Effect of in-flight particle oxidation on the phase evolution of HVOF NiTiZrSiSn bulk amorphous coating

Abstract

Bulk amorphous NiTiZrSiSn feedstock was sprayed using a high velocity oxy-fuel spraying process. In order to evaluate the amorphous formability in view of chemical instability, the oxygen to hydrogen gas flow rate ratio was changed at the constant total gas flow rate. Flame gas enthalpy was increased with the O2/H2 gas ratio. In-flight particle melting state was improved with the increase of oxidizer flow rate. Accordingly, porosity as well as unmelted particle size and density were decreased. For the amorphous phase fraction within an as-sprayed coating, it was decreased with the oxygen flow rate. From the thermal analysis and X-ray diffraction, the effects of O2/H2 gas ratio on the crystallization of HVOF sprayed bulk amorphous NiTiZrSiSn could be divided into two categories. One is due to the solid-state crystallization during flight along the low gas enthalpy flame. The other results from the destabilization of bulk amorphous feedstock due to the chemical composition change resulting from in-flight particle oxidation at the higher oxygen gas flow rate. Oxidation degree was different from the O2/H2 gas ratio and it had influences on the glass formability [ΔTx] and crystalline phase. Increase of oxide phase fraction was parallel to that of γ-Ni solid solution.