The effect of MoAlB substitutions on the oxidation behavior of ZrB2-SiC Composites at 1600 °C

ZrB2-SiC ceramics are potential candidates for thermal protection materials for re-entry and hypersonic vehicles. Phases are typically added to ZrB2-SiC to enhance the oxidation resistance of the material. MoAlB is an attractive nanolaminated ternary boride compound. Due to its damage tolerance, crack healing ability, and good oxidation resistance, MoAlB is a promising material for high-temperature applications. Therefore, the effect of MoAlB substitution on the oxidation of ZrB2-SiC at 1600 °C for 10 h was evaluated. Five samples with different MoAlB contents (0 vol%, 4 vol%, 8 vol%, 12 vol%, 16 vol%) were prepared. The results indicate that ZrB2-SiC-8vol%MoAlB exhibits improved oxidation resistance compared to ZrB2-SiC. After being oxidized at 1600 °C for 10 h, the thickness of the oxide layer on ZrB2-SiC-8vol%MoAlB was significantly smaller than that on ZrB2-SiC. The thickness of the oxide layer in ZrB2-SiC was 250 μm, while the thickness of the oxide layer in ZrB2-SiC-8vol%MoAlB was 108 μm. The formation of dendritic crystals on the surface of ZrB2-SiC-4vol%MoAlB destroys the oxide layer, leading to a decrease in the oxidation resistance of material. The mass gain of ZrB2-SiC-12vol%MoAlB and ZrB2-SiC-16vol%MoAlB is greater, and the oxidation layer is thicker, indicating a lower oxidation resistance.


Introduction
The need for non-ablative thermal protection systems for sharp leading edges in hypersonic vehicles has been a key driver for recent research on ultra-high temperature ceramics (UHTCs) [1].ZrB 2 , from this category of advanced materials, possesses a series of excellent characteristics including chemical stability, a high melting point, thermal conductivity, hardness, and a low theoretical density [2,3].ZrB 2 is promising candidate for ultrahigh temperature ceramics for hypersonic applications.However, the oxidation resistance of ZrB 2 is very poor at temperatures above 1100 °C due to the volatilization of B 2 O 3 , and the porous ZrO 2 layer cannot provide sufficient protection [4][5][6][7][8].It was found that the addition of SiC improved oxidation resistance by promoting the formation of borosilicate glass [2,3].This borosilicate glass provides oxidation protection due to its high melting point and low vapor pressure, effectively filling the pores [4].
The oxidation behaviors of ZrB 2 -SiC composites in air have been well defined previously [9][10][11].ZrB 2 -SiC exhibits passive oxidation behavior with parabolic kinetics over a wide temperature range [4].However, the presence of bubbles and a porous SiC-depleted layer adversely affects the oxidation resistance of ZrB 2 -SiC.S. Gangireddy et al found that ZrB 2 -SiC composites formed bubbles after a certain period of oxidation at 1450 °C [12].Patel et al found that significant bubbles were observed when ZrB 2 -SiC ceramics were oxidized at 1650 °C for 30 min [13].The presence of bubbles causes the oxide layer of the sample to thicken [14].The active oxidation of SiC produces a porous depleted layer [15].The presence of bubbles and pores creates a pathway for oxygen to enter, which is detrimental to the oxidation resistance of material.
Several phase additions have also been considered and tested with the goal of improving the oxidation resistance of ZrB 2 -SiC.Most of these studies attempted to modify the microstructure and composition [4,14].MoAlB is an attractive nanolaminated ternary boride compound within a group of MAB phases, where M represents a transition metal, A represents either Al or Zn, and B represents boron [16].The nanolaminated structure endows MoAlB with damage tolerance, moderate electrical and thermal conductivity, crack healing ability, and good resistance to oxidation [17][18][19][20][21][22][23].With these properties it is a promising material for high temperature applications.It has been reported that MoAlB exhibits a good oxidation resistance at high temperatures due to the formation of a protective Al 2 O 3 layer [23][24][25].MoAlB exhibits excellent oxidation resistance above 1200 °C, comparable to that of Ti 2 AlC and Cr 2 AlC MAX phases [26][27][28].Xu et al [21] found that after five cycles (50 h) at 1600 °C, MoAlB exhibited a sample surface covered by Al 2 O 3 .The dense Al 2 O 3 layer did not detach or crack.The results showed that MoAlB had high oxidation resistance at 1600 °C.
Taking this into account, MoAlB is promising to be a suitable phase for enhancing the oxidation resistance of ZrB 2 -SiC.While the oxidation resistance of ZrB 2 -SiC composites has been extensively studied over a range of temperatures, the oxidation behavior of ZrB 2 -SiC composites with small amounts of MoAlB substitutions has not been systematically studied.The main purpose of this study is to investigate the high-temperature oxidation behavior of ZrB 2 -SiC-MoAlB at 1600 °C in air.The composites were prepared using spark plasma sintering (SPS).

