Optimization of 3D inlay mode of “Z-pins like” V 0.9 -Si 0.1 rods and their improvement effect on the anti-ablative performance of C/C-ZrC-SiC

: Neoteric “Z-pins like” vanadium 0.9 -silicon 0.1 rods (V 0.9 -Si 0.1 rods) as polybasic multiphase oxide compensators were prepared to improve the anti-ablation of C/C-ZrC-SiC over 2500 °C. The microstructure and improvement effect on the anti-ablation of different C/C-ZrC-SiC surface were investigated. Results show that the density of “Z-pins like” V 0.9 -Si 0.1 rod was effectively improved after adding Si. When ablation time was less than 180 s, “Z-pins like” V 0.9 -Si 0.1 rods perpendicular to non-woven cloth layer presented the best improvement of the anti-ablation performance by liquid/gaseous multicomponent oxide compensation. However, when ablation time was greater than 180 s, “Z-pins like” V 0.9 -Si 0.1 rods perpendicular to non-woven cloth layer increased another harm that weaken the anti-ablative performance of C/C-ZrC-SiC, namely, the corrosion damage of oxide layer on the matrix surface caused by excessive oxide melt. Therefore, there is a best improvement of the anti-ablation performance of “Z-pins like” V 0.9 -Si 0.1 rods parallel to non-woven cloth layer. Synthesis of large areas of highly oriented, very long silicon


Ablation experiment
The ablation resistance of two "Z-pins like" V0.9-Si0.1 rod-reinforced C/C-ZrC-SiC composites were tested by oxy-acetylene ablation test for 180 s and 240 s (according to the GJB323A-96 standard [31,32]). The pressures and fluxes of acetylene and oxygen were 0.095 MPa and 0.696 L/s and 0.4 MPa and 1.960 L/s respectively. During the test, an infrared thermometer (error of ±0.75%, Raytek MR1SCSF) was indicated that the highest temperature of C/C-ZrC-SiC and "Z-pins like" V0.9-Si0.1 rod-reinforced C/C-ZrC-SiC composites were 2650 ± 10 ℃ and 2455 ± 10 ℃ at the distance of 22 mm between the torch nozzle and the sample surfaces, respectively. The mass ablation rates and linear ablation rate of two samples were Where Rm is the mass ablation rate, Δm is the mass change of the sample. RL1 and RL2 are the linear ablation rate of C/C-ZrC-SiC and "Z-pins like" rods in the sample, which are measured separately. ΔL1 and ΔL2 are the thickness change of "Z-pins like" rods and matrix respectively, and t is the ablation time.

Characterization
The phase compositions of ZCC-1 and ZCC-2 before and after ablation were measured by D/mas 2550vb+18KW rotating target X-ray diffraction analyzer (XRD, Rigaku Co.). Their interface between the "Z-pins like" rod and C/C-ZrC-SiC substrate, surface microstructures of ZCC-1 and ZCC-2 before and after ablation were characterized by scanning electron microscopy (SEM, Fei Na Pro X) with energy dispersion spectroscopy (EDS). And the elements distribution of the "Z-pins like" V0.9-Si0.1 rod after sintering were analyzed by electron probe microanalysis (EPMA, JEOL CO., Jxa8230).

Results and discussion
3.1 Different surface ablating microstructure of C/C-ZrC-SiC inspired optimization for the 3D inlay mode of "Z-pins like" V0.9-Si0.1 rods Due to the porosity of non-woven cloth layers are much lower than that of short-cut web layers, the content of ceramic of the former is lower than that of the latter. Therefore, when ablation direction is perpendicular or parallel to non-woven cloth layer, the phase distribution on the ablative surface is greatly different.
