slides for coating techniques part 1 gfe schmalkalden uni

1

Principles of Coating Technology I 1

GFE Schmalkalden e.V.

Principles of Coating Technology

Part I - Basics 1. Motivation 2. Tribology and Wear 3. Surface properties and characterization 4. Pre-and post-treatment 5. Deposition methods - Overview

Part II – Deposition methods 6. Painting 7. Electroplating and anodic oxidation



1. Motivation

2


GFE Schmalkalden e.V. Motivation

4.5 % of the national product of industrialized nations

>50 Mrd € are material and energy loss by wear and corrosion on metallic parts

5 % of sales

are used for repair and maintenance (in wear intensive industry 10-15%)

10 % of production costs are maintenance costs

10 % of wear parts in industrial equipment

are involved in 70% of failures and disorders

50 % of wear loss are preventable

Some Numbers



Loss of material and energy by wear and corrosion must be avoided

• Preventable maintanence

„to act is better than to react“

• Using functional material for machine function

wear and corrosion protection as design elements

Surface protection by functional coating and surface technology

3



Source: Siemens AG

Costs (€) for annual production of 1 Mio units

tools

maintanance

Coating

Without coating

BALINIT®- coating

31.800 15.700

17.200

800

total 16.500 49.000

Annual coating costs (200 different models)

800 x 200 =

Savings per annual (200 different models)

32.500 x 200 =

160.000

6,5 Mio

Motivation

Example: Production of phones



Industrial Application

Wear protection in mining

Coating of wear parts

Print roll (printing industry)

Power generation (turbine blades)

4



Using coated „band saw“ to separate submarine „Kursk“ in 2001



Hip implant

Synchronrings

EUROFLAMM

Engine block

Household devices

Motivation

Application samples of coated parts

5



Application samples of coated parts

Industry Coated components

aerospace Landing gear, airbrakes

Industrial gas turbines Blades, abrasive coatings, cumbustion coatings

engines Cylinder holes, wear coatings, Synchronrings

Paper, printing, steel industry Printing and transportation rolls

Oil and gas equipment Pump engines, seals, shafts, compressor shafts

Medicine Implants, x-ray targets

Textile machines rolls

Other industries Components and parts

Consumer goods Flat irons, writing utensils, frying pan

On-site maintenance Steam generator, paper rolls, gas turbines



2. Tribology and Wear

6


GFE Schmalkalden e.V. Tribology and Wear

Loss of material and energy by wear and corrosion must be avoided

enviroment

intermediary

load

Counter body

Base body

velocity

Tribological system metal mineral Liquid gaseous

liquid solid gaseous

metal mineral plasic elastomere

liqud gas dust temperarture

Protection of the surface by coating system or surface modification Consideration of the tribological system



