Date of Completion

7-28-2016

Embargo Period

7-22-2017

Advisors

Douglas Adams, John Agar, Avinash Bidra, Patchanee Rungruanganunt

Field of Study

Dental Science

Degree

Master of Dental Science

Open Access

Campus Access

Abstract

Objective: Main aim was to explore a novel surface treatment for ceramics that creates surface features allowing any cement to enhance any ceramic in resisting crack opening displacement and 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 hoped that rapid heating and cooling of ceramic surfaces can leave compressive surface stresses, thus increasing its strength. Additionally, it was hypothesized that this novel treatment may cure or eliminate existing surface flaws.

Materials and Methods: this project first screened laser-treated surfaces by biaxial strength testing. Differently sized and spaced features were investigated for their influence on flexural strengths. Features were developed in 0.5 mm thick tabs of In-Ceram YZ (Vita Zahnfabrik, Bad Säckingen, Germany) by the Laser Applications Laboratory at Trumpf Laser Und Systemtechnique GmbH, Ditzingen, Germany. Dimensional and visual details of each surface treatment were provided by Trumpf using optical laser scanning confocal microscopy. An IR ps (picoseconds laser) based on a Disk- and Fiber technology (TruMicro 5000 series, Trumpf, USA) was used with characteristics of 6ps pulse duration, IR 1030nm wavelength and 50W average power. Laser treated tabs were compared with as-received and sand-blasted surface treatments. Based on flexural strength testing results, laser treatment resulting in 22 μm spot size, 5 μm depth and 60 μm distance was chosen as the treatment of choice. 22 laser treated In-Ceram YZ specimens were tested. This was compared with sandblasting using 50 μm Al2O3 particles (aluminum oxide blasting compound, 50 μm/240 grit fine grain size; Ivoclar Vivadent, Inc., Amherst, NY) at 2.0 bar pressure from a distance of 10 mm parallel and perpendicular to the tabs’ surface for 10 seconds. 21 sand-blasted In-Ceram YZ specimens were tested. All specimens were cemented to bases of a dentin-like material. Each specimen was cyclically loaded by 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. If cracked, the next specimen was cycled at a lower load; if not cracked, at a higher load (step size of 25 N).

Results: Comparison of flexural strength values between groups with features of varying distance showed that there was no statistically difference between them. Comparison of strength values between groups with features of varying depth showed that specimens with 22 μm deep features showed significantly lower strength than specimens with features of 3 μm or 15 μm depth. Overall, laser treated groups showed decreased flexural strength than sandblasted or as-received specimens. Two-dimensional fit of flexural strength results of groups with different feature depths lead to the observation that strength started to deteriorate after depth increased beyond 5-10 μm. Since flexural strength testing was done as a screening procedure to chose the best characteristics for laser surface treatment, we chose to perform fatigue testing on specimens with 5 μm deep features. Fatigue testing showed that the sandblasted group was significantly stronger than the laser treated group.

Conclusions: The laser surface treatment envisioned in this study was meant to provide protection against crack opening displacement without weakening dental ceramics. Flexural strength of specimens with surface features varying in depth and spacing was significantly lower than strength of sandblasted and as-received specimens. Laser treatment resulting in 20 μm spot size, 5 μm depth and 60 μm distance was chosen as the treatment of choice for fatigue testing and compared with sandblasted specimens. The laser treatment lead to decreased fatigue strength when compared to sandblasting. Laser treatment did not have the expected result, most probably due to undetected subsurface damage.

Major Advisor

J. Robert Kelly

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