Composites particulate

The all or nothing feature of metal powder composites is very much a feature of conductive composite systems. In order to understand this behaviour, most theories borrow from percolation theory (Broadbent and Hamersley, 1957), which was originally developed as a model for predicting fluid permeation through porous media. The percolation model is based on having a medium 

Unfortunately, the percolation approach is also not really able to predict accurately the critical volume fraction in real composites, because so many different factors, like filler shape, size, distribution and particle agglomeration, come into play. Lux (1993) has reviewed the various percolation models that have been proposed in theoretical treatments of the problem. 

Two other approaches have been taken to modelling the conductivity of composites, effective medium theories (Landauer, 1978) and computer simulation. In the effective medium approach the properties of the composite are determined by a combination of the properties of the two components. Treating a composite containing spherical inclusions as a series combination of slabs of the component materials leads to the Maxwell-Wagner relations, see Section 3.6.1. Treating the composite as a mixture of spherical particles with a broad size distribution in order to minimise voids leads to the equation  

Computer simulations have also been used to model the properties of composites for specific particle distributions and geometry, e.g. spherical particles with either a bimodal size distribution (Fu et ai, 1999) or an 

The characteristic threshold behaviour of conductive composites is an unsatisfactory feature for many applications, both because the poor mechanical properties incurred at concentrations where conduction is obtained cannot be tolerated, and because the high level of conductivity above the threshold is not required or even not desirable. Thus a common requirement is for an antistatic material which has good plastics properties, sufficient conductivity to allow charges to leak away, and sufficient resistivity to prevent dangerous shocks to personnel who may become accidentally connected to mains electrical supplies through it. Unfortunately, the very steep slope of the conductivity versus filler concentration curve in the region of the threshold makes it very difficult to manufacture materials with reliable intermediate conductivities. 

This chapter aims to describe the principles behind the processing, microstructural development and properties of particulate ceramic composites and to illustrate these using experimental results. The main emphasis is on examples where the addition of particulates to a ceramic matrix causes new mechanisms to operate that give an improvement in properties greater than would be expected from a rule of mixtures . The chapter concentrates almost exclusively on structural composites, since this is where most work has been done to date. Particulate nanocomposites are included in the chapter, since the important examples described are currently at the coarse end of the nanoscale , and the principles underpinning their properties seem to be a simple extension of those relevant to the microcomposites with which the rest of the chapter is concerned. 

The next section describes the processing and microstructural development of particulate composites, and is followed by a section on thermal residual stresses. These stresses are often the most obvious consequence of adding second-phase particles to a matrix and can have a profound effect on properties. Factors determining the toughness, strength and wear resistance of particulate composites are then considered in turn, and the chapter concludes with an assessment of possible future developments in this area. 

Unlike fibre- or whisker-reinforced composites, particulate composites have the advantage of being compatible with conventional powder processing, and in many cases can be pressurelessly sintered. As with other ceramic microstructures, a myriad of other ingenious fabrication routes have also been reported, but these are too numerous and system-specific to describe here. This section merely outlines the main points of powder processing where the production of composites in chemically compatible systems (i.e. those in which the components do not react chemically with one another) differs from that of monolithic ceramics. 

The addition of fine particles that are much more refractory than the matrix has a different effect. The outstanding example of this behaviour is in alumina/SiC nanocomposites [9]. The SiC/Al203 interface may have a low 

This striking property of the Al203/SiC interface can be understood in terms of the observation of Ashby and Centamore [14] that the more refractory of two phases at an interface (the covalently bonded SiC in this case) controls the interface reaction because in general atoms in both phases must be involved in the reaction. The majority of the Al203/SiC interfaces in the nanocomposites have been observed to be free of any glassy phase, the presence of which would presumably allow alumina to be removed or deposited at the interface without the involvement of the SiC, and consequently much more rapidly. The introduction of an interfacial layer may be the source of the ability of sintering aids such as Y203 to enable these materials to be pressurelessly sintered [15, 16] (Fig. 4.2). 

A particularly important area of fundamental research and technology hinges upon the control of the adhesion between the polymer matrix and the filler. Completely unbonded particles appear to have the same effect as voids in many types of behaviour. The quality of the interface and its response during deformation and as well as its sensitivity to environmental changes are of major interest. Nelson and Hancock, and Kendall have examined the consequences of interfacial slip in composites and underlined its importance. In general terms this aspect of composite technology has very much in common with the design of structural adhesive Joints and hence the appropriate references of the previous section are of value. 

