ABSTRACT：ZrTiN gradient coated tools were developed on cemented carbide substrates by pulsed bias assisted deposition of multi arc ion plating combined with ion beam assisted deposition technique. Friction and wear behavior, cutting performance and wear mechanism were proceed to the further and systematic research, as well as the design theory, gradient structure, deposition parameters, mechanical properties and micro-structures of the coatings.
Solid solution strengthening mechanism of ternary nitride like TiAIN, ZrTiN and TiCrN were analyzed. Chemical reactions among ZrTiN coating, cemented carbide substrate and metal adhesion layer were analyzed and calculated by Gibbs free energy change of reaction. The results show that ZrTiN is a stable solid solution based on ZrN;there is no chemical reaction between coating and substrate; chemical reaction of Ti adhesion layer and substrate forms the combination of compounds at the interface.Physical compatibility of ZrTiN coating and substrate are investigated, and the distribution of thermal residual stress of coating is analyzed detailedly. The results indicate that the optimum substrate material matched with ZrTiN coating is YT15;thermal residual stress of the coating increases with increasing depositing temperature,while decreases with increasing coating thickness; TiZr adhesion layer can effectively reduce the thermal residual stress, especially shear stress.
Based on the type of coating-substrate interface and adhesion mechanism, the factors affecting adhesive strength were analyzed. In order to improve the binding strength, different structures of ZrTiN coatings are designed: single layer ZrTiN coating (SZT); composition gradient adhesion layer coating (TGC); coating with gradient distribution of the coefficient of thermal expansion (EGC); coating with gradient distribution of adhesive strength & hardness (ZGC).
ZrTiN coatings were deposited on cemented carbide substrates by pulsed bias assisted deposition of multi arc ion plating combined with ion beam assisted deposition technique. During the depositing, the basic parameters were target current, nitrogen flow rate, deposition temperature, deposition time and negative bias voltage. The effects and laws of these parameters on microscopic structures and properties of the coatings were investigated systematically. The results reveal: the target current shows obvious impact on the atomic percent of Zr/Ti and crystallographic lattice constant of ZrTiN coatings; the nitrogen flow rate increases or the deposition temperature raises will lead to the preferred orientation of ZrTiN changing from (111) to (200),accompanied with the increase of grain sizes; the negative bias voltage shows the most obvious impact on the preferred orientation of ZrTiN: as the negative bias voltage increases, the preferred orientation of ZrTiN changes from (111) to (200),ultimately (220),at the same time, the quality of the coating surface decreases. The film on YT15 substrate exhibits a dense and fine grained structure with the elements distributing quite evenly, and the combination of film and substrate is compact. The properties of the films on YT15 substrate are much better than YG8 substrate. The suitable deposition parameters for coating on YT15 substrate are: negative bias voltage 250 V,Zr target current 110 A, Ti target current 70 A, nitrogen flow rate 140 seem, deposition time 60?90 min, deposition temperature 200℃。
Based on YT15 cemented carbides, SZT, TGC, EGC and ZGC coated tools were prepared, and then properties and microstructures of these four kinds of coated tools were analyzed. The results indicate that SZT coated tools show the highest surface hardness of 32.6 GPa,and EGC coated tools with the highest adhesive strength of 84.6 N; the coating thickness is among 2.62?2.73 (im; all the crystal structures of the four show (111) preferred orientation. The element diffusion of the coating to the substrate is obvious,while the element diffusion of the substrate to the coating is relatively slight;the interfacial bonding of ZrTiN and YT15 substrate is combining effect of mechanical toothing, element diffusion and formation of compounds.
A further research which regarded to the friction and wear behavior of the ZrTiN coatings displays that both the friction coefficient and the wear rate of ZrTiN coatings against 40Cr hardened steel decrease with the increase of atomic ratio of Zr:Ti, and the influence of N element content is not distinct. If the applied load increases, the friction coefficient of the coating increases while the wear rate decreases; if the sliding speed increases, both the friction coefficient and the wear rate decrease. When at a low applied loads of friction, the form of the wear mechanism of the coatings performs a mild abrasive wear and flake,at the same time SZT and EGC coatings with high surface hardness show better wear resistance; when at a heavy applied loads of friction, the wear mechanism displays a state of abrasive wear,and the form of this performs brittle fracture and flake, meanwhile TGC and EGC coatings with better toughness and adhesive strength show better wear resistance. The form of wear mechanism of the coatings at low sliding speed performs adhesive wear and brittle fracture, moreover TGC and EGC coatings with higher toughness and adhesive strength show better wear resistance; the wear mechanism is mild abrasive wear at high sliding speed, and EGC coating with the gradient distribution of the coefficient of thermal expansion shows the best wear resistance, as the thermal stress of the coating is small and the thermal injury is mild.
Experiments which used the ZrTiN coated tools of dry cutting of 40Cr hardened steel were carried out, compared with YT15 tools. The results indicate that the effect of ZrTiN coated tool on cutting force is not obvious,while the coating can significantly reduce the cutting temperature; the thermal barrier effect of ZrTiN coating is obvious,especially in high speed cutting. When the cutting distance is 3600 m, the machined surface roughness of coated tools is significantly lower than YT15, while the machined surface roughness has more consistencies. Flank wear of the tool is the form of abrasive wear and boundary wear; ZGC coating has high surface hardness and adhesive strength,and the multilayer structure can effectively suppress the crack diffusion, so the amount of flank wear is the smallest. The order of ascending the flank wear is ZGC <EGC〈TGC<SZT<YT15. Rake wear of the tool is the form of crater wear and adhesive wear;EGC coating with the gradient distribution of the coefficient of thermal expansion can effectively reduce the thermal stress and the thermal injury of the coating, and the crater wear is the slightest. The order of ascending the crater wear zone area is EGC<ZGC<TGC < SZT< YT15. The main reasons for the failure of ZrTiN coating are brittle fracture and flake caused by the initiation and propagation of the crack; ZrTiN gradient coatings play an important role in blocking and deflecting the cracks, moreover they can effectively improve the life of the coating.
Key words: Cutting tool; Coated cemented carbide; ZrTiN coating; Gradient coating;Friction and wear
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