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Comparative research on dry sliding wear of hot-dip aluminized and uncoated AISI H13 steel
Wear 344-345 (2015) 22–31
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Comparative research on dry sliding wear of hot-dip aluminized and uncoated AISI H13 steel Q.Y. Zhang a, Y. Zhou a, J.Q. Liu b, K.M. Chen a, J.G. Mo b, X.H. Cui a, S.Q. Wang a,n a b
School of Materials Science and Engineering, Jiangsu University, 212013 Zhenjiang, China Suzhou Universal Group Technology Co., Ltd., 215156 Suzhou, China
art ic l e i nf o
a b s t r a c t
Article history: Received 29 June 2015 Received in revised form 26 October 2015 Accepted 29 October 2015
In order to improve the elevated-temperature wear resistance of steels, an aluminized coating consisting of FeAl and Fe3Al was prepared on AISI H13 steel by hot-dip aluminizing and subsequently hightemperature diffusion. Dry sliding wear tests were conducted for the aluminized and uncoated AISI H13 steel against AISI M2 steel at room temperature and 600 °C, respectively. Compared with the uncoated H13 steel, the aluminized steel possessed excellent elevated-temperature wear resistance, in spite of worse room-temperature wear performance. As the load increased, the wear rate of the aluminized steel rapidly increased at room temperature, but maintained extraordinarily low values at 600 °C. On the contrary, with an increase of load, the wear rate of H13 steel marginally increased at room temperature, but rapidly increased at 600 °C. The aluminized steel seemed to be suitable to the applications involving elevated-temperature sliding. Interestingly, the excellent elevated-temperature wear performance of the aluminized steel was noticed to be attributed to the protection of the thin tribo-oxide layer with a strong support from high thermal-strength intermetallic compound. & 2015 Elsevier B.V. All rights reserved.
Keywords: Sliding wear Intermetallic Steel Tribochemistry Electron microscopy
1. Introduction AISI H13 steel is a widely used hot-working die steel because of its high strength and ductility, good tempering resistance and moderate cost [1]. However, thermal wear, caused by the alternate stress, ambient and frictional heating on the working surface, is considered to be a leading failure mode. It drastically reduces the service life of dies, resulting in an enormous increase of industrial costs. Therefore, it is urgent to enhance the elevated-temperature wear performance of die steels. The coating technology is considered to be a good solution to effectively improve elevated-temperature wear resistance of H13 steel, that is, high temperature wear-resistance material is produced on the surface of die steels. The iron aluminides are well known to be one of the most advantageous materials owing to their low cost, excellent resistance of oxidation and corrosion, high strength-to-weight ratio, great elastic modulus and superior elevated-temperature strength [2–5]. Moreover, the iron aluminides are considered to be the promising tribological materials and appropriately substitutes for special steels or Ni-based superalloys in aggressive environments [6,7]. n
Corresponding author. Tel.: þ 86 511 88797618; fax: þ86 511 88791919. E-mail addresses: [email protected] (X.H. Cui), [email protected] (S.Q. Wang). http://dx.doi.org/10.1016/j.wear.2015.10.017 0043-1648/& 2015 Elsevier B.V. All rights reserved.
According to Fe–Al binary phase diagram, there exist five types of intermetallic compounds (Fe3Al, FeAl, FeAl2, Fe2Al5, and FeAl3 phases) [8]. But not all phases can be applied as the coating to improve elevated-temperature wear resistance of steels. Compared with Fe3Al and FeAl, cracks are preferential to nucleate and grow in the Al-rich phases (FeAl2, Fe2Al5, and FeAl3) at room temperature. Hence, they were always reported to be problematic in the reliable application due to their brittleness [9,10]. Moistureinduced environmental embrittlement, weak grain boundaries and vacancy hardening and embrittlement are noticed to be the three major causes of low tensile ductility and brittle cleavage fracture of Fe–Al alloy at room temperature [11,12]. In order to improve their poor ductility, massive researches were performed for the alloying and compositing of Fe–Al intermetallics through the addition of alloy elements and second-phase particles, such as Cr, C, B, Mo, Cu, TiC, WC, Al2O3, MoS2 [7,13–21]. Fe3Al and FeAl, a high iron-content phases, were once thought to be of inherent brittleness [2,8,10,11]. However, the occurrence of anomalous yield stress of Fe3Al and FeAl at elevated-temperature [22,23] indicated that FeAl and Fe3Al possesses moderate ductility and strength to use as a protective coating to improve the elevated-temperature wear resistance of steels. But in the area of friction and wear, the elevatedtemperature performances of Fe–Al binary phases were usually ignored. Especially, the researches concerning dry sliding wear characteristics of Fe–Al as coating are sparsely reported.
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In the present research, the aluminized coating, containing FeAl and Fe3Al, was prepared on AISI H13 steel via hot-dip aluminizing and subsequently high temperature diffusion treatment. Dry sliding wear tests against AISI M2 steel were conducted for an aluminized and an uncoated AISI H13 steel at room temperature and 600 °C. The purpose of this study is to investigate the wear characteristics of Fe–Al coating at room and elevated temperature. Compared with AISI H13 steel, the wear mechanism and application feasibility of Fe–Al coating were explored.
2. Experimental procedure A commercial AISI H13 die steel (Fe–0.41% C–5.23% Cr–1.15% Mo–0.92% V) as the substrate was machined into a pie-shaped specimen with a dimension of Ø70 12 mm2. The surface of the substrate was polished to 0.45 μm in Ra. After being descaled in 8% hydrochloric acid solution, rinsed with water, degreased in acetone with an ultrasonic cleaner and finally dried, the pretreated H13 steel was immerged into a melting bath of high pure aluminum (Al Z 99.99 wt%) for 5 min at 750 °C, which was heated with a well-type resistance furnace. The encased steel was then taken out at a modest speed and cooled in air. Subsequently, a diffusion annealing treatment for the encased steel was carried out and cooled in a ZT-12-11 type vacuum induction furnace. According to the previous studies [10,24], the brittle phases of FeAl3, FeAl2 and Fe2Al5 initially generated in the interface of steel substrate and liquid aluminum at 600–800 °C. But FeAl and Fe3Al with high fracture resistance were preferentially formed at 1000 °C and above. Therefore, the parameters of diffusion treatment were determined to be at 1000 °C for 5 h. The aluminized coating was
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obtained after the removal of residual pure aluminum and trace alumina by a saturated solution of sodium hydroxide. More detailed preparation process was presented elsewhere [10,24]. For the uncoated H13 steel, heat treatment was carried out by austenizing at 1040 °C for 20 min, quenching in oil and tempering for 2 h at 200 °C to achieve a hardness of HRC 50-55. The microstructural characteristics of the aluminized and uncoated H13 steels are shown in Figs. 1 and 2a. Dry sliding wear tests were performed in air on an MG-2000 type pin-on-disc elevated-temperature wear tester. The aluminized and uncoated H13 steel was machined into pins with the dimensions of 6 mm in diameter and 12 mm in height, respectively. Because of high hardness at elevated temperature, a commercial AISI M2 high speed steel (Fe–0.8% C–3.9% Cr–4.4% Mo– 5.6% W–1.75% V–0.2% Si) was selected as the mating material. M2 steel was austenitized at 1180 °C, oil quenched and then tempered three times at 540 °C for 2 h to obtain tempered martensite plus carbides (Fig. 2b) with an average hardness of HRC 62-64. The sliding parameters were determined as follows: 25, 200, 400 and 600 °C for the ambient temperature; 50–300 N with an interval of 50 N for the normal load; 1 m/s for the sliding velocity; 1200 m for sliding distance. The wear loss of pins was determined by mass measurement before and after tests by an electric balance with an accuracy of 0.01 mg. The wear rate (W) was calculated by the equation: W¼ ΔM/ρL, where ΔM is the weight loss, ρ is the density of materials, L is the sliding distance. For the aluminized steel, the density of FeAl (5.37 g/cm3) [9,25] was adopted to calculate the wear rate. The mean value of three repeated tests was used as the final result. The phases, compositions, and morphologies of the aluminized coating and worn specimens were examined using X-ray diffractometer (XRD), energy dispersive
Fig. 1. Microstructural characteristics of aluminized coating on H13 steel: (a) XRD pattern for surface, (b) cross-section morphology, (c) EDS analysis of spectrum 1 and (d) spectrum 2.
