Continuous Stiffness Measurement Nanoindentation Research Articles

Investigation of dynamic micromechanical properties of biodegradable elastic material by continuous stiffness measurement analysis

Micromechanical properties of polymeric materials play a critical role in various biological applications in terms of their biocompatibility and mechanical durability. Apart from material properties such as modulus and density, viscoelastic properties play a crucial role during the design and fabrication of devices. Here, we investigated the viscoelastic properties of poly (glycerol sebacate) (PGS), a widely used bioresorbable elastic material, through the nanoindentation technique, configured by the continuous stiffness measurement (CSM) method at frequencies from 10 Hz to 50 Hz. The results revealed that the storage modulus (E’) depends on the test frequency and cannot be ignored as the results showed significant changes. Additionally, increasing the curing temperature of PGS specimens between 150 to 170°C allows modifying the storage modulus of samples between 0.52 MPa and 1.05 MPa at 10 Hz. The results were also confirmed using the dynamic mechanical measurements to validate the reliability of the CSM nanoindentation technique.

A multi-technique and multi-scale analysis of the thermal degradation of PEEK in laser heating

The present work studies the thermal degradation of laser-heated poly-ether-ether-ketone (PEEK) as the heating duration increases. Its damage morphology, chemical composition, crystallinity content, and mechanical properties are examined with optical microscopy, attenuated total reflection-Fourier transform infrared spectroscopy, differential scanning calorimetry, Raman spectroscopy, and continuous stiffness measurement nanoindentation. The applicability of those methods in detecting the thermal degradation of laser-heated PEEK and assessing the induced thermal damage is highlighted. Results show that short-time laser heating acts as an annealing process that improves the crystallinity and hardness on the affected surface of PEEK by up to 5.1% and 10.8% respectively. With a further increase in the heating duration, surface carbonisation occurs and a char layer is formed. Surface carbonisation is associated with the thermal limits of PEEK in laser heating decreasing by up to 50% its hardness and by 45% its indentation modulus. Finally, the char layer is found to act as a shielding mechanism that protects the bulk PEEK from the applied thermal load, resulting in mostly superficial thermally induced damage.

Effects of a porous boron nitride nanofiber on nano-mechanical properties in shallow surface of WC-Si3N4 composite

The nanoindentation technique has been applied to materials research for decades, such as WC, Si3N4, and other ceramics or composites. However, nano-mechanical properties in shallow surfaces were often unstable, especially for ceramics-based composites. In this paper, the mechanical properties of WC-10 wt.% Si3N4 without and with the addition of ultrafine porous boron nitride nanofiber were investigated by nanoindentation continuous stiffness measurement in this study. The effects of the porous fiber on elastic modulus and hardness were revealed, which led to more stable and regular profiles of the elastic modulus and hardness versus displacement curves. The curves were cut out and shown before the 30 nm displacement of the tip, which displayed the regularity of the profiles caused by the porous fiber more directly. The values were statistically analyzed at intervals of 10 nm after 30 nm displacement, and then lower standard deviations were observed. The different Si3N4 cracks made from Vickers indentation on the composites were photographed and associated with the regularity of the curves after the fiber addition. It’s considered that the regularity effects on mechanical properties of the porous boron nitride nanofiber on ceramics composites can be helpful in thin film studies, which are also in difficulties of mechanical properties scattering on the nanometer scale using continuous stiffness measurement.

Practical method to determine the effective zero-point of indentation depth for continuous stiffness measurement nanoindentation test with Berkovich tip

The zero-point of indentation depth in nanoindentation or depth-sensing instrumented indentation tests should be precisely set to evaluate the indentation hardness and indentation elastic modulus of materials to be tested, especially at shallow depths. A critical contact stiffness value has been widely used to determine the zero-point in nanoindentation tests with a Berkovich tip using the continuous stiffness measurement (CSM) method. However, this criterion occasionally gives an inadequate zero-point owing to the surface roughness of materials, the vibration of the testing system, and the flaws of the CSM method at shallow depth. This study proposes a practical method to determine the effective zero-point of indentation depth, which was obtained linearly at the zero-point of contact stiffness and extrapolated from the depth-dependent contact stiffness values, except for those at initially unstable contact depths. The proposed method enables nanoindentation tests to obtain a constant indentation elastic modulus and low deviation of nanoindentation hardness of homogenously fused silica and metallic materials, which provides an efficient way to obtain more accurate test data.

