JOURNAL OF MATERIALS SCIENCE LETTERS 14 (1995) 744-747 Hydroxyapatite sintering characteristics: correlation with powder morphology by high-resolution microscopy A. J. RUYS, C. C. SORRELL Department of Ceramic Engineering, University of New South Wales, PO Box 1, Kensington, NSW 2033, Australia A. BRANDWOOD, B. K. MILTHORPE Centre for Biomedical Engineering, University of New South Wales, PO Box I, Kensington, NSW 2033, Australia Hydroxyapatite Cal0(PO4)6(OH)2 (HAp) is a hydrated calcium phosphate ceramic that closely resembles the chemical composition of bone mineral and is therefore one of the small group of bone- replacement materials classed as bioactive, i.e. capable of bonding chemically with bone [1]. This group includes calcium phosphate ceramics, which, although chemically similar to HAp, are generally more prone to in vitro dissolution [2], and certain calcium phosphate glass compositions [1]. Thus HAp is a prime candidate for bone replacement applications. HAp ceramics are prepared by forming a chemical precipitate of HAp by either the metathesis method [3], the acid-base method [4], or the metal alkoxide method [5], followed by sintering. HAp implants to be used in load-bearing sites must be sintered at a temperature that will optimize their strength. For most ceramics, this would require simply sintering to the point of maximum density. However, dense HAp has an unusual strength-temperature interre- lationship, previously characterized by the authors [6], in that the maximum strength occurs before the attainment of maximum density and closed porosity. The term "dense HAp" is defined here as HAp that has no open porosity, namely >95% dense [7], which is >3.0gcm -3 in the case of HAp. The strength drop after pore closure occurs because the dehydroxylation vapour pressure can exceed the mechanical strength of the solid, thereby degrading the microstructure during sintering [6]. Reversible dehydroxylation occurs at all elevated temperatures and above -800 °C, occurs at a signifi- cant rate [6]. At the decomposition temperature (generally -1200-1400 °C), irreversible dehydroxy- lation occurs and the HAp structure collapses, producing anhydrous calcium phosphates [6], which are biodegradable [2]. The temperature-density sintering curve of dense HAp follows a sigmoidal pattern, attaining a plateau density at specific temperature-density coordinates (here defined as Tplateau, Dplateau) [6, 8, 9]. Optimization of Tplateau (minimization) and Dplatea u (maximization) will re- duce both the reversible dehydroxylation vapour pressure and the likelihood of decomposition. It is well known that the sintering characteristics of a powder are related to surface area, although it is 744 not necessarily a simple relationship. Six different HAp powders encompassing a broad range of specific surface area values were used comparatively to evaluate the sintering-surface area correlation, including five precalcined commercially available powders suitable for thermal spraying applications (Plasma Biotal Ltd.: Px, P81, P88, P120, P149) and an uncalcined powder (defined as UC) produced by the authors using the acid-base method [4]. High-resolution microscopy of these six powders using a field emission scanning electron microscope (FESEM: Hitachi $900, resolution limit 0.8 nm at 30 kV) revealed that the particles of each calcined powder consisted of agglomerates of nanoparticles, while the uncalcined powder consisted of unagglom- erated nanoparticles. The correlation between these ultrastructural morphological features of HAp powders and the associated surface area and sinter- ing characteristics has not previously been investi- gated. The literature appears to contain no high- resolution SEM investigations of calcined or uncal- cined HAp, only transmission electron microscopy (TEM/STEM) and conventional SEM investiga- tions. Specific surface area as a physical quantity was measured using the BET method (UNSW Phlosorb). Morphological assessment by FESEM required the development of specialized sample preparation techniques in order to view the porous non-conducting mineral particles using a minimal amount of conductive surface coating to optimize conductivity and resolution. This was achieved by depositing the particles from dilute ethanol suspen- sions (suspension concentration was critical and specific to each powder type) on to copper supports, drying, and coating with a 3 nm thick layer of chromium. A magnification of 40 000x at 10 kV was found to be optimal for a comparative overview of the six powders since it was sufficiently high to show the finest nanoparticles but low enough to give a full view of the calcined agglomerates. Evaluation of the sintering characteristics in- volved the generation of sintering curves for each powder. Test pellets were cold-pressed (no binder) at 80 MPa, placed on a coarse granulated HAp substrate, and sintered for l h at a range of temperatures from 900 ° to 1400 °C in 50 °C steps 0261-8028 © 1995 Chapman & Hall