Reduction In Surface Hardness Research Articles

Novel photocatalysis-assisted mechanical polishing of laser cladding cobalt-based alloy using TiO2 nanoparticles

This paper presents a novel photocatalysis-assisted mechanical polishing method for cobalt-based alloy cladding layers using TiO2 nanoparticles. By leveraging the active oxygen species generated by the photocatalyst under illuminated conditions, surface oxidation reactions on cobalt-based alloys are initiated, thereby enhancing material removal efficiency. The underlying principles of photocatalytic oxidation are elucidated, particularly the promotion of oxidation by •OH when it interacts with the metal surface, leading to the formation of a CoO oxide film on the cladding layer surface and a subsequent reduction in surface hardness. An experimental platform was established, and research findings identified an etching time of 60 min and a TiO2 concentration of 10 wt% as optimal process parameters. Comparative analysis with pure mechanical polishing and chemical mechanical polishing revealed that photocatalysis-assisted mechanical polishing yielded superior surface roughness of 60 nm and a material removal rate of 63.8 μm/min.

Mastering textural control in multi-polysaccharide gels: Effect of κ-carrageenan, konjac glucomannan, locust bean gum, low-acyl gellan gum, and sodium alginate

To comprehend the intricate interplay of five common food polysaccharides, κ-Carrageenan (KC), konjac glucomannan (KGM), locust bean gum (LBG), low-acyl gellan gum (LAG), and sodium alginate (SA), within composite polysaccharide gels, widely employed for textural modulation and flavor enhancement. This study systematically modulates the quantities of these five polysaccharides to yield six distinct multi-polysaccharide gels. The unique impact of each polysaccharide on the overall quality of composite gels were studied by thermostability, microstructure, water-holding capacity (WHC), texture, and sensory attributes. The findings unequivocally manifest the phenomenon of thermoreversible gelation in all composite gels, except for the KC-devoid sample, which displayed an inability to solidify. Notably, KGM, LBG, and LAG emerged as pivotal enhancers of the network structure in these composite gels, while SA was identified as a promotor of layered structure, resulting in a reduction of surface hardness. Leveraging principal component analysis (PCA) to analyzed 14 critical evaluation parameters of the five multi-polysaccharide gels, revealing the order as follows: KC > KGM > SA > LAG > LBG. These findings would imparts valuable insights into the pragmatic utilization of multi-polysaccharide gels for the development of food products (e.g. Bobo balls in milk tea) with tailored textural and sensory attributes.

Surface roughness effects on electrochemical anodization behavior of binderless tungsten carbide and oxide boundary generation

Electrochemical anodization has been utilized to reduce the surface hardness of Tungsten Carbide (WC) so that to increase its machinability. In this study, experiments have been conducted to illustrate the impact of surface roughness on electrochemical anodization of binderless WC. Anodization was performed on binderless WC that had different surface roughness by applying a constant voltage and utilizing Sodium Hydroxide (NaOH) as the electrolyte. Oxide generation mechanism during the anodization process has been analyzed in detail, through the investigation of current behavior, surface characterization, surface hardness reduction, surface profile change. It has been observed that during the anodization process, multiple peaks in current curves were produced as a result of discharge channels, induced by the cavities at oxide-substrate interface. When the electric barrier is broken, there was an increase in the formation of oxide layers. Hardness decreased significantly across all substrate conditions. Pristine WC substrate had a robust hardness of 33.34 GPa, while non-polished, semi-polished, and polished substrates had 8.26, 9.545, and 9.411 GPa, respectively. These findings demonstrate the significant impact of anodization on WC's mechanical characteristics, resulting in a 71.6 % decrease in hardness across substrate states. A hypothesis has been presented to describe the overall anodization behavior and boundary oxide generation mechanism of binderless WC. Polished WC substrates have lower surface roughness, indicating more controlled and homogeneous anodization as compared to non-polished and semi-polished WC substrates. The present study enhances the understanding of the impact of surface roughness on the electrochemical anodization of binderless WC, which will further facilitate its incorporation with machining techniques.

