Surface Treatment

1. ED19P27 CORROSION AND
SURFACE ENGINEERING
Unit-4
SURFACE TREATMENTS

2. Wear Resistant Coatings
• Various coating techniques available for modifying the surface to improve
its tribological characteristics are,

3. Classifications
• Coating Techniques Dependent on Vacuum or Gas at Very Low Pressure.
– Physical vapour deposition (PVD)
– Chemical vapour deposition (CVD)
– Ion implantation.
• Coating Processes Requiring Localized Sources of IntenseHeat.
– Surface welding
– Thermal spraying
– Laser hardening or surface melting

4. Cont..
• Coating Processes Based on Deposition in the Solid State.
– Explosive Cladding or explosive bonding.
– Friction Surfacing

5. Physical Vapour Deposition
• This process is used to apply coatings by condensation of vapours in a
vacuum.
• The extremely clean conditions created by vacuum and glow discharge
result in near perfect adhesion between the atoms of coating material and
the atoms of the substrate.
• Porosity is also suppressed by the absence of dirtinclusions.
• Virtually any metal which do not undergo dissociation can be easily
deposited onto substrates of virtually any material.

6. • Physical vapour deposition consists of three majortechniques:
– Evaporation,
– Ion-plating
– Sputtering.
Cont..

7. Evaporation
• This is a relatively simple and cheap process and is used to deposit coatings
up to 1 mm thick.
• During the process of evaporation the coating material is vaporized by
heating to a temperature of about 1000 - 2000°C in a vacuum typically 10-6
to 1 Pa.
• The source material can be heated by,
– materials with a low melting point - electrical resistance.
– materials with a high melting point – need higher power density
methods, e.g. electron beam heating.

8. • The substrate is also pre-heated to a temperature of about 200 - 1600°C.
• Atoms in the form of vapour travel in straight lines from the coating source
towards the substrate where condensation takes place.
• The collisions between the source material atoms and the ambient gas
atoms reduce their kinetic energy.
• To minimize these collisions the source to substrate distance is adjusted so
that it is less than the free path of gas atoms, e.g. about 0.15 - 0.45 m.
Cont..

9. Cont..
Schematic diagram of the evaporation process.

10. • Disadvantages:
– Because of the low kinetic energy of the vapour the coatings produced
during the evaporation exhibit low adhesion and therefore are less
desirable for tribological applications compared to other vacuum based
deposition processes.
– Because the atoms of vapour travel in straight lines to the substrate, this
results in a ‘shadowing effect’ for surfaces which do not directly face
the coating source and common engineering components such as
spheres, gears, moulds and valve bodies.
Cont..

11. Ion-plating
• It is a process in which a phenomenon known as ‘glow discharge’ is
utilized.
• If an electric potential is applied between two electrodes immersed in gas at
reduced pressure, a stable passage of current is possible. The gas between
the electrodes becomes luminescent hence the term ‘glow discharge’.
• When sufficient voltage is applied the coating material can be transferred
from the ‘source’ electrode to the ‘target’ electrode which contains the
substrate.

12. • Prior to ion-plating the substrate is subjected to high-energy inert gas
(argon) ion bombardment causing a removal of surface impurities which is
beneficial since it results in better adhesion.
• The most important aspect of ion-plating which distinguishes this process
from the others is the modification of the microstructure and composition
of the deposit caused by ion bombardment.
• Ion plating processes can be classified into two generalcategories:
– Glow discharge ion plating conducted in a low vacuum of 0.5 to 10 Pa.
– Ion beam ion plating performed in a high vacuum of 10-5 to 10-2Pa.
Cont..

13. Cont..
Schematic diagram of the ion-plating process.

14. • Disadvantages:
– The inert gas ion bombardment is continued without interruptions
causes an undesirable effect of decreasing deposition rates since some
of the deposited material is removed in the process. Therefore for the
coating to form the deposition rate must exceed the sputteringrate.
– The heating of the substrate by intense gas bombardment may also
cause some problems.
Cont..

