Knoop Microhardness Measurements Research Articles

Effect of thickness on shrinkage stress and bottom-to-top hardness ratio of conventional and bulk-fill composites.

This study evaluated the effect of the material thickness on shrinkage stress and bottom-to-top hardness ratio of conventional and bulk-fill composites. Six commercial composites were selected based on their different technologies: Two conventional (C1, C2), two high-viscosity bulk-fill (HVB1, HVB2), and two low-viscosity bulk-fill (LVB1, LVB2). Shrinkage stress was analyzed for five specimens with 2mm thickness (C-factor 0.75 and volume 24mm3 ) and five specimens with 4mm thickness (C-factor 0.375 and volume 48mm3 ) for 300s in a universal testing machine. Bottom-to-top hardness ratio values were obtained from Knoop microhardness measurements in specimens with 2- and 4-mm thickness (n=5). Thickness increase resulted in significantly higher shrinkage stress for all materials with the exception of HVB2 and LVB1. C1, C2, HVB2, and LVB1 showed lower bottom-to-top hardness ratios at 4mm than at 2mm. Only LVB2 presented a bottom-to-top hardness ratio lower than 80% at 2mm, while HVB1 surpassed this threshold at 4mm of depth. The results suggest that the increase of composite thickness affected the shrinkage stress values. Also, thickness increase resulted in lower bottom-to-top hardness ratio. HVB1 showed better behavior than other bulk-fill materials, with low stress and adequate bottom-to-top hardness ratio at 4mm thickness.

Anti-demineralizing protective effects on enamel identified in experimental and commercial restorative materials with functional fillers

The aim of this study was to investigate whether experimental and commercial dental restorative materials with functional fillers can exert a protective anti-demineralizing effect on enamel that is not immediately adjacent to the restoration. Four experimental resin composites with bioactive glass and three commercial restorative materials were investigated. Enamel blocks were incubated in a lactic acid solution (pH = 4.0) at a standardized distance (5 mm) from cured specimens of restorative materials. The lactic acid solution was replenished every 4 days up to a total of 32 days. Surfaces of enamel blocks were periodically evaluated by Knoop microhardness measurements and scanning electron microscopy. The protective effect of restorative materials against acid was identified as enamel microhardness remaining unchanged for a certain number of 4-day acid addition cycles. Additionally, the pH of the immersion medium was measured. While enamel microhardness in the control group was maintained for 1 acid addition cycle (4 days), restorative materials postponed enamel softening for 2–5 cycles (8–20 days). The materials capable of exerting a stronger alkalizing effect provided longer-lasting enamel protection. The protective and alkalizing effects of experimental composites improved with higher amounts of bioactive glass and were better for conventional bioactive glass 45S5 compared to a fluoride-containing bioactive glass. Scanning electron micrographs evidenced the protective effect of restorative materials by showing a delayed appearance of an etching pattern on the enamel surface. A remotely-acting anti-demineralizing protective effect on enamel was identified in experimental composites functionalized with two types of bioactive glass, as well as in three commercial ion-releasing restorative materials.

Remineralization of Artificial Dentin Lesion In vitro using Dental Nanomaterials.

Background:Teeth in the human body are the most mineralized tissue, which contain both organic and inorganic components. Demineralization and remineralization of teeth occur continuously, but demineralization causes structural loss of a tooth.Aim:This study was done to find the effect of ceramic by adding mono-n-Dodecyl phosphate to ceramic in dentin remineralization.Materials and Methods:A total of sixty permanent intact tooth specimens were randomly divided into four equal groups: Group 1: control group, Group 2: dentin was etched and restored with plain ceramic restoration, Group 3: etched dentin restored with ceramic containing 2% mono-n-Dodecyl phosphate, and Group 4: etched dentin restored with ceramic containing 5% mono-n-Dodecyl phosphate. Each sample was immersed completely in simulated body fluid and was kept in an incubator at 37°C to simulate the human body environment. Knoop microhardness measurements were recorded at 10, 20, and 38 days.Results:Knoop microhardness of dentin (KHN) reduced to 32.6% after dentin was etched. No significant difference was found between group 2, 3 and 4 after 10 days. KHN value was increased and showed significant changes restored with group 2 and 3, group 3 showed nonsignificant changes.Conclusion:Through this study, we found that Ceramic can be used as a remineralizing agent to restore marginal dentine around of the cavity and root lesions from secondary caries.

