Effect of chrome plating and varying hardness on the fretting fatigue life of AISI D2 components

Wear 418–419 (2019) 215–225

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Effect of chrome plating and varying hardness on the fretting fatigue life of AISI D2 components

T

Furqan Mukhtara, Faisal Qayyumb, , Zeeshan Anjuma,c, Masood Shaha,d ⁎

a

Fracture Mechanics and Fatigue Laboratory, Mechanical Engineering Department, University of Engineering and Technology, Taxila, Punjab, Pakistan Institut für Metallformung, Technische Universität Bergakademie Freiberg, Bernhard-von-Cotta-Str. 4, D-09599 Sachsen, Germany c Department of Mechanical Engineering, Swedish College of Engineering and Technology, Wah Cantt, Punjab, Pakistan d Université de Toulouse, INSA, UPS, Mines Albi, ISAE, ICA (Institut Clément Ader) Route de Tiellet, Campus Jarlard, Albi, France b

ARTICLE INFO

ABSTRACT

Keywords: Fretting fatigue AISI D2 Hardness Chrome plating Pin in dovetail Crack initiation and propagation

Effect of commercial grade chrome plating on the fretting fatigue life of AISI D2 is investigated using recently developed inverted pin-in-dovetail configuration fixture. Chrome plated and non-plated specimens having hardness of 40, 43, 52, 57 and 62 HRC were tested. All samples were tested at 7.5 kN tension-tension axial load (R = 0.1) at a constant frequency of 20 Hz and were observed under optical microscope after specific number of cycles to record crack initiation and crack propagation. It was observed that chrome plating is ineffective when component hardness is low but improves the fretting fatigue life by 60% when component hardness is 62 HRC. SEM images give an insight of how chrome plating plays an effective role in improving fretting fatigue life.

1. Introduction Fretting fatigue failures relate to the damage in mechanical components which occur due to micro slip between joints when subjected to the cyclic loadings. Such failures are most common in turbine disc and blade dovetail assemblies, riveted and bolted joints, aircraft lap joints and biomedical implants [1,2]. Fretting fatigue is the combined effect of wear, corrosion and cyclic loadings which causes significant reduction in the service life of the component as compared with components undergoing simple fatigue [3]. Fretting causes localized contact stresses (surface and sub-surface) at the interface of the components and a region of high tensile stress tangential to the surface at the edge of contact and fretting cracks are originated at the edges of contact [4,5]. Like simple fatigue, the major proportion of total fretting fatigue life of the components comprises of crack nucleation and growth of short cracks [2,6,7]. A number of factors (upto 50) affect the fretting fatigue life of the components which majorly include the contact configuration, material properties, slip amplitude, load, frequency, surface conditions, friction and hardness [8–10]. Increase in the material hardness, induction of compressive residual stresses and decrease in coefficient of friction are some major techniques used to improve the fretting fatigue life of surfaces in contact [11]. The dominant wear mechanism in AISI D2 specimens with hardness from 55 to 62 HRC hardness is the adhesive wear while at less hardness

the dominant wear mechanism is abrasive in nature. Above 62 HRC hardness, material becomes more brittle, which causes more cracks and delamination wear [12]. An increase in hardness causes less plastic deformation, decrease in coefficient of friction and a variation in slip magnitude between the contacting bodies during fretting [13]. Many researchers in the past have reported the failure mechanisms observed in different kind of tool steels which are commercially used [14,15]. AISI D2 finds great use in the manufacturing industry especially in the manufacturing of dies due to its high hardness, strength and wear properties. Many researchers have tried to evaluate the failure in materials [16–19] and wear performance of AISI D2 under different loading conditions [20,21]. Wear and tribological properties of tool steels can be enhanced by the application of heat treatment and surface treatments like boronizing and chrome plating [22–24]. Chrome coating is most common surface coating techniques in industries because of its ability to withstand high strains in different loading conditions and also due to the fact that it reduces the coefficient of friction between mating parts and thus increases the fretting fatigue life of surfaces in contact [25–27]. Hard chromium coatings like brush chrome plating provide an increase in hardness, corrosion resistance, wear resistance and fatigue strength of the components undergoing fretting [11,28,29]. Chemical Vapour Deposition (CVD) and Physical Vapour Deposition (PVD) are the two most commonly used techniques worldwide for thin hard

