Spring-seals (C-seals, S-seals, U-seals) use metal spring energizers (stainless or Inconel) with PTFE, PEEK, or modified TFM jackets for metal-to-metal sealing in liner hangers and subsea wellheads at 350°C. https://globalsealstech.com/product-category/oil-gas-seals/downhole-completion-tools-seals/