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Fig. 2. Microstructure of the uncoated H13 steel (a) and treated M2 steel (b).
spectroscopy (EDS), and scanning electron microscopy (SEM). The hardness of the steels after heat treatment was determined by an HR-150A type Rockwell apparatus. The microhardness of the coating and worn subsurfaces, was also measured using an HVS1000 type digital microhardness tester with a load of 0.49 N and a hold time of 15 s.
3. Results and analysis 3.1. Microstructure of aluminized and uncoated H13 steel Fig. 1 presents the microstructural characteristics of the aluminized H13 steel. It was identified by XRD patterns that the coating consisted of FeAl and Fe3Al (Fig. 1a). The two phases seemed to be roughly confirmed owing to their semblable and overlapped diffraction peaks. The cross-section morphology of the aluminized coating is given in Fig. 1(b). The intermetallic coating surpassed 300 μm of thickness, and was made up of two distinct layers divided by a small quantity of pores or cavities at the interface. EDS regional analysis semiquantitatively verified that the atomic ratios of these layers were approximate to the stoichiometric of FeAl and Fe3Al, respectively. The outer and inner layers, therefore, were identified to be FeAl and Fe3Al, respectively. Both of FeAl and Fe3Al were noticed to be relatively compacted and continuous. EDS line analysis demonstrated that the content of Al element gradually decreased from the outside to the inside layer, but the opposite way for Fe element. It can be deduced that iron atoms moved outwards and aluminum ones moved inwards during the high-temperature diffusion. The previous researches [10,24] reported that FeAl and Fe3Al phases were converted from the initial tongue-shaped Fe2Al5 phase through high temperature diffusion. When the initial Fe2Al5 formed and grew into the substrate along the C-axis direction, the pores inevitably formed because of the diffusion rate discrepancy between Fe and Al atoms, namely the Kirkendall effect [26]. The interface between the Fe–Al phase and substrate became smoother after Fe2Al5 was turned into FeAl and Fe3Al. Because dispersive carbide particles in the substrate did not react with aluminum at all, most of them gathered together around the interface between Fe–Al coating and substrate (Fig. 1b) and some entered the coating during diffusion. Thus, FeAl and Fe3Al would be strengthened because of the existence of these carbide particles. The microhardness values of FeAl and Fe3Al reached 580 HV and 420 HV, respectively, which were slightly higher than those in the work of Kobayashi [10]. The uncoated H13 steel was austenitized at 1040 °C for 20 min, oil quenched, tempered at 200 °C for 2 h, and cooled in air. Tempered martensite plus a small quantity of retained austenite was achieved for H13 steel, as is illustrated in Fig. 2a. In this case, the
Fig. 3. Wear rate of aluminized and uncoated H13 steel as a function of load at 25 and 600 °C.
hardness of the uncoated H13 steel reached HRC 50-55. Clearly, the hardness values of the aluminized and uncoated H13 steel approached each other. 3.2. Wear rates of aluminized and uncoated H13 steel The wear rate of the aluminized and uncoated H13 steel is shown in Fig. 3. At 25 °C, the wear rate of the aluminized H13 steel increased linearly and rapidly as a function of load, whereas it slightly increased for the uncoated H13 steel. As the temperature was elevated to 600 °C, the wear rate of the aluminized H13 steel was substantially reduced, and maintained extremely lower values, even invariably under severer loads. On the contrary, the wear rate of the uncoated H13 steel increased catastrophically as a function of the load. It is clear that the aluminized and uncoated H13 steel revealed totally different wear behavior. The aluminized H13 steel presented excellent wear resistance at 600 °C, but worse one at room temperature. Conversely, the uncoated H13 steel possessed better wear resistance at room temperature, but extremely poor elevated-temperature wear resistance at 600 °C. It is commonplace that medium or low alloy steels have good wear performance at room temperature, but worse elevated-temperature one. The aluminized coating seemed to provide a good way to improve elevated-temperature wear performance of steels. 3.3. XRD analysis and morphology of worn surfaces The XRD patterns of worn surfaces for the aluminized and uncoated H13 steel sliding under 200 N at 25 and 600 °C are
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delaminated region seemed to be slightly increased (Fig. 5a4). Interestingly, fish-scale traces unexpectedly appeared in delaminated pits, which was different from the typical morphology of oxidative wear. This implied that the tough delamination existed, which differed from the brittle delamination of tribo-oxides in steels [27]. The fish-scale traces were mainly made up of Fe, Al and O (Fig. 6c), among whom the atomic ratio between Fe and Al was close to 1:1. It can be deduced that the fish-scale traces appeared at the outer aluminized layer, accompanied with the formation of some oxides. For the uncoated steel, the black tribo-oxide layer partially covered the worn surface with more delaminated regions under 150 N (Fig. 5b3). As the load reached 250 N, the black tribo-oxide layer almost disappeared and the deeper and wider grooves emerged on the smoother worn surface (Fig. 5b4). 3.4. Cross-section analysis of worn surfaces and subsurface Fig. 4. XRD patterns of worn surfaces for aluminized and uncoated H13 steel under 200 N.