Correspondence to: Institut für Metallformung, TU Bergakademie Freiberg, Bernhard-von-Cotta-Strasse, 4, 09599 Freiberg, Sachsen, Germany. E-mail addresses: [email protected] (F. Mukhtar), [email protected] (F. Qayyum), [email protected] (Z. Anjum), [email protected] (M. Shah). ⁎

https://doi.org/10.1016/j.wear.2018.12.001 Received 31 May 2018; Received in revised form 22 November 2018; Accepted 1 December 2018 Available online 03 December 2018 0043-1648/ © 2018 Elsevier B.V. All rights reserved.

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Nomenclature

HRC MTS UTS

Acronyms AISI ASTM SEM EDM BSE SE SWT CT

Rockwell Hardness Material Testing System Ultimate Tensile Strength

List of symbols

American Iron and Steel Institute American Society for Testing and Material Scanning Electron Microscope Electric Discharge Machining Backscattered Electrons (Imaging mode) Secondary Electrons (Imaging mode) Smith Watson Topper Compact Tension

R μ L ao af

chrome coating. Pack method for chromium carbide coating on AISI D2 has also been shown to have effectively improve the surface properties [24]. All the plating and coating techniques employed by most of the researchers in the past are either very expensive or not viable at industrial scale. Although from literature review it is evident that coating improves the fretting fatigue life of components, yet not much research work has been reported on the investigation of the effect of chrome plating (commercial grade) and hardness combined, on the fretting fatigue life of AISI D2 (commercial material for die manufacturing) components. The aim of this research was to improve the fretting fatigue life of AISI D2 components by using chrome plating and see the effect of hardness in combination. In this research, fretting fatigue strength of AISI D2 specimens having different hardness in contact with carbide rod has been investigated. Pin-in-dovetail contact configuration has been used and the tests were carried out using the newly developed fixture developed by Anjum et al. [30]. The effect of chrome plating and varying hardness on the fretting behaviour has also been analysed. Details about the force distribution and stress evolution in the component using static elastic analysis approach is provided for the used fretting fixture. Multiple micro cracks were observed and detailed analysis was carried out using SEM.

Stress ratio Co-efficient of friction Contact length Initial crack length Final crack length

2. Methodology of research 2.1. Fretting fatigue tests 2.1.1. Specimen preparation Final shape and geometry of manufactured 2 mm thick specimen is schematically shown in Fig. 1. For specimen preparation, 2.5 mm thick slices were cut from AISI D2 rod having 150 mm diameter. Chemical composition of AISI D2 rods as provided by the supplier is shown in Table 1. The slices were cut in longitudinal direction using horizontal bend saw. The rectangular specimens of 75 mm × 45 mm were cut from the slices using milling machine. The specimens were heat treated to obtain desired hardness. The specimens were kept in furnace at 1000 °C for 10 min and air quenched to obtain maximum hardness (62–64 HRC). They were later tempered and air cooled to obtain varying hardness values. A total of 10 specimens were heat treated in five different hardness ranges i.e. 40 ± 0.5, 43 ± 0.5, 52 ± 0.5, 57 ± 0.5, 62 ± 0.5 HRC. The hardness of the samples was measured at multiple location on a sample using Rockwell Hardness Tester at 150 kg load to ensure the accuracy of the reported results. After the heat treatment, specimens were grounded to the final thickness of 2 mm. To ensure the dimensional accuracy and repeatability, all the samples were clamped and the internal geometric

Fig. 1. Schematic diagram of fretting test specimen (all dimensions are in mm). 216

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Table 1 Chemical composition of AISI D2 (Provided by the supplier). Composition

