Fracture toughness of t’ ZrO2 stabilised with MO1.5 (M =Y, Yb & Gd) for thermal barrier application

Abstract

Thermal Barrier coatings (TBC's) protect the gas turbine blades at high temperature exposure. The t' phase is metastable and slowly transforms to the high-temperature equilibrium state consisting of tetragonal (t) and cubic (c) during high temperature exposure. A comparative study of the phase stability and fracture toughness of monolithic Y, Yb and Gd stabilised zirconia with composition 8 mol%MO 1.5 was studied. Powders were prepared by co-precipitation method with crystallite size ~15 nm. Pressureless sintering, Spark plasma sintering (SPS) at 1250°C for 10 min was carried out to produce compacts with >96% relative density for fracture toughness measurements XRD and Raman studies revealed the the initial phase was t'. After sintering, no spontaneous monoclinic phase formed during cooling. The fracture toughness of the sintered pellets of different composition was measured. The results were analysed in terms of the effect of phase constitution on fracture toughness. The role of ferroelastic toughening in these materials was explored.

Phase and fracture toughness evolution during isothermal annealing of spark plasma sintered zirconia co-doped with Yb, Gd and Nd oxides

“…7b) are single phase t . 10,11,39 The indentation fracture toughness of spark plasma sintered 8YbSZ which was annealed at 1250 • C for 192 h and t 8YSZ pellets which were sintered at 1250 • C in air and isothermally annealed at the same temperature up to 192 h can be directly compared with the present 8QSZ results. 10,11 The 8QSZ 3 h sample has 35% and 32% higher toughness than that of 8YbSZ 0 h (87 J m −2 ) and 8YSZ 0 h samples (91 J m −2 ), respectively.…”

“…Though the ferroelastic switching strains of the co-doped 8QSZ material, pressureless sintered 8YSZ, SPS 8YbSZ and 8GdSZ are comparable, considerable difference, however, was observed in the toughness values. 39 It is clear that c/a ratio is not the deciding factor in the toughness of t zirconia. Another important aspect is the change in indentation toughness with thermal exposure in 8QSZ is different from 8YSZ and 8YbSZ.…”

“…7b. 10,11,39 The possible reasons for this observation are discussed later. The fracture toughness of the 6h sample was measured as 231 ± 71 J m −2 , the highest in this study.…”

“…The switching strain for SPS 8YbSZ, 8GdSZ processed under similar conditions as 8QSZ is 0.0078, 0.0095. 10,11,39 Though it is known that the c/a ratio increases as the tetragonal phase becomes lean in stabilizers, the dependence of the critical stress on composition is not known. The importance of this aspect is highlighted by the fact that for similar values of c/a, the 8QSZ has a significantly higher toughness than the 8YSZ.…”

“…The importance of this aspect is highlighted by the fact that for similar values of c/a, the 8QSZ has a significantly higher toughness than the 8YSZ. 11,39 As aging progressed, change in tetragonal phase composition and coarsening of the microstructure might have contributed to the decrease in toughness. The decrease in toughness may also be due to the precipitation and growth of cubic phase, as this would lead to a decrease in the ferroelastically active volume.…”