XPS Characterizations of CAD/CAM Lithium-disilicate-based Glass-ceramics after HF Etching

Objectives: The surfaces of four lithium disilicate glass ceramics (LDGC) were characterized at the nanolevel. The goal was to detect the chemical alteration of the surface on etching with hydrofluoric acid (HF).
Methods: The four LDGC tested were Celtra Duo, IPS e.max CAD, Straumann n!ce and Vita Suprinity. Four blocks of each LDGC were sectioned to ~1 mm thicknesses. The requirement for firing, or not, was carried out following the manufacturer’s recommendations. The samples were then divided in two groups: a control and an etched group. Etching was carried out using a solution of 5% HF for 20 s, rinsed for 20 s and dried for 10 s. The atomic percentages of the first atomic layers were probed using X-ray photoelectron spectroscopy (XPS) in survey mode (n=12). The oxygen and silicon peaks (O1s and Si2p) were then analyzed in high resolution mode. The samples were also characterized using scanning electronic microscopy at high magnification (100KX).
Results: XPS showed the amounts of the major elements, Si, O, and Li, were different on etching. For all samples, trace elements such as P, Zn, Y, Na, and Sr, disappeared on glass phase dissolution. Zr and Al percentages varied, based on the LDGC analyzed. High resolution spectra of the O1s and Si2p peaks showed that the chemical environments were qualitatively different in all samples. Following etching, SEM images showed needle-like crystals for IPS e.max CAD and n!ce, and also nanometer-sized rod-like crystals for Celtra Duo and Vita Suprinity.
Conclusions: Acid etching, using 5% HF for 20 s, modifies not only the topographic structure, but also the chemical composition of the LDGC surface.