Investigation of Ni/Al2O3 nacre-like composite through hot pressing of freeze-cast foamsMaxime Garnier1,2, Fernando Reyes2, Eric Maire3, David Dunand2, Andreas Mortensen1

1École Polytechnique Fédérale de Lausanne, Materials Science and Engineering Department, Laboratory of Mechanical Metallurgy, CH-1015 Lausanne, Switzerland2Northwestern University, Materials Science and Engineering Department, Structural Metallic Materials Group, IL 60208 Evanston, USA3INSA Lyon, MATEIS, CNRS UMR 5510, F-69621, Villeurbanne, France

Why mimicking nacre ?The development of materials that combine high strength and toughness is crucial for structural applications where catastrophic failures can not be tolerated. Despite being mainly composed of brittle materials, nacre, found in seashells, exhibits remarkable damage-tolerant properties. It is composed at 95vol.% of aragonite platelets that are aligned on a long-range order and bonded by a thin organic layer. For this reason, the structure of nacre is referred to as a «brick-and-mortar» structure. Its hierarchical structure exhibits several toughening mechanisms which enable its toughness to be as high as three orders of magnitude (in energy terms) greater than either its components. Hence, it makes natural nacre an ideal case study to achieve damage-tolerant materials.

Electroless nickel (EN) platingThis method enables the deposition of nickel onto non-conductive surfaces such as alumina. The deposition was realised onto alumina platelets to provide a uniform and continuous distribution of nickel throughout the composite. It was achieved by immersing the platelets into three successive chemical solutions, respectively for sensitisation, activation and deposition. The sensitisation and activation steps make their surface catalytic thanks to the adsorption of tin and palladium ions. The last solution contains nickel ions and a reducing agent that promotes their reduction and leads to the deposition of nickel onto the alumina platelets.

Freeze-castingFreeze casting consists of freezing a liquid suspension, where the solidified liquid is then sublimated to obtain a porous scaffold that is sintered for densification. The liquid suspension was realised with water as solvent because it leads to a lamellar structure which is convenient for further pressing. In addition, the lamellar growth enables to align the nickel-coated alumina platelets trapped in-between the ice lamellae. Due to the low amount of nickel deposited via the electroless nickel deposition, the nickel content of the final composite was further increased by adding nickel oxide nanoparticles into the liquid suspension. The soli-dification was forced by placing the liquid suspension on top of a cooled copper plate at -20°C for 2h at a cooling rate of 10°C/min. As the orientation of the ice lamellae is defined upon nucleation which occurs randomly on the cold surface, a wedge was used to constrain the ice to grow paralell to each others. The utilisation of a wedge enables to confine the nucleation and affect the thermal gradient during solidification which lead to a bi-directional scaf-fold. Sublimation was performed in a freeze-dryer where the pressure was decreased to 1.10-6 atm while the tempe-rature was maintained at -50°C for 48h.

Reduction/sinteringAfter freeze-casting, the consituents are weakly bonded together as they were simply trapped together in-between ice-lamellae. The structure is therefore sintered to bond the particle together which provides its structural stability. Sintering was realised in a hydrogen furnace to reduce the nickel oxide at 1050°C for 4 hours.

ShapingThe samples were shaped in a cylinder with the composite lamellae perpendicular to its longitudinal axis. This step was realised manually by using sandpapers. It enabled to press the cylinder while keeping the lamellae alignment and therefore the alignment of the platelets.

Hot pressingThe macroporosity of the structure was removed by pres-sing the cylinder along its axis to achieve a brick-and-mortar structure. Pressing was realised under a pressure of 60MPa for 2 hours at 1200°C. During this final step, sintering between the components gave the composite its optimal mechanical properties.

ObjectivesThis project investigated the development of a nacre-like composite made of micron-size alumina platelets coated with nickel to achieve a damage-tolerant composite that can withstand elevated temperature. The goal of using coated platelets was to achieve a brick-and-mortar structure where the alumina platelets act as the bricks, to provide strength to the composite, while the nickel acts as the mortar, to toughen the structure. The development of the composite emphasizes the two milestones:

• Feasibility assessment of a manufacturing process for damage-tolerant nacre-like metal matrix composite

• Investigatation of the toughening mechanism in a Ni-Al2O3 brick-and-mortar structure

