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Al2O3 Graded Coatings on Aluminium Alloy Deposited by the Fluidized Bed (FB) Technique:

Film Formation and Mechanical Performance

M. Barletta1

1Dipartimento di Ingegneria Meccanica, Università di Roma - “Tor Vergata”, Via del Politecnico, 1

– 00133, Rome, Italy

Abstract

Al2O3 coatings have been deposited onto an Al alloy by the fluidized bed (FB) technique using

alumina powder. Film formation through a cold deposition process and its growth kinetics have

been investigated by varying the deposition time. This allowed us to establish how the morphology,

microstructure, hardness, scratch resistance, and adhesion strength of the Al2O3 films were

progressively imprinted. The fluidized bed (FB) process led to the deposition of a good-looking and

well-adhered graded Al2O3 coating, which was found to be progressively richer in Al2O3 on moving

from the interface with the Al alloy towards the outermost layers. The resulting Al2O3 coatings have

been shown to produce a consistent improvement of the overall mechanical and tribological

performance of the Al alloy, thus leading to the build-up of an overlying hard and tough protective

layer.

Key Words: Fluidized Bed (FB); Al2O3 Powder; Graded Coatings.

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1. Introduction

As weight-saving, stiff, high strength-to-density and elastic modulus-to-density ratio material,

aluminium and aluminium alloys are becoming increasingly important in several industrial fields

[1]. Yet, their poor tribological behaviour restrains severely their applications whenever resistance

to sliding contact is required [2, 3]. Accordingly, the development of protective coatings on Al

alloys is a matter of outmost importance.

Ceramic materials are widely used as hard coatings on metals for their wear and corrosion

resistance features. Al2O3 films are among the most used coatings in mechanics, microelectronics,

and optics because of their unique thermo-mechanical, chemical, electrical, and optical properties

[4]. Several techniques are available for the deposition of Al2O3 films on Al substrates, such as sol-

gel, PVD, CVD, conventional and hard anodizing, as well as high-velocity and plasma-spray

processes [58]. In most of them, heat or electricity is the energy source that drives the deposition

process. Of the various surface treatments available for aluminium alloys, anodizing is the most

common [9-10]. Conventional anodizing treatments are carried out in a sulphuric or chromic acid

bath at about 50 °C: above a thin, dense barrier layer, a thicker layer with very small columnar

pores is formed. These pores are usually sealed (by treatments in hot water or in aqueous solutions)

for enhanced corrosion resistance, although this impairs the layer hardness. Alternatively, hard

anodizing treatments, performed at low temperature (≤5 °C), produce a very hard and dense layer

(free of columnar pores), which needs no sealing and provides optimized protection against wear

[9-10]. Anodized films offer interesting properties but have some significant drawbacks: anodized

films are intrinsically quite brittle; their maximum thickness is limited and process times are often

quite long (several hours) [10]; anodizing treatments requires a careful surface pre-treatment; the

outcome of anodizing treatments is dependent on the alloy composition [9, 10], with some alloys

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being ill-suited for anodizing. Finally, anodizing treatments are generally considered a high

environmental impact technology.

Alternatively, high-performance coatings can be produced by thermal-spraying or vacuum (PVD

and/or CVD) technologies. These techniques are less sensitive to the nature of the substrate, they do

not require complex surface pre-treatments, and they enable a flexible choice of coating thickness

and material [11-13]. Nevertheless, they are discontinuous process requiring the usage of high-cost

and time-consuming equipments, the need to dispose of potentially hazardous exhaust emissions

and the employment of skilled technicians. As a matter of fact, such drawbacks erode the market

competiveness of all those processes.

Fluidized Bed (FB) technology is a simple, efficient and environment friendly process, which has

been widely used to carry out surface treatments as aluminizing, chromizing, nitriding, carburizing,

carbonitriding and boronizing [14-17] as well as to deposit single and multi-element, hard and/or

corrosion resistant coatings [18]. FB technology is well known and its theoretical and practical

aspects are described elsewhere in great detail [19]. Briefly, FB technology makes use of different

sized particles pre-loaded in a conical or cylindrical reactor with a bottom frit or distribution plate

through which a fluidizing gas is supplied. At low flow rate, the particles stand nearly still in the

bed. Increasing the flow rate, the weight of the particles is balanced and they become suspended in

the gas flow and assume a fluid-like behaviour, with outstanding heat and mass transfer rate.

Further increase in the flow rate causes the particles to be entrained and elutriated out of the reactor.

Coating processes in FB technology is mostly performed by using reactive or non reactive particles

suspended inside a carrier gas with or without reacting vapour species [20]. In the most emerging

technique, the particles of the bed acts themselves as a source of the material to be deposited as well

as an efficient heat and mass transfer medium. The substrate to be coated is dipped in the bed and a

mix of carrier and reactive gas species is used to fluidize the particles. At high temperature, the

reactive coating species are generated in situ in the bed. When the reactive coating species reach the

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surface of the substrate, they disproportionate and/or reduce, thus depositing a coating [20]. Whilst

apparently simple, this coating process is suited only to heat resistant alloys, as common deposition

temperature ranges from 800 to 950 °C.

At low (ambient) temperature, the mechanism by which a hard coating can be deposited on a softer

metal substrate by FB technology is completely different [21-22]. The fluidizing gas and the

particles are both inert and there are no reactions inside the bed. The coating is achieved by cold

deposition, which means by repeated impacts of hard and sharp particles on the softer metal

substrate. The pushing force toward the deposition is therefore the kinetic energy of the fluidized

particles and the gap in the hardness (i.e, and in the overall mechanical properties) between the

particles and the substrate itself. Following each effective impact, a small portion of the particles

can detach and remain trapped inside the outermost layers of the impinged metal substrate. The

reiteration of this phenomenon can give rise to the formation of a relatively thick (generally, few

microns) and tough coating [21-22]. In this respect, cold deposition of Al2O3 films on Al alloy by

FB technology can be pursued using Al2O3 powder as fluidizing particles, which lead to the

formation of a continuous and hard film, extremely helpful in increasing the surface properties and

the fatigue resistance of the coated surface [22]. In comparison to other deposition processes, cold

deposition by FB technology has been shown to offer many advantages, such as the possibility of

operating at low temperature, the capacity to deposit virtually any kind of powder material onto a

softer substrate, and the ability to cover complex shapes with minimum requirements. Moreover,

the system is well known to be safe, environmentally friendly, easy to automate and operate, and

characterized by low investment and running costs [22].