Oxidation test
Samples were placed on ZrO 2 crucibles.Oxidation tests were carried out in a box furnace (ZHX-36163, Tianjin Zhonghuan Electric Furnace Co. LTD, China) at 1600 °C for 2 h, 4 h, 6 h, and 10 h, respectively.The heating rate was 5 °C min −1 , and the sample was cooled with the furnace after the holding period.Samples were weighed before and after the test.

Characterization
The densities were measured using the Archimedes method according to the ASTM standard B962-15.The phase compositions of samples before and after oxidation were analyzed by x-ray diffraction analysis (XRD, D8 advanced, Bruker, Germany).The morphologies of surfaces and cross-section of samples were characterized with a Scanning Electron Microscope (SEM, Philips S-4800, Hitachi Ltd, Japan) equipped with an energy dispersive x-ray spectroscopic system (EDS, X-Max, Oxford, UK).The thickness of the oxide layer was measured using Image J software to assess the oxidation resistance of the material.The thickness of the oxide scale was determined by measuring the thickness at 10 areas or more, along the scale.

Microstructures before oxidation
Figure 1 shows the morphology of the sintered sample.The Archimedes method was used to measure the density of the SPS sintered billets, which ranged from 97% to 98%. Figure 2 presents the XRD results for the prepared composites.The composites are primarily composed of ZrB 2 (PDF 34-0423), SiC (PDF 72-0018) and MoAlB (PDF 72-1277), which also contains MoB (PDF 06-0644).MoB is found in the XRD patterns, which probably indicates that decomposition occurred in MoAlB.When Kota et al [29] conducted DTA experiments on MoAlB in an Ar atmosphere, they found that the decomposition of MoAlB was reversible.The significant peak areas of endothermic and exothermic peaks were similar, with a small separation.At the same time, the XRD patterns before and after DTA were similar, consisting mainly of MoAlB.Therefore, MoAlB decomposes into MoB and liquid Al during heating, and MoAlB reforms during cooling.The simplified reaction that occurs in this system is: MoAlB ↔ Al (liquid) + MoB (T = 1708 K).Xu et al [21] prepared MoAlB bulk materials by hot pressing at 1600 °C with a pressure of 60 MPa for 1 h.The heating rate was 10 °C min −1 , and the cooling rate was 15 °C min −1 .After hot pressing, the mass of MoB was 5.25 wt%, primarily composed of MoAlB.Therefore, both MoAlB and MoB can be found in the XRD pattern of the sintered sample.The roles of both MoB and MoAlB are considered in this article.Figure 3 shows the microstructure of the sintered samples.The surfaces of all samples after sintering consist of both dark and bright features.Based on the distribution of the elements in the EDS map, the bright features mainly consist of Zr, while the dark features mainly consist of Si.Combined with the composition of the samples, the bright features are identified as ZrB 2 , while the dark features are

