Microstructural and Wear Characteristics of High Velocity Oxygen Fuel (HVOF) Sprayed NiCrBSi–SiC Composite Coating on SAE 1030 Steel

In this paper, wear properties of NiCrBSi–SiC coatings were investigated using the ball-on-disk wear test. In experimental study, NiCrBSi–SiC powders were sprayed using a high-velocity oxygen fuel technique on an SAE 1030 steel substrate. Powder mixtures with different weight mixing ratios, NiCrBSi + 10 wt% SiC, NiCrBSi + 20 wt% SiC and NiCrBSi + 40 wt% SiC coatings were prepared. The deposited coatings are compared in terms of their phase composition, microstructure and hardness. It is proved that the degree of mixing of the NiCrBSi and SiC components in the powder has a massive effect on the phase composition, microstructure and hardness of the coatings. Wear tests were conducted on both the uncoated and coated substrates at same normal load, speed, and wear distance. It has been determined that the coated substrates exhibit a very good tribological performance in comparison to the uncoated substrate. The increase in the adhesive wear resistance provided by the coating has been attributed to the presence of a large amount of dispersed Ni and Cr carbide and/or borides in the Ni matrix.


Introduction
High-velocity oxygen fuel (HVOF) is a coating deposition process whereby a powder coating material is heated rapidly in a hot gaseous medium. Simultaneously the powder material is then projected at a high particle velocity onto a prepared substrate surface where it builds up to produce the desired coating [1][2][3]. High-velocity oxygen fuel (HVOF) sprayed coatings have been used widely throughout the years of the last decade mainly in industrial applications, aerospace, and power plants, because the coatings express low porosity and oxide content, high hardness and high adhesion [4][5][6][7][8][9][10][11][12]. The main advantage of HVOF compared to other thermal spray techniques is the ability to accelerate the melted powder particles of the feedstock material at a relatively large velocity [13,14]. The HVOF thermal spraying process has shown to be one of the best methods for depositing conventional Ni-based and NiCr feedstock powders, because the hypersonic velocity of the flame shortens the time of interaction between the powder and the flame [15]. These effects in conjunction with the relatively low temperature (as compared to plasma-based techniques) result in less decomposition of the carbide particles during spraying.
Nickel-based alloys are applied extensively in a number of applications both because of their outstanding wear and corrosion resistance at high temperatures and their relative low cost [16][17][18]. They have high strength and hardness and good corrosion resistance due to the addition of chromium [19]. Boron brings down the melting temperature and helps in the formation of hard phases. Silicon is added to increase self-fluxing properties. Carbon produces carbides with high hardness levels that improve the wear resistance of coatings [20,21]. Considerable research studies were carried out to examine the HVOF coatings [22][23][24][25][26][27]. Wear and corrosion resistance of NiCrBSi coating deposited using AC-HVOF technique was investigated by Liu et al. [24]. They determined that the excellent wear properties of the Ni-based coating were tested in a dry sliding wear test.    resistance of the Ni-based coating was found higher than that of the stainless steel substrate material due to the passive film-forming effect of Cr. The hot corrosion behavior of NiCrBSi coatings deposited on Ni-and Fe-based superalloys was investigated by Sidhu et al. [25]. They showed that structure of the as sprayed NiCrBSi coating mainly consisted of γ -nickel solid solution containing small fraction of Cr 7 C 3 , Ni 3 B phases and NiCr 2 O 4 spinel oxides. However, they found that the hot corrosion resistance imparted by NiCrBSi coatings may be attributed to the formation of oxides of silicon, chromium, nickel and spinels of nickel and chromium in the molten salt environment at 900 • C. The influence of microstructural and mechanical properties and wear resistance of HVOF-sprayed WC-Co and WC-Ni coatings were examined by Berger et al. [26]. They found that the resistance to erosive wear was improved when cobalt was used as binder metal. The influence of spray parameters on the microstructure and mechanical properties of Colmonoy 88 alloy HVOF coating were studied by Sosa et al. [27]. They determined that the microstructure consists of increase in the unmelted particles volume fraction and the development of interlamellar microcracks as the spraying distance increases, leading to a decrease in the elastic modulus of the coatings. The the adhesion evaluation of different interlayers such as Co-Cr, Ni-Cr (80-20) HVOF (High Velocity Oxy-Fuel) thermally sprayed coatings and Ni-plating between the cermet-based WC-Co-Cr coatings were examined by Hadad et al. [28]. They indicated that the electrochemically deposited interlayer Ni-plating provides the highest adhesion to cermet coating within the multilayered structured coatings.
