57

SOME PRELIMINARY EVALUATIONS OF BLACK COATING ON ALUMINIUM

AA2219 ALLOY PRODUCED BY PLASMA ELECTROLYTIC OXIDATION (PEO)

PROCESS FOR SPACE APPLICATIONS

S. Shrestha (1)

, A. Merstallinger (2)

, D. Sickert (3)

and B. D. Dunn (4)

(1) TWI Ltd, Granta Park, Great Abington, Cambridge CB1 6AL, United Kingdom, Email: suman.shrestha@twi.co.uk (2) ARC Seibersdorf research GmbH, A-2444 Seibersdorf, Austria, Email: andreas.merstallinger@arcs.ac.at

(3) Dresden University of Technology, Germany, Email: Daniel.Sickert@mailbox.tu-dresden.de (4) European Space Agency, Keplerlaan 1, NL-2201AZ Noordwijk ZH, The Netherlands, Email: Barrie.Dunn@esa.int

ABSTRACT

This paper describes the results of a study of a black

coating produced on aluminium AA2219 alloy using a

process that involves creation of a hard ceramic oxide

layer on the surface of the alloy by plasma electrolytic

oxidation (PEO) known as the ‘KERONITE�’ process.

Coating microstructure has been examined and the

coating characteristics such as porosity, hardness,

adhesion and phase composition were measured. The

thermo-optical properties such as solar absorptance

‘Ds’ and normal infrared emittance ‘Hn-IR’ of the coating

were measured in the ‘as-prepared’ condition and after

environmental exposures to humidity, thermal cycling

and UV-radiation in vacuum and to thermal shock.

Comparison was made with alternative coatings

produced using standard black anodising processes.

The study also looked at the cold welding and friction

behaviours of the coated alloy in vacuum and in an

ambient laboratory environment. Standard spacecraft

materials tests were conducted on the coated disc

against an AISI 52100 steel ball and also against a

coated pin using a pin-on-disc apparatus. Parameters

such as friction coefficient and wear depth were

measured and the cold welding behaviours were

investigated. Test results were compared with the data

generated for NiCr plated and anodised coatings.

Corrosion performance was assessed using a salt spray

exposure test and using an accelerated electrochemical

test method. In addition, the study looked at the effect

of post coating sealing with a sol-gel solution.

1. INTRODUCTION

Coating application by electrolytic methods such as

anodising has been extensively used for improving the

wear (abrasion) and corrosion resistance of aluminium

alloys. The porous nature of anodised coatings allows

production of coloured coatings by deposition of

organic dyestuffs or metallic pigments [1]. For space

applications, additional coating characteristics such as

black colour effect to improve thermo-optical

properties e.g. solar absorptance, infrared emittance

and improved surface characteristics to resist against

cold welding are considered necessary. A widely used

coating technique to date for space applications is

black anodising that has been considered to provide

aluminium alloys with suitable thermo-optical

properties and acceptable resistance to corrosion, wear

and cold welding. However, the conventional black

anodising process is considered environmentally not

safe and the black colour finish for long-term

exposures is not stable. Thus, there exists a strong need

to identify new coating processes that can meet the

requirements of recent environmental legislation and

the continual drive for better coating performance. The

‘KERONITE�’ process is one of the relatively new

environmentally safe electrolytic coating processes that

is applicable to light metals and their alloys in

particular Al and Mg.

The ‘KERONITE’ process involves the application of

a modulated voltage to the component in an electrolytic

bath agitated using compressed air. The voltage is

sufficiently high to create intense plasma due to micro­

arc generation at the component surface. This results in

oxidation of the component surface (plasma

electrolytic oxidation) as well as elemental co­

deposition from the electrolyte solution, which creates

a hard ceramic oxide layer on the substrate alloy. A

typical Keronite coating consists of a porous outer

layer and a low porosity main layer that is the bulk of

the coating. The top porous layer is often removed by

polishing to expose the main layer. The electrolyte is a

low concentration alkaline solution of proprietary

composition. Similar coating processes also known as

micro arc oxidation (MAO) have been reported

elsewhere [2]

2. EXPERIMENTAL APPROACH

2.1 Materials and coating characteristics

An aluminium alloy type AA2219 of composition bal

Al, 6.6Cu, 0.3Mn, 0.1Si, 0.01Mg, 0.1Fe, 0.1V and

0.1Zr was selected in this work as a substrate material.