Characterization of strain amplitude-dependent behavior of hardness and indentation size effect of SS400 structural steel

In this paper, the continuous stiffness measurement (CSM) indentation is employed to investigate fatigue mechanical properties of structural steel under cyclic loading. For this purpose, several representative analytical approaches were introduced to estimate the basic mechanical properties including Young’s modulus and indentation hardness from the characteristics of the loading/unloading curves. Several experiments including CSM nanoindentation, low-cycle fatigue experiment for four strain amplitude levels, optical microscope (OM), and transmission electron microscopy (TEM) examinations were conducted to observe the variation characteristics of mechanical properties at the microscale and their micro-mechanisms. The microstructural evolution of the specimens deformed by the low-cycle fatigue was observed using the OM and TEM examinations. The standard nanoindentation experiments were then performed at different strain rate levels to characterize the influences of strain rate indentation on hardness of the material. The micro-mechanisms established based on the microstructural evolution and strain gradient plasticity theory were introduced to be responsible for the variation of indentation hardness under cyclic loading. Finally, the indentation size effect (ISE) phenomenon in SS400 structural steel was investigated and explained through the strain gradient plasticity theory regarding geometrically necessary dislocations underneath the indenter tip. The experimental results can be used for practical designs as well as for understanding the fatigue behavior of SS400 structural steel.
 Keywords:
 cyclic loading; fatigue; nanoindentation; indentation size effect; strain rate sensitivity; structural steel.

Experimental study to investigate microstructure and continuous strain rate sensitivity of structural steel weld zone using nanoindentation

In this study, a series of experiments consisting of continuous stiffness measurement (CSM) nanoindentation experiments, optical microscope (OM), atomic force microscopy (AFM) examinations, and finite element (FE) analysis were performed to study the microstructures, the indentation size effects, and the rate-dependent behavior of mechanical properties in three phases of SM490 structural steel weld joint. The microstructures of base metal (BM), heat-affected zone (HAZ), and weld metal (WM) were observed using OM examinations. The size-dependent behaviors of mechanical properties in BM, HAZ, and WM were characterized and interpreted through the strain gradient theory. The CSM nanoindentation experiments were carried out in the strain rate range of 0.01–0.1 s−1 to investigate the rate-dependent behavior of indentation hardness and continuous strain rate sensitivity. The results indicated that indentation hardness depended on not only the indentation size but also the strain rate of the indentation level. The strain rate sensitivity (SRS) of BM, HAZ, and WM showed the depth-dependent behavior. The SRS of WM is quite high, over 0.4 at the indentation depth of 200 nm, quickly drops to 0.1, and finally is around 0.0298 at large indents. Similarly, the SRS behavior in the case of HAZ is the same, however, the SRS values at larger indents are higher than those obtained in the WM region. At larger depths, the SRS values of the WM region are lower than those of BM and HAZ, while BM has the highest SRS value compared with WM and HAZ. The continuous SRSs of WM, HAZ, and BM are attributed to the change in the dislocation behaviors during the indentation process. The results of the present study can be used to access and understand the rate- and the size-dependent behaviors in structural steel weld zone.

Nano-indentation and avalanches in compressed porous SiO2

The “nano-indentation continuous stiffness measurement technique” has been employed to analyze the failure dynamics of mesoporous SiO2 based material (Vycor). The depth dependence of the indentation hardness (H), elastic modulus (E), and elastoplastic parameter (S2/P) shows crackling noise, which has been analyzed to monitor the jerky strain release. The noise is power law distributed with exponents near ∼1.5 over several decades, confirming avalanche criticality. This value is in good agreement with literature results obtained by other techniques and with earthquake statistics.

Mechanical Properties of Uranium Silicides by Nanoindentation and Finite Elements Modeling

Three methods were used to measure the mechanical properties of $${\text{U}}_{3}{\text{Si}}$$ , $${\text{U}}_3{\text{Si}}_{2}$$ , and USi. Quasi-static and continuous stiffness measurement nanoindentation were used to determine hardness and Young’s modulus, and microindentation was used to evaluate the bulk hardness. Hardness and Young’s modulus of the three U-Si compounds were both observed to increase with Si content. Finally, finite elements modelling was used to validate the nanoindentation data calculated for $${\text{U}}_{3}{\text{Si}}_{2}$$ and estimate its yield strength.

