Increase In Indentation Hardness Research Articles

Elaboration and Experimental Characterizations of Copper-Filled Polyamide Micro-Composites for Tribological Applications

Polyamide 66 (PA66) has been used for dynamic bearing applications due to its good wear and abrasion resistance, hardness, and rigidity. PA66/copper micro-composites were studied with respect to micro-mechanical, tribological, and structural properties. A mixing step followed by injection molding was used to develop the different composites: PA66+5 wt.% Cu, PA66+10 wt.% Cu, and PA66+15 wt.% Cu. The morphological aspects of the composites were studied using scanning electron microscopy and microtomography. Good dispersion and adhesion of Cu particles across the matrix were also seen. DSC analysis showed a slight improvement in the % of crystallinity and thermal characteristics of the composites, particularly with 5 wt.% filler. Additional crystallization enhanced the tensile performance of the composites, including the modulus, elongation at break, and tensile strength. Nanoindentation tests also indicated an increase in indentation hardness and elastic modulus as a function of the filler fraction. A pin-on-disk tribometer was used to study the friction and wear properties of neat PA66 and copper-filled PA66 composites. It was found that the composite with 5 weight percent copper had the best wear resistance. A progressive decrease in the friction coefficient was also seen. Copper filler increases hardness and may effectively reduce the temperature at contact interfaces during rotating cycles.

Enhancing the lifetime and vacuum tribological performance of PVD-MoS2 coatings by nitrogen modification

As a solid lubricant, MoS2 is used for reducing friction and wear in machine elements under challenging operating conditions. In application, service life is thus a decisive factor that depends on the coating. Here, the effect of gaseous nitrogen modification on the performance of physical vapor deposited MoS2 coatings has been studied under vacuum conditions. The coatings were modified chemically or in their structural design by varying the bias voltage alternately. Tribologically induced changes were identified in short-running and long-running intermittent ball-on-disc tests, while the lifetime was evaluated in ball-on-disc tests until coating failure, with a coefficient of friction value exceeding 0.3 defined as the failure criterion. Furthermore, the microstructure was investigated before and after tribological loading. It appeared from scanning electronic microscopic imaging, that nitrogen modification reduced the initial degree of porosity and material compacting occurred after a short tribological loading. Nanoindentation measurements in the wear track of the compacted coating supported these results by a significant increase in indentation hardness and indentation modulus. Overall, the improvement in lubrication performance due to nitrogen modification led to a significant extension of service life, whereas the chemical and structural modification exhibited the highest service life of the coating.

Effect of Indentation Load on Nanomechanical Properties Measured in a Multiphase Boride Layer.

The study investigated the dependence of the indentation load on nanomechanical properties for a gas-borided layer produced on Inconel 600-alloy. During the measurements, the indentation load range from 10 mN to 500 mN was used. Three types of tested areas, differing in the concentration of chromium, were examined. The increase in chromium concentration was accompanied by an increase in indentation hardness and Young’s modulus. Simultaneously, the increase in the indentation load resulted in a decrease in the indentation hardness and Young’s modulus, for each type of the tested area. The presence of the indentation size effect was analyzed using four models: Meyer’s law, Hays and Kendall model, Li and Bradt model, Nix and Gao model. For all tested areas, good agreement with the Meyer’s law was obtained. However, areas with a higher chromium concentration were more susceptible to indentation size effect (ISE). The proportional specimen resistance (PSR) model was used to describe the plastic-elastic behavior of the tested materials, as well as to detect the presence of ISE. It was found that the increase in chromium concentration in the tested area was accompanied by a greater tendency to elastic deformation during nanoindentation.

Impact Resistance and Properties of (Cr,Al,Si)N Coatings Deposited by Gas Flow Sputtering with Pulsed DC Supply

To increase the erosion and corrosion resistance of compressor blades, hard (Cr,Al,Si)N coatings are used. Physical vapor deposition (PVD) is a well‐known method for the deposition of nitride hard coatings. The application of pulse technique in power supply of conventional PVD technologies like magnetron sputtering shows a great potential for increasing the durability and lifetime of different tools and components. However, one disadvantage of conventional PVD is the line‐of‐sight characteristic. This is eliminated by means of the convection‐driven transport mechanism in high speed physical vapor deposition (HS‐PVD) technology, based on gas flow sputtering. Herein, the influence of gas flow sputtering with bipolar pulsed direct current (DC) power supply on coating properties and impact resistance of (Cr,Al,Si)N/substrate compound is investigated. Thereby, (Cr,Al,Si)N coatings are deposited onto X3CrNiMo13‐4 by HS‐PVD using continuous DC and bipolar pulsed DC supply. Nanoindentation analyses show an increase in indentation hardness using pulsed DC compared with continuous DC. Moreover, the lowest imprint depth and degree of cohesive and adhesive failures are observed in this case. These release a high potential for pulsed power supply in HS‐PVD, resulting in denser morphology, higher hardness and impact resistance compared with DC.

