Wear Mitigation by the Use of
Surface Coatings
by Stephen Huse
Introduction
Description of the Problem
The purpose of this project is to become familiar with common wear mechanisms, wear
process variables, and mitigating undesired wear by the use of surface coatings.
The following questions are addressed in this paper:
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What wear mechanisms are commonly found in applications such as metal
forming and manufacturing?
What variables are involved in these wear mechanisms?
What variables can be controlled in order to reduce wear, specifically by the use
of coatings
What common surface coating processes are available to reduce wear?
Background Overview
This section provides a summary of the answers to the above questions. For more
detailed discussion, see Theory and Methodology.
Common wear mechanisms are abrasive and adhesive wear. Abrasive wear is
common in mining, earth moving, farming, and mineral processing in factories.
Abrasion occurs when surface asperities physically interfere with motion. Adhesive
wear is common in metal, ceramic, and polymer contact in machinery. Adhesion occurs
when two similar surfaces create “cold weld” bonding between asperities, resisting
relative motion.
Different variables involved in wear are: friction, temperature, surface hardness, surface
roughness, surface material, environmental considerations, and amount of relative
surface travel. Some of these variables are defined in the design of the process and
cannot be changed, such as ambient temperature and surface travel, but some can be
controlled by the use of surface coatings.
Surface coatings reduce abrasive and adhesive wear by greatly increasing the surface
hardness of the substrate and by reducing friction. Increasing the hardness of a surface
decreases the amount of wear occurring on a surface. Also, since two solid surfaces in
direct contact will tend to wear the surface with lower hardness, raising tooling surface
hardness will tend to wear the worked material instead of the tooling. Surface coatings
can also reduce wear by reducing adhesion when a dissimilar coating material is added
between two identical material surfaces.
Surface coatings adhere hard material such as ceramic, diamond, and titanium carbide
to a substrate in order to provide a hardened surface layer. Some of the processes to
apply coatings include physical vapor deposition, chemical vapor deposition,
electrodeposition, thermal spray, and diamond in a nickel alloy matrix.
Theory and Methodology
Wear Mechanisms
The wear mechanisms discussed for this project involve plastic displacement and
removal of material due to contact of two solid surfaces with relative motion between the
surfaces. Contact forces and pressures create deformation of the material surfaces
through physical interference and adhesion of the two surfaces through surface
bonding.
These wear mechanisms are defined by the interaction of the surfaces such as abrasive
involving mechanical interference, adhesive involving surface bonding, erosive involving
particle impingement, and fatigue involving repetitive motion crack growth. For this
paper, abrasive and adhesive wear will be focused on.
Wear can also be classified in others ways such as whether the surfaces are lubricated
and what the motion of the surfaces is. The type of relative motion between the
surfaces can be impact contact normal to the surfaces, sliding contact tangential to the
surfaces, and rolling contact.
Abrasive Wear
Abrasive wear is common in mining, earth moving, farming, and mineral processing
such as cement factories. Abrasion generally involves hard particles moving across a
relatively softer metal surface producing gouges in the surface. These particles can be
embedded on a surface creating two-body wear or trapped between two surfaces
creating three-body wear. The material of the metal surface is then displaced or
removed by the abrasion particles. This damages the surface eventually leading to
reduced efficiency or failure of the equipment.
The displacement can take different forms. One form is called ploughing where the
material is plastically deformed and pushed to the side.
Ploughing
(Reproduced from surface.mat.ethz.ch)
Another is cutting. Cutting is different in that the displaced metal is removed and
remains in front of the abrasive particle.
Cutting
(Reproduced from surface.mat.ethz.ch)
One other method of abrasive wear is brittle cracking, which occurs in less ductile
materials.
Brittle Cracking
(Reproduced from surface.met.ethz.ch)
These types of wear are commonly found where abrasive wear is occurring. Using
surface coatings adds a hard layer to the metal surfaces and thereby reduces these
abrasion mechanisms from occurring.
Adhesive Wear
Adhesive wear is common in metal, ceramic, and polymer contact in machinery. When
two similar surfaces come into contact, bonds are formed between the surfaces, and
material may transfer from one surface to another. The contact at low wear rates
initially acts as abrasion, deforming asperities until the surface oxide layer is removed.