C

Mn

Si

Cr

Ni

Mo

Fe

Weight %

1.4–1.6

0.6

0.6

11–13

0.3

0.7–1.2

Balance

features were machined using wire EDM which has machining accuracy of ~5 µm. The dovetail groove in all samples was then polished with water proof abrasive and polish sheets of p150, p300, p600, p800, p1000 and p1200. “Rugo Surf” surface roughness tester was used to ensure 0.2 µm roughness inside the dovetail of all specimens. Out of 10 samples, 5 samples having 40 ± 0.5, 43 ± 0.5, 52 ± 0.5, 57 ± 0.5, 62 ± 0.5 HRC hardness were electroplated. To prepare samples for electroplating vapour degreasing was done by hanging the samples over 50–60 °C hot tricholoroethylene container for 30 min. After vapour degreasing all the samples were shield with brown packing tape to prevent chrome plating at unwanted areas. Chrome plating bath solution was prepared by mixing 25 g/l of chromic acid (solid form) and 4 g/l sulphuric acid (concentrated) in distilled water. The degreased and shielded samples were made cathode and lead anode was used during chrome plating. Current density of 40 A/dm2 and temperature of 50–60 °C were maintained throughout the process. Application of constant potential difference for 90 min resulted in 30 µm thick chrome plating on the inner wall of dovetail cavity. To make sure that the surface finish of non-plated and chrome plated samples is same, the samples were polished using abrasive paper of p1000 and p1200. The complete process chart for sample preparation is presented in Fig. 2.

Fig. 3. Fretting fatigue testing fixture.

2.1.3. Experimentation Fretting fatigue tests for all samples were performed using MTS 810 with force control parameters. Specimens were clamped in the fixture by carbide rods passing through the dovetail and circular hole. Constant amplitude tension-tension axial load of 7.5 kN (R=0.1) was applied in all tests, at constant frequency of 20 Hz. All tests were performed at temperature of ~24 °C and humidity of ~13 g/m3. Due to many initiating and propagating cracks, the nomenclature for crack representation is adopted in the article is schematically presented in Fig. 4. A1, A2 and B1,B2 denote the major cracks which appeared in all the specimens, but in harder samples more than 4 cracks were observed. Cracks below A1,B1 were termed as A0,B0 and cracks above A2,B2 were termed as A3,B3. The nomenclature of crack numbering is also schematically presented in View A of Fig. 4. Fretting describes a tribological contact situation and loading under the influence of a normal load, acting perpendicularly on the specimen which is subjected to a vibration. A simple static analysis for approximating the normal and tangential forces acting on the specimen surface at the point of contact can be calculated as a very simple geometric construction as shown in Fig. 5. An axial tension-tension cyclic load (F) of 7.5 kN (R=0.1) was applied in all experiments. The maximum and minimum magnitudes of normal force (FN) on either side of dovetail joint were 1.28 kN and 0.128 kN respectively, while the maximum and minimum magnitudes of tangential force (FT) on either side of dovetail joint during testing were 3.5 kN and 0.35 kN respectively. The Hertzian

2.1.2. Test fixture A newly developed fretting fatigue test fixture developed by Anjum et al. [30] which is shown in Fig. 3 was used in this research work due to its simplicity and ease of use. Commercially available tungsten carbide rods of 12 mm diameter were used in all experiments as contacting body. These rods have hardness of ~81–83 HRC and Young's modulus of approximately 530–700 GPa [7]. Under the current test conditions (Fmax=7.5 kN) it was visually observed that there was no yielding or wear of the carbide rod surface during testing and therefore, only the AISI D2 specimen surface was studied in this research work.