The aim of this work was to prepare Ni-based coatings by the HVOF technique and to study their microstructure and properties. The surface properties, such as microstructure and hardness of NiCrBSi-SiC coatings prepared by HVOF coating process have been investigated. In particular, the friction and wear behaviors for the NiCrBSi-SiC coatings have been analyzed by comparison with each other.

Experimental Procedures
A mixture of a NiCrBSi powder with an average grain size of 60 µm and SiC powder with an average grain size of 68 µm powder was used as spray material. XRD analysis results of the powders are described in Fig. 1a and b. From the morphology of the powder given in Fig. 2, NiCrBSi powders can be observed that the powder completely consists of spherical particles ( Fig. 2a) while SiC powder reveals the angular shape of the ceramic particles (Fig. 2c). The chemical composition of all the materials is summarized in Table 1.
The substrates were pre-cleaned in acetone for 5 min, and then blast-cleaned by 60 mesh aluminum oxides for 5 min to Oxygen gas pressure (bar) 7 Fuel gas (C 3 H 8 ) pressure (bar) 5 Compressed air pressure (bar) 5 Powder feed rate (g/ min) 55 Spray distance (mm) 180-200 Gun speed (mm/s) 3 Carrier gas (N 2 ) pressure (bar) 5   Turkey). Figure 3 shows the schematic diagram of the coating system. The spraying parameters optimized to produce the coatings on the steel substrate are summarised in Propane  Table 3. Coatings were then air-cooled. Samples prepared for metallography examinations after coating process were polished with conventional metallography processes after mechanical treatments. Samples polished were etched with ferric chloride etching agent (25 g FeCl 3 +25 ml HCl+100 ml H 2 O). For each coating, microstructure and phase were investigated using a scanning electron microscope (SEM) and X-ray diffraction (XRD). Surface hardness was measured using an Instron Wolpert tester with a load of 1.96 N and a loading time of 10 s, and porosity was determined by analyzing images photographed by an optical microscope.
The friction and wear behavior of the coatings were characterized by a linear reciprocating motion with CSM Tribometer at room temperature with a relative humidity of 25-30 % at the dry sliding conditions. Al 2 O 3 ball (diameter 3 mm) was used as the counter body. All tests were performed under a load of 3 N, a maximum linear speed of 2.5 cm/s, and 4 mm linear distance. Sliding distance was selected and fixed at 50 m. The coefficient of friction was recorded automatically during the tests (acquisition rate 10 Hz). After the wear test, the depth profile of wear trace was measured using a surface profilometer and then wear rates were calculated.

Results and Discussion
The SEM micrographs at the cross-section of HVOF-sprayed coatings are shown in Fig. 4. The thicknesses of the coatings were measured from the SEM micrographs, taken along the cross-section of the mounted samples. All coatings have a thickness of about 250 µm and bond well to the substrate (can be seen in Fig. 4a-c).
