new advances in dental ceramics have led to the use of yttrium - stabilized tetragonal zirconia polycrystals ( y - tzp ) as a substitute for fixed pfm dental restorations .
zirconia has shown several important features that make it suitable for dental applications , including : ideal mechanical properties , good biocompatibility , high strength and esthetics [ 35 ] .
although mechanical properties are very important , clinical success depends on the strength and durability of the bond of resin cement to ceramic substrates and teeth to prevent marginal discoloration , recurrent caries and failure .
one problem in this respect is the low capacity of resin bonding to zirconia ceramic ; which is related to glass - free composition of zirconia .
different surface treatments have been recommended in order to increase surface roughness and obtain better mechanical bond strength .
airborne particle abrasion has been proposed to facilitate the micromechanical retention between the y - tzp ceramics and resin cements .
this method increases the surface roughness and bonding area and changes the superficial energy and wettability .
air abrasion may cause flaws and defects that may act as initiators of cracks and compromise the mechanical properties and long - term durability of restorations [ 810 ] .
laser irradiation is another technique to increase surface roughness and improve adhesion between ceramics and resin cements .
laser is one of the most common and effective electromagnetic wave sources in the infrared spectrum .
because the co2 laser emission wavelength is absorbed by the ceramics , this device is suitable for ceramic surface treatment [ 11 , 12 ] .
one study stated that zirconia ceramic is the selective material for irradiation by nd : yag laser due to toughness and low thermal conductivity .
er : yag laser is a solid state laser that uses suspended er   ions in a yttrium - aluminum - garnet ( yag ) crystalline matrix to provide electrons for excitation .
er : yag lasing occurs in 2.94 m wavelength that is well absorbed by hydroxyapatite and water .
of course , the control of the laser settings ( power , pulse and duration ) is very important in order to prevent damage to the zirconia surface .
all the above procedures emphasize increasing the surface area to strengthen bonding and durability of the restorations .
although several surface treatments have been proposed , selection of the most appropriate method is still a challenge .
the objective of this study was to characterize the surface morphology and roughness of zirconia surface after different laser irradiations including nd : yag , co2 and er : yag and compare them with air - particle abrasion .
our hypothesis was that different surface treatments have obvious effects on the patterns of surface roughness .
first , 77 blocks of presintered zirconia ( ice zirkon , zirkonzahn gmbh , bruneck , italy ) measuring 10  10  2 mm were made using a diamond blade mounted in mecatome t201 cutting saw machine ( presi .
all samples were sintered according to the instructions of the manufacturer , and then embedded in acrylic resin in the metal molds .
next , the upper surface was polished with 800 , 1000 and 1200 grit silicon carbide disks under running water .
then , the specimens were randomly divided into 11 groups ( n=7 ) and received the following surface treatments : 
  er : yag laser with output power of 1.5 w(4.39 j / cm) er : yag laser with output power of 2 w(5.85 j / cm) er : yag laser with output power of 2.5 w(7.32 j / cm) nd : yag laser with output power of 1.5 w(111.96 j / cm) nd : yag laser with output power of 2 w(149.28 j / cm) nd : yag laser with output power of 2.5 w(186.6 j / cm) co2 laser with output power of 3 w(2.29 j / cm) co2 laser with output power of 4 w(3.05 j / cm) co2 laser with output power of 5 w(3.82 j / cm) air abrasion with 50 al2o3 particles , perpendicular to the surface , from a distance of 10 mm for a period of 10 sec at 2.8 bar pressure controls ( no treatment )  er : yag laser with output power of 1.5 w(4.39 j / cm )  er : yag laser with output power of 2 w(5.85 j / cm )  er : yag laser with output power of 2.5 w(7.32 j / cm )  nd : yag laser with output power of 1.5 w(111.96 j / cm )  nd : yag laser with output power of 2 w(149.28 j / cm )  nd : yag laser with output power of 2.5 w(186.6 j / cm )  co2 laser with output power of 3 w(2.29 j / cm )  co2 laser with output power of 4 w(3.05 j / cm )  co2 laser with output power of 5 w(3.82 j / cm )  air abrasion with 50 al2o3 particles , perpendicular to the surface , from a distance of 10 mm for a period of 10 sec at 2.8 bar pressure  controls ( no treatment ) the er : yag laser ( smart 2940d , deka laser , florence , italy ) was used with a wavelength of 2940 nm , frequency of 10 hz and pulse duration of 230sec ( very short pulse ) .
