•Bovine serum albumin showed different adsorption behaviours on different deformed metal surfaces.•The subsurface can affect the protein adsorption by changing the chemical composition and semiconductivity of passive film.•Crystallization and residual are the key factors to affect the protein adsorption.The adsorption of proteins has great influence on the biocompatibility, lubrication and corrosion properties of implantable metals. The subsurface microstructure of metals can be easily changed during the manufacturing or service processes and it is easily ignored. The same chemical composition but with various surface microstructures can result in very different adsorption behaviour. In this study, mechanical polishing, electrochemical polishing and shot peening methods were used to generate different gradient deformed surfaces for CoCrMo alloys. The increase of the residual compressive stress and the decrease of grains to a nano-size on the deformed subsurface can effectively enhance the activity of metal atoms and improve the element diffusion, that is helpful in forming a more compact passive film. This can increase the contents of oxides on the surface, and then enhance the electrostatic force and increase the surface’s positive charge density as well as adsorption sites. All of the above facilitated the adsorption of the negatively charged albumin onto the alloy surface.Download high-res image (85KB)Download full-size image

CoCrMo alloys have been widely used in metal-on-metal (MoM) hip replacements due to their superior wear and corrosion resistance properties. However, metal ions like Co2+ and Cr3+, or even Cr6+ released from CoCrMo hip prostheses can induce macrophage apoptotic vs. necrotic mortality and damage the surrounding tissues. Simultaneously, osteolysis induced by the wear debris can be a cause of failure. Nano wear debris is more active than the bulk material, due to its small size. In this study, to accurately analyse the fresh wear debris retrieved from the hip simulator and the interaction between the particles and tribocorrosion of CoCrMo, wear debris was observed without protein digest, using a combined experimental approach involving the employment of TEM and ICP-MS. The results suggest that nanoscale wear debris generated from a hip simulator in bovine serum albumin (BSA) lubrication was Cr-rich, containing crystalline and amorphous structures; meanwhile, without any proteins, the wear particles mostly had an hcp-Co crystalline structure.

•The referential dissolution of the carbide boundary area accelerates tribocorrosion.•The stacking faults accumulate at the carbide boundaries resulting in micro-cracks.•The carbide cluster can decrease the Volta potential of the surrounding matrix.In order to enhance the wear resistance of the Co-based biomedical alloy, hard phase carbides were introduced into the alloy matrix. However, torn-off carbides can lead to abrasive wear and may increase the wear factor. In this study, the behaviour of carbides in the tribocorrosion processes of CoCrMo alloys was investigated. Scanning Kelvin probe force microscope (SKPFM) was used to evaluate the relative nobility of the carbides and their surrounding areas. The experimental results indicated that carbides can inhibit and block the movement of the stacking fault under plastic deformation. Carbides would be torn from the matrix during the tribocorrosion process, due to the preferential dissolution of the boundary area under the action of galvanic corrosion and tribological contact.

•Albumin can enhance the corrosion rate in static conditions for CoCrMo alloy.•By forming a complex layer, albumin can protect the worn surface from corrosion.•SKPFM is very useful to determine the corrosion trend of the tested biomaterials.Once biomaterials are implanted in patients, the first reaction that occurs on the surface is the adsorption of biomolecules such as proteins, amino acids, etc. For load-bearing surfaces, some adsorbed proteins can be removed by relative motion (tribology) and some adsorbed proteins can be denatured due to tribochemical reactions. Although the effect of proteins on the corrosion behaviors of metals has been studied, the local reaction induced by the protein adsorption under tribological contact at the micro level is still unknown. The adsorption of bovine serum albumin (BSA) on CoCrMo alloy surfaces and the tribocorrosion behaviors were studied by AFM and SKPFM. The results showed that adsorbed BSA molecules could decrease the work function and promote the corrosion process for CoCrMo alloys. In the wear track, the albumin denatured, and changed the surface potential as time progressed.

•The equiaxed grain zone was the most ductile part of the weldment.•Large size precipitates were the preferential microcrack nucleation sites.•Hydrogen-induced cracking in the weld depends on the applied stress value.Precipitation strengthened austenitic stainless steels are widely used in hydrogen related applications. However, their applications may face hydrogen damage resulting in hydrogen-induced delayed failure. Results show that the weld is more sensitive to fracture and hydrogen-induced failure than the matrix. High density curved dislocations, abundant of large size precipitates and considerable γ′ precipitates coarsening are found in the weld. Large size precipitates are found to be major hydrogen traps and preferential microcrack nucleation sites. The γ′ precipitates coarsening make the weld more ductile than the matrix. With the decrease of the applied stress, hydrogen-induced cracking mechanism in the weld changes from brittle transgranular fracture to brittle intergranular fracture.

