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Al2O3 composites
Journal Pre-proof Effect of heat treatments on the mechanical and tribological properties of electrodeposited Fe–W/Al2O3 composites Antonio Mulone, Aliona Nicolenco, Naroa Imaz, Jordina Fornell, Jordi Sort, Uta Klement PII:
S0043-1648(19)31702-8
DOI:
https://doi.org/10.1016/j.wear.2020.203232
Reference:
WEA 203232
To appear in:
Wear
Received Date: 21 November 2019 Revised Date:
6 February 2020
Accepted Date: 6 February 2020
Please cite this article as: A. Mulone, A. Nicolenco, N. Imaz, J. Fornell, J. Sort, U. Klement, Effect of heat treatments on the mechanical and tribological properties of electrodeposited Fe–W/Al2O3 composites, Wear (2020), doi: https://doi.org/10.1016/j.wear.2020.203232. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Author statement Antonio Mulone: Conceptualization, Methodology, Investigation, Visualization, WritingOriginal Draft, Writing-Review & Editing. Aliona Nicolenco: Investigation, Resources, Writing-Review & Editing. Naroa Imaz: Investigation, Validation, Writing-Review & Editing. Jordina Fornell: Investigation, Validation, Writing-Review & Editing. Jordi Sort: Supervision, Writing-Review & Editing. Uta Klement: Supervision, Writing-Review & Editing.
Effect of heat treatments on the mechanical and tribological properties of electrodeposited FeW/Al2O3 composites Antonio Mulonea, Aliona Nicolencob ,c, Naroa Imazd, Jordina Fornellb, Jordi Sortb, e, Uta Klementa a
Chalmers University of Technology, Department of Industrial and Materials Science, SE-412 96
Gothenburg, Sweden b c
Institute of physics, Chisinau MD-2028, Moldova
d e
Departament de Física, Universitat Autònoma de Barcelona, Bellaterra E-08193, Spain
CIDETEC, Paseo Miramón 196, E-20014 Donostia-San Sebastián, Spain
Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluís Companys 23, Barcelona E-08010,
Spain
Abstract In this study, the influence of heat treatment on the mechanical and tribological properties of electrodeposited Fe-W/Al2O3 composite coatings is studied. The properties of the as-deposited and annealed composites are compared with those of electrodeposited hard chromium coatings. The amorphous structure of the Fe-W matrix transforms into a mixed amorphous-crystalline structure upon annealing at 600 ⁰C for 1 hour. The observed microstructural transformations result in a substantial increase of both the hardness and the reduced Young’s modulus of the Fe-W/Al2O3 composite coatings, reaching values of 16.3 GPa and 191.7 GPa, respectively. The results on the wear resistance studied under dry friction using ball-on-disc sliding tests show that a low wear rate is obtained for both as-deposited and annealed composite coatings, i.e. ∼1.5 x 10-6 mm3/Nm. In contrast, the heat treatments are detrimental for both the hardness and wear resistance of hard chromium coatings. As a consequence, the mechanical and wear properties of the electrodeposited Fe-W/Al2O3 composite coatings, especially after annealing, are superior to the properties of hard chromium coatings. Hence, Fe-W/Al2O3 composite coatings can be considered as a valid and sustainable alternative to hard chromium coatings, particularly in applications where these materials may be exposed to high temperatures.
Keywords: electrodeposition; iron-tungsten alloys; composite coatings; wear resistance; heat treatment;
1
1
Introduction
In the recent years, electrodeposited Fe-W and Fe-W/Al2O3 composite coatings have been studied and evaluated as an effective sustainable alternative to electrodeposited hard chromium coatings [1,2]. Chrome coatings are largely applied in aerospace and automotive applications [3,4] owing to their outstanding properties, e.g. high hardness, as well as wear and corrosion resistance. The production of hard chromium coatings, however, involves the use of extremely toxic compounds such as Cr6+ [5,6], which can significantly affect human beings, plants and microbial life. Since compounds containing hexavalent chromium have been added to the Annex XIV (i.e. European “Authorization List” that limits the import and the use of a compound under very strict rules), many studies have been performed with the aim to find an environmentally friendly alternative to electrodeposited hard chromium coatings. Trivalent chromium plating (i.e. Cr3+), for example, is less harmful and it is considered as a promising technology, but its application is still limited due to thickness and surface quality of the obtained coatings [7]. Other studies on electrodeposited Cr-free alloys and composites such as Ni and Co-based coatings (particularly with W or P) have shown to be characterized with a higher hardness and wear resistance as compared to hard chromium coatings [8–14]. However, the proposed Ni and Co-based coatings do not always represent a valid environmentally friendly alternative. Cobalt salts are targeted as “substance of very high concern” as cancerogenic compounds and the use of Ni is discouraged as it is listed as highly allergenic [15]. Thus, exploring new environmentally friendly alternatives to hard chromium coatings constitutes a topic still under intense investigation. Among various electrodeposited alloys proposed as alternatives to Cr6+ coatings, Fe-W is gaining considerable attention because it can be electrodeposited from environmentally friendly and thermodynamically stable electrolytes [16]. In addition, this system exhibits tunable composition and structure and the resulting hardness and thermal stability can exceed those of similar Co- or Nibased alloys [17]. Indeed, Fe-W alloys with a W content above 17 at.% are characterized with high hardness, i.e. about 9–11 GPa [18]. Furthermore, the hardness of Fe-W coatings can be substantially increased by heat treatments. In particular, a maximum hardness value of 16.5 GPa was obtained for W-rich Fe-W coatings (i.e. 24 at.% of W) after annealing for one hour at 600 ⁰C [19]. Wear analyses performed in dry conditions on as-deposited Fe-W coatings showed that the coatings were characterized by a rather low wear resistance [20]. The severe tribo-oxidation mechanism observed in Fe-W coatings during dry sliding tests was found to be the main cause of their low wear resistance. Abrasive iron oxide particles were formed during the sliding tests, leading to high values of the coefficient of friction (COF), i.e. varying from 0.8 to 1.2 (sliding against corundum counterbody), and to high wear track volumes, i.e. ∼12 x 10-6 mm3/Nm [1,20]. A remarkable improvement in the wear resistance was obtained with the co-deposition of Al2O3 sub-micron particles within the 2
Fe-W metal matrix [2]. The co-deposited alumina particles reduce the contact area of the Fe-W matrix which is affected by tribo-oxidation, resulting in a lower COF and wear rate. Among the different Fe-W/Al2O3 coatings tested, the composite coating deposited with the highest amount of Al2O3 particles, i.e. 12 vol.%, was characterized with the lowest COF, i.e. 0.6, and wear rate, i.e. 1.8 x10-6 mm3/Nm. However, while increasing the wear resistance, the co-deposition of 12 vol.% alumina particles also resulted in a slight reduction of both the hardness and reduced elastic modulus of the Fe-W/Al2O3 composite as compared to pure Fe-W (with no particles). Such decrease in the mechanical performance of composite coatings is often caused by porosity and a weak particle-matrix interface strength [21]. Heat treatments have proven to be beneficial for the mechanical and wear properties of different electrodeposited coatings [22–24]. Previous studies have also shown the beneficial effects of heat treatments on the bond strength of the reinforcement-matrix interface in composites [25]. The aim of this paper is to study the effect of heat treatment on the mechanical properties and wear resistance of Fe-W/Al2O3 composite coatings and to compare them with the wear resistance and mechanical resistance of hard chromium coatings. The possibility to tailor the microstructure, and consequently the material properties, through proper heat treatments is investigated. The synthesis of Fe-W/Al2O3 composite coatings with a combination of good mechanical properties and high wear resistance can be considered as an important step towards the development of competitive and sustainable alternatives for coatings produced using environmentally hazardous processes, such as plating with hexavalent chromium.
