The ZrO₂ Formation in ZrB₂/SiC Composite Irradiated by Laser

Abstract

In order to clearly understand the details of ZrO₂ formation during ablation, high intensity continuous laser was chosen to irradiate ZrB₂/SiC. The results reveal that there are two different modes of ZrO₂ formation depending on whether liquid SiO₂ is present. When liquid SiO₂ is present, ZrO₂ generated by the oxidation of ZrB₂ is firstly dissolved into SiO₂. Then, ZrO₂ will precipitate again, the temperature will decrease and the SiO₂ will evaporate. Otherwise, the ZrB₂ will be oxidized to ZrO₂ directly.

1. Introduction

Ultra high temperature ceramics (UHTCs) refer to a class of refractory materials with melting temperatures in excess of 3000 °C, such as diborides and carbides of transition metals [1,2]. As a member of UHTCs, zirconium diboride (ZrB₂) has been widely attractive for decades. Because of its excellent properties, such as high melting temperature, high thermal conduction, excellent mechanical properties, etc., ZrB₂ can be operated as leading edges in hypersonic vehicles.

According to previous research, ZrB₂/SiC exhibited excellent ablation resistance against oxyacetylene torch, arc jet or plasma arc [3,4,5]. No obvious mass loss or macroscopic damage appears. Some products, like SiO₂ and ZrO₂, can be detected ona microscopic scale, and have been proved to be helpful to improve ZrB₂/SiC’s ablation resistance [6,7]. However, the upper surface of the sample is wholly heated by the above ablation method, and we could only obtain the information of phase and microstructure until the ablated sample cools completely. Longer cooling time contributes to ablation transformation. So some important characterized information has been concealed or disappeared during the cooling process. Researchers only observed that the ablated layer of ZrB₂/SiC was composed by ZrO₂ skeleton and liquid SiO₂ [8,9]. The real process and details of the formation of oxidation product, especially ZrO₂, are still unknown.

If the information of phase and microstructure under high temperature can be preserved by fast cooling, the real transformation will be easily observed at room temperature. Because of thistheory, the high intensity laser ablation method with rapid heating rate was utilized. The local temperature of ZrB₂/SiC at the spot center can reach thousands of degrees instantaneously at the beginning of laser irradiating. When laser loading stops, the sample can cool down rapidly to room temperature (which is similar to quenching), since the heating area is very small and the emissivity and conductivity of ZrB₂/SiC are very high [10,11].

In this paper, ZrB₂/20 vol % SiC composite (donated as ZS hereafter) was prepared and irradiated by high intensity continuous laser. The phase and microstructure evolution of ZrB₂/SiC was investigated.

2. Results and Discussion

Figure 1 shows the polished surface microstructure of the as-sintered material. ZrB₂ with grain size of about 5 μm is present as a grey matrix, while the dark SiC homogeneously distributes in the matrix.

Figure 1. The microstructure of as-sintered ZrB₂/SiC composite.

Figure 1. The microstructure of as-sintered ZrB₂/SiC composite.

The X-ray diffraction (XRD) result of the ZrB₂/SiC surface after laser ablation for 20 s is shown in Figure 2. According to the XRD, m-ZrO₂ and ZrB₂ are the major phases on the surface. It reveals that some ZrB₂ are oxidized into ZrO₂ while parts of ZrB₂ are not oxidized at all. No peaks of SiO₂ can be observed on the spectrum, because SiO₂ is amorphous on the surface. However, small amounts of ZrSiO₄ aredetected, as shown in the spectrum. This proves that SiO₂ forms, and that parts of ZrO₂ dissolve into SiO₂ to form ZrSiO₄.

Figure 2. X-ray diffraction (XRD) of the sample surface afterablation for 20 s.

Figure 2. X-ray diffraction (XRD) of the sample surface afterablation for 20 s.

The surface macrostructure of ZrB₂/SiC after laser ablation for 20 s is shown in Figure 3. Since the local heating of the laser induces extremely high temperature at the spot center, a great temperature gradient is generated along the radial direction, shown bythe arrow in Figure 3. Significant differences in ablation behavior between area 1 and area 2 were detected.

Figure 3. The surface macrostructure of the sample after ablation for 20 s.

Figure 3. The surface macrostructure of the sample after ablation for 20 s.

The surface morphology of area 1 after laser ablation is shown in Figure 4. From Figure 4a, the surface performs a porous structure. The energy dispersive spectroscopy (EDS) result reveals that only ZrO₂ exists on the surface; neither ZrB₂ nor SiC can be detected. A typical flushing morphology is clearly shown as Figure 4b. It should be attributed to the liquid splashing and the following rapid solidification. This means that the oxidized ZrO₂ in this region has been totally melted during laser ablation. Meanwhile, SiC is totally decomposed because the temperature here is higher than its decomposition point of 2300 °C.

Figure 4. The microscopic morphology of area 1 (a) with a typical flushing morphology (b).

Figure 4. The microscopic morphology of area 1 (a) with a typical flushing morphology (b).

The different microstructure in area 2 is shown in Figure 5. According to Figure 5a, lots of white or dark grey particles implant in amorphous substance. Based on the EDS result as shown in Table 1, the particles are ZrO₂ while the amorphous substance is SiO₂. This also can be confirmed by the XRD result shown in Figure 2. The region shown in Figure 5b is farther fromthe spot center than the region shown in Figure 5c. As can be seen in Figure 5b, very fine sub-micron ZrO₂ distributes evenly in amorphous SiO₂. With the temperature increases from left to right in Figure 5b, amorphous SiO₂ reduces gradually, while the quantity and size of ZrO₂ particles increase gradually. In Figure 5c, amorphous SiO₂ is much lower, even almost disappearing at the right side, which results from the evaporation of SiO₂. Moreover, ZrO₂ grains close to the spot center grow gradually, and can even be seen to sinter obviously.

