Knoop Hardness Number Research Articles

Influence of Chemical Degradation on the Surface Properties of Nano Restorative Materials

The aim of this in vitro study was to investigate the effect of chemical degradation on the surface roughness (Ra) and hardness (Knoop hardness number [KHN]) of nano restorative materials. Disc-shaped specimens (5-mm diameter; 2-mm thick) of Filtek Z350 and TPH Spectrum composites and the Vitremer and Ketac Nano light-curing glass ionomer cements were prepared according to the manufacturers' instructions. After 24 hours, polishing procedures were performed and initial measurements of Ra and KHN were taken. The specimens were divided into 12 groups (n=10) according to material and storage media: artificial saliva, orange juice, and Coca-Cola. After 30 days of storage, the specimens were reevaluated for Ra and KHN. The pH values of the storage media were measured weekly. Data were tested for significant differences by repeated-measures three-way analysis of variance and Tukey tests (p<0.05). Composites were found to present lower roughness values and higher hardness values than the ionomeric materials under all storage conditions. After degradation, the KHN of all experimental samples decreased significantly, while the Ra of the ionomeric materials increased, depending on the media, with a markedly negative impact of Coca-Cola and orange juice. There was no difference among the storage media for Filtek Z350 with regard to the KHN values. Nanofillers did not show any influence on the roughness and hardness of resin-modified glass ionomer cements and resin composites concerning their degradation resistance.

Do new matrix formulations improve resin composite resistance to degradation processes?

The aim of this study was to determine the degradation resistance of three new formulations-silorane-, Ormocer- and dimer-acid-based materials-and compare them to the traditional dimethacrylate-based materials. One silorane- (Filtek P90, P90), one Ormocer- (Ceram-X, CX), one dimer-acid- (N'Durance, ND) and two dimethacrylate-based (Filtek P60, P60; Tetric Ceram, TC) materials were investigated. Water sorption (Wsp) and solubility (Wsl) were determined after the materials were immersed in water for 28 days. Knoop hardness (KH) was determined before and after 24 h immersion in pure ethanol. The flexural-strength (FS) was determined by the bending test after one-week storage in a dry environment or after one-week immersion in pure ethanol. Data were submitted to analysis of variance (ANOVA) and Tukey's test (95%). The three new formulations showed lower Wsp than the dimethacrylate-based formulation. CX (0.50 ± 0.17%) and ND (0.72 ± 0.19%) exhibited the lowest Wsp, whereas P90 (0.02 ± 0.03%) and P60 (0.04 ± 0.03%) showed the lowest Wsl. All resins showed reduced Knoop hardness number (KHN) after ethanol immersion. P60 presented the lowest decrease in KH value (19 ± 5%). TC (48 ± 3%) and P90 (39 ± 9%) showed the highest KHN decrease after ethanol storage. The FS of CX, ND and TC were affected by ethanol storage. The new formulations did not improve the degradation resistance, as compared with the traditional methacrylate-based materials.

Impact of the light-curing source and curing time on the degree of conversion and hardness of a composite

Adequate physical properties of the resinous materials are related to clinical longevity of adhesive restorations. The aim of this investigation was to assess the impact of light-curing source and curing time on the degree of conversion (DC) and Knoop hardness number (KHN) of a composite resin. Circular specimens (5 x 2 mm) were carried out (n = 7) of the Filtek Z250 (3M ESPE) composite. The specimens were light-cured by quartz-halogen-tungsten (QTH) XL 3000 (3M ESPE, 450 mW/cm2) or light-emitting diode (LED) Bluephase 16i (Vivadent, 1390 mW/cm2) for 20, 40, or 60 s. After 24 h, absorption spectra of composite were obtained using Spectrum 100 Optica (Perkin Elmer) FT-IR spectrometer in order to calculate the DC and, KHN was performed in the HMV-2T (Shimadzu) microhardness tester under 50-g load for 15 s dwell time. DC and KHN data were subjected to 2-way ANOVA and Tukey’s test at a pre-set alpha of 0.05. The LED showed highest DC and KHN values than QTH (p &lt; 0.05). The increase of curing time improved the DC and KHN, all curing times with statistical difference (p &lt; 0.05). The use of light-curing units with high irradiance and/or the time of cure increased may improve the physical properties of resin-based materials.