Effect of Lemon Juice, Apple Cider Vinegar and Green Tea on Microhardness of Nanohybrid resin composite: An In-vitro study

This study aimed to evaluate effect of Lemon Juice, Apple Cider Vinegar and Green Tea on microhardness of nanohybrid resin composite using vickers microhardness tester. Materials and methodology: Forty specimens of nanohybrid resin composite (3M ESPE Filtek Z350 XT) were prepared and polished the prepared discs using sandpaper. Before immersion of prepared composite discs in prepared solutions the baseline data of microhardness of each prepared composite discs were recorded by using Vicker’s microhardness tester. The prepared composite discs were divided into 4 groups (n=10) and alternatively immersed in 25 ml of prepared solutions namely Green tea, Apple Cider Vinegar, Lemon Juice and Artificial saliva for 5 sec and then in 25 ml of artificial saliva for another 5 sec and the procedure was repeated for 10 cycles. After following the above mentioned procedure, artificial saliva was used as storage media for 24 hours. For the next 28 days, this entire process was repeated. The microhardness test was carried out prior to immersion and then at intervals of 7, 14, 21 & 28 days after immersion in respective solutions. Results: Two-way analysis of variance (ANOVA) with repeated measurements was carried out for the obtained data. Artificial Saliva demonstrated highest microhardness values at all time points, followed by Green Tea and Lemon Juice. Apple Cider Vinegar showed the least microhardness values of all the four groups tested. Conclusion: All the beverages significantly reduced surface microhardness of nanohybrid resin composite, especially at the end of 28 days. Immersion in apple cider vinegar caused more reduction in surface hardness over time when compared to other beverages. The outcome on microhardness of nanohybrid resin composite by these beverages on depended upon time the latter was exposed and also the chemical composition of beverages. Keywords: Apple Cider Vinegar, Green Tea, Lemon Juice, Microhardness, Nanohybrid resin composite, Vickers Microhardness Tester

Effect of machining operations on mechanical properties, surface integrity and corrosion resistance of tungsten heavy alloy

Tungsten heavy alloys (WHAs) are metal-metal matrix composites which are suitable candidates for defence and nuclear applications. In order to meet the geometric complexity and dimensional tolerance of products made with these WHAs, secondary machining operations are necessary. The effect of three set of machining operations (wire EDM, rough turning and finish turning) on mechanical properties, surface residual stress, surface and sub-surface hardness and corrosion resistance are explored in this work. Uniaxial tensile test specimens were prepared with two different sets of feed rate and depth of cut corresponding to rough and finish turning. For the specimens prepared with harsher conditions around 20% reduction in elongation was noticed though there were no significant changes in yield strength or ultimate tensile strength. Finish turning resulted in a surface residual stress in the order of 700 MPa, compressive in nature. Though compressive, the magnitudes of the surface residual stress were significantly reduced for the other two operations. Turning operations were found to work harden the surface while WEDM caused a reduction in surface hardness. Subsequent variations in the subsurface hardness were also observed reaching upto a depth in the range of 50–150 µm. Surfaces produced under different machining conditions were exposed to chloride solution to examine the impact of surface integrity on the corrosion behaviour and the correlation among the two are investigated. Rough turned surfaces and wire EDM surfaces have shown reduced corrosion resistance.

Effect of in-office bleaching agent on the surface roughness and microhardness of nanofilled and nanohybrid composite resins.

Background: To compare the surface roughness and microhardness of Ceram.x® SphereTEC™ one and Filtek Z350 XT after in-office bleaching with Pola office. Methods: Twenty samples each of (10 mm diameter and 2 mm height) Ceram.x® SphereTEC™ one and Filtek Z350 XT were prepared. The samples were subjected to three bleaching sessions with 35% hydrogen peroxide (Pola office) with a seven-day interval between each session. Surface roughness and microhardness of the prepared samples prior to and after the bleaching regimen were measured using a profilometer and Vickers hardness tester, respectively. Results: A significant reduction (p <0.001) in the surface hardness of Filtek Z350 XT from 27.67 ± 2.10 to 17.83 ± 1.36 Vickers hardness number (VHN) was observed after the bleaching whereas no significant reduction in surface hardness was observed with Ceram.x® SphereTEC™ one. The adjusted mean (estimated marginal mean) microhardness after bleaching for Ceram.x ® SphereTEC™ one (35.79 ± 1.45) was significantly higher than Filtek Z350 XT (19.54 ± 1.45) (p < 0.001). However, in-office bleaching of these materials did not significantly alter their surface roughness. Conclusions: In office-bleaching with 35% hydrogen peroxide can reduce the microhardness of nanofilled composite. However, the surface roughness was not influenced by the bleaching procedure in both nanohybrid and nanofilled composite resin materials.

