Discrepancies in the hardness data and the role of grinding‐induced surface effects for a porous zirconate ceramic

The conventionally sintered La₂Zr₂O₇ (LZ) ceramic surfaces illustrated notable structural and microstructural changes post grinding and polishing operations. X‐ray diffraction studies confirmed presence of in‐plane compressive strain on polished surfaces; no phase segregation/separation was noted at surface or bulk level. The scanning electron micrographs revealed that the LZ ceramic grains on the polished top surfaces accommodated this strain by changing their size distribution from unimodal (observed for fractured and unpolished LZ surfaces with average size of 1.2 ± 0.5 μm) to a bimodal (with two modes occurring at 270 nm and 840 nm, respectively) distribution. Possibly, this grain size refinement led the pathway for obtaining surprising high hardness when probed via nanoindentation (13.8 GPa at 4 mN load). This hardness data made this 85% dense LZ sample comparable to its high density (>98%) counterpart (13.8 GPa). The hardness values were noted to have a strong dependence on the penetration depth (h_max). For h_max values beyond 500 nm (and loads≥30 mN), the hardness values depicted a significant decline (by one order of magnitude) and lowered down to 1.8 GPa for 30 mN load. The strain‐induced microstructural changes and the strong dependence of the hardness data (with h_max) paves the path for an in‐depth understanding of the mechanism by which the grinding‐induced strain is modifying the grain structures; especially for the smaller (mode I) grains on the polished LZ surface.     

Nanoindentation of diamond,graphite and fullerene films

The recently developed method of nanoindentation is applied to various forms of carbon materials with different mechanical properties, namely diamond, graphite and fullerite films. A diamond indenter was used and its actual shape determined by scanning force microscopy with a calibration grid. Nanoindentation performed on different surfaces of synthetic diamond turned out to be completely elastic with no plastic contributions. From the slope of the force–depth curve the Young's modulus as well as the hardness were obtained reflecting a very large hardness of 95 GPa and 117 GPa for the {100} and {111} crystal surfaces, respectively. Investigation of a layered material such as highly oriented pyrolytic graphite again showed elastic deformation for small indentation depths but as the load increased, the induced stress became sufficient to break the layers after which again an elastic deformation occurred. The Young’s modulus was calculated to be 10.5 GPa for indentation in a direction perpendicular to the layers. Plastic deformation of a thin fullerite film during the indentation process takes place in the softer material of a molecular crystalline solid formed by C₆₀ molecules. The hardness values of 0.24 GPa and 0.21 GPa for these films grown by layer epitaxy and island growth on mica and glass, respectively, vary with the morphology of the C₆₀ films. In addition to the experimental work, molecular dynamics simulations of the indentation process have been performed to see how the tip–crystal interaction turns into an elastic deformation of atomic layers, the creation of defects and nanocracks. The simulations are performed for both graphite and diamond but, because of computing power limitations, for indentation depths an order of magnitude smaller than the experiment and over indentation times several orders of magnitude smaller. The simulations capture the main experimental features of the nanoindentation process showing the elastic deformation that takes place in both materials. However, if the speed of indentation is increased, the simulations indicate that permanent displacements of atoms are possible and permanent deformation of the material takes place.     

Micro-scale evolution of mechanical properties of glass-ceramic sealant for solid oxide fuel/electrolysis cells

The structural integrity of the sealant is critical for the reliability of solid oxide cells (SOCs) stacks. In this study, elastic modulus (E), hardness (H) and fracture toughness (K_IC) of a rapid crystallizing glass of BaO–CaO–SiO₂ system termed “sealant G” are reported as determined using an indentation test method at room temperature. A wide range of indentation loads (1 mN–10 N) was used to investigate the load-dependency of these mechanical properties. Values of 95 ± 12 GPa, 5.8 ± 0.2 GPa and 1.15 ± 0.07 MPa m^0.5 were derived for E, H and K_IC using the most suitable indentation loads. An application relevant annealing treatment of 500 h at 800 °C does not lead to a significant change of the mechanical properties. Potential self-healing behavior of the sealant has also been studied by electron microscopy, based on heat treatment of samples with indentation-induced cracks for 70 h at 850 °C. Although the sealant G is considered to be fully crystallized, evidence indicates that its cracks can be healed even in the absence of a dead load.     