The purpose of the present investigation has therefore been to evaluate the principal mechanisms

involved in the formation of the Al2O3 films on the underlying Al alloy. Particular emphasis has

been placed on the determination of the film growth kinetics and how this issue relates to the

morphology, microstructure, hardness, scratch resistance, and adhesion strength of the growing

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films. A practical interpretation to explain the establishment of a graded structure of the Al2O3 film

from its surface to the underlying aluminium substrate is also provided.

2. Experimental

2.1 Materials

Commercial AA 6082 T6 aluminium sheets were used as metal substrates. Their nominal

composition is reported in Table 1. Standard samples, 80 mm wide and 100 mm long, were

obtained from 1 mm thick aluminium sheets. Further cylindrical samples, 5 mm height and 30 mm

diameter, were specifically produced for tribological tests. Before fluidized bed (FB) deposition, the

samples were cleaned with common solvents for the removal of oil, grease, loose metal chips, and

other contaminants.

Al2O3 powder (supplied by Smyris Abrasivi Srl), with an average mesh size of 16 (~1.2 mm

diameter) and a shape factor of 0.67, was used as the standard FB medium. Before being employed

in the FB process, the alumina powder was sieved, using a standard 120 mesh sieve, in order to

remove all of the fines. A maximum working time of 4 hours was allowed for sieved alumina

powder as a medium in FB processing. Thereafter, the exhausted alumina powder was discarded

and fresh powder was provided to guarantee the uniformity of the FB process over time.

2.2 Equipment

After surface cleaning, the aluminium samples were submitted to the FB deposition process. The

system employed for fluidizing solid particles with an air flux is shown schematically in Figure 1. It

consists of four main sections: (i) the homogenization section, filled with porous materials

(typically Raschig’s rings) and designed to produce a uniform flux through the entire cross-section

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of the system; (ii) the porous-plate distributor, made from a 1 mm thick stainless steel mesh with a

pore size of 30 m, narrow enough to sustain the alumina powder when not fluidized, and

concurrently large enough to ensure the entrance of the fluidization air into the column without

modifying the velocity distribution produced by the homogenization section; (iii) the fluidization

column, 1.2 m in height with a square cross-section of 400 mm and made from stainless steel, with

a large window on each side to make the fluidization process visible during the treatment; and (iv)

the blower (Mapro model Turbotron), 37 kW maximum power, used to feed a purified air flux free

from oil and moisture into the inlet section of the FB under strictly monitored process conditions. A

standard flowmeter with a 24 V output and an inverter (Mitsubishi model FR-A-540-30k) were

respectively used to read the current value of the air flux and to regulate its correction in order to

keep the flow rate constant throughout the treatment. A pressure probe, a hygrometer, and a set of

thermocouples were also used to monitor the deposition parameters and to ensure constant

environmental conditions, thereby guaranteeing the best reproducibility of the experimental

settings.

Al2O3 powder was used as the fluidizing material with a static bed height (i.e. a ‘fixed’ bed) of 230

mm. During FB processing, the aluminium samples, kept in the inner part of the fluidization

column, as shown in Figure 1, were exposed to the strikes of the fluidized Al2O3 powder.

Processing times ranging from a few minutes up to 16 hours were investigated. In this way, a

progressive change in the surface topography (form, waviness, and roughness), in the surface

properties (hardness, residual stress, and density of dislocations), as well as in toughness and

adhesion of the Al2O3 coating, was obtained.

The aluminium samples were clamped on a horizontal shaft rotating at ~50 rpm so that they were

exposed to repeated impacts by the incoming alumina powder. A system comprising a digital

counter provided with a 24 V output and an inverter was used to monitor the rotating speed of the

aluminium samples and to keep it constant throughout the process.

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The experimental schedule is summarized in Table 2.

2.3 Characterization tests

The morphology, thickness, microstructure, hardness, scratch adhesion and wear resistance of the

Al2O3 coatings were characterized. Images of film morphologies and cross-sections were obtained

using a field emission gun scanning electron microscope (FEG-SEM Leo model Supra 35) using

Secondary Electron (SE) and Back-Scatter Electron (BSE) detectors. The 3D morphology of the

Al2O3 coatings was obtained with a TaylorHobson surface topography system (model TalySurf

CLI 2000) using a non-contact 300 m chromatic aberration length (CLA) HE gauge, recording

4000 patterns with a lateral resolution of 1 m for each sample so as to cover a representative area

(16 mm2) of the entire surface structure. The surface morphology was then examined by using the

TalyMap software Release 3.1.

Aluminium samples were also characterized before and after 4h FB Al2O3 deposition by /2 and

grazing incidence ( = 1°) X-Ray Diffraction (XRD) with a Philips X’Pert Pro diffractometer,

equipped with a plane mono-chromator using Cu K radiation ( = 1.5418 Å). Semi-quantitative

information about the chemical composition of the as-deposited coatings was obtained by energy-

dispersive X-ray spectroscopy (EDXS, Oxford Instruments Ltd., model Inca 300).

Depth profiles of the aluminium samples after 4h FB Al2O3 deposition were obtained by using a RF

Glow Discharge Optical Emission Spectrometer (GD-Profiler Horiba Jobin Yvon) in an argon

atmosphere (800 Pa) by applying an RF of 13.56 MHz, 25 to 40 W power and a sampling time of

0.01 s. The emission lines available and used for depth profiling were: Al, 396.15 nm; O, 130.2 nm.

Other emission lines used include: Cr, 425.43 nm; Si, 288.16 nm; Mn, 257.61 nm; Cu, 327.40 nm;

Fe, 371.9 nm.

Depth-sensing micro-indentation (depth-sensing Vickers micro-indenter, C.S.M. Instruments) was

used for instrumented micro-hardness evaluation in constant depth mode. The indentation depths

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investigated were 0.25, 0.5, 1, 2, 3, 5 and 10 m (0.1 N/min loading and unloading rates, 15 s

loading time, Poisson’s ratio assumed to be 0.3). A standard micro-hardness test (micro-Vickers

indenter) with loads ranging from 0.25 N to 10 N and a loading time of 10 s was used to

qualitatively assess the mechanical behaviour of the alumina coatings under a series of increasing

loads.

Scratch tests were first performed on aluminium substrates FB treated at different deposition time

(30, 60, 120 and 240 min). Tests were performed in progressive mode using a Rockwell conical

(60°) diamond stylus with tips of radius 200 μm (micro-scratch tester, C.S.M. Instruments).

