Optimization of In Situ CO₂ Oxidation Temperature for Hydrogen-Resistant ZrO₂ Films on Zirconium Hydride

Abstract

Zirconium hydride is susceptible to dehydrogenation at elevated temperatures. In this study, zirconium hydride was oxidized by in situ oxidation in a CO₂ atmosphere at temperatures ranging from 550 to 700 °C for 10 h. The morphology, elemental distribution, phase structure, and hydrogen barrier performance of the resulting oxide films were systematically characterized using SEM, EDS, XRD, film adhesion and microhardness tests, and dehydrogenation experiments. At 550–600 °C, the formed oxide films are thin and non-uniform, containing numerous micropores and cracks, which results in limited hydrogen barrier performance. When the oxidation temperature is increased to 650 °C, a better balance between the oxidation reaction and diffusion processes is achieved. This leads to the formation of a dense, continuous, and uniform ZrO₂ film with strong adhesion to the substrate. As a result, the initial dehydrogenation temperature increases to 660 °C, while both the dehydrogenation rate and cumulative hydrogen release are significantly reduced, indicating the best overall hydrogen resistance. However, further increasing the oxidation temperature to 700 °C causes an excessively high oxidation rate, which introduces large growth and thermal stresses. These stresses promote the formation of microcracks in the oxide film, weaken the interfacial bonding strength, and consequently reduce the hydrogen barrier performance. The results demonstrate that the hydrogen permeation resistance of the oxide film is mainly governed by film compactness, defect evolution, and interfacial integrity. Based on these findings, 650 °C is identified as the optimal processing temperature for producing a high-quality hydrogen-resistant ZrO₂ film on zirconium hydride under a CO₂ atmosphere.

1. Introduction

Zirconium is an excellent neutron moderator material, primarily due to its low neutron absorption cross-section and high neutron scattering cross-section. In addition, collisions between neutrons and hydrogen atoms effectively reduce neutron velocity [1]. From a microstructural perspective, the lattice of zirconium can accommodate a large number of hydrogen atoms, resulting in a high hydrogen density in zirconium hydride, and, consequently, a significantly enhanced neutron-slowing efficiency [2]. Owing to its high hydrogen density, superior moderation efficiency, and good stability, zirconium hydride has become an important candidate material for solid neutron moderators in space nuclear power systems [3]. However, when zirconium hydride is used in high-temperature environments, such as those encountered in space nuclear power systems, it faces a critical challenge: a high equilibrium hydrogen partial pressure leads to hydrogen loss if not properly controlled. Hydrogen precipitation reduces the hydrogen-to-zirconium atomic ratio, thereby significantly degrading the neutron moderation efficiency and shortening the service life of the material [4]. Therefore, mitigating hydrogen loss from zirconium hydride at elevated temperatures is essential for its practical engineering applications.

Previous studies have shown that [1,5,6,7,8,9] the dehydrogenation process of metal hydrides consists of five main stages: hydrogen diffusion to the reaction interface within the hydride phase; phase decomposition of the hydride at the interface, producing mobile hydrogen atoms; diffusion of hydrogen through the decomposition products toward the material surface; desorption of hydrogen from the surface into the gas phase; and recombination of hydrogen atoms to form hydrogen molecules. Currently, two main approaches are used to suppress hydrogen loss from zirconium hydride at high temperatures. One approach involves alloying, where additional elements are introduced to form more stable hydrides. The other approach employs surface modification techniques to create a physical hydrogen barrier layer on the surface of zirconium hydride, thereby reducing hydrogen permeation [4,10]. Among these methods, surface treatment has minimal impact on neutron moderation performance and offers good process controllability, making it a promising strategy for addressing high-temperature hydrogen loss.

Recent efforts have focused on developing hydrogen permeation barrier coatings on zirconium-based substrates. For instance, Wang et al. reported the fabrication of multiphase ZrO₂ coatings on ZrH_1.8 via micro-arc oxidation, demonstrating markedly improved hydrogen resistance due to optimized phase distribution and film structure [11]. Li et al. further enhanced hydrogen blocking by introducing yttria pre-layer prior to oxidation, which significantly stabilized the tetragonal ZrO₂ phase [12]. Complementary studies using sol–gel approaches have shown that precursor composition critically affects coating density and barrier effectiveness [13]. Additionally, rare-earth-doped zirconia films have been demonstrated to suppress phase transformation-induced cracking, further reinforcing hydrogen barrier performance [14]. In another study, micro-arc oxidation was applied to AZ91 magnesium alloy using different voltages in a dual-electrolyte system. The results showed that the thermal conductivity of the resulting oxidation film was nearly 30% lower than that of the substrate, and that thicker and denser films could be formed, leading to improved material performance [15].

