Strength Of Resin Research Articles

Assessment of surface hardness and impact strength of denture base resins reinforced with silver–titanium dioxide and silver–zirconium dioxide nanoparticles: In vitro study

Abstract Polymethyl methacrylate (PMMA) is frequently utilised for fabricating denture bases due to its perfect qualities. However, a significant issue with this resin is the occurrence of frequent fractures caused by heavy chewing power, resulting in early cracks and fractures during clinical use. This study investigates the influence of silver, titanium dioxide, and silver zirconia nanoparticles on the surface hardness and impact strength of self-cured denture base. The samples were categorised into four categories according to the incorporation of different nanoparticles. The samples were divided into three subgroups based on the nanoparticle content: 0.1, 0.3, and 0.5% of TiO2 and ZrO2. Each group had a set ratio of 0.3% Ag as an antibacterial agent. Except for the fourth group (Group D), a combination of 0.05, 0.15, and 0.25% of TiO2 and ZrO2, along with 0.3% Ag, was utilised to investigate their collective impact. The Shore D hardness and Charpy test were employed to quantify the surface hardness and impact strength, respectively. The samples were subjected to X-ray diffraction analysis and field emission-scanning electron microscopy to characterise nanoparticles and ascertain the structure of acrylic samples. All nanoparticle-modified samples showed a substantial improvement in surface hardness compared to the control group. The maximum hardness value was seen in the samples containing 0.3% Ag–0.3% TiO2 and 0.3% Ag–0.5% ZrO2. The samples treated with 0.3% Ag and 0.3% TiO2 showed the maximum impact strength. The incorporation of Ag and ZrO2 hinders the ability to withstand impact strength. The samples treated with 0.3% Ag, 0.15% TiO2, and 0.15% ZrO2 exhibited an augmentation in impact strength. Modified samples in all groups showed a colour change, which required colour modifiers.

Influence of 3D printing system, postpolymerization and aging protocols on resin flexural strength and dimensional stability for printing occlusal splints, models and temporary restorations.

Investigate the effect of different postpolymerization protocols, aging, and 3D printing systems on the flexural strength (σ), dimensional stability, and roughness of resins used to fabricate occlusal splints, dental models, and temporary restorations. 180 bars (25 × 2 x 2mm-ISO 4049) of each type of resin (T-Temporary/Cosmos Temp, Yller; OS-Occlusal splint/Cosmos Splint, Yller; MO - Models/ Cosmos Model, Yller) were printed and divided into 12 groups (n = 15) according to the factors: "Postpolymerization" (Ctr - Control; UV - Ultraviolet oven and MW - Microwave); "Printer" (SLA- stereolithography (Forms 2/Formslab); LCD- liquid crystal display (FlashForge Foto 6.0/FlashForge)) and "Aging" (TC - 10,000 thermocycling cycles and Without). Each bar was measured with a digital caliper at 11 points before and after postpolymerization to evaluate dimensional stability. The samples were subjected to the σ test (100Kgf;1mm/min). Data was evaluated using Three- and Two-way ANOVA, and Tukey's test (5%). Weibull analysis, Scanning Electron Microscopic and optical profilometry was performed. LCD printing system and UV oven postpolymerization exhibited the highest σ (P < .05). The groups printed in SLA and post-polymerized in microwave ovens showed the greatest variations in their dimensions, for the occlusal splint resin, the OS-SLA-MW group (-4.29 ± 3.15)A showed a shrinkage of 40.2%. The resins for models (3.31 ± 0.66)A and temporary (-2.06 ± 1.52)A showed a shrinkage of 33% and 20.6%, respectively. LCD printing with UV light postpolymerization was the most effective method for resins used in occlusal splints, dental models, and temporary restorations. SLA printing with UV postpolymerization showed the most significant dimensional changes, leading to shrinkage in occlusal splint resins, while model resins and temporary restorations expanded. Resins for 3D printing should ideally be post-polymerized with UV light and printed using LCD technology, as this approach results in better mechanical properties and less dimensional change compared to microwave oven post-polymerization.

