Knoop Indentation Research Articles

Restoring Enamel Strength: A Knoop Hardness Number Evaluation of Remineralizing Toothpastes.

Aim This investigation compares and assesses the microhardness of human dental enamel after the use of two types of widely available remineralizing toothpaste. Methodology Thirty extracted anterior incisors were chosen as study samples and split into three groups: Control (Group 1), Enafix (EX-Group 2), and Sensodyne Repair (SR-Group 3). All of the sample groups were assessed using a Knoop indenter at baseline, after demineralization, and then again after remineralization. Then, to compare the outcomes for the variation in Knoop hardness, a one-way analysis of variance (ANOVA) was employed. Results The Control group exhibited the highest mean microhardness (472.00±17.783), indicating superior enamel integrity and the least variability. The EX group showed a lower mean microhardness of 340.40±40.368, demonstrating effective remineralization but with greater variability. Statistical analysis using ANOVA revealed highly significant differences between the groups, with an F-statistic of 104.292 and a p-value of 0.000, indicating that the variations in microhardness among the groups were statistically significant. Conclusion Both SRand EX toothpaste can effectively help dental enamel regain its hardness and remineralization following demineralization with slightly superior activity from EX.

Correlation between fluoride release, surface hardness and diametral tensile strength of restorative glass ionomer cements.

The aim of this study was to determine if there is a correlation between fluoride release, surface hardness, and diametral tensile strength of restorative glass ionomer cements (GICs). Conventional (Riva Self Cure) and resin-modified (Riva Light Cure) GICs were used. Thirty-four samples (ø 6 x 3 mm) were prepared for each cement. The kinetics of fluoride release (n=4) was evaluated over 28 days using a fluoride-selective electrode (ISE 4010-C00). The analysis of surface hardness (n=10) was performed using a microhardness tester (Shimadzu HMV-2000, Japan) with a Knoop indenter and a load of 25 gf for 30 seconds. The diametral tensile strength test (n=10) was conducted on a universal testing machine at a speed of 0.75 mm/min. Fluoride release data were analyzed by two-way repeated measures ANOVA and Bonferroni post hoc test, while independent t-test was used for other analyses (α=0.05). Overall, the groups showed higher fluoride release until day 7 and a progressive decrease until day 28. On day 1 and day 21, Riva Self Cure showed a higher level of release than Riva Light Cure (p=0.026). Riva Light Cure showed higher diametral tensile strength (p<0.0001) and surface hardness (p=0.034) than Riva Self Cure. A negative correlation was found, indicating that higher fluoride release is associated with lower surface hardness and diametral tensile strength. Fluoride release and mechanical performance are related properties of GICs, and these properties exhibit different values depending on the type of material. Resin-modified GIC release less fluoride but exhibit better mechanical performance compared to conventional GIC. Key words:Diametral Tensile Strength, Fluoride, Glass Ionomer Cement, Surface Hardness.

Micromechanical characterization of zirconia and silicon nitride ceramics using indentation and scratch methods

Micromechanical characterization of brittle ceramics remains challenging due to the difficulty in sample preparation required by conventional tests. The micromechanical properties (e.g., elastic modulus EIT, indentation hardness HIT, and fracture toughness KIC) of ZrO2 and Si3N4 were investigated by indentation and scratch tests with different indenters (e.g., Vickers, Knoop, and Berkovich indenters) under various loads. The ratio of the true hardness by the Vickers indenter over that by the Knoop indenter is approximately a constant of 1.13 and independent of material. Elastic recovery of residual imprint by the Knoop indenter was also used to assess elastic modulus of ZrO2 and Si3N4, and modified equations were proposed. The length of Vickers indenter-induced cracking was found to be proportional to the applied load under large loads, and a modified formula for calculating fracture toughness was proposed for ZrO2 and Si3N4, which have relatively large fracture toughness of about 9 MPa⋅m1/2. The critical void volume fraction f* in the nanoindentation energy-based methods was found to linearly decrease with the exponent mf of the power-law equation used for curve fitting of the degradation of HIT or EIT with hmax. The determination of fracture toughness by scratch methodology was found to be affected by mechanical properties of material, indenter geometry, and data range.

