Localized adhesion mechanisms in an assembly

One of the great challenges of modern times is to reduce the weight of objects. In the automotive, aerospace or naval sectors, this is the result of an approach to reduce energy consumption. Indeed, lighter means of transport induce a reduced consumption of energy (fuel, electricity).
To achieve this, several solutions exist, such as the miniaturization of electronic components, the use of alloys (magnesium, aluminum), thermoplastics or lighter composite materials... Another possibility consists in gluing the elements to be assembled rather than screwing or welding them together.

Adhesion between two materials is a science in itself. In an assembly, a multitude of combinations must be considered.

In aerospace, solid propulsion engines are made up of a thermal protection and a propellant. A thin layer of glue called binder holds the assembly together and requires a high level of requirement concerning :

• the inter-material bonding of the assembly: thermal protection (TP)/binder and binder/propergol
• binder/filler adhesion within the propellant, which is highly charged with oxidizing nanoparticles of various sizes and chemical natures

To achieve these performance objectives, the R&D teams must have a high level of physical, chemical and mechanical knowledge of the strategic areas of the assembly, such as the macro and micro interfaces between the oxidizing charges and the propellant matrix.

The AFM will provide information on the mechanical properties of the materials in the vicinity of the interfaces of the PT/liner/propergol assembly. The ToF-SIMS will provide information on chemical interactions and migration phenomena between materials.

These two analytical techniques give access to a better knowledge of the adhesion mechanisms in very localized areas (a few tens to several hundreds of nm) on either side of the two interfaces of the assembly.
 

Atomic Force Microscopy (AFM) is a technique that allows to visualize with a nanometric resolution the three-dimensional morphology of the surface of a material, and to map some of its properties (adhesive, mechanical, magnetic, electrical, ...). The principle of AFM is based on the measurement of the different interaction forces (ionic repulsion forces, Van-der-Waals forces, electrostatic forces, etc...) between the atoms of the surface of the sample to be observed and the atoms of a nanometric probe tip, fixed under a flexible microlever.

Different types of probes/microlevers can be used to obtain a qualification and a quantification of the various physical properties of the surface.

The Peak Force Tapping and QNM modes allow the simultaneous acquisition of 3D topography and mechanical parameters. The force applied on the tip is controlled in order to preserve the integrity of the sample and the AFM tip. The approach-retraction curves (2 kHz) are obtained (force spectroscopy) and analyzed in real time in order to extract the mechanical parameters (Young's modulus, tip-surface adhesion, deformation...). In QNM mode, the measurements become quantitative after calibration of the tip.

ToF-SIMS allows to detect with a very high sensitivity traces of elements up to ppb and molecules up to femtomole.

A pulsed source of mono or multi-atomic primary ions (Ga⁺, Bin⁺, Au⁺, C₆₀⁺, ...) with an energy of a few keV bombards the sample surface. The secondary ions resulting from the interaction between the primary ions and the sample are then focused and accelerated with the same kinetic energy towards the time-of-flight analyzer which separates them according to their m/z ratio with a very good mass resolution (ΔM/M > 10 000 at mass 28).

TESCAN ANALYTICS has nearly 30 years of expertise in the use of AFM and ToF-SIMS on all types of materials. With the latest generation of instruments, our team of experts works with all industrial sectors.

Figure 1: Binder/thermal protection interface with adhesion problem (PF QNM)

Figure 3: Binder/thermal protection interface without adhesion problems
 

The adhesion of the different layers is an essential property for the good combustion of solid propellant in rocket engines. When a macroscopic adhesion defect is observed, an investigation of the mechanical properties at the micrometer scale can provide solutions.

Figure 1 shows the presence of an interphase at the interface binder (right)/thermal protection (left) which displays a smooth topography and a high Young's modulus. It can be schematized as a dotted cut along the two assembled layers. This defect weakens the adhesion between the bonder and the thermal protection, making the assembly unusable.

A comparison of the results obtained via the Peak Force QNM mode of AFM and by nanoindentation was carried out on different propellant formulations (Table 1). The measurements acquired by the two techniques correspond, the AFM can thus be used in replacement of a nanoindenter on isotropic materials.

A complementary analysis of the linker/thermal protection interface using ToF-SIMS (Figure 2) showed the increased presence of the cross-linking agent used in the linker formulation. This migration at the interface with the thermal protection is accompanied by a local over-crosslinking of the linker and explains the higher Young's modulus measured by AFM. 

A modification of the linker formulation allowed to correct the adhesion defect and to obtain an interface as in figure 3.

The use of physicochemical investigation techniques at the nanometric scale, such as AFM and ToF-SIMS, gives access to complementary and precise information in localized interfacial zones for a better understanding of the mechanical behaviors observed at the macroscopic scale.

The understanding of bonding mechanisms in macro assemblies and/or charge/bond adhesion requires observations at the local submicron scale.

For more analysis applications with AFM or ToF-SIMS, ask us for information.

IR or XPS can also provide useful information for the understanding of adhesion mechanisms and/or the analysis of adhesion defects at less localized levels.