effect of hydrothermal, chemical, and mechanical degradation on flexural strength and phase transformation of ground, glazed, and polished zirconia

Objectives: evaluation of the effect of grinding on flexural strength of zirconia after low temperature degradation (ltd) and ph-cycling.materials and methods: sixty-four bar-shaped specimens of yttria-stabilized tetragonal polycrystalline zirconia were milled, sintered, wet-polished, and divided into 8 groups (n=8). the four control groups were not aged while artificial aging was performed in the 4 experimental groups in three steps including  ltd in steam for 40h, ph-cycling, and tooth brushing for artificial aging. all groups underwent surface preparations as follows: standard polishing without surface treatment (sp), grinding with a blue-yellow band diamond instrument (gr); grinding with a diamond rotary instrument (dri) and then over-glazing (gl); grinding with a dri followed by two-step intraoral polishing (po); standard polishing and aging (sp-ag); grinding and aging (gr-ag), grinding, over-glazing and aging (gl-ag); and grinding, polishing and aging (po-ag). monoclinic content was assessed in one specimen of each group by x-ray diffraction (xrd). the 3-point flexural strength test was performed in a universal testing machine. the results were analyzed with two-way anova and tukey’s test (α=0.05).results: mean flexural strength (mpa) was significantly higher in groups gr and po compared to group sp (both, p<0.0001) and group gl (both, p<0.0001). in xrd analyses, the highest monoclinic phase before aging was observed in group gr (12.6%), and after aging in group gr-ag (51.2%).conclusion: grinding and polishing increased the flexural strength, while glazing did not exhibit any significant effect on this parameter.  furthermore, aging did not adversely affect flexural strength.