Effect of carbon fibers modification on joint adhesion steel-epoxy resin-aluminum

The combination of high-performance steel with aluminum or aluminum alloys shows a wide range of application in high technologies field like aerospace, smart materials, etc. The problem in using of adhesive bonding technology for these substrates is the weak mechanical occlusion between steel/aluminum and common coatings. In this study, different methods of treatment of carbon fiber were employed to enhance the adhesion strength in steel-epoxy resin filled with carbon fiber-aluminum triplex. The treatment of carbon fibers with 3-glicidyloxypropyl-trimetoxysilane after their oxidation in 65 wt.% nitric acid leads to the additives which improve the adhesion strength of epoxy resins at 15 wt.% contain on 67.5% in comparison with neat epoxy up to 20 MPa. This result can be explained by the change of failure mechanism from “cohesive’ to “adhesive” that leads to an increase of adhesion strength to steel and aluminum. The SEM study shown that 60–80 nm thick two-layer interphase formed between carbon fiber and epoxy matrix. The mechanical strength of this interphase governs the mechanical and adhesion performance of steel-epoxy resin-aluminum triplex. Taking into account the mechanical properties and adhesive strength of these composites, it emerges as a potential candidate for advanced materials.

Effect of mechanical and chemical surface treatments on the repairing of milled and 3D-printed denture bases.

Ensuring a strong bond between chairside autopolymerized acrylic resin to denture base is essential for denture repair and reline procedures. However, there is no established protocol to enhance bond strength between autopolymerizing resin and computer-aided design and computer-aided manufacturing (CAD-CAM) denture base materials. The purpose of this study was to determine shear bond strength of CAD-CAM denture bases and autopolymerizing acrylic resin after mechanical and chemical surface treatments compared with heat-polymerized acrylic resin. Heat-polymerized, milled, and 3-dimensional (3D) printed denture bases were divided into 4 surface treatment protocols: none (control), airborne-particle abrasion (APA), tetrahydrofuran, and Vitacoll application. Autopolymerizing acrylic resin cylinders were bonded to denture surface. Shear bond strength and failure modes were determined after thermocycling. Denture base surfaces were assessed for surface roughness, surface morphology, and microhardness before and after surface treatment. Data was analyzed using two-way ANOVA and multiple comparison tests. The results showed that APA significantly increased shear bond strength and surface roughness of all denture base materials. Tetrahydrofuran and Vitacoll application improved shear bond strength of heat-polymerized acrylic resin, but did not reach the level achieved by APA. Conversely, tetrahydrofuran application improved bond strength of 3D-printed denture to the level of APA. Tetrahydrofuran and Vitacoll application significantly reduced denture base hardness, compared with control and APA. In conclusion, mechanical surface treatment using APA enhances the adhesion of autopolymerizing acrylic resin to heat-polymerized and CAD-CAM denture bases. Tetrahydrofuran and Vitacoll chemical surface treatment improved adhesion to heat-polymerized acrylic resin, with only tetrahydrofuran enhancing bond strength of 3D-printed denture to the level of APA. Without surface treatment, the highest bond strength was shown in 3D-printed denture base material.

In Situ Synthesis and Characterizations of a Strontium-Substituted Dicalcium Phosphate Anhydrous/Hydroxyapatite Biphasic Whisker and Its Properties Evaluation.

Dicalcium phosphate anhydrous (DCPA) presents good biomineralization ability, the strontium element is known for superior bone affinity, and a whisker possesses good mechanical strength; all these are beneficial for improving the drawbacks of hydroxyapatite (HAP) like weaker mechanical properties, poor biomineralization, and slower degradation/absorption. Therefore, a homogeneous precipitation was adopted to synthesize Sr-substituted and DCPA and HAP coexisting whiskers. The composition, structure, and morphology based on urea dosage and substitution content were characterized, and the roles of DCPA, Sr, and whisker shape were investigated. It turned out that Sr-DCPA/HAP biphasic products contained about 19% DCPA and 81% HAP, and both phases occupied the outer and inner parts of the whisker, respectively. Increasing the urea dosage made the morphology transform from a sea urchin shape to fiber clusters and then whiskers, while Sr substitution brought the whisker back to the porous microsphere shape. Only 5% of Sr content and 15 g of urea could maintain the whisker shape. Sr could promote the proliferation of MC3T3-E1 cells even at a higher extract concentration of 10 mg/mL. The cells stayed in a healthy state whether cocultured with the whisker or the microsphere. The unstable DCPA combined with the decreased crystallinity brought by Sr doping contributed to shortening the apatite deposition period to within 7 days. The whisker morphology enhanced the compressive strength of acrylic resin, and the apatite layer helped to reduce the strength loss during soaking. The Sr-DCPA/HAP biphasic whisker with enhanced overall properties possessed more promising potential for biomedical application.

