Yes, zirconia can be milled. This process is commonly performed using CAD/CAM systems, which are utilized for the fabrication of various dental prosthetic restorations such as crowns and veneers. Zirconia blocks used in these systems are available in both fully sintered and pre-sintered forms.
The pre-sintered zirconia, also known as "green-state" zirconia, is initially soft and malleable, making it unsuitable for milling until it undergoes a sintering process. This process involves heating the material to a specific temperature, which hardens it into a chalk-like state suitable for milling. The material density in the pre-sintered state is approximately 40% to 50% of its maximum theoretical density.
Fully sintered zirconia, on the other hand, has a lower volume fraction of pores, greater strength, and improved resistance to hydrothermal aging. It can be milled directly to the final desired dimensions without the need for further heat treatment, which could cause dimensional changes. However, the high strength of fully sintered blocks leads to longer milling times and rapid wear of machining tools.
Despite the need for post-milling sintering and accounting for sintering shrinkage, pre-sintered blocks are commonly used in CAD/CAM systems due to their shorter milling times, easier processing, and higher productivity.
Milling machines for zirconia can be categorized into wet type, dry type, and combined type. Wet type machines use coolant or water to remove excess material and reduce heat during milling, making them suitable for hard materials. Dry type machines use air for material removal and are ideal for softer materials like zirconia. Combined type machines offer the flexibility to use both wet and dry methods, providing versatility in processing various dental restoration materials but requiring a higher upfront investment.
After milling, zirconia undergoes sintering, a critical process that fuses materials together without liquefaction, significantly affecting the final mechanical strength and properties of the zirconia product.
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