When forming metal alloys metal bonds are applied, in polymeric materials covalent bonds are applied and in ceramic systems ionic or ion-covalent bonds are applied. Formation of composite materials is based on a mutual synergy effect (i.e. cooperation) of surfaces of the involved phases (matrices, reinforcement) at their interface.
An important precondition for good connections of the connected phases is their mutual adhesion. In order to stabilize their connection the reinforcement matrix must be wetted. Wetting may be applied by physical or chemical sorption, or also by a chemical reaction of the matrix with the surface of the reinforcement.
The last case is undesirable because it leads either to a distortion of the reinforcement or secondly to the formation of a brittle interphase, which deteriorates the properties of the composite.
To be a perfectly compact composite the matrix must well wet the reinforcement. This is achieved if the energy of the free surface of the reinforcement is high and the surface energy of the matrix is minimized as possible.
Wettability is defined by a so-called contact angle θ (wetting angle), which originates at the interface between a liquid and gas with the surface of the solid phase. A good wettability condition is met by a low contact angle, i.e. the value of cos θ is close to one. Otherwise it is poor wettability. The easiest way to prepare composites is having the matrix in the liquid state.
Classification of composites
According to the matrix material
- Metal Matrix Composites (reinforced metals, cermets, alloys) – MMC
- Polymer Matrix Composites based on macromolecular substances (reinforced polymers) – PMC
- Ceramic Matrix Composites (ceramic and other inorganic composites: ceramics, glass, carbon) – CMC
According to the structure or geometry of the reinforcement dispersive composites – contain very fine particles, dispersion
- particulate composites (particulate, granular) – contain larger particles of regular shapes (spheres, platelets) or irregular shapes
- fibrous composites – contain long or short fibers that may be oriented or unoriented, be of various origins (glass, carbon, polymeric, textile, etc.)
In practice, composites are classified according to the geometric shape on a particulate and fiber composites.
Particulate composites :
Particles as a reinforcing phase are dispersed in a matrix and do not own an aggregate structure. They may have a spherical, lamellar, rod or irregular shape where one dimension of such a reinforcement unit does not significantly exceed the dimensions of others. A metal, polymeric, or ceramic matrix is used. The most often used particles are: oxides, nitrides, carbides and borides.
Particles in the composite limit the development of the plastic deformation in the matrix material, and participate in the transmission of stress, but to a much lesser extent than the fibers in the fiberous composites.
Particle reinforcement (filler) is used to improve the properties of matrix materials, for example to modify thermal and electrical conductivity and the behaviour at high temperatures, to reduce friction, increase wear resistance, improve machinability, increase hardness, and reduce contraction.
Generally speaking, from the mechanical point of view, the particles in the matrix of the composite cause strengthening
Dispersive strengthening is typicalprimarily for the metal matrix composites – such a composite has a greater tensile modulus, higher thermal stability and reduced contraction.
Granular strengthening is based on separate particles of different sizes and theirdifferent volume share in the matrix. Optimum strengthening occurs at approximately the same particle size and their even distribution.
Such composites are used particularly to obtain specific combinations of performance and not only to increase their strength. A concrete system with a crushed aggregate can be given as an example of granular reinforcement.
Fibrous composites :
For fiberous composites the units of reinforcement (fibers) in one direction are significantly larger than in other directions.
Synergistic interaction (synergy) between solid and rigid fibers with a ductile or brittle matrix allows for the construction of composites with high strength, stiffness and toughness. These composites have the widest range of application.
Basic Types of Composites :
Generally (speaking) almost every material used today is a composite. Materials in a pure form are almost unknown. A strict definition determining what exactly is or is not a composite does not existing, it is always a matter of perspective and criteria for assessing the material. Composites can be divided into two basic groups:
- materials with a paint or finished surface
- layered (laminated) materials
- associated materials (sandwiches)
- fiber reinforced systems
- granular systems (large particles)
- penetrated systems
Properties defining a composite :
- the characteristics of the phases – mechanical properties, isotropy and anisotropy of phases,
- the volume of the phases – the geometry and arrangement of phases in the system
- the interaction with a surrounding environment
- the history of a material – it should be taken into account how phase composites were created, material aging.
