Investigation On The Strengthening-toughening And The Tribological Behavior For The WB_x Films

With the development of modern industry,the service environment of machine is becoming more serious,and the requirements for the comprehensive property of protective materials are also getting higher.The improvement of a single property often cannot meet the needs of practical applications.Especially for the surface of crucial moving parts such as bearings in advanced equipment,having high hardness,high toughness and lubrication characteristics effectively can effectively reduce friction and wear and reduce energy consumption.Improving substrate performance by thin-film technology is an effective means to improve the surface properties of moving parts.Therefore,the development of high-hardness,high-toughness,low-friction,and wear-resistant films has become an urgent pursuit in the field of surface engineering.Transition metal borides films have received extensive attention in the field of mechanical protection due to their excellent properties such as high hardness,good thermal stability,and high melting point.However,the increase in hardness is often accompanied by a decrease in toughness.How to make the film maintain high hardness while maintaining high toughness is a challenge.As the content of boron changes,tungsten boride has diverse structures and significantly different performance.Therefore,this work takes the tungsten boride film as the research object.The microstructure is optimized by controlling the boron content.It is expected that the nanocomposite films composed of tungsten boride with different phase structures can be constructed by one-step method.And the mechanical properties for the films can be optimized by adjusting the ratio of the two phases.And then,the tribological behavior of the composite structure film in atmospheric environment and base oil environment is further studied.The main research contents and conclusions are as follows:1.The effects of deposition process on structure,mechanical and tribological properties of tungsten boride(WB_x)films.When the substrate bias voltage is-60 V,as the deposition temperature gradually increases from room temperature(RT)to 400°C,600°C and reaches 800°C,the film structure evolves from the initial amorphous structure to single-phase W₂B,and then to dual-phase of W₂B and WB₂,and finally obtaines the single-phase WB₂.The significant structural change induces the evolution of the mechanical properties of the material:With the increase of deposition temperature,the hard phase WB₂gradually becomes the main phase of the film.Therefore,the hardness of the film increases linearly.When the film consists entirely of WB₂(the deposition temperature was 800?),the hardness reached the maximum(34.31±1.56 Gpa).Nonetheless,the film at this time has a very unstable friction coefficient and wear-through failure occurs quickly during the friction process.While the film consist of W₂B and WB₂dual-phase structure exhibits the lowest friction coefficient(0.47)and lowest wear rate(5.8×10^-17m3/Nm).Next,we further optimized the structure and properties of the WB_xfilms by changing the substrate bias.As the bias voltage increases from 0 to-240 V,the B/W ratio in the tungsten boride film decreased from 1.2 to 0.8.At the same time,the structure of the WB_xfilm gradually changes from the coexistence of W₂B and WB₂dual-phase to W₂B single-phase structure.However,due to the increase of film compressive stress and the solid solution strengthening effect of B atoms,the film hardness shows a trend of first increase and then decrease.The hardness increases from 28.5±1.6 to 32.7±1.9Gpa,and finally drops to 25.7±1.1 Gpa at-240 V.The film hardness reaches maximum at-120 V.At this time,due to the increase in the proportion of the more metallic W₂B phase in the film,the toughness of the film is also greatly improved.Finally,the film also exhibits the lowest friction coefficient(0.43)and ultra-low wear rate(5.2×10^-18m3/Nm).In this part of the work,the ratio of hard phase and tough phase in the tungsten boride film is controlled by optimizing the deposition process,so as to the film achieves strengthening-toughening.And an ultra-low wear rate of 5.2×10^-18m3/Nm is obtained in the friction of the atmospheric environment.2.The tribological behavior and lubrication mechanism of tungsten boride films in base oil.On the basis of the research in the first part,we carried out a large number of friction experiments on the WB_xfilm with the best mechanical and tribological properties after structural optimization in the neopentyl glycol dioleate(NPGD).It was found that in the range of wide load(1?9 N),the films experienced 100,000cycles of long-term friction all show ultra-low friction coefficient(much less than 0.1).Especially under the conditions of a load of 5 N and a rotational speed of 3 cm/s,the film achieves kilometer-level superlubricity(friction coefficient as low as 0.001)and shows an ultra-low wear rate(6.31×10^-20m~3/Nm).The main reason for the superlubricity and ultra-low wear of the film is that the molecular chain of NPGD oil breaks during the friction process,releasing hydrocarbon,hydroxyl,alkenyl and other groups and adsorbing on the friction interface to form an aromatic passivation layer,which effectively reduces the direct contact between the friction pairs.This work achieves ultra-low friction and ultra-low wear in the form of solid-liquid composite lubrication by combining a solid film with excellent mechanical and tribological properties with a base oil.More importantly,this method effectively replaces the use of oil lubricating additives,avoids environmental pollution and other problems.It provides data basis and design ideas for the further exploration and development of the solid lubricating film materials that can operate under maintenance-free and environment-friendly conditions.