This article briefly introduces a method to improve the life of boron nitride nozzles, derived from a research paper by Li Bei and Guo Fengping. By depositing a Si coating inside the BN nozzle and then treating it at a high temperature in a nitrogen atmosphere, a dense Si3N4 coating with a certain thickness is finally formed inside the nozzle. Si3N4 has a Mohs hardness of 9-9.5 and a thermal expansion coefficient of 2.75×10-6/°C, which is more compatible with hexagonal BN matrix. Si3N4 Silicon Nitride ceramic has excellent high temperature mechanics, erosion resistance, wear resistance, corrosion resistance, and oxidation resistance. It can protect BN nozzles from oxidation and ensures that the life of the nozzle is greatly improved.
In the process of spraying Fe-based amorphous alloy strips (containing Fe, Si, B), the hexagonal boron nitride nozzle is in contact with the high temperature (> 1,300°C) and high pressure melt, and the surface is oxidized seriously, and the formed boron oxide (B2O3) is a glassy melt, this material softens at 450°C and becomes a viscous liquid at 600°C. Under the impact of high-pressure melt, boron oxide is easily blown away, and the inner diameter of the nozzle rapidly expands, which leads to failure. In order to solve this problem, Li Bei and Guo Fengping proposed the following method.
In order to extend the service life of the BN ceramic nozzle, by depositing a Si coating inside the BN nozzle and then treating it at a high temperature in a nitrogen atmosphere, a change reaction occurs: 3Si+2N2 = a -Si3N4. Finally, a dense layer of Si3N4 coating with a certain thickness is formed inside the nozzle, the coating affinity is better, and the expansion coefficient is relatively consistent. The deposition amount of Si3N4 coating is 0.15~0.20mm, larger deposition amount will make the coating easy to fall off, smaller deposition amount is not resistant enough to erosion. The processing temperature is 900~1,000°C. The processing time is 0.5~1.5 hours to ensure the curing effect.