As Fig. 2(a) shown, when ablation direction is perpendicular to non-woven cloth layer (Sample CC-1), ceramic distribution of ablative surface is relatively continuous ( Fig. 2(a)A), but bare carbon fiber tows still expose on there ( Fig. 2(a)B). However, when ablation direction is parallel to non-woven cloth layer (Sample CC-2), the bare carbon fiber-rich non-woven cloth layer and ceramic-rich short-cut web layer on the ablative surface are arranged with each other, and the continuity of ceramic distribution is relatively low (Fig. 2(d)). After ablating at 2654 ℃ for 180 s, although ZrO2 layer formed in the ablative center of sample CC-1 is relatively continuous, which is full of holes and cracks with different sizes (Fig. 2(b)). And the ablative edge area of the exposed carbon fiber tows are ablated seriously (Fig. 2(c)). However, due to a large number of regularly exposed non-woven cloth layer on the ablative surface of sample 2, many regularly arranged grooves are formed in the ablative central area and the edge area. Finally, the oxide layer on the entire sample CC-2 surface is porous and discontinuous. In order to increase the density of oxide layer at all areas and enhance the ablation resistance of C/C-ZrC-SiC, a novel binary "Z-pins like" V0.9-Si0.1 rods with liquid/gaseous multicomponent oxide compensation effect were designed and fabricated on C/C-ZrC-SiC surface. For these rods of a particular component, the morphology and phase composition of the oxide layer on the matrix surface will directly affect the spreading rate of the compensating melt, and then effect the ablation resistance improvement of the composites. The affecting mechanism of the oxide density of matrix on the spreading behavior of compensating oxide melt is explained using the models illustrated in Fig. 3. The spreading of compensating oxide melt is divided into two stages: the first one is the spreading of the matrix surface around the structure (Stage Ⅰ in Fig. 3), and the second is the spreading away from the structure (Stage Ⅱ in Fig. 3).
When "Z-pins like" V0.9-Si0.1 rods are perpendicular to non-woven cloth layer ( Fig. 1(d1)), the density and continuity of ZrO2-rich layer on the substrate surface ( Fig.   2(b)) are higher than that in the parallel direction ( Fig. 1(d2) and Fig. 2(e)). The flow resistance of the compensating oxide melt of the first is lower than that of the latter.
In the second stage, when "Z-pins like" V0.9-Si0.1 rods are perpendicular to non-woven cloth layer, the compensating oxide melt still flows on the continuous matrix oxide layer (Stage Ⅱ in Fig. 3(a)). And the continuous ablation pits formed by the oxidation of exposed carbon fibers in the short-cut web layer are conducive to the flow and aggregation of oxide melt. However, when "Z-pins like" V0.9-Si0.1 rods are parallel to non-woven cloth layer, the regular grooves formed by the oxidation of the short-cut web layer act as an obstacle to the oxide melt flow in the second stage (Stage Ⅱ in Fig. 3(b)).
Therefore, when "Z-pins like" V0.9-Si0.1 rods are perpendicular to the non-woven cloth layer ( Fig. 1(d1)) of the the ablation surface or parallel ( Fig. 1(d2)), the spreading effect of compensating oxide melt and the improvement effect of the structure on the ablation resistance of the composite all need test and contrastively analyse, and then optimize the three-dimensional embedding mode of "Z-pins like" V0.9-Si0.1 rods. The sintered "Z-pins like" V0.9-Si0.1 rods were taken out from C/C-ZrC-SiC and detected by XRD. Results show that "Z-pins like" V0.9-Si0.1 rod is chiefly composed of V5Si3, VC and V2C (Fig. 4)). The reason for the generation of ZrO2 is that original V particles are oxidized during the filling process of vanadium powder and the crushing process of "Z-pins like" rod (Eq. (4)). And then, abundant C is provided by decomposition of forming agent or matrix, which reduces VO to form V and O2 during sintering (Eq. (5)). V particles have high affinity to the C and Si, and form VC under 800-1000 ℃ and V5Si3 under 1160-1260 ℃ respectively [33][34][35]. Finally, V particles are carbonized (Eqs. (6) and (7)) and siliconized (Eqs. (8) and (9)) , and O2 reacts with ZrC to form ZrO2 (Eq. (10)). As Fig. 5(a) shown, the density of "Z-pins like" V0.9-Si0.1 rod is significantly improved comparing with "Z-pins like" V rod [17]. The holes and gaps between each V particle are filled with Si melt generated during sintering. Wherein, the white phase is VC, the gray phase is V2C, and the gray phase is V5Si3 ( Fig. 5(b)). "Z-pins like" V0.9-Si0.1 rod is closely bonded with the ceramic region and the carbon fiber tow region of the matrix (Fig. 5(c)). (c) The interface between "Z-pins like" V 0.9 -Si 0.1 rod and ZrC-SiC ceramics and carbon fibers. It is attributed to that V particles preferentially react with C to form VC and V2C, and then to react with the liquid phase Si to form V5Si3 and V3Si. 3.3 Ablation property and macro-profiles of "Z-pins like" V0.9-Si0.1 rods reinforced C/C-ZrC-SiC composites.
