Vapor Deposition

Chemical Vapor Deposition (CVD) is a coating process involving gaseous chemical compounds transported to a reaction chamber, activated thermally close to the prepared substrate, and made to react to form a solid deposit on the surface.

Carbides, nitrides, borides, and oxide coatings are just some of the desired products, formed by a metal halide vapor accompanied by a reactive gas species.

In comparison with other vapor deposition processes, like ion plating (where both substrate and deposited film are bombarded with high energy particles to affect the interfacial region and deposited film) CVD coatings can be comparatively more dense, higher purity, higher-strength materials, that penetrate and uniformly cover intricate, complex shapes. ‘Throwing power’ is exceptional.

The sequence of events typically begins with reactants being diffused to a surface where adsorption will occur, followed by chemical reaction, surface motion, and, ultimately, lattice creation. The most important rate-limiting steps are mass transport control and kinetic energy.

Thickness typically ranges from 0.0002 to 0.050 inch. Compatible substrate can include high-speed steels, stainless steels, and cemented carbides, based on reaction temperatures normally ranging from 1500 F to 2200 F.Plasma Enhanced (PECVD) or Plasma Assisted (PACVD) thin film coatings are defined as ways to react constituents of the vapor phase to form a solid film when assisted by electric discharge. Here, the gas molecules are mainly disassociated by the impact of electrons to form highly active neutral, radical, and ionic species. Together, the increased number of ions, electrons, fragmented molecules, and excited sub-atomic particles form a film through increased bombardment. Consequently, temperature requirements can easily be reduced below 550 F, yet with excellent adherence, uniformity, and properties.

Physical Vapor Deposition (PVD) comprises an array of vacuum processes where materials are essentially physically removed from a source through sputtering or evaporation, moved through a vacuum by the energy of the vaporized particles, and condensed as a film onto the working surface.

Over the years, there have been a number of descriptive terms for PVD coating processes including sputtering by magnetron, planar diode or triode, DC or radio frequency, electron beam or arc evaporation, and others.

Typical thickness ranges from 1 to 200 micro inch (0.03 to 5 micrometer). Titanium nitride (TiN) and other transition-metal carbides and nitrides are the most recognized.

These related processes are "line of sight". Advantages include lower reaction temperatures (350 F to 900 F) and, practically speaking, a multitude of comparable surface characteristics. This includes extreme hardness, wear resistance, low friction, corrosion resistance, and non wetting or non stick properties (using diamond-like coatings, also known as "DLC").

Remember that, for any of these materials, your base metal must resist plastic deformation more than the coating. Otherwise, stress fracture of the film can result.

Greater demands for engineered products, higher standards called for performance, are all driving the need for new, emerging combination materials. Vapor deposition products now include ‘polymeric’ qualities while others offer lamellar structure performance. Both have truly crossed the boundary of traditional market segments for general wear relating to punches,dies, and metal forming tools.

In fact, most of these products are non-objectionable with FDA, based on their low likelihood of abrasion and migration. And many are showing acceptance in the medical industry (biocompatibility status) for both short-term and long-term implant device.

Once the surface wear requirements are clearly understood (adhesive, fretting, abrasive, impact, chemical, or curious combination thereof) the proper design criteria and material selection can be met.

Today, the most promising breakthrough forms of vapor deposition products, and their evolutionary outlook, are the multilayer coating technology.

These will ultimately prove to advance reliable choices for friction, lubrication, and wear technology.

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