The influence of microstructural orientation on fracture toughness in (V, Al)N and (V, Al)(O, N) coatings as measured by microcantilever bending

Abstract

Since protective transition metal (oxy)nitride coatings are widely used, understanding of the mechanisms linking microstructure to their fracture behaviour is required to optimise wear resistance, while maintaining fracture toughness. To assess this interconnection, beam bending was performed using microcantilevers oriented parallel and at 90° to the growth direction. Furthermore, the tests were applied to favour normal bending and shear fracture. Coatings were synthesised by both direct current magnetron sputtering (DCMS) as well as high power pulsed magnetron sputtering (HPPMS). Here, we show that the fracture toughness depends on the alignment of the grains and loading directions. Furthermore, an improved fracture toughness was found in coatings produced by HPPMS, when microstructural defects, such as underdense regions in DCMS deposited coatings can be excluded. We propose indices based on fracture and mechanical properties to rank those coatings. Here, the HPPMS deposited oxynitride showed the best combination of mechanical properties and fracture toughness. Graphical abstract Principle of measuring the effects of microstructure and process route on the fracture toughness via microcantilever bending.

Ductile–brittle indentation fracture transitions in hard coatings

“…A parameter called P 50 , that is, the load at 50 nm depth for all the cases was measured and tabulated to correlate the different tests. The penetration depths for both loading and unloading at 1 mN load represented as h L (1) and h UL (1) were also taken into consideration. A higher penetration depth (load) causes dislocation-based creep.…”

“…The formation of small crystals may cause an increase in hardness due to enhanced dislocation motion which includes dislocation climbs giving rise to strain hardening, but strictly speaking, may or may not lead to an increase in fracture toughness in the same way. 1 Although there is evidence of phenomena such as crack deflection to account for an increase in toughness, for those positions that offer an initial high resistance to penetration, the chances of fracture increase as the initial high stress imposed by the indenter gets arrested in the material showing no signs of fracture initially until the applied stress imposed by the indenter exceeds the fracture toughness (K C ) threshold value.…”

“…These crystals naturally possess high hardness and are good electrical and thermal insulators. [1][2][3] Diamond is the hardest and talc is the softest material known to mankind. These hard coatings with their various combination of phases have major surface engineering applications in terms of possessing resistance to wear and abrasion properties as well as being corrosion resistant and sustainable to harsh operating conditions, Novel inclusion of elements such as V, Nb, Y, and even some rare earth metals such as Eu have been tried to obtain phases sufficing any technical requirement.…”