The influence of higher surface hardness on fretting fatigue life of hard anodized aerospace AL7075-T6 alloy

Materials Science & Engineering A 560 (2013) 377–387

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The influence of higher surface hardness on fretting fatigue life of hard anodized aerospace AL7075-T6 alloy Ahmed A.D. Sarhan a,b, E. Zalnezhad a,c,n, M. Hamdi a a

Center of Advanced Manufacturing and Material Processing, Department of Engineering Design and Manufacture, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia b Department of Mechanical Engineering, Faculty of Engineering, Assiut University, Assiut 71516, Egypt c Faculty of Engineering, Islamic Azad University, Chalous Branch, Iran

a r t i c l e i n f o

abstract

Article history: Received 14 June 2012 Received in revised form 20 August 2012 Accepted 22 September 2012 Available online 27 September 2012

In this research work, the influence of higher surface hardness on the fretting fatigue life of hard anodized aerospace AL7075-T6 alloy was investigated. An optimization of the parameters of hard anodized Al7075-T6 alloy to obtain higher surface hardness was presented. Confirmation test was carried out to show the improvement after using the best parameter combination attained from the optimization process. The result shows that hard anodized coating (surface with hardness of 360 HV and thickness of around 17 mm) can significantly improve fretting fatigue life of specimens at low stress, while at high stress, the extent of fretting fatigue life decreases. Hard anodized coating with higher thickness (29 mm) and high surface hardness (393 HV) decreased the fretting fatigue life of specimens at stresses beyond 200 MPa, this mainly attributed to the coating brittleness and micro-cracking. & 2012 Elsevier B.V. All rights reserved.

Keywords: Aluminum7075-T6 alloy Hard anodized coating Surface hardness Fretting fatigue

1. Introduction Fretting fatigue occurs when a component is subjected to fatigue load while it is in contact with a mating component in a presence of contact load. This causes oscillatory micro-sliding at the contact surface, which encourages earlier crack initiation and thereby leads to premature, catastrophic material failure [1]. Fretting fatigue is a common occurrence in a multitude of components or structures involving riveted joints, bolted joints, wire ropes, and dovetail joints of gas turbine engines [2]. Aircraft engines, fuselage, automobile parts, and energy saving strategies in general, have promoted the interest and research in the field of lightweight materials, typically on alloys based on aluminum. Aluminum itself does not provide sufficient mechanical strength for structural parts. Aluminum alloy 7075-T6, which is used in this research work, has low specific weight and high strength to weight ratio as well as high electrical and thermal conductance. This alloy is widely used in industry and particularly in aircraft structure and pressure vessels [3–6]. Therefore improvements in surface properties are required in practical applications, especially when aluminum is in contact with other

n Corresponding author at: Center of Advanced Manufacturing and Material Processing, Department of Engineering Design and Manufacture, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia. Tel.: þ60 147256741; fax: þ 60 979675330. E-mail address: [email protected] (E. Zalnezhad).

0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.09.082

parts. Some researchers have been worked on fatigue of aluminum alloys with treatments include shot peening, laser shock peening, and low plasticity burnishing. The major benefit of these surface treatments is to modify the mechanical properties of the surface and near surface region through the induced plasticity, leaving the treated region in compressive residual stress states [7–12]. Depending on the nature of the treatment, the compressive residual layer might extend to several hundred microns into the depth of the material [13]. However, in order to diminish the deleterious effects of fretting fatigue, aerospace components have to be surface treated to increase the wear resistance. surface coatings such as physical vapor deposition (PVD), chemical vapor deposition (CVD), and electrophoretic deposition (EPD) are widely used to improve the wear resistance of materials especially on aluminum alloys because of their low wear resistance. Hard anodized coating is one of the most effective techniques which are used for this purpose. This coating is also promising in terms of the possibility of achieving high hardness, strength, and simultaneously good protective and decorative surface properties. Anodizing is an electrochemical process for producing stable oxide films on the surface of metals. Anodic coating can be produced on aluminum by using a wide variety of electrolytes with AC, DC, or a combination of both, in order to increase the hardness of metals. Some aluminum alloys do not accept hard anodized coating equally well. Hard anodized alloys with high copper or silicon content tend to be porous and not very hard.