For the HVOF-sprayed nickel-based coating, the typical coating thicknesses are in the range of 250-300 µm as suggested by Sidhu [29]. All coatings were completely crackfree. Moreover, porosity value in the HVOF coatings is also low. The HVOF coating shows a very homogeneous microstructure and a porosity of less than 1 %. Adherence between substrate and coating seems to be good with a low presence of either cracks or voids in the interface. Porosity is completely eliminated after the fusion process. A low quantity of unmelted particles is shown in Fig. 4c. Figure 5 depicts the optical micrograph of the surface of the coatings. The coatings have a uniform microstructure ( Fig. 5a and b). Wang et al. [30] study stated that most NiCrBSi particles were completely melted under the present spray conditions. Therefore, the current test results confirm that HVOF coating deposited with the droplets melted sufficiently will not form effective adhesion to a smooth substrate surface. Some limited porosity is visible as dark contrast spots, but generally the coatings have dense structures (Fig. 5b and c). The uniform microstructure of the coatings indicates that a high proportion of the feedstock powder appeared to have melted prior to impact on the substrate. In Fig. 5c, for the coating-3, the results of EDS analysis of the light, grey coloured structure, which appears intensively at the particle borders, are given in Fig. 6.
According to the analysis results, the structure appearing in the sediment form on the particle borders caused solidification which contains predominantly C, Cr and Fe in small amounts. The fact that there is O element in small amounts in the structure indicates that the surface has a trend of oxidation. It has resulted from the XRD analysis in Fig. 7 that the possibility of Ni 3 Si 2 , Cr 3 Ni 2 Si and Cr 13 Ni 5 Si 2 phase presence in the structure, which is intensively seen in the particle borders and occurs in the sample labelled with coating-3, is high. Some dark areas that appeared in the coating structure or at the coating substrate interface may be the inclusions.
The SEM micrographs taken from the sample labelled with coating-2 in high resolution and EDS analysis are also given in Fig. 7. As can be seen from Fig. 7a, there exist structures based on Ni matrix material, which are randomly scattered and relatively darker grey coloured. This fact is similar in the coating-3 (Fig. 6c). In Fig. 7a, there are three different structures in SEM microstructure taken from the centre of coating-3 with higher magnification. Based on Ni matrix material, there are structures randomly scattered in, which are relatively dark grey and black. EDS analyses belonging to these structures are given in Fig. 7b, c and d. As the matrix material involves 19.08 %C, 15.04 %Si, 4.9 %P, 2.97 %Cr, 0.87 %Fe, 57.07 %Ni (Fig. 7b) in atomic scale, the atomic structure with dark grey colour come to occurrence of solidification with the composition given as follows; 4.04 %B, 44.13 %C, 2.19 %Si, 33.72 %Cr, 0.4 %Fe, 15.51 %Ni (Fig. 7c). According to XRD results given in Fig. 8, the possibility of the fact that this dark grey coloured structure is CrB 2 , Cr 3 Ni 2 Si and/or Cr 7 C 3 is high. The similar findings are achieved by Abdi and Labaili [31] and Planche et al. [32]. EDS analysis of the dark structure marked with number 3 in Fig. 7a is given in Fig. 7d. This structure exists in all three samples. The probability of being Cr 7 C 3 of this structure is rather high. In case of spraying onto the coating surface, the mixture of NiCrBSi and SiC coating powder decomposes due to high temperature, and then forms new phases by re-solidi-fying during the fusing to the base material. For the structure to be Cr 7 C 3 complies with the XRD results given in Fig. 8. Figure 8 shows the XRD patterns for the HVOF-sprayed NiCrBSi-SiC coatings and it is evident that the coatings mainly consist of γ -nickel-based face-centred cubic (fcc) structure as a principal phase. This situation is a feature common in all the nickel-based coated [24][25][26][27][28][29][30][31][32][33]. XRD patterns of the coatings have also revealed the presence of very low intensity peaks of Ni 4 B 3 , BNi 2 , BNi 3 , SiC and Cr 7 C 3 phases. Furthermore, very weak peaks, indexed as belonging to Cr 3 Ni 2 Si, Cr 13 Ni 5 Si 2 and CrB 2 are identified.