Hydrogen effect on the deformation evolution process in situ detected by nanoindentation continuous stiffness measurement

Nanoindentation continuous stiffness measurement has been used to study the hydrogen effect on deformation in stable 310S and metastable 304 stainless steels at room temperature. Both stainless steels are gaseous hydrogen-charged. It is first found that the extent of hydrogen effect on the mechanical property is dependent on the deformation stage. Hydrogen decreases the elastic modulus and increases the hardness in the initial stage of plastic deformation, while the hydrogen effect becomes weaker and remains nearly constant with further plastic deformation, attributing to the evolution from mobile dislocation to immobile dislocation. α′ martensite increases the elastic modulus and hardness. α′ martensite counteracts the hydrogen effect on elastic modulus, while the superposition of hydrogen and α′ martensite increases the measured hardness greatly.

Variation of the strain rate during CSM nanoindentation of glassy polymers and its implication on indentation size effect

The indentation strain rate is currently assumed to remain unvaried during continuous stiffness measurement (CSM) nanoindentation where P/P is imposed to remain constant. To probe the validity of this assumption for the nanoindentation of glassy polymers, a series of experiments have been performed at different set P/P values on poly(methyl methacrylate) and polycarbonate using CSM technique. It is firstly shown that the actual P/P value changes drastically at shallow indentation depths and it takes a considerable depth, which is material independent, for this parameter to attain a stabilized value. Furthermore, the strain rate is measured directly as the descent rate of the indenter divided by its instantaneous depth ( h/h), and indirectly via considering the variations of the load and hardness during the test. Both of these approaches reveal that the strain rate is considerably larger at shallow depths, and the depth beyond which it becomes constant is material and P/P ratio dependent. Finally, by considering the relationship between the hardness and strain rate, it is observed that although the strain rate variation alters the hardness, its contribution is not able to justify the observed indentation size effect; hence, other contributing factors for this phenomenon are discussed for their possible effects. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2179–2187

Comparison of methods for the mechanical characterization of polymers for MEMS applications

This paper gives an overview of a series of experiments conducted in order to characterize the mechanical behavior of two photodefinable epoxies. In particular, it focuses on the stiffness of the materials. The (complex) modulus is measured using five different methods: conventional nanoindentation, continuous stiffness measurement nanoindentation, nanoindentation on MEM structures, dynamic tensile tests and static tensile tests. The measured moduli range from 0.9 to 7.4 GPa for SU-8 and from 1 to 4.1 GPa for Epoclad, depending on the type of experiment and the loading speeds used in the experiments. Results reveal that conventional nanoindentation is not well optimized for polymer characterization and that there is a need for stiffness measurements using more fundamental approaches like a (dynamic) tensile test.

Determination of the True Young's Modulus of Pb(Zr0.52Ti0.48)O3 Films by Nanoindentation: Effects of Film Orientation and Substrate

PZT films were deposited by the sol‐gel method on platinized silicon substrates with silicon nitride and silicon oxide structural layer materials. The crystalline orientations of the PZT films were controlled by both a chelating agent and pyrolysis temperature. A nanoindentation CSM (continuous stiffness measurement) technique was utilized to characterize the Young's moduli of these PZT films. It was found that the measured moduli of PZT films on the two types of substrates showed similar orientation dependence (E001 > E111>E(110,111)) but distinct values, which indicated a clear substrate effect. By using a new model of film indentation, we were able to decouple the effects of film orientation and structural layer type on the Young's modulus.

Investigation of the Crystalline Orientations and Substrates Dependence on Mechanical Properties of PZT Thin Films by Nanoindentation

AbstractIn this paper, we investigated the effects of the substrates and crystalline orientations on the mechanical properties of Pb(Zr0.52Ti0.48)O3 thin films. The PZT thin films were deposited by sol-gel method on platinized silicon substrates with different types of layer materials such as silicon nitride and silicon oxide. The crystalline orientations of PZT thin films were controlled by combined parameters of a chelating agent and pyrolysis temperature. A nanoindentation CSM (continuous stiffness measurement) technique was employed to characterize the mechanical properties of those PZT thin films. It was observed that (001/100)-oriented films show a higher Young’s modulus compared to films with mixed orientations of (110) and (111), indicating a clear dependence on film orientation. The influence of substrates on the mechanical properties of PZT thin films was also characterized. Finally, no significant influence of the film thickness was found on the mechanical properties of films thicker than 200 nm.