Fig. 2. Flow chart of specimen manufacturing. 217

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Fig. 4. Schematic diagram of yielded surface and cracking zones in dovetail joint.

and image processing tools were used. The Olympus BX51 metallurgical microscope is equipped with 2 megapixel DP20 digital camera which takes pictures at resolution of 1200 × 1600. The images are then stored and analysed live using Olympus Stream image processing software. In current work, the crack lengths were measured using this technique. The TESCAN VEGA 3 SEM was used to analyse the failed samples after 100k fretting fatigue cycles. Image-J [32] was used to analyse and measure some complex features of the tested samples which are reported in this article. 3. Results 3.1. Fretting fatigue tests The hardness and chrome plating has an influence on cycles required for crack initiation, sites of crack initiation, number of cracks generated in the specimen, inter crack distance and plastic deformation zone visible as contact scar. Initially under the action of fretting load the specimen deforms plastically at the contact region with the carbide rod and then cracks are initiated principally in Mode II at the edges of contact as shown schematically in Fig. 4. As the hardness of the specimen increases, the brittleness increases and specimens depict less plastic deformation, reduction in friction, which reduces the size of contact scar [33] and inter crack distance. The light microscope images in Fig. 7 give a general idea for comparison of contact scar size with varying hardness of 40, 52, and 62 HRC. The area shown in these images correspond to View A in Fig. 4. The decreasing size of contact scar and inter crack distance can be clearly observed.

Fig. 5. Schematic diagram explaining the force component acting on the contact region.

contact pressure model for elastic bodies [31] for cylinder-on-flat contact was used. The Hertzian Pressure for this load case which was calculated to be 2.54 GPa. Static analysis of the contact configuration to determine the stress distribution due to applied load was carried out using commercial FEM software ABAQUS. The details of the simulation modelling and the results of the study are presented in Supplementary Data Annex D. Crack initiation and propagation in both chrome plated and nonplated specimens with different hardness was observed using metallurgical microscope by dis-assembling the specimen after regular intervals of 20k, 35k, 50k, 70k, 90k and 100k loading cycles respectively. Olympus BX-51 metallurgical microscope equipped with Olympus DP20 high resolution digital camera was used to observe, measure and record the propagating cracks during the experimentation. The experimentation was carried up to 100k fatigue cycles for all samples because cracks stop propagating or propagate so slow that it is not detectable by light microscope, therefore we called this condition as “crack arrest”. Microscopic images of initiation and propagation in nonplated (40 HRC) specimen of crack-A1 are shown in Fig. 6. TESCAN VEGA 3 SEM was also used for the in depth study of fractured surface, which is discussed later in the article.

3.1.1. Crack initiation The cracks initiate at the edges of the contact where shear stress is maximum in Mode II [34]. The observed trend in our experiments was that in all non-plated specimens the cracks initiated before 20,000 cycles, whereas, in chrome plated specimens the cracks generally initiated after 20,000 cycles. The lower cracks (A1 and B1) initiate and propagate faster than upper cracks (A2 and B2) in all specimens is principally due to the test contact configuration. Cracks are generated on both sides of dovetail joint and at the both edges of the contact scar. Average crack lengths of lower cracks at 20,000 cycles for both plated and non-plated specimens at different hardness are shown in Fig. 8. Trend lines have been plotted to give an idea about the effect of hardness and chrome plating on average crack length, although it is hard to conclude concrete results and to be sure for the obtained scattered dataset. The dashed trend line for the average crack length in non-plated samples shows that it remains consistent with increase in hardness. The dotted

2.1.4. Data analysis For analysis and processing of results, different software packages 218

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Fig. 6. Micrographs of crack (A1) propagation in a non-plated (40 HRC) specimen after (a) 20k cycles (b) 35k cycles (c) 50k cycles (d) 70k cycles (e) 90k cycles and (f) 100k cycles.

trend line for chrome plated samples shows that with increasing hardness the average crack length drops significantly. It is important to mention here that no cracks were observed in plated 52 HRC sample after 20,000 cycles and therefore the value is marked as zero. The detailed crack propagation history of this sample can be seen in Fig. 2(c), Annex A of Supplementary data. While the overall length of the cracks is observed to be decreasing in Fig. 8. An increase in number of cracks was observed with the increase in hardness both in plated and non-plated specimens, the trend is shown in Fig. 9. These cracks appear between the contact scar edges (also shown schematically in Fig. 4), and are discussed in the later part of the article.

