Understanding reliability in memristive devices requires identifying the mechanisms that drive degradation under electrical stress. We investigate failure pathways in TiN/Ti/HfO₂/W memristors using ramped voltage stress to study the influence of polarity, environment, and device scaling. These factors define distinct degradation regimes. In large-area devices (5×5 µm²), stable resistive switching is achieved, and failure is dominated by filamentary processes within the active region. Under ambient conditions, high electrical stress produces polarity‑dependent damage in the top electrode, including characteristic worm‑like patterns linked to localized hard breakdown and redox reactions involving oxygen and moisture. In vacuum, breakdown still occurs but without visible electrode damage, demonstrating that environmental species enable chemically assisted degradation. In scaled devices (2×2 µm²), the forming step directly triggers electro‑thermal failure of the top metal lines before stable switching can be established. Increased current density and current crowding lead to strong Joule heating, causing thermal runaway in the TiN/Ti electrode. This results in fissures, localized melting, and rupture of the metal lines, while the W bottom electrode remains intact due to its higher thermal stability. These electro‑thermal effects appear even in vacuum but intensify under ambient conditions, where redox activity lowers the failure threshold. Overall, the results reveal two distinct failure regimes: filament‑driven electrochemical degradation in larger devices and electrode‑limited electro‑thermal breakdown in scaled devices. The transition arises from increased current density and reduced thermal dissipation at smaller dimensions. These findings highlight the importance of electrode design, thermal management, and environmental control for ensuring reliable memristor operation at reduced scales