the spot size was 1 mm and the handpiece was used at a distance of 4 mm ( focusing distance of the handpiece ) above the surface ( noncontact mode ) for 10 sec accompanied by water and air spray .
the nd : yag laser ( fidelis , fotona , slovenia ) was used at a wavelength of 1064 nm , frequency of 10hz and pulse duration of 100 sec ( very short pulse ) .
total surfaces of the block were scanned with 320 fiber about 1 mm above the surface for 1 min .
the co2 laser ( smart us20d , deka laser , florence , italy ) was used at a wavelength of 10600 nm , in continuous mode , 1 mm above the surface for 1min .
surface roughness was assessed by profilometry ( t-8000 , hommelwerke , jenoptik , germany ) .
the diamond tip of the device with a speed of 0.15 mm / s and load of 4 n was passed through the surface of the samples .
the average surface roughness ( ra - micron ) and other parameters such as rku and rsk in treated surfaces were determined .
surface roughness ( ra ) , a quantitative calculation of the relative roughness of a linear profile or area , was expressed as a single numeric parameter ( ra ) .
ra does not make a distinction between the grooves and prominences and provides no information about the three - dimensional structure .
rsk or skewness , the symmetry measure of the profile about the mean line , was characterized as positive or negative .
the next parameter rku or kurtosis was the sharpness measure of the surface profile and took the value 3 for the normal distribution .all parameters were analyzed using one - way anova and post - hoc multiple comparisons by tukey s test .
leven s test confirmed the equality of variances ( p value > 0.05 ) . to evaluate the surface morphology ,
the same samples used for profilometry were coated with gold - palladium ( quorum technologies polaron sc7620 , newhaven , east sussex , uk ) and observed under sem ( jsm-6610 lv scanning electron microscope , jeol usa ) with 50x , 500x and 5000x magnifications and 15 kv voltage .
surface roughness was assessed by profilometry ( t-8000 , hommelwerke , jenoptik , germany ) .
the diamond tip of the device with a speed of 0.15 mm / s and load of 4 n was passed through the surface of the samples .
the average surface roughness ( ra - micron ) and other parameters such as rku and rsk in treated surfaces were determined .
surface roughness ( ra ) , a quantitative calculation of the relative roughness of a linear profile or area , was expressed as a single numeric parameter ( ra ) .
ra does not make a distinction between the grooves and prominences and provides no information about the three - dimensional structure .
rsk or skewness , the symmetry measure of the profile about the mean line , was characterized as positive or negative .
the next parameter rku or kurtosis was the sharpness measure of the surface profile and took the value 3 for the normal distribution .all parameters were analyzed using one - way anova and post - hoc multiple comparisons by tukey s test .
to evaluate the surface morphology , the same samples used for profilometry were coated with gold - palladium ( quorum technologies polaron sc7620 , newhaven , east sussex , uk ) and observed under sem ( jsm-6610 lv scanning electron microscope , jeol usa ) with 50x , 500x and 5000x magnifications and 15 kv voltage .
the anova showed that the surface roughness ( ra ) was significantly different between groups ( p < 0.001 ) .
ra in nd : yag group with output power of 2 and 2.5 w was higher than other groups ( p < 0.05 ) .after that , co2 laser with output power of 4 and 5w and nd : yag laser with power of 1.5w showed higher ra values ; however , there was no significant difference between these groups .
the lowest surface roughness ( ra ) was in the control group ( table 1 ) .
there was no significant difference between groups in the rku values ( p > 0.05 ) .
all of them showed rku over 3 , expressing more pointed peaks and valleys in the rough profile .
rsk values were positive in the samples treated with 1.5 , 2 and 2.5w nd : yag and 3 , 4 and 5 w co2 lasers ; indicating the sharp prominences and depressions of the surface .
in sem images , non - treated surfaces of ytzp showed grooves and irregularities due to the polishing process ( figure 1 ) .
treated surfaces by al2o3 air - particle abrasion ( 50 ) indicated roughness , grooves and irregularities , uniformly throughout the surface ( figure 2 ) .
the er : yag laser with power of 1.5w created a rough surface with irregularities ( figure 3a ) .
surface roughness increased by enhancing output power to 2w ; also , the surface became more uniform ( figure 3b ) .
the er : yag laser irradiation at power of 2.5w reduced surface roughness and depth of irregularities ( figure 3c ) .