Biochemical characterisation of carious dentine zones using Raman spectroscopy

ObjectiveCarious tissue discrimination in clinical operative caries management relies traditionally on the subjective hardness of carious dentine. Biochemical alterations within the lesion have the potential to discriminate the lesion zones objectively. This study aimed to determine the correlation between the biochemical proportions of amide I and phosphate moieties as these are the most prominent peaks found in dentine with the Knoop microhardness of carious dentine zones, using non-contact Raman spectroscopy. The null hypothesis investigated was that there was no correlation between Raman peak ratios, amide I: phosphateν1, and the Knoop microhardness within specific zones of a carious lesion. Methods423 scan points from 20 carious dentine lesion samples examined using high-resolution Raman spectroscopy. The peak ratio of the characteristic vibration mode of amide I (1650 cm−1) and phosphate (960 cm−1) bands were calculated, following a straight line path through the lesion to the pulp and correlated to corresponding Knoop microhardness measurements. ResultsUsing logistic regression analysis, clear correlations were found between the Knoop microhardness and Raman peak ratio cut-off values between caries-infected and caries-affected dentine (81.5 % sensitivity / 92.7 % specificity), with a lower specificity (2.7 %) found between caries-affected and sound dentine. ConclusionThis study concluded that non-contact Raman spectroscopy can be used in vitro to discriminate objectively between the different zones of a carious dentine lesion at high resolution, using the Raman peak ratios, amide I : phosphate ν1. Clinical SignificanceSpecific biochemical alterations have the potential to be used in-vitro and in-vivo to identify the end-point of selective carious lesion excavation.

Chemo-mechanical characterization of carious dentine using Raman microscopy and Knoop microhardness.

One of the aims in the clinical operative management of dental carious lesions is to remove selectively the highly infected and structurally denatured dentine tissue, while retaining the deeper, repairable affected and intact, healthy tissues for long-term mechanical strength. The present study examined the correlation of chemical functional groups and the microhardness through the different depths of a carious lesion using Raman spectroscopy and Knoop microhardness testing. The null hypothesis investigated was that there was no correlation between Raman peak ratios (amide I : phosphate ν1) and equivalent Knoop microhardness measurements. Ten freshly extracted human permanent teeth with carious dentine lesions were sectioned and examined using high-resolution Raman microscopy. The ratio of absorbency at the amide I and phosphate bands were calculated from 139 scan points through the depth of the lesions and correlated with 139 juxtaposed Knoop microhardness indentations. The results indicated a high correlation (p < 0.01) between the peak ratio and the equivalent Knoop hardness within carious dentine lesions. This study concluded that Raman spectroscopy can be used as a non-invasive analytical technology for in vitro studies to discriminate the hardness of carious dentine layers using the peak ratio as an alternative to the invasive, mechanical Knoop hardness test.

Analysis of residual stress in 1.4539 austenitic steel joints welded with TIG method

The article compares the results of tests of residual stress determined based on Knoop microhardness measurements and obtained experimentally with the use of an x-ray diffractometer. Distribution of residual stress in the weld after strengthening of the surface layer, resulting from shot peening, was specified. A method of residual stress determination proposed by Oppel, based on Knoop microhardness distribution, was applied. An analysis of residual stress of 1.4539 austenitic steel welded joints, made with the use of TIG method and additionally strengthened with shot peening of the surface, showed good agreement of the results obtained both with the sin2y method and based on the microhardness measurement. The highest compression stress has occurred in a so-called Belayev point, approximately of 35 ÷ 40 μm from the surface. Keywords: residual stress, welded joints, TIG, Knoop microhardness, shot peening.

