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|Novel Machinable Ceramics|
|Joe Doyle, Materials Ireland Research Centre, University of Limerick, Ireland|
Why Machinable Ceramics?
Although ceramics have excellent abrasion resistance and thermomechanical properties (creep resistance, hardness…) as well as good chemical stability and corrosion resistance, they are generally difficult to shape. Sintering processes do not enable complex shapes to be formed with precise dimensions. Machining of ceramics is difficult and ultrasonic erosion, high velocity water jets containing abrasive slurries and laser cutting have not found widespread use in commercial production. CAD-CAM machining is possible with diamond tooling, but is very expensive and time consuming and applications are limited to high added value areas such as machined dental porcelains for crowns and inlays. However, dental porcelains tend to chip during machining because of their relatively low fracture toughness and strength. Clearly there exists a need for ceramics which can be easily machined without the risk of chipping.
Industrial Machinable Ceramics
Machinable glass-ceramics based on fluormica phases were developed in the early 1970s by Corning Inc. in the USA. Machinability arises from the randomly orientated mica crystals with a « house of cards » structure allowing cracks to readily propagate between the mica planes but hindering crack propagation across the layers. However, at that time, CAD-CAM technology systems, whilst technically excellent, were too expensive. The glass-ceramic developed was called Macor® and its composition was based on a magnesium-fluoro-alumino-silicate glass.
Due to falling costs CAD-CAM machining is an increasingly used technology. As a result Macor® is now a commercially important machinable ceramic and is used in many advanced applications: precision electrical insulators, vacuum feed throughs, microwave tube windows, in optoelectronics and scientific equipment and in over 200 parts on the space shuttle.
In addition to machinability, Macor® possesses attractive properties such as high dielectric strength (40 kV/nm) and very low gas permeability. The general properties of Macor® are summarised in Table 1.
Table 1: Summary of the properties of Macor®
However, Macor® is difficult to produce because of problems with volatilisation of fluorine, in the form of silicon tetrafluoride, during manufacture. This makes the material quite expensive, which precludes its use for many applications. Its relatively poor strength and fracture toughness, due to easily cleaved mica crystals, are also too low for many applications.
Machined Dental Ceramics
Cast dental restorations rarely achieve correct occlusal contacts because fabrication and contouring of the ceramic-tooth contact surface is difficult. However, correct contact surfaces can be readily achieved by CAD-CAM machining. Macor® was initially introduced to the dental area by Corning for CAD-CAM machined cores which were overglazed to produce crowns. They subsequently adapted the Macor® composition for lost wax die casting and launched Dicor®, which has a small share of the dental ceramics market.
Currently most CAD-CAM systems in the dental area use diamond tipped tooling and dental porcelain as the ceramic material. One system utilises a slightly modified Dicor® composition, Dicor MGC®. The machinability of the dental porcelains and Dicor MGC® is not ideal and they are prone to edge chipping during machining. Furthermore, for many applications both dental porcelains and Dicor MGC® lack sufficient fracture toughness, with each material having fracture toughness values in the range 1.0 – 1.4 MPam½. Chipping and surface cracking of machined crowns and inlays adversely effects their strength. Thus, a machinable glass-ceramic with improved fracture toughness and strength is required.
Improving Mechanical Properties
Macor® is based on fluorine phlogopite (KMg3AlSi3O10F2) crystals, whereas Dicor® is based on a tetrasilicic mica (KMg2.5Si4O10F2). In these systems the mica crystals are randomly oriented in a « house of cards » structure with potassium ions between the mica layers effectively binding the structure. Machinability is achieved by offering preferential cleavage between the mica planes, but hindering crack propagation across the mica layers. To improve fracture toughness it is necessary to make cleavage more difficult through partial or complete replacement of potassium.
Whilst mica based ceramics have been commercially available for over twenty years the fundamental problem of loss of silicon tetrafluoride during melting of the glass has not been addressed. Furthermore, loss of silicon tetrafluoride and its subsequent hydrolysis to hydrofluoric acid and silica has prevented any thorough systematic study of the important relationship between glass composition and crystal phase formation.
In a recently completed EU funded research programme (NOVMAC), the twin problems of costly production due to difficulties with volatilisation of fluorine and low fracture toughness of existing machinable fluoromicas have been addressed. The project involved two partners from the Euroceram regions (Materials Ireland, Shannon, Ireland and BCRC, Mons, Belgium) as well as industrial partners in the UK and the Netherlands and academic partners in the UK.
This project has shown that by replacing potassium with barium in fluoromica glass-ceramics, strength and fracture toughness were doubled and improved machinability was achieved. Although cleavage was still the preferred fracture mechanism it was rendered more difficult by replacement of potassium ions by barium ions in the layers between the mica planes. Thus, improved mechanical properties were obtained while maintaining machinability.
Previous studies on high fluorine content fluoro-alumino-silicate glasses for dental glass-ionomer cements indicated that the problem of silicon tetrafluoride loss during melting of the glass could be avoided by applying fluorine retention rules. The NOVMAC project showed that the rules can also be applied to the fluoromica systems. This enabled a thorough study of nucleation and crystallisation in fluoromica based glass-ceramics for the first time. Such fundamental and basic studies should lead to ease of commercial production, reduced cost, improved machinability, reduced machining time and further improved mechanical properties.
In NOVMAC, glass-ceramic compositions and heat treatments were tailored to give optimum « house of cards » microstructures as evidenced by elongated mica crystals of high aspect ratio with a high volume fraction and a high degree of interconnectivity. Ultimately this delivered highly machinable glass-ceramics, some of which were more machinable than Macor.
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