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 are the wear mechanisms 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 is the effect of surface coatings on the mechanisms of wear?
In addition, a comparison of surface effects in FEA will be conducted.
Background Overview
This section provides a summary of the answers to the above questions. For more
detailed discussion, see Theory and Methodology.
The most common wear types for industrial machining are abrasive and adhesive wear.
Abrasive wear is common in mining, earth moving, farming, and mineral processing
such as cement 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,
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 process defined and cannot be changed,
such as 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 reducing friction. The hardness of a surface is directly
related to the amount of wear occurring to the surface, also when two solid surfaces are
in direct contact, the surface with lower hardness will tend wear instead of the harder
surface. Surface coatings also reduce adhesive wear by reducing adhesion when
dissimilar coating material is added to one of two identical material surfaces.
Surface coatings add material that is naturally hard, such as ceramic or diamond, in
order to provide a protective layer to the underlying substrate. Processes to apply
coatings include physical vapor deposition, chemical vapor deposition,
electrodeposition, thermal spray, and nickel alloy matrix with diamond impregnation.
Theory and Methodology
Wear Mechanisms
Wear is the removal or plastic displacement 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.
Different types of wear can be classified in multiple ways such as whether the surfaces
are lubricated and what the mechanism of the surfaces is. The “mechanism” of the
surfaces is the type of relative motion between the surfaces such as impact contact
normal to the surfaces, sliding contact tangential to the surfaces, and rolling contact.
Another way to classify wear is by the qualities of the two surfaces, such as dry/wet,
metal/non-metallic, sliding/rolling, etc.
Wear can alternatively be classified 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 the
purpose of this paper abrasive and adhesive wear will be focused on.
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 are the types of wear that are commonly found for abrasive wear. using surface
coatings adds a hard layer to the metal surfaces and thereby reduces these abrasion
mechanisms from occurring as the metal surface is no longer the softer surface.
Adhesive Wear
Adhesive wear is common in metal, ceramic, and polymer contact in machinery. When
two similar surfaces come into contact, bonds are formed, and material may transfer
from one surface to another. The contact initially acts as abrasion, deforming asperities
until the surface oxide layer is removed resulting in adhesive bonding between the
substrates. As the relative surface motion continues, the asperities are broken, not
always along the bonded interface, and material is transferred, and eventually becomes
loose wear particles.
It is not fully known how the 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 partly dependent upon the amount of friction between
the surfaces. 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 better 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
in the Archard Equation which applies to sliding wear:
Q
KWL
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 material
properties including friction coefficients. Typical values are 10-8 for mild wear and 10-2
for severe wear. Mild wear occurs when the metal oxide layer is not penetrated.
Severe wear occurs when the oxide layer is displaced and there is metal to metal
adhesion.
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. Rolling contact
and impact contact do cause wear such as 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 or surface coatings. Reduction in
friction by the separation of the solid surfaces using lubrication results in much less
wear occurring. For tooling applications, separation of the surfaces is not possible
which leaves reduced friction coatings.
Results and Discussion
Surface Coatings
Surface coatings are beneficial for reducing wear in abrasive and adhesive processes.
Common types of wear are abrasive 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, high
temperatures, types of substrate surfaces that the coating is adhering to, level of
adhesion desired, and complex geometries.
Some of the types of 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 ceramic spray
reaches temperatures of 18,000° F and is 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, resulting in
condensation of the plasma-phase spray 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. Less friction
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 a coating. Because of the chemical compound bonding, the
adherence of the 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. Heavy
forming operations are more common for CVD coating, but it is also sometimes 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, also known as synthetic diamonds.
These diamonds are more economical to produce than the high pressure process to
make synthetic diamonds.
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 the
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.
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. These types of coatings can have increase in hardness or reduced
friction properties.
Composite Diamond Coatings are particles of diamond held in place by a nickel alloy
matrix. The resultant 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
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