Ablation behaviour of C/C-HfC-SiC composites prepared by joint route of precursor infiltration and pyrolysis and gaseous silicon infiltration

Jiaping ZHANG, Yuepeng XIN, Running WANG, Qiangang FU

Shaanxi Key Laboratory of Fiber Reinforced Light Composite Materials, Northwestern Polytechnical University,Xi’an 710072, China

KEYWORDS

Abstract C/C-SiC-HfC composites were fabricated by using Precursor Infiltration and Pyrolysis(PIP) combined with Gaseous Silicon Infiltration (GSI) process.Different GSI temperatures(1900 ℃and 2100 °C) were selected.The combination of PIP and GSI could significantly reduce the preparation time of the composites.The morphology displaying a rich-Si layer was formed on the surface of the composites prepared at GSI 2100°C.Ablation performance of the composites was investigated by oxyacetylene torch.The results showed that after ablation for 120 s,compared to the composites prepared by PIP +1900°C GSI, the linear and mass ablation rates of the composites fabricated by PIP + 2100 °C GSI were decreased from 8.05 μm/s to 5.06 μm/s and from 1.61 mg/s to 1.03 mg/s, respectively.The coverage of the rich-Si surface layer promoted the generation of more SiO2 during ablation,which not only benefited for decreasing the surface temperature but also contributed to the formation of H-Si-O glass and the HfO2 skeleton, thus better resisting the denudation of the oxyacetylene torch.

1.Introduction

With the development of the aerospace and aircraft industry,new requirements such as high strength,low density,low thermal expansion, high thermal stability, etc., are raised in the selection of high thermal structural materials.1–5Carbon/Carbon (C/C) composites, which are carbon matrix composites using carbon fibers as the reinforcements, are regarded as promising materials that are highly appropriate to meet these demands.6–9However, poor oxidation resistance at high temperature (>500 °C) limits their applications.Ultra-High Temperature Ceramics (UHTCs)10–13including transition metal carbides, nitrides and borides possess high melting points and excellent oxidation/ablation resistance properties,which makes them very promising for applications in extreme aerodynamic heating environments.However, bulk UHTCs are susceptible to catastrophic failure caused by their inherent brittleness and poor thermal shock resistance.14,15The integration of C/C and UHTCs to fabricate C/C-UHTCs could mitigate the limits from each other, leading to multiple benefits.8,15–20

Hafnium Carbides (HfC), as the typical representative of UHTCs, has a high melting point (3890 °C), good chemical and high-temperature stability.The introduction of Silicon Carbide(SiC)can further enhance its oxidation resistant property in a wide temperature range.21–23As a result,many methods have been developed to introduce HfC-SiC into C/C composites,24–28such as Reactive Melt Infiltration (RMI),24Precursor Infiltration and Pyrolysis(PIP)25and slurry impregnation.28Among them,PIP is the most commonly used,which has the advantages of near-neat shape manufacturing, lowtemperature processing and the tailor of the matrix through proper selection of the pyrolysis cycle.25,27Li KZ et al.prepared C/C-HfC-SiC with a density of 2.38 g/cm3by PIP.25It should be noted that repeated cycles are required for infiltration and densification of the C/C-HfC-SiC, and each cycle can be time-consuming.Additionally, cracks and pores are easily formed due to the shrinkage of the precursor during pyrolysis at high temperatures.

To shorten the preparation period and increase the densification efficiency, the combination of PIP with other methods has been successively investigated.17,20,29Thanks to its fast processing/cost-effectiveness, RMI2,18,19representing the infiltration of a porous carbon fiber reinforcement with molten metal is adopted.Yano et al.obtained C/C-HfC-SiC composites fabricated through HfSi2melt impregnation,30in which HfC and SiC were formed by the reaction between HfSi2and carbon.However, the reaction between the molten metal with the C matrix is difficult to control,which can easily block the pores and then result in limited penetration thickness.Compared with RMI, gas infiltration has a better ability to penetrate into small pores in the composites,31–33and then can effectively avoid the shortcoming caused by the blockage of liquid phase reaction which usually took place during RMI.Li QG et al.34fabricated the Cf/ZrC-SiC composites by vapor silicon infiltration.The density and porosity of the composites were 2.25 g/cm3and 6% respectively.To our knowledge, the study of C/C-HfC-SiC composites prepared by the combined method of PIP+Gaseous Silicon Infiltration(GSI) is limited.