Perhaps the largest volume commercial production of particle-filled polymers is carbon- or graphite-filled rubbers. Carbon blacks are widely used in natural and synthetic rubbers and convey significant improvements in modulus, abrasion resistance, and tear strength as well as additional thermal and electrical conductivity. Carbon blacks are uniquely efficient in these respects the reasons for this are still the subject of debate. A very recent review by Rigbi is an excellent compilation of current theory and experiment. An earlier review by Medalia  

Pearce, D. H. Richards, and D. Thompson. Synthesis of Flexibilised Epoxy Resins in Adhesion-3 , ed. K. W. Allen, Applied Science Publishers, London, 1978. 

Finally, mention should be made of the wide use and study of particle-filled foams, particularly those based upon polyurethanes.  

However some systems, such 2 TiB2-Zr02, show good 

A wider variety of all non-oxide composites has been made by hot pressing (Table 7) than by sintering. While some of these composites have 

Composite Hot pressing conditions ic Flexure strength MPam Investigator 

Sato et Considerable development was undertaken on composites of HfB2 and especially ZrB2 with SIC + C by hot pressing relatively coarse (10 M v) powders at 1800 at high pressure (840 These composites have been produced commercially. More recently, composites of TaN + ZrB2, with either ZrN or WC have been hot pressed (2100 C for 30 min. with 18 MPa in N2 for wear studles ). 

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Fig. 14. Reinforcement and crack tip kiteractions ki a particulate composite (a) coarse particles ki a strong particle—matrix kiterface, and (b) fine particles ki...

Particulate Composites. In addition to the geometric obstacles to mixing dissimilar shape and size particles, there are also chemical barriers that must be overcome in processing composites, requiring novel techniques. 

Plastic laminated sheets produced in 1913 led to the formation of the Formica Products Company and the commercial introduction, in 1931, of decorative laminates consisting of a urea—formaldehyde surface on an unrefined (kraft) paper core impregnated with phenoHc resin and compressed and heated between poHshed steel platens (8,10). The decorative surface laminates are usually about 1.6 mm thick and bonded to wood (a natural composite), plywood (another laminate), or particle board (a particulate composite). Since 1937, the surface layer of most decorative laminates has been fabricated with melamine—formaldehyde, which can be prepared with mineral fiUers, thus offering improved heat and moisture resistance and allowing a wide range of decorative effects (10,11). 

Particulate Composites. These composites encompass a wide range of materials. As the word particulate suggests, the reinforcing phase is often spherical or at least has dimensions of similar order ia all directions. Examples are concrete, filled polymers (18), soHd rocket propellants, and metal and ceramic particles ia metal matrices (1). 

Sohd rocket propellants represent a very special case of a particulate composite ia which inorganic propellant particles, about 75% by volume, are bound ia an organic matrix such as polyurethane. An essential requirement is that the composite be uniform to promote a steady burning reaction (1). Further examples of particulate composites are those with metal matrices and iaclude cermets, which consist of ceramic particles ia a metal matrix, and dispersion hardened alloys, ia which the particles may be metal oxides or intermetallic compounds with smaller diameters and lower volume fractions than those ia cermets (1). The general nature of particulate reinforcement is such that the resulting composite material is macroscopicaHy isotropic. 

Fig. 19.10. A cermet is a particulate composite of a ceramic (WC) in a metal (Co). A crock in the ceramic is arrested by plasticity in the cobalt.

Concrete is a particulate composite of stone and sand, held together by an adhesive. The adhesive is usually a cement paste (used also as an adhesive to join bricks or stones), but asphalt or even polymers can be used to give special concretes. In this chapter we examine three cement pastes the primitive pozzolana the widespread Portland cement and the newer, and somewhat discredited, high-alumina cement. And we consider the properties of the principal cement-based composite, concrete. The chemistry will be unfamiliar, but it is not difficult. The properties are exactly those expected of a ceramic containing a high density of flaws. 

Fig. 20.5. Concrete is a particulate composite of aggregate (60% by volume) in a matrix of hardened cement paste.

Cheapest of all are the particulate composites. Aggregate plus cement gives concrete, and the composite is cheaper (per unit volume) than the cement itself. Polymers can be filled with sand, silica flour, or glass particles, increasing the stiffness and wear-resistance, and often reducing the price. And one particulate composite, tungsten-carbide particles in cobalt (known as "cemented carbide" or "hard metal"), is the basis of the heavy-duty cutting tool industry. 

We now examine the properties of fibrous and particulate composites and foams in a little more detail. With these materials, more than any other, properties can be designed-in the characteristics of the material itself can be engineered. 