Influence of Laser Irradiation on Pits and Fissures: AnIn SituStudy

The aim of this in situ study was to analyze the influence of the Er:YAG, Nd:YAG, and CO(2) lasers on the enamel acid resistance of pits and fissures. The laser tissue interaction has been studied as a method of preventing occlusal caries. Thirteen volunteers wore palatal acrylic appliances containing human occlusal enamel blocks that were divided into four groups (G1, control; G2, Er:YAG; G3, Nd:YAG; G4, CO(2)). Each palatal acrylic appliance was used in the four studied groups and was used for 14 consecutive days. A sucrose solution was applied to the specimens six times per day. The specimens were then sectioned in half, and a microhardness test was applied. The other halves were analyzed using polarized light microscopy to measure the caries-like lesion areas, and a morphological analysis was conducted using a scanning electron microscope (SEM). For the statistical analysis of the data obtained from the microhardness test (Knoop hardness number. [KHN]) (α=5%), Fisher's exact test was performed, and the group means were as follows: G1, 247±71; G2, 258±70; G3, 272±73; and G4, 298±56. The results demonstrated that the control group was significantly different from G3 and G4, which presented higher microhardness values. The Wilcoxon signed-rank test was used to analyze the data obtained from the caries-lesion area measurements (mm(2)) (α=5%) (G1, 0.01±1.08; G2, 0.13±0.18; G3, 0.05±0.17; and G4, 0.09±0.22). The results no showed significant differences among the groups in this analysis. Based on the results from the present study, it may be concluded that the CO(2) and Nd:YAG lasers increased the enamel acid resistance in pits and fissures.

Acceleration of curing of resin composite at the bottom surface using slow-start curing methods

The aim of this study was to evaluate the effect of two slow-start curing methods on acceleration of the curing of resin composite specimens at the bottom surface. The light-cured resin composite was polymerized using one of three curing techniques: (1) 600 mW/cm(2) for 60 s, (2) 270 mW/cm(2) for 10 s+0-s interval+600 mW/cm(2) for 50 s, and (3) 270 mW/cm(2) for 10 s+5-s interval+600 mW/cm(2) for 50 s. After light curing, Knoop hardness number was measured at the top and bottom surfaces of the resin specimens. The slow-start curing method with the 5-s interval caused greater acceleration of curing of the resin composite at the bottom surface of the specimens than the slow-start curing method with the 0-s interval. The light-cured resin composite, which had increased contrast ratios during polymerization, showed acceleration of curing at the bottom surface.

The Effect of the Addition of Tricalcium Phosphate to 5% Sodium Fluoride Varnishes on the Microhardness of Enamel of Primary Teeth

Aim. The aim of this study was to compare the effect of pH cycling on the microhardness of the enamel of primary human teeth treated with a conventional brown Sodium Fluoride (5% NaF) Varnish to those treated with a white Fluoride Varnish (5% NaF) enhanced with functionalized tricalcium phosphate (fTCP). Materials and Methods. Ninety extracted caries-free primary incisors were washed in a detergent and divided into three groups; group A received no treatment, teeth in group B were coated with Sodium Fluoride (5% NaF) Varnish, while teeth in group C were coated with 5% NaF varnish enhanced with functionalized tricalcium phosphate (fTCP). After ten days of pH cycling, the surface microhardness of the teeth was measured using a Knoop indenter. Results. The mean Knoop hardness number (KHN) of 5% NaF with fTCP was greater than that of 5% NaF alone while the control group had the lowest mean KHN. Conclusion. The results of this study suggest that the use of an additive such as fTCP to a fluoride varnish significantly improves the protective ability of the varnish on primary teeth in vitro.

