Study On Preparation And Properties Of TiB₂ Coatings

The development and application of coating technology have played a critical role in improving the tool performance and promoting the progress of cutting technology.The coated cutting tools have become an important direction for the development of modern tools.Hard coatings can be roughly classified into metal nitride coatings,metal carbide coatings,metal oxide coatings,metal boride coatings,and other compound coatings.Compared to nitride and carbide coatings,the study of boride coatings is relatively rare.TiB₂ has received extensive attention because of its excellent physicochemical properties.In this paper,TiB₂ coatings were prepared by pulsed magnetron sputtering technology.The influences of negative bias voltage,different transition layers,and C addition on microstructure and mechanical properties of TiB₂ coatings have been studied.The main research results are as follows:1)TiB₂ coatings prepared under all biases（-50^-300 V）exhibit hexagonal phases and show apreferred（001）orientation;as the bias voltage increases,the diffraction intensity of（001）peak increased firstly and then decreased.As the bias voltage increases,the surface structure of the coating tends to be uniform,the columnar structure in the coating gradually refines,the gap between the columnar structures decreases,and the coating becomes denser;As the bias voltage increased,the growth structure of the coatings became dense and compact due to the high energy of the sputtered species at high bias voltage,which resulted in the improvement of the hardness of the TiB₂ coatings.The TiB₂ coating deposited at-200 V exhibited the highest hardness value of 47.5 GPa.However,when the bias voltage increased to-300V,the hardness of the coatings decreased instead.This may be related to the ion bombardment.More enhanced ion bombardment with high bias voltage induce more defects,which would reduce the mechanical properties of the coating.2)Depositing the transition layer by arc ion plating technique can effectively improve the adhesion strength of the coating,In the absence of a significant decrease in the hardness of the coating（the hardness of the coating is maintained above 35GPa）,the adhesion strength of the coating can reach 54 N after adding the transition layer.However,under the same process conditions,the adhesion strength of the transition-free TiB₂ coating is only 17 N.After the transition layer is added,the toughness of the TiB₂ coating is also improved.Among them,The TiB₂ coating with CrN and Cr/CrN transition layer exhibits the best toughness,and the toughness of the single TiB₂ coating without the transition layer is the worst;The machining performance of the coated milling cutters was also studied.The cutting life of coated cutters was significantly higher than that of uncoated cutters,in which the cutting life of Cr/TiB₂ and CrN/TiB₂ coated cutters was at 4 times the uncoated cutter,Cr/CrN/TiB₂ and Cr/CrN/Al Ti N/TiB₂ coated cutters have a cutting life that is three times that of uncoated cutters.3)The increase of C content in the coating inhibits the growth of TiB₂ grains and refines the TiB₂ grains.The C forms DLC phases and Ti（B,C）₂ phases in the coating.The content of Ti（B,C）₂ phase increases as the C content increases;C content has a significant effect on the hardness of TiB₂ coating.the hardness of the coating increased firstly and then decreased as the C content in the coating increase,The coating with 13.9 at.%C exhibited the highest hardness value of 43.9 GPa.When the C content exceeds 13.9 at.%,the coating hardness drops sharply due to the increase of the soft phase in the coating,The hardness of the coating dropped to 21.9 GPa when the C content reach 24 at.%.As the C content increase,the toughness of the coating inproved.When the C content was 13.9 at.%,the toughness of the coating was the best.However,increasing the C content further decreases the toughness of the coating.In addition,the increase of C content in the coating hinders the diffusion of O element in the coating,thereby improving the oxidation performance of the coating.