Rapidly solidified neodymia-stabilised zirconia coatings prepared by DC plasma spraying

Stabilisation of the phases in zirconia alloys is pivotal in their applications as advanced structural ceramics. Stress-induced transformation toughening is dependent on the characteristics of the metastable tetragonal zirconia phase. Rare-earth oxides are known to stabilise the cubic and tetragonal phases. This paper presents the preparation of neodymia-stabilised zirconia powders and their coatings by plasma spraying. The amount of neodymia ranged from 2 mol.% to 10 mol.%. The feedstock for plasma-sprayed coatings was prepared by a combination of mechanical milling and thermal plasma melt and solidification processes. Phase analysis of the plasma rapidly solidified powders and plasma-sprayed coatings was determined through X-ray diffraction (XRD). Results show that the phases in the plasma-sprayed coatings have better stability than the plasma-spheroidised powders, and demonstrates the efficacy of "double melting" in achieving enforced solid solution of the tetragonal phase and chemical homogeneity in the rapidly solidified material. The tetragonal phase is stabilised when 2 to 7 mol.% neodymia is added. There is evidence that suggests the formation of an enforced solid solution of the tetragonal phase in the compositional range 4-7 mol.% Nd₂O₃. This extended solid solution series was found to be rather unstable upon heating and the t→m transformation was observed during cooling. Transformation to the cubic phase occurred after heat treatment at 1400 °C for 10 h. However, at 8 mol.% and 10 mol.% neodymia (as-sprayed coatings) the cubic phase is stabilised. Heat treatment at high temperature (1400 °C) showed that there could be some amounts of the non-transformable t′ phase. The amount of monoclinic phase in the coatings is lower than in the feedstock powder, suggesting that further stabilisation of the tetragonal phase (which reflected better chemical homogeneity) is achievable with plasma-spray deposition.