Cross-sectional microhardness of bovine enamel subjected to three paediatric liquid oral medicines: an in vitro study

This study aimed to investigate the in vitro effects of three paediatric liquid oral medicines on bovine dental enamel subsurfaces under pH cycling conditions. Bovine enamel blocks were evaluated for surface hardness at baseline for sample selection. 52 intact bovine enamel blocks (16mm(2)) were randomly divided into four groups (n=13) according to the immersion treatments: G1: antibiotic (Klaricid®), G2: antihistamine (Claritin®), G3: antihistamine (Dimetapp®) and G4: control (de-ionised water). The blocks were submitted to pH cycling treatments twice a day for 12 days. The medicines were evaluated for pH, viscosity, and concentration of calcium, phosphate and fluoride. After the treatment period, cross-sectional microhardness (CSMH) measurements of the enamel blocks were taken and the data, expressed in Knoop hardness number (kg/mm(2)) was used to calculate the ΔS. ANOVA followed by the Tukey test were used for statistical analyses (p<0.05). The antibiotic Klaricid® showed the highest concentration of fluoride, calcium and phosphate. Considering pH and viscosity, the following pattern was observed according to the treatment group: G4>G1>G2>G3 and G1>G2>G3>G4 respectively. Regarding the demineralisation pattern, the following results were observed: G4>G3>G2>G1. Compared to the control, the antibiotic and both the antihistamines provoked less demineralisation of the enamel blocks (p<0.05). Antibiotic G1 (Klaricid®) presented an in vitro protective effect against acid attacks probably due to its mineral content and viscosity.

Influence of photoactivation method and mold for restoration on the Knoop hardness of resin composite restorations

The aim of this study was to evaluate in vitro the Knoop hardness in the top and bottom of composite photo activated by different methods when different mold materials were used. Z250 (3M ESPE) and XL2500 halogen unit (3M ESPE) were used. For hardness test, conical restorations were made in extracted bovine incisors (tooth mold) and also metal mold (approximately 2mm top diameter × 1.5mm bottom diameter × 2mm in height). Different photoactivation methods were tested: high-intensity continuous (HIC), low-intensity continuous (LIC), soft-start, or pulse-delay (PD), with constant radiant exposure. Knoop readings were performed on top and bottom restoration surfaces. Data were submitted to two-way ANOVA and Tukey's test (p = 0.05). On the top, regardless of the mold used, no significant difference in the Knoop hardness (Knoop hardness number, in kilograms-force per square millimeter) was observed between the photoactivation methods. On the bottom surface, the photoactivation method HIC shows higher means of hardness than LIC when tooth and metal were used. Significant differences of hardness on the top and in the bottom were detected between tooth and metal. The photoactivation method LIC and the material mold can interfere in the hardness values of composite restorations.

X-Ray Microtomography Study to Validate the Efficacies of Caries Removal in Primary Molars by Hand Excavation and Chemo-Mechanical Technique

Background/Aims: Mechanical removal of carious dentine based on perceived hardness is subjective and tends to be excessively destructive; chemo-mechanical techniques have been proposed as being more objective and conservative. The aims of the present study are to use X-ray microtomography (XMT/micro-CT) to determine the three-dimensional mineral concentration distribution in sound, carious and excavated dentine using hand excavation (HE) and a chemo-mechanical, Carisolv (CS), removal technique for primary molars, and to compare the volume of sound dentine removed in order to validate the efficacies of these two techniques. Methods: Twenty-one primary molars with open carious cavities were hemisected. The carious tissue in one half was then removed by HE and the other by CS. XMT scans were taken before and after caries removal. After alignment, subtracted XMT images from the two scans revealing the tissues removed were generated, from which mineral distributions were determined, and volumes of sound dentine removed by each technique were calculated. Results: It was found that the sound dentine removed by HE and CS techniques accounted for 4.0 and 2.1% of total tissues removed, respectively. The mean cut-off linear attenuation coefficients at 40 keV to which HE and CS excavated to were 1.27 and 1.09 cm^–1, respectively. The corresponding Knoop hardness number for the cut-off for CS was 25 kg·mm^–2. Conclusion: It is concluded that using XMT, CS is validated to be more conservative than HE and preserves a layer of partially demineralised dentine with a mineral concentration >0.97 g·cm^–3.

