Boriding of Laser-Clad Inconel 718 Coatings for Enhanced Wear Resistance

: Nickel-based superalloys are particularly suitable for applications under corrosive conditions. Economic advantages can be achieved by limiting the use of materials to the surface region. Furthermore, the tribological property proﬁle can be signiﬁcantly improved by surface hardening. In the present study, the possibility of a process combination comprising a coating and a surface hardening technology was investigated. For this purpose, Inconel 718 coatings were applied to austenitic stainless steel by laser cladding. Subsequently, a thermochemical surface hardening by boriding was carried out. Scanning electron microscopic (SEM) examinations were performed to evaluate the microstructure. The phase composition was determined by means of X-ray diffraction (XRD) for the different states of the coating system. The inﬂuence of thermochemical hardening was investigated for different wear conditions. The increase in microhardness and wear resistance clearly demonstrates the utilization potential of the presented process combination. increase in wear resistance under ball-on-disk and reciprocating wear conditions. The results conﬁrm the great potential of the presented process combination of laser cladding and surface hardening.


Introduction
Nickel-based superalloys are high-performance materials with a unique property profile [1][2][3][4][5][6][7][8]. The high material costs only justify their use where alternative material concepts cannot offer a sufficient service life or protection. Coating technologies enable an economical alternative to traditional monolithic materials. Laser cladding as a coating process is particularly suitable for such a purpose [4,[9][10][11]. By creating a metallurgical bond between the cladding and the substrate, high adhesive strengths can be achieved. In addition, a compact coating structure is produced, which reliably protects the substrate material from corrosive attack. However, nickel-based superalloys are prone to tribological attack due to their ductile matrix [7,8,[12][13][14][15][16][17]. Inconel 718 is widely used as feedstock material. The tribological properties of laser-clad coatings have already been investigated by several groups [18,19]. For a corrosive and tribological load collective, adaptive solution concepts are necessary for the component surface. In the field of functionalization of structural materials, thermochemical surface hardening methods have become well established. Compared to alternative processes, boriding offers the highest achievable values with regard to surface hardness and wear resistance [20][21][22][23][24][25][26][27][28][29]. The process variant of powder-pack boriding is particularly suitable for this purpose [23][24][25][26][27][28][29][30][31]. An adapted process routine allows for homogeneous surface layers to be formed without silicidation in the case of nickel-based alloys. A process combination consisting of a coating step, as well as surface layer hardening, can provide economic advantages, especially in comparison to monolithic component design. Such a process combination is currently unexplored in the case of laser-clad, nickel-based superalloys.
However, a series of studies on the thermochemical surface hardening of coating systems have proven the fundamental feasibility of this process combination. Initial investigations were carried out on high-velocity oxy-fuel (HVOF) and plasma-sprayed AISI 316L coating systems [32,33]. Further investigations on the surface hardening of cold gas sprayed coatings confirmed the results [34,35]. By carburizing, nitriding, and nitrocarburizing at low process temperatures, the formation of an expanded austenitic phase could be achieved. This has led to a significant increase in wear resistance [32][33][34][35][36][37][38]. Investigations on plasma nitriding of austenitic layers produced by direct laser metal deposition and laser powder-bed fusion could also confirm the improvement potential [39,40]. First investigations on the thermochemical surface hardening of powder feedstock materials for coating processes were carried out [41]. In addition to the methods for interstitial surface hardening, various possibilities of precipitation hardening by nitriding were also considered [38]. In comparison to solid solution hardening, a different diffusion behavior was demonstrated. First investigations showed the suitability of a powder-pack boriding treatment for improving the wear resistance of thermally sprayed coating systems [42,43]. Laser cladding causes a higher thermal load on the substrate material compared to thermal spraying. Nevertheless, laser-clad coatings are characterized by a higher corrosion resistance due to their compact structure.
In the present study, the concept of a process combination consisting of a coating step by laser cladding and subsequent surface hardening by boriding was investigated using the alloy Inconel 718. The conducted feasibility study clearly showed the potential of such a process combination.