3.1.2. Crack propagation Fretting fatigue cracks propagate in Mode I and their propagation is perpendicular to the surface on which the fretting load is applied [30,35,36]. In our specific contact configuration, the trend of propagating lower cracks is more for being in tension whereas, the upper cracks propagate less for being in compression during the loading cycle. Propagation of lower cracks is greater than the upper cracks which undergo compression. Due to the gradual decrease in stress intensity factor, cracks stop propagating after certain number of cycles under the same magnitude of applied load [30]. This crack arrest is mainly due to this contact configuration. Non-plated specimens with higher hardness values have comparatively smaller crack lengths than the specimens with lower hardness values but are larger in number. 219

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Fig. 7. Decrease in plastic deformation with the increase in hardness at 100k cycles (a) 40 HRC (b) 52 HRC and (c) 62 HRC.

Fig. 8. Effect of hardness on the average crack lengths of lower cracks in both plated and non-plated specimens at 20, 000 cycles.

Fig. 9. Increase in number of cracks with the increase in hardness.

It has been investigated previously that chrome plating has a hardness of ~66 HRC [37], this is higher than the hardness of all the set of specimens which were tested in the current study. When subjected to fretting fatigue loads, chrome plating acts as a barrier by absorbing all energy and disintegrating under external fretting fatigue loads. That is why chrome plated specimens have relatively smaller crack lengths than non-plated specimens. Chrome plating improves surface integrity of the components by improving surface finish and reducing friction

coefficient during contact [29,38,39]. It also acts as a barrier for the fretting fatigue cracks to initiate in the substrate. Due to this multi fold action of chrome plating, it helps in improving the fretting fatigue life of components. Generally, the very thin chrome plating (which in the present case was 30 µm) is very hard (~66 HRC [37]). When the substrate is soft (40 HRC), the difference of chrome plating and substrate hardness is 40%, in this case, large deformation occurs in substrate under applied cyclic 220

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load whereas the plating layer disintegrates after lesser deformation. Therefore, when the substrate is soft, the chrome plated and non-plated samples show similar behaviour under fretting fatigue load. When the substrate is harder (62 HRC), the difference of chrome plating and substrate hardness is 6%. The substrate and coating undergo similar deformation due to applied load and hence the chrome plating retains on the surface for a greater number of cycles. In this case the life of chrome plated sample is observed to be higher than nonplated sample. The detailed crack propagation history of individual cracks in presented in Supplementary Data Annexure A. Fig. 10 presents the average results for each sample. It is shows that varying hardness has almost no effect on average ao (crack lengths at 20,000 cycles) and average af (crack lengths at 100,000 cycles) in non-plated samples (dotted lines slope remains constant). Whereas, when samples are chrome plated and tested for fretting fatigue crack initiation and propagation under same testing conditions, the results are almost same as non-plated samples for 40HRC but for 62 HRC samples average ao drops by 56% and average af after 100k cycles drops by 40% when (dashed lines slope decreases). Hence, it is observed that chrome plating effectively increases the fretting fatigue life of components when the base is hard. SEM micrographs support this phenomenon.

images are helpful in identifying the precipitates by showing a clear contrast between chemically different constituents. In our case, distribution and size of carbides (dark spots) in the steel matrix (light area) can be clearly seen in Fig. 12. It has been reported [40] that the chemistry of carbides present in AISI D2 steel is M7C3. They are nonhomogeneously distributed and their size varies. Fig. 12 (a) is constructed to show the two stage crack propagation due to varying stress regime along the propagation path. After initiating due to high shear stress, the cracks propagate perpendicular to the surface, through brittle carbides and ductile steel matrix. Due to increase in crack length the stress intensity at crack tip decreases [30]. In second zone the stresses at crack tip are high enough to trigger brittle failure and cracking of carbide and the crack propagation is path is determined by the failed carbide rich zone, it can be observed in the constructed Fig. 12(a) showing the crack tip in detail in the inset with failed carbides around it. This kind of two stage crack propagation phenomenon is observed in all samples. Another phenomenon of carbide spalling is also observed on the surface in all samples. Spalling of carbides on surface of specimens in the plastic deformation zone occurs because they have very high yield strength as compares to the soft steel matrix. Due to large difference of yield strength, the carbides “fall of” the surface when plastic deformation in matrix occurs.