illustrated in Fig. 4. At 25 °C, only Fe peaks appeared on worn surfaces of both the aluminized and uncoated H13 steel, without any Fe–Al phases. This was right for the steel, but seemed to be unreasonable for the aluminized steel. While at 600 °C, besides weakened peaks of Fe, there existed many peaks of Fe2O3 and Fe3O4 for the uncoated H13 steel. But FeAl and FeAl3 were noticed to be predominated phases for the aluminized H13 steel; additionally, some Fe2O3 and Al2O3 appeared on worn surfaces. It can be deduced that tribo-oxidation occurred for both the aluminized and uncoated H13 steel at elevated temperature. Morphologies of worn surfaces for the aluminized and uncoated H13 steel are shown in Fig. 5. At 25 °C and 50 N, the aluminized H13 steel presented a different morphology from that of the uncoated H13 steel (Fig. 5a1 and b1). The former worn surface was characterized with brittle fractured traces and fragmented particles besides trace strip-like protrusion regions (Fig. 5a1). These protrusion regions seemed to be tribo-layers, but readily destroyed under the action of load. On the contrary, the worn surface of the uncoated H13 steel revealed plastic tearing, fish-scale adhesion and grooves were revealed on worn surfaces (Fig. 5b2). Clearly, in this case, the wear of the aluminized and uncoated H13 steel was of brittle and plastic characteristic, respectively. As the load increased to 250 N, the worn surfaces of the aluminized and uncoated H13 steel became almost the same and presented adhesive traces (plastic deformation, tearing, material transfer) and grooves, as shown in Fig. 5a2 and b2. Clearly, the aluminized steel represented brittle fracture of intermetallic compound at lower load, compared with plastically shearing and tearing of the uncoated H13 steel. While at higher load, the plastic wear characteristic resulted from the steel regardless of the aluminized and uncoated H13 steel. In this case, the aluminized coating seemed to be worn off completely; EDS results indicated that there was no aluminum (Fig. 6a). For the uncoated steel, on matter what the load was, adhesive trace, plastic tearing and grooves were revealed on worn surfaces (Fig. 5b1 and b2). The worn extent intensified as the load increased (Fig. 5b2). As the temperature was elevated to 600 °C, a special morphology appeared in the aluminized and uncoated H13 steel with black, smooth regions and gray, rough delaminated regions, which was similar to the typical morphology of oxidative wear, reported by our previous research on steels [27]. The black, smooth regions should be tribo-oxide layers. For the aluminized H13 steel, the black and smooth tribo-layers were noticed to cover completely the worn surfaces with some delaminated regions (Fig. 5a3 and a4). EDS analysis demonstrated that the tribo-layer consisted of multiple elements, such as O, Fe, Al, Cr, Mo and W (Fig. 6b). It is clear that the tribo-layer was a tribo-oxide layer. Under 250 N, the amount, area and deepness of the
Fig. 7 shows the cross-section morphology of worn subsurfaces of the aluminized and uncoated H13 steel. Regardless of material, temperature and load, more or less tribo-layers existed on the worn surface. At 25 °C, the tribo-layers of the aluminized and uncoated H13 steel seemed to be essentially metallic, which was proved through the analysis of EDS (Fig. 8a) and XRD results (Fig. 4). For the aluminized H13 steel, most of the coating was almost worn off and merely trace coating was noticed to remain in the deformed matrix at subsurface under of 150 N (Fig. 6a1). With the increase of load, there existed no coating but a thin tribo-layer containing no oxides formed on the deformed matrix (about 10 μm). Thus the aluminized and uncoated H13 steel presented the same wear characteristics. However, at 600 °C, the aluminized and uncoated H13 steel represented remarkably different cross-section morphologies of worn subsurfaces. For the aluminized steel, 1–2 μm-thick tribo-layer was noticed to form on the intermetallic coating (Fig. 7a3 and a4) and contained some oxygen and other elements (Cr, W, Mo) identified by EDS (Fig. 8b). It is clear that the tribo-layer was a mechanically mixed tribo-oxide layer rather than an integrated oxide layer. Under higher loads, the tribo-oxide layer became more compacted and continuous. The plastic deformation of the intermetallic coating was confirmed from the elongated second-phase particles, which was characterized as FeAl2 phase in our previous study [24]. However, the uncoated H13 steel presented totally different morphologies of tribo-oxide layer from the aluminized H13 steel. A multi-layer tribo-layer was observed, as shown in Fig. 7(b3 and b4). Many cracks were noticed to emerge in the underlying interface of tribo-layer and substrate, thus tribo-oxide layers readily fractured and delaminated. Under a severer load (250 N), the multi-layer tribo-layer was thickened to be a lamellar structure. The multi-layer tribo-oxide layer was reported to result in severe wear [27]. Fig. 9 shows the microhardness distribution curves on the crosssection of worn subsurfaces for the aluminized and uncoated H13 steel sliding at 25 and 600 °C under 200 N. At 25 °C, the hardness of the aluminized and uncoated steel was approached each other, and kept the original value of the steel with tiny fluctuation, about 500– 575 HV. At 600 °C, the hardness of the uncoated steel dropped sharply to 300 HV or so. However, the aluminized H13 steel retained high hardness, especially at the outmost part. Three-part variations were revealed in the hardness distribution of the aluminized H13 steel. In the outer part near the worn surface, the hardness slightly increased to 550–600 HV, compared with that at 25 °C. Unexpectedly, the hardness abruptly decreased to 425–475 HV in the middle part. The hardness restored to the original hardness value of 500– 575 HV in the inner part. The hardness variations of three parts were considered to just correspond to the microstructure: FeAl, Fe3Al and the substrate, respectively. This meant that after wear, the Fe–Al coating and the substrate almost remained intact.
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Fig. 5. Morphologies of worn surfaces of the aluminized H13 steel: 50 N (a1), 250 N (a2) at 25 °C, 150 N (a3), 250 N and (a4) at 600 °C; the uncoated H13 steel: 150 N(b1), 250 N (b2) at 25 °C, 150 N (b3), and 250 N(b4) at 600 °C.
4. Discussion 4.1. Evaluation of the wear performance of aluminized and uncoated H13 steel Steels, as widely used structural materials in engineering applications, normally possess good mechanical properties at room temperature. However, as the temperature increased, they will be thermally softened to lose its strength. The low strength of steels inevitably result in severe wear at elevated-temperature. As
reported by many researches [27–30], high ambient temperature raised the wear rate and degraded wear resistance of steels. The aluminized coating on steel was expected to change the situation. Fig. 10 compares the wear performance of the aluminized and uncoated H13 steel as a function of temperature, in which the histograms of the wear rate under 150 and 250 N at 25–600 °C were plotted. Because the wear resistance was inversely proportional to the wear rate, the reciprocal value of wear rate can be simplified as the corresponding wear resistance. The demarcation of mild and severe wear was considered to be 5 10 6 mm3/mm,
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Fig. 6. EDS Regional analysis in Fig. 5: (a) EDS1, (b) EDS2 and (c) EDS3.
which was put forwards by Zhang and Alpas [31]. It can be noticed that at 600 °C for the uncoated steel and at 25 °C for the aluminized steel, the severe wear prevailed and the wear rate markedly increased by one or two orders of magnitude with an increase of load. The poor wear resistance of the uncoated and the aluminized steel in corresponding conditions seemed to be essential shortcoming. Except for the above conditions, the uncoated and aluminized steel seemed to fall in mild wear and was suitable to use in sliding wear. As shown in Fig. 10, the wear resistance of the uncoated H13 steel started to substantially decrease at 200 °C, further deteriorated as the temperature was elevated and the extremely poor wear resistance was presented at 600 °C. On the contrary, for the aluminized steel, in spite of poor wear resistance at room temperature, the wear rate retained extraordinarily low and stable values at elevated temperature, especially at 400 and 600 °C. As long as the ambient temperature reached 200 °C, its excellent wear resistance was presented. It is clear that such an excellent high-temperature wear performance was not possessed by the uncoated H13 steel, like the majority of steels [27–30]. Consequently, at elevated temperature (200–600 °C), the aluminized coatings can completely offset notoriously poor wear resistance of steels and widely apply in the service conditions involving thermal friction and wear. 4.2. Effect of tribo-layers and substrate materials on wear behavior and mechanism The formation of tribo-layers is an inevitable phenomenon during the sliding of metals. Kerridge [32] carried out an elegant experiment with radioactive steels and confirmed that the elements in tribo-layers resulted from the sliding couple and environment. Rigney [33] considered the formation process of tribolayers as the mechanically alloying of wear debris with environmental elements (especially oxygen) under pressure. Table 1 and Fig. 11 illustrate that there were also material loss of the harder M2 steel. It can be thought that the constituent of tribo-layers partly came from counterfaces. As illustrated in Figs. 7 and 8, the tribolayer was indeed a mixture of metal particles and more or less oxide ones. When the tribo-layers existed, the wear behavior and mechanism undoubtedly mainly associated with tribo-layers and subsurface matrix. However, whether the tribo-layers affected the wear behavior and mechanism or not is supposed to be decided by their ingredient [34]. As the tribo-layers were no protective, the wear behavior and mechanism of metal alloy was merely decided by the properties of metal alloy itself at subsurface. As the tribolayers was protective, the original wear behavior and mechanism of metal alloy would be changed, which not only relied on the characteristics of tribo-layers, but also on the properties of metal alloy itself at subsurface.