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Table 1 The list of the aluminum alloys which should be avoided to hard anodizing. Difficult Al alloys for hard anodizing 2011 2017 2024 7075 Cast and wrought alloys with Cu 44% or Si 47%

Table 1 lists some of the aluminum alloys that are particularly troublesome and should be avoided [14–16]. With the advent of new technologies such as vacuum processing, the hard anodizing problem can be obviated. Pure aluminum coating on the AL7075-T6 using magnetron sputtering technique is a method to generate possibility for hard anodizing performance. The sputter process has almost no restrictions in the target materials ranging from pure metals where a DC power supply can be used, to semiconductors and isolators which require an RF power supply or pulsed DC. Deposition can be carried out in either non-reactive (inert gas only) or reactive (inert and reactive gas) discharges with single or multielemental targets. To improve surface hardness, optimizing the effect of hard anodized coating parameters is required for optimum surface hardness [17,18]. The conventional method is to use the ‘‘trial and error’’ approach, which is actually very time consuming due to the requirement of a large number of experiments. Hence, a reliable systematic approach for optimizing coating parameters is thus required [19]. Taguchi optimization method is an efficient, effective, reliable and simpler approach, in which the response parameters affecting surface hardness can be optimized. The steps in the Taguchi method include: selecting the orthogonal array (OA) according to the number of controllable factors, running experiments based on that OA, analyzing data, identifying the optimum parameters, and conducting verification tests with the optimal levels of all parameters. The aim of this present work is to investigate the effect of higher surface hardness on fretting fatigue performance of hard anodized specimen by conducting a series of rotary bending fatigue tests. For this reason, first of all, the specimens were coated with pure aluminum using magnetron sputtering technique. Second, the effects of surface modification (hard anodized coating) to optimize the process voltage, temperature, solution concentration and time on hardness, were studied using the Taguchi method. Taguchi’s method uses a statistical measure of performance called signal-to noise ratio (S/N), which is logarithmic function of desired output to serve as objective functions for optimization. The S/N ratio considers both the mean and variability into account. It is defined as the ratio of the mean (signal) to the standard deviation (noise). The ratio depends on the quality characteristic of the product/process to be optimized [20–23].

Table 2 Parameters and levels used in the experiment. Parameters

A B C D

Experimental condition levels

Voltage (V) Temperature(1C) Solution concentration (%) Time (min)

1

2

3

4

10 0 5 30

20 5 10 60

30 10 15 90

40 25 20 120

Table 3 Standard L16 (44) orthogonal array. Experiment

Parameters combination

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

A

B

C

D

1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4

1 2 3 4 1 2 1 4 1 2 3 4 3 4 1 4

1 2 3 4 2 3 3 2 3 4 1 2 4 3 2 2

1 2 3 4 3 2 3 2 4 3 2 1 2 1 4 3

Table 4 Chemical composition of AL 7075-T6. Cu

Si

Mg

Cr

Zn

Mn

1.85

0.47

1.8

0.28

4.6

0.06

Fig. 1. Drawings of the fretting fatigue specimen.

3. Experimental details 3.1. Specimen material and geometry

2. Design of experiment The most important stage in experimental design using the Taguchi approach lies in the selection of control parameters and identifying the orthogonal array (OA) [24]. This experiment comprised four parameters with four levels each, so the fractional factors design used was a standard L16 (44) orthogonal array, which was chosen due to its capability to check the interactions among parameters. The factors and levels were assigned as in Table 2, while Table 3 shows the sixteen experiments along with the combination details of the experiment condition levels for each control parameter (A–D).

Aluminum 7075-T6 alloy was used in this investigation. The material composition was obtained using the EDX apparatus, as illustrated in Table 4. From a number of tensile tests, the yield stress and ultimate strength of Al7075-T6 were obtained, sy ¼520 MPa and sut ¼590 MPa, respectively. Two types (uncoated and hard anodized) were employed for the fretting fatigue test. First, the specimens were machined with an initial surface roughness of Ra ¼0.6 70.1 mm by lathe turning (CNC Lathe Machine, Miyano, BNC-42C5, Japan). The round shape specimens used in this work, were prepared in accordance with the ISO standard [25,26]. A drawing and dimensions of the fretting fatigue specimen are given in Fig. 1.

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3.2. Sample preparation for hard anodized coating All samples for anodizing were initially coated by an AL target with a purity of 99.99% using a magnetron spattering machine. The surface of all samples for aluminum coating were polished with 800–2000 grit SiC paper, after which the samples were surface mirrored by diamond liquid and the substrates were ultrasonically cleaned in acetone for 14 min, thoroughly rinsed with distilled water and dried using nitrogen gas to avoid contamination. An SG Control Engineering Pte Ltd series magnetron sputtering system was used to experimentally deposit thin films of metal. This system contained 600 W RF and 1200 W DC generators with 4  12 in. electrodes 15 cm away from the target. To easily sputter the metals, DC generators were designed. The substrate carrier was circular and rotatable at various speeds for the required co-sputtering deposition. The chamber was evacuated to below 2  10  5 Torr before the argon gas for sputtering was introduced. Here, a constant sputtering pressure of 5.2  10  3 Torr. The pure aluminum coating process parameters employed in the present work are shown in Table 5. The adhesion of pure aluminum films to the substrate was determined using a Micro Material Ltd, Wrexham, UK. The data was stored in a digital computer and displayed on a screen. Subsequently, all pure aluminum coated specimens were subject to the electrochemical conversion of the hard anodizing process using the conditions shown in Tables 2 and 3. At the beginning, substrate cleaning was required to remove all unwanted surface contamination in surface preparation for further processing. Substrate surface finish was created by etching with a hot sodium hydroxide solution which removed minor surface imperfections. To remove surface oxides known as smut, which is a combination of inter metallic, metal and metal oxides remaining on the surface after cleaning/etching, an aqueous solution containing an oxidizing inorganic acid, phosphoric and sulfuric acids, simple and complex fluoride ions, an organic carboxylic acid with 1–10 carbon atoms and manganese in its oxidation state was used. Finally, a near-mirror finish was created with a concentrated mixture of phosphoric and nitric acids which chemically smoothed the surface. After cleaning, pure aluminum coated specimen (substrate) was suspended in an electrolytic bath (sulfuric acid at different

