Wave-transmitting Si₃N₄ ceramics is one of the main materials for making the antenna window. The tool-workpiece friction characteristics in milling are closely related to the machined surface morphology. Through the tool-workpiece contact relationship analysis, the friction contact area is mainly focused on the cutting edge flank surface and the workpiece surface. The friction mechanism of the contact area under different processing mechanisms has been analyzed. It comes to conclusions that the friction mechanism is adhesive friction, plough friction of tool surface roughness crest and rolling friction of powder for the plastic domain processing, while it also includes the plough friction of the broken ceramics particles for the brittle domain processing. The friction coefficient calculation formula has been proposed with the existing friction theory and the analysis results have been verified with the experimental data. It shows that the critical cutting depth of the transition mechanism for the wave-transmitting Si₃N₄ ceramics is in the range of 0.3 to 0.4 mm. The maximum error rate of the friction coefficient formula is 20.46%, which can reflect the evolution of the friction characteristic to some extent. In the initial machining mechanism transition stage, the friction coefficient decreases, and the influence of friction coefficient on surface roughness follows the third order polynomial distribution. This study provides the reference for improving the surface quality of wave-transmitting Si₃N₄ ceramics milling.