Failure Mode of Ceramic-reinforced Glass Ionomer Under Hertzian Indentation

Objectives: To explore the application of a Hertzian indentation test to glass ionomer (GI) and investigate the effects of GI thickness (t) and substrate elastic modulus (E_s) on the failure mode. Materials and Methods: 10 mm diameter ceramic-reinforced GI discs (Amalgomer CR, AHL, UK), elastic modulus, E_c = 13 GPa, were prepared for two series of experiments. In the first, they were tested resting freely on reinforced nylon substrate discs (10 mm diameter, 5 mm thickness, E_s=10 GPa), varying t from 0.4 to 7.0 mm. In the second, 2 mm thick GI specimens were tested on various substrates (ABS, polycarbonate, reinforced nylon, polyaramide / vinylester composite, aluminium alloy 6061, stainless steel) with E_s ranging from 2 to 220 GPa. GI surfaces were ground level under running water to 1220 grit. Specimens were stored in artificial saliva at 37°C for 7 d while testing was at 23°C, wet. Centre-point load was applied by a 20 mm diameter hard steel ball at 0.2 mm/min cross-head speed. Load at the first crack (P) was recorded; fracture surfaces were observed under SEM to identify the failure mode. Results: For GI on nylon, P increased with t from 163 ± 38 N for 0.4 mm to 2446 ± 558 N for 7.0 mm. Bottom-initiated radial cracking (RC) was observed to be the main failure mode; continuing loading led to the occurrence of top Hertzian cone cracking (CC). For GI on various substrates, when E_c : E_s ≥ 1, P increased slightly with E_s; For E_c : E_s < 1, the main failure mode changed abruptly from RC to CC, P increased by about 5 times and then remained constant. Conclusions: The Hertzian indentation test can be successfully extended for use on GI. The failure mode is controlled by thickness and the substrate elastic modulus.