The Effect of Sintering Temperature on the Phase Composition, Microstructure, and Mechanical Properties of Yttria-Stabilized Zirconia

Abstract

It is known that the yttria-stabilized zirconia (YSZ) material has superior thermal, mechanical, and electrical properties. This material is used for manufacturing products and components of air heaters, hydrogen reformers, cracking furnaces, fired heaters, etc. This work is aimed at searching for the optimal sintering mode of YSZ ceramics that provides a high crack growth resistance. Beam specimens of ZrO2 ceramics doped with 6, 7, and 8 mol% Y2O3 (hereinafter: 6YSZ, 7YSZ, and 8YSZ) were prepared using a conventional sintering technique. Four sintering temperatures (1450 °C, 1500 °C, 1550 °C, and 1600 °C) were used for the 6YSZ series and two sintering temperatures (1550 °C and 1600 °C) were used for the 7YSZ and 8YSZ series. The series of sintered specimens were ground and polished to reach a good surface quality. Several mechanical tests of the materials were performed, namely, the microhardness test, fracture toughness test by the indentation method, and single-edge notch beam (SENB) test under three-point bending. Based on XRD analysis, the phase balance (percentages of tetragonal, cubic, and monoclinic ZrO2 phases) of each composition was substantiated. The morphology of the fracture surfaces of specimens after both the fracture toughness tests was studied in relation to the mechanical behavior of the specimens and the microstructure of corresponding materials. SEM-EDX analysis was used for microstructural characterization. It was found that both the yttria percentage and sintering temperature affect the mechanical behavior of the ceramics. Optimal chemical composition and sintering temperature were determined for the studied series of ceramics. The maximum transformation toughening effect was revealed for ZrO2-6 mol% Y2O3 ceramics during indentation. However, in the case of a SENB test, the maximum transformation toughening effect in the crack tip vicinity was found in ZrO2-7 mol% Y2O3 ceramics. The conditions for obtaining YSZ ceramics with high fracture toughness are discussed.

The Effect of Yttria Content on Microstructure, Strength, and Fracture Behavior of Yttria-Stabilized Zirconia

“…At least three dependences related to these elements can be observed on the graph: (i) the oxygen concentration decreases and the yttrium percentage increases with an increase in yttria content from 3 to 5%; (ii) the oxygen content is almost equal to this of yttrium with yttria content varying from 6 to 8%; and (iii) both the oxygen and yttrium concentrations change suddenly with the yttria content in the bulk material composition varying from 5 to 6 mol%—these allow us to assume a global transition from one to another mechanism of phase stabilization. To confirm this assumption, the phase balance variation with yttria content in the studied materials is plotted in Figure 8 on the bases of the XRD analysis [ 33 , 34 ]. The tetragonal phase (t-ZrO 2 ) showed the maximum concentration at 4% yttria.…”

“…The propensity to brittle fracture of YSZ ceramics stabilized by the various amounts of yttria was evaluated in References [ 33 , 34 ], based on the study of phase composition, microstructure, and fracture micromechanisms. Among YSZ ceramics containing from 3 to 5 mol% Y 2 O 3 , 5YSZ ceramics sintered at 1450 °C for 2 h exhibited the highest fracture toughness.…”

“…The fracture toughness of this material was found to correlate with a high percentage of the tetragonal ZrO 2 [ 33 ]. By comparing the mechanical behaviors of YSZ ceramics containing from 6 to 8 mol% Y 2 O 3 , it was found that 7YSZ ceramics sintered at 1600 °C for 2 h have the highest level of fracture toughness [ 34 ]. The above results are consistent with the mechanical characteristics of yttria-stabilized zirconia presented in Reference [ 35 ].…”

Impact of the material and sintering protocol, layer thickness, and thermomechanical aging on the two-body wear and fracture load of 4Y-TZP crowns

“…While the grain size of 4Y-TZP increased with an increase in sintering temperature and led to a reduced Vickers hardness [ 17 ], biaxial flexural strength was not affected [ 13 , 17 ]. The impact of the sintering parameters on the phase content of the zirconia is, as of yet, not fully understood, with an increase of the tetragonal phase and a simultaneous decrease of the monoclinic phase with a temperature increase from 1450 to 1600 °C being reported for 6Y-TZP, while a reversed trend was described for 7Y-TZP and 8Y-TZP, namely a decrease in the tetragonal and an increase in the monoclinic and cubic phase [ 40 ]. To allow a deeper understanding of the observed findings, especially with regard to the contradictory results regarding the reliability of the sintered restorations, additional investigations analyzing the grain size, phase content, and mechanical properties of a wide variety of 4Y-TZP materials in dependence on the sintering parameters are warranted.…”

Preparation and cutting performance study of YSZ-toughened PcBN superhard tools

“…to ZrO 2 , the phase transformation process of tetragonal zirconia during cooling can be inhibited, which makes the material matrix disperse sub-stable t-ZrO 2 at room temperature, and when the applied load is applied, the sub-stable t-ZrO 2 will undergo the phase transformation process of t → m by stress to produce volume expansion, which inhibits the expansion of cracks within the material matrix under the action of external forces, so that the strength and toughness of the material can be improved. 21–23…”