Microstructural characteristics of SiC-B₄C reinforced laser alloying composite coatings

1 Introduction

Titanium and its alloys are extensively used in the aeronautical, marine and chemical industries owing to their specific properties, such as high strength, excellent corrosion, oxidation and high-temperature resistance. Nevertheless, the application of titanium alloys under severe wear and friction conditions is highly restricted because of their poor tribological properties, such as high friction coefficient and low hardness [1, 2]. Laser alloying is a promising technique for improving the wear, fatigue and corrosion resistance of machined titanium alloys [3].

SiC is an advanced ceramic that has been developed recently, which demonstrates good performance characteristics such as corrosion and wear resistance [4, 5]. Al-Sn alloys are widely used for sliding bearing applications owing to their good load-carrying capacity, fatigue resistance and wear resistance. Homogeneous and dispersed distribution of fine Sn, which is the lubrication phase in the Al matrix, is beneficial to friction and wear behavior [6]. Addition of Y₂O₃ was beneficial in refining the microstructure and also reducing the internal stress of the laser alloying coating, thus preventing the formation of microcracks. In this study, the SiC-B₄C reinforced alloying coating was selected, which exhibited an excellent wear resistance. Through repeated experiments, it was found that Mo plays an important role in refining the microstructure, leading to an improvement of the wear resistance of the laser alloying coating. This paper will discuss the microstructural characteristics of the SiC-B₄C reinforced laser alloying composite coating.

2 Experimental

A 5-kW continuous CO₂ laser TJ-HL-T5000 laser equipment (Wuhan Unity Laser Limited Company, Wuhan, China) was used to prepare the alloying coating on the substrate. In this study, Ti-3Al-2V samples (10 mm×10 mm×35 mm or 10 mm×10 mm×10 mm) were used, and the thickness of the preplaced layer was 0.7 mm. The alloying surfaces were ground with emery paper to remove oxide scale and rinsed with alcohol before laser alloying. Alloy powders consisting of Al (≥99.5% purity, 20–150 μm), Sn (≥99.5% purity, 50–150 μm), B₄C (≥98.5% purity, 50–150 μm), Mo (≥99.5% purity, 50–200 μm), SiC (≥99.5% purity, 50–100 μm) and Y₂O₃ (≥99.5% purity, 50–100 μm) were used for the laser alloying, and water glass was used as a binder to form a layer. The process parameters of laser alloying are as follows: laser power P=1 kW, scanning velocity V=3–7.5 mm/s and laser beam diameter D=4.5 mm. During the laser alloying process, surface oxidation was prevented by argon with a flow rate of 40 l/min. An overlap of 35% between successive tracks was selected. The composition (in wt%) of the preplaced powders used in this experiment was 7SiC-20B₄C-56Al-6Sn-10Mo-1Y₂O₃.

The wear resistance of the composite coating was tested with an SFT-2M disc wear tester (Beijing Western lofty Technology Limited Company, Beijing, China). An HV-1000 microsclerometer (Shanghai Zhongyan Instrument Manufacturing Plant, Shanghai, China) was used to test the microhardness of the composite coating. The microstructural morphologies of such coating were analyzed by JSM-6500F scanning electron microscopy (SEM) (Jeol, Osaka, Japan) using an S-520 SEM and by energy-dispersive spectroscopy (EDS).

3 Results and analysis

3.1 Microstructure analysis

As shown in Figure 1A, metallurgical bonding between the composite coating and substrate was obtained. The block/stick-shaped precipitates were produced in the bottom coating. In fact, during the laser alloying process, most of the B₄C decomposed into B and C, which further reacted with Ti in the molten pool, leading to the formation of TiC and titanium borides. Also, the bottom coating was nearest to the substrate, so the Ti density was largest in this location, favoring the production of stick-shaped TiB precipitates [7]. In addition, during the alloying process, Al and Si were able to react with oxygen in the air, leading to the formation of Al₂O₃ and SiO₂. With the addition of Y₂O₃, SiO₂-Al₂O₃-Y₂O₃ glassy phases were produced in the alloying coating [8]. Mo is an active element, and thus during the laser alloying process, Mo reacted with a portion of the SiC and Ti-Al in the molten pool, leading to the formation of Mo₅(Si, Al, Ti)₃C [9]. Mo addition was able to decrease the glassy phase content, which was beneficial in decreasing the brittleness, and also refined the microstructure of the alloying coating, leading to an improvement of the wear resistance [10]. It is noted that fine block-shaped Mo₅(Si, Al, Ti)₃C precipitates are produced in the boundary of the crystals (see Figure 1B). As a result of the fine grain strengthening, the wear resistance and microhardness of the coating increase significantly.

Figure 1

SEM micrographs of the laser alloying coating: (A) the bonding zone and (B) the alloying zone.