Experimental investigation on thermal properties of carbon nanotubes/zinc silicate composites prepared by powder processing

Carbon nanotubes as fillers in ceramic matrix composites have been widely explored to enhance the thermal properties due to its excellent thermal transport mechanism. Zinc silicate (ZS) is a crystalline ceramic with good fluorescence properties but low thermal diffusivity and effusivity. The introduction of CNTs into ZS is a good approach to improve the thermal transport parameters of the ceramic. Carbon nanotubes/zinc silicate (CNTs/ZS) composites with various CNTs mass fractions, ranging from 0 to 3.0 wt%, were fabricated using a combination of powder processing and pressureless sintering. The measured surface hardness, alternating current (AC) conductivity and thermal diffusivity of CNTs-free ZS composites are 285 HV, 2.31 × 10−7 Sm−1, and 0.25 mm2s−1, respectively. Paradoxically, the composite's surface hardness and thermal diffusivity decreased by 90.3 % and 72.1 %, respectively, when 3.0 wt% CNTs were added, while the AC conductivity increased by 105 times. The reduced bulk density and the high degree of phonon scattering in grain boundaries led to a reduction in surface hardness and thermal diffusivity. In addition, the micro-morphology observations revealed that the CNTs aggregation rate increased proportionally with the CNTs mass fraction. The CNTs aggregation formed a soft phase at the composite's surface and a phonon scattering site, which further degraded the material's mechanical and thermal properties. The enhanced AC conductivity of the composites was attributable to the introduction of an efficient free electron flow path formed by the CNTs network. The combination technique of powder processing and pressureless sintering provided a fabrication route to prepare the CNTs/ZS composite with minimal complexity and expense. Through this fabrication method, the results allowed the CNTs-ceramic research path to be navigated in the better direction and served the scientific community in the development of micro-optoelectronic and passive heat sinks.

Effect of vital bleaching on surface roughness and microhardness of nanofilled and nanohybrid composite resins

Background: To compare the surface roughness and microhardness of Ceram.x® SphereTEC™ one and Filtek Z350 XT after in-office vital bleaching with Pola office. Methods: A total of 20 samples of 10 mm diameter and 2 mm height of Ceram.x® SphereTEC™ one and Filtek Z350 XT were prepared. The samples were subjected to two bleaching sessions with 35% hydrogen peroxide (Pola office) with a seven-day interval between each session. Surface roughness and microhardness of the prepared samples prior to and after the bleaching regimen were measured using a profilometer and Vickers hardness tester, respectively. Results: A significant reduction (p &lt;0.001) in the surface hardness of Filtek Z350 XT from 26.67 ± 2.10 to 17.83 ± 1.36 Vickers hardness number (VHN) was observed after the bleaching whereas no significant reduction in surface hardness was observed with Ceram.x® SphereTEC™ one. However, in-office bleaching of these materials did not significantly alter their surface roughness. Conclusions: The effect of in-office bleaching on the surface hardness of resin composites seem to vary due to variations in the composition of dental composites such as filler loading. Among the composites tested, in office bleaching of Ceram.x® SphereTEC™ did not result in significant changes in its surface hardness and roughness.

The Effect of Gastric Acid on the Surface Properties of Different Universal Composites: A SEM Study.

The aim of this study is to compare surface roughness and microhardness changes of three monochromatic (Omnichroma, Vittra Unique, and Charisma Diamond One) and three universal shade (Neo Spectra ST, G-ænial A'CHORD, and Nova Compo C) resin composites after exposure to simulated gastric acid. A total of 144 disc-shaped specimens (24 discs of each composite resin) were prepared using plexiglass molds (R = 5 mm × h = 2 mm) for evaluation from each material. Specimens of each material were divided into two main groups for the evaluation of the microhardness and surface roughness and also two subgroups for 7- and 14-day exposures to simulated gastric acid. Initial microhardness and surface roughness measurements of all samples were measured before immersion (T1) followed by exposing the samples to simulated gastric acid for 7 days (T2) and 14 days (T3), and then, the microhardness and roughness measurements were repeated. Randomly selected specimens of each material for each of the time intervals (T1, T2, and T3) were evaluated with scanning electron microscopy (SEM). One-way ANOVA revealed that the surface roughness and hardness values of all tested composite resin restorative materials show no statistically significant difference for the initial (T1) value (p > 0.05). Regarding the 7th day (T2) and 14th day (T3) surface roughness and microhardness value of all composites, there are statistically significant differences between the groups (p < 0.05), while there was no statistically significant difference between the surface hardness reduction percentage between the time intervals (p > 0.05). As a result of this in vitro study, increase in surface roughness and decrease in microhardness of the tested universal composite materials when exposed to simulated gastric acid were statistically significant.