2 2.1
Experimental Electrodeposition of Fe-W/Al2O3 and chromium coatings
Fe-W/Al2O3 coatings were electrodeposited according to the procedure described in our previous work [2]. The following composition of base electrolyte was used: 1 M glycolic acid, 0.3 M citric acid, 0.1 M Fe2(SO4)3 and 0.3 M Na2WO. The alumina particles (Alfa Aesar 42572) were added into the electrolyte with a concentration of 100 g L−1. The electrolyte pH was adjusted to 7.0 by adding NaOH. The suspension was then stirred at 300 rpm for 24 h to hydrate the particles. Before the electrodeposition of the coatings, the suspension was ultrasonically agitated for 10 minutes to avoid the agglomeration of alumina particles. The majority of the particles embedded in the coating are characterized by a mean diameter between 50 and 150 nm, as measured with image analysis (ImageJ software) [2]. Electrodeposition was carried out using a standard three-electrode cell. A 4 cm2 brass plate served as a working electrode, a platinized titanium mesh as a counter electrode, and 3
an Ag/AgCl/KCl (sat) as the reference electrode. The brass substrates were decreased using a commercial alkaline bath to remove possible contaminations from the surface and then activated in 2 M H2SO4 solution. A nickel seed layer was deposited from a chloride bath at 65 °C, 30 mA cm-2 for 1 minute to improve the adhesion of the composite with the substrate. The electrodeposition of Fe-W/Al2O3 coatings was performed at 65 °C applying a constant cathodic current density of 40 mA cm-2 for 1 hour, which resulted in 20 ± 2 µm thick layer. The volume of the electrolyte was maintained 200 mL and the solution was mechanically stirred at 250 rpm. The electrodeposition of chromium coating was performed using a three-cell composed of: (i) polished carbon steel as a working electrode, (ii) Ag/AgCl/Cl− (3M) as a reference electrode, and (iii) Lead-8% tin as a counter electrode. The electrolyte contained 250 g L−1 CrO3 and 2.5 g L−1 H2SO4, the volume was maintained 400 mL, while keeping a temperature of 51 ⁰C. The electrodeposition was performed by applying a cathodic current density of 300 mA·cm−2 for 1 hour and 30 minutes. Cr coatings were deposited with a thickness of 20 ± 1 µm and surface roughness (Ra) of 220 ± 22 nm.
2.2
Coating characterization
To analyze the surface morphology of the deposited coatings, a Leo 1550 Gemini Scanning Electron Microscope (SEM) with a field emission gun was used. The chemical composition of the coatings was measured at the surface and along the cross-section of the samples by Energy Dispersive X-ray Spectroscopy (EDS). The volume fraction of co-deposited Al2O3 particles in the deposits was determined by performing image analyses (ImageJ software) on the cross-sections of the coatings. The surface roughness was measured using a WYKO RST-PLUS optical profilometer. The heat treatments were performed in the furnace of a NETZSCH 402 C dilatometer, keeping the sample at 600 ⁰C for 1 hour. High purity Ar 6.0 was used and the annealing temperature was reached with a heating rate of 10 ⁰C/min. After the heat treatments, the samples were kept inside the furnace until reaching room temperature. X-rays diffraction (XRD) analyses were performed using a Bruker AXS D8 advance diffractometer with Cr Kα radiation (λ = 2.28970 Å) and the equipment was operated at 40 kV and 50 mA. The annealing temperature of 600 ℃ was selected based on previous findings that showed a maximum in the hardness of Fe-W coatings after annealing at 600 ℃ [19]. A NHT2 Nanoindentation Tester from Anton-Paar was used to evaluate the mechanical properties, i.e. hardness and reduced elastic modulus. The measurements were performed along the cross-section of the deposits, under load-control mode and applying a load of 25 mN with a Berkovich pyramidal-shaped diamond tip. This load is low enough to avoid the influence of the embedding resin and the substrate, but it is high enough to embrace a representative area of the 4
coating (including particles and matrix). A total of fifteen indentations were performed on each sample and the results are expressed as the average of all the tests. For nanoindentation measurements, the cross-section of the samples was polished down to a 1 µm finishing. The hardness (H) and reduced elastic modulus (Er) of the analyzed samples are obtained from the loaddisplacement curves as according to the Oliver and Pharr method [26]. The tribological properties of the coatings were measured using a ball-on-disc tribometer under dry sliding conditions, at room temperature and ~55% of relative humidity. An alumina ball with a diameter of 6 mm was used as a counter-body, due to its high stability and resistance to abrasion and oxidation. Thanks to such properties, alumina counter-bodies are commonly employed in hard ball/hard coating systems [27,28]. Also, being Fe-W alloys affected by tribo-oxidation, the use of a counter-body less resistant to oxidation, e.g. steel ball, would increase the possibility to introduce wear impurities (i.e. debris and oxides) into the system. The size of the ball was selected considering the size of the sample (i.e. 2 x 2 cm), and to allow to run multiple tests per each specimen. The wear tests were conducted applying 2 N load which results in an initial Hertzian contact pressure of 665.3 MPa, assuming a “sphere on plane” configuration, where the elastic modulus, Er, and the Poisson ratio for corundum ball were 210 GPa and 0.22, respectively, and for FeW/Al2O3 coatings were 0.27 (in approximation) and 122 GPa, respectively [2]. The sliding speed was 4 cm·s−1, and the sliding distance was 500 m with a rotation diameter of 2.5 mm. The applied wear parameters (i.e. load, sliding speed and alumina counter-body) do not intend to simulate the condition of a specific wear-critical application. Therefore, such parameters were selected in consideration of previously published works on the wear resistance of Fe-W alloys [1,2,20] and other Fe-group metals coatings suggested as alternatives to Cr replacement (included in the Table S1 as supplementary material). In particular, the applied load and sliding speed were selected in order to reduce the formation of cracks and debris upon sliding which can largely affect the correct evaluation of the wear track profiles, and thus of the wear rates [1]. The influence of 1, 2 and 5 N applied load on the crack and debris formation during wear tests on Fe-W alloys is discussed in detail in [20]. To ensure the reproducibility of the results, three wear tests per specimen were performed. A confocal optical microscope (Leica, model DCM3D, Wetzlar, Germany) was used to measure the depth profiles of the wear tracks. The wear rate of the tested coatings was calculated from the equation: (1)
=
where A is the cross-section area of wear track (mm2), l is the length of the wear track (mm), F is the load (N) and D is the sliding distance (m). 5
3 3.1
Results and discussions Structural Characterization of As-Deposited and Annealed Fe-W/Al2O3 Coatings
The surface and cross-section of the as-deposited and annealed Fe-W/Al2O3 coatings were studied by SEM and representative micrographs are shown in Fig. 1. The alumina particles appear uniformly distributed both on the surface and across the thickness of the as-deposited coatings, appearing as darker particles in Figs. 1a and 1b. The volume percentage of the co-deposited alumina particles is 10.7 ± 0.4%, as calculated from image analysis. Thus, in the following, the samples will be designated as Fe-W/11%Al2O3. The Fe-W matrix had a fixed composition of 22 ± 0.6 at.% of W. After annealing for 1 hour at 600 ⁰C, small white precipitates are found at the surface of the composite coating (Fig. 1c). EDS point analysis reveals these granules are Fe-rich.
Figure 1. Secondary electrons images of the surface (a) and the cross-section (b) of the Fe-W/11%Al2O3 coating in the as-deposited condition, and of the surface (c) and cross-section (d) of the Fe-W/11%Al2O3 coating after annealing at 600 ℃.
