den Jmj Jaap Toonder

Measuring mechanical properties of coatings: a methodology applied to nano-particle-filled sol–gel coatings on glass

The main aim of this paper is to demonstrate the practical use of nano-indentation and scratch testing in determining mechanical properties of thin coatings. We place our emphasis on how information obtained using both techniques can be combined to give a more complete representation of the properties of a coating–substrate system. Part I of the paper gives an overview of approaches to determine mechanical properties of thin coatings that have been proposed in the literature, and develops them further to be useful tools in the analysis of coatings. This results in methods for measuring the mechanical properties of thin coatings. We particularly emphasise the determination of the elastic modulus, hardness, coating and interfacial fracture toughness and residual stress using indentation and scratch testing. Part II of the paper illustrates the application of these methods to relatively soft coatings of methyltrimethoxysilane (MTMS) filled with colloidal silica or alumina particles on glass. The coatings were prepared using a sol–gel process. We report results of the dependence of the mechanical properties on the filler particle content, illustrating that microstructural changes can also be tracked using these techniques. The effects of the nature and volume fraction of the filler particles are discussed.

Drag reduction by polymer additives in a turbulent pipe flow: numerical and laboratory experiments

In order to study the roles of stress anisotropy and of elasticity in the mechanism of drag reduction by polymer additives we investigate a turbulent pipe flow of a dilute polymer solution. The investigation is carried out by means of direct numerical simulation (DNS) and laser Doppler velocimetry (LDV). In our DNS two different models are used to describe the effects of polymers on the flow. The first is a constitutive equation based on Batchelors theory of elongated particles suspended in a Newtonian solvent which models the viscous anisotropic effects caused by the polymer orientation. The second is an extension of the first model with an elastic component, and can be interpreted as an anisotropic Maxwell model. The LDV experiments have been carried out in a recirculating pipe flow facility in which we have used a solution of water and 20 w.p.p.m. Superfloc A110. Turbulence statistics up to the fourth moment, as well as power spectra of various velocity components, have been measured. The results of the drag-reduced flow are first compared with those of a standard turbulent pipe flow of water at the same friction velocity at a Reynolds number of Re τ ≈1035. Next the results of the numerical simulation and of the measurements are compared in order to elucidate the role of polymers in the phenomenon of drag reduction. For the case of the viscous anisotropic polymer model, almost all turbulence statistics and power spectra calculated agree in a qualitative sense with the measurements. The addition of elastic effects, on the other hand, has an adverse effect on the drag reduction, i.e. the viscoelastic polymer model shows less drag reduction than the anisotropic model without elasticity. Moreover, for the case of the viscoelastic model not all turbulence statistics show the right behaviour. On the basis of these results, we propose that the viscous anisotropic stresses introduced by extended polymers play a key role in the mechanism of drag reduction by polymer additives.

Elastic modulus, indentation pressure and fracture toughness of hybrid coatings on glass

Abstract The indentation load-displacement behavior of an organic-inorganic hybrid coating was tested using a Berkovich indenter in an attempt to offer a simple and fast method to analyze the mechanical properties of a coating. The coatings were deposited using a spin-coating technique. The elastic modulus and the indentation pressure as a measure of the hardness were determined on the basis of the load-displacement curve. The effects of the coating thickness and the coating preparation conditions were investigated. Cracks, delamination and chipping were observed and were used to assess the fracture toughness of the coating and the interface. Elastic modulus, indentation pressure and the fracture toughness were dependent on the time elapsed before application of the coating fluid and on the curing temperature.

Nature-inspired microfluidic propulsion using magnetic actuation

In this work we mimic the efficient propulsion mechanism of natural cilia by magnetically actuating thin films in a cyclic but non-reciprocating manner. By simultaneously solving the elastodynamic, magnetostatic, and fluid mechanics equations, we show that the amount of fluid propelled is proportional to the area swept by the cilia. By using the intricate interplay between film magnetization and applied field we are able to generate a pronounced asymmetry and associated flow. We delineate the functional response of the system in terms of three dimensionless parameters that capture the relative contribution of elastic, inertial, viscous, and magnetic forces.

A micro-indentation method for probing the craze-initiation stress in glassy polymers

Initiation of a localised plastic zone is numerically simulated using a constitutive model that incorporates an accurate description of the post-yield behaviour with the important phenomena of strain softening and strain hardening. Subsequent nucleation of voids in the deformation zone is detected using a hydrostatic stress criterion. This criterion is identified and validated. A micro-indenter with a sapphire sphere is used to produce indents that are later examined with an optical microscope. These observations lead to a critical force at which crazes are initiated in polystyrene. Combining these experiments with a numerical study shows that the loading part of indentation can be accurately predicted. A critical hydrostatic stress of 40 MPa is extracted from the numerical model by monitoring of the local stress field at the moment that the indentation force reaches the experimentally determined force level at which crazes were found to initiate. This criterion is validated by indentations on samples with different thermal histories, and at various loading rates. Finally, the influence of network density on the value of the hydrostatic stress criterion is investigated by indentation of blends of polystyrene and poly(2,6-dimethyl-1,4-phenylene-oxide). It is shown that the critical hydrostatic stress increases with network density.

