The application of new wear-resistant ceramic materials in industrial equipment and pipelines is becoming increasingly widespread due to their excellent performance. By replacing ordinary metal materials, the service life and continuous production capacity of equipment have been greatly improved. Nowadays, wear-resistant ceramic components in the field of engineering ceramics both domestically and internationally are mostly made of materials such as alumina, zirconia, silicon carbide, and silicon nitride.

Among them, alumina ceramics have good wear resistance, corrosion resistance, mechanical properties, excellent production capacity, and are very affordable. They are quite suitable for industrial applications and have become one of the most commonly used wear-resistant materials in this field. They can be seen everywhere in wild and unrestrained situations such as ore crushing and processing systems, raw material grinding systems, and high-speed cutting.

The Secret of Wear Resistance of Alumina Ceramics

Evans has conducted a systematic study on the factors affecting the wear rate of ceramic materials and found that the hardness and fracture toughness of ceramic materials are the key factors affecting their wear rate, and ceramic materials with high hardness and fracture toughness have lower wear rates. Scholars from various countries have conducted extensive research on improving the hardness and fracture toughness of ceramic materials, which can be analyzed in the following aspects:

① Ceramic grain size

There are two types of alumina ceramic materials: single-phase ceramics and multiphase ceramics (i.e., adding a second phase to the matrix). In the field of research on the correlation between grain size and ceramic tribological properties, researchers mainly investigated the influence of grain size of the matrix phase (or second phase) on ceramic tribological properties.

Roy et al. studied the friction and wear performance of submicron and micrometer scale single-phase alumina ceramics in biological environments and found that the wear rate of submicron ceramics in bovine serum albumin environment was much lower than that of micrometer scale ceramics, and the grain pull-out and grain boundary microcracks of submicron ceramics were significantly less than those of coarse-grained alumina ceramics. Sedlacek et al. investigated the effect of different alumina matrix grain sizes on wear performance, with alumina matrix grain sizes ranging from 0.8 to 4 μ The variation between m and the second phase SiC is at the nanoscale. Research has shown that the wear resistance of alumina matrix in submicron size is better than that of nanocomposite ceramics with micrometer grain size; When the matrix grains are at the submicron scale, there is no significant relationship between wear resistance and fracture toughness, while the wear rate of alumina composite ceramics with a matrix at the micrometer scale decreases with increasing hardness.

Obviously, from the above examples, it can be seen that refining grains can effectively help improve the uniformity of material structure, including increasing material density and reducing material defects.

② Second phase material

In the field of research on the tribological properties of alumina multiphase ceramics, component compounding, which forms composite materials by adding various second phases, particles (or whiskers), is also the main way to improve the tribological (or cutting) properties of alumina ceramics. According to different influencing mechanisms, it can be divided into several types, including the second phase self-lubricating mechanism, the second phase grain boundary strengthening effect, and the second phase frictional chemical reaction mechanism.

● Second phase self-lubricating mechanism. Introducing second phase solid lubricants such as graphite, CaF2, PbWO4, MoS2, BN, and soft metals into the Al2O3 ceramic matrix can effectively reduce the friction coefficient of the material and improve its tribological properties. 10% CaF2 solid lubricant was introduced into the Al2O3/TiC composite ceramic matrix, and CaF2 was extruded and coated onto the friction surface to form a self-lubricating film. The self-lubricating film can effectively prevent the adhesion between the material and the friction pair, reduce the friction coefficient, and play a self-lubricating role.

The second phase grain boundary enhancement effect. Introducing a second phase (mainly particles and whiskers) into the alumina ceramic matrix, utilizing the difference in thermal expansion coefficient between dispersed particles and the matrix material, generates residual stress during the cooling process of material preparation, achieving the effect of strengthening grain boundaries. When cracks propagate along grain boundaries, they not only need to overcome the inherent grain boundary energy of the matrix material, but also the additional energy brought by residual compressive stress, thereby increasing the crack propagation resistance.

On the other hand, due to the smaller thermal expansion coefficient of the second phase particles compared to the matrix, volume effects will occur during material cooling, leading to microcracks around the second phase particles, inducing crack deflection and consuming more energy for crack propagation; In addition, the second phase particles are generally approximately spherical in shape, which passivates the crack tip, reducing stress concentration and preventing crack propagation, thereby improving the frictional properties of the material.

The second phase frictional chemical reaction mechanism. The mechanism of the second phase frictional chemical reaction refers to the chemical reaction between the second phase doped in the Al2O3 matrix and the gas in the air (mainly oxygen) or with the friction pair material during friction, producing a lubricating film and reducing the friction coefficient of the material, thereby improving the frictional properties of the material.

