Barium fluorphlogopite based machinable glass ceramics for maxillofacial applications

Objectives: Commercial fluormica glass-ceramics such as Macor and Dicor (Corning, USA) are based on potassium micas. Their machinability is a result of the mica crystal structure and the weak bonding between the layers. Barium fluorphlogopite micas have potential as alternatives to Macor and Dicor in maxillofacial applications because the barium phase is known to impart high strength and fracture toughness. However, little is known about their fine machinability. The objective of this study is to evaluate the machinability of two barium fluorphlogopite compositions, LG228 and LG254, to determine their suitability for maxillofacial applications.

Methods: LG228(8SiO2-1.5Al2O3-3.75MgO-2.25MgF2-BaO) was fired at 1395°C (90mins) and cerammed at 1175°C (300mins) and LG254 (8SiO2-3Al2O3-1.0Li2O-2MgO-3MgF2-BaO) was fired at 1430°C (60 mins) and cerammed at 1125°C (240 mins). The machinability of the materials was evaluated by two methods; 1) a conventional CNC milling technique which evaluates the degree of smoothness of a drill tip travelling through a block of the glass ceramic; the smoother the tip's progress, the greater the machinability and 2) from the machinability parameter, n, based on hardness measurements, H (measured from ten indents on the surface), where n= 0.643- 0.122H.

Results: 1) From the CNC technique, LG228 deflected the drill tip less, inferring that the material was more machinable. Examining the surfaces that were in contact with the drill, there was evidence of smearing on the surface of the LG254, but a clean microstructure was evident in LG228 suggesting high machinability. 2) For LG254, n=0.33 and for LG228, n=0.42.

Conclusions: Positive n values indicate both materials are machinable but the higher value, coupled with the better result in the CNC test, showed that LG228 was the more machinable glass ceramic.