lecture 10

what is a base material

an element with metallic character that is prone to oxide formation and corrosion under normal influence

what is corrosion

the physical destruction of a metal or alloy in oral cavity conditions

what is tarnish (passivating layer)

a surface layer formed on a metal surface (most often by oxides) in which the metal loses its lustre and often darkens.

change does not penetrate in depth

what are important base metals

nickel, cobalt, iron, titanium and chromium

other base metals that often enter into the composition of alloys are...

copper, zinc, tin, gallium

classification of alloys according to mechanical properties

type 1 - soft = inlays

type 2 - medium-hard = onlay, overlay, crown

type 3 - hard = crowns, partial dentures up to 3 units

type 4 - very hard = multi bridge units, partial dental frameworks

1932 historical importance of alloys, according to classification

type 1 = VHN 60-90

type 2 = VHN 90-120

type 3 = VHN 120-150

type 4 = VHN > 150

what are base metal casting alloys used for

all-metal fixed restorations (rare and usually post and core)

frameworks of partial dentures

metal-ceramic constructions

technical features related to base casting alloys

casting conditions

castability

finishing procedures

oxide layer

casting conditions/ requirements

high mpt - 1150-1500 deg

acetylene torch

high freq current for melting

cast with phosphate or ethyl silicate investment materials

castability

the property of molten metal to accurately fill fine details in mould

low density - almost 2x than noble alloys

diff construction of casting pins

finishing procedures

non noble = high hardness

used with a veneering material

dust released is potentially hazardous

oxide layer

makes the connection to veneering layer

important in metal-ceramic layers

can require sandblasting and additional oxide baking

2 main groups of base alloys according to composition

Co/Cr = used for larger restorations that require a high modulus of elasticity + force required for plastic deformation

Ni/Cr = mainly in smaller restorations

--> with beryllium

--> without beryllium

what are functions of beryllium

increases thinness of alloys

improves their structure

works with beryllium

containing alloys and should be carried out in a well ventilated area

vapours are extremely dangerous however

mechanical properties of base metal casting alloys

modulus of elasticity is > or equal to 2, relative to noble alloys

stiffness is proportional to elastic modulus and width of connector area

latter is in cubic proportion to height of connector

2x higher modulus of elasticity allows a reduction in diameter of 20.6%

base metal alloys for removable partial dentures

4 main metallic components:

- major connector

- occlusal rests + reciprocal arms

- denture base mesh

- clasps

all but clasps need to be stiff --> basically not bend with applied force

cold working defn

permanent deformation induced by the application of mechanical force at a temp lower than the recrystallisation temp

includes metals and alloys casted in a particular shape + intended for subsequent cold working:

--> cutting

--> drilling

--> drawing

--> rolling

--> bending

---> twisting

yield is increased

ductility is reduced

crystal imperfections

theoretical and actual strengths of poly-crystalline materials differ significantly due to the presence of imperfections in their crystal structure

point and linear defects

point defects

highly localised distortions in crystal lattice arrangement

characteristic of metals

Important due to impact on larger defects

interatomic - usually due to small atoms (H,B,C,N,O)

promote diffusion

size of replacement atom can be larger or smaller

linear defect

dislocation of an entire atomic layer under the influence of an external force

slip occurs along the edge of a partial plane or in the form of a helicoid

linear defects in poly-crystalline alloys

PC possess mechanisms by which slip is limited

any structure diff from standard arrangement in lattice is a barrier to dislocation

twinning - what is it

plastic deformation without dislocation

occurs when alloys solidify or when pressure is applied

force produced > than dislocation force

changes shape of object but not relative position of atoms in the crystal lattice

mirrored position

metal and alloys for cold working - stainless steel

orthodontic wires

bent wire clasps in PD

endodontic instruments for RCT

metal crowns in paed patients

bending in case of a beam attached on one side (cantilever)

shows principle action of ortho arches

useful working range determined by elastic stress in material at the limit of proportionality

metal and alloys for cold working - carbon steel

major phase - iron-carbon binary phase

defined as a binary mixture between 2 elements in which carbon does not exceed 2.1% by weight

at room temp, pure iron has a face-centred cubic crystal lattice

at 912 deg it transforms to body centred

ferrite = 0.02% body centred crystal lattice

solid soln in a face centred lattice reaches up to 2.1%

binary (iron- carbon) containing 0.8% carbon is cooled slowly in austenite phase to 723 def, a pearlite is formed

pearlite is characterised by a combo of ferrite and iron carbide lamellae (FC3)