15. Sputtering
• Sputtering is based on dislodging and ejecting the atoms from the coating
material by bombardment of high-energy ions of heavy inert or reactive
gases, usually argon.
• In sputtering the coating material is not evaporated and instead, ionized
argon gas is used to dislodge individual atoms of the coatingsubstance.
• In glow-discharge sputtering a coating material is placed in a vacuum
chamber which is evacuated to 10-5 to 10-3 Pa and then back-filled with a
working gas to a pressure of 0.5 to 10 Pa. The substrate is positioned in
front of the target so that it intercepts the flux of dislodgedatoms.

16. Cont..
Schematic diagram of the sputtering process.

17. • Advantages:
– Since the coating material is transformed into the vapour phase by
mechanical (momentum exchange) rather than a chemical or thermal
process, virtually any material can be coated.
– Sputtering is that substances which decompose at elevated temperatures
can be sputtered.
– Substrate heating during the coating process is usuallynegligible.
Cont..

18. • Disadvantages:
– The coating material arrives at the substrate with far less energy than in
ion-plating so that a distinct boundary between film and substrate is
formed.
– The condensation process is critical to coating quality and unless
optimized by the appropriate selection of coating rate, argon gas
pressure and bias voltage, it may result in a porous crystal structure
with poor wear resistance.
Cont..

19. Chemical Vapour Deposition
• In this process the coating material is formed by volatilization from either a
liquid or a solid feed.
• The vapour is forced to flow by a pressure difference or the action of the
carrier gas toward the substrate surface.
• The coating is obtained either by thermal decomposition or chemical
reaction.
• The chemical reactions usually take place in the temperature range between
150 - 2200°C at pressures ranging from 50 Pa to atmosphericpressure.
• During the process the coating material is deposited, atom by atom, on the
hot substrate.

20. • Since the vapour will condense on any relatively cool surface that it
contacts, all parts of the deposition system must be at least as hot as the
vapour source.
• The substrate is usually heated by electric resistance, inductance or infrared
heating.
Cont..
Schematic diagram of the CVD process.

21. • Advantages:
– The CVD process at low pressure allows the deposition of coatings
with superior quality and uniformity over a large substrate area at high
deposition rates.
• Disadvantages:
– Although CVD coatings usually exhibit excellent adhesion, the
requirements of high substrate temperature limit their applications to
substrates which can withstand these high temperatures.
Cont..

22. Physical-Chemical Vapour Deposition
• This is a hybrid process which utilizes glow discharge to activate the CVD
process.
• In this process the techniques of forming solid deposits by initiating
chemical reactions in a gas with an electrical discharge are utilized.
• Coating can be applied at significantly lower substrate temperatures, of
about 100 - 600°C, because of the ability of high-energy electrons
produced by glow discharge to break chemical bonds and thus promote
chemical reactions.
• Working pressures ranging from 1 to 500 Pa.

23. • During this process a diamond coating can be produced from carbon in
methane or in acetylene.
• Amorphous diamond-like coatings in vacuum can attain a coefficient of
friction as low as 0.006.
• It is broadly referred to as ‘plasma enhanced chemical vapour deposition’
(PECVD) or ‘plasma assisted chemical vapour deposition’(PACVD).
Cont..

24. Cont..
Schematic diagram of the PECVD process.

25. Ion Implantation
• If sufficient electrical potential is applied then the plasma can be converted
to a directed beam which is aimed at the material to be coated allowing the
controlled introduction of the coating material into the surface of the
substrate. This process is known as ion implantation.
• During the process of ion implantation, ions of elements are propelled with
high energy at the specimen surface and penetrate the surface of the
substrate.
• This is done by means of high-energy ion beams containing the coating
material in a vacuum typically in the range 10-3 to 10-4 Pa.

26. • The mass of implanted ions is limited by time, therefore compared to other
surfaces, the layers of ion-implanted surfaces are very shallow, about 0.01
to 0.5 μm.
Cont..
Schematic diagram of the ion implantation process.

27. • Advantages:
– Despite the thinness of the modified layer, a long lasting reduction in
friction and wear can be obtained.
– The treatment is very clean and the deposited layers very thin, hence
the tolerances are maintained and the precision of the component is not
distorted.
– The ion implantation is carried out at low temperatures.
– The technique allows for the implantation of metallic and non-metallic
coating materials into metals, cermets, ceramics or even polymers.
Cont..