————————————————————————————————————

Proceedings of the 9th International Symposium on Materials in a Space Environment Noordwijk, The Netherlands, 16-20 June 2003 (ESA SP-540, September 2003)

58

Coatings to a thickness of 75-80Pm were prepared at

Keronite Ltd on disc specimens of various sizes cut

from a 6mm rolled AA2219 plate. The disc specimens

were ground to 600-grit finish using SiC paper and the

edges of the discs were rounded to 2mm radius prior to

coating deposition. One face of the coated specimen

was polished to a final coating thickness of about 60-

70Pm after removing a top porous layer. This involved

manual polishing of the coating face successively on

360 and 600 grit SiC papers followed by final

polishing in 1Pm diamond slurry to Ra ~0.04Pm. The

coating after polishing was rinsed in running water and

degreased with acetone. These are referred to ‘as­

prepared’ coatings. In addition, the coatings were

subjected to post sealing by dipping in a colourless sol­

gel solution followed by subsequent further polishing

to remove the top sol-gel layer. This resulted in the

exposure of the Keronite surface while the pores

remained impregnated and sealed with sol-gel. The

latter coatings are referred to ‘sealed’ coatings.

Coating cross sections were prepared, ready for

examination using standard metallographic techniques

to a 1Pm diamond polishing. A scanning electron

microscope (SEM) was used for coating microstructure

characterisation. Coating porosity was measured from

the polished cross-section using the In2ViewCat-Pro

image analyser connected to an optical microscope.

Coating hardness was measured from the polished

cross section using a DURAMIN Vickers diamond

pyramid micro-indenter from Struers Ltd and using a

50g indentation load. Determination of various

crystalline phases in the coating was undertaken using

the X-ray diffraction (XRD) technique by collecting

spectrum from the coating surface using CuKD

radiation.

Coating adhesion measurements were undertaken

following the guidelines as described in the ASTM

standard C633-79 (Re-approved 1993) ‘Standard test

method for adhesion of cohesive strength of flame

sprayed coatings’. The test method was slightly

modified from the ASTM C633-79 and consisted of

bonding a steel dolly to the coated face of the test

specimen (for this only one face was coated) and

another identical steel dolly bonded to the rear

uncoated face of the test specimen. The test specimen

was sandwiched between the two steel dollies using an

epoxy film adhesive, type FM73 and heat cured at

120°C. The bonded test specimens were then

individually placed in a tensile loading machine

(Dartec) with self-aligning devices. Tensile load was

increased at 1mm.min-1 and the load at failure

recorded. The failure stress was calculated and the

nature of failure, whether at the coating/substrate

interface, coating cohesion or in the adhesive, was

examined. Adhesion measurements were taken also

from the coated specimens that had been subjected to

336 hours of salt spray exposure.

The coating peel-off test followed the procedure as

described in the European Cooperation for Space

Standardization ECSS-Q-70-13A ‘Measurement of the

peel and pull-off strength of coatings and finishes using

pressure sensitive tapes’ [3]. This test method is based

on the controlled peeling of a pressure sensitive tape

from the sample surface (�25mm) using a tensile

loading machine. The maximum failure stress was

recorded and the sample surface as well as the tape was

examined to look at particles detached from the coating

surface that may have adhered to the tape. The tape

used was double sided 3M-pressure sensitive tape type

600 (670g.cm-1). Details of the test procedure are

described in ECSS-Q-70-13A [3]. This method was

used to look at dust generation from the as-prepared

coating surface and the coating that had been exposed

to thermal shock.

2.2 Environmental exposure

Humidity

During the humidity test coated samples were exposed

for one week to 95% relative humidity (RH) at 50°C.

