Chemical and mechanical stability of an ion-exchanged lithium disilicate glass in artificial saliva.

Multi-component lithium disilicate (LD) glasses were ion-exchanged in a pure or mixed nitrate salt bath. The surface morphologies, mechanical properties, chemical stability and ion leaching of ion-exchanged LD glasses before and after storage in artificial saliva for 21 days were investigated. It can be found that chemical stability of ion-exchanged LD glass was temperature-dependent. The residual compressive stress induced by ion-exchange increased the chemical potential of alkali ions in glass, and the ion-exchanged LD glass, especially 235 °C/64 h group, chemical stability in artificial saliva for 21 days were deteriorated. Back-exchange treatment could relax the stress on the outermost layer of the ion-exchanged LD glass without deteriorating its strengthening effect, and back-exchanged LD glass presented good chemical and mechanical stability in artificial saliva. The results might help to enhance the service stability of ion-exchanged LD glass-ceramics in the oral condition.

Improving early running-in wear characteristics for dental lithium disilicate glass-ceramics by ion-exchange.

OBJECTIVES: This study examined the effects of Li+→Na+ ion-exchange on the early wear performance of dental lithium disilicate (LD) glass-ceramics. METHODS: Specimens with different shapes were prepared using IPS e.max Press as the LD glass-ceramics. Ion-exchange was conducted by placing polished specimens in molten salt containing 25% NaNO3 and 75% KNO3 at 385 °C for 16 or 64 h. The ion-exchanged specimens were analyzed using X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) to investigate the structure and the elemental distribution. Thereafter, the specimens were tested for flexural strength, Vickers hardness, and fracture resistance. A portion of the specimens were tested with a pin-on-disk tribometer with 10 N for 40 × 104 wear cycles in artificial saliva. Wear analysis of the specimens was performed using a 3D profilometer and analyzed with one-way analyses of variance and Tukey's post hoc pairwise comparisons. Worn surfaces were examined with scanning electron microscopy. RESULTS: The LD glass-ceramics exhibited strong time-dependent wear behavior, with typical running-in and steady wear stages. Ion-exchange treatments at 385 °C for 16 h and 64 h both enhanced the mechanical properties and decreased the wear rates of early running-in wear stage. The early wear performance of specimens treated with ion-exchange for long time (64 h) was improved significantly. CONCLUSION: A thicker ion-exchange layer may be obtained by processing ion-exchange for a long time. This protocol improves the early wear performance of the glass-ceramics effectively. CLINICAL SIGNIFICANCE: Dental restorations may fail prematurely due to excessive wear. It is important to improve the early wear performance of the glass-ceramics. Ion-exchange has the potential to strengthen dental LD glass-ceramics. Understanding the effect of ion-exchange on the early wear performance of glass-ceramics provides insight improving the early wear performance of these restorations.

Effect of Li+/Na+ exchange on mechanical behavior and biological activity of lithium disilicate glass-ceramic.

Lithium disilicate (LD) glass-ceramics with a stoichiometric composition were ion-exchanged in pure NaNO3 or mixed NaNO3 + KNO3 molten salt baths below the glass transition temperature (Tg). The microstructures, surface morphologies, mechanical properties and bioactivities of the ion-exchanged glass-ceramics were studied in detail. It was found that the strength and toughness of LD glass-ceramic could be enhanced from 175 MPa to 0.96 MPa m1/2 before ion-exchange to 546 MPa and 4.31 MPa m1/2 respectively under a lowered ion-exchange temperature because the less stress relaxation. In addition, a gradient of Na+ rich layer in the surface of glass-ceramic was induced by Li+/Na+ exchange, which could be beneficial to the formation of HA (Hydroxyapatite) with nano-size porous after soaking in SBF (Simulated Body Fluid) solution and exhibited better bioactivity compared with the original LD glass-ceramic. The results might provide a reference for the strengthening and biological activation of LD glass-ceramics in bone restoration applications.

Improved reliability of mechanical behavior for a thermal tempered lithium disilicate glass-ceramic by regulating the cooling rate.

