The difference in densification between the pressureless sintering and hot pressing sintering lies in their distinct densification mechanisms.
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Pressure-assisted Sintering (e.g., Hot Pressing Sintering) -
Primary Driving Force:
External mechanical pressure. Sustained pressure is applied throughout the sintering process. It forces powder particles to compact, breaks up agglomerates, and fills pores via mechanisms such as plastic flow and creep. This is defined as mechanical densification.
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Advantages:
It effectively inhibits grain growth (since pressure facilitates densification rather than grain boundary migration). Moreover, it enables the production of sintered parts with near-theoretical density at lower temperatures and in shorter durations. For hard-to-sinter materials (e.g., ceramics with strong covalent bonds), the application of pressure is almost an essential prerequisite for achieving high densification.
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Pressureless Sintering -
Primary Driving Force: Reduction of powder particle surface energy. By using refined powder raw materials (e.g., nanopowders, spherical powders), optimized sintering aids, and controlling specific sintering atmospheres, a stable and effective liquid phase is generated at high temperatures. This liquid phase draws solid particles together through capillary force, and realizes mass transfer, pore filling and elimination via the dissolution-precipitation mechanism. This is referred to as
chemical/physicochemical densification.
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