Processing of calcium metaphosphate-based glass-ceramic coatings on alumina
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Processing of calcium metaphosphate-based glass-ceramic coatings on alumina K. J. Vaidya and L. F. Francis Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455-0132 (Received 16 December 1994; accepted 3 October 1995)
Dense, crystalline, glass-ceramic coatings containing calcium metaphosphate and aluminum phosphate were prepared on aluminum oxide substrates by a three-step method. The processing involved glass (40 mol % CaO, 10 mol % Al2 O3 , 50 mol % P2 O5 ) formation, deposition of a glass particle coating, and heat treatment to sinter the glass and crystallize the phosphates. Sintering and microstructure evolution were influenced by wet coating thickness, heat-treatment temperature, time, and heating rate. Heat treatment for 1 h at 725 ±C using a 50 ±Cymin heating rate was found to give a dense, crack-free coating. The resultant coating microstructure has spherulitic morphology (0.3 mm size) with aluminum phosphate in the center of the spherulite. The hardness of the fully crystallized glass-ceramic coating was ,5.2 GPa.
I. INTRODUCTION
Calcium phosphates, such as hydroxyapatite Ca10 (PO4 )6 (OH)2 (HA), are increasingly used as bulk prosthesis materials1 and as coatings on metals and ceramics.2 Many applications take advantage of the ceramics’s compatibility with existing bone tissue. Hydroxyapatite implants with controlled porosity enable bone tissue ingrowth, resulting in firm attachment of the prosthesis to the bone.3 Bioactive HA coatings also promote bone tissue formation around the coating.4 While porous calcium phosphate coatings have received much attention, dense coatings also have importance in biomedical applications, particularly in restorative dentistry. For example, long-term clinical evaluation of titanium dental implants with dense HA coatings has shown firm bone-implant attachment and reduced healing time.5 The research described in this paper demonstrates preparation of dense, crystalline calcium phosphate-based coatings on alumina. HA coatings are typically applied using vapor phase deposition techniques such as plasma-spraying6 , laser ablation,7 and sputtering.8 The high temperatures involved in these processes result in partially glassy coatings containing tricalcium phosphate Ca3 (PO4 )2 (TCP) in addition to the desired crystalline HA. These multiphase coatings have inferior strength due to the brittle glassy phase and a high dissolution rate due to the presence of TCP. HA coatings may be prepared by deposition of slurry onto a substrate followed by drying and heat treatment. Our initial attempts at this method were not successful as coatings were too porous after low temperature heat treatment (,1000 ±C) and temperatures greater than 100
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J. Mater. Res., Vol. 11, No. 1, Jan 1996
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1100 ±C resulted in decomposition of HA and formation of TCP. Considering the problems associated with current vapor phase coating techniques and the t
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