Effect of the reversal of the layer thickness ratio in the CrN/CrCN multilayer coating on cavitation-induced degradation

Abstract

CrN/CrCN multilayer coatings with reversal ratios of layer thickness (1:2 and 2:1) and uncoated 1.4021 steel substrate were tested against cavitation erosion. Mechanical properties of the coatings were investigated. The CrN/CrCN coating with a layer thickness ratio of 2:1 (CrN_2–1 coating) had higher hardness, H, Young's modulus, E, and adhesion, i.e. LC2 force (the friction force in a scratch test at which the coating is completely removed) than the CrN/CrCN coating with a layer thickness ratio of 1:2 (CrN_1–2 coating). However, the CrN_2–1 coating had lower stiffness (the H/E and H3/E2 ratios) and LC1 force (the friction force at which the first coating failure occurs). Cavitation tests were carried out using a tunnel with a system of barricade exciters. Both coatings improved cavitation erosion resistance. The erosion rate of all tested materials started with the highest erosion rates which then decreased. After the cavitation tests, the erosion rate of 1.4021 steel was 50% higher than the erosion rate of the CrN 1–2 coating and 122% greater than the erosion rate of the CrN_2–1 coating. The H·LC2/E parameter better than the H/E ratio correlated with the volume loss of the tested coatings in cavitation erosion tests. Deep cracks and pits, and the dense network of cracks were the reason for the higher roughness of uncoated steel compared to coated steel. The maximum Ra roughness parameter of 1.4021 steel was from 60% to about 100% bigger than those obtained for coatings. Thicker layers of CrN in the multilayer CrN/CrCN coating led to a reduction in the density and size of pits due to a higher ability to absorb the impact energy compared to the coating with the thicker CrCN layers.