Table 5 The pure aluminum coating parameters. DC power (w)

Temperature (1C)

DC bias voltage

Time (h)

350

200

75

6

DC power supply

379

concentrations) as an anode, and current was passed through the bath, so oxygen was produced at the anode surface. The equipment used in the anodizing process comprised a power supply, electrolytic solution, anode (substrate material) and cathode (stainless steel) as shown in Fig. 2. The oxygen reacted with the substrate to form a thin oxide layer of durable and abrasion resistant while at the same time hydrogen was formed at the cathode. The anodizing process produced an oxide layer (coating) which was uniform, much harder and denser than natural oxidation. In addition, the hard anodizing coating layers increased the melting point of the substrate surface from approximately 650 1C to approximately 2000 1C, which is sufficient to ensure the maintenance of mechanical properties at higher temperatures. 3.3. Fretting fatigue testing preparation The specimens were gripped and loaded rotationally in a rotating bending fretting fatigue test apparatus developed inhouse (Fig. 3). By adjusting the load screw on a fretting ring with a torque driver, the normal contact load between the contact pads and specimen was controlled, as shown in Fig. 3(a) and (b). Fretting fatigue pads were fabricated from AISI 4140 steel plate with hardness of 346 HV. Substrate material (179 HV) is softer than the pads, in contrast to hard anodized coating (360 HV) which is harder than the pads. A ring-type load cell and bridge-type fretting pads were designed and manufactured to simulate fretting fatigue conditions. The friction force, created by normal force and a sliding movement between the specimen and pads, along with the friction coefficient, was measured by a friction test machine. The friction coefficient between pads (AISI 4140 steel) and AL7075-T6 was calculated around 0.607. The friction force was determined from the relation F¼ mP, in which P is the contact load calculated by the ring shaped load cell and F is the friction force measured with the friction test machine. The fretting fatigue tests were carried out at a constant average contact pressure of 100 MPa. When a fatigue specimen is subjected to cyclic stresses, fretting between the contact pads and specimen is generated. The samples used for the fretting fatigue test were uncoated and hard anodized with AL7075-T6. Plain and fretting fatigue testing were carried out at room temperature in a two-point loading, rotating bending machine (R¼  1) under constant stress amplitude at a rotational speed of 3000 rpm. The nominal maximum cyclic stress was set at a value that was expected to result in a fatigue life between 104 and 107 cycles, and tests were stopped if the specimen did not fail at 1  107 cycles. The three dimensional (3D) model of fretting fatigue specimen, pads and bearing were modeled according to the ISO-1143:2010

Acid sulfuric solution Fig. 2. Schematic of hard anodizing process.

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5

A pair of springs

Fretting ring

to apply the load Chuck

Friction pad

Fig. 3. Schematic of fretting fatigue test rig. (a) Drawing of fretting pad, (b) Fretting ring and (c) Rotating bending fretting fatigue test machine.

(E) in ABAQUS 6.10-1 (ABAQUS Inc., Providence, RI, USA). The specimen was meshed by 3D quadratic tetrahedral element. Mesh sizes decreased in order to carry out the convergence test and mesh independency to achieve the higher accuracy of results in ABAQUS. The fretting specimen was considered as linear elastic material with young modulus of 72 GPa. The Poisson’s ratio was considered constant and equal to 0.3. The bearing and pads considered as rigid material. Surface to surface contact with the small sliding considered between the pads and specimen, and bearings and specimen. The friction coefficient of 0.3 assigned to the interfaces of pads and specimen. However, the interaction in the specimen and bearings was frictionless. The specimen movement was restricted in axial direction. Meanwhile, bearings and pads movement was limited in axial and perpendicular directions. The loads magnitude which applied on the bearings and pads were 150 and 100 Mpa respectively.

