title: Effect of surface acid etching on the biaxial flexural strength of two hot-pressed glass ceramics.
abstract: OBJECTIVE
  The purpose of this study was to assess the effect of surface acid etching on the biaxial flexural strength of two hot-pressed glass ceramics reinforced by leucite or lithium disilicate crystals.
  
  
  METHODS
  Forty glass ceramic disks (14-mm diameter, 2-mm thick) consisting of 20 leucite-based ceramic disks (IPS Empress) and 20 lithia disilicate-based ceramic (IPS Empress 2) were produced by hot-pressing technique. All specimens were polished and then cleaned ultrasonically in distilled water. Ten specimens of each ceramic group were then etched with 9% hydrofluoric (HF) acid gel for 2 minutes and cleaned ultrasonically again. The biaxial flexural strength was measured by the piston-on-three-ball test in a universal testing machine. Data based on ten specimens in each group were analyzed by two-way ANOVA (alpha= 0.05). Microstructure of ceramic surfaces before and after acid etching was also examined by a scanning electron microscope.
  
  
  RESULTS
  The mean biaxial flexural strength values for each group tested were (in MPa): nonetched IPS Empress = 118.6 +/- 25.5; etched IPS Empress = 102.9 +/- 15.4; nonetched IPS Empress 2 = 283.0 +/- 48.5; and etched IPS Empress 2 = 250.6 +/- 34.6. The results showed that the etching process reduced the biaxial flexural strengths significantly for both ceramic types (p= 0.025). No significant interaction between the ceramic type and etching process was found (p= 0.407).
  
  
  CONCLUSIONS
  From the results, it was concluded that surface HF acid etching could have a weakening effect on hot-pressed leucite or lithia disilicate-based glass ceramic systems.
authors.affiliation: Research Center for Science and Technology in Medicine, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran. hoshmand@sina.tums.ac.ir
authors.forename: Tabassom
authors.initials: T
authors.lastname: Hooshmand
authors.forename: Shaghayegh
authors.initials: S
authors.lastname: Parvizi
authors.forename: Alireza
authors.initials: A
authors.lastname: Keshvad
mesh_terms: D000134:Acid Etching, Dental; D000538:Aluminum Silicates; D000704:Analysis of Variance; D003776:Dental Porcelain / Q000737:chemistry; D003799:Dental Stress Analysis; D006358:Hot Temperature; D006858:Hydrofluoric Acid; D018020:Lithium Compounds; D008422:Materials Testing; D018583:Pliability; D013314:Stress, Mechanical; D013499:Surface Properties; D013718:Tensile Strength
chemical_list: D000538:Aluminum Silicates; C118557:Empress 2; C075815:IPS-Empress ceramic; D018020:Lithium Compounds; D003776:Dental Porcelain; D006858:Hydrofluoric Acid