Linearly increasing load of 0.1 to 15 N, scratch length of 3mm and load rate of 5 N/min were set.

Scratch tests were subsequently performed on untreated aluminium substrates and after 4h FB

Al2O3 deposition using a Rockwell conical (60°) diamond stylus with tips of radius 100, 200 and

800 μm (micro-scratch tester, C.S.M. Instruments). For each sample scratch tests in “progressive”

mode, i.e. with linearly increasing load (0.1 N to 15 N, 3 mm length, 5 N/min loading rate), were

carried out. Then, five scratches for each sample in “continuous” mode, i.e. at 10 N constant load

and different scratch speeds (0.2, 1, 5, 25, 100 mm/min), were carried out. Figure 2 shows the

location of the scratch patterns after using the 800 mm tip radius indenter on aluminium substrates

after 4h FB Al2O3 deposition. During each scratch test, the indenter first profiled the surface with a

very small load (pre-scan). Then, the indenter went back to the starting location and applied the load

according to the above prescribed conditions. Finally, the indenter went back to the starting location

and re-profiled the surface where the scratch was produced with a very small load (post-scan).

During the pre-scan, the system software stored the starting surface profile, which was finally

subtracted from the loaded scratch scan profile to determine the depth of surface penetration, dP.

During the post-scan, the magnitude of the residual scratch ditch, dR and the extent of immediate

recovery (dP−dR) could be evaluated. Being the distance between the indenter and the evaluation

unit measured in terms of the movement of the translation table, positional values of load and

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penetration could be addressed to the residual deformation at the same position. Normal and

tangential forces, as well as friction coefficients, were also monitored on-line during the scratch

test. Finally, it is worthwhile remarking that at least three scratches in progressive mode were

carried out on each coating and the minimum distance between scratches was 4 mm. For this reason

the results should be representative of the average response over greater surfaces. The scratch tests

were carried out at about 20 °C and 40% RH.

After the scratching procedure, all the scratch patterns were visually inspected by means of an

optical microscope and FEG-SEM, and their 3D profiles were acquired by non-contact 3D

profilometry. For the latter, using a non-contact 3 mm CLA HE gauge of the TalySurf CLI 2000, a

number of patterns with a lateral spacing of 1 m were recorded for each sample so as to cover the

whole area of the scratch patterns. The 3D surface morphology of each pattern (was then examined

by using the TalyMap software Release 3.1.

Ball-on-disk tribological tests (Pin-on-Disk Tribometer, CSM Instruments) were performed on as-

deposited alumina coatings (after 4h FB) and on the untreated aluminium samples, using 6-mm

diameter stainless steel (100Cr6) balls as counterparts. Test conditions include 1 to 5 N normal

load, 0.05 m/s relative sliding speed, 100 to 500 m overall sliding distance; 21±2 °C temperature

and 56±2 % relative humidity. 10 Hz linear reciprocating tests were performed under the same

loading conditions, with a wear pattern 6 mm long and a sliding distance of 500 m.

Wear patterns were first visually inspected by FEG-SEM. Then, wear rates were measured using a

non-contact 3 mm CLA HE gauge of the TalySurf CLI 2000. A number of patterns with a lateral

spacing of 2 m were recorded for each sample so as to cover the whole area of the scratch patterns.

The 3D surface morphology of each pattern was then examined by using the TalyMap software

Release 3.1.

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3. Results and discussion

3.1 Al2O3 film deposition

The impact of the fluidized Al2O3 powder on the aluminium samples during FB processing caused

fragmentation of the powder due to its brittleness and the consequent embedding of smaller but

harder Al2O3 particles into the softer metal surface. As a result of the progressive accumulation of

Al2O3 particles on the aluminium surface, the growth of a thin Al2O3 coating was observed.

Figure 3 shows the trend in mass growth of aluminium samples with respect to processing time.

Fast growth of the Al2O3 film was detected during the first minutes of the FB process, and thereafter

the Al2O3 film approached an almost asymptotic thickness after ~200 min. A longer processing time

was found to cause only minor changes in mass growth. Therefore, progressive saturation of the

external layers of the aluminium substrate with embedded Al2O3 particles can be hypothesized. In

fact, samples obtained after 15 min of deposition showed large uncovered areas (zone A in Figure

4) together with areas in which the film grew more rapidly (zone B in Figure 4). A processing time

of least 30 min was necessary to attain full coverage of the aluminium substrate (Figure 5).

Nevertheless, with increasing processing time, a slowing down of the growth rate was observed

because to some extent the already deposited Al2O3 film acted as a barrier to further deposition of

Al2O3, being considerably harder than the starting aluminium substrate.

Figure 6 shows SEM images, at different magnifications, of an Al2O3 coating obtained after 240

min of FB deposition (sample 4). A dense and compact film adhered to the Al surface was attained.

Loose Al2O3 debris minimally adhered to the growing Al2O3 film can also be observed, evidencing

how the FB deposition of further alumina particles onto the already deposited alumina can be more

and more troublesome. Surface defects within the Al2O3 coating were sometimes observed, in

particular when long processing times were applied. Newly incoming Al2O3 fragments can impinge

on some of the outermost weakly adhered Al2O3, thus causing its removal rather than its

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consolidation onto the underlying layers. Figure 7 exemplifies the aforementioned phenomenon,

showing the presence of typical defects of an Al2O3 coating deposited over a very long FB

deposition time. In particular, Figures 7a and 7b display the presence of uncovered or partially

covered portions of the aluminium substrates after very long FB (240 min) deposition time, whilst

Figure 7c shows the presence of small cracks in the outermost layers. The cracks are about 50 nm

wide, that means very small if they are compared with typical defectiveness which could arise on

alumina coatings deposited by anodizing treatments [10, 23-24] or thermal spraying [25].

Anodization and thermal spraying can often lead to high defective Al2O3 coatings, characterized by

the presence of both vertical and transversal cracks. In the severest scenarios, such cracks can be

rather wide (around 1 m or even larger) or through-thickness and they can compromise the

adhesion and wear resistance of the coatings [26]. In this respect, many studies have revealed that

the failure of the thermally sprayed coatings occurs readily from the interfaces among lamellae and

at the interfaces between the coating and the substrate whenever localized loads such as in abrasive

wear [27] or erosion [28], fretting wear [29] and fracture mechanics test [30] are applied.

