Instrumented Micro-indentation Tests Research Articles

Investigation of high-temperature plastic deformation using instrumented microindentation tests. Part II: The deformation of Al-based particulate reinforced composites at 473 K to 833 K

Constant-load pyramidal indentation tests were performed from 473 K to 833 K on P/M fabricated 2024 aluminum reinforced with either SiO2, SiC, or Al2O3 particles to investigate the influence of particulate reinforcement on the high-temperature plastic deformation process during indentation. The composites all displayed larger apparent threshold stress σth than the previously reported unreinforced P/M 2024 aluminum alloy. SEM investigation of the indentations indicated that the indentation process is accompanied by considerable cracking and interfacial debonding of the reinforcing particles, the extent of which increases with increasing temperature. The magnitude of σth was largest for the Al2O3 reinforced composite and this is attributed to the load-transfer that occurs when the indenter contacts the reinforcing particles and the superior high-temperature interfacial strength of this composite. The apparent activation energy ΔG0 of the indentation strain rate increased from 0.25µb3 at 473 K to 0.60µb3 at 833 K. These values are within the expected range for weak particles and dislocation-dislocation interactions but are lower than the previously reported Δ0 of the unreinforced P/M 2024 alloy. We conclude that the low indentation strain rate of the particulate reinforced composites is the result of the load transfer due to the presence of the reinforcements and its affect on increasing the σth. The low values of ΔG0 are consistent with our observation that extensive particle cracking and interfacial debonding occur in the reinforced material during indentation.

Application of instrumented micro-indentations to ‘in situ’ mechanical characterization of wooden structures: Part II—Analysis of different species

Flat punch instrumented micro-indentation tests were performed and analysed by using an original approach to assess strength characteristics of different wooden species. The quality of the perpendicular compression strength-indentation load correlation, using this particular type of indenter, was investigated. It is found that some characteristic force values of the instrumented micro-indentation test are very well correlated to perpendicular compression ultimate strength. In this work, the characterization on pinewood was first led, by using highly selected ‘clear’ and ‘straight grained’ materials which, at the same time, were not affected by bad orientation of annual ring as well as by a moisture level higher than 12%. Then, in order to generalize the experimental procedure, a broadening of the method to generic ‘clear’ and ‘straight grained’ pinewood, fir wood and larch wood samples, characterized by a variable moisture level and annual ring orientation, was performed. The interpretation of the load over displacement trace, the development of an experimental set-up, the execution of tests on various materials and the estimation of reliability of correlations by a strict statistic approach are also discussed in detail.

Determination of Young's modulus of electroplated nickel

Electroplated nickel is a promising material to realise movable structures for micro-electro-mechanical systems (MEMS) applications. The determination of mechanical parameters are of significant interest for their optimum design. Nickel was electroplated using a nickelsulfamate electrolyte under pulse-plating conditions. Young's modulus was determined by comparison of different measuring techniques: laser-acoustic method, instrumented microindentation test as well as resonance frequency measurements of laterally and vertically swinging cantilevers. For nickel layers deposited with a mean current density of 2 mA/cm 2, Young's modulus values of about 205 GPa were observed. The values decreased to about 165 GPa when increasing the mean current density to 20 mA/cm 2. This behaviour can be explained by texture effects.

Local mechanical characterisation and modelling of the interfacial behaviour in Hi-Nicalon/BN/α-Si3N4 ceramic matrix composites by way of instrumented microindentation tests

Microindentation tests (spherical indentation) have been used to evaluate local mechanical properties of Hi-Nicalon fibre reinforced silicon nitride ceramic matrix composite. This technique gave a satisfactory estimate of in-situ longitudinal Young’s modulus of both the matrix and the fibres which correlates quite well with 3-points bend tests. By means of the same technique, but with a different indenter (pyramidal), the interfacial behaviour has been studied. When varying the maximum applied load, Fmax, and considering a constant interfacial shear stress, the presence of Poisson’s effect has been detected. Based on Coulomb’s law of friction, a model has been proposed to extract frictional stress values adjusted for Poisson’s effect. The advantages of this model are the explicit integration of all contributions to the clamping stress at the fibre-matrix interface as well as the use of measured topography properties of Hi-Nicalon fibres.

Influence of sample deformation and porosity on mechanical properties by instrumented microindentation technique

General equations are derived for evaluating the contributions of the non-penetration displacements to the apparent Young's modulus and hardness measured in an instrumented microindentation test. An idealized disk model is proposed to simulate the roles of porosity and sample displacements in determining Young's modulus and hardness from the indentation curve. The model is then applied to experimental data obtained from the indentation tests completed with separated and bonded yttria-stabilized zirconia (YSZ) thermal barrier coating layers. It is shown that in an instrumented microindentation test of a thin material plate, large scatter in the mechanical properties may originate from non-penetration displacements which usually increase with decreasing sample thickness. Predictions are also made for the apparent E-H values and show reasonable agreement with the measured data.