Assessment of the Erosive Potential of Mineral Waters in Bovine Dental Enamel

Background: High intake of acidic foods and beverages has been often associated with the onset of dental erosive wear. Objetive: This study in vitro assessed the pH of different mineral waters marketed in Brazil and their effects on the properties and surface of dental enamel. Methods: Forty-eight bovine incisor specimens were divided into four groups (n=12): CG-control group, PeG-Perrier, PrG-Prata, and SLG-São Lourenço. The immersion cycles were performed after analysis of the pH of the waters, for 5 days (5 minutes in mineral water and 60 minutes in artificial saliva). Knoop micro-hardness was assessed by means of three indentations with a load of 50kgf for 15 seconds, and surface roughness with a cut off of 0.25mm. The data were analysed using Student's t -test, ANOVA, and Tukey test, with a significance level of 5%. Results: The groups of waters with lower pH (Perrier ® and São Lourenço ® ) exhibited a reduction in Knoop micro-hardness ( p &lt;0.0001) and an increase in surface roughness ( p =0.04 and p =0.004, respectively). The Prata water group did not exhibit significant changes in Knoop micro-hardness ( p =0.07) and surface roughness ( p =0.26). Conclusion: Mineral waters with a pH below the critical value can lead to a reduction in surface hardness and roughness in the bovine enamel.

Surface Microhardness Of Microhybrid And Nanocomposite After Storage In Mouth Washes.

Dental composites are aesthetic direct restorative material. However, the effect of mouthwashes on the durability of the material is controversial. This study evaluated and compared the influence of mouthwash composition on the surface hardness of nanofilled (Z350XT) and microhybrid (P60) resin composites. Comparative in-vitro study was conducted over 6 months at Multan Medical & Dental College. Sixty-four disc-shape specimens of each {nanofilled (Z350XT) and microhybrid (P60)} resin composite were prepared and stored in distilled water at 37°C for 24 hours. The baseline microhardness reading (To) was recorded by Vickers micro-hardness tester. Samples were then randomly divided into four groups (n=16) and stored in Listerine Cool Mint, Colgate Plax, Clinica and distilled water (control). The hardness test was repeated after 12 hours and 24 hours of storage. Nanocomposite (Z350XT) had statistically (p<0.01) higher surface hardness. A significant reduction (p≤0.05) in microhardness was observed after immersion of samples in mouthwashes. The reduction in surface hardness was dependent on the immersion time and composition of mouthwashes. Listerine Cool Mint (alcohol-based mouthwash) had greatest degradation effect. Mouth rinses negatively impacted the surface microhardness of the tested resin-based materials. Alcohol-based mouthwashes had greater potential for reducing microhardness. Microhybrid composite appears to be a more suitable material for restoring teeth in patients accustomed to using regular mouthwashes.

The Surface Characteristics, Microstructure and Mechanical Properties of PEEK Printed by Fused Deposition Modeling with Different Raster Angles.

Additive manufacturing provides a novel and robust way to prepare medical product with anatomic matched geometry and tailored mechanical performance. In this study, the surface characteristics, microstructure, and mechanical properties of fused deposition modeling (FDM) prepared polyether-ether-ketone (PEEK) were systematically studied. During the FDM process, the crystal unit cell and thermal attribute of PEEK material remained unchanged, whereas the surface layer generally became more hydrophilic with an obvious reduction in surface hardness. Raster angle has a significant effect on the mechanical strength but not on the failure mechanism. In practice, FDM fabricated PEEK acted more like a laminate rather than a unified structure. Its main failure mechanism was correlated to the internal voids. The results show that horizontal infill orientation with 30° raster angle is promising for a better comprehensive mechanical performance, and the corresponding tensile, flexural, and shear strengths are (76.5 ± 1.4) MPa, (149.7 ± 3.0) MPa, and (55.5 ± 1.8) MPa, respectively. The findings of this study provide guidelines for FDM-PEEK to enable its realization in applications such as orthopedic implants.

