Introduction
Translucent glass ceramics consisting of nanocrystalline ZrO₂ grains in an amorphous SiO₂ matrix have shown high flexural strength in combination with good fracture toughness [1]. Thermal treatments well below the sintering temperature can affect the microstructure and phase composition [2]. This study explores the effect of annealing on fracture strength, fracture toughness, microstrain, and phase composition.
Material and methods
Using spark plasma sintering at 1050 °C, ceramic discs were made from amorphous powder with the composition 65%ZrO₂-35%SiO₂ (molar ratio). The discs were analyzed with X-ray diffraction before and after being subjected to annealing at 650 °C (3 samples) and 1050 °C (3 samples) for 0.5 h. Additional samples were annealed also for 2 h. Rietveld refinement gave information about the chemical phase composition, grain size, and microstrain. Flexural strength, Vickers hardness, fracture toughness, and Young’s modulus were analyzed and methods such as the piston-on-three-ball test, microhardness testing, analysis of the crack length, and nanoindentation were used.
Results
On average, 91 weight% of the ZrO₂ was in the tetragonal phase after sintering with grains having an average size of 37 nm. Annealing resulted in only a minor change of the phase composition and the amount of tetragonal phase in different samples was not correlated to any of the investigated mechanical parameters. An increase in crystallite size was observed as a result of annealing and 0.5 h at 1050 °C resulted in a size increase of up to 9%. The average flexural strength after sintering was 698 MPa and was not significantly affected for samples annealed at 0.5 h at 650 °C, but an annealing time of 2 h resulted in a reduction of the flexural strength by 10%. Annealing for 0.5 h at 1050 °C reduced the flexural strength by 22%. The reduction in flexural strength correlated to the observed increase in crystallite size in the different samples. Also, Young’s modulus decreased with increasing crystallite size. There was a slight decrease in the microstrain as a result of annealing and within the different annealing groups, samples with higher microstrain most often had a lower flexural strength. The fracture toughness was also considerably reduced as a result of annealing.
Conclusion
This study shows that both the flexural strength and fracture toughness of nanocrystalline ZrO₂-SiO₂ glass ceramics made by spark plasma sintering can be reduced considerably as a result of thermal treatments. A temperature as low as 650 °C affects the mechanical properties and this must be taken into consideration in an application where glass ceramics are exposed to a temperature load.
References
[1] L. Fu et al., ‘Ultrastrong Translucent Glass Ceramic with Nanocrystalline, Biomimetic Structure’, Nano Lett., vol. 18, no. 11, pp. 7146–7154, 2018, doi: 10.1021/acs.nanolett.8b03220.
[2] L. Fu, B. Li, G. Xu, J. Huang, H. Engqvist, and W. Xia, ‘Size-driven phase transformation and microstructure evolution of ZrO₂ nanocrystallites associated with thermal treatments’, J. Eur. Ceram. Soc., vol. 41, no. 11, pp. 5624–5633, Sep. 2021, doi: 10.1016/j.jeurceramsoc.2021.04.058.