In situ reaction technology is an effective surface modification approach in which appropriate reactions and treatment temperatures are selected to induce chemical reactions that generate ceramic phases on the metal surface. This method is relatively simple, and the resulting films exhibit strong interfacial bonding with the substrate. Moreover, the structure and properties of the oxide film can be precisely controlled by adjusting the reaction gas introduction time, pressure, and heat treatment temperature during the in situ oxidation process.

In this context, the present study aims to systematically investigate the influence of in situ CO₂ oxidation temperature on the growth behavior, microstructural characteristics, mechanical properties, and hydrogen permeation resistance of ZrO₂ films formed on zirconium hydride. Although previous studies have reported various surface coating strategies to improve hydrogen barrier performance, a clear understanding of how oxidation temperature governs the coupling relationship between film growth kinetics, defect evolution, interfacial integrity, and hydrogen resistance under a CO₂ atmosphere remains limited. Therefore, this work focuses on identifying the optimal oxidation temperature within the range of 550–700 °C and clarifying the underlying structure–property relationship. The novelty of this study lies in establishing a comprehensive correlation between oxidation temperature, oxide film compactness, interfacial adhesion, stress evolution, and dehydrogenation behavior, thereby providing both mechanistic insight and a practical process window for fabricating high-quality hydrogen-resistant ZrO₂ films on zirconium hydride.

2. Materials and Methods

2.1. Materials

The matrix material used in this study was zirconium hydride with an H/Zr atomic ratio of 1.8. The zirconium hydride was cut into disk-shaped samples with dimensions of Φ20 mm × 2 mm using a CNC wire-cutting machine. After washing, the samples were sequentially ground using SiC waterproof abrasive papers. The polished samples were then ultrasonically cleaned in acetone and deionized water, followed by rinsing with ethanol to remove surface grease and impurities. Finally, the samples were dried using a hot-air blower and prepared for subsequent treatment.

2.2. Film Preparation

Figure 1 illustrates the experimental procedure for the in situ oxidation of zirconium hydride and the subsequent dehydrogenation tests. The pretreated zirconium hydride samples were placed in a tube furnace for oxidation. Prior to heating, the furnace chamber was evacuated to a pressure below 5 Pa, after which CO₂ gas was introduced until the pressure reached 0.1 MPa. This evacuation-purging process was repeated three to four times to minimize the influence of residual gases. After introducing CO₂, the gas flow rate was maintained at 100 mL/min. The tube furnace was then heated to the target temperature and held for isothermal oxidation.

2.3. Characterization Methods

The surface and cross-sectional morphologies of the in situ oxide films were examined using a high-resolution field-emission scanning electron microscope (FEI Apreo 2S, Shanghai, China). Energy-dispersive spectrometry (EDS) was performed using an Oxford Instruments Ultim Max 4.0 (Oxford Instruments, Shanghai, China). Film thickness was measured at five different positions on each side of the sample using a coating thickness gauge (MiniTest745). The adhesion strength between the oxide film and the zirconium hydride substrate was evaluated using an automatic scratch tester (WS-2005, Lanzhou, China), with measurements conducted at three locations on each side of the sample. Surface hardness was measured using a Vickers microhardness tester (HVS-1000A, Beijing, China) with a load of 200 g and a dwell time of 15 s. Five indentation points were selected on each side of the sample, and the hardness values were determined by manually measuring the indentation diagonals. The phase composition of the oxide film was analyzed using a PHILIPS APD automatic powder X-ray diffractometer with Cu Kα. The scanning step size was set to 0.02°.

2.4. Dehydrogenation Test

Hydrogen loss from zirconium hydride before and after oxidation was evaluated using a tube furnace (OTF-1200X, Heifei, China) coupled with a four-stage mass spectrometer (HPR-20 EGA, Heifei, China). Both untreated and in situ oxidized zirconium hydride samples were heated from 25 °C to 750 °C at a heating rate of 5 °C/min in a pure Ar atmosphere. The hydrogen barrier performance of the oxide film was assessed by continuously monitoring changes in hydrogen partial pressure using the mass spectrometer. In addition, the stability of the oxide film was evaluated under a CO₂ atmosphere. The oxidized zirconium hydride samples were held at 650 °C for 15 h, during which the hydrogen partial pressure evolution was recorded to assess long-term hydrogen resistance.

3. Results

3.1. SEM Surface Analysis

SEM observations of the surface morphology of zirconium hydride oxidized at different temperatures for 10 h in a CO₂ atmosphere reveal a strong temperature dependence of the oxide film morphology. As shown in Figure 2a–d, a black-gray oxide film formed on the surface of all samples, indicating that significant oxidation occurred at all experimental temperatures.