Rapidly-Dissolving Silver-Containing Bioactive Glasses for Cariostatic Applications

A novel bioactive glass series containing incremental amounts of silver oxide was synthesized, ground down, and subsequently incorporated into a dentifrice for the purpose of reducing the incidence of dental caries and lesion formation. Three glasses were synthesized using the melt quench route: Si-Control (70SiO2–12CaO–3P2O5–15Na2O, mol %), Si-02 and Si-05, where 0.2 and 0.5 mol % Ag2O were substituted, respectively, for SiO2 in Si-Control. The glasses were then ground, sieved, characterized, and dissolved in Tris buffer solution (pH = 7.30) for 6, 12, and 24 h, with the pH of the resultant solution being recorded and the ions that were released into solution quantified. Samples of each glass were subsequently embedded into a non-fluoridated, commercially available toothpaste which was then used to brush resin-mounted lamb molars which, up to the point of testing, had been stored in a 1.0 M HCl solution. Knoop microhardness measurements of the molars were recorded before and after brushing to determine the presence of remineralization on the surface of the teeth (surface hardness loss of 37%, 35%, and 34% for Si-Control, Si-02 and Si-05, respectively, after 24 h). Four oral cavity bacterial strains were isolated through swabs of the inner cheek, gums, and teeth surfaces of three volunteers, and placed on agar discs. Of each glass, 0.5 g was placed onto the discs, and the resultant inhibition zones were measured after 6, 12, and 24 h. Si-05 performed better than Si-02 on two strains after 24 h, while exhibiting similar behavior for the remaining two strains after 24 h; the largest inhibition zone measured was 2.8 mm, for Si-05 after 12 h. Si-Control exhibited no antibacterial effect at any time point, providing evidence for the role of silver oxide as the antibacterial component of these glasses.

A Novel Enamel and Dentin Etching Protocol Using α-hydroxy Glycolic Acid: Surface Property, Etching Pattern, and Bond Strength Studies.

To determine the use of α-hydroxy glycolic acid (GA) as a surface pretreatment for dental restorative applications. The etching pattern of GA pretreatment of dental hard tissues was assessed by surface microhardness and scanning electron microscopy (SEM). The effectiveness of GA surface etching on the enamel and dentin resin bond strengths was assessed using two etchant application modes (rubbing and no rubbing) and three adhesive systems (Single Bond [SB], One Step Plus [OSP], and Scotchbond Universal [SBU]). Knoop microhardness measurements were carried out on polished enamel and dentin surfaces before and after treatment with 35% GA, 35% phosphoric acid (PA), or distilled water (control group) for 30 seconds. The microtensile bond strength test was carried out on enamel and dentin. Ultrastructural analysis of the surface and interfacial interaction was qualitatively accomplished using SEM. Etching with either PA or GA significantly decreased the enamel microhardness, with GA being significantly less aggressive than PA ( p<0.001), while both acids showed similar decreases in dentin microhardness ( p=0.810). SEM revealed similar etching patterns of GA and PA, while apparently a thinner hybrid layer was observed for GA groups. In dentin, the bond strengths were statistically similar between PA and GA groups, regardless of the etchant application mode ( p>0.05). However, rubbing of GA enhanced the bond strength to enamel. PA and GA significantly increased the SBU bond strength to enamel when compared to SB and OSP ( p<0.05). GA effectively etched enamel and dentin surfaces, resulting in bond strength values similar to those associated with traditional PA. GA is a suitable enamel and dentin surface etchant for adhesive restorative procedures.

An investigation into the effect of a resin infiltrant on the micromechanical properties of hypomineralised enamel.

