TEM observations of wear mechanisms of TiAlCrN and TiAlN/CrN coatings grown by combined steered-arc/unbalanced magnetron deposition

The dry sliding wear of monolayer TiAlCrN and TiAlCrYN and multilayer TiAlN/CrN coatings has been investigated against a BM2 tool steel counterface using a ring on block configuration at 91 N, 0.42 m/s. The coatings were deposited on a BM2 tool steel substrate by combined steered-arc/unbalanced-magnetron deposition. The wear rate of the multilayer was superior to the monolayer, although both provide a substantial improvement compared with the wear behaviour of the base BM2 tool steel (e.g., wear rate = 6.1 x 10(-4) mm(3)/m for the BM2 tool steel; 3.98 x 10(-5) mm(3)/m for the TiAlCrN monolayer and 2.58 x 10(-5) mm(3)/m for the TiAlN\CrN multilayer). Wear of the coatings occurred by several mechanisms, fine scale (< 200 nm) detachment in the early stages and micron scale detachment associated with cracking in the coating in the later stages. Detailed transmission electron microscopy of cross-sections of the worn surface indicated that two dominant types of cracking were present within the coating: (1) cracking perpendicular to the coating surface, often along columnar grain boundaries, typically running through the entire coating; (2) cracking approximately parallel to the worn surface, extending across several columnar grains. For the multilayers, there was no evidence that the spalling was induced by decohesion along the interface of the multilayers. Limited surface deformation was detected at the worn surface of the TiAlCrYN and TiAlCrN, but not at the worn surface of the TiAlN/CrN, tested under identical conditions. In contrast, the uncoated BM2 tool steel worn surface exhibited extensive plastic deformation. The relationship between wear mechanism and coating structure is discussed. (C) 1999 Published by Elsevier Science S.A. All rights reserved.