UNDERSTANDING CERAMICS

Post 181 – by Gautam Shah 

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Enamel ware Bath Tub Clawfoot_bathtub

A ceramic may be defined as a product manufactured by the heat treatment to a material or mix of materials. Ceramics are generally inorganic and non metallic solids in nature, however, small but appreciably affective metal compounds may be present in them. Ceramics are mainly crystalline, but could also be partly crystalline structured or like a glass -amorphous (non-crystalline) in nature.

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Ceramic – Enamelled Hospital-ware

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Ceramics are produced for many different uses. Ceramic products such as pottery, bricks etc. are formed or shaped before firing, while other ceramic products such as cement and glass are formed after firing. Some ceramics are used in their manufactured form, while others are crushed or powdered before use. Pozzolana is a natural ceramic powder, produced by the lava activity. Cements are ceramic products, where initially a ceramic called clinker is produced and then powdered. Surkhi is a burnt clay product, used to replace cement in low strength structures. Ceramics like bricks, roofing tiles, flooring tiles, cooking pots etc. can be produced cheaply at all geographic locations, from all sorts of clays. Earthenware ceramics made with very plastic or wet material are very porous. Bricks, water pots, planters etc. are conceived for this quality.

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Terracotta Roofing tiles

Ceramic materials are used in toilet products such as the water closets, bath tubs, wash basins, soil pipes, etc. These items have vitreous and a non-porous surface with good gloss and excellent abrasion resistance. Similarly household tableware and oven-ware in addition have good thermal shock resistance and low thermal expansion.

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Most ceramics are dielectric and except for ferrite, generally lack magnetic properties. Ceramics products’ properties like better thermal insulation, fire resistance, electric insulation and very high density make them the ideal material in electric, electronics and super conductivity field.

SiSiC

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Ceramics unlike metals do not oxidize and are fairly stable in extreme environments, so are used for storage of alkaline and acidic substances in industries. Ceramics withstand extremely high operating temperatures, with high radiation resistance, which make them suitable for engine and turbine parts. High abrasion resistance and the capacity to withstand high temperature are two prime qualities, why ceramics are used for saturated steam and super heated gas nozzles and burner ends.

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Refractory products have superior resistance to high temperatures and thermal shock, and are used in applications such as furnace linings. Ceramics’ creep resistance or ability to withstand high stress at elevated temperature is better then any material yet produced and its applications include turbine wheels, jet engines, dies etc.

Si3N4bearings

Speciality ceramics are much harder than metals, and can easily cut steel and glass. Speciality ceramics because of their extremely fine-gained poly crystalline micro structures are almost free of residual pores and defects. Such materials are extremely hard and cannot easily be machined or cut, except by a laser. Toughened zirconium ceramics are used in the home as non magnetic scissors and kitchen knives, cutting tool bits, cutter edges, etc. Silicon Nitride ceramic’s engine parts can run more efficiently than do parts made of nickel based super alloys. Engine parts including combustion chamber walls, cylinder liners and heads, piston crowns and intake / exhaust parts can be made of ceramic. Ceramic bearings can operate at high speed without lubricants.

Tantalum_capacitors

Capacitors -high-end ceramics for electric and electronics field

Body implants of ceramics in orthopaedic surgery have superior wear and erosion resistant characteristics compared with other materials. Ceramic implants for human body show not only high strength to weight ratio, but where required, these ceramics can be deliberately made porous, enabling regenerating bone to grow into and bond with the implant.

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Body implant -Ceramic teeth

Ceramic air tiles consisting of an open cellular micro structure of extremely fine coated silica fibres, so loosely packed, that the tiles consist of 95% air, as lightweight as cotton wool, are used in rocketry. The ceramic tiles not only withstand temperature of 1500 C (above melting point of steel) but insulate the space module within. Micro-porous ceramics’ candles are used for filtration.

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Ceramic products show various levels of glassification and depending on the raw materials used and the process of crystallization. Higher glassification levels impart better density, low porosity and in most cases better homogeneity.

Chapter Dogdae Jar Republic Of Korea Traditional

Earthenware are soft porous ceramic products fired at low temperatures (950 -1050 °C) Stoneware fired at higher temperatures (1100 -1300 °C ), show fusing of the clay body, and are fairly dense and non-porous. Porcelains are almost glass like bodies, fairly homogeneous, dense and non-porous (fired at 1200 -1,400 °C). Glass (1400 -1650 °C ) is a formation of non crystalline solid, referred to as super-cooled liquid.

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FILLERS and COMPOSITES

Post 169 – by Gautam Shah

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Forms of Glass fiber used as Fillers

Fillers are inevitable constituents of composites. Fillers provide body, reinforcement, and impart special properties to the new material. Fillers have many forms, such as fine particulates, staple fibre (whiskers or short fibres), filaments (long or continuous fibres), unwoven (felt) and woven fabrics, knit textiles, aggregates, and sheets. Filler materials are natural (wood, plant, hair), minerals (asbestos, sand, stones, powders), and man-made (polymers, metals, ceramics) materials.

