Dynamic shrinkage behavior of hydroxyapatite and glass-reinforced hydroxyapatites

Hydroxyapatite (HA) has generated a great deal of interest over recent years as a surgical implant material [1]. This is attributed to its bioactive nature which stems from the chemical similarity it has with the inorganic phase of natural bone [2–6]. Biologically, HA can serve as a means by which tissue, such as bone, can regenerate, and can be developed in various forms to serve this purpose either as a scaffold, coating or as a monolithic block [3]. However, mechanically, HA is limited due to its low load bearing capacity [1]. Recent studies have shown that the introduction of phosphate based glasses as a sintering aid is known to reinforce HA mechanically [2]. Furthermore, phosphate glasses are known to facilitate the decomposition of the HA phase to β-tricalcium phosphate (β-TCP) and can also lead to further inversion of the β-TCP phase to α-tricalcium phosphate (α-TCP) [2–5]. These phase changes can be brought about by ionic substitutions between the glass and HA that occur during liquid phase sintering, to which the extent of substitution can depend on the chemical nature of the glass [2, 4]. Some glasses may contain mineral ions such as Na+ or Mg2+ which could be responsible for structural changes in the HA lattice during the sintering process. Physical considerations must be highlighted to explain the effect these phosphate glasses have on HA. Phosphate glasses, when incorporated into HA, melt at lower temperatures compared with HA and can act to increase densification by enhancing the sintering mechanisms [2, 3, 6]. Although this process is known to improve the material mechanically it was important in this study to emphasize how this may be correlated to changes in density, porosity and in particular, shrinkage. A glass was produced with a composition of 32 mol% CaO, 23 mol% Na2O and 45 mol% P2O5. The starting reagents used to prepare the glass consisted of NaHPO4, P2O5 and CaCO3. These were mixed thoroughly and placed in a platinum crucible, melted at 1000 ◦C for 1 h, and then poured onto a steel plate and allowed to cool. The resulting glass was then ground to a fine powder using an agate grinder. The glass (5 or 10 g) powder was placed in a porcelain mill pot and milled dry for 24 h. HA (supplied by Plasma Biotal Ltd. UK) was then added to the mill pot at either 195 or 190 g to give glass additions of 2.5 and 5 wt% glass additions respectively (see Table I). Methanol (300 ml) was also added and the mixture was then wet milled for a further 24 h. The resulting slip was then dried at 70 ◦C and the dry powder was then sieved to 75 μm. Four grams of powder were placed into a steel die and uniaxially pressed at 20 tons using a hydraulic press to give 30 mm compacted discs. The discs were fired at a heating rate of 4 ◦C min−1 to 1200, 1250, 1300 and 1350 ◦C. The specimens were held at the appropriate temperature for 1 h then furnace cooled. The dimensions of the sintered discs were approximately 25 mm in diameter and 2.5 mm in thickness. The shrinkage behavior of the HA compacts in the various atmospheres was studied using a push rod type dilatometer (Netzch 402E). The shrinkage was measured in the axial direction. The sample support, Netchze measuring unit, and displaceable furnace of the dilatometer were mounted horizontally. The length change measurements were made by a transducer, which was maintained at a constant temperature by means of water circulation from a constant temperature bath. The accuracy of the measurement change in length was within ±0.1 mm. The temperature was measured using a calibrated thermocouple, which was placed directly above the sample. A small force of 0.2 N was applied to the sample through the push rod in order to maintain contact with the sample. The selection of the temperature program was controlled by a computer via a data acquisition system. Correction was applied to the expansion of the system by making a run under identical conditions using a standard sample (Al2O3). The shrinkage behavior was monitored from 0 to 1350 ◦C with a heating rate of 4 ◦C min−1. Density measurements were performed on each sintered specimen using the Archimedes principle. The measurements were carried out in distilled water. These were then compared with the theoretical densities calculated from the Rietveld analysis to give the porosity. Samples taken from the flexure test were ground to a fine powder, placed in a specimen holder and then analyzed on a Philips PW 1780 diffractometer with Ni filtered Cu Kα radiation ( Kα1 = 1.5406 A, Kα2 = 1.5444 A) at 40 kV and 30 mA. The data were collected with a scintillation counter between 10 ◦ < 2θ < 90 ◦