Analysis of Sintering Brittleness in Dry-Pressed Ceramics: A Multidimensional Perspective from Raw Materials to Manufacturing Processes

Ceramic materials are widely used in industry due to their excellent high-temperature resistance and corrosion resistance. However, dry-pressed ceramics often exhibit significant brittleness after sintering, which severely impacts their mechanical performance and service life. The root causes of this brittleness are multifaceted: inherent defects in raw materials, process parameter influences, and the effects of equipment and molds all play critical roles. In this article, we will explore the origins of ceramic brittleness from multiple scales and factors, providing a comprehensive understanding to guide improvements.

1. Raw Material Factors: Microdefects as the Origin

The properties of raw powders—such as particle size distribution, impurities, agglomeration, and formulation design—are fundamental to the final ceramic’s toughness.

• Uneven Particle Size Distribution:

When the D90/D10 ratio exceeds 5, powder exhibits a bimodal or multimodal distribution. This uneven packing leads to microstress imbalances during pressing, resulting in anisotropic shrinkage paths. During sintering, differential shrinkage at grain boundaries causes stress concentrations, promoting fracture along grain boundaries.

• High Agglomerate Content:

Insufficient milling or surface modification can result in agglomerates exceeding 15%. These clusters are difficult to decompose during sintering, creating secondary pores and unreacted particles, which serve as crack initiation sites and reduce fracture toughness.

• Impurities:

Elements like Fe and Na exceeding 0.3 wt% can form low-melting-point secondary phases or brittle intergranular phases at high temperatures. These weaken grain boundaries and increase the risk of brittle fracture.

• Formulation Defects:

Uneven distribution of sintering aids (coefficient of variation > 0.25) causes abnormal liquid phase distribution at grain boundaries—local excesses weaken the interface, while deficiencies hinder grain growth. Improper proportions of boundary phases (<5% or >15%) can also impair toughness—too little results in poor bonding, too much leads to excessive grain refinement and brittleness.

• Volatile Components:

Volatile oxides like PbO volatilize at temperatures above 800°C at rates exceeding 2%/h, causing compositional deviations and microstructural defects, ultimately decreasing material toughness.

Summary: Interactions among multiscale defects and impurities in raw materials lead to intergranular fractures post-sintering, reducing toughness by 20-40%.

2. Process Parameters: Key to Microstructural Control

Process parameters directly influence the microstructure and stress distribution within the ceramic, playing a pivotal role in mitigating brittleness.

• Pressing Pressure:

 Insufficient pressure results in high porosity and lower density; excessive pressure can cause interfacial delamination and microcracks, serving as crack initiation points.

• Heating Rate:

At around 900°C (β-transformation point), rapid heating (>5°C/min) induces thermal stresses, abnormal grain growth, and entrapment of pores, leading to microcracks and reduced toughness.

• Holding Time:

Less than 120 minutes may prevent complete densification and pore elimination, facilitating crack propagation. Extending to 180 minutes improves sealing and toughness, but overly long durations can cause excessive grain growth, adversely affecting properties.

• Cooling Rate:

Cooling faster than 3°C/min introduces residual stresses and phase imbalance, resulting in brittle microstructures and diminished plasticity and fatigue life.

Summary: Precise control of heating, soaking, and cooling parameters ensures uniform microstructure and optimal grain size, significantly enhancing ceramic toughness.

3. Equipment and Mold: The Invisible Killers of Microdefects

The condition and precision of manufacturing equipment and molds are crucial in determining the internal quality of ceramic billets.

• Mold Wear:

Long-term use causes dimensional deviations (>0.1mm) and surface roughness, leading to asymmetric structures and uneven density distribution, which promote stress concentration and cracking during sintering.

• Pressing Machine Settings:

Pressure fluctuations (>5%) and slight variations in holding time (~3 seconds) result in uneven pore and residual stress distribution, fostering microcracks.

• Temperature Control of Equipment:

Temperature deviations over 15°C or fluctuations in oxygen partial pressure can lead to uneven grain growth and weakened interfaces, increasing crack susceptibility.

Summary: Regular maintenance and precise control of equipment parameters are essential to ensure the toughness and reliability of ceramic products.

Conclusión

The brittleness of ceramics is a result of complex interactions across multiple factors and scales—from microdefects in raw materials to process optimization and equipment maintenance. A systematic approach to analyzing and refining each stage is vital for reducing brittleness, enhancing mechanical performance, and expanding the application potential of dry-pressed ceramics.

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