Direct manufacture of wear resistant and corrosion resistant coatings for the paper, mining and materials industries

At the present time there are various thermal spray coatings in use for wear and corrosion protection, these coatings contain inherent porosity which could be detrimental to their performance. Plasma sprayed chromium oxide coatings are currently used within the paper, mining and materials industries; they provide good wear and corrosion resistance, although they are still susceptible to failure. This work focuses on thermal sprayed nickel based coatings deposited using flame spray and High Velocity Oxy-Fuel (HVOF) techniques. They were then processed with a flame or laser heat treatment process called fusing to create a dense, pore free and strongly bonded microstructure. This is to overcome the shortcomings present in the chromium oxide thermal spray coatings to provide a good wear and corrosion resistant coating. Laser cladding of the Ni-based coatings was also performed to observe the potential use in depositing these coatings. Dense, pore free microstructures were produced with laser cladding process. A coating adhesion testing method was developed to determine the bonding strength of these fused coatings (thermal sprayed followed by fusing process). The coatings were placed in a salt spray fog corrosion chamber to observe their corrosion resistance; to rank and compare them to coatings currently used in the paper and mining and materials industries. The same coatings were subjected to a pin-on-disk wear test to observe their wear resistance performance. Corrosion testing showed that flame and laser fused coatings had good corrosion resistance along with Cr2O3. The majority of coatings had performed well and the coatings that failed showed corrosion had occurred at the coating and substrate interface, where corrosion had initiated at the sample edges. Wear testing demonstrated the low wear of chromium oxide coatings, while it was possible to obtain comparable wear results with laser fused nickel based coatings. Some flame fused coatings demonstrated high wear due to pull out of tungsten carbide particles present in the coating. The study shows that nickel based fused coatings demonstrated very high bonding strength as well as showing good corrosion and wear resistance. These coatings can replace the currently used chromium oxide coatings. Laser fusing was able to provide coatings that could compare with the wear resistance of chromium oxide. Laser cladding coatings showed a favourable microstructure that could also be used to replace current coatings, however further testing is required before they can be utilised in industry.