Study On Titanium Diboride Matrix Composite Ceramic Tool Materials And Tool Failure Mechanisms

Titanium diboride matrix composite materials are difficult to be sintered. The flexural strength and fracture toughness of the materials are very low. Aiming at the disadvantages of the materials, the materials with high performance have been successfully developed according to the chemical and physical compatibility. Under the liquid-phase hot-pressing technology, the high-performance titanium diboride matrix ceramic tool materials such as TiB2-25wt.%TiC-WC(TTW7), TiB2-WC(BW3) and TiB2-WC-TiC(TWTN3), were fabricated by adding metal phases and hard phases. The metal phases were Ni, Co,(Ni, Mo),(Co, Mo),(Ni, Ti), and (Co, Ti) respectively. The hard phases were TiC and WC. The content of metal, the physical compatibility between the metal phases and the hard phases, the densification mechanism during the sintering process, the mechanical properties, the micro structure, toughening mechanisms of the materials, the cutting performance and failure mechanisms of tools were investigated.Additives and compositions of the titanium diboride matrix ceramic tool materials were determined according to the design purpose and design principles. The additives were TiC and WC. According to the model for the metal phase coating the hard phases, the metal phase volume fractionφ∈[3.5%,13%] can be calculated. The smaller the size of the hard phase is, the bigger the difference of the thermal expansion coefficient between the metal phases and the hard phases is. The result is obtained by calculating the physical compatibility of the metal phases and the hard phases.The densification process was investigated, and the densification equations of the initial and final sintering stages were established. It is shown that the densification during the initial sintering stage is determined by the formation of the metal liquid, the dissolution of the ceramic particle, the shrinkage of pores, the grain recomposition, and the densification rate has been affected by the physical properties of liquid-phase, external compressive pressure, particle size and the wetting angle between liquid-phase and particle. The relative density during the final sintering stage increases with an increase in the size of grains(b), the content of the metal(λ1), sintering time(t) and the density of the liquid(ρL) and decreases with an increase in the dynamic viscosity coefficient of the liquid(μ) and the height of the samples(h). The effects of external compressive pressure(P) and sintering temperature(T) on the relative density during the final sintering stage were not obvious.Sintering temperatures, sintering times, the contents of the metals and the hard phases were optimized by the orthogonal test method. Titanium diboride matrix ceramic tool materials were frabricated according to the optimal sintering process. The effects of the metal phases and the contents of hard phases on the mechanical properties of the materials were investigated and the fabrication processes of the cutting tool such as TTW7, BW3and TWTN3were made.TiB2-25wt.%TiC-WC(TTW7) composite was fabricated successfully. The contents of TiB2-25wt.%TiC, WC and Ni were72wt.%,20wt.%and8wt.%, respectively. The matrix and WC grains had a good chemical compatibility in the composite that was sintered at1650℃. The mechanical properties increased with a decrease in the defects such as coarse grains and pores when the content of WC increases. The mechanical properties reduced when the content of WC was more than20wt.%. The optimal mechanical properties of TiB2-25wt.%TiC-20wt.%WC composite were955.7MPa of flexural strength,7.5MPa·m1/2of fracture toughness and23.5GPa of Vickers hardness.TiB2-WC(BW3) composite was fabricated successfully. The contents of TiB2, WC and (Ni, Mo) were72wt.%,20wt.%and8wt.%, respectively. TiB2and WC grains had a good chemical compatibility in the composite that was sintered at1650℃. The defects including coarse grains, pores and brittle phases had an important effect on the the mechanical properties. TiB2-WC-Co composites had more defects such as coarse grains and pores in the micro structure than TiB2-WC-Ni and TiB2-WC-(Ni, Mo) composites, so the mechanical properties of the composites were very low. TiB2-WC-(Ni, Mo) composite has a good micro structure, so the composite has optimal mechanical properties that were1307.OMPa of flexural strength,8.2MPa·m1/2of fracture toughness and22.7GPa of Vickers hardness.TiB2-TiC-WC(TWTN3) composite was fabricated successfully. The contents of TiB2, WC, Ni,(Ni, Mo) and TiC were42wt.%,20wt.%,8wt.%,8wt.%and30wt.%, respectively. TiB2, WC and TiC grains had a good chemical compatibility in the composite that was sintered at1650℃. The micro structure and mechanical properties of the composite were investigated. There are many pores and coarse grains in the composite when the TiC content is10wt.%. The quantity of the pores and the coarse grains reduced gradually with an increase in the content of TiC. The defects of micro structure disappeared in TiB2-WC-TiC-Ni and TiB2-WC-TiC-(Ni, Mo) composite when the TiC content is30wt.%. The mechanical properties increased when the defcts reduced. The optimal mechanical properties of TiB2-WC-TiC-Ni composite were996.6MPa of flexural strength,7.6MPa·m1/2of fracture toughness and23.6GPa of Vickers hardness.Toughening mechanisms of TTW7, BW3and TWTN3composite cutting tool materials were studied. The metal phase dispersed at the interface of the ceramic phase, which formed rings and particles in the composite. The main toughening mechanisms are crack bridging and deflection, pulling out of WC grain, the change of fracture mode and solid solution.Compared to the commercial SG4ceramic tool, the cutting performance and failure mechanisms of the cutting tools such as TTW7, BW3and TWTN3in continuous machining hardened Cr12MoV mold steel, hastelloy alloy C-276and stainless steel1Cr18Ni9Ti were investigated. The wear mechanisms of the ceramic tools were analyzed. The wear resistance of the ceramic tools was very poor in high cutting speed. There were many small scraps that adhered to the finished surface when these tools were used to machine hardened Cr12MoV mold steel and hastelloy alloy C-276without cutting fluid, but there were no small scraps on the surface when they were used to machine the materials with cutting fluid. Under the proper cutting speed, the quality of the finished surface was very good when these tools were used to machine stainless steel1Cr18Ni9Ti without cutting fluid. The wear resistance is BW3>SG4>TTW7>TWTN3when continuous machining hardened Cr12MoV mold steel with cutting fluid under the cutting conditions that the cutting speed is60m/min, the feed rate is0.1mm/r and the cutting depth is0.3mm. The main wear patterns are tool flank and rake wear as well as the main wear mechanisms were adhesive wear and abrasive wear. The wear resistance is BW3>SG4>TTW7>TWTN3when continuous machining hastelloy alloy C-276with cutting fluid under the cutting conditions that the cutting speed is60m/min, the feed rate is0.1mm/r and the cutting depth is0.2mm. The main wear patterns are tool flank and rake wear as well as the main wear mechanisms were adhesive wear and abrasive wear. The wear resistance is TTW7>SG4>TWTN3>BW3when continuous machining stainless steel1Cr18Ni9Ti without cutting fluid under the cutting conditions that the cutting speed is80m/min, the feed rate is0.1mm/r and the cutting depth is0.1mm. The main wear patterns are tool flank and rake wear as well as the main wear mechanisms were adhesive wear, abrasive wear and diffusing wear.