Flexural Strength of Nanohybrid Composite Resin: An in-vitro Study

In the present study, the Flexural strength of nanohybrid composite was compared with conventional microfilled composite resin in vitro. Thirty nanohyrid and 30 conventional microfilled composite resins were subjected to Flexural strength measurement as follows; a teflon mold (4 mm in diameter and 6 mm in depth) was filled with materials, light cured through the glass slides for 40 sec top and bottom surfaces. Then the specimens were taken out from the mold, again light curing for 40 sec, stored in dry condition and placed into a Flexural strength tester and loaded until fracture of the sample. Student t-test was performed to assess the differences between two groups and a value of p &gt;0.05 was considered as statistically significance. The results showed that the means ± SD Flexural strength of nanobybrid and conventional microfilled composite resin were 118.70 ± 3.90 Mpa and 77.43 ± 2.65 Mpa, respectively and the differences were statistically significant. It can be concluded that nanohybrid is superior to conventional microfilled composite resin in respect to Flexural strenth in class II restorations of permanent molar teeth.

Assessing the impact of Ag: TiO2 and Ag: ZrO2 nanoparticles on the flexural strength of self-cured denture base resin

Abstract This study investigates the effect of silver, titanium dioxide, and silver, Zirconium dioxide nanoparticles on the flexural strength of self-cured denture base resin. The samples were divided into four groups according to the addition of different nanoparticles to the self-cured acrylic PMMA. Control group A, reinforcement group B (Ag + TiO2), reinforcement group C (Ag + ZrO2) and reinforcement group D (Ag + TiO2 + ZrO2). The samples were again divided into three subgroups according to the nanoparticulate addition ratio of 0.1, 0.3 and 0.5 wt.% TiO2 and ZrO2 each with a fixed ratio of 0.3wt.% Ag as antimicrobial. Excluding group D addition, it was 0.05, 0.15, 0.25 wt.% for both TiO2 and ZrO2 with 0.3 wt.% Ag. After immersing the samples in distilled water for 48h, the flexural strength was measured using the three-point bend test. Evaluation of flexural strength showed a significant decrease with an increasing percentage of nanoparticles, except the increase in the percentage of additions of ZrO2 showed an increase in flexural strength. The samples underwent X-ray diffraction examination and FESEM to describe nanoparticles and examine the structure of acrylic samples. X-ray diffraction revealed the absence of mistake diffraction, indicating high crystal structure purity. In addition, images from the scanning electron microscope showed a homogeneous distribution of nanoparticles within the acrylic structure. Maximum flexural strength was seen in the 0.3 wt.% Ag-0.15 wt.% TiO2-15 wt.% ZrO2 and minimum in 0.3 wt.% Ag -0.5 wt.% TiO2. The modified samples also exhibited colour changes. We conclude flexural strength value depends on the percentage of additions and type of nanoparticles.

Evaluation of flexural strength of dental occlusal splints resins

Evaluation of flexural strength of dental occlusal splints resins

Polymerization techniques on strength of denture base resins

Polymerization techniques on strength of denture base resins

Shear Bond Strength of Five Different Repair Systems to Three Different Ceramic Materials.

The purpose of the current study was to investigate the shear bond strength of composite resin in five different ceramic repair systems for three different ceramic materials. 225 monolithic zirconia (Zr) and lithium disilicate (LDS) and cobalt chromium (CoCr) specimens with feldspathic porcelain (FP) veneer were fabricated (N=75 per material). The specimens underwent thermo-cycling and were randomly divided into five groups for the following intra-oral repair systems (n=15): 1) Clearfil Ceramic Primer Plus, Kuraray Noritake, Japan; 2) Ibond Intraoral Repair Kit, Kulzer, Germany; 3) VOCO Cimara, VOCO GmbH Germany; 4) Ivoclar Vivadent, Ceramic Repair System Kit, IvoclarVivadent, Liechtenstein; 5) Ultradent Porcelain Repair Kit, Ultradent Products Inc, USA. After surface conditioning, composite was applied using a cylindrical mold and a second round of thermocycling was performed. Each specimen was then subjected to shear load until failure. Microshear bond strength was compared using analysis of variance (one-way ANOVA, Tukey's test, a <0.05). Type of porcelain repair kit significantly affected the shear bond strength. For Zr, the repair systems from Kulzer and VOCO had significantly higher shear bond strength than the other systems (p<0.001) without significant difference between Kulzer and VOCO. For the LDS and CoCr with FP, hydrofluoric acid had the highest shear bond strength (p<0.001). For zirconia, a 10-Methacryloyloxydecyl dihydrogen phosphate (10-MDP) system should be used. The results also suggest that a cleaning bur may be beneficial. For glass-based ceramics and feldspathic porcelain, etching with hydrofluoric acid remains the product of choice.

