HYDROFLUORIC ACID ETCHING PATTERNS AND SURFACE MORPHOLOGY OF THREE SILICA-BASED CERAMIC MATERIALS

Purpose: This study investigates the hydrofluoric acid etching patterns and the surface morphology of three types of glass-ceramic materials. Materials and methods: Hydrofluoric acid etching patterns and surface morphology changes of three silica-based ceramics - EX-3, E.max press, and Heraceram press were studied. Fourteen rectangular experimental bodies with a cross-section of 2x2mm and a length of 5-7 mm were divided into two groups(n=7): polished (con-trol group) and etched (5% hydrofluoric acid). Etching time was 120s in EX-3 (EXHF) and Heraceram press (HHF) groups and 20s in E.max press group (EHF). Surface morphology was observed under a scanning electron microscope at magnifications ranging from x50 to x2700. Results: The presence of numerous pores and channels as a result of the etching procedure was observed in all ceramic materials. In the EXHF group, irregularly shaped structures and canals with a width of 10-20µm characterize the ceramic surface. At higher magnifications, microwells with a size of 1-2 µm and channels with a length of 5-10 µm were observed. In the EHF group, lithium disilicate crystals with different orientations and the glassy phase dissolved between them can be seen. Channels with a wavy course and a length of about 5 µm are also observed. In the HHF group, 5-20µm cavernous-like formations surrounded by partially dissolved channels can be seen. Conclusions: Each ceramic material showed different etching patterns due to the material’s composition and the distribution of the crystalline and vitreous phases.


INTRODUCTION
Dental ceramic is the most widely used dental material [1,2]. It provides high aesthetics and functional durability in all-ceramic and porcelain fused to metal restorations [3].Recent progress in chemistry led to the development of many ceramic materials [4]. Some of them are indicated for all-ceramic inlays, onlays, overlays, veneers, full crowns, or frameworks; others could be used as a veneering material. When high aesthetic performance is desired, all-ceramic restorations are a suitable choice. They provide excellent mechanical and optical properties [5,6]. Glass ceramics show excellent aesthetic properties and weaker mechanical characteristics compared to polycrystalline ceramics. After resin cement bonding, the flexural strength of glass-ceramic increases; thus, adhesive bonding is necessary for restoration longevity [7].
Hydrofluoric acid (HF) etching is a crucial step when bonding glass ceramics to tooth structures. It alters surface roughness surface free energy and provides better micromechanical retention [8]. These factors are essential for providing a strong and durable resin ceramic bond. Etching is an essential step when recently developed hybrid ceramics and resin nanoceramic materials are used [9]. Etching patterns differ according to the material choice, HF concentration, and etching time [10,11].
TheHF-treated surface described by the various authors has numerous, evenly distributed channels and pores that become larger and deeper with the increased acid action time [12][13][14]. Menget al.
This study aims to investigate the HF etching patterns of three types of glass-ceramic materials.

