MEASUREMENT AND CONTROL OF INTERFACE STRENGTH OF RF MAGNETRON-SPUTTERED CA-PO COATINGS ON TI-6A1-4V SUBSTRATES USING A LASER SPALLATION TECHNIQUE

In previous studies, an RF magnetron sputter technique was developed f or the production of thin calcium phosphate coatings. Several depositi on parameters were found to influence the growth rate and the stoichio metry of the coatings. For instance, deposition with additional oxygen pressure decreased the Ca to P ratio of the coatings to 1.67. For app lication of these coatings on actual Ti implants, it is important to k now their adhesive tensile strength with the implant surface and how i t varies with the deposition parameters. Motivated by these goals, the aim of this study was to measure the adhesive tensile strength of the se coatings with Ti substrates and to study its variation with the dep osition parameters. Since most mechanical characterization methods are incapable of providing a direct measure of the interface's fundamenta l strength, a novel laser spallation experiment was used to accomplish this task. In this experiment,a compressive stress pulse is generated on the back side of a substrate by impinging a 3-ns long Nd:YAG laser pulse. The stress pulse propagates through the substrate and is refle cted into a tensile stress wave from the free surface of the coating d eposited on its front surface. The returning tensile pulse pries off t he coating if its amplitude is high enough. The peak interface tensile stress is computed by using the optically recorded free surface displ acement of the coating. Because interface decohesion is accomplished a t a strain rate of almost 10(7) s(-1), all inelastic processes essenti ally are suppressed and the measured value essentially is the intrinsi c tensile strength of the interface. Tensile strength values in the ra nge of 500-900 MPa were recorded for the interfaces between sputter-de posited calcium-phosphate coatings and Ti substrates. To confirm the l ocus of failure, the spalled spots were examined using SEM and EDS. Th e variation in the measured values was related to the changes in the d eposition conditions. (C) 1998 John Wiley & Sons, Inc. J Biomed Mater Res, 41, 624-632, 1998.