Surface modification of ASTM F75 cobalt-chromium alloy for biomedical applications

Among the metallic biomaterials, cobalt-chromium alloys are widely used in production of load bearing orthopedic implants especially in our country due to good combination of their high strength and improved corrosion resistance as well as their machinability. Despite their superior properties, service life of the implants made of these materials is limited to 10-15 years. Development of localized corrosion, micro-wear and dissolution of their surfaces in the human body adversely affect their performance and may lead to early failure of the implants. In addition, surface of this alloys is not classified as bioactive. Bioactivity is interaction of implant material with body tissue at good conditions. Material, which helps growing body tissue on itself with no toxic effect, can be called as bioactive. A biomaterial must be bioactive, because any toxic reactions on body tissue near the implant causes re-implantation. Re-implantation operations are very painful and costly. So this reasons make people life more difficult and uncomfortable. For this reasons improving cobalt- chromium alloys direction of surface bioactivity is one of the actual topics of the world. They use surface modification techniques like PVD or flame spraying to coating cobalt-chrome alloys surface more bioactive systems. In our study, we prefer using cold gas dynamic spraying and thermal oxidation processes combination to improve bioactivity of cobalt-chromium surfaces. Titanium is one of the most biocompatible material in metallic biomaterials. The oxide film, which occurs titanium and its alloys surface, has good bioactivity and good corrosion resistance. But this oxide film, which is, occurs at titanium and its alloys surface natively, have poor mechanical properties and they are easily fractured under fretting and sliding wear conditions. For this reason lots of study about developing mechanically and chemically more stable oxide film on the surface of titanium and its alloys are working. Micro arc oxidation, anodic oxidation and thermal oxidation are very favorite topics for developing titanium oxide films on titanium and its alloys. In thermal oxidation, oxidation occurs at temperatures above 200°C, promotes the development of a crystalline oxide film. Increasing temperature induces the formation of a thicker oxide layer, which is accompaniment with oxygen diffusion zone. At this zone titanium dissolves oxygen amounts of maximum 30 at. %. Hardness of this zone increases with oxygen percent dissolving in titanium. So this oxygen diffusion zone also acts like transition zone between soft titanium and hard titanium oxide layer. Cold gas dynamic spraying process is a relatively new technique used for the surface modification of materials like metals, composites and polymers. This technique discovered by Professor Papyrin et al. in the mid 1980's to be alternative way to thermal spraying process. During thermal spraying process, powders were melted to form the coating but during cold spray process, coating formation depends on the plastic deformation of particles. Plastically deformation of particles occurred by the factor of kinetic energy obtained from high particle velocity. Particle and process gas temperature during cold spray process does not reach to the melting point of coating particles. So that is why this system called cold gas dynamic spraying. Due to low process temperature, oxide free coating can be obtained by this technique especially for easily oxidize materials such as copper and titanium. All of the particle strikes to the substrate surface are not result in deposition as a result of plastic deformation. During the impact of particles on substrate, lots of different characteristic can be seen. These characteristics can be categorized as sticking or erosion at low velocities, ballistic effect at high velocities and cold spray deposition at medium velocities In this study, cobalt-chromium alloy (ASTM F75) coated with commercially pure titanium and titanium matrix composite reinforced by Zn, ZnO, ZrO2 and Ag by cold gas dynamic spray process. Subsequently, coated alloys thermally oxidized at 600 °C for 60 hours in order to obtain a thick, stable, well adhered, bioactive and antibacterial titanium based oxide layer. In the frame of this project, multilayered coatings comprising an inner titanium (titanium matrix composite) layer and an outer titanium oxide layer produced. Main purpose of the study is to improve bioactive and antibacterial properties of cobalt chromium materials without sacrificing their wear resistance. Characterizations of the titanium and particle reinforced titanium matrix composite coatings on cobalt-chromium alloys made by microstructural surveys, surface mapping analyses, hardness measurements, roughness analyses and X-Ray diffractions. Triboligical behaviors of the coatings investigated by ball on disc wear tests technique with different forces. Additionally, antibacterial and bioactivity tests done to determine bioactivity