Apatite-coated bio-inert implants can provide well-behaved bone substitutes in bioactive and mechanical properties. In this work, citric acid (CiA) was added into a dipping solution of 1.67Ca(NO3)(2)-PO(OH)(x)(OEt)(3-x) to investigate its effect on wettability, apatite formation, and film morphology. The dipping solution with a CiA/Ca ratio of 0.5 led to a good wettability on alumina substrates, formation of oxy-hydroxyapatite phase in the film after 800 degreesC heat treatment, rough or porous surface morphology, and adhesive strength up to 10 MPa. CiA in the dipping solution has a strong gelation ability and gives rise to corresponding changes in the film. (C) 2002 Elsevier Science B.V. All rights reserved.