3.2 Surface morphology and chemical composition

Al2O3 films are generally characterized by a rough morphology, with micro-peaks and valleys all

over the surface. This can be rationalized by considering the deposition mechanism inferred in the

previous section. The angular Al2O3 fragments impact the softer and ductile substrate, deform it,

and remain adhered, thus generating a highly corrugated and uneven morphology. Figure 8 shows

the roughness parameters of the aluminium substrate and Al2O3 coating, as determined by

profilometry. The average roughness Ra and Rz of the coatings were typically found to be in the

ranges ~0.450.68 m and ~3.24.8 m, respectively, i.e. higher than those of the uncoated surface

(Ra < 0.4 m, Rz < 3 m). Only sample 3 unexpectedly exhibited a somewhat smoother surface.

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The inferior surface finish achieved on the FB-treated samples is apparently in contrast with data

reported by one of the authors in a previous work [31], in which the FB process was inferred to be

able to remarkably improve the surface finish of a similar aluminium alloy. Nonetheless, in that

case, the impacts of the alumina particles on the aluminium surface were characterized by very low

(i.e., acute) impact angles. Consequently, the alumina particles exerted a sort of cutting action on

the peaks characteristic of the aluminium surface, thereby producing a progressively better overall

quality of the surface finish (Ra < 0.2 m). In the present case, the impacts between the alumina

particles and the aluminium surface are mostly normal (i.e., almost at right angles). Therefore,

beyond the accumulation of Al2O3 fragments at the metal interface, a kind of micro-grooving of the

exposed substrate, even with the generation of local deep valleys, is caused by the repeated impacts

of the Al2O3 powder. On the other hand, the observed surface morphology is also due to the growth

characteristic of the Al2O3 coatings, that is, a progressive superimposition of small fragments,

which, owing to their nature (angular geometry), cannot produce smooth surface finishes.

Figure 9 report X-ray diffractograms of untreated aluminium substrates and after 4h FB Al2O3

deposition. /2 X-Ray Diffraction (XRD) shows the presence of small Al2O3 peaks after FB

deposition process. Yet, the intensity of Al2O3 peaks tends to dramatically increase when grazing

incidence tests are performed (=1.0°), that is, when the material sampled mostly belong to the

outermost layer of the film and, therefore, richer in Al2O3. According to [32-33], the X-Ray

penetration depths of XRD in /2and grazing incidence analysis (=1.0°) can be calculated using

the following formulas:

sin2

1ln xRt

(1)

sinsin

sinsin

2

1ln

xR

t (2)

where is the linear attenuation coefficient, the incident beam angle to the surface, the angle of

diffraction beam to the surface, Rx is the intensity of the diffraction beam between infinite thickness

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and thin film sample (0.935 is the most common value used [34]). By substituting each value in Eq.

1 and 2, the X-ray penetration depths were estimated in the range 10-80 m for /2 analysis and 3-

5 m for grazing incidence analysis (=1.0°), that means well below the measured thickness of the

Al2O3 film after 4h FB deposition.

Figure 10 shows a cross-section of an Al2O3 coating obtained after 4h FB deposition. The coating

was clearly discernible using the BSE detector of the SEM apparatus, with the darker colour of the

film compared with that of the bulk aluminium. The coating thickness can be approximately

estimated to be > 10 µm.

Figure 11 shows the EDXS oxygen distribution over the coating cross-section (red dots). The

oxygen concentration appears to decrease on going from the outermost layers of the surface to the

bulk coating. Quantitative EDXS data shown in Figure 12, recorded at the locations indicated in

Figure 11, confirm these results. Oxygen concentration was found to decrease progressively from

the top (about 30%) towards the bottom (close to 0%). Thus, the maximum oxygen concentration

was found at the top of the Al2O3 coating. This can be rationalized by considering the inferred

formation mechanism of the Al2O3 coating. During the initial stage of the FB deposition process,

the harder Al2O3 particles impinge on the surface of the softer and ductile aluminium substrate. The

substrate tends to be locally highly deformed under the action of the incoming Al2O3 particles.

Some fragments of Al2O3 particles can penetrate inside the external layer of the aluminium

substrate and form the first layer of material. This yields a kind of composite with the aluminium

alloy, in which the aluminium alloy is the metal matrix and the embedded fragments of Al2O3

particles serve as the reinforcement (filler). As has been established, on increasing the FB

processing time, further Al2O3 particles impact on the surface of the freshly formed composite

material, pushing the harder fragments of Al2O3 deeper inside the aluminium matrix and leaving

more fragments of Al2O3 to deposit on the outermost layer of the composite. In this way, an

outermost and newly forming layer of the composite material richer in Al2O3, that is, richer in terms

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of the percentage of reinforcement inside the metal matrix, is superimposed onto an innermost layer

of composite material poorer in Al2O3. The iteration of such phenomena generates a graded

material, which is progressively poorer in Al2O3 on going from the coating surface toward the

interface with the aluminium substrate. Of course, this phenomenon is self-limiting and this

determines a threshold to the maximum attainable thickness of the Al2O3 coatings by FB deposition.

In fact, in agreement with the discussion in the previous section, after a very long deposition time,

the newly incoming Al2O3 particles find an outermost layer of composite material that is very rich

in the reinforcement, which acts as a barrier to further embedding of Al2O3 fragments. At the same

time, the incoming Al2O3 particles are projected towards the coating at a pressure that is insufficient

to keep on pushing the already consolidated Al2O3 fragments deeper inside the innermost portion

(bulk) of the aluminium substrate. Therefore, a sort of saturation effect can be inferred, whereby the

outermost layer of the coating is made essentially from Al2O3 and does not allow the embedding of

further Al2O3 splinters nor the movement of the already adhered Al2O3 splinters to within deeper

layers of the aluminium substrate.

The resulting Al2O3 coating can therefore be inferred to be like a type of ‘graded’ coating

characterized by external layers made essentially from Al2O3 superimposed on layers made of

Al2O3 particles embedded in the Al substrate, with the concentration of Al2O3 decreasing from the

top to the bottom of the modified layer.

The inferred gradient-like structure of the Al2O3 coating was confirmed by compositional depth

profiling with GDOES. Figure 13 reports atomic % of Al and O across the outermost 15 m of the

Al2O3 coated substrate after 4h FB deposition. A progressive decrease in oxygen concentration can

be revealed going from the surface, where its amount almost matches the stoichiometric amount of

oxygen inside the alumina, to the inner part of the Al2O3 film, where, after nearly 10 micron, it

approaches a very low asymptotic value (< 5%, and ascribable to the natural oxidation of the cross-

sectioned aluminium sample). Similarly, the aluminium content is minimum at the surface and its

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value is that expected according to the stoichiometry of its oxide. The aluminium content tends to

progressively decrease across the thickness of the film up to reach very high concentration starting

from 10 mm below the surface. The concurrent steady decrease in oxygen concentration and

increase in aluminium concentration across the thickness of the film supports that FB technology is

able to deposit gradient Al2O3 film onto the underlying aluminium substrate, with the outermost

layer being mostly composed of Al2O3.

