Multiscale materials design of hard coatings for improved fracture resistance and thermal stability

Abstract: Physical vapor deposited hard coatings comprised of cubic (c) transition metal (TM)-Al-N, and (TM)-Si-N are the current workhorse materials for a large number of metal cutting and wear resistant applications to fight against the extreme conditions of temperature and stress simultaneously. In spite of a high degree of sophistication in terms of material choice and microstructural design, a lower fracture resistance and limited thermal stability of the coatings remains a technological challenge in the field. The lower fracture resistance of the coating is an inherent material property. Limited thermal stability in the TM-Al-N system is associated with the transformation of metastable c -AlN to its stable wurtzite (w)-AlN phase at a temperature above 900 oC resulting an undesirable hardness drop.The current work shows how to overcome these challenges by manipulating the coating material at different length scales, i.e. microstructure, crystal and interface structure, and alloy design. The endeavor of multiscale materials design is achieved by converging a deeper material and process knowledge to result specific structural modification over multiple length scales by alloying transition metal nitrides with AlN and SiNx as following.Microstructure variation is achieved in ZrN coating by alloying it with SiNx, where the surface segregated SiNx breaks down the columnar structure and evolves a selforganized nanocomposite structure with a hardness variation from 37 ±2 GPa to 26 ±1 GPa. The indentation induced fracture studies reveal crack deflection for the columnar coating, likely along the column boundaries. The crack deflection offers additional energy dissipative mechanisms that make the columnar structured coating more fracture resistant, which is not the case for the nanocomposite coating in spite of its lower hardness.Crystal structure of AlN is varied between stable wurtzite structure to metastable cubic structure in the ZrAlN alloy by adapting a multilayer structure and tuning the layer thickness. The multilayer consisting c-AlN layer shows a hardness of 34 ±1 GPa and a twofold enhancement in the critical force to cause an indentation induced surface crack compared to the multilayer containing w-AlN in spite of a lower hardness for the later case. The higher fracture resistance is discovered to be caused by stress- induced transformation of AlN from its metastable cubic structure to its thermodynamically stable wurtzite structure associated with a molar volume expansion of 20% that builds up local compressive stress zones delaying the onset and propagation of the cracks. This is in fact the first experimental data point for the stress-induced transformation toughening in a hard coating.The current work also demonstrates a concept of improving the thermal stability of the TM-Al-N by modifying the interface structure between w-AlN and c-TMN. A popular belief in the field is that AlN in its stable wurtzite structure is detrimental to coating hardness, and hence the current material design strategy is to force AlN in metastable cubic phase that confines the application temperature (~ 900 oC). In contrast, here it is shown that the w-AlN offers a high hardness provided if it is grown (semi-)coherent to c-TMN. This is experimentally shown for the multilayer system of TiN/ZrAlN. The interface structure between the c-TiN, c-ZrN and w-AlN is transformed from incoherent to (semi-)coherent structure by tuning the growth conditions under a favorable crystallographic template. Furthermore, the low energy (semi-) coherent interface structure between w-AlN and c- TiN, c- ZrN display a high thermal stability, causing a high and more stable hardness up to an annealing temperature of 1150 oC with a value of 34± 1.5 GPa. This value is 50 % higher compared to the state-of-the-art monolithic and multilayered Ti-Al-N and Zr-Al-N coating containing incoherent w-AlN.Finally, an entropy based alloy design concept is explored to form a thermodynamically stable solid solution in the TM-Al-N material system that has a positive enthalpy of mixing. Multi-principal element alloys of (AlTiVCrNb)N are formed in a near ideal cubic solid solution. The high configurational entropy in the alloy is predicted to overcome positive enthalpy of mixing, there by an entropy stabilized solid solution formation is expected at a temperature above 1000 K. However, at elevated temperature, optimization between the minimization of interaction energy and maximization of configurational randomness causes precipitation of AlN in its stable wurtzite structure and the cubic solid solution is only confined between TiN, CrN, VN and NbN that have a low enthalpy of mixing.In summary, this work provides technological solutions to the two outstanding issues in the field. A significant enhancement in fracture resistance of the coating is achieved with appropriate material choice and microstructural design by invoking crack deflection and stress induced transformation toughening mechanisms. A remarkable thermal stability enhancement of the TM-Al-N coating is achieved by a new structural archetype consisting c-TMN and thermodynamically stable w-AlN with a low energy (semi-)coherent interface structure.