Physical properties and microstructures of a BN-NiCoCrAlY laser amorphous-nanocrystal reinforced composite coating

1 Introduction

Among recent advances in the formation of advanced, excellent performance materials is the concept of amorphous and nanocrystalline metals. The laser cladding technique has provided important opportunities in the realization of amorphous-nanocrystalline coatings on metals [1, 2]. Amorphous alloys are multi-component and approximate deep-eutectic alloys that offer extraordinarily high hardness, elastic limit, and corrosion resistance, and the production of nano-materials can further refine the microstructures of laser clad coatings, and also reduce the tendency of cracking [3, 4]. Superhard materials can be divided into two categories: intrinsic, such as BN or diamond, and extrinsic, whose mechanical properties are determined by an artificially created microstructure [5]. Current researchers mainly focus on the development of new Zr-based or Fe-based bulk metallic glasses. However, little effort has been devoted to the effect of the NiCoCrAlY alloy system on the formation mechanism of amorphous and nanocrystals in laser clad coatings.

Experimental results indicated that laser cladding of NiCoCrAlY-BN mixed powders on an important aeronautical material TA15 titanium alloy can form an amorphous-nanocrystal reinforced coating. During the cladding process, a large number of elements, such as Zr, Mo, and Si, entered into the molten pool from the substrate due to the dilution effect, which changed the microstructure performance of the laser clad coating, and a series of amorphous alloys with high glass-forming ability in Si-, Zr-, or Y-based alloy systems were produced in such a coating [6–8]. In this study, the physical properties and formation mechanism of the NiCoCrAlY-BN laser amorphous-nanocrystal reinforced coating were investigated in detail. This research provided an essential experimental and theoretical basis to promote the application of the laser cladding technique in the aerospace field.

2 Materials and methods

Chemical compositions of the TA15 alloy were as follows (wt%): 6.06Al, 2.08Mo, 1.32V, 1.86Zr, 0.09Fe, 0.08Si, 0.05C, 0.07O, and balance Ti. The materials used in this experiment – the TA15 samples, size: 10 mm×10 mm×10 mm – were abraded with abrasive paper prior to the coating operation. The pre-placed powders of NiCoCrAlY (≥99.5% purity, 50–150 μm) and BN (≥99.5% purity, 150–250 μm) were used for laser cladding. The thickness of the pre-placed coating was approximately 0.7 mm. A cross-flow CO₂ laser was employed to melt the surface of the samples. During the cladding process, argon gas at a pressure of 0.4 MPa was fed through a nozzle which was coaxial with the laser beam. The process parameters of laser alloying were as follows: laser power p=1.1 kW, scanning velocity V=3–7.5 mm/s, and the laser beam diameter D=4 mm. An overlap of 35% between successive tracks was selected. The composition (wt%) of the pre-placed powders used in this experiment was 80NiCoCrAlY-20BN.

Metallographic samples were prepared using standard mechanical polishing procedures and then etched in a solution of HF, HNO₃, and H₂O in a volume ratio of 1:1:5 to reveal the growth morphologies of the compounds in such a laser clad coating. The microstructural morphologies of the coating were analyzed by means of a QUANTA200 scanning electron microscope (SEM) (Fei, Amsterdam, Netherlands) and a JEM-2100 high-resolution transmission electron microscope (HRTEM) (Japan Electron Optics Laboratory Co. Ltd, Osaka, Japan). The phase constituent was determined by X-ray diffraction (XRD) using D/MAX-RC equipment (Rigaku Corporation, Tokyo, Japan).

3 Results and analysis

As shown in Figure 1A, a metallurgical combination was obtained between such a composite coating and a TA15 substrate, and a large amount of fine block-/stick-shape precipitates were produced in the bottom coating. As shown in Figure 1B, boride stick-shape precipitates were present in such a coating, increasing the micro-hardness of the coating. In fact, due to the dilution effect, a large quantity of Ti entered into the molten pool from the substrate, which was favorable to the formation of TiB stick-shape precipitates; moreover, due to the sufficiently rapid heating and cooling character of a laser cladding technique, a number of TiB showed one-dimensional nanostructures; TiB₂ refined the boundaries of the crystals and usually gathered at the grain boundaries of the laser clad coatings, which exhibited as a small cluster [9]. It was also observed that nano-size particles were produced in the coating matrix (see Figure 1C). During the cladding process, N reacted with Ti in the molten pool, leading to the formation of TiN. It was known that as laser cladding is a rapid heating and cooling technique, a number of TiN ceramics did not have enough time to grow up, forming fine block-shape morphologies (see Figure 1D).

Figure 1

SEM micrographs of the composite coating: (A) bonding zone, (B) the borides, (C) nano-size particles, and (D) the fine TiN precipitates.

Figure 2A shows that nano-size particles were produced in such a composite coating. In fact, when the elements with short atomic radii, such as B and N, were added in the laser clad coating, they were able to lodge in the interval of the Bernal structure, leading to the enhancement of the irregular stacking of the alloy liquid phase structure and the complexity of the topological structure. Thus, this atomic (B and N) packing model distortion in the short program range led the solid/liquid interfacial energy to increase and also decrease the atomic diffusion capacity, which promoted the production of amorphous/nanocrystalline phases [10].

