Environmentally assisted failures in metallic and composite materials like corrosion and hygrothermal attack are described here.

1. Environmentally Assisted
Failures in Metallic and
Composite Materials – Case
Studies
Padmanabhan Krishnan
SMEC
VIT University, Vellore.

2. 2
Mythology

3. 3
Bottom up and
Top down Philosophy

4. Conventional Materials
4

5. What is corrosion
• Corrosion can be defined as the reaction of a material with its environment.
The problem of corrosion arises in various environments ranging from urban
and marine atmospheres to industrial chemical plant installations. It is a major
factor governing the design and operation of plant and equipment as it
reduces their useful life and can often result in unscheduled shutdowns or, in
some cases, it cause failure. The control of corrosion presents a considerable
challenge to engineers and, in spite of our best efforts, the annual costs of
corrosion damage and corrosion related service failures run into many millions
of pounds. However, there is scope to reduce this cost burden by making
improvements in materials selection, methods of protection, design and in-
service monitoring.
• In aqueous environments, corrosion may occur as uniform (general) or non-
uniform (local) attack. Uniform corrosion results in general wastage, is
reasonably easy to inspect and to predict from weight loss experiments or
electrochemical data. Local corrosion can take a number of various forms and
is much less predictable. It can result in more serious damage to structures.

6. Corrosion Mechanisms
 Modern corrosion science has its roots in electrochemistry and
metallurgy. Electrochemistry contributes an understanding of the
mechanism that is basic to the corrosion of all metallic objects. Metallurgy
provides knowledge of the characteristics of metals and their alloys as
well as the methods of combining the various metals and working them
into the desired shapes. Corrosion can proceed by several different
mechanisms, including:
 · Rusting
 · Pitting
 · Galvanic attack
 · Intergranular attack
 · Leaching (selective corrosion)
 · Corrosion and erosion
 · Stress corrosion cracking (SCC)
 · Corrosion fatigue
 · Hydrogen damage

7. Corrosion Types -
Corrosion Forms
 Uniform Corrosion
 Concentration Cell Corrosion
 Intergranular Corrosion
 Atmospheric Corrosion
 Fretting Corrosion
 Dealloiyng
 Microbiological Corrosion - MIC
 Galvanic Corrosion or Bimetallic Corrosion
 Pitting Corrosion - Crevice Corrosion
 Stress Corrosion Cracking
 Cavitation Corrosion
 Hydrogen Embitterment
 Immunity
 Corrosion Fatigue

8. UNIFORM ATTACK
 This is the most common form of corrosion.
 A chemical reaction (or electrochemical reaction) occurs over entire exposed surface (or
large areas) more or less uniformly.
 Not usually serious and is typically predictable from simple tests.
 Can be designed “around” by specifying an adequate CORROSION ALLOWANCE for
the expected lifetime of the component.

9. Concentration Cell Corrosion
 Concentration cell corrosion is corrosion that is accelerated by differences in
environment between separated areas on a single metal
 A difference in environment between sites on a single metal can also result
in increased electrochemical activity.

10. Intergranular Corrosion
 Intergranular corrosion is a selective attack of a metal at or adjacent to grain
boundaries.
 Intergranular attack caused by high grain boundary energies or impurities at
the grain boundaries results in attack with a grainy residue and rough
surface. Under high magnification, the individual grains are often visible.
Intergranular attack of aluminum alloys is associated with pitting or other
localized attack.

11. Fretting Corrosion
 Fretting corrosion is an attack that is accelerated by the oscillatory relative
motion of contacting surfaces.
1. Fretting was common in riveted joints on ships and other riveted structures where
cyclic loads were experienced, but this has largely been eliminated through welded
construction. Fretting is, however, still encountered in bolted joints and flanges where
there is not enough bolt tension to eliminate movement in the joint. Thermal expansion
with frequent cycling can also result in fretting attack. Any combination of corrosion and
wear will almost always be worse than the action of either one separately.

12. Dealloying Corrosion
 Dealloying is the selective corrosion of one or more components of a solid
solution alloy. It is also called parting, selective leaching or selective
attack. Common dealloying examples are decarburization,
decobaltification, denickelification, dezincification, and graphitic corrosion or
graphitization.
Graphitic corrosion of a gray cast iron valve

14. Bimetallic Corrosion
 Bimetallic corrosion is a localised mechanism by which metals can be preferentially
corroded. This form of corrosion has the potential to attack junctions of metals, or
regions where one construction metal is changed to another.