At room temperature, low ambient temperature and limited frictional heating would hardly oxidize metal surface and debris. In this case, the compositions of tribo-layers mainly resulted from the sliding pin and counterface; the formed tribo-layers were nooxide tribo-layers. It is clear that no-oxide tribo-layers predominantly presented metallic characteristics. In addition, tribolayers formed at room temperature were usually thin, discontinuous and unsound. During sliding, the tribo-layers readily delaminated under the action of load. Even if this metallic tribolayer was retained on worn surfaces, they seemed not to affect the original wear behavior and mechanism of metal alloys because of their lower hardness similar to the metal alloy itself. Hence, the metal–metal contact and adhesion was retained. In this case, the wear performance mainly depended on the metal alloy itself, regardless of the steel or intermetallic. In this case, high friction coefficients were obtained during sliding in Table 1, regardless of the existence of the aluminized coating or not. For the uncoated H13 steel, the worn surfaces were plowed by harder counterface. Adhesive traces and grooves appeared on worn surfaces. Clearly, adhesive and abrasive wear was the main mechanism for the uncoated H13 steel at 25 °C. Because of high strength and toughness, H13 steel resisted extensive plastic deformation and fracture under wear, thus resulting in a relatively low wear rate. On the contrary, for the aluminized coating, environmental embrittlement seriously weakened the roomtemperature ductility and caused brittle cleavage fracture [11,12]. Under the normal load and transverse shearing force, the worn surfaces of the aluminized coating presented brittle fracture and fragmentized particles. It is clear that fatigue wear prevailed in sliding at 25 °C. Compared with intermetallic, high wear resistance of H13 steel was attributed to its high plasticity and toughness at room temperature. At 600 °C, higher ambient temperature and frictional heat decreased the free energy of oxidation, enormously accelerated the reactions between oxygen and metal. A large amount of oxidative particles and debris would be produced, accumulated and sintered to form the tribo-oxide layer on worn surfaces. Usually, it was thick, continuous and sound, and supposed to have the loadcarrying capacity [35]. Because of many oxides, tribo-oxide layer possessed high hardness and ceramic characteristics. When the tribo-oxide layer existed, the metal–metal contact was avoided. In this case, tribo-oxide layers undoubtedly change the original wear behavior and mechanism. As the substrate metal alloy supported them, tribo-oxide layers would reduce wear. On the contrary, as the substrate metal alloy did not support them, tribo-oxide layers would accelerate wear because of their severe delamination. Kato confirmed the protective role of oxides by introducing fine Fe2O3 particles onto rubbing interfaces of carbon steels [36]. It seemed that more oxides would result in better wear resistance (Fig. 10) and lower friction coefficient (Table 1). However, in the
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Fig. 7. Morphologies of cross-section of worn subsurfaces for the aluminized steel: 150 N (a1), 250 N (a2) at 25 °C, 150 N (a3), 250 N and (a4) at 600 °C; for the uncoated H13 steel: 150 N (b1), 250 N (b2) at 25 °C, 150 N (b3), 250 N and (b4) at 600 °C.
present study, more tribo-oxides did not lower the wear rate of H13 steel. According to our previous work [27,29,35], the prerequisite for the protective role of tribo-oxides was that the underlying substrate should possess the mechanical strength enough to support tribo-oxide layers. The mechanical strength of materials could be roughly evaluated by their hardness. After sliding at 600 °C, the hardness of the uncoated H13 steel had markedly dropped from 500–575 HV to 300 HV or so (Fig. 9). This meant that a severe thermal softening occurred. In this case, massive plastic deformation occurred in thermally softened
substrate at subsurfaces, thus the bond between tribo-oxides and substrate could be readily broken under transverse shearing force and normal load. Finally, tribo-oxide layer would be delaminated. In this case, tribo-oxide layer lost protective or load-carried function. The cracks would nucleate and propagate in substrate during sliding. At elevated-temperature, oxygen readily permeated into the subsurface along cracks. The multi-layer oxides layer would be formed because of large plastic deformation and many oxides. The existence of the multi-layer oxide layers caused massive delamination of oxides and matrix, thus the wear rate rapidly
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Fig. 8. EDS regional analysis in Fig. 7: (a) EDS1, (b) EDS2, (c) EDS3, (d) EDS4, and (e) EDS5.
Fig. 9. Microhardness distribution of cross-section of worn surfaces for the aluminized and uncoated H13 steel under 200 N.
rose. The delamination of tribo-oxide layers also appeared in counterface of M2 steel (Fig. 11b2). The predominated mechanism was a mild-to-severe wear transition of oxidative wear. For the aluminized steel, in spite of a thin layer (1–2 μm), the tribo-oxide layer was compact and continuous. It should be protective and took effect in reducing wear. The predominated oxide in tribolayer was Fe2O3, which was reported to act as a solid lubricant in reducing friction and wear [36], as shown in Table 1. More importantly, the aluminized H13 steel (the aluminized coating) preserved its originally high hardness after sliding at 600 °C. As reported, FeAl and Fe3Al intermetallic compound possessed an anomalous yield stress and reached the highest value in a high-temperature range of 400–600 °C as a result of hardening by thermal vacancies [22,23]. The toughness would be also improved up to 800 °C [2], which was evidenced by fish-scale adhesion in Fig. 5(a4). Hence, the aluminized coating presented a high thermal strength and toughness during
Fig. 10. Wear rate of the aluminized and uncoated H13 steel at 25, 200, 400 and 600 °C.
Table 1 Average friction coefficient and wear rate of M2 steel at 150 N. Temperature (°C) Average friction coefficient
25 600
Wear rate of M2 steel ( 10 6 mm3/mm)
Aluminized steel
Uncoated steel Aluminized steel
Uncoated steel
0.82 7 0.08 0.54 7 0.05
0.73 70.06 0.6770.05
1.59 7 0.31 16.667 1.82
0.83 7 0.18 4.30 7 0.46
sliding at 600 °C. Moreover, the 300 μm-thick coating separated the steel matrix and counterface, which inhibited the excessive increase of bulk temperature by frictional heat. As shown in Fig. 9, the thermal softening of H13 steel matrix did not occur. Therefore, the excellent
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Fig. 11. Morphology of worn surfaces of M2 steel as mating material at 150 N: sliding against the aluminized steel at 25 °C (a1) and 600 °C (a2); the uncoated steel at 25 °C (b1) and 600 °C (b2).
elevated-temperature wear performance of the aluminized steel can be suggested to be attributed to the protection of the thin tribo-oxide layer with a strong support from high thermal-strength intermetallic compound. According to the wear characteristics, oxidative mild wear was the main mechanism at 600 °C.
5. Conclusions
(1) The aluminized coating, consisting of FeAl and Fe3Al, on H13 steel was prepared by hot-dip aluminizing and subsequently hightemperature diffusion. Compared with the uncoated H13 steel, the aluminized steel possessed excellent elevated-temperature wear resistance, in spite of worse room-temperature wear performance. The aluminized steel seemed to be suitable to the applications involving elevated-temperature sliding. (2) The excellent elevated-temperature wear performance of the aluminized steel was suggested to be attributed to the protection of the thin tribo-oxide layer with a strong support from the intermetallic compound with high thermal strength and toughness.
Acknowledgments Financial support of our work by National Natural Science Foundation of China (No. 51071078), the Research and Innovation Project for College Graduates of Jiangsu Province (No. KYLX-1031) and Jiangsu Province Key Laboratory of High-end Structural Materials (No. hsm1403) are gratefully acknowledged.
