18 Prakt. Met. Sonderband 52 (2018) 
3. SEM = SEM — On the Blessings of Field Emission 
There are a large number of varieties of electron optics for SEMs on the market. The most 
common device, used in almost every industrial laboratory concerned with failure analysis, 
is the tungsten hairpin cathode, with its thermal emission electron source. This is a very 
sturdy, robust and reliable design. It is the workhorse of the fractographer. Every lab 
should have one. In these robust devices that do not require a high-quality vacuum, large 
and even dirty, corroded and greasy fracture surfaces can be studied as a first step in the 
failure investigation. Here, regions of interest (ROI) are identified that are later studied in 
more detail in more advanced devices. In some failures, very tiny fracture features might 
become important for the determination of the metallurgical root case of failure. For 
instance, if the question is whether a gas turbine moving blade failed by low cycle fatigue 
(LCF) or high cycle fatigue (HCF) fracture, the striation spacing could become the deciding 
factor. Even though there is no clear differentiation, no clear border that could be drawn 
between the two, it is however possible to see some trends. As a rule of thumb, if the 
striation spacing is far in the sub-um range, measuring only between some tens to some 
hundreds of nm, it is more likely that the part failed by HCF rather than LCF. For such a 
decision to make, one has to be able to resolve all fatigue striations that may be present 
on the fracture surface. If the very small striations in between coarser ones cannot be 
resolved by a tungsten hairpin electron gun, the failure analyst may actually come to the 
wrong conclusion as to the mode of failure. If he or she, however, has a field emission gun 
at their disposal, resolving power much increases (Fig. 4). It is not difficult with such 
devices to capture high-resolution images. With today's modern electron optics, not much 
beam parameter optimisation is required, making working with field emitters generally 
faster, which can be a big advantage in industrial laboratory settings. Also, and maybe 
even more important, the signal-to-noise ratio of a field emission gun is far superior 
compared with a tungsten hairpin device (Figs. 5-7). 
It should be emphasised that the advantages of field emission guns will become all the 
more important with the advent of additive manufacturing, since some microstructural 
constituents, such as gamma prime precipitates in selectively laser melted nickel-base 
superalloy components, might be an order of magnitude smaller than those known from 
conventional manufacturing routes, e.g. casting, forging and rolling (Fig. 5). By the same 
token, it should be noted that the improvement of lateral resolution of energy dispersive X- 
ray spectrometry in a field emission scanning electron microscope comes in handy when 
additively manufactured parts have to be characterised (Fig. 6). 
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Fig. 5: Bimodal Gamma Prime Precipitates in Rene 80 Laser Micro Cladding. oo 
Aged at 1120°C 05h /1080°C 1h