Figure 2

SEM micrograph of nanocrystals (A); HRTEM morphologies of the laser clad coating: TiB₂ (B), TiN and Ti₃Al (C), and the amorphous zone (D).

The HRTEM image indicated that fine granular-like precipitates were produced in the selected location, which corresponded to the (101) crystal of TiB₂ (see Figure 2B). TiN and Ti₃Al high-resolution lattices were also produced, which all corresponded to their (200) crystal plane (see Figure 2C). As shown in Figures 2C and D, the irregular amorphous spots were present in the diffraction pattern of the composite coating, which proved the existence of amorphous phases. The fact is that laser cladding is one of the surface amorphization technologies due to the sufficiently rapid heating and cooling that inhibits long-range diffusion and avoids crystallization [11, 12]; in addition, it should be mentioned that the Y₂O₃ addition was also beneficial in producing the amorphous phases, which suppressed the crystallization of the materials to a certain extent [13].

The X-ray diffraction pattern result indicated that the laser clad coating mainly consisted of γ-Co/Ni, Ti₃Al, TiN, TiB₂, and TiB (see Figure 3). Due to the dilution effect of the TA15 substrate on the laser molten pool, a large quantity of Ti entered into the pool from the substrate; a Ti-rich condition can be obtained in some location, favoring the formation of Ti₃Al and TiB [14]. The broad diffraction peaks appeared at 2θ=34–43° proved the existence of amorphous phases. During the laser cladding process, a large number of Zr and Si entered into the molten pool from the substrate due to the dilution effect, and a large number of Ni, Co, Y, and B were also included in the pre-placed powders. The fact is that there were a series of amorphous alloys with high glass-forming ability in Zr-, Y-, Co-, Ni-, and Si-based alloy systems. Thus, after the solidification process, a number of amorphous phases were produced in such a coating; on the other hand, the glass-forming alloy compositions were close to eutectic, which implies a (relatively) low melting point, i.e., the production of eutectics, such as Ti-Co and Ti-Si, in such a coating also greatly promoted the formation of amorphous phases [8].

Figure 3

XRD diagram of the NiCoCrAlY-BN laser clad coating.

Figures 4A and B show the test location and its corresponded electron diffraction pattern, respectively. Figure 4B shows three small spots in the direction of the white line in the selected area electron diffraction (SAED) pattern that revealed that thin rods were produced in such a coating; the amorphous ring and the incomplete diffraction spots in the TiB₂ polycrystalline ring revealed that a portion of the amorphous phases just began to crystallize when the laser molten pool had completed the solidification process [15].

Figure 4

TEM selected location (A) and its electron diffraction pattern (B); SEM micrograph of octagon-shape precipitate (C) and the differential thermal analysis (DNA) curve of such a laser clad coating (D).

It was interesting to note that an octagon-shape precipitate was produced in such a coating (see Figure 4C). In fact, most of TiB₂ exhibited the fine acicular morphology due to the preferential growth of TiB₂ along the C axis ({0001} direction) during the rapid cooling process of laser cladding [16]. It was considered that hard particles, such as TiN, had become the nucleation point during the forming process of such octagon-shape precipitates, which promoted TiB₂ to grow in the different direction.

The differential thermal analysis test result of the laser clad coating indicated that three exothermic peaks existed (see Figure 4D). It was noted that two weak exothermic peaks (peaks 1 and 2) were present near 550°C, which corresponded to the crystalline peaks of amorphous phases. An exothermic broad peak (peak 3) was present near 200°C, which corresponded to the structural relaxation. It was noticed that the two exothermic peaks near 550°C were weak and broad, indicating that the amorphous content was low, and its crystallization process was influenced by the nanocrystals. The mixture of the nanocrystals and amorphous phases broadened the exothermic peak. The nucleation and growth of the amorphous phases were retarded by the nanocrystalline phases to a certain extent during the crystallization process of the amorphous phases, and the nanocrystalline phases were stable compared with the amorphous phases [17]. Under the hinder of the nanocrystalline phases, the amorphous phases in such a coating were not free to grow up during the crystallization process, which steered clear of these nanocrystalline phases. Thus, more energy was needed, which broadened the exothermic peaks.

4 Conclusions

In summary, laser cladding of NiCoCrAlY-BN mixed powders on a TA15 alloy substrate can form an amorphous-nanocrystal reinforced composite coating. Such a coating mainly consists of Zr-, Si- and Y-based amorphous phases and crystalline phases, such as γ-Co/Ni, Ti₃Al, TiN, TiB₂ and TiB. Due to the sufficiently rapid heating and cooling character of the laser cladding technique, a lot of the nanocrystals were produced in such a coating. The formation of eutectics greatly promoted the formation of amorphous phases. The elements with short atomic radii, such as B and N, were able to lodge in the interval of the Bernal structure, which also promoted the production of amorphous/nanocrystalline phases, and a portion of the amorphous phases just began to crystallize when the laser molten pool had completed the solidification process. The nucleation and growth of the amorphous phases were retarded by the nanocrystalline phases to a certain extent during the crystallization process.