Materials and Methods
For the experimental procedure, a gas atomized Inconel 718 powder (Praxair NI-202-3) with a particle size of d90 = 45 µm and d10 = 16 µm was used. The coating was applied to a round blank of grade AISI 316L with a thickness of 6 mm and a diameter of 100 mm. Prior to the coating process, the blanks were cleaned with alcohol. The coating was performed with a high-speed laser cladding system, consisting of a TRUMPF BEO D70 Optic, which was equipped with the prototype of a TRUMPF 7-ray nozzle, and a TRUMPF TruDisk 6001 laser source. The powder was fed during the deposition process using a plate feeder. The feed rate, which results from the path velocity and the trace offset, was controlled by the axial rotation of a conventional industrial robot. The synchronization of both systems by the robot controller allowed for the implementation of a constant feed rate depending on the diameter during the entire coating process. The processing of the powder was carried out according to the parameters stated in Table 1. In order to remove the surface roughness, a grinding treatment and subsequent polishing were performed according to standard metallographic procedures. The thermochemical treatment by powder-pack boriding was carried out using a process routine developed for nickel alloys. In the present study, 50-100 nm B 4 C powders were chosen, since boronizing powder mixtures often allow for boronizing to be achieved in a shorter time for the same boronizing temperature compared to micron-sized powders [44]. The process parameters are shown in Table 2. The feedstock powder and the laser-clad coatings were subjected to X-ray fluorescence (XRF) analysis in a Fischerscope X-ray XAN device (Helmut Fischer, Sindelfingen, Germany) to determine the chemical composition in three comparative measurements. An acceleration voltage of 30 kV, a current of 1 mA, a Ni primary filter, and a collimator diameter of 0.6 mm were applied.
Metallographic cross-sections of the coating were prepared according to a metallographic standard routine. Etching was performed using a solution consisting of 50 mL HCl and 150 mL HNO 3 with an etching time of 15 s. The microstructure of the coating was investigated by scanning electron microscopy (SEM) using an LEO 1455VP (Zeiss, Oberkochen, Germany) microscope. The morphology, microstructure, and chemical composition were investigated by means of secondary and backscattered electron imaging.
A surface microhardness measurement was carried out by nanoindentation. For this, a Fischerscope HM 2000 XYp (Helmut Fischer GmbH, Sindelfingen, Germany) with a Vickers tip was used for progressive measurement with a load of 10 mN. Borosilicate glass BK7 was used as the calibration standard. At least ten single measurements were considered for the calculation of the average microhardness and the standard deviation.
Crystallographic studies were carried out by XRD measurements using a D8 DIS-COVER diffractometer from the Bruker Corporation (Billerica, MA, USA). Monochromatic Co-Kα X-ray radiation was utilized for all measurements. A voltage of 40 kV, a current of 40 mA, a sampling rate of 24 • h −1 , and a diffraction angle range of 20 • to 130 • was used. The X-ray sensor used was a LynxEye-XE detector, the aperture size after the beam gun was 2 mm, and a polycap was used to obtain a parallel beam emission. The data was evaluated using Rietveld refinement. Elemental analyses of the borided layers with depth resolution were performed by glow discharge spectroscopy (GDOS) with a GDA 750 (SpectrumaAnalytik GmbH, Hof, Germany). For the measurements, a 2.5 mm anode, operating under 800 V, and 25 mA under 3 hPa Ar pressure were used.
Furthermore, detailed studies to determine the influence on the tribological properties were conducted. The sliding wear behavior was investigated utilizing a ball-on-disk test, using the Tetra basalt tester (Tetra, Ilmenau, Germany). For the investigation of the wear behavior under reciprocating conditions, a Wazau SVT 40 device (Wazau, Berlin, Germany) was used. The wear test parameters are summarized in Table 3. The wear depth of the ball-on-disk test was determined by tactile measurements utilizing a Hommel-Etamic T8000 device (Jenoptik, Villingen-Schwenningen, Germany).
For the evaluation of the reciprocating wear tests, a Keyence VK-X200 laser scanning microscope (Keyence, Osaka, Japan) was used.

Results
The chemical composition was determined for the feedstock powder, as well as the coating in the as-clad state, the as-ground state, and both borided states by XRF spectrometry. Five measurements were conducted for each state. The normalized values of the main alloying elements are shown in Table 4. The results of the XRF measurement reveal a good correlation between the chemical composition of the Inconel 718 powder feedstock and the coating system. This consistency, which was already achieved in the as-clad state, confirms the process quality. The results could be confirmed in the polished condition. XRF measurements within GDOS calottes demonstrated an excellent agreement of the chemical composition to the polished state. This confirmed a homogeneous chemical composition without elemental gradations over almost the entire coating thickness. Consequently, mixing between the substrate and coating material, as well as the vaporization of constituents, was largely prevented. Due to diffusion enrichment with boron, the ratios of the alloying elements in the immediate surface area shifted in relation to each other. Nickel increased significantly with the increasing treatment time compared to chromium and iron. The average thickness of the coating was approximately 500 µm. After mechanical finishing, the thickness was reduced to about 230 µm. The transition zone was crack-free and ensured good metallurgical bonding. Limited intermixing appeared only within the immediate interfacial area.
