Coatings Tribology

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Coatings Tribology

• Wei-Yu Ho
• Dept. Materials Science Engineering
• MingDao University

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Tribological behaviour of composite

• Under abrasive and sliding wear situations,
  composites containing a high volume fraction of
  hard reinforcement particles exhibited high wear
  resistance.

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oil in lubrication systems

• The oil in lubrication systems usually contains
  particle contamination. The experimental results
  of Nilsson et al. show that abrasive wear caused
  by hard particles can significantly change the
  surface profile of both washers and rollers in
  roller bearings. Lubricant particle contamination
  can result from either generated or ingested
  particles.

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run-in period of rolling and sliding

• During the run-in period of rolling and sliding
  components, particle generation can be very high
  even for a clean system with clean components. It
  is therefore important that filtration be
  efficient during this period. Experimental
  results show that 1 h of filtration with a 3µm
  filter during the run-in of a roller bearing can
  reduce both the mass loss and the number of
  self-generated particles by a factor of 10.

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Abrasive scratches can be seen over the entire
contact surfaces of all components.
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Abrasive wear
Abrasive wear covers generally two types of
situations. In both cases wear occurs by the
plowing-out of softer material of a given volume
by the harder indenters of an abrasive surface.
In the first instance a rough, harder abrasive
surface slides against a softer metal surface.
In the second case abrasion is caused by loose
hard particles sliding between the rubbing
abrasive and metal surfaces.
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Effect of abrasive grit dimension

• The size of an abrasive grit had a direct
  influence on the associated wear mechanism.
• a ratio term, w / r, where w was the groove width
  and r was the radius of the spherical tip of the
  grit particle.
• Depending on this ratio, the abrasive particle
  would either plastically deform the surface or
  cut it.

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• During abrasive wear, the extent of the
  plastically strained region below an abraded
  surface depended on the abrasive grit size and
  the applied load. The overall depth of the
  plastic deformation was linearly related to the
  applied load and the grit dimensions. They
  reported that the energy expended in plastic
  deformation of the wearing material to form
  grooves and deform the surface accounted for
  almost all the external work done for all grit
  sizes in abrasion wear.

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abrasive wear resistance was influenced by the
size of the reinforcing particles, matrix
hardness and the abrasive grit size.
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influence of hardness

• the role of second phase particles in providing
  localized areas of high stress concentrations
  that influenced flow stress and wear rate. The
  highest wear resistance was obtained in
  microstructures associated with fine, well
  dispersed semi-coherent particles, The influence
  of carbides on wear resistance depended or, their
  hardness relative to the matrix hardness.

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the composite with spherical reinforcing
particles shows the lowest wear loss
the spherical one shows lower abrasivity without
large cracks existing in the matrix.
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volume fraction of reinforcement

• Increasing the volume fraction of reinforcement
  can result in a decrease in wear loss. However,
  there exists a critical volume fraction, above
  which the wear loss increases. The critical
  volume fraction changes with the size ratio as
  the size ratio decreases, the critical volume
  fraction shifts to a higher level.

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spacing between adjacent reinforcements

• The size and volume fraction of reinforcement
  change the spacing between adjacent
  reinforcements. An appropriate spacing may
  effectively reduce wear loss, which is also
  influenced by the size of abrasive particles or
  the size ratio. Adding dispersed fine
  reinforcements in matrix is an effective approach
  to increase the wear resistance of a composite.

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Aluminium pressure die-casting

• Aluminium pressure die-casting is widely applied
  for the high-volume production of high-quality
  castings, e.g. used in the automotive industry.
• mechanisms claimed to limit die lifetime, the
  so-called washout phenomenon,
• corrosion, erosion and soldering as a result of
  the exposure of the tool steel to liquid
  aluminium.
• thermal fatigue caused by thermal cycling of the
  dies.

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Aluminium alloys

• Aluminium alloys are the most commonly utilized
  materials in composite fabrication e.g. 2000,
  5000, 6000 and 7000 alloy series. The
  reinforcement phase is generally one of the
  following continuous boron or graphite fibres,
  or "hard particles such as SiC and AI2O3 in
  discontinuous particulate or whisker morphology.
  The volume fraction of reinforced particles or
  whiskers is generally within the range of 10 -30.

