ICMCTF 2012! - CD-Lab Application Oriented Coating Development

Environment
Tribofilm
Deposition
temperature
Counter material
Coating
Composition
Thickness
Hardness
Stress
Substrate
Topography
Wear resistance
Low friction Desired properties
Thermal and
chemical resistance
Contact pressure
Loading cycle
Sliding speed Contact conditions
Temperature
To avoid premature coating failure due to cracking and fragmentation, the hardness of the substrate has to be at least as high
as that of the counter surface. If a coating is applied to a substrate material that is softer than the counter surface, it either must
be very thick or very ductile.
One has also to ensure that the substrate will withstand the deposition temperature without softening. Unfortunately, this limits
the number of candidate substrate materials for many promising coatings. For example, the common ball-bearing steel AISI
52100 softens at temperatures below 200°C. For Al and Mg alloys, the number of potential coatings is even more limited.
Another important issue is adhesion of the coating to the substrate. One of the most important factors to consider for hard and
brittle coatings is the presence of internal stresses. Tensile stresses are usually detrimental, but reasonably high compressive
stresses are beneficial since they improve coating cohesion and increase its hardness and wear resistance. However, they may
deteriorate the adhesion. The combined influence of compressive stresses in the coating and substrate geometry on practical
adhesion has been estimated using simple geometrical models. Excessively high compressive stresses in combination with
unfavourable geometry may cause spontaneous flaking.
In sliding applications for tools or mechanical components, not only are friction and wear resistance important; it is also necessary
to avoid adhesion of the counter material to the coating surface. This phenomenon is often termed galling. Modern
development of forming and cutting tools aims at the highest possible galling resistance. An important question arises; is
galling related to chemical or topographical properties or both? It is well known that certain materials like austenitic stainless
steels or Ti-, Al-, or Ni-alloys are difficult to machine and deform plastically because of their strong tendency to adhere to the
tool surface. This also makes them poor materials in sliding components. Thus, special effort is required for the development
of surfaces to use against these “difficult” counter materials. Generally, the smoother the surface of a tool or component, the
better is its galling resistance.
In on-going in situ experiments in the SEM, the galling resistance of finely polished tool steel surfaces are compared with
equally finely polished surfaces of DLC coatings by repeatedly sliding against a tip of austenitic stainless steel. For the DLC
coatings, the friction coefficient remains ~0.15 for the entire series of 10 consecutive sliding passes, whereas for the tool steels,
the coefficient increases from 0.15 during the first pass to ~0.8 after only 4 passes. Intentional scratches in a DLC coating affect
friction only during two passes; the coefficient returns to 0.15 at the third pass as the scratches become partially filled and their
ridges worn off. In this example, the DLC coatings act to preserve the low initial friction level by avoiding galling while, in
addition, inhibiting local scratch damage.
One track in the current development of active coatings involves the design of coatings whose wear resistance and/or friction
properties improve during use. A way of achieving this is to design materials and conditions in which the coating, the surrounding
medium, and/or the counter material react chemically or combine mechanically to form tribofilms with the desired
properties. The prerequisites for low friction in dry or boundary-lubricated sliding contact combine high hardness with low
shear resistance. Well-known materials with these properties are diamond in humid atmosphere or water, MoS 2, WS 2, and ZnO
in dry sliding.
Another approach is coatings that change composition during tribological loading. For example, TiC x-based coatings alloyed
with excess amounts of Al or Ni lose graphitic carbon when loaded in tribological contact. In addition, the graphite aligns with
crystallographic easy shear planes parallel to the sliding direction.
Finally, there is a trend today to design tribological materials and coatings such that they, together with the surrounding medium
and/or the counter material, will form new materials in the form of tribofilms that can lower the friction.
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