ICMCTF2015 Session F5: Coatings for Compliant Substrates

A common problem across various industries is to design interfacial adhesion at a certain level and then attain it via tailoring of atomic-scale microstructure with the aid of an adhesion metrology tool capable of measuring the intrinsic tensile strength or fracture energy of the interface. Once an optimized microstructure has been attained, a remaining problem is to predict the degradation of the interfacial adhesion upon exposure to deleterious service conditions. Solutions to each of these problems require expertise across various disciplines. Our research has tackled them for a wide range of interfacial systems. The talk will review these accomplishments.

The talk will start with fracture mechanics-based calculations of the required interfacial strength and toughness needed for carrying out intended function for selected engineering components. The attainment of the calculated interfacial properties through atomic-scale tailoring of the interfacial microstructure by a combination of heat treatment and interfacial segregants will be demonstrated. To achieve the desired microstructure, use of TEM and other surface science tools to characterize the interface will be discussed. To guide the development and optimization of the interfacial microstructure, the intrinsic tensile strength and intrinsic fracture energy of the interface were measured using a novel laser spallation experiment and a double cantilever beam experiment at cryogenic temperature. In the laser spallation setup, a laser-generated stress wave in the substrate pries off the film deposited on its front surface. The transient free surface velocity of the film upon wave reflection is interferometrically recorded and used in a wave mechanics simulation to calculate the tensile strength of the decohered interface. Because of the ultra-short duration of the stress wave, all plastic deformations that usually accompany the coating decohesion process are largely suppressed such that the measured value can be regarded as intrinsic to the interface system, including defects, if any.

Specific results for several interface systems of interest to the industry will be presented. These include: interfaces involving thermal barrier coatings and metal substrates; dielectric, metal, and polymer films deposited on nitride and oxide surfaces of silicon for device applications; interfaces between Cu and Pb-free solders for packaging applications; and metal/polyurea interfaces in blast-resistant structures.

The talk will end by demonstrating how the laser spallation experiment can be utilized to obtain extremely high strain rate properties of polymeric films for developing future blast resistant coatings.

There is currently a great interest in developing flexible conducting electrodes. Such electrodes are used in most electronic devices ranging from displays to solar cells to flexible bio-sensors.

To date most of these electrode components are fabricated using expensive vacuum based techniques such as sputtering, and are based on transparent conducting oxides (TCO’s). These oxides are not entirely compatible with flexible substrates under the application of relatively large mechanical stresses. Therefore, there is a need to explore novel low-cost and large-area fabrication methods to deposit ductile conducting materials alternative to brittle TCO’s.

This work focuses on Ag patterns fabricated on flexible polyethylene naphthalate substrates utilizing a nozzle-based robotic printing approach. Such lithography-free method minimizes material waste by printing exact amounts of solution-derived inks on digitally predefined substrate locations. Additionally, it allows a broad feature size range, from a few μm to a few mm, and a variety of ink viscosities allowing control of printed shapes.

In this study, we investigate the synthesis and direct writing of Ag inks, patterned-on-flex as lines and grid lattices of various sizes in the μm scale. Microstructural characterization is performed using microscopy, and X-ray diffraction. The optical, electrical, and mechanical properties of the patterns are assessed as a function of processing parameters and ink properties. Specifically, optical transmission measurements are correlated to line/grid spacing. Electrical properties are assessed through Hall effect measurements and are linked to ink heat treatment temperatures. The mechanical performance of the flexible patterns is investigated using a micromechanical tester under monotonic and cyclic tensile loading conditions up to a few thousand cycles. During mechanical characterization, the patterns’ electrical resistance is monitored in-situ using a digital multimeter. Resulting variations are associated with potential mechanical failure modes observed.

It is believed that direct writing of solution-based conductive patterns on compliant substrates may hold the key in developing the next generation of truly flexible optoelectronic devices.

The Atomic Layer Deposition (ALD) technique enables the deposition of ultrathin coatings by sequential exposure of a solid surface to pulses of precursor vapor and a reactant gas [1]. The self-limiting nature of the surface reactions ensures precise film thickness control and excellent conformality, even on complex 3D substrates [2]. These unique advantages render ALD a promising technique for applications where nanoscale coatings are required on fibrous or porous substrates, such as fuel cells, batteries, catalytic surfaces, filtration devices, membranes, etc. The successful conformal coating of 3D substrates requires, however, a careful optimization of the growth parameters. This is especially true for plasma enhanced (PE-)ALD processes. In PE-ALD, the reactant gas is activated in a plasma, generating radicals that are more reactive towards the growing film [3]. Consequently, PE-ALD often allows for deposition in a lower temperature range, and results in higher growth rates and improved film purity. However, because radicals can recombine on the sidewalls of high aspect ratio features, good conformality is often indicated as a challenge for PE-ALD.

This work explores the conformality of thermal and PE-ALD on fibrous and nanoporous materials. In a first study, we deposited Al₂O₃ on nonwoven polyester fibers and evaluated the conformality as a function of precursor exposure time and fiber density [4]. While longer exposure times resulted in deposition deeper inside the nonwoven for the thermal ALD process, the O₂ plasma-based process was found to have a very limited penetration. In a second study, we investigated ALD of TiO₂ in mesoporous titania films with sub-10 nm pore sizes [5]. Novel in situ synchrotron-based x-ray fluorescence and scattering techniques were developed to obtain cycle-per-cycle information on the material uptake and densification of the porous film. The results demonstrated the ability of thermal ALD to tune the diameter of nanopores down to the molecular level.

In addition, results will be presented where the unique advantages of ALD are exploited to functionalize the surface of porous polymers and inorganic materials. A first example concerns the modification of PTFE by PE-ALD of Al₂O₃ to provide a hydrophilic effect to the “teflon” material. Other examples demonstrate the ability of ALD to introduce catalytic sites in mesoporous silica materials [2].

[1] Puurunen, J. Appl. Phys. 97, 121301 (2005).

[2] Detavernier et al., Chem. Soc. Rev. 40, 5242 (2011).

[3] Profijt et al., J. Vac. Sci. Technol. A 29, 050801 (2011).

[4] Musschoot et al., Surf. Coat. Technol. 206, 4511 (2012).

[5] Dendooven et al., Chem. Mater. 24, 1992 (2012).

Alumina thick and thin films can be used for a variety of purposes, including the semiconductors, electronics, substrate and dielectric, piezoelectric and ferromagnetic devices. The films can be produced by conventional methods such as tape casting and roll compaction from ceramic slurry. However, these methods offer limited part geometries and cause defects in the film structure. Plasma electrolytic oxidation is a novel surface engineering technology, allowing relatively thin and thick oxide coatings to be formed on metals. Alumina ceramics were produced by conversion of a rectangular shape aluminium foil (15 × 15 × 0.05 mm) using a plasma electrolytic oxidation technique. To achieve better understanding of the coating formation on the foil substrates the PEO process was carried out for 6 minutes in electrolytes containing various proportions of alkali, silicate and phosphate salts. The coating thickness, morphology and phase composition were analysed using SEM and XRD techniques. The effect of the electrolyte contents on the current density, surface morphology and phase composition of oxide layers were studied. Preliminary results indicate that the percentage of conversion to alumina reaches ~46%. The increase of potassium hydroxide concentration leads the discharge phenomena to occur at lower voltages. Moreover the addition of silicate in the electrolyte can increase the activity of discharge as well as coating surface roughness. It was found that the pore size increases with increasing the electrolyte concentration, which may affect coating properties. Oxide layers generated in phosphate solutions show pancake-like morphology, while the volcano-like structures are observed in the coatings prepared in silicate rich electrolytes.