At lower oxidation temperatures (Figure 2a,b), the oxide film surface is rough and non-uniform, with numerous holes and microcracks. This suggests that the oxidation reaction rate does not adequately match the oxygen diffusion process, leading to preferential local growth and the formation of oxide regions with uneven thickness. The presence of holes provides fast diffusion pathways for hydrogen atoms, which is detrimental to hydrogen resistance. Although the film continuity improves slightly compared with Figure 2a, obvious cracks and pore defects are still observed locally in Figure 2b. These defects compromise the structural integrity of the film and are an important cause of barrier failure.

When the oxidation temperature is increased to 650 °C, the surface morphology of the oxide film is significantly improved. The film becomes more uniform, continuous, and compact, with almost no holes or penetrating cracks. This improvement can be attributed to enhanced oxygen diffusion rate and a more complete oxidation reaction, which promote the formation of a ZrO₂ layer with uniform thickness and a well-developed structure. In addition, the moderate temperature helps balance growth stress and thermal stress within the film, facilitating defect healing during formation and providing an effective barrier to hydrogen diffusion.

However, when the temperature is further increased to 700 °C, although the oxide film remains generally continuous, a large number of microcracks appear on the surface, with localized crack concentration. These defects are characterized by micropores and microcracks associated with discharge channels and thermal stress during cooling [16,17]. The formation of these defects is mainly attributed to two factors. First, a phase transformation from T-ZrO₂ to M-ZrO₂ occurs during oxide formation, leading to volume expansion and stress relaxation [18,19]. Second, the excessively high oxidation rate introduces substantial growth and thermal stresses, which accumulate in the zirconia coating during cooling [20]. This stress accumulation ultimately causes brittle cracking of the coating, thereby degrading its compactness and interface stability.

3.2. EDS Analysis

Figure 3 presents the surface morphology and elemental distributions of zirconium hydride oxidized in a CO₂ atmosphere at different temperatures for 10 h. The results show that oxidation temperature plays a decisive role in the uniformity and compositional homogeneity of the oxide film. At 550 °C and 600 °C, a large number of microcracks and pores are observed on the film surface. The distribution signals of Zr and O fluctuate strongly and exhibit poor correlation, indicating insufficient and non-uniform oxidation. At these lower temperatures, the oxygen diffusion rate is limited and the oxidation process is kinetically constrained, making it difficult to form a continuous and effective protective layer.

At 650 °C, the film surface becomes relatively smooth, and the Zr and O elements exhibit uniform and continuous distributions, indicating that a well-developed ZrO₂ layer is formed at this temperature. This suggests that an appropriate oxidation temperature provides optimal thermodynamic driving force and kinetic conditions, resulting in an oxide film with superior integrity and performance. When the temperature is increased to 700 °C, the surface elemental distribution remains more uniform than at 550 °C and 600 °C, and the signal intensity of the C element increases Figure 4. The element content of the surface EDS spectrum of the oxide film did not change much. However, despite the improved elemental uniformity, 700 °C oxidation process localized surface defects reappear, leading to an overall deterioration in hydrogen barrier performance. In the present work, the improved hydrogen resistance is primarily attributed to film densification and interfacial integrity rather than direct evidence of chemical trapping.

Based on the cross-sectional SEM images shown in Figure 5a–d and the corresponding EDS maps of Zr, O, and C, the microstructural evolution of the oxide film at different oxidation temperatures can be clearly identified. The Zr signal remains strong in the substrate and is reduced near the surface due to oxidation, while a pronounced enrichment of O in the surface layer confirms the formation of ZrO₂ adjacent to the matrix. At 550 °C and 600 °C, the O distribution is thin and non-uniform, reflecting a low oxygen diffusion rate and resulting in a thin oxide film with local compositional fluctuations. This discontinuous and porous structure provides preferential diffusion pathways for hydrogen, which explains the poor hydrogen barrier performance observed at these temperatures. At 650 °C, the oxygen diffusion capability is significantly enhanced, leading to a marked increase in O signal intensity and a uniform distribution across the oxide layer. As a result, a ZrO₂ protective layer with moderate thickness, dense microstructure, and a well-defined interface is formed. This optimized microstructure corresponds to the best hydrogen barrier performance.

When the oxidation temperature is further increased to 700 °C, the interfacial chemical state deteriorates. Although the oxide film becomes thicker, the EDS maps reveal an overall increase in carbon intensity, with pronounced carbon enrichment at the interface between the oxide film and the substrate. Previous studies have reported that hydrogen may interact with oxygen- and carbon-containing species within oxide films. In the present work, the improved hydrogen resistance is primarily attributed to film densification and interfacial integrity rather than direct evidence of chemical trapping. At 700 °C, although excessively high temperatures accelerate oxide film growth, they also introduce detrimental effects that compromise interfacial integrity.

