Simulation of a Dental Tribological ... -...

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1 www.mikrotribologiecentrum.de © Fraunhofer-Institut für Werkstoffmechanik IWM Simulation of a Dental Tribological System at a Microscopic Scale Christian Nutto, Sandra Sarembe, Matthias Scherge, Claas Bierwisch Fraunhofer IWM, MikroTribologie Centrum, Germany

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Simulation of a Dental Tribological System at a Microscopic Scale

Christian Nutto, Sandra Sarembe, Matthias Scherge, Claas BierwischFraunhofer IWM, MikroTribologie Centrum, Germany

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Outline

Motivation for the investigation of an oral hygiene -μtribological system

Experimental and numerical approach

Testing the parameter space of the „toothbrush – toothpaste – enamel“ system

Comparison between simulations and experiments

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Motivation

The abrasive character of toothpaste is dependent on hardness, volume filling factor and size of the abrasive particles (Relative Dentin Abrasion (RDA) value)

The addition of abrasive particles in the toothpaste is based solely on empirical data.

Tribological investigations of the cleansing effect of toothpastes on the surface of the enamel are very limited

The introduction of numerical models with a predictive power for abrasive suspensions could yield an important contribution for the development of dental hygiene products.

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Microtribological investigations

Reduction of the complex tribological system toothbrush – toothpaste – enamel to a microscopic level in order to investigate the interactions between the basic components:

Experimental characterisation of the friction and wear behavior -tribological investigationsμ

Development of smoothed particle hydrodynamic (SPH) simulations for the numerical investigation of the interactions between basic components of the tribosystem

200 µm

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Experimental approachThe Setup

Experimenta tribological system: Filament – Toothpaste-Slurry – Enamel

For the determination of the coefficient of friction, (µ), a toothbrush filament is mounted on the force transducer of a tribometer (Basalt PT, Tetra) or on a nanoindentor (Nano Indenter G200, Agilent)

While being forced on a rotatory movement over the enamel, a normal force Fn [mN] is applied in the vertical

direction on the filament

Determination of the tangential force Ft

Enamel

FilamentSlurry

n

t

FF

µ =

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Numerical approachParameter space

Suspension:• Volume filling factor• Rheologic character

of carrier fluid• Viscosity

Abrasive particles:• Size• Shape

Filament:• Shape• Velocity• Tilt angle of filament

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Numerical approachParameter space

Suspension:• Volume filling factor• Rheology of carrier

fluid• Viscosity

Workpiece: • Young‘s modulus:

7.5 kPa• Poisson‘s ratio:

0.3

Abrasive particles:• Size• Shape

Filament:• Shape• Velocity• Angle between

Filament and Enamel

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Numerical approachNumerical model – Geometrical setup

Abrasive Particle: Diameter 50 mμ Resolution: 10 mμ Shape: Sphere

v

Filament: Diameter 200 mμ Round tip Path-controlled:

• V=0.15m/s

1mm

0.4mm

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ResultsDisplaying all components

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Results – Stress distributionParameter study: viscosity

Investigation of the induced stress into the enamal as an indication of the abrasive process

Investigation of the behavior of the involved abrasive particles

1mPas1mPas

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Results – Stress distributionParameter study: viscosityHigher viscosity:

20mPas20mPas

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Results – Stress distributionParameter study: viscosity - Comparison

20mPas20mPas1mPas1mPas

• Lower capture probability of abrasive particles for lower viscosities• Higher viscosity yields larger induced stress into the enamal by the abrasive

particles• Discrete stress input outside of the sliding wear track

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ResultsSliding wear track in the experiment

Wear track after 8000 cycles of a cylindric filament embedded within a toothpaste slurry on the enamel.

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ResultsViscosity dependend pressure distribution

1mPas1mPas

20mPas20mPas

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Experimental resultsAre there stick-slip effects?

Wear track [ m]μ

Toothpaste (RDA 30)

stick-slip effect

Enamel

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ResultsGeometrical setup – new model

v

FN

Improved model: Geometrical shape of enamel:

(600x450x200) mμ Filament movement:

Force-Path-controlled

Suspension: Volume fraction of abrasive

particles: 20% Ø = 50 m μ Viscosity: 1mPas

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Resultsstick-slip effect in the simulations

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ResultsVariation of tilt angle

• Inclined filament possesses higher capture probability of abrasive particles and hence higher risk of abrasive processes on the enamel

0° 0° 30° 30°

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Summary

Improved knowledge about the fluid mechanical mechanisms in toothpaste slurrys due to the comparsion of experimental and numerical investigations

Gained insight due to microscopical simulations and experiments: Higher viscosities of the toothpaste slurry raises the induced stress into

the enamel by the abrasive particles Numerical investigations support the idea about a broad stress

distribution across the wear track Larger tilt angle raises capture probability of abrasive particles by the

filament and hence increases abrasion Suggested stick-slip effects seen in experiments is supported by the

numerical investigations