As shown by X-ray diffraction (XRD) analysis (Fig. 2), the main microstructural transformation occurring upon annealing at 600 ⁰C is the crystallization of α-Fe. In the as-deposited state, because of the high amount of co-deposited W, the structure of the Fe-W matrix is amorphous. This can be observed from the broad shoulder starting at 2θ ∼ 60 ⁰ in the XRD spectrum. The effect of W and alumina content on the as-deposited structure is discussed in detail elsewhere [2,17]. A large fraction of the amorphous structure is preserved upon annealing at 600 ⁰C (∼ 85%, as calculated by dividing the area of the crystalline peaks by the total area of the XRD spectrum) due to the high thermal stability of the Fe-W metal matrix [17]. Also, the microstructural transformation observed 6
after annealing at 600 ⁰C (Fig. 2) is consistent with the previous finding regarding the annealing of amorphous Fe-W coatings [17]. Hence, the presence of the co-deposited alumina particles seems not to have an influence on the thermal stability and microstructural transformation of the Fe-W matrix. As a matter of fact, crystallographic transformations of alumina are not expected at 600 ⁰C [29]. However, a slight increase in the surface roughness (Ra) was measured after the annealing treatment, i.e. from 135 to178 nm, due to the formation of α-Fe precipitates. For the sake of comparison, dense chromium coatings were also electrodeposited. Surface morphology and cross-section images of hard Cr coatings are shown in Fig. 3 in the as-deposited state (a-b) and after annealing at 600 ⁰C (c-d). As shown in Fig. 3, cracks are found both in the surface and along the cross-sections of the Cr samples. The formation of the cracks can be related to the stresses formed in the coatings due to the significant hydrogen evolution occurring during electrodeposition of hard chrome [30].
Figure 2. X-ray diffraction patterns of Fe-W/-11%Al2O3coatings in as-deposited state, and after annealing at 600 ⁰C for 1 hour.
7
Figure 3. Secondary electrons images of the surface (a) and the cross-section (b) of hard Cr coating in the as-deposited condition, and of the surface (c) and cross-section (d) of the hard Cr coating after annealing at 600 ℃.
3.2
Mechanical Properties and Wear Resistance of As-Deposited and Annealed Fe-W/11%Al2O3 and hard Cr Coatings
The mechanical properties, i.e. hardness and reduced elastic modulus, of the as-deposited and annealed Fe-W/11%Al2O3 and the hard Cr coatings were assessed by nanoindentation analyses. Representative load-displacement curves are shown in Fig. 4 and the H and Er results are shown in Fig. 5. As shown in Fig. 5, the hardness of the Fe-W/11%Al2O3 composite coating considerably increases after the annealing treatment, reaching the value of 16.3 GPa. Considering that alumina is not affected by the annealing process, such significant increase in the hardness of the composite can be attributed to the structural transformation occurring in the Fe-W matrix upon heat treatment: the precipitation of fine α-Fe crystallites and the formation of a mixed amorphous-nanocrystalline structure. In fact, a similar hardness was obtained for Fe-24at.%W coatings after annealing at 600 ⁰C which also resulted in the formation of a mixed amorphous-nanocrystalline structure [19]. This reasoning is also strengthened by previous studies on Fe-based metallic glasses [31]. As the authors observed from TEM analysis, annealing of such materials resulted in the precipitation of fine crystallites within an amorphous matrix, leading to an increase of the mechanical and wear properties of the alloy [31]. Moreover, the increase in Er after annealing treatments, see Fig. 5b, can
8
be related to the structural relaxation and the density increase of the material which occurs when annealing amorphous alloys [32].
Figure 4. Load versus penetration depth curve measured for the as-deposited and annealed Fe-W/11%Al2O3. The insert shows a representative SEM image of an indent imprint on the cross-section of the Fe-W/11%Al2O3 coatings.
Figure 5. Hardness (a) and reduced elastic modulus (b) of Fe-W/11%Al2O3 and hard Cr coatings in the as-deposited state and after annealing at 600 ⁰C, respectively.
9
In contrast to what is observed for Fe-W/11%Al2O3 composite coatings, the hardness of the hard Cr coating substantially decreases after the heat treatment, see Fig. 5a. Such decrease in hardness in heat-treated metallic thin films is quite common and it can be related to grain growth and, in this case in particular, to the formation of larger cracks [33,34], as it can be also noticed from Fig. 3a and 3c. To study the effect of heat treatments on the wear resistance of Fe-W/11%Al2O3 composite and hard Cr coatings, sliding tests were performed using a ball-on-disk technique. The variation of the coefficient of friction (COF) as a function of sliding distance of the as-deposited and annealed FeW/11%Al2O3 and Cr coatings is shown in Fig. 6. The influence of the annealing treatment on the COF of both composite and Cr coatings appears rather limited. For the as-deposited FeW/11%Al2O3 coating the COF increases rapidly and, after a running period of 650 s, it stabilizes at an average value of ∼0.7 (see Fig. 6a). The COF increase during the first stages of the sliding test can be related to the formation of third body particles which increase the asperity contact between the two sliding bodies. For the heat treated Fe-W/11%Al2O3 coating, the contact between the alumina counter-body and the sample occurs initially through the α-Fe grains formed at the surface of the sample (see Fig. 1c). These grains are relatively soft, thus a lower COF is observed up to a running period of 689 s. Afterwards, when the alumina counter-body is in contact with the bulk of the coating the COF rises to ∼ 0.8 and then it stabilizes around ∼ 0.7.
Figure 6. Coefficient of friction evolution for the as-deposited and annealed Fe-W/11%Al2O3coatings (a), and for as-deposited and annealed hard Cr coatings (b).
The COF variations for the as-deposited and annealed hard Cr coatings appear quite similar with an average value of ∼ 0.6, see Fig. 6b. However, a sudden increase in the COF is visible for the annealed specimen. Such phenomenon can be related to the brittle nature of the Cr coatings. A sharp increase in the COF has been observed when brittle fracture occurs leading to the formation 10
of fine debris [35]. The higher surface roughness after annealing the Cr coatings (Ra= 260 nm) could also influence the observed COF trend.
Figure 7. Secondary electrons images of the wear track of the Fe-W/11%Al2O3 coatings in the as-deposited state (a), after annealing at 600 ◦C (b), as well as of hard Cr coatings in the as-deposited state (c), and after annealing at 600 ◦C (d), respectively. The SEM micrograph of the wear track of the hard Cr coating after annealing is including an insert of the cracked wear track acquired at higher magnification. Areas where EDS point analyses were performed are indicated by numbers (1 to 7).
The wear tracks of the studied samples were analyzed by SEM and the results are shown in Fig. 7. Chemical analyses were performed with EDS point analysis in the spots indicated in Fig. 7 and the results are summarized in Table 1. For the Fe-W/11%Al2O3 composite coatings, traces of adherent oxide were found along the wear tracks (e.g. spectrum 2 and 3 indicated in Fig. 7a) or agglomerated at the side of the wear track (e.g. spectrum 5 indicated in Fig. 7b). The adherent oxide film is not distributed homogeneously, i.e. in some areas the surface of the composite coating is still visible (e.g. spectrum 1 indicated in Fig. 7a, and spectrum 4 indicated in Fig. 7b). As shown by the EDS results in Table 1, in the areas where the oxide film and the debris are present the oxygen increases up to ~60 at.%. For the evaluation of light elements such as oxygen, EDS point analysis is qualitative rather than quantitative. Also, it is hard to evaluate the precise quantity of oxygen considering the contribution from the co-deposited alumina particles. However, the results from the spectra acquired at the location of an adherent oxide in the as-deposited sample (i.e. spectrum 2 and 3) are characterized with a Fe to W ratio which is almost identical to the Fe to W ratio of the non11
tested samples, i.e. ∼ 3.8. This indicates that the oxides formed during the wear tests are mixed FeW-oxides. In the case of the annealed Fe-W/11%Al2O3 sample, the data acquired from the spectrum 5 show a slightly higher Fe to W ratio, i.e. ∼ 4.7. Here, the wear debris are richer in Fe due to the formation of α-Fe granules at the surface of the annealed coating, see Fig. 1c. The formation of such oxides is related to the tribo-oxidation mechanism: the oxidation of the coatings during the sliding test is an intrinsic response of the material to recover from the thermal energy generated during dry friction. Previous studies already highlighted that the wear mechanism of electrodeposited Fe-W coatings in dry sliding conditions is mainly characterized by oxidative and adhesive mechanisms [20,36,37]. Table 1. Chemical composition obtained by point analysis when using Energy Dispersive X-ray Spectroscopy (EDS) at locations shown in Figure 7.
