Effect of heat treatment and nitrogen atmosphere during post-curing on mechanical properties of 3D-printed orthodontic aligners.

OBJECTIVES: Three-dimensional (3D)-printed aligners present a promising orthodontic treatment modality, whose clinical success largely depends on the material's mechanical properties. The aim of this study was to evaluate the mechanical properties of resin-made 3D-printed aligners and assess the effect of two different post-curing conditions. MATERIALS AND METHODS: Forty dumbbell-shaped specimens and 40 resin aligners were 3D-printed and divided into four equal groups according to post-curing conditions: presence or absence of oxygen during post-curing and water heat treatment at 85°C for 15 s or none. Samples from the central incisor of the aligner (n = 5/group) were studied by Attenuated Total Reflection Fourier-transform infrared spectroscopy (ATR-FTIR). The dumbbell-shaped specimens were loaded up to fracture under tensile mode and yield strength, ultimate tensile strength, elastic and plastic strain were calculated. The first mandibular molar area from 3D-printed aligners (n = 10/group) was cut and embedded in acrylic resin and then underwent metallographic grinding and polishing followed by instrumented indentation testing to determine the following mechanical properties: Martens hardness, indentation modulus, elastic index, and indentation relaxation. After descriptive statistics, differences according to each post-curing protocol, as well as their combination, were analyzed with linear regression modeling at a 5% significance level. RESULTS: All groups showed identical ATR-FTIR spectra, while no statistically significant effects were seen for either post-curing protocol (N2 presence and heat treatment) or their combination (P > .05 in all instances). CONCLUSIONS: The mechanical properties of 3D-printed resin aligners were not considerably affected either by post-curing in N2 atmosphere or heat treatment.

Comparative analysis of mechanical properties of orthodontic aligners produced by different contemporary 3D printers.

OBJECTIVE: The aim of this study was to compare the mechanical properties of orthodontic aligners among different commercially available 3D printing devices. MATERIALS AND METHODS: Five 3D printers (Ka:rv LP 550, Swinwon; "KAR"), (L120, Dazz 3D; "L12"), (MiiCraft 125, Miicraft Jena; "MIC"), (Slash 2, Uniz; "SLS") and (Pro 95, SprintRay; "PRO") were used to prepare orthodontic aligners with dental resin (Tera Harz TC-85DAW, Graphy). The central incisors of each aligner were cut, prepared and evaluated in terms of Martens-Hardness (HM), indentation-modulus (EIT ) and elastic-index (ηIT ) as per ISO14577-1:2002. Force-indentation curves were recorded and differences among printers were checked with generalized linear regressions (alpha=5%). RESULTS: Statistically significant differences were seen for all mechanical properties (P < .05), which were in descending order: HM (N/mm2 ) as median (Interquartile Range [IQR]): SLS 108.5 (106.0-112.0), L12 103.0 (102.0-107.0), KAR 101.5 (97.5-103.0), MIC 100.0 (97.5-101.5) and PRO 94.0 (93.0-96.0); EIT (MPa) as mean (Standard Deviation [SD]): SLS 2696.3 (124.7), L12 2627.8 (73.5), MIC 2566.2 (125.1), KAR 2565.0 (130.2) and PRO 2491.2 (53.3); and ηIT (%) as median (IQR): SLS 32.8 (32.3-33.1), L12 31.6 (30.8-32.3), KAR 31.3 (30.9-31.9), MIC 30.5 (29.9-31.2) and PRO 29.5 (29.1-30.0). Additionally, significant differences existed between liquid crystal display (LCD) and digital light processing (DLP) printers for HM (P < .001), EIT (P = .002) and ηIT (P < .001), with aligners from the former having higher values than aligners from the latter printer. CONCLUSION: Under the limitations of this study, it may be concluded that the mechanical properties of 3D-printed orthodontic aligners are dependent on the 3D printer used, and thus, differences in their clinical efficacy are anticipated.

Multiscale analysis of mechanical behavior of multilayer steel structures fabricated by wire and arc additive manufacturing.

