Abrasive Wear Performance of Fe 2 B Layers Applied on Steel Substrates

The resistance of dry abrasive wear in Fe 2 B layer deposited on AISI D2 and 1040 steel substrates, using the powder-pack boriding process, was evaluated. The boriding process was carried out at temperatures of 1220 and 1320 K for a time of 8 h. A Rockwell hardness tester was used to assess the Daimler-Benz adhesion test. The abrasive wear tests were carried out in dry conditions according to the ASTM G65 test standard. The test parameters used were a sand flow of 400 g/min, a nominal rubber wheel constant rotation of 200 rpm, a load of 122 N, and a sliding distance of 716.28 m. The type of abrasive used was steel round grit with a grain size of 260 μ m and a hardness of 1100 HV. The total time for each test was 30 min, removing the specimens every 5 min to determine the amount of mass loss using an analytical balance (sensitivity of 0.0001 g). The average value of volume loss and wear rates is reported. Optical microscopy and SEM were carried out in order to identify the wear mechanisms. The wear mechanisms presented in this study were two-body abrasive wear, pitting action, and plastic deformation.


Introduction
Abrasive wear occurs when a hard particle slides on a surface, causing loss of material. This type of wear depends on factors such as hardness, roughness, and particle geometry [1][2][3][4].
On the other hand, some works have been developed using boron coatings as anti-wear material. Boronizing is a thermo-diffusion process in which boron atoms, due to their small diameter and high mobility at elevated temperatures, diffuse into a metal surface and form intermetallic compounds with atoms of base metal [11]. Abrasive wear tests were carried out using boronizing on SAE 1010, 1040, D2, and 304 steels [12]. It was seen that boronizing improved the wear strengths considerably. The best abrasive wear strengths were obtained in boronizing for 8 h at 900°C for SAE 1010 and SAE 1040 steels, 4 h at 900°C for D2 steel, and 6 h at 900°C for 304 steel. In another work [13], abrasive wear resistance of boride layers on Fe-15Cr alloy was studied. It was found that the dry abrasive wear resistance of borided alloy samples was around 45 times greater than that of non-borided ones. In another study [14], the micro-abrasive wear of boride layers on AISI D2 tool steel was investigated. Some results indicated that wear resistance of the borided samples was superior to the hardened, uncoated AISI D2 steel. According to literature [15], wear resistance of boronized steels in abrasive wear conditions depends on the phase composition and hardness of the layer and its stress state, but the hardness of abrasive particles also has a significant importance on the wear speed.
The objective of this work was to evaluate the resistance of dry abrasive wear in Fe 2 B layer deposited on AISI D2 and 1040 steel substrates without a previous heat treatment (hardened and tempered) using the powder-pack boriding process. The substrate materials were selected in order to compare the wear abrasion behavior of a plain carbon (1040) versus a high-carbon, high-chromium steel (D2). AISI 1040 steel is frequently cold drawn to specified physical properties for use without heat treatment for some practical applications such as cylinder head studs. AISI D2 tool steel has desirable properties such as abrasion resistance, high hardness, and no deforming characteristics, and is used in lamination and stamping dies, shear blades, master tools, etc. The wear resistance of D2 tool steel is approximately eight times that of plain carbon steels, so also was of interest to know if this great difference in wear behavior could increase or decrease in borided conditions.

Test specimens
The specimens had a rectangular shape with dimensions of 50 × 25 mm and 10 mm in thickness. The chemical composition of the AISI D2 and 1040 steels is shown in Table 1 [16,17].
The boriding process used in the specimens of AISI D2 and 1040 steel substrates was the same as reported in a previous work [18]. The only difference is that for this work, two boriding temperatures were employed (1220 and 1320 K).
It is important to mention that, based on the Fe-B phase diagram and the high iron content of the AISI D2 and 1040 steels [19], in addition to the diffusion of boron at high temperatures (1220 and 1320 K) and the treatment time of 8 h, the formation of a Fe 2 B monolayer is ensured under the conditions of the boriding process proposed in this work. Because the Fe-B phase is formed on the surface of the sample, which generates the Fe-B/Fe interface between the Fe-B phase and the steel, this allows the gradual formation of the Fe 2 B phase that grows when the thickness of the boride is increased and at the same time the Fe-B phase decreases. At the

Test procedure
The tests were performed according to the ASTM G65 test standard [21]. Figure 1 shows the experimental rig and a simplified schematic diagram of the dry/sand rubber wheel apparatus used in this research work [22].
The test parameters used were a sand flow of 400 g/min, a nominal rubber wheel constant rotation of 200 rpm, a load of 122 N, and a total sliding distance of 716.28 m, using a 228.6 mm diameter wheel rotating. The wheel consists of a steel disk with an outer layer of neoprene rubber tire molded to its periphery with hardness A60. As the rubber wheel reduces in diameter, the number of wheel revolutions was adjusted to equal the sliding distance of the new wheel. The type of abrasive used was steel round grit with a grain size of 260 μm and a hardness of 1100 HV. The total time for each test was 30 min, removing the specimens every 5 min to determine the amount of mass loss using an analytical balance (sensitivity of 0.0001 g). Before the overall tests were performed, the specimens were cleaned by washing in ethanol in an ultrasonic bath (Fisherbrand 11020).
The average value of volume loss (V), wear rates (Q ), and wear coefficients (k) are reported. Optical microscopy and SEM were carried out on the damaged surfaces in order to identify the wear mechanisms. Additionally, the profiles of the wear scars are presented using a Mitutoyo Surftest Profilometer.

