Effects of multiple firings on mechanical properties and resin bonding of lithium disilicate glass-ceramic

Effects of repetitive firings on mechanical, optical, and phase formation changes of ceramic materials

STATEMENT OF PROBLEM

Studies are sparse on how rapid heating and cooling negatively affect optical properties, including color and transparency, and mechanical properties, including hardness and durability, that affect esthetics and shorten the clinical usage period of the ceramics.

PURPOSE

The purpose of this in vitro study was to determine the effects of repeated firing on the color difference, mechanical properties, and phase formation of different ceramic materials.

MATERIAL AND METHODS

A total of 160 disks (12×1.35 mm) were produced from 4 different ceramic materials: lithium disilicate glass-ceramic, zirconia-reinforced lithium silicate ceramic, zirconia core, and monolithic zirconia. Specimens from all groups were divided into 4 groups (n=10) with a different number of veneer porcelain firings (1 to 4) by simple randomization. After the firings, color measurement, X-ray diffraction analysis, environmental scanning electron micrograph analysis, surface roughness evaluation, Vickers hardness evaluation, and biaxial flexural strength tests were performed. Data were analyzed with analysis of variance (2-way ANOVA) (α=.05).

RESULTS

Repeated firing did not affect the flexural strength of the specimens in any of the groups (P>.05), while the color, surface roughness, and surface hardness were significantly affected (P<.05). The specimens fired 4 times showed the highest mean Vickers hardness and ΔE00 values but the lowest mean surface roughness values. Zirconia core specimens showed the highest mean ΔE00 and flexural strength values, and lithium disilicate glass-ceramic specimens had the highest mean Vickers hardness values.

CONCLUSIONS

The increase in the number of firings affected the color, mechanical properties, and phase formation of the specimens; this change differed according to the ceramic tested.

Long-term effect of firing protocols on surface roughness and flexural strength of lithium disilicate glass-ceramic

This study evaluated the effect of different firing protocols on the surface roughness and flexural strength of CAD/CAM lithium disilicate glass-ceramic (LD) after aging methods. Forty-two LD bars of 16 x 4 x 2 mm (IPS e-max CAD, Ivoclar) were randomly separated into two groups according to firing protocols: Single firing-Staining, glazing, and crystallization in a single step; Multiple firings-Crystallization+First staining+Firing+Second staining+Firing+Glazing+Firing. After protocols, initial surface roughness readings were taken (Surfcorder SE1700, Kosakalab). Samples were then randomly separated into three groups (n=7) according to the aging methods they were submitted: Thermomechanical cycling (TMC, ER System, Erios, 1,200,000 cycles, 0.3 MPa, 2 Hz and 5°C/37°C/55°C, 30 s swell time); Simulated toothbrushing (STB, Pepsodent, MAVTEC, 73,000 cycles), and Control (no aging). Final surface roughness readings were done, and samples were submitted to a three-point bending test (OM100, Odeme Dental Research) and fractographic analysis by scanning electron microscopy (EVO-MA10, ZEISS). Data were analyzed (2-way ANOVA, (α=.05). There was no difference (p>.05) in the flexural strength between the firing protocols, regardless of the aging method. STB decreased the flexural strength of samples submitted to multiple firings, different from control (p<.05). Without aging (Control), before TMC, and after STB, LD had lower surface roughness when submitted to multiple firings than to single firing (p<.05). The firing protocols did not affect the flexural strength or the surface roughness of the lithium disilicate glass-ceramic, even after aging. However, toothbrushing negatively affected the flexural strength and smoothed the surface of the ceramic submitted to multiple firings.

