Surface engineering is the process of altering the properties of the surface and the region near the surface in a desirable way. This can be achieved through overlay processes or surface modification processes.
Overlay processes entail adding a material to the surface, which then covers the substrate material, rendering it undetectable on the surface. Surface modification processes, on the other hand, change the surface properties while maintaining the presence of the substrate material on the surface. For instance, in the case of aluminium anodisation, the anodic aluminium electrode of an electrolysis cell reacts with oxygen to produce a thick oxide layer on the surface of the aluminium.
Different surface modification processes have their pros, cons, and practical applications.
Surface modification processes can be utilised to alter the substrate surface before depositing a coating or film. For instance, a surface modification process can be used to alter the characteristics of an overlay coating. An example is utilising shot peening to densify and put an aircraft turbine blade's sputter-deposited coating into compressive stress. Conversely, plasma nitriding (ion-nitriding) can harden a steel surface before applying a hard coating to a physical vapour deposition (PVD) process.
Often, the properties of thin films are affected by the properties of the underlying material (substrate) and can vary through the thickness of the film. The initial step is to select the appropriate application, consider the reasons behind requiring PVD coatings on those substrates and determine the applications and tasks where those substrates will be applied, such as cutting, moulding or precision components.
Coating materials are durable and lubricative but are relatively thin, usually ranging from a few nanometers to tens of microns.
This means the influence of coatings on material performance is limited to surface-level or near-surface-level occurrences on the coated substrate. Therefore, the substrate material must be compatible with the kind of stress or environment it will experience when it comes to functional or performance-based applications.
Few more examples:
The presence of "macro-columnar morphology" on substrate surfaces can lead to less dense PVD coatings on rough surfaces. In contrast, smooth surfaces tend to produce denser coatings due to the absence of such morphology. Mechanical polishing is a commonly employed technique to achieve surface smoothing.
Certain edge-forming processes, like shearing and grinding, produce a burr or thin metal protrusion on the edge. Deburring, which involves removing the burr, can be accomplished through abrasion, laser vaporisation, or "flash deburring." The latter technique employs a thermal pulse generated by an exploding gas-oxygen mixture to heat and vaporise the thin metal protrusions.
To effectively protect against corrosion, coatings must exhibit inertness to the surrounding environment and be compatible with the substrate material. Failure to meet these requirements may result in galvanic reactions and subsequent substrate degradation.
In high-temperature applications, it's essential for the substrate material to withstand the heat generated during part and tool operation. For instance, if we apply a coating that can withstand 550°C to a heat-treated and tempered steel tool that operates at 375°C, the PVD process won't raise the operating temperature of the part or device to 550°C. In this case, the substrate material is the weak link. It's important to note that the maximum operating temperature of a coating should not be confused with the PVD processing temperature used to create that coating.
Some glasses contain nucleating agents that allow the material to be formed as a glass, then heat treatment allows crystallisation so the mirror becomes a crystalline ceramic (ceramic glasses).
Heat treatment is a standard procedure in the industry for components and tools involving heating, quenching, and tempering operations. These processes can generate residues that may necessitate removal through subsequent treatments like grinding and polishing. It's critical to note that the surface byproducts must be eliminated before performing PVD processing. Moreover, it's imperative to conduct the PVD process at temperatures lower than the tempering temperature of the substrate material. Avoid thermochemical processes such as black oxide and nitriding. Plasma nitriding followed by PVD coating within the same chamber and without breaking vacuum is an exception.
Hardening by thermal diffusion: Carbonitriding can enhance the surface of ferrous materials by diffusing carbon and nitrogen. Nitrogen has a faster diffusion rate than carbon, forming a nitrogen-rich layer beneath the carbonitrided layer. Quenching this layer can increase the fatigue strength of the carbonitrided layer. For hardening, any material containing a constituent that forms a stable boride, such as Fe2B, CrB2, MoB, or NiB2, can undergo boronizing.
The diverse range of mechanical applications demands different coatings to combat various forms of wear, including abrasive, adhesive, erosion, and tribo-oxidation. In addition to these coatings, substrates with suitable properties, such as adequate hardness and elastic modulus, are crucial to provide proper support during operation.
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