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Control the color

A whole industry has grown up around PVD tool coatings because of the success of physical vapour deposition (PVD) technology over the past four decades. This industry is a result of the early pioneers in this field.

A gas is delivered into the coating chamber, where it combines with the vaporized metal species to produce the required compound. This is how most PVD hard coating procedures work. Furthermore, all modern coating processes for successful hard coating layers today involves ion-supported hard coating deposition. Ion irradiation during deposition ensures a hard covering that is completely thick and well-adhered.

All substrates undergo a sputter etch prior to deposition to remove any oxide layer, and when high-speed steel equipment is employed, heat is applied to raise the substrate temperature to the region of 425 to 550° C.

Four major types of equipment are now being used to deposit PVD tool coatings, and they all fall under the umbrella term of ion plating.

  • The way the raw material is vaporized is what differentiates the four varieties

  • either by evaporation or sputtering

  • the way that the plasma is created

  • and the density and energies of ions, electrons, and gas atoms that constitute the plasma.

The four PVD hard coating methods include

  • balanced and unbalanced magnetron sputtering

  • cathodic arc deposition

  • plasma aided chemical vapor deposition, and

  • electron beam evaporation.

These four PVD methods have many things in common, but they also differ greatly, and these variances can affect the kinds of films that can be deposited in these systems.

Sputter deposition

Sputter deposition is one of the most expensive and complicated processes. Sputter deposition, on the other hand, allows for more versatility in the types of materials that may be deposited as well as more control over the composition of multielement films.

The requirements for decorative PVD coatings are getting more and more stringent:

- appealing color

- high hardness and wear resistance

- color constancy over time

- corrosion resistance, etc.

Quality standards are getting stricter while production costs are always going down.

Magnetron sputtering undoubtedly has a prominent position among all PVD deposition options, offering significant advantages over rival techniques

This technology allows for the coating of substrates that are less temperature-resistant at relatively low deposition temperatures (usually 170°C), such as plastics, elements of watches especially springs, structurally-hardened materials, porous materials, etc.)

Introduction of reactive nitrogen gas

The process parameters, including the flow of nitrogen utilized as a reactive gas, the chamber's overall pressure, the Ar/N2 ratio, the substrate temperature, and the substrate bias voltage, affect the color of the deposited films.

The nitrogen flow is unquestionably the most crucial of the listed variables since it causes the coatings' color to change from grey (from titanium metal deposition) to yellow when the stoichiometric TiN composition is reached.

Depending on the available ion bombardment energy, the deposition of TiN films by sputtering in balanced and unbalanced magnetron modes has demonstrated highly diverse results.

The most often employed nitrides in decorative applications are undoubtedly titanium and zirconium. Thanks to their great resistance to wear, they have been utilized for a long time in the functional applications as coatings used for machining tools and molds.

Due to their tint resembling gold, they are widely employed in the decorative industry. The accompanying figure given below for PVD coatings deposited by cathodic arc show the colors of TiN and ZrN. The coatings become more yellow and reddish and lose brightness over a specific nitrogen flow, whereupon the color of the coatings becomes darker, more yellow, and reddish with a rise in nitrogen.

The color coordinates ‘L’, ‘a’ and ‘b’ of TiN and ZrN as a function of nitrogen gas flow for the industrial PVD coatings deposited by cathodic arc

The color coordinates ‘L’, ‘a’ and ‘b’ of TiN and ZrN as a function of nitrogen gas flow for the industrial PVD coatings deposited by cathodic arc

Figure curtesy: Thin Solid Films, vol. 415, no. 1-2, pp. 187–194, 2002

Oxygen's effect on the layers

The amount of oxygen incorporated into the transition element nitrides' lattice can also have a significant impact on color. Because oxides form before nitrides do thermodynamically, oxygen can be found on every surface of these nitrides. A large increase in the red hue and a significant fall in brilliance are the principal effects of oxidation on TiN or ZrN coatings.

Industrial sputtering deposition, which begins with heating the chamber to release the chemical species adsorbed on the walls, is a good example of how this effect can be seen. At the commencement of heating, the pressure rises quickly to a maximum value that is equal to the maximum concentration of water in the chamber as determined by a mass spectrometer.

Since oxygen is always adsorbed on the walls and traces of residual water are always present even after extensive heating, this oxygen is likely to be integrated into the lattice during the deposition of nitrides and will probably change the color of the coatings.

We can see why a PVD facility's level of cleanliness is essential to the quality of the golden PVD coatings.

In many cases, it is even impossible to compare the characteristics of layers deposited in tiny lab facilities with those of coatings produced in large industrial facilities. Due to the relatively low surface area of the chamber walls in a small PVD plant, the preheating stage is not required. We advocate using a facility exclusively for nitride deposition when making products since contamination from other reactive gases, such as O2, CO2, and C2H2, can eventually cause the formed TiN or ZrN layers to lose their brilliance and mechanical properties.

Ion bombardment's impact on color coatings

Ion bombardment is a crucial factor in determining how to develop nitride layers with the best possible mechanical and optical qualities. In general, we may state that a strong ion bombardment encourages the creation of nitrides, especially TiN, in a significant way. To maximize ion bombardment during deposition, several approaches are available:

  • Argon pressure: Low argon pressure in the chamber increases the deposition of nitrides by reducing the energy losses of argon ions from inelastic shocks. A large mean free path encourages atoms and ions to travel farther distances without losing energy after collisions with other atoms or ions.

  • Substrate temperature: Adatom mobility on the substrate surface is favored by a high substrate temperature, facilitating the production of the reactive molecule. However, there are situations when the temperature is restricted owing to the substrate’s ability to withstand heat.

  • Substrate bias: This crucial variable has an immediate bearing on the kinetic energy of the ions' kinetic energy of the ions impacting the sample surface during deposition directly impacts the kinetic energy of the ions striking the sample surface However, this densification of the microstructure usually comes at the expense of an increase in the internal stresses in the layer for all hard PVD coatings. Compressive internal stresses that are acceptable are always advantageous since they increase the layers' toughness and hardness. The adherence of the layer to the substrate is restricted and the coating may potentially shatter if they are too high.

The bias saturation current is often a valuable indicator of the intensity of ion bombardment, and for example for TiN, a density of bias current must be high to obtain a dense coating with a relatively low bias voltage. Sputtering depositions of TiN have been carried out with different types of magnetron configurations, and another face-to-face cathodes industrial facility with two different types of magnetrons: balanced with mirror geometry and unbalanced with opposed geometry.

Beitrag: Blog2_Post
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