When the oxide layer is removed faster than it is replaced, adhesion begins. As the
wear rate continues to increase, heat generated by friction will result in a faster
replacement of the oxide layer, faster than it is removed by wear, and adhesion no
longer occurs.
The presence of adhesive bonding sharply increases the wear rate. Relative surface
motion breaks adhesive bonds, not always along the original interface, and material can
be transferred between surfaces. The transferred material can then detach and
become loose wear particles. It is not fully known how the adhesive surface interactions
are based on bulk material properties, but some of the known dependencies are crystal
structure, crystal orientation, and cohesive strength.
Designing contact with dissimilar materials will reduce adhesion. Surface coatings are
one way of creating dissimilar surface contact without changing the substrate materials.
For example, ceramic coating can be applied to a surface that was previously in steel
on steel contact to create ceramic on steel contact.
Friction Equation
Abrasive and adhesive wear are indirectly dependent on the amount of friction between
the surfaces. Friction is also a concern since it creates heat.
The coefficients of static and kinetic friction are difficult to accurately replicate for given
settings, but in general, rough surfaces tend to have more wear with more friction due to
more surface asperities mechanically interfering. Also, surfaces that adhere tend to
have more friction due to the shear stress needed to break the adhesion bonds.
A general equation for the force of friction is the following: F f   s ,k N . Note that friction
is independent of the apparent area of contact.
μ is the static or kinetic coefficient of friction depending on if the surfaces are not moving
or in relative motion. Typical ranges are from 0.03 to 0.7. The coefficient of friction
depends on many factors such as wear-in history, temperature, mechanism of motion,
environmental influences, surface finishes, surface coatings, and surface materials.
N is the force acting normal to the plane of the interface between the two surfaces.
Wear Equation
The friction and the volume of material displaced or removed by wear are simply related
KWL
in the Archard Equation which applies to sliding wear: Q 
H
Q is the total volume of material removed from the surfaces through wear.
K is a wear constant that indicates the severity of wear and is dependent on many
material properties including friction coefficients. Typical values are 10-8 for mild wear
and 10-2 for severe wear. Mild wear occurs at lower loads when the metal oxide layer is
not removed completely. Severe wear can occur when the oxide layer is removed and
there is adhesive wear present.
W is the normal load.
L is the relative tangential distance traveled between the surfaces for abrasion and
adhesion. Rolling contact and impact contact which are normal to the surface have L=0
and would not be predicted to cause wear using the Archard equation. However, rolling
contact and impact contact do cause wear in the form of fatigue cracking and erosion.
H is the surface hardness of the softer surface and is generally accepted as three times
the yield strength. Hardness is changeable by the use of surface finishing such as work
hardening and by the use of surface coatings. It is seen that surface hardness is
inversely proportional to wear volume.
Reduction of K is possible with the use of lubricants and surface coatings. Reduction in
friction by the separation of the solid surfaces using lubrication results in much less
wear occurring, but for machining applications, separation of the surfaces is not
possible. Surface coatings can be used to reduce friction in machining applications.
Results and Discussion
Surface Coatings
Surface coatings are beneficial for reducing abrasive and adhesive wear. Types of
wear that surface coatings can help mitigate include abrasive wear, adhesive wear,
corrosive wear, chemical wear, galling, erosive wear, and oxidizing wear. There are
many types of coatings depending on the type of wear experienced, level of hardness
desired, environmental considerations, ambient temperatures, types of substrate
surfaces that the coating is adhering to, level of adhesion desired, and complexity of the
part geometry.
Some of the coatings with very hard surfaces applied for abrasive wear resistance are
PVD (Physical Vapor Deposition) deposited on the substrate under vacuum, CVD
(Chemical Vapor Deposition), Chromium plating by electrodeposition, thermal spray
coatings, and composite diamond coatings.
Physical Vapor Deposition
PVD coatings are thin films deposited on the substrate with vacuum deposition
methods. The coating is vaporized by a variety of means such as plasma torch,
cathodic arc, and electron beam or physically separated by magnetic “sputter”
technology pulling the plasma discharge to the substrate. The newest method of PVD
was developed by NASA in 2010 and uses plasma spraying to apply very thin ceramic
films down to 10μm on ceramic and metal substrates. The plasma-phase gas reaches
temperatures of 18,000° F and melts the input ceramic powder while being transported
to the work piece where it will condense on the cooler substrate material.
