Nanostructure Design And Properties Of TiSiN-based Nitride Hard Coatings

Hard coatings significantly improve cutting tools’performance and have become an important development direction of advanced cutting tools.Physical vapor deposition nitride coatings have good hardness and chemical inertness and are widely used for the wear protection of cemented carbide tools.TiSiN coatings have attracted much attention due to their properties of ultra-high hardness,which can exceed 40 GPa.However,TiSiN coatings obtain considerable residual stress and insufficient high-temperature wear resistance,limiting their application in cutting tools.This thesis intends to design and deposit high-performance TiSiN-based coatings with high hardness,high wear resistance,low stress,and low friction by combining nano-multilayer structure construction and alloying methods.The main research results obtained are as follows:（1）The TiN/SiN_x nano-multilayer coating deposited by hybrid arc/sputtering technology has a columnar crystal growth morphology,and SiN_x is epitaxially grown on the TiN.The mechanical properties of TiN/SiN_x coatings are affected by the modulation period.The hardness reaches the maximum（H=33 GPa）when the modulation period is 6 nm.In addition,the residual stress of TiN/SiN_x coatings is 1.5 GPa lower than that of the arc-evaporated TiSiN coating,while the fracture toughness and hardness are also lower(H_TiSiN=36 GPa).The friction coefficient of TiN/SiN_x coatings at room temperature increases with the decrease of the modulation period（from 0.59 to 0.95）.The wear resistance of TiN/SiN_x coatings is comparable to the TiSiN coating(wear rate of 5.0×10^-6 mm~3/N·m),showing abrasive wear and oxidative wear.In contrast,the wear resistance of TiN/SiN_x coating at 400℃ is lower than TiSiN but higher than TiN.（2）All TiSiN/TiAlN nano-multilayer coatings deposited by cathodic arc technology exhibit columnar crystal growth.The TiSiN/TiAlN nano-multilayer with a reduced modulation period of 8.5 nm and modulation ratio of 1:2.3（thickness ratio of TiAlN to TiSiN sublayer）shows a noticeable increase in hardness to 36.8 GPa.The modulation geometry impacts TiSiN/TiAlN nano-multilayers’wear behavior through two aspects.Firstly,the mechanical enhancement effectively promotes the wear resistance of coatings at room temperature.Secondly,the chemical variation changes the wear mechanism at high temperatures.For the situation at 600℃,the combined action of adhesive,abrasive,and oxidative wear gives rise to dispersive and small oxide patches,adhered to sliding surfaces for TiSiN/TiAlN coatings with a higher modulation ratio of,for instance,2.3:1.This tribolayer contributes to the lower wear volume loss than the case at room temperature for the TiSiN/TiAlN with a modulation period of 29 nm and a modulation ratio of 2.3:1.In contrast,the TiSiN/TiAlN coating with a lower modulation ratio of 1:2.3 exhibits mild abrasive and oxidative wear at 600℃.（3）The growth morphologies of the TiSiNbN/TiAlN and TiSiMoN/TiAlN nano-multilayer coatings deposited by cathodic arc evaporation are consistent with the TiSiN/TiAlN.The hardness of TiSiNbN/TiAlN coating decreased to 31 GPa due to the doping of Nb.And the effect of Modoping on the hardness of coating was not pronounced.At room temperature,the addition of Moreduces the friction coefficient to 0.52 and reduces the wear rate of the coating to 3×10^-7mm~3/N·m.The wear rates of TiSiNbN/TiAlN and TiSiMoN/TiAlN nano-multilayer coatings are both 1.0×10^-7 mm~3/N·m at 300℃,which is two orders of magnitude lower than that of TiSiN/TiAlN.With the test temperature rising to 600℃,the friction coefficient of TiSiMoN/TiAlN nano-multilayer coating decreased to 0.46.Surface oxides formed during friction greatly influence the friction coefficient of multilayer coatings.There are abrasive wear and oxidative wear in the friction process.