Nanoparticle Loading Level and Properties of Experimental Hybrid Resin Luting Agents

This study investigated the influence of nanoparticle loading level on properties of experimental hybrid resin luting agents. Silanated 2-μm barium borosilicate glass microparticles and 7-nm silica nanoparticles were used. Five materials were obtained by loading a photocurable Bis-GMA/TEGDMA co-monomer with a total mass fraction of 60% inorganic fillers. The mass fraction of nanoparticles was set at 0% (control), 1% (G1), 2.5% (G2.5), 5% (G5), or 10% (G10). The properties evaluated were flexural strength (σ) and modulus (E(f) ), Knoop hardness number (KHN), and film thickness (FT). Dispersion/interaction of the particles with the resin phase was assessed by scanning electron microscopy (SEM). Data were submitted to statistical analysis (5%). For σ, G1 > G2.5 = G5 = G10, and control > G10. For E(f) , G2.5 > control = G1 > G5 > G10. For KHN, G5 = G10 > control = G1 = G2.5. For FT, G10 = G5 > control = G1, and G10 > G2.5. Incorporation of nanoparticles was associated with observation of clusters in the SEM analysis. The clusters were more frequent for higher nanoparticle loadings. Modest incorporation of nanoparticles may improve the properties of resin luting materials. Nanofiller mass fractions above 2.5% should, however, be avoided because they may be detrimental to the properties of the resin luting agents.

Properties of particulate resin‐luting agents with phosphate and carboxylic functional methacrylates as coupling agents

AbstractThis study investigated properties of resin‐luting agents using silane [3‐(trimethoxysilyl)propyl methacrylate (TSPM)], phosphoric acid methacrylate (PAM) [mono/bis(methacryloyloxyethyl (di)hydrogen phosphate)], or carboxylic acid methacrylate (CAM) [mono‐2‐(methacryloyloxy)ethyl maleate] as coupling agents between the inorganic and organic phases. Ba‐B‐Al‐Si microparticles (3 μm) and SiO2 nanoparticles (7 nm) were coated with TSPM, PAM, or CAM (control = no filler coating). A Bis‐GMA/triethyleneglycol dimethacrylate comonomer was loaded with 60% mass of inorganic fillers. The properties evaluated were degree of CC conversion (DC), flexural strength (σ), and modulus (Ef), Knoop hardness number (KHN), and film thickness (FT). Dispersion/interaction of the particles with the resin phase was assessed by scanning electron microscopy (SEM). No significant differences in DC were observed. For σ and Ef, TSPM &gt; CAM &gt; Control &gt; PAM. For KHN, TSPM &gt; CAM &gt; PAM = Control. For FT, TSPM &lt; Control &lt; CAM &lt; PAM. The SEM analysis revealed clustering of nanoparticles for all groups and better organic–inorganic phases interaction for TSPM and CAM. The use of TSPM generated agents with improved properties as compared with the acidic methacrylates, with CAM showing better performance than PAM. The use of PAM generated agents with properties usually poorer compared with the material with no coupling agent. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Carbon dioxide laser and bonding materials reduce enamel demineralization around orthodontic brackets