•Hydrogen-induced failure always occurs at the weld not at the base material.•The normalized threshold stress decreased with the increase of tc.•The diffusion coefficient of hydrogen in the weld is about 1.13 × 10−11 cm2/s.The safety of precipitation strengthened austenitic stainless steels used for hydrogen storage tanks is of great interest. However, their application may face hydrogen damage resulting in hydrogen-induced delayed failure. Results show that over-loading and hydrogen-induced failure always occur at the weld part of the alloy. Hydrogen damage such as microcracks could be observed on the surface of the matrix and the weld during charging even without any applied stress. Hydrogen-induced failure occurred during charging under constant load and the normalized threshold stress decreased exponentially with increasing defined time tc. It is shown that the threshold stress with no hydrogen-induced failure occurring for expected service life, i.e. forty years, was 713 MPa. Therefore under the service stress, which is less than the threshold stress, 713 MPa, the safety factor of the hydrogen storage tank for hydrogen-induced fracture is great enough to indicate the tank to last for the entire designed service time.

The second generation Metal-on-Metal (MoM) hip replacements have been considered as an alternative to commonly used Polyethylene-on-Metal (PoM) joint prostheses due to polyethylene wear debris induced osteolysis. However, the role of corrosion and the biofilm formed under tribological contact are still not fully understood. Enhanced metal ion concentrations have been reported widely from hair, blood and urine samples of patients who received metal hip replacements and in isolated cases when abnormally high levels have caused adverse local tissue reactions. An understanding of the origin of metal ions is really important in order to design alloys for reduced ion release. Reciprocating pin-on-plate wear tester is a standard instrument to assess the interaction of corrosion and wear. However, more realistic hip simulator can provide a better understanding of tribocorrosion process for hip implants. It is very important to instrument the conventional hip simulator to enable electrochemical measurements. In this study, simple reciprocating pin-on-plate wear tests and hip simulator tests were compared. It was found that metal ions originated from two sources: (a) a depassivation of the contacting surfaces due to tribology (rubbing) and (b) corrosion of nano-sized wear particles generated from the contacting surfaces.Highlights► By integrating a hip simulator, corrosion behavior of hip components can be assessed in real time. ► Mechanical wear of hip implants can be accelerated by electrochemical corrosion, vice versa. ► The reduction of metallic ions and total material loss can be attributed to a tribofilm formation. ► Tribofilm can reduce friction and act as a pseudo-passive layer to protect the bearing surfaces.

Due to the reduced wear compared to the metal-on-polyethylene implant, second generation metal-on-metal hip prostheses have been widely used as the replacement in younger patients in recent years. Osteolysis induced by polyethylene wear debris was a major concern with metal-on-polyethylene hip replacements. In metal-on-metal total joint replacements, however, there has been concern about the incidents of pseudo tumours as a result of the production of very fine wear debris and the associated production of metallic ions of Co and Cr. The origins of the metallic ions may be from two potential sources: from the bearing surface and from the dissolution of wear debris produced by the tribological action or the production of ions by depassivation of the CoCr alloys.Although there has been extensive work on simulation of wear processes in hip joint replacements through hip simulators running over prolonged periods and mapping the wear rates, to date there have been several attempts to measure the interactions (biotribocorrosion) between corrosion and tribology in-situ in simulated body fluids using a hip simulator. This paper describes the instrumentation of an integrated hip joint simulator to provide electrochemical measurements in real-time. The open circuit potential and polarization experiment results are reported and the importance of these measurements to gain an understanding of the origins of metal ions and to complement the wealth of wear data available is discussed.Highlights► By integrating a hip simulator, tribocorrosion behaviour of hip components can be assessed. ► Mechanical wear of hip implants can be accelerated by electrochemical corrosion, vice versa. ► The reduction of metallic ions and total material loss can be attributed to a tribofilm formation.

Orthopedic prostheses are lubricated by a pseudosynovial fluid that contains proteins. Under regular movements, bearing surfaces would suffer wear and corrosion. More importantly, their interaction controls the material degradation process. Nanocrystalline layer was found on the surface of CoCrMo alloy surface after tribocorrosion tests. Tribocorrosion tests were taken in 0.9% NaCl and 0.9% NaCl with 1% bovine serum albumin (BSA) solution. Small angle X-ray Scattering was applied to measure the size distribution of the nano-crystals. As a general conclusion, proteins can absorb on prosthesis materials and act as a lubricant during sliding. The negative charge distribution on the material surface can promote the adsorption of protein. The average size of the nano-crystals on the bearing surface was 5 nm.

•Accelerated electrochemical corrosion results in severer plastic deformation with finer grains.•Lower applied potential can increase protein adsorption on sample surfaces.•The tribo-film decreases the shear stresses and relief subsurface deformation.•Tribocorrosion induced passive film can suppress the annihilation of stacking faults.The subsurface microstructures of metallic implants play a key role in bio-tribocorrosion. Due to wear or change of local environment, the implant surface can have inhomogeneous electrochemical corrosion properties. In this work, the effect of electrochemical corrosion conditions on the subsurface microstructure evolution of CoCrMo alloys for artificial joints was investigated. Transmission electron microscope (TEM) was employed to observe the subsurface microstructures of worn areas at different applied potentials in a simulated physiological solution. The results showed that applied potentials could affect the severity of the subsurface deformation not only by changing the surface passivation but also affecting the adsorption of protein on the alloy surface.