Investigation On Preparation And Properties Of WB₂-based Hard Coatings Deposited By Magnetron Sputtering

As a candidate for novel protective hard coatings with broad applications,WB2 coating has received extensive attention in recent years.Among all the WB2-based hard coatings,AlB2-type WB2 coating,which has high hardness,high wear resistance,high chemical stability,has been wildly followed with interests.However,the typical shortcomings of boride coatings,such as poor toughness and insufficient bonding strength,limit the further application of AlB2-type WB2 coatings in industry.However,little attention has been paid to amorphous WB2 coatings,which have better toughness.In this thesis,Al-doped AlB2-type WB2 coatings,Si-doped AlB2-type WB2 coatings and amorphous WB2 coatings,amorphous WB2/Ti multilayer coatings and amorphous WB2/Si3N4 multilayer coatings are deposited on YG8 cemented carbide substrates and Si substrates by magnetron sputtering.Effects of element doping amount of nanocomposite coatings,modulation ratio and bilayer number of multilayer coatings on the structure and properties of WB2-based hard coatings are studied.Effects of mechanical properties of the coatings on the wear resistance and wear mechanisms are systematically analyzed.For Al-doped AlB2-type WB2 coatings,it is found that as the Al content increases,the preferred orientation of the coating changes from(001)to(101),and gradually transforms into amorphous state.With a small amount of Al content,W-Al-B coating reaches the highest hardness of 34.4 GPa.The doping of an appropriate amount of Al element can improve the bonding strength of the coating by releasing the stress generated during the movement of the indenter.The amorphous Al2O3 phase formed by the oxidation of partially doped Al element in the coating by O2 in the background vacuum can reduce the friction coefficient of the coating.Combined with the selflubricating effect and the high hardness of the coating,W-Al-B coating with 2.2 at.%Al element has the lowest wear rate of 2.2×10-7 mm3/Nm.By comparing the Si-doped AlB2-type WB2 coating and amorphous WB2 coating,it is found that for the crystalline W-Si-B coating,with the increase of Si content,the crystallinities of the coatings increase first and then decrease,and then the coating changed into an amorphous state.At the same time,due to the nanocomposite structure,both hardness and fracture toughness of the coatings increase,and the highest hardness of the coating reaches 35.5 GPa.The doping of Si element will reduce the bonding strength of the coating.Crystalline W-Si-B coating with Si content of 4.7 at.%has the highest wear resistance due to its high hardness and fracture toughness.For amorphous WB2 coating,the doping of Si element has little influence on the coating hardness due to the absence of crystal structure.The hardness increases slightly due to the formation of Si3N4 phase with higher hardness in the coating,but the fracture toughness and bonding strength of the coatings decrease with the doping of Si element.The wear rate of amorphous WB2 coating is in the same level as that of AlB2-type WB2 coating due to its high fracture toughness.With the doping of Si element,the wear resistance of amorphous W-Si-B coatings decreases gradually.For amorphous WB2/Ti multilayer coating,it is found that with the fixed bilayer number of 10,the hardness of the coatings increases with the increment of modulation ratio,and the maximum hardness was 19.2 GPa,which is lower than that of amorphous WB2 coating.At the same time,the fracture toughness of the coating decreases.As the modulation ratio is fixed to 5,with the increase of the bilayer number,both hardness and fracture toughness of the coatings increase owing to the interface effect,and the wear resistance of the coating increases.WB2/Ti multilayer coating with the modulation ratio of 5 and bilayer number 50 reaches the lowest wear rate of 1.76×10-7 mm3/Nm.However,as the bilayer number increases to 70,the comprehensive performance of the coating is reduced due to the lack of interface effect.For amorphous WB2/Si3N4 multilayer coatings,it is found that some N elements could be detected in the WB2 sublayers due to the diffusion of interlayer elements and the residual N2 gas,therefore the hardness of WB2 sublayers decreases while the fracture toughness increases.The interface strength between WB2 sublayers and Si3N4 sublayer is relatively low due to their small differences.as the number of modulation cycles is fixed to 5,with the decrease of bilayer number,the hardness of the coatings increases gradually and the fracture toughness decreases.WB2/Si3N4 coating with the modulation ratio of 3 has the lowest wear rate due to its relatively balanced hardness and fracture toughness.While the modulation ratio is fixed to 3,due to the low interface strength between the sublayers,with the increase of bilayer numbers,the comprehensive performance of the coatings decreases due to the lack of interface effect.