PS-PVD Ceramic Coating
(reproduced from nasa.gov)
The temperature of the vaporized coating is much higher than the substrate. The
plasma-phase spray condenses when it contacts the substrate. This condensation
creates bonding to the substrate. To promote stronger bonding, reactive gases such as
oxygen and nitrogen may be added. This will also alter the composition of the coating
and its tribological properties. The resulting hardness of PVD coatings is much harder
than untreated tool steel. Another benefit to PVD coatings is lowering the coefficient of
friction. PVD coating results in less force required when using a tool, less adhesion of
the worked material to the tool due to reduced chemical affinity, reduced burr creation,
less heat generated in friction, and improved surface finish of the worked material.
One major disadvantage of PVD coatings is that line-of-sight transfer is needed,
resulting in poor coverage of complex geometries.
Chemical Vapor Deposition
CVD Coatings differ from PVD coatings in that the process does not occur under
vacuum, but under a controlled atmospheric environment. Substrate temperature for
the CVD process is elevated to about 2,000° F, higher than the substrate temperature
for PVD coating. Thin film coatings are deposited due to chemical reactions of the
gaseous atmosphere chemicals, and the heated substrate.
One common atmospheric chemical is gaseous titanium chloride, TiCl4. This can be
mixed with nitrogen and hydrogen to create titanium nitride: 2TiCl4 + N2 + 4H2 1000°C
→ 2TiN + 8HCl. Titanium chloride also forms titanium carbide when mixed with
methane and hydrogen: TiCl4 + CH4 + H2 1000° C → TiC + 4HCl + H2. These
compounds are chemically bonded to the substrate to create the coating. Because of
the chemical compound bonding, the adherence of CVD coating is superior to other
types of “cold” substrate coatings such as PVD. The better adhesion means that CVD
coatings are more suitable for heavy forming operations such as bending, punches,
drawing, trimming, and others that require good adhesion to the substrate under heavy
loading. Heavy forming operations are a more common application for CVD coatings,
but it is also used for machine tooling coatings.
Another unique application for CVD coatings is worth noting. By slightly increasing
pressure, it is possible to grow carbon crystals on a substrate. These crystals are
synthetic diamonds. The CVD coating process is a more economical way to make
synthetic diamonds than other high pressure processes.
Other Coatings
Chromium plating also known as Chrome plating is applied by electrodeposition. This
results in a surface that has reduced friction and good corrosion resistance due to
chromium being relatively unreactive. Chrome plating is used in a large number of
applications and is beneficial for wear reduction due to reduced friction and increase in
surface hardness in addition to the common use for corrosion resistance.
Thermal spray coatings are similar to PVD coatings except that they are not applied
under a vacuum. The substrate can be initially heated to remove contaminants and
oxides leaving the bulk material exposed. Then the coating is heated and sprayed onto
the substrate. Thermal spray coatings increase hardness and reduced friction.
Composite Diamond Coatings are particles of diamond held in place by a nickel alloy
matrix. The resulting hardness is very high. Along with being wear resistant, CDC
coatings are also temperature and environment resistant.
Reference List
http://www.enduracoatings.com/hardness.html
http://www.richterprecision.com/pvd-coatings.html
http://www.nasa.gov/topics/technology/features/ceramic_coatings.html
http://www.richterprecision.com/cvd-coatings.html
http://www.cvd-diamond.com/synthesis_en.htm
http://www.diamondinnovations.com/SiteCollectionDocuments/Micron/CDC.pdf
http://www.surface.mat.ethz.ch/education/courses/surfaces_interfaces_and_their_applic
ations_II/SIandAII_Ch1_Wear_Course
http://www.ewp.rpi.edu/hartford/~ernesto/F2012/FWM/Notes/ch06.pdf
http://www.expresspolymlett.com/articles/EPL-0000031_article.pdf
http://www.tstcoatings.com/plasma_spray.html
M.J. Neale, The Tribology Handbook 2nd Ed. (Ch. D), Butterworth, London, 1995
T.A. Stolarski, Tribology in Machine Design (Ch. 2.8-2.12), Butterworth, London, 1990
J.R. Davis, Surface Engineering for Corrosion and Wear Resistance (Ch. 3 and 8),
ASM, London, 2001