Coatings and Surface Modification of Alloys for Tribo-Corrosion Applications

This review of the tribocorrosion of coatings and surface modifications covers nearly 195 papers and reviews that have been published in the past 15 years, as compared to only 37 works published up to 2007, which were the subject of a previous review published in 2007. It shows that the research into the subject area is vibrant and growing, to cover emerging deposition, surface modification and testing techniques as well as environmental influences and modelling developments. This growth reflects the need for machines to operate in harsh environments coupled with requirements for increased service life, lower running costs and improved safety factors. Research has also reacted to the need for multifunctional coating surfaces as well as functionally graded systems with regard to depth. The review covers a range of coating types designed for a wide range of potential applications. The emerging technologies are seen to be molten-, solution-, PVD- and PEO-based coatings, with CVD coatings being a less popular solution. There is a growing research interest in duplex surface engineering and coating systems. Surface performance shows a strong playoff between wear, friction and corrosion rates, often with antagonistic relationships and complicated interactions between multiple mechanisms at different scale lengths within tribocorrosion contacts. The tribologically induced stresses are seen to drive damage propagation and accelerate corrosion either within the coating or at the coating coating–substrate interface. This places a focus on coating defect density. The environment (such as pH, DO2, CO2, salinity and temperature) is also shown to have a strong influence on tribocorrosion performance. Coating and surface modification solutions being developed for tribocorrosion applications include a whole range of electrodeposited coatings, hard and tough coatings and high-impedance coatings such as doped diamond-like carbon. Hybrid and multilayered coatings are also being used to control damage penetration into the coating (to increase toughness) and to manage stresses. A particular focus involves the combination of various treatment techniques. The review also shows the importance of the microstructure, the active phases that are dissolved and the critical role of surface films and their composition (oxide or passive) in tribocorrosion performance which, although discovered for bulk materials, is equally applicable to coating performance. New techniques show methods for revealing the response of surfaces to tribocorrosion (i.e., scanning electrochemical microscopy). Modelling tribocorrosion has yet to embrace the full range of coatings and the fact that some coatings/environments result in reduced wear and thus are antagonistic rather than synergistic. The actual synergistic/antagonistic mechanisms are not well understood, making them difficult to model

Tribological Performance of Artificial Joints

Joint replacement is a very successful medical treatment. However, the survivorship of the implants could be adversely affected due to the loss of materials in the form of particles or ions as the bearing surfaces articulate against earch other. The consequent tissue and immune response to the wear products, remain one of the key factors of their failure. Tribology has been defined as the science and technology of interacting surfaces in relative motion and all related wear products (e.g., particles, ions, etc.). Over the last few decades, in an attempt to understand and improve joint replacement technology, the tribological performance of several material combinations have been studied experimentally and assessed clinically. In addition, research has focused on the biological effects and long term consequences of wear products. Improvements have been made in manufacturing processes, precision engineering capabilities, device designs and materials properties in order to minimize wear and friction and maximize component longevity in vivo

Finite element analysis of stress distribution within metal-on-metal joint replacements

Demand for joint replacements is rising in Australia, driven by a sharp increase in the number of joint problems associated with population aging and obesity. In artificial joints, delamination or failure within the coatings occurs when the stress reaches a critical level, resulting in large wear debris particles appearing on the contact surface between the head and the cup. The process has been described as due to a stress-corrosion-cracking mechanism. Under the same loading, stress increases when the contact area decreases, which happens in the vicinity of wear debris. As such, once wear debris is generated, a catastrophic process could be initiated, resulting in more stress-corrosion-cracking. As such, acquiring a strong coating that will not fail is highly desirable for the applications of hip joint replacement. Failure in a coating layer is normally initiated by excessive local tensile or shear stress; therefore, it is important to clarify the stress distribution within the coating layer under different loading conditions, which is necessary for improving the load-carrying capability of the coating. Unlike previous studies, the multilayer diamond-like carbon (DLC) coatings having high elastic modulus and hardness were analysed in this work. Under normal contact conditions, plastic deformation occurs in contacting materials when the contact pressure is greater than the hardness of the materials. Therefore, high hardness coatings can resist plastic deformation to avoid failure of the coating; in addition, multilayer coatings can decrease stress concentration to avoid cracking. The purpose of this study is to determine whether DLC multilayer coatings can improve the property of the coating against potential cracking in the coating. It has been shown that structurally graded coatings had effect on reducing the contact-induced stress among all the factors considered. It is anticipated that the multilayer design parameters will be important to understand the stress distribution within metal-on-metal (MOM) hip replacements

Non-resorbable glass fibre-reinforced composite with porous surface as bone substitute material: Experimental studies in vitro and in vivo focused on bone-implant interface

