Formation of Fe–Al Intermetallic Compound Layer by AIH-FPP and its Effect on Tribological Properties of Stainless Steel

Abstract

In order to increase the surface hardness and improve the tribological properties of stainless steel, Fe–Al intermetallic compound layers were created by atmospheric-controlled induction-heating fine particle peening (AIH-FPP). The surface microstructure of the stainless steel treated with AIH-FPP was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and micro-Vickers hardness testing. In addition, the tribological properties were investigated by reciprocating ball-on-disk wear tests. When high-speed steel particles coated with thin aluminum layers were used as the shot particles, Fe–Al intermetallic compound layers were formed at the surfaces of the stainless steel with increased heating time during AIH-FPP. This is because aluminum was transferred from the shot particles, and the temperature at the treated surface increased as a result of a combustion synthesis reaction. When the heating temperature after FPP was increased, the transferred aluminum and nitrogen in the atmosphere reacted, which resulted in the formation of aluminum nitrides in addition to Fe–Al intermetallic compounds. The tribological properties of the stainless steel were improved by AIH-FPP since high-hardness layers were created. The results indicate that the formation of an Fe–Al intermetallic compound layer with high hardness by AIH-FPP is effective for modifying the tribological properties of the stainless steel within a relatively short span of time.

1. Introduction

Austenitic stainless steels are extensively used in various industries such as those in chemical and medical fields because of their high corrosion resistance. However, the stainless steels generally exhibit poor tribological properties, such as high friction coefficient and low wear resistance.

One promising technique for overcoming this drawback is surface modification. Aluminizing, by such methods as hot-dipping using molten aluminum^1,2) and subsequent heat treatment,³⁾ deposition of thin aluminum foil and subsequent heat treatment,⁴⁾ cladding,⁵⁾ pack cementation,⁶⁾ cold spray,⁷⁾ explosive welding⁸⁾ and slurry aluminizing,⁹⁾ is widely employed as an effective modification technique. Although these aluminizing techniques can create Fe–Al intermetallic compound layers on the steel surfaces,^{1,2,3,4,5,6,7,8,9)} these techniques have some disadvantages. For example, non-reacted aluminum with low hardness can remain on the surface of the hot-dipped steel.^1,2) Long treatment times lasting several hours or multiple step treatments are required to form compound layers with high hardness.^3,4,5,6,7,8) There is a report on the formation of the Kirkendall pores in the created compound layers.⁹⁾

Atmospheric-controlled induction-heating fine particle peening (AIH-FPP), a surface modification technique proposed by the authors, is expected to solve the problems encountered in conventional aluminizing techniques. In AIH-FPP, an induction-heated metal is subjected to fine particle peening (FPP) under an arbitrary atmosphere. Layers containing intermetallic compounds such as Ni–Al¹⁰⁾ and Ti–Al^11,12,13,14) without non-reacted metallic elements can be formed on the surfaces of the carbon steel^10,11) or titanium alloy^12,13,14) within a few minutes. The modified layers improve the wear resistance and high-temperature oxidation resistance of the substrates.^10,11,13) Possible mechanisms for the rapid formation of the intermetallic compound layer include the occurrence of combustion synthesis reactions between the elements of the shot particles and substrate¹³⁾ or diffusion of the shot particle elements transferred to the surface into the substrate.¹⁴⁾ Based on the previous studies, the application of AIH-FPP with shot particles containing aluminum is expected to form an Fe–Al intermetallic compound layer on stainless steel surfaces rapidly, and the tribological properties of the steel would be improved.

The aim of this study is to characterize the surface-modified layer formed on an austenitic stainless steel by AIH-FPP with shot particles containing aluminum and to reveal the effect of the modified layer on the tribological properties of the steel.

2. Experimental Procedures

2.1. Specimen Preparation

AISI 304 austenitic stainless steel was used as a sample material. Steel rods with a diameter of 15 mm were machined into disk-shaped specimens with a thickness of 3 mm. One end face of each specimen was polished using emery paper (#240–#1200).

AIH-FPP was carried out on the polished surfaces of the specimens. Figure 1 shows a schematic illustration of the AIH-FPP system.¹⁵⁾ The equipment consists of an FPP nozzle and an induction-heating (IH) coil in a sealed chamber. The atmosphere inside the chamber can be replaced by supplying gas through the FPP nozzle. The oxygen concentration was measured by an oxygen meter and confirmed to be 0.0 vol.%.

