Machinable calcium pyrophosphate glass-ceramics
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Mitsuo Niinomi Department of Production Systems Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi 411-8580, Japan (Received 15 June 2000; accepted 5 January 2001)
Glass-ceramics containing a large amount of calcium pyrophosphate (–Ca2P2O7) crystal were prepared via crystallization and sintering; compacts of a calcium phosphate glass powders with a composition of 60CaO ⭈ 30P2O5 ⭈ 5TiO2 ⭈ 5Na2O (in mol%) were heated for 3–6 h at 850 °C in air. The compacts were densified by the viscous flow of the glassy phases during heating. The calcium phosphate glass-ceramics were found to show a bending strength of approximately 100 MPa, and they were easy to machine, as confirmed by a drilling test using a conventional carbide tool. The machinability is thought to arise from the microstructure consisting predominantly of the interlocking and platelike –Ca2P2O7 precipitated in the glass.
I. INTRODUCTION
Calcium phosphate ceramics such as hydroxyapatite or -tricalcium phosphate show high biological compatibility and safety in living tissues. These ceramics, however, do not have the desirable performances in mechanical properties such as bending strength, fracture toughness, or machinability. Attempts have been made to improve the strength and toughness of the ceramics by introducing fibers or particles into the matrix phase.1–4 However, very little work has been reported on the improvement of the machinability, to the best of our knowledge. If the ceramics could be machined using conventional tools, they would be used for various applications in surgical or dental fields. Mica-containing glass-ceramics such as MACOR威 (Corning Inc., Corning, NY) have been known to show good machinability,5 that is, the ability to be cut, sharpened, planed off, or scraped off using conventional tools. Preparation of bioactive, machinable glass-ceramics have been reported by the precipitation of apatite and mica crystals in a glass during heat treatment.6 The easy machinability of the ceramics resulted from the cleavage of mica crystals precipitated in the glass.5–7 Materials containing no mica crystals such as CaO–Al2O3–Y2O3–SiO2 glass-ceramics 8 or 40.2CaO–39.1P 2 O 5 –7.4Al 2 O 3 – 5.5TiO2–7.8SiO2 glass-ceramics9 have been reported also to be easy to machine although the exact mechanism for the machinability is not clearly understood.
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J. Mater. Res., Vol. 16, No. 3, Mar 2001 Downloaded: 21 Mar 2015
In the present work we report, for the first time, machinable SiO2-free glass-ceramics containing a large amount of calcium pyrophosphate (–Ca2P2O7; hereafter denoted by CPP) crystals. Calcium pyrophosphate ceramics have been known to have bioactivity, which is the ability to form a chemical bond directly with natural bone.10,11 The materials discussed in the present work are expected to have potential applications in the area of bioceramics. II. EXPERIMENTAL PROCEDURE
In our earlier reports,12–14 calcium phosphate glasses in the pyrophos
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