Duplex Coatings - Lesson 3: Duplex coatings and their layer properties & performance
In this lesson, we will look at the result of plasma nitriding. Two layers form, each with their own characteristics – some of which are very much desired and others are not.
The effect of plasma nitriding in detail
During plasma nitriding, the nitrided surface layer grows into the base material. The result consists of two layers:
Compound layer
“white layer”
Diffusion layer
You can see them in Figure 1. The compound layer has a typical thickness between 5 µm and 25 µm. It can be divided into two more layers, by chemical composition: the γ’ compound layer (Fe4N nitrides, cubic) and ε compound layer (Fe2-3N, hexagonal). The ε compound layer contains 8 to 11 mass.-% nitrogen and the γ’ compound layer contains 5.7 to 6.1 mass.-% nitrogen.
The underlying diffusion layer can stretch from 10 µm to a depth of 800 µm. In the diffusion layer, you can find nitrogen atoms that are interstitially embedded in the iron solid solution (which cause solid solution hardening), as well as fine nitrides (which cause precipitation hardening). The mechanical properties of the diffusion layer are controlled by the these two hardening mechanisms. The type, shape and distribution of the precipitates (special nitrides) of the alloying elements (e.g., Cr, Mo and V in D2 cold work steel), have a significant impact here.
Typical Layer Structure After Plasma Nitriding
Two layers with very different properties
The compound and diffusion layer both have a broad variety of properties, which is why plasma nitriding covers a wider range of applications. See Figure 2 for a list of layer properties and their influence on the tool performance. The most important property of the compound layer is its high hardness, which directly results in high abrasive wear resistance. Common hardness values that can be reached with plasma nitriding are 700 HV0.01 to 900 HV0.01 for unalloyed steels and 1,000 HV0.01 to 1,300 HV0.01 for alloyed steels.
Layer Properties & Performance
[source: Lietke, D.; Huchel, U.: Wärmebehandlung von Eisenwerkstoffen, Nitrieren und Nitrocarburieren, 2005, Expert, ISBN 8169-2416-6]
For duplex coating, Ionbond reduces the compound layer
Not all layers that result from a plasma nitriding treatment are equally desirable. For duplex processes, in which a PVD coating is deposited on the functional surface of the tool after plasma nitriding, formation of a compound layer has several strong downsides. Although it has high hardness, the ceramic character of the layer means it is brittle and susceptible to crack formation. In addition, the electrically insulating effect of the ceramic compound layer makes it more difficult to create good adhesion of the PVD coating to the plasma nitrided tool.
Therefore, Ionbond eliminates or minimizes compound layer formation during plasma nitriding for duplex coatings. With the latest technology and optimized plasma nitriding process parameters, we are able to offer plasma nitriding on all common tool steels while creating compound layers far below 2 µm. In this way, we can economically produce duplex coatings with excellent adhesion on our customers’ tools.
In our next lesson, we will dive into the first of two ways to apply a duplex coating: the in-situ duplex process. See you there!
Ionbond Summer School
Discover all our lessons on Duplex Coatings
Lesson 1: The correct heat treatment of tool steels
Available on 31. July 2024
Duplex coatings are especially suited for forming and molding tools. Before we can get started with that, though, we need to take one step back and look at how your tool has been treated before it arrives at Ionbond – whether it has had the correct heat treatment.
Lesson 2: Plasma nitriding of tool steels
Available on 7. August 2024
We are diving into the plasma nitriding process, a nitriding method with high reproducibility and close tolerances in the treatment result. Discover what it is exactly!
Lesson 3: Duplex coatings and their layer properties & performance
Available on 14. August 2024
We will look at the result of plasma nitriding. Two layers form, each with their own characteristics – some of which are very much desired and others are not.
Lesson 4: Shallow plasma nitriding plus Ionbond™ 35 & Ionbond™ 90
Available on 21. August 2024
We are going into one of the two types of duplex coating we will discuss. These two ways are differentiated by the depth of the plasma nitriding layer. In this lesson: shallow plasma nitriding.
Lesson 5: Deep plasma nitriding plus Ionbond™ 90
Available on 28. August 2024
In Lesson 4 we looked at shallow plasma nitriding. Now, we are going to look into duplex coatings for the most demanding applications, which require much deeper plasma nitriding layers.
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Dr. Ing. Tobias Brögelmann
Globaler Segmentleiter Umformung und Formgebung
Dr. Ing. Tobias Brögelmann
Globaler Segmentleiter Umformung und Formgebung