Mechanical properties and microstructure of TiB2 ceramic influenced by ZrB2 additive

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Microstructural development and mechanical properties of reactive hot pressed nickel-aided TiB2-SiC ceramics

TiB₂-SiC composites with different amounts of Ni (0, 2 and 5 wt.%) added as sintering aid were fabricated by reactive hot pressing (RHP). The mechanical properties were assessed under ambient conditions and the flexural strength was further tested in the temperature range of 700–1000 °C. The microstructures of the composites were characterized by a scanning electron microscope (SEM), transmission electron microscope (TEM) and energy-dispersive spectrometer (EDS). The flexural strength degradation mechanism occurring at elevated temperatures was studied. Addition of a moderate amount of Ni led to an improvement of the mechanical properties at room temperature. For the investigated ceramic composites, TiB₂-SiC-5 wt.% Ni sample showed significantly enhanced mechanical properties, i.e., a flexural strength of 1121 ± 31 MPa, a fracture toughness of 7.9 ± 0.58 MPa·m^1/2, a hardness of 21.3 ± 0.62 GPa, and a relative density of 98.6 ± 1.2%. Ni distributed along grain boundaries improved the interface strength. The improved fracture toughness was ascribed to crack deflection, grain rupture and crack shielding effect of Ni. A substantial strength degradation occurred at elevated temperatures, which was attributed to softening of the grain boundaries, surface oxidation and sliding of grain boundaries. The elastic modulus was found to decrease with increasing temperature.     

Characterization of hot pressed SiC whisker reinforced TiB2 based composites

TiB₂–SiC ceramic composites, with different contents of SiC whiskers (SiC_w), as a ceramic sinter-additive, were prepared by the hot pressing process at 1850 °C for 2 h under a pressure of 20 MPa. For comparison, a monolithic TiB₂ ceramic was also fabricated under the identical temperature, pressure, atmosphere, and holding time by the hot pressing process. The effects of fabrication process and SiC whiskers on microstructural features, phase evolution and mechanical properties were investigated. Hardness measurements revealed an initial increase in hardness for TiB₂–SiC compared to TiB₂. Also the improvement of the fracture toughness was attributed to the toughening and strengthening effects of SiC whiskers such as crack deflection. The results showed that promoted densification of TiB₂–SiC ceramic composites is due to addition of SiC whiskers and reduction of oxide impurities by reacting with SiC whiskers and removing them from the surface layer of TiB₂ particles. The reaction between TiB₂ particles and SiC whiskers led to in-situ formation of TiC phase in the matrix as well. In general, it is concluded that the sinterability of TiB₂-based composites was remarkably improved by introducing SiC whiskers compared to the single phase TiB₂ ceramic.     

Anisotropic elastic constants and thermal expansivities in monocrystal CrB2, TiB2, and ZrB2

The elastic constants and thermal expansivities in monocrystals of three transition-metal diborides with the AlB₂ structure, CrB₂, TiB₂, and ZrB₂, have been investigated in the temperature ranges from 300 to 1373 K and from 300 to 1073 K. The anisotropic parameters deduced from the measured elastic constants and thermal expansivities indicate that of the three diborides, the anisotropy is the most and least significant in CrB₂ and ZrB₂, respectively. The factors determining the significance in anisotropy in atomic bonding in AlB₂-type diborides are analyzed by an approach similar to the valence-force-field method and are discussed in terms of the deformation of the electronic charge around the metal atoms occurring to fit themselves in the (0 0 0 1) basal plane.     

热压C-SiC-B4C-TiB2复合材料的组织与力学性能

以磷片石墨Cfg,SiC,B4C和TiO2为原料,热压合成C-SiC-B4C-TiB2复合材料,研究不同Cfg含量和热压温度对复合材料显微组织和力学性能的影响规律.结果表明烧结过程中TiO2与B4C反应原位生成TiB2;复合材料的密度和抗弯强度随着热压温度的升高而增加,却随着Cfg含量的增加而降低,随着热压温度的升高和Cfg含量的增加,复合材料的断裂韧性则提高;在2 000 ℃,25 MPa下热压时,Cfg含量为20%(质量分数)的复合材料其体积密度为2.81 g/cm3,抗弯强度为236.7 MPa,断裂韧性为5.3 Mpa·m1/2,Cfg含量为65%含量的复合材料的体积密度为2.42 g/cm3、抗弯强度为103.6 MPa、断裂韧性为8.1 Mpa·m1/2;复合材料的致密化程度和陶瓷晶粒随热压温度的升高而增大,复合材料中Cfg层状分布结构随Cfg含量的增加更加明显;复合材料中Cfg弱界面分层诱导韧化作用及第二相TiB2和陶瓷基体热膨胀系数不匹配所产生的残余应力导致的裂纹偏转作用是复合材料断裂韧性提高的主要原因.     