Spectrum
Fe (at.%)
W (at.%)
Al (at.%)
O (at.%)
Cr (at.%)
1
47.1
12.6
2.4
37.9
/
2
27.3
7.2
2.5
63
/
3
24.8
6.5
3.4
65.3
/
4
56
14.4
5.6
24
/
5
23.3
5
3.9
67.8
/
6
/
/
/
24.4
75.6
7
2
/
/
61.4
36.6
Figure 7c shows the SEM micrographs from the wear track of the as-deposited Cr coating. Here, the presence of abrasion grooves along the wear track suggests that the wear mechanism is mainly abrasive. Wear debris is found in the wear track which is composed of chromium oxide, as shown from spectrum 6, see Table 1. In the area not affected by the wear test, the amount of co-deposited O is found to be lower than 3 at.%. The wear track of the annealed Cr coating appears substantially wider as compared to the wear track of the as-deposited sample (notice the different scale bar in Fig. 7d). Also, the wear track appears oxidized and cracked (see the insert in Fig. 7d). In the proximity of the cracks, EDS analysis shows the presence of iron (see spectrum 7), which indicates that the specimen interaction volume is reaching the steel substrate.
12
Figure 8. Wear rate of Fe-W/11%Al2O3 and hard Cr coatings in the as-deposited state and after annealing at 600 ⁰C (a). Representative wear track profiles of as-deposited and annealed Fe-W/11%Al2O3 (b and c), and of as-deposited and annealed Cr coatings (d and e).
Figure 8 shows the wear rate of the as-deposited and annealed Fe-W/11%Al2O3 composite and hard Cr coatings, together with representative depth profiles of the wear tracks. The wear rate was calculated using Equation (1), considering for each sample an average area of the wear track (A). The results reveal that the wear rate of Fe-W/11%Al2O3 composite in both as-deposited and annealed state is significantly lower than the wear rate of as-deposited hard Cr. Also, the wear rate results clearly illustrate the detrimental effect of the heat treatment on the wear resistance of the hard Cr coating, leading to a substantial increase in the wear rate reaching the value of ∼24 x10-6 mm3/Nm. Many studies have indicated the importance of the mechanical properties for a good wear resistance, showing in particular how a high hardness to Young´s modulus ratio (H/E) generally implies a good wear resistance [38,39]. It is assumed that materials with high H/E ratio should better resist the plastic deformation and thus result in higher wear resistance. The H/Er ratios for the Fe-W/11%Al2O3 and Cr coatings, before and after annealing, are reported in Table 2. The values are calculated from the nanoindentation results. The H/Er of the as-deposited and annealed FeW/11%Al2O3 is higher than the H/Er of the Cr coatings. This is in agreement with the higher wear resistance of the composite coatings as compared to Cr coatings. Also, the dramatic increase observed for the wear rate of hard Cr after annealing can be related to the decrease of the H/Er. As shown in Table 2, a 57% reduction of the H/Er ratio for the Cr coatings is observed after annealing at 600 ℃. For Fe-W/11%Al2O3 coatings, despite the substantial increase in the hardness after annealing, the wear rate of the annealed sample is in principal unchanged and identical to the asdeposited coating. This could be related to the fact that the relative increase in the H/Er after annealing (18%) is not high enough to strongly impact the wear resistance. Also, it has been shown that the tribological behavior of the Fe-W system is rather complex, being strongly affected by 13
tribo-oxidation mechanism. For example, previously studied Fe-W/3%Al2O3 and Fe-W/6%Al2O3 coatings characterized with high mechanical properties (e.g. H/Er) resulted in a moderate wear resistance. In that case, a reduction in wear rate (and tribo-oxidation) was observed in presence of higher amounts (i.e. 12 vol.%) of co-deposited alumina particles [2]. Table 2. Summary of the values of hardness, reduced elastic modulus and hardness to reduced elastic modulus ratio for as-deposited and annealed Fe-W/11%Al2O3 and hard Cr coatings.
Sample
H (GPa)
Er (GPa)
H/Er
As-deposited Fe-W/11%Al2O3
8.8
122
0.072
Annealed Fe-W/11%Al2O3
16.3
191.7
0.085
As-deposited Hard Cr
11.9
224
0.053
Annealed hard Cr
5.8
250
0.023
As shown from the hardness and wear results presented in this work, heat treatments of FeW/11%Al2O3 composite coatings can be applied to further enhance the mechanical properties of the coating while preserving the wear resistance. The annealed Fe-W/11%Al2O3 composite coating is characterized with high hardness as a result of the partial crystallization of the amorphous Fe-W matrix. The high wear resistance of the composite is preserved due to the presence of co-deposited Al2O3 particles, which are beneficial in reducing the tribo-oxidation mechanism.
4
Summary and conclusions
The effect of heat treatment on the mechanical properties and on the wear resistance of FeW/11%Al2O3 has been studied and compared to hard chromium coating. After annealing at 600 ⁰C for 1 hour, the amorphous structure of the Fe-W matrix transforms into a mixed amorphouscrystalline structure containing α-Fe crystallites. The co-deposited Al2O3 particles seem not to be affected by the annealing treatment. The mixed amorphous-crystalline structure obtained after annealing is found to be the main factor responsible of the substantial increase in the hardness of the composite coatings, i.e. the increase from 8.8 GPa in the as-deposited films up to 16.3 GPa which is substantially higher of the hardness of hard Cr coatings. The wear resistance of the Fe-W/11%Al2O3 is preserved upon annealing, i.e. the COF and wear rate for the as-deposited and annealed coatings are similar. In contrast, the heat treatment has shown to be detrimental for both hardness and wear resistance of hard Cr coatings. The heat treatment of hard Cr coatings resulted in an abrupt decrease of the hardness and wear resistance of the coatings. In particular, a 57% reduction of the H/Er ratio
14
upon annealing resulted in a substantial increase of the wear rate from 3.8 x10-6 mm3/Nm to 24.2 x10-6 mm3/Nm. When comparing the performances of Cr and Fe-W/11%Al2O3 coatings, especially the annealed FeW/11%Al2O3 composites show higher hardness and lower wear rate, i.e. 1.5 x10-6 mm3/Nm. As shown in this work, annealing of Fe-W/11%Al2O3 coatings is a useful strategy to obtain coatings with a combination of high hardness and high wear resistance. The investigated composite coatings can indeed be considered as a possible sustainable alternative to hard chromium coatings, whose production is often hazardous and not environmentally friendly. 5
Acknowledgements
This work was supported by the Spanish Government (MAT2017-86357-C3-1-R, MAT201786357-C3-2-R and associated FEDER), the Generalitat de Catalunya (2017-SGR-292), Basque government (ELKARTEK, No. KK-2018/00108) and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement nº 665919, and H2020-MSCA-RISE-2017 SMARTELECTRODES project (No.778357). J.F. acknowledges the “Juan de la Cierva” (IJCI-2015-27030) contract by the Spanish Government.