In this work, C/C-HfC-SiC composites were fabricated by PIP combined with GSI.Effect of GSI temperature on the microstructure and ablation resistant property of C/C-HfCSiC composites was investigated, and the corresponding ablation mechanism was revealed.

2.Experimental procedure

2.1.Composites preparation

Fig.1 shows the general preparation process of C/C-HfC-SiC composites.T300 PAN-based carbon fiber was employed to fabricate the 2.5D preform by alternatively stacked weftless plies and short-cut-fiber webs by a needle punching technique(Yixing Tianniao High Technology Co.Ltd., Jiangsu, China).The fiber volume content was about 20%–25%.The preform was densified by Isothermal Chemical Vapor Infiltration(ICVI) to prepare the C/C composites with the density of 0.8–0.9 g/cm3.During ICVI, CH4was used as the reactant to obtain the Pyrocarbon (PyC).Then HfC-SiC was introduced into the low-density C/C composites by PIP.Polycarbosilane (National University of Defense Technology,Changsha, China) and organic hafnium-containing polymer(Institute of Process Engineering, Chinese Academy of Sciences,Beijing,China)were used as SiC and HfC precursors,respectively.The precursors of HfC and SiC were mixed at a weight ratio of 1∶2.The density of the initial HfC-SiC modified C/C composites was about 1.20 g/cm3with a porosity of 20.12% (measured by ImageJ software).After that, the samples were cut into 11 mm × 11 mm × 7 mm size cubic, ultrasonic cleaned for 0.5–1.0 h and dried at 70–100 °C for 5–9 h.Then the samples were subjected to GSI treatment in a graphite crucible(with the size of ?90 mm×60 mm).The added amount of silicon pieces (Fanrui Composites Research Institute Co.Ltd, China) was about 1/2 of the volume of the crucible.The samples were fixed on a porous graphite plate which was placed above the silicon pieces.The graphite crucible was heated to 1900 °C and 2100 °C, respectively.The holding time was 0.5–2.0 h.The final densities of the HfC-SiC modified C/C composites prepared at the GSI temperatures of 1900 °C and 2100 °C were 2.06 g/cm3and 2.22 g/cm3, and the corresponding porosities were 6.78%and 7.36%,respectively.To facilitate the following discussion,the HfC-SiC modified C/C composites (C/C-HS) prepared at the GSI temperatures of 1900 °C and 2100 °C were noted as C/C-HS-1 and C/C-HS-2, respectively.

2.2.Characterization methods

The ablation test was performed using an oxyacetylene torch,which was a reliable, high throughput and easy-operative labscale ablation screening test for investigating candidate aerospace materials.Fig.2 shows the ablation experimental setup.The instrumentation mainly consists of an oxyacetylene torch gun and sample stage (Fig.2(a)).The pressures of O2and C2H2were 0.400 MPa and 0.095 MPa,and the fluxes were 0.244 L/s and 0.167 L/s.The diameter of the nozzle was 2 mm.The ablation angle was 90°,and the distance between the sample and the oxyacetylene torch was 10 mm.An infrared thermometer (Raytek MR1SCSF) was employed to record the sample surface temperature.The size of the ablated sample was 11 mm × 11 mm × 7 mm.A graphite mold was used to fix the cubic sample during ablation (Fig.2(c)).Two exposure times, 60 s and 120 s, were set for the ablation test.Fig.2(b)reveals the testing process.During ablation, the materials would suffer oxygen-rich environments accompanied by high heat fluxes, high temperatures and gas scouring.The mass and thickness of the samples were measured before and after the ablation test respectively.Five points were selected for the thickness measurement.After ablation,the linear and mass ablation rates of the composites were calculated by

Fig.1 Schematic presentation of preparation of C/C-HS composites using PIP combined with GSI.

Fig.2 Ablation test of composites by oxyacetylene torch.

where Rlrepresents the linear ablation rate; Rmrepresents the mass ablation rate; d1and d2are the thickness at the center region of the sample before and after ablation; m1and m2are the sample mass before and after ablation; t is the ablation time.

The morphologies and compositions of the composites were analyzed by Scanning Electron Microscope (SEM, TESCAN VEGA3) and Energy Dispersive Spectroscopy (EDS).

