Laser Cladding and Anti-Laser Ablative Behaviour of Si-SiC-MoSi 2 -ZrB 2 Composite Coatings on Carbon Surface

A Si-SiC-MoSi 2 -ZrB 2 antioxidation composite coating was fabricated by laser cladding technology on the graphite matrix surface, and laser ablation was carried out on the coating surface to evaluate the anti-ablation capability of the coating surface. The phase transformation, microstructure reconstruction and the change of binding strength between the coating and the matrix during the laser cladding and ablation were investigated. The coating of Si-SiC-MoSi 2 series modi�ed by 40wt% ZrB 2 had the lowest weight loss rate and the highest coating bonding strength. The oxide mostly existed in amorphous form after laser cladding, and the semi-melting particles in laser path lap zone on the coating surface disappeared. The oxide layer peeled off and Mo-Zr-Si-O composite glass phase was formed on the coating surface. The internal microstructure of the coating co-existed with non-oriented �ne needle-like dendrites and irregular particles, which improved the coating strength. After laser ablation, the coating modi�ed by 40wt% ZrB 2 had lower weight loss rate and higher scratch bonding strength compared with the unmodi�ed coating of Si-SiC-MoSi 2 series, which indicated that the ablation resistance of the coating was improved by the formation of continuous oxide layer on the surface.


Introduction
Carbon materials are ideal high temperature materials in aerospace and other industrial elds because of their excellent material properties such as high melting point, low density, with high strength and high modulus, high heat stability, low thermal expansion coe cient, stable friction coe cient, high thermal conductivity, heat resistant corrosion resistance and other excellent performances. Even at high temperature (2200℃) under inert atmosphere, it still remain good mechanical properties comparable to room temperature [1][2][3] , thus in the aerospace, nuclear, chemical, and other elds it has a broad application prospect [4][5][6] . However, in the air atmosphere, the untreated carbon materials oxidized at 450℃, and the rate increased rapidly with the increase of temperature [7][8] .
In general, matrix modi cation technology and coating technology are mainly used to improve the hightemperature antioxidant performance of C materials, so that it can continue to work effectively under high temperature oxygen atmosphere. Among them, coating technology is more practical because it doesn't need to sacri ce the excellent performance of the material, and the produced material has high usage temperature, long service life, and can be designed for a variety of composite coatings according to the actual needs [9][10] . The coating of silica-based materials has excellent antioxidant performance under high temperature environment. Among them, Si [11][12] , MoSi 2 [13][14] , SiC [15] , as common raw material of silicabased antioxidant coating, can be used for the antioxidant coating on the surface of C materials. In addition, as an ultra-high temperature ceramic, ZrB 2 has excellent high-temperature strength stability, antioxidant performance and chemical erosion resistance, and shows excellent ablative performance in C materials [16][17][18] . However, the traditional preparation method of antioxidant coating has some disadvantages such as long production cycle, insu cient binding force and easy formation of pores. As a new coating preparation technology, laser cladding has many advantages, such as compact structure, good metallurgical combination between cladding material and substrate material, controllable morphology of cladding material and small dilution degree of substrate coating.
In this work, the anti-oxidation coating of Si-SiC-MoSi 2 -ZrB 2 was prepared by laser cladding technology, and the anti-laser ablation performance was characterized by laser ablation method. The phase and microstructure of the coating after laser cladding were analyzed by means of quality change, XRD, SEM and EDS.

