Effect of Substrate Elastic Modulus on Failure Behaviour of GIC

Objectives: To investigate the effect of substrate elastic modulus (E) on load-bearing capacity and failure mode of glass ionomer cement under Hertzian indentation as a simplified mimic of the cuspal loading of restorations.

Methods: Discs of 2 mm thickness, 10 mm diameter, 8 to 18 replicates, were fabricated for ceramic-reinforced glass ionomer cement (GIC, Amalgomer, Advanced Healthcare, UK). Testing was at 23 °C, wet, after 7 d storage at 37 °C in artificial saliva, using a 20 mm diameter hard steel ball. GIC discs were tested resting on each of eleven substrate materials of varying E. E for the substrates (E_s) and the GIC (E_c) were measured separately under Hertzian indentation. Load at first failure was detected acoustically; mode was determined visually. At least 1/3 of specimens in each case were examined under scanning electronic microscopy for corroboration.

Results: E_s ranged from 0.09 ~ 210 GPa, E_c was 7.7 GPa. Failure load generally increased with E_s value at first and then levelled off. The turning point was at Ec/Es = ~1. Bottom-initiated radial cracking was the principal failure mode for relatively soft substrates (Ec/Es > 1), while top conical cracking usually occurred for relatively hard substrates (Ec/Es < 1). Conclusions: For bi-layer structures, the elastic modulus ratio of coating and substrate is a controlling factor for both failure load and mode of the coating material under Hertzian indentation test. It follows that the stiffness of the cavity floor can affect service behaviour.