Microstructure and mechanical properties of hot isostatically pressed cermets with TiN coatings

To increase the adhesion strength between the coating and the substrate, sintered Ti（C,N）-based cermets were selected and deposited with monolayer TiN using a multiarc ion-plating technique; subsequently, hot isostatic pressing （HIPhag） treatment was performed at 1000℃ using nitrogen pressure up to 110 MPa. The mechanical properties of cermets after a coating process and subsequent HIPing treatment have been evaluated with respect to the hardness, the residual stress, and the coating adhesion. The results show that atter the HIPing process, there was a higher increase ha critical load ha the TiN-coated cermets with lower surface roughness compared with those with higher surface roughness. In all cases, the residual stress was found to be compressive. The effects of substrate surface roughness and posttreatment on the adhesion strength of the coatings were thus investigated. It was also fotmd that the HIPing posttreatment process is well suited for hacreasing the adhesion strength between the coating and the substrate.     

A model for the oxidation of ZrB2, HfB2 and TiB2

A mechanistic model that interprets the oxidation behavior of the diborides of Zr, Hf and Ti in the temperature range of ∼1000–1800 °C was formulated. Available thermodynamic data and literature data for vapor pressures and diffusivities were used to evaluate the model. Good correspondence was obtained between theory and experiments for weight gain, recession and scale thickness as functions of temperature and oxygen partial pressure. At temperatures below about 1400 °C, the rate-limiting step is the diffusion of dissolved oxygen through a film of liquid boria in capillaries at the base of the oxidation product. At higher temperatures, the boria is lost by evaporation, and the oxidation rate is limited by Knudsen diffusion of molecular oxygen through the capillaries between nearly columnar blocks of the oxide, MO₂.     

原位TiB增强ZrB2-SiC接头的界面组织和力学性能

以Ag-Cu共晶箔和Ti箔的叠层箔片为中间层,实现了ZrB2-SiC复合陶瓷自身的连接。扫描电镜和能谱分析表明:液态钎料中的Ti能够与ZrB2反应,在陶瓷表面原位生成了具有定向分布的TiB晶须,接头的典型界面结构为ZS/TiB(Ag(s,s))/TiCu(AgCu4Zr)/Ag(s,s)/TiCu(AgCu4Zr)/TiB(Ag(s,s))/ZS。研究了不同温度下接头的组织演化规律,发现ZrB2在液态钎料中的分解温度为860℃,TiB晶须的生成温度为880℃。接头在900℃下保温10 min获得最高抗剪强度134 MPa。较高的强度主要得益于原位TiB晶须阵列对陶瓷表面应力的调节,以及形成陶瓷向焊缝中心的梯度过渡。     

Sliding wear evaluation of hot isostatically pressed thermal spray ceramet coatings

The principal aim of this study was to compare the sliding wear performance of as-sprayed and Hot Isostatically Pressed (HIPed) thermal spray cermet (WC-12Co) coatings. Results indicate that HIPing technique can be successfully applied to post-treat thermal spray cermet coatings for improved sliding wear performance, not only in terms of coating wear, but also in terms of the total volume loss for test couples. WC-12Co coatings sprayed by a HVOF system were deposited on SUJ-2 bearing steel substrate and then encapsulated and HIPed at 850 °C for one hour. A high frequency reciprocating ball on plate rig was used to measure the sliding wear resistance of these coatings in dry conditions under steel and ceramic contact configurations at two different loads. Results are discussed in terms of coating microstructure, microhardness, fracture toughness and residual stress evaluations. Microstructural investigations indicate fundamental changes in grain morphology, whereas x-ray diffraction revealed beneficial transformations in phase composition of these coatings during the HIPing post treatment. The effects of these microstructural changes on the physical properties and wear resistance are discussed.     

