Si3N4 + SiO2 based wave-transparent ceramics are popular due to their excellent hardness, good flexural strength, and superior dielectric properties. A recent study explored the use of compression molding to produce green pellets, followed by sintering. However, further investigation into the friction, wear, and water erosion characteristics should be explored to provide the final shape on the green and sintered pellets. Therefore, this study investigates the friction, wear, and water erosion behaviour of Si3N4 + SiO2 ceramic green compact GP1 and GP2 (compression molded at 305 MPa and 1912 MPa, respectively) and their sintered pellets SP1 and SP2 (sintered at 1500 °C). The wear and friction behaviour of the green pellets were studied under various loads (5 N and 10 N), while the sintered samples were tested at 5, 10, and 20 N, using ball-on-disk tribology in dry sliding conditions. The results show that the wear rate rises with an increase in the applied load. GP1 showed a high wear rate (165.0 ×10−4 mm3/N m), coefficient of friction (COF) of 0.36, and lower Hertzian contact stress (2.49 ×107 N/mm2) under 10 N load. Additionally, GP1 exhibited a higher erosion rate (143.61 mm3/min) with continuous erosion traces. In contrast, the sintered ceramics SP2 had an insignificant wear rate (4.55 ×10−4 mm3/N m) and erosion rate (15.83 mm3/min) with discontinuous erosion traces due to having higher hardness. Fractographic analysis showed that the primary wear mechanisms were abrasive wear, grain pull-out, crack propagation, debris formation, and surface fatigue. Additionally, the wear debris size decreased from green to sintered ceramics. HRTEM results confirmed the non-stoichiometric composition of mixed phases (Si3N4, SiO2, SiC, and Si2N2O) in the wear debris of both samples SP1 and SP2. In water jet erosion, the primary wear mechanisms identified were crack propagation, particle pull-out, striations, delamination, brittle fracture, and crater formation.