15. MElec-Ch5 - 15
Galvanic Corrosion
Understanding Galvanic Corrosion
• Causes
• Results
• Galvanic Series of Metals
• Additional Notes

16. MElec-Ch5 - 16
Causes
• Requires
 Two different metals (electrodes)
 Immersed in current-carrying solution (electrolyte)
 Interconnected by a current-carrying conductor

17. MElec-Ch5 - 17
Results
New Zinc
(for 1” diameter shaft)
of Galvanic Corrosion
Old Zinc after 8 months
(for 1” diameter shaft)

18. MElec-Ch5 - 18
Galvanic Scale of Metals
What is the voltage
difference between
Zinc (Zn) and Copper
(Cu)?
What is more noble
than Stainless Steel
(Passive)?
An. 0.67v
An. Graphite

19. MElec-Ch5 - 19
Signs of Galvanic Corrosion
• Blistering of paint
 1st Warning Sign
• Formation of powdery
substance
 2nd Warning Sign
• Pitting of metal
 Too late
 Severe Galvanic Corrosion
• Don’t treat the symptom,
fix the problem

20. MElec-Ch5 - 20
Galvanic Corrosion
Controlling Galvanic Corrosion
• Types of Metal
• Area of Metals
• Self-Destroying Metals
• Use of Sacrificial Anodes
• Indirect Cathodic Protection
• Resistance of an Electrical Path
• Between boats

21. MElec-Ch5 - 21
Types of Metal
• Copper, bronze and copper-nickel are
compatible
• Avoid bronze propeller on plain steel shaft
• Stainless steel shaft with bronze prop may
be used
 Need zinc washer and/or zinc prop nut
 Avoid graphite grease

22. MElec-Ch5 - 22
Self-Destroying Metals
• Brass (an alloy of copper and zinc)
 Zinc will corrode away in sea water, leaving a
copper sponge
• Stainless steel hose clamps with different
metal take-up screws
• Stainless steel should be non-magnetic
 If magnetic, it will corrode

23. MElec-Ch5 - 23
Use of Sacrificial Anodes
• Made from active metals
 Magnesium, zinc or aluminum
• Corrosive action occurs on the expendable
metal anode
• Bolted to the metal they are to protect
• Never painted
• Replaced when half-corroded or annually
Shaft Prop Nut Rudder

24. Pitting & Crevice Corrosion
 What is Pitting Corrosion?
Under certain specific conditions, particularly involving chlorides (such as sodium
chloride in sea water) and exacerbated by elevated temperatures, small pits can form
in the surface of the metal. Dependent upon both the environment and the metal
itself these small pits may continue to grow, and if they do can lead to perforation,
while the majority of the metal surface may still be totally unaffected.
What is Crevice Corrosion?
Crevice Corrosion can be thought of as a special case of pitting corrosion, but one where the
initial "pit" is provided by an external feature; examples of these features are sharp at corners,
overlapping metal surfaces, non-metallic gaskets or incomplete weld penetration. To function as a
corrosion site a crevice has to be of sufficient width to permit entry of the corrodent, but
sufficiently narrow to ensure that the corrodent remains stagnant. Accordingly crevice corrosion
usually occurs in gaps a few micrometers wide, and is not found in grooves or slots in which
circulation of the corrodent is possible

26. Stress Corrosion Cracking
 Stress corrosion cracking is the intergranular or transgranular cracking
of a material due to the combined action of tensile stress and a specific
environment.


27. Cavitation Corrosion
 Cavitation corrosion is corrosion that is enhanced through the
formation and collapse of gas or vapor bubbles at or near the
metal surface.
 Under high velocity flow conditions, particularly when the flow is turbulent,
areas is high and low pressure will be induced. In areas of low pressure, gas
and vapor bubbles will be produced. When these bubbles move to an area of
higher pressure, they collapse and their implosion creates a pressure wave
that can remove protective films and cause increased corrosion.