Material And Methods
The graphite samples with dimensions of 10×10×1mm were used as substrate in this work. It should be sanded until the surface is bright. Then cleaned it with deionized water and placed in an oven at 60℃ for 30 minutes. After that, roughened it with sandblasting machine. After sandblasting, the surface residual particles were washed with anhydrous ethanol and deionized water by ultrasonic cleaning, and then dried in a 60℃ oven for 30min.
The prefabricated powder used in this work were prepared from Si, SiC, MoSi 2 and ZrB 2 powders. The raw materials were weighted according to a certain mass ratio and then mixed by TJGN high-energy planetary ball mill with 600RPM for 2h. After the end, rinsed and poured into the beaker with alcohol, and then putted in a 60℃ oven to dry for 10-12h.
Weighed 3.28g of deionized water and 0.72g of raw powder placed in a 10mL beaker, added 300μL of alcohol and stirred with a magnetic stirrer for 20min. Then the beaker was placed in an ultrasonic cleaner for ultrasonic oscillation for 10min to disperse agglomerated powder particles. Then, placed the graphite samples in a rotator, used a pipette to draw 100μL solution from the 10mL beaker onto the surface of the samples and rotate the coating. The samples were then removed and placed in a petri dish to dry naturally. Finally, the anti-oxidation coating of Si-SiC-MoSi 2 -ZrB 2 was obtained through laser cladding using a CO 2 laser (JQ1390), which was also used for laser ablation.
The weight loss of samples was measured by an analytical balance (Shanghai Youke Instrument Co., LTD., FB224). The coating adhesion was characterized by the scratch tester (Lanzhou Zhongke Kaihua Technology Development Co., LTD., WS-2005). The composition of the powders was measured by X-ray diffraction(XRD, BrukerD8-ADVANCE, German). The morphology of the powders was examined by tungsten lament scanning electron microscope(SEM, Hitachi High-tech SU3500, Japan).

Results And Discussion
3.1 Quality change of the coating and contrast of scratch adhesion via laser cladding As shown in Fig.1, after ZrB 2 coating is modi ed, the weight loss rate slightly increases after laser cladding. When the content of ZrB 2 is 40wt%, the coating has a small weight loss rate, which is still a little higher than that of Si-SiC-MoSi 2 coating. The mass loss is partially due to the self-volatilization of Si and oxidation products, such as MoO 3 and B 2 O 3 when the temperature exceeds 1155℃. In addition, the air in the hole of the coatings expanded when being heated by laser and eventually burst, which caused some powders taken away from the coating' s surface during laser cladding process. The scratch adhesion of coating after coating modi ed by 40 wt% ZrB 2 was increased about 10.87% compared with that before, because ZrB 2 particles changed the structure of ternary coating to form disordered dendrites and particle mixture structures, which improved the adhesion of coating. However, the ZrB 2 on the surface would react with oxygen to form volatile B 2 O 3 phase, which makes it unable to ll the holes and cracks in the coating.
Also, the high temperature during the laser cladding process would increase the oxidation and volatilization of Si and MoSi 2 phases, which introduced some defects on the coating, such as holes and cracks. This resulted in the decrease of coating adhesion, which is also the reason why the scratch adhesion of the coating coated with 20wt% ZrB 2 is low.

Microstructure and phase composition of laser cladding coating
As shown in g.2, with the increase of ZrB 2 phase content, the intensity of diffraction peak increases continuously, while the intensity of other phases decreases. After laser cladding, the coating of Si-SiC-MoSi 2 series modi ed by 40wt% ZrB 2 has obvious amorphous diffraction peak, because the surface oxidized Si-O compound exists in the form of amorphous glass layer due to rapid cooling. The oxide layer will prevent oxygen from diffusing into the coating and improve the coating's antioxidant performance.
As one can clearly see in Fig.3(a), the holes on the surface of the coating are the defects left by mass transfer during the solidi cation process of the molten pool, but there are no obvious cracks. The semimelting particles disappear in the lap zone of laser path, and the number of holes decreases, and the surface atness of the coating is better. Fig.3(b) shows the surface of the uncoated oxide layer, which can be seen that the layered oxide layer is the surface layer exfoliated during the process of gas escape.
As shown in Fig.3(c), there are signs of the overall peeling of the loose surface oxide layer. After measurement, the thickness of the peeling layer is about 7μm. The thickness of the oxide layer is limited due to the decrease of oxygen diffusion rate by itself. The whole stripping is mainly caused by the obstruction of gas escaping during laser cladding in this area. Fig.3(d) shows the dense structure after the surface oxide layer is exfoliated. The improvement of surface density and atness and the reduction of the number of holes can improve the adhesion of coating scratches. Combined with EDS results, it can be seen that the stripping layer is mainly loose Si-O compounds, and the content of Mo and Zr elements is extremely low. This is due to the rapid rise and fall of temperature in the laser cladding process, the oxidation reaction between Si and SiC forms Si-O compound, and MoO 3 phase has a low boiling point and is easy to volatilize.
As shown in Fig.4, the coating after laser cladding includes surface Si-O glass layer, laser cladding coating and carbon substrate, the average thickness of laser cladding coating is about 31.5 μm. After laser cladding, the coating showed obvious mixed state of unoriented dendrites and irregular particles, and three different regions could be seen in Fig.4(b). Region I was the mixing region of unoriented dendrites and irregular particles, while region II was mainly the mixing region of irregular particles, and region III was mainly the mixing region of dendrites and a few particles. The characteristics of the region III can be seen more clearly in Fig.4(c), the dendrites are cross distributed and have no obvious orientation. The results of EDS showed that the distribution of Mo and Si was almost overlapped, while the distribution of Zr was characterized by local enrichment and granular dispersion. O was mainly enriched on the surface of the coating, which was caused by the rapid oxidation of the cladding material during laser cladding. The surface glass layer hindered the further diffusion of O element into the coating, thus the antioxidant capacity of the coating could be improved.
Combined with the grain distribution and element distribution, it could be determined that MoSi 2 phase was needle like dendrite after laser cladding, while ZrB 2 phase was granular dispersed in the coating. This was because a large number of ZrB 2 phase existed as particles in the molten pool, leading to heterogeneous nucleation of MoSi 2 phase attached to ZrB 2 particles in the solidi cation process, which greatly improved nucleation rate and re ne grains. At the same time, due to the in uence of temperature gradient and composition of coating, MoSi 2 phase growed in a dendrite manner. However, due to the difference of laser heat ow, surface tension and viscous force in the molten pool, the ZrB 2 particles moved irregularly and destroyed the dendrite oriented growth state. Moreover, the existence of particles would also restrict the growth of dendrites. Finally, most of MoSi 2 growed into ne non-oriented dendrites.
The toughness of the coating could be improved by the non-oriented growth of ne dendrites and the dispersion of ZrB 2 as the second phase.