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Die casting products in automobile industry
Surface Coatings Technology 201 (2007) 56285632
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Aluminum die casting tools for cylinder blocks
The predominant failure mechanisms of die casting
tools are thermal fatigue by thermal shock
stress, abrasion, adhesion, erosion, hydrogen
embrittlement, and corrosion by liquid metal
melts. As a consequence of the failure mechanisms
the tools show only a limited lifetime.
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Aluminum space frame technology
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automotive lightweight construction

• The automotive lightweight construction is a
  growing sector, because of the ecological
  necessity to save fuel and to increase the load
  capacity of vehicles. From that background
  previous structural parts made of steel were
  replaced by aluminum or magnesium components.

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Lubricants issue
Aluminium pressure diecasting is a frequently
applied manufacturing process for the
near-netshape production of precision parts.
State-of-the-art production techniques still
demand considerable amounts of die lubricants.
However, the side effects of these lubricants
include a decrease in surface quality of the cast
part as well as a lengthening of the casting
cycle time. The job quality decreases, and last
but not least, the disposal of waste and exhaust
during the production and application of
lubricants is ecologically risky.
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The deposition of wear-resistant coatings offers
a way to mininimize the use of lubricants in the
casting process and simultaneously to increase
the service life of the diecating tools.
Plasma-assisted chemical vapour deposition
(PACVD) was chosen as the most suitable
deposition method for this application. PACVD
therefore permits the coating of tools with
complicated geometries. The deposition
temperatures range below the tempering
temperatures of the hot work steels typical of
diecasting tools.
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The coatings deposited have to withstand the
complex loads occurring at the surface of Al
diecasting tools. These loads are composed, for
example, of thermal shock, abrasive wear,
adhesive wear and corrosion by liquid aluminium.
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tensile crack pattern

• A tensile crack pattern was observed because of
  the mismatch of thermal expansion coefficients of
  coating and substrate. These cracks seem to often
  initiate at droplets formed during deposition
  (compare Fig. 1b).
• The number of cracks increases with increasing
  hardness of the coating.

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The suitability of PACVD coatings for diecasting
tools was evaluated using the example of cores
for the production of induction pipes. Fig. 1
shows a TiN- and a Ti(B,N)-coated core with the
corresponding cast product.
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Wetting behaviour
Selected TiN, TiN/TiC and Ti(B,N) coatings were
analysed with regard to their wettability with
the Al alloy used in the field tests. The wetting
angles determined were between 174 and 177. The
majority of the coatings did not show any wetting
at all. Consequently, adhesion between Al and the
protective coating is not one of the predominant
failure mechanisms in die casting.
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Corrosion resistance
After 6 h of dip testing in the Al melt, the
samples showed only small traces of pitting
corrosion. The coatings were able to prevent a
reaction between the aluminium melt and the
steel. In the case of mechanical damage of the
coating before dipping, aluminium penetrated into
the steel and deteriorated the layersubstrate
interface of the surrounding areas. The layer
material itself still proved to be
corrosion-proof.
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diecasting field tests
During the diecasting field test, the amount of
lubricant agent was decreased continuously. Fig.
2 displays the composition of the spray before
and after study. The amount of lubricant could be
reduced by 97, i.e. to less than 1 of the spray.
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Under these tightened conditions, an uncoated
dummy core had a lifetime of approximately 100
casts. Fig. 3 shows the maximum number of casts
before the criterion of failure occurred.
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Fig. shows a Ti(C,N) coated die used for pressure
die casting of an oil pump housing made of
AlSi9Cu3. Compared to salt-bath nitrided cores,
the increase in lifetime was between 100 for
Ti(B,N) coatings and more than 300 for Ti(C,N)
coatings.
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powder injection molding (PIM) process
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Sheet metal forming

• In sheet metal forming, the main wear mechanisms
  have been identified as adhesive wear due to the
  high loads applied, abrasive wear, e.g. by highly
  strain hardened wear debris, and mechanical
  fatigue due to cyclic loading.

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• Important task in surface engineering for sheet
  metal forming processes is the reduction of
  frictional forces between metal sheet and die
  which is normally done using lubricants.
• For example, in sheet metal forming of a
  cold-strip steel, the lifetime of an uncoated
  AISI A11 cold-work steel was approximately 2000
  using brush lubrication every 20 strokes.
  Applying a PACVD Ti(C,N) coating to the same
  tool, the lifetime could be increased to 26 000
  strokes whereas brush lubrication was reduced to
  every 50 strokes.