3.3. XRD Analysis of the Oxide Film

Figure 6 shows the X-ray diffraction (XRD) patterns of the oxide films formed on zirconium hydride after oxidation at temperatures ranging from 550 °C to 700 °C. The main diffraction peaks can be indexed to three phases: monoclinic zirconia (M-ZrO₂), tetragonal zirconia (T-ZrO₂), and zirconium hydride (ZrH_1.8) from the substrate. Weak diffraction peaks corresponding to T-ZrO₂ are observed at approximately 31° and 50°, which is consistent with previous reports [21,22,23]. At lower oxidation temperatures (550 °C and 600 °C), the XRD patterns are dominated by the characteristic peaks of M-ZrO₂, with the strongest peak clearly appearing at around 28°. At these temperatures, the surface oxide layer is relatively thin, allowing X-rays to penetrate into the substrate; consequently, the diffraction peaks of ZrH_1.8 are also clearly detected. In particular, at 550 °C, the diffraction peaks of T-ZrO₂ are extremely weak and nearly indistinguishable, indicating that the oxide formed at low temperatures consists mainly of the thermodynamically stable monoclinic phase.

As the oxidation temperature increases from 600 °C to 700 °C, both the phase composition and crystallinity of the oxide films change significantly. The diffraction peak intensities of the oxide phases (M-ZrO₂ and T-ZrO₂) increase with increasing temperature. Although M-ZrO₂ remains the dominant phase throughout the entire temperature range, the characteristic T-ZrO₂ peak at approximately 31° becomes clearly distinguishable at 650 °C and further intensifies at 700 °C. The coexistence of monoclinic and tetragonal zirconia at higher temperatures may be attributed to the increased thermal stability of the tetragonal phase and the stress at the film–substrate interface, which can stabilize a small fraction of T-ZrO₂ [24,25,26]. Meanwhile, the relative intensity of the ZrH_1.8 substrate peaks decreases markedly with increasing temperature as the oxide peaks become stronger. This trend further confirms the formation of a thicker oxide layer at higher temperatures, which effectively attenuates the diffraction signal from the underlying zirconium hydride matrix. For the sample oxidized at 700 °C, two additional diffraction signals become noticeable at approximately 60° and 70° (2θ). These peaks can be indexed to high-angle reflections of monoclinic and partially tetragonal ZrO₂. Their appearance is mainly attributed to enhanced crystallinity and grain growth at elevated temperature, which makes previously weak reflections more distinguishable. No evidence of secondary phases such as zirconium carbide or suboxides was detected.

3.4. Hydrogen Resistance Test

Figure 7 shows the hydrogen partial pressure–temperature curves obtained during the dehydrogenation of zirconium hydride samples oxidized in a CO₂ atmosphere for 10 h at different temperatures (550 °C, 600 °C, 650 °C, and 700 °C), measuring in an inert high-purity Ar atmosphere. The temperature at which dehydrogenation begins—defined as the inflection point where the hydrogen partial pressure increases sharply—is a key indicator of the hydrogen barrier performance of the oxide film. A higher dehydrogenation onset temperature corresponds to a stronger hydrogen barrier effect.

As shown in Figure 7a, the dehydrogenation onset temperatures vary with oxidation temperature. The onset temperatures for samples oxidized at 550 °C, 600 °C, 650 °C, and 700 °C are approximately 612 °C, 623 °C, 660 °C, and 632 °C, respectively. Among these, the sample oxidized at 650 °C exhibits the highest onset temperature, indicating that the oxide film formed under this condition provides the strongest inhibition of hydrogen release. This trend is consistent with the microstructural and EDS analyses of the oxide films.

Figure 7a within the oxidation temperature range of 550–650 °C, the continuous increase in the dehydrogenation onset temperature (from 612 °C to 660 °C) directly reflects the progressive improvement in the structural integrity of the oxide film. With increasing oxidation temperature, the film becomes denser and more uniform (SEM analysis Figure 2a–d), its thickness increases moderately (thickness and weight-gain analysis, and the adhesion between the film and the substrate is significantly enhanced (mechanical property analysis). In particular, the oxide film formed at 650 °C consists of a dense and uniform ZrO₂ layer with strong interfacial bonding, which effectively suppresses the outward diffusion of hydrogen from the matrix.