Determination of the elastic modulus and hardness of sol–gel coatings on glass: influence of indenter geometry

Indentations have been carried out on methyltrimethoxysilane coated float glass by using a spherical and a Berkovich indenter. The composite hardness as well as the effective elastic modulus were determined as a function of indentation depth for coatings of a thickness 0.5, 2 and 4 μm. Using the Berkovich indenter, the coating exhibited radial cracking (only for the two thicker coatings), delamination and chipping, whereas no cracking occurred during indentations using a spherical indenter. For the experiments in which the coatings exhibited radial cracking, we measured a constant composite hardness over the whole depth range instead of the expected increase. The elastic modulus, on the other hand, turned out to be insensitive to the radial cracking. All results show the expected increase of the modulus with indentation depth. The spherical indenter showed a much steeper rise of the effective elastic modulus with indentation depth than the (sharp) Berkovich indenter. By re-scaling the results with respect to contact area instead of indentation depth, the Berkovich and the sphere elastic moduli are comparable. Only at the point where delamination occurred in the Berkovich indentations, the re-scaled results of the two indenter geometries start to deviate.

Numerical simulation of flat-tip micro-indentation of glassy polymers: influence of loading speed and thermodynamic state

Flat-tip micro-indentation tests were performed on quenched and annealed polymer glasses at various loading speeds. The results were analyzed using an elasto-viscoplastic constitutive model that captures the intrinsic deformation characteristics of a polymer glass: a strain-rate dependent yield stress, strain softening and strain hardening. The advantage of this model is that changes in yield stress due to physical aging are captured in a single parameter. The two materials studied (polycarbonate (PC) and poly(methyl methacrylate) (PMMA)) were both selected for the specific rate-dependence of the yield stress that they display at room temperature. Within the range of strain rates experimentally covered, the yield stress of PC increases linearly with the logarithm of strain rate, whereas, for PMMA, a characteristic change in slope can be observed at higher strain rates. We demonstrate that, given the proper definition of the viscosity function, the flat-tip indentation response at different indentation speeds can be described accurately for both materials. Moreover, it is shown that the model captures the mechanical response on the microscopic scale (indentation) as well as on the macroscopic scale with the same parameter set. This offers promising possibilities of extracting mechanical properties of polymer glasses directly from indentation experiments.

High-Q integrated RF passives and micromechanical capacitors on silicon

The PASSI/spl trade/ technology platform is described for the integration of low-loss inductors, capacitors, and MEMS on high-ohmic Si substrates. Using this platform the board space area taken up by e.g. impedance matching circuits can be reduced by 50%. The losses of passives induced by the semi-conducting Si substrate can effectively be suppressed using a combination of surface amorphisation and e-beam irradiation. The incorporation of MEM tuneable capacitors in high-Q inductor-capacitor networks is demonstrated.

Quantitative assessment and prediction of contact area development during spherical tip indentation of glassy polymers

This paper describes the development of the contact area during indentation of polycarbonate. The contact area was measured in situ using an instrumented indentation microscope and compared with numerical simulations using an elasto-plastic constitutive model. The parameters in the model were obtained using macroscopic tests. Indentations were performed on samples with different thermal histories and at different speeds. For all cases, the numerical model correctly predicted the development of the contact area during indentation. For increasing strain rates, the contact area decreased at equal indentation depths. Annealing the samples resulted in a smaller contact area at equal indentation depth. Using only numerical simulation, it was also shown that pile-up around the indenter resulted from localization effects and was, thus, promoted by strain-softening properties of the indented material. Strain hardening, on the other hand, will tend to promote sink-in. Finally, we performed simulations of load relaxation during indentation. The results indicate that about 40% of the total observed relaxation may be assigned to plastic effects.

Foil-to-Foil System Integration Through Capillary Self-Alignment Directed by Laser Patterning

This paper introduces a new integration technology for cost-effective high-precision mechanical and electrical integration of mesoscopic functional foil components onto foil substrates. The foil-to-foil assembly process is based on topological surface structuring via laser patterning that enables accurate capillarity-driven self-alignment of foil dies. The concurrent establishment of high-yield electrical interconnections is obtained through conductive adhesives. The foil surface energy controls the acceptance window of initial offsets for optimal self-alignment performance. The proposed topological patterning and system design enable alignment accuracies for centimeter-sized foil dies as high as 15 μm, barely influenced by the evaporation of the assembly liquid and curing of the conductive paste. Full foil-to-foil system integration is demonstrated through the electrically functional assembly of an array of Au-sputtered capacitive humidity sensors onto a patterned base foil circuitry.