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 friction-wear-violence, and anti-sticking behaviour.

Limitation on thin coating application

• Costly process due to heating of the substrate
• Limited thickness of PVD and CVD coatings
• High hardness and stiffness of the substrate producing coating failures (poor adhesion cracking, spalling) such as corrosion problems when appropriate if the wear requires corrosion

Coating failure

If the coating wears down gradually at about constant rate, a significant proclamation in life can be expected as compared to that of the uncoated material machining concepts. 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 excessive mechanics in adhesive loading. But a cracking on a soft substrate, a fracture can spread. Once the coating has fractured, fragments are peeled off. Then is produced a powder. The rate of detachment increases due to extreme contact pressures at the edge of the sand.

Composition includes carbides, oxides or friction valley of coefficients: 0.4-0.9 dry (static). WC/, MoS₂, DLC can be classified as low-friction coatings.

A tribological coating has to provide a perfect friction behaviour (low, even, on stable friction levels) and a high wear resistance. A sufficient adhesion between coating and substrate and a sufficient load-carrying capacity (or ability to resist tribological loads without premature failures) are prerequisites.

Repeated livings, exposition, microstructure, hardness 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 to microscopic asperity contacts.

In steady, increasing or strong contact with low friction. The lower shear strong contact at low Hertzian contact radius.

The group of requirements that usually relate to surface fatigue include nonconforming contact surfaces (gears). Here, the low shear stress is below the thickness of DLC coatings. A thin coating is effective in reducing the wear process. In sliding contact between conformal surfaces (piston/cylinder, gears), the load is distributed over a large nominal area instead zone.

The contact stresses are localized to the areas of asperity contact. The probability of improving friction and wear prepared by applying the thin coating is significant for higher. An outstanding advantage, the coefficient of friction approaches the surface when friction increases.

On a friction coefficient >0.3, it is confined to the contact surface so a thin coating is subject to wear.

Topography

To minimize the max contact strength, the roughness grade should be smooth. The inhomogeneous substrate preparation might be responsible for increasing the adhesion and increasing the production adhesion of coatings, which in turn: on the other hand, coatings roughness can be beneficial for oil retention. Thin coatings are preferred to reduce the risk of delamination when thick are high loaded structures.

Load-carrying capacity

1. A thick hard coating relieves the substrate by "hosting" over
2. A thin, high-yield coatings can spread the load over a larger area on the substrate.
3. A thin soft coating yields giving a flexer, less concentrated load distribution.

Coating structures

Graded coatings

They improve the load-carrying capacity by offering smoothed transitions in mechanical properties from those of the hard and stiff coating 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 ones at the coating-substrate interface.

Examples: enriched and columnized layers.

Duplex coatings

Wear-resistant coatings are relatively brittle and can be successfully applied only to hard and stiff 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.

Surface technology coatings for tribological application

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

Limitation in thin coating application

• Cracks open → delving to the substrate
• Quite thickness of PVD and CVD coatings
• High hardness and stiffness of the substrate
• Premature coating failures (poor adhesion, cracking, spalling) can be catastrophic
• Hard coatings can aggravate the wear
• Galvanic corrosion

Coating failure

If the coating wears down gradually at low rate, a significant prolongation in life can be achieved as compared to that of the uncoated material. Increasing coating thickness - protective effect due to damping the wave in (non-drawing shocks). 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 excessive mechanical or thermal loading by initiation of cracks in an exact substrate or fracture and spall. Once the coating has fractured, fragments are peeled off. This is progressive, and the rate of detachment increases due to the extreme contact pressure at the edge of the scratch.

Deposition... includes carbides, oxides → friction value of coefficients: 0.4-0.9 (dry & damp). TiC, N/C, DLC can be classified as low-friction coatings → dry sliding friction coefficients: 0.05-0.25.

Properties

A tribological coating has to provide a preferred friction behaviour (low signal average friction value) and a high wear resistance. A sufficient adhesion between coating and substrate and a sufficient loading-carrying capacity (→ ability to transfer tribological loads without premature failure) are prerequisites. Repeatedly (shocks, exposition, microstructure, residual stress, toughness) are determined by the process, the material, and the topography of the substrate.

Contact stresses

Hertzian contact stresses are related to the macroscopic contact geometry, other kinds of stresses are associated with microscopic asperity contacts.

In steady sliding contact, or strong contact with low friction, the wax shear stress occurs at a bulk Hertzian contact radius.

The group of requirements that usually relate to surface fatigue includes non-conforming contact surfaces (gears, etc.). Here, the wax shear stress is below the thickness of SVd, PVd coatings → a thin coating is effective in reducing the wax shear...

In sliding contact between conformal separate (piston/cylinder, gears, ...), the load is distributed over a large nominal contact area. The contact stresses are localized in the areas of asperity contact. The probability of improving friction and wear properties by applying a thin coating is significant. Higher max on sliding attitude, the coefficient of friction approaches the surface when friction increases. For a friction coefficient greater 0.3, it is confined to the contact surface, so a thin coating is sufficient.

Topography

To minimize the wax contact stress under the asperity, the coating surface should be smoother. The fine rough substrate preparation may be possible with grinding or honing. The adhesion increases if reproduction of contacts with irregular wax contact stresses. On the other hand, crater roughening can be beneficial for oil retention. Thin coatings are preferred to reduce the risk of delamination when there are high internal field structures.

