Assessment of biaxial flexural strength (BFS) and modulus (BFM)

BFS and BFM of all materials (n = 5) were determined under a ball-on-ring testing jig with a mechanical testing frame (AGSX, Shimadzu, Kyoto, Japan). Firstly, disc specimens were prepared according to the protocol of the previous study. The specimens were immersed in 10 ml of deionized water for 24 and 2 weeks in an incubator (37 °C). Then, the specimens were placed on the jig under the testing frame. The load cell and crosshead speed for the test were 500 N and 1 mm/min, respectively. The fracture load was recorded (N). The BFS and BFM were calculated using the equation 2 and 3, respectively.

where F refers to the maximum load (N), d is the specimen’s thickness (m), r is the radius of circular support (m), ν is Poisson’s ratio (0.3) [23, 24], ΔHΔWc is the rate of change of load with regard to central deflection (N/m), β_c is the center deflection junction (0.5024) [25], and q is the ratio of support radius to the radius of the disc. The method for calculating β_c was provided in the previous study [25]. The fracture surface of the representative specimen from each material was sputter-coated with Au (Q150R ES, Quorum Technologies, East Sussex, UK) using a current of 23 mA for 45 s. The fracture surface was then examined under SEM (JSM 7800 F, JEOL, Tokyo, Japan).