3.1.3. Contact Scar and crack growth Fig. 11 presents the SEM images of tilted specimens to show the contact scar on B side of 40 HRC and 62 HRC samples after 100k cycles. Fig. 11 (a) and (b) for non-plated, 40 HRC and 62 HRC fretted specimens show that with the increase in hardness the width of contact decreases from 2.7 mm to 1.7 mm. The debris during the fretting wear adheres on the softer body of the two contacting bodies. This debris retention is a complex phenomenon as reported by previous researchers as well [13,33]. Often it stops the wear from the softer body and acts as an abrasive body to wear the harder body and sometimes it separates the two bodies i.e. protects the underlying softer body to avoid any further wear. In this research work, it was found that at 40 HRC the specimen has the less debris retention and more relative slip in both plated and non-plated specimens as shown in Fig. 11(a) and (c). Whereas at 62 HRC, in both plated and non-plated specimens as shown in Fig. 11(b) and (d), more debris retention was observed protecting the lower softer surface from further wear by reducing their relative slip. Increase in hardness and coating has been reported to decrease the coefficient of friction by several researchers [38,39] while debris retention phenomenon is also present in our research work causing slipstick at 62HRC.

4. Discussion In the present study, fretting fatigue crack initiation and propagation in AISI D2 chrome plated and non-plated specimens of varying hardness was investigated. Fretting fatigue tests generally are conducted using sphere-on-flat, cylinder-on-flat and flat-on-flat surface configurations [4,41,42] which generalizes the results and does not effectively account for the complicated geometry of the moving parts. In this research, the tests were carried out using Anjum et al.’s [30] newly developed fixture of pin-in-dovetail configuration which is more close to the realistic contact configuration in moving machine parts (i.e. between gas turbine blades and shaft). It was shown that how varying the hardness of the components alone or studying the effect of chrome plating alone on the fretting fatigue life of components can be misleading. Commercial grade chrome plating (which is inexpensive and is a viable solution for large industrial dies and components) has been shown to effectively increase the fretting fatigue life of high hardness AISI D2 components. Due to the use of pin-in-dovetail fretting fatigue test configuration, in depth analysis of test specimens was possible. The study reveals different zones of crack propagation behaviour in different specimens. Previously, researchers have shown the implication of chromium and chromium carbide coating on improving the wear and fretting properties of different materials [24,28]. Chromium carbide is extremely hard and its deposition is achieved by PVD, CVD and Ion implantation techniques [39]. The bond between the coating and surface

3.1.4. Fretting fatigue crack path Detailed analysis of the characteristic crack propagation path was carried out. Fig. 12 presents BSE images obtained from SEM for the crack B1 in 40 and 62 HRC plated and non-plated AISI D2 samples. BSE

Fig. 10. Comparison of initial and final crack lengths in fretting fatigue tests of non-plated and chrome plated AISI D2 samples. 221