4. Hard anodizing results and data analysis for higher surface hardness 4.1. Pure aluminum thin film to substrate adhesion strength results The pure aluminum thin film-to-substrate adhesion strength was measured quantitatively using a scratch tester (Micro Material Ltd, Wrexham, UK). An image accompanied by graphs (depth and load versus distance) are shown in Fig. 4. A diamond indenter (Rockwell type) of 25 mm radius applied an initial load of zero onto a sample at a sliding velocity of 5 mm/s. The load was increased gradually by 9.2 mN/s. The scratch length during the scratch test was 800 mm. In this test, critical load (Lc) could be used to calculate adhesion strength. In order to obtain the magnitude of the critical load, acoustic signal, friction curve, and microscope observation was utilized. Acoustic signal produced by the film delamination

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Table 6 The measured surface hardness and calculated (S/N) ratio.

Failure point

Experiment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

641.23µm

846.21µm

12000

Failure point

10000

Depth (nm)

381

8000 6000

Measured surface hardness (HV) 1st

2nd

3rd

241 266 259 185 289 266 361 181 338 179 230 198 201 214 239 185

236 273 247 179 297 273 343 200 355 165 235 183 181 211 221 202

218 237 243 180 268 255 375 188 328 204 246 199 220 229 253 176

Average surface hardness (HV)

Calculated S/N ratio

230 245 250 181 285 265 360 190 341 183 237 193 201 218 238 187

4.723 4.76 4.796 4.515 4.91 4.846 5.113 4.557 5.063 4.523 4.75 4.571 4.606 4.677 4.573 4.544

4000

surface hardness of hard anodized coated AL7075-T6 rises with the increase of coating thickness. The maximum surface hardness achieved in the experiment was 360 HV, which compared to substrate (AL7075-T6 alloy) surface hardness, 101.12% improvement is obtained.

849

743

796

690

637

531

584

478

425

319

372

213

266

-2000

160

1

54

0

107

2000

Distance (µm) 2500

Load (mN)

2000

4.3. Data analysis for higher surface hardness

Failure point

1500 1000

851

801

751

701

651

601

551

501

451

401

351

301

251

201

151

1

51

0

101

500

Distance (µm) Fig. 4. Scratch force (adhesion) testing on a coated sample and the critical load accompany with their force and depth versus distance graphs.

could be used to characterize the critical load (Lc) and show the failure character of pure aluminum coating on AL7075-T6. The failure load was about 2035 mN at a distance of roughly 641 mm. 4.2. Hard anodizing results After orthogonal array identification, the next step in the Taguchi optimization method was running the experiment based on the respective OA shown in Table 3. The hardness surface layers were measured using micro-hardness equipment (HMV micro-hardness tester Shimadzu, Japan). The measurement was repeated three times and the measured values are summarized in Table 6. The layers were characterized using scanning electron microscopy (FE/SEM-FEG), focused ion beam technique (Quanta FEG250, Netherlands). Fig. 5 illustrates a typical example of a hard anodized coating, where SEM indicates that the coating structure is columnar. There are two types of coatings, the first of which is pure aluminum on the substrate and the second layer is hard anodizing coating in two directions (inside and outside the pure aluminum coating). Hard anodized coating thickness at different conditions is illustrated in Table 7. It seems that the

The procedure following the experimental runs was to analyze data in order to optimize the parameters and identify which process parameters are statistically significant. Data analysis was conducted using signal to noise (S/N) response analysis. The method for calculating S/N ratio is classified into three main categories, depending on whether the desired quality characteristics are smaller the better, larger the better, or nominal the better. In the case of surface hardness, the larger values were required. The equation for calculating the S/N ratio for the larger the better characteristic (in dB) is the following   1 X1 S=N ¼ 10 log ð1Þ 2 n y where y is the observed data, and n is the number of observations. For each type of characteristics, with the above S/N ratio transformation, the higher the S/N ratio, the better is the result. The S/N values function as a performance measurement to develop processes insensitive to noise factors. The degree of predictable performance of a product or process in the presence of noise factors could be defined from S/N ratios. Table 6 shows the calculated S/N ratio for surface hardness, while Table 8 provides the S/N response values for the measured data which is plotted as shown in Fig. 6. As an example of S/N response calculation, Ai in Fig. 6 is the average of all S/N ratios in Table 6 that have the same experimental level (i) under A (Table 3). In this case, (i) is equal to 1, 2, 3 or 4, corresponding to four parameter levels. Similarly, the S/N response values were calculated for Bi, Ci and Di. The desired ‘‘higher the better’’ criteria implies that the highest S/N would reflect the best response, which results in the lowest noise influence on the machine setup. This is the criteria employed in this study to determine the optimal coating parameters for the highest surface hardness. As seen in Fig. 6 and according to the higher (S/N) response base, the voltage (factor A), temperature (factor B), solution concentration (factor C), and time (factor D) were significant in determining the best surface hardness.

