Metals and alloys for biomedical applications

Transcription

1 Metals and alloys for biomedical applications Fundamental of Materials Engineering Chedtha Puncreobutr Department of Metallurgical Engineering Chulalongkorn University

2 Biomaterials and applications 2

3 Crystal Structures Atom Packing in Metals 4

4 Tensile properties of metals 5

5 Stress Shielding 6

6 Ion concentrations in body 7

7 Corrosion of metal 8

8 Commonly used metals 9

9 Metals as Biomaterials Mostly used metals and alloys Stainless Steels temporary implants e.g. plate, screw Type 304 Type 316 Type 316L Titanium alloys Ti-6A%Al-4%V implants and dental Ti-Ni (55%Ni and 45%Ti) vascular stent (shape memory alloy) Cobalt base alloys CoCrMo alloy - dental and artificial joints CoNiCrMo alloy - stems for prostheses for heavy loaded joints such as knee and hip 10

10 Stainless steel 11

11 Fe-C phase diagram Steel is an alloy consisting mostly of iron, with carbon content up to 1.4% of its weight. Alloys with higher carbon content than this are known as cast iron because of their lower melting point 12

12 Type of steels Carbon steels Classified as low, medium and high: 1. Low-carbon steel or mild steel, < 0.3%C, bolts, nuts and sheet plates. 2. Medium-carbon steel, 0.3% ~ 0.6%C, machinery, automotive and agricultural equipment. 3. High-carbon steel, > 0.60% C, springs, cutlery, cable. Stainless steels Characterized by their corrosion resistance, high strength and ductility, and high chromium content. Stainless as a film of chromium oxide protects the metal from corrosion 13

13 Stainless steel 14

14 Stainless steel 15

15 Austenitic steel AISI 316L is commonly used in implants (316 = Mo-containing and L = low carbon) Ni stabilises the austenitic microstructure of steel (note: allergic reactions) Cr-containing steel produces a thin and relatively durable passivating oxide layer Mo has a strong positive effect on pitting and crevice corrosion resistance in chloride-containing solutions 16

16 Austenitic steel % chromium 8% nikel % chromium 10% nikel 2% molybdenum CHROMIUM- luster+durability+prevent rust NIKEL-hardness+strength MOLYBDENUM- help resist chlorides corrosion 17

17 Chromium oxide passivating film 18

18 Steel 19

19 Cobalt alloys Because of their excellent mechanical properties, high corrosion resistance, and high wear resistance, Co-Cr alloys have been recognized as effective metallic biomaterials. Co-Cr-Mo alloy (Vitallium) was developed in 1930s. Cr increases corrosion resistance Mo is added to produce finer grains which results in higher strength after casting or forging 20

20 Cobalt alloys Pure Co undergoes an allotropic transformation at 690 K from the hightemperature γ-phase with the FCC structure to the low-temperature ε-phase with the HCP structure 21

21 Cast and wrought cobalt-based alloys 22

22 CoCr: pros & cons 23

23 Titanium Titanium is the fourth most abundant of structural metals and is the ninth most abundant element on the earth. Not found in its free, pure metal form in nature but as oxides such as ilmenite (FeTiO 3 ); rutile (TiO 2 ); arizonite (Fe 2 Ti 3 O 9 ); perovskite (CaTiO 3 ) and titanite (CaTiSiO 5 ). 24

24 Combination of physical and metallurgical properties Unique set of characteristics creates intrinsic value 26

25 Exceptional strength-to-weight ratio Titanium s excellence tensile and yield strength combined with low-density results in the highest strength-to-weight ratio of any of today s structural metals 27

26 Low Density Density of Titanium is roughly 56% that of stainless steels and half that of copper and nickel alloys, means greater metal volume per pound compared to other materials. In conjunction with its strength, this often means components can be made smaller and/or lighter. This effectively offsets its higher perpound cost, especially when life cycle costs are also considered (resistance to corrosion) 28

27 Excellence Corrosion Titanium is a reactive metal, meaning it spontaneously forms a natural oxide film Highly adherent film, Repairs itself instantly Titanium TiO 2 Non magnetic thus it minimises electro-magnetic interference 29

28 Low modulus means excellence flexibility Discussion point: stress shielding Elastic Modulus (GPa) 30

29 Extraction of Titanium The production of ductile, high purity titanium still proved to be difficult, because of the strong tendency of this metal to react with oxygen and nitrogen. There are some commercial methods for producing titanium like: sodium reduction process (or Hunter process); direct oxygen reduction process; electrolytic process. But, the most famous titanium production method is Kroll process. spongy and porous, titanium sponge 2Mg + TiCl 4 2MgCl 2 + Ti It is removed from the titanium by distillation under very low pressure at a high temperature 32

30 Ti-O phase diagram 33

31 Crystal Structure of Titanium The crystal structure of titanium at ambient temperature and pressure is close packed hexagonal (α) with a c/a ratio of

32 Crystal Structure of Titanium Pure titanium crystalline structure undergoes a transformation from hcp (α at lower temperature) to bcc (β at higher temperature) by increasing the temperature up to 882 C Complete transformation from one into another crystal structure is called Allotropic transformation; the respective transformation temperature is called the Transus temperature. HCP BCC 35

33 Crystal Structure of Titanium The existence of the two different crystal structures and the corresponding allotropic transformation temperature is of central importance since they are the basis for the large variety of properties achieved by titanium alloys Both plastic deformation and diffusion rate are closely connected with the respective crystal structure. In addition, the hexagonal crystal lattice causes a distinctive anisotropy of mechanical behaviour. The Young s modulus of titanium single crystals consistently varies between 145 GPa for a load vertical to the basal plane and only 100 GPa parallel to this plane. 36

34 Slip system The planes and directions of highly dense packed atoms are energetically most favourable for plastic deformation. The number of slip systems is determined by the number of slip planes multiplied by the number of slip directions. Slip system is only 3 for the hcp structure while it is 12 for the bcc lattice. This phenomenon also explains the limited plastic deformability of the hcp titanium compared to the bcc titanium 37

35 Types of Titanium alloys Titanium has an electron valence number of two, three or four, and any element with a different valence number can promote the formation of the alpha or beta phases Alpha stabilisers Elements with a lower valence number than titanium promote the formation of -Ti. Aluminium and Oxygen Beta stabilisers Elements with a higher electron valence number promote the formation of the -Ti. Vanadium, Molybdenum, Chromium and Copper Neutral elements Same valence number as titanium, not stabilise any phase 38

36 Ti alloying 39

37 Types of Titanium alloys Commercially Pure (CP) Titanium Less expensive, more corrosion resistance, lower in strength, high ductility, non-heat-treatable Alpha Titanium alloys Low to medium strength, good toughness, optimum high temperature creep strength and oxidation resistance, non-heat-treatable Beta Titanium alloys High strength, good creep resistance to intermediate temperatures, heat-treatable Alpha + Beta Titanium alloys Medium to high strength, heat-treatable Ti-6Al-4V being a general purpose titanium alloys 40

38 Alpha+Beta Titanium alloy Microstructure of + -Ti alloy. The -phase is the dark region and the - phase the light region 41

39 Titanium alloys vs other materials 42

40 Corrosion Resistance 43

41 Biocompatibility Physical and chemical properties of primary corrosion products of metallic biomaterials Survival curves of L132 cells in the presence of different metal powder suspensions 44

42 Bioadhesion (osseointegration) 45

43 Dental implant 46

44 Coronary Stents Ni-Ti alloys or Nitinol 48

45 Shape memory alloys 49

46 Shape memory alloys : Principle 51