model works with identity pairs but not with contact pairs

Any tips on what I am doing wrong in the attached axisymmetric model involving a titanium part with a rounded top that supports a nylon liner that fits inside a titanium "cap"? An approximate image of the structure being modeled is attached as a .jpg. I'm idealizing the top of the rounded center portion as a semi-circle that's swept through the 360 degrees of the axisymmetric model to simulate the top of this part. I'm calling it a "ring" here. The boundary of the cross section of the ring that's not in contact with the nylon is held fixed in my model. Meanwhile, a downward force is applied to the top of the titanium cap. The sketch is not quite representing the exact geometry, but the Comsol geometry captures what I want to explore.

The model with identity pairs works OK, see first attached .mph file. However, when I put in contact pairs in the boundaries shown in these models (i.e., at the interface of the nylon and the Ti cap, and at the interface between the metal "ring" and the nylon), the model does not work; see last attached .mph file. Just FYI, this model works fine when I simply use a "union" command to construct the entire model, e.g., with all interfaces connected. But I would like to test the effects of friction at the interfaces noted, hence the attempts with assembly and contact pairs, etc.

I get various error messages depending on how I tweak the model, including errors such as "segmented solver steps do not include all components", etc. Sometimes there is a convergence error.

Any suggestions?

Thanks in advance.