Indentation fracture of 4H-SiC single crystal

Understanding the deformation and fracture behavior of single crystal silicon carbide (SiC) under a single particle is of scientific and practical importance for the micromachining of single crystal SiC. In this work, we investigate the indentation fracture of a 4H-SiC single crystal wafer with the indentation direction perpendicular to (0001) plane, using Berkovich, Vickers, and Knoop indenters. All the indentations create radial cracks for the normal loads used in this work. Both the Vickers and Knoop hardness tests produce lateral cracks. The fracture toughness calculated from the Vickers hardness tests is smaller than the ones calculated from the Knoop hardness tests and the Berkovich indentations under large indentation loads. The fracture toughness of 4H-SiC single crystal decreases with the increase of the indentation load under small indentation loads for the Berkovich indentations, revealing the phenomenon of surface hardening. The surface energy calculated from the radial cracks is comparable to the results reported in the literature. We establish an analytical expression of the total energy dissipation of a single indentation cycle, including the contribution of indentation-induced cracking, which offers an approach to calculate the ratio of the energy dissipation due to cracking to the energy dissipation due to plastic deformation and determine the dominant deformation process controlling the indentation deformation. The analysis of the energy dissipation for the indentations of the 4H-SiC single crystal by the Berkovich indenter reveals that the energy dissipation from the indentation-induced cracking is significantly larger than (more than eight times) that from plastic deformation for the indentation loads larger than 75 mN.

Nondestructive testing method of residual stresses in metallic materials based on Knoop indentation technique

ABSTRACT In the process of manufacturing and shaping critical mechanical structures, the inevitable result is the creation of non-uniform residual stresses (RS). These residual stresses significantly affect the mechanical properties, fatigue strength, and resistance to stress corrosion cracking in the structures, ultimately posing risks to their safety and operational lifespan. Furthermore, obtaining residual stress for equipment in service continues to be a challenging endeavour. Hence, the development of an in-situ testing method for measuring residual stresses is crucial to ensure the safe operation and extended lifespan of structures. In this study, we used finite element analysis to thoroughly examine the connection between residual stresses, indentation loading curves, and material mechanical properties. We then developed a biaxial residual stress testing method suitable applicable to part of mechanical properties, utilising the Knoop indentation technique. The reliability of method is experimentally validated using nuclear pipe material 316 L and 304 stainless steels. The results indicate that this method enables the transformation of biaxial residual stresses into two orthogonal uniaxial residual stresses. By conducting a pair of orthogonal indentation tests, biaxial residual stresses can be accurately determined, with errors mostly within a 16% margin. This method fulfils the criteria for non-destructive testing in engineering, providing ease of use and in-situ applicability.

Mechanical properties of Al2O3–LaPO4 composites with eutectic microstructure produced by flash sintering

AbstractThe mechanical properties of Al2O3–LaPO4 composites with varying microstructures produced by flash sintering and conventional sintering are evaluated. Specifically, Vickers and Knoop hardness values were measured and calculated for different resultant microstructures, including eutectic microstructures with varying layer thickness, polycrystalline (noneutectic) microstructures, and single‐phase samples of Al2O3, LaPO4, and 8YSZ. The findings indicate that eutectic microstructures exhibited higher hardness values than polycrystalline counterparts on the flash‐sintered sample. However, the hardness values of eutectic microstructures with varying layer thicknesses show no significant or systematic variation. The grain size, indentation size, eutectic colony size, indentation shape (elastic recovery in Knoop indentations), and crack propagation pathways in the indented samples are also discussed. Overall, the results suggest that Al2O3–LaPO4 eutectic composites have higher hardness than their polycrystalline counterparts and have great potential as abradable coatings with high machinability and durability.

