用纳米压痕法分析溶剂对聚醚醚酮(PEEK)力学性能的影响

Poly (ether ether ketone)(PEEK) is a high-performance semi-crystalline thermoplastic polymer.Exposure of the polymeric surface to solvents can have a strong effect like softening/swelling of polymeric network or dissolution.In this study,nano-indentation analysis was performed to study the effect of acetone on the surface mechanical properties of PEEK using different exposure time.The experiments were performed with a constant loading rate (10 nm/s) to a maximum indentation displacement (1000 nm).A 30-second hold segment was included at the maximum load to account for any creep effects followed by an unloading segment to 80% unloading.The indentation hardness and the elastic modulus were computed as a continuous function of the penetration displacement in the continuous stiffness mode (CSM) indentation.The experimental data showed that the peak load decreased from ~5.2 mN to ~1.7 mN as exposure time in solvent environment increased from 0 to 18 days.The elastic modulus and the hardness of PEEK samples also displayed a decreasing trend as a function of exposure time in the solvent environment.Two empirical models were used to fit the experimental data of hardness as a function of exposure time which showed a good agreement with the experimental values.     

ESSENTIAL WORK OF FRACTURE AND THE PHASE TRANSFORMATION IN β-iPP

The possibility of phase transformation toughening is demonstrated by the example of the β-modification isotactic polypropylene (β-iPP), which undergoes the β–α transformation (i.e., from hexagonal to monoclinic) during mechanical loading. The β–α recrystallization was examined on essential work of fracture (EWF) specimens with two processing conditions. Differential scanning calorimetry and wide-angle x-ray diffraction demonstrate the occurrence of this β–α transformation. Some structural changes can be identified in EWF specimens by using nano-indentation machine. Toughness of the α- and β-iPP is studied and compared with the EWF concept by using static-loaded deeply double-edge-notched tensile specimens. The main effect of the β–α transformation is a large increase in the specific plastic work consumed in the necked zone.     

Effects of experimental variables on PMMA nano-indentation measurements

This work aims to investigate the effects of experimental variables on nano-indentation measurements on PMMA. A wide range of conditions, including different load levels, loading rates, holding times and unloading rates were employed to examine the sensitivity of nano-indentation measurements to the selected experimental variables. The test results indicate that the elastic modulus and hardness of PMMA are approximately load-level invariant. However, they are sensitive to the loading rate, holding time and unloading rate. Both elastic modulus and hardness increase with increasing loading rate, while increasing holding time leads to decreasing elastic modulus and hardness. Moreover, the unloading rate has almost no obvious effect on the hardness of PMMA, while the opposite is true for elastic modulus.     

The structure, surface topography and mechanical properties of Si-C-N films fabricated by RF and DC magnetron sputtering

Silicon carbon nitride thin films were deposited on Co-Cr alloy under varying deposition conditions such as sputtering power and the partial pressure ratio of N₂ to Ar by radio frequency and direct current magnetron sputtering techniques. The chemical bonding configurations, surface topography and hardness were characterized by means of X-ray photoelectron spectroscopy, atomic force microscopy and nano-indentation technique. The sputtering power exhibited important influence on the film composition, chemical bonding configurations and surface topography, the electro-negativity had primary effects on chemical bonding configurations at low sputtering power. A progressive densification of the film microstructure occurring with the carbon fraction was increased. The films prepared by RF magnetron sputtering, the relative content of the Si-N bond in the films increased with the sputtering power increased, and Si-C and Si-Si were easily detachable, and C-O, N-N and N-O on the film volatile by ion bombardment which takes place very frequently during the film formation process. With the increase of sputtering power, the films became smoother and with finer particle growth. The hardness varied between 6 GPa and 11.23 GPa depending on the partial pressure ratio of N₂ to Ar. The tribological characterization of Co-Cr alloy with Si-C-N coating sliding against UHMWPE counter-surface in fetal bovine serum, shows that the wear resistance of the Si-C-N coated Co-Cr alloy/UHMWPE sliding pair show much favourable improvement over that of uncoated Co-Cr alloy/UHMWPE sliding pair. This study is important for the development of advanced coatings with tailored mechanical and tribological properties.     

A constitutive material model for a commercial PMMA bone cement using a combination of nano-indentation test and finite element analysis

To obtain accurate mechanical properties of an orthopedic Polymethylmethacrylate (PMMA-based) bone cement, nanomechanical testing was performed. Due to visoelastic characteristics of this polymer-like bone cement, the mechanical properties cannot be identified using conventional indentation methods. A well-known two-layer viscoplasticity model was selected and simulated in a finite element solver. A complete analogical study between the results of the finite element simulation and the experimental data was made to reach the best optimized parameters for the selected model. It is shown that the proposed model can be used to obtain the constitutive material relationship for polymeric materials.     

Analysis of Solvent Effect on Mechanical Properties of Poly(ether ether ketone) Using Nano-indentation

Poly (ether ether ketone)(PEEK) is a high-performance semi-crystalline thermoplastic polymer.Exposure of the polymeric surface to solvents can have a strong effect like softening/swelling of polymeric network or dissolution.In this study, nano-indentation analysis was performed to study the effect of acetone on the surface mechanical properties of PEEK using different exposure time.The experiments were performed with a constant loading rate (10 nm/s) to a maximum indentation displacement (1000 nm).A 30-second hold segment was included at the maximum load to account for any creep effects followed by an unloading segment to 80% unloading.The indentation hardness and the elastic modulus were computed as a continuous function of the penetration displacement in the continuous stiffness mode (CSM) indentation.The experimental data showed that the peak load decreased from ~5.2 mN to ~1.7 mN as exposure time in solvent environment increased from 0 to 18 days.The elastic modulus and the hardness of PEEK samples also displayed a decreasing trend as a function of exposure time in the solvent environment.Two empirical models were used to fit the experimental data of hardness as a function of exposure time which showed a good agreement with the experimental values.     