3.Results and discussion

3.1.Microstructure

Fig.3 shows the microstructure of the C/C-HS composites after only PIP.2.5D carbon fiber felts fabricated by alternatively stacked non-woven layers and short-cut carbon fiber webs are adopted as reinforcements.It is evident that the pores in non-woven webs are almost filled, while the pores in shortcut carbon fiber webs are still in large size (Fig.3(a)).During ICVI, the PyC was prone to deposit in the tiny pores in the non-woven webs, while the pores in short-cut webs were relatively larger and difficult to be filled,so the short-cut webs provide more space for the infiltration of the precursor during PIP.From the magnified view (Fig.3(b)), XRD (Fig.3(c))and EDS analysis (Fig.3(d)), it is confirmed that HfC-SiC ceramic phases are mainly distributed in the pores of the short-cut fiber webs.Due to its lower mass ratio in the mixed precursor, the content of HfC in the composites is lower than that of SiC after PIP.

Fig.4 shows the surface morphologies of the C/C-HS composites after further densification by GSI at different temperatures.The surface of C/C-HS-1 is covered by a discontinuous layer(Fig.4(a)),which is accumulated by ceramic grains of different sizes.As the GSI temperature increases, C/C-HS-2 presents distinct morphologies.The grains grew up and connected, resulting in the coverage of a continuous layer(Fig.4(b)).The two different surface morphologies are related to their different GSI temperatures, which affect the diffusion as well as the chemical reaction of gas Si.

Fig.3 Microstructure, XRD pattern and EDS analysis of C/C-HS after only PIP.

Fig.5 reveals the cross-section and EDS analysis of the C/C-HS-1.A relatively compact morphology is obtained although some small holes still exist (Figs.5(a) and (b)), indicating that gas Si possesses good infiltration ability and fills the pores in the short-cut webs effectively.Combined with the XRD pattern and EDS analysis (Figs.5(c) and (e)), the white and grey phases could be deduced to be HfC and SiC + Si respectively (Fig.5(b)).Compared with C/C-HS-1, the GSI temperature of C/C-HS-2 is higher which could result in a much more rapid diffusion/reaction rate.Fig.6 exhibits the cross-section and EDS analysis of the C/C-HS-2.It is found that the cross-section is covered by a continuous layer(Figs.6(a) and (b)), which is in good agreement with the surface morphology (Fig.4(b)).The coverage of the continuous layer is expected to block the path for gas Si for further penetrating the composites with a deeper depth.This can be confirmed by the XRD pattern and EDS mapping analysis(Figs.6(c) and (d)).It is observed that Si mainly concentrates at the outer layer,and the Hf distributes more in the middle of the sample.The holes and cracks gradually appear with the distance away from the surface.

3.2.Ablation properties

Fig.5 Cross-section morphologies, XRD pattern and EDS analysis of C/C-HS-1.

Fig.6 Cross-section morphologies, XRD pattern and EDS analysis of C/C-HS-2.

C/C-HS-1 and C/C-HS-2 were ablated using oxyacetylene torch for 60 s and 120 s respectively.Oxyacetylene torch is an efficient tool often used to simulate extremely high temperatures that the thermal-structural components encountered in extreme aero-thermal environments.During ablation, the samples were exposed to a high-temperature oxidizing environment accompanied by gas stream scouring, a coupled process like oxidation, volatilization and denudation would take place on the surface of C/C-HS-1 and C/C-HS-2, which was responsible for their morphology changes and ablation rates.For comparison, the composites prepared with only PIP were also evaluated.The measured results of linear/mass ablation rates and ablation temperatures are summarized in Table 1.It can be found that after GSI,the ablation properties of the composites are significantly enhanced, and as the GSI temperature increases, the linear/mass ablation rates and the maximum surface temperatures of the sample further decrease.After ablation for 120 s,compared to C/C-HS-1,the linear and mass ablation rates of C/C-HS-2 were decreased from 8.05 μm/s to 5.06 μm/s and 1.61 mg/s to 1.03 mg/s, respectively.The maximum ablation temperature of C/C-HS-2 is the lowest(about 1950 °C), indicating the rich-Si surface layer is beneficial for taking the heat away and decreasing the ablation temperature.In addition, it is also found that the mass loss of C/C-HS-2 after 60 s ablation is the lowest, indicating the weight loss through erosion and evaporation is offset by the weight increase.As the ablation continues,mass loss increases,indicating the mass gain by the generation of ablation products is difficult to compensate for the consumption.

