Effect of elastic modulus mismatch on failure behaviour of glass ionomer cement under Hertzian indentation

To investigate the effect of the substrate: coating elastic modulus ratio on the failure behaviour of glass ionomer cement (GIC) under Hertzian indentation.

GIC (Amalgomer CR, Advanced Healthcare, Tonbridge, Kent, UK) discs, 2 mm thick and 10 mm diameter, were randomly divided into 11 groups, 10-22 specimens for each group, and stored at 37 °C in artificial saliva for 7 days. Discs were tested resting freely on 11 substrate materials at 23 °C in air by loading centrally through a 20 mm diameter hard steel ball at a crosshead speed of 0.2 mm/min. The elastic moduli of the substrates (E(s)) and the coating GIC (E(c)) were first measured with a method similar to that of the Hertzian indentation, except for the three harder materials: amalgam, Al6061 and stainless steel 304. The failure load at the first crack was recorded with the aid of acoustic emission detection. Fracture surfaces were observed by SEM.

E(s) values ranged from 0.09 to 210 GPa; E(c) was 7.7 GPa. Failure load generally increased with E(s) at first and then became relatively stable; the turning point occurred for E(s)/E(c)=1. Bottom surface-initiated radial cracking was the principal failure mode on softer substrates (E(s)/E(c)<1), while top surface conical cracking usually occurred on harder substrates (E(s)/E(c)≥1).

The elastic modulus ratio, substrate: coating, is a controlling factor for both the failure mode and load for the coating material under Hertzian indentation.