28. Hydrogen Embrittlement
 Hydrogen embrittlement is the severe loss of ductility of a metal when hydrogen has
been introduced into the metal structure.
Hydrogen can enter most metals. Due to the small size of the hydrogen atom, it can migrate
through the metal structure and cause a loss of ductility similar to that experienced in stress
corrosion cracking.
Hydrogen atoms can enter a metal either from hydrogen gas, usually at elevated temperatures,
or from atomic hydrogen that is electrolytically formed on its surface. This hydrogen can either
reduce the energy required for forming cracks under stress or can accumulate at areas of high
stress, such as crack tips, and cause pressure

29. Immunity
 Immunity is the lack of measurable attack on a metal when exposed to operational
environments.
The first form of corrosion described is the lack of attack, or immunity. This can result from the
action of two basic mechanism. Corrosion test measurements that are used to measure very low
corrosion rates must be used to validate that corrosion activity is completely absent.
Immunity can result from two basic mechanisms. In the first case, the energy content of the metal
is lower (more stable) than any of the corrosion products that could possibly form. Such metals
are commonly found in nature as metals that indicate the stability of the metallic state for these
elements.

30. Corrosion Fatigue
 Corrosion fatigue is the reduced ability of a metal to withstand repeated stress when
exposed to the combined action of stress and a corrosive environment as compared
to the effects of stress alone.
Many materials will exhibit a substantial reduction in fatigue life when exposed to a corrosive
environment. In some cases, the reduction is severe, in other cases it is less dramatic, but only a
very few materials show a fatigue resistance in a corrosive environments as great as that in dry
air.

31. Corrosion Control &
Corrosion Protection
There are four basic methods for Corrosion Control & Corrosion Protection.
1. Materials resistant to Corrosion
2. Protective coatings
3. Cathodic protection
4. Corrosion Inhibitors- Modify the operating environment.
Ways to protect metal from corrosion:
1. Galvanization is method of protecting steel & iron from rusting by coating them with a
thin layer of zinc. The galvanised is protected against rusting even if the zinc coating
is broken.
2. Painting, greasing & oiling are some commonly used methods to prevent rusting.
3. Chrome plating, anodising & making alloys are some other ways to prevent corrosion
of metals.

32. Methods of preventing the atmospheric corrosion of metals fall into two broad
categories:
i) The choice of a suitably resistant metal or alloy or combination of
materials, or the provision of a protective coating which supplements or
enhances the protection given by the air-formed oxide film on the metal
surface.
ii) The control of the environment by the exclusion of water or aggressive
contaminants, or by the introduction of a corrosion inhibitor.
Tin plating,
Galvanization,
Coating,
Rubber paints
Copper plating

33. Plastics and Composites

34. CTE and CME
• Coefficient of Thermal Expansion and
• Coefficient of Moisture Expansion
• Expansion per degree per unit moisture concentration.
• m = moisture weight/ dry composite wt.

35. Fickian and Langmuir
Behaviour

36. Moisture Absorption

37. Maximum Moisture Content

38. Variation of Tg with Moisture

39. Moisture Absorption

40. Moisture Absorption

41. HYGROTHERMAL ANALYSIS
Tgw = (0.005Mr
2-0.1Mr+1.0)Tgo
Where,
Fm = Mechanical property retention ratio,
P = Strength or stiffness after hygrothermal degradation,
Po = Reference strength or stiffness before degradation,
T = Temperature at which P is to be predicted (0C),
Tgo= Glass transition temperature for reference dry condition
Tgw=Glass transition temperature for reference wet condition at
moisture content corresponding to property P,
To = Test temperature at which Po was measured (250C),
Mr = weight percent of moisture at equilibrium

43. Increasing moisure content
Tgw Tgo
Temperature
Strength/Stiffiness
Rubbery region
Glassy region
Dry
Wet
Variation of strength with temperature
Glass Transition Temperature

44. All composite Aircrafts
Indian Hansa-All composite aircraft
VTOL Aircraft
Ref: B.K. Parida, RMVGK Rao and K. Padmanabhan , Proceedings of the third joint
National Aerospace Laboratories- Chinese Aircraft Establishment workshop on
composites, April 22-24, Bangalore, India, 1996, p9. & NAL website

45. Biomedical Composites
s
75X106 cyles of heart beat78 x 106 cycles of heart beat
over 25 years

46. Nanocomposites
The SWNT has a Tensile Modulus of 1-5 TPa and a Tensile Strength of 13-53
GPa at an elongation of 16 %. A 0. 50 Vf composite , in theory will have a tensile
strength and a modulus around half as much, assuming that the matrix is weak !
Carbon nanotubes

47. 47
Multiscale Composites in Dentistry

48. Composites in Dentistry
Acrylics, Acrylic esters, Bis-
GMA, PMMA derivatives,
Ceramic filled composites
Can you make out the
difference ?
Ref: K. Padmanabhan , Programme overview, NIST ceramics machining
consortium, I th Chapter, Gaithersburg, USA, October 8-9, 1998.