Hybrid micro-milling assisted with laser oxidation based on the hardness reduction that caused by cemented carbide oxidation

Cemented carbides have various industrial applications in many fields. The large cutting force and excessive tool wear are the most prominent issues encountered during the mechanical cutting of cemented carbide materials. The isothermal oxidation test showed that cemented carbide exhibited a surface hardness reduction of 80% and better material machinability after full oxidation owing to the formed porous oxide layer. Inspired by the hardness reduction that caused by cemented carbide oxidation, hybrid micro-milling operation was developed based on laser oxidation. It was found that cemented carbide can produce a porous oxide layer having a thickness of several microns on the material surface under laser irradiation. Additionally, traditional and hybrid micro-milling operations were performed on cemented carbide to validate the feasibility of this method. The results showed that the developed hybrid micro-milling operation was conducive to decreasing the tool wear and cutting force. Thus, it is a potential hybrid machining operation capable of achieving high-efficiency machining of cemented carbide.

Microstructure and high temperature erosion behavior of laser cladded CoCrFeNiSi high entropy alloy coating

In this study, CoCrFeNiSi high entropy alloy (HEA) coating without pores or slag defects was prepared on 304 stainless steel surface by laser cladding. Thermodynamic parameters of CoCrFeNiSi HEA were calculated. Microstructure and phase composition were analyzed by means of scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Comparative studies on hardness, high temperature erosion and oxidation performances were conducted between the CoCrFeNiSi HEA and Stellite-6 Co-based alloy coatings. The results show that laser cladding CoCrFeNiSi HEA coating was composed of single FCC structure. Typical metallurgical bonding between cladding coating and the substrate was formed. Microhardness of CoCrFeNiSi HEA coating reached to approximately 580HV0.2, which was higher than that of Co-based alloy coating (485HV0.2). There was little surface hardness reduction at 500, 600 and 700 °C for CoCrFeNiSi and Co-based alloy coatings. Erosion tests were conducted using quartz sand particles at 20, 500, 600 and 700 °C with an impact angle of 30, 60 and 90°, respectively. The HEA coating showed comparatively lower erosion rates than Co-based alloy coating under all test conditions. The erosion morphologies of both coatings showed that ploughing and micro-cutting were the two primary erosion mechanisms at oblique impact angles, while the erosion mechanism was replaced by the formation of craters and platelets at normal impact angles. A dense and continuous oxidation film could form on HEA coating at 700 °C which may protect the coating from erosion at high temperature to some extent.

Classification of Aged Epoxy Micro–Nanocomposites Through PCA- and ANN-Adopted LIBS Analysis

Epoxy micro–nanocomposite specimens are subjected to different levels of gamma irradiation and water aging to analyze their degradation behavior after aging. The plasma temperature calculated from the laser-induced breakdown spectroscopy (LIBS) spectral data tends to decrease, and the crater depth after laser ablation tends to increase with increment in the level of aging, reflecting the reduction in surface hardness of the epoxy specimens after aging. Water aging creates a more severe impact on the surface hardness of the specimens, compared to gamma irradiation, in the range studied. The normalized intensity ratio of H(I)/Si(II) is in direct correlation with the water diffusion coefficient. Machine learning techniques, such as the principal component analysis (PCA) and the artificial neural network (ANN) analysis, have been performed on LIBS spectral data in order to classify the unaged, gamma-irradiated, and water-aged specimens. The number of neurons in the hidden layer of the neural network architecture has been optimized based on classification accuracy and the number of epochs required to converge. The ANN-adopted LIBS analysis is successful with good classification accuracy compared to PCA, for categorizing the level and type of aging in epoxy micro–nanocomposites as observed in this work.