28. • Disadvantages:
– Since the coatings generated by ion implantation are very shallow it can
used only in lightly loaded contacts.
– an expensive process since the cost of the equipment and running costs
are high.
Cont..

29. Surface Welding
• In this technique the coating is deposited by melting of the coating material
onto the substrate by a gas flame, plasma arc or electric arc welding
process.
• During the welding process a portion of the substrate surface is melted and
mixed together with the coating material in the fusion zone resulting in
good bonding of the coating to the substrate.

30. • There are variety of specialized welding processes,
– Oxyfuel Gas Welding (OGW),
– Shielded Metal Arc Welding(SMAW),
– Submerged Arc Welding(SAW),
– Gas Metal Arc Welding(GMAW),
– Gas Tungsten Arc Welding(GTAW).
Cont..

31. Cont..
Schematic diagram of the welding process.

32. • Advantages:
– Welding is used in a variety of industrial applications requiring
relatively thick, wear resistant coatings ranging from about 750 [μm] to
a few millimeters.
– Welding processes can be easily automated and are capable of
depositing coatings on both small components of intricate shape and
large flat surfaces.
Cont..

33. Thermal Spraying
• This is the most versatile process of deposition of coating materials. During
this process the coating material is fed to a heating zone where it becomes
molten and then is propelled to the pre-heated substrate.
• Coating material can be supplied in the form of rod, wire or powder.
• The distance from the spraying gun to the substrate is in the range of 0.15
to 0.3m.
• The molten particles accelerated towards the substrate are cooled to a semi-
molten condition. They splatter on the substrate surface and are instantly
bonded primarily by mechanical interlocking.

34. • There are a number of techniques used to melt and propel the coating
material and the most commonly applied are:
– Flame spraying,
– Plasma spraying,
– Detonation-gun spraying,
– Electric arc spraying and others.
Cont..

35. Flame Spraying
• Flame Spraying utilizes the flame produced from combustion gases, e.g.
oxyacetylene and oxyhydrogen, to melt the coating material.
• The flame temperature is in the range of 3000 to 3500°C.
• Compressed air is fed through the annulus around the outside of the nozzle
and accelerates the molten or semi-molten particles onto the substrate.
• The process is widely used in industry, i.e. for corrosion resistantcoatings.

36. Cont..
Schematic diagram of the flame spraying process.

37. • Advantages:
– The process is relatively cheap, and is characterized by high deposition
rates.
• Disadvantages:
– The flame sprayed coatings, in general, exhibit lower bond strength and
higher porosity than the other thermally sprayed coatings.
Cont..

38. Plasma Spraying
• In plasma Spraying the coating metal is deposited as molten droplets rather
than as individual atoms or ions.
• The technique utilizes an electric arc to melt the coating material and to
propel it as a high-velocity spray onto the substrate.
• The coating material is fed to the plasma flame where it melts and is
propelled to the substrate.
• The temperature of the plasma flame is very high, e.g. up to 30,000°C.

39. Cont..
Schematic diagram of the plasma spraying process.

40. • Advantages:
– The very high particle velocity in plasma spraying compared to flame
spraying results in very good adhesion of the coating to the substrate
and a high coating density.
Cont..

41. Detonation-Gun Spraying
• The mixture of a metered amount of coating material in a powder form
with a controlled amount of oxygen and acetylene is injected into the
chamber where it is ignited.
• The powder particles are heated and accelerated at extremely high
velocities towards the substrate where they impinge.
• They are used in applications where close tolerances must be maintained,
i.e. valve components, pump plungers, compressor rods, etc.

42. Cont..
Schematic diagram of the detonation gun spraying process.

43. • Advantages:
– The coatings produced by this method exhibit higher hardness, density
and adhesion (bonding strength).
– The coating porosity is also very fine.
– Wear and corrosion resistant coatings capable of operating at elevated
temperatures are produced by this method.
• Disadvantages:
– Very hard materials cannot be coated by this process because the high
velocity gas can cause surface erosion.
Cont..

44. Electric Arc Spraying
• In this process there is no external heat source such as a gas flame or
electrically induced plasma.
• An electric arc is produced by two converging wire electrodes. Melting of
the wires occurs at the high arc temperature and molten particles are
atomized and accelerated onto the substrate by the compressed air.
• The use of an inert atomizing gas might result in improved characteristics
of some coatings by inhibiting oxidation.
• The wires are continuously fed to balance the sprayed material.