Tribo-Mechanical Properties of the Antimicrobial Low-Density Polyethylene (LDPE) Nanocomposite with Hybrid ZnO-Vermiculite-Chlorhexidine Nanofillers.

Materials made from low-density polyethylene (LDPE) in the form of packages or catheters are currently commonly applied medical devices. Antimicrobial LDPE nanocomposite materials with two types of nanofillers, zinc oxide/vermiculite (ZnO/V) and zinc oxide/vermiculite_chlorhexidine (ZnO/V_CH), were prepared by a melt-compounded procedure to enrich their controllable antimicrobial, microstructural, topographical and tribo-mechanical properties. X-ray diffraction (XRD) analysis and Fourier transform infrared spectroscopy (FTIR) revealed that the ZnO/V and ZnO/V_CH nanofillers and LDPE interacted well with each other. The influence of the nanofiller concentrations on the LDPE nanocomposite surface changes was studied through scanning electron microscopy (SEM), and the surface topology and roughness were studied using atomic force microscopy (AFM). The effect of the ZnO/V nanofiller on the increase in indentation hardness (HIT) was evaluated by AFM measurements and the Vickers microhardness (HV), which showed that as the concentration of the ZnO/V nanofiller increased, these values decreased. The ZnO/V and ZnO/V_CH nanofillers, regardless of the concentration in the LDPE matrix, slightly increased the average values of the friction coefficient (COF). The abrasion depths of the wear indicated that the LDPE_ZnO/V nanocomposite plates exhibited better wear resistance than LDPE_ZnO/V_CH. Higher HV and HIT microhardness values were measured for both nanofillers than the natural LDPE nanocomposite plate. Very positive antimicrobial activity against S. aureus and P. aeruginosa after 72 h was found for both nanofiller types.

Fracture behavior of metakaolin‐based geopolymer reinforced with carbon nanofibers

AbstractWe investigate the fracture response of metakaolin‐based geopolymer reinforced with 0.1 wt%, 0.2 wt%, and 0.5 wt% carbon nanofibers. We measure the elastoplastic response using microindentation tests. We note an increase in indentation modulus of 5%, 13%, and 21%, and an increase in indentation hardness of 9%, 18%, and 25%, respectively. We measure the fracture energy using cutting‐edge microscopic fracture tests. In our tests, a sphero‐conical diamond indenter pushes across the specimen's surface under a prescribed vertical force. We analyze the recorded penetration depth and horizontal force using nonlinear fracture mechanics and extract the fracture parameters. We find that carbon nanofibers enhance fracture resistance. The fracture toughness increases by, respectively, 38%, 40%, and 45%; meanwhile, the fracture energy increases by, respectively, 83%, 72%, and 74%. We find that carbon nanofibers lead to a densification of the microstructure. Moreover, we observe crack‐bridging mechanisms in geopolymer nanocomposites. This study is important to pave the way for novel enhanced‐performance and multifunctional structural materials.

Mechanical behavior of annealed electrochemically deposited nanocrystalline nickel-iron alloys

Abstract Nanocrystalline nickel-iron layers are produced electrochemically on copper discs by varying the current density and then annealed in a vacuum furnace at a temperature range between 200 and 800 °C. Grain size, iron content, texture and microstrain of the microstructure are primarily characterized by X-ray diffraction (XRD). Instrumented indentation tests and microbending tests for mechanical characterization are carried out. The iron contents of the investigated layers are 5.7, 8.8, 13.5 and 17.7 wt.-%. By varying the annealing temperature, the reduction of the microstrains is initiated at 200 °C and ends at a temperature of about 280 °C. Primary recrystallization starts slightly higher at 220 °C and is completed at 300 °C. With higher iron content, the indicated temperatures shift to slightly higher values. Indentation modulus, Young's modulus, indentation hardness and strength change considerably after the annealing treatment. Fracture strain at the edge, as a measure of ductility, decreases immediately after annealing at 200 °C to 0 %. Low annealing temperatures occurring before the beginning of primary recrystallization lead to an increase in indentation hardness and 0.01-% offset bending yield strength Rp0.01∗ as compared to the electrochemically deposited initial state. After annealing at high temperatures, the mechanical parameters are mostly below the initial values for electrochemical deposition. Hall-Petch (HP) behavior is observed for Rp0.01∗, both for the electrochemically deposited specimens down to almost 6 nm and for the specimens annealed at high temperatures. Specimens annealed at low temperatures deviate from the HP straight line to higher values. In this case, an increase in strength is assumed to be due to the very small nanocrystalline (nc) grain sizes, segregation at the grain boundaries and a decrease in dislocation density. Indentation hardness measurements show almost no dependence on D−0.5 for the electrochemically deposited specimens and also for annealed specimens below 30nm grain size. Above 30nm, the indentation hardness values are considerably higher than for the HP straight line. Overall, the hardness and strength values of the nc specimens, electrochemically deposited or additionally annealed, are significantly higher than those of the microcrystalline (mc) specimens.