Consideration of the tribological system Determination of the wear mechnism

Wear combination loading Wear type Mechanism

Solid/solid sliding

rolling

impact

vibrations

sliding wear

rolling wear

impact wear

vibration wear

adhesion / abrasion

adhesion / fatigue

fatigue / adhesion

fatigue

Solid/liquid flowing

impact

cavitation

droplet erosion

fatigue

fatigue

Solid / gas with solid particles

flowing

impact

sliding jet wear

impact jet wear

abrasion

fatigue

Verschleißpaarungen

VE

RS

CH

LP.C

DR

Verschleißpaarung Beanspruchung Verschleißart Mechanismus

Festkörper /Festkörper

Festkörper/Flüssigkeit

Festkörper/Gas mit Fest-stoffpartikeln

Gleiten

Rollen

Prallen

Schwingen

Strömen

Prallen

Strömen

Prallen

Gleitverschleiß

Wälzverschleiß

Stoßverschleiß

Schwingverschleiß

Kavitation

Tropfenschlag

AdhäsionAbrasion

AdhäsionErmüdung

Ermüdung

AdhäsionErmüdung

Ermüdung

Ermüdung

Abrasion

Ermüdung

Gleitstrahlverschleiß

Prallstrahlverschleiß


VE

RS

CH

LP.C

DR





Gleiten

Rollen

Prallen

Schwingen

Strömen

Prallen

Strömen

Prallen

Gleitverschleiß

Wälzverschleiß

Stoßverschleiß

Schwingverschleiß

Kavitation

Tropfenschlag

AdhäsionAbrasion

AdhäsionErmüdung

Ermüdung

AdhäsionErmüdung

Ermüdung

Ermüdung

Abrasion

Ermüdung




VE

RS

CH

LP.C

DR





Gleiten

Rollen

Prallen

Schwingen

Strömen

Prallen

Strömen

Prallen

Gleitverschleiß

Wälzverschleiß

Stoßverschleiß

Schwingverschleiß

Kavitation

Tropfenschlag

AdhäsionAbrasion

AdhäsionErmüdung

Ermüdung

AdhäsionErmüdung

Ermüdung

Ermüdung

Abrasion

Ermüdung



Tribology and Wear

7



Wear depth

Coating wear

Corrosion wear

Adhesive wear Abrasive wear

Surface fatigue wear Base material

Oxide-Reaction zone

Distubtion by forming and modified chemical composition

Adsorption layer

Outher unrelated

surface layer

Inner related surface layer



Progress of wear

Lin

eare

r V

ers

ch

leiß

be

trag

Abrasio

n

Weg, Zeit

Adhäsion

Tribochem. Reaktion

Oberflächenzerrüttung

Source: Uni Dortmund, LWT

Lin

ear

wea

r

Distance, time

8



Reduction of wear

Reduction of abrasion: high hardness with adequate ductility Hard phases in a ductile matrix Reduction of adhesion unrelated surface layer, lower adhesive bonding force material with heterogeneous structure Reduction of fatigue: high strength with high ductility avoiding of stress concentrations Reduction of thermal fatigue high thermal strength reduction of loading by thermal insulation layers Reduction of tribo-oxidation: avoiding of reactive layers



Important physical processes

Adsorption: Accumulation of liquid oder gaseous materials

(adsorbens) on the surface of solid parts (adsorbat)

Saturation of bonding states on the surface leads to the

reduction of the fee energy (steady state)

Physical adsorption

• Interaction of induced or permanent

dipoles (Van-der-Waals-forces)

• Adsorption heat 4 – 40 kJ/mol

• Process is reversible

Chemical Adsorption

• Formation of a chemical bonding

• Adsorption heat 40 – 400 kJ/mol

• Process is not reversible

Absorption: infiltration of gaes or gaseous mixtures by diffusion processes in a

condensed (solid) phase

The absorbed gas will be dissolved in a steady state at defined

temperatures and concentrations, molecules will be dissociated to

atoms

9




Adhesion: adhesive forces on the contact area of two (liquid or solid) material

• Physical and chemical adhesion • Mass attraction • Mechanical clamping

Cohesion: cohesive forces within a material or body

• Primary bonding chemical bonding

• Secondary bonding partial bonding, Van-der Waals Bonding




Wetting: Formation of a contact or boundary angle at the boundary between a solid and

a liquid

In the steady state the Young Equation is valid:

𝜎𝑆 = 𝜎𝑆𝐿 + 𝜎𝐿 ∗ cos 𝜗 𝜎𝑆 , 𝜎𝐿: surface tension of the solid and the liquid 𝜎𝑆𝐿: surface tension between solid and liquid 𝜗: contact angle

Wetting: 𝜗 < 90°

No wetting: 𝜗 > 90°

Complete wetting: 𝜎𝑆 > 𝜎𝑆𝐿 + 𝜎𝐿

10



3. Surface properties and characterization



Deposition is the application of an adherend coating of shapeless material on a component.

Surface properties and characterization

Definition of the coating deposition processes

Function of coatings are

• Decoration

• information (signals)

• corrosion protection

• wear protection

• physical effects (diffusion barrier, flame barrier, thermal isolation, electrical isolation, …)

11


GFE Schmalkalden e.V. Surface properties and characterization

Processes for surface modification

• mechanical (e.g. shot peening)

• thermal (partial laser hardening)

• thermo-mechanical (e.g. hot isostatic

pressing

• Thermo-chemical (e.g. nitriding)

• Pure metals (chromium, zinc, gold, …)

• Alloys for special applications (e.g.

corrosion)

• Anorganic, non-metallich (enamal,

ceramics…)