Resin infiltration may alter the mechanical properties of enamel hypomineralised lesions (HL); however, variable surface layer (SL) thickness may affect resin penetration. To determine the thickness of the SL of HL and to investigate the effect of resin infiltration on the mechanical properties of HL. The thickness of the SL over HL was determined using polarised light microscopy. Etching time using 15% HCl gel to remove the SL of 52 samples was determined. Selected HL and control areas of 21 teeth were infiltrated with Icon® resin infiltrant, and cross-sectional Knoop microhardness (KHN) measurements were recorded before and after resin infiltration. Ninety-six point five per cent of HL had a detectable SL with mean thickness of 58 ± 29 μm. Application of HCl for 120 s produced a mean MIH erosion depth of 58 ± 12 μm. Eleven of 21 samples had evidence of infiltration using visual examination. The infiltrant penetrated some of the HL leading to an increase in KHN (111 ± 75 KHN) which, when compared to adjacent non-infiltrated HL (96 ± 52 KHN), was not statistically significantly different (P = 0.56). There was marked variation in the SL thickness of HL. Resin infiltration of HL did not increase microhardness significantly.

Effect of a novel commercial potassium-oxalate containing tooth-desensitizing mouthrinse on the microhardness of resin composite restorative materials with different monomer compositions.

BackgroundThe effects of mouthrinses on dental resin composites have been investigated extensively. However, there is little information available regarding the effects of ‘newly developed mouthrinse’ formulations on the microhardness of different monomer based composite systems. Therefore, the aim of this study was to investigate the effect of a novel potassium-oxalate containing desensitizing mouthrinse on the microhardness of different monomer based composite materials.Material and MethodsA hundred and twenty specimens (6mm in diameter and 2mm in height) were prepared for composite resin groups (methacrylate based, DX-511 monomer based and silorane monomer based) and for storage solution groups (artificial saliva and potassium oxalate-containing tooth-desensitizing mouthrinse). After allowing post-polimerization the baseline Knoop microhardness measurements for all specimens were recorded. The specimens were stored in 20 mL mouthwash and artificial saliva for 12 hours at 37ºC. The post-immersion microhardness values of all specimens were also recorded. Data were subjected to ANOVA/Scheffe’s test at a significance level of 0.05. The intra group (pre and post immersion values) comparison of the mean microhardness values of the specimens was done using Wilcoxon signed rank test.ResultsThe microhardness of the silorane based composite was not affected significantly (p>0.05). The hardness values of the DX-511 monomer based composite and the methacrylate based composite exhibited a slight but not significant microhardness change compared to the baseline values (p>0.05).ConclusionsStudies reported that the effect of mouthrinses on microhardness changes of composite resins may be material dependent, and the hardness change susceptibility of a restorative material may be attributed to its resin matrix or filler type. However, dental monomers as well as the oral care products have an ever-evolving technology and future studies should consider newer products. Potassium oxalate containing mouthrinses, especially alcohol-free ones, may be used safely with dental composites with newly developed low-shrink monomer compositions. Key words:Microhardness, monomers, mouthrinses, potassium-oxalate.

Effect of different storage conditions on the physical properties of bleached enamel: An in vitro vs. in situ study

ObjectivesEvaluate the effect of different storage conditions on bleached enamel using Knoop microhardness (KHN) and colour variation. MethodsForty-eight tooth blocks were divided into four groups (n=12), based on storage media (SM): purified water (PW), artificial saliva (AS), natural saliva (NS), in situ (IS). Three whitening sessions were carried out using 35% hydrogen peroxide, with a week interval. Colour and KHN measurements were taken before the samples were placed in the SM (t1), after 24h in the SM (t2), and after 24h at the end of the bleaching treatment (t3). Two extra samples from each group were analysed using a scanning electron microscope (SEM). KHN results were analysed by PROC-MIXED and Tukey–Kramer test (α=0.05), and colour changes were evaluated using ΔL, Δa, Δb, ΔE between the different times Δ1(t1−t2), Δ2(t2−t3) using the Kruskal–Wallis test and Dunn's test (α=0.05). ResultsSignificant statistical difference was noted in KHN at t3, with the lowest values found for PW. As for colour analysis in ΔE2 and Δb2, IS showed values that were statistically lower when compared to AS. Likewise, there were differences between PW and AS in relation to IS when evaluating ΔL2. In addition, NS showed similar values to IS. ConclusionsThe storage conditions had different effects on the physical properties of bleached enamel. NS was the only SM that showed similar behaviour to IS. Clinical SignificanceNS proved an effective SM in the protection and recovery of damage caused by bleaching and is a viable SM for in vitro studies.

