Repetition: Adhesion Mechanisms

a) Mechanical interlockingb) Monolayer/monolayerc) Chemical bondingd) Diffusione) Psedo diffusion due to augmented energy input

(hyperthermal particles)

Repetition: Adhesion Measurement

a) Substrateb) Coatingc) Glued stud

Attention: the following ucertainties may be present:+ Reaction coating/glue + Diffusion of glue into coating+ After diffusion: reaction glue/

substrate+ Lack of adhesion glue/coating

Repetition: Structure Zone ModelsGrowth phases of a coating

Nucleation on substrate

Partial coalescence and interface formation

Total coalescence and poly-crystal formation

Growth of polycrystal grains

Structure zone models yield a qualitative image of film growth, morphology and crystallography in dependence on the coating parameters.

Repetition: Growth Mechanisms

Zone Mechanism Char. feature

1: T/TM<0,2

T: T/TM<0,4

2: T/TM<0,8

3: T/TM>0,8

Shadowing

Particle energy

Surface diffusion

Volume diffusion

Fibers, pores

Nano grains

Columnar crystlites

3d - Grains

Repetition : Thickness Measurement I

Amd

S ⋅ρ=

d = ThicknessρρρρS= DensityA = Substrate surface

Attention:The coating density, ρρρρS, is in most cases different from the bulk density, ρρρρD.

Repetition : Thickness Measurement IIPrinziple:Two beaminterference

Example:Multiple coatingOptical thickness:

alReOptical dndn

⋅=

⇒λ→λ

Friction and Wear

Friction:No removal of material

Wear:Removal of material associated with weight loss

F =mgg Fg

FgFgF=µr

Fg

nr FF µ=No dependence on the

extension of the interacting surfaces!

=>microscopic interaction unclear!µ = Friction coefficient

0 < µ < 4 - 5(!); µ is notconfined to values smaller 1

Friction and Wear: Measurement

Wear:+ All above methods with analysis of transfer

films and abraded coatings+ Abrasion measurement by thickness control+ Slurry-abrasion+ Special test rigs

Friction:+ Linear load (scratch-test)+ Pin on disc+ Disc on disc+ Special tribometers

Micro Hardness

Defined by the residual deformation of a material due to the penetration of an (ideally) undeformable test body.

Test body material:+ Diamond

Test body geometries:+ Vickers: Pyramid with diagonal vs. height 1:7+ Knoop: three sided pyramid+ Rockwell: sphere+ Wedge

Test loads: + 10-5 – 2 N

Micro Hardness: Test GeometryUltra micro hardness-tester, Vickers geometry:

a) Strain gageb) Samplec) Double springd) Coile) Clutchf) Base plate

Test body

This type of hardness tester can easily be implemented into an optical or a scanning electron microscope.

Micro hardness impressions

NanoindenterThe Nanoindenter also allows the determination of the elastic (reversible) deformation (i. e. of the elastic modulus) of the sample.

In the case of coatings care has to be taken that the indentation depth of the test body is less than 1/3 of the film thickness.

Only under this condition the influence of the substrate can be neglected.

Penetration depthResidual deformation

load

Elastic

modulu

s

unload

Forc

e

Non-Destructive Hardness MeasurementHertzian contact:

w(r) corresponds to the indentation depth of the test body.G and νννν result from the elastic constants of the sample:

r

z

0 w(r)

w(r)=f(F,G, )νF

Point force acting onto an ideally elastic half-plain:

G...Shear modulusνννν...Poisson ratio

G c= 44

ν =+

cc c

12

44 122( )

Determination of Elastic Constants

Elastic constants can be determined by the measurement of the sound velocity of longitudinalund transversal vibrational modes wthin a solid body.

Surface Acoustic WavesExciatation of longitudinal and transversal surface modes by a defined laser pulse:

From the runtime of the wave package the sound velocity can be determined. From this the elastic constants can be deduced.The excitation of surface wavesallows the application of this principle to thin films.