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in these techniques is extremely strong [43] which almost eliminated the possibility of disintegration of coating during slip. In the present study, commercial grade electrolytic chrome plating was done to deposit a 30 µm thick chrome layer on the contact surface of the samples. Due to the intrinsic limitation of this process of chromium deposition, the coating and surface bond is not as strong as in PVD [44] and CVD. Because of the high brittleness, the chrome layer quickly disintegrates due to large plastic deformation in the contact zone when the specimen is soft (40 HRC). During this disintegration of the chrome plating layer, sub sized particles stick to the surface, they affect the surface integrity of the component and hence reduce the fretting fatigue life. With increasing hardness of the components, chrome plating retains for a greater number of cycles and hence keeps the surface integrity high, friction coefficient low, and improves the fretting fatigue life of the components. Chrome plating in 62 HRC sample was observed to have increased the fretting fatigue life by as much as 60%. Researchers are divided on the implication of hardness of the mating bodies during fretting fatigue. Some researchers noted that

when there is a large difference in the hardness of the contacting bodies, initially wear occurs in the softer surface and with time the debris sticks to the surface and wear of hard surface starts [45,46]. Some other researchers noted that during such interaction of bodies having large difference in hardness, only the softer surface erodes and no effect of wear on hard surface was observed [47]. In the present study non-plated and chrome plated (hardness of almost 66 HRC) AISI D2 samples of varying hardness i.e. 40, 43, 52, 57 and 62 HRC hardness were tested. During tests carbide pin was used because of its extremely high hardness (almost 85 HRC). Hence, the difference in hardness between contacting surface is large. It was observed that there were no scratches or contact marks on the carbide pins. The same set of pins was used to test all samples, which accounts for almost 10,000k cycles. As effect of hardness has not been reported before for pin-in-dovetail configuration, it is interesting to note that in current contact configuration, in soft specimens (40 HRC), cracks initiate only at the lower side of the contact scars. This is because in soft specimens due to excessive plastic deformation surface integrity significantly reduces,

Fig. 11. SE SEM micrographs of slip planes produced during fretting fatigue (a) Non-plated 40 HRC (b) Non-plated 62 HRC (c) Chrome plated 40 HRC (d) Chrome plated 62 HRC. 222

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Fig. 11. (continued)

friction coefficient increases, which results in stick and slip phenomena between the two contacting bodies [13]. Due to such biased load on one side of the contact scar cracks initiate and propagate only on the lower side of contact scar. With increase in hardness, brittleness of AISI D2 increases [12]. The increase in hardness results in contact scar size reduction which in turn results in concentration of applied shear stresses in a very narrow zone. With increasing component hardness, the surface integrity of the component also increases which results in reduction of friction coefficient and pure slip condition between two contacting surfaces. This reduction in size of contact scar (concentration of applied shear stresses in a narrow region) and better slip results in reduction in inter crack distance, therefore, at high hardness of 62 HRC, crack appear not only at the edges of the contact scar but also at the intermittent positions.

configuration. Following conclusions are drawn from the current study: 1. Variation in hardness in non-plated specimens has no significant effect on limiting crack initiation and propagation under fretting fatigue. 2. The number of microcracks initiated increases with the increase in hardness for both plated and non-plated specimens, whereas maximum crack length decreases. 3. Increasing hardness of AISI D2 results in reduction of the contact scar size, due to which, the applied shear stress on the surface is localized in a narrower region and a greater number of initiating fretting fatigue crack are observed. 4. At hardness of 62 HRC, chrome plating effectively improves the surface integrity of AISI D2 components and improves crack initiation life by 60% and crack propagation by 50%. 5. For this specific pin in dovetail test configuration, crack propagation can be divided in High Stress Regime Zone, where the crack propagate perpendicular to the surface and Low Stress Regime Zone, where the crack propagation is influenced by brittle fracturing and

5. Conclusions In this research, non-plated and chrome plated samples of different hardness were tested under fretting loads and pin-in-dovetail 223

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Fig. 12. BSE SEM micrographs of B1 cracks produced during fretting fatigue (a) Non-plated 40 HRC (b) Non-plated 62 HRC (c) Chrome plated 40 HRC (d) Chrome plated 62 HRC.

spalling of carbides.

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Acknowledgements This research work was funded and supported by Department of Mechanical Engineering, University of Engineering and Technology, Taxila 47050, Pakistan, which is gratefully acknowledged. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations, however, any specific dataset can be shared with the readers upon request. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at https://doi.org/10.1016/j.wear.2018.12.001. 224

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