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Abrasives are natural or artificial substances of great hardness used in mechanical processing. Their use is known since ancient times as evidenced by the emery deposits of the Greek island of Naxos and the pumice deposits of the Aeolian Islands. The characteristics that distinguish an abrasive are the high hardness, very low fragility and crystalline nature. They have countless uses for countless materials, depending on which changes the support, some applications are: sharpening, cutting, abrasive soaps, abrasive pastes, etc.
The best known natural abrasives are quartz, corundum, silica, pumice, sandstone, diamond, emery, kieselguhr, garnet. Among the artificial ones there are aluminum oxides, chromium oxides, iron oxides, boron azide, silicon carbide, glass, boron carbide.
Known since ancient times (sandstone and pumice stone used to sharpen weapons and tools), abrasives are now widely used in industry and natural abrasives have been joined by artificial ones. In abrasives it is very important to determine the abrasive capacity based on criteria of hardness, toughness, chemical inertia and in some cases thermal conductivity.
Hardness is defined by the Mohs scale and in technical terms by the Kneep scale. Toughness is defined as the ability to resist applied forces during use. Generally, it is determined by grinding the abrasive: the greater the amount of grinding caused, the lower the toughness; this characteristic can often be modified chemically, for example by adding 3% titanium oxide to an alumina-based abrasive, with the result of transforming a brittle material into a tough one.
The chemical inertia is a very important factor because the high temperatures that are reached during the abrasion favor possible chemical reactions between abrasives and material being processed. The thermal conductivity is decisive because an abrasive that is not very conductive heats up very little during abrasion and the agglomerating substance does not carbonize, so the abrasive does not deform or break.
The natural abrasives that have industrial interest are: diamond, one of the hardest substances, used for drilling chisels, for grinding wheels and as a means of reactivating grinding wheels; emery, a mixture of alumina, magnetite and hematite, used for grinding wheels and as an additive to cements for the construction of non-slip floors; pumice, used in lithography and for the preparation of metal surfaces; quartz, used for the preparation of metal surfaces and as an additive for abrasive soaps; apatite, calcite, tripoli, and talc, used for degreasing and cleaning surfaces.
Artificial abrasives include several products among which the most important are: silicon carbide (carborundum act for grinding ferrous and nonferrous materials; tungsten carbide, act for cutting plastics and rubbers; aluminum oxide, whose toughness can be chemically increased and which is produced in the form of small spherulites by sintering finely ground bauxite; Synthetic diamonds; boron carbide, in the powdered state or shaped into grinding wheels, used for machining very hard materials such as tungsten carbide or tantalum carbide; for polishing stainless steel, platinum and chrome surfaces, Cr2O3∤Fe2O3 is used.
Abrasives to be used must be embedded in an agglomerating mass and then processed so as to give them the most suitable shape to do the job (grinding wheel, skid, etc.). The binders used for agglomeration are mostly ceramic or glass, but they are also based on plastic resins and rubbers.
The most important characteristic of abrasives is hardness and there are various methods to measure it. The oldest is represented by the Mohs scale, easy to apply and specific for minerals: it consists of a succession of 10 mineral species where the one that follows is able to scratch the mineral that precedes it. This scale is approximate and non-linear so other hardness scales have been introduced, including the Knoop Scale, which expresses the measure of hardness in kg/mm2 and is particularly suitable for brittle and very hard materials.
The measurement of hardness is performed by means of durometers, an instrument that presses with a certain pressure a diamond tip (in order not to be deformable) in the material whose hardness is sought. The numerical ratio between the applied load (weight in kg) and the maximum section of the engraving (length in mm) produces the hardness value (kg/mm2).
Another important factor to consider is the chemical nature, as it characterizes the behavior of the abrasive depending on the contact material. As we are in working conditions with high temperature and kinetic energy, all endothermic chemical reactions are favored. An example is the reaction that takes place at the contact of silicon carbide with iron: A SiC + 4Fe → FeSi + Fe3C
Furthermore, both iron and silicon carbide are oxidizable in normal atmosphere. Therefore, besides hardness, it is necessary to consider the chemical nature of the abrasive, so, referring to the case previously mentioned, silicon carbide is not used for ferrous materials but it is excellent for glass. On the contrary, alumina is not suitable for glass grinding but is excellent for iron.
Always referring to alumina, the oxygen contained in the atmosphere helps in grinding operations; in fact, the formation of iron oxide prevents the detached chips from welding to the metal or to the abrasive itself; on the contrary, inert gases such as argon, nitrogen and carbon dioxide hinder abrasion. In general, sulfur and chlorinated compounds have an antioxidant action against metals and therefore used in abrasive processes for the latter.
Toughness or agglomeration
Another very important factor is the mechanical resistance or toughness of an abrasive, in those obtained synthetically this parameter is variable without excessively altering the chemical composition of the abrasive itself; in the case of alumina it can be made tougher by adding titanium dioxide or zirconium. On the contrary for the natural ones it is necessary to search for another one or deposits with different compositions. The mechanical resistance is a factor not to be underestimated, because during the grinding operations the abrasive grains can be flattened or rounded, making the abrasive less effective. In order to avoid this, a hardness such as to allow the breaking of the abrasive granules with the consequent creation of new edges is sought, finally the detachment of the granules now exhausted is sought to allow the emergence of new ones still intact and suitable for processing; in the case of flexible media with powder abrasives this is also entrusted to the glues or resins that hold the abrasive to the media.
From this we deduce that during the abrasive operations heat and chips/powders are produced from the abrasive and the abraded material. Lubricants are used to reduce the heat, which promotes chemical reactions, and to remove the chips produced. The oldest is water but it is now rarely used in purity and only for the processing of glass, ceramics and plastics, for other processes are used mixtures of water and antioxidant additives or mixtures of water / oil or oil mixtures. The most effective oils are the oils of mineral origin or animal but often use mixtures of them depending on operational needs and costs. In addition, in case of processing of metals are used organic chlorinated or sulfur additives to prevent reheating of metal particles removed.
Finally, for the processing of soft materials, lubricants with waxes and solid greases are used, which prevent clogging of the abrasive, i.e. the covering of the abrasive surface by the abraded material to form a layer that prevents contact between the abrasive granules and the material being processed.
A final but no less important influencing factor is the grit of an abrasive, that is, the average diameter of its particles or grains. The grain size of an abrasive is classified using an international scale in which each value on the scale corresponds to a certain average value of the grains and the number of meshes per linear inch of the sieve used to screen the grains. For extremely fine grains (< 50 µm), the water sedimentation method is used. In this international scale, the grain size value is inversely proportional to the average grain diameter, i.e. a high grain size value corresponds to a finer grain diameter.
Grit affects the fineness of machining and the roughness of the surface, since these parameters are governed by the operating speed (in the case of a grinding wheel, rotational speed) and the grain size; a higher grain size (smaller grain diameter) corresponds to a lower roughness and greater fineness, as does a higher operating speed.
The roughness of a surface or degree of finish is determined with the profilometer or roughness meter which measures the deviation of the points of the actual surface compared to an ideal smooth surface, expressed as root mean square (RMS). Therefore, a higher fineness corresponds to a lower profilometer value.