Nanoindentation of WC–Co hardmetals

WC–Co cemented carbide has been investigated using instrumented indentation with maximum applied loads from 0.1 to 10 mN. The hardness and indentation modulus of individual phases and the influence of crystallographic orientation of WC on the hardness and indentation modulus have been studied. The hardness of the Co binder was approximately 10 GPa and that of WC grains up to 50 GPa with relatively large scatter under the indentation load of 1 mN. Investigation of the role of crystallographic orientation of WC grains on hardness at 10 mN load revealed average values of H_ITbasal = 40.4 GPa (E_ITbasal = 674 GPa) and H_ITprismatic = 32.8 GPa (E_Itprismatic = 542 GPa), respectively. The scatter in the measured values at low indentation loads is caused by the effects of surface and sub-surface characteristics (residual stress, damaged region) and at higher loads by “mix-phase” volume below the indenter.     

Nanoscale elastic-plastic deformation and stress distributions of the C plane of sapphire single crystal during nanoindentation

The nanoscale elastic-plastic characteristics of the C plane of sapphire single crystal were studied by ultra-low nanoindentation loads with a Berkovich indenter within the indentation depth less than 60 nm. The smaller the loading rate is, the greater the corresponding critical pop-in loads and the width of pop-in extension become. It is shown that hardness obviously exhibits the indentation size effect (ISE), which is 46.7 ± 15 GPa at the ISE region and is equal to 27.5 ± 2 GPa at the non-ISE region. The indentation modulus of the C plane decreases with increasing the indentation depth and equals 420.6 ± 20 GPa at the steady-state when the indentation depth exceeds 60 nm. Based on the Schmidt law, Hertzian contact theory and crystallography, the possibilities of activation of primary slip systems indented on the C surface and the distributions of critical resolved shear stresses on the slip plane were analyzed.     

Hardness of polycrystalline SiO2 coesite

We measured elastic moduli and hardness of polycrystalline SiO₂ coesite. Translucent polycrystalline bulk coesite with a grain size of about 10 micrometers was fabricated at 8 GPa and 1600°C using a Kawai-type multianvil apparatus. The obtained bulk and shear moduli are 94(1) and 60.2(3) GPa, respectively. The resulting Vickers and Knoop hardness values are 10.9(7) and 9.6(4) GPa, respectively, at an indentation load of 4.9 N. Coesite is as hard as other fourfold coordinated silica materials such as quartz and densified silica glasses. The hardness values of coesite and the fourfold coordinated silica materials are about one-third of those of sixfold coordinated silica materials, stishovite, and seifertite, which are the hardest known oxides.     

Investigation of mechanical and creep properties of polypyrrole by depth-sensing indentation

In this study, we used depth-sensing indentation (DSI) technique to investigate some mechanical properties (reduced elastic modulus, indentation hardness, and creep) of polypyrrole (PPy) conducting polymer obtained with different support electrolyte concentration. The influence of support electrolyte concentration on these parameters was also determined. The order of doping degree of the samples was determined by cyclic voltammetry. The indentation load–displacement curves of the samples were obtained under different peak load levels with a 70 s holding time at maximum load. Reduced elastic modulus and hardness values were determined by analysis of these curves using the Feng–Ngan (F–N) and Tang–Ngan (T–N) methods, respectively. Both reduced elastic modulus (E _r) and indentation hardness (H) exhibited significant peak load dependence, i.e., indentation size effect (ISE). It was found that both E _r and H values decreased as the support electrolyte concentration was increased. This was explained by an increase in the free volume as the doping degree was raised. The creep behavior of the samples was monitored from the load holding segment of the load–unload curves. It was found that creep increases with the increasing support electrolyte concentration.     

Boro/carbothermal reduction co-synthesis of dual-phase high-entropy boride-carbide ceramics

Dense, dual-phase (Cr,Hf,Nb,Ta,Ti,Zr)B₂-(Cr,Hf,Nb,Ta,Ti,Zr)C ceramics were synthesized by boro/carbothermal reduction of oxides and densified by spark plasma sintering. The high-entropy carbide content was about 14.5 wt%. Grain growth was suppressed by the pinning effect of the two-phase ceramic, which resulted in average grain sizes of 2.7 ± 1.3 µm for the high-entropy boride phase and 1.6 ± 0.7 µm for the high-entropy carbide phase. Vickers hardness values increased from 25.2 ± 1.1 GPa for an indentation load of 9.81 N to 38.9 ± 2.5 GPa for an indentation load of 0.49 N due to the indentation size effect. Boro/carbothermal reduction is a facile process for the synthesis and densification of dual-phase high entropy boride-carbide ceramics with both different combinations of transition metals and different proportions of boride and carbide phases.     

Nanoindentation pop-in in oxides at room temperature: Dislocation activation or crack formation?