As Fig. 6 shown, When "Z-pins like" V0.9-Si0.1 is perpendicular to the non-woven cloth layers ( Fig. 7(a) and (b)), the spreading area of the oxide formed by "Z-pins like" V0.9-Si0.1 rods towards the matrix is much larger than that of the horizontal action on non-woven cloth layers (Fig. 7(c) and (d)). Moreover, when the ablation time is extended to 240 s, ZCC-1 begins to be eroded by excess oxide melt ( Fig. 7(b)). According to the spread area of oxidation products of "Z-pins like" V0.9-Si0.1 rod and the color depth of surface oxide layer, the surface ablative morphology of each sample is divided into three areas, namely, the ablation center, the transition and the edge.  As XRD patterns of two samples after ablation for 180 s and 240 s shown However, when "Z-pins like" V0.9-Si0.1 rods parallel to non-woven cloth layer, the diffraction peaks intensity of quartz SiO2 is stronger than that on the ZCC-1 surface, and other phases all basically remain the same.

Ablative microstructure of ZCC-1
In order to explore the evolution of the surface ablation microstructure of ZCC-1 after ablating for 180 s in accordance with the ablating center, transition, inner edge and outer edge shown in Fig. 7(a) is detected. The yellow arrow in Fig. 7(a) represents the representation sequence. Microstructure of all regions marked with capital letters in Fig. 7(a) are selected to detect surface and interface of "Z-pins like" V0.9-Si0.1 rod (Areas A, C, E and G in Fig. 7(a)), as well as C/C-ZrC-SiC matrix surface far away from "Z-pins like" V0.9-Si0.1 rod (Area B, D, F and H in Fig. 7(a)). In addition, the ablative center and transition of ZCC-1 ablated for 240 s (Fig. 7(b)) are characterized and analyzed. composite layer (Fig. 9(c)). This is because V5Si3 from "Z-pins like" V0.9-Si0.1 rod preferentially oxidizes, and generates a large amount of SiO2 to reduce the central ablation temperature by its evaporation. Meanwhile, the consumption of SiO2 is much lower than its production. Finally, C/C-ZrC-SiC surface still retain a handful of SiO2.
However, due to a large number of oxide melts producing, ZrO2 skeleton on the matrix surface will be washed. By amplifying the agglomerated ZrO2 sphere, it can be clarified that the compensating vanadium oxide generated during the high temperature ablation process shows radial flow after bypassing the agglomerated ZrO2 sphere ( Fig.   9(b)). Meanwhile, the interface between "Z-pins like" V0.9-Si0.1 rod and C/C-ZrC-SiC generates thermal stress Crack. As the transitional ablating SEM picture shown in Fig. 10(a), SiO2 is also residual on the surface of "Z-pins like" V0.9-Si0.1 rod, and its content is higher than that in the center. In addition, some tetragonal ZrO2 particles are found as the impact of V element ( Fig. 10(a)) [36]. However, no Zr element is found in "Z-pins like" V0.9-Si0.1 rod by EPMA (Fig. (5)). Therefore, the main reason for the appearance of ZrO2 on "Z-pins like" V0.9-Si0.1 rod surface is that the compensating oxide melt carries ZrO2 particles in the flow process. It is worth mentioning that extensive SiO2 film appear on the substrate surface that closes to or far from "Z-pins like" V0.9-Si0.1 rod. There are three typical topographical features appearing on the substrate surface near "Z-pins like" V0.9-Si0.1 rod ( Fig. 10(b)). The first one is agglomerated ZrO2 spheres ( Fig. 10(c)) which are surrounded by V2O5. The second is a mixed region of SiO2 and ZrO2 (Fig. 10(d)B), where SiO2 is uniformly filled between ZrO2 particles.