TiB2和TiB弹性性质的理论计算

采用基于密度函数理论的赝势平面波方法和广义梯度近似对TiB2和TiB化合物的弹性性质和电子结构进行了理论计算,并用Voigt-Reuss-Hill方法计算得到多晶体的弹性模量和切变模量。结果表明：TiB2和TiB的弹性模量分别为599 GPa和443 GPa,切变模量分别为268.5 GPa和193.5 GPa,Pugh定律和泊松比等经验判据表明TiB2比TiB脆性更大;并从这两种化合物的电子结构对其弹性性质的差异进行了讨论。     

热挤压原位合成TiB_2/6351Al复合材料的组织性能

利用金相显微镜、扫描电镜、透射电镜和万能拉伸试验机等测试手段,研究了原位合成TiB2(wt,8%)/6351Al复合材料在热挤压前后的显微组织及室温拉伸性能。结果表明,热挤压变形有助于增强颗粒在基体合金中均匀分布,热挤压后TiB2颗粒与基体界面结合良好,未发现界面处开裂;热挤压变形时TiB2颗粒周围的基体合金中形成复杂的位错;基体合金发生再结晶和回复形成完整的等轴晶和亚结构,显微组织得到细化,基体合金再结晶的主要形核方式为增强体颗粒引起位错塞积区形核,亚晶吞并长大形核及应变诱发晶界迁移形核。热挤压复合材料基体合金具有较强的[111]织构。与铸态相比,热挤压后复合材料的屈服强度Rp0.2、抗拉强度Rm、伸长率A及布氏硬度显著提高。复合材料断口特征由热挤压前的韧性和沿晶的混合断裂,转变为以韧性断裂为主。     

Mechanical properties of Ti-6Al-4V/TiB composites with randomly oriented and aligned TiB reinforcements

In situ Ti-6Al-4V/TiB discontinuously reinforced composites, containing 20 and 40% of TiB whiskers by volume, were produced by blending Ti, Al/V, and TiB₂ powders. The consolidated powder blends were annealed to transform the TiB₂ particles to TiB. The microstructural evolution of the composite was studied as a function of heat treatment duration at 1100, 1200, 1300 and 1400 °C. The mechanical properties of Ti-6Al-4V/TiB composites were established in tension and compression at room temperature and 300 °C, and by resonant ultrasound spectroscopy (RUS), for the two volume fractions of TiB, and for randomly oriented and aligned arrays of TiB whiskers. The average Young’s modulus of the composite with 20% of randomly oriented TiB whiskers was 153 GPa, compared to 109 GPa for unreinforced Ti-6Al-4V. The average Young’s modulus of composites with 20 and 40% of aligned TiB whiskers was measured along the extrusion axis as 169 and 205 GPa, respectively. The stiffness of TiB whiskers was determined from bulk measurements with the Halpin-Tsai equation to be 482 GPa. Yield and ultimate strengths near 1200 MPa were measured. The strength and ductility of the materials were limited in the present study by non-optimal matrix microstructure and inadequate particulate distribution, and approaches for properties improvements are provided.     

热压烧结工艺对超韧Al2O3-TiC-Co复合陶瓷性能的影响

热压烧结化学镀法获得的Al2O3-Co TiC-Co复合粉体制备超韧Al2O3-TiC-Co复合陶瓷.探讨了烧结温度、烧结气氛和分级保温时间对复合陶瓷微观形貌和力学性能的影响.结果表明,烧结温度为1550℃、真空保护并在750℃和1200℃各保温10 min制备的复合陶瓷力学性能最佳.采用最佳烧结工艺制备的Al2O3-TiC-8%Co复合陶瓷的平均抗弯强度和断裂韧性分别达到782MPa和8.0 MPa·m1/2.     

ZrB2基层状复合材料的制备与性能研究

以SiC颗粒和纳米SiC晶须复合增韧的ZrB2为基体层，以金属Mo为界面层，采用轧膜成型和热压烧结的方法，在1950℃，1h，25MPa压力／Ar气氛的条件下，成功制备了ZrB2／Mo层状复合材料。结果表明：制备的ZrB2／Mo层状复合材料的室温断裂韧性可达9．3±0．21MPa·m^1/2：通过对Mo界面层的合金化可使其抗弯强度达到400±36MPa，并且减弱了Mo层的室温脆化，克服了层状材料开裂现象。其主要增韧机制包括裂纹分叉钝化、裂纹偏转、裂纹沿界面层并行扩展等。Mo与ZrB2基体层发生界面反应生成MoB，ZrB以及Mo5SiB2，从而形成了强结合界面，影响了层状结构强韧化优越性的发挥。     