Influence of nanoporous titanium niobium alloy surfaces produced via hydrogen peroxide oxidative etching on the osteogenic differentiation of human mesenchymal stromal cells

Titanium niobium alloys exhibit a lower stiffness compared to Ti6Al4V, the ‘gold standard’ for load-bearing bone implants. Thus, the critical mismatch in stiffness between the implant and adjacent bone tissue could be addressed with TiNb alloys and thereby reduce stress shielding, which can result in bone resorption and subsequent implant loosening; however, the cellular response on the specific material is crucial for sufficient osseointegration. We therefore hypothesize that the response of human mesenchymal stromal cells (hMSC) and osteoblast-like cells on Ti45Nb surfaces can be improved by a novel nanoporous surface structure. For this purpose, an etching technique using hydrogen peroxide electrolyte solution was applied to Ti45Nb. The treated surfaces were characterized using SEM, LSM, AFM, nanoindentation, and contact angle measurements. Cell culture experiments using hMCS and MG-63 were conducted. The H2O2 treatment resulted in surface nanopores, an increase in surface wettability and a reduction in surface hardness. The proliferation of MG-63 was enhanced on TiNb45 compared to Ti6Al4V. MG-63 focal adhesion complexes were detected on all Ti45Nb surfaces, whereas the nanostructures notably increased the cell area and decreased cell solidity, indicating stimulated cell spreading and pseudopodia formation. Alizarin red stainings indicated that the nanoporous surfaces stimulated the osteogenic differentiation of hMSC. It can be concluded that the proposed surface treatment could potentially help to stimulate the osseointegration behaviour of the advantageous low stiff Ti45Nb alloy.

Radiotherapy Reduces Microhardness and Mineral and Organic Composition, and Changes the Morphology of Primary Teeth: An in vitro Study

Objective: We aimed to evaluate whether radiotherapy causes changes in the mineral composition, hardness, and morphology of enamel and dentin of primary teeth. Materials and Methods: Thirty specimens of primary teeth were subjected to radiotherapy. At baseline and after 1,080, 2,160, and 3,060 cGy, the specimens were subjected to microhardness, FT-Raman spectroscopy, and scanning electron microscopy (SEM) analysis. The pH of artificial saliva was determined, as were the calcium and phosphate concentrations. The data were subjected to the Shapiro-Wilk normality test, showed a nonnormal distribution, and were compared by the Kruskal-Wallis test. Results: The results showed that the microhardness of the enamel surface decreased after 2,160 cGy (281.5 ± 58 kgf/mm²) when compared to baseline (323.6 ± 59.5 kgf/mm²) (p = 0.045). For dentin, the surface hardness decreased after 1,080 cGy (34.9 ± 11.4 kgf/mm²) and 2,160 cGy (26 ± 3.5 kgf/mm²) when compared to baseline (56.5 ± 7.7 kgf/mm²) (p < 0.0001). The mineral and organic contents of phosphate (p < 0.0001), carbonate (p < 0.0001), amide (p = 0.0002), and hydrocarbons (p = 0.0031) of enamel decreased after 3,060 cGy (5,178 ± 1,082, 3,868 ± 524, 999 ± 180, and 959 ± 168 kgf/mm², respectively). For dentin, we noticed a growing increase in phosphate v2, amide, and hydrocarbon content after 1,080 cGy (8,210 ± 2,599, 5,730 ± 1,818, and 6,118 ± 1,807 kgf/mm², respectively) and 2,160 cGy (1,0071 ± 2,547, 7,746 ± 1,916, and 8,280 ± 2,079 kgf/mm², respectively) and a reduction after 3,060 cGy (6,782 ± 2,175, 3,558 ± 1,884, and 3,565 ± 1,867 kgf/mm², respectively) (p < 0.0001). SEM images showed cracks on enamel and degradation of peritubular dentin. Conclusion: We concluded that radiotherapy caused a reduction in surface hardness, changed mineral and organic composition, and promoted morphological changes on the enamel and dentin of primary teeth.

Effect of Remineralizing Gels on Microhardness, Color and Wear Susceptibility of Bleached Enamel.

