CoCr-alloys

The most common overcoats, however, are sputtered carbons. Their role in disk corrosion has been described in contradicting ways. Whereas Garrison [141] clearly observed that carbon, like Rh, can enhance galvanic corrosion, Smallen et al. [131] believe that carbon decreases corrosion by preventing lateral growth of corrosion products. Results of similar tests are sometimes contradictory Nagao et al. [145] have shown an improvement of the corrosion resistance of carbon-coated CoCr alloys on T/H test (with either SOz gas or NaCl mist), whereas Black [146] finds that pyrolitic carbon over a CoCrMo alloy results in elevated corrosion rates. 

Stent Platform 316L Close cell 316L Open cell CoCr alloy Open cell SS/Ta/SS Sandwich L605 Holes in strut... 

Total knee replacements can be implanted with or without cement, the latter relying on porous coating for fixation. The femoral components are typically made of CoCr alloy and the monolithic tibial components are made of UHMWPE. In modular components, the tibial polyethylene component assembles onto a titanium alloy tibial tray. The patellar component is made of UHMWPE, and a titanium ahoy back is added to components designed for uncemented use. The relatively small size of the pateUar component compared to the forces that travel through the extensor mechanism, and the small area of bone available for anchorage of the prosthesis, make the pateUa vulnerable. 

Although a variety of metallic biomaterials have been employed in joint arthroplasty, alloys of cobalt, 28% chromium, and 6% molybdenum (CoCr) are viewed as the gold standard for use in MOM bearings. CoCr alloy is also considered the gold standard as a femoral head material for articulations against conventional as well as highly crosslinked UHMWPE (Muratoglu and Kurtz 2002, Sauer and Anthony 1998). CoCr alloys (e.g., Vitallium ) have been used for hip replacements since 1938, when the biomaterial was employed in the Smith-Petersen mold arthroplasty (Smith-Petersen 1948). 

Minimum American Society for Testing and Materials Specifications for Mechanical Properties of Cast, Forged, and Wrought CoCr Alloys for Use in Implants... 

Between the 1950s and 1970s, pioneering British surgeons like McKee (1966) and Ring (1968) developed MOM joint replacements fabricated from CoCr alloy. 

However, nanostructured CoCrMo implants still possess a potential risk of longterm toxicity caused by nanoscale wear debris from the articulating surfaces [30], especially in the case of metal-on-metal implants. In a recent study, cytotoxic and genotoxic effects of nanoparticles and micron-sized particles of CoCr alloy were... 

Tsaousi A, Jones E, Case CP. The in vitro genotoxicity of orthopaedic ceramic (A1203) and metal (CoCr alloy) particles. Mutat Res 2010 697 1-9. 

Patients with total hip replacements by implants of stainless steel or of CoCr alloys who experienced difficulties after two to fifteen years due to a loosening of the prosthesis and/or allergic reactions to Cr, Co or Ni were found to have an increased content of these elements in their urine, plasma and blood. Already fifteen months after removal the contents were excessive in these fluids [9]. 

After insertion of wires of different metals into the epiphyseal region of rabbits and an exposure time of fifteen months, the histology showed different results. With materials of inert or biocompatible behaviour the cells in the vicinity of the implant were still supplied with blood, while the cells in the neighbourhood of toxic materials underwent an inflammatory reaction and died. A few elements (Cr, Co, Ni and V) have toxic effects and also have a relatively low polarization resistance. Ti and its alloys, Nb and Ta, which have a high polarization resistance, exhibit an inert behaviour. In between the materials were found which are capsu-lated. The results also show that not only the corrosion behaviour provided by the polarization resistance is responsible for the biocompatibility of the material exposed to the tissue. The steel 316L and the CoCr alloy, which have a polarization resistance similar to that of titanium, are encapsulated by a tissue membrane and their behaviour is not inert [13]. 

Since CoCr-alloys have a low thermal conductivity, a high shear strength and a high work hardening rate and since they contain hard intemetallic compounds, they are expensive to machine because high cutting forces must be applied. 

For turning CoCr alloys, tungsten carbide tools are usually applied. Ceramic coated carbide, boron nitride and high speed steels can also be used. Water soluble oils are applied as cutting fluids. Table lb.5 gives the turning parameters for CoCr-alloys. 

Drilling of CoCr-alloys is accomplished by similar methods. The speeds and feeds must be reduced because of the slower cooling rate and lubriant efficiency. 

Milling of CoCr-alloys requires, like turning, higher forces and climb milling is preferred. Cuts deeper than 1.5 mm are not possible. Besides high-speed cutters carbide tools can also be used. Table lb.7 gives the milling parameters for CoCr-alloys. 

Table lb.5 Turning parameters of cast and wrought CoCr-alloys (Ref. 11)... 

Grinding of CoCr-alloys requires good control of the parameters because of the sensitivity of the alloys to microcracks and because of alterations in the microstructure. 

Brazing of CoCr-alloys can be performed either in a hydrogen atmosphere or in vacuum. As filler metals Ni- or Co-base alloys or Au-Pd-alloys can be used. In order to achieve a better wetting by the filler metals an electroplating or flashing with Ni is performed. Copper filler metals should be avoided because of the danger of an embrittlement of the seam. Table lb.8 shows the compositions of a typical Co-base filler metal. 

Table lb.9 Recommended parameters for the welding of CoCr-alloys by different processes (Ref. 12)... 

CoCr-alloys are welded in the solution-treated condition with electrodes of a similar composition to the base metal. CoCr-alloys are more difficult to weld than wrought CoCr-alloys. Table lb.9 gives recommended parameters for the welding of CoCr-alloys by different processes. 

Table lb.l4 High cycle fatigue strength of various CoCr alloys (Ref. 16,17)... 

Table lb.l6 High cycle fatigue strength of hip endoprostheses of CoCr alloys, measured in 0.9% NaCl solution at 37°C. Testing conditions similar to DIN 58840 (simulation of a loosened shaft, 50 mm free length) (Ref. 19)... 

Table lb.18 Repassivation time in 0.9% NaCl and breakdown potential in Hank s solution of CoCr alloys (Ref. 21)... 

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