3D Joule heating model: failed to converge

Hi,

I am trying to simulate the joule heating of Chromium, and to the heating of Silicon Nitride layer that is covering the Chromium metal.
When I compute the model, the error message is 'Returned solution failed to converge'

When I added the 'Surface to Ambient Radiation', the error message is
Failed to evaluate variable.
- Variable: material.parameter.epsilon_rad
- Geometry: 1
- Boundary: 3
Failed to evaluate variable.
- Variable: mod1.jh.epsilon_rad
- Defined as: material.parameter.epsilon_rad
Failed to evaluate expression.
- Expression: d((mod1.jh.epsilon_rad*sigma_const*(mod1.jh.Tamb^4-mod1.T^4)*test(mod1.T))*(dvol),{test@2})

Could anyone give some advice on how to get rid of the error? Thank you in advance!