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Comparative research on dry sliding wear of hot-dip aluminized and uncoated AISI H13 steel Q.Y. Zhang a, Y. Zhou a, J.Q. Liu b, K.M. Chen a, J.G. Mo b, X.H. Cui a, S.Q. Wang a,n a b
School of Materials Science and Engineering, Jiangsu University, 212013 Zhenjiang, China Suzhou Universal Group Technology Co., Ltd., 215156 Suzhou, China
art ic l e i nf o
a b s t r a c t
Article history: Received 29 June 2015 Received in revised form 26 October 2015 Accepted 29 October 2015
In order to improve the elevated-temperature wear resistance of steels, an aluminized coating consisting of FeAl and Fe3Al was prepared on AISI H13 steel by hot-dip aluminizing and subsequently hightemperature diffusion. Dry sliding wear tests were conducted for the aluminized and uncoated AISI H13 steel against AISI M2 steel at room temperature and 600 °C, respectively. Compared with the uncoated H13 steel, the aluminized steel possessed excellent elevated-temperature wear resistance, in spite of worse room-temperature wear performance. As the load increased, the wear rate of the aluminized steel rapidly increased at room temperature, but maintained extraordinarily low values at 600 °C. On the contrary, with an increase of load, the wear rate of H13 steel marginally increased at room temperature, but rapidly increased at 600 °C. The aluminized steel seemed to be suitable to the applications involving elevated-temperature sliding. Interestingly, the excellent elevated-temperature wear performance of the aluminized steel was noticed to be attributed to the protection of the thin tribo-oxide layer with a strong support from high thermal-strength intermetallic compound. & 2015 Elsevier B.V. All rights reserved.
Keywords: Sliding wear Intermetallic Steel Tribochemistry Electron microscopy
1. Introduction AISI H13 steel is a widely used hot-working die steel because of its high strength and ductility, good tempering resistance and moderate cost [1]. However, thermal wear, caused by the alternate stress, ambient and frictional heating on the working surface, is considered to be a leading failure mode. It drastically reduces the service life of dies, resulting in an enormous increase of industrial costs. Therefore, it is urgent to enhance the elevated-temperature wear performance of die steels. The coating technology is considered to be a good solution to effectively improve elevated-temperature wear resistance of H13 steel, that is, high temperature wear-resistance material is produced on the surface of die steels. The iron aluminides are well known to be one of the most advantageous materials owing to their low cost, excellent resistance of oxidation and corrosion, high strength-to-weight ratio, great elastic modulus and superior elevated-temperature strength [2–5]. Moreover, the iron aluminides are considered to be the promising tribological materials and appropriately substitutes for special steels or Ni-based superalloys in aggressive environments [6,7]. n
Corresponding author. Tel.: þ 86 511 88797618; fax: þ86 511 88791919. E-mail addresses: [email protected] (X.H. Cui), [email protected] (S.Q. Wang). http://dx.doi.org/10.1016/j.wear.2015.10.017 0043-1648/& 2015 Elsevier B.V. All rights reserved.
According to Fe–Al binary phase diagram, there exist five types of intermetallic compounds (Fe3Al, FeAl, FeAl2, Fe2Al5, and FeAl3 phases) [8]. But not all phases can be applied as the coating to improve elevated-temperature wear resistance of steels. Compared with Fe3Al and FeAl, cracks are preferential to nucleate and grow in the Al-rich phases (FeAl2, Fe2Al5, and FeAl3) at room temperature. Hence, they were always reported to be problematic in the reliable application due to their brittleness [9,10]. Moistureinduced environmental embrittlement, weak grain boundaries and vacancy hardening and embrittlement are noticed to be the three major causes of low tensile ductility and brittle cleavage fracture of Fe–Al alloy at room temperature [11,12]. In order to improve their poor ductility, massive researches were performed for the alloying and compositing of Fe–Al intermetallics through the addition of alloy elements and second-phase particles, such as Cr, C, B, Mo, Cu, TiC, WC, Al2O3, MoS2 [7,13–21]. Fe3Al and FeAl, a high iron-content phases, were once thought to be of inherent brittleness [2,8,10,11]. However, the occurrence of anomalous yield stress of Fe3Al and FeAl at elevated-temperature [22,23] indicated that FeAl and Fe3Al possesses moderate ductility and strength to use as a protective coating to improve the elevated-temperature wear resistance of steels. But in the area of friction and wear, the elevatedtemperature performances of Fe–Al binary phases were usually ignored. Especially, the researches concerning dry sliding wear characteristics of Fe–Al as coating are sparsely reported.
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In the present research, the aluminized coating, containing FeAl and Fe3Al, was prepared on AISI H13 steel via hot-dip aluminizing and subsequently high temperature diffusion treatment. Dry sliding wear tests against AISI M2 steel were conducted for an aluminized and an uncoated AISI H13 steel at room temperature and 600 °C. The purpose of this study is to investigate the wear characteristics of Fe–Al coating at room and elevated temperature. Compared with AISI H13 steel, the wear mechanism and application feasibility of Fe–Al coating were explored.
2. Experimental procedure A commercial AISI H13 die steel (Fe–0.41% C–5.23% Cr–1.15% Mo–0.92% V) as the substrate was machined into a pie-shaped specimen with a dimension of Ø70 12 mm2. The surface of the substrate was polished to 0.45 μm in Ra. After being descaled in 8% hydrochloric acid solution, rinsed with water, degreased in acetone with an ultrasonic cleaner and finally dried, the pretreated H13 steel was immerged into a melting bath of high pure aluminum (Al Z 99.99 wt%) for 5 min at 750 °C, which was heated with a well-type resistance furnace. The encased steel was then taken out at a modest speed and cooled in air. Subsequently, a diffusion annealing treatment for the encased steel was carried out and cooled in a ZT-12-11 type vacuum induction furnace. According to the previous studies [10,24], the brittle phases of FeAl3, FeAl2 and Fe2Al5 initially generated in the interface of steel substrate and liquid aluminum at 600–800 °C. But FeAl and Fe3Al with high fracture resistance were preferentially formed at 1000 °C and above. Therefore, the parameters of diffusion treatment were determined to be at 1000 °C for 5 h. The aluminized coating was
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obtained after the removal of residual pure aluminum and trace alumina by a saturated solution of sodium hydroxide. More detailed preparation process was presented elsewhere [10,24]. For the uncoated H13 steel, heat treatment was carried out by austenizing at 1040 °C for 20 min, quenching in oil and tempering for 2 h at 200 °C to achieve a hardness of HRC 50-55. The microstructural characteristics of the aluminized and uncoated H13 steels are shown in Figs. 1 and 2a. Dry sliding wear tests were performed in air on an MG-2000 type pin-on-disc elevated-temperature wear tester. The aluminized and uncoated H13 steel was machined into pins with the dimensions of 6 mm in diameter and 12 mm in height, respectively. Because of high hardness at elevated temperature, a commercial AISI M2 high speed steel (Fe–0.8% C–3.9% Cr–4.4% Mo– 5.6% W–1.75% V–0.2% Si) was selected as the mating material. M2 steel was austenitized at 1180 °C, oil quenched and then tempered three times at 540 °C for 2 h to obtain tempered martensite plus carbides (Fig. 2b) with an average hardness of HRC 62-64. The sliding parameters were determined as follows: 25, 200, 400 and 600 °C for the ambient temperature; 50–300 N with an interval of 50 N for the normal load; 1 m/s for the sliding velocity; 1200 m for sliding distance. The wear loss of pins was determined by mass measurement before and after tests by an electric balance with an accuracy of 0.01 mg. The wear rate (W) was calculated by the equation: W¼ ΔM/ρL, where ΔM is the weight loss, ρ is the density of materials, L is the sliding distance. For the aluminized steel, the density of FeAl (5.37 g/cm3) [9,25] was adopted to calculate the wear rate. The mean value of three repeated tests was used as the final result. The phases, compositions, and morphologies of the aluminized coating and worn specimens were examined using X-ray diffractometer (XRD), energy dispersive
Fig. 1. Microstructural characteristics of aluminized coating on H13 steel: (a) XRD pattern for surface, (b) cross-section morphology, (c) EDS analysis of spectrum 1 and (d) spectrum 2.
