Lecture 9 - Metallic Biomaterials Flashcards
Why Metals?
Mechanical Profile - failure modes, modulus, strength, modes of deformation (plastic)
Why Metal Alloys?
- Fine-tune mechanical profile
- Reduce Corrosion
- Exceptions: Au, Pt, CP Ti
Applications of Metallic Biomaterials
- Fracture plates
- Tibia rods and intramedullary nails (center cavity of bone shafts - storage of bone marrow)
- Bone screws
- Joint replacements
Types of Metallic Biomaterials
Main: - Stainless steels - Co-based alloys - Ti-based alloys Others: - Copper - Gold - Platinum Degradable: - Mg alloys (quick degradation profile)
Goal of Metallic Biomaterials
Replace or repair diseased or damaged organs
Use of Stainless Steels
Removable or low commodity devices:
- plates
- screws
- pins
- replacement joints
Types of Stainless Steels
- ASTM F138
- ASTM F139
- Austenitic (FCC)
- 316L
316L Stainless Steel
- Low C content (< 0.03%)
- Fe (60-65 wt.%)
- Cr (17-19 wt.%)
- Ni (12-14 wt.%)
- Minor additions: N, Mn, Mo, P, Si, S
- do not want cementite (brittle), maintain austenite
- 0% Ni (Ferrite), 5% Ni (Duplex), >8% Ni (Austenite)
Why Cr Alloying Addition in 316L?
- Forms Cr2O3 surface oxide (passivating layer - preventing interaction between aqueous fluid and base metal)
- Corrosion resistant
- Strongly adhered to base metal
- Con: stabilizes weaker ferritic phase (BCC) than austenite
Why Mo Alloying Addition in 316L?
- Resists pitting corrosion
- Con: stabilizes weaker ferritic phase (BCC) than austenite
Why Ni Alloying Addition in 316L?
- Corrosion resistant
- Work hardening
- Austenitic stabilizer to counter Cr & Mo effects
Why low C Alloying Addition in 316L?
- Don’t want to form carbides
- If > 0.03% C, Cr23C3
- Precipitate at grain boundary (forms at gb because free space between unaligned grains)
- Depletes grain boundary of Cr, decreases Cr2O3 layer
Alloying Additions of 316L
Balance between corrosion (Cr, Mo) and mechanical (Ni, C) properties
Carbides
- Not great mechanically
- Deplete grain boundary zones of Cr2O3 passivating layer
- Loss of individual grains
- Release abrasive particles
Corrosion
More preferential at grain boundaries because higher energy
Stainless Steel: ASTM Standards
- Single phase austenite (FCC)
- No carbides (act as sites of stress concentrators)
- ASTM grain size 6 or finer (100um or less)
- Hall-Petch relationship
- Additional processing (cold working)