Effect of HfC Content on the Elevated-Temperature Ablation Behavior of W-HfC Composites

Abstract

The effects of HfC content on the ablation resistance of W-HfC composites were systematically studied. The oxy-acetylene flame ablation test was conducted at 2800 °C. Post-ablation samples were characterized via XRD, section morphology, and EDS. W-10HfC showed the best ablation resistance, with a linear ablation rate of just 0.0175 mm/s. This enhanced performance is attributed to the formation of a dense HfW₂O₈ oxide layer with negative thermal expansion properties, reinforced by uniformly dispersed blocky HfO₂ particles. However, excessive HfC content induces a stratified oxide structure. The thermal expansion coefficient mismatch between HfW₂O₈ and HfO₂ causes microcrack formation, ultimately degrading ablation resistance. These findings establish critical guidelines for HfC content optimization in W-HfC composite design.

1. Introduction

Tungsten (W), as a typical refractory metal, exhibits outstanding properties, including a high melting point (3410 °C), high elastic modulus, thermal shock resistance, superior high-temperature strength, and excellent corrosion resistance [1,2,3,4,5]. Owing to these characteristics, W and its alloys have been extensively utilized in aerospace and nuclear industries as a critical material for high-temperature structural components, such as engine parts, rocket nozzles, and nuclear fusion reactors [6,7,8]. However, tungsten exhibits intrinsic limitations, including a high ductile-to-brittle transition temperature that induces low-temperature embrittlement [9,10,11]. The strength of pure tungsten significantly degrades with increasing temperature, with its tensile strength at 1000 °C reduced by 60–80% compared to room temperature [12]. Additionally, the severe oxidation of pure tungsten at high temperatures constitutes a critical restriction for engineering applications [13].

To effectively overcome these limitations, researchers have significantly enhanced the performance of W matrix composites through strategies such as alloying element doping, grain boundary engineering, and hierarchical structural design [14,15,16]. The incorporation of secondary-phase particles, such as rare-earth oxides, refractory metal nitrides, and carbide ceramic particles within the tungsten matrix, inhibits grain boundary migration. Thereby refining grain size and elevating the recrystallization temperature. This approach improves both the strength and ductility of tungsten-based alloys while enhancing their high-temperature performance and stability [17,18,19,20,21].

Compared to oxide particles, carbide reinforcements with higher melting points are ideal strengthening phases for tungsten-based composites [22]. The currently developed W-MeC composites possess an attractive combination of mechanical, physical, and chemical characteristics in the temperature region of 20–1600 °C compared with monolithic W and, especially, show unusual high-temperature strength and super ablation resistance at ultra-high temperatures [23,24]. Hafnium carbide (HfC), characterized by its ultrahigh melting point (3900 °C) and low thermal conductivity (20 W/m·K), is recognized as a superior candidate for enhancing the high-temperature performance of tungsten matrix composites. HfC dispersion-strengthened tungsten demonstrates significant enhancements in yield strength, compressive strength, and hardness [25]. Studies further reveal that HfC particles react with oxygen impurities to form HfO₂ [26]. Moreover, the comparable coefficients of thermal expansion between HfC ceramics and the tungsten matrix enable improved high-temperature stability without compromising room-temperature ductility. These attributes establish W-HfC composites as a highly promising refractory material [27].

Ablation resistance is critical for materials serving in ultra-high-temperature environments. Studies have shown that W-HfC composites exhibit superior ablation resistance compared to pure W, with enhanced performance at 2200 °C as HfC content increases [25]. This improvement arises from the formation of a dense HfO₂ oxide layer during ablation, which inhibits oxygen and heat flux penetration into the W matrix. However, inadequate interfacial bonding between HfC and the tungsten matrix, coupled with the formation of complex oxides exhibiting negative thermal expansion coefficients during ablation, introduces significant challenges. Thermal expansion mismatch among oxidation products promotes microcrack initiation, accelerating oxidative degradation and ultimately compromising ablation resistance. Current research lacks systematic investigations on the ablation behavior of W-HfC composites at elevated temperatures and the formation mechanisms of composite oxides during ablation, as well as the influence of HfC content on ablation resistance and associated mechanisms, which remain unclear.

