Nanoindentation-induced delamination of submicron polymeric coatings

An elastic model is developed to estimate the interfacial strength between a submicron surface coating and a compliant substrate. The analysis uses a shear-lag model and assumes the plane-stress state in the surface coating. The critical indentation load for the indentation-induced delamination of the coating from the substrate increases with the third power of the indentation depth and is a linear function of the reciprocal of the coating thickness. The indentation-induced delamination of SR399 ultrathin surface coatings over acrylic substrate has been evaluated, using the nanonindentation technique for coating thicknesses of 47, 125, 220 and 3000 nm. For the submicron coatings, the dependence of the critical indentation load on the coating thickness supports the elastic model. The interfacial strength is found to be 46.9 MPa. In contrast, the polymeric coating of 3000 nm displays multiple “excursions” in the loading curve, and the critical indentation load is a linear function of the indentation depth.     

Molecular dynamics investigation on the atomic-scale indentation and friction behaviors between diamond tips and copper substrate

Classical molecular dynamic (MD) simulations are used to investigate the atomic-scale indentation and friction behaviors of the spherical diamond(111) or diamond(001) tip in contact with a flat copper(001) substrate. In the indentation simulations, six radii ranging from 5 to 30 nm are adopted for each tip and the contact radius is examined as a function of normal load. The results demonstrate that the contact radii calculated from the MD simulation always deviate from the continuum theory predictions and the deviation varies with the tip surface atomic structure, tip radius, and normal load. Furthermore, the atomic-scale friction behaviors are investigated using 10 nm and 30 nm diamond(111) tips sliding over the copper(001) surface with a variety of loads. Apparent atomic stick–slip behavior is observed on such ordered but incommensurate contact interface; moreover, it does not disappear with increasing tip radius. It is also revealed that the friction versus load relationship is approximately linear, which is not in agreement with the continuum theory predictions and many reported atomic force microscope (AFM) experiments.     

Nanotribological and nanomechanical properties of 5–80 nm tetrahedral amorphous carbon films on silicon

Nanoscratch testing has been used to investigate the tribological behaviour of 5, 20, 60 and 80 nm tetrahedral amorphous carbon (ta-C) thin films deposited on silicon by the filtered cathodic vacuum arc method. The nanoscratch behaviour of the films was found to depend on the film thickness, with 60 and 80 nm films undergoing border cracking and then at higher critical load a dramatic delamination event. 5 and 20 nm films have a lower critical load for onset of border cracks but do not undergo a clear dramatic failure, and instead are increasingly worn/ploughed through until film removal as confirmed by microscopic analysis. This is consistent with the thinner films having lower stress and reduced load-carrying ability. Nanoindentation confirms that the thicker films have enhanced load support and higher measured composite (film + substrate) hardness. The 80 nm film in particular can retain appreciable load support whilst deformed during indentation, as shown by its ability to alter the critical loads for contact-induced phase transformations in the Silicon substrate during unloading.     

Microstructure and mechanical properties of Al2O3–YSZ spherical polycrystalline composites

The mechanical behavior and microstructure of highly densified, spherically shaped, polycrystalline Al₂O₃–YSZ composites, processed from pseudoboehmite powders by sol–gel is reported here. Processing was carried out by combining nanometric sized α-Al₂O₃ (120 nm) seeds and YSZ particles of tetragonal structure. The YSZ particles were homogeneously distributed in a coarse-grained matrix of alumina, both inside grains and along grain boundaries. Fracture surfaces, achieved by impact tests showed toughening effects of the zirconia particles. The tetragonality of the YSZ phase stability even after fracture events and fracture toughness measurements by Vickers indentation, where the crack tip interacts with YSZ particles, are all provided and discussed. The local mechanical properties, such as elastic modulus, indentation hardness and the onset of plastic deformation or fracture contact pressure of both YSZ particles and the Al₂O₃ matrix were quantified by nanoindentation. Evidence of coercive contact pressure was observed in YSZ from indentation stress–strain curves.     