Shear-Bond Strength of Composite Resins Repaired with Injectable Composite

Shear-Bond Strength of Composite Resins Repaired with Injectable Composite

Bond strengths of resins between dentures and titanium cylinders

Bond strengths of resins between dentures and titanium cylinders

Shear bond strength of a composite resin restoration in primary teeth following cavity preparation using laser- an in-vitro study.

To evaluate and compare the shear bond strength of composite resin restorations in primary teeth, following cavity preparation with both traditional dental burs and laser irradiation. One hundred primary molars extracted from the children visiting our department were collected and randomly divided into five groups (A-E) with 20 teeth in each group. In groups A, B, C, D, and E the teeth samples were etched with phosphoric acid, Er; YAG laser followed by acid etching, Er, Cr: YSGG laser followed by acid etching, Er; YAG laser etching only and Er, Cr: YSGG laser etching only, respectively. Following, all the samples were restored with composite resin and subjected to 500 cycles of thermocycling. The shear bond strength of the resin composite was analyzed. The type of fractures was also noted. Data obtained were subjected to statistical analysis. The mean value of shear bond strength of Group A, B, C, D, and E was 17.562 ± 0.810, 15.928 ± 0.415, 14.964 ± 0.566, 11.833 ± 0.533 and 11.187 ± 0.517, respectively. Adhesive failure was most commonly seen in all the groups. The phosphoric acid etching remains a highly effective technique for pre-treating dentin in composite resin restorations. The shear strength of composite resin to the dentin of laser-prepared cavity in primary teeth can be improved by the addition of acid etching.

Comparison of Flexural Strength and Wear of Injectable, Flowable and Paste Composite Resins.

(1) Objectives: This study investigated and compared the wear and flexural strength of two highly filled (injectable), one flowable and one paste composite. (2) Methods: Two highly filled flowable composites (G-aenial Universal Injectable and Beautifil Plus F00), a paste composite (Empress Direct) and a conventional flowable (Tetric EvoFlow) were tested. A two-body wear test was carried out using 10 disc-shaped samples from each group, which were subjected to 200,000 wear machine cycles to simulate wear, followed by Scanning Electron Microscope analysis. Flexural strength was tested using a three-point bend test using 15 beam samples for each of the four groups. Values were statistically compared using one-way analysis of variance (ANOVA) for flexural strength and a Kruskal-Wallis test for wear. (3) Results: The median volume loss for G-aenial Universal Injectable and Beautifil Plus F00 was statistically lower than that of both Empress Direct and Tetric EvoFlow. For flexural strength the two highly filled flowable composites both exhibited statistically higher mean flexural strength values compared to Empress Direct (p < 0.004) and Tetric Evoflow (p < 0.001). There were no statistically significant differences in the values of wear and flexural strength between the two highly filled flowable composites. (4) Conclusions/significance: Highly filled flowable composite resins with nano filler particles outperformed a conventional flowable and a paste composite resin in terms of wear resistance and flexural strength, and may be suitable to use in occlusal, load-bearing areas.

Effect of Addition of Metal Oxide Nanoparticles on the Strength of Heat-Cured Denture Base Resins: Protocol for Systematic Review and Meta-Analysis of In Vitro Studies.