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Hard anodizing Pure aluminum

Substrate

Fig. 5. SEM micrograph of hardanodizingon AL7075-T6 at voltage of 20 V, temperature 0 1C, solution concentrate 15%, and time 90 min.

400

Table 7 The thickness of hard anodize coating on different samples.

Fretting fatigue Bending stress (MPa)

350

Specimen Hard anodize coating thickness(mm) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

9.10 12.30 16.20 8.30 15.45 13.60 17.50 11.70 16.65 8.70 12.40 9.81 10.40 8.80 10.50 9.30

A B C D

Coating parameters

200 150

50 1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

No. of cycle to failure Fig. 7. S/N curve of plain fatigue for uncoated specimens.

S/N response

Voltage Temperature Solution concentration Time

250

100

Table 8 The (S/N) response values for surface hardness. Symbol

Plain fatigue

300

Level 1

Level 2

Level 3

Level 4

4.6985 4.9124 4.7563 4.657

4.8565 4.71 4.7625 4.7038

4.7267 4.72 4.899 4.7771

4.645 4.5728 4.548 4.7773

The voltage parameter level (A2), temperature level (B1), solution concentration level (C3) and time level (D4) appear to be the best choices for obtaining a high surface hardness value. Therefore, the optimal combination for achieving a high surface hardness value is A2B1C3D4 within the tested range. At this level, the confirmation test was carried out using the best parameters combination to validate the finding. The surface hardness obtained from this confirmation test was 393 HV, showing an improvement of 9.2% compared with the highest surface hardness value obtained during the experiments shown in Table 6. Therefore, an overall improvement of 119.55% in surface hardness was achieved as opposed to uncoated specimens (which had surface hardness values of 179 and 393 HV).

5. Fretting fatigue test results 5

In order to investigate the fretting fatigue life of hard anodized AL7075-T6 alloy at higher surface hardness, a number of experiments were performed. The experiments were conducted for a stress ratio of R¼ 1, 50 Hz at a constant contact force of 100 Mpa and working stress amplitudes of 150–300 MPa. The relationship between stress amplitude and the number of cycles to failure for all the conditions analyzed is defined by Eq. (2) [27].

S/N response values

4.9 4.8 4.7 4.6 4.5

Voltage (V) Temprature (˚C) Solution concentration% Time (min)

4.4 4.3 10 20 30 40

0

5

10 25

5

10 15 20 30 60 90 120

Hard anodize coating parameters lavel Fig. 6. The S/N response graph for surface hardness.

S ¼ ANbf

ð2Þ

where S is stress amplitude, A is fatigue strength coefficient, b is fatigue strength exponent and Nf is the number of cycles to failure. The S–Nf curve was obtained by the least square fitting relationship in Eq. (2) [27]. Each data point on the S/N curve represents the average of five specimens tested under identical conditions. Fig. 7 shows the number of cycles to failure versus

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400

Bending stress (MPa)

Uncoated

350

Hardanodized 360HV

300 250 200 150 100 50 1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

No. of cycle to failure 400

Bending stress (MPa)

Uncoated

350

Hardanodized 393HV

300

383

which gives rise to fracture of specimen when it is sufficiently weakened by the crack zone development. By viewing the failed surface at high magnification, the striations due to each stress cycle can be seen as in Figs. 10 and 11(b), which show the crack surface of failed aluminum 7075-T6 at 40  magnification along with a representation of the stress cycle pattern that failed it. The occasional large amplitude stress cycles show up as larger striations than the more frequent small amplitude once, indicating that higher stress amplitudes cause larger crack growth per cycle [28]. Fig. 12 shows the SEM micrograph of a hard anodized coated specimen with hardness of 360 HV after fracture at stress of 220 MPa. In Fig. 12(b), the pure aluminum thin film hard anodized coating and substrate can be clearly seen. An SEM micrograph of optimum hard anodized coated (393 HV) specimen with thickness of 29 mm at a stress of 220 MPa and friction pressure of 100 Mpa is illustrated in Fig. 13. Meanwhile, Fig. 13(a) shows several pits around the specimen and interior cracks under the fretting fatigue. Fig. 13(b) depicts the crack propagation under fretting fatigue.