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Fig. 2. Microstructure of the uncoated H13 steel (a) and treated M2 steel (b).
spectroscopy (EDS), and scanning electron microscopy (SEM). The hardness of the steels after heat treatment was determined by an HR-150A type Rockwell apparatus. The microhardness of the coating and worn subsurfaces, was also measured using an HVS1000 type digital microhardness tester with a load of 0.49 N and a hold time of 15 s.
3. Results and analysis 3.1. Microstructure of aluminized and uncoated H13 steel Fig. 1 presents the microstructural characteristics of the aluminized H13 steel. It was identified by XRD patterns that the coating consisted of FeAl and Fe3Al (Fig. 1a). The two phases seemed to be roughly confirmed owing to their semblable and overlapped diffraction peaks. The cross-section morphology of the aluminized coating is given in Fig. 1(b). The intermetallic coating surpassed 300 μm of thickness, and was made up of two distinct layers divided by a small quantity of pores or cavities at the interface. EDS regional analysis semiquantitatively verified that the atomic ratios of these layers were approximate to the stoichiometric of FeAl and Fe3Al, respectively. The outer and inner layers, therefore, were identified to be FeAl and Fe3Al, respectively. Both of FeAl and Fe3Al were noticed to be relatively compacted and continuous. EDS line analysis demonstrated that the content of Al element gradually decreased from the outside to the inside layer, but the opposite way for Fe element. It can be deduced that iron atoms moved outwards and aluminum ones moved inwards during the high-temperature diffusion. The previous researches [10,24] reported that FeAl and Fe3Al phases were converted from the initial tongue-shaped Fe2Al5 phase through high temperature diffusion. When the initial Fe2Al5 formed and grew into the substrate along the C-axis direction, the pores inevitably formed because of the diffusion rate discrepancy between Fe and Al atoms, namely the Kirkendall effect [26]. The interface between the Fe–Al phase and substrate became smoother after Fe2Al5 was turned into FeAl and Fe3Al. Because dispersive carbide particles in the substrate did not react with aluminum at all, most of them gathered together around the interface between Fe–Al coating and substrate (Fig. 1b) and some entered the coating during diffusion. Thus, FeAl and Fe3Al would be strengthened because of the existence of these carbide particles. The microhardness values of FeAl and Fe3Al reached 580 HV and 420 HV, respectively, which were slightly higher than those in the work of Kobayashi [10]. The uncoated H13 steel was austenitized at 1040 °C for 20 min, oil quenched, tempered at 200 °C for 2 h, and cooled in air. Tempered martensite plus a small quantity of retained austenite was achieved for H13 steel, as is illustrated in Fig. 2a. In this case, the
Fig. 3. Wear rate of aluminized and uncoated H13 steel as a function of load at 25 and 600 °C.
hardness of the uncoated H13 steel reached HRC 50-55. Clearly, the hardness values of the aluminized and uncoated H13 steel approached each other. 3.2. Wear rates of aluminized and uncoated H13 steel The wear rate of the aluminized and uncoated H13 steel is shown in Fig. 3. At 25 °C, the wear rate of the aluminized H13 steel increased linearly and rapidly as a function of load, whereas it slightly increased for the uncoated H13 steel. As the temperature was elevated to 600 °C, the wear rate of the aluminized H13 steel was substantially reduced, and maintained extremely lower values, even invariably under severer loads. On the contrary, the wear rate of the uncoated H13 steel increased catastrophically as a function of the load. It is clear that the aluminized and uncoated H13 steel revealed totally different wear behavior. The aluminized H13 steel presented excellent wear resistance at 600 °C, but worse one at room temperature. Conversely, the uncoated H13 steel possessed better wear resistance at room temperature, but extremely poor elevated-temperature wear resistance at 600 °C. It is commonplace that medium or low alloy steels have good wear performance at room temperature, but worse elevated-temperature one. The aluminized coating seemed to provide a good way to improve elevated-temperature wear performance of steels. 3.3. XRD analysis and morphology of worn surfaces The XRD patterns of worn surfaces for the aluminized and uncoated H13 steel sliding under 200 N at 25 and 600 °C are
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delaminated region seemed to be slightly increased (Fig. 5a4). Interestingly, fish-scale traces unexpectedly appeared in delaminated pits, which was different from the typical morphology of oxidative wear. This implied that the tough delamination existed, which differed from the brittle delamination of tribo-oxides in steels [27]. The fish-scale traces were mainly made up of Fe, Al and O (Fig. 6c), among whom the atomic ratio between Fe and Al was close to 1:1. It can be deduced that the fish-scale traces appeared at the outer aluminized layer, accompanied with the formation of some oxides. For the uncoated steel, the black tribo-oxide layer partially covered the worn surface with more delaminated regions under 150 N (Fig. 5b3). As the load reached 250 N, the black tribo-oxide layer almost disappeared and the deeper and wider grooves emerged on the smoother worn surface (Fig. 5b4). 3.4. Cross-section analysis of worn surfaces and subsurface Fig. 4. XRD patterns of worn surfaces for aluminized and uncoated H13 steel under 200 N.
