What is the "self-bonding" effect of thermal spray primer materials

       Under the action of the high temperature heat source of the thermal spraying flame, the different components of the coating material can undergo an exothermic chemical reaction, so that the coating and the substrate form a micro-metallurgical bond. The most typical and widely used "self-bonding" primer material is nickel-aluminum composite powder.



The bonding strength of the coating and the substrate mainly depends on the contact temperature between the substrate and the sprayed particles, the molten state of the particles and the impact force applied to the particles during the spraying process. In the process system, the main factor that can be considered is to increase the contact temperature. The exothermic properties of the primer material are designed on this premise.



In general, the contact temperature in turn depends on the matrix at the moment the coating is formed, the particle temperature, and the thermophysical properties of both. During the thermal spraying process, it is impossible for the metal substrate to be preheated to a very high temperature, not exceeding 100~200°C in principle, otherwise the surface will be severely oxidized, hindering the combination of the coating and the substrate material. Relying on a heat source to increase the temperature of the particles has little effect, because the time for the particles to fly to the substrate is only a few thousandths of a second, and the heating time is too short. Therefore, it is the most effective way to contribute heat by means of the chemical reaction of the particles themselves.



There are two kinds of large self-adhesive primer coating materials: one is high melting point metals, such as tungsten, molybdenum, etc., which have a high heat content in the molten state (tungsten is 29.4kJ/mol, molybdenum is 21kJ/mol) mol); the other is a composite powder material with exothermic properties, such as Ni-Al powder. They undergo an exothermic reaction between Ni and Al during the thermal spraying process. This exothermic reaction can continue for 0.003~0.005s after the powder particles reach the surface of the substrate, resulting in a strong micro-metallurgy between the coating and the substrate. combine.



It is known that there are more than 100 pairs of elements that emit a lot of heat in their mutual reactions, of which the ideal is one or more of Al and Ni, Co, Cr, Mo, W, Nb, and Ti. The most commonly used is the exothermic reaction between Al and elements such as Ni or Mo, such as Ni-Al powder, Ni-Mo-Al powder.