Attività

Abrasive Material

Blasting is carried out with abrasive materials such as chilled iron grit, steel or aluminium oxide grit. Sand or other substances containing free silica must not be used, as anyone exposed to dust from it could develop silicosis.76

In a factory, blasting operations should be carried out in a suitable enclosure or room to protect other personnel from injury and nearby machinery from damage. The ‘blast room’ should be provided with an efficient system of exhaust ventilation, preferably of the down-draught type. During the blasting operation super abrasive material rebounds from the surface of the article with a high velocity. Consequently the operator must be given special protective clothing such as gloves, apron and leggings. A helmet supplied with fresh air at a positive pressure is also necessary to protect the blaster from both flying particles and harmful dust.82

Because of the friction between the finely divided particles of grit and the blasting hose and nozzle, discharges of static electricity occasionally take place. It is advisable to earth the blasting hose and nozzle.

The sand blasting technique is based on blasting an abrasive material in granular, powdered or other form through a nozzle at very high speed and pressure onto specific areas of the garment surface to give the desired abraded look. A straighter surface and less effect can be obtained with the sand blasting process than with the sanding process, and sand blasting can be done in less time. For this reason, it is more advantageous in terms of costs. However, silicon grains that are located in the sand can cause silicosis disease. The sand blasting process is now prohibited in most countries because of its negative effect on human health (Suglobal Tekstil, 2013; Paul and Naik, 1997a; Paul and Pardeshi, 2003).

Very hard surfaces can be studied by the abrasion of the surface with a sheet of abrasive material, such as silicon carbide or carborundum paper. At this point a number of different methods may be used to analyze the abraded material. In essence, any solid sampling technique that is capable of handling fine powders—KBr pellet, diffuse reflectance, ATR, photoacoustic, etc.—may be used to study the material. An interesting variant is to use diffuse reflectance to study the abrasive (see the reference to the silicon carbide method in Section 4) for the residual material.

Materials used as abrasives include both natural minerals and synthetic products. Abrasive materials can be considered as cutting tools with geometrically unspecified cutting edges that are characterized by high hardness, sharp edges, and good cutting ability. The sharpness of abrasive grains may be described in terms of edge radius and apex angle. As grain size increases, the percentage of sharp apex angles decreases, indicating a deterioration of grain cutting ability. In addition, cutting ability depends on specific features such as grain structure and cleavage, which are connected with the ability of cutting grains to regenerate new sharp cutting edges and points.

The choice of abrasive for a particular application may be based on durability tests involving impact strength, fatigue compression strength, dynamic friability, and resistance to spalling which occurs under the influence of single or cyclic thermal stress.

The abrasives industry is largely based on five abrasive materials; three are considered to be conventional abrasives, namely silicon carbide (SiC), aluminum oxide (alumina, Al2O3), and garnet. The other two, namely diamond and cubic boron nitride (CBN), are termed superabrasives.

A primary requirement of a good abrasive flow is that it should be very hard; but hardness is not the only requirement of an abrasive. The requirements of a good abrasive are discussed below. The decision to employ a particular abrasive will be based on various criteria relating to workpiece material, specified geometry, and removal conditions.

Cutting fluids should be used wherever possible in grinding to achieve high material removal rates coupled with low wear of the grinding wheel.

Mechanical polishing is done on a rotating disk covered with a felt, and is sprayed with a very fine abrasive materials (e.g., aluminum oxide or magnesium oxide). Because of, not creating grooves on the samples, brush or the felt that is employed here, should be thick and have no hard particles especially dust. At the entire period of polishing, it is necessary to smear the felt with suspended aluminum dust in water frequently. Grading of the aluminum oxide particles is different and we usually polish hard materials such as steel and cast iron with coarser aluminum powder rather than the soft metals like aluminum and lead. The alumina is in two forms: allotropic alpha (hexagonal) and gamma (cubic). The polishing properties of these two forms are different, so that alpha alumina acts faster in abrasion of metals and is more suitable for rough polishing, while gamma alumina prepares a polished surface with the high quality, so it is apt for the final polishing. Sometimes, it is utilized magnesium oxide for polishing aluminum and its alloys. This substance absorbs the carbonic gas of the air and produces carbonate. Therefore, aluminum oxide is usually used in the metallurgy laboratory. A disk with a velvet coating and diamond powder is being used to obtain a mostly polished surface and devoid of any grooves. Often, it is better to use an appropriate lubricant such as alcohol for synthetic diamond paste, to remaining free or rising up the power of cutting powder of diamond particles, and also to increase the life of velvet coating.