• Organic (pintings, polymers)

• Compounds

Materials for surface modification



Coating application fields

Coating of

new parts

optics

Repair coatings

High

Temperature

protection

Oxidation

protection Wear

protection Corrosion

protection

Elcetrical

properties

Bio-

activities

Heat

insulating

decoration

Bear-

ring

12



Relevant surface properties

primary

• Chemical composition

• Phase composition

• Structure and microstructure

• Residual stress

• Surface roughness

secondary

• elasticity

• hardness

• strength, fatigue strength

• friction and sliding properties

• corrosion resistance

• wear resistance

• optical properties (colour, coverage)

• electrical / thermal conductivity

Important: characterization of the properties



key property: coating adhesion

adhesion mechanism:

• Mechanical clamping

• Adhesion

• Diffusion

• Chemical bonding

• Electrostatical forces

adhesion is influenced by

• Surface energy

• Material properties (e.g. strength,

conductivity, …)

• Surface material interaction

• Bonding mechanism

• Residual stresses (High residual

stresses lead to coating delamination

• …

Adhesion > coating strength Adhesion < coating strength

13



Measurement of coating properties

• coating thickness • coating adghesion strength • Hardness • Wear resitance at diferent loads • Friction coefficient • corrosion properties • Thermal properties • Mechanical properties (ductility,

elasitcity, stresses, … • Electrical properties (conductiviy,

resistance, … • Optical properties (color, brillance, …) • Surface roughness • … • …

Methods for measurements

• Metallographic investigations

• Corrosion behaviour

• Wear behaviour

• Thermal behaviour

• Mechanical behaviour

• Optical behaviour

• …

• …

Destructive and nondestructive tests are possible



Methods for measurements

Mechanical values of coating Bond strength test (<80 MPa) cupping test bending test measurement of residual stresses test of fatigue strength creep behaviour thermal shock test

Metallographic evaluation structure and microstructure micro- / macro- hardness phase boundaries Interface coating / substrate pores and pore distribution roughness

corrosion behaviour salt-spray test, thermal test, current density potential test

Wear behaviour Taber-Abraser-Test; pin on disc test; vibrational wear ….

Extremely high number of other tests with or without standards

14



Measurement examples Measurement of coating thickness d - Simple and cheap procedure - Fast measurement - Destructive method

parameters: grinding time ball diameter Preparation of a calotte grinding - friction pair coating / ball) - Measurement of the diameter

of the grinded calotte



Measurement examples Measurement of wear behaviour - Destructive method - Pin-On-dic method - Determination of the friction and

the wear coefficient

• Methode Ball with defined load Interaction with a coated surface Friction between ball and surface Measurement of Wear trace; Determination friction coefficient :(FR = FN * µR)

• Parameter: Normal force [N] Rotational speed [s-1 ; m-1] Speed [m/s ; m/min] Trace diameterLaufspurdurchmesser [mm] Friction length [m] rotation Friction time [min]

15



Measurement examples Measurement of coating hardness • Micro harndess messurement • Measurement of the indentation of an

indenter (diamond pyramide) with difined load

• Determination of hardness by relation of geometriy and load



Measurement examples Measurement of coating adhesion strenth • Scratch-Test • Diamond indenter will be moved with a defined

load along the coated surface • At critcal load: coating delamination or crack

formation • Determination and evaluation of the scratch

Moving direction of the probe

Acoustic emission sensor

Diamond indenter

Depth sensor

16



Measurement examples Measurement of roughness • Mechanical method (tactile scanning) • Measuring sensor are moved with

constant speed along the surface • Recording of the different roughness

values

R a = 0.2 - 0.45 µm R a = 0.07 - 0.15 µm



4. Pre-and post-treatment

17



Process steps during coating deposition

1. Pre-treatment - cleaning - Surface activation - roughening

2. Coating deposition - Determination of coating technology and parameters - Surface protection

3. Post-treatment

- Homogenization of the coating - Additional improvement of the properties

4. Measurement of coating quality g - Mechanical values - metallographic values - Wear and corrosion

Pre- and Posttreatment



Pre- Treatment


Cleaning

- removal of dirt - removal of oils and greases - removal of paints

Blasting

- roughening - decontamination - activation

Blasting materials

- sand (improper due silkose) - corund (Al2O3) - grit - pellets - carbides

Ultrasonic assited claning - removal of blasting residues

18



Post- Treatment


Objective: homogenization of the coating or the surface near areas to improve the

properties (e.g. corrosion resitance, wear resistance)