The Toothbrushing Effects on Surface Properties and Color Stability of CAD/CAM and Pressable Ceramic Fixed Restorations-An In Vitro Study

Pressable ceramic restorations have been introduced and investigated, and found comparable to CAD/CAM ceramic in terms of mechanical properties; however, the effect of toothbrushing on the pressable ceramic has not been thoroughly investigated. The objective of the current study was to assess the effect of artificial toothbrushing simulation on the surface roughness, microhardness, and color stability of different ceramic materials. Three lithium disilicate-based ceramics (IPS Emax CAD [EC], IPS Emax Press [EP]; (Ivoclar Vivadent AG), and LiSi Press [LP] (GC Corp, Tokyo, Japan)) were examined. For each ceramic material, eight bar-shaped specimens were prepared and subjected to 10,000 brushing cycles. Surface roughness, microhardness, and color stability (∆E) were measured before and after brushing. Scanning electron microscopy (SEM) was used for surface profile analysis. The results were analyzed using one-way ANOVA, Tukey's post hoc test, and paired sample t-test α = 0.05. The findings revealed a non-significant decrease in the surface roughness of EC, EP, and LP groups (p > 0.05), and both LP and EP have the lowest surface roughness values (0.64 ± 0.13, 0.64 ± 0.08 µm) after brushing, respectively. Toothbrushing showed a decrease in the microhardness of the three groups: EC and LP, p < 0.001; EP, p = 0.012). EP showed the lowest hardness value after brushing (862.45 ± 273.83). No significant changes (∆E) were observed in all groups (p > 0.05); however, the EC group was found to be considerably affected by color changes, in comparison to the EC and LP groups. Toothbrushing had no effect on surface roughness and color stability of all tested materials, but it decreased the microhardness. Material type, surface treatments, and glazing of ceramic materials contributed to the surface changes in the ceramic materials, necessitating further investigations in terms of the toothbrushing effect with different glazing as variables.

The effect of prolonged holding time on the mechanical property and microstructural property of lithium disilicate glass-ceramic

Repeat firing produces uncertainty about stabilizing lithium disilicate glass-ceramic (LDGC) material properties, even though prolonged holding time can enhance the mechanical property of LDGC during a single firing cycle. However, the effect of prolonged holding time and repeat firing on the mechanical property and microstructure of LDGC is not fully understood. In the present study, three groups of LDGC material were created: (i) extension of holding time (7 vs. 14 vs. 28 min) at 780-800 °C; (ii) holding time extension (7 vs. 14 min) and dual sintering at 800-820 °C, respectively; (iii) dual sintering with prolonged holding time (7 vs. 14 min) at 820-840 °C. The nano-indenter test revealed that prolonged holding time (14 and 28 min) promoted the enhancement of LDGC hardness and Young's modulus. X-ray photoelectron spectroscopy, X-ray diffraction and Fourier transform infrared spectroscopy confirmed that prolonged holding time increased and stabilized LD phase in LDGC, as well as induced residual compressive stress. Scanning electron microscopy showed that prolonged holding time increased LD crystal grains homogeneously and facilitated LDGC to form dense interlocking structure without enlarging crystal size grains significantly. In contrast, LDGC that dual sintered alone at 820-840 °C possessed inferior mechanical properties, coupled with heterogeneous crystal phases, residual tensile stress, and melted crystals grains in the porous microstructure. Interestingly, these deteriorated properties of LDGC caused by dual sintering alone could be counteracted by prolonging the holding time. Nevertheless, the LDGC materials displayed an excellent biocompatibility throughout the study. This study identified that prolonged holding time during repeated firing cycles stabilized LD phase and crystal grain size of LDGC, thus enhanced the mechanical properties, which provided a new insight to extend the repeat fired restoration longevity of LDGC. Graphical abstract.

Effect of multiple firings on surface roughness and flexural strength of CAD-CAM ceramics

STATEMENT OF PROBLEM

Knowledge on the effect of multiple firings on surface roughness and the flexural strength of different types of monolithic computer-aided design and computer-aided manufacturing (CAD-CAM) ceramics is limited.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of multiple firings on the surface roughness and flexural strength of 4 different CAD-CAM ceramics after thermocycling.