Strain Hardening of Phases in High‐Alloy CrMnNi Steel as a Consequence of Pre‐Deformation Studied by Nanoindentation

Metastable high‐alloy CrMnNi steels exhibit a martensitic phase transformation from austenite via an intermediate hexagonal phase arranged in deformation bands − often referred to as ϵ‐martensite − into α’‐martensite, resulting in the well‐known transformation induced plasticity (TRIP) effect. The hardness of individual microstructural constituents (austenite, ϵ‐martensite, α’‐martensite) is studied by nanoindentation in a scanning electron microscope. The indentation hardness of the austenite in both its undeformed state and after tensile deformation, with elongation of up to 14%, is measured in order to study the influence of plastic deformation as well as of the grain orientation of the austenite. The microstructure after deformation is characterized by means of electron backscatter diffraction. Load–displacement curves are recorded in both a pre‐deformed specimen and in an undeformed specimen, with both having similar grain orientations relative to the axis of indentation. Pronounced pop‐in events are observed for undeformed austenite and ϵ‐martensite. Indentation hardness values are determined for all microstructural constituents and compared to the undeformed state in order to obtain information on the influence of the orientation of austenitic grains and the plastic deformation. A significant increase in indentation hardness is observed, while some tendencies for orientation dependence are found. In addition, pop‐in events occurring in ϵ‐martensite are analyzed using transmission electron microscopy (TEM). α’‐martensite is found underneath such indents, and is formed during the indentation process.

Mechanical properties of single-crystal tungsten irradiated in a mixed spectrum fission reactor

To collect data for fusion applications and understand the basic properties of tungsten, single-crystal tungsten was neutron irradiated in the mixed-spectrum High Flux Isotope Reactor at the Oak Ridge National Laboratory at temperatures of 90–830 °C to fast fluences of 0.01–9 × 1025 n/m2 (E > 0.1 MeV). For tensile tests at room temperature of the irradiated material in both orientations, <110> and <100> tensile orientation, initially there was strengthening that peaked at 0.02 dpa and was followed by progressive modulus of toughness reduction, approaching zero at higher doses. For all irradiation temperatures, in elevated temperature tensile tests there was a distinct transition from ductile-to-brittle behavior between 0.1 and 0.4 dpa, accompanied by an increase in indentation hardness. The ductile-to-brittle transition with increasing dose was particularly critical because it presented as a total loss in elongation and modulus of toughness. The critical transition dose was well below the dose (∼1 dpa) where irradiation-induced precipitates are visible in the TEM. The extent of Vickers microhardness increase was significant at higher doses and did not depend on irradiation temperature or crystal orientation, reaching 12.9 GPa after 2.8 dpa. The significant mechanical property degradation above 0.1 dpa is believed to have been caused by the accumulation of irradiation-induced clusters and eventually precipitates of the transmutation elements Re and Os.

Wear Resistance of Cu/Ag Multilayers: A Microscopic Study.

The microscopic wear behavior of copper-silver multilayer samples was studied by performing sliding wear tests using a tribo-indenter. Multilayers with an average composition of Cu90Ag10 and Ag layer thicknesses ranging from 2 to 20 nm were grown by magnetron sputtering. For reference, a homogeneous Cu90Ag10 solid solution film was similarly grown. The thin films were subjected to two-dimensional wear tests by rastering a cono-spherical diamond indenter under loads of 100-400 μN for 1-20 consecutive passes or cycles. The wear volumes were determined by atomic force microscopy. Characterization of the specimens employed nanoindentation, nanoscratch, and transmission electron microscopy (TEM). Wear rates were found to reach steady state after five cycles or less. The hardness values of the as-grown and worn samples both increased with decreasing thickness of the Cu and Ag layers, whereas the steady-state wear rates decreased. Notably, the wear resistance increased faster than the corresponding increase in indentation hardness, indicating a deviation from the Archard's law. An inverse relationship between the wear rate and hardness was, however, recovered when using scratch hardness, suggesting that scratch hardness is a better predictor of wear resistance. Characterization of subsurface wear microstructures by TEM revealed that forced chemical mixing and dissolution of layers occur to a depth of ≈40 to 50 nm, stabilizing a chemically homogeneous solid solution below the wear surface. Comparative wear tests on thicker multilayers revealed that Cu/Ag interfaces reduced the wear rate significantly, thus helping to rationalize the high wear resistance of thin multilayers.