Fig. 1.

Schematic illustration of the AIH-FPP system.

Table 1 summarizes the AIH-FPP conditions. The purity of the used nitrogen gas was 99.99%. The heating time and the temperature after FPP were varied to examine their effects on the formation of the surface-modified layer on the stainless steel. The treatment temperatures were selected to avoid the sensitization temperature of the stainless steel. Figure 2 provides the thermal histories during respective AIH-FPP experiments. The applied shot particles were prepared by mechanical milling of high-speed steel particles and pure aluminum particles. By mechanical milling of particles with low hardness and high hardness, a thin layer of the low hardness material can be created at the surface of the high hardness particle.^16,17,18) The details of the preparation of the shot particle can be found in our previous study.¹⁴⁾ Scanning electron microscopy (SEM) images and elemental maps obtained using energy dispersive X-ray spectroscopy (EDX) are shown in Fig. 3. High-speed steel particles coated with thin aluminum layers were formed. Appropriate amounts of aluminum can be transferred from the shot particles to the specimen surface during AIH-FPP using these particles.¹⁴⁾ The treatment temperature of AIH-FPP was determined using a K-type thermocouple welded to the surface of the specimen and controlled by adjusting the power applied to the IH inverter. The IH coil consisted of a single turn and its internal diameter was 30 mm.

Table 1. AIH-FPP conditions.

Atmosphere	N2
Shot particle	High-speed steel/aluminum mechanical milling particle
Particle supply rate	1 g/s
Peening time	30 s
Heating time after FPP	0, 180, 300 s	300 s
Treatment temperature	673 K	During FPP: 673 K
		After FPP: 673, 973, 1173 K
Gas flow rate	50 L/min
Nozzle distance	100 mm
Internal diameter of nozzle	4 mm

Fig. 2.

Thermal histories during AIH-FPP for (a) various heating times after FPP and (b) various heating temperatures after FPP.

Fig. 3.

SEM micrographs and EDX maps of the particles used for AIH-FPP. (Online version in color.)

2.2. Characterization of Surface-modified Layer

The cross-sections of the prepared specimens were observed using SEM and EDX. Characterization of the cross-sections of the specimen surfaces was carried out after cutting, molding with resin, and polishing to a mirror finish using emery paper (#240–#1200) and diamond slurry. The compounds formed on the specimens were determined by X-ray diffraction (XRD) using Cu Kα radiation (wavelength 0.154 nm), a diffraction angle 2θ of 30°–80°, a voltage of 40 kV and a current of 40 mA. The XRD profiles were obtained from the entire surface of the specimens. The hardness distribution at the cross-sections of the specimens was measured using a micro-Vickers hardness tester. The applied indentation load was 0.49 N and the load holding time was 20 s during the hardness measurements.

To investigate the effect of AIH-FPP on the tribological properties, reciprocating ball-on-disk wear tests were performed at ambient temperature in air. Table 2 presents the test conditions. The tests were performed for the untreated specimen and the specimens treated with AIH-FPP heating at 673 and 1173 K for 300 s after FPP at 673 K for 30 s. In order to obtain the friction coefficient, the force generated due to the contact of the specimen with alumina ball (counter material) during the tests was measured using a load sensor connected to the jig for the alumina ball. The measured load was divided by the applied load (1 N), and the friction coefficient was obtained. The mass of the specimens was measured using an electric balance before and after the tests to determine the wear losses. The wear tracks formed on the specimens during the tests were observed using SEM. The cross-sectional profiles of the wear tracks were obtained using laser microscopy.

Table 2. Conditions of reciprocating ball-on-disk wear test.

Opposite material	Alumina ball (φ3 mm)
Load	1 N
Sliding stroke	8 mm
Sliding speed	10 mm/s
Number of sliding cycles	14000

3. Results

3.1. Characterization of the Surface Modified Layer Formed by AIH-FPP

Figure 4 shows the cross-sectional SEM micrographs and EDX maps of specimens treated via AIH-FPP for different heating times after FPP. Al was detected at the surfaces of all specimens. This is because aluminum contained in the shot particle surfaces was transferred to the surface of the specimen during the FPP process.^14,19) The thickness of the Al transfer layer was approximately 20 μm, and was the same for all specimens. This means that the amount of transferred Al was almost the same in each case due to the same peening time being used. In the region where Al was detected, Fe was also detected. Therefore, there is a possibility that the Fe and Al reacted, and their intermetallic compounds were formed.