15
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20
Highlights • • • •
Partial crystallization of the amorphous Fe-W/11%Al2O3 coating upon annealing Substantial increase in the hardness and modulus of annealed Fe-W/11%Al2O3 coating Abrupt decrease in the hardness and wear resistance of annealed hard Cr coating Annealed Fe-W/11%Al2O3 coating shows superior wear resistance than hard Cr coating
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0043-1648(19)31702-8
DOI:
https://doi.org/10.1016/j.wear.2020.203232
Reference:
WEA 203232
To appear in:
Wear
Received Date: 21 November 2019 Revised Date:
6 February 2020
Accepted Date: 6 February 2020
Please cite this article as: A. Mulone, A. Nicolenco, N. Imaz, J. Fornell, J. Sort, U. Klement, Effect of heat treatments on the mechanical and tribological properties of electrodeposited Fe–W/Al2O3 composites, Wear (2020), doi: https://doi.org/10.1016/j.wear.2020.203232. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Author statement Antonio Mulone: Conceptualization, Methodology, Investigation, Visualization, WritingOriginal Draft, Writing-Review & Editing. Aliona Nicolenco: Investigation, Resources, Writing-Review & Editing. Naroa Imaz: Investigation, Validation, Writing-Review & Editing. Jordina Fornell: Investigation, Validation, Writing-Review & Editing. Jordi Sort: Supervision, Writing-Review & Editing. Uta Klement: Supervision, Writing-Review & Editing.
Effect of heat treatments on the mechanical and tribological properties of electrodeposited FeW/Al2O3 composites Antonio Mulonea, Aliona Nicolencob ,c, Naroa Imazd, Jordina Fornellb, Jordi Sortb, e, Uta Klementa a
Chalmers University of Technology, Department of Industrial and Materials Science, SE-412 96
Gothenburg, Sweden b c
Institute of physics, Chisinau MD-2028, Moldova
d e
Departament de Física, Universitat Autònoma de Barcelona, Bellaterra E-08193, Spain
CIDETEC, Paseo Miramón 196, E-20014 Donostia-San Sebastián, Spain
Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluís Companys 23, Barcelona E-08010,
Spain
Abstract In this study, the influence of heat treatment on the mechanical and tribological properties of electrodeposited Fe-W/Al2O3 composite coatings is studied. The properties of the as-deposited and annealed composites are compared with those of electrodeposited hard chromium coatings. The amorphous structure of the Fe-W matrix transforms into a mixed amorphous-crystalline structure upon annealing at 600 ⁰C for 1 hour. The observed microstructural transformations result in a substantial increase of both the hardness and the reduced Young’s modulus of the Fe-W/Al2O3 composite coatings, reaching values of 16.3 GPa and 191.7 GPa, respectively. The results on the wear resistance studied under dry friction using ball-on-disc sliding tests show that a low wear rate is obtained for both as-deposited and annealed composite coatings, i.e. ∼1.5 x 10-6 mm3/Nm. In contrast, the heat treatments are detrimental for both the hardness and wear resistance of hard chromium coatings. As a consequence, the mechanical and wear properties of the electrodeposited Fe-W/Al2O3 composite coatings, especially after annealing, are superior to the properties of hard chromium coatings. Hence, Fe-W/Al2O3 composite coatings can be considered as a valid and sustainable alternative to hard chromium coatings, particularly in applications where these materials may be exposed to high temperatures.
Keywords: electrodeposition; iron-tungsten alloys; composite coatings; wear resistance; heat treatment;
1
1
Introduction
In the recent years, electrodeposited Fe-W and Fe-W/Al2O3 composite coatings have been studied and evaluated as an effective sustainable alternative to electrodeposited hard chromium coatings [1,2]. Chrome coatings are largely applied in aerospace and automotive applications [3,4] owing to their outstanding properties, e.g. high hardness, as well as wear and corrosion resistance. The production of hard chromium coatings, however, involves the use of extremely toxic compounds such as Cr6+ [5,6], which can significantly affect human beings, plants and microbial life. Since compounds containing hexavalent chromium have been added to the Annex XIV (i.e. European “Authorization List” that limits the import and the use of a compound under very strict rules), many studies have been performed with the aim to find an environmentally friendly alternative to electrodeposited hard chromium coatings. Trivalent chromium plating (i.e. Cr3+), for example, is less harmful and it is considered as a promising technology, but its application is still limited due to thickness and surface quality of the obtained coatings [7]. Other studies on electrodeposited Cr-free alloys and composites such as Ni and Co-based coatings (particularly with W or P) have shown to be characterized with a higher hardness and wear resistance as compared to hard chromium coatings [8–14]. However, the proposed Ni and Co-based coatings do not always represent a valid environmentally friendly alternative. Cobalt salts are targeted as “substance of very high concern” as cancerogenic compounds and the use of Ni is discouraged as it is listed as highly allergenic [15]. Thus, exploring new environmentally friendly alternatives to hard chromium coatings constitutes a topic still under intense investigation. Among various electrodeposited alloys proposed as alternatives to Cr6+ coatings, Fe-W is gaining considerable attention because it can be electrodeposited from environmentally friendly and thermodynamically stable electrolytes [16]. In addition, this system exhibits tunable composition and structure and the resulting hardness and thermal stability can exceed those of similar Co- or Nibased alloys [17]. Indeed, Fe-W alloys with a W content above 17 at.% are characterized with high hardness, i.e. about 9–11 GPa [18]. Furthermore, the hardness of Fe-W coatings can be substantially increased by heat treatments. In particular, a maximum hardness value of 16.5 GPa was obtained for W-rich Fe-W coatings (i.e. 24 at.% of W) after annealing for one hour at 600 ⁰C [19]. Wear analyses performed in dry conditions on as-deposited Fe-W coatings showed that the coatings were characterized by a rather low wear resistance [20]. The severe tribo-oxidation mechanism observed in Fe-W coatings during dry sliding tests was found to be the main cause of their low wear resistance. Abrasive iron oxide particles were formed during the sliding tests, leading to high values of the coefficient of friction (COF), i.e. varying from 0.8 to 1.2 (sliding against corundum counterbody), and to high wear track volumes, i.e. ∼12 x 10-6 mm3/Nm [1,20]. A remarkable improvement in the wear resistance was obtained with the co-deposition of Al2O3 sub-micron particles within the 2
Fe-W metal matrix [2]. The co-deposited alumina particles reduce the contact area of the Fe-W matrix which is affected by tribo-oxidation, resulting in a lower COF and wear rate. Among the different Fe-W/Al2O3 coatings tested, the composite coating deposited with the highest amount of Al2O3 particles, i.e. 12 vol.%, was characterized with the lowest COF, i.e. 0.6, and wear rate, i.e. 1.8 x10-6 mm3/Nm. However, while increasing the wear resistance, the co-deposition of 12 vol.% alumina particles also resulted in a slight reduction of both the hardness and reduced elastic modulus of the Fe-W/Al2O3 composite as compared to pure Fe-W (with no particles). Such decrease in the mechanical performance of composite coatings is often caused by porosity and a weak particle-matrix interface strength [21]. Heat treatments have proven to be beneficial for the mechanical and wear properties of different electrodeposited coatings [22–24]. Previous studies have also shown the beneficial effects of heat treatments on the bond strength of the reinforcement-matrix interface in composites [25]. The aim of this paper is to study the effect of heat treatment on the mechanical properties and wear resistance of Fe-W/Al2O3 composite coatings and to compare them with the wear resistance and mechanical resistance of hard chromium coatings. The possibility to tailor the microstructure, and consequently the material properties, through proper heat treatments is investigated. The synthesis of Fe-W/Al2O3 composite coatings with a combination of good mechanical properties and high wear resistance can be considered as an important step towards the development of competitive and sustainable alternatives for coatings produced using environmentally hazardous processes, such as plating with hexavalent chromium.
