Evaluation of the strength and adherence of soft cobalt-base and hard TiN coatings on 4340 low alloy steel

Abstract

Soft T-800 and SF-6 cobalt-base alloy coatings and hard TiN coatings were deposited by cathodic arc plasma PVD on 4340 and chromium-plated 4340 low alloy steel specimens. The cohesive load and adherence of the coatings to the substrate were evaluated with an automated scratch testing apparatus and their abrasive wear resistance was measured with a pin-on-disk tribometer. The soft coatings adhere strongly to 4340. During testing transverse cracks initiate within the coating at the cohesive load and propagate into the substrate without delamination. These coatings also adhere strongly to chromium-preplated 4340. During testing pre-existing cracks within the chromium layer propagate across the coating and the substrate at a lower load and spacing is smaller than in nonplated specimens. Soft coatings processed at a lower pressure are stronger. Hard TiN coatings on 4340 or chromium-preplated 4340 exhibit approximately the same cohesive and critical loads. The wear resistance of coatings on 4340 specimens is higher than on chromium-preplanted 4340 specimens. For the specimens and experimental conditions used herein the nitride coatings exhibit a higher wear resistance than T-800 coatings which, in turn, are more wear resistant than SF-6 coatings.