Crack Propagation Resistance of a-Al 2 O 3 Reinforced Pulsed Laser-Deposited Hydroxyapatite Coating on 316 Stainless Steel SHUBHRA BAJPAI, 1 ANKUR GUPTA, 2 SIDDHARTHA KUMAR PRADHAN, 1 TAPENDU MANDAL, 3 and KANTESH BALANI 3,4 1.—Surface Engineering Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013, India. 2.—Advanced Material Materials Processing and Analysis Center, Department of Materials Science and Engineering, University of Central Florida, Orlando, FL 32816, USA. 3.—Biomaterials Processing and Characterization Laboratory, Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur 208016, India. 4.—e-mail: kbalani@iitk.ac.in Hydroxyapatite (HA) is a widely used bioceramic known for its chemical similarity with that of bone and teeth (Ca/P ratio of 1.67). But, owing to its extreme brittleness, a-Al 2 O 3 is reinforced with HA and processed as a coating via pulsed laser deposition (PLD). Reinforcement of a-Al 2 O 3 (50 wt.%) in HA via PLD on 316L steel substrate has shown modulus increase by 4% and hardness increase by 78%, and an improved adhesion strength of 14.2 N (improvement by 118%). Micro-scratching has shown an increase in the coefficient-of-friction from 0.05 (pure HA) to 0.17 (with 50 wt.% Al 2 O 3 ) with enhancement in the crack propagation resistance (CPR) up to 4.5 times. Strong adherence of PLD HA–Al 2 O 3 coatings (4.5 times than that of HA coating) is attributed to efficient release of stored tensile strain energy (17 9 10 3 J/m 2 ) in HA–Al 2 O 3 composites, making it a potential damage- tolerant bone-replacement surface coating. INTRODUCTION The use of hydroxyapatite (HA: Ca 10 (PO 4 ) 6 (OH) 2 , Ca/P ratio = 1.67) as a biomaterial coating is well established. It was first carried out in 1984 when the pulsed laser deposition (PLD) technique evolved and the first hydroxyapatite coating by this tech- nique was reported in 1991. 1 The PLD technique has been shown to be a good method for depositing amorphous or crystalline, dense or porous films while maintaining the complex stoichiometry by controlling the laser parameters. 2–4 More recently, because of the extremely small heat-affected zone and limited interactions that are associated with PLD, its applications have been extended to the areas of micro-electro-mechanical systems, nano- structure medical devices, biosensors, ceramic nanocomposite film and engineered hard tissue coatings. 3,5–8 Several other methods such as plasma spraying, dip and spin coating, sol–gel, ion-beam sputtering and electrophoretic deposition have been employed to deposit HA coatings. 1,9–13 Among them, plasma spraying has found much more industrial acceptance and has, therefore, been investigated intensively. 1,14–16 In order to retain structural integrity, HA coatings need to be crystalline in nature. The present clinical coating contains about 70% crystalline HA. 1 But plasma-sprayed HA coatings possess low crystallin- ity (54%), 14 which leads to uncontrollable dissolu- tion into some unwanted phases such as tetra calcium phosphate (TTCP), a- and b-tri-calcium phosphate (a-TCP, b-TCP), and exhibits insufficient mechanical properties like low fracture toughness (0.6 MPa m 1/2 ). 17–21 The inability of plasma spraying to produce thin coatings (less than 0.5 lm) 22 is another drawback, as thin HA coatings are favorable for achieving the high fatigue resistance required for a sufficient life time of bone implants. There are some common methods applied to address the above-mentioned issues, i.e. to improve toughness and adhesion of HA films which include: (1) achieving uniform distribution of alloying elements and nanoparticles throughout the HA JOM, Vol. 66, No. 10, 2014 DOI: 10.1007/s11837-014-1152-3 Ó 2014 The Minerals, Metals & Materials Society (Published online September 23, 2014) 2095