The materials that play a cutting role in grinding, lapping and polishing are collectively called abrasives. The earliest abrasives used by people are all natural abrasives. With the deeper research on abrasives, people found that almost all natural abrasives contain alumina components. After trying to synthesize them, it was found that man-made alumina abrasives had better performance than natural ones.

Development history of alumina abrasives

The development history of alumina abrasives

In 1981, the United States 3M company first introduced a new type of abrasive called “Cubitron”, with a very uniform microcrystalline structure, its toughness is 2.3 times higher than ordinary alumina, the grinding capacity is based on the different workpiece, higher than ordinary alumina 1 ~ 3 times, easy to maintain the sharpness of the cutting edge. After that, the abrasive named SG developed by Norton Company came into people’s view.

Characteristics of ceramic alumina abrasives (SG abrasives)

α-Al2O3 is a alumina structure, which has the most stable structure among all the phases of aluminum oxide and belongs to the tripartite crystal system. Compared with ordinary alumina abrasives, ceramic alumina abrasives have more outstanding characteristics.

Physical properties of some abrasives

Fine grain size and uniform structure. Ceramic alumina abrasive because of its sol-gel process, the preparation process, the distribution of raw materials is uniform, the reaction is complete, less impurities, the required sintering temperature is low, so the resulting abrasive microstructure is uniform, fine grain size.

Good toughness, long service life. Ceramic alumina abrasive toughness is significantly higher than ordinary fused alumina abrasive, is 1.5~2 times of the latter, grinding heat is small, low grinding temperature, wheel shape retention is good, high durability, easy to obtain high precision and size and shape consistency when applied to precision grinding.

Good self-sharpening, high grinding efficiency. Ceramic alumina abrasive can constantly expose new micro cutting edge in the cutting process, and the abrasive grain is always in the state of sharpness, so it can maintain stable grinding performance, and it can be used for large depth of cut, large feed, and heavy load grinding. Less frequent dressing of the grinding wheel, in order to effectively improve the production efficiency and greatly reduce the production cost.

Good versatility and high cost performance. It can be used for dry grinding and wet grinding, cooled by water or oil, and can be used for grinding ferrous or non-ferrous metals, which makes up for the shortcomings of CBN and diamond in grinding, and the price is much lower than both of them, and the maintenance is easy, and it does not need special equipment, and the frequency of maintenance is less, so it is easy to realize the automated production.

Ceramic alumina abrasive precursor synthesis process

Ceramic alumina abrasives are generally obtained by drying, granulating and sintering the precursor of aluminum oxide such as aluminum hydroxide or proposed thin hydrotalcite. Among them, the synthesis methods of precursors mainly include solid-phase method and liquid-phase method.

Liquid-phase method has become one of the most widely used ceramic alumina abrasive preparation methods due to a series of advantages such as simple preparation process, low energy consumption and low environmental pollution, among which the sol-gel method is most favored by researchers.

For the first time the sol-gel process applied to alumina abrasive synthesis process of the United States 3M company in the patent US4314827 introduced the technology: the use of Al2O3-H2O micropowder (Boehmite) for aluminum raw materials, the first raw materials and water mixed into a suspension, and then added to the gum solvent (eg, HNO3, HCl, or acetic acid solution, etc.) to make it a Stable hydrosol; then add modifier to promote its gelation, modifier is generally selected metal oxides or its salt solution (such as MgO, ZnO, ZrO2, TiO2, etc.), the formation of the gel drying and curing, and then crushing granulation into the required shape and size of the particles, calcined (volumetric shrinkage rate of 20 ~ 40%) to become the abrasive.

In addition, according to the type of raw materials used can be sol-gel method can be divided into organic metal salt sol-gel method and inorganic salt sol-gel method. The outstanding advantage of using metal alcohol salt sol-gel method to prepare alumina materials is that it is easy to realize doping, and the prepared nanomaterials have good uniformity and high purity. However, the use of metal-alcohol salts as raw materials makes the synthesis process costly. Meanwhile, the alcohol salt gelation process is slow and the synthesis period is long, which is not conducive to the realization of industrial production. And inorganic salt as raw material sol-gel technology, on the one hand, the price of raw materials is lower, less harmful to the human body, on the other hand, the preparation process is simple, the requirements of the equipment is not as high as the metal alcohol salt, the reaction can be carried out at room temperature, which greatly reduces the cost of production and facilitates the promotion of, but at the same time, the purity is lower, the stability of sol-gel is poorer, and it is often necessary to join the additives to improve its stability.