EN plating• 1vol.% of nickel was deposited onto alumina• 0.02vol.% of phosphorus as sodium hypophosphite

was used as reducing agent• Layer by layer growth led to nickel clusters• Limiting rinsing after sensitisation and activation en-

hanced the deposition• Complete dewetting above 1300°C

Freeze-casting• Long-range order alignment in the cross section

(±20° misorientations)• Coarsening of the ice lamellae during solidification

leads to composite lamellae coarsening• The structure wavelength can be tuned by changing

the freezing front velocity that is affected by the free-zing conditions

• Partial alignment of the platelets within the lamellae• Random domain orientation achieved with standard

freeze-casting as seen in the top section• Realisation of a bi-directional scaffold by affecting

the thermal gradient with a wedge • The cooling rate and the freezing temperature affect

the wedge efficiency

Composites• 3 compositions were realised: alumina with 1vol.%,

10vol.% and 30vol.% of nickel• Addition of nickel oxide in the slurry leads to nickel

agglomerates after reduction that gets larger as the nickel content increases

• Inter-platelets spacing leads to high porosity (~30%) for composite with low nickel content (1vol.% and 10vol.%)

• Non-continuous distribution of mortar for nickel content of 1vol.% and 10vol.%

• Increasing the nickel content to 30vol.% reduces the porosity to 16% and provides a continuous mortar layer

• Hot pressing further improves the platelets alignment• Nickel enables to bond alumina at low temperature

(1200°C)• Large agglomerates in the freeze-cast structure leads

to inhomogeneous mortar thickness

AcknowledgementsI acknoweldge Prof. David Dunand for welcoming me in his research group and for his precious guidance throughout the development of this project. I especially thank Prof. Andreas Mortensen for this opportunity and his support along this work. Many thanks to Eric Maire, the Mateis group, the Dunand group and the thermoelectrics group.Zeno Karl Schindler foundation for supporting this project.

Conclusions• Ni-Al2O3 metal matrix composites were successfully made by combining freeze-casting and hot pressing. A high volume fraction of ceramic was achieved

by mimicking a nacre-like structure with alumina platelets and nickel content of 1vol.%, 10vol.% and 30vol.%.• Deposition of Ni-P alloy onto the alumina platelets was realised by electroless nickel deposition to provide a continuous layer of nickel. The presence of a

continuous mortar layer in brick-and-mortar structures is a critical feature that enables to toughen the structure by releasing high stresses.• The orientation of the ice lamellae was controlled by affecting the thermal gradient with a silicone wedge during solidification of the freeze-cast structure.

It enables to achieve a long-range order lamellar structure. Freeze-casting was used to partially align micron-size platelets within the lamellae.• Despite the high porosity of the structure, metal matrix composite with 10vol.% of nickel exhibits toughening mechanisms such as crack-bridging. The

asperities created by the deposition on top the platelets form inter-platelet bridges that maintain the structure strength by limiting the deformation.

March 22nd, 2017Contact: maxime.garnier@epfl.ch

2500 μm2500 μm

500 nm

Natural nacre [1]

Freeze-cast lamellae Pressed composite

Sn

Al2O3

Sn-Pd Ni

Al2O3Al2O3Al2O3

Sensitisation DepositionActivation

Solidification

Sublimation & Reduction

Shaping

Hot pressing

P

P

WedgeCopper plate Growing ice

Ni-coated Al2O3 platelets

Ice dendrite

Composite lamella

Ni mortar

Al2O3 brick

NiO nanoparticles

Ni nanoparticles

90°

0°180°

270°

Orientation map (cross section) Optical cross section

Standard freeze-casting(Top section)

Freeze-casting with a wedge(Top section)

Results

Pure alumina platelets Ni-coated alumina platelets

30vol.% Ni

10vol.% Ni 10vol.% Ni

30vol.% Ni

1vol.% Ni1vol.% Ni

• Toughening mechanisms qualitatively observed in a composite with 10vol.% of nickel

• Crack path follows the alignment of the platelets at high energy while it is deviated at low energy

• Platelets pull-out leads to tortuous crack path • Friction between platelets and the mortar lowers the

crack-driving force• Nickel coating provides asperities on the surface of

each platelet that toughen and strengthen the com-posite as it creates interlamellae bridges similar to natural nacre

Process

Reference[1] Porter et al., Biomimetic materials by freeze casting, JOM, 2013

5μm

500nm ∆T

P∆T