Altering the structure of the enamel surface around the orthodontic bracket by reducing its content of carbonate and phosphate resulting from application of CO(2) laser may represent a more effective strategy in preventing caries in this region. This study aimed at determining whether irradiation with a CO(2) laser combined with fluoride-releasing bonding material could reduce enamel demineralization around orthodontic brackets subjected to cariogenic challenge. Ninety bovine enamel slabs were divided into five groups (n = 18): non-inoculated brain-heart infusion broth group, non-fluoride-releasing composite resin (NFRCR--control group), resin-modified glass ionomer cement (RMGIC), CO(2) laser + Transbond (L+NFRCR) and CO(2) laser + Fuji (L+RMGIC). Slabs were submitted to a 5-day microbiological caries model. The Streptococcus mutans biofilm formed on the slabs was biochemically and microbiologically analysed, and the enamel Knoop hardness number (KHN) around the brackets was determined. The data were analysed by ANOVA and Tukey tests (α = 0.05). Biochemical and microbiological analyses of the biofilm revealed no statistically significant differences among the groups. Lased groups presented the highest KHN means, which statistically differed from NFRCR; however, no difference was found between these lased groups. RMGIC did not differ from NFRCR which presented the lowest KHN mean. The CO(2) laser (λ = 10.6 μm; 10.0 J/cm(2) per pulse) use with or without F-bonding materials was effective in inhibiting demineralization around orthodontic brackets. However, no additional effect was found when the enamel was treated with the combination of CO(2) laser and an F-releasing material.

Erosive effects of beverages in the presence or absence of caries simulation by acidogenic challenge on human primary enamel: an in vitro study

To evaluate in vitro the erosive effects of beverages in the presence or absence of caries simulation (acidogenic challenge) on the microhardness of primary enamel. Forty human primary teeth were submitted to the erosive effects: 3 × 20-min-long daily immersion in fresh orange juice (orange group), strawberry yogurt drink (yog group), or cola soft drink (cola group) separately or in combination with acidogenic challenge (pH cycling for 10 days). Specimens were also submitted to acidogenic challenge alone, and in the negative control group specimens were not submitted to any treatment. Mineral loss was evaluated by cross-sectional microhardness determination. The data (Knoop hardness numbers, KHN) were subjected to 2-way analysis of variance and Tukey's post hoc test (α = 0.05%). All the test beverages significantly reduced the sample cross-sectional enamel hardness (KHN ± SD, 235.93 ± 18.15, 257.23 ± 21.79, and 253.23 ± 13.86 in the orange, yog, and cola groups, respectively) compared to samples in the negative control group (290.27 ± 3.92). In vitro acidogenic challenge exacerbated the mineral loss induced by all beverages (166.02 ± 4.28, 190.43 ± 17.55, and 198.39 ± 21.39 in the orange, yog, and cola groups combined to acidogenic challenge, respectively) compared to acidogenic challenge alone. All beverages exhibited erosive effects on primary enamel. Simulated caries challenge considerably exacerbated the enamel softening of primary teeth.

Evaluation of the effects of the bonding agent on acid-etched human enamel demineralization: in situ study

This in situ study evaluated the influence of the bonding agent on demineralization of acid-etched human enamel. Enamel blocks obtained from 20 human molars (four blocks per tooth) were divided into five groups. For the control groups, healthy control (HC, no treatment) and acid-etched control (EC, etching with 35 per cent phosphoric acid for 20 seconds), the specimens were obtained by sectioning in half, one block from each tooth. For the experimental groups, experimental etched group (EE), experimental Concise™ group (CE), and experimental Transbond™ group (TE), all specimens were etched with 35 per cent phosphoric acid for 20 seconds (EE) and treated with Concise™ enamel bond (CE) and Transbond™ XT primer (TE). Specimens from the experimental groups were fixed at intra-oral appliances wore by 10 volunteers and exposed to oral environment. After 28 days, the specimens were sectioned in the mesial-distal direction and the enamel cross-sectional Knoop hardness number (KHN, 50 g, 15 seconds) was measured at the cervical and occlusal regions at 25, 50, and 75 μm from the enamel surfaces. Data were analysed by analysis of variance (ANOVA) and Tukey honestly significantly different test for multiple comparisons (α = 0.05). Enamel KHN (P < 0.05) was recovered only in the Concise-treated group (CE; P < 0.05). When considering depth measurements, KHN results were 25 > 50 > 75 μm (P < 0.05). None of the groups exposed to the intra-oral environment for 28 days have recovered completely the enamel microhardness, but the order of improvement was better in the CE group than the TE group, the latter being only slightly better than the EE group.

