Coatings Tribology

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Coatings Tribology

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

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Rolling contact fatigue (RCF)

• Rolling contact fatigue (RCF) is responsible for
  the failure of rolling element bearings, gears,
  camshafts and may be defined as cracking or
  pitting/delamination limited to the near-surface
  layer of bodies in rolling/sliding contact.

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RCF failure

• the subsurface-initiated RCF
• fatigue crack iniliation is caused by shear
  stresses generated by the macroscopic contact and
  usually occurs in the subsurface region
  corresponding to the maximum intensity of these
  stresses
• the near-surface initiated RCF
• fatigue initiation is presumably caused by
  small-scale contact stress perturbations
  generated by surface roughness or by small
  abrasive particles that may be present in the
  lubricant.

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Fatigue of railway wheels and rails under rolling
contact

• Rolling contact fatigue (RCF) of railway
  components is a most crucial subject.
• Non-catastrophic RCF failures are of importance
  since they cause unplanned maintenance which
  eventually causes decreased capacity and delays
  in the train traffic.

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Schematic sketch of plastic deformation of the
surface material in a railway wheel. The dashed
lines indicate material planes before and after
deformation.
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Schematic representation of growth of surface
initiated fatigue cracks in wheels. Once
initiated, the crack will deviate to an almost
radial direction. At a depth of about half a
millimeter, the crack will tend to deviate (or
branch) towards a circumferential growth. Final
fracture will typically occur as deattachment of
a piece of the surface material when the cracks
deviate towards the surface.
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(a) Roughness of a material surface on different
scales. (b) Loading of an asperity causing a cone
crack.
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the near-surface initiated RCF

• The near-surface initiated RCF can be slowed down
  dramatically or even prevented by reducing the
  surface roughness and improving lubricant
  filtering, or by increasing the thickness of the
  lubricant film separating the contacting
  surfaces.
• Unfortunately. In many applications it is
  impossible or impractical to achieve a
  sufficiently high surface finish and lubricant
  cleanness to prevent surface-initiated RCF of the
  rolling elements, and it is not always possible
  to maintain a sufficiently thick lubricant film.

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one possible solution

• one possible solution is to deposit a hard
  tribological coating on one or both of the
  contacting surfaces of rolling elements, such as
  bearing races and balls (rollers). In principle,
  such surface treatment can protect the near
  surface material layers from the contact stress
  spikes produced by surface roughness, and thus
  inhibit the near-surface RCF initiation and
  prolong the RCF life of coated rolling elements

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RCF performance of PVD coatings
It was seen that at lower stress levels, the pre-
treatment and the surface roughness of the
coatings had a significant influence on the
fatigue life. However, at high contact stresses,
there was little influence from these two
parameters. In isolating the fatigue pits which
formed as a result of RCF testing, it was noted
that the number of fatigue pits usually increased
with decreasing stress levels. This was however,
only noted with a low hardness of coating. An
increase in the coating hardness had an opposite
effect.
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the thickness of the coating
TiN coatings were tested using a two-disc
machine. The results from this test indicated
that relatively thick coatings could not protect
the substrate material, whereas the thin coatings
improved the rolling contact fatigue life of the
specimens. The optimum coating thickness for the
TiN coatings was considered as 0.25µm.
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Nanocomposite TiNy/SiNx coatings

• SiNx has an amorphous structure stable up to 1100
  0C
• a nanocomposite coating made of a mixture of
  nanocrystalline TiN and amorphous Si3N4 attains
  hardness gt50 GPa and is resistant against
  oxidation in air up to 800 0C.

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Surface and Coatings Technology 154 (2002) 152161
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Nanoscratch resistance of TiNy/SiNx coatings as a
function of SiNx layer thickness
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AISI52100 alloy steel

• AISI52100 alloy steel is widely used as rolling
  contact bearing material in the aerospace,
  nuclear, automotive and other special industries.
  It attracts people's great interest owning to its
  high compressive strength, low cost, good wear
  resistance, and excellent corrosion resistance in
  oxidation and acid atmospheres.

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PVD process

• Requirements for rolling contact bearing
  applications are not only high hardness and high
  fatigue resistance but also high dimensional
  stability and strong adhesion during production
  and usage of finished parts. In addition,
  AISI52100 bearing steel has relatively poor
  resistance to softening at elevated temperature
  (400 F), so conventional surface modification
  methods hardly satisfy the above demands
  simultaneously.

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plasma immersion ion implantation and deposition
(PIIID) technique

• The mechanical and fatigue properties of the
  AISI52100 bearing steel, can be greatly improved
  by plasma immersion ion implantation and
  deposition (PIIID) technique. The rolling
  contact fatigue (RCF) life of all treated samples
  is prolonged, and the maximum value is 108.5 h,
  increased by 6.5 times. Two kinds of fatigue
  damage mode, surface fatigue wear and adhesive
  delamination of TiC film are discussed.