If austenite is cooled quickly, (quenching) martensite is formed

martensite is characterised by a body centred tetragonal crystal lattice

martensite has high hardness, strength and brittleness

phase formation is due to atom displacements rather than diffusion

phase is meta-stable + on heating converts to austenite --> known as tempering --> hardness and britlleness is reduced

metal and alloys for cold working - ferrite steel

cheap

good corrosion resistance

dont allow hardness inc by heat and cold treatment

not widely used in dent

metal and alloys for cold working martensite steel

undergoes heat treatment

leads to significant inc in hardness and proportionality limit

used for surgical and cutting instruments

metal and alloys for cold working - austenite steel

addition of Ni stabilises structure on cooling

highest corrosion resistance

good ductility

inc strength after cold working

fine grain structure

easy cold working

titanium and titanium alloys

high mpt (1668)

explosive reactivity with oxygen above 900 deg

special casting equipment

Zr-Mg investment mat

low density - difficult casting

uses a combo of centrifugal forces, vacuum and positive pressure

reacts with investment mat and forms a very hard coating later

alpha casting up to 150 microns thick

Vickers hardness is variable: 200 at surface, ~650 at 25μm depth

requires special machining tools

Commercially pure titanium

divided into 4 classes based on impurity content by ASTM Standard F67

^^ C, H, Fe, O ^^

class 1-4 have same N C and H content of 0.03, 0.08 and 0.15 respectively

class 1 has 0.2 Fe

class 2 and 3 have 0.3 Fe

class 4 has 0.5 Fe

class 1 has 0.18 O

class 2 has 0.25 O

class 3 has 0.35 O

class 4 has 0.4 O

modulus of elasticity is similar to that of enamel and noble alloys but lower than base alloys

Oxygen increases strength and fatigue resistance

titanium and titanium alloys

undergoes allotropic and polymorphic transformation at a temp of 882 def

<882 = alpha form (hexagonal with closely spaced crystals)

>882 = beta form (face-centred with inc ductility)

addition of stabilisers determines availability of 5 titanium alloy types:

alpha, near alpha

alpha-beta

near beta, beta

what are stabilisers for alpha alloys

aluminium, carbon, nitrogen and gallium

they raise temp for transformation of alpha to beta on heating

stabilisers for beta alloys

molybdenum, cobalt, nickel, niobium, copper, palladium, tantalum, vanadium

lower temp for phase transformation for beta to alpha on cooling

what do the "near alloys" form

near alpha = min amount of beta phase on heating

near beta = min amount of alpha phase on cooling

alpha-beta = meta stable and both phases occur at room temp. they are suitable for heat treatment

what is the most used alloy in dentistry

Ti-6Al-4V

is an alpha-beta alloy

vanadium in high dose is toxic

aluminium associated with neurological problems

alternative to V is Nb - same proportional atomic ratio forms = Ti6-Al7-Nb

both are acceptable for biomed applications

exhibit similar mechanical properties

their modulus of elasticity is comparable to type 4 alloys

retain more plaque due to electrostatic loading of oxide layer

change colour especially when highly alkaline denture cleaning soln applied

nickel titanium alloys for cold working

introduced in 1970 as Nitinol

55% Ni, 45% Ti

austenitic structure = complex body centred cubic crystal lattice

martensitic structure = mix of monoclinic, triclinic and hexagonal

nickel titanium alloys for cold working - properties

difficult bending in clinical conditions

high tensile strength

high ductility

low modulus of elasticity

superer (pseudo) elasticity of Ni-Ti alloys for ortho wires

transformation btwn austenitic+martensic forms

Austenitic form = high temp and low pressure (stress)

martensitic form = low temp anf high pressure (tension)

twinning = below elastic limit is reversible

bending moment vs angular deflection graph

a-b = initial elastic deformation

b-c austenitic to martensitic transformation

c- 10% elastic deformation

c-d - additional elastic and plastic deformation

e-f determines phase transformation from martensitic to austenitic structure

f-g - an angular offset into austenitic phase occurs

g is not equal to a due to permanent deformation occurring in segments c-d

b-c = upper limit (plateau) of superelasticity

e-f = lower limit (plateau) of superelasticity

memory of Ni-Ti alloys for orthodontic wires

transformation between austenitic and martensitic forms in Ni-Ti - temp and stress:

AUSTENITIC FORM

high temp

low pressure (stress)

MARTENSITIC FORM

low temp

high pressure

(stress)

pairing process - reversible below elastic limit

fixed arch shape at 480 deg.

on cooling to room temp - twinned martensitic structure with shape retention.

deformed martensite under bending

Ni-Ti alloys for endodontic instruments

low modulus of elasticity allows the manufacture of tools for RCT

with super-elasticity and transformable austenite

less austenite compared to ortho wires

manufactured by cutting - cutting edge defects - premature tool fracture

Ni-Ti alloys for endodontic instruments - R phase

is the alloy deformed into a rhombohedral cubic austenite shape

is an intermediate phase between austenite and martensite

only phase that allows the fabrication of Ni-Ti tools by bending

lower modulus of elasticity than austenite

Ni-Ti alloys for endodontic instruments - evolution

M-WIRE

introduced in 2007

contains large amount of stable martensite

created by thermo-mech process

inc fatigue strength = more RCT without fracture

CM WIRE (controlled memory)

presented in 2010

contains less nickel 52% instead of 54-57 - allows for controlled memory so tool stays bent essentially

3x higher fracture resistance

beta-titanium alloys for cold working - ortho wires

principle components of first ortho alloy contains 79% Ti, 6% Zr, 4% tin

molybdenum is stabilising element at room temp

first commercial alloy - titanium-molybdenum alloy

suitable for welding

nickel free

high surface roughness - so care is needed in cold working

additional materials and method for fabrication of indirect restorations

electroplating - applying thin layer of metal onto surface of restoration

sintering - used in ceramic production

spark erosion - manufacturing tools, shaping and cutting crowns, bridges for precise fit

celay system

milling (CAD/CAM subtraction tech)

3D printing (CAD/CAM addition tech) -