In this study, tungsten matrix composites reinforced with different mass fractions of HfC were fabricated via powder metallurgy and the spark plasma sintering (SPS) process. The mass fraction-dependent ablation resistance evolution and oxidation–ablation mechanisms of W-HfC composites under extreme thermal loading were systematically studied and clarified. This work provides a valuable experimental and theoretical basis for the development of tungsten-based composites for high-temperature aerospace components.

2. Materials and Methods

Tungsten powders (99.95%, 1–3 μm) and HfC powder (99.9%, 1–3 μm) were used to fabricate the composites. Four W-HfC composites with different HfC contents (5, 10, 15, and 20 wt%) were prepared and labeled as W-5HfC, W-10HfC, W-15HfC, and W-20HfC, respectively. Both powders were dry ball-milled by a ball milling machine using a ball-to-powder mass ratio of 4:1. The mixture was processed at a rotation speed of 300 rpm for 8 h under Ar, with intermittent pauses of 10 min after every 30 min of milling to prevent overheating. The ball-milling process used tungsten carbide balls. The microstructures of W powder, HfC powder, and the mixed powder are shown in Figure 1. The composite powders were consolidated by SPS (20T-6-IV, Shanghai Chenhua Technology Co., Ltd., Shanghai, China) in a graphite mold at 2000 °C for 15 min in vacuum under a pressure of 40 MPa. The heating rate was maintained at 100 °C min⁻¹ up to the sintering temperature. Figure 2 shows the ram displacement and temperature curves during the ablation process. The sample size is 40 mm in diameter and 5 mm in thickness, and the relative densities of the composites were measured using Archimedes’ method.

Ablation tests were carried out using a plasma flame at 2800 °C. The temperature at the ablation center of the samples was measured by using a non-contact infrared thermometer. The temperature fluctuation during the ablation process is within ±25 °C. The ablation temperature was maintained and controlled through the automatic adjustment of the ablation distance, and the ablation distance was about 30 mm. The plasma jet was generated by ionizing argon and hydrogen in a direct current plasma torch, with gas flow rates of 45 L/min and 5 L/min, respectively. The inner diameter of the nozzle was 6 mm. The specimen was firmly held in its position in a graphite holder. The linear ablative rate was calculated based on the thickness and burn-through time. The microstructure and elemental distribution of the ablated samples were analyzed using a scanning electron microscope (SEM, TESCAN MIRA3) equipped with energy-dispersive X-ray spectroscopy (EDS) and operated in backscattered electron (BSE) mode. Phase identification of the specimens was performed using X-ray diffraction (XRD, 10°/min, 20–70° range). The partial oxidation products on the surface of the ablated samples were removed, ground into powder using an alumina mortar, and then subjected to XRD testing. The ablated samples were radially cut by diamond wire cutting to facilitate the observation of the cross-sectional morphology. The cross-section of the samples was polished by mechanical and vibration polishing.

3. Results and Discussion

3.1. Ablation Properties

The high-temperature ablation behavior of W-HfC composites was investigated. Figure 3 reveals the post-ablation surface morphologies of four W-HfC variants subjected to 2800 °C oxy-acetylene flame testing. The ablation process involves synergistic thermomechanical loading from both extreme temperatures and high-pressure conditions induced by high-velocity gas flow scouring, resulting in coupled oxidative and erosive degradation [28]. Gas flow dynamics from the central impingement zone toward peripheral regions caused mechanical perforation at the sample core. Surface analysis identified a continuous oxide layer with localized yellowish oxidation products near the central region. While W-5HfC and W-10HfC specimens maintained structural integrity without discernible cracking, W-15HfC and W-20HfC exhibited radial crack propagation from the core to periphery, leading to catastrophic fracture.

Table 1. Relative density, thickness, burn-through time, and linear ablative rate of W-HfC composites.