Asymmetric indentation: problem and solution

Vickers and Knoop testers are the most common tools used to measure the hardness of materials. However, a very small tilt in the sample surface even in the order of a fraction of a degree produces a noticeable asymmetry, which affects the accuracy of the measured hardness. In this investigation, a mathematical approach has been used to correct asymmetry in the Vickers and Knoop indentations in both metallic and ceramic materials. Measurements were taken for metals such as aluminium 6061(Al-6061), 304 stainless steel as well as various zirconia toughened ceramic materials including tetragonal zirconia doped with: 2.5 mol% Y2O3 (TZ2.5Y), 3 mol% Y2O3 (TZ3Y) and the composite containing 20 wt% alumina (TZ3Y20A), all prepared from commercial powders. A hardness tester equipped with Vickers and Knoop indenters was used for hardness and elastic modulus determination. Optical and scanning electron microscopes have been used to get the indentation micrographs. The method enabled accurate determination of both hardness and elastic modulus from corrected Vickers and Knoop indentations. The hardness and elastic modulus values obtained in this study are in good agreement with reported data for similar materials. The results obtained in this study have been successfully validated using the Zwick reference block. The developed method is readily applicable for the most widely used Vickers hardness machines for the correction of asymmetric indentations if existing, consequently leading to accurate determination of the hardness.

Properties of Ni-B/B Composite Coatings Produced by Chemical Reduction

The paper presents the results of research on composite Ni-B coatings with embedded particles of boron produced by a chemical reduction method and their influence on the mechanical, tribological, and corrosion properties of produced materials. Boron was used in two forms: an alloy component and nanoparticles incorporated into the Ni-B matrix. The coatings were deposited from a bath at different concentrations of dispersion phase. The paper also presents the characteristics of the used boron particles. Results of morphology and surface topography (SEM) as well as the structure (XRD) of the produced coatings are presented. The mechanical properties of the produced coatings, like Knoop hardness, Martens hardness, and indentation hardness, as well as the modulus of elasticity, were determined. The tribological tests were carried out using the ball-on-disc method. Electrochemical corrosion tests were conducted by the potentiodynamic method. The adhesion of the coatings to the substrate was evaluated by the scratch test method. Consequently, this investigation yielded a novel material in the form of Ni-B/B composite coatings, exhibiting significantly improved mechanical and tribological properties compared to Ni-B coatings without embedded particles of the dispersion phase.

Maintenance of enamel properties after bleaching with high-concentrated hydrogen-peroxide gel containing calcium polyphosphate sub-microparticles.

To assessed the physical and chemical properties of human-enamel after treatment with an experimental bleaching gel containing 35%-hydrogen peroxide (HP) and calcium polyphosphate sub-microparticles (CaPP). Enamel/dentin specimens (4 × 4 × 3mm) were obtained (n = 120) and allocated to different groups: control (saliva only); experimental (HP35%); commercial (whiteness-HP-Maxx); CaPP0.5% (HP35% + CaPP0.5wt%); CaPP1.5% (HP35% + CaPP1.5wt%). Three sessions were performed. The specimens' color was assessed using a spectrophotometer and the color (ΔE/ΔE00) and bleaching index (ΔWID) determined. The surface roughness and microhardness were assessed with a roughness tester and Knoop indenter. Raman spectroscopy was performed to obtain the ratios between the areas under the 431, 580, and 1070cm-1 and the 960cm-1 bands (430:960, 580:960, 1070:960). Kruskal-Wallis and Dunn compared the color, Ra, and SMH data. The Raman data was analyzed with Kruskal-Wallis and Dunn (α = 5%). The ΔE, ΔE00, and ΔWID were similar among the bleached groups (p > 0.05). The roughness was not different between the groups (p > 0.05). After the 3rd session, CaPP0.5% had higher microhardness than the experimental (p < 0.05). The 1070:960 was higher in the experimental than in the CaPP1.5% and control (p < 0.05). In human enamel, CaPP did not alter the bleaching effectiveness or roughness, and additionally, CaPP-containing gels increased the microhardness and preserved the mineral content when compared to the experimental without CaPP. Experimental bleaching gels containing calcium polyphosphate sub-microparticles as a mineral source reduce the mineral content alteration and superficial microhardness reduction, known potential side effects of the in-office bleaching treatments.

Micromechanical characterization of a reduced-activation ferritic/martensitic steel by instrumented indentation and scratch methods

Micromechanical properties of a reduced-activation ferritic/martensitic steel CLF-1 were characterized by nanoindentation tests with Berkovich indenter, microhardness tests with Vickers and Knoop indenters, and microscratch tests with Berkovich and spherical indenters to investigate the effects of different indenters. Elastic modulus and indentation hardness obtained by various methodologies such as energy-based and displacement-based approaches are consistent with those obtained by Oliver & Pharr method. The reasonable fracture toughness of CLF-1 steel can also be obtained by energy-based nanoindentation method based on the deterioration of indentation hardness with the critical void volume fraction of 0.49.