To evaluate the effect of a remineralizing gel combining fluoride and calcium silicate/phosphate or a sodium fluoride gel on bleached enamel microhardness, color, and wear susceptibility. Two hundred forty bovine enamel-dentin samples were prepared. Baseline analysis of Knoop microhardness, color coordinates (L*a*b*), and surface profile were performed. According to the baseline microhardness values, specimens were stratified into six groups (n=40): NC (negative control)-no treatment; BL (positive control)-bleaching with 40% hydrogen peroxide gel (Opalescence Boost, Ultradent); BL/Rs-bleaching + application of calcium silicate/phosphate gel (Regenerate Serum, Unilever - Rs); Rs/BL-Rs + bleaching; Rs/BL/Rs-Rs + bleaching + Rs; and BL/F-bleaching + 2% sodium fluoride gel. After the treatment described for each group, color change (ΔE) and microhardness were evaluated again. To evaluate abrasion susceptibility, samples were randomly divided into two subgroups, according to the toothpaste used (Cp-Close Up or Rt-Regenerate), and underwent 100,000 brushing strokes. The profile of each sample was evaluated and the mean wear calculated. The data were analyzed by ANOVA and Tukey tests. All bleached groups showed a significant reduction of microhardness in relation to the negative control. The groups treated with remineralizing gels showed a significantly higher microhardness and less wear than the positive control, although nonsignificant differences were observed among them. Nonsignificant differences in ΔE were found among bleached groups. The groups brushed with Regenerate toothpaste showed significantly less wear than those brushed with Close Up toothpaste. The remineralizing gels did not interfere with bleaching efficacy. However, all the treatments minimized the surface hardness reduction caused by the bleaching procedure and enamel loss after abrasion. Regenerate toothpaste resulted in less enamel abrasion.

In vitro simulation of erosive challenges to human enamel using a novel artificial mouth.

This in vitro work reports upon the design, build and operation of an artificial environment (Saltus) that sought to simulate the process of in vivo dental erosion upon human enamel. A novel testing environment, housed 8 erosion testing substrate specimens, that on separate occasions were subject to 4 different experimental diets, of increasing erosive challenge, simulating the consumption of an acidic beverage. Each set of specimens was subjected to one of the experimental diets only. These were liquid only and administered the test beverage over a standardized range of volumes and durations. Flow of both artificial unstimulated and stimulated saliva was maintained throughout and the effects upon the substrates were measured by profilometry, surface microhardness determination and chemical analysis of the saliva and beverage mixture for traces of Calcium and Phosphate ions. The overall trend of surface hardness reduction, depth of surface loss and ion loss across the diets increased in proportion to the severity of insult. Accepting the limitations of this study Saltus appeared to perform well as an environment in which to simulate and assess dental erosion using parameters defined by previous in vivo observations of human drinking behaviour. The authors however acknowledge that in vitro testing can never replicate fully the in vivo situation.

Influences of Light-emitting Diode Illumination Bleaching Technique on Nanohardness of Computer-aided Design and Computer-aided Manufacturing Ceramic Restorative Materials

This study evaluated the effect of light-emitting diode (LED) illumination bleaching technique on the surface nanohardness of various computer-aided design and computer-aided manufacturing (CAD/CAM) ceramic materials. Twenty disk-shaped samples (width, length, and thickness = 10, 15, and 2 mm) were prepared from each of the ceramic materials for CAD/CAM, including Lava™ Ultimate (LV), Vita Enamic® (En) IPS e.max® CAD (Me), inCoris® TZI (IC), and Prettau® zirconia (Pr). The samples from each type of ceramic were randomly divided into two groups based on the different bleaching techniques to be used on them, using 35% hydrogen peroxide with and without LED illumination. The ceramic disk samples were bleached according to the manufacturer's instruction. Surface hardness test was performed before and after bleaching using nanohardness tester with a Berkovich diamond indenter. The respective Vickers hardness number upon no bleaching and bleaching without or with LED illumination [mean ± standard deviation (SD)] for each type of ceramic were as follows: 102.52 ± 2.09, 101.04 ± 1.18, and 98.17 ± 1.15 for LV groups; 274.96 ± 5.41, 271.29 ± 5.94, and 268.20 ± 7.02 for En groups; 640.74 ± 31.02, 631.70 ± 22.38, and 582.32 ± 33.88 for Me groups; 1,442.09 ± 35.07, 1,431.32 ± 28.80, and 1,336.51 ± 34.03 for IC groups; and 1,383.82 ± 33.87, 1,343.51 ± 38.75, and 1,295.96 ± 31.29 for Pr groups. The results indicated surface hardness reduction following the bleaching procedure of varying degrees for different ceramic materials. Analysis of variance (ANOVA) revealed a significant reduction in surface hardness due to the effect of bleaching technique, ceramic material, and the interaction between bleaching technique and ceramic material (p < 0.05). Bleaching resulted in a diminution of the surface hardness of dental ceramic for CAD/CAM. Using 35% hydrogen peroxide bleaching agent with LED illumination exhibited more reduction in surface hardness of dental ceramic than what was observed without LED illumination. Clinicians should consider protection of the existing restoration while bleaching.