3.3 Mechanical characterization

As regards hardness values, as measured through depth-sensing micro-indentation tests at 0.25 m

below the coating surface, it can be noted (Figure 14) that they increase with the deposition time

from about 2.5 GPa for the sample after 30 min FB processing to about 7.0 GPa for the samples

obtained after longer processing times (> 60 min). The measured hardness values are very low if

compared, for example, to high hardness alumina coatings deposited by gas tunnel or low power

plasma spraying, where hardness of 13 to 15 GPa can be achieved [35-36]. Similarly, conventional

thermally spraying processes can lead to alumina coatings whose hardness average 8 to 11 GPa

[25], still rather larger than the values measured after FB deposition. To the contrary, hardness of

anodic alumina coatings can also be definitely lower, averaging 2.5 to 4 GPa [10, 23]. Therefore,

FB can lead to the deposition of comparatively harder protective layers.

These low micro-hardness results are probably due to two main reasons. Firstly, the influence of the

substrate must be taken into account, even if the test was performed only at about 10% or less of the

measured thickness of the alumina layer, which is particularly relevant for the very thin layer

deposited in 30 min. Secondly, the peculiar microstructure of the coatings, formed by the

continuous superposition of brittle alumina particles and debris at room temperature, cannot be

compared to a fully dense physical- or chemical-vapour-deposited film, and thus its micro-hardness

is much lower than in the mentioned cases, but higher than in the case of bare aluminium alloy or

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even of most of the alumina films produced by means of anodizing treatments. The experimental

results were found to be subject to significant deviations. Nonetheless, a high degree of uncertainty

in the data was to be expected, with depth-sensing micro-indentation tests being performed on as-

deposited coatings presenting highly irregular morphologies, thereby significantly influencing the

test procedure.

The trend of the hardness with the depth below the surface is, as expected, decreasing (Figure 14).

Three different branches can be identified: a first branch at very low depth (<0.5 m) where the

hardness of the Al2O3 coatings approaches their maximum values whatever the FB deposition time.

A second branch at intermediate depth (1-3 m) where a steep decreasing in the coatings hardness

can be observed. A third branch (> 5 m) where the hardness nearly approaches the typical

hardness of the underlying aluminium substrate. Such trend is not unlikely to occur in hard ceramic

coatings or oxides deposited onto softer metal substrates. Korsunski et al. [37] were the first to

propose such trends, showing how the composite hardness of a hard coating deposited onto a softer

substrate can be described by the use of a limited number of fitting parameters, including the

indentation depth (independent variable), the film thickness, the substrate hardness (constants), the

film hardness and a fitting parameter related to the coating fracture energy.

Figure 15 reports a FEG-SEM observation of a FB deposited Al2O3 coating after standard Vickers’

test. The Al2O3 coating was found to follow the plastic deformation of the aluminium substrates

under the indenter pressure and neither small cracks nor coating delamination could be clearly

discerned. Increasing the applied load (Figure 16), first appearance of small localized cracks can be

perceived around the edge of the Vickers’ nearly squared impression. Further, first delamination

phenomena can be detected inside the Vickers’ impression. In fact, the darker zone, which can be

clearly seen at the very bottom of the Vickers’ impression, is due to the onset of the Al2O3 coating

delamination phenomena. The coating delamination was found to cause the local detachment or

displacement of the overlying alumina coatings and the concurrent appearance of the underlying

MATS-09-1104, Barletta, Page 17

aluminium substrate. Accordingly, the Al2O3 coatings did not exhibit the brittle behaviour

commonly found for similar films grown by different deposition techniques [6, 10, 23, 25].

However, a maximum allowable load of the Al2O3 coating can be defined as the load beyond that

coating detachment or delamination phenomena take place, thus leaving the underlying aluminium

substrate exposed (unprotected). Maximum allowed load of the Al2O3 coatings could be easily

determined for each of the aluminium samples treated for different FB processing times by SEM

imaging of the Vickers’ impression at the different applied loads. Table 3 shows the relevant

experimental results. A consistent trend in the maximum allowable loads in dependence on FB

processing time may be noted, with the samples treated for 30 min and 240 min having maximum

allowable loads of 25 g and 100 g, respectively.

3.4 Scratch and wear tests

The results of scratch tests performed on the coated aluminium substrates obtained after different

processing times confirmed that once a suitable FB deposition time had been reached, an increasing

scratch resistance was imparted to the sample surface. During the tests, the friction force of the

sliding diamond indenter against the alumina layer was also recorded. The samples treated for 30

min showed larger friction forces as a result of their irregularly deposited alumina coatings (Figure

17). This result was surely affected by the contact conditions between the scratch indenter and the

substrate surface. Increasing the FB deposition time resulted in lower friction forces. The difference

in friction force between the samples treated for 30 min and those treated for longer was more

obviously apparent when larger loads were applied during progressive scratch tests. In fact, the

sparse Al2O3 coating typical of the samples FB processed for 30 min showed its limit when

progressively harsher contact conditions between the substrate and indenters were reached.

However, even under higher loads, neither film delamination nor the initiation and propagation of

cracks from the scratched zone was observed, as shown by FEG-SEM analysis and the scratch

MATS-09-1104, Barletta, Page 18

patterns (Figure 18) of all of the examined samples. These results can be ascribed to the inferred

‘graded’ structure of the Al2O3 coatings. Under the action of the advancing scratching geometry, the

graded coatings behave more like a semi-ductile metal matrix composite (MMC) than like a brittle

ceramic material and this is a result of the presence of the aluminium matrix at different

concentrations. Specifically, the coating is richer in aluminium matrix on going towards the

innermost layers. This permits unmatched interfacial adhesion between the innermost layers of

Al2O3 coatings rich in Al and the aluminium substrate. Moreover, the Al2O3 coatings display their

optimal ductility when submitted to higher scratch loads, whereupon the indenter penetrates deep

inside the coating material and finds its innermost layers, which are progressively richer in

aluminium matrix and so more prone to withstand the action of the scratching geometry without

exhibiting brittle failure.

