Mechanical And Cutting Properties Of Bilayer And Nano-multilayer Structures Of TiSiN/TiAlN Coatings

The difficult-to-machine materials have been widely used in high-tech equipment applications.More superior properties of the cutting tool are requested in the modern manufacturing industry.Coating technology is an effective way to improve the cutting performance of cutting tools.The multilayer nano-coating shows excellent superhard properties and oxidation resistance,so it is widely used in the machining of hard-to-cut materials.However,it is found that the nano-coatings show some limitations in the cutting process.The various structures of coatings would exhibit different advantages according to the different cutting conditions.Moreover,the performance evaluation and failure mechanisms of coatings are not investigated completely.Therefore,it is important to establish the relationship between coating properties and cutting performance.This could be the key point to study the failure mechanisms of coating and develop advanced coating technology.In this paper,the mechanical properties and failure mechanisms of bilayer TiSiN/TiAlN coating and multilayer TiSiN/TiAlN nano-coating under the quasi-static loading and dynamic cyclic impact loading were investigated.The cutting performance of these two coatings was also studied to find the correlation between mechanical properties and cutting performance of the coatings.Thus,it is conductive to achieve the high efficiency and precision machining by optimizing coating structures according to the different cutting conditions.The following results were obtained:(1)The bilayer TiSiN/TiAlN coating and multilayer TiSiN/TiAlN nano-coating were deposited.The oxidation resistance and thermal fatigue impact resistance of coatings were studied.It was found that the oxidation temperatures of bilayer TiSiN/TiAlN coating and multilayer TiSiN/TiAlN nano-coating were about 600?and 800?,respectively.The bilayer TiSiN/TiAlN coating suffered spalling failure under the thermal fatigue impact generated by the pulse laser system.However,no spalling occurred on the multilayer TiSi N/TiAlN nano-coating surface under the same conditions of thermal fatigue impact.The thermal properties of multilayerTiSiN/TiAlN nano-coating were better than that of the bilayer TiSiN/TiAlN coating.(2)The mechanical properties and failure mechanisms of coatings under the quasi-static loading and scratch loading were investigated.In addition,a mechanical model of stress distribution under indentation tests was established according to the results of numerical simulations,and then 3D crack propagation mechanisms of coatings were proposed.The results show that the hardnesses of bilayer TiSiN/TiAlN coating and multilayer TiSiN/TiAlN nano-coating were about 33.2GPa and 36.8GPa,respectively.The multiple interface structure in nano-coating may act as an improvement of hardness,relaxant of high stresses and inhibition of crack propagation.So the multilayer TiSiN/TiAlN nano-coating showed better properties of the resistance to deformation,stress concentration and crack propagation.The friction coefficients between bilayer TiSiN/TiAlN coating and multilayer TiSiN/TiAlN nano-coating with diamond were about 0.135 and 0.104,respectively.The critical loads of cohesive failure and adhesive failure of multilayer TiSiN/TiAlN nano-coating were 55.518 N and 114.241 N respectively,which were higher than that of the bilayer TiSi N/TiAlN coating.Thus,the tribological and adhesion properties of multilayer TiSiN/TiAlN nano-coating were also superior,compared with those of bilayer coating.(3)The new testing methods of performance evaluation under dynamic cyclic impact loading were proposed.The dynamic mechanical properties and fatigue failure mechanisms of coatings under the low-frequency(0.25Hz),high-frequency(20Hz)and ultra-high-frequency(20kHz)cyclic impact were investigated.The results showed that multilayer TiSiN/TiAlN nano-coating appeared excellent resistance to fatigue fracture under both the low-frequency and high-frequency cyclic impact conditions.However,for the ultra-high-frequency cyclic impact tests,it found that multilayer nano-coating exhibited fewer cracks and better properties of fatigue resistance at a low impact load,but larger spalling areas and worse properties of fatigue resistance at a high impact load.Further,the crack propagation,interfacial delamination and fracture failure of coatings were more susceptible to the dynamic impact loading than the quasi-static loading.The impact loading could induce more coating spalling compared with the quasi-static loading.(4)Cutting performance and wear mechanisms of coated cutting tools were investigated.The correlation between mechanical properties and cutting performance of coatings was established.The results show that coating suffered high-frequency cyclic impact due to the effect of tool cutting into the workpiece during the milling of titanium alloy(interrupted cutting).The impact frequency generated by the milling processes was 10–300Hz.The main coating failure modes were wear and chipping,and the tool suffered crater wear and edge break due to the exposure of the substrate.In addition,multilayer TiSi N/TiAlN nano-coating showed better milling performance under different feed rates,which was consistent with the fatigue resistance of coating in the high-frequency cyclic impact tests.In the turning of titanium alloy(continuous cutting),segmented chip formation leading to the generation of cyclic cutting forces and tool stress with much higher frequency.It could be calculated that the serrated chips produced at a frequency of about10–35kHz.This induced the fatigue with ultra-high-frequency impact on coating.The main failure modes of the coating were wear and chipping.It found that multilayer TiSiN/TiAlN nano-coating exhibited better turning performance at a low feed rate,but worse turning performance at a high feed rate.This was consistent with the fatigue resistance of coating in the ultra-high-frequency cyclic impact tests.