In contrast, the dehydrogenation onset temperature of the sample oxidized at 700 °C decreases rather than increases. This behavior is consistent with the potential problems identified in previous characterizations. Although the oxide film formed at 700 °C exhibits the greatest thickness and highest hardness, the excessively high oxidation temperature introduces severe thermal and growth stresses. These stresses lead to the formation of microcracks and interfacial damage, manifested by reduced adhesion strength and increased surface defects. Such defects provide fast diffusion pathways for hydrogen, thereby weakening the hydrogen barrier function and causing hydrogen release at relatively lower temperatures.

Figure 7b shows the tangent slopes of the hydrogen partial pressure-temperature curves at 650 °C, which are used to estimate the dehydrogenation rate constant K. The calculated values are k1 (550 °C) = 1.04123 × 10⁻¹⁰, k2 (600 °C) = 7.71135 × 10⁻¹¹, k3 (700 °C) = 5.73854 × 10⁻¹¹, and k4 (650 °C) = 1.81272 × 10⁻¹¹. Overall, the dehydrogenation rate decreases with increasing oxidation temperature. The sample oxidized at 550 °C exhibits the highest dehydrogenation rate, while those oxidized at 650 °C and 700 °C show significantly reduced rates. This indicates that the oxide films formed at lower temperatures provide limited hydrogen resistance, whereas the denser films formed at higher temperatures effectively suppress hydrogen release.

Figure 8 presents the hydrogen partial pressure–time curves for samples held at 650 °C for 15 h in a CO₂ atmosphere, reflecting the hydrogen barrier stability of oxide films prepared at different oxidation temperatures. During the initial heating stage up to 650 °C and the onset of isothermal holding (left of the dashed line), the hydrogen partial pressure increased rapidly for all samples, indicating a transient dehydrogenation process. This transient peak is mainly associated with the accelerated diffusion of hydrogen in the matrix caused by rapid temperature increase and the initial response of the oxide film to hydrogen release. Significant differences are observed in the peak hydrogen partial pressures among the samples. The sample pre-oxidized at 550 °C shows the highest peak, followed by those oxidized at 600 °C and 700 °C, whereas the sample oxidized at 650 °C exhibits the lowest peak. This result indicates that oxide films formed at 650 °C provide a stronger suppression of transient dehydrogenation.

Figure 8. Hydrogen partial pressure–time curves measured at 650 °C for 15 h in a CO₂ atmosphere.

Figure 8. Hydrogen partial pressure–time curves measured at 650 °C for 15 h in a CO₂ atmosphere.

After the transient peak, the hydrogen partial pressure gradually decreases and enters a steady-state stage, where dehydrogenation is mainly controlled by diffusion. The cumulative hydrogen release was quantified by integrating the dehydrogenation curves (S). The calculated values are S_{550 °C} = 1.67779 × 10⁻¹⁰ Pa, S_{600 °C} = 1.52487 × 10⁻¹⁰ Pa, S_{650 °C} = 8.34207 × 10⁻¹¹ Pa, and S_{700 °C} = 1.23963 × 10⁻¹⁰ Pa. The sample oxidized at 650 °C exhibits the lowest cumulative hydrogen release, confirming its superior hydrogen barrier performance during prolonged high-temperature exposure.

Combined with the SEM, EDS, and XRD results, these findings demonstrate that oxidation at 650 °C promotes the formation of a dense, continuous, and stable ZrO₂ film on the surface of zirconium hydride. This film not only effectively suppresses transient dehydrogenation during initial heating but also significantly reduces long-term steady-state hydrogen release. In contrast, oxide films formed at 550 °C are thinner and defect-rich, while those formed at 700 °C suffer from microcracking due to excessive oxidation and thermal stress, both of which are detrimental to long-term hydrogen resistance.

3.5. Adhesion and Hardness Tests of the Oxide Film

Figure 9 summarizes the results of the adhesion and microhardness tests of the oxide films. It is evident that oxidation temperature exerts a significant and non-linear influence on the mechanical properties of the films. The adhesion strength and hardness exhibit distinctly different evolution trends as the oxidation temperature changes.

Figure 9. Adhesion strength and Hardness of oxide films formed at different oxidation temperatures.

Figure 9. Adhesion strength and Hardness of oxide films formed at different oxidation temperatures.

The adhesion test results indicate that the interfacial bonding strength between the oxide film and the zirconium hydride substrate is highly sensitive to oxidation temperature. As the temperature increases from 550 °C to 650 °C, the adhesion strength rises markedly from approximately 8 N to about 18 N, reaching its maximum value. This improvement is consistent with previous studies, as a moderate oxidation rate within this temperature range promotes the formation of a dense, uniform ZrO₂ film with good interfacial compatibility. Moreover, the thermal stresses generated during oxidation remain within the tolerance of the film–substrate interface, avoiding significant interfacial damage.