Higher magnification SEM images by SE and BSE detectors of the last part of the scratch pattern of

the Al2O3 coatings after 240 min FB deposition confirm how neither cracks nor delamination

phenomena take place (Figure 19).

Examination of the 3D maps of scratch patterns in Figure 20 shows how longer deposition times

lead to more scratch-resistant coatings, thus supporting the previous experimental findings. In

particular, the experimental data presented in Tables 4 and 5 show that longer deposition times lead

to coatings characterized by less extensive scratch patterns, that is, with smaller piles and ditches. In

fact, the volume of the ditch is reduced about 2.5-fold on increasing the processing time from 30 to

240 min. Besides, films deposited by longer FB processes are characterized by a scratch behaviour

in which the amount of pile-up is reduced about three-fold and is mainly caused by the plastic

deformability of the unprotected or less protected aluminium alloy substrate, or, more accurately, by

the deformability of the innermost layers of the coating that are richer in the ductile aluminium

matrix.

MATS-09-1104, Barletta, Page 19

Figure 21 displays the scratch behaviour of 4h FB treated and untreated aluminium substrates after

progressive load scratch tests using both the 200 and 800 m radius indenters. Examining the

deformation behaviour of the sample during the application of the scratch load (i.e., penetration

depth) and after releasing it (i.e. residual depth), minimum differences between the Al2O3 coated

and uncoated samples can be emphasized, particularly when the 800 m radius indenter is used and

the maximum penetration depth reached (6 to 8 m) is, therefore, lower than the expected thickness

of the ‘graded’ Al2O3 film. This should not surprise as the scratch resistance of a thin coating is

strictly correlated to the capability of the underlying substrate to sustain it, thus withstanding the

action of the scratch indenter and refraining the coating from collapsing under the action of a

concentrated load [3]. In the present case, the aluminium substrate is not able to sustain the

overlying Al2O3 coating and, under the action of the scratch indenter, it tends to plastically deform.

Accordingly, similar deformation behaviour is detected for both Al2O3 coated and uncoated

substrates.

SEM images of the residual scratch pattern and some details of the scratch bottom taken at higher

magnification are reported in Figure 22. The ability of the Al2O3 coating to follow the underlying

aluminium substrates during the scratch tests is confirmed, whatever the choice of the tip radius of

the scratch geometry. There is no evidence of brittle failure neither when the 200 m tip radius

indenter is used and penetration depths well above the maximum expected thickness of the Al2O3

coating are approached to (i.e., ~13-18 m, during the loaded scratch scan). This result is only

apparently surprising. In fact, Al2O3 coating is well known to be rather brittle and, in any case, it

should not be able to follow the deformation of a ductile metal under the action of a very

concentrated load, as the load applied by a scratch indenter can be. Yet, the Al2O3 coating here

investigated is a gradient coating in which layers of material progressively richer in Al2O3 and

poorer in Al are superimposed onto a ductile aluminium substrate. This confers an unexpected

ductility to the Al2O3 coating, thus making it able to nearly plastically deform under the action of

MATS-09-1104, Barletta, Page 20

the scratch indenter, or, more properly, to follow the deformation of the underneath aluminium

substrate. Figures 22c to 22f also show the action of the advancing scratch indenter tends to

compact the Al2O3 coating. Moreover, the higher specific pressure of the 200 m tip radius indenter

is able to produce an even stronger compaction of the outermost layers of the Al2O3 coating, with

only minimum porosity being still visible at the very bottom of the scratch pattern (Figure 22 e).

An attentive analysis of the penetration and residual depth trends shows that some significant

differences in the trends of the Al2O3 coated and uncoated aluminium substrate can be noticed at

high load (> 12.5 N), when the 200 m tip radius indenter is used. Aluminium samples deforms

slightly more during the application of the scratch load and recover slightly less (~12 m residual

depth at 15 N scratch load vs. ~10 m for the Al2O3 coated substrates). This different behaviour can

be more likely ascribed to the extra-scratch resistance the Al2O3 coating is able to confer to the

softer aluminium substrate, which are more apparent at any time higher scratch loads are involved.

Significant differences can be also noticed by examining the trends of the tangential force and

friction coefficient in Figure 21. When scratch tests are performed by using the 800 m tip radius

indenter, the Al2O3 coatings originate higher friction and, accordingly, higher tangential forces. This

can be ascribed to the rougher morphology of the Al2O3 coated substrate, which is particularly

influential when low specific loads are used during the scratching procedure and, therefore, low

penetration depths are involved. When the 200 m tip radius indenter is used, an opposite scenario

can be highlighted. This time, the uncoated samples are characterized by a higher friction

coefficient and larger tangential forces are correspondingly involved, with such phenomena being

particularly apparent at higher scratch load (>12.5 N). However, as said before, at high load, the

aluminium substrate is slightly less scratch resistant and, above all, less prone to recover after the

cessation of the scratch load. This means that deeper layers of the material are involved during the

scratch test and the indenter will inevitably encounter more resistance along its pattern.

MATS-09-1104, Barletta, Page 21

Although the large difference in the deposition process and in the main characteristics of the Al2O3

coatings deposited by FB, their scratch resistance could be usefully compared with similar data

available on black anodic films deposited on aluminium substrates [38-39]. Goueffon et al.

measured the coating adhesion by using a Revetest Instrument with a Rockwell diamond stylus, 200

m tip radius. They found a very brittle behaviour of the anodic films, with coating debris

distributed along the whole scratch pattern [38]. Furthermore, critical loads of ~7 N (onset of

coating failure) were measured on coatings ~20 m thick [39]. As said before, it is extremely

complicated a direct comparison between the scratch resistance of FB deposited Al2O3 coatings and

black anodic films. Yet, it is worthwhile emphasizing how, under similar testing conditions, the first

ones are able to withstand scratch load of 15 N, at least. Furthermore, they did not exhibit brittle

behaviour even under the harshest testing conditions. To the contrary, black anodic films do exhibit

a definitely lower scratch resistance, joined with a higher brittleness even at very low applied load.

Continuous load scratch tests revealed the sensitivity of the Al2O3 coating to the scratch speed is

very low (Figure 23). In fact, the recovery capability of the Al2O3 coating is always approximately

the same apart from the scratch speed set. This is quite unusual for a potential brittle coating, whose

capability to withstand to a dynamic solicitation is function of the speed at which the load is

applied. Yet, as said before, the Al2O3 coating here investigated is a ‘graded’ coating and, therefore,

it explicates a rather ductile behaviour.