Multiple cohesive cracking during nanoindentation in a hard W-C coating/steel substrate system by FEM

Stress evolution and subsequent cohesive cracking in the hard and stiff W-C coating on steel substrate during nanoindentation have been investigated using finite element modelling (FEM) and eXtended FEM (XFEM). The FEM simulations showed that the maximum principal stresses in the studied system were tensile and always located in the coating. They evolved in several stages. At indentation depths below 15% of the relative indentation depth, the maximum principal tensile stresses of ∼3 GPa developed at the top surface of the coating along the indenter/coating interface. At relative depths range 15–60%, the maximum tensile stresses of ∼6–8 GPa concentrated under the indenter tip in the coating along the interface with the substrate. At relative depths exceeding 60%, the maximum stresses gradually increased up to 10 GPa and they were located in the sink-in zone outside the indent as well as below the indenter tip. The first and subsequent cohesive cracks developed when the maximum tensile stresses in the sink-in zone at the top surface of the coating (and at the coating/substrate interface under the indenter) repeatedly reached the ultimate tensile strength of the coating. The hardness profile as well as cohesive cracking is controlled by the deformation of the substrate defined by the ration of the yield stresses of the coating and substrate. Very good correlation between the experimentally obtained cracks and multiple cracks predicted by XFEM confirmed the ability of the applied modelling in the prediction of fracture behavior of the studied coating/substrate system.     

Adhesion of diamond coatings on steel and copper with a titanium interlayer

Adherent diamond coatings on steel and copper were obtained by using a titanium interlayer. The adhesion of the coatings was evaluated by scratch tests and micro-indentation tests. The diamond coating on steel exhibited a much higher critical load than on copper, as revealed by the scratch tests. However, an observation on the back of the scratch-delaminated film and on the corresponding substrate surface showed that the detachment occurred between the diamond film and the titanium interlayer. Therefore, the difference in the critical scratch load is due mainly to a substrate effect, making it difficult to compare the adhesion of different coatings.On the other hand, Knoop indentation tests showed interesting results: a small indentation load causes round spallation in the film with no observable crack. An exponential sink-in deformation under the indentation is proposed, y=−a exp(−bx). The coating adhesion is considered to be equivalent to the deformation stress at the edge of the spallation zone. The adhesion of diamond coatings on steel and copper with a titanium interlayer is evaluated quantitatively using this model. Furthermore, a thermal quench method is proposed to estimate the coating adhesion. The results found are in agreement with the indentation model.     

A molecular dynamics simulation of the mechanical characteristics of a C60-filled carbon nanotube under nanoindentation using various carbon nanotube tips

The mechanical characteristics of a single-walled carbon nanotube (SWCNT) filled with C₆₀ fullerene subject to nanoindentation is studied using molecular dynamics (MD) simulations. The effects of temperature, indentation velocity, adhesion, and tip sizes were evaluated. The simulated results clearly show that the exerted load, Young’s modulus, elastic energy, and plastic energy decrease significantly with increasing temperature and decreasing indentation velocity and tip size. C₆₀ fullerenes can effectively increase the mechanical strength of a SWCNT because they act as a “barrier” to resist the radial deformation, as well as an inner wall in a double-walled carbon nanotube. With the same indentation depth, the ratio of elastic energy to plastic energy for a material gradually increases with the increase in the radius of the tip. This indicates that the elastic recovery of a material is better when the tip has a larger radius.     

Mechanical properties of a-C:H multilayer films

Multilayered amorphous hydrogenated carbon (a-C:H) films consisting of alternating sublayers with different mechanical properties have been deposited by an electron cyclotron resonance microwave-plasma chemical vapor deposition (ECR MP-CVD) system and modulating substrate bias voltage. The mechanical properties of the multilayer films were determined using nanoindentation and nanoscratch experiments with reference to single a-C:H layers of which the multilayer structure were composed. In nanoindentation tests, the relationship between the film hardness and indentation depth has been obtained over an indentation depth range of 20–500 nm. Since the films tend to fracture under high load in nanoindentation tests, their critical fracture loads were determined. The critical loads for fracturing the multilayered a-C:H films were higher than those of single a-C:H layers. The nanoscratch tests also showed that the multilayered a-C:H films required a higher critical load for scratching fracture. This study implies that the mechanical properties of a-C:H film can be improved by engineering suitable multilayer structures.     

Tensile properties of ultrathin double-walled carbon nanotube membranes

Direct tensile tests of double walled carbon nanotube (DWCNT) membranes with thickness of 40–80 nm were performed using a micro-stress-strain puller. The tensile strength and Young’s modulus are 4.8E2–8.4E2 MPa and 4.4–8.8 GPa, respectively. The deformation and fracture processes were analyzed using the stress vs. strain curves, and SEM observations of the fracture surface of a membrane. The membrane experienced elastic strain and plastic strain during tensile-loading to fracture, and the plastic process is due to the real plastic deformation of the membrane and the slippage between the DWCNT bundles. Cracks occur and spread during the tensile test which causes the membrane to be mangled. With these excellent mechanical properties, the DWCNT membranes can be used in nanotube-reinforced composites.     