treated surfaces by co2 laser with power of 3w showed several fine and wide crack lines ( figure 4a ) .
in addition , the circular shape prominences could be seen over the surface produced by the motion of laser irradiation . with increasing power ,
the rate of irregularities and surface roughness increased ( figures 4b and 4c ) . treating samples by nd : yag laser at a power of 1.5w created surface microcracks , prominences and porosities ;
nd : yag laser irradiation at 2 and 2.5w power increased these changes ( figures 5b and 5c ) .
the anova showed that the surface roughness ( ra ) was significantly different between groups ( p < 0.001 ) .
ra in nd : yag group with output power of 2 and 2.5 w was higher than other groups ( p < 0.05 ) .after that , co2 laser with output power of 4 and 5w and nd : yag laser with power of 1.5w showed higher ra values ; however , there was no significant difference between these groups .
the lowest surface roughness ( ra ) was in the control group ( table 1 ) .
there was no significant difference between groups in the rku values ( p > 0.05 ) .
all of them showed rku over 3 , expressing more pointed peaks and valleys in the rough profile .
rsk values were positive in the samples treated with 1.5 , 2 and 2.5w nd : yag and 3 , 4 and 5 w co2 lasers ; indicating the sharp prominences and depressions of the surface .
in sem images , non - treated surfaces of ytzp showed grooves and irregularities due to the polishing process ( figure 1 ) .
treated surfaces by al2o3 air - particle abrasion ( 50 ) indicated roughness , grooves and irregularities , uniformly throughout the surface ( figure 2 ) .
the er : yag laser with power of 1.5w created a rough surface with irregularities ( figure 3a ) .
surface roughness increased by enhancing output power to 2w ; also , the surface became more uniform ( figure 3b ) .
the er : yag laser irradiation at power of 2.5w reduced surface roughness and depth of irregularities ( figure 3c ) . treated surfaces by co2 laser with power of 3w showed several fine and wide crack lines ( figure 4a ) .
in addition , the circular shape prominences could be seen over the surface produced by the motion of laser irradiation . with increasing power ,
the rate of irregularities and surface roughness increased ( figures 4b and 4c ) . treating samples by nd : yag laser at a power of 1.5w created surface microcracks , prominences and porosities ;
nd : yag laser irradiation at 2 and 2.5w power increased these changes ( figures 5b and 5c ) .
the aim of this study was to evaluate the surface roughness and morphology of y - tzp samples treated with different powers of various lasers ( co2 , er : yag and nd : yag ) and compare them with al2o3 air - particle abrasion .
sandblasting with aluminum oxide particles is a general practice that increases resin bond strength due to creating surface roughness and undercuts .
air abrasion creates flaws and roughness on the surface due to phase transformation by the higher impact energy of particles .
in addition , airborne particle abrasion of restorations has the potential to remove significant amounts of substrates ; therefore , it can affect clinical adaptation and may have negative impact on the retention of restorations [ 1719 ] .
sem images of the control group were almost smooth except for some scratch - like lines and small pits that were created by the polishing sandpaper . in this study , sem micrographs showed that the application of air abrasion with al2o3 particles leads to apparent retentive pits and scratches ; although , there was no significant difference in ra between treated surfaces with air - particle abrasion and er : yag laser . in this study ,
the high temperature due to co2 laser irradiation effectively facilitates localized surface melting and creates micro - cracks on the surface .
increased ra values of treated y - tzp by co2 laser in the present study confirm sem observations .
different reasons are expressed for the formation of these cracks after co2 laser irradiation . according to gromann et al , superficial emission of atoms ,
ions and electrons occurs during the temperature rise in the ceramic surface as the result of the absorption of laser beam . due to the photoionization produced by the irradiation ,
physical plasma is created and its formation is accompanied by extremely high pressure and temperature variations in the ranges of 10,00050,000 k. this can cause a lot of physical stress during the rehardening of the ceramic surface .
in addition , two different phases are formed : one with a crystalline structure and another with an amorphous structure ; each with a different thermal expansion coefficient .
once the material starts to harden , regions of internal mechanical stress are created between the two phases leading to the formation of micro - cracks [ 20 , 21 ] .
stubinger also reported that the main part of the absorbed energy was used during melting and only a small portion was spent for heating the mass of material .
this feature causes rapid cooling of the molten layer as soon as irradiation is removed and as a result , cracks are formed .
