Role of Surface Defect Density on the Corrosion Properties ... Workshop...

Role of Surface Defect Density on the Corrosion Properties of PVD Coatings

Darja Kek Merl

Department of Thin Films and SurfacesJožef Stefan Institute

Jamova 39, 1000 Ljubljana, Slovenia

IJS Institute Jozef Stefan


Table of contents

•Motivation•PVD Coatings•Corrosion resistance of PVD coating•Origin Growth defects in PVD coatings•Stylus profilometry •Focused ion beam•Conclusions

Motivation

BIOMATERIALS and IMPLANTS

Corrosion resistance?Biocompatibility?

SURFACE R&DCoating(s) with betterproperties than substrate

Solution?INCREASE

OF LIFE TIME

Metal, metal alloys

•corrosion resistance•tribological properties•other functional properties (i.e.,biocompatibility, antibacterial)



Introduction PVD coatings

to protect the surface of the material while leaving the bulk intactBasic idea

unprotected material

covered by a coating

•chemical methodes (organic coating, ceramic coatings..) •electrochemical methodes•surface engineering methods•etc.

Type of coatings


Surface engineering methodsPROTECTIVE COATINGS

CVD THERMAL DEPOSITION• plasma• spray• low-pressure plasma• explosion• electric arc• plasma arc

• electrochemical (galvanic)

• electroless

THERMAL

SURFACE MODIFICATION

FROM VAPOR PHASE FROM SOLID PHASE

• sputtering• evaporation• ion plating• reactive

deposition

PVD

FROM SOLUTION

WELDING

• ion implantation• ion mixing

• fire• electric arc• plasma arc

CHEMICAL IMPROVEMENT

• classical• low-

pressure• laser• electron

BY DIFFUSION BY INSERTION

PACVD• plasma CVD• reactive

pulsed plasma

• plasma polimerization

MICROSTRUCTURAL IMPROVEMENT

MECHANICAL• induction quenching• plasma quenching• laser quenching• quenching by electron

beam

• cementing• nitriding• carbonitriding• plasma nitriding• borizing• chromatizing

• cold forming• sand-blasting

PVD – physical vapor depositionCVD – chemical vapor depositionPACVD – plasma assisted chemical vapor deposition


PVD coatings: basics

Coatings are chemicaly identicalto target material (i.e, Ti, Ti-Al,Cr)

Reactive sputtering

Coatings are nitrides, carbides, oxides of target material due to reaction with gas; N2 , O2 , C2 H2

–+

plas

ma

gene

rato

r

cathode(target)

anode

plasma

sputteredmaterial

subs

trate co

atin

g

argoninlet

argonions

reac. gasinlet

–

+

plas

ma

gene

rato

r

cathode(target)

anode

plasma

sputteredmaterial

subs

trate co

atin

gargon

inlet

argonions

Sputtering

PVD coatings: basics

high hardnessgood wear resistance chemical inertness low coefficient of friction good biocompatibility

Properties of PVD coatings Problems of coatings?

substrate

coating

• adhesion• defect density

porosity?corrosion resistance ?

defect




Table of contents


load

homogenous corosion

metal

attack

selective corrosion

porous Cu

brass (Cu-Zn)

Oxide layer

pit

pit corrosion

Crevise corrosion

narrow opening

erosive corrosion

flow

metalremoval

stress corrosion

stress

fatigue corrosion

cracks

stress

intercrystalline corrosion

intercrystalline attack

Types of corrosion

Corrosion is defined as a chemical and/or electrochemical reaction of metal with the environment, where the solid, soluble or gaseous products are formed.

There are different mechanisms of corrosion:

•uniform corrosion that takes place across the exposed surface

•local corrosion (e.g. pitting, crevise, selective), take place at specific places on the surface

•intercrystalline corrosion at grain boundaries

•stress corrosion cracking, observed on material exposed to a constant tensile stress in a corrosive environment


e.g. Zn on Fe e.g. CrN on Fe; Ag on Fe

Types of coatings

covered by a coating

Ideal case real cases

As deposited

After exposedto corrosive media

More electroneg.coatingthen base material

Less electroneg.coatingthen base material

Corrosion testing by electrochemical methodes

• 3-electrode corrosion cell;WE electrode is multilayer structure substrate/coating/electrolyte

•corrosive media; •Hanks solution; bioapplication•0.1M NaCl solution; general corrosion

Methodes:• potentiodynamic measurements (PD)at dE/dt = 1 mV/s

•electrochemical impedance spectroscopy at opent circuit potential


Potentiodynamic polarisation: measurement princip

Data from PD measurement:•Ecorr•jcorr •Eb


Voltage is applied on (WE) and the current response is measured that occurs as a result of electrochemical reactions between the electrode and corrosive media.

log j

Pot

entia

l (E

) / V

EbdE/dt = 1 mV/s

E corr

jcorr

passive region-stable oxide film

transpassive region-after corrosion products have been formed

active region-dissolution

cathodic region

Potentiodynamic polarisation: results

•corrosive media; simulated physiolagical solution (Hanks) at room T; dE/dt = 1 mV/s