Traditional thermal tempering was applied to lithium disilicate (LD) glass-ceramic specimens with bar-like and disc-like shapes. The tempering process was conducted by heating the specimens to a temperature below the dynamic softening point, and then rapid cooling in silicone oil with different temperatures ranging from room-temperature to 300 °C to regulate the cooling rate. Effect of the oil-temperature on mechanical behavior of the tempered glass-ceramic was investigated. For the tempering at the lower oil-temperature (e.g., at room-temperature), it was found that the LD glass-ceramic specimens with both the bar-like and disc-like shapes could be remarkably strengthen and toughen, however, obvious anisotropy in fracture toughness was displayed by the specimens with the bar-like shape. With increasing the oil-temperature up to 250 °C, the mechanical anisotropy of the bar-like specimens could be significantly alleviated without much loss of the strengthening effect. The results can provide references for improving reliability of mechanical behavior for the tempered LD glass-ceramic by regulating the cooling condition according to specimen geometry.

Improved reliability of wear performance for a fluorapatite veneering porcelain by ion-exchange strengthening and toughening.

OBJECTIVES: Purpose of the present study was to explore the improvement of wear performance for a nano fluorapatite veneering porcelain by ion-exchange. METHODS: Bar and disk specimens were prepared by IPS e.max Ceram as the nano fluorapatite veneering porcelain. Ion-exchange was performed in a melted KNO3 bath at two temperatures for different time-periods. After the ion-exchange, the bars were tested for flexural strength, surface Vickers hardness and HIF toughness, the disks were tested for wear performance paired with zirconia antagonist using a pin-on-disk tribometer with 10 N for 70✕104 wear cycles in artificial saliva. Wear analysis of the porcelain and zirconia was performed with a 3D profilometer. The microstructure and worn surface morphology were examined with scanning electron microscopy. One-way analysis of variance and Tukey's post-hoc pairwise comparison were used to analyse the wear data. RESULTS: The nano fluorapatite veneering porcelain before ion-exchange presented strong time-dependent wear behavior. Furthermore, wear rate of the original porcelain exhibited a very large standard deviation in the running-in wear stage, which was correlated with highly inhomogeneous distribution of the characteristic fluorapatite crystals in the microstructure. After ion-exchange at 350 °C and 380 °C, especially after the processing at 350 °C for 128 h, the wear rates of both running-in and steady wear stages could be significantly decreased. More importantly, the standard deviation of wear rate in the running-in wear stage could be remarkably reduced after the treatment. The improved reliability of wear performance was attributed to the strengthening and toughening effects of the ion-exchange processing. CONCLUSION: For the fluorapatite veneering porcelain, the ion-exchange protocol to obtain an ion-exchange layer with less stress relaxation and a considerable depth could strengthen and toughen the porcelain; as a result, the reliability of wear performance could be remarkably improved.

Strong time-dependence for strengthening a lithium disilicate parent glass and the corresponding glass-ceramic by Li+/Na+ exchange.

Chemical strengthening of a lithium disilicate (LD) parent glass with stoichiometric composition and the corresponding glass-ceramic by Li+/Na+ exchange was investigated. The ion-exchange was performed in a molten NaNO3 salt bath at 380 °C below the glass transition temperature over a wide time range from 3.75 to 480 min. It was found that strength of both the glass and glass-ceramic could be significantly enhanced after very short time treatment (3.75-15 min); however, significant strength decreases occurred similarly for the glass and glass-ceramic with further increasing the treatment time to obtain a greater ion-exchanged depth. Morphology analysis of the ion-exchanged surfaces revealed that the over-long treatment would induce surface failure for the glass, or LD crystal micro-cracking in the ion-exchanged region for the glass-ceramic. The strong time-dependence of the strengthening was attributed to dual effects of the ion-exchange. Besides the conventional "stuffing effect" of ion-exchange, the thermal expansion increase of surface layer induced by Li+/Na+ exchange might be a critical factor to be considered for strengthening the LD glass and glass-ceramic. With prolonging the processing time, the dual effects would change the surface residual stress from compressive into tensile, resulting in surface failure and undesirable strength degradation.

Improving the wear performance of feldspathic veneering porcelain by ion-exchange strengthening.

