Multiscale materials design of hard coatings for improved fracture resistance and thermal stability
Physical vapor deposited hard coatings comprised of cubic (c) transition metal (TM)-Al-N, and (TM)-Si-N are the current work horse materials for a large number of metal cutting and wear resistant applicatíons to light against the extreme conditions of temperature and stress simultaneously. In spite...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2016 |
| País: | España |
| Institución: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/457422 |
| Acceso en línea: | http://hdl.handle.net/10803/457422 https://dx.doi.org/10.5821/dissertation-2117-111492 |
| Access Level: | acceso abierto |
| Palabra clave: | Àrees temàtiques de la UPC::Enginyeria dels materials 620 66 |
| Sumario: | Physical vapor deposited hard coatings comprised of cubic (c) transition metal (TM)-Al-N, and (TM)-Si-N are the current work horse materials for a large number of metal cutting and wear resistant applicatíons to light against the extreme conditions of temperature and stress simultaneously. In spite of a high degree of sophisticatíon 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 ofthe coating is an inherent material property. Limited thermal stability in the TM-Al-N system is associated with the transformation of metastable c -AIN to its stable wurtzite (w)-AIN phase ata temperature above 900 oC resulting an undesirable hardness drop. The current work shows how to overcome these challenges by manipulaling 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 AIN and SiNxs 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 self-organized nanocomposite structure with a hardness variation from 37 ±2 GPa to 26 ±1 GPa. The indentation induced fracture studies reveal crack deflection for the colum nar coating, likely a long the coiumn boundaries. The crack deflection olfers additional energy dissipative mechanisms that make the columnar structured coating more fracture resistant, which is not the case fur the nanocomposite coating in spite of its lower hardness. Crystal structure of AIN is variad between stable wurtzite structure to metastable cubic structure in the ZrAIN alloy by adapting a mullilayer structure and tuning the layerthickness. The multilayer consisting c-AIN layer shows a hardness of 34 ±1 GPa anda twofold enhancement in the critica! force to cause an indentation induced surface crack compared to the multilayer containing w-AIN in spite of a lower hardness for the later case. The higher fracture resistance is discovered to be ca u sed by stress- induced transformation of /IJN from its metastable cubic structure to its thermodynamically stable wurtzite structure associated with a molar volume expansion of20% that builds up local compressive stress zones delay;ng the onset and propagation of the cracks. This is in fact the first experím en tal data point for the stress-induced transfurmation toughening in a hard coatíng. The current work also demonstrates a concept of im proving the thermal stabilíty ofTM-Al-N by m odifying the interface structure between w-AIN and c-TMN. A popular belief in the field is that AIN in lis stable wurtzite structure is detrimental to coating hardness, and hence the curren! material design strategy Is to force AIN in metas table cubic phase that confines the application temperature (- 900 oC). In contrast, here it is shown that the w-AIN offers a high hardness provided if it is grown (semi-)coherent to c-TMN. This is experimentally shown for lhe multilayer system ofTiN/ZrAIN. The interface structure between the c-TiN, c-ZrN and w-AIN is transformed from incoherent to (semi-)coherent structure bytuning the growth conditions under a favorable crystallographic template. Furthennore, the low energy(semi-) coherent interface structure between w-AIN 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 of34± 1.5 GPa. This value is 50 % higher comparad to the state-of-the-art monolithic and multilayered Ti-/IJ -N and Zr-Al-N coating containing incoherent w-AIN. Finally, an entropy based alloy design concept is explorad to form a thermodynamic |
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