Steel and Nitriding: XPS vs SEM – Which is the Correct Tool to Use?


This is the second in a series of blog posts on X-ray photoelectron spectroscopy (XPS) by Dr. Jerry LaSalle, an expert in XPS surface analysis and physical and powder metallurgy. Dr. LaSalle’s areas of concentration include stainless steel passivation qualification, metallurgical failure analysis, additive manufacturing, and powder metallurgy consultation.

XPS determines nitrogen chemistry and depth of penetration in sub-micron surface layers.

SEM or XPS?

When it comes to examining the surface layer of a material, several excellent techniques are available that can deliver the required results. However, determining which of those techniques will give you the exact information you need requires not only instrumentation expertise, but a strong understanding of the material, as well. Many useful engineering properties of a material are governed by a thin surface layer which is often chemically distinct from its bulk. This layer can be extremely thin, often only a few molecular layers thick, or it may consist of elements difficult to detect by standard methods.

While scanning electron microscopy (SEM) is a widely used technique for examining micrometer-sized features in a material, it can’t chemically resolve thin sub-micron surface features. The SEM also does not have the level of sensitivity needed to identify low levels of light elements such as carbon or nitrogen—an important requirement when examining nitrided steel. In contrast, X-ray photoelectron spectroscopy (XPS) is extremely sensitive to very thin surface layers on the order of 10 to 100 atomic layers, and is sensitive to light elements. This makes it the ideal technique to determine low surface concentrations in general, and for nitrided steels in particular.

Nitriding graphic 1

Figure 1. Advantages of XPS compared to SEM in detecting thin surface zones.

Nitrogen, Steel, XPS and Sensitivity

Nitriding and carburizing are widely practiced surface treatments for steel. In both processes, the respective atomic species diffuse into the metal, creating a case-hardened surface that improves the steel’s performance. Managing the chemical composition and depth of penetration prevents the formation of undesirable surface layers that may cause the steel to become brittle and spall.

When nitriding, determining the nitrogen chemistry and the depth of penetration is difficult, if not impossible, using the SEM, because of the SEM’s beam spreading pattern and its relative insensitivity to nitrogen. XPS readily detects nitrogen as well as its chemical bonding state, and can characterize the composition as a function of depth by ion sputtering away the surface while continuing to gather a spectrum. Additionally, special sample substrates are not required. They can be metallic, polymeric, or biological, and XPS does not require conductive materials.

Figure 2 is an optical micrograph of the steel alloy 4140. The bulk of the alloy consists of a martensitic phase, which is the large grayish area on the right. There is also a very thin white layer visible to its left on the surface of the sample. When this thin layer was examined using an SEM/EDS spot, no evidence of nitrogen was observed. However, when that same sample’s surface was analyzed by XPS, Figure 3, with ion sputtering and a depth profile analysis, the results clearly showed a high concentration of nitrogen at the surface that drops dramatically at a depth of 4000 nm (4 micrometers). This example pointedly displays the advantage of determining surface chemistry using XPS as compared to SEM.

nitriding graphic 2

Figure 2. Optical micrograph of 4140 steel. A thin white layer is visible on the surface of the cross section. The SEM/EDS results showed only an iron signal for that layer, but XPS revealed a high concentration of nitrogen,  as shown in Figure 3.

 

nitriding graphic 3

Figure 3. The XPS depth profile of nitride powder metallurgy, 4140 steel with nitrogen peaking at approximately 1500 nm and ending at 4000 nm. This level of nitrogen is not normally detectable using the SEM.

XPS: Not Just for Nitriding!

XPS analysis of nitriding is just one example illustrating this technique’s ability to detect surface chemistry. There are innumerable situations in which it is the technique of choice for determining the diffusion, either intentional or accidental, of chemical species at and just below the surface of a material. Some typical examples of when XPS can examine surface composition and sensitivity include identifying the composition or contaminants in:

  1. Corrosion products
  2. Stains
  3. Deposited layers
  4. Diffused layers
  5. Coatings

If you believe XPS analysis would benefit your project, please contact the experts at RJ Lee Group.