Mechanism And Experimental Research On Fabricating Hydroxyapatite Coating By Cold Spraying

Hydroxyapatite (HA) is a calcium phosphate ceramics, classified as a bioactive materialchemically similar to the mineral component of bones and hard tissue in mammals, which isproven to be well suitable for orthopaedic, dental and maxillofacial applications due to itsability to integrate into bone structures and support bone ingrowth without breaking down ordissolving. HA is generally used as a coating material for common metallic implant materialssuch as titanium (Ti) and titanium alloy Ti-6Al-4V. Covered by a layer of HA, the implantsare much better accepted by surrounding tissues.Various deposition methods for forming an HA coating on Ti and Ti-based substrateshave been reported, including plasma spraying , ion-beam assisted deposition , pulsed laserdeposition , high velocity oxy-fuel (HVOF) spraying , electrophoretic deposition , sol-gelprocess , etc. At present, plasma spraying, as a thermal spraying method, is widely used in theindustrial practice to deposit HA coatings onto metallic implants. However, the HA layersdeposited by plasma spraying process have several disadvantages, e.g. micro-cracks, pooradhesion between the coating and substrate due to the delamination of HA coating from themetal implant, phase changes due to extremely high-temperature exposure, non-uniformity inthe coating density, and improper microstructural control, which could result in failure of theimplanted system .In recent years, cold spraying, as a new emerging coating technology, has attracted aworldwide interest. It has many advantages over the conventional thermal spray. In coldspraying process, spray particles (1-50 m in size) experiencing little change in microstructureand little oxidation or decomposition are accelerated by a supersonic jet of compressed gas toa high velocity (300-1200 m/s) and impact on a substrate at a temperature that is always lowerthan the melting point of the material, resulting in a coating formation constructed fromcompletely solid particles. The phenomena inherent to thermal spraying at high temperatures,such as oxidation and phase transformation, can be avoided. The most important parameterfor a spray particle in cold spraying is its velocity. It is generally accepted that there exists acritical velocity for successful particle deposition with a given spray material. A high pressuregas is usually necessary to accelerate the majority of spray particles to a velocity higher thanthe critical velocity through a convergent-divergent de Laval nozzle. Only the particlesmoving at a velocity higher than the critical one can be deposited to produce a coating.Therefore, it is essential to understand the accelerating behavior of a spray particle. At thisstage, the research about the cold spraying is focused on the formation of metal coatings such as Cu, Al, Ni, Fe, Ti and their alloys .This paper employs the cold spraying method to the fabrication of HA ceramic coatingsin order to meet the demand of HA coating formation with strong cohesive strength, goodadhesion to the substrate, remaining crystal structure and high chemical purity. Although ithas been experimentally proved that HA coatings can be fabricated by cold spraying method,the HA particle acceleration behavior and the actual bonding mechanism have not yet beenelucidated.To fabricating HA coating on titanium and titanium alloy substrates, a cold sprayingsystem is established in this paper. The system consists of the high-compressive gas supplier,HA powder feeder, spray gun, substrate heater, moving platform with X, Y, Z stages andcontrol system. The developed cold spraying system fabricate HA coatings on the heatedtitanium and titanium alloy substrates, which is different from the thermal spraying or coldsparying with heating the accelerating gas. Nozzle geometry is optimized by investigating itseffect on the particle accelerating behavior. The titanium alloy substrate is heated byelectromagnetic induction. The micro feeding of HA powders is realized through theprogressive wave generated by utilizing a single piezoelectric actuator and the PMMA tubewith appropriate damping characteristics. The moving platform with X, Y, Z stages is used tocontrol the position of the substrate and the stand-off distance.Numerical investigation on particle velocity in cold spraying of HA coating is carried outusing a computational fluid dynamics (CFD) program, FLUENT in this paper. By arrangingthe simulation conditions with Taguchi method and analyzing the simulated results withanalysis of variance (ANOVA) method, it is revealed that the throat diameter and exitdiameter of the nozzle show a highly significant influence on the HA particle velocity whilethe length of straight pipe of nozzle and the standoff distance show a little influence on theHA particle velocity. Expansion ratio is introduced to describe the influence of thecombination of throat diameter and exit diameter on the HA particle velocity and the optimalexpansion ratio is found to lie between 1.5 and 4. Variations of the throat diameter, the exitdiameter, the gas pressure and the particle size are considered for the simulation of HAparticle acceleration under different cold spraying conditions. The simulation results showthat the HA particle is accelerated remarkably to a high velocity for the combinations of throatdiameter and exit diameter whose expansion ratios lie within the optimal range of 1.5–4basically. The HA particle velocity increases noticeably with the increasing gas pressure. Andthe HA particle velocity increases with the decreasing particle size, but for the particle withthe size smaller than 5 m, the bow shock makes its velocity decelerated steeply. The suitableHA particle size is about 5–20 m for the particle accelerated to the enough impact velocitybefore it reaches the substrate.Numerical investigations on impact pressure of HA coating in cold spraying were carriedout using a computational program, ABAQUS/Explicit in this paper. The pressure in the form of impact energy is essential for the deposition mechanism of HA particles. By arranging thesimulation conditions with Taguchi method and analyzing the simulated results with a visualanalysis method, it is revealed that the particle velocity show a highly significant influence onthe shock pressure while the diameter of particles show a little influence on the shock pressure.Variations of the particle velocity and substrate temperature are considered for the simulationof HA particle impacting on the substrate. The simulation results show that the pressureincreases noticeably with the increasing particle velocity. And the substrate temperature has aneffect on the pressure at the interface between the particle and the substrate.HA coatings are fabricated experimentally by the developed cold spraying system in thispaper. The commercially available HA powders with spherical morphology and sizes rangingfrom 5 m to 10 m are used as feedstock. Titanium plate heated by electromagnetic inductionis used as a substrate and sandblasted prior to spraying. Air is used as an accelerating gas.Hydroxyapatite coatings that preserve the chemical composition and microstructure offeedstock hydroxyapatite powders are obtained. Experiments of HA coating fabrication provethe validity of the developed system in this paper.The author would like to acknowledge the financial support for this investigation fromthe National Natural Science Foundation of China (Grant No. 50875109) and Research Fundfor the Doctoral Program of Higher Education of China (Grant No. 20070183101).