In the current in vitro work, 60 pre-sintered commercial dental zirconia blocks (Cercon, Dentsply, Amherst, N.Y.) with dimensions of 4 × 4 × 4 mm were ground for 60 s via 600 and 800 Struers RotoPol 11 silicon carbide grit abrasive (Struers A/S, Rodovre, Denmark), followed by cleaning in a i600B steam cleaner (Italy ELT) for 10 minutes using ethanol (96%) and subsequently air-drying for 30 s. In accordance with an earlier identical work  and regarding the Bonferroni formula for determination of the sample size, the number of specimens in each group was determined as n = 15. The sampling method was non-randomized in this study. However, the allocation of specimens to each group was randomized.
Group 1 consisted of zirconia blocks sandblasted with Al2O3 particles (50 µm) under a pressure 4 kg/cm2 and a distance of 10 mm for 15 s in a MESTRA sandblast apparatus (TALLERES MESTRAITUA S.L, Espana), followed by cleaning via ethanol (96%) for 10 mine in ultrasonic cleaner. Group 2 consisted of zirconia blocks whose surface were coated via HA thermal protocol so that a slurry solution was prepared by adding 10 gr of nano particle HA powder (less than 100 nm) (Merck, Germany) to distilled water (50 cc). Then, 1gr of poly vinyl alcohol (Merck, Germany) was also added as the binder of the suspension, followed by heating on a magnet stirrer with 1000 rpm at 100°C for 60 min to achieve a uniform suspension. At last, the zirconia blocks were placed into in this slurry at angle of 45° for 5 s.
Group 3 consisted of the zirconia blocks sandblasted with the same condition as described in group 1 and then layered with a coating of HA following the same procedure as described in group 2.
Group 4 consisted of the zirconia blocks whose surfaces were first exposed to Er,Cr: YSGG laser irradiation with the following details: wavelength of 2.78 µm, and 140-µs pulse duration with a 20-Hz repetition rate and 4-W output power. The laser optical fiber (in a diameter of 600 µm and a length of 6 mm) with the gold handpiece was positioned to the surface at a distance of 10mm perpendicularly and moved in a sweeping manner manually within a 30-s exposure time over the whole area. Continuous water (55%)-air (65%) flow were applied while performing irradiation. After laser treatment, the surface was coated with HA as explained for group 2.
Subsequent to the surface treatments, all the blocks in groups 2, 3, and 4 were sintered following this protocol: ambient temperature to 300°C for 10 minutes, 300–600°C for10 minutes, 600–900°C for 30 minutes, 900–1200°C for 40 minutes, maintenance at 1200°C for 120 minutes and cooling for annealing, employing a CWF Furnace (Keison Products, UK). Following the surface treatment in all the study groups, a mold was selected to be Tygon Norton Performance Plastic tubes (with an inner diameter of 0.8 mm and a height of 1 mm (Cleveland, OH, USA) for the Panavia F2.0 cement bonding (Kuraray Medical Inc.) to prepared surfaces. Then, the tubes were removed with a heated sharp scalpel. All samples were placed into distilled water, for 24 hours incubated (Incubator, Model PL-455G PecoPooya Electronic Co.) at 37°C, and positioned in a micro-tensile tester (Bisco Inc., USA) for the determination of micro-shear bond strength. A metal loop (0.2 mm thick; Ligatur, dentsply GAC_SOF) was put around the cement cylinder in the bonding site for the exerted tensile load conversion to shear load through vertical soldering of casting molds to jig, as seen in Fig. 1. The load extent at failure (crosshead speed = 0.5 mm/min) was determined, followed by computing the values of micro-shear bond strength in accordance with the equation of S (MPa) = F (N)/A (mm). Following the shear experiment, a SEM was used to determine the fractured surfaces for achieving the failure variants that included Cohesive failure, Adhesive failure and Mixed failure
Statistical analysis method
To analyze data the SPSS software version 21.0 was used. The statistical comparison of the bond strength was conducted with one-sided analysis of variance: ANOVA among the 4 groups. In the present study, the type 1 error rate (ɑ or p value) was considered equal to 0.05.