Effects of fiber length and volume fraction on the reinforcement of calcium phosphate cement

A self-setting calcium phosphate cement (CPC) transforms into solid hydroxy apatite during setting at body temperature, and has been used in a number o f medical and dental procedures. However, the inferior mechanical propertie s of CPC prohibits its use in unsupported defects, stress-bearing locations or reconstruction of thin bones. The aim of the present study was to stren gthen CPC with fiber reinforcement, to examine the effect of fiber length a nd volume fraction, and to investigate the reinforcement mechanisms. Previo us studies employed either short fibers for random distributions, or contin uous fibers that were as long as the specimen size with preferred orientati ons such as unidirectional alignment. In the present study, a novel methodo logy was developed in which fibers several times longer than the specimen m old size were randomly mixed with the CPC paste to approximate the isotropy associated with short fibers, and at the same time achieve the high reinfo rcement efficacy associated with continuous fibers. Carbon fibers of 8 mum diameter were used with fiber lengths ranging from 3 mm to 200 mm, and fibe r volume fraction from 1.9% to 9.5%. A three-point flexural test was used t o fracture the specimens. Scanning electron microscopy was used to examine crack-fiber interactions and specimen fracture surfaces. The composite cont aining fibers of 75 mm in length at a volume fraction of 5.7% achieved a fl exural strength about 4 times, and work-of-fracture 100 times, greater than the unreinforced CPC. It is concluded that randomly mixing the CPC paste w ith carbon fibers that were several times longer than the specimen mold siz e resulted in substantial improvements in strength and fracture resistance; the reinforcement mechanisms were crack bridging and fiber pullout; and fi ber length and volume fraction were key microstructural parameters that det ermined the cement properties. (C) 2001 Kluwer Academic Publishers.