A multiscale tribological study of nacre : Evidence of wear nanomechanisms controlled by the frictional dissipated power

Sheet nacre is a hybrid biocomposite with a multiscale structure, including nanograins of CaCO3 (97% wt.% – 40 nm in size) and two organic matrices: (i) the “interlamellar” mainly composed of β-chitin and proteins, and (ii) the “intracrystalline” mainly composed by silk-fibroin-like proteins. This material is currently studied as small prostheses with its tribological behaviour. In this work, the latter is studied by varying the frictional dissipated power from few nW to several hundreds mW, in order to study the various responses of the different nacre’s components, independently. Results reveal various dissipative mechanisms vs. dissipated frictional power: organic thin film lubrication, tablet’s elastoplastic deformations, stick-slip phenomenon and/or multiscale wear processes, including various thermo-mechanical processes (i.e., mineral phase transformation, organics melting and friction-induced nanoshocks process on a large range). All these mechanisms are controlled by the multiscale structure of nacre – and especially by its both matrices and respective orientation vs. the sliding direction

Mechanical properties of the elemental nanocomponents of nacre structure

Sheet nacre is a nanocomposite with a multiscale structure displaying a lamellar “bricks and mortar” microarchitecture. In this latter, the brick refer to aragonite platelets and the mortar to a soft organic biopolymer. However, it appears that each brick is also a nanocomposite constituted as CaCO3 nanoparticles reinforced organic composite material. What is the role of this “intracrystalline” organic phase in the deformation of platelet? How does this nanostructure control the mechanical behaviour of sheet nacre at the macroscale? To answer these questions, the mechanical properties of each nanocomponents are successively investigated and computed using spherical and sharp nanoindentation tests combined with a structural model of the organomineral platelets built from AFM investigations

Evaluation of the real contact area in three-body dry friction by micro-thermal analysis

Many tribological properties and wear mechanisms occurring on the micro-and nanoscale are strongly controlled by the so-called real contact area (Ar) which is a small fraction of the nominal or apparent contact area (Aa). The determination of Ar is often based on either (i) a geometrical approach describing the real geometry of contacting surfaces or (ii) a mechanical approach involving contact mechanics and physical-mechanical properties. In addition some experimental methods have also been attempted but they generally do not take into account the presence of third body at the interface—i.e. the wear debris trapped within the contact. In this paper we propose an experimental approach to estimate the dynamic real contact area from the operating parameters (Fn, v, T) and the tribological responses (μ, Ft) in presence of third body. A scanning thermal microscope (SThM) is used for determining both the thermal conductivity of the third body and the relationship between the contact temperature and the thermal power really dissipated at the micro-asperity level. These results are combined with a thermal model of the macro-tribocontact for computing the real contact area and the real contact pressure. Validation of these results is carried out using a classical Greenwood Williamson model and finite element models built from the real AFM maps

Nano-Composite Structure of Nacre Biocrystal.

Pucon - ChiliIntermittent-Contact Atomic Force Microscopy with phase detection imaging reveals a nanostructure within the tablet (Pinctada maxima). A continuous organic framework divides each tablet into nanograins. Their mean extension is 45nm. Transmission electron microscopy performed in the darkfield mode evidences that intracrystalline matrix is highly crystallized and responds like a ‘single crystal'. The organic matrix is continuous inside the tablet, mineral phase is thus finely divided but behaves in the same time as a single crystal. It is proposed that each tablet results from the coherent aggregation of nanograins keeping strictly the same crystallographic orientation thanks to an hetero-epitaxy mechanism

Multi-axis MEMS force sensor for measuring friction components involved in dexterous micromanipulation: Design and optimization.

International audienceAt the nanoscale and for particular applications such as dexterous micro-manipulation, two Degrees of Freedom nanotribometers are no longer adequate for studying and characterizing the contacts. This paper deals with the specifications and working principle of a new multi-axis friction sensor designed for nanotribological testing applied to this purpose in order to extract each contribution independently (ie, sliding, rolling and spin motion). It is composed ofa central platform with a fixed ball and surrounded by a compliant table. Its sensing ability is based on piezoresistivity: four sets of piezoresistors are symmetrically distributed at the root of four central beams. Finite Elements Method simulations are performed to find the optimal dimensions of the sensor. As results, this sensor could measure independently normal and friction forces in the range of 1 mN and 100 μN, respectively and the three rotation components. Estimated crosstalk is lower than 1 % with a good sensitivity

Development of a new nanotribometer with multi asperity contact.