and antibacterial behaviors of the thermally oxidized coatings. As a starting ASTM F75 cobalt-chromium alloy coated with cold gas dynamic spray process with five different coating composition. This coatings consist of (%w) %95 Ti-%5Al, %90Ti-%5Al-%5Zn, %90Ti-%5Al-%5ZnO, %90Ti-%5Al- %5ZrO2, %90 Ti-%5Al -%5Ag. High titanium percent in coatings used for provide bioactivity and other components (Zn, ZnO, ZrO2, Ag) used to give antibacterial effect to coatings. After producing coatings with cold gas dynamic spray process, thermal oxidation treatments were conducted at 600 °C for 60 hour at normal atmospheric condition Characterization of coatings started with cross-section microstructure analyses. Cross-sections of samples examined with optical and scanning electron microscope. This analyses show that all the coatings have some porousity at their structre and there is an oxide layer on the surface of the coatings. This oxide layer has nearly 3 µm thickness. Oxide is cracked at some places; it might be reason of stress occurring during thermal oxidation process. Surface mapping analyses show that there is a homogeneous distribution of third components (Zn, ZnO, ZrO2, Ag) in titanium matrix except silver. Silver particles accumulated at some places like a line in titanium matrix. This might be the reason of particle sizes of silver powders. XRD analyses pointed out that after thermal oxidation treatment all of the coatings have alfa titanium, rutile phase of titanium dioxide (TiO2) and aluminum oxide (Al2O3) at their structure. Roughness measurements carried out for all the coatings with contact profilometer. Roughness measurements carried out for investigation effect of thermal oxidation on titanium based coatings roughness. Surface roughness measurements shows that roughness increased 5-6 times after thermal oxidation process. Average surface roughness of the coatings after thermal oxidation are approximately 0,5-0,6 micrometer. Having a rough surface like these values is one of the most important feature a biomaterial must have. Hardness measurements done with deep sense Vickers hardness technique with 10 mN force. All the coatings shows 920-970 average HV values. These results can be commented as there is no change in hardness with adding other components like Zn, ZnO, ZrO2 and Ag to coating composition. İndentation depth graphs shows Vickers indenter gone nearly 600 nanometers depth on the surface. We had 3 micrometers oxidation layer, so indenter did not pass this layer when measuring harness values. Wear tests were conducted untreated cobalt-chromium alloy and coatings with 2-3-4N force at distance of 25m. Depth-width profiles of wear tracks could not take by contact profilometer. Because wear tracks created on coating surface did not depth enough to measure on profilometer. Optical microscope and electron microscope pictures of the wear tracks shows that there is only sliding between abrasive ball and surface of the coatings. It might be the reason of oxide-oxide interaction alumina abrasive ball and oxide layer. Comparative friction coefficient graphs also showing coatings decreases the friction coefficients of cobalt-chromium alloys. If we compare coatings with each other, the coating with including Zinc additive shows best performance. Zinc particles in the coating acts like a lubricant so this coating have very low friction coefficient. Decreasing friction coefficient of the surface results with decreasing abrasion of the surface. Biological performance of the coatings against untreated cobalt-chromium alloy conducted with in-vitro bioactivity tests and antibacterial tests. In-vitro bioactivity tests were carried out by soaking the untreated cobalt-chromium alloy and coated samples in the SBF solution for three weeks at 36,5 °C. At the end of three weeks surfaces of the samples examined by SEM. SEM photographs showed that there is a flat-like precipitations both untreated cobalt-chromium and coated samples surface. These precipitates on the samples surface were composed of fine flakes with some amount of Ca and P as revealed by EDS analysis. Morphology and EDS analyses of this precipitates can be commented as they are apatite structre. If we compare untreated cobalt-chromium alloy and coatings bioactivity, they both have some apatite precipitates on the surface. But after three weeks, apatite forming on the surface of untreated cobalt-chromium alloy cracked and rised from the surface. So it can be commanted as there is not enough holding between surface and apatite layer. İf we examine coated samples, apatite layer has good holdig on the coatings surface after three weeks. This results shows we have improved surface bioactivity of cobalt-chromium alloy with this coatings. Antibacterial tests examined by E.Coli bacterias. Antibacterial affects of the coatings calculated as a referance of untreated cobalt-chromium alloy. Best antibacterial performance showed by the coating, which is reinforced with silver particles. This coating showed %98 antibacterial effect against untreated cobalt-chromium alloy.