Silicon nitride (Si₃N₄) ceramic materials are widely used in industries due to their excellent high temperature resistance and corrosion resistance. With the increasing application of Si₃N₄ ceramic materials, the research on the service in various extreme environments has attracted recent attention, especially the corrosion resistance of Si₃N₄ ceramics in corrosive liquids (i.e., the corrosion behavior of HF on Si₃N₄ ceramics).

Si₃N₄ ceramics have a superior acid corrosion resistance. A prolonged contact with HF solution leads to a decrease in the stability of Si₃N₄ ceramics, thus damaging the overall function. For porous Si₃N₄ ceramics, its unique structure aggravates the corrosion of acid solution. In some special applications, porous Si₃N₄ ceramics are required to have sufficient strength and acid resistance to ensure application stability and life. However, the existing reports mostly focus on Si₃N₄ composite ceramics or dense Si₃N₄ ceramics with sintering aids. Little work on the preparation of Si₃N₄ ceramics using pure Si powder as a raw material for nitridation sintering, specifically the corrosion behavior of porous Si₃N₄ ceramics in HF, has not been reported yet. In this work, Si₃N₄ ceramics were prepared via nitridation reaction sintering using a pure Si powder as a raw material without any sintering additives. The acid corrosion resistance of Si₃N₄ ceramic materials with a porosity of up to 30% prepared was investigated to explore the corrosion behaviors of and time on Si₃N₄ ceramic samples at different HF concentrations.

High-purity porous Si₃N₄ ceramics with a porosity of 30% were prepared via nitridation reaction sintering using Si powder with a purity of 99.99% (in mass fraction) as a raw material. After cold isostatic pressing at 200 MPa, the ceramics were processed into the samples with appropriate sizes, and their density, flexural strength, and phase composition were analyzed. The HF solutions with different mass fractions of 20%, 30%, and 40% were prepared, respectively. In the first group, the same amount of HF solution with different mass fractions was taken, put into plastic containers with the ceramic samples, and corroded at room temperature for 200 h. In the second group, the HF solution with a mass fraction of 40% was used as an etching solution, and the samples were placed for corrosion. The corrosion time was 100, 200, 300 h, and 400 h, respectively. Each group of samples was sealed and placed in dark. Afterwards, the samples were cleaned, dried and tested, and the changes of mass, density phase composition and other properties before and after the corrosion were analyzed.

As the mass fraction of HF solution increases from 20% to 40%, the ceramic sample reacts with HF to form a corrosion product (NH₄)₂SiF₆ phase, which attaches to the surface of the sample to form a loose corrosion layer, and gradually fells off and dissolves in the acid solution, resulting in an increase in the mass loss rate. Simultaneously, the increase of pores caused by corrosion reaction also results in a higher porosity, lower density, and reduced flexural strength of the ceramic samples as the HF concentration increases. The change in the microstructures of the samples before and after the corrosion indicates that the Si₃N₄ ceramic sample is immersed in HF solution to produce (NH₄)₂SiF₆, which causes the loss of Si and N, leading to an increased porosity and a reduced density within the sample, and thereby degrading the performance of the ceramic sample.

The mass loss rate and porosity of ceramic samples increase, while density and flexural strength decrease as the corrosion time increases from 100 h to 400 h. From the perspective of corrosion rate, the corrosion rate decreases after 200-h corrosion. This is because the corrosion products of (NH₄)₂SiF₆ formed on the sample surface can block a direct contact between HF and the ceramic sample at a constant HF solution concentration. The extension of the corrosion time does not lead to an increase in the corrosion rate, indicating that the corrosion layer plays a significant role in maintaining the stability of Si₃N₄ ceramics in a corrosive environment.

Si₃N₄ ceramic materials prepared by nitridation reaction sintering with pure Si powder were corroded in HF solution for 200 h. The results showed that the mass, density and bending strength of ceramic samples decreased gradually, and the porosity increased with the increase of HF concentration and corrosion time. The Si₃N₄ ceramic sample was immersed in HF solution and reacted with HF to form a phase of (NH₄)₂SiF₆, leading to a loose corrosion layer on the surface of the sample. The mass loss and density of Si₃N₄ ceramic samples were reduced due to the corrosion reaction, and the presence of the corrosion layer reduced the corrosion rate, indicating that the high-purity porous Si₃N₄ ceramics were more stable in HF solution, and could maintain a better corrosion-resistant performance.