1E-11 1E-10 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0,01 0,1

-0,5

0,0

0,5

1,0

1,5

2,0

E /V

vs

Ag/A

gCl

j/Acm-2

SS-316L

Ti/SS

3 kV2 kV

1 kV

DLC/Ti/SS

Ecorr more nobel material

icorrhigher corrosion resistance

Case: SS316L/Ti/DLC

substrate

coating

defect


20μm

1E-10 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

-1.0

-0.5

0.0

0.5

1.0

1.5TiAlN

CrCN

CrN mild steel (MS)E

/ V v

s A

g/A

gCl

j /Acm-2

Potentiodynamic polarisation: results

PVD hard coatings:

Ecorr more nobel material

icorrhigher corrosion resistance

corrosive media (general corrosion) 0.1M NaCldE/dt = 1 mV/s

Cases: (CrN, CrCN,TiAlN)/MS

substrate


20μm

Electrochemical Impedance spectroscopy (EIS): princip

Principe of measurements:

AC potential applied

AC current responce

appl

ied

pote

ntia

l

current response

Impedance= AC resistor=Z(jω)

substrate

coating

defect


20μm

0 20 40 60 80 100 1200

20

40

60

80

100

120

1h2h4h10h24h48h96h

-ZIm

/kΩ

ZRe/kΩ

0 1 2 3 40

1

2

3

4

Rp

Rpbetter corrosion properties

time Rp

Rpicorr

case:TiAlN/steel

Corrosion testing by EIS: results

Nyquist diagram

substrate

coating

defect


20μm

10-3 10-2 10-1 100 101 102 103 104 105 1060

20

40

60

80

1h2h4h10h24h48h96h

1h2h4h10h24h48h96h

-The

ta

ν/Hz

10-3 10-2 10-1 100 101 102 103 104 105 106101

102

103

104

105

|Z|/Ω

ν/Hz

elec

troly

teN

aCl s

olut

ion

Ccoat

ReleCdl

RcorrRcoatdefect-pore

coating

substratemild steel

elec

troly

teN

aCl s

olut

ion

Ccoat

ReleCdl

RcorrRcoat

Ccoat

ReleCdl

RcorrRcoat

ReleCdl

RcorrRcoatdefect-pore

coating

substratemild steel

defect-pore

coating

substratemild steel

Bode-Bode Diagram

Equvalent circuit - physical model

Case:TiAlN/steel

Corrosion resistance EIS: results

substrate

coating

defect


20μm

Imersion time/ h

Ecorr/mV

Qcoatμ/cm2s-nΩ

n Rcoat/ Ωcm2

Qdlμ/cm2s-nΩ

n Rcorr

/Ωcm2Rp

/Ωcm2

CrN/steel 1 -485 25.25 0.87 4450.2 14.54 0.80 4.64*103 9.09*103

10 -541 28.16 0.88 141.3 185.55 0.54 6.21*103 6.35*103

24 -565 63.12 0.81 139.7 339.23 0.56 4.72*103 4.87*103

48 -567 100.53 0.77 86.9 572.46 0.54 3.67*103 3.76*103

100 -570 256.98 0.67 90.6 834.55 0.57 2.29*103 2.39*103

Corrosion resistance EIS results

Values of fitted parameters (R, C) from equivalent circuit

A-1hA-96hB-1hB-96hC-1hC-96h

10-3 10-2 10-1 100 101 102 103 104 105 106101

102

103

104

105

106

107

|Z|/ Ω

ν/Hz

substrate

Impedance response of 316L/Ti/DLC-films in Hank solution; Bode-Bode Plot

10-3 10-2 10-1 100 101 102 103 104 105 1060

20

40

60

80

A-1hA-96hB-1hB-96hC-1hC-96h

substrate

-The

taν/Hz



Impedance response of 316L/Cr/DLC‐films in Hank solution

0 2 4 6 80

2

4

6

8-Z

Im/Ω

cm

2

ZRe/Ω cm2 1kV-1h 1kV-96h3kV-1h3kV-96 2kV,N2-1h2kV, N2-96h

-ZIm

/ MΩ

cm

2

ZRe/ MΩ cm2

0 100 200 300 4000

200

400

600

800

0 10 20 30 40 500

20

40

nn jCQ −−= )( ω

n=1, Q reduces to an ideal capacitor

QDLC

RDLC

Rele

RDLC/electrolyte

QDLC/electroyte

ε=7.5



Impedance response of multilayer structure 316L/Ti/DLC/electrolyte

Fitting with 2RQ term to determine resistance and capacitance properties of film,and film/electrolyte interface

nn jCQ −−= )( ω

n=1, Q reduces to an ideal capacitor

substrate

DLC film

TiQDLC

RDLC

Rele

RDLC/electrolyte

QDLC/electroyte

DLCelectroyte

IJS, Institute Jozef Stefan

Corrosion resistance EIS results

substrate


20μm

• PVD coating i.e. transitional metal (Cr, Ti)- nitrides bellong to the nobel materials, thefore they are chemical inert

• At the defects, pitting corrosion is formed due to galvanic cell between substrate and more nobel coatings

•Changeing the EIS spectra (reducing R, increasing icorr) with time is due to reaction take place at the substrate/electrolyte interface

Corrosion resistance of PVD coatings

substrate

coating

defect


20μm

All PVD coatings are microporous. Pores and other defects in the coating limit their corrosion resistance.

