Effect of air-abrasion at pre- and/or post-sintered stage and hydrothermal aging on surface roughness, phase transformation, and flexural strength of multilayered monolithic zirconia.

This study aimed to evaluate the effect of air-abrasion/sintering order and autoclave aging on the surface roughness (Ra), phase transformation, and biaxial flexural strength (BFS) of monolithic zirconia. A total of 104 monolithic zirconia specimens (Katana ML) were divided into eight groups according to airborne-particle abrasion protocols and hydrothermal aging: control (non-aged: C-, aged: C+), air-abrasion before sintering (BS-, BS+), air-abrasion after sintering (AS-, AS+), and air-abrasion before and after sintering (BAS-, BAS+). A steam autoclave was used for accelerated aging, and Ra values were measured with a surface profilometer. All specimens were analyzed by X-ray diffraction to determine any phase transformation on the zirconia surface. BFS was measured by using the piston-on-three-balls method. Scanning electron microscopy and atomic force microscopy were performed on one specimen per group. BS and BAS groups showed higher Ra values compared with groups C and AS. The aging process significantly increased the monoclinic phase content of all specimens. Lower monoclinic levels were found in AS+ and BAS+ compared with other aged groups. The AS groups exhibited higher flexural strength values relative to control groups, whereas BS groups exhibited significantly lower flexural strength values (p < .05). There was no reduction in flexural strength by using the BAS protocol. Air-abrasion of zirconia at the pre-sintered stage only is not recommended in clinical use because of the remarkable decrease in flexural strength.