properties in a wide temperature range
- Disadvantages of ceramic composites
- the final product of ceramics cannot be subsequently modified by any thermal or mechanical processing
- low fracture toughness
- difficult reproducibility of complex shapes
- difficult bonding of ceramics with each other and with other materials as well
- Ceramic matrix composite requirements
- to increase the toughness of the ceramic matrix
- to even the strength – especially tensile strength, to suppress the dependence of the ceramic matrix strength on the type of stress and the dimensions of the components
- to increase the abrasion resistance in extreme
- to increase heat resistance in extreme conditions
- to improve the machinability of the ceramic matrix
Particulate ceramic matrix composites
For the reinforcement of the ceramic matrix there are suitable only synthetically prepared ceramic powders based on oxides, nitrides, or carbides respectively borides. Matrices have a similar base; oxides, carbides, nitrides, borides, or graphite as well.
These include the composites systems:
Ceramics – ceramics
Ceramics – metal
Composites based on graphite
Ceramics – ceramics
In ceramics – the ceramic system there is a secondary ceramic phase dispersed in the base of a ceramic matrix. For example in the Si3N4 matrix there are dispersed particles of SiC or B4C. Both phases shall contribute to the improvement of properties for use on the component in the course of applications in aircraft and rocket technology.
Ceramics – metal
These systems are generally referred to as cermets. They are oxide or carbide-based (Al2O3, WC, TiC). As cermet is considered a composite material with the content of a ceramic component of more than 50 %, the rest is metal.
Their properties are a combination of both materials, i.e. good thermal and electrical conductivity, heat resistance, chemical stability. The main areas of their use are the rocketry and aviation industry, electronics, nuclear engineering, mechanical engineering and metallurgy.
Oxide-based cermets (Al2O3-Cr, Al2O3-Fe, Al2O3-W) are used for cutting tools, the thermocouple sleeves etc.
Carbide-based cermets (WC-Co) are used for components with a very high surface hardness, valves, nozzles, accurate gauges.
The systems of ceramics – metal and composites based on graphite are prepared via processes such as sintering or skeletal type of systems where the matrix and the secondary phase are mechanically penetrated and create a so-called skeletal structure.
Composites based on graphite are represented by a graphite skeleton impregnated with metal. Currently, it is the most widely used composite. They have very good mechanical properties and are chemically resistant. They are used for sliding bearings, sealing rings, (the slides of pantographs) on trolleybuses, trams or trains.
Fiber ceramic matrix composites
Ceramic matrices are brittle, have poor fracture toughness, but on the other hand, they have excellent high temperature properties and chemical resistance. To increase the fracture toughness there are fibers with a higher modulus of elasticity than that of ceramics introduced into the ceramic matrix. Fibers that are used are metal (tungsten, molybdenum), ceramic (SiC) and carbon. The strongest types are metallic and ceramic whiskers.
When using whiskers, the matrix is formed by ceramics based on Al2O3 or ZrO2 and dispersion is formed by whiskers, mostly SiC. There are newly used also Al2O3 whiskers. The composite has high tensile and compressive strength at both normal and high temperatures.
Whiskers increase resistance to creep and erosion, and reduce the coefficient of friction.
Short fibers are prepared for example from SiC, Si-Ti-O-C, Al2O3, Si3N4 and other materials. They are used to increase the fracture toughness and reduce the creep of conventional ceramic matrices from oxide or non-oxide ceramics.
Carbon or graphite fibers can be incorporated into a matrix of a suitable polymer which is subsequently graphitized. A basic semi-finished product with a plastic matrix is produced by conventional methods for the preparation of PMC and further graphitized. Graphitization is a relatively difficult process because fibers must not be degraded during the course. These composites are used for example in the most demanding parts of space shuttles (the leading edge). A carbon fiber must be (also) often protected against oxidation by SiC coating.
Ceramic coatings are applied in difficult operating stress, in an aggressive environment and while working at increased or high temperatures. They provide higher wear resistance, avoid fatigue failure and also serve as an effective heat and corrosion barrier. Commonly used technological processes for their preparation include CVD and thermal spraying.
Typical compounds – nitrides (TiN, Si3N4), carbides (TiC, WC, and SiC) and oxides (Al2O3, TiO2, ZrO2, and MgO).
Glass matrix composites
As a substrate for glass matrix composites there is used in most cases flat glass, which is either surface treated, layered or the composition of the glass is modified from which it arises.
- Composites substrate production Flat glass drawing – Fourcault method
Molten glass is a melt of silica sand, dolomite, limestone, soda and other components. Its specific composition can be modified as needed. The types of used glass can be found in the Chapter 3.2 Composites production, fibers production. The principle of this method is based on melting the charge in the melting aggregate, a fire-clay discharge is lowered into the bath of molten glass, where its slot creates the base of a glass sheet due to buoyancy. A sheet of glass is constantly drawn in a shaft, which serves as a lehr (for cooling), the glass strip is pulled upwardly and subsequently cut off according to the requirements.
