Metallic Biomaterials Flashcards
Examples of metallic biomaterial applications
Heart valves
Spinal fusion devices
Dental implants
Vascular stents
Stress Shielding
Reduction in local stresses in an adjacent component due to a load bearing implant taking a portion of the tensile load or bending moment
What can be a problem with stress shielding?
Bones adapt to mechanical loading
If an implant takes some load of the bone, the bone will suffer structural and mechanical consequences as a result of adaptation
Loss of bone mass similar to osteoporosis
What is the structure of 316L stainless steel (SS316L)? What is the usage time?
Austenitic crystal structure (FCC)
Only recommended for temporary usage due to tendency to corrode in body (due to high stress levels)
Properties can be improved through surface mods and chemical passivation
Why is Chromium added in metallic stainless steel biomaterials?
Add corrosion resistance by forming strongly adherent surface oxide passive film
Also stabilizes any weak BCC (ferritic) phase that exists
Why is nickel in stainless steel biomaterials? What are the issues?
Added to stainless steel to stabilize the austenitic phase
Limited amount due to adverse hypersensitivity issues
Carbon in stainless steel biomaterials
Too high wt% leads to likelihood of carbides forming at the grain boundaries, which reduces formation of Cr2O3
Leads to corrosion induced failures
Why are N and Mo added to stainless steel biomaterials?
N increases strength and reduces corrosion
Mo reduces pitting corrosion
Effect of alloying solute elements in crystal structure of metal?
Replacing solute (Fe) atoms creates distortion of crystal lattice Difficulty in movement and increase in strength due to solid solution strengthening Smaller solutes fit within the interstitials of larger atoms, increase strength through interaction between solute atoms and dislocations
Effect of cold working on metallic biomaterials
Higher strength and hardness with preferred grain orientation
Reduces ability to undergo further plastic deformation
Why can’t implantable stainless steel be strengthened by annealing?
Heating alloy reduces dislocations, decreasing strength
General properties of Co-Cr alloys
Cr reduces corrosion (due to Cr2O3 film) and increases strength properties
Added Mo produces finer grains, resulting in higher strengths after casting and forging
Higher modulus than Ss and Ti alloys
Fatigue properties suitable for long term applications
Difficult to machine - must be processed through casting or powder metallurgy
Investment casting of Co-Cr-Mo
Alloy is melted and poured into ceramic molds
Molds are made by fabricating a wax pattern
Pattern is coated with ceramic, which holds the shape
Wax is burned off, leaving a mold
Microstructural features of Investment casting Co-Cr-Mo alloys
Larger grain size - decreased strength
Ceramic particles or micro-cracks due to uneven cooling and shrinkage can result in stress concentration and possibly early failure
Non equilibrium cooling leads to unequal carbide distribution, Cr depletion in oxide layer, and corrosion
Powder metallurgy
Hot Isostatic pressing
Atomization of powders in inert Argon atmosphere
Powders are compacted, sintered, and forged into final shape
Better yield and fatigue strength than casting
Better distribution of smaller grains and carbides