And the last one is a thicker SiO2 layer completely covering on ZrO2 skeleton ( Fig.   10(d)C).
On the matrix surface far from "Z-pins like" V0.9-Si0.1 rod, there is a homogeneous dense SiO2-V2O5 composite film attaching some agglomerated ZrO2 spheres ( Fig. 10(e)) and remaining unfilled holes. All reunited ZrO2 spheres are tightly bonded to a SiO2-V2O5-ZrO2 oxides layer. And the ZrO2 skeleton is always looming at the bottom of SiO2 film ( Fig. 10(f)). These fully demonstrate that the oxides act compensating oxide in this region are both SiO2 and V2O3 during ablation.
After the compensating of two glassy oxide melts, the protective oxide layer on substrate surface is more dense.
In conclusion, abundant SiO2 remains in the transition region, which sufficiently prove that the temperature in this region is lower than ablation center. Moreover, SiO2 melt from "Z-pins like" V0.9-Si0.1 rods can adequately play the oxide compensation effect, so as to ensure the densification of the oxide layer at the mid-temperature. In the ablation edge, due to the completely different morphology of oxide film, it can be divided into two distinct areas to analyze. They are the inner edge region nearing the ablation transition ( Fig. 11(a-c)) and the outer edge away from ( Fig.   11(d-f)), respectively. The main reason for the two topographical features is the different effect of gaseous SiO2 compensation.
In the inner edge area, the oxide layer on "Z-pins like" V0.9-Si0.1 rod surface is composed of gray V2O5 and dark gray SiO2 (Fig. 11(a)). SiO2 is largely enriched in the marginal area of "Z-pins like" V0.9-Si0.1 rod, and only a small amount of SiO2 spreading to C/C-ZrC-SiC matrix surface. This fully indicates that SiO2 preferentially melts and acts the oxide compensating effect. C/C-ZrC-SiC far away from "Z-pins like" V0.9-Si0.1 rod is covered with a dense cauliflower-shape SiO2 molten layer decorated with V2O5 ( Fig. 11(b)).
In the outer edge area, C/C-ZrC-SiC surface near or away from "Z-pins like" V0.9-Si0.1 rod are all covered with a dense SiO2 layer (Fig. 11(c) and (e)). No oxide melt from "Z-pins like" V0.9-Si0.1 rod spreads to substrate (Fig. 11(c)), which certifies that the protective SiO2 layer generates from gaseous SiO2 and SiO by VLS mechanism and OAG model [27]. The exposed carbon fiber bundles are substantially free of damage, and their surface are all covered with a protective SiO2 layer which distributes many SiO2 nanoparticles (Fig. 11(d)). The holes between carbon fibers are also sealed (Fig. 11(d)). Simultaneously, there is a pyknotic oxide layer deposited by SiO2 and SiO vapor on the substrate surface away from "Z-pins like" V0.9-Si0.1 rod.
Plentiful SiO2 nanoparticles distributed on the SiO2 film and some are almost integrated with SiO2 layer (Fig. 11(e)). These phenomenons fully clarify the formation process of SiO2 film in the ablated edge, which undergoes three processes of nucleation of SiO2 nanoparticles, growth of SiO2 nanoparticles, and aggregation of spherical SiO2 into a dense film [37][38][39]. These can also effectively prove that when Si elements are added to "Z-pins like" rods, sufficient SiO2 and SiO vapor can be provided to the ablative edge area of during the ablation process, so as to effectively avoid oxidative damage of exposed carbon fibers. After ablating for 240 s, the ablative central spacing between two "Z-pins like" V0.9-Si0.1 rods reduced from the original 4 mm to 0.847 mm (Fig. 12(a)). This is mainly due to the surface matrix of ZCC-1 begins to suffer from melting erosion by excess oxide melt generated from "Z-pins like" V0.9-Si0.1 rods. As Fig. 12(b) shown, there is no residual SiO2, only a dense V2O5 layer with square ZrO2 particles. In summary, after adding Silicon, the "Z-pins like" rod can compensate different oxides in different areas of C/C-ZrC-SiC surface, and promote the formation of a dense gradient oxide protective layer in all regions of matrix. However, when the ablation time increases to 240 s, excessive compensating oxide melt will damage the matrix by melting corrosion, which is unfavourable for the improvement of the anti-ablation performance.