However, when the oxidation temperature is further increased to 700 °C, the adhesion strength decreases sharply to approximately 10 N. This degradation results from the combined effects of excessive oxidation and thermal mismatch stress. Excessive oxidation leads to rapid film thickening and the accumulation of growth-induced stresses. More critically, the difference between zirconia and the zirconium hydride substrate generates substantial residual thermal stress during cooling. When this stress exceeds the interfacial bonding strength, it initiates and propagates interfacial microcracks, significantly weakening the film–substrate adhesion. This mechanism also explains the increased surface defects and reduced uniformity observed in the 700 °C sample (Figure 2d), as interfacial damage extends to the film surface.

In contrast to the pronounced fluctuation in adhesion strength, the microhardness of the oxide films increases monotonically with oxidation temperature. The hardness rises from approximately 213 HV at 550 °C to about 271 HV at 700 °C. The continuous increase can be attributed to two synergistic strengthening mechanisms. First, higher oxidation temperatures promote more complete and uniform oxidation, resulting in a dense, high-purity ZrO₂ ceramic layer whose intrinsic hardness is significantly higher than that of the porous films formed at lower temperatures. Second, elevated temperatures can induce phase transformations in zirconia. Above approximately 600 °C, zirconia may partially transform from the monoclinic phase (m-ZrO₂) to the tetragonal phase (t-ZrO₂), which generally exhibits higher hardness and fracture toughness. The combined effects of densification and phase transformation contribute to the observed increase in film hardness. Excessive oxidation can generate significant growth stresses within the ZrO₂ layer and thermal mismatch stresses between ZrO₂ and the ZrH_1.8 substrate due to differences in thermal expansion and oxide growth kinetics. Such stress accumulation during high-temperature oxidation has been shown to promote microcrack initiation and propagation and degrade interfacial adhesion, consistent with observations in oxide scale growth and thermal barrier coating systems [27,28].

3.6. Oxidation Film Weight Gain and Thickness Tests

Figure 10 presents the measured weight gain percentage and thickness of the oxide films, clearly illustrating their quantitative dependence on oxidation temperature. These results demonstrate that oxidation temperature plays a decisive role in oxidation kinetics and film growth behavior.

Figure 10. Mass gain and thickness of oxide films formed at different oxidation temperatures.

Figure 10. Mass gain and thickness of oxide films formed at different oxidation temperatures.

Overall, both the mass gain and the oxide film thickness increase monotonically with increasing oxidation temperature, which is consistent with the general behavior of high-temperature oxidation governed by solid-state diffusion. As the temperature rises, the diffusion coefficient of oxygen ions in the zirconium matrix and the growing oxide layer increases significantly, thereby accelerating the interfacial chemical reaction rate and promoting continuous oxide film growth. Specifically, the film thickness increases steadily from approximately 1.8 μm at 550 °C to about 5.5 μm at 700 °C. Meanwhile, the mass gain percentage increases from roughly 0.04% at 550 °C to about 0.11% at 700 °C. Combined with the cross-sectional EDS results, these observations confirm that higher temperatures lead to more extensive oxidation, with a greater amount of oxygen incorporated into the surface to form ZrO₂.

A pronounced increase in both mass gain and film thickness is observed when the oxidation temperature rises from 650 °C to 700 °C, with the film thickness increasing by approximately 0.5 μm. When considered together with the increased defect density observed in the surface morphology at 700 °C (Figure 2d), this behavior provides insight into the underlying oxidation mechanism. Below 650 °C, oxide growth is mainly controlled by oxygen ion diffusion through a dense ZrO₂ layer, resulting in relatively uniform and gradual film growth. In contrast, at 700 °C, the oxidation rate increases substantially. Although the oxide film thickens rapidly, the resulting layer becomes relatively loose, porous, and non-uniform. In addition, the interfacial bonding strength deteriorates, as evidenced by the sharp decrease in adhesion strength at 700 °C. Consequently, despite the increased thickness, the protective effectiveness of the oxide film as a hydrogen barrier is compromised.

Although the oxide film thickness increases significantly at 700 °C due to enhanced oxidation kinetics, the excessively rapid growth results in stress accumulation and structural degradation. The presence of surface microcracks (Figure 2d) and reduced adhesion strength (Figure 9) indicates that the thick oxide layer formed at 700 °C is mechanically unstable. This demonstrates that oxide film performance is not solely governed by thickness, but rather by the balance between growth rate, stress evolution, and structural integrity.