2 2.1
Experimental Electrodeposition of Fe-W/Al2O3 and chromium coatings
Fe-W/Al2O3 coatings were electrodeposited according to the procedure described in our previous work [2]. The following composition of base electrolyte was used: 1 M glycolic acid, 0.3 M citric acid, 0.1 M Fe2(SO4)3 and 0.3 M Na2WO. The alumina particles (Alfa Aesar 42572) were added into the electrolyte with a concentration of 100 g L−1. The electrolyte pH was adjusted to 7.0 by adding NaOH. The suspension was then stirred at 300 rpm for 24 h to hydrate the particles. Before the electrodeposition of the coatings, the suspension was ultrasonically agitated for 10 minutes to avoid the agglomeration of alumina particles. The majority of the particles embedded in the coating are characterized by a mean diameter between 50 and 150 nm, as measured with image analysis (ImageJ software) [2]. Electrodeposition was carried out using a standard three-electrode cell. A 4 cm2 brass plate served as a working electrode, a platinized titanium mesh as a counter electrode, and 3
an Ag/AgCl/KCl (sat) as the reference electrode. The brass substrates were decreased using a commercial alkaline bath to remove possible contaminations from the surface and then activated in 2 M H2SO4 solution. A nickel seed layer was deposited from a chloride bath at 65 °C, 30 mA cm-2 for 1 minute to improve the adhesion of the composite with the substrate. The electrodeposition of Fe-W/Al2O3 coatings was performed at 65 °C applying a constant cathodic current density of 40 mA cm-2 for 1 hour, which resulted in 20 ± 2 µm thick layer. The volume of the electrolyte was maintained 200 mL and the solution was mechanically stirred at 250 rpm. The electrodeposition of chromium coating was performed using a three-cell composed of: (i) polished carbon steel as a working electrode, (ii) Ag/AgCl/Cl− (3M) as a reference electrode, and (iii) Lead-8% tin as a counter electrode. The electrolyte contained 250 g L−1 CrO3 and 2.5 g L−1 H2SO4, the volume was maintained 400 mL, while keeping a temperature of 51 ⁰C. The electrodeposition was performed by applying a cathodic current density of 300 mA·cm−2 for 1 hour and 30 minutes. Cr coatings were deposited with a thickness of 20 ± 1 µm and surface roughness (Ra) of 220 ± 22 nm.
2.2
Coating characterization
To analyze the surface morphology of the deposited coatings, a Leo 1550 Gemini Scanning Electron Microscope (SEM) with a field emission gun was used. The chemical composition of the coatings was measured at the surface and along the cross-section of the samples by Energy Dispersive X-ray Spectroscopy (EDS). The volume fraction of co-deposited Al2O3 particles in the deposits was determined by performing image analyses (ImageJ software) on the cross-sections of the coatings. The surface roughness was measured using a WYKO RST-PLUS optical profilometer. The heat treatments were performed in the furnace of a NETZSCH 402 C dilatometer, keeping the sample at 600 ⁰C for 1 hour. High purity Ar 6.0 was used and the annealing temperature was reached with a heating rate of 10 ⁰C/min. After the heat treatments, the samples were kept inside the furnace until reaching room temperature. X-rays diffraction (XRD) analyses were performed using a Bruker AXS D8 advance diffractometer with Cr Kα radiation (λ = 2.28970 Å) and the equipment was operated at 40 kV and 50 mA. The annealing temperature of 600 ℃ was selected based on previous findings that showed a maximum in the hardness of Fe-W coatings after annealing at 600 ℃ [19]. A NHT2 Nanoindentation Tester from Anton-Paar was used to evaluate the mechanical properties, i.e. hardness and reduced elastic modulus. The measurements were performed along the cross-section of the deposits, under load-control mode and applying a load of 25 mN with a Berkovich pyramidal-shaped diamond tip. This load is low enough to avoid the influence of the embedding resin and the substrate, but it is high enough to embrace a representative area of the 4
coating (including particles and matrix). A total of fifteen indentations were performed on each sample and the results are expressed as the average of all the tests. For nanoindentation measurements, the cross-section of the samples was polished down to a 1 µm finishing. The hardness (H) and reduced elastic modulus (Er) of the analyzed samples are obtained from the loaddisplacement curves as according to the Oliver and Pharr method [26]. The tribological properties of the coatings were measured using a ball-on-disc tribometer under dry sliding conditions, at room temperature and ~55% of relative humidity. An alumina ball with a diameter of 6 mm was used as a counter-body, due to its high stability and resistance to abrasion and oxidation. Thanks to such properties, alumina counter-bodies are commonly employed in hard ball/hard coating systems [27,28]. Also, being Fe-W alloys affected by tribo-oxidation, the use of a counter-body less resistant to oxidation, e.g. steel ball, would increase the possibility to introduce wear impurities (i.e. debris and oxides) into the system. The size of the ball was selected considering the size of the sample (i.e. 2 x 2 cm), and to allow to run multiple tests per each specimen. The wear tests were conducted applying 2 N load which results in an initial Hertzian contact pressure of 665.3 MPa, assuming a “sphere on plane” configuration, where the elastic modulus, Er, and the Poisson ratio for corundum ball were 210 GPa and 0.22, respectively, and for FeW/Al2O3 coatings were 0.27 (in approximation) and 122 GPa, respectively [2]. The sliding speed was 4 cm·s−1, and the sliding distance was 500 m with a rotation diameter of 2.5 mm. The applied wear parameters (i.e. load, sliding speed and alumina counter-body) do not intend to simulate the condition of a specific wear-critical application. Therefore, such parameters were selected in consideration of previously published works on the wear resistance of Fe-W alloys [1,2,20] and other Fe-group metals coatings suggested as alternatives to Cr replacement (included in the Table S1 as supplementary material). In particular, the applied load and sliding speed were selected in order to reduce the formation of cracks and debris upon sliding which can largely affect the correct evaluation of the wear track profiles, and thus of the wear rates [1]. The influence of 1, 2 and 5 N applied load on the crack and debris formation during wear tests on Fe-W alloys is discussed in detail in [20]. To ensure the reproducibility of the results, three wear tests per specimen were performed. A confocal optical microscope (Leica, model DCM3D, Wetzlar, Germany) was used to measure the depth profiles of the wear tracks. The wear rate of the tested coatings was calculated from the equation: (1)
=
where A is the cross-section area of wear track (mm2), l is the length of the wear track (mm), F is the load (N) and D is the sliding distance (m). 5
3 3.1
Results and discussions Structural Characterization of As-Deposited and Annealed Fe-W/Al2O3 Coatings
The surface and cross-section of the as-deposited and annealed Fe-W/Al2O3 coatings were studied by SEM and representative micrographs are shown in Fig. 1. The alumina particles appear uniformly distributed both on the surface and across the thickness of the as-deposited coatings, appearing as darker particles in Figs. 1a and 1b. The volume percentage of the co-deposited alumina particles is 10.7 ± 0.4%, as calculated from image analysis. Thus, in the following, the samples will be designated as Fe-W/11%Al2O3. The Fe-W matrix had a fixed composition of 22 ± 0.6 at.% of W. After annealing for 1 hour at 600 ⁰C, small white precipitates are found at the surface of the composite coating (Fig. 1c). EDS point analysis reveals these granules are Fe-rich.
Figure 1. Secondary electrons images of the surface (a) and the cross-section (b) of the Fe-W/11%Al2O3 coating in the as-deposited condition, and of the surface (c) and cross-section (d) of the Fe-W/11%Al2O3 coating after annealing at 600 ℃.
