Surface Technology - riassunti parte 2

Surface Technology

Coatings for Tribological Application

The bulk material is selected to meet the demands for stiffness, strength, formability, cost etc., and coating is added to improve surface properties such as fracture wear resistance and anti-sticking behaviour.

Limitation in thin coating application:

• Deposition process -> heating of the substrate
• Certain thickness of PVD and CVD coatings
• High hardness of PVD coatings
• Premature coating failure (poor adhesion, cracking, spalling) could be dangerous
• Fragments can aggravate the wear -> dynamic consequences

Coating failure: if the coating wears down quickly and reaches a significant proclamation in the application -> experienced on comparing to that of the uncoated material.

Thicker coating: more thickness -> stronger & effective (to damp the wear in gradual and steady). However, thickness is restricted by internal stresses which rise during deposition.

Cracking and spalling of the coating may be the result of occasional or repeated extensive deformation of either the coating or a thickening layer on a soft substrate or a fracture of hard small-sized wear particles -> fractured fragments are peeled off. When it spreads and spalls, the risk of delamination increases due to extensive contact pressure at the edges.

Coatings on nitride carbides, oxides -> friction value of coefficient: 0.4-0.9

• Tough coatings -> TiN, TiC, MoS₂, DLC can be classified as low-friction coatings with friction coefficients: 0.05-0.25
• Prop. ens. in tribological applications provide a preferred friction behaviour (e.g. low on stable friction and wear) and high wear resistance. A sufficient adhesion between coating and substrate and a sufficient load-carrying capacity (-> ability to resist high-loads) coexists without premature failure in application.

Properties of thickness, composition, microstructure: surface stress, roughness, are determined by the process, the material, and the topography of the substrate.

Hertzian contact stresses are related to the macroscopic contact geometry. Other kinds of stresses are associated with microscopic surface contacts.

• In steady, high s, or sliding contacts with low friction: the max shear stress occurs at a depth of Hertzian content radius.
• The group of requirements that might fail due to surface fatigue includes: non-conforming contact surfaces (gears, ...). Here, the max shear stress is located: the thickness of the solid PVD coatings. -> a thin coating is effective in reducing the peak contact stress in sliding contact between conformal surfaces (piston-cylinder), Gears, ..., the load is distributed over a large volume as a textured area. The contact diameter only occupied to the area of apparent contact. The probability of improving fracture and wear properties by applying a thin coating is significantly higher. In all sliding contacts, the coefficient of friction approaches the surface when friction increases. For a friction coefficient ~0.3, it's confined to the contact surface -> thin coating is sufficient. Topography: to minimize the max contact stress in the operation, the coating surface should be smooth. The thin-film substrate preparation must be precision engraved -> loosening improves how the stress at the sides and corners with high residual stresses.
• On the other hand, a certain roughness can be beneficial for oil retention. Thin coatings are preferred to reduce the risk of delamination when there are high mechanical stresses.

Load-carrying capacity:

• A thick hard coating relieves the substrate by "flushing" stress.
• A thin, high-yielding coating can spread the load over a large area on the substrate, e.g. 3) a thin soft coating yields giving a flexible and less concentrated load distribution.

Coating Structures

• Graded Coatings They improve the load-carrying capacity by offering smoother transitions in mechanical properties from those of the hard and stiff coatings to those of the softer and ductile substrate. The contact load can be distributed over a larger area, reducing the max contact stresses: and the one at the coating-substrate interface. Examples: nitride and carbonized layers.
• Duplex Coatings Wear-resistant coatings are relatively brittle and can be successfully applied only to stiff and hard substrate materials. On softer ones, an intermediate layer acting as a mechanical support for the coating is required: for steel and Ti alloys nitriding is used.
• Multilayer Coatings Consist of periodically repeated sequences of layers of two or more materials.