- reduction of porosity

- Smoothing of the surface

- Improvement coating adhesion

- Reduction of residual stresses

- Improvement of coating hardness and ductility

- Closing of cracks



Post- Treatment


Thermal Post-Treatment: flame (z.B. acetylen/ oxygen)

arc (tungsten inertgas weld method)

Laser beam

Electron beam

induction

Thermo-mechanical hot isostatic pressing (HIP)

Mechanical final expanding hammering shot peening simultaneous spraying and peening

19



Thermal Post-Treatment


Fuly or partial remelting of the surface with and energy source (flame, laser beam, …) for

- Smoothing of rough surfaces

- Dissolving of unwantet phases

- Creation of additional hard phases

- Hardening of thin layers

Example: Laser remelting

Bewegungsrichtung

Laserstrahl

Bauteil

Moving direction of the probe

Laser beam

Coated part

Requirements and parameters:

• Avoiding of cracks due to different thermal expansion coefficients

• Pre-heating possible

• Adjustment of process parameters

• Feed rate

• Laser power

• Laser focus point

• Shielding gas

• …





Heat treated Ti-coating with reaction zone

Electron beam surface remelted NiCrAl coating

Laser remelted TiMo coating

20





glue

coating

Probe

Adhesive tensile strength

According to DIN EN 582

w i e g e s p r i t z t u m g e s c h m o l -

z e n m i t

A u f m i s c h u n g

u m g e s c h m o l -

z e n m i t

R e a k t i o n s -

z o n e

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

Ad

hesiv

e t

en

sil

e s

tren

gth

[M

Pa]

w i e g e s p r i t z t u m g e s c h m o l -

z e n m i t

A u f m i s c h u n g

u m g e s c h m o l -

z e n m i t

R e a k t i o n s -

z o n e

5 7

3 8 , 5

> 6 5

As sprayed remelted fusion

remelted Reaction zone

coating

substrate

modification of the adhesive strength by different thermal post treatment processes - Remelting without reaction zone: adhesive strength is reduced - Remeling with reactive zone: increasing of adhesive strength



Thermo-mechanical Post-Treatment


Hot isostatic pressing (HIP)

- Material will be loaded with high temperature and high pressure in one process step

- Pressure carrier is inert gas

- HIP will be used for compacting of poros structures (eg sintering of ceramics)

Processes during HIP

• diffusion

• creeping

Modification dring hip

• Structural compacting, removal of pores

• Phase formation within the coating

• Grain growth wihin coating

21



Thermo-mechanical Post-Treatment


Hot isostatic pressing (HIP) - Example

Production of metal matrix composites (MMC)

- Twisting of fibres on a base body

- Deposition of the metalic matrix

- Hot isostatic pressing

Using for brake drums, cylinder sleeves, drive shafts

Spritzpistole

Verbundwerkstoff

Faser

Stützkörper(geschliffen)



mechanical Post-Treatment


Shot peening

Objective of mechanical post-treatment • compacting • Reduction of surface roughness

Characteristics of shot peening • Ball shaped blasting material off metal, ceramic, glass • Process similar to sand blasting • Velocity of blasting material 15-150 m/s

Simultaneous shot peening • Compacting of the whole coating • Optimization of corrosion resistance • Induction of residual compressive stresses

Thermal sprayed Ni20Cr coating before and after shot peening

22



5. Deposition methods - Overview


GFE Schmalkalden e.V. Deposition methods - Overview

Classification of Deposition methods

Deposition by

welding

Deposition by

soldering

Deposition

from gaseous or voporized

state

Deposition from ionized

state by electrolytical or chemical deposition

Deposition

from solids or powders

Deposition

from liquid or pasty state

PVD / CVD Galvanic methods

Anodic methods

Powder coating with polymers or metalls

Painting Hot dipping

23


GFE Schmalkalden e.V. Deposition methods - Overview

Important methods

Deposition by welding

• Fusion welding (autogenously, metal inert gas, tungsten inert gas plasma welding with wire or powder, submerged , laser welding )