Laser pulseat time t0

Surface wavepackage

Piezoelectr.transducer

Hardness: Important Influences

The follwing material parameters may influence hardness:

+ Sress state+ Temperature+ Grain size+ Impurities+ Degree of deformation

Mechanical Properties: Spatial Resolution

By Scanning Force Microscopy the following mechanical (surface) properties can be determined spatially resolved on the nanometer scale:

+ Elastic modulus+ Hardness + Adhesion strength

This is possible by the so-called force spectroscopy

Force-Distance curves:

a/b: Approach

b/c: “Snap-on”

c/d: Repulsive region

d/f: Pull-back

e: Zero transient

f/g: Detatchment of tip

g/h: Force free pull-back

Pulsed Force Mode

free cantileveroscillation

Repated recording of force/distance curves during an AFM-scan with electronic analysis:

Topography

Adhesion

Stiffness

Polymer chains:Force-distance curve:

ArtifactsImportant artifact of force spectroscopy: Formation of a water meniscus between tip and surface under regular envronmental conditions.

Preventive measures:+ Work under dry nitrogen+ Work in liquids+ Work under inert gas+ Work under HV

The meniscus primarily modifies the values for the adhesion of the tip to the surface due to the high surface tension of water.

AFM-tip

Water meniscus

DuctilityBulk material: breaking strain εεεεb [%]

Thin film: 3-point-bending test

0

0ZB l

ll −=ε

lZ = Sample length at breaking pointl0 = Length of uncharged sampleεεεεB = Breaking strain [%]

dR2d100

B +≅ε d = Film thickness

R = Radius of curvature for first crack formation

Cracks

StressesKinds of stress:

Mechanical Stress:

σ σ σ σ= + +MECH T I

MECHσCan be triggered by clamping the substrate and subsequent relaxation

Thermal stress:Triggered by different coefficients of thermal expansion (CTE) of substrate and coating

)TT)((E MBUSST −α−α=σES ... Elastic modulus coatingααααS ... CTE, coatingααααU ... CTE. substrateTB ... Coating temperatureTM ... Temperature of stress measurement

Stresses and Film Structure

Intrinsic stress:

Iσ Intrinsic stresses are a direct consequence of the coating structure and the deposition conditions.

Tensile stressCompressive stressVariable

Tensile stress

Compressive stress

Intrinsic Stress: Sputtering

Stress Measurement: FundamentalsCurved Substrate:

Tensile stress Compressive Stress

a) Substrateb) Coatingc) Reference plate

Total stress σσσσ of a thin film:

−

ν−=σ

2s1sFs

2ss

R1

R1

d)1(6dE

ES ... Elastic Modulus substrateννννS ... Poisson Ratio substratedS ... Substrate thicknessdF ... Film thicknessRS1, RS2 ... Radius of curvature before/after coating, respectively

Stress Measurement: Interference Optics

a) Substrateb) Coatingc) Reference plate

(plane glass)d) Beam dividere) Light sourcef) to acquisition optics

DM ... Diameter m-th Newton-fringeDN ... Diameter n-th Newton-fringeλλλλ ... Wavelength of incident light

RD Dm ns

m n=−

−

2 2

4λ( )

Stress Measurement: Geometric Optics

a) Coated substrateb) Glass plate with reflecting coatingc) Beam dividerd) Displaye) Image uncoated substratef) Image coated substrateg) Incident light

y ... Sample diametery+ ... Image diameter uncoated sampley' ... Image diameter coated sampleD ... Distance sample/display

RyD

y y=

− +

2'

Stress Measurement: Cantilever

Principle:

CoatingSubstrate

Geometry:

l

α

2α

∆

δ

RS

l

H

H)2tan( ∆≅α

∆≅α

Htana

21

SRlsin =α≅α

∆≅

∆

≅ lH2

Htana

l2RS

Neglections and assumptions:a) no lateral displacement of cantileverb) no vertical displacement of cantilever(δδδδ)c) low ∆∆∆∆/H

Compressive stress

Tensile stress

Stress Measurement: X-RaysPrinciple:

Measurement of the global deformationof the elementary cell by:+ Interstitials+ Vacancies

Advantages:+ Non-destructive+ In Situ possible

Disadvantages:Numerous error sources:+ Lattice defects+ Dislocations+ Impurities+ Impurity phases