nd : yag laser irradiation is recommended to change the ceramic surface due to the formation of a glazed superficial layer . in the present study
, results of sem micrographs and anova confirmed that the treated samples with nd : yag laser had higher ra than the other groups and indicated uniform surface cracks and molten regions .
like the co2 laser process , increasing temperature during the laser irradiation can also cause thermal melting of the ceramic surface .
another challenge is that nd : yag laser treated surfaces are characterized by a carbonized layer with silver pigments .
this feature can prevent the use of nd : yag laser in the esthetic zones .
cavalcanti et al . evaluated the surface roughness of y - tzp ceramics ( cercon , procera ) after different treatments .
they used different parameters of er : yag laser ( 200 , 400 and 600 mj ) and expressed that the lower energy intensity ( 200mj ) could be a potential method for the surface treatment of zirconia .
they also concluded that the higher intensities of laser are not suitable for treatment due to severe damage to the ceramic surface . in our study , one - way anova showed no significant difference between er : yag laser irradiation and air abrasion groups .
however , in sem micrographs , treated samples with er : yag at 2.5w showed reduction in the amount of irregularities and undercuts due to homogenous ablation of the surface .
rsk or skewness is the symmetry measure of the profile about the mean line and is characterized as positive or negative .
negative skewness indicates a plateau like profile and positive skewness confirms the peaks ( figure 6 ) .
rsk values were positive in treated samples with nd : yag and co2 lasers ; indicating prominences on the surfaces .
as the levels of rsk approach zero , we encounter more normal distribution of peak and plateau - like areas . the next parameter was rku or kurtosis .
this is a measure of the sharpness of the surface profile and takes the value 3 for the normal distribution .
rku > 3 indicates that the surface is dominated by sharp peaks ( spiky ) ; whereas , the peaks are bumpy if rku is lower than 3 ( figure 6 ) . the differences in rku between groups were not statistically significant ( p > 0.005 ) and all of them showed rku > 3 ; indicating sharp prominences and depressions .
all the above - mentioned processes rely on increasing the surface area for bonding and creating a stronger and more durable restoration . however , the range of changes on the surface of ceramics depends greatly upon the energy intensity of laser irradiation and type of ceramic .
we concluded that the different surface treatments have significant effects on the patterns of surface roughness .
also , we found that er : yag laser could be a suitable alternative for airborne particle abrasion ; but , treating the zirconia surface by nd : yag and co2 lasers is very destructive , generating a lot of heat that may adversely affect the adhesion and mechanical properties .
according to this study , all treated surfaces showed higher roughness than the control group .
treated surfaces by er : yag laser and air abrasion showed similar surface roughness ; thus , this laser can be a suitable substitute for air abrasion . increasing the output power of nd :
yag and co2 lasers increased the roughness ; but it did not apply to er : yag laser .
nd : yag and co2 lasers were destructive on the zirconia surfaces since they created extensive cracks and high temperatures .
 objective : the aim of this study was to assess the surface of yttrium - stabilized tetragonal zirconia ( y - tzp ) after surface treatment with lasers and airborne - particle abrasion.materials and methods : first , 77 samples of presintered zirconia blocks measuring 10  10  2 mm were made , sintered and polished . 
 then , they were randomly divided into 11 groups ( n=7 ) and received surface treatments namely , er : yag laser irradiation with output power of 1.5 , 2 and 2.5 w , nd : yag laser with output power of 1.5 , 2 and 2.5 w , co2 laser with output power of 3 , 4 and 5 w , al2o3 airborne - particle abrasion ( 50 ) and no treatment ( controls ) . 
 following treatment , the parameters of surface roughness such as ra , rku and rsk were evaluated using a digital profilometer and surface examination was done by sem.results:according to anova and tukey s test , the mean surface roughness ( ra ) after nd : yag laser irradiation at 2 and 2.5 w was significantly higher than other groups . 
 roughness increased with increasing output power of nd : yag and co2 lasers . 
 treated surfaces by er : yag laser and air abrasion showed similar surface roughness . 
 sem micrographs showed small microcracks in specimens irradiated with nd : yag and co2 lasers.conclusion:nd:yag laser created a rough surface on the zirconia ceramic with many microcracks ; therefore , its use is not recommended . 
 air abrasion method can be used with er : yag laser irradiation for the treatment of zirconia ceramic . 
INTRODUCTION
MATERIALS AND METHODS
Evaluation of surface roughness:
SEM:
RESULTS
Mean surface roughness: (
Results of SEM:
DISCUSSION
CONCLUSION