After laser cladding, thermochemical boriding treatments were carried out. Figure 1a,b shows exemplary SEM images of the coating after boriding using backscattered electron imaging. Despite the deviation from the intended composition, a homogeneous precipitation layer formed for both states. The thickness of the boride layer after a treatment of 2 h is at least 22 µm. No cracks, pores, or other structural defects were visible. The smooth transition from the boride layer to the Inconel 718 cladding ensured strong adhesion between the two layers. Thermochemical diffusion coatings often provide better adhesion with the substrate compared to faster coating application techniques, such as thermal spray coating [45]. At an increased boriding duration of 8 h, an increase in diffusion depth to almost 65 µm was observed. Additionally, irregularities were detected on the surface, which increased with the duration of the boriding treatment. Three areas could be distinguished in the cross-sectional microstructures. The topmost double-phase boride layer displayed a homogeneous structure. The diffusion zone underneath the boride layer exhibited an irregular morphology and contained grain-boundary precipitates. The precipitates extended far along the grain boundaries until they reached the core of the Inconel 718 cladding, which was unaffected by the diffusion of the boron. In order to identify the elemental distribution within the boride layer and the diffusion zone, GDOS measurements were carried out. The results of the qualitative analyses are shown in Figure 2. Depending on boriding time, a constant plateau for boron intensity was observed, which is characteristic of the boride layer. Beyond a certain depth, the boron intensity decreased abruptly. This corresponded to the diffusion zone containing the precipitates as detected in the SEM. In the immediate surface, increased nickel intensities could be detected, especially in the case of the 8 h boriding treatment. This result is in line with the results of the XRF measurements. However, within the borided layer, the gradient between the main alloying elements diminished. Independent of the treatment time, a simultaneous but gradual increase in the relative intensities of the main alloying elements was observed, starting from a depth of about 20 µ m. The GDOS measurements complement the characterizations of the structure of the boride layer, which was also analyzed by SEM. The phase composition was determined by XRD measurements. The diffraction patterns of the coatings are shown in Figure 3 as a function of boriding time. In the as-clad state, the coating exhibited a single-phase microstructure consisting of the gamma (γ) phase with a face-centered cubic (fcc) crystal structure. Successful inert gas control in the process was confirmed by the absence of oxides on the surface. The results of the two borided samples confirmed the successful formation of a boride layer. BNi2 was detected In order to identify the elemental distribution within the boride layer and the diffusion zone, GDOS measurements were carried out. The results of the qualitative analyses are shown in Figure 2. Depending on boriding time, a constant plateau for boron intensity was observed, which is characteristic of the boride layer. Beyond a certain depth, the boron intensity decreased abruptly. This corresponded to the diffusion zone containing the precipitates as detected in the SEM. In the immediate surface, increased nickel intensities could be detected, especially in the case of the 8 h boriding treatment. This result is in line with the results of the XRF measurements. However, within the borided layer, the gradient between the main alloying elements diminished. Independent of the treatment time, a simultaneous but gradual increase in the relative intensities of the main alloying elements was observed, starting from a depth of about 20 µm. The GDOS measurements complement the characterizations of the structure of the boride layer, which was also analyzed by SEM. In order to identify the elemental distribution within the boride layer and the diffusion zone, GDOS measurements were carried out. The results of the qualitative analyses are shown in Figure 2. Depending on boriding time, a constant plateau for boron intensity was observed, which is characteristic of the boride layer. Beyond a certain depth, the boron intensity decreased abruptly. This corresponded to the diffusion zone containing the precipitates as detected in the SEM. In the immediate surface, increased nickel intensities could be detected, especially in the case of the 8 h boriding treatment. This result is in line with the results of the XRF measurements. However, within the borided layer, the gradient between the main alloying elements diminished. Independent of the treatment time, a simultaneous but gradual increase in the relative intensities of the main alloying elements was observed, starting from a depth of about 20 µ m. The GDOS measurements complement the characterizations of the structure of the boride layer, which was also analyzed by SEM. The phase composition was determined by XRD measurements. The diffraction patterns of the coatings are shown in Figure 3 as a function of boriding time. In the as-clad state, the coating exhibited a single-phase microstructure consisting of the gamma (γ) phase with a face-centered cubic (fcc) crystal structure. Successful inert gas control in the process was confirmed by the absence of oxides on the surface. The results of the two borided samples confirmed the successful formation of a boride layer. BNi2 was detected The phase composition was determined by XRD measurements. The diffraction patterns of the coatings are shown in Figure 3 as a function of boriding time. In the as-clad state, the coating exhibited a single-phase microstructure consisting of the gamma (γ) phase with a face-centered cubic (fcc) crystal structure. Successful inert gas control in the process was confirmed by the absence of oxides on the surface. The results of the two borided samples confirmed the successful formation of a boride layer. BNi 2 was detected as the main boride phase in both (2 h and 8 h) cases. FeB could be detected as an additional phase and was detectable at significantly lower intensities for both treatment durations. With increasing boriding time, the intensity of the BNi 2 phase increased significantly, which hinted at an increase in the boride layer thickness. The characteristic peaks of FeB occurred more intensely at higher angles for 8 h of treatment, indicating a high depth of detection.