Fe 2 B layer hardness
A load of 100 g was used to evaluate the hardness of Fe 2 B layers with a Vickers indenter. The variation of the hardness, depending on the depth of layers, is shown in Figure 2. Also, the roughness of specimens was obtained with a Mitutoyo Surftest Profilometer, see Table 2.   Table 2.
Properties of the specimens. Figure 3 shows the cross-sectional view of SEM micrographs. A zigzag teeth shape is observed in both steels. This columnar shape comes from the direction in which diffusion is preferred, and the boride is of stronger (002) texture [23]. The presence of this typical morphology for good adhesion between coating and substrate is necessary.

SEM, X-ray diffraction, and EDS
The boriding is a diffusive process highly anisotropic [24]. In Figure 3, it was observed that the boride on the surface of AISI D2 steel presents a columnar morphology; in the case of boride formed on the surface of AISI 1040 mold steel is observed a dense structure due to alloying elements it has. Depending on the conditions of processing time, temperature, and chemical composition of substrates, the depth obtained of the boride layer was an interval of 10-60 μm (Figure 2). It was observed that the depth of borides formed on AISI D2 is more homogeneous than that of AISI 1040.
The results of X-ray diffraction studies are presented in Figure 4. The XRD analysis shows well-defined peaks at 42.67 and 45.11° confirming Fe 2 B phase. Also, the presence of chromium boride (CrB) phase in the borided AISI D2 steel was determined. This is due to the significant presence of chromium in AISI D2 steel as an alloying element [25,26]; apparently, during powder-pack boriding, it reacted with boron atoms and formed a little intermediate phase of CrB.

Fe 2 B layer adhesion test
A Cientec Rockwell hardness tester model 200HR-150 was used to assess the Daimler-Benz adhesion tests [27]. Figure 6 shows the indentations on the surfaces. For the AISI D2 steel, some small cracks and no visible delamination are observed (Figure 6a and c), and the adhesion strength quality is related to HF1 map [28]. In the case of AISI 1040 steel (Figure 6b and d), microcracks and small delamination are observed, and the adhesion category belongs to the HF4 level.

Wear profile
The abrasion tests carried out caused wear damage on surfaces. The wear profiles were measured using a Mitutoyo Surftest profilometer and are shown in Figure 7. The results are compatible with the volume loss (Figure 8), where 1040 steel borided at 1220 K has the greatest wear volume and the D2 steels at 1320 K had the minor wear.

Volume loss
The volume loss was obtained for all the borided and unborided steel substrates. These data were calculated using Eq. (1). The mass loss was obtained weighing the specimens before and after the test. The graph of the Figure 8 shows that the AISI D2 steel borided at 1320 K exhibited a higher wear resistance compared to the other specimens. The results also show the great difference in volume loss between borided and unborided steels. The reason that the D2 steel had a greater wear resistance is due most likely to the mechanical properties conferred by a high content of C and Cr, which is higher than in the 1040 steel (see Table 1). According to Figure 8, the abrasion wear resistance of borided D2 tool steel is approximately 16 times greater than borided 1040 plain carbon steel. This could justify the use of this tool steel for abrasion wear applications when it is borided to the conditions used in this work, without a previous heat treatment, as usual. These results also indicate that its core strength was not affected due to high temperatures of powder-pack boriding treatment.

Wear rates (Q ) and wear coefficients (k)
The wear rates were obtained from Eq. (2).
where Q = wear rate (mm 3 /m), V = volume loss (mm 3 ), and d = sliding distance (m). Figure 9 shows the wear rates obtained every 716 m of the borided and unborided D2 and 1040 steels. AISI D2 borided steels at 1320 and 1220 K had the best performance against the dry abrasive wear conditions. It was due to its good mechanical properties and chemical composition. Additionally, the adhesion tests on this steel showed an excellent performance. On the other hand, the 1040 steel borided at 1320 K had a good behavior, almost similar to the D2 steel. For the AISI 1040 steels borided at 1220 K, an abnormal value was observed at a sliding distance of 716 m, where the wear rate had a great increase. This performance was mainly due to the running in period of the test. In both steels, as expected, the wear damage was more severe on the unborided specimens (Figure 10a and b). In the case of the borided steels, AISI 1040 at 1220 K (Figure 10c) showed the greater damage and the AISI D2 borided at 1320 K (Figure 10d) showed the lower damage. This was in accordance to the results of the wear profiles showed in Figure 7 and wear rates showed in Figure 9.
In the case of borided steels, the main wear mechanism observed in the wear scars was the two-body abrasive wear due to the presence of parallel lines to the sliding direction. These parallel lines were produced by wear debris acting as indenters causing, in some cases, depth grooves. Figure 11a-d shows the SEM micrographs of the wear damage of the borided steels. In the case of AISI 1040 steel borided at 1220 K (Figure 11a), wear debris was observed, which was derived from the three-body abrasion situation, where hard particles were trapped between the two sliding surfaces. Also, severe pitting action was observed, caused by the particles of material and abrasive particles. Finally,     plastic deformation occurred due mainly to the sliding action. Figure 11b (AISI D2 steel at 1220 K) shows a typical effect of abrasive wear, where sharp asperities plowed the surface, causing permanent deformation. This plowing action produced a groove in the softer material. Also, parallel lines were observed caused by wear debris probably inlaid in the rubber wheel. In the case of AISI 1040 steel borided at 1320 K (Figure 11c), severe micropitting action and plastic deformation were observed. Finally, in the case of the AISI D2 steel at 1320 K, wear debris and pitting action were observed, but to a lesser degree. This matches with the results of wear rates obtained and indicated in Figure 9.

Conclusions
1. The formation of a Fe 2 B monolayer is ensured under the conditions of the process of boron powder-pack proposed in this work. The Fe-B phase is formed on the surface of the sample, which generated the Fe-B/Fe interface between the Fe-B phase and the steel, which allowed the gradual formation of the Fe 2 B phase that grows when the thickness of the boride is increased and at the same time the Fe-B phase decreases.