JOURNAL OF MATERIALS SCIENCE: MATERIALS IN MEDICINE 10 (1999) 401±409 Interfacial bond strength of electrophoretically deposited hydroxyapatite coatings on metals M. WEI*, A. J. RUYS { , M. V. SWAIN { , S. H. KIM*, B. K. MILTHORPE }, C. C. SORRELL* *School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia { Biomaterials Science Unit, University of Sydney, Sydney, New South Wales 2052, Australia { Centre for Advanced Materials Technology, Department of Mechanical & Mechatronic Engineering, University of Sydney, NSW 2006, Australia } Graduate School for Biomedical Engineering, University of NSW, Sydney, NSW 2052, Australia Hydroxyapatite (HAp) coatings were deposited onto substrates of metal biomaterials (Ti, Ti6Al4V, and 316L stainless steel) by electrophoretic deposition (EPD). Only ultra-high surface area HAp powder, prepared by the metathesis method 10CaNO 3  2  6NH 4  2 HPO 4  8NH 4 OH, could produce dense coatings when sintered at 875±1000  C. Single EPD coatings cracked during sintering owing to the 15±18% sintering shrinkage, but the HAp did not decompose. The use of dual coatings (coat, sinter, coat, sinter) resolved the cracking problem. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) inspection revealed that the second coating ®lled in the ``valleys'' in the cracks of the ®rst coating. The interfacial shear strength of the dual coatings was found, by ASTM F1044-87, to be * 12 MPa on a titanium substrate and * 22 MPa on 316L stainless steel, comparing quite favorably with the 34 MPa benchmark (the shear strength of bovine cortical bone was found to be 34 MPa). Stainless steel gave the better result since a-316L (20.5 mm mK  1 )> a-HAp ( * 14 mm mK  1 ), resulting in residual compressive stresses in the coating, whereas a- titanium ( * 10.3 mm mK  1 )< a-HAp, resulting in residual tensile stresses in the coating. # 1999 Kluwer Academic Publishers 1. Introduction 1.1. Hydroxyapatite coatings Deposition of bioactive coatings of hydroxyapatite (HAp) onto the surface of metal implants is a relatively recent development in clinical orthopaedics [1]. These bioactive surface ®lms are typically tens of micrometers thick. Thermal spraying [2] is the most developed process for depositing these HAp coatings and thermally sprayed implants have been used in clinical practice for some years now. Thermal spraying is a costly procedure and it is a line-of-sight process and is therefore not always ideal for coating implants of complex shape or morphology (mesh, macropores, etc.). However, thermal spraying has been the method of choice because with thermal spraying, deposition and densi®cation of the HAp coatings occur simultaneously, whereas most of the other coating methods require a subsequent densi®cation stage that involves heating the coated implant to sinter the HAp coating. The sintering temperature of HAp is generally above 1150  C. 1.2. Electrophoretic deposition Electrophoretic deposition (EPD) [3] is a low-cost ¯exible coating process, and, being a non-line-of-sight coating process, it can be used to deposit even coatings on substrates of complex shape or surface morphology. Furthermore, EPD can produce coatings of a wide range of thicknesses, from < 1 mm to > 100 mm, with a high degree of control over coating thickness and morphology. As for many other ceramic coating techniques, EPD- coated implants need a subsequent densi®cation stage in order to sinter the coating. This requirement poses something of a dilemma. If the thermal expansion coef®cient (a) of the ceramic coating is lower than for the substrate, then the coating is placed in compression on cooling, and if higher, then the coating is placed in tension on cooling. Ideally, the thermal expansion coef®cient of the coating and substrate should be very similar with a-coating slightly lower than a-substrate, since this will result in weak compressive residual stresses in the coating, which will inhibit cracking. {Corresponding author: Dr Andrew J. Ruys. Email: a.ruys@mech.eng.usyd.edu.au. 0957±4530 # 1999 Kluwer Academic Publishers 401