Electromigration effect on intermetallic growth and Young’s modulus in SAC solder joint

Solid-state intermetallic compound (IMC) growth behavior plays and important role in solder joint reliability of electronic packaging assemblies. The directional impact of electromigration (EM) on the growth of interfacial IMCs in Ni/SAC/Ni, Cu/SAC/Ni single BGA ball solder joint, and fine pitch ball-grid-array (FPBGA) at the anode and cathode sides is reported in this study. When the solder joint was subjected to a current density of 5,000 A/cm2 at 125°C or 150°C, IMC layer growth on the anode interface was faster than that on the cathode interface, and both were faster than isothermal aging due to the Joule heating effect. The EM affects the IMC growth rate, as well as the composition and mechanical properties. The Young’s modulus and hardness were measured by the nanoindentation continuous stiffness measurement (CSM) from planar IMC surfaces after EM exposure. Different values were observed at the anode and cathode. The energy-dispersive x-ray (EDX) line scan analysis was conducted at the interface from the cathode to anode to study the presence of species; Ni was found in the anode IMC at SAC/Cu in the Ni/SAC/Cu joint, but not detected when the current was reverse. Electron-probe microanalysis (EPMA) measurement on the Ni/SAC/Ni specimen also confirmed the polarized Ni and Cu distributions in cathode and anode IMCs, which were (Ni0.57Cu0.43)3Sn4 and (Cu0.73Ni0.27)6Sn5, respectively. Thus, the Young’s moduli of the IMC are 141 and 175 GPa, respectively.

Nanoindentation on SnAgCu lead-free solder joints and analysis

The lead-free SnAgCu (SAC) solder joint on copper pad with organic solderability preservative (Cu-OSP) and electroless nickel and immersion gold (ENIG) subjected to thermal testing leads to intermetallic growth. It causes corresponding reliability concerns at the interface. Nanoindentation characterization on SnAgCu solder alloy, intermetallic compounds (IMCs), and the substrates subjected to thermal aging is reported. The modulus and hardness of thin IMC layers were measured by nanoindentation continuous stiffness measurement (CSM) from planar IMC surface. When SAC/Ni(Au) solder joints were subject to thermal aging, the Young’s modulus of the NiCuSn IMC at the SAC/ENIG specimen changed from 207 GPa to 146 GPa with different aging times up to 500 h. The hardness decreased from 10.0 GPa to 7.3 GPa. For the SAC/Cu-OSP reaction couple, the Young’s modulus of Cu6Sn5 stayed constant at 97.0 GPa and hardness about 5.7 GPa. Electron-probe microanalysis (EPMA) was used to thermal aging. The creep effect on the measured result was analyzed when measuring SnAgCu solder; it was found that the indentation penetration, and thus the hardness, is loading rate dependent. With the proposed constant P/P experiment, a constant indentation strain rate h/h and hardness could be achieved. The log-log plot of indentation strain rate versus hardness for the data from the constant P/P experiments yields a slope of 7.52. With the optimized test method and CSM Technique, the Modulus of SAC387 solder alloy and all the layers in a solder joint were investigated.

A Method to Extract the Intrinsic Mechanical Properties of Soft Metallic Thin Films Based on Nanoindentation Continuous Stiffness Measurement Technique

In order to determine accurately the intrinsic hardness of the soft metallic thin film on a hard substrate using nanoindentation, a proper methodology irrespective of several important effects the Oliver-Pharr method concerns is described. First, the original analysis data such as the load, P, and contact stiffness, S, as a function of the indentation depth, h, are acquired by means of the continuous stiffness measurement (CSM) technique. By CSM, the complicating effects including indentation creep behaviour of metal materials as well as thermal drift on the measured results are avoided effectively. Then, the hardness of film-only is calculated via a material characteristic parameter, P/S2, which is independent of the contact area, A, based on the constant modulus assumption method. In this way, the influences of the substrate contribution and material pile-up behaviour needn't be accounted for. Guided by above ideas, moreover, a 504 nm Au film on the glass substrate system was chosen to study. The results show that the hardness of Au thin film is 1.6±1 GPa, which agree well with the literature. While the composite hardness measured by Oliver-Pharr method is between 2∼3GPa, obviously, which is overestimated. This implies the present methodology is a more accurate and simple way for extracting the true hardness of the soft metallic thin films.

