Nano-mechanical Properties and Corrosion of MarkII CAD/CAM Glass-ceramic

Objectives: To understand the basis for the clinical retention of excellent aesthetics and minimal abrasivity of a feldspathic porcelain-based millable ceramic (MarkII, Vita Zahnfabrik, Germany). Despite more than 25 years successful clinical use, minimal investigation of the basic micro-structure, composition, nano-mechanical properties and abrasive nature under simulated oral conditions exists. Our hypothesis was that the hardness of the various phases, were equal to or less than that of enamel.
Methods: An X-ray Diffraction (XRD) analysis determined its different phases. Atomic force microscopy (AFM)-based nanoindenter system using a Berkovich indenter with 4000µN load was used to indent the glass and crystalline phases on a polished Mark II, to determine the elastic modulus and hardness. Scratch tests (250 to 2000µN) and Fourier Transform Infrared /Attenuated Total Reflectance Spectroscopy (FT-IR/ATR) (400 and 4000 were conducted pre- and post-exposure to a corrosive environment (pH 4, at 55°C for 2 months) that mimics oral conditions, to assess surface changes. Scanning electron microscopy (SEM) imaging of the scratching deformation and elemental analysis (EDS) of different phases were conducted.
Results: XRD and nanoindentation revealed substantial glass phase (~65 vol%) and five crystalline phases (~35 vol%) with higher average elastic modulus and hardness values for the crystalline phases collectively (Table 1). SEM and EDS revealed a less soluble group of similar elemental composition (Si, O, Al, K>Na) and a more soluble crystalline phase (less Si, O, more Al, Na>K, Ca). Scratch tests revealed major reduction in chipping with pH~4 solution sample. FTIR-ATR analysis revealed a hydrated silica-gel layer (Si.xO) on pH~4 exposed sample.
Conclusions: Hardness results of the different phases were greater than reported for enamel (~5.3GPa) at similar loads, hence invalidating our hypothesis. Formation of a soft thin surface hydrated-layer by partial dissolution, may render Mark II less abrasive to the opposing dentition.