Macroscopic morphology and phase composition
Figure 4 shows the macrographs of the oxidized ZrB 2 -SiC-MoAlB containing 0%, 4%, 8%, 12%, and 16% MoAlB at 1600 °C.The macrographs of ZSM12 and ZSM16 appear similar, and extensive bubble formation is observed on these two samples after oxidation.A significant number of bubbles form on the surface of ZS after 6 h of oxidation, and the size of the bubbles is greater for ZS after 10 h of oxidation compared to the others.The oxidized surfaces of ZSM4 and ZSM8 are relatively intact, with few pores and bubbles.The formation of bubbles is caused by the release of gases.The gas evolves from the sample during oxidation, disrupting the scale.Bubbles were observed and reported by multiple authors as a result of gas generation [13].
Figure 5 presents the XRD results for the oxidized surfaces of these samples.The diffractograms from ZS, ZSM4, and ZSM8 are dominated by the peaks of ZrSiO 4 and ZrO 2 .The diffractograms from ZSM12 and ZSM16 are dominated by the peaks of mullite (3Al 2 O 3 .2SiO 2 ), with little ZrO 2 detected.To investigate the mechanism of oxidation behavior, the phase composition and microstructure of scales were characterized.

Oxidation reactions and surface micromorphology
Figure 6 shows the micrographs of ZrB 2 -SiC containing 0, 4, 8, 12, and 16 vol% MoAlB after oxidation at 1600 °C for 2 h.Many bubbles can be observed on the surface of ZSM12 and ZSM16, and there are also numerous holes caused by grain erosion.The XRD patterns of ZSM4 and ZSM8 reveal multiple peaks corresponding to ZrO 2 (e.g., 2θ = 32.847°,36.715°,41.224°, etc).However, in the XRD patterns of ZSM12 and ZSM16, 3Al 2 O 3 •2SiO 2 is identified as the predominant phase.Peaks with a high intensity of ZrO 2 are only observed at 2θ = 32.847°and36.715°, while peaks with low intensity are found at 2θ = 40.191°,41.903°, and 57.903°.At the same time, the peak with the highest intensity in the XRD pattern corresponds to 3Al 2 O 3 •2SiO 2 (2θ = 41.205°).Additionally, other peaks with high intensity are observed for 3Al 2 O 3 •2SiO 2 at 2θ = 27.421°,36.111°,43.234°, etc.Few peaks of ZrO 2 can be found in ZSM12 and ZSM16.Therefore, it is considered that the shed grain is ZrO 2 .The surface oxide of ZSM4 exhibits dendrite morphology.The morphology of the crystals is influenced by the growth environment.During energy-dispersive x-ray spectroscopy (EDS) of the oxidized surfaces, the elemental analysis does not detect any boron (B) elements, as boron cannot be identified in the standard energy spectra.Only a very small amount of Mo content is detected on the surface of ZSM4, with an atomic percentage of Mo element 0.01%.However, the atomic percentage of elemental Mo in ZSM8 is 0. As a result, there may be a difference in the Mo content of ZSM4 and ZSM8.The sources of Mo elements are MoAlB and MoB, which undergo oxidation to form MoO 3 (Reaction 1 and Reaction 2).MoO 3 transitions from the liquid phase to the gas phase at 800 °C [22].Therefore, MoO 3 should be almost completely converted to gas at 1600 °C.A reduction in viscosity will result in enhanced diffusivity according to the Stokes-Einstein equation [30].The SiO   The weight change curves of ZS, ZSM4, ZSM8, ZSM12, and ZSM16 after oxidation in air at a temperature of 1600 °C are presented in figure 7. The five materials exhibit similar oxidation behavior.The weight gains of the five materials increase as the oxidation time increases.However, throughout the entire experiment, ZS, ZSM4, and ZSM8 exhibit lower weight gains compared to ZSM12 and ZSM16, indicating a slower oxidation rate for the former.The difference in mass increase per unit area between ZS, ZSM4, and ZSM8 is not significant enough to determine the difference in performance.ZSM12 and ZSM16 exhibit numerous bubbles on their surfaces.The presence of bubbles creates a pathway for oxygen to enter, intensifying the oxidation of ZSM12 and ZSM16 and resulting in increased mass gain.The oxidation of these composites is likely to involve multiple reactions.The formation of ZrO 2 , SiO 2 , B 2 O 3 , and Al 2 O 3 leads to an increase in mass, whereas the active oxidation of SiC and the evaporation of B 2 O 3 lead to a decrease in mass.Figure 7 reflects the net effect of these reactions.Therefore, a microstructural analysis of the oxidized cross-sections is conducted to investigate the oxidation behavior of these samples.