45. Cont..
Schematic diagram of electric arc spraying process.

46. • Advantages:
– The adhesion achieved during this process is higher than that of flame
sprayed coatings under comparable conditions.
– Since there is no flame touching the substrate like in the other thermal
spraying processes, the substrate heating is lower.
– Coatings of mixed metals, e.g. copper and stainless steel, can be
produced.
Cont..

47. Laser Surface Hardening and Alloying
• Laser hardening is a form of thermal hardening where a high power laser
beam is scanned over a surface to cause melting to a limited depth.
• Rapid cooling of the surface by the unheated substrate results in a hard
quenched microstructure with a fine grain size formed on re-solidification.
• Surface alloying is also possible if the surface of the substrate is pre-coated
with the alloying element or the alloying element is fed into the path of the
laser beam. This process is also known as laser cladding.
• Because of the very large temperature gradients mixing of the molten
material is intense.

48. Cont..
Schematic diagram of the laser surface alloying process.

49. • Advantages:
– A strong bond between the modified layer and the substrate is formed
since the substrate is never exposed to any atmospheric contaminants.
• Disadvantages:
– The area coverage by this process is relatively slow.
– The overlap areas between successive laser passes have inferior
properties and microstructure.
Cont..

50. Application of Coatings and Surface Treatments
in Wear and Friction Control
• There is a wide range of coating techniques and careful selection of the
appropriate coating material and method is a pre-requisite for an effective
coating.
• If the prime objective is to reduce friction then a solid lubricant coating
should be selected and the coating method will, in most cases, be either
sputtering or a combination of painting and baking.
• To suppress wear by the application of coatings, it is first necessary to
determine the mechanism of wear occurring. Each type of coating is most
effective at preventing a few specific wear mechanisms.

51. Cont..
Basic characteristics of coatings in terms of wear control.

52. Characteristics of Wear Resistant Coatings
• Studies of wear resistant coatings reveal that hard coatings are most
effective in suppressing abrasive wear.
• It can be seen from Figure that the abrasive wear rate declines to a
negligible value once a PVD coating of titanium nitride, which is
characterized by extremely high hardness, is employed.
Example of the resistance of a hard coating, TiN, to abrasion

53. • It was also found that thin films of ceramics such as titanium nitride are
quite effective in suppressing adhesive wear in poorly lubricated and high
stress contacts.
• For example, when a cutting tool is coated with titanium nitride, adhesion
and seizure between the tool and metal chip does not occur even when
cutting is performed in a vacuum.
• Titanium and its alloys are notorious for their susceptibility to seizure in
dry sliding, and implantation by nitrogen ions reduces the coefficients of
friction in dry sliding to a value as low as 0.15.
• Fretting wear can be mitigated by the use of hard coatings, e.g. carbides,
especially at small amplitudes of fretting movement.
Cont..

54. • Wear-resistant coatings can be as vulnerable to oxidative wear as
monolithic metal substrates.
• For example, copper causes rapid wear of cutting tools coated with
titanium nitride, titanium carbide or a combination of both compounds. It
was found that the primary cause of rapid wear of the titanium nitride and
carbide coatings is a catalytic effect of copper on the oxidation of the
nitride and carbide to titanium oxide which is then rapidly worn away.
Cont..

55. DLC coating
• The acronym DLC means Diamond - Like Carbon.
• The DLC coating combines high hardness, low coefficient of friction and
this makes it very useful for increasing the efficiency of parts of machines,
tools and micro-gears.
DLC coating on sintered carbide

56. • The coefficient of friction is very low, ranging between 0.1 and 0.2, ie
much lower than all of the coatings used in tools sector.
• It is used in the electronics industry, the heads of the Hard Disk in jewelry
and industrial tooling. As the product is biocompatible can be used for
implants inside the human body.
Cont..