as the main boride phase in both (2 h and 8 h) cases. FeB could be detected as an additional phase and was detectable at significantly lower intensities for both treatment durations. With increasing boriding time, the intensity of the BNi2 phase increased significantly, which hinted at an increase in the boride layer thickness. The characteristic peaks of FeB occurred more intensely at higher angles for 8 h of treatment, indicating a high depth of detection. A successful formation of a precipitation layer was possible by diffusion enrichment with boron. To investigate the influence of the treatment condition on the resulting properties, surface hardness measurements have been conducted. The results are summarized in Table 5. For both treatment regimes, the resulting hardness exceeded 1600 HV0.001 and was significantly higher than the reference hardness of Inconel 718 with 377 HV0.001. Although a higher average microhardness was determined for a longer duration of the thermochemical treatment, no clear trend can be derived due to the high standard deviation. The main reason for this was structural defects within the precipitation layer. The wear behavior was investigated using ball-on-disk and reciprocating ball-onplane tests. The results are summarized in Figure 4. Compared to the untreated condition, boriding significantly improved the wear resistance under both wear conditions. In the ball-on-disk test, the borided coating offered almost full protection under the test conditions used for sliding wear. Significant improvements could also be seen in the reciprocating wear test. The maximum wear depth of the borided coating was in the range of the thickness of the boride layer. A successful formation of a precipitation layer was possible by diffusion enrichment with boron. To investigate the influence of the treatment condition on the resulting properties, surface hardness measurements have been conducted. The results are summarized in Table 5. For both treatment regimes, the resulting hardness exceeded 1600 HV0.001 and was significantly higher than the reference hardness of Inconel 718 with 377 HV0.001. Although a higher average microhardness was determined for a longer duration of the thermochemical treatment, no clear trend can be derived due to the high standard deviation. The main reason for this was structural defects within the precipitation layer. The wear behavior was investigated using ball-on-disk and reciprocating ball-onplane tests. The results are summarized in Figure 4. Compared to the untreated condition, boriding significantly improved the wear resistance under both wear conditions. In the ball-on-disk test, the borided coating offered almost full protection under the test conditions used for sliding wear. Significant improvements could also be seen in the reciprocating wear test. The maximum wear depth of the borided coating was in the range of the thickness of the boride layer.

Conclusions
A novel process combination involving the surface hardening of laser-clad Inconel 718 coatings by powder-pack boriding was investigated. The microstructural characteristics, as well as the elemental distribution within the borided surface layers and the tribological properties under different wear conditions, were compared. Despite the single pass in the coating process, XRF investigations have shown that mixing with the substrate material could be largely prevented. The composition of the coating surface also showed only slight deviations from the target composition, indicating low oxidation. Diffusion enrichment with boron resulted in enrichment with nickel in the surface layer region. For both 2 h and 8 h tests, the formation of a homogeneous and double-phase boride layer could be detected. The nickel-rich BNi2 phase showed the highest intensity. With increasing treatment time, an increase in diffusion depth could be detected. A significant increase in microhardness could be achieved by the thermochemical treatment. Compared with the untreated condition, this resulted in a significant increase in wear resistance under ball-on-disk and reciprocating wear conditions. The results confirm the great potential of the presented process combination of laser cladding and surface hardening.

Conclusions
A novel process combination involving the surface hardening of laser-clad Inconel 718 coatings by powder-pack boriding was investigated. The microstructural characteristics, as well as the elemental distribution within the borided surface layers and the tribological properties under different wear conditions, were compared. Despite the single pass in the coating process, XRF investigations have shown that mixing with the substrate material could be largely prevented. The composition of the coating surface also showed only slight deviations from the target composition, indicating low oxidation. Diffusion enrichment with boron resulted in enrichment with nickel in the surface layer region. For both 2 h and 8 h tests, the formation of a homogeneous and double-phase boride layer could be detected. The nickel-rich BNi 2 phase showed the highest intensity. With increasing treatment time, an increase in diffusion depth could be detected. A significant increase in microhardness could be achieved by the thermochemical treatment. Compared with the untreated condition, this resulted in a significant increase in wear resistance under ball-on-disk and reciprocating wear conditions. The results confirm the great potential of the presented process combination of laser cladding and surface hardening.