MATERIAL AND METHODS

Four different CAD-CAM ceramics (Lava All Zirconia 3-mol yttria tetragonal zirconia polycrystal [3-YTZP] [Z], VITA SUPRINITY [S], IPS e.max CAD [EX], IPS Empress CAD [E]) (n=33) were wet-sectioned to form rectangular 18×4×1.2-mm specimens. After glaze application, the specimens were divided into 3 subgroups according to the number of firings (1, 2, and 4) (n=11). The specimens were thermocycled (10 000 cycles), and surface roughness and flexural strength values were measured (n=10). One additional specimen from each group was analyzed by using scanning electron microscopy (SEM). Data were analyzed by 2-way analysis of variance (ANOVA) and the Tukey honestly significance difference (HSD) test (α=.05).

RESULTS

According to the 2-way ANOVA, the material, number of firings, and the interaction between the material and number of firings affected the surface roughness (P<.001). For flexural strength, material (P<.001) and number of firings (P<.039) were found significant. Multiple firings (2 or 4 firings) affected the surface roughness of E (P<.001). Regardless of the number of firings, the Z material had the highest flexural strength (P<.001). Four firings affected the flexural strength values only for the Z material (P≤.005).

CONCLUSIONS

CAD-CAM ceramic type affected the surface roughness and flexural strength values. The surface roughness of E was lower when fired 2 or 4 times than when fired once. The flexural strength of Z was lower when fired 4 times than when fired once.

Effect of firing cycle and aging on long-term chemical degradation of monolithic CAD-CAM ceramics

STATEMENT OF PROBLEM

Previous studies have shown the susceptibility of dental ceramics to degradation when subjected to certain media. However, knowledge on the effect of repeated firings and thermocycling on the ion elution of computer-aided design and computer-aided manufacturing (CAD-CAM) ceramics is lacking.

PURPOSE

The purpose of this in vitro study was to compare the effect of repeated firings on the ion elution of CAD-CAM materials before and after thermocycling.

MATERIAL AND METHODS

Bar-shaped specimens were prepared from 4 different CAD-CAM materials (monolithic zirconia [Z], zirconia-reinforced lithium silicate glass-ceramic [S], lithium disilicate glass-ceramic [EX], and leucite-reinforced glass-ceramic [E]) and divided into 3 groups according to the number of repeated glaze firings (1 firing [1F], 2 firings [2F], and 4 firings [4F]). Specimens were placed into deionized water (pH 7.4) and stored at 37 °C for 168 hours. Inductively coupled plasma-optic emission spectrophotometry (ICP-OES) was used to measure the baseline values of the eluted ions in immersion. The specimens were then subjected to thermocycling. Then, surface roughness (R_a) and ion elution values were measured. The Kruskal-Wallis and Mann-Whitney U tests were used to analyze the ion elution data before and after thermocycling, and the effect of thermocycling on ion elution was assessed by the Wilcoxon signed rank test. R_a data were analyzed with 2-way analysis of variance (ANOVA) and the Tukey honestly significant difference tests (α=.05).

RESULTS

Elution of some ions varied depending on the material-firing pair before (Al, As, B, Ba, Cr, Cu, Li, Mg, Na, P, and Zn) and after (Al, As, B, Ba, Be, Ca, Cd, Co, Cr, Cu, K, Li, Mg, Na, P, Y, and Zn) thermocycling. Before thermocycling, all firing groups within each material showed a similar number of significantly higher eluted ions. After thermocycling, the number of significantly higher eluted ions decreased in all materials, except for EX. The effect of thermocycling on the ion elution of the 1F group of Z (Al, Be, Ca, Cd, Co, Cr, Cu, K, Li, P, Y, and Zn), S (As, Be, Cd, Co, Cr, K, P, and Y), EX (B, Cu, and P), and E (B and Ba); 2F group of Z (Al, Be, Ca, Co, Cr, Cu, K, Li, P, and Y), S (Be, Cd, Co, K, Li, and Y), EX (P), and E (P); 4F group of Z (Al, As, Ba, Be, Ca, Cd, Co, Cr, Cu, K, Li, P, and Y), S (Al, Be, Cd, Co, Cr, Li, Mg, and Y), EX (Be, Ca, Cd, Co, K, Y, and Zn), and E (Ca and P) was nonsignificant (P≥.051). The interaction between material and repeated firings (P<.001) had a significant effect on R_a. For 1F groups, E showed the highest R_a (P≤.003), while Z had higher R_a than S (P=.009). For 2F groups, Z had higher R_a than S (P=.01). The differences among 4F groups were nonsignificant (P≥.677). An increased number of repeated firings (2F and 4F) decreased the R_a of E (P<.001).