The equipment used was a Heraeus Vötsch humidity

chamber.

Thermal cycling in vacuum

The thermal cycling test was performed according to

ECSS-Q-70-04A [4]. It comprised 100 cycles of

exposure from +100 to –100°C in vacuum. The heating

and cooling rate was 10°C·min-1 with the dwell time of

10 minutes at the maximum and minimum

temperatures.

UV-exposure in vacuum

The UV-exposure was carried out in a vacuum

chamber at a pressure between 10-6-10-7mbar and at a

temperature of 22-25°C. The coating sample was

irradiated using a Hamamatsu deuterium lamp having a

wavelength spectrum from 115 - 400nm. The distance

of UV-source to the coating surface was about 370mm.

The exposure period was 167 hours, which is about

6513 equivalent sun hours (ESH). Details of this

procedure are described in the literature [5].

Thermal shock

Two separate thermal shock exposure tests were

undertaken. The first exposure test was conducted at

the European Space Agency, which comprised of

transferring the coated specimens from a heated oven

(+50°C) to a bath of liquid nitrogen (-196°C). This was

repeated 10 times with a dwell time of 10 minutes at

each temperature. The second exposure test was

performed at TWI Ltd. This consisted of alternate

immersion of the coated specimens at two temperature

extremes: a boiling de-ionised water bath maintained at

+100°C and a liquid nitrogen bath at -196°C. The

coating specimens were manually transferred and

immersed 50 times in each bath with a dwell time of

five minutes in each bath. The maximum time during

the transfer between the two baths was 10 seconds. The

specimens were degreased with alcohol before and

after the thermal shock tests followed by air-drying.

Salt spray exposure

The corrosion performance of the coated and uncoated

test specimens was examined by exposing the

specimens to a salt spray environment following the

guidelines described in the ASTM standard B117-97

up to 2000 hours of exposure period. The testing

comprised of exposing the coating surface up to 2000

hours to a 5wt% NaCl solution (at 35°C) atomised to

create a fog within an enclosed chamber. Changes to

the coating surface were recorded following periodic

observations at 24, 336, 1000 and 2000 hours. The

surface quality was given a rating number in

accordance with ASTM D1654-92 ‘Evaluation of

painted or coated specimens subjected to corrosive

environments - procedure B’. Corrosion resistance to a

minimum of 336-hour test duration is defined in the

Aerospace Materials Specification AMS 2470J (R)

‘Anodic treatment of aluminium alloys - chromic acid

process’ as acceptance criteria [6].

Electrochemical corrosion

The electrochemical corrosion behaviour was studied

using an accelerated potentiodynamic test (anodic

polarisation) in a de-aerated 3.5% NaCl solution of pH

8 at 25°C using a standard three-electrode test method

in an Avesta cell. The coated surface was exposed to

the electrolyte and the rest potential ‘Ecorr’ (also known

as the free corrosion potential) was allowed to stabilise

for one hour prior to anodic polarisation. The potential

value was measured using a reference saturated

calomel electrode (SCE) and a plot of the current

density as a function to the polarisation potential was

recorded using a platinum auxiliary electrode. More

detail of the test procedure can be found elsewhere [7].

The collected polarisation plots were used to compare

the electrochemical corrosion behaviour of the coatings

with uncoated alloys and the integrity of coatings

before and after a thermal shock exposure.

2.3 Measurement of thermo-optical properties

Solar absorptance ‘DS’

The absolute reflectivity spectrum of the specimen was

measured using a Cary-500 UV-VIS-NIR photo­

spectrometer with an integrating sphere accessory and

corrected with the help of a reference standard. The

solar reflection coefficient was calculated by

59

integrating the measured absolute reflectivity spectra

over the solar energy spectrum [8] within the

considered wavelength range from 250 - 2500nm. This

wavelength band comprises 97% of the energy radiated

by the sun. The result of this integration was divided

by the total solar energy thus providing the ratio of

reflected to incident solar energy (RS). This can be

translated into the solar absorptance ‘.S’ by subtracting

it from unity for completely opaque samples, i.e. if the

transmittance equals zero. More details on this can be

obtained in the references [9, 10].