Ablative microstructure of ZCC-2
The microstructure of the ablative central and marginal regions of ZCC-2 is characterized by yellow arrows in Fig. 7(c). The surface and interface microstructures of "Z-pins like" V0.9-Si0.1 rod (Areas A and C in Fig. 7(c)), and C/C-ZrC-SiC surface far away from "Z-pins like" V0.9-Si0.1 rod (Area B and D in Fig. 6(c)) are characterized and analyzed.
After ablating for 180 s, there is a small amount of consumption of "Z-pins like" V0.9-Si0.1 rod in ablation center. Some micro grooves are formed after extensive oxidative damage of bare non-woven cloth layers, and the ZrO2 skeleton layer on C/C-ZrC-SiC surface is very loose and discontinuous ( Fig. 13(a)). In addition, there is no radial V2O5, only a uniformly dense mixed fusion V2O5-SiO2 layer formed on the surface of "Z-pins like" V0.9-Si0.1 rod (Fig. 13(b)). As the enlarging picture of area B in Fig. 13(a) shown, despite the exposed non-woven cloth layers encounter oxidative damage, a dense SiO2 layer from "Z-pins like" V0.9-Si0.1 rod is covered on its surface ( Fig. 13(c)). Meanwhile, the holes in the ZrO2 skeleton nearing the interface are partly filled with compensating oxides from "Z-pins like" V0.9-Si0.1 rod (Fig. 13(d)).
However, the oxide melt compensation of "Z-pins like" V0.9-Si0.1 rod is poor on the matrix surface far away from it, and a loose and heterogeneous ZrO2-SiO2 layer is still formed on the matrix surface. The main reason why the integrity and compactness of the oxide layer on ZCC-2 are lower than that of ZCC-1 is that the oxide layer formed by the initial oxidation of C/C-ZrC-SiC matrix in ZCC-2 is looser than that of ZCC-1.
Therefore, the oxidation products of "Z-pins like" V0.9-Si0.1 rod are difficult to spread to the substrate surface. After ablating for 240 s, ZCC-2 has better ablative performance than ZCC-1 (Table 1). In the ablating center, C/C-ZrC-SiC around "Z-pins like" V0.9-Si0.1 rod is not damaged by melting corrosion (Fig. 14(a)). The content of SiO2 on "Z-pins like" V0.9-Si0.1 rod surface is reduced, and the radial and needle V2O5 phases have been highlight ( Fig. 14(b)). In addition, the density of ZrO2 layer on C/C-ZrC-SiC surface away from "Z-pins like" V0.9-Si0.1 rod is denser than the sample after ablated for 180 s ( Fig. 14(c)). The gaps between two ZrO2 particles are sealed with radiating V2O5 from "Z-pins like" V0.9-Si0.1 rod (Fig. 14(d)). No exposed carbon fibers are found in the microscopic grooves (Fig. 14(e)), which are all covered with a dense SiO2-rich layer ( Fig. 14(f)). This fully proves that "Z-pins like" V0.9-Si0.1 rod can effectively play its role in oxide melt compensation at this stage.  Fig. 13, only a loose ZrO2-SiO2 layer (( Fig. 15(a)). And the bare carbon fibers are damaged serious ( Fig. 15(b)). Clearly, when "Z-pins like" V0.9-Si0.1 rods are parallel to non-woven cloth layer, the spreading rate of glass SiO2 melt from themselves towards C/C-ZrC-SiC is relatively slow, resulting in relatively small spreading area of exposed SiO2 melt, and ultimately leading to a decrease in the content of vapor phase SiO2 used for depositing SiO2 layer in the marginal area. To sum up, when the ablation time is no longer than 180 s, the improvement of anti-ablation performance from "Z-pins like" V0.9-Si0.1 rod perpendicular to the direction of non-woven cloth layer is the best. Since the C/C-ZrC-SiC surface can form a continuous ZrO2-rich layer, the flow resistance of compensation oxide melt formed by "Z-pins like" V0.9-Si0.1 rod is relatively small, which can effectively Due to C/C-ZrC-SiC surfaces of ZCC-2 exposes a lots of non-woven cloth layers, which rapidly oxidize to generate micro grooves (Eq. 13). Therefore, only a discontinuous porous ZrO2-rich layer is generated here (Eq. 14, 15). However, the porous ZrO2-rich layer on C/C-ZrC-SiC surface of ZCC-1 is relatively denser and more continuous.