Metal oxide nanoparticle-reinforced polymethyl methacrylate (PMMA) has been shown to improve mechanical properties, such as strength. Different types of metal oxide nanoparticles have been used previously, but the comparative effect on the strength of heat-cured denture base resins remains unclear. This is a protocol for a systematic review and meta-analysis that will aim to pool evidence to compare and analyze the effects of the addition of different metal oxide nanoparticles, with varied sizes and concentrations, on the strength (flexural, impact, transverse, compressive tensile strength, and fracture toughness) of heat-cured PMMA. In addition, this review aims to analyze methodological factors, such as adherence to testing and sample-making guidelines, and the effects of surface treatments of the nanoparticles on the strength of heat-cured denture base resins. The protocol has been registered in the Open Science Framework. Search strategies to identify studies on the effect of metal oxide nanoparticles on the strength of heat-cured PMMA were developed by the subject matter expert in library science. Following this, a systematic search of 5 electronic databases (PubMed [NCBI], Scopus [Elsevier], Cochrane Library [Wiley], CINAHL Plus with Full Text [EBSCO], and Dimensions Free Web App) was conducted to retrieve in vitro studies published in English from January 2012 to October 2023. Along with this citation chasing, other online sources and gray literature were also searched. Furthermore, papers will be screened, and appropriate data elements will be extracted in a standardized manner. A risk-of-bias assessment will be performed using a modified Cochrane Risk of Bias Tool. A meta-analysis will be performed using a random-effects model. Search in databases resulted in 1837 papers, of which 1752 were duplicates, leaving 85 records that were screened for titles and abstracts based on the eligibility criteria. A similar search run on other online sources identified 129 papers that will be further analyzed for inclusion. The study was initiated in November 2023 and research questions and search strategies were formulated. The proposed study is expected to be completed by December 2024. This systematic review will comprehensively analyze the effects of the incorporation of metal oxide nanoparticles in heat-cured denture base resins on the strength of the material. We anticipate gaining a deeper understanding of the effects and method of use of metal oxide nanoparticles to improve the strength of PMMA denture base resins. PRR1-10.2196/59999.

Effect of curing behaviors on performances of phthalonitrile resins: A deep molecular dynamics exploration with experiment

The curing process and mechanism of high-performance phthalonitrile (PN) thermosetting resin are crucial for its excellent mechanical and thermal-oxidation properties. The complicated polymerization reactions of PN resins were effectively controlled to obtain three types of major cured products (triazine, polyindoline and phthalocyanine). The corresponding cross-linking evolution processes and atomistic microstructural models were dynamically constructed and visualized through a creatively self-compiled procedure. Molecular dynamics simulations combined with experiment were properly conducted on two typical PN resins to illustrate the relationship between cross-linked microstructures and performance. The cross-linking degree and types of final products are positively correlated with the mechanical strength of PN resin. The hydrogen bonds present in the cross-linked structure mainly consolidate the cured networks of PN resin and the unique phenolic hydroxyl groups in PN75 enhance the hydrogen bonding contribution. The dominant pyrolysis gases were experimentally detected and distinguished as H2O, NH3, HCN, CH4, CO2 and CO during the thermal pyrolysis process of PN resin as the temperature rose. This work provides a new approach to probe the curing process and generated species, and further tailor the mechanical behavior and thermal-oxidation properties of resins or composites.

Shear bond strength of resin to additively manufactured zirconia with different surface treatments

BackgroundAdditive manufacturing is increasingly being utilized for dental restorations, but debonding remains a common issue with zirconia restorations. In particular, there is limited research on the bonding properties between additively manufactured (AM) zirconia and resin cement. AimThis study aimed to evaluate and compare the shear bond strength (SBS) of resin to milled and AM zirconia, and to investigate the effect of different surface treatments. Material and methodsMilled and AM zirconia specimens were subjected to various surface treatment methods: wet-polished group (MW&AW), where milled or AM zirconia was wet-polished with no additional treatment; primer group (MP&AP), where wet-polished zirconia was treated with primer; glass spray group (MG&AG), where wet-polished zirconia was coated with glass; and the bare group (AB), consisting of unpolished AM zirconia with preserved surface texture. Shear bond strength, surface roughness, morphology, and elemental distribution were analyzed using a universal mechanical testing machine, laser scanning microscope, scanning electron microscope, and micro X-ray fluorescence spectrometer, respectively. ResultsThe untreated groups showed the lowest SBS values (MW=6.82±2.35 MPa, AW=10.86±3.79 MPa), while the highest SBS values were observed in the glass coating groups (MG=23.06±3.86 MPa, AG=25.96±5.60 MPa). There was no significant difference in SBS between milled and AM zirconia with the same surface treatment. Additionally, the bare AM zirconia group exhibited slightly higher SBS than that of the wet-polished AM zirconia group. ConclusionsSurface treatments significantly enhance the shear bond strength between AM zirconia and resin, achieving bond strength levels comparable to those of milled zirconia. In addition, the unique surface textures of AM zirconia, enabled by its design and manufacturing flexibility, hold potential to further enhance bond properties.