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  • Straw, hair, coir, hemp, jute, papyrus, rice-husk etc., have been mixed with clay to form bricks. Sand, ash, and mineral dust were added to mud to reduce the plasticity for plaster work. Wood planks were glued together to form block board or plywood like construction in 15th C BC.
  • Man-made materials include: Fiberglass, quartz, Kevlar, Dyneema or carbon fibre, graphite, carbon-graphite, silicon carbide, titanium carbide, aluminium oxide, boron, coated boron, boron carbide, alumina, alumina-silica, niobium-titanium, niobium-tin, etc.

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Fillers Particles (of 10 t0 250 μm in diameter) help block the movement of dislocations in the composites and provide distinctive strength properties. Staple fibres used as fillers have high length to a diameter ratio, and are generally in their random orientations, whereas filaments are used for high performance structural applications and are prearranged (for a particular structural use) before introduction of matrix, or in certain cases a fixing compound.

Depending on the load conditions, fibre arrangements for reinforcement are random, unidirectional (aligned in a single direction), or multi-directional (oriented in two or three directions). Continuous fibres are more efficient at resisting loads than are short ones, but it is more difficult to fabricate complex shapes from materials containing continuous fibres, than from short-fibre or particle-reinforced materials.

Insulator_railways

Ceramic-Metal composite -Electric Insulator

Particles (fillers) of one material are dispersed in another material (matrix) in many different ways. 1 -Particles are mixed in a liquid phase of the matrix, and allowed to harden to a solid phase, 2 -particles are allowed to grow in the matrix, or 3 -articles are pressed into the matrix and inter-diffusion is encouraged by mechanical working or other energy input.

Particulate fillers in ceramic matrices enhance characteristics such as electrical conductivity, thermal conductivity, thermal expansion, and hardness. Particles of Alumina, Silicon carbide and Boron nitride embedded in a polymer matrix and formed as abrading tools are used for grinding and polishing stone floors, tools etc. Carbon black (as powder) added to vulcanised rubber provides hardness and toughness for automobile tyres. The rubber is further reinforced with metal, rayon, polyester and other threads as continuous fibre filler.

Silver_mica_capacitors

High end Ceramic composites

  • High-performance ceramic composites are strengthened with filaments that are bundled into yarns. Each yarn, strand or tow may contain thousands of filaments, each of which with a diameter of approximately 10 micrometers (0.01 millimetres).

Often components are formed that are strong in all directions, by creating a three-dimensional lattice of filler component. The filler component itself could be a composite material.

 

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Fillers affect the quality of a composite. Fillers are usually combined with ductile matrix materials, such as metals and polymers, to make them stiffer. Fillers are added to brittle-matrix materials like ceramics to increase toughness. The length-to diameter ratio of the fibre, the strength of the bond between the fibre and the matrix, and the amounts of fibre are variables that affect the mechanical properties. It is important to have a high length-to-diameter aspect ratio so that the applied load is effectively transferred from the matrix to the fibre.

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A variety of reinforcements can be used, including particles, whiskers (very fine single crystals), discontinuous fibres (short), continuous fibres, and textiles preform (made by braiding, weaving, or knitting fibres together in specified designs).

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  • Glass is the most common and inexpensive fibre and is usually use for the reinforcement of polymer matrices. Glass has a high tensile strength and fairly low density (2.5 g/cc).
  • Carbon-graphite: In advance composites, carbon fibres are the material of choice. Carbon is a very light element, with a density of about 2.3 g/cc and its stiffness is considerable higher than glass. Carbon fibres can have up to 3 times the stiffness of steel and up to 15 times the strength of construction steel. The graphitic structure is preferred to the diamond-like crystalline forms for making carbon fibre because the graphietic structure is made of densely packed hexagonal layers, stacked in a lamellar style. This structure results in mechanical and thermal properties are highly anisotropic and this gives component designers the ability to control the strength and stiffness of components by varying the orientation of the fibre.
  • Polymers: A variety of polymer materials are used as filler material for composites. The strong covalent bonds of polymers offer tailor-made properties in the form of bristles, whiskers, staple fibres, filaments, yarns or tows, spun yarns, threads, ropes, unwoven and woven fabrics, knitted compositions. Nylons, polyesters, rayon, acrylic, Kevlar and many other fibres are used for composite formation.

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  • Ceramics: Ceramic fibres made from materials such as Alumina and Silicon carbides are used in very high temperature applications, and also where environmental attacks are severe. Tungsten-boron filaments, Ceramics have poor properties in tension and shear, so most applications as reinforcement are in the particulate form.

  • Abrasives_-_Contour_Grinding
  • Metallic fibres: Metallic fibres have high strengths but since their density is very high they are of little use in weight critical applications. Drawing very thin metallic fibres (less than 100 microns) is also very expensive.

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Following are some of the earlier posts on related Topics

1   MATRIX and COMPOSITES

2   COMPOSITES Part – 1

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