Table 1. The energy dispersive spectroscopy (EDS) results of area B and C in Figure 2.

Elements	Area B		Area C
	Weight Ratio %	Atomic Ratio %	Weight Ratio %	Atomic Ratio %
O K	36.32	67.85	26.52	66.30
Si K	15.32	16.31	1.51	2.15
Zr L	48.36	15.85	71.97	31.55

Table 1. The energy dispersive spectroscopy (EDS) results of area B and C in Figure 2.

Elements	Area B		Area C
	Weight Ratio %	Atomic Ratio %	Weight Ratio %	Atomic Ratio %
O K	36.32	67.85	26.52	66.30
Si K	15.32	16.31	1.51	2.15
Zr L	48.36	15.85	71.97	31.55

Figure 5. The microscopic morphology of area 2 (a) and the further amplified morphology of region B (b) and C (c).

Figure 5. The microscopic morphology of area 2 (a) and the further amplified morphology of region B (b) and C (c).

According to the equilibrium phase diagram of ZrO₂-SiO₂ shown as Figure 6 [12], the melting temperature (2700 °C) of ZrO₂ can be decreased by liquid SiO₂, while ZrO₂ has high solubility, about 43.3% in SiO₂ at 2235 °C. With increasing temperature, the solubility of ZrO₂ will increase gradually. When laser begins to irradiate on the material, the surface temperature soars up rapidly. A large number of ZrB₂ are oxidized into ZrO₂ between 600–700 °C. Meanwhile, SiC transforms into SiO₂ at 1200 °C, and SiO₂ exhibits as a liquid above 1400 °C. When the temperature approaches 1687 °C, the oxidized ZrO₂ begins to dissolve into SiO₂. SiO₂ is unstable when the temperature is higher than 1800 °C [13]. The evaporation of SiO₂ at high temperature makes the ZrO₂ solubility decrease, so the extremely fine ZrO₂ grains precipitate. With the evaporation proceeding, the solubility of ZrO₂ dissolved into SiO₂ significantly decreases. A lot of ZrO₂ will precipitate from liquid SiO₂ to form the morphology shown in Figure 5b. As precipitated ZrO₂ is still subject to intense high temperature, fine ZrO₂ grains with strong activity gradually grow up and sinter together for a short time to form the morphology as shown in Figure 5c. Therefore, the processing of ZrO₂ dissolution and precipitation is the main transformation mechanism during ablation when liquid SiO₂ is present.

Figure 6. Thermal equilibrium phase diagram of SiO₂-ZrO₂.

Figure 6. Thermal equilibrium phase diagram of SiO₂-ZrO₂.

The surface microstructure of ZrB₂/SiC at the edge is shown in Figure 7. The temperature is lower than 1200 °C in this region away from the spot center. SiC oxidation and liquid SiO₂ are not observed on the surface. The morphology of ZrB₂ has changed because of oxidation. A lot of nano-crystals closely pack on the surface. This is proved to be ZrO₂ according to the EDS result. This indicates that ZrB₂ is directly oxidized into nano-ZrO₂. The results reveal that there is another formation mechanism of ZrO₂ during ablation when liquid SiO₂ is absent.

Figure 7. The morphology of ZrB₂/SiC at the edge after ablation (a) with nano ZrO₂ crystals packing on the surface (b).

Figure 7. The morphology of ZrB₂/SiC at the edge after ablation (a) with nano ZrO₂ crystals packing on the surface (b).

3. Experimental Section

The ZS composite was prepared using commercial powders listed as follows: ZrB₂ (Alfa Aesar Ward Hill, MA, USA, 99.0%, particlesize ~50 μm); SiC (Alfa Aesar, 99.5%, particlesize 1–2 μm). The raw powders were accurately weighted by volume ratioof 4:1 and milled for 6 h in ethanol using zirconia milling media. The mixture powders were sintered by spark plasma sintering technology (SPS, DR. SINTER type 3.20, Fuji Electronic Industrial Co. Ltd., Kanagawa, Japan) at 1750 °C for 5 min with a rate of 200 °C/min. An axial pressure of 50 MPa was applied during the whole process.

The laser irradiation experiment in this paper was carried out by using ytterbium laser system (YLS-2000) (IPG Photonics Co. Ltd., Pittsfiels, MA, USA), with 1070 nm wave-length in atmospheric environment. The spot size of the Gaussianlaser was set as about 10 mm. The power density at the spot center reached 20 MW/m² in Gaussian laser, and the duration time was 20 s. The dimensions of the ablation specimen were Φ25 mm × 3 mm.

The phase of the sample surface after ablation was detected by X-ray diffraction (XRD, X’pert PRO MPD, PANalytical B.V. Co. Ltd., Amsterdam, The Netherlands, Cu Kα). The surface microstructures of the sample before and afterablation were examined by scanning electron microscope (SEM, Philips S-4800, Hitachi Ltd., Yokohama, Japan). The composition of the sample was identified by energy dispersive spectroscopy (EDS, Oxford Instruments Co. Ltd., Oxfordshire, UK).

4. Conclusions

The ZrO₂ formation in ZrB₂/SiC composite during ablation was investigated in this paper. A characterized phenomenon at elevated temperature is preserved by using high intensity continuous laser with the rapid heating rate to irradiate. Two mechanisms of the ZrO₂ formation are directly obtained, depending on whether liquid SiO₂ is present. When liquid SiO₂ is present, oxidized ZrO₂ firstly dissolves into SiO₂. Fine ZrO₂ grains precipitate with the evaporation of liquid SiO₂, and then grow, and even sinter together. Otherwise, the ZrB₂ will be oxidized into ZrO₂ directly.