Microstructure and Phase Stability of Yttria‐Doped Tetragonal Zirconia Polycrystals Heat Treated in Nitrogen Atmosphere

Abstract

The low-temperature degradation of zirconia (ZrO 2 ) that was doped with 3 mol% yttria (Y 2 O 3 ) (3Y-TZP) was prevented by the heat treatment of sintered specimens in nitrogen. The heat treatment of sintered specimens resulted in a surface layer that was stabilized by nitrogen ions, whereas the interior was only slightly affected by the heat treatment. X-ray diffractometry and transmission electron microscopy analyses revealed that the stabilized surface layer consisted of cubic grains with tetragonal precipitates. Although the presence of the surface layer decreased the strength of the sintered 3Y-TZP, the strength of nitrified specimens was maintained when low-temperature annealing was applied.

Sintering and characterisation of nano-sized yttria-stabilised zirconia

“…Fracture toughness of 2.5 to 14 MPa m with hardness of 11 to 13.5 GPa were obtained when sintering took place in the temperature range 1400° to 1500°C [2,13,18,28,29,30],…”

“…To overcome the low-density problems, hot isostatic pressing (HIP) has become a popular technique to increase density without excessive grain growth [2,18,28]. More recently, high-quality powders have become available which enable densities > 98% of theoretical to be achieved at 1400°C, with a resulting grain size < 0.5 p.m [11,15,16,29].…”

Phase stability and microstructure evolution of yttria-stabilized zirconia during firing in a reducing atmosphere

“…The change of color for zirconia specimens after sintering in a reducing atmosphere had also been reported in previous studies [19,23]. The reducing atmosphere used in the present study is generated by firing the specimens within a graphite powder bed.…”

“…Besides using the stabilizers, the oxygen vacancy can also be created by heat-treating zirconia in an oxygen-lean environment. For examples, the solution of nitrogen into zirconia [11,19,21,22] and the use of flowing hydrogen [23] and carbon monoxide [24] can induce extra oxygen vacancy. In view of the applications for TBC and SOFC, the phase stability of YSZ in a reducing environment is an important issue.…”

“…The XRD patterns for t, c and t 0 phases are very similar to each other. Nevertheless, the presence of t 0 phase can be identified by using the X-ray diffraction (XRD) technique in the 2u range from 728 to 768 [17][18][19].…”