Normal infrared emittance ‘Hn-IR’

The normal infrared reflectivity (Rn-IR) of the coating

surface was measured using a Gier-Dunkle DB100

infrared reflectometer. According to Kirchhoff’s

radiation law the infrared emittance of a material

equals its infrared absorption when maintained at the

same temperature. Furthermore the amount of

radiation, not reflected by non-transparent materials,

equals the absorbed portion of radiation. The

measurement device consisted mainly of an internal

thermal source that emits infrared radiation towards the

coating surface and a detector for the amount of

radiation reflected. Applying the above-mentioned

relations this value is translated into the normal

infrared emittance ‘0n-IR’ by subtracting it from unity.

More detailed description of the physical basics can be

found in the literatures [9, 10].

2.4 Cold welding and friction behaviour

For the cyclic impact adhesion test, a contact was made

between a vertically moving pin. The pin was either

AISI 52100 bearing steel of nominal composition bal

Fe, 1C, 0.3Si, 0.4Mn, 1.6Cr, 0.3Ni, 0.3Cu or coated

aluminium alloy having a radiused contact area against

a fixed flat disc (coated or uncoated aluminium alloy)

for about 5000 cycles. Each cycle consisted of three

steps: i) an impact loading; ii) a static load held for 10

seconds at a vacuum pressure of less than 5.10-8mbar;

and iii) unloading with measurement of the separation

force, i.e. adhesion force.

For the cyclic fretting adhesion test, a contact (without

impact) was made followed by a static load, which is

held for 10 seconds at a vacuum pressure of less than

5.10-7mbar. During this time fretting is applied: the pin

was moved with a sinusoidal frequency at 210Hz and a

stroke of 50Pm in each cycle for a total of 5000 cycles.

After stopping fretting, the force required to separate

the pin from the disc was measured. Tests were

conducted on the coated and uncoated test specimens.

The uncoated test specimens were fine ground to

surface roughness of Ra < 0.03Pm. The coated face

was in the polished condition to Ra ~ 0.04Pm.

60

Sliding wear tests were conducted using a vacuum pin-

on-disc tribometer manufactured by CSEM. The pin

(ball) was allowed to slide on the disc surface in

unidirectional movement at a sliding speed of 0.1ms-1

and a sliding distance of 1000m. Vacuum pressure was

maintained at less than 10-6mbar and the temperature at

25qC. The ball diameter was 7mm and the load was

10N. More details on the testing procedures are given

in the literatures [11, 12]. In addition, sliding wear tests

were undertaken also in an ambient laboratory

environment (22r2qC, relative humidity 35-45%) with

a test load of 10N and sliding speed of 0.1ms-1 against

a steel ball (AISI 52100) of diameter 10mm for a

1000m sliding distance. This followed the guidelines

described in ASTM G99-95a ‘Standard test method for

wear testing with a pin-on-disc apparatus’. Friction

coefficients and wear track depths where possible were

measured.

3. RESULTS AND DISCUSSIONS

3.1 Coating characterisation prior to exposure

A backscattered SEM image of a sealed coating cross

section is shown in Fig.1, which shows a relatively

dense coating. This image is of the main coating layer

after removing the top porous layer (not shown), which

also reveals a microstructure comprising of

interlocking grains about 20-30Pm in size with

occasional very small-sized pores within the grains and

a few very fine microcracks. The coating porosity was

measured at about <3%.

Mounting compound

Coating

Substrate AA2219

Fig.1. SEM backscattered image of the sealed Keronite

coating cross-section on AA2219 alloy.

A backscattered SEM image of a sealed coating surface

is shown in Fig.2. The image in Fig.2 shows the

presence of porosity on the polished surface. The

image in Fig.2 also shows the presence of a lighter

contrast phase that lies at the boundaries between the

interlocking grains. The lighter contrast in the

backscattered SEM image is indicative of this phase

being of a higher atomic number element.

3

2 1

Fig.2. SEM backscattered image of a polished surface

of the sealed Keronite coating on AA2219 alloy.