4. Discussion

4.1. Membrane Growth Kinetics

The high-temperature oxidation of zirconium hydride in a CO₂ atmosphere is a multi-step, coupled reaction system involving dehydrogenation, gas–solid reactions, and solid-state diffusion. The growth kinetics of the oxide film can be described by the reaction sequence shown below [29].

During the initial heating stage, zirconium hydride undergoes a dehydrogenation reaction (Equation (1)):

$$Zr{H}_x = Zr + {\displaystyle \frac{x}{2}}{H}_2$$

(1)

This reaction produces an active metallic zirconium matrix for subsequent oxidation. The released hydrogen can further react reversibly with CO₂ according to Equation (2):

$${\mathrm{H}}_2+\mathrm{C}{\mathrm{O}}_2\leftrightharpoons \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$

(2)

At high temperatures, Reaction (2) generates H₂O, which exhibits higher oxidation activity toward zirconium than CO₂. As a result, the reaction between H₂O and Zr accelerates the formation of the ZrO₂ layer, leading to a relatively rapid oxide film growth rate in the early stage of oxidation. With continued oxidation, zirconium undergoes direct reactions with CO₂, as described in Equations (3) and (4):

$$\mathrm{Z}\mathrm{r}+2\mathrm{C}{\mathrm{O}}_2=2\mathrm{C}\mathrm{O}+\mathrm{Z}\mathrm{r}{\mathrm{O}}_2$$

(3)

$$\mathrm{Z}\mathrm{r}+\mathrm{C}{\mathrm{O}}_2=\mathrm{Z}\mathrm{r}{\mathrm{O}}_2+\mathrm{C}$$

(4)

These reactions dominate the subsequent growth of the oxide film. The generated CO escapes into the gas phase, while a portion of carbon is deposited at the oxide-metal interface. However, excessive or non-uniform carbon deposition may lead to local stress concentrations, promoting the formation of microcracks. Such defects can adversely affect the long-term structural integrity and stability of the oxide film.

4.2. Generation of Film Defects and Countermeasures

Zirconium hydride samples oxidized at temperatures ranging from 550 to 700 °C were systematically analyzed. The results show that the performance of the surface oxide layer is strongly dependent on the type and density of defects within the film. Defect formation is mainly governed by two key factors: oxidation kinetics and the evolution of thermal stress. Accordingly, effective countermeasures should focus on controlling these two aspects.

In the lower temperature range (550–600 °C), where the oxidation reaction is relatively insufficient, the dominant defects are micro-pores and compositional inhomogeneity. This behavior arises because the diffusion rate of oxygen in the zirconium matrix is limited at lower temperatures, preventing uniform and complete oxidation across the entire surface. As a result, locally under-oxidized regions form structural weak points. In addition, volatile by-products generated during oxidation (such as hydrogen) may escape through the growing film, leaving behind pores. These microvoids disrupt film continuity and provide pathways for hydrogen permeation, thereby degrading the hydrogen-barrier performance. These defects can be mitigated by optimizing the oxidation kinetics. In practice, this may involve moderately increasing the oxidation temperature to approximately 650 °C. Additional strategies include surface pretreatment prior to oxidation (e.g., mechanical polishing to remove pre-existing surface defects) and the use of a staged oxidation process. In such a process, an initial low-temperature step promotes the formation of a continuous oxide layer, followed by a high-temperature step to enhance film densification. This approach enables the formation of a more uniform and compact oxide film while ensuring sufficient reaction completion.

At excessively high oxidation temperatures (e.g., 700 °C), the dominant defects shift to microcracking and weakened interfacial bonding. This degradation results from the combined effects of two types of stress. First, rapid oxide growth generates significant intrinsic growth stress within the film. Second, large thermal stresses develop during cooling due to the mismatch in thermal expansion coefficients between zirconia and the zirconium hydride substrate. When these stresses exceed the mechanical tolerance of the oxide layer, crack initiation and propagation occur, potentially leading to localized interfacial debonding. To address these issues, the key strategy is effective stress management and interfacial strengthening. This can be achieved by carefully controlling the heating and cooling rates, particularly by employing slow cooling to reduce peak thermal stresses. In addition, introducing trace amounts of inert gas (Ar, N₂) into the oxidation atmosphere or controlling the partial pressure of reactive gases can moderately reduce the oxidation rate at high temperatures, thereby alleviating growth-induced stress. Furthermore, designing a compositional gradient transition layer or introducing a ductile interlayer between the substrate and the oxide film can improve mechanical compatibility at the interface and enhance the overall crack resistance of the system.

