Cause Analysis of Pore Formation and Densification Measures in Alumina Ceramics Preparation

Abstract

Alumina Ceramics Are widely used in mechanical seals, electronic integrated circuit substrates, chemical valves, and biomedical artificial joints due to their high hardness (typically up to Mohs hardness level 9), excellent wear resistance, corrosion resistance, and good insulation properties (volume resistivity > 10¹⁴ Ω·m). However, pores generated during the preparation process—even closed pores with a volume fraction of only 2%—can reduce the flexural strength of the material by approximately 20% and significantly affect its dielectric strength and corrosion resistance. This paper systematically analyzes the primary mechanisms of pore formation in alumina ceramics and explores a series of effective densification measures from multiple dimensions, including raw material treatment, forming processes, sintering regimes, and the use of additives. Supported by specific experimental data and practical cases, the study provides a theoretical basis and process guidance for achieving high-performance alumina ceramics with a porosity below 0.5%.

Keywords

Alumina ceramics; Porosity; Densification; Sintering process; Forming technology; Additives

1. Problem Background and Challenges With the advancement of high-end equipment manufacturing toward high precision and reliability, the performance requirements for Structural Ceramics have become increasingly stringent. For alumina ceramics, when used in high-speed bearings or high-voltage insulating components, internal micron/submicron-sized pores become the "Achilles' heel" of the material. Research shows that when porosity increases from 0.5% to 5%, the flexural strength of the material can drop sharply from 380 MPa to approximately 240 MPa. These pores act as stress concentration points in stress fields, easily initiating microcrack propagation. In electric fields, they cause local field distortion, reducing insulation withstand voltage by more than 30%. In acidic/alkaline corrosive environments, open pores become fast channels for corrosive media infiltration, accelerating material failure. Therefore, achieving near-full densification of alumina ceramics (density ≥ 3.92 g/cm³, theoretical density 3.98 g/cm³) has become critical to overcoming their performance limitations.

2. Preparation Process and Pore Formation Scenario Analysis Taking the preparation of a typical 99% alumina ceramic substrate as an example: First, alumina powder with an average particle size of 0.5 μm is mixed with polyvinyl alcohol binder, glycerol plasticizer, and distilled water to form a uniform slurry with 75 wt% solid content via planetary ball milling (300 rpm, 4 h). The slurry is then shaped into 0.5 mm thick green tapes via tape casting or into disc-shaped green bodies via dry pressing at 100 MPa. Finally, the green bodies are sintered in an air atmosphere furnace: heated at 2°C/min to 600°C to remove the binder, then at 5°C/min to 1650°C and held for 2 h to complete sintering.

Pores can be introduced at almost every stage of this process:

Raw material stage: If powders with a wide particle size distribution (e.g., 0.1–10 μm) or hard agglomerates are used, inter-particle voids form in the green body, reducing the initial density to below 50% of the theoretical value.

Forming stage: In dry pressing, uneven pressure application (e.g., uniaxial pressing) can cause density variations of more than 10% in different regions, leaving numerous open pores in low-pressure areas. In tape casting, poor leveling or insufficient deaeration of the slurry can trap bubbles tens of micrometers in size.

Sintering stage: Rapid heating (e.g., >10°C/min) causes violent binder decomposition, forming networked pore channels. If the maximum temperature is too low (e.g., <1600°C), surface diffusion-dominated mass transport cannot effectively eliminate small pores. Conversely, excessively high temperatures or prolonged holding times (e.g., >1800°C or 5 h) can cause abnormal grain growth, trapping pores as closed pores.

3. Comprehensive Densification Solutions and Technical Measures 3.1 Raw Powder Optimization and Pretreatment Using submicron alumina powder synthesized via chemical co-precipitation (average particle size 0.2 μm, specific surface area 8 m²/g) significantly enhances sintering activity compared to traditional mechanically crushed powder. For example, one study reduced agglomerate size from 3 μm to below 0.5 μm by introducing 0.3 wt% oleic acid as a dispersant and wet ball milling with zirconia media for 12 h, increasing the dry-pressed green density from 1.85 g/cm³ to 2.15 g/cm³.

3.2 Precise Control of Forming Processes Isostatic pressing: Applying 200 MPa cold isostatic pressing (CIP) to pre-pressed green bodies increases the green density to over 60% of the theoretical value, with a density uniformity error of <1%.

Injection molding and tape casting: Using a multi-component binder system (e.g., PW+PE+SA) and a two-step debinding process—heating at 0.5°C/min to 400°C to remove low-molecular-weight components, then at 1°C/min to 600°C to remove residual polymers—effectively prevents pore and crack formation.

3.3 Sintering Regime Design and Advanced Sintering Techniques Atmosphere pressure-assisted sintering: Hot isostatic pressing (HIP) post-treatment (1400°C, 100 MPa Ar) reduces residual porosity from 1.5% in conventional sintering to below 0.1%. For instance, HIP-treated alumina ceramic bearing balls exhibit three times the fatigue life of untreated ones.

Spark plasma sintering (SPS): Holding at 1500°C under 50 MPa pressure for 5 min achieves >99.5% relative density, with grain size maintained below 1 μm, effectively preventing pore entrapment at grain boundaries.

Optimized multi-stage sintering curve: Experiments show that slow heating at 1°C/min in the 600–1200°C range facilitates organic decomposition and gas release; faster heating at 5°C/min in the 1200–1550°C range accelerates diffusion; and slow heating at 0.5°C/min above 1550°C promotes pore elimination, ultimately reducing porosity to 0.8%.

3.4 Scientific Selection of Sintering Additives Adding 0.5 wt% MgO inhibits grain boundary migration, preventing pore entrapment. Incorporating 1–3 wt% SiO₂+MgO forms a magnesium aluminosilicate liquid phase at grain boundaries, enabling densification via liquid-phase sintering at lower temperatures (1500–1550°C). For example, adding 1% TiO₂ reduces the sintering temperature of alumina by approximately 100°C while achieving a density of 3.90 g/cm³.

3.5 Sintering Atmosphere Control Sintering in a hydrogen atmosphere reduces oxide impurities on the Al₂O₃ surface, promoting grain boundary migration. Vacuum sintering (10⁻³ Pa) facilitates the removal of residual gases, making it particularly suitable for large components with thicknesses exceeding 20 mm.

4. Summary and Outlook Reducing porosity in alumina ceramics is a systematic engineering challenge involving multiple coupled factors. Practice shows that optimizing a single process is insufficient; a comprehensive strategy encompassing "powder pretreatment + forming homogenization + sintering enhancement + additive synergy" is essential:

Selecting highly reactive nano/submicron powders and improving their dispersibility.

Enhancing green body uniformity via CIP or optimized tape casting.

Choosing SPS/HIP or precisely controlled pressureless sintering based on product shape and performance requirements.

Rational incorporation of additives like MgO and rare-earth oxides to activate grain boundaries or form transient liquid phases.

Employing reducing or vacuum atmospheres to suppress closed-pore formation.

Through these integrated measures, high-reliability alumina ceramics with porosity below 0.5% and three-point flexural strength exceeding 400 MPa can be prepared, meeting the urgent demand for high-performance ceramics in semiconductor equipment, aerospace, and other advanced fields.