With the ablation time prolonging, Tabl is higher than the melting temperature of SiO2 but lower than that of V2O3, SiO2 from the oxidation product of "Z-pins like" V0.9-Si0.1 rods and SiC performs self-sealing effect. Therefore, C/C-ZrC-SiC surface of ZCC-1 is covered with a dense ZrO2-SiO2 layer. However, under the hindrances of the micro-pores and grooves of ZrO2 layer, the flow rate of SiO2 melt generated on the ZCC-2 surface slows down, and the surface oxide layer density is lower than that of ZCC-1.
When Tabl continues to rise to about 2200 ℃, SiO2 on ZCC-1 and ZCC-2 surface evaporates largely. However, at this time, "Z-pins like" V0.9-Si0.1 rods can form moderate V2O3 (with high melting boiling point) melt and SiO2 (with low melting point) melt that is close to the demand. Under the scouring action of high-speed airflow, these melts quickly spread to C/C-ZrC-SiC surface, so as to effectively conduct oxide melt compensation to ensure oxide layer density on ZCC-1 surface, and avoid mechanical denudation of oxide layer. Meanwhile, micro grooves of ZCC-2 surface start to be filled, and the further oxidation of the non-woven cloth layer will be completely prevented.
However, When the temperature continues to rise, the content of compensating oxide melt generated by "Z-pins like" V0.9-Si0.1 rods is higher than the filling content of defects such as holes and cracks on the ZCC-1 surface, then the surface of ZCC-1 will suffer from superfluous melt erosion and form ablation pit. But because ZCC-2 surface forms many micro grooves in the initial oxidation, which can consume excess oxide melt. Therefore, no high temperature oxide melt concentration area in the ablation center of ZCC-2, and the melting corrosion will be avoided.
During the cooling process, all of the gaseous SiO and SiO2 are deposited to generate a dense SiO2 film on the ablating edge via VLS and OAG mechanisms.
Finally, V2O3 will continue to oxidize, and ZCC-1 (ZCC-2) surface are all covered a dense gradient oxide in plane, which is combined with V2O5-rich layer of ablation center, V2O5-SiO2 composite layer of ablation transition and SiO2 layer of ablation edge (V2O5-ZrO2 layer of ablation center, V2O5-SiO2-ZrO2 layer of ablation transition and SiO2 layer of ablation edge).

Conclusions
In this paper, the microstructure of C/C-ZrC-SiC composite reinforced by "Z-pins like" V0.9-Si0.1 rods was studied. The difference of ablation performance of "Z-pins like" V0.9-Si0.1 rods acting on the different C/C-ZrC-SiC surface, the evolution of ablated surface micro-morphology, and the difference of oxide compensation behavior of "Z-pins like" V0.9-Si0.1 rod were studied. On this basis, a new anti-ablative mechanism, namely liquid/gaseous multicomponent oxide compensation mechanism, is proposed. The main conclusions are as follows: (1) After adding silicon, liquid silicon was produced to assist the forming of "Z-pins like" V0.9-Si0.1 rods during the sintering, and the density of "Z-pins like" V0.9-Si0.1 rod was improved.
(2) When the ablation time was less than 180 s, there is a best improvement of the anti-ablation performance of "Z-pins like" V0.9-Si0.1 rods perpendicular to non-woven cloth layer, and "Z-pins like" V0.9-Si0.1 rod gave full play to the oxide compensation effects. However, when the ablation time was greater than 180 s, there is a best improvement of the anti-ablation performance of "Z-pins like" V0.9-Si0.1 rods parallel to non-woven cloth layer. However, when "Z-pins like" V0.9-Si0.1 rods is perpendicular to non-woven cloth layer, they increased another harm that weaken the anti-ablative performance of C/C-ZrC-SiC composite, namely, the corrosion damage of oxide layer on the matrix surface caused by excessive oxide melt.