• Pressure welding (roll cladding or explosive plating)

Deposition by brazing

• fusion soldering (with gas, metal inert gas, plasma, oven)

Deposition from gaseous or voporized state

• Physical Vapor Deposition

• Chemical Vapor Deposition

• Combination of PVD and CVD methods

Deposition from ionized state

• Galvanic methods with and without an external current generator

• anodic methods



method Diffused element -

Media

temperature [°C]

Aufkohlen C Gas, Paste, Pulver, Salzbad 800 - 1050

Carbonitrieren C, N Gas, Plasma, Salzbad 600 - 930

Nitrieren N (H) Gas, Plasma 350 - 550

Nitrocarburieren N, C (O, H) Gas, P lasma, Pulver, Salzbad 350 - 600

Oxidieren O Gas, Salzbad 150 - 550

Oxinitrieren N, O Gas ~ 500

Sulfidieren S Salzbad 200

Sulfonitrieren N, S Gas (Plasma) ~ 600

Sulfonitrocarburieren N, C, S Salzbad (Plasma) 570 - 580

Borieren B Gas, Paste, Plasma, Pulver 800 - 1000

Vanadieren V Pulver, Salzbad 850 - 1100

Chromieren Cr Gas, Pulver, Salzbad 900 - 1200

Chromvanadieren Cr, V Pulver 1000

Niobieren Nb Pulve r 1000 - 1100

Alitieren Al Gas, Pulver, Salzbad ~ 1200

Silizieren Si Pulver 930 - 1200

Stannieren Sn galv. Überzug 580

Manganieren Mn Pulver 1000 - 1100

Important thermochemical methods

Deposition methods - Overview

24



Built-up-welding 6 mm

CVD / PVD 0,001 - 0,1 mm

chemical Ni 0,03 - 0,3 mm

galvanic Cr 0,01 - 0,5 mm

thermal spraying 0,05 - 3 mm

build up soldering 0,1 - 1 mm

build up welding 2 - 20 mm

roll cladding 2 - 12 mm

TiN-

coating

PVD

Thermal spraying

50 µm

APS-

Al2O3

Typical coating thickness of some methods




10 -5

10 -4

10 -3

10 -2

10 -1

1 10 100

galvanic Cr

Coating thickness [mm]

vaporation

sputtering

Ion plating

CVD / PECVD

plasmapolymerisation

galvanic Ni

Chemical Ni

Flame spraying

Plasma spraying

Arc spraying

Build-up welding

Roll cladding

Hot dipping

Typical coating thickness of some methods


25



Thermal spray

0.1 1 10 100 1000 10000

1000

800

600

400

200

0

Coating thickness (mm)

Su

bstr

ate

tem

pe

ratu

re (

°C)

PVD

II

Thermal spraying

Build-up welding

Chemical methods

(II = Ion implantation

CVD

coating thickness and substrate temperature of some methods




Part II – Deposition methods 6. Painting 7. Electroplating and anodic oxidation

26


GFE Schmalkalden e.V. Painting

Pre-tratment

Mechanical:

• Brushing

• Blasting

Chemical:

• Etching

• lubricating

Painting systems are based on:

• pre-treatment is normally ncessary

• Using of mechanical or chemical oricesses

• Water

• Alcohole

• Organic solvents (Trichlorethylen, Toluol, …)



Mostly used industral method of painting: spray painting

Compressed air spraying - Using nozzle with defined geometry - High velocity of paint droplets - Formation of a droplet jet

Airless-nozzle Finespray-nozzle Spritzlackieren

- Spaying of the material by high pressure, high velocity or elcectrostatic fields - Ball shaped droplets are accelerated toward to the substrate

27



Compressed air nozzle

- Using compressed air

- High velocity differenz destroyed paint surface

Airless nozzle

- High pressure of the paint

- Expansion of the paint after the nozzle exit

- Priniple is used in sprays

Electrostatic nozzle

- Mechanical nebulization of the paint

- Electrostatic acceleration of the paint to the substrate

- Extremly low paint losses

nozzle geometries



Dipping

Cathodic hot dipping

- Using a dipping bath

- Dipping of the components into the bath until a fully wetting

- Hardening of the paints at the air or in a stove

- Hot dipping processes are use for mass production

http://www.youtube.com/watch?v=UQfDGNUIkr8

28



Modifications of dipping process

Electrical hot dipping Conventional hot dipping:

• only wetting of the surface

• no additional solvents

Electrical hot dipping

• Chemical modification of the paint paint droplets

• Coagulation of paint droplets on the surface

• Hardening by heat treatment

Surface of the Component



Paint particles in aqueous solvent

Paint droplets coagulated on the

surface

Paint surface after hardening

Advantages of the dipping process Disadvantages of the dipping process

• Good automatable

• Complete painting of the components

(cavities, beadings,…)

• Low consumption of paint

• Irregular paint surface possible

• High investment costs

• High quantity of paint necessary („dipping bath“)



Hot dipping

Modifications of hot dipping process

• Zinc coating (450 – 530° C)

(galvanizing)

• Hot dip tinning (300°C)

• Aluminum coating (700°C)

• Lead coating (380 °C)

Pre treatment of the hot dipping process

• Librication in an alcalic bath

• Etching with salt acid or sulfurid acid for a

metallic surface

• Purging of the surface to remove formed salts

• Tratment with fluxes of zinc chloried or

aluminum chloride

• Precision cleaning and drying

• dipping of e metallic part in a molten metal bath

• Formation of solid or liquid reaction products at the boundaries

• After removing: solidification of the adhere pint layer

29



Hot dipping – zinc coating

Wet zinc coating

• Removal of etched parts without drying

• Flux is floated on the bath surface

• Parts are dived through flux

Hot-dip galvanizing

• Diffucion between iron and zinc

• Formation of an alloy layer of iron and zinc

(intermerallic fe-Zn phases)

• Deposition of a pure zinc layer during

removal of parts

• Corrosion improvement by formation of oxidic and carbonatic passivation layers



Powder coating

• Processing of solvent-free, dry and fre-flowing powder of metals or plastic

Plastics:

• Epoxy resin (EP)

• Epoxy /polyester resin (EP/PES)

• Polyester resin (PES)

• Polyacrylic resin (PAC)

• Polyuretan PUR)

• Polyethylen (PE)

• Polyamide (PA)

• Ethylen-Vinylalkohol-Copolymerisat

• Thermosetting resing

• plastomere

Pre treatment of the powder coating process

• different pre-treatment processes

depending on application and material

• Degreasing

• Etching

• Phosphatizing (steel parts)

• Chromating (aluminum parts)

• Blasting with corund

30



Powder coating - Powder spray coating

• Up to 100 % of material will be used

• Coating thickness 40 - 120 µm

• Specific paint sprayer with continously powder feeding and deed unit

• Processing gas: compressed air

• Electrodes generating a potential difference between sprayer and grounded part

• Powder particles (10 bis 80 µm) will be charged and accelerated by compressed air and electrostatic potential

• Condenstation of the particles on the surface

Continous oven

chain conveyor

Pre treatment unit

Podwer spray chamber

High-voltage generator with powder feeder

Poder recycling unit

give in give out



Powder coating – whirl sintering

• Coating thickness 200 - 500 µm (in special cases 1000 µm)

• Component are pre-heated to sintering temperature

• Dipping the component into the whirl sintering bath

• Wetting of the component by floating powder particles

• After short dipping melting of the particles to a solid coating

• Cooling on air or in water (on air lead to a smoother surface)

• Useful for small components and large number of pieces

• Good automation

air

Powder cloud

Porous Intermediate

layer

http://www.youtube.com/watch?v=wuDF43V3mOQ

http://www.youtube.com/watch?v=gnS_A55aVFc

31



Powder coating – rotation sintering

• Shake or rotation sintering

• Internal coatings in pipes

• Pre-heating to sinter temperature

• Filling with powder

• Processing time 7- 10 s

• Homogenous layer by heating and shaking

give out

pipes

Heating

cooling

Powderdeposition

Smoothing



Powder coating – powders and powder properties

properties unit Powder type

PE PVC PA EP EVAL

Pre treatment: blasting yes yes yes yes yes

Etching / phosphatizing (without passivation)