As shown by X-ray diffraction (XRD) analysis (Fig. 2), the main microstructural transformation occurring upon annealing at 600 ⁰C is the crystallization of α-Fe. In the as-deposited state, because of the high amount of co-deposited W, the structure of the Fe-W matrix is amorphous. This can be observed from the broad shoulder starting at 2θ ∼ 60 ⁰ in the XRD spectrum. The effect of W and alumina content on the as-deposited structure is discussed in detail elsewhere [2,17]. A large fraction of the amorphous structure is preserved upon annealing at 600 ⁰C (∼ 85%, as calculated by dividing the area of the crystalline peaks by the total area of the XRD spectrum) due to the high thermal stability of the Fe-W metal matrix [17]. Also, the microstructural transformation observed 6
after annealing at 600 ⁰C (Fig. 2) is consistent with the previous finding regarding the annealing of amorphous Fe-W coatings [17]. Hence, the presence of the co-deposited alumina particles seems not to have an influence on the thermal stability and microstructural transformation of the Fe-W matrix. As a matter of fact, crystallographic transformations of alumina are not expected at 600 ⁰C [29]. However, a slight increase in the surface roughness (Ra) was measured after the annealing treatment, i.e. from 135 to178 nm, due to the formation of α-Fe precipitates. For the sake of comparison, dense chromium coatings were also electrodeposited. Surface morphology and cross-section images of hard Cr coatings are shown in Fig. 3 in the as-deposited state (a-b) and after annealing at 600 ⁰C (c-d). As shown in Fig. 3, cracks are found both in the surface and along the cross-sections of the Cr samples. The formation of the cracks can be related to the stresses formed in the coatings due to the significant hydrogen evolution occurring during electrodeposition of hard chrome [30].
Figure 2. X-ray diffraction patterns of Fe-W/-11%Al2O3coatings in as-deposited state, and after annealing at 600 ⁰C for 1 hour.
7
Figure 3. Secondary electrons images of the surface (a) and the cross-section (b) of hard Cr coating in the as-deposited condition, and of the surface (c) and cross-section (d) of the hard Cr coating after annealing at 600 ℃.
3.2
Mechanical Properties and Wear Resistance of As-Deposited and Annealed Fe-W/11%Al2O3 and hard Cr Coatings
The mechanical properties, i.e. hardness and reduced elastic modulus, of the as-deposited and annealed Fe-W/11%Al2O3 and the hard Cr coatings were assessed by nanoindentation analyses. Representative load-displacement curves are shown in Fig. 4 and the H and Er results are shown in Fig. 5. As shown in Fig. 5, the hardness of the Fe-W/11%Al2O3 composite coating considerably increases after the annealing treatment, reaching the value of 16.3 GPa. Considering that alumina is not affected by the annealing process, such significant increase in the hardness of the composite can be attributed to the structural transformation occurring in the Fe-W matrix upon heat treatment: the precipitation of fine α-Fe crystallites and the formation of a mixed amorphous-nanocrystalline structure. In fact, a similar hardness was obtained for Fe-24at.%W coatings after annealing at 600 ⁰C which also resulted in the formation of a mixed amorphous-nanocrystalline structure [19]. This reasoning is also strengthened by previous studies on Fe-based metallic glasses [31]. As the authors observed from TEM analysis, annealing of such materials resulted in the precipitation of fine crystallites within an amorphous matrix, leading to an increase of the mechanical and wear properties of the alloy [31]. Moreover, the increase in Er after annealing treatments, see Fig. 5b, can
8
be related to the structural relaxation and the density increase of the material which occurs when annealing amorphous alloys [32].
Figure 4. Load versus penetration depth curve measured for the as-deposited and annealed Fe-W/11%Al2O3. The insert shows a representative SEM image of an indent imprint on the cross-section of the Fe-W/11%Al2O3 coatings.
Figure 5. Hardness (a) and reduced elastic modulus (b) of Fe-W/11%Al2O3 and hard Cr coatings in the as-deposited state and after annealing at 600 ⁰C, respectively.
9
In contrast to what is observed for Fe-W/11%Al2O3 composite coatings, the hardness of the hard Cr coating substantially decreases after the heat treatment, see Fig. 5a. Such decrease in hardness in heat-treated metallic thin films is quite common and it can be related to grain growth and, in this case in particular, to the formation of larger cracks [33,34], as it can be also noticed from Fig. 3a and 3c. To study the effect of heat treatments on the wear resistance of Fe-W/11%Al2O3 composite and hard Cr coatings, sliding tests were performed using a ball-on-disk technique. The variation of the coefficient of friction (COF) as a function of sliding distance of the as-deposited and annealed FeW/11%Al2O3 and Cr coatings is shown in Fig. 6. The influence of the annealing treatment on the COF of both composite and Cr coatings appears rather limited. For the as-deposited FeW/11%Al2O3 coating the COF increases rapidly and, after a running period of 650 s, it stabilizes at an average value of ∼0.7 (see Fig. 6a). The COF increase during the first stages of the sliding test can be related to the formation of third body particles which increase the asperity contact between the two sliding bodies. For the heat treated Fe-W/11%Al2O3 coating, the contact between the alumina counter-body and the sample occurs initially through the α-Fe grains formed at the surface of the sample (see Fig. 1c). These grains are relatively soft, thus a lower COF is observed up to a running period of 689 s. Afterwards, when the alumina counter-body is in contact with the bulk of the coating the COF rises to ∼ 0.8 and then it stabilizes around ∼ 0.7.
Figure 6. Coefficient of friction evolution for the as-deposited and annealed Fe-W/11%Al2O3coatings (a), and for as-deposited and annealed hard Cr coatings (b).
The COF variations for the as-deposited and annealed hard Cr coatings appear quite similar with an average value of ∼ 0.6, see Fig. 6b. However, a sudden increase in the COF is visible for the annealed specimen. Such phenomenon can be related to the brittle nature of the Cr coatings. A sharp increase in the COF has been observed when brittle fracture occurs leading to the formation 10
of fine debris [35]. The higher surface roughness after annealing the Cr coatings (Ra= 260 nm) could also influence the observed COF trend.
Figure 7. Secondary electrons images of the wear track of the Fe-W/11%Al2O3 coatings in the as-deposited state (a), after annealing at 600 ◦C (b), as well as of hard Cr coatings in the as-deposited state (c), and after annealing at 600 ◦C (d), respectively. The SEM micrograph of the wear track of the hard Cr coating after annealing is including an insert of the cracked wear track acquired at higher magnification. Areas where EDS point analyses were performed are indicated by numbers (1 to 7).
The wear tracks of the studied samples were analyzed by SEM and the results are shown in Fig. 7. Chemical analyses were performed with EDS point analysis in the spots indicated in Fig. 7 and the results are summarized in Table 1. For the Fe-W/11%Al2O3 composite coatings, traces of adherent oxide were found along the wear tracks (e.g. spectrum 2 and 3 indicated in Fig. 7a) or agglomerated at the side of the wear track (e.g. spectrum 5 indicated in Fig. 7b). The adherent oxide film is not distributed homogeneously, i.e. in some areas the surface of the composite coating is still visible (e.g. spectrum 1 indicated in Fig. 7a, and spectrum 4 indicated in Fig. 7b). As shown by the EDS results in Table 1, in the areas where the oxide film and the debris are present the oxygen increases up to ~60 at.%. For the evaluation of light elements such as oxygen, EDS point analysis is qualitative rather than quantitative. Also, it is hard to evaluate the precise quantity of oxygen considering the contribution from the co-deposited alumina particles. However, the results from the spectra acquired at the location of an adherent oxide in the as-deposited sample (i.e. spectrum 2 and 3) are characterized with a Fe to W ratio which is almost identical to the Fe to W ratio of the non11
tested samples, i.e. ∼ 3.8. This indicates that the oxides formed during the wear tests are mixed FeW-oxides. In the case of the annealed Fe-W/11%Al2O3 sample, the data acquired from the spectrum 5 show a slightly higher Fe to W ratio, i.e. ∼ 4.7. Here, the wear debris are richer in Fe due to the formation of α-Fe granules at the surface of the annealed coating, see Fig. 1c. The formation of such oxides is related to the tribo-oxidation mechanism: the oxidation of the coatings during the sliding test is an intrinsic response of the material to recover from the thermal energy generated during dry friction. Previous studies already highlighted that the wear mechanism of electrodeposited Fe-W coatings in dry sliding conditions is mainly characterized by oxidative and adhesive mechanisms [20,36,37]. Table 1. Chemical composition obtained by point analysis when using Energy Dispersive X-ray Spectroscopy (EDS) at locations shown in Figure 7.
