Near-surface Mechanical Properties Research Articles

Surface topography affects the nanoindentation data

The near-surface mechanical properties of thin films as well as bulk materials are amongst the key parameters important for their application, and instrumented nanoindentation is a standard technique for determining these mechanical properties. However, it is known that the surface topography of the characterized materials may affect the nanoindentation data when a sharp indenter for small penetration depths (displacements) is used. A thin film of hydrogenated amorphous silicon carbide with a thickness of 1.0 μm was deposited on a silicon wafer by plasma-enhanced chemical vapour deposition. The cyclic nanoindentation was used to construct a depth profile of mechanical properties for the flat surface (0.5 nm roughness) of the thin film, which made it possible to determine its modulus of elasticity of 83 GPa and hardness of 8.6 GPa unaffected by the silicon substrate. Grains with a spherical cap geometry with a typical radius of 0.5 µm and a height of 60 nm are distributed along the flat surface of the film. The grains have the same mechanical properties as the deposited film. Depth profiles of mechanical properties were determined for different types of contact between the Berkovich indenter with a radius of 50 nm and the selected grain (grain top, grain foot, two or three grains); i.e. for these measurements the following applied - the radius of the tip curvature was less than grain radii (RBerkovich < Rgrain). Residual imprints after nanoindentation measurements were carefully observed by atomic force microscopy and scanning electron microscopy. The near-surface mechanical properties were significantly affected by the surface topography, and the determined modulus of elasticity and hardness were crucially under- or overestimated in the range of 50% to 100% compared to the real values. The nature of these deviations was discussed. The solution is to use cyclic nanoindentation performed on the flat surfaces or on the top of grains, followed by extrapolation of the depth profiles to the zero-contact depth (film surface).

Investigation of Chemomechanical Effects on Sapphire Surfaces Modified by Ion-Implantation-Induced Carbon Impurities

Modification of the chemomechanical behaviour of the surface of sapphire by ion implantation to improve its near-surface mechanical properties has been investigated. 300 keV Ti+ ions at various doses were implanted and the concentration and damage profiles characterised using Rutherford Backscattering (RBS). At high doses (≥ 3 × 1016 Ti+ cm−2), a surface amorphous layer is formed due to implantation-induced damage. Nanoindentation was used to determine the hardness behaviour of the ion-implanted layer. Hardness increases at low implantation doses, associated with implantation-induced damage, but it is also observed that chemomechanical softening of the surface is reduced due to the removal of adsorbed water. In situ Raman scattering measurements demonstrate this removal at low doses and the re-establishment of the adsorbed water layer at high doses. The adsorption process is changed due to the introduction of carbon into the sapphire surface during implantation. For the optimum-implanted dose, the water readsorption does not recur even several years after the implantation treatment was first carried out. The loss of water adsorption is related to the formation of a non-polar carbonaceous layer on the sapphire surface by cracking of back-streamed diffusion pump oil deposited on the sample surface by inelastic collisions with the ion beam. Based on this study, it is concluded that ion implantation with an appropriate ion species and dose can control the chemomechanical effect and improve the hardness of ceramics, such as sapphire.

Structural state awareness through integration of global dynamic and local material behavior

Structural health monitoring and nondestructive inspection techniques typically assess the lifecycle and reliability of high-value aerospace, mechanical, and civil systems. Maintenance and inspection intervals are typically time-based and dependent on the structural health monitoring/nondestructive inspection technique to detect macroscale damage resulting from fatigue or environmental damage. The current work proposes an integrated materials-structures-dynamics approach for providing state awareness of structural health. The proposed approach shifts the conventional structural health monitoring/nondestructive inspection focus of searching for cracks to a health state awareness based on tracking changes in the energetics of the materials-structures-dynamics states. Energy variations are tracked in a cantilevered structure exposed to nonlinear harmonic oscillation, where the strain energy of the beam was derived and used to determine a health state index. Nanoindentation was used to probe the near-surface mechanical properties of the beam to characterize local material variations as a function of fatigue cycles. A nonlinear ultrasonic approach was considered in order to connect the local material behavior changes to the variations in the dynamic performance of the beam. The intent of the investigation was to connect the traditionally detached materials, structural, and dynamics approaches to structural health monitoring/nondestructive inspection, while providing a framework for enabling damage precursor detection.

Effects of water immersion and humid weathering on the near‐surface mechanical properties of phosphate laser glass

AbstractIn this paper, the effect of water immersion and humid weathering on the near‐surface mechanical properties of phosphate laser (PL) glass was investigated using nanoindentation. The results indicate that, in the water immersion condition, the reduced modulus and nano‐hardness of PL glass decrease first, then increase and finally keeps unchanged with the increase in immersion duration; however, in the humid weathering condition, they decrease monotonously. The reaction mechanism occurring between water molecules and the glass network, especially during the later stage of the reaction process, determines the near‐surface mechanical properties and their differences when exposed to water and/or humid air. The results in this paper provide additional insight into the nano‐mechanics of glass surfaces, which also help understand the surface alteration process of phosphate laser glass during machining, storage, and serving in wet environments.

Characterising the early stages of crack development in environment-assisted cracking

ABSTRACTAn overview is given of recent NPL research to characterise the early stages of crack development in environment-assisted cracking, focusing on the nature of the crack precursors, the pit-to-crack transition, and the impact of solution chemistry on growth rate in the small and long crack growth rate regime. The impact on corrosion and cracking of surface grinding-induced changes in near-surface material and mechanical properties is highlighted, noting that awareness of this aspect in the corrosion community still remains limited. A major feature is the remarkable insight into the evolution of small surface cracks, from dealloyed layers and from corrosion pits, provided by advanced 3D imaging. Finally, further evidence for the concept of the solution conductivity-dependent crack-size effect in stress corrosion cracking and corrosion fatigue is put forward.

The existence of a lateral size effect and the relationship between indentation and scratch hardness in copper

Indentation size effects (ISEs) are well known in static indentation of materials that deform by dislocation-based mechanisms. However, whilst instrumented indentation techniques have become rapidly established as a means of determining the near-surface mechanical properties of materials, scratch testing has been much less widely used. Hardness is used in wear models as a proxy for the yield stress, and the design of materials and hard coatings has often sought to exploit size-derived performance enhancements through length-scale engineering. Yet, it is not known directly whether (or not) length-scale effects also apply to scratch (and thus wear) performance at small scales, or what the functional form of this effect is. This work directly demonstrates that there is a lateral size effect (LSE) and shows that there are questions to be answered if the use of hardness as an indicator of wear performance is to remain valid. We report on constant load scratch experiments using a Berkovich indenter on single-crystal, annealed copper, using a range of applied normal forces and compare results from three scratch hardness calculation methods to indentation hardness (ISO 14577:2002) measured on the same sample at the same loads. Scratch tests were performed with the Berkovich indenter aligned either edge forward or face forward to the scratch direction. In all cases, we demonstrate that there is a very significant (approximate factor of two) effect of scratch size (an LSE) on scratch hardness. The results also show that the deformation mechanisms occurring in scratch tests are different to those occurring beneath a static indentation and that different mechanisms dominated for different stylus orientations (face-forward vs. edge-forward orientation). This is, to our knowledge, the first direct demonstration of an LSE akin to the ISE in metallic materials. The results have significant implications for using static indentation as a predictor of deformation during wear processes.

Propagation characteristics of laser-generated surface acoustic waves in composite plate with gradient changes of near-surface viscous moduli

Taking into account the gradient change of the near-surface viscous property, we develop a finite element model of laser-generated surface acoustic wave in composite plate. The propagation characteristics of the surface acoustic wave in the composite plate are studied in detail, and the influences of the near-surface viscous modulus, thickness, and Lam constant on the attenuation characteristics of the surface acoustic wave are discussed. In addition, the propagation characteristics of the surface acoustic wave are verified by the theoretical calculations of the dispersion and attenuation curves. The results show that the near-surface viscous modulus and thickness are related to the attenuations of the surface shear wave and the Rayleigh wave, but have no influence on the propagation velocity. Furthermore, the imaginary part of the Lam constant has great influence on the attenuations of the surface shear wave and the Rayleigh wave, whereas the imaginary part of has no effect on the attenuation characteristics of the two waves, which indicates that the attenuation of the surface acoustic wave is related to the near-surface shear viscous modulus. The study gives theoretical basis for evaluating the near-surface mechanical properties of the composite plates by the laser ultrasound technique. Besides, it provides a feasible way to study the surface micro-cracks on the composite plates.

Is the second harmonic method applicable for thin films mechanical properties characterization by nanoindentation?

The second harmonic method is a dynamic indentation technique independent of the direct indentation depth measurement. It can be used to determine near-surface mechanical properties of bulk materials more precisely than classical dynamic nanoindentation. In this paper, the second harmonic method is extended to the measurement of the mechanical properties of thin poly(methyl methacrylate) (PMMA) layers deposited onto silicon wafers. It is shown that this new technique gives precise results at small depths (less than 100 nm), even for films with a thickness lower than 500 nm, which was not possible to achieve with the classical continuous stiffness measurement method. However, experimental and numerical results obtained both with classical nanoindentation and second harmonic methods differ at high indentation depth. Using finite element (FE) simulations and AFM measurements, it is shown that the contact depth calculation with classical models can explain this difference.