57. Understanding
Nanocoating
Technology
ESTC May 2013 Las
Vegas, NV
CherylTulkoff & Dr.Craig Hillman
ctulkoff@dfrsolutions.com
chillman@dfrsolutions.com

58. Introductio
n
• Explosion in new
coating technologies
over the past 24
months
• Reached an apex
at Consumer
Electronics Show
in Jan. 2012
• Drivers
– Moisture proofing
– Oxygen
barrier

59. • Successful adoption of these coating technologies
can lead to improved performance and market
differentiation
– Inappropriate adoption can drive higher failure rates
and alienate customers
• Obtaining relevant information can be difficult
– Information is often segmented
– Focus on opportunity, not risks
• Sources are either marketing mush or confusing,
scientific studies
– Where is the practical advice?
New
Technologies

60. • Provide a clear understanding of the
current state of the technology
• What is the technology?
• What are the benefits?
• What are the risks?
Motivatio
n

61. • Super Hydrophobicity
• Atomic Layer
Deposition
• Multi-Layer Barrier
Films
Nanocoating
Technology

62. • Definition: Wetting angle far
greater than the 90 degrees
typically defined as
hydrophobic
– Can create barriers far
more resistant to
humidity and
condensation than
standard conformal
coatings
• How to get there?
– Deposit materials with existing high surface
tension
Super
Hydrophobicity

63. • Three companies currently focused on
the electronics market
• The key technology for each company is
the process, not necessarily the
materials
Nanodeposition of High
Surface Tension
Materials

64. • GVD: Founded in 2001. Spinoff from MIT
– Key technology is initiated chemical vapor deposition
(iCVD) and PTFE and Silicone
• Porton Plasma Innovations (P2I): Founded in
2004. Spin off from UK MOD Laboratory and
Durham University
– Key technology is pulsed plasma and halogenated
polymer
coatings (specifically, fluorocarbon)
• Semblant: Founded in 2009. Spin off from Ipex
Capital
Nanocoating
Companies

65. • Hydrophobicity tends to be driven by number and
length of the fluorocarbon groups and the
concentration of these groups on the surface
• The key points to each technology are similar
– All assisted chemical vapor deposition (CVD) processes
– Room Temperature Deposition Process
– Low Vacuum Requirements
– Variety of Potential Coating Materials (with primary focus
on
fluorocarbons)
• Differentiation is how they breakdown the
monomer before deposition
Process
Technology

66. Process
Differentiation
• iCVD uses a
chemical initiator to
breakdown the
monomer
• Plasma-Enhanced
CVD (PECVD) uses
plasma to
breakdown the
monomer
http://web.mit.edu/gleason-
lab/research.htm
Courtesy of Semblant
Ltd.

67. • These are truly nanocoatings
– Minimum Parylene thickness tends to be above
one micron (necessary to be pinhole free)
– These coatings can be pinhole free at 100 nm or
lower
• Nanocoating allows for
– Optical Transparency
– RF Transparency
– Reworkability
Benefits (especially
compared to
Parylene)

68. Optically
Transparent
Courtesy of
P2i

69. LLCR
Measurements
Current
(A) V
comp (V)
Connector Male
Connector
Female
0.01
0.1
34way Pin Header Au over nickel contact (TE Connectivity 1-
215307-7) 34way Socket Header Au over nickel contact (TE
Connectivity1-826632-7)
Benefits: No
Masking
1
2
3
4
5
6
7
32
33
34
Curren
t
Sourc
e
10mA
Nano
Voltmete
r
34
way
header
pins
34 way
header
Socket
Rcontac
t
=
V
measur
e
/10x10-
3
Test
Circuit
0
3
2
1
6
5
4
8
7
9
0 10 30 40
Resistance
(mOhms)
20
Pin
Number
Low Level Contact
Resistance
Plasma
Coated
Uncoated
Courtesy of Semblant
Ltd.

70. • Voltage Breakdown
– Levels tend to be lower compared to existing coatings (acrylic,
urethane, silicone)
– Can be an issue in terms of MIL and IPC specifications
• Optically Transparent
– Inspection is challenging
• Cost
– Likely more expensive than common wet coatings
– However, major cell phone manufacturer claims significant ROI based
on drop in warranty costs
• Throughput
– Batch process. Coating times tend to be 10 to 30 minutes, depending
upon
desired thickness
– However, being used in high volume manufacturing
Risks
?

