TANG Jianhua ^{1, 2}, XU Xiuzhu ³

(1. School of Intelligent Manufacturing, Changzhou Vocational Institute of Textile and Garment, Changzhou 213164, Jiangsu, China; 2. Jiangsu Engineering Research Center for Intelligent Manufacturing Technology of Carbon Fiber and Advanced Material, Changzhou 213164, Jiangsu, China; 3. School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, China)

Extended abstract:

[Background and purposes] Ceramic capacitive pressure sensors are widely used in automotive and aerospace fields, due to their excellent corrosion resistance and thermal stability. However, with the rapid development of new energy vehicles, service environments have become increasingly harsh, particularly involving coupled high-humidity and elevated-temperature. In practical applications, alumina ceramic diaphragms may suffer from premature failure caused by stress corrosion cracking (SCC) associated with subcritical crack growth (SCG), even when the applied stress remains well below the nominal fracture strength. Existing studies still lack quantitative descriptions of critical water activity and stress thresholds under hydro-thermal coupling conditions. Therefore, this study is aimed to systematically investigate the mechanical degradation mechanism of 96% alumina ceramic diaphragms in hydro-thermal environments, thus establishing quantitative reliability thresholds to support engineering design.

[Methods] Pressure-sensitive diaphragms were made of 96% alumina ceramic with a specific focus on the grain boundary glass phase. A comprehensive experimental platform was built to simulate the hydraulic brake system environment using pure water, DOT3 brake fluid and their mixtures (5%–80% water content). The experimental design comprised three dimensions. (1) Pressurized Immersion Tests: samples were subjected to constant fluid pressures for 15 days in different media to evaluate the medium-stress coupling effect. (2) Temperature Accelerated Tests: experiments were conducted at 25 ℃, 50 ℃ and 80 ℃ to evaluate the influence of temperature on crack growth kinetics. (3) Mechanical Characterization: Residual strength of the diaphragms was measured using the "Ball-on-Ring" biaxial flexural strength test method according to the ASTM F394-78 standard. The loading rate was constant at 0.25 mm·min⁻¹. Subsequently, to reveal the degradation mechanism, microstructural evolution was examined by using scanning electron microscopy (SEM), while energy-dispersive spectroscopy (EDS) mapping was used to characterize the elemental distribution at grain boundaries. In addition, X-ray photoelectron spectroscopy (XPS) was employed to compare the chemical state evolution of surface-related oxygen and silicon species before and after hydro-thermal exposure. The apparent activation energy (Q) of the corrosion reaction was calculated using the Arrhenius equation based on strength loss data. Furthermore, a two-parameter Weibull statistical analysis was performed to evaluate the changes in failure probability distribution. Finally, long-term fatigue tests (1500 h) were conducted at 80 ℃ at different stresses to verify the engineering safety threshold.

[Results] There is a significant non-linear dependence of mechanical properties on environmental factors. Firstly, regarding the medium effect, pure water caused a severe strength reduction of approximately 15%, as compared with the air environment, indicating that polar water molecules play a dominant role in inducing degradation. In contrast, anhydrous DOT3 brake fluid suppressed strength degradation, with a slight strength increase of approximately 8%. A critical water content of approximately 10% in brake fluid was identified, below which the failure sensitivity of the ceramic diaphragm remained comparable to that under atmospheric conditions. Secondly, regarding the temperature effect, elevated temperatures significantly accelerated the degradation process. Based on the Arrhenius plot of the natural logarithm of strength loss versus the reciprocal of temperature (1/T), the apparent activation energy (Q) was calculated to be 51.9 kJ·mol⁻¹. This value falls within the typical energy range reported for hydrolysis-related reactions in silicate glass networks, suggesting that the degradation process is closely associated with chemical instability of the grain boundary glass phase rather than direct corrosion of alumina grains. EDS mapping confirmed the enrichment of Si-containing glassy phases along grain boundaries, while XPS analysis revealed an increased contribution of non-bridging or hydroxyl-related oxygen species after hydro-thermal exposure, indicating a clear chemical state evolution of the glass phase. Thirdly, the Weibull statistical analysis revealed that water corrosion deteriorates not only the material's strength but also their reliability consistency. The characteristic strength (σ0) decreased from 436.2 MPa in air to 373.5 MPa in pure water. More importantly, the Weibull modulus (m) decreased from 23.05 to 20.61. Although an m value of 20.61 still represents relatively good consistency, the reduction indicates that hydro-thermal exposure introduces additional scatter into the defect population. Finally, the long-term fatigue tests validated the reliability threshold. At a working pressure of 2.5 MPa (corresponding to a maximum von Mises stress of 107 MPa), no failure occurred in any sample after loading for 1500 h, even in the harshest pure water environment at 80 ℃. Calculation based on the Weibull parameters indicates that the safe stress for 0.1% failure probability is approximately 267 MPa, providing a safety factor of about 2.5 for the proposed 107 MPa threshold.

[Conclusions] This study systematically clarifies the degradation behavior of alumina ceramic diaphragms under hydro-thermal coupling conditions and provides quantitative design guidance. (1) Degradation mechanism: mechanical degradation is governed by the synergistic effects of moisture and temperature. Water molecules preferentially interact with the grain boundary glass phase, leading to chemical state evolution of the silicate network, with an apparent activation energy of 51.9 kJ·mol⁻¹. (2) Environmental threshold: water content of approximately 10% in brake fluid is identified as a critical threshold, beyond which the risk of stress corrosion cracking increases significantly. (3) Design threshold: a stress level of 107 MPa is established as a conservative safety threshold for long-term operation. It is recommended that the maximum working stress in ceramic pressure sensor diaphragms should be strictly controlled below this value to ensure reliable service throughout the full vehicle life cycle.

Key words: ceramic pressure transducer; alumina ceramic; critical stress; subcritical crack growth; biaxial flexural strength