1 - Al2O3; 2 - Cu containing phase; 3 - Sealant.

Spot EDX analyses taken from the areas in Fig.2

showed that area ‘1’ comprised of predominantly ‘Al’

and ‘O’ peaks suggesting it to be aluminium oxide. The

lighter contrast phase ‘2’ at the grain boundaries were

found to be highly rich in ‘Cu’ and such could have

come from the copper precipitates of the substrate alloy

that do not participate in the oxidation process. The

darker phase ‘3’ was found to be rich in ‘Si’ content

and was believed to be impregnated sealant. A cross

section through the specimen edge is shown in Fig.3.

The coating appeared well retained over the edge with

no visible cracking over the tight radius.

g at

edge

200 microns

Substrate AA2219

Coatin

Fig.3. SEM image showing good and uniform Keronite

coating retention at the substrate edge.

The X-ray diffraction pattern collected from the

coating surface is presented in Fig.4, which suggested

that the Keronite coating comprised predominately D and some J and G-Al2O3 crystalline phases. This has

resulted in a very high coating hardness at an average

value of about 1369HV with the values ranging

between 1225-1524HV. The hardness measured on the

substrate alloy was only about 140HV. This is

indicative of the coating hardness being about 10 times

higher than the substrate alloy.

61

20 40 60 80[q2T] 0

]

900

1600

2500

3600

4900

400

100

6400

[Cou

nts

Fig.4. XRD patterns of the Keronite coating.

The mean tensile stress to failure during the pull-off

test from four coating specimens was recorded at

greater than 28MPa with the failure occurring mainly

within the adhesive. It is expected the coating would

have adhesion in excess of 30MPa. The results of the

peel-off test for the sealed coatings showed the primary

mode of failure was adhesive tape separation from the

coating without any particle removal.

3.2 Coating characterisation after environmental

exposures

Examination of surfaces of the sealed coatings after the

thermal shock exposure did not show any visible

coating degradation. However, there was a thin band of

the coating removal only at the edge of the disc

specimen thus exposing the underlying substrate. This

would have been expected due to stress concentrations

at the specimen edge.

The values of adhesion measured for the coatings after

the thermal shock were in excess of 25MPa and the

mode of failure occurring primarily within the

adhesive. Adhesion of the coatings after 336-hour salt

spray exposure was measured and was in excess of

30MPa with failure in the adhesive. The coating peel­

off test after the thermal shock resulted in the tape

separation without any debris being seen on the peeled

adhesive tape. The average coating hardness measured

from the cross section of the specimen that had been

exposed in a salt spray environment for 2000 hours

displayed about 1500HV. These results demonstrate

that the coating did not show any loss of its integrity

and retained near original microstructure with good

adhesion and high hardness after the environmental

exposures.

3.3 Corrosion behaviour

Exposure of the as-prepared coating (unsealed) to the

salt spray environment displayed severe corrosion

product seen on the coating surface just after 120

hours. This corrosion attack was in the form of blisters

and extended pits with pit diameters about 1mm were

observed on the coating surface. Surface appearance of

the ‘as-prepared’ coating (unsealed) after 120 hours of

salt spray is shown in Fig.5. Such would have been

expected from the as-prepared (unsealed) coating

having porosity and interconnected path for the

corrosive media to penetrate into the aluminium

substrate.

5mm

Pitting and

blisters

Fig.5. Surface of the as-prepared (unsealed) Keronite

coating on AA2219 alloy after 120 hours of salt spray.

Further salt spray exposure tests were undertaken for

the coated specimens sealed with sol-gel, which

displayed significant improvement in the corrosion

resistance afforded by the sealed coating to the

underlying aluminium substrate. The sealed coating

surface displayed significant protection in the salt

spray environment with no visible corrosion observed

and was rated ‘10’ (the highest rating) after 336 hours

of exposure. On further exposure up to 2000 hours, the

surface of the sealed coating displayed only a very

slight corrosion attack on the exposed surface and was

still rated ‘9’ at 2000 hours. In contrast to this, the

uncoated surface of the AA2219 alloy exhibited very

poor performance in the salt spray environment when

examined at 24 hours and was rated as zero. Surface

appearances of the uncoated alloy and the sealed

coating after 2000 hours of salt spray are shown in

Fig.6. A coating section though the sealed coating after

exposure to 2000 hours in a salt spray environment is

shown in Fig.7, which displays no visible deterioration

of the coating or the substrate underneath.