The mechanical behavior of multilayer steel structures fabricated via wire and arc additive manufacturing (WAAM) has been investigated from the multiscale perspective. The multimaterial WAAM approach can control a heterogeneous structure and improve its mechanical properties. In this study, WAAM equipment based on plasma arc welding was used to fabricate two pairs of single- and duplex-phase multilayer steel structures using austenitic and martensitic stainless steel wires. The heterogeneity of these structures was characterized through micro-indentation tests. In addition, tensile tests of the multilayer structures were conducted to evaluate the effect of heterogeneity on macroscopic material properties. The deformation behavior of the heterogeneous multilayer steel structures was investigated by comparison with the finite element simulations of tensile tests in which the finite element models were created according to the estimated local elastoplastic properties from the results of micro-indentation tests. The micro-indentation tests revealed that the local mechanical properties significantly change during WAAM in cases where martensitic stainless steel wire was used. Additionally, strain-induced transformation plasticity was particularly observed in duplex cases, caused by the metastable austenite phase formed according to the thermal history and through the mixing of alloy elements. Thus, the heterogeneity of the multilayer steel structures became more complicated than its design, and consequently, its macroscopic mechanical properties exceeded the upper and lower bounds of a micromechanic estimation. The results show the potential to fabricate a structure having a unique mechanical behavior via the multimaterial WAAM approach.

Instrumented Cylindrical Punch Indentation of Solid Nitrogen at 30-40 K.

In support of NASA's Triton Hopper project, mechanical response data for solid nitrogen are needed for concept validation and development. Available mechanical properties data is sparse with only three known indentation measurements existing between 30 and 40 K. To generate more data, a custom instrumented hardness tester was developed to interface with a cryostat. The system was used to conduct cylindrical punch indentation testing at Triton-relevant thermodynamic conditions. Pressure versus displacement curves and hardness values were obtained. In the experiments the hardness ranged between about 2 kg/mm2 and 0.5 kg/mm2 in the aforementioned temperature range. A suspected brittle fracture is observed at lower temperatures in the range.

Water-induced Effects on the Hardness and Modulus of Contemporary Sealants Derived from Instrumented Indentation Testing (IIT).

AIM: To compare the mechanical properties of four different types of contemporary fissure sealants before and after water storage employing a modern instrumented indentation testing (IIT) method. MATERIALS AND METHODS: Four different types of materials used in everyday practice were deliberately selected. Fissurit (FIS) is a highly filled resin, Embrace (EMB) is a bisphenol A (BPA)-free unfilled resin, Helioseal (HEL) is an unfilled resin, and Riva Protect (RIV) is a glass-ionomer material. Six cylindrical specimens from each material were prepared (h: 3 mm, Ø: 15 mm), and Martens hardness (HM), elastic modulus (EIT), elastic index (ηIT), and Vickers hardness (HV) were determined employing an IIT machine according to International Organization for Standardization (ISO) 14577:2015. Then, the samples were stored in water at 37°C for 48 hours and measured again at the same surface. The mechanical properties tested (HM, EIT, ηIT, and HV) were statistically analyzed by two-way repeated measurements analysis of variance (ANOVA) employing materials and conditions as discriminating variables. Statistically significant differences were identified by Tukey's post hoc multiple comparison test. In all cases, a 95% level of significance was set (p = 0.05). RESULTS: Statistically significant differences in selected mechanical properties were allocated among materials tested. The artificial aging had a detrimental effect on HM, EIT, and HV apart from ηIT for FIS, EMB, and HEL. In contrast, no significant differences were identified for RIV before and after water storage for all aforementioned properties apart from ηIT. CONCLUSION: Significant differences were identified in mechanical properties among materials tested and thus differences in their clinical behavior are anticipated. CLINICAL SIGNIFICANCE: This study contributes to the understanding of the mechanical properties of different dental sealants with respect to water contact, which may influence the choice by the therapist.

Effect of printing orientation on mechanical properties of 3D-printed orthodontic aligners.

