Investigation Of Thermal Stability, Microstructure As Well As Mechanical Property And Cutting Performance Of Ti-Al-N Based Hard Coatings

Ti-Al-N coatings have become the most widely used coating for dry machining due to their high temperature oxidation resistance and excellent mechanical peroperties.The chemical composition of Ti-Al-N coatings basically determines their structure and properties.For Al contents exceeding the cubic solid solubility,a mixed cubic-NaCl and hexagonal-ZnS(h-AlN) structure is formed,which results in reduced coating properties.The Al content of Ti-Al-N coatings has a significant effect on their structure and properties.The composition of the coating deposited using sputtering technology can deviate from that of the corresponding target,which makes it difficult to control the composition of sputtered film.Recently,increasing attention has been concentrated on the thermal stability of Ti-Al-N coatings.The transformation of(c) TiAlN into the c-TiN and c-AlN at elevated temperature of～1000℃results in a drop of mechanical properties.However,this transformation involves an intermediate step of c-AlN by spiondal decomposition,and concomitant increase of the hardness occurs.Nevertheless,there is only limited information on the age-hardening of Ti-Al-N coatings so far,especially for its industrial application.The cutting temperature during high speed and dry cutting exceeds the decomposition temperature(～1000℃) of Ti-Al-N.Therefore,a considerable effort was done by alloying Ti-Al-N based coatings with several elements to improve their mechanical properties and thermal stability.Here,we study the compositional and structural evolution of sputtered Ti-Al-N coatings with different deposition parameters allowing for the different loss states of sputtered Ti,and Al atoms.Additionally,the thermal stability and age-hardening of Ti-Al-N coatings with variable Al content and structure are investigated.Furthermore,we also focus on the age-hardening application of Ti-Al-N coatings on cutting application.The incorporation of Si and Zr into Ti-Al-N coatings significantly improves their mechanical properties and thermal stability.And the commercial process of Ti-Al-Si-N coatings after adjustment of their structure is realized.This thesis is comprised of the following five parts:1.The different poisoning state of the Ti and Al particles of Ti_0.5Al_0.5 target in addition to scattering and angular losses causes a significant compositional and structural modification during preparation of Ti-Al-N with different deposition parameters.This mechanism makes it possible to compositional control of sputtered Ti-Al-N.The decreased surface-diffusion process of deposited atoms by adjustment of deposition parameters allows for the higher Al solid solution in TiN.2.The decomposition of Ti-Al-N coating into the stable phases c-TiN and h-AlN involves an intermediate step associated with the formation step of c-AlN.Ti-Al-N coating undergoes spinodal decomposition with the formation of nm-sized cubic TiN and AlN domains resulting in an increase of hardness.The spiondal decomposition and age-hardening of Ti-Al-N coating have an affinity with Al content,where Ti-Al-N coating with higher Al content permits the occurrence of spiondal decomposition and failure at lower temperature.The turning and milling performance of coated inserts can be optimized by a post-deposition annealing treatment, where the coating undergoes age-hardening,combined with a cooling condition allowing for high cohesion and fracture toughness of the cemented carbide.The latter is essential during milling.The improvement during milling is only obtained if the cooling after annealing at 900℃is performed in three steps.The temperature is decreased from 900 to 800℃by a vacuum furnace cooling,from 800 to 700℃by Ar-flow cooling, and further to RT again vacuum furnace cooling.3.The higher hardness and improved thermal stability are obtained by incorporation of Si into Ti-Al-N coatings due to the formation of special three-dimensional net structure consisting of nanocrystalline(nc) TiAlN encapsulated in an amorphous(a) Si₃N₄ matrix phase.Such a strucutre hinders the generation of h-AlN during high temperature annealing.The spiondal decomposition of nc-TiAlN/a-Si₃N₄ coatings also causes the age-hardening,where the hardness of～42.4 GPa after annealing of (Ti_0.45Al_0.55)₀.93 Si_0.07 coatings at 1100℃is acquired.4.Ti-Al-N coatings alloyed with Zr significantly improve the thermal stability,where Ti-Al-Zr-N coatings allow the occurrence of spiondal decomposition at lower temperature and also retard the decomposition of h-AlN at higher temperature.The incorporation of Zr into Ti-Al-N coatings widens the temperature region of age-hardening and is benefit to its high temperature application.5.The application of Ti-Al-N and Ti-Al-Si-N coatings on cutting tools is investigated.The performance of coated tools is further improved by multilayer structure,and thus realizes the commercial process.The interfaces of TiN/TiAlN multilayers increase the hardness and improve the cohesion strength with substrate.Therefore,the TiN/TiAlN multilayer coated inserts behaves better turning and milling performances compared to TiN and Ti-Al-N coated inserts.The turning performance of nc-TiAlN/a-Si₃N₄ coated inserts can be increased due to the higher hardness and improved thermal stability, However,a decrease in milling properties is obtained owe to their higher stress,brittleness and worse adhesion between coating and substrate.The structural adjustment of Ti-Al-Si-N coating into TiAlN-TiAlSiN bilayer and TiAlN/TiAlSiN multilayer coatings further improves the machining performance including both turning and milling.