Figure 24 reports SEM images of the residual scratch patterns after testing the Al2O3 coating with a

100 m tip radius indenter. This time, the Al2O3 coating is not able to withstand the action of the

scratch indenter and, under the very high specific load, it is removed from the underlying substrate

and ‘ductily’ displaced sideways and in front of the scratch pattern by the advancing scratching

geometry. Figure 25 shows, at different level of magnification, the displacement of the Al2O3

coating sideways (Figure 25c) and in front of the scratch pattern (Figure 25d) after continuous load

scratch test, with some debris of coating material being still clamped to the substrate surface.

MATS-09-1104, Barletta, Page 22

Tribological performance of the Al2O3 coated (4h FB treated) and uncoated aluminium substrates

was tested by both pin-on-disk and linear reciprocating. Al2O3 coatings offer a significant extra-

protection to the underlying aluminium substrate under both testing conditions. In particular, at 2.5

N as applied load, the Al2O3 coating is not worn out after pin-on-disk and linear reciprocating test

(Figure 26a). Therefore, no appreciable material loss can be estimated after the wear tests. To the

contrary, some material is detached from the stainless steel counterpart and it is embedded on the

coating surface. This is clearly shown by SEM images of the pin-on-disk wear pattern in Figure 27,

reported at different magnification (i.e., the zone with the brighter material spread over the circular

wear track). Under even lighter testing condition (1 N), the unprotected aluminium substrate is

severely worn out. Significant wear volume can be measured after both pin-on-disk and linear

reciprocating tests, as Figure 26b clearly shows. Pin-on-disk tests produce a wear volume of 0.235

mm3 after 100 m sliding distance (~27 m, maximum depth of the worn area) and a wear volume of

0.513mm3 after 500 m sliding distance (~37 m, maximum depth of the worn area). Linear

reciprocating test produces a wear volume of 0.133 mm3 after 500m sliding distance (~68 m,

maximum depth of the worn area). Increasing the load at 5N, the Al2O3 coating (after 4h FB

deposition) fails. SEM images in Figure 28 display a detail of the wear pattern, with the Al2O3

coating clearly detached from the underlying aluminium substrates and displaced sideways. Some

debris along the wear pattern is clearly visible, as well.

Comparing the wear resistance of the Al2O3 coatings deposited by FB and coatings belonging to

similar classes (i.e., anodic films on aluminium alloys) can be troublesome. Bensalah et al. tested

the tribological behaviour of anodic films on aluminium alloy (AA 1050) by an automatic lapping

machine, sand paper (320 grit, SiC) as counterpart, a 5N distributed load (0.0125 N/mm2 pressure)

and 20 rpm rotating speed [40]. Although the very soft loading conditions, the anodic films

underwent significant wear, with weight loss by abrasion, averaging 15 to 60 mg/min [40]. On the

other hand, Bolelli et al. evaluated the wear resistance of rather thick anodic films (>50 m)

MATS-09-1104, Barletta, Page 23

achieved by conventional and hard anodizing treatments on the same aluminium alloy (AA6082T6)

here investigated [41]. Testing conditions included 10 N load, 0.2 m/s sliding speed and 3 mm

diameter WC-Co (6% wt.) counterpart. Conventional sealed anodic films did fail almost

instantaneously and the coating was completely removed after 1000 m. Instead, anodic films

deposited by hard anodization were more wear resistant and characterized by a sliding wear rate of

~10-5

mm3/Nm. In any case, SEM images of wear tracks of the hard anodic films revealed they

were severely cracked, with evidence of coatings delamination and cracks extending out of the

worn area. In view of that, although a direct comparison of the tribological behaviour of Al2O3

coatings deposited by FB and anodic films on aluminium alloys is quite complicated, the available

experimental data validate the hypothesis that FB technology could be a viable method to deposit a

wear resistant coating on aluminium alloys, which can confer extra-protection and, at least, assure

tribological properties comparable with the competing anodic films.

MATS-09-1104, Barletta, Page 24

4. Conclusions

Al2O3 coatings on AA6082 T6 alloy have been produced by the Fluidized Bed (FB) technology,

and they have been characterized in terms of microstructure and mechanical features.

The FB process was found to coat aluminium alloy substrates with a tough and well-adherent Al2O3

coating. Surface morphology was found to be strongly affected by the FB processing time. FEG

observation of the cross-section revealed a coating thickness of about 10 μm.

The structure of the Al2O3 coatings was found to be characterized by a varying concentration of

Al2O3 within an aluminium matrix. Therefore, the coatings were assessed as being graded in nature,

with the innermost layers being poorer in Al2O3, the concentration of which becomes progressively

higher on moving towards the outermost layers.

The mechanical performance of the Al2O3 coatings on the aluminium substrates was tested by both

indentation under different loading conditions and by scratch and wear tests. Micro-hardness tests

demonstrated that the hardness of the aluminium alloy was increased by the presence of the alumina

layer, even if the absolute values were rather low. A nearly constant value of about 7.0 GPa was

attained for samples obtained after a processing time of 60 min or more, while samples processed

for 30 min were still too defective (2.5 GPa) and thin. Finally, the Al2O3 coatings displayed good

deformability under the action of advancing scratching geometry as a result of their mixed

composition comprising harder Al2O3 fragments and the ductile Al matrix. This makes the Al2O3

coatings more resistant to the action of the scratching indenter, without the occurrence of brittle

failure. Moreover, the Al2O3 coatings were found to allow better adhesion strength, with scratch

patterns being characterized by progressively lower piles and ditches whenever longer FB

processing times were applied. Indeed, scratch resistance of Al2O3 coatings after 4h FB deposition

time was found to be, at least, 15 N, even if a sharp 200 m tip radius indenter is used during the

scratch procedure.

MATS-09-1104, Barletta, Page 25

The rather ductile behaviour of the FB deposited Al2O3 coatings was confirmed by the constant load

scratch tests, which demonstrated the lack of sensitivity of the scratch response to the scratch speed.

This is rather unusual for a brittle material like Al2O3 and, at ambient temperature, rather typical for

a ductile metal, whose mechanical performance is not much affected by the deformation rate (i.e.,

rate at which the load is applied). Failure of the FB deposited Al2O3 coatings by scratch test is

identified by using a sharp 100 m tip radius indenter, with the coating material displaced sideways

and in front of advancing scratching geometry. Wear resistance of FB deposited Al2O3 coatings is

rather high provided that mild load are applied (<5 N). Therefore, FB deposited Al2O3 coatings

confer extra-wear protection to the underlying aluminium substrates.