4.3. Hydrogen Resistance Mechanism

The surface oxide layer formed by high-temperature oxidation of zirconium hydride in a CO₂ atmosphere plays a critical role in inhibiting hydrogen permeation. The hydrogen-barrier capability of this oxide layer primarily depends on the integrity of its microstructure and the quality of interfacial bonding with the substrate. Because the growth mechanism and final structure of the oxide film vary significantly with oxidation temperature, the effectiveness of the film as a hydrogen diffusion barrier is strongly temperature-dependent.

Hydrogen permeation in solid materials involves several sequential processes, including adsorption, dissociation, diffusion, recombination, and desorption. Within the oxide layer, hydrogen transport follows an atomic diffusion mechanism [30]. At the microscopic scale, hydrogen atoms produced by molecular dissociation can chemically interact with oxygen ions in the oxide lattice to form O-H bonds, which impede hydrogen migration [31]. This trapping mechanism effectively retains hydrogen atoms within the oxide film and suppresses further diffusion [32]. Previous studies have shown that dense oxide films composed of mixed monoclinic and tetragonal phases exhibit excellent hydrogen permeation resistance [32,33].

In the oxidation temperature range of 550–650 °C, increasing temperature promotes a more complete oxidation reaction, resulting in a progressively denser zirconia layer and a reduction in surface defects. This dense microstructure significantly extends the diffusion path of hydrogen atoms within the oxide film and increases the activation energy required for hydrogen diffusion. Among the investigated conditions, the oxide layer formed at 650 °C exhibits the best hydrogen resistance, with the highest dehydrogenation onset temperature of 660 °C. This superior performance is attributed not only to the high density of the oxide film but also to strong chemical bonding and good mechanical interlocking between the film and the zirconium hydride substrate. Such a well-integrated interface effectively suppresses hydrogen accumulation and rapid transport along the interface.

When the oxidation temperature is further increased to 700 °C, the hydrogen resistance of the oxide layer deteriorates despite the continued increase in film thickness. The degradation arises mainly from two factors. First, the excessively high oxidation temperature accelerates oxide growth, leading to the accumulation of significant intrinsic growth stress within the film. Second, the mismatch in thermal expansion coefficients between zirconia and the zirconium hydride substrate becomes more pronounced at elevated temperatures, generating substantial thermal mismatch stress during cooling. The combined effect of these stresses promotes the formation of microcracks in the oxide layer and weakens interfacial bonding. These defects act as fast diffusion pathways for hydrogen, markedly reducing the barrier effectiveness of the oxide film and causing the dehydrogenation onset temperature to decrease to 633 °C.

5. Conclusions

In this study, the growth behavior, structural evolution, mechanical properties, and hydrogen resistance of oxide films formed on zirconium hydride via in situ CO₂ oxidation at 550–700 °C were systematically investigated. Based on comprehensive microstructural characterization and performance evaluation, the following conclusions can be drawn:

• Oxidation temperature strongly influences oxide growth kinetics and structural integrity. At 550–600 °C, limited oxygen diffusion and sluggish reaction kinetics result in thin, non-uniform oxide films containing microcracks and pores, which provide preferential diffusion pathways for hydrogen.
• At 650 °C, a favorable balance between oxidation rate and stress evolution is achieved. SEM and EDS analyses confirm the formation of a dense and continuous ZrO₂ layer with a well-defined interface. XRD results reveal the coexistence of monoclinic and tetragonal phases, which may contribute to enhanced mechanical stability. This optimized microstructure leads to the highest adhesion strength and the highest dehydrogenation onset temperature (660 °C), and the lowest hydrogen release rate.
• Although further increasing the oxidation temperature to 700 °C significantly increases oxide thickness and mass gain due to accelerated oxidation kinetics, excessive growth stress and thermal mismatch stress induce microcracking and interfacial degradation. As a result, hydrogen resistance deteriorates despite increased film thickness.
• The hydrogen barrier performance of the oxide film does not increase monotonically with thickness. Instead, it is governed by the combined effects of film compactness, defect evolution, phase structure, and interfacial integrity. These findings establish 650 °C as the optimal processing temperature for producing a mechanically stable and hydrogen-resistant ZrO₂ film under a CO₂ atmosphere.

This work provides a systematic structure–property correlation framework for optimizing in situ oxide films on zirconium hydride for high-temperature hydrogen retention applications. Although the present study demonstrates that in situ CO₂ oxidation at 650 °C can effectively enhance hydrogen permeation resistance of zirconium hydride, further investigations are required before practical application in nuclear power systems. Future work should focus on evaluating irradiation stability under neutron exposure, corrosion resistance in reactor-relevant coolant environments, long-term thermal cycling durability, and the coupled effects of stress and hydrogen diffusion. These studies will provide a more comprehensive understanding of the coating reliability under realistic service conditions and help bridge the gap between laboratory optimization and engineering application.