Qualitative assessment of surface topography of XeCl laser etched Ni-Cr alloy.

Objectives: The aims of this study were to compare the mode of debonding (adhesive or cohesive) of laser-etched Ni-Cr alloy (Wiron 99) to a standard sandblasting technique, and to a combination of sandblasting and laser etching. The etched surfaces were examined using optical and electron microscopy techniques.

Methods: Five groups of randomly allocated Ni-Cr cylinders were prepared: Group I-sandblasted; Groups II-IV-laser etched at three fluences (3.0, 4.0 and 4.9 J/cm2), Group V-combination sandblasting and laser etching (4.0 J/cm2). Within groups specimens were bonded into pairs (N=8) with composite luting cement (Panavia 21) prior to tensile debonding. Optical and scanning electron microscopy were performed both prior to and following debonding.

Results: An oxide layer was observed on sample surfaces before surface preparation by lasing or abrasion. Sandblasting removed the oxidation layer, which low and medium laser fluences alone failed to fully ablate. The highest (4.9 J/cm2) laser fluence melted the surface of the Ni-Cr alloy, covering the oxidation layer. A combination of sandblasting and lasing at 4.0 J/cm2 allowed full removal of the surface oxidation layer due to abrasion, followed by roughening of the surface with the laser. This resulted in greater mean tensile debonding strengths [Murray, A, Attrill, D, Dickinson, M. Dent Mater, 2004, In press] and a tendency for cohesive rather than adhesive interface failures. Groups I-IV underwent partial adhesive failure at both surfaces. Group V, in the main, underwent cohesive failure with a layer of adhesive remaining on both surfaces.

Significance: Laser etching of Ni-Cr alloy in combination with sandblasting optimises bonding to composite luting agents, resulting in a high proportion of cohesive failures when compared to sandblasting or lasing alone.