Surface Change of Zirconia by Nd:YAG Laser Irradiation

Objectives: The aim of this study is to investigate the surface change of zirconia by Nd-YAG laser irradiation.

Methods: Cercon (DeguDent) and NANOZR (MEW) were used as zirconia substrate material. Nd:YAG laser (Desktop Laser, Dentaurum) was used as irradiator. The surface was characterized by scanning electron microscopy and X-ray diffractometry. Element distribution map was observed by wave dispersion X-ray spectroscopy.

Results: After the Nd:YAG laser irradiation, blackened circles were clearly formed and the color changed area increased with the acceleration voltage. In the case of focus 0, penetrated holes were formed and the size of hole increased with the acceleration voltage. SEM observation revealed that many cracks were formed in the blackened area of the zirconia surface. After firing at 1000˚C for 5 min, the blackened area of zirconia disappeared and recovered to the original color, but cracks still remained. Element distribution map of Cercon showed that zirconium increased, and both oxygen and yttrium decreased in the blackened area in comparison to the surrounding matrix without irradiation. In the blackened area of NANOZR, zirconium increased, whereas oxygen, cerium and aluminum decreased. Therefore, It seems that the blackening of zirconia by Nd:YAG laser irradiation depended on the reduction in oxygen content and the cracks were formed due to the solidification from the melt within the blackened area, namely irradiated area.

Conclusions: Nd:YAG laser irradiation formed many cracks and reduced the content amount of yttrium, cerium and aluminum on the surface of zirconia. It possibly results into the reduction of mechanical strength. Therefore, it is concluded that Laser welding should not be undertaken for zirconia.

Acknowledgment: This study was partially supported by a Grant-in-Aid for Scientific Research from the Japan Society the Promotion of Science (No.18390521).