| It has always been an an insurmountable technical challenge to increasing the toughness of hard coatings.The Friedel oscillation of valence electrons was the very limit which made it difficult to toughen nano-multilayer or nanocomposite hard coatings which were heterogeneous.In this paper,a series of homogeneous NbN coatings were deposited by using Modulated Pulsed Power Magnetron Sputtering(MPPMS).Under the control of the nitrogen flow ratio,micropulse duty cycle in strong ionization stage,charging voltage and bias voltage,the phase composition and microstructure of NbN coatings were studied.The mechanical properties of NbN coatings were further analyzed by nanoindentation test,scanning electron microscope and transmission electron microscopy.The conditions,the hardness and toughness of nanocomposite NbN coatings had been simultaneously enhanced,had been clarified.The main conclusions were as follows:1)The variation of MPPMS discharge parameters were important parameters influencing the phase composition and structure of the deposited NbN coatings.With the increase of nitrogen flow ratio,micropulse duty cycle and charging voltage,the peak power increased from 0.4 kW to 1.3 kW,the content of ?'-NbN phase in NbN coating decreased,and the content of ?-NbN phase increased.A bias voltage was applied with a peak power of 0.9 kW and the NbN coating consisted of a ?'-NbN single phase structure.And as the peak power increased,the columnar structure of all coatings became denser and denser.2)All the NbN coatings were composed of cubic ?-NbN phase and hexagonal ?'-NbN phase.With the nitrogen flow ratio increased from 15% to 30%,the structure of NbN coatings changed from a cubic ?-NbN phase-based structure to a hexagonal ?'-NbN phase.With the change of micropulse duty cycle and charging voltage,the preferred orientation of the NbN coatings were ?'-NbN(100)and ?-NbN(200).When the micropulse duty cycle and the charging voltage were in the intermediate parameters,the content of ?-NbN phase in the NbN coatings were low.While applying the bias voltage,the NbN coatings were mainly composed of ?'-NbN phase.3)With the increase of nitrogen flow ratio from 15% to 30%,the hardness of the coatings increased from 30.9 GPa to 36.3 GPa and then decreased to 29.4 GPa.The residual stress increased from 0.47 GPa to 1.93 GPa and then decreased to 1.01 GPa.The toughness of the coatings gradually decreased.With the duty cycle increased from 41% to 47%,the hardness of the coatings increases from 21 GPa to 28 GPa,and the residual stress increased from 1.25 GPa to 19.3 GPa.The toughness of the coatings increased.With the charging voltage increase from 360 V to 380 V,the residual stress of the coatings was between 1.38 GPa and 2.76 GPa.The hardness and toughness had similar changes with the coatings deposited by the different micropulse duty cycle.Applying bias voltage,the NbN coatings consisted of ?'-NbN phase.The hardness of the coating decreased from 36 GPa to 26 GPa,and the residual stress increased from 1.93 GPa to 3.38 GPa.The toughness of the coating generally shown a downward trend.4)It was shown that the hardness and toughness of the NbN coatings,which were composited of the cubic ?-NbN phase and the hexagonal ?'-NbN phase,were simultaneously increased by adjusting the nitrogen flow ratio and bias voltage.The result was also shown that the dense structure of the NbN coatings and the compressive residual stress of the coatings were also important factors influencing the hardness and toughness of the coatings by controling micropulse duty cycles and charging voltages.By comparing the four typical process parameters of MPPMS on the phase composition,microstructure and mechanical properties of NbN coatings,the results were shown that the key factor to obtain hardness and toughness simultaneously increased of NbN coatings were to form duplex phase structures.The compactness and residual stress of the coatings also had an effect on the improvement of its toughness. |
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