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Micropitting
Micropitting is a form of surface contact fatigue
encountered in bearings and gears, under
lubricating conditions, which leads to their
premature failure. It can occur with all heat
treatments applied to gears and with both,
synthetic and mineral lubricants and after a
relatively short period of operationin some
cases, after less than a million cycles, gears
need to be replaced due to the increased noise
and vibrations caused by the deviation of the
tooth profile as a result of micropitting.
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Problem

• Extensive investigations into micropitting have
  been carried out during the last decades but the
  micropitting phenomenon remains unpredictable,
  difficult to control, and the complete mechanism
  is unknown.
• Experimental observations show that the rougher a
  surface is the more prone it is to micropitting.
  Surface asperities act as stress raisers and
  surface initiated cracks originate in the
  asperities.

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operating temperature
Micropitting can occur at moderate loads, below
the pitting endurance limit and, it can cause
damage after short running times. The operating
temperature mainly affects the lubrication
conditions (i.e., the lubricant viscosity and the
friction coefficient). An increase in the
operating temperature results in a decrease of
the lubricant viscosity and the lubricant film
thickness and thus, an increase in contact and
the probability of micropitting occurrence.
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operating speed
An increase in the operating speed improves the
formation of the lubricating film but also
increases the operating temperature. Therefore,
high operating speeds may promote micropitting.
The initiation period of micropitting decreases
as the sliding speed is reduced. It was found
that micropitting occurs most readily at speeds
in the range of 410 m/s but micropitting may
occur even at low contact stress because of the
effect of sliding.
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phase transformation
Little or no attention has been paid to possible
effects of phase transformation occurring in
gears undergoing micropitting. Recently, it has
been shown that the decay of martensite also
occurs in specimens subjected to rolling/sliding
loading (both discs and gears) affected by
micropitting. The decay of martensite gives rise
to preferential sites for crack nucleation and
propagation. The micropitting mechanisms
suggested previously are explained in terms of
lubricant pressure effects inside the crack or
slip line field theory but with no reference to
the steel microstructure.
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micropitting measurement
Wear 258 (2005) 15101524
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micropitting measurement
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increase the speed limit

• Coating the races or the balls is another
  approach towards improving the behaviour of the
  tribological system of bearing partners and
  lubricant.
• To increase the speed limit of machine tool
  spindles by improving hybrid bearings through
  coating the raceways. While conventional spindle
  bearing systems are not able to run at speed
  characteristics above 1.75 x l06 mm min-1 (n x
  dm rotational speed times mean bearing
  diameter), the coated bearings have been
  successfully tested increasing this limit.

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Limit of coating process

• The main restriction for the coating process was
  not to exceed the critical temperature of 160C
  for the lOOCr6 steel. Otherwise the temperature
  would cause a change in microstructure and thus
  loss in hardness. Therefore a PVD process had to
  be developed first enabling bearing components to
  be coated with good adhesion below 160C.
  Afterwards parts for tribological tests were
  coated with different coatings to select those
  with the best tribological performance (good
  adhesion, low friction and low wear under
  lubricated conditions).

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Selection of coatings in grease lubricated
conditions by means of ball-on-disc machine
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Ball-on-rod testing machine
Four batches of experiments have been carried out
with stop times of 100, 270, 350 and 1000 million
cycles, respectively. In the first batch of
experiments (stop time 100 million cycles) the
lifetime of CrN and MO coatings were determined.
These coatings failed by pitting beneath
the coating, thus having much lower duration
than Ti-derivative coatings that only presented
micropitting. The harder coatings may inhibit
ploughing.
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An additional coating of the races with CrN
further improves this behaviour so that a
rotational speed of 18 000 rpm has been achieved.
The almost linear increase of the steady state
temperature with the rotational speed underlines
that the bearing would be able to run at even
higher speeds if higher temperatures were
acceptable or the system could be cooled down.
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rotary compressors
Wear 221 (1998)7785
Increase of wear and friction on those components
will induce greater power consumption and shorter
life of the compressor.
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Surface coatings on the vane surface

• the vane material, which is made of SKH51 (high
  speed tool steel)
• Titanium nitride TiN. was deposited by two
  different methods of Arc Ion Plating (TiN(I)) and
  RF Magnetron sputtering (TiN(II)) in order to
  evaluate the effects of deposition method.
• DLC coating was manufactured by Dual Ion Beam
  sputtering, a method commonly used for coating
  cutting tools.

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Wear scar width of the vane tip
Wear 221 (1998)7785
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Friction coefficient for various coatings
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WC/C and TiN showed somewhat better wear and
friction performance than the others tested.
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Notes

• The worn surface showed extensive plastic
  deformation, along with formation of the deep and
  wide grooves.
• The worn surface of the TiAlN coating was
  characterized by irregular and sharp edges of
  contact along with small grooves (Fig. 10c).
• The good tribological performance of WC/C coating
  can also be related to the generation of a
  protective film between the mating surfaces
  during sliding,