PURPOSE: The purpose of this study was to assess differences in the fundamental mechanical properties of resin-made three-dimensional (3D) printed orthodontic aligners according to the printing orientation. METHODS: Twenty resin 3D-printed dumbbell-shaped specimens and 20 orthodontic aligners were fabricated and postcured in nitrogen. Half of the specimens and aligners were built in horizontal (H), the other half in vertical (V) directions. The dumbbell-shaped specimens were loaded in a tensile testing machine, while parts of the aligners were embedded in acrylic resin, ground, polished, and then underwent instrumented indentation testing (IIT). Mechanical properties that were assessed included the yield strength (YS), breaking strength (BS), plastic strain (ε), Martens hardness (HM), indentation modulus (EIT), elastic index (ηIT), and indentation relaxation (RIT). Data were analyzed statistically with independent t­tests or Mann-Whitney tests at α = 5%. RESULTS: No significant differences were found between specimens or aligners printed either in a horizontal or a vertical direction (P > 0.05 in all instances). Overall, the 3D-printed aligners showed acceptable mechanical propertied in terms of YS (mean 19.2 MPa; standard deviation [SD] 1.7 MPa), BS (mean 19.6 MPa; SD 1.2 MPa), ε (mean 77%; SD 11%), HM (median 89.0 N/mm2; interquartile range [IQR] 84.5-90.0 NN/m2), EIT (median 2670.5 MPa; IQR 2645.0-2726.0 MPa), ηIT (median 27.5%; IQR 25.9-28.1%), and RIT (mean 65.1%; SD 3.5%). CONCLUSION: Printing direction seemed to have no effect on the mechanical properties of 3D-printed resin aligners, which are promising for orthodontic use.

Inverse Method to Determine Parameters for Time-Dependent and Cyclic Plastic Material Behavior from Instrumented Indentation Tests.

Indentation is a versatile method to assess the hardness of different materials along with their elastic properties. Recently, powerful approaches have been developed to determine further material properties, like yield strength, ultimate tensile strength, work-hardening rate, and even cyclic plastic properties, by a combination of indentation testing and computer simulations. The basic idea of these approaches is to simulate the indentation with known process parameters and to iteratively optimize the initially unknown material properties until just a minimum error between numerical and experimental results is achieved. In this work, we have developed a protocol for instrumented indentation tests and a procedure for the inverse analysis of the experimental data to obtain material parameters for time-dependent viscoplastic material behavior and kinematic and isotropic work-hardening. We assume the elastic material properties and the initial yield strength to be known because these values can be determined independently from indentation tests. Two optimization strategies were performed and compared for identification of the material parameters. The new inverse method for spherical indentation has been successfully applied to martensitic steel.

Nanoscale and Tensile-Like Properties by an Instrumented Indentation Test on PBF-LB SS 316L Steel.

The mechanical properties of a defect-free laser melting (PBF-LB) deposit of an AISI 316L steel alloy were assessed by means of an instrumented indentation test (IIT), at both the macro- and nano-scales. The inherent non-equilibrium microstructure of the alloy was chemically homogenous and consisted of equiaxed grains and large-elongated grains (under the optical microscope) with irregular outlines composed of a much finer internal cell structure (under the scanning electron microscope). Berkovich and Vickers indenters were used to assess the indentation properties across individual grains (nano) and over multiple grains (macro), respectively. The nano-indentation over the X-Y plane revealed nearly constant indentation modulus across an individual grain but variable on average within different grains whose value depended on the relative orientation of the individual grain. The macro-indentation test was conducted to analyze the tensile-like properties of the polycrystalline SS 316L alloy over the X-Y and Y-Z planes. The macro-indentation test provided a reliable estimate of the ultimate tensile strength (UTS-like) of the alloy. Other indentation properties gave inconsistent results, and a post factum analysis was, therefore, conducted, by means of a new approach, to account for the presence of residual stresses. The already existing indentation data were supplemented with new repeated indentation tests to conduct a detailed analysis of the relaxation ability of compressive and tensile residual stresses. The developed methodology allows the effect of residual stresses and the reliability of measured macro-indentation properties to be examined as a function of a small group of indentation parameters.

In vitro evaluation of CAD/CAM composite materials.