Investigation on the fabrication and mechanism of thermally conductive bismaleimide composites with low dielectric constant via a branched crosslink structure

In order to address the challenges associated with the low mechanical strength and inadequate thermal conductivity of bismaleimide resin(BMI), trifunctional maleimide (TMI) was synthesized by reacting toluene diisocyanate (TDI trimer) with maleic anhydride (MA) in this study and reacted with diallyl bisphenol A (DBA) and BMI to prepare BMI-TMI resin with a micro-branching structure. This well-designed micro-branching structure resulted in an 8.8 % decrease in the dielectric constant by increasing the free volume of BMI. Additionally, the micro-branching structure endowed BMI resin with improved mechanical properties, as evidenced by a 140 % increase in bending strength and a 149 % increase in impact strength of the cured product. Similarly, utilizing the free volume in micro-branched polymers to construct voids in molecular chain segments facilitates better dispersion of aluminium oxide (Al2O3) thermal conductive fillers, forming a continuous thermal conductive network and providing a high-speed channel for phonon transport, thereby significantly improving the thermal conductivity of the composite material.

RESISTÊNCIA DA UNIÃO DE SISTEMAS ADESIVOS À DENTINA: INFLUÊNCIA DO HIPOCLORITO DE SÓDIO

The modification of the root canal system is of paramount importance to the success of endodontic treatment. This study aimed to evaluate the influence of sodium hypochlorite at concentrations of 2.5% and 4.5% in times of 30 and 60 minutes on the bond strength of composite resin to dentin. We used for this study teeth 20, which were divided into five groups of four elements, each element of the experimental group was immersed in a characteristic time and concentration of sodium hypochlorite of their group and the control immersed in saline. Later there were three composite restorations on each tooth, totaling a sample of 60 restorations. Each restaurant had its bonding to dentin submitted to microshear test, and the results were analyzed through statistical tests: the normal, anova and multiple variations. No significant influence on the adhesion of composite resins to dentin at a concentration of 2.5% at times of 30 and 60 minutes, and concentration of 4.5% in 30 minutes time. There was only influences the concentration 4.5% after a time of 60 minutes. It is concluded that sodium hypochlorite at 4.5% in contact with the dentine for a time of 60 minutes because influence on the bond strength of resin composite, which may cause recontamination of the channel system.

The effect of adding ZrO2 nanoparticles on the transverse strength and hardness of microwave-cured acrylic and heat-cured acrylic denture base materials

Background: One drawback of acrylic denture base materials is their liability to fracture, requiring methods to increase fracture resistance. Adding nanoparticles (NPs) represented one of these methods. Purpose: The objectives of this study are to evaluate and compare transverse strength and hardness when adding zirconium oxide nanoparticles (ZrO2 NPs) at concentrations of 0%, 3%, and 5% to heat-cured acrylic denture base materials (Ivoclare, Major) and to microwave-cured acrylic (Acron MC). Methods: Transverse strength was tested with a Universal Testing Machine (GESTER, Fujian, China), while hardness tests were conducted by using a Shore-D hardness durometer (Show, China). The 90 samples were prepared and then divided into three groups for each material. Attenuated total reflectance–Fourier transform infrared (ATR-FTIR) spectroscopy was used to analyze the microstructure. The samples were prepared following the manufacturer’s instructions for each material. Results: The results revealed that the addition of ZrO2 NPs (3%, 5%) improves the transverse strength and hardness of polymethyl methacrylate acrylic resin for both types (microwave-cured and heat-cured acrylic resins). The addition of ZrO2 NPs at 3% concentration shows the highest values for both transverse strength and hardness. The ATR-FTIR confirms no structural chemical changes with the addition of ZrO2 NPs. Conclusion: The study concludes that the incorporation of ZrO2 NPs (3%, 5%) into microwave-cured and heat-cured acrylic resins improves transverse strength and hardness.