Available online at www.sciencedirect.com Journal of the European Ceramic Society 31 (2011) 2065–2071 Porous hydroxyapatite coating on strong ceramic substrate fabricated by low density slip coating-deposition and coating-substrate co-sintering Jing-Zhou Yang a,b , Rumana Sultana a , Xiao-Zhi Hu a,∗ , Zhao-Hui Huang b a School of Mechanical and Chemical Engineering, University of Western Australia, Perth, WA 6009, Australia b School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, PR China Received 3 February 2011; received in revised form 13 April 2011; accepted 17 May 2011 Abstract This study aims at developing a process technique, which can deposit porous scaffold-like hydroxyapatite (HA) coatings on strong ceramic substrates. As a first trial, micro-porous HA coatings on strong zirconia-based substrates are fabricated by the following technique—consisting of low-density HA-slip coating-deposition on the micro-porous substrates pre-sintered at 900 ◦ C, and coating-substrate co-sintering at 1300 ◦ C. The final co-sintering process ensures a strong bonding between the HA coating and the zirconia-based substrate after minimizing the mismatch in thermal expansion coefficients by adding alumina in HA coating and HA in zirconia-based substrate. The presence of porosity in the HA coating also reduces the mismatch. HA decomposition during the co-sintering process is discussed. © 2011 Elsevier Ltd. All rights reserved. Keywords: Functionally graded coatings; C. Mechanical properties; D. Apatite; D. ZrO 2 ; E. Biomedical application 1. Introduction Hydroxyapatite (HA), in particular HA scaffolds, have attracted increasing attention in recent years for potential bone implant applications because they are bio-resorbable and osteoactive. The key concern for such applications is the low bending strength of HA scaffolds due to their open pore struc- tures as even the strength of dense HA is only around 100 MPa or less or about one-third of the upper bending strength limit of nat- ural bones. 1 To overcome the concern on low bending strength of man-made HA, dense HA coatings on strong ceramic or metal substrates have been explored to combine the attractive bio- properties of HA coating with the strength of strong substrates. For instance, various coating processes including sol–gel, 2,3 laser and induction plasma, 4 slurry/suspension dipping, 5,6 ther- mal/plasma spray 7 and electrophoretic deposition 8 have been attempted. A sol–gel derived HA coating on a ceramic sub- strate is normally very thin, e.g. less than 5 m, which limits its applications. Fully sintered zirconia and alumina have been also used as substrates in the slurry dipping or sol–gel deposi- ∗ Corresponding author. Tel.: +61 8 6488 2812; fax: +61 8 6488 1024. E-mail address: xhu@mech.uwa.edu.au (X.-Z. Hu). tion processing, 5,9,10 and decomposition of HA is not an issue as HA coating is typically formed at temperature less than 1000 ◦ C. However, a strong ceramic bond between HA coating and dense ceramic substrate may not be achieved easily, and the coating thickness is also limited because of the fully dense substrates. Despite of the development of those compromising HA com- posite designs, the problem of low load-bearing capacity of HA scaffolds remains unresolved. Pure HA has been sintered from nano-powders from 900 ◦ C up to around 1300 ◦ C. 11–16 Such a temperature range is also use- ful to nano-sized zirconia and alumina powders. For instance, the upper limit of 1300 ◦ C can be used as the sintering tem- perature for nano-sized zirconia and alumina powders, and the lower limit of 900 ◦ C can be used as the pre-sintering tem- perature so that pre-sintered green zirconia and alumina parts can be green machined. In fact, it has been a common practice that ultra-fine grained zirconia (3Y-ZTP) blocks pre-sintered at around 1000 ◦ C are green-machined to the required dental crown geometries at dental labs, followed by dental porcelains coatings and final firing. 17,18 In this study, we explore a new processing method, which can potentially combine the bio-properties of HA scaffolds and the load bearing capacity of zirconia. The method can be summarized as follows: HA coatings with graded porosity are 0955-2219/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2011.05.025