Corrosion resistance of PVD coating

coating defect


20μm

Corrosion resistance of hard coatings can be improved by reducing of surface defect density; therefore the origin of defects should be analysed.

2 μm

1 μm1 µm1 µm 1 μm

2μm10 μm


•Motivation•PVD Coatings•Corrosion resistance of PVD coating•Origin of growth defects in PVD coatings•Stylus profilometry •Focused ion beam•Conclusions


Table of contents


Introduction to growth defects

200 nm 200 nm

Growth defect features

conical growthpoor adhesion to the matrix (delamination)decrease film properties

TiAlN/CrN on hard metal

Intoduction to growth defects: defects origin

coating defect


20μm

substrate imperfections

substrate contamination

film contamination(flaking, dust, abrasion, arcing) defect removal and regrowth


after polishing after ion etching after deposition

Case: tool steel ASP30 + TiAlN coating

Substrate imperfections


Substrate imperfections

10 µm

1 µm

500 nm

VC

steel

TiAlN

1 µmHM

AlTiN/TiN

1 µmD2

TiAlN

1 µmD2

TiAlN

valleys peaks

improperpolishing

pinholeformation

carbideinclusions

Substrate contamination

dustafter etchingshade effect

voids


1 µm HM

CrN/TiAlN

1 µmCrN/TiAlN

D2 1 µmCrN/TiAlN

D2

5 µm

500 nm

1 µm

Film contamination


200 nm

Defect removal and regrowth


1 µm

1 µm

Growth defect study

What is the concentration of growth defects?What is the structure of growth defects?

density ≈ 100 mm–2



Table of contents

stylus witha diamondtip

measurementdirection

sample

Stylus profilometry: experimental

0

0,1

0,2

0,3

0,4

0,5

0 0,2 0,4 0,6 0,8 1[mm]

[µm

]


0,6

A common technique for evaluation of roughness


sharp peak narrow trough

Features:insensitive to material type or colorslowproblems in sharp edges

Optical alternative:problem with dustsize of defects comparable to λ



resolution x: 0.2 µm

Y: 1 µmZ: few

nm

tip radius: 2 µmscanning area: 1 mm × 1 mmscanning time: 1.5 h

Acquired topography

growthdefect

film surface

x

y

substrate

film

growthdefect



Statistics

roughnessheight distribution of defectsnumber of peaks, valleys

Stylus profilometry: results

µm

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

10 0.2 0.4 0.6 0.8 1 mm

mm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

density ≈ 100 mm–2



Table of contents

etching by Ga ions

A relatively new technique for observation of cross-sectionUsually integrated in a SEM

untreated surface observation by electrons

Focused ion beam: experimental


Focused ion beam: experimental

10 µmfilmsubstr.

growth defectionetching

film

growth defect

substrate seed

substrate

film

growthdefect

Sample

FIB trough etching

SEM analysisIJS Institute Jozef Stefan

Focused ion beam: results

5 µmIJS Institute Jozef Stefan

CrN on Fe

IJS, Institute Jozef Stefan


5 µm 5 µm

CrNNi

Al alloy

CrN

2 µm 2 µm

TiAlN

steel

Fe

case 1CrN / Al alloy

case 2TiAlN / steel


20 µm

2 µm 5 µm2 µm

5 µm2 µm

5µm10 µm

5 µm

a b

c d


TiAlN/on steel



Table of contents


Conclusions


Potentiodynamic measurementsCorrosion potentialCorrosion current density(corrosion rate)Break down potential Destructive methode

Electrochemical impedance spectroscopy

Corrosion potential, corrosion rateProperties of coating/electrolyte and substrate/electrolyte interfaceModeling neccessaryNon-destructive methode

Take-home messagePVD coatings are microporous, pore and defects limit the corrosion resistanceRcorr can be improved by: interlayer, thickness, multilayer, nanocomposite etc.

Conclusions


Stylus profilometrygrowth defect densitystatistical nature time consuming

Focused ion beamanalysis of one defect onlycomplete structure of the growth defectEDS: chemical composition of the seed

Take-home message

do not underestimate the role of growth defect in PVD coatingscarefully prepare the substrates and keep the lab hygene

Growth Defects in PVD coating


Thank you for your attention!

Jožef Stefan Institute, Department of Thin Films and SurfacesPeter Panjan, Miha

Čekada, Matjaž

Panjan, Srečko Paskvale, Peter Gselman

University of Maribor, Faculty of Mechanical EngineeringFranc Zupanič, Tonica

Bončina

National Institute of ChemistryGregor Kapun

Acknowledgements

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Role of Surface Defect Density on the Corrosion Properties ... Workshop...

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