4 Ways to Reduce Porosity of Thermal Spray Coatings
1. Using self-sealing effect of material itself
Some spray materials have a self-sealing effect. It has good bond to substrate, and the coating is dense. For example Ni-B-Si, Ni-Cr-B-Si, Co-Cr-W-B-Si coatings has B, Si elements which have much better oxygen binding ability than the metallic content. When the alloy powder heated to 1000 ~ 1200 ℃, Si, B elements react with oxygen or with Ni, Co oxide to form boron oxide and silicon oxide. After cooling down, they can form a non-porous coating.
2. Taking advantage of additional elements
The use of certain additional elements can effectively reduce the coating porosity. It is found that rare earth elements in the alumina coating and the iron-based coating can effectively reduce the porosity. The addition of 0.8% rare earth ferrosilicon to the plasma sprayed Al2O3 powder can improve the coating corrosion resistance and grain refinement, reduce the porosity and improve the bonding strength of the coating. Some reputed colleges in China have successfully developed a rare earth-containing cored wire, effectively reducing the porosity. Numerous articles indicate that the addition of 13% TiO2 to the Al2O3 coating, although lower in hardness than the pure Al2O3 coating, has lower porosity, good density and higher bond strength and therefore shows better wear resistance.
3. Utilizing the function of ceramic oxides
Some mixed ceramic oxides are able to form coatings with low porosity at high temperatures. The higher melting oxides form a porous coating skeleton, while the lower melting oxides fuse and firmly adhere to the pores of the former oxide. The lower melting oxide also forms a solid solution with the higher melting oxide and thus forms a complete dense coating. Good examples are MgO-TiO2 coating, and Al2O3-Cr2O3 coating.
4. Sealing with self-corrosion products
Some coating materials can use their own corrosion products for coating sealing. It is found that Al(OH)3, which is passivated and corroded by Al coating, adheres to the surface of coating in the early stage of seawater exposure, which limits the infiltration of corrosion factors. With the extension of time, the pores of the Al coating are almost completely enclosed by them, so the coating shows excellent shielding and effectively extends the service life of the Al coating.