Composites	Relative Density (%)	Thickness (mm)	Burn-Through Time (s)	Linear Ablative Rate (mm/s)
W-5HfC	98.2	5.1	245	0.0208
W-10HfC	99.9	5.3	302	0.0175
W-15HfC	99.9	5.1	241	0.0211
W-20HfC	97.1	5.2	224	0.0232

presents the relative density, thickness, burn-through time, and linear ablative rates of fabricated W-HfC composites. Among the four samples, W-10HfC and W-15HfC exhibited superior densification. Notably, W-10HfC demonstrated the minimum linear ablation rate (0.0175 mm/s), indicating optimal ablation resistance under 2800 °C oxy-acetylene flame exposure. The linear ablative rate of W-HfC composites exhibits a non-monotonic dependence on the increase in HfC content, initially decreasing and then increasing. This trend reveals a critical optimization threshold, where ablation resistance is best at intermediate HfC concentrations.

The XRD diffraction pattern presented in Figure 4 indicates that the oxidation products generated by W-HfC composites during the ablation process primarily consist of WO₃, HfO₂, and the composite oxide HfW₂O₈. This suggests that W and HfC phases within the composite are both oxidized to form an oxide layer throughout the ablation process.

During ablation, tungsten undergoes oxidation to form WO₃, exhibiting a bright yellow coloration as observed in the central region of specimens in Figure 3. This porous oxide phase (melting point ≈ 1470 °C) melts and migrates toward the outer area of the surface [25]. HfC additions promote the formation of beneficial HfO₂ and HfW₂O₈ phases, constituting the protective surface oxide layer shown in Figure 3. Notably, the progressive increase in HfC content correlates with a marked reduction in surface WO₃ concentration, indicating that the oxidation product of W changes from WO₃ to HfW₂O₈ with the increase in HfC content. The enhanced ablation resistance of W-HfC composites is attributed to the formation of a dense HfO₂-HfW₂O₈ composite oxide scale, which seals surface pores and suppresses both inward oxygen permeation and outward metallic species migration during ablation. Ideally, sufficient HfC additions enable complete oxide scale coverage to maximize oxidation–ablation resistance. However, exceeding the 10 wt% HfC threshold induces a marked decline in ablation resistance concurrent with the emergence of ablation-induced crack propagation in post-test specimens. This performance degradation likely stems from HfC-induced embrittlement, where excessive HfC additions reduce fracture toughness, rendering the composite susceptible to thermomechanical stress cracking under coupled high temperature and high-velocity gas flow, which eventually led to a decline in ablation resistance [29].

3.2. Ablated Microstructure

Figure 5 presents section BSE micrographs and EDS elemental mapping of the ablated interface in W-HfC composites. The BSE images reveal a bilayered structure: the lower region corresponds to the bulk W-HfC material, while the upper region constitutes the oxide layer. Within the bulk material, the light-gray matrix is identified as W, and dark-gray particulates represent HfC reinforcements. EDS analysis confirms compositional segregation in the oxide layer, with W-rich phases (primarily HfW₂O₈ and minor WO₃) and Hf-rich phases (HfO₂) coexisting. The oxide scale across all W-HfC composites comprises a mixture of WO₃, HfW₂O₈, and HfO₂.

Figure 5 indicates that W-HfC composites with different HfC contents formed different oxide layer structures during the ablation process. EDS mapping reveals distinct phase distributions: W-5HfC primarily forms the composite oxide HfW₂O₈ with limited HfO₂ particles. The oxide layer exhibits internal cracking, while the outer surface transforms into a granular structure under high-temperature and high-velocity gas flow scouring, indicating insufficient HfO₂ formation to protect the matrix at low HfC additions. In contrast, W-10HfC develops a dense oxide layer comprising an HfW₂O₈ matrix uniformly reinforced with monolithic HfO₂ particles. The synergistic combination of HfW₂O₈ (exhibiting negative thermal expansion) and HfO₂ (exhibiting exceptional high-temperature stability and oxidation resistance) ensures tight interfacial bonding, resulting in optimal ablation resistance at 2800 °C. With increasing HfC content, W-15HfC and W-20HfC develop a stratified oxide structure where HfO₂ aggregates into an outer protective layer, while HfW₂O₈ predominantly resides in the inner oxide region, with a small part diffusing outward into the HfO₂ layer. The thermal expansion coefficient mismatch between HfO₂ and HfW₂O₈ induces deleterious microcracks within the oxide layer, accelerating oxygen inward diffusion, thereby increasing the linear ablation rate and reducing the ablation resistance.