Quality change of the coating and contrast of scratch adhesion via laser ablation
It can be seen from Fig.5 that the weight loss rate of the coating modi ed by 20wt% ZrB 2 increased after laser ablation, but the coating modi ed by 40wt%ZrB 2 has the lowest weight loss rate. The main reason is that ZrB 2 is oxidized during laser ablation, and the continuous formation and volatilization of B 2 O 3 phase would aggravate the peeling off of the surface glass layer. Therefore, the mass loss of low content ZrB 2 coating would increase. However, the ZrO 2 phase produced at the same time has a high melting point, which can weaken the disturbance of laser heat ow on the coating surface and slow down the coating weight loss. Moreover, the oxygen partial pressure of ZrO 2 phase is low, and the ZrO 2 phase produced by oxidation will disperse on the coating surface to protect the coating. In addition, the surface of 40wt% ZrB 2 modi ed coating after laser cladding was relatively at and the number of holes was less, which would also have a positive effect on the laser ablation performance.
In the other hand, the scratch adhesion force of 20wt% ZrB 2 phase modi ed coating decreased compared with that of unmodi ed coating, while scratch adhesion force of 40wt% ZrB 2 phase modi ed coating was increased. This indicated that the addition of 40wt% ZrB 2 signi cantly improved the laser ablation resistance of the coating, while the 20wt% ZrB 2 addition did not improve the laser ablation resistance of the coating, mainly because there are a lot of cracks and oxide layer peeling off from the coating surface in the process of continuous laser ablation. The cracks weakened the coating strength and reduced the coating adhesion, while the continuous peeling of the oxide layer aggravated the continuous oxidation of the coating and had a negative impact on the adhesion of the coating. When the content of ZrB 2 phase was increased to 40wt%, ZrB 2 phase was oxidized in the process of continuous laser ablation to form continuous ZrO 2 phase with high melting point and low oxygen pressure, which protected the coating from continuous oxidation. With the laser ablation time increasing, coating on the surface effectively inhibited the peeling off of the oxide layer and the internal microstructure reconstructs to form a "Semimelting ice structure" of Zr-Mo-Si-O from the oxidation products of the coating, which further improved the laser ablation resistance of the coating.