On the response of Ti–6Al–4V and Ti–6Al–7Nb alloys to a Nitron-100 treatment

Titanium alloys have been widely used in automotive, biomedical and aerospace industries due to their high strength-to-weight ratio, outstanding corrosion resistance and biocompatibility. Although α–β alloys such as Ti–6Al–4V (workhorse of aerospace industry) and Ti–6Al–7Nb (much used for surgical implants) exhibit a good combination of mechanical properties, their tribological properties are still limited as they tend to seize and gall when in contact with other surfaces. In this paper, the response of Ti–6Al–4V and Ti–6Al–7Nb α–β alloys to a Nitron-100 treatment is investigated. This treatment, which enhances the load-bearing capacity of titanium alloys, consists of two sequential processes: plasma nitriding using a glow discharge under triode configuration and deposition of a TiN coating. Scanning electron microscopy (SEM), optical surface profilometry (OSP), Knoop microhardness measurements and glow discharge optical emission spectroscopy (GDOES) were used to characterise both titanium alloy materials prior to and after the Nitron-100 treatment. Although the Nitron-100 treatment significantly improved the load-bearing capacity of both alloys, the Ti–6Al–4V alloy exhibited a deeper hardened case but lower surface hardness than the Ti–6Al–7Nb alloy for identical processing conditions. Their different response to the Nitron-100 treatment could be attributed to the different chemical compositions (replacement of vanadium by niobium) of these two-phase (α–β) titanium alloys containing the same amount (i.e. 3.6at.%) of niobium or vanadium. Due to the larger β-phase stabilising effect of niobium, a higher fraction of β phase resulted in the Ti–6Al–7Nb alloy at the nitriding temperature (750°C). Under the Nitron-100 treatment conditions used in this work, results indicate that Nitron 100-treated Ti–6Al–4V alloy (evidenced by its higher load-bearing capacity) may be preferred to be used in engineering applications involving high contact loads (e.g. aerospace applications such as aero-bearings). Nitron 100-treated Ti–6Al–7Nb alloy may be more suitable in applications requiring high surface hardness but relatively lower contact loads (e.g. biomedical applications).

Effect of fluoride gels on microhardness and surface roughness of bleached enamel.

The effect of bleaching treatments containing added calcium and combined with neutral or acidic fluoride gels on tooth enamel was investigated in vitro through Knoop microhardness (KHN) and surface roughness (SR) measurements. A total of 60 bovine incisors were tested, including 30 for SR measurements and 30 for KHN measurements. The specimens were divided into 12 groups and subjected to a bleaching agent with hydrogen peroxide 35% (Whiteness HP 35% Maxx, FGM) or hydrogen peroxide 35% with calcium (Whiteness HP 35% Blue Calcium, FGM) and a fluoride treatment flugel acidulated phosphate fluoride (APF) or flugel neutral fluoride (NF). Control specimens were submitted to bleaching treatments without fluoride. Microhardness tests were performed using a Knoop indentor. Roughness measurements were obtained using a roughness analyzer. Measurements were obtained before and after treatment. The specimens were stored in distilled water at 37 °C between treatments. The results were analyzed using descriptive and inferential statistics. Treatments using APF combined with 35% HP caused a decrease in microhardness, while NF combined with HP 35% Ca increased the enamel hardness. Fluoride gels did not alter the SR of the bleached enamel.

