Study On Advanced Titanium Diboride Matrix Composite Ceramic Tool Materials And Cutting Performance

Aiming at the disadvantages of titanium diboride matrix composite ceramic materials, a new kind of titanium diboride matrix composite ceramic tool materials with high mechanical properties has been successfully developed according to the chemical, physical and sintering compatibility. Under the liquid-phase hot-pressing technology, the titanium diboride matrix composite ceramic tool materials, TiB₂-TiN, TiB₂-Al₂O₃ and TiB₂-TiN-Al₂O₃, were fabricated by adding the particles TiN and Al₂O₃ into TiB₂ with Ni and Mo as sintering aids. The hot-pressing technology and sintering theory, the mechanical properties and toughening mechanisms, the oxdiation behavior and theory at high temperature, the thermal-shock resistance and the cutting performance of the titanium diboride matrix composite ceramic tool materials were studied.The composite ceramic tool materials, TiB₂-TiN, TiB₂-Al₂O₃ and TiB₂-TiN-Al₂O₃, were fabricated by the optimized hot-pressing technology, and effect of additives on the density was studied. It is shown that the TiN and Al₂O₃ particles can promote the sintering process of the TiB₂ ceramic tool materials. The relative density of TiB₂-TiN increases consistently with an increase in the content of TiN and gets up to 98.87% when the addition content of TiN is 30vol%. A small amount of Al₂O₃ can distinctly improve the density of the TiB₂ ceramic tool materials, but more content of Al₂O₃, exceeding 30vol%, is not beneficial to improving the density of the TiB₂ ceramic tool materials. The densification process was studied, and the densification equations of the initial and final sintering stages were established. It is shown that the densification during the initial stage of sintering is determined by the grain accommodation, 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. When the wetting angle between liquid-phase and particle is smaller than that between liquid-phase and matrix, particle can promote the sintering process of the TiB₂ ceramic tool materials, and the more amounts and the faster of the densification rate. Whereas, particle is not beneficial to the sintering of the TiB₂ ceramic tool materials when the wetting angle between liquid-phase and particle is equal to that between liquid-phase and matrix. The process of densification during the final sintering stage is determined by liquid flow and solid diffusion, and the densification rate has been affected by grain-boundary diffusion coefficient (D), grain-boundary width (W), atom volume (α³) and surface tension of the particle. The density increases consistently with the increase in sintering time, and the -ln(1-ρ) is linear with the sintering time. The relationship between the -ln(1-ρ) and the particle amounts is a parabola, that is to say, the densification of TiB₂ matrix composite ceramic tool materials first slightly and then sharply increases with an increase in the content of the additives.The mechanical properties, microstructure and toughening mechanisms of the TiB₂ matrix composite ceramic tool materials were studied. The TiB₂ matrix composite ceramic tool materials, BT30, BA30 and BT30A10, were fabricated successfully. Their average flexural strength are 1240MPa, 915MPa and 1036MPa, the average fracture toughness are 7.43MPa·m^1/2, 7.00MPa·m^1/2 and 7.8MPa·m^1/2, the average Vickers hardness are 20.47GPa, 21.42GPa and20.33GPa, respectively. The main toughening mechanisms are metal bridging, cleavage vein in the fracture surface and crack shielding caused by dislocations. The mechanics model of bridging by metal particle and metal ring was established. The model indicates that the toughening value by metal particle is 5.09 times higher than that by metal ring at the same amount of metal.The oxidation behavior of the TiB₂ matrix composite ceramic tool materials, TiB₂-TiN, TiB₂-Al₂O₃ and TiB₂-TiN-Al₂O₃, at the temperature of 700℃, 800℃and 1000℃was investigated systematically. It is shown that the oxidation gain of TiB₂-TiN increases with an increases in the content of TiN, and that of TiB₂-Al₂O₃ and TiB₂-TiN-Al₂O₃ decreases with an increase in the content of Al₂O₃. The oxidation of TiB₂-TiN and TiB₂-TiN-Al₂O₃ occurs at 700℃, and that of TiB₂-Al₂O₃ begins at 800℃. The oxidation energy of the BT30 is the least one among the BT30, BA30 and BT30A10, and that of BA30 is the highest one at the temperature range from 800℃to 1000℃. After an oxidation time of 50h at the temperature of 800℃, TiO₂ appears at the surface of the BT30 and BT30A10, it covers the materials surface and surrounds the TiB₂ particles, which prevents the oxidation from occurring still. Light oxidation of the BA30 occurs and the TiO₂ does not surround the TiB₂. The flexural strength of the BT30, BA30 and BT30A10 is more than 750MPa after the oxidation and can act as tool still. After an oxidation time of 50h at the temperature of 1000℃, the TiN of the BT30 and BT30A10 and the TiB₂ of BA30 have been reacted completely. The flexural strength of the BT30, BA30 and BT30A10 is too low to act as tool.The thermal-shock resistance and R-curve behavior of the BT30, BA30 and BT30A10 were investigated, and the prediction model of flexural strength was established. The parameter of crack propagation after thermal-shock was proposed. It is shown that the critical difference in temperature of the BT30, BA30 and BT30A10 is 750℃, 800℃and 750℃, the holding rate of flexural strength is 26.37%, 50.17% and 33.78%, the parameter of the R-curve is 0.038, 0.139 and 0.087, respectively. The holding rate of flexural strength after a single thermal-shock is a constant relating to the parameter of the R-curve, and the bigger of the parameter the bigger of flexural strength.Compared to the commercial SG4 ceramic tool partly, the cutting performance of the BT30, BA30 and BT30A10 in continuous machining hardened 40Cr alloy steel, hardened 45^# steel, stainless steel lCrl8Ni9Ti, and in intermittent machining hardened 45^# steel was studied. The wear mechanisms of the TiB₂ composite ceramic tool materials were analyzed. It is shown that the wear resistance ability is BA30>BT30 when continuous machining hardened 40Cr alloy steel, and the difference between BA30 and SG4 is not significant. The main wear patterns are tool flank and rake wear as well as the main wear mechanism is abrasive wear. The wear resistance ability is BA30>BT30A10>BT30 when continuous machining hardened 45^# steel. The main wear patterns are tool flank and rake wear as well as the main wear mechanism is abrasive wear in low speed, the main wear mechanism of the BA30 and BT30A10 is abrasive wear in high speed, the main wear mechanism of the BT30 is oxidation wear on rake face and abrasive wear on flank face in high speed. The BA30 and BT30A10 are not suitable to machining stainless steel lCrl8Ni9Ti, however the wear resistance of the BT30 is strong. The wear resistance of the tool with a rake angle of 0°is superior to that with a rake angle of -5°. The wear patters are tool flank and rake wear as well as the wear mechanism of tool flank is abrasive wear, and that of the tool rake is diffusing wear. The fracture resistance of the BT30 is superior to that of the BA30 and BT30A10 when intermittent machining hardened 45^# steel, and the fracture mechanism is mechanical fracture in low speed and high depth, thermal fracture in high speed and low depth, mechanical and thermal fracture in medial speed and depth.