The question “what is the real fracture toughness of ceramics” has been a challenge for past three decades and is still an open question to the ceramic community. In this work, the indentation fracture resistance (IFR) and toughness measurements of several bearing grade silicon nitride are performed using standardized methods such as Single Edge V-notch Beam (SEVNB), Surface Crack in Flexure (SCF) and Single-Edged Precracked Beam (SEPB) and two exploratory methods for studying the effect of R- curve, i.e. as “Compliance” method performed on bending bars, but also “ball-on-three balls” (B3B) variant of the SCF method on discs extracted from balls.

The resulting values were between 4 - 7 MPa√m for all material grades, depending on method, specimen (preparation) & crack size. It is clearly shown that IFR is always higher than the fracture toughness measured by any other method, but with different calibration factors depending on the material grade / characteristics. The observation that SEVNB values are (always) lower than SCF & SEPB suggests the presence of an R-curve behavior for all materials.

Generally bearing grade silicon nitrides are always showing R-curve behavior, but depending on the microstructure, the plateau value was reached at clearly different crack length increments.

In order to explain the differences in fracture toughness between material grades the effect of the R-curve behavior can be used as an additional parameter. Results were quite variable and seemed to depend upon the test method, i.e. crack growth rate, the crack growth stability, and R-curve phenomena contributed to the data variability.

Values measured using compliance method are always lower values from the SCF & SEPB method – also for long cracks. The reason lies in the slow loading rate of 0.02 μm/s (in order to obtain stable crack growth) that applies to the compliance method. Subcritical crack growth - just before the partial unloading - is clearly reducing the observed fracture toughness.