Fig. 25.1. (a) When loaded along the fibre direction the fibres and matrix of a continuous-fibre composite suffer equal strains, (b) When loaded across the fibre direction, the fibres and matrix see roughly equal stress particulate composites ore the some. ( ) A 0-90° laminate has high and low modulus directions a 0-45-90-135° laminate is nearly isotropic. 

Particulate composites are made by blending silica flour, glass beads, even sand into a polymer during processing. 

Particulate composite materials that are composed of particles in a matrix... 

Particulate composite materials consist of particles of one or more materials suspended in a matrix of another material. The particles can be either metallic or nonmetallic as can the matrix. The four possible combinations of these constituents are described in the following paragraphs. 

Numerous multiphase composite materials exhibit more than one characteristic of the various classes, fibrous, laminated, or particulate composite materials, just discussed. For example, reinforced concrete is both particulate (because the concrete is composed of gravel in a cement-paste binder) and fibrous (because of the steel reinforcement). 

The mechanics of materials approach to the micromechanics of material stiffnesses is discussed in Section 3.2. There, simple approximations to the engineering constants E., E2, arid orthotropic material are introduced. In Section 3.3, the elasticity approach to the micromechanics of material stiffnesses is addressed. Bounding techniques, exact solutions, the concept of contiguity, and the Halpin-Tsai approximate equations are all examined. Next, the various approaches to prediction of stiffness are compared in Section 3.4 with experimental data for both particulate composite materials and fiber-reinforced composite materials. Parallel to the study of the micromechanics of material stiffnesses is the micromechanics of material strengths which is introduced in Section 3.5. There, mechanics of materials predictions of tensile and compressive strengths are described. 

Note that the expressions for E., and v.,2 are the generally accepted rule-of-mixtures results. The Halpin-Tsai equations are equally applicable to fiber, ribbon, or particulate composites. For example, Halpin and... 

COMPARISON OF APPROACHES TO STIFFNESS 3.4.1 Particulate Composite Materials... 

The composites have been classified in general under the following heads particulate composites, fibrous composites, and laminated composites [1]. 

Particulate composites consist of particles dispersed in a matrix. These particles are divided into two classes, skeletal and flakes. The first one consists of continuous skeletal structures filled with one or more additional materials. Flakes consist generally of flat flakes oriented parallel to each other. These particles may have any... 

By the term particulate composites we are referring to composites reinforced with particles having dimensions of the same order of magnitude. Particulate composites are produced from a polymeric matrix, into which a suitable metal powder has been dispersed, and exhibit highly improved mechanical properties, better electrical and thermal conductivity than either phase, lower thermal expansivity, and improved dimensional stability and behaviour at elevated temperatures. 

Fig. la and b. Principal sections of the Hashin two-phase model and its respective three-layer unfolding model for a typical particulate composite... 

Then, for a particulate composite, consisting of a polymeric matrix and an elastic filler, it is possible by the previously described method to evaluate the mechanical and thermal properties, as well as the volume fraction of the mesophase. The mesophase is also expected to exhibit a viscoelastic behaviour. The composite consists, therefore, of three phases, out of which one is elastic and two viscoelastic. 

The epoxy matrix was filled with iron particles of average diameter df = 150 pm at a volume fraction uf = 0.05. The mechanical and thermal behaviour of the particulate composite was studied in Ref. 8), which gave the following values ... 

Fig. 3. (a) Thermal expansion coefficients a for the inclusion (f), matrix (m), mesophase (i) and composite (c) of a typical iron-epoxy particulate composite, with 5 percent volume fraction for the inclusions, versus temperature, (b) the reduced longitudinal expansion of the same elements, normalized to the unit-length versus temperature (diameter of inclusions df = 150 pm)... 

Thus, in the three-layer model, with the intermediate layer having variable physical properties (and perhaps also chemical), subscripts f, i, m and c denote quantities corresponding to the filler, mesophase, matrix and composite respectively. It is easy to establish for the representative volume element (RVE) of a particulate composite, consisting of a cluster of three concentric spheres, that the following relations hold ... 

Lipatov 111 has indicated that the following relation holds between a weight constant X, defining the mesophase volume-fraction Uj, and the jumps of the heat capacity AC1 of the filled-composite and AC of the unfilled polymer for particulate composites ... 

In order now to define the radius r of the spherical layer corresponding to the mesophase, we express it as t = (rf + Ar) and we modify the respective relation given by Lipatov 11 for particulates to the appropriate relation for cylindrical inclusions. For the cases of particulate composites it was shown that the following relation holds ... 

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