<p>I've seen many references to a FDM print being weakest in the Z axis, due to poor bonding between layers compared to the extruded walls. </p>

<p>Thinking about optimising this for a specific material (excluding temperature and geometry), is there an optimum layer height? It seems obvious that too thick a layer will give less compression and maybe less heat transfer into the layer below (so 0.3 with a 0.4mm nozzle might be expected to be a bit weak). Is there a single break point (i.e. less than half the nozzle is good), or are super fine layers either good or bad?</p>

<p>I'm specifically using PLA at the moment, in case different materials have different behaviour in this respect.</p>

<p>I am <strong>not</strong> asking how to model the strength of layer bonds or how to take that into account when designing a part.</p>

My3dmatter.com performed a
series of tests
with PLA, using "a universal testing machine". They conclude:
Layer height influences the strength of a printed part when it becomes
  thin. A printed part at 0.1mm shows a max stress of only 29MPa, as
  opposed to 35MPa for 0.2mm (21% increase).
Past 0.2mm, the max stress remains fairly constant around 36 MPa (we
  confirmed this conclusion with an extra test at 0.4mm, not shown here
  because it was not part of the same batch).
Note: It is recommended to read the full article to comprehend the complexity of the subject matter.