no no no yes yes

Adhesion layer yes yes yes yes yes

Processing temperature °C 320-360 270-360 270-360 200 180-360

Post treatmend yes yes yes yes no

Minimal sheet thickness mm 1 1 1 - 0,5

Shore hardnees 70 80 95 95 85

Melting tange °C 105-110 70-150 186 - 105-108

Thermal expansion K-1 2,5 x 10-5 8 x 10-5 12 x 10-5 4 x 10-5 13 x 10-5

Heat conductivity W/Km 0,35 0,15 0,29 0,15 0,28

Spez. heat kJ/kgK 2,3 0,98 2,4 1,7 1,9

Water absorption (24 Std. RT) % 0 0,2 0,8-1,5 0,3-1,5 0,2

Chemical stability good moderate moderate moderate good

Weather resistance bad moderate moderate bad good

32



7. Electroplating and anodic oxidation


GFE Schmalkalden e.V. Electroplating and anodic oxidation

electroplating

Electroplating (wtih external energy source)

• Using an electrolytic bath

• Part will be used as cathode (metallic layer)

• Metallic ions of electrolyte are transfered

by the external electrical field to the

cathode

• Metallic ions are reduced to metal atoms

• Oxidation of the remianing of anions on

the anode

• Anode is composed from coating material

KathodeAnode

Metallionen

Non-uniform coating thickness at cavities and on edges due to concentration of electical field lines

Current free electroplating

• Ion exchange method (exchange of ion between anode and solved ions)

• Reduction method (deposition of solved ions by reduction of the electrolyte)

• Contact method (short time contact by an ignobly material)

33



electroplating

• Metals (steel, copper and copper alloys, zinc, aluminum and aluminum alloys

• Plastics

• ceramics

• Electrical non conductive material must be pre-coated with a conductive coating (e.g. Au, Cu, Al)

Pre-tratment is nessecary

• Removal of rust and scale salts, lubricants, oils, soaps, paints

• Removal of coarse conatamination: etching, grinding, burning

• Removal of fine contaminations: olishingdegreasing, deoxidation

Electroplating and anodic oxidation



electroplating: some basics

ions

• Positve or negative charged particles

• Formation of ions by dissoziation of electrolytes

• Cations: metallic ions (Na+) ; anion: acid radicals (Cl-)

Complex ions

• Ions of more atoms

Reaction on the anode

• Dissolvabel anode: anode will be oxidized

• Insolvabe abbode: parts of the electrolyte will be aoxidized; formation of gases

34



Electroplating: some basics

electrolyte

Electrolytes are acid or alcalic liquid or non-liquid solvents with ions

• acid: metal sulfates or chlorides

• Alcalic: complex cyanides or metal oxygen combinations

Components of the electrolyte

• Metal carrier (salts)

• Conducting salts (for high current densities)

• Buffer materials (for constant pH value)

• Wetting agents ( for dissolving dirt)

• Brightener (organic materials for a more unifcorm and fain grained coating)

• Defoaming agent

• Supenser (to influence electical fields)

• Complex creator (to mask unwanted metals



Electroplating: some basics

Cathodic deposition:

• Faraday equation to determine deposition rate

• Deposition rate can be used to control deposition rate

Fz

MtIm G

th

a) b) c)

Grundwerkstoff

Beschichtung

Coating characterisitics

• nonunifom coating thickness distribution

• Specification of minimum thickness necessary

• Reduction of surface roughness by deposition: higher deposition rate within holes

Mg atomic mass of the metal

z ionic charge

F Faradya constant (96.496 C/mol)

mth theoretical deposit mass

I current

t time

35



Methods of deposition

Bath electroplating

• for large components

• Expensive equipment

• High current for deposition

Exampe of

bath electroplating

Trommel or cavity electroplating

• For mass production (loose material)

• Rotating boxes (trommel shape)

• cathodic current supply by the loose material

• No bright surface possible



Cathodic nickel deposition

Substrate materials:

• Steel

• Zinc and zinc alloys

• Copper and copper alloys (often adhesion layer for nickel coatings)

electrolytes

• Matt nickel (WATTS-electrolytes) (coating hardness 155 -200 HV)