Fig. 4.

SEM micrographs and EDX maps of longitudinal cross-sections of specimens treated with AIH-FPP for heating times of 0, 180 and 300 s. (Online version in color.)

To identify the compounds formed on the specimens, XRD analyses were carried out, and Fig. 5 shows the analysis results. The untreated specimen exhibited diffraction peaks resulting from austenite (Fe (γ)) and martensite (Fe (α’)). The martensite peak is attributed to the deformation-induced martensite formed during the polishing process. For the specimen treated via AIH-FPP for a heating time of 0 s, that is, with only FPP at 673 K for 30 s, peaks caused by Al were mainly observed, although minor peaks of Fe–Al intermetallic compounds (Fe₂Al₅) were also detected. Meanwhile, for the specimens heated for 180 or 300 s after FPP, peaks related to Fe–Al intermetallic compounds and Al-diffused ferrite (Fe(Al)) were exhibited. The results indicate that Fe–Al intermetallic compound layers can be created using the AIH-FPP system by increasing the heating time after FPP.

Fig. 5.

XRD patterns of the untreated specimen and specimens treated with AIH-FPP for heating times of 0, 180 and 300 s.

Figure 6 shows the cross-sectional distribution of the Vickers hardness of the specimens treated via AIH-FPP for different heating times after FPP. High-hardness layers were formed at the surfaces of all specimens. The maximum hardness value was approximately 650 HV, and the hardness value decreased with increasing distance from the surface.

Fig. 6.

Distribution of Vickers hardness for specimens treated with AIH-FPP for heating times of (a) 0, (b) 180 and (c) 300 s.

Figure 7 shows SEM micrographs and EDX maps of the specimens treated via AIH-FPP at different heating temperatures after FPP. The data for the specimen heated at 673 K are the same as for that heated for 300 s, as in Fig. 4. The thickness of the layer wherein Al could be detected was significantly reduced with increasing heating temperature, and a greater amount of Al was detected inside the substrate. This is because Al diffusion into the substrate was accelerated by heating at high temperatures. The particles containing Al were also observed in the specimen treated with AIH-FPP heating at 1173 K after FPP.

Fig. 7.

SEM micrographs and EDX maps of longitudinal cross-sections of specimens treated with AIH-FPP at temperatures of 673, 973 and 1173 K after FPP. (Online version in color.)

Figure 8 shows the XRD patterns obtained from the specimens treated via AIH-FPP at different heating temperatures after FPP. In the specimen heated at 973 K, the peak intensities of the Fe(Al) increased relative to those of the specimen heated at 673 K. This result agrees with the results of the EDX analyses (Fig. 7), thereby revealing the diffusion of Al into the substrate. The specimen heated at 1173 K exhibited weak peaks related to aluminum nitride (AlN) in addition to the peaks of the Fe–Al intermetallic compound and Al-diffused ferrite.

Fig. 8.

XRD patterns of specimens treated with AIH-FPP at temperatures of 673, 973 and 1173 K after FPP.

The elemental concentration was measured at the longitudinal section of the specimen treated via AIH-FPP, with heating at 1173 K after FPP. Figure 9 shows the results thereof. A high concentration of Al was detected at the outermost surface of the specimen with a certain level of N and a low concentration of Fe. The Al and N concentrations tended to decrease, whereas the Fe concentration gradually increased with increasing distance from the surface.

Fig. 9.

Elemental concentration for the specimen treated with AIH-FPP at temperature of 1173 K after FPP as a function of distance from the surface.

Figure 10 shows the cross-sectional distribution of the Vickers hardness of the specimens treated via AIH-FPP at different heating temperatures after FPP. When the heating temperature was increased to 973 or 1173 K, as shown respectively in Figs. 10(b) and 10(c), the maximum hardness value at these conditions decreased when compared with that at 673 K, as in Fig. 10(a). However, the thickness of the hardened layer increased with increasing heating temperature.

Fig. 10.

Distribution of Vickers hardness for specimens treated with AIH-FPP at temperatures of (a) 673, (b) 973 and (c) 1173 K after FPP.

The results presented in this section indicate that surface-modified layers, including Fe–Al intermetallic compounds and aluminum nitride with high hardness, were formed as a result of AIH-FPP, and the characteristics of the modified layers differed with varying AIH-FPP conditions.