Structural classification of composites:
The composite as a whole is formed by a superstructure. The structure of the individual phases, which are contained in the composite is called an infrastructure. The term microstructure is designed to describe the structure of the individual substances that are contained in each phase of the composite.
Specifically cement concrete. This means the structural arrangement of cement concrete as a system is described by a superstructure, which is formed by the infrastructure of the binder, liquid phase (pores) and fillers. The binder is composed of a variety of minerals and each mineral has its own microstructure. The same is obviously true for every grain of the filler. The third infrastructure (pores) may be formed by a gas or liquid.
Cement concrete = superstructure – the main determining factor of the composite, which tells us how many distinct phases are there in the system, how are they arranged, etc.
Aggregate, binder and pores = infrastructure speaks about the shapes of individual phases, their sizes, and how they touch.
Minerals in the binders, liquids and gases in the pores = microstructure says for example, from what the molecules of individual minerals are in phases, what is the nature of those minerals.
- Composites according to the number of stages in a superstructure
- single-structured – systems with one matrix and one or more added phases
- two or multi-structured – systems with two or more matrices, where each maycontain one or more added stages
A multistructure usually provides the significantly better properties of composites.
- Composites in accordance with the condition of an interphase boundary
- conjugated systems – by chemical and physical forces at the interface the materialremains rigid even under external forces; by the phase that reinforces the system are those further divided as:
o granular – axial dimensions of the particles are not very different (aggregate) o fibrillar – where there is one dimension prevailing (fiber)
o lamellar – one dimension is suppressed compared to the other two (plate)
· disjugated systems – bonds between phases are so weak that the system has onlylimited consistency (loose materials, earth/soil) – not a composite
Superstructure evaluation according to the reinforcement condition
segregated reinforcement – reinforcement particles are not in direct contact, they donot create their own infrastructure, reinforcement is a discontinuous phase
aggregated reinforcement– the individual particles are in direct contact, they cancreate their own infrastructure – they are somehow arranged, reinforcement is a continuous phase
Dividing composites according to a superstructure
Composites Type I – the space they occupy is completely filled (without the presence of a liquid phase)
Composites Type II · composites Type III
Composites Type I
A system that does not contain pores. This system is composed of matrices and fillers and their mutual relationship is that the filler particles are segregated, i.e. they do not touch. It is a system where there is not too much of a filler. An example of composite type I is a polymeric matrix with added particles such as ash, without pores. A second common example is the glass matrix composite, non-porous again.
Two-component system: Vk = Vm + Vf
Composites Type II
A system, that is already three-phase, consisting of a matrix, filler and pores. In such a system there are little pores, less matrix, and it contains mostly filler. As the filler takes up most of the whole system, it is aggregated. And because the system contains little pores, they are segregated. An example of composite type II is concrete.
Three-component system: Vk = Vm + Vf + Vv
Composites Type III
This system is again a three-phase, therefore it contains a matrix, filler and pores. In this case it is again a little matrix, but a lot of filler and pores. Both the filler and the pores are aggregated. These pores are even and continuous. If the pores are completely continuous, the system is inconsistent. It follows that it is a material with good insulating properties, such as plasterboard.
Three-component system: Vk = Vm + Vf + Vv
Principles and material selection problems:
The correct choice of material for a given application is extremely important and in the process of a new product development it is the key.
The correct choice of material affects the utility properties of the future product – whether it will perform the function for which it was designed.
Inappropriately or less suitably selected material can significantly be reflected as:
- technological problems during production
- increase in the cost of production and thus in the price of the final product
- a negative impact on the environment.
