A particle has no long dimension. Particle composites consist of particles of one material dispersed in a matrix of a second material. Particles may have any shape or size, but are generally spherical, ellipsoidal, polyhedral, or irregular in shape. They may be added to a liquid matrix that later solidifies; grown in place by a reaction such as agehardening; or they may be pressed together and then inter-diffused via a powder process. The particles may be treated to be made compatible with the matrix, or they may be incorporated without such treatment. Particles are most often used to extend the strength or other properties of inexpensive materials by the addition of other materials.
Portland cement is an example of a large particle reinforced composite. Dispersion strengthened composites have much smaller particle sizes whose interactions with the matrix can be seen at the molecular level. These particle-matrix interactions at the molecular level increase the overall strength of the composite.
Particle reinforcing in composites is a less effective means of strengthening than fibre reinforcement. Particulate reinforced composites achieve gains in stiffness primarily, but also can achieve increases in strength and toughness. In all cases the improvements are less than would be achieved in a fibre reinforced composite.
Particulate reinforced composites find applications where high levels of wear resistance are required such as road surfaces. The hardness of cement is increased significantly by adding gravel as a reinforcing filler.
The principal advantage of particle reinforced composites is their low cost and ease of production and forming.
There is a great need for materials with special properties with emergence of new technologies. However, conventional engineering materials are unable to meet this requirement of special properties like high strength and low density materials for aircraft applications. Thus, emerged new class of engineering materials – composites. Unfortunately, there is no widely accepted definition for a composite material. For the purpose of this module, the following definition is adopted: any multiphase material that is artificially made and exhibits a significant proportion of the properties of the constituent phases. The constituent phases of a composite are usually of macro sized portions, differ in form and chemical composition and essentially insoluble in each other. Composites are, thus, made by combining two distinct engineering materials in most cases; one is called matrix that is continuous and surrounds the other phase – dispersed phase. The properties of composites are a function of the properties of the constituent phases, their relative amounts, and size-and-shape of dispersed phase. Millions of combinations of materials are possible and thus so number of composite materials. For ease of recognition, composite materials are classified based on different criteria like: (1) type of matrix material – metal matrix composites, polymer matrix composites and ceramic matrix composites (2) size-and-shape of dispersed phase – particle-reinforced composites, fiber-reinforced composites and structural composites. It is understandable that properties of composite materials are nothing but improved version of properties of matrix materials due to presence of dispersed phase. However, engineers need to understand the mechanics involved in achieving the better properties. Hence the following sections highlight the mechanics of composites, which depend on size-and shape of dispersed phase.
This class of composites is most widely used composites mainly because they are widely available and cheap. They are again two kinds: dispersion-strengthened and particulate- reinforced composites. These two classes are distinguishable based upon strengthening mechanism – dispersion-strengthened composites and particulate composites. In dispersion-strengthened composites, particles are comparatively smaller, and are of 0.01-0.1μm in size. Here the strengthening occurs at atomic/molecular level i.e. mechanism of strengthening is similar to that for precipitation hardening in metals where matrix bears the major portion of an applied load, while dispersoids hinder/impede the motion of dislocations. Examples: thoria (ThO2) dispersed Ni-alloys (TD Ni-alloys) with high-temperature strength; SAP (sintered aluminium powder) – where aluminium matrix is dispersed with extremely small flakes of alumina (Al2O3).
Particulate composites are other class of particle-reinforced composites. These contain large amounts of comparatively coarse particles. These composites are designed to produce unusual combinations of properties rather than to improve the strength.
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