Yield Locus of a Cold Rolled and Solution Treated Nimonic-263 Alloy Sheet, Determined Using Asymmetrical Knoop Micro Hardness Indenter

The Von Mises or perhaps the Tresca criterion is adequate for predicting the onset of yielding under combined stress loading for isotropic materials. However, this prediction is not so straight forward for anisotropic materials in which the magnitude of tensile and compressive yield stresses are different along the different orientations. Such directionality in yield behavior of anisotropic materials may strongly depend on the nature and degree of crystallographic texture. Texture driven yield surfaces are the representatives of the states of plane stress (tension–tension, tension–compression and compression–compression) in thin wall tube by externally applied forces and internal pressure. The applications of yield surfaces are far too wide and particularly have direct relevance to the metal forming, such as cold rolling. A simple procedure based on Knoop microhardness measurements has been proposed in the literature for determining the plane stress yield surface of the sheet materials. In the present study, an attempt has been made to describe this technique and the procedure to determine the yield locus using Knoop microhardness indenter and compare the yield surface anisotropy determined with the experimentally observed tensile properties in the in-plane directions and thus establish the usefulness of this technique in case of a cold rolled and solution treated Nimonic-263 alloy sheet of 1.0 mm thickness in solution treated condition.

Microhardness of nanofilled composite resin light-cured by LED or QTH units with different times

Aim: To evaluate the influence of light-curing units and light-curing time on the microhardness of a nanofilled composite resin. Methods: Forty-five composite resin (Z350 - 3M) specimens were randomly prepared using Teflon ring molds (4.0 mm internal diameter and 2 mm depth) and divided into nine experimental groups (n=5): three polymerization units (conventional - 450 mW/ cm2 ; 2nd generation LED - 1100 mW/cm2 ; and 3rd generation LED - 700 mW/cm2 ) and three lightcuring times (20 s, 40 s, and 60 s). All specimens were polymerized with the light-curing tip positioned 8 mm far from the top surface of the specimen. After 24 h, Knoop microhardness measurements were made on the top and bottom surfaces of the specimen, with a load of 10 g for 10 s. Five indentations were made on each surface. All results were analyzed statistically by subdivided parcel ANOVA (Split-Plot) and Tukey’s tests (p<0.05). Results: There were no statistically significant differences for the polymerization unit and light-curing time factors in either top or bottom surface. For all experimental conditions, the top surfaces showed greater hardness than the bottom surfaces (p<0.0001). Conclusions: The mode of polymerization and the lightcuring time did not affect the hardness of the nanofilled composite resin, and increasing the lightcuring time did not improve the hardness of the bottom surface of the composite resin.

Non-monotonous refractive index changes recorded in a phosphate glass optical fibre using 248nm, 500fs laser radiation

Results are presented on the photosensitivity behavior of a phosphate glass optical fibre using 248nm, 500fs laser radiation. Bragg grating exposures performed using a double phase mask interferometer and peak intensities of 0.37TW/cm2, revealed that grating growth becomes non-monotonic in terms of average and modulated refractive index changes, resembling the Type IIA photosensitivity behavior. Average refractive index changes greater than 10−3 were measured after accumulated energy density doses of 6.5KJ/cm2. The Bragg gratings inscribed maintained significant part of their strength up to 377°C. Exposed and side-polished fibre samples were subjected to Knoop micro-hardness measurements for revealing that the irradiated glass undergoes significant volume dilation.

Preparation and mechanical characterization of dense and porous zirconia produced by gel casting with gelatin as a gelling agent

A modified gel casting procedure based on a natural gelatin for food industry and commercial polyethylene spheres as pore formers was successfully exploited to produce dense and porous ceramic bodies made of yttria stabilized tetragonal zirconia polycrystal (Y-TZP). Vickers and Knoop microhardness, elastic modulus and fracture toughness measurements on dense samples obtained by experimental investigation closely matched results found in the literature for similar materials. However, after a careful analysis of obtained results, no indentation size effect and a lower scattering of experimental data from low load indentations were observed, in comparison with literature. Mechanical testing of porous samples (with reproducible values of porosity of about 40%) evidenced a high scattering of compressive strength values, suggesting that the uneven distribution of cavities in the material or the presence of defects from the agglomeration of pore forming agents could have a more direct influence on the mechanical properties of such materials than the absolute value of porosity.

