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Coatings Tribology


									    Coatings Tribology

           Wei-Yu Ho
Dept. Materials Science & Engineering
         MingDao University
  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
                  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.
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
Schematic sketch of plastic deformation of the surface material in a
railway wheel. The dashed lines indicate material planes before and
after deformation.
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.
(a) Roughness of a material surface on different scales. (b) Loading of an
asperity causing a cone crack.
 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
• 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.
       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
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.
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.
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 >50
  GPa and is resistant against oxidation in
  air up to 800 0C.
Surface and Coatings Technology 154 (2002) 152–161
Nanoscratch resistance of
TiNy/SiNx coatings as a
function of SiNx layer
        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.
              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
plasma immersion ion implantation
and deposition (PIII&D) technique
• The mechanical and fatigue properties of the
  AISI52100 bearing steel, can be greatly
  improved by plasma immersion ion implantation
  and deposition (PIII&D) 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.
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 operation—in 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.
• 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.
         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.
              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 4–10 m/s but micropitting may occur even at low contact
stress because of the effect of sliding.
          phase transformation
Little or no attention has been paid to possible effects of
phase transformation occurring in gears undergoing
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.
micropitting measurement

               Wear 258 (2005) 1510–1524
micropitting measurement
       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.
        Limit of coating process
• The main restriction for the coating process was not to
  exceed the critical temperature of 160°C 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
  160°C. 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).
Selection of coatings in grease lubricated
conditions by means of ball-on-disc machine
  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
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
          rotary compressors

                                               Wear 221 (1998)77–85

Increase of wear and friction on those components will induce
greater power consumption and shorter life of the compressor.
  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.
Wear scar width of the vane tip

                      Wear 221 (1998)77–85
Friction coefficient for various
WC/C and TiN showed
somewhat better wear
and friction
performance than the
others tested.
• 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,

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