OBJECTIVES: The purpose of this in vitro study was to evaluate the microstructural, elemental and mechanical properties of contemporary computer-aided-design/computer-aided manufacturing (CAD/CAM) resin based composite (RBC) materials. METHODS: Six CAD/CAM RBC materials [Brilliant CRIOS (Coltene Whaledent AG), Cerasmart (GC), Lava Ultimate (3M ESPE), Tetric CAD (Ivoclar Vivadent), Shofu Block HC (Shofu), Grandio Blocs (VOCO GmbH)] were tested. Ten rectangular blocks (14 Χ 12 Χ 18 mm) for each material, after metallographic grinding and polishing, were subjected to Instrumented Indentation Testing (ΙIT). Martens Hardness (HM), Indentation Elastic Modulus (EIT), Elastic (ηIT) and Creep indices (CIT) were determined according to formulas provided by ISO 14577. The diagonal length of each indentation was measured and HV was determined. The results of HM, EIT, ηΙΤ, HV, and CIT were statistically analyzed by one-way ANOVA and Tukey post hoc test employing the material as a discriminating variable (a = 0.05), while the possible correlations were determined by Spearman's correlation test. One specimen from each group was examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). RESULTS: Backscattered Electron images and EDX analysis demonstrated differences in size, shape and type of fillers along with elemental composition among materials tested. Statistical significant differences were identified for all mechanical properties tested. Grandio Blocs had the significantly higher HM (953±7 N/mm2), HV (136±1) and EIT (23±1 GPa) followed by Lava Ultimate (ΗM=674±25 N/mm2, HV=105±2, EIT=15±1 GPa). Elastic index ranged from 41% to 52%, with Shofu Block demonstrating the significantly highest ηIT (52 ± 1%) values. Cerasmart had significantly higher CIT value (8.4 ± 0.1%) than all other materials tested, while Grandio Blocs and Lava Ultimate had the lowest ones. Spearman's correlation revealed that all mechanical properties tested exhibited correlations with each other, apart from ηΙΤ. CONCLUSIONS: The results showed that the CAD/CAM materials tested have differences in their microstructure, elemental composition and mechanical properties. CLINICAL SIGNIFICANCE: The RBCs tested showed significant differences in mechanical properties and thus differences in clinical performance are anticipated. RBCs with increased filler loading had the most favorable combination of hardness, elastic modulus and creep index indicating that these materials may have better clinical performance under intraoral loading conditions.

Mechanical Properties of Structural Components in Hastelloy X Joints Brazed with Ni-Pd-Cr-B-Si Alloy.

The brazing of structural high-temperature-resistant nickel alloys is a predominant method in manufacturing jet engines in the aircraft industry. Ni-Cr-base brazing filler metals (BFMs) containing B and Si as the melting point depressants are used for this purpose. The presence of the latter can lead to the formation of brittle constituents in the joints, decreasing their strength, toughness and creep resistance. The structures of Hastelloy X nickel superalloy joints brazed with Palnicro 36M BFM are presented in this paper along with the mechanical properties of their particular phases as a function of brazing time. Indentation hardness, Martens hardness, reduced modulus and creep coefficient were measured using the instrumented indentation method. The elastic part of the indentation work was also calculated. Pd forms an unlimited solution with Ni, but its high content in BFM does not fundamentally change the general joint structure known from other Ni-superalloy-Ni-BFM systems. However, new Pd-containing phases are emerging. The hardest components were Ni-B and Cr-B boride phases and Pd-Ni-Si phase in MZ and the boundary of DAZ and BM. MZ reduces the plasticity of a joint to the highest extent. The hardness of particular parts in the joints and the elastic portion of the indentation work decreased with the increase in brazing time, while the reduced modulus of the indentation contact and indentation creep increased. The results of indentation creep measurements indicate that all structural components of the joints were less susceptible to creep than the parent material at room temperature.

Microstructure and Mechanical Properties of Spark Plasma Sintered CoCrFeNiNbX High-Entropy Alloys with Si Addition.

Three mechanically alloyed (MA) and spark plasma sintered (SPS) CoCrFeNiNbX (X = 5, 20, and 35 at.%) alloys with an addition of 5 at.% of SiC were investigated. The face-centered cubic (FCC) high-entropy solid solution, NbC carbides, and hexagonal Laves phase already developed during MA. In addition, the SPS compacting led to the formation of oxide particles in all alloys, and the Cr7C3 carbides in the Nb5 alloy. The fraction of the FCC solid solution decreased with increasing Nb concentration at the expense of the NbC carbide and the Laves phase. Long-term annealing at 800 °C led to the disappearance of the Cr7C3 carbide in the Nb5 alloy, and new oxides-Ni6Nb6O, Cr2O3, and CrNbO4-were formed. At laboratory temperature, the Nb5 alloy, containing only the FCC matrix and carbide particles, was relatively strong and very ductile. At a higher Nb content (Nb20 and Nb35), the alloys became brittle. After annealing for 100 h at 800 °C, the Nb5 alloy conserved its plasticity and the Nb20 and Nb35 alloys maintained or even increased their brittleness. When tested at 800 °C, the Nb5 and Nb20 alloys deformed almost identically (CYS ~450 MPa, UTS ~500 MPa, plasticity ~18%), whereas the Nb35 alloy was much stronger (CYS of 1695 MPa, UCS of 1817 MPa) and preserved comparable plasticity.

