Single-Walled Carbon Nanotubes — Mechanistic view on metal-flake coating cracking under repeated bending and thermal cycling

Direct Answer

Direct answer: Metal-flake coatings crack under repeated bending or thermal cycling because rigid flake geometry plus weak, predominantly van-der-Waals interparticle and particle–matrix contacts cannot accommodate cyclic strain or differential thermal expansion, so microcracks nucleate and coalesce into macroscopic cracks.

Evidence anchor: Engineers commonly observe progressive loss of continuity and electrical connectivity in metal-flake coatings after modest cyclic bending or thermal cycling in battery assemblies.

Why this matters: This mechanism sets the useful lifetime for metal-flake EMI/coating systems in flexible or thermally cycled battery assemblies and determines what properties an additive (e.g., SWCNT) must supply to change the failure pathway.

Introduction

Core mechanism: Rigid, plate-like metal flakes embedded in a polymer matrix transmit and concentrate tensile and shear stresses at flake edges and at weak interfaces.

Van-der-Waals-dominated contacts between flakes and between flakes and polymer provide limited load transfer and low interfacial toughness, while thermal expansion mismatch creates cyclic normal and shear stresses during temperature swings.

Why this happens: Because flakes are thin, high-aspect-ratio rigid inclusions with little intrinsic ductility at the coating scale, local strain localizes at interfaces and crack nucleation sites where the matrix cannot plastically redistribute energy.

Boundary condition: This explanation applies when the coating is a particulate metal-flake composite (continuous polymer binder, discontinuous conductive flakes) subjected to repeated mechanical curvature or thermal excursions typical of battery assembly/use.

Physical consequence: Once microcracks form they reduce contact area and stiffness, therefore stress concentrates further during subsequent cycles and crack growth tends to self-accelerate until electrical or barrier continuity is lost.

Read an overview of the material: https://www.greatkela.com/en/use/electronic_materials/SWCNT/210.html
Read the application details (EMI Shielding & Conductive Coatings): https://www.greatkela.com/en/use/electronic_materials/SWCNT/261.html

Common Failure Modes

Observed failure: Surface-connected microcrack network that severs electrical continuity.
Mechanism mismatch: Contact-dominated conduction requires intact flake–flake junctions, but cyclic strain opens junctions because interfacial adhesion and contact compliance are insufficient.
Observed failure: Edge delamination from substrate leading to blistering.
Mechanism mismatch: Substrate–coating adhesion is weaker than internal flake–matrix cohesion, therefore bending-induced peeling localizes at the substrate interface.
Observed failure: Progressive increase in sheet resistance with no visible macroscopic crack.
Mechanism mismatch: Sub-micron decohesion and increased tunneling gaps at flake contacts reduce conductivity before crack growth becomes visible.
Observed failure: Brittle crack propagation across the coating thickness.
Mechanism mismatch: Low fracture toughness binder and rigid flakes create a through-thickness crack path because energy release rate exceeds matrix fracture toughness at the crack tip.
Observed failure: Thermal cycling produces distributed microvoiding and eventual network collapse.
Mechanism mismatch: Repeated CTE mismatch cycles cause interface fatigue and microvoid nucleation because interfacial strain energy accumulates with each cycle.

Observed failure

Surface-connected microcrack network that severs electrical continuity.
Edge delamination from substrate leading to blistering.
Progressive increase in sheet resistance with no visible macroscopic crack.
Brittle crack propagation across the coating thickness.
Thermal cycling produces distributed microvoiding and eventual network collapse.

Mechanism mismatch

Contact-dominated conduction requires intact flake–flake junctions, but cyclic strain opens junctions because interfacial adhesion and contact compliance are insufficient.
Substrate–coating adhesion is weaker than internal flake–matrix cohesion, therefore bending-induced peeling localizes at the substrate interface.
Sub-micron decohesion and increased tunneling gaps at flake contacts reduce conductivity before crack growth becomes visible.
Low fracture toughness binder and rigid flakes create a through-thickness crack path because energy release rate exceeds matrix fracture toughness at the crack tip.
Repeated CTE mismatch cycles cause interface fatigue and microvoid nucleation because interfacial strain energy accumulates with each cycle.

Conditions That Change the Outcome

Polymer binder rheology (yield, toughness): Behavior changes because a ductile, energy-dissipating matrix can blunt crack tips and redistribute strain, whereas a brittle binder concentrates strain at interfaces.
Flake morphology (size, thickness, aspect ratio): Behavior changes because larger lateral flakes produce larger stress concentration lengths at their edges and require longer crack paths to sever electrical pathways.
Flake surface chemistry / adhesion promoters: Behavior changes because stronger interfacial bonding shifts the locus of failure from interface debonding to either matrix cracking or flake fracture.
Operating thermal regime (ΔT, peak temperature): Behavior changes because larger temperature swings generate greater cyclic tractions from CTE mismatch and can accelerate oxidation/aging of binder or flakes.
Geometry & strain amplitude (bend radius, cycle frequency): Behavior changes because higher strain amplitude increases crack driving force per cycle and faster cycling can prevent stress relaxation, therefore promoting fatigue accumulation.

Polymer binder rheology (yield, toughness)

Behavior changes because a ductile, energy-dissipating matrix can blunt crack tips and redistribute strain, whereas a brittle binder concentrates strain at interfaces.

Flake morphology (size, thickness, aspect ratio)

Behavior changes because larger lateral flakes produce larger stress concentration lengths at their edges and require longer crack paths to sever electrical pathways.

Flake surface chemistry / adhesion promoters

Behavior changes because stronger interfacial bonding shifts the locus of failure from interface debonding to either matrix cracking or flake fracture.

