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A Review of Surface Treatments

This column will discuss a brief comparison between low- to high-temperature-deposition methods of surface-modification treatments in relation to thermal-diffusion treatments (Fig. 1). The low- to high-temperature-deposition methods of surface treatments include:

  • Electrodeposition
  • Thermal spraying (melted material sprayed onto a substrate steel)
  • Painting (powder coating)
  • Chemical treatments
  • Electroplating

Electrodeposition can be defined as an electro-metallic e-coating such as electro-painting. This method utilizes a paint powder and a depositing powder (at low temperature) by making use of an electric-charge system with the workpiece as the cathode and the paint powder the anode. Electrical current is used to deposit paint on the surface. It is similar (in principle) to electroplating.

Electroplating is a procedure that deposits a protective coating using a warm to hot liquid solution of the solute anode material onto a cathodic component that is immersed in the solute liquid for deposition. Electroplating is usually used to cover a less-expensive metal with a more-expensive metal or to cover a corrosive metal with a less-corrosive metal.

The major problem with electroplating is that parts are generally in a toxic liquid solution that requires good pre-cleaning of the component (cathode). The liquid-metal-carrying agent and the effluent necessitate extreme care in terms of personal operator safety.

A metallic anode is generally manufactured of the coating metal – a single metallic element (e.g., chromium, nickel, cadmium, copper) – to be deposited onto the cathodic components. Plating is performed to:

  • Improve wear resistance
  • Resist corrosion
  • Improve physical properties such as torsional strength, tensile strength and impact strength
  • Protect against indentation

Fig. 1. Simple categorization of surface treatments (courtesy of PMIC)

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Some alloys can be electro deposited, however, notably brass and solder. Plated "alloys" are not true alloys (i.e., solid solutions) but rather discrete tiny crystals of the metals being plated. In the case of plated solder, it is sometimes deemed necessary to have a "true alloy," and the plated solder is melted to allow the tin and lead to combine to form a true alloy. The true alloy is more corrosion resistant than the “as-plated” alloy.

High-Temperature Thin-Film Hard Coatings

Deposition processes are able to improve the following:

  • Very high-temperature operating conditions
  • High wear-resistant conditions
  • Dimensional stability
  • Productivity and reduction of manufacturing costs

Types of Depositions

The two deposition methods of extreme hard coatings are chemical vapor deposition and physical vapor deposition. Be aware that these procedures require high-temperature metallurgical processing conditions.

Chemical vapor deposition (CVD) requires high temperature in the presence of a suitable gas. The metallic processing gas decomposes and releases the metallic deposition material onto the substrate metal (generally steel).

Physical vapor deposition (PVD) requires a metal vapor to be produced for the surface deposition. This will react with the component to form a gas that will form the hard surface coating.

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Fig. 3. Simple categorization of surface treatments (courtesy of PMIC)

Fig. 2. Simple categorization of surface treatments (courtesy of PMIC)

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Nitriding

Nitriding is a thermal-chemical diffusion treatment that forms stable nitrides within the surface of a component. The purpose of the procedure is to generally transform the steel by diffusing nitrogen into the surface. This is accomplished with a carburizing/ferritic nitrocarburizing process at low thermal-processing temperatures, which are below the A1 transition line of the iron-carbon equilibrium diagram (Fig. 4).

The nitriding procedure generally takes place at processing temperatures that are below the A1 line from 840-1130°F and, generally, without a quench procedure. Nitrogen is diffused into the steel surface. It is first dissolved into the iron matrix. If the nitrogen concentration exceeds the solubility limit of 2.5% by weight, a single or multiphase nitride layer is formed.

If only molecular nitrogen diffuses into the surface of the steel, the process is referred to as nitriding. If at the same time carbon diffuses into the surface of the steel as a result of the addition of a carbon source into the nitriding process gas, however, the process is now known as nitrocarburizing and generally known as ferritic nitrocarburizing.

Fig. 4. Iron-carbon equilibrium diagram (courtesy of PMIC)

Derivative Nitriding Processes

The derivative nitriding processes are generally applied to low-alloy steels – ferritic nitrocarburizing and a combination treatment of nitriding followed by controlled surface oxidation. Figure 6 is a metallurgical processing method of improving both surface hardness and corrosion resistance.

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Fig. 6. Schematic diagram of FNC plus additive mediums (courtesy of PMIC)

Fig. 5. Nitriding methods (courtesy of PMIC)

Because of the addition of carbon into the process-gas stream, the epsilon compound layer is formed quite rapidly. With the addition of an oxygen source at the end of the nitrocarburizing treatment, a homogenous layer of iron oxide will be formed that will assist in corrosion resistance.

When using the plasma-assisted process technique, the pre-cleaning (commonly known as sputter cleaning or simply explained as surface cleaning by ionic bombardment) could be likened to atomic shot blasting.

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Fig. 7. General benefits of FNC procedure (courtesy of PMIC)

Fig. 8. End-mill cutters, Russia

Fig. 10. Illustration of nitride networking, which can lead to premature failure (courtesy of PMIC – Heat Treatment Training Notes)

Thin-Film Hard Coating of Cutters and Engineering Tooling

This procedure can work extremely well combined with the nitriding procedure.

  • Pre-harden and temper of the cutter to produce a cross-sectional hardness traverse of approximately 64 HRC (800 HV) up to 66 HRC (830 HV)
  • Nitride (by diffusion) to 68 HRC (880-940 HV)
  • Thin-film hard coat (deposition treatment and not diffusion) to a surface-hardness value of approximately 1,800-2,100 HV

Fig. 9. Tree illustrating surface-modification processes (courtesy of PMIC – Heat Treatment Training Notes)

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Fig. 11. Sketch (not to scale) of the final nitride structure (courtesy of PMIC - Heat Treatment Training Notes)

David Pye
Pye Metallurgical International Consulting

David Pye, 911 Backspin Court, Newport News, Va., is a contributing writer.

He can be contacted at tel: 1-757-968-1007; e-mail: pye_d@ymail.com; web: www.heat-treatment-metallurgy.com.

All figures/graphics provided by the author.

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