Extensive corrosion No visible corrosion

a) b)

10mm10mm

Fig.6. Surface of: a) uncoated; b) sealed Keronite

coating on AA2219 after 2000 hours of salt spray.

g comp

62

Substrate AA2219

Coating

Mountin ound

Fig.7. Cross-section of the sealed Keronite coating on

AA2219 alloy after 2000 hours of salt spray.

Anodic polarisation plots from the electrochemical

tests are shown in Fig.8. The uncoated 2219 alloy

displayed an immediate rapid increase of the current

density on slightly increasing the potential from its rest

potential ‘Ecorr’. This immediate rise of the anodic

current density over a small positive potential range

from the rest potential is indicative of very rapid

corrosion occurring at the surface of the uncoated

alloy. The coated alloy displayed a large potential

range (passivity) positive from the Ecorr, where the

current density was stable and low at 0.2PA.cm-2,

indicative of very little corrosion occurring on the

coating/substrate system and a good level of barrier

protection afforded to the underlying substrate. Higher

currents would be expected if the salt solution

penetrated into the underlying substrate via cracks and

pores.

Current density, uA/cm2

750

500

250

0

-250

-500

-750

i

i

loy

Po

ten

tia

l, m

V S

CE Coat ng

Coat ng after

thermal shock

Uncoated al

0.001 0.01 0.1 1 10 100 1000

Fig.8. Anodic polarisation plots (forward scans) from

the uncoated AA2219 alloy, sealed Keronite coating

before and after the thermal shock exposure.

Anodic polarisation of the coating after exposure to

thermal shock displayed the stable current density

region being shifted to about 2PA.cm-2. This was taken

to indicate that possible fine cracks might have resulted

during the thermal shock, which possibly had led to a

slightly higher corrosion rate from the substrate.

However, this value was still low and a large potential

region at stable current density < 5PA.cm-2 would

mean the coating was still effective as a barrier to

minimise the passage of the corrosive electrolyte to the

underlying aluminium substrate.

3.4 Thermo-optical properties

The mean solar absorptance ‘DS’ and infrared

emittance ‘Hn-IR’ values measured from 12 as-prepared

Keronite coatings (unsealed) prior to environmental

exposures are given in Table 1. The mean ‘DS’ about

0.89 (r0.01) and ‘Hn-IR’ ranging between 0.64-0.76 with

a mean value of 0.72 was measured.

The ‘DS’ and ‘Hn-IR’ values after exposures to high

humidity, thermal cycling in vacuum, humidity

followed by thermal cycling in vacuum, UV radiation

and thermal shock between temperature extremes of

+50 and -196qC are given in Table 1. The data in Table

1 showed a good stability of the thermo-optical

properties retained by the Keronite coating after the

environmental exposures. Visual examination of the

coating surfaces did not show any coating degradation

after environmental exposures to humidity, thermal

cycling and UV radiation. The coating subjected to

thermal shock had some coating spalling only from the

specimen edge and believed to be due to stress

concentrations. No coating damage was observed on

the main exposed coating surface after the thermal

shock test. The thermo-optical properties after the

thermal shock exposure were stable and similar to the

coatings prior to the exposure.

‘H

Table 1. Solar absorptance ‘DS’ and infrared emittance

n-IR’ of the coatings

Coatings and

environment DS Hn-IR DS/Hn-IR Ref.