Influence of the shot-peening intensity on the structure and near-surface mechanical properties of Ti40Zr10Cu38Pd12 bulk metallic glass

Shot-peening (SP) changes the near-surface structure and mechanical properties of a Ti40Zr10Cu38Pd12 bulk metallic glass. Near the surface, the hardness, Young's modulus, and elastic strain limit are all reduced. Measurements of the heat of relaxation show that an exceptionally high stored energy of cold work can be induced, implying a large increase in free volume. At the highest SP intensity there is partial nanocrystallization enabled by the increased free volume and not by the increase in temperature.

Investigation of Near-Surface Mechanical Properties of Materials Using Atomic Force Microscopy

As ultra-thin films or small-scale structures become widely used in electronics and biology, knowledge concerning their near-surface mechanical properties of the materials is increasingly important. Atomic force microscopy (AFM) is employed to determine near-surface elastic modulus via force-penetration curves acquired during indentation. Samples include polydimethylsiloxane (PDMS), parylene, mica, and single-crystal silicon, and indentations are performed with single-crystal silicon and silicon nitride AFM tips. An analysis algorithm based on the secant modulus method is proposed to extract the true penetration curves from the experimental displacement curves. The penetration data is then analyzed in terms of Hertzian model to estimate the elastic modulus. Three concerns in applying nanoscale AFM indentation to the measurement of the elastic modulus of an ultra-thin material are addressed. First, the effect of the lateral force caused by the inclined angle of the cantilevered probe is investigated theoretically and by numerical simulation. A second concern is local plastic deformation induced by a sharp probe tip. In this case, numerical results show a relatively small effect on the force-penetration curves if the plastic deformation is limited to the central area below the probe tip. The deviation of the elastic-plastic simulation from the elastic estimation depends on the yield strength of the material. Finally, the effect of stiffness matching between the AFM probe and the sample is a key issue that is studied numerically, and appropriate stiffness matching criteria are suggested.

Mechanical and Structural Properties of Fluorine-Ion-Implanted Boron Suboxide

Results on a systematic study on the effects of ion implantation on the near-surface mechanical and structural properties of boron suboxide (B6O) prepared by uniaxial hot pressing are reviewed. 150 keV fluorine ions at fluences of up to5.0×1016 ions/cm2were implanted into the ultrahard ceramic material at room temperature and characterized using Raman spectroscopy, atomic force microscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Evidence of ion-beam-assisted nucleation of novel clusteredBxOyFzparticles by ion implantation is revealed. In addition, obtained results also reveal that fluorine implantation into the B6O specimen leads to an overall degradation of near-surface mechanical properties with increasing fluorine fluence. Implications of these observations in the creation of amorphous near-surface layers by high-dose ion implantation are discussed in this paper.

Mechanical properties of 3D carbon/carbon composites by nanoindentation technique

Nanoindentation tests were conducted to investigate the near-surface mechanical properties of the individual components (fiber and matrix) for three-dimensional reinforced carbon/carbon composites (3D C/C). Optical microscope and polarizing light microscope were used to characterize the microstructure of 3D C/C. The microscopy results show that large number of pores and cracks exist at both bundle/matrix interface and pitch carbon matrix. These defects have important effect on the mechanical behavior of 3D C/C. The in situ properties for components of 3D C/C were acquired by nanoindentation technique. Relative to the matrix sample, the fiber samples have more larger values for modulus, stiffness and hardness. However, there is no significant difference of modulus and stiffness among fiber samples with different directions.

Mechanical properties of plasma polymer film evaluated by conventional and alternative nanoindentation techniques

A plasma-polymerized tetravinylsilane film of about 1 μm thickness was prepared under steady-state deposition conditions on polished silicon wafer using plasma-enhanced chemical vapor deposition. Near-surface mechanical properties such as the reduced modulus and hardness of the plasma polymer film were investigated using conventional depth-sensing nanoindentation as well as load-partial-unload (cyclic) nanoindentation. The study found an influence of the dwell time and loading/unloading rate on the determined values of the reduced modulus, hardness, and contact depth.

Surface hydration and nanoindentation of silicate glasses

The near-surface mechanical properties of glasses and differences in mechanical behaviour between high and low durability silicate glasses are investigated. Nanoindentation is used to examine the effect of hydration on the near-surface mechanical properties of silicate glasses with varying degrees of chemical durability. It is shown that hydration has little if any effect on high durability glasses even at long immersion times, whereas in low durable glasses hydration reduces the near-surface mechanical properties significantly and the thickness of hydrated layer may exceed the indentation depth. In addition an attempt is made to measure the thickness and mechanical properties of hydrated layer in low durability glasses where influence of the substrate is negligible.