250 200

6. Discussion

150

6.1. The effect of hard anodizing parameters on surface hardness

100 50 1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

No. of cycle to failure Fig. 8. Comparison of S/N curve of fretting fatigue for uncoated and hard anodized specimen.(a) S/N curve of fretting fatigue for uncoated and hard anodized specimens (360HV) and (b) S/N curve of fretting fatigue for uncoated and optimum hard anodized parameters specimens (393HV).

stress for plain fatigue and uncoated fretting fatigue. Clearly, fretting drastically decreases specimen fatigue life. Fig. 8(a) and (b) illustrate the fretting fatigue life of uncoated and hard anodized specimens at 360 and 393 HV respectively, under the S/N curve. According to Fig. 8(a) the fretting fatigue life of hard anodized specimens with hardness of 360 HV improved in comparison to uncoated specimens at low bending stress, while at high bending stress from 250 to 300 MPa the results were reversed. However, Fig. 8(b) shows the comparison between the fretting fatigue life of an uncoated specimen and a hard anodized specimen with the highest surface hardness (393 HV) obtained using the parameters combination A2B1C3D4, the fretting fatigue lives of hard anodized specimens are improved only at low bending stresses from 150 to 200 MPa. By increasing the bending stress from 200 to 300 MPa, the fretting fatigue lives of hard anodized specimens (hardness of 393 HV) decreased. A study of Fig. 8(a) and (b) indicates that the fretting fatigue life of a hard anodized specimen (360 HV) improved even at bending stress of 220 MPa compared to hard anodized specimen with a hardness value of 393 HV. Fig. 9(a)–(d) show the moment and stress graphs accompanied by finite element analysis using ABAQUS software for the friction pads placed and not placed on the specimen. Fig. 9(d) also indicates that the maximum stress is under the lower friction pad where the specimen is under bending load. A combination of stress concentration and stress distribution can provide earlier fatigue failure. Fracture surfaces of tested specimens were examined using optical microscopy. Two typical results of fractured surface and the cross sections of uncoated and hard anodized specimens are illustrated in Figs. 10 and 11. These figures clearly indicate that the fracture surface consists of two quite distinct regions: a fatigue zone created by crack propagation and a tensile region

The selection of hard anodizing conditions is essential for fabricating composite thin films. The most important parameters affecting deposition rate are voltage (v), temperature (1C), solution concentration (%), and time (min). The effect of hard anodized coating on AL7075-T6 was observed at different conditions for the best surface hardness with the combination A2B1C3D4. Fretting fatigue tests were carried out on uncoated, hard anodized and optimum, hard anodized specimens at the best condition of anodizing achieved through optimization. From the experimental and Taguchi optimization results, as it can be seen in Fig. 6 the surface hardness of hard anodized specimens was low when voltage was 10 V. Voltage below this amount produced soft, porous and thin films. By increasing the voltage from 10 to 20 V, the specimens surface hardness increased when temperature was kept constant at 0 1C, while by further increasing voltage upto 40 V surface hardness decreased again. This phenomenon is attributed to the fact that at low voltage, the movement of ions is slow and less oxygen ions separate from the cathode so less aluminum oxide can be constructed on the surface of AL7075-T6 aluminum coated both inside and outside. With increasing voltage, the film formed more quickly with relatively less dissolution by the electrolyte, resulting in harder and less porous film. At very high voltage, there is a tendency for ‘‘burning’’ to occur, which is the development of excessively high current flow rate through local areas leading to overheating. On the other hand, by increasing the temperature, surface hardness decreased, meaning that the best hardness was achieved at 0 1C. The effect of an increase in electrolyte temperature was directly proportional with the increase in the rate of dissolution of the anodic film resulting in thinner, more porous and softer films. Thus, low temperatures are used to produce hard coatings, normally in combination with high current density and vigorous agitation. If temperature is increased further, the maximum thickness is reduced due to the elevated electrolyte dissolving power [17,29,30]. Solution concentration also plays a significant role in hard anodized coating by providing high harness values. Mixing oxygen and aluminum the surface is become ceramic and the surface hardness of hard anodized samples are increased with increasing solution concentration from 5 to 15%; by further increasing solution concentration upto 20% surface hardness decreased as seen in Fig. 6, this is may because the surface was more porous. The effect of increasing solution concentration on the coating characteristics was

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Pulley for applying the bending load

Fig. 9. Schematic of parallel specimen with two points loading accompanies with moment, stress graphs at slender region and analysis of stress in different situation. (a) Schematic of specimen-two points loading, (b) Distribution of stress in the narrow part of specimen, (c) Stress concentration under friction pads without applying loads (F) and (d) Combination of stress distribution and concentration on specimen.

similar to that of temperature increase; the effect of temperature is perhaps more important than that of concentration. Higher concentration restricts the maximum film thickness due to the elevated dissolving power of the concentrate solutions. By increasing time from 30 to 120 min while the solution was at roughly 15% concentration, surface hardness increased as it can be seen in Fig. 6; this is attributed to the creation of an aluminum oxide hard film, which can make the coating film thicker. However, by additionally increasing the solution concentration as well as time, specimen surface hardness potentially decreases.