illustrated in Fig. 4. At 25 °C, only Fe peaks appeared on worn surfaces of both the aluminized and uncoated H13 steel, without any Fe–Al phases. This was right for the steel, but seemed to be unreasonable for the aluminized steel. While at 600 °C, besides weakened peaks of Fe, there existed many peaks of Fe2O3 and Fe3O4 for the uncoated H13 steel. But FeAl and FeAl3 were noticed to be predominated phases for the aluminized H13 steel; additionally, some Fe2O3 and Al2O3 appeared on worn surfaces. It can be deduced that tribo-oxidation occurred for both the aluminized and uncoated H13 steel at elevated temperature. Morphologies of worn surfaces for the aluminized and uncoated H13 steel are shown in Fig. 5. At 25 °C and 50 N, the aluminized H13 steel presented a different morphology from that of the uncoated H13 steel (Fig. 5a1 and b1). The former worn surface was characterized with brittle fractured traces and fragmented particles besides trace strip-like protrusion regions (Fig. 5a1). These protrusion regions seemed to be tribo-layers, but readily destroyed under the action of load. On the contrary, the worn surface of the uncoated H13 steel revealed plastic tearing, fish-scale adhesion and grooves were revealed on worn surfaces (Fig. 5b2). Clearly, in this case, the wear of the aluminized and uncoated H13 steel was of brittle and plastic characteristic, respectively. As the load increased to 250 N, the worn surfaces of the aluminized and uncoated H13 steel became almost the same and presented adhesive traces (plastic deformation, tearing, material transfer) and grooves, as shown in Fig. 5a2 and b2. Clearly, the aluminized steel represented brittle fracture of intermetallic compound at lower load, compared with plastically shearing and tearing of the uncoated H13 steel. While at higher load, the plastic wear characteristic resulted from the steel regardless of the aluminized and uncoated H13 steel. In this case, the aluminized coating seemed to be worn off completely; EDS results indicated that there was no aluminum (Fig. 6a). For the uncoated steel, on matter what the load was, adhesive trace, plastic tearing and grooves were revealed on worn surfaces (Fig. 5b1 and b2). The worn extent intensified as the load increased (Fig. 5b2). As the temperature was elevated to 600 °C, a special morphology appeared in the aluminized and uncoated H13 steel with black, smooth regions and gray, rough delaminated regions, which was similar to the typical morphology of oxidative wear, reported by our previous research on steels [27]. The black, smooth regions should be tribo-oxide layers. For the aluminized H13 steel, the black and smooth tribo-layers were noticed to cover completely the worn surfaces with some delaminated regions (Fig. 5a3 and a4). EDS analysis demonstrated that the tribo-layer consisted of multiple elements, such as O, Fe, Al, Cr, Mo and W (Fig. 6b). It is clear that the tribo-layer was a tribo-oxide layer. Under 250 N, the amount, area and deepness of the
Fig. 7 shows the cross-section morphology of worn subsurfaces of the aluminized and uncoated H13 steel. Regardless of material, temperature and load, more or less tribo-layers existed on the worn surface. At 25 °C, the tribo-layers of the aluminized and uncoated H13 steel seemed to be essentially metallic, which was proved through the analysis of EDS (Fig. 8a) and XRD results (Fig. 4). For the aluminized H13 steel, most of the coating was almost worn off and merely trace coating was noticed to remain in the deformed matrix at subsurface under of 150 N (Fig. 6a1). With the increase of load, there existed no coating but a thin tribo-layer containing no oxides formed on the deformed matrix (about 10 μm). Thus the aluminized and uncoated H13 steel presented the same wear characteristics. However, at 600 °C, the aluminized and uncoated H13 steel represented remarkably different cross-section morphologies of worn subsurfaces. For the aluminized steel, 1–2 μm-thick tribo-layer was noticed to form on the intermetallic coating (Fig. 7a3 and a4) and contained some oxygen and other elements (Cr, W, Mo) identified by EDS (Fig. 8b). It is clear that the tribo-layer was a mechanically mixed tribo-oxide layer rather than an integrated oxide layer. Under higher loads, the tribo-oxide layer became more compacted and continuous. The plastic deformation of the intermetallic coating was confirmed from the elongated second-phase particles, which was characterized as FeAl2 phase in our previous study [24]. However, the uncoated H13 steel presented totally different morphologies of tribo-oxide layer from the aluminized H13 steel. A multi-layer tribo-layer was observed, as shown in Fig. 7(b3 and b4). Many cracks were noticed to emerge in the underlying interface of tribo-layer and substrate, thus tribo-oxide layers readily fractured and delaminated. Under a severer load (250 N), the multi-layer tribo-layer was thickened to be a lamellar structure. The multi-layer tribo-oxide layer was reported to result in severe wear [27]. Fig. 9 shows the microhardness distribution curves on the crosssection of worn subsurfaces for the aluminized and uncoated H13 steel sliding at 25 and 600 °C under 200 N. At 25 °C, the hardness of the aluminized and uncoated steel was approached each other, and kept the original value of the steel with tiny fluctuation, about 500– 575 HV. At 600 °C, the hardness of the uncoated steel dropped sharply to 300 HV or so. However, the aluminized H13 steel retained high hardness, especially at the outmost part. Three-part variations were revealed in the hardness distribution of the aluminized H13 steel. In the outer part near the worn surface, the hardness slightly increased to 550–600 HV, compared with that at 25 °C. Unexpectedly, the hardness abruptly decreased to 425–475 HV in the middle part. The hardness restored to the original hardness value of 500– 575 HV in the inner part. The hardness variations of three parts were considered to just correspond to the microstructure: FeAl, Fe3Al and the substrate, respectively. This meant that after wear, the Fe–Al coating and the substrate almost remained intact.
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Fig. 5. Morphologies of worn surfaces of the aluminized H13 steel: 50 N (a1), 250 N (a2) at 25 °C, 150 N (a3), 250 N and (a4) at 600 °C; the uncoated H13 steel: 150 N(b1), 250 N (b2) at 25 °C, 150 N (b3), and 250 N(b4) at 600 °C.
4. Discussion 4.1. Evaluation of the wear performance of aluminized and uncoated H13 steel Steels, as widely used structural materials in engineering applications, normally possess good mechanical properties at room temperature. However, as the temperature increased, they will be thermally softened to lose its strength. The low strength of steels inevitably result in severe wear at elevated-temperature. As
reported by many researches [27–30], high ambient temperature raised the wear rate and degraded wear resistance of steels. The aluminized coating on steel was expected to change the situation. Fig. 10 compares the wear performance of the aluminized and uncoated H13 steel as a function of temperature, in which the histograms of the wear rate under 150 and 250 N at 25–600 °C were plotted. Because the wear resistance was inversely proportional to the wear rate, the reciprocal value of wear rate can be simplified as the corresponding wear resistance. The demarcation of mild and severe wear was considered to be 5 10 6 mm3/mm,
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Fig. 6. EDS Regional analysis in Fig. 5: (a) EDS1, (b) EDS2 and (c) EDS3.
which was put forwards by Zhang and Alpas [31]. It can be noticed that at 600 °C for the uncoated steel and at 25 °C for the aluminized steel, the severe wear prevailed and the wear rate markedly increased by one or two orders of magnitude with an increase of load. The poor wear resistance of the uncoated and the aluminized steel in corresponding conditions seemed to be essential shortcoming. Except for the above conditions, the uncoated and aluminized steel seemed to fall in mild wear and was suitable to use in sliding wear. As shown in Fig. 10, the wear resistance of the uncoated H13 steel started to substantially decrease at 200 °C, further deteriorated as the temperature was elevated and the extremely poor wear resistance was presented at 600 °C. On the contrary, for the aluminized steel, in spite of poor wear resistance at room temperature, the wear rate retained extraordinarily low and stable values at elevated temperature, especially at 400 and 600 °C. As long as the ambient temperature reached 200 °C, its excellent wear resistance was presented. It is clear that such an excellent high-temperature wear performance was not possessed by the uncoated H13 steel, like the majority of steels [27–30]. Consequently, at elevated temperature (200–600 °C), the aluminized coatings can completely offset notoriously poor wear resistance of steels and widely apply in the service conditions involving thermal friction and wear. 4.2. Effect of tribo-layers and substrate materials on wear behavior and mechanism The formation of tribo-layers is an inevitable phenomenon during the sliding of metals. Kerridge [32] carried out an elegant experiment with radioactive steels and confirmed that the elements in tribo-layers resulted from the sliding couple and environment. Rigney [33] considered the formation process of tribolayers as the mechanically alloying of wear debris with environmental elements (especially oxygen) under pressure. Table 1 and Fig. 11 illustrate that there were also material loss of the harder M2 steel. It can be thought that the constituent of tribo-layers partly came from counterfaces. As illustrated in Figs. 7 and 8, the tribolayer was indeed a mixture of metal particles and more or less oxide ones. When the tribo-layers existed, the wear behavior and mechanism undoubtedly mainly associated with tribo-layers and subsurface matrix. However, whether the tribo-layers affected the wear behavior and mechanism or not is supposed to be decided by their ingredient [34]. As the tribo-layers were no protective, the wear behavior and mechanism of metal alloy was merely decided by the properties of metal alloy itself at subsurface. As the tribolayers was protective, the original wear behavior and mechanism of metal alloy would be changed, which not only relied on the characteristics of tribo-layers, but also on the properties of metal alloy itself at subsurface.