Real time TEM observation of alumina ceramic nano-particles during compression

The behavior of alumina nano-particles taken from a commercial powder is investigated during in situ compression experiments in a transmission electron microscope (TEM). Small particles of 40 nm in diameter can undergo severe plastic deformation without failure, whereas brittle fracture is observed for 120 nm sized nano-particles. This is evidence of a critical size under which alumina, at least in the form of nano-particles, cannot be considered as brittle materials even at room temperature and a direct observation of the grinding limit generally observed during ball milling.     

Viscoelastic effects on the scratch resistance of polymers: relationship between mechanical properties and scratch properties at various temperatures

Scratch durability of polymer surfaces and coatings is becoming critical for the increasing use of these materials in new applications, replacing other materials with harder surfaces.

Scratch resistance of polymers has been the subject of numerous studies, which have led to specific definitions for plastic deformation characterization and fracture resistance during scratch testing. Viscoelastic and viscoplastic behavior during a scratch process have been related to dynamic mechanical properties that can be measured via dynamic nano-indentation testing. Yet, the understanding of the origin of the fracture process of a polymer during scratch remains approximate. Parameters like tip shape and size, scratch velocity and loading rate, applied strain and strain rates, have been considered critical parameters for the fracture process, but no correlation has been clearly established.

The goal of this work is to define and analyze scratch parameters that relate to mechanical properties. The evolution of scratch resistance parameters as a function of temperature and strain rate, compared to the evolution of dynamic mechanical properties obtained from indentation and uniaxial tensile tests over a range of temperature for poly(methyl methacrylate) (PMMA) helped in identifying a correlation between the tensile stress–strain behavior and scratch fracture toughness.

This correlation brings a new understanding of the origin of the fracture mechanisms during a scratch process. In particular, it is demonstrated that the characteristic strain applied by the indenter is a most relevant parameter to describe the fracture resistance during a scratch process, independently of the indenter geometry.     

Nanoindentation behavior of ultrathin polymeric films

Measurement of the mechanical properties of nanoscale polymeric films is important for the fabrication and design of nanoscale layered materials. Nanoindentation was used to study the viscoelastic deformation of low modulus, ultrathin polymeric films with thicknesses of 47, 125 and 3000 nm on a high modulus substrate. The nominal reduced contact modulus increases with the indentation load and penetration depth due to the effect of substrate, which is quantitatively in agreement with an elastic contact model. The flow of the nanoscale films subjected to constant indentation loads is shear-thinning and can be described by a linear relation between the indentation depth and time with the stress exponent of 1/2.     

Nanoscratching of Optical Glass Surfaces Near the Elastic–Plastic Load Boundary to Mimic the Mechanics of Polishing Particles

The nanomechanical deformations on glass surfaces near the elastic–plastic load boundary have been measured on various glasses by nanoscratching using an atomic force microscope (AFM) to mimic the mechanical interactions of polishing particles during optical polishing. Nanoscratches were created in air and aqueous environments using a 150‐nm radius diamond‐coated tip on polished fused silica, borosilicate, and phosphate glass surfaces; the topology of the nanoscratches were then characterized by AFM. Using load ranges expected on slurry particles during glass polishing (0.05–200 μN), plastic‐type scratches were observed with depths in the nm range. Nanoscratching in air generally showed deeper & narrower scratches with more pileup compared to nanoscratching in water, especially on fused silica glass. The critical load needed to observe plastic deformation was determined to range from 0.2–1.2 μN for the three glasses. For phosphate glass, the load dependence of the removal depth was consistent with that expected from Hertzian mechanics. However, for fused silica and borosilicate glass in this load range, the deformation depth showed a weak dependence with load. Using a sub‐T_g annealing technique, material relaxation was observed on the nanoscratches, suggesting that a significant fraction of the deformation was due to densification on fused silica and borosilicate glass. Repeated nanoscratching at the same location was utilized for determining the effective incremental plastic removal depth. The incremental removal depth decreased with increase in number of passes, stabilizing after ~10 passes. In water, the removal depths were determined as 0.3–0.55 nm/pass for fused silica, 0.85 nm/pass for borosilicate glass, and 2.4 nm/pass for phosphate glass. The combined nanoscratching results were utilized to define the composite removal function (i.e., removal depth) for a single polishing particle as a function of load, spanning the chemical to the plastic removal regimes. This removal function serves as an important set of parameters in understanding material removal during polishing and the resulting workpiece surface roughness.     