Choice of material
The choice of material is a complex process where it is necessary to consider such factors as:
- materials costs
- production cost
- energy and raw material demands (material intensity)
- the possible impact of material choice on the environment – production and consumer cycle
- the possibility of material recycling
The relationship of material, technology and product
The function of the product (its components), structure (its shape), material and technology interact and therefore the choice of material for the product cannot be carried out independently of the technology.
Parameters and requirements during designing the product
- the selection of individual (most suitable) components with regard to their future performance
- determining the compatibility of components – each phase in the composite must maintain its positive qualities, ingredients (components) must not damage each other
- finding an appropriate geometrical form for each phase – stronger parts to be elongated (fibers, strips, belts etc.), while the weaker phase should be wrapping the stronger one and bring individual fibers together into a single structure
- phases in the composite should be distributed so that they can work together
- conditions in which the future composite will function – temperature, abrasion, etc.
The less similar associated phases are, the more difficult it is integrating them into a whole. In the opposite case the result is a composite with excellent cooperation between these phases, and therefore with excellent properties.
Production of fiber composites
The main problem is to insert the fibers into the matrix avoiding mechanical damage and to maintain the uniform distribution, directionality and coherence of fibers. Fibers may be relatively easy to break (mechanically) during each manipulation, especially non-metallic fibers (glass, ceramics) are sensitive as their strength depends greatly on the surface integrity. With fiber bundles we need to ensure wetting of each fiber, e.g. by means of wetting agents.
Production of particulate composites
Production of particulate composites is considerably simpler, grains are not as sensitive and therefore intensive manual mixing can be used. It is necessary to ensure complete coating of all grains by a matrix, force out the air and ensure maximum homogeneity of a mixture, and further to prevent particle sedimentation, while the viscosity of the matrix is important (as well). In some cases, sedimentation is used especially when we want to obtain a material with a graduated percentage of filler.
The basis is selection in accordance to the desired/required mechanical, physical and chemical properties. It is necessary to insure contact of the two components, therefore:
- the matrix must wet the reinforcing phase
- the matrix must withstand a surrounding often aggressive environment
- it must have the ability to deform under load
- it must restrict the development of cracks
Composites production, fibers production
During production it is necessary to ensure the following conditions in particular:
- the distribution of the reinforcing phase in the volume is even
- the possibility of putting(in) layers with a random orientation of fibers
- a good connection of reinforcement and/with a matrix
- the possibility of changing the volume of the reinforcement
- the possibility of heat treatment after production
- the simplicity and effectiveness of production
Methods of composite production
The procedure for the production of composites is divided according to the state of the matrices during its application on the frame in the following way:
- production of composites with a matrix in the solid state
- production of composites with a matrix in the liquid state (melts, solution)
- Production of composites with a matrix in the solid state
- pressing/press forming and hot rolling
- plasma injection
- powder methods
Pressing or hot-rolling are mainly used for producing composites with metal matrices, single layer or multilayer.
Pressing / press forming
While pressing, the reinforcement (of fibers) is placed between the metal foils and inserted between heated press boards, where due to the action of pressure and temperature both components connect diffusively. Pressing takes place in a vacuum or in a protective (modified) atmosphere. By the compression of several single-layer parts of the multi-layer composite are formed.
The fibers are inserted between two metal foils which pass between the pressure rollers at elevated temperatures.
Plasma injections are used for the production of composites with a metal or ceramic matrix. Reinforcing fibers are covered in layers by plasma spraying and the resulting layers are subsequently connected by pressing (ceramic matrix), or hot forming in a vacuum (a metal matrix).
This method is applied in the production of composites with metal, ceramic and polymeric matrices. The base is the application of the powder metal, ceramic or polymer matrix onto reinforced fibers (by electrostatic forces, in water suspension, etc.), compaction into a shape and then the subsequent sintering at high pressures and temperatures. Whiskers (metallic or ceramic) may be mixed into a powder matrix. The most important application of a powder method is in the production of composites with ceramic matrices. It is the basis for the production of composites with a skeletal matrix.
