Phase transformation of Nb2O5 during the formation of flame sprayed coatings and its influence on the adhesion strength, abrasive wear and slurry erosive wear

Abstract

The development of research related to the characterization of niobium pentoxide (Nb2O5) has been of great interest in the scientific and commercial community due to promising performances in bio-electrochemical applications, advanced catalysts and corrosive environments. In this work, layers of 250 µm, 350 µm and 550 µm of Nb2O5 were deposited by low-speed flame spray on NiAl-bond coated AISI 1020 steel substrates. The phase transformations of Nb2O5, which occur during the formation of the layers, were studied as a function of deposited thickness and further correlated with adhesion, abrasion and slurry erosion resistance. X-ray diffraction analyzes of the coatings were performed for the study of the phases in the process. Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) with dispersive energy microanalysis (EDS) were used to help microstructural analysis. The adhesive strength of the layers was evaluated by ASTM C633-13 and correlated with abrasion (ASTM G65-16) and slurry erosion wear resistance. From the microscopy analysis, a dense layer was observed with good interconnection at the interface Nb2O5-NiAl, with presence of pores typical of the thermal spray process. The XRD diffractograms of the niobium oxide coated samples show similar results regardless of the coating thickness (250, 350 and 550 µm) presenting superior compatibility with the hexagonal Nb2O5 and the monoclinic NbO2.46 (Nb22O54). The significantly higher intensities of the hexagonal Nb2O5 peaks suggest that it is the predominant phase. The thinner coatings, 250 and 350 µm, had higher adhesive strength, with a mean fracture stress of 16.12 MPa and 15.27 MPa, respectively. Nb2O5 thinner coatings showed also higher abrasive wear resistance when compared to "D2" tool steel and "H13" chrome tool steel. The thicker coatings (550 µm) had a mean adhesive strength of 13.73 MPa. From the abrasive wear tests, thicker coatings showed a greater volume loss of 41.18 mm3. The 550 µm coating presented higher slurry erosion resistance at a 30° particle impact angle, with a lower volume loss (12.93 mm3).