Steel in all its alloys is the most common material for which customers ask us to perform a root cause analysis in case of failure. However, from time to time, there are more exotic materials among them. One such case will be discussed here, more specifically a failure analysis on tungsten carbide. This is one of the hardest materials known, except for diamond.
In this case, the part to investigate was a component of a paint crusher drum. This part must be able to absorb a high vibration and shock load during operation, keep its shape, and not chip off during the process. Hence the selection of tungsten carbide. In order to achieve high quality products, contact with the environment must be avoided at all times. So the part must also seal well against its counterpart during the entire production process.
At a certain moment long before the expected part replacement time, the customer detected one such part did not perform well due to the loss of its sealing function. To understand why the seal was lost, the piece was offered to TÜV AUSTRIA Belgium for root cause analysis.
The first visual observation shows that the part has lost its normal luster and has turned dull. The dull layer itself was well adhering to the surface. Using the SEM-EDX scanning technique on this surface, it was proven the dull layer is constructed mainly out of tungsten, with minor elements such as carbon, oxygen and sodium.
Figure 3: SEM-EDX spectrum of dull surface layer
Further analysis involved a destructive cross section sampling of the ring to examine the structure of the layer and the bulk. Tungsten carbide being a hard and strong material, the classical sawing, grinding and polishing techniques could not be used. Using Wire Electrical Discharge Machining (EDM) and a dedicated polishing and etching technique, the structure of the layer could be revealed.
The image clearly shows the bulk material (bright gray area), and two distinct layers on top. The bulk material is dense, as expected in order to guarantee a good seal during operations. However, the two top layers are clearly porous, creating gaps for air to infiltrate, causing the loss of sealing.
The customer confirmed the surface was shiny upon installation of the ring, so the porous structure must have been created during operations, cleaning, … . Further SEM-EDX analysis reveals the base material consists out of tungsten, carbon and cobalt. The latter is a typical binder element in components made of tungsten carbide. Compared to the surface scan, it is clear the cobalt has been removed from the surface layer.
Figure 5: SEM-EDX spectrum of the bulk material
Based on these results product components, production conditions, cleaning chemicals or other process aids will be screened on their potential to leach out cobalt from this piece, creating the sponge structure and, hence, loose the capacity to seal off. Performing failure analyses like this one provides customers valuable information towards identifying root causes for failed components. At TÜV AUSTRIA Belgium, we can help you out, even for the “hardest” cases.
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