• Bright nickel (coating hardness 380 – 480 HV)

• Electrolyte composition influences coating properties (structure, hardness, optics)

Exampe of

Hard nickel cylinder

36



Cathodic chrome deposition

Coating properties

• High hardness

• High wear and corrosion resistance

• Reduced friction coefficient

• Reduced affinity to adhesion

• High gloss

Application

• Decorative coatings (thickness < 2 mm)

• Functional coatings on barrels, cylinders, forms, … (tickness up to 500 mm, with microcracks)

Used cylinder

Chrome plated cylinder

High gloss chrome cylinder



Cathodic chrome deposition

Substrate materials:

• Steel

• Cast steel

• Copper and copper alloys

electrolytes

• Chromimtrioxide CrO3 (acutely poisonous)

• Anorganic acid as catalyzer(H2SO4, HF)

• Cr3+ ions

• Wetting agent

• Aluminum

• Nickel

• High stiftness of the substrate necessary (due to the brittleness of chromim)

Hard chrome coating: higher CrO3 content

Bright chrome: lower CrO3 content

37



Cathodic zinc deposition

applications

• Corrosion protection on plates, bands, pipes, small parts, …

• On iron based materials cahodic corrosion protction

• Non-decorative coating (no brightness possible)

Coating properties

• thin and uniform coatings

• Fine coating microstructure

• Limited possibility for bright surfaces

• No pre-treatment necessary

• No influence on the substrate

Process modifications

• Hot-dip galvanizing

• Sealing of chlinch connections

• For large components and baths

• Spray galvanization

• Flexible application on site (repair)



Electroplating of plastics

Generation of conductive surfaces necessary by

• Electroless metal plating without external power

• Graphite dust coating

• Electro-paint

applications

• Thin coatings (thickness < 0,5 mm) for gloss properties

• Thick coatings (thickness ~10 mm) show higher strength

• Bad adhesion properties (coating delamination due residual stresses possible)

• Adhesion optimization by mechanical or chemical treatment

• Uniform coating thickness at edges and within cavities

Metallionen +Reduktionsmittel

38



Electroplating methods

Fe

Fe2+

CuSO4

Cu2+

e-

e-

• Reduction of a solved metal (electrolyte)

• Oxidation of an ignoble metal potential difference between solvent and metal is the driving force

• Potential difference influences coating structure

Dipping method

• ignoble metal will be coated

• Inner shortcut

Contact method

• Ignoble metal as electron donator

• Current by outher contact of disssolving and coated metal

Fe2+

SO4

2-

Cu2+

2Al3+

Cu2+

Al

e-

e-

Fe

e-

e-

Cu2+

Stromfluss

Kontakt-material

e-

e-

e-

e-

+ -



Electroless nickel depsotion

• Reduction of a Ni2+ ions by solved reducing agents (elcetron donator)

• Reducing agents: natriumhypophospite, natriumborhydrite

• Inclusion of phosphor (max. 13 %) and Boron (max. 6 %) in nickel coatings possible

Deposition conditions

• Using acid baths

• Working temperature 80-95°C

• Addionally using of NaOH, nickel salt and reducing agents

39



Electroless nickel depsotion

Applications

• High wear and corrosion resistance for steel copper and copper alloys

• Higher contur accuracy

• Chemical stability

Properties

• Amorphous or fine crystalline structure with moderate hardness

• Dispersion hardening by heat treatment

• With higher phosphor content :

• Higher electrical resistivity, ductility and corrosion resistance

• Lower hardness and wear resistance



Advantages and disadvantages

• Thin layer

• only metallic layers

• Coatings with multilayer and graded structure possible

• Dense coatings depending on coating material)

• High effort of pre-treatment (etching)

• Lower adhesion at local loads (egg-shell effect)

• During deposition inclusion of hydrogen into the substrate heat treatment necessary

• Limited possibility to deposit very complex geometries

• Limited contour accuracy at deposition with external current sources

40



slides for coating techniques part 1 gfe schmalkalden uni

Documents

coating balinit coating

gfe schmalkalden

functional coating

annual coating costs

wear coatings

surface technology

wear loss of material

wear intensive industry