The development of load-bearing osseous implant with desired mechanical and surface properties in order to promote incorporation with bone and to eliminate risk of bone resorption and implant failure is a very challenging task. Bone formation and resoption processes depend on the mechanical environment. Certain stress/strain conditions are required to promote new bone growth and to prevent bone mass loss. Conventional metallic implants with high stiffness carry most of the load and the surrounding bone becomes virtually unloaded and inactive. Fibre-reinforced composites offer an interesting alternative to metallic implants, because their mechanical properties can be tailored to be equal to those of bone, by the careful selection of matrix polymer, type of fibres, fibre volume fraction, orientation and length. Successful load transfer at bone-implant interface requires proper fixation between the bone and implant. One promising method to promote fixation is to prepare implants with porous surface. Bone ingrowth into porous surface structure stabilises the system and improves clinical success of the implant. The experimental part of this work was focused on polymethyl methacrylate (PMMA) -based composites with dense load-bearing core and porous surface. Three-dimensionally randomly orientated chopped glass fibres were used to reinforce the composite. A method to fabricate those composites was developed by a solvent treatment technique and some characterisations concerning the functionality of the surface structure were made in vitro and in vivo. Scanning electron microscope observations revealed that the pore size and interconnective porous architecture of the surface layer of the fibre-reinforced composite (FRC) could be optimal for bone ingrowth. Microhardness measurements showed that the solvent treatment did not have an effect on the mechanical properties of the load-bearing core. A push-out test, using dental stone as a bone model material, revealed that short glass fibre-reinforced porous surface layer is strong enough to carry load. Unreacted monomers can cause the chemical necrosis of the tissue, but the levels of leachable resisidual monomers were considerably lower than those found in chemically cured fibre-reinforced dentures and in modified acrylic bone cements. Animal experiments proved that surface porous FRC implant can enhance fixation between bone and FRC. New bone ingrowth into the pores was detected and strong interlocking between bone and the implant was achieved.Siirretty Doriast

Ceramic coatings for Cervical Total Disc Replacement

Surgical interventions for the treatment of chronic neck pain, which affects 330 million people globally, include fusion and cervical total disc replacement (CTDR). Most of the currently clinically available CTDRs designs include a metal-on-polymer (MoP) bearing. Numerous studies suggest that MoP CTDRs are associated with issues similar to those affecting other MoP joint replacement devices, including excessive wear and wear particle-related inflammation and osteolysis. The aim of this study was to investigate the biotribology of a novel metal-on-metal (MoM) design of cervical total disc replacement device in its pristine form and coated with chromium nitride or silicon nitride, in order to understand the influence of loading conditions upon the tribological performance of the implant, and to investigate biological effects of the wear debris produced by the implants. To achieve this, a series of studies were carried out. Chromium nitride and silicon nitride coatings have been characterised for their mechanical properties, chemical composition and surface finish. Whilst some of the experiments showed minor differences between the mechanical properties and adhesion of the coatings, there was no indication of significant differences between the chromium nitride and silicon nitride coated samples. Functional testing in the six-station spine wear simulator showed that MoM CTDRs produced wear volumes significantly lower than those of the commercially available MoP devices. The wear volumes were reduced further by three-fold, following testing under altered ISO-18192-1:2011 kinematics, whereby, reduced ranges of motions were applied. Whilst the silicon nitride coated CTDRs failed catastrophically early in the test, chromium nitride coated CTDRs produced an eight-fold reduction in wear volumes, when compared to the pristine devices tested under the same conditions. Investigation of potential biological effects of the particles generated in wear testing showed that that high concentrations (5-50µm3 per cell) of CoCrMo particles resulted in significant reduction of cell viability of the L929 fibroblast cells, but not the dural fibroblasts, which were used in this study. No ceramic coating particles, at any concentrations, caused significant reduction of cell viability. In summary, results presented in this thesis showed that whilst the MoM CTDR device exhibited significantly lower wear rates than those of the commercially available MoP devices, the cytotoxic wear particles could potentially lead to adverse biological reactions, particularly in patients with metal hypersensitivity, and lead to devastating consequences similar to those of failed MoM THRs. Currently, the consequences of similar failure, leading to metalosis or pseudotumour formation in the vicinity of the spinal cord are unknown. During the investigation of the ceramic coatings, it was also found that chromium nitride ceramic coating could not only lower wear rates further, but it also has the potential to reduce the cytotoxic potential of the wear particles

Biotribology of Artificial Hip Joints

Tribology is the science of interacting surfaces; when these surfaces are in a biological system, it is called as biotribology. With the increasing rate of joint replacement operations and need for artificial prosthesis, biotribology is becoming a very important and rapidly growing branch of tribology. Based on this fact, in this chapter, basic tribological concepts are presented in terms of friction, lubrication, and wear; then with these fundamental backgrounds the biotribological behavior of natural and artificial hip joints are discussed in detail. Moreover, material pairs that are used in artificial joint replacements and the application of surface modification for the enhancement of the tribological properties of these materials are handled. Furthermore, the determination of tribological behavior of joint materials such as wear, coefficient of friction, friction torque, and frictional heating by using conventional techniques and hip joint simulator are discussed. Finally, the measurement and analysis of wear in both retrieved prosthesis and experimental studies are discussed referring the latest research articles