Microstructural analysis of the cross-sections
Figure 8 shows the cross-section microstructures of these composites.There are four different layers in the oxide scale: (1) a SiO 2 -rich outer layer, (2) a sub-layer of crystalline ZrO 2 , containing little SiO 2 , (3) a SiC-depleted layer, and (4) an unaltered material.As a result of oxidation, the sample undergoes volume expansion, causing the liquid phase to concentrate in the surface layer, resulting in the appearance of an outer layer as a liquid phase.Figure 8 illustrates the division between the sub-layer and SiC-depleted layer of ZS, ZSM4, and ZSM8.Since it is difficult to distinguish between the sub-layer and SiC-depleted layer of ZSM12 and ZSM16, and the analysis does not necessitate the separation of the two layers, they are not divided.The thickness of different oxide layers was measured by Image J. ZSM12 and ZSM16 exhibit a thicker oxide scale compared to ZS, ZSM4, and ZSM8.Even after 2 h of oxidation, the thickness of the oxide layer exceeds 100 μm.Compared to ZS, ZSM12 and ZSM16 have worse oxidation resistance.The oxide layer of ZSM4 is also very thick.This is due to the disruption of the surface caused by the dendritic crystals, which promote the penetration of oxygen.
There is no significant difference in the thickness of the oxide layer between ZSM8 and ZS after oxidation at 1600 °C for 2 and 4 h.After 2 h of oxidation, the thickness of the oxide layer of ZS is 96 μm, and that of ZSM8 is 78 μm.After 4 h of oxidation, the thickness of the oxide layer of ZS is 110 μm, while that of ZSM8 is 105 μm.Due to the presence of large bubbles, the thickness of the outer layer of ZS varies significantly and is particularly small in some areas.Therefore, the thickness of the outer layer of ZS oxidation after 6 h and 10 h is not discussed.After 10 h of oxidation, ZSM8 exhibits significantly better oxidation resistance than ZS.The thickness of the sub-  layer of ZS is 175 μm, while that of ZSM8 is 53 μm.The thickness of the SiC-depleted layer of ZS is 75 μm, while that of ZSM8 is 55 μm.It can be clearly seen from figure 4 that the bubbles present on the surface of ZS after being oxidized at 1600 °C for 10 h are significantly large.The presence of these bubbles creates a pathway for oxygen to enter.Therefore, the thickness of the oxide layer of ZS significantly increases after 10 h of oxidation.
As shown in Reaction 5, the products produced by the active oxidation of SiC are all gases.Therefore, the active oxidation of SiC significantly contributes to the decrease in weight per unit area of the sample.After 6 h of oxidation, the thickness of the SiC-depleted layer of ZS is 29 μm.After 10 h of oxidation, the thickness of the SiC-depleted layer of ZS increases to 75 μm, indicating a significant increase in the thickness of the SiC-depleted layer.ZSM4 exhibits a significantly thick SiC-depleted layer (greater than 90 μm) throughout the entire oxidation process.Therefore, even though the thickness of oxide layer of ZS and ZSM4 is substantial, the weight per unit area does not increase significantly.
ZSM8 exhibits the highest oxidation resistance among all the samples.The change in oxidation performance can be attributed to the presence of Al 2 O 3 formed during MoAlB oxidation, which reduces the viscosity of the scale.Changing the viscosity of the borosilicate glass phase in ZrB 2 -SiC ceramics is a common method for altering the oxidation resistance of ZrB 2 -SiC ceramics.Adding AlN to ZrB 2 -SiC can reduce the viscosity of the glass phase.This allows the glass phase to quickly fill the pores, form a continuous oxide layer, and improve the oxidation resistance of ZrB 2 -SiC [30,31].TaB 2 is added to ZrB 2 -SiC to enhance the viscosity of the glass phase, decrease the rate of oxygen penetration on the oxide surface, and improve the oxidation resistance of ZrB 2 -SiC [32].In conclusion, increasing the viscosity of the glass phase is beneficial for inhibiting the oxygen diffusion, while decreasing the viscosity of the glass phase is advantageous for enhancing the mobility of the glass phase to fill the pores.Therefore, increasing or decreasing the viscosity of the glass phase may benefit the oxidation resistance of the material, as long as it can provide better surface coverage while oxygen penetration is not as fast.
