Heat densification methods includes: hot pressing, hot isostatic pressing, etc. As a matter of fact, hot squeezing and hot casting also belong to heat densification, but they are not widely used in ceramic production. Heat densification has high cost, but production rate is low, but for ceramics that need high performance while hard to get sintered, this method is the most widely used, because when press in high temperature, heat densification is helpful for for viscous and plastic flow, so it’s conductive to densification, therefore product those almost without hole can be obtained. In powder metallurgy field, this method is also called “all densification process”. Meanwhile, process time is short and temperature is low, grain growth is well controlled, so quality of final product can be improved.

1.（Hot Pressing, HP）

Hot pressing is combining forming and sintering in the same process, since compared with metal, special ceramics are harder to get formed and sintered, so hot pressing is often used to produce. Special ceramics like Al2O3, BeO, BN, Si3N4, SiC are usually produced by hot pressing, and for this method, advantage is that powder is pressed under thermoplastic condition, resistance of formation is small, which benefits plastic flow. Since pressure is low (only 1/10 of cold pressing) and process time is short, grain growth is well controlled, as a result, for the final product can have density closed to theoretical value, meanwhile, grain size is small and has very good microscopic structure. For many pure ceramic product, it’s impossible to get densified product by cold pressing, only hot pressing and hot isostatic pressing can be used. For hot pressing, the disadvantage is production rate is low, and cost is high.

2.（Hot Isostatic Pressing,HIP）

Hot isostatic pressing, HIP for short.

The same as hot pressing, hot isostatic pressing is also an important forming method for special ceramics. Compared with cold isostatic pressing, in which material of case is rubber, metal or ceramic case (low-carbon steel, Ni, Mo, glass, etc.) is used, and for the medium, nitrogen or argon is used, instead of liquid like engine oil. In HIP, advantages of hot pressing and cold isostatic pressing are combined together, with lower forming temperature, while get densified product with good performance. For instance, sintering of Al2O3 in normal pressure need higher than 1800℃, but for hot pressing under 20MPa, only requires 1500℃, and for HIP under 400MPa, densifed product can be obtained under 1000℃.

The disadvantage of HIP is that device is expensive and production rate is low.