Sintering of ceramic alumina abrasive

Sintering is the most important part of the preparation of ceramic alumina abrasives. Selecting a reasonable sintering system is not only related to the effectiveness of the previous product preparation process, but also has an important impact on the microstructure and density of the abrasive. This in turn affects the performance of the product, such as grindability, strength, toughness and hardness. Ceramic alumina abrasive sintering mainly solid atmospheric pressure sintering, hot pressure sintering, hot isostatic pressure sintering, microwave sintering, discharge plasma sintering and so on.

Solid state atmospheric pressure sintering process

Atmospheric pressure sintering process is carried out in the absence of external driving force, the sintering driving force mainly from the ceramic powder surface free energy changes, that is, the powder surface energy decreases, the reduction of surface area. Because of the low sintering driving force, so the atmospheric pressure sintering rate is low, it is difficult to reach the theoretical density. Usually atmospheric pressure solid-phase sintering of ceramic alumina abrasive grain size is larger, the sintering temperature is higher, the sintering cycle is long, energy consumption.

Hot press sintering process

Hot pressure sintering is heated at high temperatures in the powder at the same time applying unidirectional axial stress, so that the densification of the sintered body mainly rely on the role of the applied pressure and the completion of the migration of substances. Hot pressing sintering can be divided into vacuum hot pressing sintering, atmosphere hot pressing sintering and continuous hot pressing sintering. Hot press sintering can reduce the sintering temperature, inhibit grain growth, but due to unidirectional pressure, resulting in uneven distribution of stress in the billet, the final density distribution in the sintered body is not uniform, and hot press sintering equipment is expensive, high cost.

Isostatic sintering process

The basic principle of hot isostatic pressing is that the sintered material is first encapsulated in the glass, and then in the heating process to apply the balanced gas pressure of each phase, with the help of high temperature and high pressure together to make the material densification, so that the material microstructure is more uniform. Hot isostatic sintering can obtain complex shapes of products. However, hot isostatic sintering requires encapsulation or pre-sintering of the billet, and the pressure conditions are relatively harsh, so the actual operation is difficult.

Microwave sintering process

Microwave sintering is the use of microwave interaction with the medium, due to dielectric loss and make the surface of the ceramic blank and the internal heating of a sintering method. Compared with the traditional sintering method, microwave sintering has the advantages of internal heating, rapid sintering, refining the material organization, improving the material properties and high efficiency and energy saving. It is a promising sintering method for nanoceramic materials. However, the microwave sintering method can not be used for the sintering of all ceramic materials, because some ceramic materials themselves are not suitable for microwave sintering, as well as microwave sintering of abrasive samples are prone to yield bending and cracking, and temperature uniformity in the sintering process needs to be improved. The reaction mechanism of microwave sintering of ceramic alumina abrasives needs to be further studied in depth.

Plasma Discharge Sintering

Plasma discharge sintering, also known as electric field-assisted sintering, is a pressure-assisted plasma rapid sintering technique that utilizes pulsed electron discharges to instantaneously generate high temperatures between powder particles. It is characterized by the following features: through the strict control of temperature, the pulsed electronic heating generated by the localized plasma between the powder particles does not lead to grain coarsening, and the integrity of the initial structure can be maintained; the synergistic use of electrical energy and mechanical pressure can rapidly integrate nanoparticles without affecting the initial particle size of the powder; the pulsed-discharge plasma generated between the powder particles helps to remove the surface oxidative layer, increase the surface activity, and promote sintering.

Summary

The ceramic alumina abrasives were prepared using a sol-gel process, which theoretically ensures the homogeneity of the chemical composition of the system. Moreover, the sintering process is different from the traditional molten alumina abrasives, which on the other hand ensures that the abrasive grains are sintered polycrystalline and the sintering temperature is lowered, thus improving the toughness under the premise of unchanged hardness. Since the preparation process of ceramic alumina abrasives is different from that of conventional alumina abrasives, it results in a different microstructure from that of ordinary aluminum oxide abrasives. Therefore, ceramic alumina abrasives show many characteristics different from ordinary alumina-based abrasives in terms of grinding performance, with the advantages of high strength, high toughness, good self-sharpening, high grinding efficiency, long service life and so on. The emergence of ceramic alumina abrasives has been favored by the mechanical, automotive and aerospace industries, filling the gap between ordinary abrasives and super-hard abrasives, and is regarded as a revolutionary new generation of alumina-based abrasives with broad prospects for development.

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