Production of composites with a matrix in the liquid state
The methods of producing composites with a liquid matrix are the most commonly used. The basic condition for producing and obtaining the optimal properties of composites are:
- good wettability of reinforcement by a matrix material
- minimum development of chemical reactions at the interface of the matrix – reinforcement
Production/manufacturing methods – the most commonly used and that is in connection with the technology–forming,casting, injection molding etc.
- impregnation of the reinforcement-matrix liquid
- infiltration-gas phase
Impregnation of the reinforcement by a matrix in the liquid state
It is used in the production of composites based on thermoset (polymers), thermoplastic and metal matrices. The principle is based on (comprises) the saturation of a shaped reinforcement (fibers, fabrics, mats) with a corresponding liquid matrix in the form and subsequent solidification (thermosets by hardening, thermoplastics by solvent evaporation and metals by crystallization).
At the gas-phase infiltration the reinforcement (of continuous fibers) is saturated by vapours arising from the thermal decomposition of chemical compounds – so-called matrix precursor. There is a condensation of saturated vapour on the reinforcement (fibers) and a solid phase forms.
Casting technology (e.g. pressure) is used mainly in the manufacture of metalcomposites with short fibers (possibly with whiskers) or polymer composites.
Forming technologies are suitable for the production of shaped parts, profiles, panels,etc. The oldest technology is a hot pressing. More recent technologies include extrusion and injection moulding.
Production of glass fibers
The base of glass fibers is carbon SiO2. In a glass furnace, fresh molten glass is produced from a batch, or rather glass beads or frits are melted. The glass is then drawn (becoming fibrous) by nozzles located in the bottom of the furnace.
The fiber diameter is typically 1 mm, but it can be regulated by stretching the glass stream flowing from the furnace. The final diameter of the fiber is given by the difference between the flow speed of molten glass and the speed of extracting the individual fibers.
After removal from the furnaces, monofilaments are surface-treated with lubricants, grouped into strands and wound onto a coil. Before use, they are usually spun using one of textile technologies into the desired shape.
Production of graphite and carbon fiber
Graphite and carbon fiber are mainly made of fibers of PAN (polyacrylonitrile fibers), and from novoloid fibers (phenol-aldehyde fibers). The production of PAN fibers has three basic steps:
- stabilization – 200-300°C,an oxidizing atmosphere, and the fiber is undermechanical stress, it will cross-link macromolecules with oxygen bridges; fiber turns black and becomes infusible.
- carbonization – 1200-1500°C,an inert atmosphere N2, the decomposition ofmacromolecules, hydrogen removal, the reduction of oxygen and nitrogen, 80-95% mass is carbon; fiber achieves the highest strength
- graphitization – 1800-3000°C, aninert atmosphere of N2 and Ar, a further increaseof the carbon content, the similar conversion of the recrystallization of a given graphite is taking place; the increase of fiber stiffness; due to the growth of the graphite crystals and the increase of defectiveness, fiber strength decreases
Carbon fibers are protected against abrasion (they are more fragile than glass, and in order to reduce adsorption of the gases on their surfaces) by using polymeric coatings. It is also necessary to increase surface reactivity towards binding agents, of the matrix and therefore it is necessary to roughen the surface of the fibers by etching for example.
Production of boron fibers
Boron fibers are prepared by the CVD method (Chemical Vapor Deposition). This method is based on reducing the gas mixture (H2 a BCl3) that is fed into the reactor due to high temperature.
The substrate is a thin tungsten filament. Boron trichloride is reduced with a hydrogen forming elemental boron, which is deposited on the substrate surface.
Production of SiC fibers
Continuous SiC fibers are produced, like boron fibers using the CVD method. The source of silicon and carbon are gaseous alkylsilanes (CH3SiCl3), which are reduced in the reactor with hydrogen. Silicon carbide settles on the carbon substrate.
Short fibers of SiC – whiskers are produced for example from rice hulls, which are annealed in an inert atmosphere, wherein there is the decomposition of organic substances and the conversion of SiO2 present in the shells into SiC.