Determining the Influence of the Processing Temperature by Injection and of the Subsequent Pressure on the Surface�s Hardness and Indentation Modulus of the Products Made of HDPE, PMMA, PC+ABS through Nanoindentation - G-Series Basic Hardness Modulus at a Depth Method

The first part of the paper presents the influence of the processing temperature by injection of HDPE, of PMMA, and PC+ABS blend on the indentation hardness and on the indentation modulus, when other factors that can influence the injection remain unchanged. The second part of the paper presents the influence of subsequent pressure by injection of HDPE, PMMA, and PC+ABS blend on the indentation hardness and on the indentation modulus, when the other factors remain unchanged. The HDPE samples were obtained at the following injection temperatures: 180, 190, 200, 210, and 220�C, and at the following subsequent pressures: 800 bar, 900 bar, 1000 bar, 1100 bar, and 1200 bar. The PMMA samples were obtained at the following injection temperatures: 220, 230, 240, 250, and 260�C, and at the following subsequent pressures: 450, 550, 650 , 750, and 850 bar. The PC+ABS samples were obtained at the following injection temperatures: 230, 240, 250, 260, and 270�C, and at the following subsequent pressures: 500 bar, 600 bar, 700, 800, and 900 bar. The G-Series Basic Hardness Modulus at a Depth method was used to obtain the indentation hardness and the indentation modulus. It was observed that by increasing the processing temperature and subsequent pressure, in the case of HDPE, leads to an increase in indentation hardness and in indentation modulus. It was observed that increasing the processing temperature by injection in the case of PMMA, from 220 to 250�C, leads to a slight increase in indentation hardness and in indentation modulus, whereas increasing the subsequent pressure of PMMA, from 450 bar to 850 bar, leads to a slight decrease in indentation hardness and in the indentation modulus. Increasing the processing temperature by injection in the case of PC+ABS, from 230 to 250�C, leads to a slight increase in indentation hardness and in indentation modulus. By further increasing the processing temperature by injection, from 250 to 270�C, leads to a decrease in indentation hardness and in the indentation modulus. Alternatively, increasing the subsequent pressure from 500 bar to 900 bar leads to not only a decrease in indentation hardness but also to a decrease in the indentation modulus.

In situ monitoring and depth-resolved characterization of wet wear of silicon carbide

Wear of ceramic materials is a complex process and requires characterization methods capable of resolving the complexity of the degradation mechanisms in both time and spatial resolution. In this study, we investigated wet wear of silicon carbide (SiC) ceramics by means of depth-resolving electron microscopy, nanoindentation, and Raman spectroscopy. While these methods allow us to address changes in microstructure and chemistry, additional information on subsurface effects are gained (e.g., indentation modulus and hardness). Here, we see a decrease in elastic modulus and increase in indentation hardness leading to an improved H/E ratio beneficial to the overall wear performance. In addition to post mortem analysis, we present first data of online monitoring the wear loss. This was realized by constantly analyzing the Si concentration of water via optical emission spectrometry. A linear mass loss was found after an initial run-in phase.

Material-dependent representative plastic strain for the prediction of indentation hardness

The definition of representative plastic strain induced by a Vickers indent has received considerable attention in recent years. Previous reports have attempted to define a universal value that was independent of a material’s plastic response. However, the work presented here will show that a material-dependent representative plastic strain is valid in the conversion of flow stress to indentation hardness. This representative plastic strain is the volume average plastic strain within the plastic zone of Vickers indentation. The increase in indentation hardness within the plastic zones of macro-indents was experimentally determined by micro-Vickers indentation and then compared with that predicted by finite element modeling, which utilizes the proposed representative plastic strain. It was further shown that the representative plastic strain defined here is independent of yield strength, elastic modulus and magnitude of prior plastic deformation for both linear and power law strain hardening materials.