MATERIALS AND METHODS
Fourteen rectangular experimental bodies with a cross-section of 2x2mm and a length of 5-7 mm were made from each type of the following dental ceramics: Heraceram press (press ceramics, HeraeusKulzer GmbH, Hanau, Germany), EX-3 (synthetic feldspar veneer porcelain, Kuraray Noritake Dental, Japan), E.max press -lithium disilicate press ceramics (IvoclarVivadent, Schaan, Lichtenstein). Additive silicone matrices (Elite Double 8, Zhårmack Technical, BadiaPolesine, Italy) were utilized to make the ceramic rods. Wax rods prototypes were prepared for the two pressable ceramics. They were packed in refractory die material and pressed according to the manufacturer's recommendations after the wax hardening (Fig. 1). All steps were performed by the same operator. Heraceram Press wax rods (cross-section of 2x2mm and a length of 10mm) were packed in Heravest press (HeraeusKulzer GmbH, Hanau, Germany) -200 gr. powder a liquid: distilled water ratio 40ml: 10ml, according to the manufacturer's instructions. After 30 minutes, the coupling was released from the silicone mold and placed for 40 minutes in a muffle furnace (VOP, Bulgaria), preheated to 850ºC. Ceramic Master ceramics press furnace (VOP, Bulgaria) was used according to the manufacturer's instructions. The samples were released from the mold. They were cleaned from the investment material with 30 µm silica powder at a 2.5 Bar pressure. The samples werecut with a diamond disk mounted on a dental micromotor to a standard size -crosssection 2x2 mm and length 5-7 mm.
E.max press wax rods were made following the same procedure. The wax prototypes were packed in a refractory die material IPS Press Vest Speed (IvoclarVivadent, Schaan, Lichtenstein ) in 200-gram couplings at a liquid: distilled water ratio of 32ml: 22ml according to the manufacturer's instructions. The couplings were heated in a muffle furnace at a temperature of 850°C for 60 minutes. The pressing process followed the manufacturer's instructions: starting temperature 700ºC, temperature rise rate 60ºC/min., T 930ºC, H 25 min., V1 500ºC, V2 930ºC. The subsequent stages of sample preparation were the same as those already described in the preparation of the samples by Heraceram press. EX-3 ceramic powder was mixed with distilled water to obtain a homogeneous mass. By successive application and condensation of the ceramic in the silicone mold, bodies with a cross-section of 2x2mm and a length of 23mm were formed. They were carefully removed from the silicone matrix and placed on a heat-resistant plate, then sintered at the following temperature: drying 10 min., starting temperature 600ºC, vacuum start at 600ºC, growth rate 45ºC/min., Vacuum shut-off at 940ºC, baking temperature at 950ºC, cooling 4 min. Corrective sintering was performed at the following temperature regime: drying 7 min., Starting temperature 600ºC, vacuum start at 600ºC, growth rate 45ºC/ min., Vacuum shutdown at 910ºC, baking temperature 920ºC, cooling 4 min. The samples werecut with a diamond disk to a standard size -section 2x2 mm and length 5-7 mm.
One side of the test specimens was polished with three degrees of abrasiveness to obtain a mirror surface. The final treatment was performed with Drendel + Zweiling polishing rubber (LOT 060907).
The samples of each ceramic material were divided into two groups. The first group (n=7) was treated with 5% HF (IPS Ceramic Etching Gel, IvoclarVivadent, Schaan, Lichtenstein) as follows: Heraceram Press and EX-3 for 120 sec and E.max Press for 20 sec. (groups EHF, EXHF, HHF). The second group (n=7) received no furder treatment after polishing (groups EP, EXP, HP). After processing, the samples from both groups of the three types of ceramics were cleaned in an ultrasonic bath with distilled water for 10 minutes. They were stored in a dust-free environment until scanning electron microscope (SEM) examination.
The samples' examination was carried out in a research laboratory at the Faculty of Geology, MGU "St. Ivan Rilski"-Sofia. The test specimens were mounted on special plates and then coated with carbon to provide electrical conductivity in a JEOL JEC-530 apparatus (Jeol Ltd., Tokyo, Japan). The plates were fixed in the respective holders and observed on an SEM JEOL JSM-6010PLUS / LA (Jeol Ltd., Tokyo, Japan, Fig. 2).