49. Composites in Prosthodontics
Tooth is a functionally graded
composite material with enamel
and dentin. In the third maxillary
molar the occlusal stress can
be 2-3 MPa.
The masticatory heavy chewing
stress will be around 193 MPa.
A composite restorative must with
stand this with an FOS and with
constant hygrothermal attack.

50. 50
Multi Scale Composites for
Dentistry
 Bis GMA, UDMA, Methacrylic Esters contain
glassy particles that are mostly less than 1
micron in size
 Esters and acrylates/ceramic filler (barium
alumina silica glass, glassy microfillers, 0.1 to
10 microns size) restorative composites.
 Multi scale composites are also useful as
luting cements, crown and bridge materials
and cements and veneer materials.

51. 51
Wear Data for Restoratives
 Aesthetics, Shade, Reliability
 Most of the composites show a logarithmic wear
rate, linear wear rate is unwelcome !
 Wear of less than 200 microns in 10 years is
acceptable.
 Wear volume will be 0.5 to 0.8 cu. mm per
annum, enamel vs restoratives.
 Coeff of Friction is ~ 0.1 to 0.35.

52. The need to revise standards
52
K. Padmanbhan,

53. Natural and Accelerated Ageing

54. Microsystems Packaging
Roadmap

55. TYPES OF PACKAGES
IC
package
materials
Plastic
Thin film (Tape carrier)
Ceramic
IC
assembly
SMT (Surface mount technology)
PTH (Pin through hole)
DCA (Direct chip attach)
IC
interconnection
Peripheral (Quad flat package)
Partial area array
Area array (Ball grid array)

56. FUNCTIONS OF PACKAGES
1. Shut out environmental
influences
2. Enable electrical connectivity
3. Dissipate heat
4. Improve handling
& assembly

57. Applications of IC Packages
 Mobile Phones
 PDA s
 Automotive Electronics
 Digital Cameras and Camcorders
 TVs and Monitors
 Signal Transmission and Reception
ICs
 Aviation, Marine and Biotechnology

58. Analysis
 Electric-Thermal Analysis
 Thermal – Structural Analysis
 Materials used
 Lead Frame and Leads - Copper
 Wire bonding - Gold
 Die - Silicon
 Adhesive for lead frame
and die - Silver Epoxy
 Encapsulation - Carbon black and fused silica
particles filled epoxy and
phenolic resin

59. Thermal Shock Test (JESD22 –
A106B)
 Purpose of this test is to determine the resistance of the part to sudden
exposures of extreme changes in temperature and alternate exposures to
these extremes as well as its ability to withstand cyclical stresses
 Here the IC packages are baked in an oven for 125ºC/24 Hrs and the
temperature is spiked to 260ºC for lead free product and 240ºC for
leaded product for 5 to 10 minutes.
 If the baking temperature is higher than the glass transition temperature
at this extreme heat the package tends to delaminate or fail. This failure
or delamination can be viewed using SAM (Scanning Acoustic
Microscopy)

60. Hygrothermal Behaviour
 Plastic packages are known for environmental attacks
that reduce their function.
 Mechanical properties degrade over time !
 Moisture plays havoc at elevated temperatures, in the
presence of voids, defects and in low Tg plastics .
 Diffusion and osmotic pressure are the driving
mechanisms for hygrothermal attack
 Evaluation methods and surface preservation
 Important in marine, biomedical, aeronautical,
electronic and automobile applications

61. Temperature Vs Property Retention
Ratio Plot

62. Temperature Vs Strength
Plot of 9220ZHF 10L

63. Temperature Vs Strength
Plot of EME G770HCD

64. DIE SHEAR TEST

65. Die shear test for 4x4mm 24L
TQLMP
Maximum permissible shear stress ( shear strength)
[τxy]max = shear force
shear area
Shear force : 10 kgf (actual measured value)
Shear area : 4.5732mm2
Maximum permissible shear stress
( shear strength)
[τxy]max = 10* 9.81
4.5732
= 21.875N/mm2
Actual shear stress< Shear Strength
(3.25 < 21.875)N/mm2 with encapsulation
(1.301 < 21.875)N/mm2 for without encapsulation
So our design is safe.