Hardness Measurements and Interface Behavior of SiC-B4C-Si Multiple Phase Particulate Composites Made with Melt Infiltration and Additive Manufacturing

Reaction-bonded SiC-B4C-Si ceramic composites were binder jet 3D-printed and subsequently pressureless-melt-infiltrated with molten Si. The addition of B4C aided the Si infiltration to produce a highly dense composite. The microstructures and phases of these composites were examined. The measured hardness values of each constituent with Vickers and nanoindentation matched the bulk values, and the macro-hardness values with Knoop and spherical indentation represented the bulk, composite hardness values of all three phases together, which was close to a rule of mixtures value. For particulate-based composites, this is a significant finding. The interfacial bonds of SiC and Si were imaged using scanning transmission electron microscopy to view intimacy, whereas the crack propagation was examined with carefully placed indents. This work demonstrated that pressureless melt infiltration with a reactive particle provides a method to shape non-wetting reaction-bonded ceramic composites with limited shrinkage and high density and provides insights into the mechanical behavior with numerous indentation techniques.

Determination of biaxial residual stresses via indentation energy difference method

<p indent="0mm">In this study, a novel method is proposed to estimate biaxial residual stress (RS) by using the indentation energy difference between the stressed and the stress-free samples. A Knoop indenter is applied along the directions of principal stresses, and the indentation energy obtained from the load-depth (<italic>F</italic>-<italic>h</italic>) curves is used as the calculation parameter without observing the indentation morphology. A work-energy transformation model is constructed to explain the change in the <italic>F</italic>-<italic>h</italic> curve with the RS in terms of energy. Then, the finite element analysis of different power-law hardening materials under different RS combinations is implemented to study the energy evolution. Consequently, the formula for calculating the indentation energy difference under a uniaxial RS is established. Meanwhile, the indentation energy difference of the Knoop indenter exhibits superposition and direction characteristics, which can be used to convert the indentation energy difference under a biaxial RS into the sum of that under two equivalent uniaxial stresses. Through this conversion, the two stress components can be calculated from the two indentation energy difference formulas. An indentation testing device is developed to verify the proposed method. The results obtained using the proposed method agree well with those obtained using the indentation strain-gage method.

Nano-hydroxyapatite-induced remineralization of artificial white spot lesions after bleaching treatment with 10% carbamide peroxide.

To assessed in vitro the effect of nanohydroxyapatite (n-HA) to improve the aesthetic appearance and microhardness of white spot lesions (WSL) when associated with a low-concentration bleaching agent (carbamide peroxide-CP10%). Enamel/dentin specimens (n=60) of 5 × 5 × 2.2 mm were prepared, of these, 48 were submitted to pH-cycling to create artificial WSL. Subsequently, these were allocated into five groups (n=12): n-HA; n-HA + CP10%; CP10%; WSL control (WSLC ); sound control (Sound). The color was assessed at baseline, pre-treatment, and post-treatment using a spectrophotometer, and the color (ΔE/ΔE00 ) and whiteness index (ΔWID) alterations were determined. The enamel cross-sectional microhardness (CSMH) was evaluated (post-treatment) with a Knoop indenter, 25gf/5s, 20-200 μm. The data was analyzed through generalized linear models (α=5%). ΔE and ΔE00 were significantly higher for the bleached groups (n-HA + CP10% and CP10%), and the n-HA was higher than the WSLC group (p < 0.05). ΔWID was significantly higher for the bleached groups (p < 0.05). The CSMH values were significantly higher in the sound group than in the n-HA, CP10%, and WSLC groups (p < 0.05). The WSLC had lower microhardness than the n-HA + CP10% and sound groups (p < 0.05). n-HA is suitable to remineralize and recover the color of the WSL. However, its association with CP10% maintains the esthetical outcome while increasing its in-depth remineralizing effect. Considering the aesthetic and functional repercussions of the WSL persistence, treatments that tend to improve its physical appearance and reinforce its weakened substructure in a non-invasive way are ideal. For this associating low-concentration, bleaching agents to the remineralizing treatments is promising to treat this type of lesions.

