Sintering of the Ultrahigh Pressure Densified Hydroxyapatite Monolithic
Total Page：16
File Type：pdf, Size：1020Kb
Journal of MATERIALS RESEARCH Welcome Comments Help Sintering of the ultrahigh pressure densified hydroxyapatite monolithic xerogels J´an Majling, Peter Zn´aik, Angela Palov´a, and Stefan Svet´ık Department of Ceramics, Slovak Technical University, 812 37 Bratislava, Slovak Republic Stefan Koval´ık Department of Materials Science and Technologies, Slovak Technical University, 812 37 Bratislava, Slovak Republic Dinesh K. Agrawal and Rustum Roy Intercollege Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 16 February 1996; accepted 28 June 1996) Dense and translucent ceramics were prepared by sintering of cylindrical preforms of hydroxyapatite extruded from xerogels. Extruded specimens were dried as monoliths and then consolidated by applying cold isostatic pressure, ranging from 500 to 1500 MPa. Upon heating the samples began to densify at 610 ±C, and the densification/sintering was completed at 870 ±C as was evidenced by the dilatometry plot indicating no further shrinkage. The sintered specimens thus formed were translucent in appearance. Further heating of the samples up to 1200 ±C resulted in their “bloating” or creation of pores in the originally dense matrix. Pore creation within the structure is reproducible, it proceeds from the surface to the interior of the sample, and its spreading can be thermally controlled. Pore evolution within the single phase dense polycrystalline material is not related to the frequently occurring phenomenon of microcracking in ceramics during cooling. I. INTRODUCTION powder). The consolidation of powder was achieved Hydroxyapatite (HAp) ceramics draw special inter- by the cold isostatic pressing (350 MPa), and sintering est for their biocompatibility. Dense apatite ceramics are of resultant pellets was accomplished in a microwave in commercial use for bone replacement in the human furnace. body. Fabrication procedures for dense hydroxyapatite Cold isostatic pressing of HAp powders by medium, ceramics are quite diversified. Most of the researchers high, and very high pressures is a most frequently have used methods involving the formation of suitable used technique to consolidate powders, in order to gel-like precipitates1,2 and then drying the obtained pow- smear the aggregates of particles. In this contribution ders. The resulting xerogels are then calcined,3,4 milled, we have examined the influence of isostatic pressures compacted,5,6 and finally subjected to sintering.7,8 (500–1500 MPa) on the densification of hydroxyapatite Aggregation of powders during drying and calcina- preforms which were subsequently sintered at a constant tion stages (dry route) causes nonuniformity in packing rate. The preforms were fabricated as monoliths by of particles in green compacts, and thus prevents full extruding the wet gel. During drying the consolidation of densification during sintering. Colloidal or sol/gel proc- particles was expected to contribute to the uniformness esses are therefore considered in order to prevent the of the green structure. aggregation of powders. A defect-free fine microstructure in a ceramic is II. EXPERIMENTAL synonymous with the optimum mechanical properties The hydroxyapatite gel (with CayP ratio of 1.8) of polycrystalline materials and is usually manifested was precipitated from 0.2 M solutions of Ca(NO3)2 and by the translucency or transparency of the respective NH4H2PO4, with pH adjusted to 12. The two solutions materials. The preparation of a translucent1 HAp ce- were mixed together and heated in a closed plastic ves- ramics was reported in which solution route assisted sel at 50 ±C for three days. The resultant suspension in the partial consolidation of colloidal particles. The was homogenized by shaking several times a day. Am- transparent HAp was obtained after hot pressing.9,10 In monia was boiled out, and the precipitate was decanted a recent report, however, it was also prepared by the several times, filtered, and thoroughly washed with dis- pressureless sintering,11 using a noncalcination route and tilled water. The wet cake was extruded using an adapted hydrothermally synthesized powder (an aggregate free syringe into small cylinders of 1.5 cm in diameter. The 198 J. Mater. Res., Vol. 12, No. 1, Jan 1997 1997 Materials Research Society J. Majling et al.: Sintering of the ultrahigh pressure densified hydroxyapatite monolithic xerogels cylinders were dried while suspended vertically at room TABLE I. Relative densities of green and sintered (at 1000 ±C) temperature, and subsequently calcined at 300 to 500 ±C samples of hydroxyapatite. for 5 h. The calcined preforms were wrapped in a thin Pressure Green density Sinter density aluminum foil and encapsulated with a flexible rubber MPa A B A B coating by dipping them into a natural latex emulsion. The encapsulated samples were isostatically