At room temperature, low ambient temperature and limited frictional heating would hardly oxidize metal surface and debris. In this case, the compositions of tribo-layers mainly resulted from the sliding pin and counterface; the formed tribo-layers were nooxide tribo-layers. It is clear that no-oxide tribo-layers predominantly presented metallic characteristics. In addition, tribolayers formed at room temperature were usually thin, discontinuous and unsound. During sliding, the tribo-layers readily delaminated under the action of load. Even if this metallic tribolayer was retained on worn surfaces, they seemed not to affect the original wear behavior and mechanism of metal alloys because of their lower hardness similar to the metal alloy itself. Hence, the metal–metal contact and adhesion was retained. In this case, the wear performance mainly depended on the metal alloy itself, regardless of the steel or intermetallic. In this case, high friction coefficients were obtained during sliding in Table 1, regardless of the existence of the aluminized coating or not. For the uncoated H13 steel, the worn surfaces were plowed by harder counterface. Adhesive traces and grooves appeared on worn surfaces. Clearly, adhesive and abrasive wear was the main mechanism for the uncoated H13 steel at 25 °C. Because of high strength and toughness, H13 steel resisted extensive plastic deformation and fracture under wear, thus resulting in a relatively low wear rate. On the contrary, for the aluminized coating, environmental embrittlement seriously weakened the roomtemperature ductility and caused brittle cleavage fracture [11,12]. Under the normal load and transverse shearing force, the worn surfaces of the aluminized coating presented brittle fracture and fragmentized particles. It is clear that fatigue wear prevailed in sliding at 25 °C. Compared with intermetallic, high wear resistance of H13 steel was attributed to its high plasticity and toughness at room temperature. At 600 °C, higher ambient temperature and frictional heat decreased the free energy of oxidation, enormously accelerated the reactions between oxygen and metal. A large amount of oxidative particles and debris would be produced, accumulated and sintered to form the tribo-oxide layer on worn surfaces. Usually, it was thick, continuous and sound, and supposed to have the loadcarrying capacity [35]. Because of many oxides, tribo-oxide layer possessed high hardness and ceramic characteristics. When the tribo-oxide layer existed, the metal–metal contact was avoided. In this case, tribo-oxide layers undoubtedly change the original wear behavior and mechanism. As the substrate metal alloy supported them, tribo-oxide layers would reduce wear. On the contrary, as the substrate metal alloy did not support them, tribo-oxide layers would accelerate wear because of their severe delamination. Kato confirmed the protective role of oxides by introducing fine Fe2O3 particles onto rubbing interfaces of carbon steels [36]. It seemed that more oxides would result in better wear resistance (Fig. 10) and lower friction coefficient (Table 1). However, in the
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Fig. 7. Morphologies of cross-section of worn subsurfaces for the aluminized steel: 150 N (a1), 250 N (a2) at 25 °C, 150 N (a3), 250 N and (a4) at 600 °C; for the uncoated H13 steel: 150 N (b1), 250 N (b2) at 25 °C, 150 N (b3), 250 N and (b4) at 600 °C.
present study, more tribo-oxides did not lower the wear rate of H13 steel. According to our previous work [27,29,35], the prerequisite for the protective role of tribo-oxides was that the underlying substrate should possess the mechanical strength enough to support tribo-oxide layers. The mechanical strength of materials could be roughly evaluated by their hardness. After sliding at 600 °C, the hardness of the uncoated H13 steel had markedly dropped from 500–575 HV to 300 HV or so (Fig. 9). This meant that a severe thermal softening occurred. In this case, massive plastic deformation occurred in thermally softened
substrate at subsurfaces, thus the bond between tribo-oxides and substrate could be readily broken under transverse shearing force and normal load. Finally, tribo-oxide layer would be delaminated. In this case, tribo-oxide layer lost protective or load-carried function. The cracks would nucleate and propagate in substrate during sliding. At elevated-temperature, oxygen readily permeated into the subsurface along cracks. The multi-layer oxides layer would be formed because of large plastic deformation and many oxides. The existence of the multi-layer oxide layers caused massive delamination of oxides and matrix, thus the wear rate rapidly
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Fig. 8. EDS regional analysis in Fig. 7: (a) EDS1, (b) EDS2, (c) EDS3, (d) EDS4, and (e) EDS5.
Fig. 9. Microhardness distribution of cross-section of worn surfaces for the aluminized and uncoated H13 steel under 200 N.
rose. The delamination of tribo-oxide layers also appeared in counterface of M2 steel (Fig. 11b2). The predominated mechanism was a mild-to-severe wear transition of oxidative wear. For the aluminized steel, in spite of a thin layer (1–2 μm), the tribo-oxide layer was compact and continuous. It should be protective and took effect in reducing wear. The predominated oxide in tribolayer was Fe2O3, which was reported to act as a solid lubricant in reducing friction and wear [36], as shown in Table 1. More importantly, the aluminized H13 steel (the aluminized coating) preserved its originally high hardness after sliding at 600 °C. As reported, FeAl and Fe3Al intermetallic compound possessed an anomalous yield stress and reached the highest value in a high-temperature range of 400–600 °C as a result of hardening by thermal vacancies [22,23]. The toughness would be also improved up to 800 °C [2], which was evidenced by fish-scale adhesion in Fig. 5(a4). Hence, the aluminized coating presented a high thermal strength and toughness during
Fig. 10. Wear rate of the aluminized and uncoated H13 steel at 25, 200, 400 and 600 °C.
Table 1 Average friction coefficient and wear rate of M2 steel at 150 N. Temperature (°C) Average friction coefficient
25 600
Wear rate of M2 steel ( 10 6 mm3/mm)
Aluminized steel
Uncoated steel Aluminized steel
Uncoated steel
0.82 7 0.08 0.54 7 0.05
0.73 70.06 0.6770.05
1.59 7 0.31 16.667 1.82
0.83 7 0.18 4.30 7 0.46
sliding at 600 °C. Moreover, the 300 μm-thick coating separated the steel matrix and counterface, which inhibited the excessive increase of bulk temperature by frictional heat. As shown in Fig. 9, the thermal softening of H13 steel matrix did not occur. Therefore, the excellent
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Fig. 11. Morphology of worn surfaces of M2 steel as mating material at 150 N: sliding against the aluminized steel at 25 °C (a1) and 600 °C (a2); the uncoated steel at 25 °C (b1) and 600 °C (b2).
elevated-temperature wear performance of the aluminized steel can be suggested to be attributed to the protection of the thin tribo-oxide layer with a strong support from high thermal-strength intermetallic compound. According to the wear characteristics, oxidative mild wear was the main mechanism at 600 °C.
5. Conclusions
(1) The aluminized coating, consisting of FeAl and Fe3Al, on H13 steel was prepared by hot-dip aluminizing and subsequently hightemperature diffusion. Compared with the uncoated H13 steel, the aluminized steel possessed excellent elevated-temperature wear resistance, in spite of worse room-temperature wear performance. The aluminized steel seemed to be suitable to the applications involving elevated-temperature sliding. (2) The excellent elevated-temperature wear performance of the aluminized steel was suggested to be attributed to the protection of the thin tribo-oxide layer with a strong support from the intermetallic compound with high thermal strength and toughness.
Acknowledgments Financial support of our work by National Natural Science Foundation of China (No. 51071078), the Research and Innovation Project for College Graduates of Jiangsu Province (No. KYLX-1031) and Jiangsu Province Key Laboratory of High-end Structural Materials (No. hsm1403) are gratefully acknowledged.
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