Most oxide ceramics are known to be brittle macroscopically at room temperature with little or no dislocation-based plasticity prior to crack propagation. Here, we demonstrate the size-dependent brittle to ductile transition in SrTiO₃ at room temperature using nanoindentation pop-in events visible as a sudden increase in displacement at nominally constant load. We identify that the indentation pop-in event in SrTiO₃ at room temperature, below a critical indenter tip radius, is dominated by dislocation-mediated plasticity. When the tip radius increases to a critical size, concurrent dislocation activation and crack formation, with the latter being the dominating process, occur during the pop-in event. Beyond the experimental examination and theoretical justification presented on SrTiO₃ as a model system, further validation on α-Al₂O₃, BaTiO₃, and TiO₂ are briefly presented and discussed. A new indentation size effect, mainly for brittle ceramics, is suggested by the competition between the dislocation-based plasticity and crack formation at small scale. Our finding complements the deformation mechanism in the nano-/microscale deformation regime involving plasticity and cracking in ceramics at room temperature to pave the road for dislocation-based mechanics and functionalities study in these materials.     

Nanoindentation with spherical tips of single crystals of YBCO textured by the Bridgman technique: Determination of indentation stress–strain curves

The mechanical properties of superconductor ceramics are of interest in the manufacture of superconducting devices. The current trend is to produce smaller devices (using, e.g., thin films), and the correct characterization of small volumes of material is critical. Nanoindentation is used to assess mechanical parameters, and several studies determine hardness and Young's modulus by sharp indentation. However, studies on the elasto-plastic transition with spherical indentation are scare. Here we used, spherical diamond tip indenter experiments to explore the elasto-plastic transition and to measure the yield strength of the orthorhombic phase of YBa₂Cu₃O_7?δ (YBCO or Y-123) at room temperature. The study was carried out for a range of monodomains on the (1 0 1)-plane for Bridgman samples. Inspection of the load–unload curves for penetration depths lower than 200 nm allows for observation of the elasto-plastic transitions. Focused ion beam (FIB) trenches showed no cracking due to the indentation, although oxygenation cracks were apparent. The mean pressure for the onset of elasto-plastic deformation is 3.5 GPa, and the elastic modulus, E, calculated using the Hertzian equations is 123.5 ± 3.4 GPa.     

Superhard and superelastic films of polymeric C60

The C₆₀ thin film deposited on steel substrate was transformed by high pressure–high temperature treatment to a superhard and superelastic material. The films were studied by Raman spectroscopy in situ at 20 GPa after heating at 300°C and ex situ after the quenching. The hardness and elastic properties of the high-pressure phases have been characterized with nanoindentation. The hardness of the films were determined to be 0.5±0.1 GPa and 61.9±9 GPa for unmodified C₆₀ and HPHT treated films, respectively. The hardness of the pressurized film is higher than for cubic BN but lower than hardness values reported for ultrahard fullerite samples prepared from powders. An interesting observation was that the HPHT treated film showed an extreme elastic response with an elastic recovery of approximately 90%.     

Effects of interlayer interactions on the nanoindentation behavior and hardness of 2:1 phyllosilicates

Six non-expandable, hydrous 2:1 phyllosilicate minerals with different layer charges, including pyrophyllite and talc from the pyrophyllite-talc group and biotite, two muscovites, and margarite from the mica group, were nanoindented to probe their nanoscale deformation behavior and hardness. Remarkably different deformation responses were obtained, which are highly dependent upon the type and strength of interlayer interactions controlled by Coulomb or van der Waals forces. Concave load–displacement curves were observed on the micas with stronger Coulomb forces as interlayer interaction, while convex curves on the other group with weaker van der Waals attractions as interlayer interaction. The hardness increases with layer charge both within each group and between the two groups, ranging from 3.86 GPa for talc to 16.34 GPa for margarite, with the highest being four times greater than the lowest. Owing to their layered structure that favors kink band formation and layer delamination, pop-ins of varied extensions occur randomly in the load–displacement curves. An observed apparent indentation size effect is attributed to the pop-in events. The dependence of hardness on interlayer interactions between 2:1 layers instead of the atomic bonds within the 2:1 layer suggests that the interlayer interactions can be used as a hardness or plastic signature for phyllosilicate minerals.     

