Boron carbide is a kind of super-hard compound in nature. In normal state, its hardness is second only to diamond and cubic boron nitride, and it is hardest at high temperature (about 1000 degrees Celsius). At the same time, boron carbide is a kind of P-type semiconductor with excellent thermal conductance. Most importantly, it is particularly resistant to acid and alkali corrosion.
Because of various advantages, the industry has been trying to use boron carbide as a coating material. But research in this area started relatively late.
Epoxy resin is A polycondensation product of epichlorohydrin and bisphenol A or polyols. Because of the chemical activity of the epoxy group, it can be opened by a variety of compounds containing active hydrogen, curing and crosslinking to form a network structure, so it is a thermosetting resin.
Therefore, the composite coating made of epoxy resin and boron carbide can complement each other and get better coating.
Because of its high hardness, boron carbide is very suitable for wear – resistant coating. However, boron carbide powder has relatively high surface energy, so if boron carbide is directly coated only, according to Lin Leiting and Wang Zhenguo’s “Boron carbide ultrafine powder agglomeration and solution”, boron carbide powder will have a strong van der Waals force, chemical bond and other forces and is easy to aggregate; Insoluble in the matrix and lose the due protective effect; According to the paper “Experiment and Theoretical Study of inductively Coupled Plasma for B4C Coating Preparation Zhou Qiujiao”, the thickness of boron carbide coating made by traditional process is not easy to control, and boron carbide molecules are easy to fall off when the coating is too thick.
In order to make boron carbide molecules evenly dispersed on the matrix, there are two main ideas: one is to directly synthesize boron carbide molecules on the matrix; Second, boron carbide plasma is sprayed onto the matrix to achieve uniform distribution of boron carbide according to the repulsion. But these two methods are not mature enough, and the process is complex and expensive. Therefore, it is very important to find a third way to synthesize boron carbide powder in industry.
The relative molecular mass of epoxy resin is not very high, the density is 1.2 kg/m; And its molecular chain has active epoxy groups, which can cross-link with a variety of materials, and has excellent adhesion. So is there a way to coat the boron carbide molecules in an epoxy resin that effectively prevents the boron carbide from accumulating, forming a composite coating, and getting the desired material? This is what this paper illustrates.
However, boron carbide powder can not be directly obtained by traditional method and then intersected with epoxy resin, because the powder particles are too coarse and need to be refined. Or boron carbide nanoparticles can be prepared by SHS method.
Nanoparticles have multiple effects.
Surface effect: The surface area of a spherical particle is proportional to the square of its diameter, and its volume is proportional to the square of its diameter, so its specific surface area (surface area/volume) is inversely proportional to its diameter. The specific surface area will increase significantly as the particle diameter decreases. For example, when the particle size is 10nm, the specific surface area is 90m2/g; When the particle size is 5nm, the specific surface area is 180m2/g. When the particle size drops to 2nm, the specific surface area jumps to 450m2/g. When the particle diameter decreases to the nanometer level, not only the number of atoms on the surface increases rapidly, but also the surface area and surface energy of the nanoparticles increase rapidly. This is mainly due to the large number of atoms on the surface, the crystal field environment and binding energy of the surface atoms are different from those of the inner atoms. Lack of adjacent atoms around the surface atoms, there are many suspended bonds, with unsaturated properties, easy to combine with other atoms and stabilize, so it has great chemical activity, crystal microparticle accompanied by the increase of such active surface atoms, its surface energy greatly increased. The activity of surface atoms not only causes the change of surface atom transport and configuration, but also the change of surface electron spin conformation and electron energy spectrum.
Volume effect: With the quantitative change of particle size, it will cause the qualitative change of particle properties under certain conditions. The change of macroscopic physical properties caused by the reduction of particle size is called small size effect. As the size of nanoparticles decreases, the specific surface area also increases significantly, which results in great changes in magnetism, internal pressure, optical absorption, thermal resistance, chemical activity, catalysis and melting point, resulting in a series of novel properties. For example, the optical absorption of metal nanoparticles is significantly increased and the plasmon resonance frequency shift of absorption peak is generated. The magnetism of small size nanoparticles is obviously different from that of bulk materials, which changes from magnetic order state to magnetic disorder state and superconducting to normal phase. The melting point of nanoparticles is significantly lower than that of large solid materials.
Although carbon tube furnace is often used for carbonic reduction of boron carbide powder (chemical reaction equations 2.1, 2.2), but the carbonic reduction of boron carbide powder by carbon tube furnace is too coarse, its diameter is generally tens of microns. And the boron carbide powder nano-filler we need needs about 1 micron particles. Therefore, the powder obtained by the traditional method can not be used to synthesize epoxy composite coating; We need the SHS reaction, the self-propagating high temperature synthesis.
Through a method called Nano Fillers, because the diameter of boron carbide particles is about one micron, the surface effect and volume effect of nano-molecules can be achieved according to the study of Song Dan’s “Preparation of functionalized boron carbide and corrosion Resistance of Epoxy composite coating”. Can be “according to the high temperature epoxy resin: curing agent to a certain mass ratio; And prepare the resin matrix by stirring for one day at room temperature. After ADDING boron carbide powder wood TO RESIN IN a certain proportion, the nano-filler can be dispersed well in resin matrix by ultrasonic stirring for half an hour with ultrasonic machine. Then the well-dispersed resin is quickly transferred to the spray gun and sprayed evenly on the P110 steel sheet after sandblasting. After spraying, the steel sheet shall be stationary at room temperature for 1h, and then solidified in the oven at the following temperature: Curing at 60℃ for 2h, curing at 80℃ for 10min, curing at 100℃ for 10min, gradually rising to 120℃ and keeping curing for 2h, and then increasing the temperature to 220℃ for 2h according to the previous stage temperature increasing method. After curing, the sample is cooled to room temperature and then the thickness of the dry film of the coating is measured by spiral micrometer.”
Accordingly, we can get the coating we need.