In addition, as the thickness of the coating increases upto 29 mm, porosity decreased and the substrate become denser. Therefore, surface hardness of hard anodized coating increased. The film closest to the metal substrate has a high density and hardness and low porosity. 6.2. Fretting fatigue and S/N curves The corresponding plain fatigue and fretting fatigue S/N curves at contact pressure of 100 MPa are displayed in Figs. 7 and 8, respectively. It is apparent that fretting has a deleterious effect on

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385

Tensile zone

Fretting zone

Location of friction pads

Fig. 10. Fracture surface and cross-section view of uncoated specimens under fretting fatigue. (a) Fracture in uncoated AL7075-T6 specimen after 2E+06 cycles at 200MPa stress and (b) Cross-section view of uncoated specimen under fretting fatigue.

Tensile zone

Fretting zone

Location of friction pads

Fig. 11. Fracture surface and cross-section view of hard anodized specimens under fretting fatigue. (a) Fracture in hard anodized AL7075-T6 specimen after 1.7,E+06 cycles at 200MPa stress with hardness of 360HV and (b) Cross-section view of hard anodized specimen under fretting fatigue.

the fretting fatigue life of AL7075-T6 in substrate and coated conditions, at all applied bending stress values. Fig. 7 shows that fatigue (plain) strength decreases with an increase in stress. Fig. 8(a) shows a comparison between the fretting fatigue life of uncoated and hard anodized specimens with a hardness of 360 HV. The reduction in fatigue strength for hard anodized coated specimens is less than untreated substrate. On the other hand, the trend of the effect of hard anodizing depends on the stress value. Hard anodized coating has an increasing effect on specimen fatigue life at low stress regions at approximately 220 MPa. It is obvious that the influence of hard anodized coating is more profound at lower stress. The lack of effect of hard anodized coating at higher stress in fretting fatigue life may be the result of early crack initiation in the hard anodizing film due to high, local stress concentration resulting from bulk stress. The increased fretting fatigue life in low-stress regions for conditions considering substrate hardness, pads material, coating thickness and hardness, and loading type in this study is likely attributed to a low friction coefficient preventing metal to metal contact, which may result in higher fretting fatigue life because of retardation of crack initiation resulting from lower stress concentration compared to the substrate. It is suggested that fretting fatigue cracks form in regions where the frictional shear stress is concentrated locally on the contact surface. Thus, the decrease in fatigue life by fretting damage is considered to be due to a decrease in crack initiation life caused by the local stress concentration due to fretting, and initial crack propagation acceleration by fretting [31]. As one of the main mechanisms of initial crack acceleration by fretting, the wedge effect where the wear debris goes into the small initial

fretting fatigue crack is considered [32]. However, if the crack is completely filled with wear debris, the effect is considered to decrease because the wear debris cannot go into the crack furthermore. The action of fretting causes considerable damage to specimen surface. Figs. 10 and 11(a) show the appearance of fretting scars on substrate and hard anodized specimens. It is observed that the extent of fretting damage to the hard anodized coating is less than that on the substrate. This effect may be caused by increased hardness due to hard anodized coating of the surface. It is clear that during plain fatigue, cracks originate randomly at one or several points around the periphery of the specimen case, while during fretting, cracks inevitably start from the same location at a point adjacent to the leading edge of the fretting areas where the bending stress and induced shear stress are the highest [33]. Crack propagation occurs from two sides resulting in the appearance of a specimen’s final fracture area, as shown in Figs. 10 and 11(b). Fig. 8(b) shows a comparison between the fretting fatigue life of uncoated and hard anodized specimens with the highest surface hardness (393 HV) using optimum parameters. Hard anodized coating with optimum parameters has an increasing effect on the fretting fatigue life of specimens in lower stress regions (upto 200 MPa) than hard anodized specimens with surface hardness of 360 HV, which is attributed to the generation of micro-cracks and brittleness of the coated area [33–35]. It is possible that during hard anodizing the aluminum-rich matrix dissolves, especially around the sediments, generating decohesion of these particles and therefore producing pitting-like irregularities along the metal–oxide interface [36]. As for the

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Deep Pits

Crack propagation

Pure aluminum

Hard anodize coating

AL7075 -T6 Crack propagation

Fig. 12. SEM micrograph of hard anodize coated specimen with hardness of 360 HV after fracture at stress of 220 MPa. (a) Overall surface fracture of hard anodized specimen and (b) Fracture of coated specimen.

Fig. 13. SEM micrograph of hard anodized specimen (hard ness 393 HV and thickness 29 mm) after fracture at 220 MPa with friction pressure of 100 Mpa. (a) Interior crack propagation and fracture surface of hard anodized specimen and (b) Crack propagation in hard anodizing thin film.