Mechanical properties of polymer‐ceramic nanocomposite coatings by depth‐sensing indentation

The mechanical properties of antimony‐doped tin oxide (ATO) nanoparticle/poly (vinyl acetate‐co‐acrylic) (PVAc‐co‐acrylic) coatings with various ATO contents were investigated using depth‐sensing indentation. These coatings were prepared from aqueous dispersions of ATO and PVAc‐co‐acrylic latex. Three types of methods, including a prolonged load holding time, analysis of the pull‐off portion of the unloading curve, and dynamic indentation, were used to characterize the mechanical properties of these composite coatings. As compared to dynamic indentation, quasistatic conventional indentation even with a prolonged load holding time and analysis of the pull‐off portion of unloading curves generate more scattered coating modulus data. This is due to the effect of creep deformation and inconsistency of the pull‐off portion dimension, respectively. The results obtained using dynamic indentation are more reliable because the technique minimizes the effect of creep deformation using a combination load including static and dynamic components. The dynamic indentation results indicate that the addition of the ATO nanoparticles made the composite coatings stiffer and more elastic solid–like. For example, the storage indentation modulus of the PVAc‐co‐acrylic coating is ～1 GPa and tan δ is ～1.6; the addition of 0.50 volume fraction of ATO increased the modulus to ～5 GPa and reduced the tan δ to ～0.01. POLYM. ENG. Sci. 45:207–216, 2005. © 2005 Society of Plastics Engineers.     

Microstructure and mechanical properties of reaction bonded B4C-SiC composites: The effect of polycarbosilane addition

Reaction bonded B₄C-SiC composites were prepared by infiltrating silicon melt into porous B₄C-SiC green preforms at 1500 °C in vacuum. The porous green preform was obtained from a mixture of polycarbosilane (PCS) and particle size graded B₄C after pre-sintering at 1600 °C. For the first time, PCS was used to adjust the phase composition and microstructure of the reaction bonded boron carbide composites. It is indicated that the addition of PCS and its content has a significant influence on the microstructure as well as the mechanical properties of the subsequent reaction bonded B₄C-SiC composites. For the B₄C-SiC composite with 5 wt% PCS added, a flexural strength of 319±12 MPa, and an elastic modulus of 402±18 GPa can be achieved, which is 23% and 15% higher than those of the composite without PCS addition, respectively. While, with the higher content of PCS addition, the mechanical properties of the composites are decreased drastically due to the large amount of residual Si agglomeration in the composites. The reaction mechanisms as well as their microstructure evolution processes correlated with the mechanical properties of the reaction bonded B₄C-SiC composites are further discussed in our work.     

Experimental investigation on the indentation hardness of precursor derived Si–B–C–N ceramics

The elasto-plastic response of the precursor derived Si–B–C–N ceramics upon contact loading was determined by depth sensing nanoindentation technique. The indentation response of as-thermolyzed Si–B–C–N ceramic was compared with the heat-treated counterpart. The as-thermolyzed ceramic was X-ray amorphous and the heat-treated ceramic was phase separated and crystallized. The hardness and reduced elastic modulus values of the as-thermolyzed ceramic were ～16 GPa and ～172 GPa, respectively. The reduction in hardness to ～9 GPa in the heat-treated ceramic was attributed to phase separation and crystallization of SiC and Si₃N₄. Furthermore, high elastic recovery with a plastic work ratio of ～0.3 was observed and ascribed to volume controlled deformation mechanism.     

Transition metal diboride-silicon carbide-boron carbide ceramics with super-high hardness and strength

Three phase boride and carbide ceramics were found to have remarkably high hardness values. Six different compositions were produced by hot pressing ternary mixtures of Group IVB transition metal diborides, SiC, and B₄C. Vickers’ hardness at 9.8 N was ~31 GPa for a ceramic containing 70 vol% TiB₂, 15 vol% SiC, and 15 vol% B₄C, increasing to ~33 GPa for a ceramic containing equal volume fractions of the three constituents. Hardness values for the ceramics containing ZrB₂ and HfB₂ were ~30% and 20% lower than the corresponding TiB₂ containing ceramics, respectively. Hardness values also increased as indentation load decreased due to the indentation size effect. At an indentation load of 0.49 N, the hardness of the previously reported ceramic containing equal volume fractions of TiB₂, SiC and B₄C was ~54 GPa, the highest of the ceramics in the present study and higher than the hardness values reported for so-called “superhard” ceramics at comparable indentation loads. The previously reported ceramic containing 70 vol% TiB₂, 15 vol% SiC, and 15 vol% B₄C also displayed the highest flexural strength of ~1.3 GPa and fracture toughness of 5.7 MPa·m^1/2, decreasing to ~0.9 GPa and 4.5 MPa·m^1/2 for a ceramic containing equal volume fractions of the constituents.     