ZrB2基层状复合材料的制备与性能研究

以SiC颗粒和纳米SiC晶须复合增韧的ZrB2为基体层,以金属Mo为界面层,采用轧膜成型和热压烧结的方法,在1950℃,1 h,25 MPa压力/Ar气氛的条件下,成功制备了ZrB2/Mo层状复合材料.结果表明:制备的ZrB2/Mo层状复合材料的室温断裂韧性可达9.3±0.21 MPa·m1/2;通过对Mo界面层的合金化可使其抗弯强度达到400±36 MPa,并且减弱了Mo层的室温脆化,克服了层状材料开裂现象.其主要增韧机制包括裂纹分叉钝化、裂纹偏转、裂纹沿界面层并行扩展等.Mo与ZrB2基体层发生界面反应生成MoB,ZrB以及Mo5SiB2,从而形成了强结合界面,影响了层状结构强韧化优越性的发挥.     

热喷涂制备TiB2涂层的研究进展

TiB2具有优良的性能和潜在的应用前景,采用粉末冶金制备致密块体材料的工艺复杂、成本高昂,限制了它的应用.涂层技术则是简化制备工艺、降低制造成本的有效手段.本文综述了热喷涂制备TiB2涂层的工艺特点以及TiB2物理化学特性对热喷涂制备工艺的影响,提出通过粉体复合技术改善TiB2粉体的物性,满足热喷涂工艺要求,从而获得高质量涂层的措施.     

Effect of ZrB2 and SiC addition on TiB2-based ceramic composites prepared by spark plasma sintering

TiB₂-based ceramic composites with different amounts of ZrB₂ and SiC were prepared by spark plasma sintering at 1700 °C with an initial pressure of 40 MPa and a holding time of 10 min. The (Ti_xZr_y)B₂ solid solution was found in the sintered TiB₂/ZrB₂/SiC composites by XRD. The microstructural and mechanical properties of the prepared samples were investigated. The composite with the addition of 30 vol.% ZrB₂ shows better comprehensive performances, and the bending strength and the fracture toughness of the composite are 780.5 MPa and 7.34 MPa m^1/2, respectively. The generation of the (Ti_xZr_y)B₂ solid solution makes the microstructures of the composites finer and more homogeneous, which has played a very important role in grain refinement and interface fusion.     

Preparation and properties of hot-pressed NbMo-matrix composites reinforced with ZrB2 particles

The NbMo-matrix composites reinforced with (0–60 vol%) ZrB₂ were fabricated by hot-pressing at 2400 °C for 10 min under a pressure of 50 MPa in dynamic vacuum in the induction heating furnace specially designed in our institute. The optimum ZrB₂ content in NbMo solid solution was determined to be 30 vol% for excellent comprehensive mechanical property. NbMo-30 vol% ZrB₂ has the highest density of 99.63%, the most uniform microstructure, high fracture toughness of 5.75 MPa m^1/2. The highest ZrB₂ concentration that reacts with NbMo solid solution is at the range of 30 to 45 vol%. The types of the formed niobium borides were decided by the original ratio of Nb to B. The distribution of Mo and Zr was mutually exclusive in low ZrB₂ content composites, however, there was Mo₂Zr in high ZrB₂ content composite. Except for NbMo-45 vol% ZrB₂, the compressive strength increased with ZrB₂ content (from 927.09 MPa to 1635.91 MPa). The Young's modulus values were directly proportional to ZrB₂ content. The fracture toughness (from 6.34 MPa m^1/2 to 3.99 MPa m^1/2) was inversely proportional to ZrB₂ content. The big residual ZrB₂ particles in high ZrB₂ content samples such as NbMo-45 vol% ZrB₂ and NbMo-60 vol% ZrB₂ was the main reason for nonhomogeneous microstructure, low density (94.09% and 94.83%, respectively) and low fracture toughness (4.58 MPa m^1/2 and 3.99 MPa m^1/2, respectively).     

Preparation and some properties of nanocrystalline ZrB2 powders

Nanocrystalline ZrB₂ has been prepared by the reaction of ZrCl₄ with NaBH₄ in the temperature range of 500–700 °C. The microstructure of the powders consists of uniform particles about 20 nm in size. The UV–vis absorption spectra and thermal oxidation behavior of the products are studied.