The secret of «wear-resistant» alumina ceramics

"Friction" is a fairly common phenomenon, which is always accompanied by human activities, such as friction when walking, friction when driving cars, and friction when running trains. The physical phenomenon caused by this relative movement seems to be very friendly to humans, but in fact it often shows a hideous side. For example, in industry, the "friction" between two objects is often harmful, and it will cause wear on the contact surface of the object, and severe wear may even make the normal working motion mechanism ineffective. In order to cope with this industrial enemy, people often use advanced ceramic materials with excellent wear resistance as a substitute for ordinary metals or plastics, or cover the surface of vulnerable workpieces with wear-resistant ceramics to improve their durability. Compared with ordinary metals or plastics, wear-resistant ceramics have the following advantages: high hardness, high strength, good wear resistance, more than 100 times higher than manganese steel, high chromium steel, high temperature resistance, acid and alkali corrosion resistance, light weight, only Half of the steel can greatly reduce the load on the equipment. Among them, alumina ceramics have become one of the most commonly used wear-resistant materials in this field because of their very affordable prices and are quite suitable for industrial applications. They are compared in ore crushing processing systems, raw material grinding systems, and high-speed cutting. "Wild and unrestrained" occasions can be seen everywhere. Since the friction and wear properties of ceramic materials are critical to their ability to serve well, if alumina ceramics "control" friction, it is necessary to understand and study the relationship between it and friction. 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 the wear rate, and the wear rate of ceramic materials with high hardness and fracture toughness is low. For the goal of improving the hardness and fracture toughness of ceramic materials, scholars from various countries have carried out a lot of research work, which can be divided into the following aspects for analysis:

1. Ceramic grain size

Alumina ceramic materials are divided into single-phase ceramics and multi-phase ceramics (that is, the second phase is added to the matrix). In the field of research on the correlation between grain size and ceramic tribological properties, researchers have mainly investigated the matrix phase (or the first phase). The effect of the grain size of the second phase) on the tribological properties of ceramics. For example, Roy et al. studied the friction and wear properties of submicron and micron single-phase alumina ceramics in a biological environment, and found that the wear rate of submicron ceramics in the bovine serum albumin environment is much lower than that of micron ceramics, and that of submicron ceramics Grain pullout and grain boundary microcracks are significantly less than coarse-grained alumina ceramics. Sedlacek et al. studied the influence of different alumina matrix grain sizes on the wear performance. The matrix alumina grain size varies from 0.8 to 4 μm, while the second phase SiC is nano-sized. Studies have shown that when the alumina matrix is ​​in the sub-micron size, the wear resistance is better than the nano-composite ceramics with the grain size of the micron size; when the matrix grains are in the sub-micron size, there is no obvious relationship between the wear resistance and the fracture toughness, and the matrix is ​​in the The wear rate of micron-scale alumina composite ceramics decreases with increasing hardness. Obviously, it can be seen from the above examples that the refinement of crystal grains can effectively help improve the uniformity of the material structure, including increasing the density of the material, reducing material defects, and so on.

2. The second phase material

In the field of research on the tribological properties of alumina composite ceramics, the composition of components, that is, the formation of composite materials by adding various second phases and particles (or whiskers) is also the main way to improve the tribological (or cutting) performance of alumina ceramics. . According to different influence mechanisms, it can be divided into several types, such as the second phase self-lubricating mechanism, the second phase grain boundary enhancement effect, and the second phase tribochemical reaction mechanism.

①Second phase self-lubricating mechanism

The introduction of graphite, CaF2, PbWO4, MoS2, BN, soft metal and other second-phase solid lubricants into the Al2O3 ceramic matrix can effectively reduce the friction factor of the material, thereby improving the tribological performance of the material. Deng Jianxin and others introduced a 10% CaF2 solid lubricant into the Al2O3/TiC composite ceramic matrix. Through cutting and friction experiments, they found that CaF2 is extruded and smeared into a self-lubricating film on the friction surface. The self-lubricating film can effectively prevent the material from interacting with The adhesion between the friction pairs reduces the friction factor and plays a self-lubricating effect.

②The second phase grain boundary enhancement

The second phase (mainly particles and whiskers) is introduced into the alumina ceramic matrix, and the difference in thermal expansion coefficient between the dispersed particles and the matrix material is used to generate residual stress during the material preparation and cooling process to achieve the effect of grain boundary enhancement. When expanding along the grain boundary, not only the inherent grain boundary energy of the matrix material must be overcome, but also the additional energy brought by the residual compressive stress must be overcome, thus increasing the crack propagation resistance; on the other hand, because the thermal expansion coefficient of the second phase particles is less than The thermal expansion coefficient of the matrix, the volume effect will be produced during the cooling process of the material, and microcracks will be generated around the second phase particles, which induces crack deflection, so that the crack propagation consumes more energy; in addition, generally the second phase particles are approximately round The spherical shape makes the crack tip dull, thereby reducing stress concentration and preventing crack propagation, thereby improving the tribological properties of the material.

③The mechanism of the second phase tribochemical reaction

The second-phase tribochemical reaction mechanism means that the second phase mixed in the Al2O3 matrix reacts with the gas in the air (mainly oxygen) or with the counter-grinding material when it rubs against the counter-grinding material to produce a lubricating film. Reduce the friction factor of the material, thereby improving the tribological properties of the material.