Load-carrying capacity

1. A thick hard coating relieves the substrate by "absorbing" stress.
2. A thin high-yielding coating can spread the load over a larger area on the substrate.
3. A thin soft coating yields giving a flexible, 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 coating to those of the softer and ductile substrate. The contact load can be distributed over a larger area, reducing the wax contact stresses and the one at the coating-substrate interface.

Examples: nitrided and carburized layers.

Duplex coatings

Wear-resistant coatings are relatively brittle and can be successfully applied only to hard and stiff substrate materials. On softer ones, an intermediate layer acting as a mechanical support for the coating is required - for steel and Ti alloys nitride is used.

Multilayer coatings

Consist of periodically repeated sequences of layers of two or more materials. The layer thickness can vary from a few nm to a few tens of nm. E.g. TiAlN films exhibit enhanced hardness when annealed due to the formation of coherent precipitates (out of mixed dislocation).

Superlattice coatings

They have coatings with a bilayer period in the range of 2-10 nm. Resistance against wear, corrosion, oxidation, high friction, etc. E.g. TiN/VN exhibit high hardness (40 GPa), low coefficient of friction (0.4), and high wear resistance. Wear occurs by coarsening of cracks parallel to the worn surface, and extending on a few layers in depth. Other examples include TiN/TiC and TiN/W₂N, etc. Generation of the high-point hardness increases superlattice effect → locking of propagating dislocations at the layer interface by the difference in the shear modulus and critical shear stress to generate the weaker dislocation loops in shear stress for ideal cathode (coercion) generation within the layers increases as the superlattice period decreases across the layer. Interface networks decrease when the interface width becomes comparable to the layer thickness.

Nanocomposite coatings

They are composed of mono-sized crystals dispersed in an amorphous phase. Hardness values of 90 GPa can be attained. The structure derived from spontaneous reaction between vapor phase and substrate, possible only in transition metals, can superimpose over each other. The coating is stable up to high T avoiding T-cohesion. Dissociation occurs in sudden pressure disturbance of the shell around mesoscale size grain (3-5 nm).

Residual stresses and fatigue

During deposition, structure results in coating nucleation growth and simultaneous formation of compressed zone. Stress due to mismatch in thermal contraction between coating and substrate, possible due to mismatching of thermal coefficients. Because measurement of adhesion properties of film is quite tricky, now compressive stress measured: X-ray diffraction can provide information on adherent small components in the coating but not residual stress can be obtained only if elastic constants are known.

Coating thickness/hardness ratio: 1:1000 or 1:10000. Film containing tensile stresses bends the substrate. Bending in the opposite direction: compressive, convexly upwards. Forces developed in the film are balanced by opposite forces in the substrate. If the film/substrate pair is more translational from moving it will elastically tend to counteract unbalanced movements.

Residual Stresses (df = (ds - αf)ΔT. αs, αf = coefficients of thermal expansion)

Shot Peening...

Electroplating... Cr... Zn...

Diffusion in solids

D = Doexp(–Q/RT) or Thermally activated process....

1. Multiple-boundary diffusion regime
2. Grain-boundary diffusion regime

D_eff = D_L + (πρ²δ / 2L)D_dislocations Dislocations are usually less supersaturated than interfaces as short-circuit diffusion paths become of relatively less significance. Exceptions are materials with large grains and very high dislocation density.

Grain growth

Assuming no impediment of boundary orientation: It is assumed that across a curved boundary between two fields, a pressure difference exists. For a two-phase interface: For a principal interface: ΔP = 2Γ/r = the pressure is higher on the side that contains the center of curvature. A random grain structure when inherently unstable and, on annealing, the unbalanced force will cause the boundaries to migrate towards their center of curvature. This eventually results in a reduction of the number of grains, increasing the mean grain size and reducing the total GB energy.

Solute-drag: Solute-drag/coarsening may impede the motion of interface and GBs. The interface can drag the solute atmosphere along with it or pass itself through. Assumptions: A randomly moving force on the remaining force (atoms forming lumps) will become the driving force on the interface motion.

Kinetics of grain growth

In a single-phase material, the rate at which the mean grain size increases with time depends on GB mobility and the driving pressure for migration. Experimental: k ~ K1^e; k=k1K

Sintering

Sintering is a kinetic process that deals with the reduction of porosity. It is expected to produce near-net shape components starting from powders.

Features: net shaping (minimum finishing operations), maintenance of high interparticular contacts, introduction of porosity during consolidation, and achievement of fine microstructure. Powders are fabricated into strong parts by different techniques and then heat treated in order to give them the required mechanical and physical properties.

Three

Solid-state sintering: simultaneous application of heat treated and transport by capillary forces. Hot-pressing: simultaneous application of panel heat. Method: By pressing a liquid and capillary contact (sweep). The mechanical transport involves several mechanisms of diffusion type. By feeling the surface energy of contact between the powder particles and fine porosities, sintering increases the entity and strength of the compounded powders.

Driving energy for sintering

This stress is tensile under a concave surface and compressive under a convex surface.

Transport mechanisms

Diffusion along surface and gas inside the neck.

Material sinks and sources

Surface energy is reduced when concave regions are filled in (neck surface pores, solid interfaces). Convex parts are sources.

Neck growth and densification

Bulk transport: Add bits together, fixing the neck across from the grain boundary; in contact regions to push them apart and create pores.

Shrinkage

During the initial stage, GBS develops at the contact regions, and the grain structure in the interface changes by grain growth or recrystallization. The microstructure is characterized by a continuous pore space with GBS.