3.2. Evaluation of the Tribological Properties of the Stainless Steel Treated with AIH-FPP

Figure 11 shows the cross-sectional profiles of the wear tracks formed on each specimen. The sizes of the wear tracks formed on the AIH-FPP-treated specimens were smaller than that on the untreated specimen. This indicates that AIH-FPP improved the wear resistance of stainless steel.

Fig. 11.

Cross-sectional profiles of the wear tracks formed on (a) the untreated specimen, and specimens treated with AIH-FPP heating at (b) 673 and (c) 1173 K after FPP.

Figure 12 shows the wear losses of the specimens during the wear tests. The specimen treated via AIH-FPP at 673 K exhibited a lower wear loss than that of the specimen treated at 1173 K. The friction coefficient measured during the tests is shown in Fig. 13. The friction coefficient of the specimen treated via AIH-FPP at 673 K was lower than that of the specimen treated at 1173 K. These data indicate that AIH-FPP at 673 K is more effective than that at 1173 K for improving the tribological properties of stainless steel.

Fig. 12.

Wear loss of the untreated specimen, and specimens treated with AIH-FPP heating at 673 and 1173 K after FPP during wear tests (n=3, mean±max., min.).

Fig. 13.

Relationship between sliding cycle and friction coefficient for specimens untreated and treated with AIH-FPP heating at 673 and 1173 K after FPP.

4. Discussion

4.1. Mechanism of the Surface Modified Layer Formation by AIH-FPP

Based on the obtained results, the mechanism of the formation of the surface-modified layer is discussed in this section. Figure 14 shows a schematic illustration of the process of formation of the surface-modified layers with a thermal history.

Fig. 14.

Schematic illustration explaining the process of the formation of surface modified layers by AIH-FPP. (Online version in color.)

When the shot particle with thin aluminum layers collided with the induction-heated substrate, fragments of aluminum are transferred to the substrate surface, as shown in Fig. 14(a). After these particle collisions and transfers of aluminum were repeated, an aluminum transfer layer with a thickness of several tens of micrometers is created (see Fig. 14(b)) as shown in Fig. 4. At this point, a part of the transferred aluminum reacts with Fe in the substrate, and Fe–Al intermetallic compounds are formed, as shown in the XRD data (Fig. 5). This trend is similar to that of the results of a previous study wherein the same treatment was performed for titanium alloy, and an aluminum transferred layer and a Ti–Al intermetallic compound layer were formed on the titanium alloy surface after FPP without subsequent heating.¹⁴⁾ The formation of the compound layer at a relatively low heating temperature within a short span of time during AIH-FPP is attributed to the occurrence of a combustion synthesis reaction between Fe and Al²⁰⁾ and to the local increase in the temperature of the treated surface.¹³⁾ According to the XRD data (Fig. 5) and hardness measurements (Fig. 6), the surface-modified layer contains Fe₂Al₅, with a relatively high hardness of 820 HV²¹⁾ and unreacted Al with lower hardness.

Through variation of the heating temperature after FPP, the surface-modified layer ultimately formed could also be varied according to the results shown in Figs. 7, 8, 9, 10. When the heating temperature is low (673 K in this study), an Al-rich Fe–Al intermetallic compound layer is mainly formed, and Al-diffused ferrite is formed beneath it, as shown in Fig. 14(c). In contrast, when the heating temperature is increased (973 K in this study), an Al-diffused ferrite layer is mainly formed (see Fig. 14(d)). This is because the diffusion of aluminum into the substrate is accelerated by increasing heating temperature. The acceleration of diffusion of shot particle element was also revealed in our previous study regarding AIH-FPP.²²⁾ When the heating temperature was further increased (1173 K in this study), aluminum nitride (AlN) was also formed, as shown in Fig. 14(e), in addition to the Fe–Al containing layer. This is because the transferred Al and N in the processing atmosphere reacted. This occurs in metals that are easily nitrided, such as titanium.^23,24) According to the elemental concentration measurement shown in Fig. 9, Al-rich compounds, such as Fe₂Al₅ and AlN, were predominantly formed at the outermost surface, while the created compounds changed to Al-diffused ferrite (Fe(Al)) in the subsurface region of the substrate. The XRD results shown in Fig. 8 reveal that Fe(Al) mainly existed in the surface layer of the specimens heated at 973 or 1173 K, exhibiting a relatively lower hardness than compounds. Thus, the specimens heated at higher temperatures exhibited lower surface hardness values than those of the specimen heated at 673 K (Fig. 10), which contained more Fe₂Al₅ with a high hardness (820 HV²¹⁾). The Al-rich particles inside the substrate heated at 1173 K (Fig. 7) were considered to be AlN because the concentrations of Al and N were similarly varied over the distance of 10 μm from the surface in Fig. 9, and Fe were not detected from the particle in Fig. 7. We inferred that AlN formed at the outermost surface diffused into the substrate; however, detailed mechanism of the formation of these particles is not clarified currently.