Bulk high-entropy hexaborides

For the first time, a group of CaB₆-typed cubic rare earth high-entropy hexaborides have been successfully fabricated into dense bulk pellets (>98.5 % in relative densities). The specimens are prepared from elemental precursors via in-situ metal-boron reactive spark plasma sintering. The sintered bulk pellets are determined to be single-phase without any detectable oxides or other secondary phases. The homogenous elemental distributions have been confirmed at both microscale and nanoscale. The Vickers microhardness are measured to be 16?18 GPa at a standard indentation load of 9.8 N. The nanoindentation hardness and Young’s moduli have been measured to be 19?22 GPa and 190?250 GPa, respectively, by nanoindentation test using a maximum load of 500 mN. The material work functions are determined to be 3.7–4.0 eV by ultraviolet photoelectron spectroscopy characterizations, which are significantly higher than that of LaB₆.     

Mechanical properties of thermally sprayed porous alumina coating by Vickers and Knoop indentation

Depending on the thermal spraying conditions, coatings obtained can present different defects, like pores, cracks and/or unmelted particles, and different surface roughnesses, that can affect the determination of the hardness and elastic modulus. The present work investigates the mechanical properties, determined by means of Knoop and Vickers indentations, of a plasma as-sprayed alumina coating, obtained with a nano-agglomerated powder sprayed using a PTF4 torch, in order to highlight how the surface defects interfere into the indentation process. As a main result, Knoop indentation compared to Vickers one gives less dispersive results (15% and 33%, respectively), that are, in addition, more representative of the coating properties. The mean values obtained are 110 ± 40 GPa for the elastic modulus and 1.75 ± 0.42 GPa for the hardness. In addition, and for the two indenter types used, multicyclic indentation has been performed because it allows a more appropriate characterization of such heterogeneous coatings due to the representation of the mechanical properties as a function of the indentation load and/or the penetration depth, leading to more reliable results according to the depth-variability of the coating microstructure.     

Effect of nonstoichiometry on mechanical properties of reactive hot‐pressed monolithic ZrCx Ceramic

The reactive hot pressing (RHP) of Zr:C powder mixture at various molar ratios (1:0.5, 1:0.6, and 1:0.67) at applied pressures of 4‐7 MPa and 1200°C resulted in dense ZrC_x ceramics. Nano‐hardness values of ZrC_x are reported to be 21‐31 GPa as “x” was varied from 0.5 to 1.0. However, indentation modulus for all ZrC_x compositions remained at ~350 GPa. Microhardness of the ZrC_x increased from 13 to 15 GPa as the stoichiometry was increased from 0.5 to 1.0. The indentation fracture toughness for ZrC_0.5 was 4 MPa m^1/2, and for ZrC_0.67 it was reduced to 3.6 MPa m^1/2. The 3‐point flexural strength for ZrC_0.5 was determined to be 386 ± 26 MPa, which decreased to 316 ± 20 MPa as the carbon content (ZrC_0.67) was increased. The dry sliding wear of ZrC_0.5 to ZrC_0.6 indicated that the coefficient of friction was increased from 0.73 to 0.86 at 5 N load and 500 m sliding distance. Further, ZrC_0.67 showed a reduction in friction coefficient of 0.81, and this was due to the increase of strong Zr–C covalent bond and unreacted graphite.     

Multi-response optimization of mechanical properties for ZrWN films grown using grey Taguchi approach

ZrWN nitride films are prepared using direct current (dc) reactive magnetron sputtering. Grey Taguchi analysis is used to determine the effect of deposition parameters (substrate plasma etching time, N₂/(N₂+Ar) flow rates, deposition time, and substrate temperature) on the microstructure and the tribological properties. An orthogonal array, signal-to-noise ratio and analysis of variance are used to determine the effects of deposition parameters. The substrates are pretreated using oxygen plasma etching. The resulting ZrWN coatings are homogeneous, very compact and completely adhered to the substrate. In the confirmation runs, using grey Taguchi analysis, the coefficient of friction decreases from 0.45?±?0.02 to 0.35?±?0.02, the corrosion potential increases from ??0.201?±?0.01 to ??0.072?±?0.01?V, the Vickers hardness increases from 23.63?±?0.07 to 24.65?±?0.05?GPa, and reduced modulus increases from 115.82?±?1.13 to 136.17?±?1.18?GPa. The ZrWN films are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Rockwell C indentation and scratch testing. The TEM pattern for the ZrWN films corresponds to the (111), (200) and (220) planes of the face center cubic structure. Samples with a ZrWN film coating are classified as HF1 and exhibit good adhesive strength. The signal of friction and the associated acoustic emission signal are analyzed, and the scratch profile is analyzed using an optical microscope. Results show that the adhesive force for the critical load Lc2 is about 76.2?±?0.5?N.