Enhanced hardness in B-doped ZnO thin films on fused quartz substrates by pulsed-laser deposition

B-doped ZnO thin films have been fabricated on fused quartz substrates using boron–ZnO mosaic target by pulsed-laser deposition technique, and the mechanical properties have been studied by nanoindentation continuous stiffness measurement technique and transmission electron microscope (TEM). Nanoindentation measurement revealed that the hardness of B-doped ZnO films, 9.32±0.90 to 12.10±1.00GPa, is much greater than that of undoped ZnO films and very close to that of traditional semiconductor Si. The mean transmittance (%) is larger than 81% in the visible range (380–780nm) for all the films, and the Hall effect measurement showed that the carrier density is around 2×1020cm−3 and the resistivity lower than 3×10−3Ωcm. TEM characteristics show undoped thin films have more amorphous area between grains while the B-doped ZnO films have thin grain boundaries. We suggest that the grain boundaries act as the strain compensation sites and the decrease in thickness of grain boundaries enhances the hardness of the B-doped ZnO films.

Comparing the Young's Modulus of Near-Frictionless Carbon Films Obtained From Different Methods

ABSTRACTThe near-frictionless carbon (NFC) thin films grown at Argonne National Lab are excellent candidates for mitigating friction and wear issues in rotating and sliding MEMS applications. In this study, NFC films with Si/Si3N4 substrates were tested with static and CSM (continuous stiffness measurement) nanoindentation methods. Furthermore, the same NFC films with 900 nm thickness were fabricated into freestanding structures for the membrane deflection experiment (MDE). The Young's modulus obtained from these three methods were compared and contrasted. The E values obtained from the nanoindentation methods were nearly constant when the indent depth was less than 100 nm (1/10 of the film thickness). Both the static and CSM indentation data indicated a clear increasing trend of the Young's modulus with the indent depth until the elastic response from the substrate started to dominate the values (depth larger than 800 nm). The average modulus obtained from static and CSM nanoindentations were 49.5 ± 4.3 GPa and 50.2 ± 1.8 GPa for as-deposited films, respectively. The values changed to 46.9 ± 5.4 GPa and 46.8 ± 2.9 GPa after microfabrication processing. The larger standard deviation values indicated the affect of the increased surface roughness to the indentation tests. The E value determined from MDE tests was 35.1 ± 2.3 GPa, which was measurably smaller those from the indentation tests. As the AFM analysis showed little/no pile-up during indentation, the difference might come from the residual stress relief, which influences the measurement, when the NFC membranes were released from the substrate.

Microfabrication and nanomechanical characterization of polymer microelectromechanical system for biological applications

Polymer microelectromechanical system (MEMS) devices are promising for biological applications such as development of biosensors and biomechanical devices. In order to develop polymer biological MEMS (BioMEMS), polymer microfabrication techniques are required, and the nanomechanics studies, including measurement of the nanomechanical properties of the polymer materials, must be carried out. This article presents the development of soft lithography based polymer BioMEMS microfabrication techniques and systematic studies on the nanomechanical characterization of the polymer thin films and beams. Poly (methylmethacrylate) (PMMA) and poly (propyl methacrylate) (PPMA) are used to make the polymer beams for MEMS integration. The hardness, elastic modulus and creep behavior of PMMA and PPMA thin films and microstructures were measured using continuous stiffness measurement nanoindentation technique, and the scratch resistance of the polymer thin films was measured using a nanoscratch technique. The elastic moduli of the polymer beams were also obtained from bending tests performed by nanoindentation. To simulate the working environment of the polymer BioMEMS, the beams were also soaked in de-ionized water or heated up to human body temperature, and the nanomechanical properties were measured.

Hardness and elastic modulus of Au/NiCr/Ta multilayers on Al 2O 3 substrate by nanoindentation continuous stiffness measurement technique

Au/NiCr/Ta soft metallic multilayered films were deposited on hard Al 2O 3 substrate by magnetron sputtering. The distribution of hardness and elastic modulus was investigated by nanoindentation continuous stiffness measurement technique. The film hardness was constant for all samples at indentation depths 1/7 ≤ h/ t ≤ 1/4, which can be considered as the true hardness. The hardness increased with increasing indentation depths at 1/4 ≤ h/ t ≤ 1. The hardness was shown to be not regular at indentation depths h/ t ≤ 1/7; this is due to the different surface roughness of films. Compared to the hardness, the measured elastic modulus was more sensitive to the substrate.