Introducing TiB2 particles into Al2O3 ceramic matrix to prepare Al2O3/TiB2 composite ceramic cutting tools, it was found during cutting experiments with 45 # quenched steel that when the cutting speed is greater than 120m/min, that is, the cutting temperature is greater than 800 ℃, TiB2 in Al2O3/TiB2 composite ceramic cutting tools reacts with oxygen to generate TiO2 and B2O3. Due to the much lower elastic modulus and hardness of TiO2 compared to the matrix material, the shear strength of the tool is reduced, resulting in a decrease in the friction coefficient of the material, reducing the adhesive wear of the tool, and improving the wear resistance of the tool.   

③ Tribological mechanism

Under different application scenarios, the frictional mechanisms exhibited by alumina ceramics are actually different, so different strengthening methods should be combined to tailor the treatment accordingly.

The wear mechanism of Al2O3/TiB2 ceramic cutting tools during low-speed dry cutting is characterized by adhesive wear and abrasive wear; In high-speed dry cutting, the wear mechanism of the tool is manifested as oxidation wear. The reaction film generated on the tool surface through frictional chemical reactions plays a solid lubrication role, improving the wear resistance of the tool. With the increase of TiB2 content and cutting speed, the anti friction and anti-wear effect of the reaction film is enhanced.

The tribological properties of Al2O3 based ceramic tool materials are related to the type of additives, and their wear resistance is in descending order: Al2O3/SiCw, Al2O3/Ti (C, N), and Al2O3/TiC; The frictional properties of the material are related to its hardness (H), elastic modulus (E), and fracture toughness (KIC). The wear rate W increases with the increase of E/H and decreases with the increase of KIC; The wear mechanism of Al2O3/TiC ceramic tool materials is mainly adhesive wear, while the wear mechanism of Al2O3/Ti (C, N) and Al2O3/SiCw ceramic tool materials is mainly abrasive wear.

The application of alumina wear-resistant ceramics

① Application in material and coal transportation systems

The equipment wear of coal and material transportation systems is mainly caused by impact force and friction, with the wear parts being separator baffles, tees, and coal chute. These parts are extremely prone to wear and even wear out. When power plants and cement plants use lined manganese steel plates, the usage time is generally about 6 months, and they are prone to coal sticking and powder blockage; Although using ultra-high molecular weight polyethylene sheets is not easy to block, their impact resistance and wear resistance are not as good as manganese steel plates, especially when there are coal powder and cement particles squeezed into and falling off at the joint between the lining plate and the steel plate. The use of alumina wear-resistant ceramics will greatly improve the service life, bring significant improvements to safety production and economic benefits.

② Application in the milling system

The wear of the coal powder production system in coal-fired power plants is mainly caused by coal powder collision and impact wear. The high-speed coal powder airflow causes severe wear and tear on the coal mill outlet, coarse and fine powder separator inlet and outlet, and primary air duct elbow of the pulverizing equipment. The same phenomenon occurs in the air duct bend of the cement plant powder selection system. The use of wear-resistant alumina ceramics will have a positive impact on actual production.

③ Application in ash and slag systems

In power plants that use hydraulic ash removal and slag discharge systems, the inlet and outlet pipes such as ash pumps, ash ditches, and nozzles are severely worn. After using wear-resistant alumina ceramics, the service life is long and the wear resistance is good, solving problems such as heavy mechanical maintenance tasks and poor environment.

④ As a grinding medium

Due to its high hardness, moderate density, wear resistance, corrosion resistance, and low price, alumina grinding balls are widely used in the grinding and processing of raw materials in industries such as cement, minerals, ceramics, electronic materials, magnetic materials, coatings, and paints. They are a high-quality grinding medium. In the construction ceramic industry, the wear efficiency of alumina grinding balls is 20% to 40% higher than that of natural flint and pebbles. With the reduction of high-quality natural ball stone resources and the high wear rate of ordinary ceramic balls, alumina grinding balls will be increasingly used by more and more manufacturers.

⑤ Oil and gas extraction

Alumina wear-resistant ceramics can adapt to harsh environments, especially those with an alumina content of over 97% (by mass), which can be used in drilling equipment for oil and natural gas. Typical applications include nozzles, valve seats, regulating devices, pump accessories, and even drill bit accessories that can vibrate in high-pressure environments, in petroleum and mud mortar, and sometimes work in the presence of acids and salts, with stricter requirements for wear resistance and corrosion resistance.