As shown in Figure 2A, most of the fine particles are produced in the middle coating. According to the EDS spectrum (see Figure 2B), mainly Al, Si, Ti, Mo and Sn are found in the test location, and a few C and O are in existence; such result indicated that the Al-Sn, Ti-Al intermetallics, Mo₅(Si, Al, Ti)₃C phases and a few of the oxides are produced in such coating. In fact, during the alloying process, when the temperature exceeded a certain value, a portion of Sn and Al melted as a result of the eutectic reaction and then smeared out along the boundaries of the matrix. During the eutectic reaction in the cooling process, the Al and Sn phases crystallized separately; the Al phase nucleated and grew from the existing Al matrix and then left the Sn phase along the boundaries of the matrix [11].

Figure 2

SEM micrographs of the laser alloying coating: (A) tested location and (B) its corresponding EDS spectrum.

As shown in Figure 3A, many of the nanoparticles were produced in the coating matrix; these showed high microhardness and were able to hinder the formation of adhesion patches and deep plowing grooves during the sliding wear process. Moreover, when the laser power increased to 1.2 kW, Mo₅(Si, Al, Ti)₃C absorbed more energy from the laser beam and exhibited fine stick-shaped morphologies (see Figure 3B). It should also be mentioned that a series of amorphous alloys with high glass-forming ability in Si- or Y-based alloy systems were obtained in such coating [12, 13]. The production of the amorphous phases was also ascribed to the sufficiently rapid heating and cooling rates of the laser alloying technique.

Figure 3

SEM micrographs of the laser alloying coating: (A) nanoparticles, and (B) Mo₅(Si, Al, Ti)₃C.

3.2 Microhardness and wear resistance

Under the action of the laser alloying composite coating, the microhardness distribution of the substrate surface is in the range of 1250–1350 HV_0.2, which is approximately five times higher than that of the substrate (approximately 260 HV_0.2). It is considered that an enhancement of wear resistance of such coating was mainly ascribed to the fine grain, amorphous and hard phase strengthening. When laser power increased from 1 to 1.2 kW, the crystals absorbed more energy from the laser beam, giving them more time to grow and leading to the formation of a coarse microstructure, so the microhardness of the alloying coating decreased (see Figure 4). Thus, in order to guarantee the quality of the alloying coating, the laser power should be controlled strictly.

Figure 4

Microhardness distributions of the coating with different laser power.

When the load was 54 N, after 50 min the wear tester revealed that the wear volume loss of titanium alloy substrate was seven times higher than that of the alloying coating (see Figure 5A). The high wear resistance is primarily attributed to the high microhardness of the alloying coating, which plays an important role in improving the wear resistance of the substrate surface. However, the characteristics of alloying coating in which its phase constituent showed excellent tribological properties and the fine grain strengthening of the titanium borides and Y₂O₃, etc. Furthermore, because of the pinning effect of the precipitates, the counterpart should overcome the hindrance of these fine and dense precipitates during the wear process [14].

Figure 5

Wear volume losses of the composite coating and the substrate (A) and worn surface of the alloying coating (B).

SEM images show the worn surface of the alloying coating. Owing to the high microhardness of the alloying coating, the hard asperities on the surface of the counterpart were hard to penetrate into, which led to an improvement of wear resistance and prevented the formation of adhesion patches and deep plowing grooves (see Figure 5B). Furthermore, under the dry-sliding wear test, the moderate-growth dispersal precipitates, such as titanium borides and Mo₅(Si, Ti, Al)₃C, may withstand the external normal load better [15]. As mentioned previously, under the action of Mo, the fine microstructure of the alloying coating is obtained, which also shows excellent properties of plasticity and toughness, leading to the formation of a smooth wear surface. Moreover, the addition of Y₂O₃ was able to improve the strength and ductility of the alloying coatings [16, 17]. Thus, under the action of the wear plate, deep peeled holes and microcracks are not observed in the worn surface of such coating.

4 Conclusion

The correct choice of laser alloying parameters provides the SiC-B₄C+Al-Sn-Mo-Y₂O₃ laser alloying composite coating on Ti-3Al-2V titanium alloy with microhardness distribution in the range of 1250–1350 HV_0.2, which is approximately five times higher than that of the substrate (about 260 HV_0.2). Mo-Y₂O₃ plays an important role in refining the microstructure of the alloying coating. Fine Mo₅(Si, Al, Ti)₃C precipitates, which show fine microstructures, are produced in the coating matrix, leading to an improvement of the wear resistance. Wear volume loss of the laser alloying coating is approximately 1/7 of a Ti-3Al-2V alloy substrate due to the actions of hard and amorphous phases and fine grain strengthening. Al-Sn alloys are also produced, which improve the wear resistance of such coating.