Fig.7 exhibits the morphologies of the C/C-HS composites with only PIP before and after ablation.Compared to the macroscopic appearance before ablation (Fig.7(a)), pits,grooves and white ablation products are found at the center region after ablation(Fig.7(b)).The center possesses the most considerable thickness change, and a large number of cavities are concentrated in this area(Fig.7(c)).Firstly,it is because of the direct erosion of oxyacetylene flame.Secondly, the highest ablation temperature results in the occurrence of the most violent reactions here.Some broken carbon fibers presenting needle-shaped features distribute desultorily on the ablated surface (Fig.7(d)).The formation of needle-shaped carbonfibers indicates that ablation is prone to take place preferentially at the fiber–matrix interface.8C/C-HS composites with only PIP possess higher porosities,which provide the diffusion channels for the oxyacetylene torch and then enhanced its denudation.During ablation, the thermal-chemical ablation will decrease the fiber strength.The consumption of carbon fiber and carbon matrix15generates CO2and CO,and the oxidation mass gain of the ceramics hardly offset the mass loss,thereby leading to an overall weight decrease (Table 1).Once they cannot withstand the shearing force, the fibers are fractured and then are distributed randomly by the denudation of the oxyacetylene torch.

Table 1 Ablation rate and maximum surface temperature of samples.

Fig.7 Ablation morphology of composites with only PIP before and after ablation for 120 s.

Fig.8 illustrates C/C-HS-1 before and after ablation.Compared to the composites that are fabricated only by PIP (Figs.7(a)and(b)),a significant color change is found on the ablated surface (Fig.8(a)).Ablation products presenting yellow and grey colors cover the surface after ablation 60 s.With the ablation time prolonged,a higher ablation temperature darkens the color of the ablation products.According to the morphology and element distribution difference(Figs.8(b)and(c)),the surface can be divided into three regions,inner layer,middle layer and outer layer, as shown in Fig.8(b).The main elements of the inner, middle and outer layers are Hf-Si-O, Hf-O-C and Si-O,respectively.Fig.9 shows the microstructures of the three regions in Fig.8(b).From Fig.9(a), the inner layer is covered by a glass layer accompanied by some spherical and clustershaped white ablation products.Micro-holes are also observed on the glass layer.According to EDS patterns (Figs.9(d) and(f)), the main ablation products are SiO2, HfO2and Hf-Si-O glass.The glass layer and the spherical white ablation products are deduced to be Hf-Si-O glass and HfO2(Fig.9(a)), respectively.Compared to the composites that are fabricated only through PIP (Figs.7(c) and (d)), the oxide layer of C/C-HS-1 formed during ablation is more condensed and more tightly adheres to the carbon fibers.The generated SiO2melted,evaporated, and volatilized during ablation since its melting point was lower than the maximum ablated temperature of 2093°C(Table 1).As a result, the heat is taken away through these processes.The remaining liquid phase SiO2flows at the ablated surface, which is beneficial for sealing the defects(holes or microcracks) and protecting the underlying matrix by obstructing oxidation gases.Hf will also dissolve into SiO2under high temperature and form Hf-Si-O glass,35,36contributing to the thermal stability improvement.Due to the low partial pressure of oxygen, the underlying matrix is oxidized with Si oxides and gas byproducts of CO2and CO.The evaporated Si oxides and gas byproducts of CO2and CO along with the gas scouring led to the formation of micro-holes on the glass.In the middle layer (Fig.9(b)), cavities and holes on the surface are noticeable.Most ablation products form tiny droplets.For the outer layer(Fig.9(c)),a relatively dense film forms with the existence of some white ablation oxides(deduced to be HfO2).SiO2is the main component of the oxide film (Figs.9(d) and (f)).HfO2is in small particles instead of gathering, which are expected to be blown from the ablation center during ablation.

Fig.8 Ablation morphology and EDS analysis of C/C-HS-1.

Fig.9 Ablation morphology and EDS analysis of different regions of C/C-HS-1 after ablation for 120 s.

Fig.10 Ablation morphology and EDS analysis of C/C-HS-2 after ablation for 120 s.

In contrast,for the C/C-HS-2,the surface presents a lighter color(Fig.10(a)).Besides the inner layer,the middle and outer regions are hard to distinguish (Fig.10(b)).The Si element is distributed evenly (Figs.10(b) and (c)).For the convenience of discussion, the surface of C/C-HS-2 after ablation 120 s is also divided into three regions, namely the inner layer, middle layer (near the inner layer) and outer layer (near the brim region).Fig.11 shows the microstructure of the noticed three regions of C/C-HS-2.After ablation,the inner layer is covered by a Hf-Si-O glass layer containing HfO2in skeleton structures(Fig.11(a)).At the middle layer (Fig.11(b)), the covered area of the glass phase increases, and the HfO2particles gradually gather and form a skeleton structure.At the outer layer(Fig.11(c)), ablation products present tiny droplet features,and the droplets are comprised of a mixture of Hf-Si-O glass and HfO2particles.Few isolated white HfO2particles are found, meaning the protective layer of the Hf-Si-O glass + HfO2skeleton in the inner layer has good stability to withstand mechanical denudation and is hardly blown away.

