| AlMgB14 is a kind of ultra-hard material. With the advantages of excellent mechanical properties, high chemical stability, low density and low cost, it is a promising candidate for application in the field of cutting tools, wear resistant coatings and key components in aerospace and military application. However, its wider applications were restricted due to the limitation of poor toughness and elevated temperature oxidation resistance. In this paper, nano TiB2 particles were added to AlMg B14 to fabricate AlMg B14-TiB2 composite to improve mechanical properties and elevated temperature oxidation resistance of AlMgB14. Moreover, in order to achieve more widely used applications, AlMgB14-TiB2 composite was bonded with metals. The paper has an important theoretical and practical significance to expand application scope of AlMg B14.In this paper, the AlMgB14 bulk, nano TiB2 toughened AlMgB14-TiB2 ultrahard composite and Ti B2/AlMgB14-TiB2 gradient composite were synthesized by the field activated and pressure assisted synthesis technology(FAPAS). AlMgB14-TiB2 composites were synthesized and bonded with Nb and Mo plates to fabricate AlMgB14-Ti B2/metal simultaneously by the field-assisted diffusion bonding process(FDB).The microhardness of the samples was tested by microhardness tester, and the crack length of indentation was measured by scanning electron microscope(SEM). The effect mechanism of reinforced phase on microhardness and fracture toughness of composites were discussed. The friction coefficients of AlMgB14 bulk and AlMgB14-30 wt.%TiB2 composite reinforced by nano TiB2 were tested by multifunctional surface experimental instrument. The wear property of AlMgB14-30 wt.%Ti B2 composite were tested by HT-1000 ball-disc reciprocating temperature friction and wear tester. The wear surface, phase structure and composition distribution were analyzed by X-ray diffractor(XRD), SEM and EDS. The tribological characteristics of the composite at different temperature and the effect mechanism of reinforced phase TiB2 on friction and wear properties of composite were discussed.Elevated temperature oxidation resistance was measured by thermal gravimetric analysis. The lattice parameters and crystal structures of AlMgB14-TiB2 composites were analyzed by first principle, and compared with theoretical values. The adsorption of oxygen molecule on the surface of the composite was numerically simulated. The adsorption energy and the state density of the model were calculated and analyzed. Microstructures and phase structures of the surface and the cross section of the oxidized samples were observed and analyzed by XRD, SEM and EDS. The elevated temperature oxidation behaviors of the composite were discussed. The microhardness and connection strength of AlMgB14-TiB2/metal composite were measured. The mechanism of diffusion bonding between AlMgB14-TiB2 and the metal plate was discussed.The results show that the lattice parameters of Al Mg B14 in Al MgB14-Ti B2 composite sintered with the process parameters of sintering temperature of 1400°C, pressure of 50 MPa and holding time of 8min were closed to Al0.75Mg0.78B14. The optimized Rwp of AlMg B14 was 3.47%, and the optimized Rwp of TiB2 was 4.95%. The lattice parameters and atomic position of TiB2 phase had a slight deviation compared with theoretical value, which illustrated that there was the squeezing between the particles and thus induced a large number of lattice distortion. The occurrence of the lattice distortion was beneficial to improve microhardness and toughness of the samples. With the increase of the content of TiB2, microhardness and fracture toughness of the composite increased linearly. The mechanical properties of the composite were the best when the content of TiB2 reached 70 wt.%. The interface of TiB2 phase and AlMgB14 phase was semi-coherent interface. The atomic mismatch of interface was 6.34%. There was a low interfacial energy structure between the two phases. The toughening mechanism of TiB2 in composite was crack deflection.The average friction coefficient of AlMgB14 bulk at room temperature was 0.5, and that of AlMg B14-30 wt.%TiB2 composite was 0.45. The friction coefficients of AlMgB14 and AlMgB14-30 wt.%TiB2 composite were similar. However, the average friction coefficient of AlMgB14-30 wt.%TiB2 composite at 300 °C was about 0.45, which was lower than that of AlMgB14 bulk of 0.65. The friction coefficient of AlMgB14-30 wt.%TiB2 composite increased with the increase of temperature below 800 °C. The friction coefficient of the composite changed in the range from 0.4 to 0.65. The wear mechanism was abrasive wear and adhesive wear. However, the friction coefficient of the composite at 800 °C decreased to 0.12 due to the self-lubrication of TiO2 film formed on the wear surface. The wear mechanism was oxidation wear.The oxidation kinetics curves of AlMgB14 bulk, AlMg B14-10 wt.%TiB2 composite and AlMgB14-30 wt.%TiB2 composite followed the compound law of line and curve. The oxidation rate of AlMgB14 bulk was maximum, while that of AlMgB14-30 wt.%TiB2 composite was minimum. The elevated temperature oxidation resistance of AlMg B14-10 wt.%TiB2 and AlMgB14-30 wt.%Ti B2 composite improved 2-4 time and 3-7 time compared with AlMgB14 bulk, respectively. AlMgB14-30 wt.%TiB2 composite had the most excellent elevated temperature oxidation resistance.The synthesis of Al MgB14-30 wt.%TiB2 composite and the connection between AlMg B14-30 wt.%Ti B2 composite and Nb(or and Mo) implemented synchronously using FDB technique at a temperature of 1400°C and pressure of 50 MPa. The thickness of the diffusion layer was 150~200μm.The main phase of diffusion layer formed between Al MgB14-30 wt.%Ti B2 composite and Nb was Nb B2. The main phase of diffusion layer formed between Al MgB14-30 wt.%TiB2 composite and Mo was MoB4. In comparison, the connection strength of Al Mg B14-30 wt.%TiB2 composite and Nb was higher than that of Al MgB14-30 wt.%Ti B2 composite and Mo.The oxidation results of TiB2/AlMgB14-TiB2 composite show that the phases formed in the oxide scale on the surface of TiB2 layer were mainly TiO and TiO2, which illustrated that Ti B2 layer was easily oxidized at both low temperature and high temperature and formed protective oxide scale on the surface. The adsorption energy of AlMgB14(010) surface and TiB2(110) surface was-18.42kJ/mol and 85.35kJ/mol, respectively. Thus, Ti B2(110) surface absorbed oxygen molecule preferentially in elevated temperature oxidation due to its stronger adsorptivity. |
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