Depth of cure and hardness of indirect composite materials polymerized with two metal halide laboratory curing units

The purpose of this study was to evaluate the depth of cure and Knoop hardness of indirect composite materials polymerized with different laboratory curing units. Five composite materials designed for fixed restoration veneer (Artglass, Ceramage, Epricord, Prossimo, and Solidex) were filled into a cylindrical mold and then light-exposed by using the respective proprietary laboratory curing unit or two metal halide curing units (Hyper LII and Twinkle X). Depth of cure was determined by a scraping technique, as described in ISO 4049. Composites also underwent Knoop hardness testing after immersion in water. The results (n = 5) were analyzed with the Kruskal-Wallis test and Dunn's multiple comparison test. For three materials (Prossimo, Artglass, and Epricord), depth of cure after polymerization with the Twinkle X unit was greater than that after polymerization with the respective proprietary units. For the Ceramage and Artglass materials, the Twinkle X unit resulted in the highest Knoop hardness number (KHN), whereas, for the Prossimo material, the Hyper LII unit resulted in the highest KHN. The metal halide units were effective in enhancing the post-polymerization properties of specific composite materials while reducing exposure time.

Comparing the effectiveness of self-curing and light curing in polymerization of dual-cured core buildup materials

The authors conducted an in vitro study to investigate the influence of various curing protocols on subsurface microhardness of dual-cured resin-based composites. The authors filled molds with four dual-cured resin-based composite core buildup materials: Clearfil DC Core Automix (Kuraray, Tokyo), Core•X flow (Dentsply DeTrey, Konstanz, Germany), LuxaCore Z-Dual (DMG, Hamburg, Germany) and MultiCore Flow (Ivoclar Vivadent, Schaan, Liechtenstein). They light cured the specimens for 20 or 60 seconds immediately after the filling procedure (time delay, 0 seconds) or after a 90-or 300-second delay. Self-cured specimens served as controls. The authors tested 15 specimens in each group. They measured the Knoop hardness number (KHN) at depths of 0.25, 2.00 and 5.50 millimeters two weeks after the initiation of polymerization. Light curing with Core•X flow, LuxaCore Z-Dual and MultiCore Flow did not result in significantly higher KHNs at any depth compared with those for the self-cured control specimens. For Clearfil DC Core Automix, immediate light curing for 60 seconds resulted in significantly higher KHNs than those for self-cured specimens only at a depth of 5.50 mm (P=.0171). The study results showed no significant differences in KHN at different depths for Core•X flow specimens, independent of the curing protocol. All other materials exhibited a significant decrease in KHN at increased depths in only one of the six light-cured groups. The results of this study show that photoactivation of the tested dual-cured materials provided no clinically relevant benefit compared with self-curing with regard to the degree of microhardness. The dominant self-curing mode allows placement of the tested buildup materials in areas inaccessible to a curing light.

Formation and characterization of aluminide coatings on alloy 800 substrate

Fe-Ni-Cr based Alloy 800 substrates were pack aluminized at 1273K for 8 hours in argon atmosphere. The cross-section of aluminized substrates indicated formation of multilayer of aluminides as revealed by microscopy, electron probe microanalysis and X-ray diffraction. The outermost layer (~150μm) was found to consist of FeAl+Fe2Al5 type phases, while the adjoining layer (~250μm) was composed of an FeAl type phase. The innermost layer (~60μm), which was a solid solution zone, was found to consist of~43at.% Fe, 38at.% Cr, 11at.% Ni containing about 6at.% Al. In microhardness test, a hardness variation of 213 to 1098 Knoop hardness number along the cross-section was obtained. Scratch test along the cross-section at load levels ranging from 0.9 to 10N with a loading rate of 30N/min showed a maximum penetration depth of 12μm indicating a good adherence of aluminide coatings.