The Nano-Jackhammer effect in probing near-surface mechanical properties

Because of its ease of implementation and insensitivity to indenter drift, dynamic indentation techniques have been frequently used to measure mechanical properties of bulk and thin film materials as a function of indenter displacement. However, the actual effect of the oscillating tip on the material response has not been examined. Recently, it has been shown that the oscillation used with dynamic indentation techniques alters the measured hardness value of ductile metallic materials, especially at depths less than 200 nm. The alteration in the hardness is due to the added energy associated with the oscillation which assists dislocation nucleation. Atomistic simulations on nickel thin films agree with experiments that more dislocations are nucleated during dynamic indents than with quasi-static indents. Through the analysis of quasi-static and dynamic indents made into nickel single crystals and thin films, a theory to describe this phenomenon is presented. This is coined the Nano-Jackhammer effect, a combination of dislocation nucleation and strain rate sensitivity caused by indentation with a superimposed dynamic oscillation.

Near-surface mechanical properties and surface morphology of hydrogenated amorphous carbon thin films

The study of the near-surface nanomechanical properties of thin films is a very ambitious task and can be accomplished by advanced surface sensitive techniques. Depth-sensing nanoindentation (NI) is a widely used technique for the nanomechanical characterization of thin films, but it has the inherent limitation of the substrate influence to the measured hardness ( H) and elastic modulus ( E). Thus, sophisticated modeling is required to determine H and E at the surface. On the other hand, surface acoustic methods seem more promising for such a study. Among them, atomic force acoustic microscopy (AFAM) is a scanning probe microscopy technique based on the resonant vibration of the AFM cantilever. In this work, we study the near-surface nanomechanical properties and the surface morphology of soft hydrogenated amorphous carbon (a-C:H) thin films. We use NI, employing the continuous stiffness measurements technique, and AFAM for the imaging of the variations of the surface mechanical properties and the accurate determination of the elastic modulus. We analyze NI data using empirical models in order to estimate near-surface E and H and the results are compared to those obtained by AFAM. From depth-sensing NI, it was found that the a-C:H thin films present “pop-in” events, which are eliminated by changing the deposition conditions e.g. increasing the negative bias voltage to the substrate ( V b). Near-surface H and E of the a-C:H thin films measured with nanoindentation were found to initially increase with increasing | V b |. Further increase of | V b| has no effect on the E and H values. Finally, by comparing the results from AFAM and NI, we conclude that the obtained values for E by NI are lower for all the a-C:H thin films.

Polishing effect on nanoindentation behavior of nylon 66 and its nanocomposites

The effect of polishing on the near-surface mechanical properties of nylon 66 (PA66) and its clay nanocomposites has been investigated by a nanoindentation technique. The results show that polishing has significantly reduced the uncertainty in determining the surface property when performing nanoindentation experiments. In addition, polishing does not introduce serious strain hardening to a non-filled PA66 system, probably indicating that the polishing effect could be ignored for homogeneous polymer systems. For heterogeneous systems, especially for the injection-molded systems reinforced by nanofiller, polishing tends to expose the inner part of the sample, which may possess slightly different properties from that of the surface layer.

Near-surface mechanical properties of amorphous polymers

Near-surface mechanical properties of amorphous polymers

Near-surface mechanical properties of amorphous polymers

Abstract Polymeric material near a free surface can have properties which deviate considerably from the bulk properties. Many researchers have reported a reduced glass transition temperature in thin polymeric films and attributed this effect to an enhanced segmental mobility near a free surface. It was also reported that sufficiently thin polymeric structures show a higher ductility than the bulk material. In this paper, we therefore investigate the hypothesis that the near-surface mechanical properties of amorphous polymers differ from the bulk properties owing to the presence of an absolute length scale. Microindentations and nanoindentations are performed on polystyrene, using a range of indenter sizes and indentation loads. In addition, numerical simulations are carried out with an advanced material model for polystyrene. A comparison between the experimental and numerical results indeed indicates that a length-scale effect is present near the surface. Simulations performed at an elevated temperature indicate that our results are consistent with the observations of a reduced T g.

Micromechanical characterisation of plasma treated polymer surfaces

It has been recently demonstrated that a physically thick (about 50 nm) interfacial region (‘interphase’) is formed between a plasma deposited film and the underlying plasma treated or untreated polymer substrate. This can have an important effect on the performance of optical coatings, protective coatings, barrier films for packaging, and others. In the present work we study the effect of N2 and He plasma treatment of polycarbonate on the surface and near-surface (composite) mechanical properties. Multi-technique mechanical testing (scratching and depth-sensing indentation) has been performed using a broad range of applied loads (1 mN–10 N). We demonstrate that low-load scratching provides a more surface-specific information on the hardness and scratch resistance than do other methods. We also point out the advantage of constant-load, multipass scratching compared to the progressive-load approach.