title: The effect of veneering and heat treatment on the flexural strength of Empress 2 ceramics.
abstract: OBJECTIVE
  The aims of the study were to test and compare the biaxial flexural strength and reliability of Empress 2 ceramics after heat treatment and the addition of the veneering material and to characterise their microstructures.
  
  
  METHODS
  Forty disc specimens (2 x 14 mm) and forty disc specimens (1 x 14 mm) were produced by heat pressing in the EP 500 press furnace. Group 1 (2 x 14 mm Empress 2 core) was as heat pressed and group 2 (2 x 14 mm Empress 2 core) was subjected to the recommended firing cycles. Groups 3 and 4 (1 x 14 mm Empress 2 core) were veneered with the dentine material and heat-treated as per group 2. Groups 1, 2 and 3 were lapped to 800 grit silicon carbide paper on the compressive surface only and group 4 on both the compressive and tensile test surfaces. Twenty disc specimens per group were tested using the biaxial flexure test at a crosshead speed of 0.15 mm/min. Specimens were characterised using X-ray diffraction (XRD) and secondary electron imaging (SEM).
  
  
  RESULTS
  Mean biaxial flexural strengths (MPa+/-SD) were group 1: 265.5+/-25.7; group 2: 251.3+/-30.2; group 3: 258.6+/-21.4 and group 4: 308.6+/-37.7. There was no statistical difference between groups 1, 2 and 3 (p>0.05), but differences for group 4 (p<0.05). XRD and SEM revealed lithium disilicate and lithium orthophosphate in the Empress 2 core material and an amorphous glass and some evidence of a crystalline phase in the dentine material. CONCLUSIONS; Veneering or heat treatment of Empress 2 ceramics did not significantly affect the mean biaxial flexural strength (p>0.05) or reliability. Surface modification of the Empress 2 core material increased the mean biaxial flexural strength (p<0.05).
authors.affiliation: Department of Conservative Dentistry, St Bartholomew's and The Royal London School of Medicine and Dentistry, QM, Turner Street, London E1 2AD, UK. m.cattell@mds.qmul.ac.uk
authors.forename: M J
authors.initials: MJ
authors.lastname: Cattell
authors.forename: R P
authors.initials: RP
authors.lastname: Palumbo
authors.forename: J C
authors.initials: JC
authors.lastname: Knowles
authors.forename: R L
authors.initials: RL
authors.lastname: Clarke
authors.forename: D Y D
authors.initials: DY
authors.lastname: Samarawickrama
mesh_terms: D017554:Carbon Compounds, Inorganic / Q000737:chemistry; D019245:Compressive Strength; D016001:Confidence Intervals; D003460:Crystallization; D003764:Dental Materials / Q000737:chemistry; D003776:Dental Porcelain / Q000737:chemistry*; D003801:Dental Veneers; D004548:Elasticity; D004577:Electron Probe Microanalysis; D005898:Glass / Q000737:chemistry; D006358:Hot Temperature; D006801:Humans; D018020:Lithium Compounds / Q000737:chemistry*; D008422:Materials Testing; D016017:Odds Ratio; D010710:Phosphates / Q000737:chemistry; D018583:Pliability; D015203:Reproducibility of Results; D017640:Silicates / Q000737:chemistry; D017655:Silicon Compounds / Q000737:chemistry; D013314:Stress, Mechanical; D013499:Surface Properties; D013718:Tensile Strength; D014961:X-Ray Diffraction
chemical_list: D017554:Carbon Compounds, Inorganic; D003764:Dental Materials; C118557:Empress 2; D018020:Lithium Compounds; D010710:Phosphates; D017640:Silicates; D017655:Silicon Compounds; D003776:Dental Porcelain; C022088:silicon carbide