Spectrum
Fe (at.%)
W (at.%)
Al (at.%)
O (at.%)
Cr (at.%)
1
47.1
12.6
2.4
37.9
/
2
27.3
7.2
2.5
63
/
3
24.8
6.5
3.4
65.3
/
4
56
14.4
5.6
24
/
5
23.3
5
3.9
67.8
/
6
/
/
/
24.4
75.6
7
2
/
/
61.4
36.6
Figure 7c shows the SEM micrographs from the wear track of the as-deposited Cr coating. Here, the presence of abrasion grooves along the wear track suggests that the wear mechanism is mainly abrasive. Wear debris is found in the wear track which is composed of chromium oxide, as shown from spectrum 6, see Table 1. In the area not affected by the wear test, the amount of co-deposited O is found to be lower than 3 at.%. The wear track of the annealed Cr coating appears substantially wider as compared to the wear track of the as-deposited sample (notice the different scale bar in Fig. 7d). Also, the wear track appears oxidized and cracked (see the insert in Fig. 7d). In the proximity of the cracks, EDS analysis shows the presence of iron (see spectrum 7), which indicates that the specimen interaction volume is reaching the steel substrate.
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Figure 8. Wear rate of Fe-W/11%Al2O3 and hard Cr coatings in the as-deposited state and after annealing at 600 ⁰C (a). Representative wear track profiles of as-deposited and annealed Fe-W/11%Al2O3 (b and c), and of as-deposited and annealed Cr coatings (d and e).
Figure 8 shows the wear rate of the as-deposited and annealed Fe-W/11%Al2O3 composite and hard Cr coatings, together with representative depth profiles of the wear tracks. The wear rate was calculated using Equation (1), considering for each sample an average area of the wear track (A). The results reveal that the wear rate of Fe-W/11%Al2O3 composite in both as-deposited and annealed state is significantly lower than the wear rate of as-deposited hard Cr. Also, the wear rate results clearly illustrate the detrimental effect of the heat treatment on the wear resistance of the hard Cr coating, leading to a substantial increase in the wear rate reaching the value of ∼24 x10-6 mm3/Nm. Many studies have indicated the importance of the mechanical properties for a good wear resistance, showing in particular how a high hardness to Young´s modulus ratio (H/E) generally implies a good wear resistance [38,39]. It is assumed that materials with high H/E ratio should better resist the plastic deformation and thus result in higher wear resistance. The H/Er ratios for the Fe-W/11%Al2O3 and Cr coatings, before and after annealing, are reported in Table 2. The values are calculated from the nanoindentation results. The H/Er of the as-deposited and annealed FeW/11%Al2O3 is higher than the H/Er of the Cr coatings. This is in agreement with the higher wear resistance of the composite coatings as compared to Cr coatings. Also, the dramatic increase observed for the wear rate of hard Cr after annealing can be related to the decrease of the H/Er. As shown in Table 2, a 57% reduction of the H/Er ratio for the Cr coatings is observed after annealing at 600 ℃. For Fe-W/11%Al2O3 coatings, despite the substantial increase in the hardness after annealing, the wear rate of the annealed sample is in principal unchanged and identical to the asdeposited coating. This could be related to the fact that the relative increase in the H/Er after annealing (18%) is not high enough to strongly impact the wear resistance. Also, it has been shown that the tribological behavior of the Fe-W system is rather complex, being strongly affected by 13
tribo-oxidation mechanism. For example, previously studied Fe-W/3%Al2O3 and Fe-W/6%Al2O3 coatings characterized with high mechanical properties (e.g. H/Er) resulted in a moderate wear resistance. In that case, a reduction in wear rate (and tribo-oxidation) was observed in presence of higher amounts (i.e. 12 vol.%) of co-deposited alumina particles [2]. Table 2. Summary of the values of hardness, reduced elastic modulus and hardness to reduced elastic modulus ratio for as-deposited and annealed Fe-W/11%Al2O3 and hard Cr coatings.
Sample
H (GPa)
Er (GPa)
H/Er
As-deposited Fe-W/11%Al2O3
8.8
122
0.072
Annealed Fe-W/11%Al2O3
16.3
191.7
0.085
As-deposited Hard Cr
11.9
224
0.053
Annealed hard Cr
5.8
250
0.023
As shown from the hardness and wear results presented in this work, heat treatments of FeW/11%Al2O3 composite coatings can be applied to further enhance the mechanical properties of the coating while preserving the wear resistance. The annealed Fe-W/11%Al2O3 composite coating is characterized with high hardness as a result of the partial crystallization of the amorphous Fe-W matrix. The high wear resistance of the composite is preserved due to the presence of co-deposited Al2O3 particles, which are beneficial in reducing the tribo-oxidation mechanism.
4
Summary and conclusions
The effect of heat treatment on the mechanical properties and on the wear resistance of FeW/11%Al2O3 has been studied and compared to hard chromium coating. After annealing at 600 ⁰C for 1 hour, the amorphous structure of the Fe-W matrix transforms into a mixed amorphouscrystalline structure containing α-Fe crystallites. The co-deposited Al2O3 particles seem not to be affected by the annealing treatment. The mixed amorphous-crystalline structure obtained after annealing is found to be the main factor responsible of the substantial increase in the hardness of the composite coatings, i.e. the increase from 8.8 GPa in the as-deposited films up to 16.3 GPa which is substantially higher of the hardness of hard Cr coatings. The wear resistance of the Fe-W/11%Al2O3 is preserved upon annealing, i.e. the COF and wear rate for the as-deposited and annealed coatings are similar. In contrast, the heat treatment has shown to be detrimental for both hardness and wear resistance of hard Cr coatings. The heat treatment of hard Cr coatings resulted in an abrupt decrease of the hardness and wear resistance of the coatings. In particular, a 57% reduction of the H/Er ratio
14
upon annealing resulted in a substantial increase of the wear rate from 3.8 x10-6 mm3/Nm to 24.2 x10-6 mm3/Nm. When comparing the performances of Cr and Fe-W/11%Al2O3 coatings, especially the annealed FeW/11%Al2O3 composites show higher hardness and lower wear rate, i.e. 1.5 x10-6 mm3/Nm. As shown in this work, annealing of Fe-W/11%Al2O3 coatings is a useful strategy to obtain coatings with a combination of high hardness and high wear resistance. The investigated composite coatings can indeed be considered as a possible sustainable alternative to hard chromium coatings, whose production is often hazardous and not environmentally friendly. 5
Acknowledgements
This work was supported by the Spanish Government (MAT2017-86357-C3-1-R, MAT201786357-C3-2-R and associated FEDER), the Generalitat de Catalunya (2017-SGR-292), Basque government (ELKARTEK, No. KK-2018/00108) and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement nº 665919, and H2020-MSCA-RISE-2017 SMARTELECTRODES project (No.778357). J.F. acknowledges the “Juan de la Cierva” (IJCI-2015-27030) contract by the Spanish Government.
15
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20
Highlights • • • •
Partial crystallization of the amorphous Fe-W/11%Al2O3 coating upon annealing Substantial increase in the hardness and modulus of annealed Fe-W/11%Al2O3 coating Abrupt decrease in the hardness and wear resistance of annealed hard Cr coating Annealed Fe-W/11%Al2O3 coating shows superior wear resistance than hard Cr coating
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: