STRUCTURE AND PROPERTIES OF CAST DENTAL ALLOYS.pptx

STRUCTURE AND PROPERTIES OF CAST DENTAL ALLOYS.pptx

1. STRUCTURE AND PROPERTIES OF CAST DENTAL ALLOYS GUIDED BY: Dr. TUSHAR TANWANI ( PROFESSOR &HOD) Dr. GAURAV TRIPATHI (PROFESSOR) Dr. GAURAV AGRAWAL (READER) Dr. SUDEEPTI SONI (READER) PRESENTED BY: Dr. SHRUTI JUMDE (PG 1 ST YEAR )

2. HISTORY OF METAL DEVELOPMENT IN DENTISTRY: 17 th century BC - Etruscans used ivory and bone supported by gold frameworks as tooth forms. 1800 - Metal restorations were made by compacting aluminium, amalgam, gold, platinum, and silver in tooth cavities. 1903 - concept of producing gold casting using the lost- wax process and a melting torch. 1907 - Taggert introduced a method for producing precise cast metal crown and bridge forms. 1970s - Several technological advantage such as CAD-CAM, electroforming, laser sintering and LASER welding.

3. INTRODUCTION

4. Metals As a class, metals are ductile and malleable and therefore exhibit elastic and plastic behavior. They are good electrical and thermal conductors. Higher in density than other classes Exhibit good toughness are opaque and can be polished to a luster. Metals may be cast, drawn, or machined to create dental restorations and instruments

5. A metal is any element that ionizes positively in solution. As a group, metals constitute nearly two thirds of the periodic table. During ionization, metals release electrons This ability to exist as free, positively charged, stable ions is a key factor in the behavior of metals and is responsible for many metallic properties that are important in dentistry

6. Another important group of elements shown are the metalloids including carbon, silicon, and boron. Although metalloids do not always form free positive ions, their conductive and electronic properties make them important components of many dental alloys

7. Atomic Structure At the atomic level, pure metals exist as crystalline arrays that are continuous in three dimensions. In these arrays, the nuclei and core electrons occupy the atomic center with the ionizable electrons floating freely among the atomic positions.

8. The mobility of the valence electrons is responsible for many properties of metals, such as electrical conductivity. It is important to note that the positively charged atomic centers are held together by the electrons and their positive charge is simultaneously neutralized by the negative electrons. Thus pure metals have no net charge

9. In array, the smallest repeating unit that captures all the relationships among atomic centers is called a unit cell. The three most common crystal lattice unit cells in dental metals and alloys Body-centered cubic cell Face-centered cubic cell Hexagonal close-packed cell

10. BODY CENTERED CUBE is noted in CHROMIUM IRON MOLYBDENUM TUNGSTEN

11. FACE CENTERED IS NOTED IN: Most pure metals - aluminum , copper, platinum, silver Alloys of gold, palladium, cobalt, and nickel

12. Titanium exhibits the more complex hexagonal close-pack array In this array, the atoms are equidistant from each other in the horizontal plane but not in the vertical.

13. All properties of metals result from the metallic crystal structure and metallic bonds. Metals have high densities that result from the efficient packing of atomic centers in the crystal lattice. Metals are electrically and thermally conductive because of the mobility of the valence electrons in the crystal lattice.

14. The opacity and reflective nature of metals result from the ability of the valence electrons to absorb and emit light. The corrosion properties of metals depend on the ability of atomic centers and electrons to be released in exchange for energy. The amount of energy required depends on the strength of the metallic force, which is related to the freedom of the valence electron and the energy that the released ion can gain by solvating in solution.

15. ALLOY: They are generally prepared by fusion of the elements above their melting points. For example,• A certain amount of chromium is added to iron, carbon, and other elements to form stainless steel, an alloy that is highly resistant to corrosion.

16. At least four factors determine the extent of solid solubility of metals. atom size , valence , chemical affinity and crystal structure. ATOM SIZE: If the sizes of two metallic atoms differ by less than approximately 15% (first noted by Hume- Rothery ), they possess a favorable size factor for solid solubility.

17. VALENCE: Metals of the same valence and size are more likely to form extensive solid solutions than are metals of different valences. CHEMICAL AFFINITY : When two metals exhibit a high degree of chemical affinity, they tend to form an intermetallic compound upon solidification rather than a solid solution.

18. USES OF METALS AND ALLOYS IN DENTISTRY: Steel alloys are commonly used for the construction of instruments and wires for orthodontics. Gold and noble alloys are used for making crowns , bridge framework , inlays and denture bases. Dental amalgam , an alloy containing mercury, is the most widely used dental filling material.

19. CLASSIFICATION OF ALLOYS : Cast dental alloys can be classified according to five categories : USE : All metal alloys Crown and bridges Metal-ceramic prosthesis Post and cores Removable partial denture implants

20. MAJOR ELEMENTS: Gold -based Palladium –based Silver-based Nickel -based Cobalt –based Titanium -based

21. NOBILITY: High noble Noble Predominantly base metal THREE PRINCIPAL ELEMENT: Au-Pd-Ag Pd-Ag-Sn Ni-Cr-Be Co-Cr-Mo Ti -Al-V Fe-Ni-Cr

22. ALLOY CLASSIFICATION BY NOBLE METAL CONTENT

23. DESIRABLE PROPERTIES OF DENTAL CASTING ALLOYS

24. FUNCTIONAL MECHANICAL PROPERTIES OF CASTING ALLOYS

25. ELASTIC MODULUS This property represents a proportional constant between stress and strain during the elastic deformation of a material. The elastic moduli of base metal alloys , excluding titanium alloys, are up to twice as high as those for some popular noble metal alloys.

26. YIELD STRENGTH Definition- Yield strength is defined as the amount of stress needed to cause 0.2% plastic deformation of the material, which is also called 0.2% offset yield strength. Yield strength reflects the capacity of a cast prosthesis to withstand mechanical stresses without permanent deformation. Why yield strength is necessary ? High yield strength Higher capacity to deal with stress Which alloys function satisfactorily in mouth ? T ensile yield strengths above 300 Mpa function satisfactorily in the mouth.

27. DUCTILITY This mechanical property represents the amount of plastic deformation under tensile stress that an alloy can undergo before it fractures. High ductility means that the amount of deformation that one can produce by adjusting a prosthesis or by burnishing a cast metal margin .

28. HARDNESS Hardness is a measure of the resistance of the surface to indentation by an object in the shape of a spherical or a diamond-shaped point. Hardness of the metal should be high enough to resist scratching and abrasion and also to maintain the smoothness of the prosthesis in the oral environment.

29. FATIGUE RESISTANCE This phenomenon occurs when a material is subjected to repeated loading and unloading below its elastic limit. When the load is above a certain threshold, it initiates cracks from surface flaws of the material. Most fractures of prostheses and restorations develop progressively over many stress cycles. Eventually a crack propagates to a critical size, and sudden fracture occurs

30. ALLOYS FOR ALL-METAL PROSTHESES 1] HIGH NOBLE AND NOBLE ALLOYS For prosthetic dental applications, it is necessary to incorporate various elements in gold to produce alloys with suitable properties. .

31. Which are these elements & why they are added?

32. Since some noble metals contain no gold and rely on palladium for corrosion resistance, these alloys are discussed in two categories: Au-based alloys Ag-Pd alloys

33. Gold-Based Alloys These alloys are generally yellow in color

34. The cast alloy must be rigid to resist flexure , possess high yield strength to prevent permanent distortion, and be ductile enough for adjustment in the clasp Changes of alloy color caused by the reduction in gold compensated for by an increase in copper, silver, and palladium Higher silver and copper content reduces the corrosion resistance