Fracture Resistance Enhancement in Hard Mo-B-C Coatings Tailored by Composition and Microstructure

Abstract

State-of-the-art protective coatings often suffer from brittleness. Therefore, the coatings are intensively sought which would simultaneously exhibit high hardness and stiffness with moderate ductility and fracture resistance. In this paper, we report on the nanostructure designing of coatings containing metal, boron, and carbon enabling the simultaneous presence of stiff boridic and carbidic bonds together with weaker metallic bonds to provide coatings with these desirable properties. Three designs are presented with different relative amounts of nanocrystalline and amorphous phases, ranging from near-amorphous to prevalently crystalline microstructure. All presented coatings exhibit an unusual combination of high fracture resistance and high hardness that cannot be achieved with state-of-the-art protective coatings. Indentation tests at high loads revealed that no cracks are present at the surface of the investigated coatings while state-of-the-art ceramic protective coatings already exhibit significant cracking. Cracks in the bulk of the presented coating are detected only when the deformation is so severe that the substrate itself fails.

The Effect of a Taper Angle on Micro-Compression Testing of Mo-B-C Coatings

“…The MoBC AM coatings exhibited a dominant broad peak centered at~38 • and second broad peak with lower relative intensity centered at~71 • . No well-defined sharp peaks implying a crystalline microstructure were observed, and the obtained diffractogram was in accordance with results typical for near-amorphous Mo-B-C coatings [16][17][18]. The diffractogram of the MoBC CR coating showed several sharp peaks with relative intensities and positions corresponding well with the reference Mo 2 BC material.…”

“…This study was devoted to experimentally evaluate the usability of empirical stress corrections theoretically calculated and presented in [ 10 ] when applied to micro-compression experiments done to near-amorphous and nanocrystalline Mo-B-C coatings. The Mo-B-C coatings show a very promising combination of high hardness and moderate ductility [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ] due to their unique microstructure and chemistry. This unusual combination of high hardness and ductility is very demanding in the field of protective coatings, and a more detailed study of their mechanical properties at micro and nanoscale can give a new insight into how the microstructure affects the properties of coatings for which the micro-compression testing is very suitable.…”

Composition, Structure and Mechanical Properties of Industrially Sputtered Ta–B–C Coatings

“…The residual imprints, in general, show clear picture frame cracking. The amount and severity of the cracks are comparable to that found in current typical commercially used protective coatings such as AlTiN or TiB 2 [63]. Therefore, the fracture resistance of the coating is on a par with the current hard protective coatings.…”

Atomic Substitution to Balance Hardness, Ductility, and Sustainability in Molybdenum Tungsten Borocarbide

“…Synthesizing Mo 0.9 W 1.1 BC using arc-melting and analyzing the products with Vickers microindentation recently revealed that this tungsten-rich composition is extremely hard, reaching the superhardness limit at low indentation load . Interestingly, the analysis of damage behavior in a Mo 2 BC coating has shown its highly ductile nature. , Using ab initio calculations, the presence of stiff carbide and boride layers in conjunction with the metallic interlayer bonding is proposed to give rise to the occurrence of high hardness while maintaining moderate ductility in Mo 2 BC . Indeed, prior computational research suggested that the valence electron concentration in X 2 BC systems (X = Mo, Ti, V, Zr, Nb, Hf, Ta, W) leads to higher ductility based on Pugh’s ratio as well as the positive Cauchy pressure .…”