3.3. Ablation Mechanisms

There are various reactions in the ablation process of W-HfC composites, written as follows [25,30]:

2W + 3O₂ → 2WO₃

(1)

W + 3H₂O → 2WO₃ + 3H₂

(2)

W + 3CO₂ → WO₃ + 3CO

(3)

2HfC + 3O₂ → 2HfO₂ + 2CO

(4)

HfC + 3CO₂ → HfO₂ + 4CO

(5)

2WO₃ + HfO₂ → HfW₂O₈

(6)

The primary oxidation product of W at high temperatures is WO₃, while HfC predominantly forms HfO₂ under similar conditions. WO₃ and HfO₂ can react via solid-state interactions to generate composite oxide HfW₂O₈ at elevated temperatures.

The proposed ablation mechanism of the composite material is illustrated in Figure 6. Tungsten initiates oxidation at approximately 400 °C, forming porous and loosely structured yellow WO₃, which partially sublimates or melts during subsequent ablation. Subsequently, HfC particles oxidize under ablation temperatures to primarily generate blocky HfO₂. For W-5HfC and W-10HfC composites, the resultant HfO₂ reacts with WO₃ at elevated temperatures to form HfW₂O₈, a composite oxide exhibiting negative thermal expansion behavior [31]. HfW₂O₈ demonstrates low thermal conductivity and enhanced thermodynamic stability under high-temperature oxygen-rich environments, effectively converting detrimental WO₃ into a protective phase. HfW₂O₈ forms a dense barrier layer that encapsulates residual blocky HfO₂ particles. The HfO₂ phase contributes superior thermophysical properties, including a high melting point (2900 °C) and low thermal conductivity (1.5 W/m·K) [32]. The synergistic combination of this dense oxide structure and uniformly distributed HfO₂ effectively impedes inward heat transfer and oxygen diffusion, thereby shielding the bulk W-HfC from further ablation damage. Consequently, moderate HfC additions (~10 wt%) optimize the ablation resistance of W-HfC composites.

For W-15HfC and W-20HfC composites, elevated HfC content drives excessive HfO₂ generation during ablation. Beyond reacting with WO₃ to form HfW₂O₈, residual HfO₂ progressively aggregates and migrates toward the surface, culminating in a bilayered oxide structure: an outer HfO₂-rich layer and an inner HfW₂O₈-dominated zone. Owing to the negative thermal expansion coefficient of HfW₂O₈, interfacial thermal expansion mismatch arises between the HfO₂ and HfW₂O₈ layers. This mismatch induces microcrack nucleation within the oxide scale during ablation, which accelerates inward heat transfer and oxygen diffusion. Consequently, intensified oxidative penetration degrades the ablation resistance of W-HfC composites.

4. Conclusions

In this study, W-HfC composites with varying HfC contents were fabricated via spark plasma sintering (SPS). Their high-temperature ablation behaviors under an oxyacetylene flame at 2800 °C were systematically investigated, and the ablation mechanisms were analyzed, leading to the following conclusions:

(1)

For insufficient HfC content (5 wt%), the dominant ablation product of W-HfC composites is HfW₂O₈, with limited HfO₂ formation during high-temperature ablation. The sparse HfO₂ distribution fails to effectively block heat flux and oxygen ingress. The W-5HfC composite exhibits a linear ablation rate of 0.0208 mm/s.

(2)

Optimal ablation resistance is achieved at 10 wt% HfC content with a minimized linear ablation rate of 0.0175 mm/s. The W-10HfC composite forms HfW₂O₈ with negative thermal expansion coefficients and uniformly dispersed HfO₂ blocks during ablation. The dense oxide layer, featuring strong interfacial bonding between HfW₂O₈ and HfO₂, effectively impedes heat and oxygen penetration, thereby enhancing ablation resistance.

(3)

Excessive HfC additions (15, 20 wt%) promote excessive HfO₂ formation, which aggregates and migrates outward, forming a layered structure with HfW₂O₈. The thermal expansion mismatch between these oxides induces microcracks, accelerating heat flux and oxygen infiltration, and degrading ablation performance.

These findings demonstrate that moderate HfC incorporation (~10 wt%) significantly enhances the ablation resistance of W-HfC composites, establishing them as promising candidate materials for rudders and nozzles of rocket motors.