Degradation of Dental Methacrylate-Based Composites in Simulated Clinical Immersion Media.

The selection of restorative materials with regard to the longevity and durability of a restoration is of crucial importance for daily dental practice and requires that the degradation of the material in the oral environment can be assessed. The aim of this study was to investigate the extent to which the mechanical properties of four (Esthet X, Ceram X, Filtek Supreme XT, and Filtek Supreme XT flow) resin-based composites (RBCs) alter during storage in saliva substitutes (artificial saliva) for 24 h and 28 days and in the context of simulated, more aggressive clinical conditions, including cycles exposure to de- and remineralization, alcohol, or salivary enzymes. For this purpose, flexural strength and modulus were determined in a three-point bending test (n = 20) followed by Weibull analysis, while quasi-static behavior was evaluated by instrumented indentation techniques. Degradation occurred in all RBCs and all aging protocols and was quantifiable at both macroscopic and microscopic levels. The postulated stabilizing effect on degradation through the incorporation of urethane-based co-monomers into the organic matrix or a higher filler loading is refuted. Even though modern RBCs show high clinical survival rates, biodegradation remains an issue that needs to be addressed.

New aesthetic in-house 3D-printed brackets: proof of concept and fundamental mechanical properties.

OBJECTIVES: Three-dimensional (3D) printing technology is an emerging manufacturing process for many orthodontic appliances, and the aim of this study was to evaluate the mechanical properties of resin-based materials as alternatives for the in-house preparation of orthodontic brackets. MATERIAL AND METHODS: Two types of 3D printed resins used for temporary (T) and permanent (P) crown fabrication were included in this study. Ten blocks from each resin were manufactured by a 3D printer and, after embedding them in acrylic resin, the samples were subjected to metallographic grinding and polishing, followed by instrumented indentation testing (IIT). Martens hardness (HM), indentation modulus (EIT), and elastic index (ηIT) were determined with a Vickers indenter recording force-indentation depth curves from each specimen. After calculating descriptive statistics, differences between material types were investigated with Wilcoxon rank sum test accounting for clustering of measurements within specimens at alpha = 5%. RESULTS: No statistically significant differences in the mechanical properties of the two tested materials were seen: HM: median 279 N/mm2 (interquartile range [IQR] 275-287 N/mm2) for T and median 279 N/mm2 (IQR 270-285 N/mm2) for P (P value = 0.63); EIT: median 5548 MPa (IQR 5425-5834 MPa) for T and median 5644 (IQR 5420-5850 MPa) for P (P value = 0.84); ηIT: median 47.1% (46.0-47.7%) for T and median 46.0% (IQR 45.4-47.8%) for P (P value = 0.24). CONCLUSIONS: Under the limitations of this study, it may be concluded that the mechanical properties of the two 3D printed resins tested are equal, and thus, no differences in their clinical performance are expected.

Characterization of the hierarchical architecture and micromechanical properties of walnut shell (Juglans regia L.).

In the present work a comprehensive characterization of the hierarchical architecture of the walnut shell (Juglans regia L.) was carried out using scanning electron microscopy (SEM), atomic force microscopy (AFM) and confocal laser scanning microscopy (CLSM). Furthermore, micromechanical properties (hardness, HIT and elastic modulus, EIT) of plant tissues were evaluated at cell wall level by applying the instrumented indentation technique (IIT). The complex architecture of the material was described in terms of four hierarchical levels (HL): endocarp (H1), plant tissues (H2), plant cells (H3) and cell wall (H4). Our findings revealed that the walnut shell consists of a multilayer structure (sclerenchyma tissue, ST; interface tissue, IT; porous tissue, PT; and flattened parenchyma tissue, FPT), where differences in the microstructure and composition of plant tissues generate parallel gradients along the cross-section. The indentation tests showed a functional gradient with a sandwich-like configuration, i.e., a lightweight and soft layer (PT, HIT = 0.04 GPa) is located between two dense and hard layers (ST, HIT = 0.33 GPa; FPT, HIT = 0.28 GPa); where additionally there is an interface between ST and PT (IT, HIT = 0.16 GPa). This configuration is a successful strategy designed by nature to improve the protection of the kernel by increasing the strength of the shell. Therefore, the walnut shell can be considered as a functionally graded material (FGM), which can be used as bioinspiration for the design of new functional synthetic materials. In addition, we proposed some structure-property-function relationships in the whole walnut shell and in each of the plant tissues.