The thickness of the hardened layer increases with increasing heating temperature. This is because Al diffusion into the stainless steel was accelerated by heating at high temperatures, which is consistent with the EDX analysis results shown in Fig. 7 and XRD analyses shown in Fig. 8. It was previously revealed that, when high-speed steel particles and chromium particles are simultaneously employed for AIH-FPP, the diffusion of the transferred chromium accelerates because of the existence of a layer with a high dislocation density.²²⁾ Therefore, in this study, Al diffusion is accelerated owing to the application of shot particles containing high-speed steel.

In stainless steels, the formation of Fe–Al intermetallic compound layer is affected by alloying elements such as Cr and Ni.²⁵⁾ However, the detailed effects were not investigated in this study and will be investigated in future studies.

4.2. Effect of AIH-FPP on the Improvement of the Wear Resistance of the Stainless Steel

The specimen treated via AIH-FPP heated at 673 K after FPP exhibited tribological properties superior to those of the specimen heated at 1173 K, as demonstrated in Section 3.2. One reason for this is the hardness distribution of the specimens. As shown in Fig. 10, the outermost surface hardness of the specimen treated at 673 K was higher than that of the specimen treated at 1173 K. The wear of the AIH-FPP-treated specimens occurred at the peaks of the rough surface, which were locally worn as a result of contact with a counter material.²⁶⁾ Therefore, the high hardness at the outermost surface is effective for modifying the tribological properties.

Haftlang et al.²⁷⁾ demonstrated that the friction coefficient of steel aluminized for 5 h was 0.756. This value is higher than that of the specimen treated with AIH-FPP in this study (Fig. 13). Although the conditions of the wear tests between this study and the study performed by Haftlang et al. differ, this comparison shows the effectiveness of the AIH-FPP in improving tribological properties within a short span of time.

Previous studies^25,28) demonstrated that Al-rich Fe-Al intermetallic compounds, such as Fe₂Al₅, which existed on the specimen treated at 673 K, underwent brittle fracture during the wear test, which resulted in deterioration of the tribological properties. However, the SEM micrographs of the wear tracks formed on the specimens (Fig. 15) show no significant brittle fractures contributing to the improvement of tribological properties. Although further studies are required on this aspect, this observation is probably the result of relatively ductile Fe(Al) being present in the surface-modified layer.

Fig. 15.

SEM micrographs of wear tracks formed on untreated specimen and specimens treated with AIH-FPP heating at 673 and 1173 K after FPP.

5. Conclusions

AIH-FPP was performed for austenitic stainless steel AISI 304 to form Fe–Al intermetallic compound layers. The effects of the heating time and temperature on the formation of the compound layers were examined. In addition, the tribological properties of the steel treated with AIH-FPP were evaluated. The main conclusions are as follows.

(1) Fe–Al intermetallic compound layers were formed on the surface of the stainless steel with increasing the heating time after FPP. The rapid formation of the compound layers was due to the occurrence of a combustion synthesis reaction between Fe from the substrate and Al from the shot particles.

(2) By increasing the heating temperature after FPP, the main phase contained in the surface layer could be varied from an Al-rich intermetallic compound to Al-diffused ferrite. In addition, aluminum nitrides were formed on the treated surface. These are because the reactions between Fe in the substrate, Al transferred from the shot particle and N in the processing atmosphere progressed. The thickness of the hardened layer increased due to the diffusion of Al, although the outermost hardness value decreased because of the variation of the formed compound.

(3) The tribological properties of the stainless steel were improved by AIH-FPP. This is owing to the formation of a surface-modified layer with high hardness. In particular, a modified layer with higher hardness at the outermost surface is effective for improving the tribological properties.

Acknowledgments

This study was supported by JSPS KAKENHI (Grant Number 20K14609) and 27th ISIJ Research Promotion Grant. The authors are grateful for their financial support. We also thank Shuya Saito (Graduate School of Keio University) for the help with the experiments.

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