In 2012, Rungruanganunt and Kelly, discounted “adhesion” in ceramic protection. They observed that sandblasted InCeram Alumina tabs, cemented with either Panavia or zinc phosphate, were equally durable in fatigue testing under water (see Fig. 1). This strongly suggests that non-adhesive cements can protect ceramics as well as “bonding” cements and that the protective effect is not due to “adhesion” but to “crack opening displacement.”
Since sandblasting is well-known to degrade strength, it was hoped to identify a novel surface treatment which creates surface features for cements to “grab” the ceramic and resist crack-opening displacement, without reducing (and hopefully increasing) the ceramic material’s strength. This novel treatment is laser machining which was used to create a matrix of shallow surface “divots”. It was also hoped that rapid heating and cooling of ceramic surfaces could leave compressive surface stresses, thus increasing its strength.
Flexural Strength Testing
Differently sized and spaced laser-created divots were created on 0.5 mm tabs of In-Ceram YZ (Vita Zahnfabrik, Bad Säckingen, Germany) . They were screened using biaxial strength testing to investigate the influence of laser treatment on flexural strength. The flexural strength of specimens that received laser treatment were lower than strength of sandblasted and as-received specimens. However based on these preliminary results, laser treatment resulting in 22 μm spot size, 5 μm depth and 60 μm distance was chosen as the treatment of choice.
Cyclic Fatigue Testing
22 In-Ceram YZ specimens received this laser treatment and were compared with 21 sand-blasted In-Ceram YZ specimens. All specimens were cemented with resin-based cement to bases of a dentin-like material (NEMA G10, woven glass-epoxy). Each specimen was cyclically loaded with a 2-mm diameter G10 piston in water. Loads ranging from 10 N to the target load were applied at a frequency of 20 Hertz for 500 000 cycles (step size of 25 N). Specimens were examined for cracking by trans-illumination (staircase sensitivity statistical design).
Fatigue testing showed that the sandblasted group was significantly more durable than the laser treated group (see Fig. 3). Fractography was to analyze the fracture surface of laser-machined ceramics (see Fig 4). Using scanning electron microscopy we observed that laser treatment created depressions with significant porosity and that failure origins were associated with the base of surface depressions.
The laser surface treatment envisioned in this study was meant to provide protection against crack opening displacement without weakening dental ceramics. The laser treatment did not have the expected result, as it lead to decreased fatigue strength when compared to sandblasted specimens. This is likely attributed to the damage of the ceramic created by the laser treatment itself. Future work may explore treatment optimization.
Authors gratefully acknowledge financial assistance from Vita Zahnfabrik and technical support from Trumpf USA for performing the laser treatments