The SEM micrographs and elemental variations across the cross-section of HVOF-coated materials are shown in Fig. 9. EDAX analysis of coating-1 (Fig. 9a) reveals that the concentration of Ni, Cr and C decreases at point 1, whereas at points 2, 3 and 4, the amounts of Cr are relatively more. Quantity of Ni only increased at point 2, but this element is at its minimal value at the point 4. EDAX analysis of coating-2 ( Fig. 9b) shows that the Ni and C elements are almost constant at point 1-5. Although quantity of Cr is higher at point 3, the concentration of Cr is the lowest at point 5. As Fig. 9c reveals, for EDAX analysis results of coating-3, the weight percentage of Si, C and O increases at point 5 where Ni decreases substantially signifying that dark contrast phase at the top of scale might be rich with oxides of Cr, C and Si. The existence of higher oxygen at point 5 might lead to the formation of Cr-and Si-rich oxide scale. The dark, black phase present at the upper point of the coating (point 5) is rich with Si and C unmelted SiC particles. It is believed that SiC grains might be retained due to inadequate time to find for the deposition and re-solidification of molten or semi-molten droplets. Both XRD analysis results (can be seen in Fig. 8) and EDAX analysis results (can be seen in Figs. 7 and 9) reveal that the major phase of NiCrBSi-SiC coating is austenite structure rich with Ni, Cr, C and Si.
The microhardness data of the coatings are shown in Fig. 10a, which shows the microhardness profiles along the cross-section of the coatings as a function of distance from the coating-substrate interface (Fig. 10b). The microhardness of the substrate is in the range 210-230 Hv. The microhardness of the coatings is found to be variable with the distance from the coating-substrate interface. This significant variation in the microhardness along the thickness of the coatings might be due to the distribution of the SiC hard phase in Nibased alloy matrix. Maximum value of about 831 Hv was obtained from the coating-3, while sample 1 showed minimum values of about 474 Hv. Microhardness for coating-2 is 750 Hv. Further, an increase in microhardness of all the substrates was observed near the coating-substrate interface.
The average coefficient of friction and wear rate values of all samples coated with NiCrBSi + SiC via the HVOF method were determined to be smaller than that of the original material (Fig. 11). Especially for the coating-3 sample to which highest amount of SiC powder was added it has decreased to half its original value. Increased hardness and grain refining of particles due to increased SiC powder quantity has affected this result. As a result of the experiments conducted under the same conditions the coefficient of friction and wear rate of low carbon SAE 1030 steel was determined to be at their highest levels. Abrasive wear is observed clearly in the EDS analysis of worn surface (Fig. 12a). Also the weak iron oxides that are formed and break up during the experiment thus increasing wear rate. Significant decreases have been obtained for the coefficients of friction and wear rate due to the Ni containing oxides (Fig. 12b and c) that form on the surface of coating-1 and -2. In coating-3 sample, adhesive (oxidative) wear has been observed due to the lubricating effect of strong oxides (Fig. 12d) that absorb the carbon which has increased due to the dissolution of SiC powder.
Tribochemical reaction between water vapor and SiC controls the tribological behaviour in these coatings. In coating-1 and -2, forms of microcracks networks that are similar to "mud cracks" can be seen. These cracks decrease from coating-1 to coating-3. These cracks can cause increase of the coefficient of friction. This result is in good agreement with Stachowiak [34].

Conclusions
1. Under the employed spray conditions, NiCrBSi and SiC mixed powders have been deposited by HVOF process to develop coatings of average 250 µm thick on SAE 1030 steel substrates. Microhardness of the coatings is found in the range 550-830 Hv, which is higher than that of the substrate material. 2. The microstructure of the as-sprayed coating has a nickel-based fcc structure as the principal phase. XRD  analysis of the coating microstructures revealed the presence of low intensity peaks of Ni 4 B 3 , BNi 2 , BNi 3 , SiC and Cr 7 C 3 phases. Furthermore, very weak peaks, indexed as belonging to Cr 3 Ni 2 Si, Cr 13 Ni 5 Si 2 and CrB 2 are identified. 3. Coefficents of friction and wear rate of all HVOF coating-applied samples were lower than that of the SAE 1030 steel. The increase in the amount of SiC powder positively influenced the coefficent of friction and the wear rate of the samples.