29 mm thick coating oxide layer, in addition to such irregularity, contains a number of radial deep cracks. This crack formation, which was not perceived in thinner coating layer, can be attributed to the difference in thermal expansion coefficients of the aluminum alloy and oxide layer [37], and to the innertensile residual stress in the coating [38–40]. Fig. 8(b) also indicates that the application of oxidation has a propensity to decrease fatigue performance at high stress. Fatigue strength reduces with increasing oxide coating thickness. For a 29 mm thick coating, a reduction of 41% was recorded at high stress in fretting fatigue strength based on a fretting fatigue life of 107 cycles, compared to the bare specimen. The fatigue test studies on oxide-coated 7050–7451 alloy carried out by Camargo et al. [37,38] showed that anodic oxidation caused a 46% decrease in high cycle fatigue strength with respect to the base material. The destructive effect of tensile residual stress was remarked, which was induced during anodic

coating, but could be restrained by applying shot peening as a pre-process. Another study performed on 7475-T6 alloy reported that fatigue strength reduces with an increase in coating thickness and about a 75% reduction was recorded for 60 mm thick coating [36]. It is generally accepted that the diminution in fretting fatigue performance of anodic oxide coated specimens is directly related to the brittle and porous nature of the coating layer and tensile residual stress induced during the hard anodizing coating process [40–42]. Hard anodized coatings of enlarged thickness offer a larger area for micro-crack growth and union. Therefore, average crack lengths can increase following a small number of cycles. As a result, thick, hard anodized coated specimens demonstrate a relatively short fatigue life, owing to the easy nucleation of fatigue cracks. Merati and Eastaugh [39,43,44] accentuated that the relatively high modulus of anodized layer is also important for boosting crack nucleation. Authors believed

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that the presence of radial deep crack in the thickest coating layer (29 mm) is the major reason for the significant reduction in fretting fatigue performance. Rateick et al. [34] specified that the irregularity is the authority for the reduction in fatigue strength. Large fatigue strength loss was observed when irregularities were existent at the fracture origins, whereas only small fatigue strength loss was detected when the irregularities were far away [34]. Monsalve et al. [35] explained similar findings who studied fatigue behavior of anodized 7075 alloy. Fig. 13 presents an SEM micrograph on the crack initiation sites of the coated specimens. At 220 MPa stress, a partial crack was detected at the coating and substrate interface. It is well known that cracks are related to interfacial shear strength. It is also likely that the presence of irregularities beneath the coating layer is another factor affecting both nucleation and propagation processes (Fig. 13(b)). Local stress concentration due to irregularities may have a provocative effect on the cracks generated on the surface of coating. Similar observations were reported in literature [30,34]. At high cyclic stresses, due to brittle coating, cracks can easily occur at the surface layer. These cracks can act as nuclei for surface cracks in the base material, while through the development of multiple cracks within the coating layer, the coating fractures. Then, cracks may grow from the interface into the substrate having relatively low hardness. Existing irregularities beneath the oxide layer may also facilitate cracking of oxide layer. It is believed that the presence of deep cracks in the thickest coating layer accelerates the crack initiation period.

7. Conclusion In this research work, the Taguchi optimization method was used with the orthogonal array of L16 (44) to obtain the most optimum parameters for the best surface hardness. The parameters in this study include voltage, temperature, solution concentration and time. Plain fatigue and fretting fatigue tests on two types of specimens, uncoated and hard anodized, were carried out to investigate the fatigue and fretting fatigue life of specimens. The following conclusions can be derived from the present study: 1. Pure aluminum coating using magnetron sputtering technique on the surface of substrate is improved the ability of AL7075T6 to accept becoming hard anodized. 2. In the hard anodized coating on Al 7075, the use of voltage (20 V), temperature (0 1C), solution concentration (15%), and time (120 min) are recommended to obtain better surface hardness for the specific test range. 3. An improvement of 119.55% in surface hardness of hard anodized AL7075-T6 with a hardness of 393 HV was achieved in comparison to an uncoated specimen. 4. Fretting drastically decreases the fatigue life of AL7075-T6 alloy. The reduction in fatigue life is attributed to the introduction of shear stress on the surface through contact between the fretting pads and the substrate. 5. Hard anodized coating with 360 HV improved the fretting fatigue life of AL7075-T6 alloy at low stress. However, toward higher stress levels, the extent of increase in fatigue life reduced and at applied bending stress of approximately 220 Mpa, it was observed that hard anodize coating in fretting fatigue life has nearly no effect. 6. Fretting fatigue life of hard anodized specimens with a hardness of 393 HV obtained from optimization by the Taguchi method improved at low stress regions upto 200 MPa, while in stress regions of more than 200 MPa the results were reversed.

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Acknowledgments The authors acknowledge the financial support of the University Malaya Research Grant (Grant no. UM.TNC2/RC/AET/ GERAN (UMRG) RG133/11AET) from the University of Malaya, Malaysia.

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