Fabrication And Performance Of Tools With WS₂-WS₂/Ti-TiSiN Soft-hard Coatings

The design solution of soft-hard coated tools which combine good lubrication of soft coatings and high wear resistance of hard coatings is proposed in this paper. The key deposition technologies of hard coatings and soft coatings are studied. WS2-WS2/Ti-TiSiN soft-hard coated tools with both antifriction and antiwear effects are developed successfully. Their tribological behaviours and cutting performance are systematically researched. Furthermore, the antifriction and antiwear mechanisms are analysed in detail.Based on the analysis of coating-substrate interface bonding machanism, the design principals of soft-hard coatings are proposed. TiSiN that has superhigh hardness is selected as the hard layer and WS2 that has superlow friction coefficient is selected as the soft layer. Gibbs free energy function method and finite element method (FEM) are utilized to analyze the chemical compatibility and physical compatibility of the coating materials system, respectively. The results show that YT15 and YS8 cemented carbides with lower thermal residual stress have a good matching with TiSiN coating; interlayer with the structure of Ti-TiN between TiSiN layer and substrates can both avoid the generation of silicides on the interface, and decrease the thermal residual stress of TiSiN coating; WS2/Ti composite layer between TiSiN and WS2 layers is conductive to enhance the bonding strength of WS2 layer. Based on the analysis above, the structure of soft-hard coatings is designed as WS2-WS2/Ti-TiSiN-substrates.TiSiN hard coatings are deposited by the combining method of multi arc ion plating (MAIP) and middle-frequency (MF) magnetron sputtering, and the influences of different depositing parameters on coating’s properties are investigated. The results indicate that Si content is decisive factor to TiSiN coating’s microstructure and hardness. Under the parameters that Si targets current of 4A, Ti target current of 65A, nitrogen flowrates of 180sccm and bias voltage of-150V, TiSiN coatings have the highest hardness of～35.6GPa. Due to the different ion rate between Ti and Si targets, Si content near the specimen edge is lower than that in the central region, which is called "edge effect". Decreasing Ti target current and bias voltage and increasing nitrogen flowrate can weaken edge effect. For weakening edge effect while ensuring good mechanical properties, the depositing parameters of TiSiN coating are optimized as that Si targets current of 4A, Ti target current of 65A, nitrogen flowrates of 180sccm and bias voltage of -80V. WS2 soft coatings are also deposited by middle-frequency magnetron sputtering and MAIP. The effect of various depositing parameters are studied and the optimum parameters are obtained:WS2 target current of 1A, depositing temperature of 200℃ and depositing time of 100min. Combining the optimum parameters of TiSiN and WS2 coatings, WS2-WS2/Ti-TiSiN soft-hard coatings are prepared. Its microstructure and mechanical properties are investigated. It is found that the deposition of soft layer on TiSiN coating significantly increase the adhesive strength.WS2-WS2/Ti-TiSiN (named WTT), WS2-TiSiN (named WT) soft-hard coatings and TiSiN hard coatings (name TN) are prepared on YT15 cemented carbides discs. Their tribological properties under dry friction are studied systematically. The results show that the friction coefficients of soft-hard coating dramatically increase after soft layer is worn out, but is still lower than JT and TN coating. Comparing to WT coating, the low friction coefficient stage of WTT coating is much longer. By measuring wear scars, it is found that the wear rate of friction balls against WT is 5%-63% percent lower than TN coating, while the reduced proportion of WTT than TN coating is >90%. There is much adhesive materials in the wear tracks of TN and WT coatings. Almost no adhesive wear occurs on WTT coatings, and lots of WS2 still exists in its wear tracks. It indicates that WTT coatings have the best lubricant and antifriction effects.Dry cutting tests on 45# hardened steel are carried out with YS8 cemented tool (JS), TiSiN coated tool (TN), WS2-TiSiN coated tool and WS2-WS2/Ti-TiSiN coated tool (WTT). Results show that WTT tools have the lowest cutting force, cutting temperature, friction coefficient at the tool-chip interface and surface roughness. Compared with TN tools, soft-hard coated tools can reduce Fe adhesion on the rake face and coating delamination on the flank face. The wear area of soft layer on WTT tools is much lower than WT tools.The antiwear and antifriction mechanisms are discussed. The excellent lubricant effect of WTT tools is beneficial from three factors:WS2 layer provides adequate lubrication to the internal friction area; the surface defects on TiSiN layer can store WS2, which would continue to provide lubrication after soft layer is worn out; WS2/T1 composite layer increases soft layer’s adhesive strength so as to prolong the lubricant effect. The high wear resistance of WTT tools is firstly ascribed to the high hardness of TiSiN layer. Secondly, the improved adhesive strength of TiSiN layer by the deposition of soft layer reduce coating flaking, further enhancing tools wear resistance.