Hardness of particles

Earlier, in Chapters 2, 4, and 5, it was pointed out that many foods contain solid particles. Thus the role of the size, shape and hardness of particles in oral perception of texture is of interest. For example, in the confectionary industry the minimum particle size that can be comprehended by the palate is said to be about 25 p,m. Further, particle sizes about 10-15 p,m are considered to be the limit of diminishing effect. On the other hand, in tooth paste, the alumina trihydrate particles with an average diameter 5-20 p,m are used and larger particles are known to contribute to gritty sensation in the mouth (Tyle, 1993). 

Tyle, P. 1993. Effect of size, shape and hardness of particles in oral texture and palatability. Acta Psychologica 84 111-118. 

For protection against toxic gases, activated carbon is supplied in granular form. Hardness of particles is essential to maintain a firmly packed bed in the canister and minimize dusting. Because of the small space available in the canister of a ga mask the adsorptive power must be concentrated in a small volume. The carbon, therefore, should be as dense as is consistent with high intrinsic adsorptive power. A rapid rate of adsorption is essential. 

The previous investigations of hard particle transport processes during laser beam dispersing have shown, that the high speed microfocus radioscopy system is a usable arrangement to observe and analyse the movements, velocities and accelerations of particles inside the molten bath. That possibility was, until now, not given by conventional techniques of process... 

In the MS approximation, for hard core particles of diameter a, one approximates the direct correlation fiinction by... 

The most conunon choice for a reference system is one with hard cores (e.g. hard spheres or hard spheroidal particles) whose equilibrium properties are necessarily independent of temperature. Although exact results are lacking in tluee dimensions, excellent approximations for the free energy and pair correlation fiinctions of hard spheres are now available to make the calculations feasible. 

Despite all the shortcomings listed above, full particle classical MD can be considered mature [84]. Even when all shortcomings will be overcome, we can now clearly delineate the limits for application. These are mainly in the size of the system and the length of the possible simulation. With the rapidly growing cheap computer memory shear size by itself is hardly a limitation several tens of thousands of particles can be handled routinely (for example, we report a simulation of a porin trimer protein embedded in a phospholipid membrane in aqueous environment with almost 70,000 particles [85] see also the contribution of K. Schulten in this symposium) and a million particles could be handled should that be desired. 

Molecular graphics representation ofihe paths generated by 32 hard spherical particles in the solid (left) and ht) phase. (Reproduced from Alder B J and T E Wainwright 1959. Studies in Molecular Dynamics. I. Method. Journal of Chemical Physics. 31. 459-466.)... 

An interesting example of a large specific surface which is wholly external in nature is provided by a dispersed aerosol composed of fine particles free of cracks and fissures. As soon as the aerosol settles out, of course, its particles come into contact with one another and form aggregates but if the particles are spherical, more particularly if the material is hard, the particle-to-particle contacts will be very small in area the interparticulate junctions will then be so weak that many of them will become broken apart during mechanical handling, or be prized open by the film of adsorbate during an adsorption experiment. In favourable cases the flocculated specimen may have so open a structure that it behaves, as far as its adsorptive properties are concerned, as a completely non-porous material. Solids of this kind are of importance because of their relevance to standard adsorption isotherms (cf. Section 2.12) which play a fundamental role in procedures for the evaluation of specific surface area and pore size distribution by adsorption methods. 

Crushing and grinding uses mechanical force to break larger particles into smaller ones. A variety of tools are used depending on the particle s size and hardness. Large particles are crushed using jaw crushers capable of reducing particles to diameters of a few millimeters. Ball mills, disk mills, and mortars and pestles are used to further reduce particle size. 

Magnesium hydroxide is white, has an average particle size of 1—10 p.m, density of 2.36 g/mL, refractive index of 1.58, and Mohs hardness of 2.00. Water loss on ignition is 31.8 wt %. Magnesium hydroxide contains 1.0 wt % Ca(OH)2 and is made by Solem Industries and Morton Thiokol (25). 

Austenitic Nitrocarburizing. This is similar to ferritic nitrocarburizing except that the temperature may extend into the austenite range. The case usually consists of hard carbonitride particles, and quenching to achieve hardening is not required. 

Newer high velocity thermal spray coating processes produce coatings in compression rather than tension because of the shot peening effect of the supersonic particles on impact. This has permitted coating as thick as 12,500 p.m without delamination as compared to older processes limited to 1,250 p.m. The reduced residence time of particles at temperature minimises decomposition of carbides present in conventional d-c plasma. This improves wear and hardness (qv) properties. 

Abrasion, a serious problem in some appHcations, requires the addition of hard-surfacing materials to points exposed to abrasive wear (12). The severity of wear depends on the nature, size, hardness, and shape of particles as well as the frequency of contact, the force exerted against the wearing parts, and sohds loading as related to feed rate and soflds concentration. 

Historically, strontium metal was produced only in very small quantities. Rapid growth of metal production occurred during the late 1980s, however, owing to use as a eutectic modifier in aluminum—silicon casting alloys. The addition of strontium changes the microstmcture of the alloy so that the siUcon is present as a fibrous stmcture, rather than as hard acicular particles. This results in improved ductility and strength in cast aluminum automotive parts such as wheels, intake manifolds, and cylinder heads. 

Wear. Ceramics generally exhibit excellent wear properties. Wear is deterrnined by a ceramic s friction and adhesion behavior, and occurs by two mechanisms adhesive wear and abrasive wear (43). Adhesive wear occurs when interfacial adhesion produces a localized Kj when the body on one side of the interface is moved relative to the other. If the strength of either of the materials is lower than the interfacial shear strength, fracture occurs. Lubricants (see Lubricants and lubrication) minimize adhesion between adj acent surfaces by providing an interlayer that shears easily. Abrasive wear occurs when one material is softer than the other. Particles originating in the harder material are introduced into the interface between the two materials and plow into and remove material from the softer material (52). Hard particles from extrinsic sources can also cause abrasive wear, and wear may occur in both of the materials depending on the hardness of the particle. 

The abrasion resistance of cobalt-base alloys generally depends on the hardness of the carbide phases and/or the metal matrix. For the complex mechanisms of soHd-particle and slurry erosion, however, generalizations cannot be made, although for the soHd-particle erosion, ductihty may be a factor. For hquid-droplet or cavitation erosion the performance of a material is largely dependent on abiUty to absorb the shock (stress) waves without microscopic fracture occurring. In cobalt-base wear alloys, it has been found that carbide volume fraction, hence, bulk hardness, has Httie effect on resistance to Hquid-droplet and cavitation erosion (32). Much more important are the properties of the matrix. 

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