Keronite coatings

(Before and after environmental exposure)

Before exposure 0.89 0.72 1.2

-After humidity 0.89 0.72 1.2

After thermal cycling 0.89 0.71 1.2

After humidity + 0.89 0.73 1.2

thermal cycling

After UV exposure 0.89 0.74 1.2

After thermal shock 0.88 0.75 1.2

Benchmark coatings

ESA PSS 01-703 0.96 0.93 1.0

5Colinal 3100 0.89 0.93 1.0

The data in Table 1 suggests that the solar absorptance

of the black Keronite coating can be similar to Colinal

3100, however, the normal infrared emittance is about

20% lower compared to the coatings produced with

black anodising processes. The PSS-01-703 coating

and Colinal 3100 are based upon the colouring of

anodized surface layers. Colouring is done either by

depositing cobalt or nickel sulphides or by

0

0

electrolytically depositing tin. These latter two

processes are considered to have environmental

concerns.

3.5 Cold welding behaviour

The data from the impact test in Fig.9 shows that the

adhesion measured after the impact tests for the

Keronite coatings against itself and against a standard

bearing steel (AISI 52100) are very low and are similar

to anodised and NiCr-plated coatings. However, there

were some differences in the damage mechanisms

between the Keronite and anodised coatings. The

Keronite coating did not display any damage to the

coating surface under impact, whereas, the anodised

coating displayed breaking of the coating surface under

impact resulting in the formation of loose debris on the

impacted surface. The aluminium alloy type AA7075

without a coating demonstrated a very high adhesion

value after the impact tests. Tests were not undertaken

on the uncoated AA2219 alloy.

37

23

58

24 A

A /

A

A /

A

A2219+Keronite / AISI 52100

A2219+Keronite AA2219+Keronite

A7075+Anodised / SS15-5PH

A7075+NiCr pl. AA7075+Anodised

A7075 / AA7075 1033

200 400 600 800 1000

Adhesion force, mN

Fig.9. Adhesion force measured during impact tests.

A

A

A /

A /

A /

7330

100

242

329

108

A7075 / AA7075

A7075+Anodised / SS15-5PH

A2219+Keronite AA2219+Keronite

A2219+Keronite AISI 52100

A7075+NiCr pl. AA7075+Anodised

1000 2000 3000 4000 5000 6000

Adhesion force, mN

Fig.10. Adhesion force measured during fretting tests.

The adhesion values measured after the fretting tests

are presented in Fig.10. The data in Fig.10 shows that

adhesion values of the Keronite coated AA2219

against a 52100 steel ball and NiCr plated AA7075

against an anodised AA7075 are similar. An

aluminium alloy without a coating resulted in

extremely high adhesion after fretting. The Keronite

63

coating surface against itself displayed a higher

adhesion than against a steel surface by about three

times.

Despite low adhesion values displayed by the Keronite

and hard anodised coatings against steel surfaces, the

Keronite coating displayed superiority over the

anodised coasting. The Keronite coating showed no

sign of coating damage, whereas, the anodised coating

displayed extensive cracking and chipping of the

coating surface under fretting environment. The

differences in the damage mechanisms between the

Keronite and anodised coating surfaces are shown in

Fig.11.

a) b)250 250

Fig.11. Surface after fretting test of: a) Keronite

coating (no visible damage) and b) anodised coating

(showing extensive cracks).

3.6 Friction and wear behaviour

The results of the friction wear tests are given in Table

2. The data in Table 2 show mean friction coefficient

and disc wear track depth after the sliding wear tests

conducted in vacuum and in an ambient laboratory

environment.

Table 2. Pin-on-disc sliding wear test data.

Disc / pin Environment

Mean

friction

coefficient

(P)

Disc wear

track

depth

(Pm) AA2219 /

AISI 52100

Vacuum

0.3 18

As-prepared Keronite

on AA2219 / AISI

52100

0.6 -

Keronite AA2219 /

Keronite AA2219

0.5 -

AA2219 /

AISI 52100

Laboratory

ambient

temperature

(22r2qC)

0.3 26

Sealed Keronite on

AA2219 / AISI

52100

0.5 (-0.8)

Thermal shocked

Keronite AA2219 /

AISI52100

0.5 (-1.0)