The effect of basecoat pigmentation on mechanical properties of an automotive basecoat/clearcoat system during weathering

The aim of this study is to elucidate the effect of basecoat pigmentation on mechanical aspects of an automotive basecoat/clearcoat (BC/CC) system during artificial weathering exposures. A silver basecoat as the most reflective and a black basecoat as the most absorptive basecoat were selected. These two extreme behaviored basecoats were chosen with hopes that other basecoats would behave somewhere between the silver and the black extremes. The structural and mechanical properties of the coatings were investigated after various weathering exposure times (0, 150, 300, 450, 600 h). Dynamic mechanical thermal analysis (DMTA) was carried out to study variations in structure and basic characteristics of the system such as cross-linking density and T_g during weathering. In order to investigate variations in mechanical properties of the system, nano indentation, nano scratch, and tensile tests were also utilized.It was found that although both silver and black systems experienced post-curing reactions (dominant at earlier stages of weathering) and degradation reactions (dominant at later stages of weathering), but basecoat pigmentation affected the post-curing and degradation reaction rates of the BC/CC system, leading to variations in mechanical properties. It was concluded that post-curing occurred to a greater extent in the black pigmented system whilst in the silver pigmented system weathering degradation was much more sever.     

A study of conditions for dislocation nucleation in coarser-than-atomistic scale models

We perform atomistic simulations of dislocation nucleation in defect free crystals in 2 and 3 dimensions during indentation with circular (2D) or spherical (3D) indenters. The kinematic structure of the theory of Field Dislocation Mechanics (FDM) is shown to allow the identification of a local feature of the atomistic velocity field in these simulations as indicative of dislocation nucleation. It predicts the precise location of the incipient spatially distributed dislocation field, as shown for the cases of the Embedded Atom Method potential for Al and the Lennard–Jones pair potential. We demonstrate the accuracy of this analysis for two crystallographic orientations in 2D and one in 3D. Apart from the accuracy in predicting the location of dislocation nucleation, the FDM based analysis also demonstrates superior performance than existing nucleation criteria in not persisting in time beyond the nucleation event, as well as differentiating between phase boundary/shear band and dislocation nucleation. Our analysis is meant to facilitate the modeling of dislocation nucleation in coarser-than-atomistic scale models of the mechanics of materials.     

Dwell fatigue in two Ti alloys: An integrated crystal plasticity and discrete dislocation study

It is a well known and important problem in the aircraft engine industry that alloy Ti-6242 shows a significant reduction in fatigue life, termed dwell debit, if a stress dwell is included in the fatigue cycle, whereas Ti-6246 does not; the mechanistic explanation for the differing dwell debit of these alloys has remained elusive for decades. In this work, crystal plasticity modelling has been utilised to extract the thermal activation energies for pinned dislocation escape for both Ti alloys based on independent experimental data. This then allows the markedly different cold creep responses of the two alloys to be captured accurately and demonstrates why the observed near-identical rate sensitivity under non-dwell loading is entirely consistent with the dwell behaviour. The activation energies determined are then utilised within a recently developed thermally-activated discrete dislocation plasticity model to predict the strain rate sensitivities of the two alloys associated with nano-indentation into basal and prism planes. It is shown that Ti-6242 experiences a strong crystallographic orientation-dependent rate sensitivity while Ti-6246 does not which is shown to agree with recently published independent measurements; the dependence of rate sensitivity on indentation slip plane is also well captured. The thermally-activated discrete dislocation plasticity model shows that the incorporation of a stress dwell in fatigue loading leads to remarkable stress redistribution from soft to hard grains in the classical cold dwell fatigue rogue grain combination in alloy Ti-6242, but that no such load shedding occurs in alloy Ti-6246. The key property controlling the behaviour is the time constant of the thermal activation process relative to that of the loading. This work provides the first mechanistic basis to explain why alloy Ti-6242 shows a dwell debit but Ti-6246 does not.     

The role of friction on sharp indentation

Frictional effects on sharp indentation of strain hardening solids are examined in this paper. The results of finite element simulations in a wide range of solids allow us to derive two simplified equations, accounting for the influence of the friction coefficient on hardness. Comparisons between the simulations and instrumented micro-indentation experiments are undertaken to ensure the validity of the former to metallic materials. Quantitative estimates of the role of friction on the development of pileup and sinking-in around the contact boundary are also given in the paper. These results provide a physical insight into the plastic flow features of distinctly different solids brought into contact with sharp indenters. Overall, the investigation shows that the amount of pileup can be used to set the range of validity of the two hardness equations indicated above. Friction has the largest influence on the contact response of solids exhibiting considerable piling-up effects (whose parameter , see text for details), whereas materials developing moderate pileup or sinking-in are less sensitive to friction. Finally, a methodology is devised to assess the influence of the friction coefficient on mechanical properties extracted through indentation experiments.