RESULTS
Each ceramic material showed different etching patterns. An evenly scratched surface was found in the EXHF group samples. HF has dissolved the vitreous phase, with the acid-unreacted areas visible in the photographs as flatter structures with an area of 5-20 µm and a highly branched course protruding over wells and channels of different sizes. The areas corresponding to the acid-dissolved sections were irregularly shaped and had a width of 10-20 µm and a complex winding course. At higher magnifications, it was noticed that in the depth of these sections, there were microwells with a size of 1-2 µm, as well as channels with a length of 5-10 µm and a width below 2 µm (Fig. 3).  Lithium disilicate crystals with different orientations and the glassy phase dissolved between them could be seen under magnification x1500 and x2700. The formed pits had an irregular shape and dimensions below 2-3 µm. Channels with a wavy course and a length of about 5 µm were also observed (Fig. 5).  A smooth defect-free surface with single traces of the polishing procedure and a vitreous phase located between them was found in the polished specimens made by E.max Press (Fig. 6). The HF treated Heraceram Press samples are shown in Fig. 7. The presence of relatively clearly formed structures with irregular oval and elliptical shapes corresponding to the areas dissolved by the acid was established even at low magnification. Under higher magnification, these were found to be cavernous-like formations. There were partially dissolved channels in many areas, which may surround an almost entirely formed cavity or were located on the forming one's borders. The caverns' dimensions were 5-20 µm. Pits with a diameter of less than 5µm located between the formed caverns were observed. Fig. 7. SEM micrographs of HF etched Hera Ceram press ceramic specimens. Magnification x50, x1000, x1500, x2700 (A, B, C, D).

DISCUSSION
HF etching altered the surface morphology in all three types of ceramic. The presence of numerous pores and channels as a result of the etching procedure was an expected SEM image, and similar changes have been observed by other authors [15][16][17]. A three-dimensional structure resembling three-dimensional dendrites or a honeycomb was described by Kato et al. [16] in sintered porcelain (VMK 68 dentin) and Meng et al. [15] in GN-I machinable ceramic. Ramakrishnaiah et al. [10] described irregular surfaces and numerous micro porosities, grooves, and striations due to the glassy phase's dissolution in five ceramic materials.
The SEM images of the studied groups in this research differ due to the composition of the material and the crystalline and vitreous phases' distribution. In the HHF group, structures resembling caverns of different sizes located in areas undissolved by the acid were found. In the E.max Press group, homogeneous ceramic dissolution with partial vitreous phase preservation around the lithium disilicate crystals was established. Irregular pits and wavy channels characterize the ceramic surface. More uniform SEM images were observed in the EXHF group.
HF concentration used in this study was 5% for all tested ceramic materials. Etching time was 120 s in HHF and EXHF and 20 s in the EHF group in accordance with other studies [15,16,18] and manufacturer's instructions. In a study by Ramakrishnaiah et al. [10], etching time of 20 s resulted in the predominant dissolution of the glassy phase and small isolated pores with irregular borders. Etching duration of 40 s increased the size of the pores, which appeared as elongated grooves. A further increase in the etching duration resulted in an extensive loss of the glassy phase and irregularly oriented crystals exposure. An increase in the etching time resulted in increased wettability or hydrophilicity of the ceramic surface [10], leading to higher resinceramic bond strength. Ten percent HF etching resulted in higher shear bond strength value of feldspathic ceramic to resin cement than the 5% HF etching [10]. Straface et al. recommended surface pretreatment of silicate ceramics with 5% or 9% HF for 15 s to 60s to achieve the highest shear bond.
[19]. Another study did not show the significance of the HF etching time and concentration on the micro shear bond strength between resin cement and ceramics [20]. On the other hand, extended overetching could lead to ceramics weakening thus, it should be adapted to the type of ceramic material [21].
Differences in surface morphology were observed between the polished specimens in this study. EHP group showed a smooth and defect-free surface. The EXP and HP group specimens were hard to polish from a practical point of view, as observed by the operator. As a result, numerous defects (mainly in the HP group) and polishing traces (EXP group) could be seen in the SEM micrographs. These findings could be explained with the material structure and could be observed in other studies [10]. Preexisting cracks and defects are the first to be affected by low HF concentrations. As a result, specific pores, furrows, and channels form. This process could explain the etching patterns in EXP and HP groups.

CONCLUSION
Each ceramic material showed different, specific etching patterns due to the material's composition and the crystalline and vitreous phases distribution.
Under low magnification, HP specimens showed a smooth polished surface. Under higher magnification, pits less than 5 mm in diameter could be seen. They are unevenly distributed and could be easily distinguished on the polished ceramic surface (Fig. 8).