Micromechanical properties of Dy3+ ion-doped (LuxY1-x)3Al5O12 (x = 0, 1/3, 1/2) single crystals by indentation and scratch tests

Three kinds of Dy3+ ion-doped (LuxY1-x)3Al5O12 (x = 0, 1/3, 1/2) single crystals fabricated by the Czochralski method with 4 at.% Dy3+ ion doping were investigated by indentation and scratch techniques under Vickers, Knoop, Berkovich, and spherical indenters to understand the influence of Lu ion on micromechanical properties and fracture behavior of Y3Al5O12 (i.e. YAG for x = 0) single crystals. The largest (or smallest) values of hardness, elastic modulus, and fracture toughness were found for x = 1/3 (or 1/2). The indentation size effect was explained by four different models with the Hays-Kendall approach being the most suitable one to determine the true hardness. Fracture toughness values of YAG crystals obtained by the Vickers hardness method agreed with those obtained by scratching with a spherical indenter based on linear elastic fracture mechanics.

Corrosion performance of TiAlVN-Ag nanocomposite coating deposited by reactive direct current magnetron sputtering

Surgical devices and tools are manufactured in AISI 420 martensitic stainless steel, due to its hardenability, adequate hardness and acceptable biocompatibility. Nevertheless, many surface modification strategies are being investigated with a view to improving this material by providing it with self-protection against the colonization of bacteria. One such strategy is the use of a TiAlVN-Ag nanocomposite coating to promote bactericidal effect in surgical devices; however, it is necessary to study the electrochemical response of such coatings to aggressive environments that simulate disinfection and sterilization processes. The aim of this work is to study the relationship between the microstructure, chemical phases and the electrochemical and corrosion response of TiAlVN-Ag coating, varying the amount of silver with the target power increase (0, 50, 70, 80 and 100 W). To study the microstructure, chemical phases, scanning and high resolution transmission electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction were used. Roughness, microhardness, residual stresses, qualitative adhesion of coatings were estimated by atomic force microscopy, Knoop indentation, profilometer and Rockwell C indentation, respectively. Electrochemical behavior and corrosion protection of coatings were studied using electrochemical impedance spectroscopy and potentiodynamic polarization. TiAlVN-Ag 50W coating showed an improvement in electrochemical behavior and in protection of the steel against corrosion and exhibiting higher hardness than that of the AISI 420 stainless steel substrates. This coating exhibited a dense microstructure and high crystallinity, indicating that the low Ag content does not affect the insulating nature of the coating matrix.

Comparative evaluation of microhardness of residual dentin in primary molars following caries removal with chemomechanical agent (BRIX 3000) and polymer burs (ss white)

Aim: To evaluate and compare microhardness of residual dentin in primary molars following caries removal with chemomechanical agent (BRIX 3000) and polymer burs (SS white). Materials and Method: Extracted maxillary or mandibular primary molars with proximal carious lesion, were selected with active carious cavities extending in to 2/3rd of dentin. The selected teeth were preserved in phosphate buffer saline solution no longer than 30 days. The selected teeth were separated into two experimental groups: group A- chemomechanical method – Brix3000 and Group B- slow speed polymer bur. After caries excavation the microhardness test was carried out on treated dentin using a Knoop indenter. Results: The results suggest a significant difference between among the groups Brix 3000â showed superior results compared to group B (Polymer burs) i.e., The results showed that the microhardness of the remaining dentine after caries excavation with Brix 3000 was found to be greater than the microhardness of the remaining dentine after caries excavation with Polymer bur. Conclusion: In conclusion of the current study, it can be stated that Brix 3000 may be utilized as a successful agent in minimally invasive caries excavation since Polymer burs showed partial removal of carious dentine.

Effect of an enamel matrix derivative (Emdogain) on the microhardness and chemical composition of human root dentin: an in vitro study

The advantage of using an Enamel matrix derivative EMD Emdogain as an intracanal medication could be a manner to strength the tooth structure, improving the physical and chemical properties of dentin. We tested, in vitro, the effect of Emdogain on the surface microhardness and chemical composition of root dentin. Ten human teeth were used to produce dentin specimens originated from the canal walls (n = 30) that remained in contact to Emdogain gel for 90 days. Baseline and 90-days after Emdogain treatment measurements were performed using Fourier Transform Infrared Spectroscopy (ATR/FTIR), Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) and Knoop indenters. The use of EMD (Emdogain) for 90 days in contact with human root canal dentin specimens did not alter the microhardness and morphology of dentin. The elemental structure of dentin was altered because there was a reduction in carbonate content.