In our day-to-day life, few manufactured products in their production process escape a finishing and/or grinding operation involving abrasive materials. Whatever their origin, abrasive minerals or abrasive tools, their economic weight is huge; more than 10 B€ for the overall abrasives and around 2B€ for the abrasives minerals markets. The other way to assess the importance of such products is the number of patents published regularly; more than 50,000 per decade since the year 2000 (Nadolny, 2014). However, these patents are firstly, mainly related to abrasive tools and secondly, focused on super abrasives – around 10,000 patents per decade these last decades – as well as microcrystalline abrasive materials using sintered abrasive ceramic grains.

For scientists the difficulty will be to evaluate what will be the final performance of their abrasives grains in the numerous end user applications, knowing that such performance is a 50/50 combination between the abrasive grain and the abrasive tool; a common approach in the industrial abrasives community.

Although abrasive minerals are never studied from a detailed academic point of view, for industrial scientists there are big challenges which require scientific knowledge across a broad range of materials science such as crystallography, high temperature chemistry, surface chemistry, tribology, technical and advanced ceramic processing etc. This is what will be seen in the following part of this chapter which is based upon industrial experience of the author.Powder blasting is not selective in terms of chemistry; hence, it will etch any material with similar elastic properties at the same rate. Unlike chemical etching (which is chemically selective), powder blasting will continue to etch through substrate materials if the mechanical properties are comparable to those of the film. For example, a silicon substrate will continue to be etched once the ceramic film has been patterned. Due to the high etch rates, it is often not possible to prevent the underlying substrate from being attacked. It is also important to note that thin layers of ductile material will not resist removal if the underlying material is brittle, as any particle impacts will cause the underlying material to fracture, thereby lifting the thin metal film off. This can be seen where thin metallic electrode layers (100–200 nm) are removed when etching through a ceramic film.

It is possible to achieve a degree of selectivity by choosing the appropriate blasting media. The rate of material removal is a function of the difference in hardness between the blasting media and the target material, as well as the kinetic energy of the blasting media. Very hard blasting media with high kinetic energy (i.e. large mass and high speed) will exhibit high removal rates. For example, very hard blasting material such as alumina will remove the majority of materials, while softer materials, such as cornstarch, show much lower material removal rates. The contrast between the two media can be seen in Figure 5.4, where both alumina and cornstarch etch through the green ceramic layer, but the cornstarch is unable to etch the underlying silicon.The powder blasting etch process is relatively anisotropic in nature as the abrasive media is directed in one direction. Some sideways etching does still occur, as the abrasive is not all traveling in one direction due to the divergent nature of the spray and random deflections of the etching media. The edge of the mask may also be deformed or abraded by the etchant, resulting in further degradation of the sidewalls.

These effects can give rise to different etch profiles as the etch area decreases in size. When relatively large-sized holes are etched, the etch front is relatively uniform with only slight edge effects. When the size of the etch hole decreases, the edge effects are extenuated and the etch front becomes less uniform. This is mainly caused by etching media being deflected off the edges of the hole and hitting the base of the depression resulting in a pronounced ‘U’-shaped profile. As the feature size decreases still further, the ability of the abrasive powder to reach the material at the base of the depression decreases significantly, leading to the cessation of the etch process. This is the limit of the achievable resolution and is related to the size of the abrasive particle; the resolution is between 5 and 10 times the particle size. Smaller particles can be used to overcome this issue, but their lower mass means that the kinetic energy available for etching also decreases unless the speed of particles is increased. At a critical point, there will be insufficient kinetic energy for etching to occur.

Ice blasting is a simple, nonabrasive, cleaning process that uses ice crystals as impact medium for removing surface contaminants without the use of chemicals, abrasive materials, high temperatures, or steam. The technology employs ordinary tap water, compressed air and electricity to create an environmentally friendly, cost-effective method for surface cleaning. Ice as a phase change blast medium has the ability to change its physical state to liquid water which flushes contaminants from the surface, leaving no solid residue after blasting. The technique can be used for cleaning surfaces, removing paint, or stripping contaminants from a surface. It can also be used to remove loose material, blips, and burrs from metal components after machining, and even softer materials, such as organic polymeric materials, including plastic and rubber components, can be processed. Compared to other blast cleaning processes, ice blasting does not accumulate ice particles in the waste and thus significantly reduces secondary waste. Applications of ice blasting range from precision cleaning of semiconductor wafers and delicate items, such as books and antiques, to removal of contaminants on a variety of substrates. Ice blasting has been effective in removing paint coatings from a variety of substrates without substrate damage, including delicate surfaces such as Kevlar and graphite-epoxy composites, as well as removing paints, grease, oil, grime, and other contaminants from a variety of glass surfaces, including windows, gauge panels, and controls.
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