The KAK AR-15 Carbine Buffer and Spring Combo is a Milspec replacement. The spring is made from Chrome Silicon. If you are looking for a silent spring, I would recommend the JP Enterprises Gen 2 Silent Captured Buffer Spring System, found by using the link below:
https://www.opticsplanet.com/jp-enterprises-gen-2-silent-captured-buffer-spring-system.html?_iv_gun-model=ar-15&_iv_additional-features=standard-version