64

The mean coefficient between an AA2219 disc and

52100 steel ball in vacuum of 0.3 was about twice less

than that was observed with the Keronite/steel or

Keronite/Keronite surfaces. Similar friction values

were measured in the laboratory environment at

ambient temperature. In all cases, the Keronite coating

was intact without any visible coating damage when in

sliding contact with steel. The only damage seen was

either on an uncoated AA2219 disc against a steel ball

or on a steel ball sliding against the Keronite coated

disc. The values in brackets in Table 2 are negative

wear i.e. depth loss on a counter steel ball. An

exposure to the thermal shock did not appear to reduce

the coating sliding wear resistance against the steel

ball. However, the Keronite coating sliding against

itself displayed some evidence of coating damage on

both the coated disc and coated ball surfaces. These are

not shown.

a) b)

-5

-10

Pm

-15

-20

-25

0 Pm

20

10

0

-10

-20

-30

Fig.12. Topography of: a) Keronite coating surface on

AA2219 alloy (shows small particles pulled out); b)

AA2219 alloy (uncoated) after sliding wear test against

AISI 52100 steel ball in vacuum (shows deep wear

track).

The surface topographies of the Keronite coated

AA2219 and uncoated AA2219 discs after sliding wear

tests against the AISI 52100 steel ball in vacuum are

shown in Fig.12. The Keronite coated surface did not

show any sign of visible coating damage, except for a

few particle pullouts. The uncoated disc displayed a

deep wear track with severe grooving effect on the disc

of about 30Pm deep and material being displaced from

the groove resulting in the formation of raised lips at

the edge of the groove.

4. CONCLUSIONS

x� The Keronite� process is capable of depositing a

uniform coating at edges and tight corners with a

typical coating thickness about 60-70Pm. The coating

on the AA2219 alloy after polishing is black in colour

with a porosity level <3%. The Keronite coating on

AA2219 alloy comprises of predominantly crystalline

D-Al2O3 that results in high coating hardness of about

1300HV. The coating adhesion can be in excess of

30MPa. The individual oxidised particles (grains) in

the coating retain good cohesion as shown by the

peel-off test.

x� The corrosion resistance of the as-prepared

Keronite coating is poor and a life of less than 120

hours is expected in a salt spray environment.

However, this poor corrosion performance can be

overcome by post coating sealing. The Keronite

coating sealed with sol-gel can provide extremely

good resistance in excess of 2000 hours without a

sign of any visible corrosion attack.

x� The Keronite coating on the AA2219 alloy can

retain its good integrity and original microstructures

with good adhesion, good cohesion and high hardness

after the environmental exposures such as salt spray

and thermal shock.

x� The solar absorptance to infrared emittance ratio

of the Keronite coating is 1.2, suggests that it can

replace the PSS-01-703 black anodised coating. The

thermo-optical properties of the Keronite coating on

AA2219 alloy will remain stable and not be affected

by environmental exposures.

x� The Keronite coating on AA2219 can prevent cold

welding under impact and fretting against AISI 52100

steel in vacuum environment. The Keronite coating

compared to anodised coating can have improved

resistance to impacts and fretting, as shown by no

visible damage to the Keronite coating after the tests.

x� The Keronite coating sliding against AISI 52100

steel will result in the wear of mainly the steel

surface. The coating however, against itself in sliding

wear condition will result in higher damage to the

coating surfaces.

x� Friction coefficients of the Keronite coating

against itself and steel are high at about >0.5.

5. ACKNOWLEDGEMENTS

The authors would like to thank Dr E Semerad and Mr

W Costin of Austrian Research Centre Seibersdorf for

undertaking fretting, impact and wear tests. Thanks are

also due to several TWI staff for undertaking various

tests and Dr A Sturgeon for helpful comments to this

paper. The authors would also like to acknowledge

DePuy International, European Space Agency,

Goodrich Actuation Systems and Keronite Ltd. for the

permission granted to use data on AA2219 alloy

generated in the TWI Group Sponsored Project funded

by them.

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treatment and finishing of aluminium and its alloys, 5th

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65

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