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Reactive Hot-Pressing and Mechanical Properties of (ZrB2-ZrO2)/BN Composites

Author: CuiLei
Tutor: WangYuJin
School: Harbin Institute of Technology
Course: Materials Science
Keywords: (ZrB2-ZrO2)/BN composites reactive hot-pressing mechanical properties microstructure friction and wear mechanism
CLC: TB332
Type: Master's thesis
Year: 2008
Downloads: 76
Quote: 1
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Abstract


(ZrB2-ZrO2)/BN composites were prepared by reactive hot-pressing at 1600℃under 30MPa for 90 min using ZrO2, B4C and C asstarting materials.Effects of sintering aids、sintering temperature and ZrB2/ZrO2 ratio on structure and properties of the composites have been systematically studied by the X-ray diffraction(XRD), scanning electron microscopy(SEM), three-point bending test, single-edge-notched beam method. The effects of applied loads, sliding velocities and ZrB2/ZrO2 ratio on tribological behaviors of (ZrB2-ZrO2)/BN composites, against GCr15 steel ball under dry sliding conditions, was revealed.Using B4C as reactant, ZrB2/BN composites were successfully achieved following the equation 2ZrO2+B4C+3C=2ZrB2+CO,.While the ZrB2/BN composites failed because of volatilization of B2O3 during sintering.The mixed sintering aids of SiO2+X promotes the densification of the composites, 8% SiO2+2%X registering as a higher relative density of 96.9% with no sintering aids and 10%SiO2, for 88% and 84% respectively.With the decrease of sintering temperature, the relative density of ZrB2/BN composites were decrease, while the mechanical properties increase remarkably Sintering at 1800℃and 1600℃, the relative density of composites were 96.9% and 95.2%, accompany by the vickers’ hardness, elastic modulus, flexture strength and fracture toughness for 1.41GPa, 120GPa, 223MPa, 3.4 MPa?m1/2 and 1.63GPa, 120GPa, 286MPa, 4.2 MPa?m1/2, respectively. Smaller of BN plates and ZrB2 grains leads the formation of s decreasing intering temperaturefrom 1800℃to 1600℃which is beneficial to strengthen the composites.With decreased ZrB2/ZrO2 ratio, the mechanical properties of 35vol%(ZrB2-ZrO2)/BN. composites increasing firstly and then decrease, reaching a maximum value at ZrB2:ZrO2=2.5:1 with the vickers’ hardness, elastic modulus, flexture strength and fracture toughness of 35%ZrO2-BN and 35%ZrO2-BN were 1.47GPa, 93GPa, 235MPa, 4.0MPa?m1/2,respectivelyUnder dry conditions against GCr15 steel ball, the coefficient of friction and wear rate of (ZrB2-ZrO2)/BN composites increased with increasing applied loads and sliding velocities. With the decrease of ZrB2/ZrO2 ratio, the wear rate increases to 5×10-5mm3/(N?m) with 35%ZrB2. The observation of worn morporlogies shows the wear mechanism of (ZrB2-ZrO2)/BN composites is particle wear.

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