Application of Macro-Instrumented Indentation Test for Superficial Residual Stress and Mechanical Properties Measurement for HY Steel Welded T-Joints.

HY-80 and HY-100 steels, widely used in constructing large ocean vessels and submarine hulls, contain mixed microstructures of tempered bainite and martensite and provide high tensile strength and toughness. Weld integrity in HY steels has been studied to verify and optimize welding conditions. In this study, the T-joint weld coupons, HY80 and HY100, were fabricated from HY-80 and HY-100 steel plates with a thickness of 30 mm as base metals by submerged-arc welding. Flux-cored arc welding was performed on an additional welding coupon consisting of HY-100 to evaluate the effect of repair welds (HY100RP). Microstructures in the heat-affected zones (HAZ) were thoroughly analyzed by optical observation. Instrumented indentation testing, taking advantage of local characterization, was applied to assess the yield strength and the residual stress of the HAZ and base regions. The maximum hardness over 400 HV was found in the HAZ due to the high volume fraction of untempered martensite microstructure. The yield strength of the weld coupons was evaluated by indentation testing, and the results showed good agreement with the uniaxial tensile test (within 10% range). The three coupons showed similar indentation residual stress profiles on the top and bottom surfaces. The stress distribution of the HY100 coupon was comparable to the results from X-ray diffraction. HY100RP demonstrated increased tensile residual stress compared to the as-welded coupon due to the effect of the repair weld (323 and 103 MPa on the top and bottom surfaces). This study verifies the wide applicability of indentation testing in evaluating yield strength and residual stress.

Comparison of Different Cermet Coatings Sprayed on Magnesium Alloy by HVOF.

In the present study, two different cermet coatings, WC-CrC-Ni and Cr3C2-NiCr, manufactured by the high-velocity oxy-fuel (HVOF) method were studied. They are labeled as follows: WC-CrC-Ni coating-WC and Cr3C2-NiCr coating-CrC. These coatings were deposited onto a magnesium alloy (AZ31) substrate. The goal of the study was to compare these two types of cermet coating, which were investigated in terms of microstructure features and selected mechanical properties, such as hardness, instrumented indentation, fracture toughness, and wear resistance. The results reveal that the WC content influenced the hardness and Young's modulus. The most noticeable effect of WC addition was observed for the wear resistance. WC coatings had a wear intensity value that was almost two times lower, equal to 6.5·10-6 mm3/N·m, whereas for CrC ones it was equal to 12.6·10-6 mm3/N·m. On the other hand, the WC coating exhibited a lower value of fracture toughness.

New method to differentiate surface damping behavior and stress absorption capacities of common CAD/CAM restorative materials.

OBJECTIVES: To assess energy dissipation capacities and surface damping abilities of different CAD/CAM restorative materials (CRMs) to characterize stress resistance during load peaks. METHODS: Using instrumented indentation testing (IIT), Martens hardness (HM) together with its elastic (ηIT) and plastic index (ηITdis) and Leeb hardness (HLD) together with its deduced energy dissipation (HLDdis) were determined for eight ceramic, eight composite, and four polymer-based materials as well as three metals. The results were compared to those of bovine enamel. Ten indentations per material were performed at room temperature (23 ± 1 °C) on two separate specimens (12.0 × 12.0 × 3.5 mm3) after water storage (24 h; 37.0 ± 1.0 °C). Hardness parameters were recorded, and data were analyzed with one-way MANOVA (Games-Howell post hoc tests, α = 0.05). Correlations between different parameters were tested (Pearson, α = 0.05). RESULTS: Independently determined HLDdis, and ηITdis values substantiated different energy dissipation characteristics of CRM, whereby a strong correlation was observed for the two datasets (r = 0.956, p = 0.011). Ceramics had the significantly lowest values (p < 0.001) while both parameters revealed the highest surface damping effects for metals (p < 0.001), followed in both cases by bovine enamel. Energy dissipation of polymer and composite CRM was in between ceramics and bovine enamel (p < 0.001), whereas only for HLDdis did both show no significant difference (p > 0.05). SIGNIFICANCE: Promising new HLDdis and ηITdis data allow a reliable differentiation of energy dissipation and surface damping capacities of CRMs. Previously published rankings of edge chipping and loss tangent results were perfectly reproduced, especially by HLDdis.