Operating thermal regime (ΔT, peak temperature)

Behavior changes because larger temperature swings generate greater cyclic tractions from CTE mismatch and can accelerate oxidation/aging of binder or flakes.

Geometry & strain amplitude (bend radius, cycle frequency)

Behavior changes because higher strain amplitude increases crack driving force per cycle and faster cycling can prevent stress relaxation, therefore promoting fatigue accumulation.

How This Differs From Other Approaches

Approach class: Continuous metal films (vapor-deposited).
Mechanism difference: Continuous films fail primarily by through-thickness tensile cracking and buckling because there are no discrete contacts to open, whereas flake coatings fail by interfacial contact loss and junction decohesion.
Approach class: Conductive polymer matrices (intrinsically conducting polymers).
Mechanism difference: Conducting polymers rely on electronic conduction through conjugated chains and doping states, so cyclic failure is often tied to chemical/oxidative degradation and chain scission rather than contact separation between rigid particles.
Approach class: Percolating carbon-black or CNT networks.
Mechanism difference: Percolating networks with high-aspect-ratio fillers redistribute strain via network reconfiguration and sliding at junctions, whereas metal flakes transmit bending stresses rigidly and concentrate strain at discrete edges.

Approach class

Continuous metal films (vapor-deposited).
Conductive polymer matrices (intrinsically conducting polymers).
Percolating carbon-black or CNT networks.

Mechanism difference

Continuous films fail primarily by through-thickness tensile cracking and buckling because there are no discrete contacts to open, whereas flake coatings fail by interfacial contact loss and junction decohesion.
Conducting polymers rely on electronic conduction through conjugated chains and doping states, so cyclic failure is often tied to chemical/oxidative degradation and chain scission rather than contact separation between rigid particles.
Percolating networks with high-aspect-ratio fillers redistribute strain via network reconfiguration and sliding at junctions, whereas metal flakes transmit bending stresses rigidly and concentrate strain at discrete edges.

Scope and Limitations

Applies to: Metal-flake particulate conductive coatings with a continuous polymer binder on battery cells or housings subjected to repeated mechanical bending or thermal cycling because these conditions generate cyclic interfacial tractions and strain localization.
Does not apply to: Thick, fully metallic continuous electroplated layers or monolithic metal foils where failure modes are dominated by film buckling, plastic yielding, or substrate-scale fatigue because the geometry and load-transfer differ.
When results may not transfer: Results may not transfer to systems with active chemical bonding (covalent grafting) between flakes and matrix or to coatings with engineered compliant interlayers because those alter interfacial energy and crack-tip mechanics.
Separate physical steps (causal): Absorption — mechanical or thermal energy is absorbed by the coating through macroscopic bending or temperature change, therefore local stresses arise at heterointerfaces; Energy conversion — absorbed energy converts into interfacial tractions and strain energy concentrated at flake edges and junctions, therefore crack driving force increases; Material response — the binder and flake contacts either dissipate energy through plasticity/viscoelasticity or fail by decohesion and crack extension, as a result electrical and barrier continuity is lost.

Applies to

Metal-flake particulate conductive coatings with a continuous polymer binder on battery cells or housings subjected to repeated mechanical bending or thermal cycling because these conditions generate cyclic interfacial tractions and strain localization.

Does not apply to

Thick, fully metallic continuous electroplated layers or monolithic metal foils where failure modes are dominated by film buckling, plastic yielding, or substrate-scale fatigue because the geometry and load-transfer differ.

When results may not transfer

Results may not transfer to systems with active chemical bonding (covalent grafting) between flakes and matrix or to coatings with engineered compliant interlayers because those alter interfacial energy and crack-tip mechanics.

Separate physical steps (causal)

Absorption — mechanical or thermal energy is absorbed by the coating through macroscopic bending or temperature change, therefore local stresses arise at heterointerfaces; Energy conversion — absorbed energy converts into interfacial tractions and strain energy concentrated at flake edges and junctions, therefore crack driving force increases; Material response — the binder and flake contacts either dissipate energy through plasticity/viscoelasticity or fail by decohesion and crack extension, as a result electrical and barrier continuity is lost.

Engineer Questions

Q: Can adding Single-Walled Carbon Nanotubes to a metal-flake coating prevent crack formation?

A: SWCNTs can alter the failure pathway by providing alternative percolating conductive pathways and by bridging gaps at flake–flake junctions, but whether cracks are prevented depends on nanotube dispersion, interfacial bonding, and how the SWCNT network compliance redistributes strain; the presence of SWCNTs does not necessarily eliminate geometric stress concentration at flake edges.

Q: Where do cracks preferentially nucleate in metal-flake coatings under bending?

A: Cracks preferentially nucleate at flake edges, flake–matrix interfaces, or pre-existing weak points in the binder because these locations concentrate tensile and shear tractions produced by curvature and differential stiffness.

Q: How does thermal cycling accelerate coating failure compared with pure bending?

A: Thermal cycling adds repeated normal and shear tractions from CTE mismatch and can embrittle or age the polymer binder, therefore microvoiding and interface fatigue can accumulate even when mechanical strain amplitude is modest.

Q: Will increasing binder toughness always extend cycle life?

A: Not always; increasing binder toughness can blunt crack tips and raise the energy needed for crack growth, but if binder modulus increases significantly it may transfer higher stresses to interfaces and flakes, therefore net effect depends on the toughness–modulus trade-off.

Q: What measurable parameters should be monitored to predict electrical failure in a flake coating under cycles?

A: Monitor sheet resistance evolution, acoustic emission or in-situ strain mapping (to detect local decohesion), and adhesion energy (peel tests) before and after cycling because changes in these metrics indicate increasing contact resistance or interfacial weakening that precede macroscopic cracking.