Effect of mixing methods on the physical properties of dental stones

Objectives This in vitro comparative study evaluated the effect of different stone mixing methods on material properties of four dental stones. Two ADA type IV stones (Silky-Rock and Snap-Stone), one type V high expansion stone (Die Keen), and one recently introduced type V specialty stone (HandiMix) were chosen for this study. Methods Forty cylindrical specimens (25 mm × 12.5 mm) were cast for each of the nine stone sub-groups and bench dried at 23 ± 2 °C for 1 and 24 h. Specimens were then tested in an Instron in tensile and compression modes at crosshead speeds of 0.5 and 1.0 mm/min, respectively. Four rectangular-shaped specimens (30 mm × 15 mm × 15 mm) of each stone type were cast and bench dried for 48 h. Knoop microhardness measurements were obtained from defined areas on each specimen for surface hardness testing using 200 g load and 20 s dwell time. A 12.6 mm 2 area was then delimited in the center of two sides of each specimen and photographed under low power magnification (40×). The average pore number per area was then determined for each specimen for surface porosity testing. The setting time and setting expansion for each stone type was recorded as well. Results ANOVA ( P < 0.001) and Ryan–Einot–Gabriel–Welsh test ( P < 0.05) showed significant differences between diametral tensile strengths and pore numbers for both stone types and mixing methods. Conclusion Within the limitations of this study, the newly introduced mixing method did not appear to have an effect on the physical properties of HandiMix stone.

Enhancement of wear and corrosion resistance of nitrogen implanted dental tools

We present a study on the surface modification of dental tools by nitrogen PIII with a view to increasing both their cutting life and corrosion resistance. The treatment temperature was selected between 350 and 500 °C, every 50 °C, being controlled by varying the pulse duration in order to determine the improvement of the material performance after implantation. The tools were additionally characterized by Knoop microhardness measurements, SEM, XDR and electrochemical corrosion tests which, in all, pointed to an abundance of the supersaturated iron phase accompanied by considerable increases in microhardness and yet, with a slightly enhanced corrosion resistance. Furthermore, the formation of the associated expanded austenitic phase appears to enhance the corrosion resistance of the untreated drills.

Change in surface hardness of enamel by a cola drink and a CPP–ACP paste

Objectives This in vitro study used surface microhardness to evaluate whether a paste containing casein phosphopeptide amorphous calcium phosphate (CPP–ACP) can reharden tooth enamel softened by a cola drink, and how different saliva-substitute solutions affect the enamel hardness. Methods Twenty-four bovine incisors, each tooth consisting of treatment and control halves, were immersed in a cola drink (Coke ®) for 8 min, then placed under a 0.4 mL/min drip with various saliva-substitute solutions. The saliva-substitute solutions were: saliva-like solution (SLS) with 1 ppm fluoride, SLS without fluoride, and Biotene ® mouthwash. CPP–ACP paste was applied to the treatment halves for 3 min at 0, 8, 24, and 36 h. Knoop microhardness measurements were performed at baseline, after the cola drink immersion, and after 24 and 48 h contact with saliva-substitute solution. Results Enamel hardness significantly decreased after immersion in cola drink (ANOVA, p < 0.05). After contact with saliva-like solutions for 48 h, those treated with CPP–ACP paste were significantly harder than those untreated regardless of the presence of 1 ppm fluoride in the saliva-like solution (ANOVA, p < 0.05). Biotene ® mouthwash significantly softened the enamel surface (ANOVA, p < 0.05). Two-way ANOVA showed significant effects of the CPP–ACP paste application and types of saliva-substitute solutions on the changes in surface hardness of the softened enamel at a significance level of 0.05. Conclusion The application of CPP–ACP paste with continuous replenishment of saliva-like solution for 48 h significantly hardened enamel softened by a cola drink. Biotene ® mouthwash softened enamel surface after 48 h contact.