Structure And Toughness Of TiB₂Based Coatings With Metal Doped

Hard coatings were widely used as protective coatings on cutting tools, mining tools, wearand corrosion resistant parts, due to their excellent properties such as: high hardness andstrength, high elastic modulus, good wear and corrosion resistance. With the development ofnew fields, the efficiency and service life of tools bring up greater challenges for the propertiesof coatings. These years, the idea of bulk WC-Co was drawn into coating materials toovercome the limitation of toughness. That is, toughen ceramic by metals doped to form aceramic/metal nanocomposite coating. However, the improvement of toughness was always atthe expense of hardness.In this paper, TiB2-Ni coatings was prepared by magnetron co-sputtered method withdifferent Ni content. The influence of Ni content to the microstructure, hardness, andtoughness properties was investigated. Moreover, TiB2-Cu and TiB2-Ag coatings with~10at.%metal content were prepared to research the toughening effect of other metals. And themicrostructure and mechanical properties of these coatings were investigated initially.The main conclusions of this paper are as follows:(1) The Ni addition led to mixed effects on the growth and grain structures. Thecolumnar growth structure was retained in all TiB2-Ni coatings. However, noticeable changesin the grain structure were observed. In particular, a cell-like nanocomposite structure, inwhich <10nm TiB2grains were fully separated by a~1nm Ni-rich layer, was obtained in the10.8at.%Ni coating. Additionally, chemical interactions were also identified between theconstituents.(2) Relatively high values of hardness were found in all TiB2-Ni coatings. Moreover,improved toughness was identified in those Ni-alloyed coatings in terms of the scratch response,indention cracking behavior, and normalized plastic depth (δH). For example, the10at.%Nicoating exhibited an enhanced toughness without sacrificing the high hardness (>35GPa).(3) Tribological tests were performed under different loads (2N or5N) on the TiB2andTiB2-10.8at.%Ni coatings to examine the coefficient of friction, wear track morphology, andthe wear rates. It was found that:(i) Under the low load (2N), the enhanced toughness led tolower wear rates, and (ii) no improvement in the wear resistance was found under the high load(5N), possibly due to the loose columnar structure.(4) Different effects on the structure and properties resulted from the additions ofcopper and silver. Enlarged grains and more pronounced columnar structure were observed inthe Cu-alloyed TiB2coating, while no crystalline Cu was identified. However, crystalline Aggrains were found in the Ag-alloyed TiB2coating, while the corresponding growth structure became less columnar. In both cases, the metal-alloyed coatings exhibited better toughnessthan the TiB2coatings, with their high hardness unaffected.