In conclusion, although a direct comparison of the scratch and wear resistance of Al2O3 coatings

deposited by FB and competing coatings (for example, anodic films on aluminium alloys) is

definitely troublesome, the available experimental findings support the general idea that FB

technology can be a simple, cheap and eco-sustainable method to deposit a scratch and wear

resistant protective thin layer on aluminium alloys.

MATS-09-1104, Barletta, Page 26

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List of Captions

Table 1. Aluminium AA 6082 T6 composition.

Table 2. Processing times and sample labels.

Table 3. Maximum allowed load vs. processing time.

Table 4. Results of 3D maps of progressive scratch patterns according to FB processing time.

Table 5. Results of 3D maps of progressive scratch patterns according to FB processing time.

Figure 1. Fluidized bed system: experimental set-up.

Figure 2. Map of the scratch strategy.

Figure 3. The growth of the Al2O3 thin film: mass accumulation vs. FB processing time.

Figure 4. Morphology of the Al2O3 thin film after 15 min of FB processing: the surface is not

completely coated.

Figure 5. Morphology of the Al2O3 thin film on sample 1 (30 min): complete surface coverage.

Figure 6. Morphology of the Al2O3 thin film on sample 4 (240 min): (a) overall view; (b) detail of

Al2O3 debris.

Figure 7. Morphology of sample 2 (60 min) after cleaning under sonification: (a) large defects, (b)

small cracks.

Figure 8. Roughness parameters of untreated and FB-treated samples.

Figure 9. /2 and grazing incidence (=1°) X-Ray Diffraction (XRD).

Figure 10. Cross-sections of sample 4 (240 min): (a) SE detector; (b) BSE detector.

Figure 11. EDXS microanalysis of cross-section: location of EDXS linear spots and EDXS smart

map.

Figure 12. Quantitative EDXS analysis of linear spots in Fig. 9.

Figure 13. GDOES compositional depth profiling of Al2O3 film after 4h FB deposition.

MATS-09-1104, Barletta, Page 31

Figure 14. Depth-Sensing Indentation at different indentation depth performed on Al2O3 coatings

deposited by FB with different deposition time.

Figure 15. Micro-Vickers hardness test: FEG-SEM indentation images of 0.5 kg load test

performed on an aluminium substrate FB-treated for 4 h at different magnifications: (a) SE detector;

(b) BSE detector.

Figure 16. Micro-Vickers hardness test: FEG-SEM indentation images of 1 kg load test performed

on an aluminium substrate FB-treated for 4 h at different magnifications: (a) lower magnification;

(b) higher magnification.

Figure 17. Progressive load scratch test: friction force trends according to scratch pattern with FB

processing time.

Figure 18. FEG-SEM images of progressive scratch patterns according to FB processing time using

the BSE detector: (a) 30 min; (b) 60 min; (c) 120 min; (d) 240 min.

Figure 19. FEG-SEM of the last part of a scratch pattern after scratch test performed on an

aluminium substrate FB-treated for 4 h: (a) SE detector; (b) BSE detector.

Figure 20. 3D maps of progressive scratch patterns according to FB processing time: (a) 30 min

overall; (b) 60 min overall; (c) 120 min overall; (d) 240 min overall.

Figure 21. Signals after progressive load scratch tests.

Figure 22. Residual scratch pattern after progressive load scratch test with 200 and 800 m tip

radius: (a) 200 m tip radius overall view; (b) 800 m tip radius overall view; (c) 200 m tip radius

high load; (d) 800 m tip radius high load; (e) 200 m tip radius, magnification of the scratch

bottom at high load; (f) 800 m tip radius, magnification of the scratch bottom at high load.

Figure 23. Residual depth after continuous load scratch tests performed at different scratch speed

Figure 24. Overall view of progressive and continuous load residual scratch patterns using an

indenter with a 100 m tip radius

MATS-09-1104, Barletta, Page 32

Figure 25. Residual scratch pattern after continuous load scratch test using an indenter with a 100

m tip radius: (a) overall view; (b) high load; (c) coating material displaced sideways; (d) coating

material displaced in front of the advancing scratch geometry.

Figure 26. Wear pattern after pin-on-disk and linear reciprocating tests at 2.5 N and different

sliding distance: (a) Al2O3 coated (after 4h FB deposition) aluminium substrate; (b) Uncoated

aluminium substrate.

Figure 27. Wear pattern after pin-on-disk at 2.5 N and 500 m sliding distance on Al2O3 coated

(after 4h FB deposition) aluminium substrate at different magnifications.

Figure 28. Wear pattern after pin-on-disk at 5 N and 500 m sliding distance on Al2O3 coated (after

4h FB deposition) aluminium substrate at different magnifications.

MATS-09-1104, Barletta, Page 33

List of Tables

Table 1. Si Fe

max

Cu

max

Mn Mg Cr

max

Zn

max

Ti

max

Others Al

Each max Total max

AA6082T6 0.71.3 0.5 0.1 0.41.0 0.61.2 0.25 0.2 0.1 0.05 0.15 Bal.

Table 2. Sample label Processing time (min.)

1 30

2 60

3 120

4 240

Table 3. Processing time (min.) Maximum allowed load (g)

30 25

60 75

120 75

240 100

Table 4. Processing

time (min)

Volume of the

ditch (m3)

Volume of the

ditch (SD) (m3)

Max depth

(m)

Max depth

(SD) (m)

Mean depth

(m)

Mean depth

(SD) (m)

30 438749 56528 9.1 0.92 3.51 0.22

60 348927 49649 9.44 0.45 3.26 0.6

120 323454 44901 7.15 0.48 3.02 0.32

240 172054 37041 7.13 0.55 2.63 0.36

Table 5. Processing

time (min)

Volume of the

pile (mm3)

Volume of the pile

(SD) (mm3)

Max height

(mm)

Max height

(SD) (m)

Mean height

(m)

Mean height

(SD) (m)

30 259242 20696 16.5 1.78 3.75 1.27

60 236614 28388 15.4 1.12 2.82 0.86

120 188216 24349 15.4 1.15 3.93 0.43

240 84480 12226 12.0 0.38 2.41 0.49

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List of Figures

Figure 1

Figure 2

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Figure 3

Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Figure 10

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Figure 11

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Figure 12

Figure 13

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Figure 14

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Figure 15

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Figure 16

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Figure 17

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Figure 18

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Figure 19

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Figure 20

Figure 21

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Figure 22

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Figure 23

Figure 24

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Figure 25

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Figure 26

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Figure 27

Figure 28