Mechanical properties of lithic raw materials from Kazakhstan: Comparing chert, shale, and porphyry.

The study of lithic raw material quality has become one of the major interpretive tools to investigate the raw material selection behaviour and its influence to the knapping technology. In order to make objective assessments of raw material quality, we need to measure their mechanical properties (e.g., fracture resistance, hardness, modulus of elasticity). However, such comprehensive investigations are lacking for the Palaeolithic of Kazakhstan. In this work, we investigate geological and archaeological lithic raw material samples of chert, porphyry, and shale collected from the Inner Asian Mountain Corridor (henceforth IAMC). Selected samples of aforementioned rocks were tested by means of Vickers and Knoop indentation methods to determine the main aspect of their mechanical properties: their indentation fracture resistance (a value closely related to fracture toughness). These tests were complemented by traditional petrographic studies to characterise the mineralogical composition and evaluate the level of impurities that could have potentially affected the mechanical properties. The results show that materials, such as porphyry possess fracture toughness values that can be compared to those of chert. Previously, porphyry was thought to be of lower quality due to the anisotropic composition and coarse feldspar and quartz phenocrysts embedded in a silica rich matrix. However, our analysis suggests that different raw materials are not different in terms of indentation fracture resistance. This work also offers first insight into the quality of archaeological porphyry that was utilised as a primary raw material at various Upper Palaeolithic sites in the Inner Asian Mountain Corridor from 47-21 ka cal BP.

A theoretical model to predict the anisotropic characteristics in grinding of diamond conical indenter

Mechanical grinding of a high-precision diamond conical indenter is difficult due to the strong grinding removal rate anisotropy. In the existing literature, theoretical models have been established to predict the strength anisotropy of diamond on the (100), (110) and (111) planes, which provide a theoretical basis for the fabrication of pyramid diamond indenters, such as Berkovich, Vickers or Knoop indenters. However, investigations on the anisotropic characteristics in processing of diamond curved surface are rarely carried out. To solve the anisotropy problem in grinding of diamond conical indenter, this work contributes a theoretical model to predict the anisotropic characteristics in grinding of diamond cone face, and provides an effective method to optimize the grinding direction for enhancing the fabrication precision. Based on a proposed grinding-ability factor model, a general method is developed to analyze the grinding ability along different directions according to the formation process of the cone face. The results suggest that the optimized mechanical grinding direction can be determined by minimizing the standard deviation of the grinding-ability factors of the crystal planes. The proposed method is validated by grinding experiments on indenters, with the <100> or <111> orientation along the indenter axis. The experimental observations confirm that the optimized grinding direction can weaken the pyramid phenomenon, yielding a roundness error of 0.12 μm. In addition, the grinding removal rate-dependent roughness petal phenomenon is suppressed, which considerably enhances the conical surface quality.

Structure and properties of dislocations and the twin boundary on (101) in β-cyclotetramethylene tetranitramine

ABSTRACT Dispersion-corrected density functional theory calculations of the generalised stacking fault energy surface of the (101) plane in monoclinic β-cyclotetramethylene tetranitramine (β-HMX) indicate that a stacking fault with an energy of approximately 130 mJ/m at the translation vector is metastable. Using the energy of the metastable stacking fault and anisotropic linear elasticity theory, we have evaluated whether dislocations with total Burgers vectors , and will split into partial dislocations on . Our results suggest that it is not favourable for the screw dislocation to split into partials but that the edge dislocation will split into partials separated by a narrow, 19.8 Å, stacking fault. Both screw and edge dislocations are predicted to split into two partials with splitting distances of about 28.7 and 43.4 Å, respectively. The screw and edge dislocation are also predicted to split into partials, with equilibrium distances of 16.3 and 43.2 Å, respectively. These results are consistent with recent analyses of Knoop indentation experiments in which the slip system is inactive whereas dislocations are glissile. Since twinning is an important deformation mechanism in β-HMX, dispersion-corrected density functional theory calculations have also been used to compute the energy of the twin boundary, for which we report a value of 163 mJ/m.