Microstructure and phase composition of coating after laser ablation
As shown in g.6, the crystal phase of the coating changed and a small amount of MoB phase formed after laser ablation. The diffraction peak intensity of Si, SiC, MoSi 2 and ZrB 2 phases in the cladding layer is signi cantly reduced, while the intensity of monoclinic ZrO 2 phase is signi cantly enhanced. It could be seen that obvious oxidation reaction, as shown in (1) -(5) [19][20][21] , would occur after laser ablation. ZrB 2 reacted with O 2 to generate ZrO 2 and B 2 O 3 , and oxidation of SiC and Si occurred as well with O 2 to form corresponding oxides.
In addition, in the early stage of laser ablation, when the temperature was between 400-800 ℃, a "pesting" reaction of MoSi 2 began to take place and oxidation products of MoO 3 and SiO 2 were formed [22] . The MoO 3 phase produced by this reaction has poor mechanical properties, and was easy to volatilize and form holes, which affected the coating performance. However, as shown in formula (4), Because of the oxide volatilization and continuous peeling of the surface coating during laser ablation, the oxide tends to form amorphous coating in the process of rapid temperature rise and fall. Therefore, in the low angle range, the peak intensity of amorphous steamed bread before laser ablation is greater than that after laser ablation. In addition, the original powder can still be detected in the XRD of the coating after laser ablation, which indicates that the coating has a certain anti laser ablation ability. 2ZrB 2 (s)+5O 2 (g)=2ZrO 2 (s)+2B 2 O 3 (s) Si(s)+O 2 (g)=SiO 2 (s) (2) 2SiC(s)+3O 2 (g)=2SiO 2 (s)+2CO(g) 4MoSi 2 (s)+5O 2 (g)+2B 2 O 3 (s)=4MoB(s)+8SiO 2 (s) (4) 2MoSi 2 (s)+7O 2 (g) =2MoO 3 (s)+4SiO 2 (s) Si(s)+ SiO 2 (s)=2SiO(g) MoO 3 (s)= MoO 3 (g) It can be seen from Fig. 7 (a) that the surface of the coating is smooth and dense even after 10 times laser ablation, which is the result of surface remelting in the process of continuous laser ablation and continuous reconstruction of the internal structure of the coating. This remelting and reconstruction process of the coating also healed the existing holes and cracks, which indicated the good resistance to laser ablation and oxidation. In Fig.7(b), there are two forms of light gray phase covering the surface layer and the phase partially covering the surface layer. In Fig.7(c), there are obvious cracks, but the crack opening presents zigzag structure. The change of crack shape requires greater crack growth formation energy. In addition, there is bifurcation and re nement phenomenon in the front of the crack. In addition, the cracks in Fig.7(d) are surrounded by "ice melting" Zr-Mo-Si-O compound, which indicates that the microstructure can hinder the crack growth.
Combined with the EDS results, it is known that the content of Mo on the surface of the material is very low. This is due to the continuous volatilization of volatile oxides such as MoO 3 during laser ablation.
The results show that the whole gray coverage area of the material surface is Zr-O compound, which helps to improve the laser ablation resistance of the coating.

Conclusion
In this work, the coating of Si-SiC-MoSi 2 series modi ed by ZrB 2 were prepared on the graphite matrix surface by laser cladding technology, and laser ablation was carried out on the coating surface. The coating modi ed by 40 wt% ZrB 2 had the lowest weight loss rate and the highest coating binding strength after laser cladding, which is better than that of unmodi ed Si:SiC:MoSi 2 coating. The oxide layer peeled off and Mo-Zr-Si-O composite glass phase was formed on the coating surface, and the internal microstructure of the coating co-existed with non-oriented ne needle-like dendrites and irregular particles, which improved the coating strength. After laser ablation, the weight loss rate of 20 wt% ZrB 2 modi ed Si-modi ed by 40 wt% ZrB 2 had lower weight loss rate and higher scratch bonding strength compared with the unmodi ed coating of Si-SiC-MoSi 2 series, which indicated that the ablation resistance of the coating was improved by the formation of continuous oxide layer on the surface. Figure 1 The weight change ratio and scratching failure load of the Si-SiC-MoSi2-ZrB2 coating modi ed by different contents of ZrB2 after laser cladding The weight change ratio and scratching failure load of the Si:SiC:MoSi2=20:16:64wt% coating modi ed by ZrB2 with different content after laser ablation Figure 6 The XRD of the Si:SiC:MoSi2=20:16:64wt% coating modi ed by 40wt% ZrB2 after laser ablation