Float glass production – Float method
The molten glass of the same composition is melted from a batch in the melting aggregate similarly as in the previous case. The principle of this method is then based on molten glass which is gradually passed from the melting furnace into the area of shaping, which takes place on the surface of the molten tin. Tin guarantees the quality surface of the bottom side of the strip-shaped flat glass, the top layer is in contact with the furnace atmosphere only, and its quality depends on the surface tension of the molten glass. After passing through the cooling furnace the glass strip is cut and modified as required.
Particulate composites with a glass matrix
- Turbid glass
- opaline glass
- opal glass
- opaque glass
There are dispersed foreign particles in the glass (crystalline, vitreous, and gaseous). These particles have a different refractive index from other glass, they scatter light and the isotropy of the glass changes. Particles cause turbidity whose level depends on the difference between refractive indexes, on the size of the particles and their quantity.
A slight turbidity provides opaline glasses (phosphates) a yellow, blue. More turbidity give opal glasses (fluorides) milk glass. Non-transparent or opaque glasses are produced by the exclusion of borides from the molten glass (larger particles), in which they are initially dissolved.
This type of glass is primarily used for aesthetic purposes-chandeliers, furniture glass, ovenproof glass (Pyrex) or jewellery.
- Coloured glass
- anti-sun glass
These are generally sun glasses and welding goggles-eye protection, colour filters or signal glass in transportation. The homogeneous ionic colours are ions of transition elements and rare earth elements that provide colouration independently of the composition of the glass. Heterogeneous colloidal colours are the respective oxides and their compounds, metal particles of Cu, Ag, Au, Pt, which when introduced into the glass mixture reduce and cause colouration.
This glass is used for eye protection glasses and goggles having iron oxides as the active colouring ingredient , which absorbs UV (Fe3+) and IČ (Fe2+) radiation, but other colouring oxides (Mn, Bi, Cr, Ti) are also introduced.
These glasses absorb 50-90 % of the light in the visible area. Welding goggles should have a green tint due to the high content of iron oxide; the blue tint is caused by Fe3+ and Cu2+ ions for lower temperatures.
It is used for eyeglasses and instrument glass in measurements (dosimetry). Photochemically active glass (Ag halides) becomes for example darker when exposed to a visible light, because it does not allow the full range of colours to pass through.
This glass is used for the windows of vehicles, the glass is coloured by iron oxides with a predominance of Fe2+ and captures the thermal component of the solar radiation.
Partially crystalline glass
Sitalls or glass ceramics are glass-formed materials which can be easily forced to partially crystallize. Therefore the basic glass matrix creates a dispersion from microcrystals of certain ingredients. The volume amount of the dispersion is usually 40-50 % and the size of crystals is in tenths of micrometers.
There are three basic methods for preparing these composites:
- The heat treatment of the glass – glass is heated for long time to a temperature above the devitrification point (the temperature at which transition of amorphous structure to a crystalline structure by diffusion of individual particles occurs, without melting) so that the formation of small crystals inside the glass emerge.
- The controlled cooling of the glass – the molten glass is cooled so slowly that during cooling it partially crystallizes.
- Glass frit sintering (glass powder) – where the frit partially crystallizes
The most frequent types of composites contain, besides the conventional nitrogen in glass, also Li2O which makes the spodumene (LiAlSi2O6) preferentially crystallize and it has a negative coefficient of thermal expansion, so the entire composite has a thermal expansion close to zero. With a content of about 80% of crystals the composite can withstand thermal shocks up to 800 K and still transmit over 80% of IR radiation. This type of composite is often used for the glass-ceramic cooktop of kitchen stoves. Similar composites are due to a minimum dimension change of temperature and are also used for astronomical mirrors.
Dispersion glass with ceramics
A particle composite can be obtained by adding particles of a substance with a higher melting point into the molten glass.
A composite with a Pyrex matrix and about 30% of the dispersion of either particles of yttrium-stabilized ZrO2 or Al2O3 platelets is used for example in fuel cells for the direct conversion of thermal energy into electrical energy.
A composite with a matrix of borosilicate glass with a dispersion of 55% of the oriented mica platelets (e.g. MACOR by FiberOptic) is a highly versatile material. Because of its low brittleness, excellent electrical insulation and applicability up to 1000°C, it is used in a high-vacuum technology, satellites and medical science as well. Despite the glass matrix and the particle dispersion it is relatively easily machinable.
Fiber composites with a glass matrix
As a replacement of special types of steel in many applications, the composite of a mat of continuous fibers, in which the molten glass is infiltrated, can be used. When borosilicate glass is used, the composite is applicable at temperatures up to 500 °C; silicon carbide or graphite fibers can be used. A composite with graphite fibers in the plane of the fibers is electrically conductive and has high thermal conductivity.
Superkanthal This is a branded composite with a matrix of pure silica glass with a dispersion of 80 % of MoSi2 particles, oxidation resistance of this material is almost equal to the platinum, yet it has considerable electrical conductivity and thermal shock resistance. It is stable up to 1400 °C, and is therefore an ideal resistive material for high-temperature applications.