Fig.11 Ablation morphology and EDS analysis of different regions of C/C-HS-2 after ablation for 120 s.

Fig.12 Schematic presentations of ablation mechanism for composites.

3.3.Ablation mechanism

Chemical ablation and mechanical denudation are the two processes that contribute the most during ablation.The mass change of the composites is determined by the competition between composites consumption and new product generation.To summarize the discussion above, a schematic drawing of the ablation process is shown in Fig.12.For C/C-HS-1(Fig.12(a)), during ablation, SiO2and HfO2are generated on the surface when it encounters the attack from the hightemperature oxidizing heat flow at the beginning of the ablation.Under temperatures above 1700 °C, the SiO2melts and volatilizes.The dissolving of the Hf element into the liquid phase SiO2increases its high thermal stability, and then a Hf-Si-O liquid oxide film is gradually formed.However, its higher ablation temperature (2093 °C) will lead to the quick evaporation of the SiO2glass and the escape of the gas byproducts (CO and CO2), resulting in the formation of pores and holes on the glass film24and then degrading its ablation performance.37These defects(holes and pores)will provide the diffusion channel for the flame and then enhance its denudation,leading to increased mass loss.Although the flowing of this glass film could seal the defects to some extent, the generated SiO2glass products are limited, which is difficult to compensate for the ablation consumption.As the ablation further continues, thermal-chemical ablation and denudation gradually consume the protective oxide layer (Fig.9(a)).In contrast,the surface of C/C-HS-2 is covered by a rich-Si layer,as shown in Fig.12(b),which is in favor of the generation of more SiO2glass.The evaporation of Si(g) + SiO2(g) could decrease the surface temperature (maximum ablation temperature is 1950 °C), which is beneficial for reducing the thermalchemical ablation and improving the thermal ability of the Hf-Si-O glass + HfO2protective layer.Though the evaporation of the SiO2glass and the escape of the gas products(SiO, CO and CO2) are also disadvantageous for the integrity of the glass film, the more generated SiO2glass is expected to seal the defects in time.The generated abundant SiO2glass could not only promotes the formation of Hf-Si-O glass by the dissolution of Hf into SiO2, but also act as the role of the quasi-liquid phase sintering additive38to promote the formation of HfO2skeleton(Fig.11(b))and then improve its stability to avoid being blown away (Fig.11(c)).With the ablation further continuing, the formed Hf-Si-O glass and HfO2skeleton could efficiently resist the attack of the oxyacetylene torch.

4.Conclusions

C/C-HfC-SiC composites were successfully fabricated by the combined method of PIP with GSI.GSI as a supplement for PIP can shorten its preparation period and increase its densification efficiency.The obtained composites fabricated by PIP + 1900 °C GSI reached a density of 2.06 g/cm3and a porosity of 6.78%, while the average density and porosity for the composites fabricated by PIP + 2100 °C GSI are 2.22 g/cm3and 7.36%.The higher porosity of the composites prepared by PIP+2100°C GSI was due to the coverage of the rich-Si layer on the surface, which blocked the infiltration of the gas Si and limited its penetration depth.The composites obtained by PIP + 2100 °C GSI showed better ablation performance.After ablated for 120 s using the oxyacetylene torch,its linear and mass ablation rates were 5.06 μm/s and 1.03 mg/s.The good ablation performance was attributed to the following reasons.Due to the existence of the rich-Si surface layer,more SiO2was generated during ablation.The evaporation of Si and SiO2was beneficial for decreasing the surface temperature and reducing the ablation.Additionally,the more generated SiO2not only contributed to the formation of Hf-Si-O glass but also played the role of quasi-liquid-phase assisted sintering, which promoted the formation of the HfO2skeleton and improved its stability to resist the denudation of the oxyacetylene torch.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (No.2021YFA0715803), the Science Center for Gas Turbine Project, China (No.P2021-A-IV-003-001), the National Natural Science Foundation of China (52002321), the Fundamental Research Funds for the Central Universities, China (No.G2022KY0609) and the Young Talent Program of Association for Science and Technology in Xi’an, China (No.095920211338).