Microstructural changes in 3Y-TZP induced by scratching and indentation

Microstructural changes produced in 3Y-TZP by contact with a Vickers indenter during indentation and scratch tests have been studied by μ-Raman spectroscopy and transmission electron microscopy. In the scratch test, the monoclinic phase distribution under the scratch track is maximal at the surface, close to the edges of the groove, where the pile-up is formed by the flow of deformed material. This distribution coincides with the plastic deformed zone observed by electron microscopy. For indentation, the highest concentration of monoclinic phase is found below the imprint at a depth approximately equal to its semi-diagonal, while plastic deformation is concentrated in a narrow region of less than 1 μm just below the indenter tip. The present results show that the microstructural changes induced by the scratch test are similar to those induced during grinding.     

Surface damage resistance and yielding of chemically strengthened silicate glasses: From normal indentation to scratch loading

We report on surface elasticity, plastic deformation and crack initiation of chemically strengthened soda-lime silicate and sodium aluminosilicate glasses during lateral indentation and scratch testing. Instrumented indentation using a normal indenter set-up corroborated previous findings on the effects of chemical strengthening on surface Young's modulus, hardness, and indentation cracking. Using lateral indentation in the elastic-plastic regime, we find a pronounced increase in the scratch hardness as a result of chemical strengthening, manifest in higher work of deformation required for creating the scratch groove. Thereby, the glass composition is found to play a stronger role than the absolute magnitude of surface compressive stress. Using a blunt conical stylus for instrumented scratch testing reveals three distinct modes of scratch-induced surface fracture, which occur during scratching or after unloading. Occasional micro-cracking caused by pre-existing surface flaws at low scratching load can be completely suppressed through chemical strengthening. The intrinsic defect resistance to microcracking is reduced as a result of ion stuffing, depending on the initial glass composition, whereas the resistance to abrasive yielding is enhanced by several hundred MPa.     

Plastic Deformation of Aluminum Oxide by Indentation and Abrasion

Transmission electron microscopy provided direct evidence that plastic deformation occurs during the room-temperature indentation and abrasion of Al₂O₃. Examination of single-crystal and polycrystalline specimens showed that high densities of dislocations are produced within the near-surface regions by mechanical polishing with a fine diamond compound (0.25 μm) and that plastic deformation by both slip and mechanical twinning occurs during the placement of Vickers microhardness indentations. The occurrence of plastic deformation in this normally brittle material is considered to be a consequence of the nature and magnitude of the local stresses developed under pointed indenters and irregularly shaped abrasive particles. Preliminary results on the effect of annealing on the retained substructure are also presented. Annealing at 900°C and higher resulted in the reduction of residual stresses through the motion of dislocations and their rearrangement into lower-energy configurations.     

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.     

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.     

DNAN力学性能分析

利用Materials Studio软件计算了DNAN和TNT晶体在常温(25℃)、常压下(105Pa)的弹性模量,预估了二者受力过程中塑性变形能力的差异;通过纳米压痕试验测试了DNAN和TNT的弹性模量及塑性变形能力;采用力学性能试验机测试了其抗压性能、抗拉性能、抗剪性能,并结合分子间作用力对强度差异的原因进行了分析。结果表明,DNAN的抗压强度为5.96MPa,抗拉强度为2.57MPa,抗剪强度为0.34MPa;TNT的抗压强度为15.57MPa,抗拉强度为2.35MPa,抗剪强度为1.8MPa;DNAN和TNT在受力过程中的弹性形变均为200nm,DNAN的塑性形变为450nm,TNT的塑性形变为1 200nm,DNAN相比于TNT更容易发生脆性断裂。     

Depassivation-repassivation behavior of a pure iron surface investigated by micro-indentation

Depassivation-repassivation behavior on a pure iron surface in borate buffer solution was examined under potentiostatic control by a micro-indentation test. Current peaks emerge during both downward and upward drives of the indenter due to depassivation which is caused by plastic deformation of the substrate but not elastic deformation and repassivation. The total electric charge of the current peaks is proportional to the maximum load. The total electric charge also increases with increase in intermission time of the indentation, indicating that the passive film is ruptured even during stress relaxation. It is estimated from the electric charge balance that 82% and 18% of the film rupture occurs during the downward drive and intermission, respectively, and that no rupture occurs during the upward drive. Furthermore, the film-ruptured area is estimated to be 80% of the plastic deformed surface area. The partial retainment of the passive film on iron suggests that the ductility of the passive film is higher than that of the substrate.