pressed in a 500 0.51 0.55 0.99 0.96 universal hydraulic press, equipped with a high pressure 1000 0.59 0.60 0.98 0.97 1500 0.66 0.62 0.96 0.90 steel vessel capable of withstanding pressures up to 2000 MPa. A preforms calcined at 300 ±C. ± The phase composition of hydroxyapatite gels was B preforms calcined at 500 C. analyzed by x-ray diffractometry (STADI, Stoe). Its specific surface area was measured by BET (Sorptomatic (1100 ±C) XRD pattern (Fig. 1). By heating the gel to 1900, Fisons). Sintering experiments were performed 1180 ±C (HT), the average crystallite size is increased, on , 1 cm long cylinders using a Netzch dilatometer as is evident by the sharper XRD peaks in the HT pat- (Model 402 E) at a heating rate of 10 ±Cymin in a static tern (Fig. 1). air atmosphere. The stress induced in the sample by the push rod of the dilatometer was 0.5 N. Bulk density of Dilatometric measurements samples was measured by Archimedes’ technique. The After further heating of samples above 1000 ±Cat microstructures (fracture surfaces) of sintered samples which shrinkage of samples was completed, an expan- were observed by SEM (Tesla BS 300). sion of the samples was observed at about 1200 ±C as is evident from their dilatometric curves (Fig. 2). III. RESULTS AND DISCUSSION The expansion occurs at a temperature that is well above the temperature at which the shrinkage of samples The precipitation procedure of the hydroxyapatite completely ceased. Figure 2 shows a typical dilatometric gel was essentially the same as the one given in the 1,12 curve (for a sample pressed at 500 MPa). The dried literature with a slight modification that in our prepa- heated preforms of extruded cylinders, which were not ration the solutions of Ca(NO3)2 and NH4H2PO4 were subjected to cold isostatic pressing, did not exhibit any simply poured together. Instantaneous massive nucle- such expansion at all. ation occurred, and the primary particles were expected A fracture surface (perpendicular to the axis of to be smaller than those obtained in the method in which the sample cylinder) of the sample, cooled to room one solution was added dropwise to the second stirred temperature from the temperature at which the expansion solution. just commenced, showed an inner dense (D) and a The precipitate was filtered, thoroughly washed, ± circumferential porous (P) zone (Fig. 3). There exists dried, and when heated at 1200 C for 0.5 h, exhibited a transitional zone between these two constant density single phase material of well-crystallized hydroxyapatite zones which is also shown in detail in Figs. 4(b) and with no trace of tri calcium phosphate. This was consid- 4(c). Figures 4(a) and 4(d) are enlargements of the D and ered as a proof that the precipitated powder was not P zones from Fig. 3. A sample that has just completed a calcium deficient hydroxyapatite as reported in the literature.2 Dried filter cake showed a specific surface area of 106.2 m2yg. The extruded wet cylinders shrank in their diameters from 1.5 cm in the wet state to approx. 0.52 cm in the dry state. Pressing to 500 MPa reduced the cylinder’s diameters further to approx. 0.39 cm. The green densities of pressed samples are listed in Table I. In the first run of experiments, the samples at sintering (dilatometric) experiments were heated only to 1000 ±C, a tempera- ture well above the temperature at which the shrinkage of samples had completed. Samples after heating to 1000 ±C were translucent with a slight ruby color. Their relative densities are given in Table I. The theoretical density of hydroxyapatite was taken as 3.156 gycc.13 Crystallite size estimated from the surface area data FIG. 1. X-ray powder diffraction patterns for samples before and after is around 15–20 nm, so the particles are very fine an expansion, LT: sample heated to 1000 ±C; HT: sample heated to which is also reflected in the line broadening in LT 1180 ±C. J. Mater. Res., Vol. 12, No. 1, Jan 1997 199 J. Majling et al.: Sintering of the ultrahigh pressure densified hydroxyapatite monolithic xerogels FIG. 2. Dilatometric curve of the hydroxyapatite sample compacted at 500 MPa. LT, HT, and MS refer to thermal curing of samples taken for x-ray analysis (Fig. 1) and for the microstructure analysis (Figs. 3 and 4). expanding exhibits larger crystals and single phase HAp. In the x-ray diffraction pattern (HT), the diffraction peaks are much sharper than the corresponding diffrac- tions of the unexpanded sample, LT (Fig. 1). The reason for the poring of the originally dense ma- trix and a corresponding expansion of sample is believed to be due to the presence and the phase transition of tricalcium phosphate (CTP).14–16 As shown in Ref. 16 FIG. 3. Microstructure of a sample in which the expansion has just commenced, exhibiting inner dense (D) and a circumferential porous such a phase transformation causes the intragranular (P) zones. fracture without creating pores.