Fracture behaviour of chemical vapour deposited and hot isostatically pressed zinc sulphide ceramics

Fracture behaviour of zinc sulphide ceramics prepared by chemical vapour deposition (CVD) followed by hot isostatic pressing (CVD + HIP) was investigated in terms of flexural strength ( σ f), plane-strain fracture toughness ( K Ic), even conditional fracture toughness ( K IQ), R-curve behaviour (variation of total fracture energy release rate, J c with crack extension, δ/ δ c) and fracture mode. The corresponding Knoop Hardness number (KHN) and its correlations to flexural strength ( σ f) are also evaluated and reported. The present study showed that the zinc sulphide (ZnS) ceramics processed by CVD exhibited higher fracture resistance compared to ZnS processed by CVD + HIP condition. This observation is principally attributed to higher grain size associated with post-CVD HIPing process. In both conditions, the ZnS materials exhibited conditional fracture toughness ( K IQ) that decreased moderately with increased crack length due to the change in fracture mode form grossly tensile to predominant shear. A constantly rising R-curve behaviour was indicated in both the materials with significant increase in total fracture energy release rate ( J c with the normalised displacement ( δ/ δ c), a parameter representing crack extension.

Load dependence of the apparent Knoop hardness of SiC ceramics in a wide range of loads

AbstractSilicon carbide (SiC) ceramics is a material with increasing use, due to its excellent mechanical properties, especially high hardness. In order to integrate this material into design process, we need to know its hardness as precise as possible. The Knoop hardness number (HK) is calculated using the expression: HK = α·F/d2, where F is the applied load, d is the long diagonal of the resulting 10indentation and a is the Knoop indenter geometrical constant. In this paper, the Knoop hardness of SiC ceramics was measured in the applied load range from 4.9 to 98.07 N. For some materials measured “apparent” hardness value decreases with increasing applied test load (normal indentation size effect – ISE), while for some materials measured “apparent” hardness increases with increasing applied test load (reverse indentation size effect – RISE). Obtained results show the measured hardness exhibits the ISE. In the literature several models are given for the phenomenon explanation. We used the following models: Meyer's law (F = K·dn), proportional specimen resistance – PSR (F = a1·d + a2·d2) and modified proportional specimen resistance – MPSR model (F = a0 + a1·d + a2·d2). Results of regression analysis for all applied models show they can all be used for ISE analysis. “True” hardness was determined based on the PSR and MPSR model (HKT = α·a2). The obtained results were similar. If the specimen surface is carefully prepared and the range of loads is wide, the a0 coefficient from MPSR model reaches small values and can be excluded. Therefore, for the calculation of SiC ceramics Knoop hardness, the simpler model (PSR) can be used.

Characterization of aluminides formed on superalloy 690 substrate

Ni-Cr-Fe based superalloy 690 substrate, pack aluminized at 1273 K, revealed formation of multilayer comprising (NiCr)Al + Cr5Al8, Ni2Al3 + Cr5Al8, NiAl and γ phases. Knoop hardness number varied from 225 to 1142 along the cross-section. Wear and friction tests on aluminized specimen were performed in dry medium using reciprocating sliding wear and friction machine with tungsten carbide ball at 15 N load with frequencies at 10, 15 and 20 Hz. The coefficient of friction, the static ones were obtained in the vicinity of 0.2, 0.3 and 0.5, while dynamic ones were 0.3, 0.4 and 0.4 respectively. For the ball, the wear rate was 1.9 × 10−6, 1.2 × 10−5 and 1.5 × 10−5 mm3/Nm, whereas the wear rate was 5.8 × 10−5, 3.8 × 10−4 and 4.6 × 10−4 mm3/Nm respectively for the aluminized specimen indicating good adherent surface coating.