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PubMed1989

[Evaluation of the effectiveness of ion exchange on strengthening of dental porcelain].

This study evaluated the effectiveness of ion exchange on strengthening of porcelain used in metal ceramic. The paste for ion exchange was painted directly on the porcelain surface, dried, and heated at 450 degrees C in the porcelain furnace. The bending strength of porcelain test samples (6.5 x 1.5 x 20 mm), the compressive strength of porcelain-fused-to-metal (PFM) crowns, which were cemented on the metal dies of upper central incisor by an adhesive luting resin 'Panavia EX', and the color change of porcelain test samples (phi 10 x 4 mm) were measured before and after ion exchange. The conclusions were: 1. It was confirmed by X-ray analysis that the compressive stress layer generated by ion exchange for 30 minutes was 150-200 microns in thickness. However, further elongation of time of ion exchange elicited no significant argumentation in the strength. 2. 34 or 57 percent increase of the mean compressive strength of PFM crowns was observed after ion exchange for 30 minutes when the load was applied in the direction of 45 degrees lingually to the tooth long axis or of the tooth long axis, respectively. 3. No significant decrease was found when parts of the incisal edge or the lingual surface of PFM crowns which were strengthened by ion exchange for 30 minutes were reduced by 200 microns. 4. There was so slight difference in the color of dental porcelain before and after ion exchange that the influence seemed to be negligible in its clinical application.

Dental Porcelain↗
PubMed1994

Stress relaxation behavior of dental porcelains at high temperatures.

OBJECTIVES: The purpose of this study was to measure the stress relaxation behavior at elevated temperatures of three experimental opaque porcelains and three experimental body porcelains. METHODS: Feldspathic porcelain formulations covering a range of thermal contraction coefficients were supplied by a dental ceramics manufacturer. Six specimens, 11 mm in diameter by 22 mm long, were fabricated for each porcelain. The specimens were tested in compression at five temperatures controlled to +/- 1 degree C in a hot stage furnace attached to a screw-type uni-axial testing machine. RESULTS: Mean values of relaxation time, tau u, and the b function were determined by a regression fit to the relation: psi (t) = exp [-(t/tau u)b]. Values of b ranged from 0.23 to 0.53 for opaque porcelain and 0.47 to 0.64 for body porcelain. Relaxation times ranged from 2.6 s to 4 x 10(4) s for the opaque porcelains and 1.5 s to 5.5 x 10(2) s for the body porcelains. A statistically significant variation of b with temperature for three of the experimental porcelains is an indication that these porcelains do not satisfy the theoretical requirements for the porcelains to be classified as thermorheologically simple. SIGNIFICANCE: A knowledge of the relaxation behavior of dental porcelains is necessary so that dental researchers can identify metal/porcelain combinations that will result in low stress values and, therefore, reduce the potential for failure from thermally induced stresses. These properties can be used in the optimization of prosthesis design to reduce the destruction of healthy tissue to accommodate the placement of the dental prosthesis.

Dental Porcelain↗