Post 169 – by Gautam Shah
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.
- 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.
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.
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.
- 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.
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.
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).
- 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.
- 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.
- 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.
Following are some of the earlier posts on related Topics