CONCLUSIONS

The effect of repeated firings and thermocycling on the chemical stability of the tested CAD-CAM materials varied. No clear trend was observed on the elution of different ions within material-firing pairs before thermocycling. However, thermocycling increased the number of significantly higher eluted ions for EX. The effect of thermocycling on the ion elution of materials varied depending on ions. Repeated firings decreased the surface roughness of E.

Effect of Consecutive Firings on the Optical and Mechanical Properties of Silicate and Lithium Disilicate Based Glass-Ceramics

PURPOSE

To evaluate the effect of multiple firings on the optical and mechanical properties of two dental CAD/CAM glass-ceramics.

MATERIALS AND METHODS

A total of 45 specimens of each lithium disilicate-LD (IPS E.max CAD, Ivoclar Vivadent) and zirconia lithium silicate-ZLS (Vita Suprinity, Vita Zahnfabrik) material were prepared in a disk shape. These specimens were divided into three groups according to two factors: "type of ceramic" (LD and ZLS) and "numbers of firings" (Control 2F-two firings, 5F-five firings and 7F-seven firings). The firing cycles were performed according to the manufacturer's recommendations. X-ray diffraction was additionally performed to determine crystalline phases in each group, spectrophotometry was used to determine color and translucency variation, and biaxial flexural strength (BFS) evaluated the mechanical behavior. The data were analyzed individually using two-way ANOVA tests and Tukey's test at α = 0.05.

RESULTS

The crystalline phases did not present any change after multiple firings for either of the analyzed materials. Both commercial materials showed a significant difference regarding translucency at 7F (p = <0.01), and ZLS presented a difference in color higher than one (ΔE > 1) at 5F and 7F. Regardless of the number of firings, LD presented a higher BFS compared to ZLS (p = <0.001), and a significant increase in BFS comparing 2F and 7F (p = <0.024).

CONCLUSION

The use of multiple firings can significantly alter the color, translucency, and mechanical strength of CAD/CAM ceramics.

Effects of multiple firing processes on the mechanical properties of lithium disilicate glass-ceramics produced by two different production techniques

STATEMENT OF PROBLEM

Repeated firings cause materials to be exposed to additional heat treatments. The effect of these additional heat treatments on the mechanical properties of lithium disilicate glass-ceramics is not fully known.

PURPOSE

The purpose of this in vitro study was to determine the effects of repeated firing on the mechanical properties of lithium disilicate glass-ceramics produced by 2 different techniques, press and computer-aided design and computer-aided manufacturing (CAD-CAM).

MATERIAL AND METHODS

Eighty rectangular (25×4×2 mm) lithium disilicate glass-ceramic specimens were used in this study, 40 produced by heat pressing and 40 by milling, and divided into 4 groups (n=10) with a different number of veneer porcelain firings (1 to 4). After firing, the Vickers hardness, flexural strength (3-point bend test), and fracture toughness were determined, and the specimens were analyzed with an environmental scanning electron micrograph. Data were analyzed with analysis of variance (ANOVA) (α=.05).

RESULTS

The repeat firing processes did not affect the flexural strength of the specimens in either group (P>.05), while the surface hardness and fracture toughness were significantly changed (P<.05).

CONCLUSIONS

Increasing the number of firings adversely affected the mechanical properties of lithium disilicate glass-ceramics.