Quantitative Representation of Mechanical Behavior of the Surface Hardening Layer in Shot-Peened Nickel-Based Superalloy.

Surface hardening treatment can usually introduce severe grain distortion with a large gradient in the surface layer. It results in mechanical properties being difficult to accurately determine through macroscopic tests due to the non-uniformity of the shot-peened material. In this study, the mechanical behavior of uniformly pre-deformed nickel-based superalloy IN718 was investigated with monotonic tensile tests and instrumented indentation tests. For the shot-peened material, the hardness distribution of the surface hardening layer after shot peening was identified through the instrumented indentation method. According to the stress-strain results of pre-deformed materials, Ramberg-Osgood model parameters could be presented with plastic deformation. Assuming the power-law relationship between hardness and plastic deformation, the plastic deformation distribution along the depth of the surface hardening layer was clarified. Based on the results, a method to identify the stress-strain relationships of hardened material at different depths was established. Finally, the finite-element simulations of the instrumented indentation test considered residual stress and strain hardening were built to verify the method presented herein. The results show that the solution to evaluate the mechanical properties of hardening layer materials in the microscopic zone is feasible, which can provide the foundation for the failure analysis of shot-peened materials.

Correlation Between the Indentation Properties and Microstructure of Dissimilar Capacitor Discharge Welded WC-Co/High-Speed Steel Joints.

The welding of cemented carbide to tool steel is a challenging task, of scientific and industrial relevance, as it combines the high level of hardness of cemented carbide with the high level of fracture toughness of steel, while reducing the shaping cost and extending the application versatility, as its tribological, toughness, thermal and chemical properties can be optimally harmonised. The already existing joining technologies often impart either insufficient toughness or poor high-temperature strength to a joint to withstand the ever-increasing severe service condition demands. In this paper, a novel capacitor discharge welding (CDW) process is investigated for the case of a butt-joint between a tungsten carbide-cobalt (WC-Co) composite rod and an AISI M35 high-speed steel (HSS) rod. The latter was shaped with a conical-ended projection to promote a high current concentration and heat at the welding zone. CDW functions by combining a direct current (DC) electric current pulse and external uniaxial pressure after a preloading step, in which only uniaxial pressure is applied. The relatively high heating and cooling rates promote a thin layer of a characteristic ultrafine microstructure that combines high strength and toughness. Morphological analysis showed that the CDW process: (a) forms a sound and net shaped joint, (b) preserves the sub-micrometric grain structure of the original WC-Co composite base materials, via a transitional layer, (c) refines the microstructure of the original martensite of the HSS base material, and (d) results in an improved corrosion resistance across a 1-mm thick layer near the weld interface on the steel side. A nano-indentation test survey determined: (e) no hardness deterioration on the HSS side of the weld zone, although (f) a slight decrease in hardness was observed across the transitional layer on the composite side. Furthermore, (g) an indication of toughness of the joint was perceived as the size of the crack induced by processing the residual stress after sample preparation was unaltered.

Microstructure and Selected Properties of Cr3C2-NiCr Coatings Obtained by HVOF on Magnesium Alloy Substrates.

In present work the Cr3C2-NiCr coating was deposited on magnesium alloy substrate with high velocity oxygen fuel (HVOF) spraying. The microstructure of the samples has been characterized by means of electron microscopy, SEM and phase composition analysis carried out. The porosity of coatings has been also estimated. Finally, tests of selected mechanical properties, such as instrumented indentation, abrasive erosion have been performed. The results of the investigations confirmed that dense, homogeneous and well-adhered Cr3C2-NiCr cermet coating is possible to obtain onto the magnesium AZ31 alloy substrate. Moreover, the coatings exhibit high resistance to erosion.