OBJECTIVES: The present study examined the effects of Na+→K+ ion-exchange on the wear performance of feldspathic veneering porcelain. METHODS: Bar and disk specimens were prepared using IPS classic as the feldspathic veneering porcelain. After ion-exchange by immersion of the specimens in melted KNO3 at two temperatures for different time-periods, the bars were tested for flexural strength and Vickers surface hardness. The disks were paired with zirconia antagonists and tested with a pin-on-disk tribometer with 10 N for 70☓104 wear cycles in artificial saliva. Wear analysis of the porcelain and zirconia was performed using 3D profilometer and analysed with one-way analysis of variance and Tukey's post-hoc pairwise comparison procedures. Worn surfaces were examined with scanning electron microscopy. RESULTS: The feldspathic veneering porcelain exhibited strong time-dependent wear behaviour, with typical running-in and steady wear stages. Ion-exchange treatments at 380 °C and 440 °C both enhanced the mechanical properties, decreased the wear rates of running-in wear and steady wear. The wear performance of porcelain treated by ion-exchange at lower temperature (380 °C) was improved significantly, especially reducing the wear rate of the running-in stage. CONCLUSION: A thicker ion-exchange layer with less stress relaxation may be obtained by ion-exchange at lower exchange temperature for a long processing time. Such a protocol improves the wear performance of the porcelain effectively. CLINICAL SIGNIFICANCE: Restorations with veneering porcelain may fail prematurely due to excessive wear. It important to improve the wear performance of the porcelain. Ion-exchange has the potential to strengthen dental veneering porcelain. Understanding the effect of ion-exchange on the wear performance of porcelain provides insight improving the wear performance of these restorations.

Understanding the mechanism for the mechanical property degradation of a lithium disilicate glass-ceramic by annealing.

Post-crystallization annealing above the Tg was applied to a lithium disilicate glass-ceramic with microstructure consisting of glassy matrix and rod-like lithium disilicate crystals, effect of the annealing on the mechanical behavior was investigated. Flexural strength and VIF toughness of the glass-ceramic unexpectedly decreased after the annealing. The mechanical behavior variation was understood based on residual micro-stress analysis. Coefficient of thermal expansion for the glassy matrix presented a tendency to increase after the annealing, which would enlarge the thermal expansion mismatch between the glassy matrix and the lithium disilicate crystals. As a result, the residual micro-stresses in the glass-ceramic abnormally increased with increasing the annealing time. The mechanical property degradation was attributed to the annealing-induced increase of the residual micro-stresses.

Antimicrobial compounds from Ceanothus americanus against oral pathogens.

During the search for antimicrobial compounds from higher plant sources, a methanol extract of Ceanothus americanus demonstrated antimicrobial activity against selected oral pathogens. Through further bioassay-guided fractionation and purification, three triterpenes (ceanothic acid, 27-hydroxy ceanothic acid and ceanothetric acid) and two flavonoids (maesopsin and maesopsin-6-O-glucoside) were identified. Among these, ceanothetric acid and maesopsin-6-O-glucoside were new compounds. Ceanothic acid and ceanothetric acid demonstrated growth inhibitory effect against Streptococcus mutans, Actinomyces viscosus, Porphyromonas gingivalis, and Prevotella intermedia with MICs ranging from 42 to 625 micrograms ml-1. Maesopsin, its glucoside, and 27-hydroxy ceanothic acid, were inactive below the concentration of 500 micrograms ml-1.

Binaphthalenone glycosides from African chewing sticks, Diospyros lycioides.

Our laboratory has engaged in the exploration of active antimicrobial principles present in chewing sticks commonly used by the African and Middle Eastern countries as a mechanical oral hygiene aid in place of tooth brushing. During this investigation, a methanol extract from the twigs of Diospyros lycioides, a Namibia tooth cleaning stick, demonstrated antimicrobial activity against common oral pathogens including Streptococcus mutans and Porphyromonas gingivalis (MICs 2.5 and 0.156 mg/mL). Subsequent fractionation and purification of this extract led to the identification of two novel binaphathalenone glycosides: 1', 2-binaphthalen-4-one-2',3-dimethyl-1,8'-epoxy-1,4',5,5',8, 8'-hexahydroxy-8-O-beta-glucopyranosyl-5'-O-beta-xylopyranosyl(1-- >6) -beta-glucopyranoside (1) and 1',2-binaphthalen-4-one-2', 3-dimethyl-1,8'-epoxy-1,4',5,5',8,8'-hexahydroxy-5', 8-di-O-beta-xylopyranosyl(1-->6)-beta-glucopyranoside (2). Their structures were established using spectroscopic techniques. Examination of the antimicrobial activity of these two compounds revealed positive but only marginal growth inhibition against the test cariogenic pathogens, S. sanguis and Streptococcus mutans.