66. Azzi-Tsai-Hill Theory
Where,
σ11 = X ( tensile / compressive) stress in MPa
σ22 = Y ( tensile / compressive) stress in MPa
τ12 = Shear stress in MPa
SLt = Longitudinal tensile strength in MPa
STt = Transverse tensile strength in MPa
SLts = in-plane shear strength in MPa

67. Tsai – Wu Failure Theory
F1 σ11 + F2 σ22 + F6 τ12+ F11 σ11
2+ F22 σ22
2+ F66 τ12
2+2 F12 σ11σ22 = 1
Where,
Other parameters / symbols appear on the previous slide,
SLc = Longitudinal compressive strength in MPa
STc = Transverse compressive strength in MPa.

68. Failure theory for 4x4mm
24L TQLMP
Normal conditioning i.e at 25°C ambience
X stress in MPa = 39.695
Y stress in MPa = 63.794
XY shear stress in MPa = 27.078
Tensile strength in MPa =185
Compressive strength in MPa = 195
Shear strength in MPa = 92.5
Tsai – Wu Failure theory: 0.203 < 1
Azzi-Tsai-Hill theory: 0.1757(Tensile) / 0.1675(compressive) < 1
Design is safe.

69. Failure theory for 4x4mm
24L TQLMP
Maximum operating temperature i.e at 61.1°C
X stress in MPa = 39.695
Y stress in MPa = 63.794
XY shear stress in MPa = 27.078
Tensile strength in MPa =145.78
Compressive strength in MPa = 153.66
Shear strength in MPa = 72.86
Tsai – Wu Failure theory: 0.318 < 1
Azzi-Tsai-Hill theory: 0.2841(Tensile) / 0.0.27 (compressive) < 1
Design is safe.

70. Failure theory for 4x4mm
24L TQLMP
Peak conditioning i.e 125°C for 24 hrs
X stress in MPa = 70.509
Y stress in MPa = 98.811
XY shear stress in MPa = 45.793
Tensile strength in MPa =27.01
Compressive strength in MPa = 28.47
Shear strength in MPa = 13.5
Tsai – Wu Failure theory: 22.29> 1
Azzi-Tsai-Hill theory: 22.047(Tensile) / 21.083(compressive) > 1
Design is unsafe.

71. SAM Picture for 24L TQLMP
before Preconditioning, with Plasma*
cleaning

72. SAM Picture of 24L TQLMP after
preconditioning, with Plasma* cleaning

73. ASTM STP D 5229 M Rule
 The MOT ( Maximum Operating Temperature) of
the material, device/component should be at
least 25 Celsius lower than the lowest Tg (
normally wet) of the material. In this case the
mould compound qualify this clause as the
maximum temperature developed due to joule
heating is 83.3 Celsius in the 64 L TQLMP. The
wet Tg of 9220ZHF 10L mould compound is
121.25 Celsius.

74. Coffin-Manson's Equation
 Fatigue is the most common failure mode in IC packaging,
 IC device dissipate heat to it’s surroundings during operation,
differential thermal expansion generates stresses in the
interconnecting structure.
 These stresses produce instantaneous elastic and plastic strain in
the material joint.
 The mechanical properties of material change strongly over the
normal temperature range of operation and moisture ingress.
 Above the room temperature, mechanical built-in stresses that
result from thermal mismatch with the other materials can relax
and vanish over time.

75. Coffin-Manson Equations
 Cyclical plastic deformations change the grain structure, weaken
the joints and can lead to fatigue.
 The time of the joint fracture depends on relative deformation
(strain), temperature and frequency of deformation.
 A simplified relationship is given by the Coffin-Manson's formula;
N0.5 × δp = constant
 Where
 N – number of cycles,
 δp -relative deformation

76. Coffin-Manson Fatigue vs CTE

77. Design Philosophy
Pyramidal Substantiation
The product ,we hope,
is reliable
The sub-assemblies and the
assemblies
are tested the least
The subcomponents and components
are tested less often
The test specimens are tested more often

78. Thank You
That one coming being,
Was covered with void,
That arose through the power of
heat
-The Rig Veda ( The Existence-10.129.03)