Humidity Affects Fractal Dimension of Fracture in a Fluorapatite Glass-Ceramic

Objectives: This study is part of a larger project to validate fractal analysis of fracture surfaces as a tool for estimating the stress intensity factor at failure for dental ceramics. It was hypothesized that specimens fractured in a low humidity environment would have greater fracture toughness (K_Ic) and fractal dimensional increment (D*) than specimens fractured in water.
Methods: Rectangular beam specimens were prepared by vibration blotting a slurry of nano-fluorapatite glass-ceramic powder (e.max Ceram, Ivoclar Vivadent) and deionized water followed by firing according to the manufacturer’s instructions. The samples were polished to dimensions of 1.2mm thickness, 4mm width, ~22mm length using a 30µm metal-bonded diamond disk followed by 15µm diamond films. A controlled flaw was created using a Knoop indentation followed by removal of 4.5X the crushed zone depth and three-point flexure in either deionized water (n=7) or dry nitrogen (n=5). The K_Ic was calculated using the surface crack in flexure (SCF) method by measuring the critical flaw dimensions according to ASTM C1421. An epoxy replica of the hackle region on each fracture surface was prepared, and a 5µmX5µm area was scanned using the atomic force microscope (Bioscope Catalyst, Bruker) at a rate of 0.592Hz. The height data were exported into a custom MathCAD script, and D* was calculated in the FRACTALS software (Russ) using the Minkowski Cover technique.
Results: The mean (95% confidence interval) K_Ic were 0.73(0.06) MPa●m½ for water environment and 0.85(0.06) MPa●m½ for dry nitrogen. The D* values were 0.171(0.001) for water environment and 0.190(0.004) for dry nitrogen. The data fit a linear model between K_Ic and the square-root of D* (R²=0.99).
Conclusions: Fractal dimension of the fracture surface increased in proportion to the fracture toughness. Fracture toughness and fractal dimension were significantly higher when fractured in dry nitrogen instead of in water (p=0.019 and p<0.001, independent t-test).