In ZrB 2 -SiC, the glass phase is composed of B 2 O 3 -SiO 2 .Since the glass phase is fluid, it can fill pores and bubbles, preventing the penetration of oxygen.The presence of B 2 O 3 contributes to reducing the viscosity of the glass phase, enabling it to flow easily and seal the surface.However, the evaporation of B 2 O 3 increases the viscosity of the glassy phase, leading to a decrease in fluidity.The increase in the viscosity of the glass phase leads to an increase in the retention of bubbles and pores, which has a negative impact on the oxidation resistance of the material.After adding MoAlB, the glass phase consists of SiO 2 -Al 2 O 3 .Since the viscosity of the glass phase decreases with an increase in Al 2 O 3 content [31], the viscosity of ZSM is lower than that of ZS.
The glass phase with good fluidity can provide better surface coverage [30,31].The fluidity of the glass phase in ZSM8 is high, which leads to the formation of a thin oxide layer and enhances the oxidation resistance of ZSM8.In figure 4, it can also be observed that the surface of ZSM8 remains intact after 10 h of oxidation, indicating the presence of a glass phase flow filling.However, a lower viscosity scale can provide better surface coverage, but at the same time, it allows for rapid oxygen permeation.As a result, ZSM12 and ZSM16 have worse oxidation resistance than ZS.In other words, adding an appropriate amount of MoAlB helps improve the oxidation resistance of ZS.
It has been observed that certain samples exhibit spalling at the interface between the oxide layer and the substrate.However, the occurrence of this spalling is found to be random in repeated experiments.This is due to the mismatch of thermal expansion coefficients between ZrO 2 (10.8 × 10 −6 K −1 ) and ZrB 2 (6.7 × 10 −6 K −1 ).As a result, porosity is more likely to occur at the interface.During cutting, the presence of interfacial defects weakens the area, making it easier to fall off.
(4) A glass phase with lower viscosity can offer improved surface coverage, but it also facilitates rapid oxygen permeation.The ideal viscosity (and therefore the composition) is determined by surface coverage and the transport of oxygen through the scale.
The MoAlB content in the ZrB 2 -SiC composite should be optimized based on the practical application requirements.There are still some problems to be solved, such as understanding the formation mechanism of the dendritic morphology after the oxidation of ZrB 2 -SiC-4vol%MoAlB.These issues require further experiments to be explained.
2 and B 2 O 3 are produced by the oxidation of SiC and ZrB 2 (Reactions 3 and 4).The glass phase is formed by B 2 O 3 , SiO 2 , and

Figure 7 .
Figure 7. Mass change of samples on oxidation in air at 1600 °C.The points represent the change in mass at this oxidation time.A parabolic curve represents the relationship between oxidation time and the mass change of a sample with different compositions.

Figure 8 .
Figure 8. Cross-sectional scanning electron micrographs of ZrB 2 -SiC-MoAlB oxidized at 1600 °C in air.The topmost line divides the outer layer and the sub-layer, while the line farthest from the top separates the SiC-depleted layer and the unaltered material.ZS, ZSM4, and ZSM8 divide the sub-layer and SiC-depleted layer.