71. • Nanocoating technology is being used by
almost every major hearing aid
manufacturer (8 million per year)
High Volume
Manufacturing
Courtesy of
P2i

72. • High aspect surfaces create very
high (>150o) wetting angles
• Rapid adoption in
other industries
– Paints, clothing, etc.
• Challenges with electronics
– Different surfaces/chemistries
can disrupt surface
modification
Where are Lotus
Leaf
Coatings?

73. • Developed in the 70s and 80s
• Sequential surface reactions
• Deposition rate of ~0.1
Atomic Layer Deposition
(ALD)
Deposit
organometallic /
hydroxyl compounds
Purg
e
Treatment to react 1st
precursor (2nd
precursor or energy)
Purg
e

74. • Designed for
inorganic
compounds
– Primarily simple
oxides and
nitrides
• Limited industrial acceptance
– Electroluminescence displays
(obsolete)
– Basic research
– Glass strengthening
– Considered for next-gen silicon
ALD
(cont.)

75. ALD
(cont.)
• Advantages
– Deposition at low
temps (80-150C)
– Not line of sight (conformal)
– Precise thickness control
– Large area
– Less stringent vacuum requirements (0.1 – 5 mbar)
– Multilayer and gradient capability (it can be tailored)
• Northrop Grumman / Lockheed Martin estimated
cost reductions of 50% vs. traditional hermetic
approaches

76. Water Vapor Transmission
Rates (WVTR) at
80C
Meyer, et. al, Appl. Phys. Lett. 96, 243308
(2010);
• Room temperature WVTR could be as
low as 8x10-7 g/m2/day (assumes Ea =

77. • How does 5 x 10-8 atm-cm3/second
(MIL requirements) compare to 8 x10-
7 g/(m2 x day)?
• For a 7.6mm x 5mm package, leak rate
can be approximately four orders of
magnitude slower
WVTR
(cont.)

78. • The process can be very slow
(100nm thickness can take over an hour)
• Not reworkable
• There have been challenges in regards to
compatibility of ALD with existing electronic
materials and manufacturing process
– Clean surfaces with similar coefficient of
thermal expansion (CTE) work best
– De-adhesion and cracking sometimes observed when
applied to metals, polymers, and surfaces with flux
residue (requires some tailoring)
Risk
s

79. • A Multi-Layer Barrier Stack to retard water vapor
and oxygen diffusion
– Patent history
includes Osram,
filed in 2001
– Similar patents filed by
Battelle Labs and
commercialized by Vitex
Systems in 1999 (Barix)
(now Samsung America)
• Flexibility and transparency
key attributes of the technology
– Index matching of materials (primary driver is organic
LEDs and thin film solar)
Multi-Layer Barrier Films:
What is the
Technology?

80. • Oxide barrier layer protects against oxygen and moisture
– Can be aluminum oxide, silicon oxide, or silicon nitride
• Polymer layer provides adhesion, smooth surface, decouples
defects
– Three to four layers required to survive 60C/90%RH for 500 hrs
– Requires permeation less than 10-6 g/m2/day
Multi-Layer Barrier
(cont.)
3 um total
thickness

81. • Requires a low temp vacuum deposition process
(PVD or CVD)
– Concerns about cost and throughput (similar to
parylene)
Multi-Layer Barrier Manufacturing
Process
Vitex
Process

82. • Recent advances use
nanotechnology to eliminate defects
– Nanoparticles seal
defects and react/retain
oxygen and moisture
– Requires fewer layers
to be effective
– Claims up
to 2300
hours
at 60C/90%RH
Multi-Layer Barrier
Advances

83. • Multi-layer barrier is believed to be the only
solution for OLEDs
– However, commercialization has been limited
– Developed over ten (10) years ago; still not in
commercial production
• Only one or two manufacturers in this space
• Cost could be high (similar to parylene) and
potential for low throughput (batch process)
• Rework is likely impossible
Risk
s

84. • There is significant opportunity for field
performance improvement and cost reduction
through the use of nanocoatings
• Requires a knowledge of the materials and
processes on the market
– Benefits vs. Risks
• With any new technology, do not rely on
standard qualification tests!
– A physics-based test plan provides the most
robust mitigation
Conclusio
n