International audienceIt is often difficult to make a connection between the tribological properties really assessed for a single asperity and the ones which involve the whole asperities in a microcontact. We propose a new apparatus development based on passive diamagnetic levitation (PDL) that can be used to study friction in microcontact with a lower range of force than with classical nanotribometers. This sensor measures micro and nanoforces. Its sensitive part is a ten centimeters long glass capillary tube used as a horizontal levitating seismic mass (20 to 80 mg). This rigid part is connected to a magnetic spring with stiffness between 0.01 and 0.03 N/m. The measurement range is 100μN with a resolution between one and five nanonewton. In order to validate this sensor in a multi asperity nanotribological context, this part is used to measure tangential friction forces generated by spherical micro-objects sliding on flat substrate. Results are compared with the ones provided by a classical multiasperity nanotribometer

Multiscale structure of nacre biomaterial: Thermomechanical behavior and wear processes

Sheet nacre is a hybrid biocomposite with a multiscale structure, including nanograins of CaCO 3 (97% wt.% – 40 nm in size) and two organic matrices: (i) the interlamellar mainly composed of β-chitin and proteins, and (ii) the intracrystalline composed by silk-fibroin-like proteins. This material is currently contemplated for the manufacture of small prostheses (e.g. rachis and dorsal vertebra prostheses) which are subjected to micro-slip or fretting motion. In this work, the tribological behaviour of nacre is studied by varying the frictional dissipated power from few nW to several hundreds mW, in order to assess the various responses of the different nacre’s components, independently. Results reveal various dissipative mechanisms vs. dissipated frictional power: organic thin film lubrication, tablet’s elastoplastic deformations, stick-slip phenomenon and/or multiscale wear processes, including various thermo-mechanical processes (i.e., mineral phase transformation, organics melting and friction-induced nanoshocks process on a large range). All these mechanisms are controlled by the multiscale and anisotropy of its structure – and especially by its both matrices and respective orientation vs. the sliding direction

Triboactive surfaces in multi-asperity nanotribology

In this work, n-octadecyltrichlorosilane (OTS) have been grafted on various micro-pillars created by Deep Reactive-Ion Etching (DRIE) of silicon wafers. This multi-architectured surfaces have then been tested with a ball-on-disc nanotribometer CSM Instruments (Fn: 3 mN, ball: Si3N4 1,5 mm) working in linear reciprocating mode, under various environmental conditions. Whereas the pillar's height is always fixed at 10 μm, their shapes and pitches are changed in order to test various wettability models – as Cassie-Baxter or Wenzel ones [4]. The Cassie-Baxter model can be applied in the densest pillars' area while the Wenzel one matches with weakest pillar's area. Since the frictional behavior of OTS monolayers is known to be thermally sensitive, the temperature of the structure is imposed during the tribological test by using a Peltier module

Nacre biocrystal thermal behavior

International audienceThe thermal behaviour of Pinctada margaritifera nacre was studied at different temperatures by means of thermal gravimetric, thermo-mechanical and Rock-Eval analyses. From the mechanical point of view nacre exhibited a complete reversible behaviour up to 230 °C. The bio-aragonite allotrope was seen to be as stable as the abiotic aragonite up to 470–500 °C. It was also evidenced that the organic phase was keeping cracking oxygen functions at temperatures as high as 650 °C. Nacre thermal behaviour could be described following four distinctive stages and discussed in comparison with previous data obtained in oxidative conditions

Multiscale structure of sheet nacre.

This work was conducted on Pinctada maxima nacre (mother of pearl) in order to understand its multiscale ordering and the role of the organic matrix in its structure. Intermittent-contact atomic force microscopy with phase detection imaging reveals a nanostructure within the tablet. A continuous organic framework divides each tablet into nanograins. Their shape is supposed to be flat with a mean extension of 45 nm. TEM performed in the darkfield mode evidences that at least part of the intracrystalline matrix is crystallized and responds like a ‘single crystal'. The tablet is a ‘hybrid composite'. The organic matrix is continuous. The mineral phase is thus finely divided still behaving as a single crystal. It is proposed that each tablet results from the coherent aggregation of nanograins keeping strictly the same crystallographic orientation thanks to a hetero-epitaxy mechanism. Finally, high-resolution TEM performed on bridges from one tablet to the next, in the overlying row, did not permit to evidence a mineral lattice but crystallized organic bridges. The same organic bridges were evidenced by SEM in the interlaminar sequence