A new synthesis reaction of Ti3SiC2 through pulse discharge sintering Ti/SiC/TiC powder

Ti₃SiC₂ was synthesized by pulse discharge sintering 4Ti/2SiC/TiC mixture powder in a temperature range of 1250–1450 °C. The purity of Ti₃SiC₂ was improved to 92 vol% at a sintering temperature of 1350 °C. The microstructure in the synthesized samples was controlled to be fine, coarse and duplex grains, depending on the sintering temperature and time.     

Ti3 SiC2 复合材料耐磨性能研究

Ti₃SiC₂是一种六方晶体结构的特殊陶瓷材料,兼具金属与陶瓷的优异性能,拥有优良的高温强度、抗氧化性及可加工性等优点,广泛应用于耐磨润滑材料。本文综述其同金属和SiC、金刚石、TiC、Al₂O₃等复合后的优异性能和广阔应用,并展望其在和金属、陶瓷、金刚石等材料复合领域的研究方向。      

Effects of TiC addition on formation of Ti3SiC2 by self-propagating high-temperature synthesis

Formation of Ti₃SiC₂ was conducted by self-propagating high-temperature synthesis (SHS) from both the elemental powder compacts of Ti:Si:C = 3:1:2 and the TiC-containing samples compressed from powder mixtures of Ti/Si/C/TiC with TiC content ranging from 4.3 to 33.3 mol%. The effect of TiC addition was studied on combustion characteristics and the degree of phase conversion. For the elemental powder compacts, with the progress of combustion wave the sample experiences substantial deformation, including axial elongation and radial contraction. The extent of sample deformation and flame-front propagation velocity were considerably reduced for the samples with TiC addition, because the dilution effect of TiC lowered the reaction temperature. Two reaction mechanisms were adopted to explain the formation of Ti₃SiC₂, one involving the reaction of a Ti–Si liquid phase with solid reactants for the elemental powder compact and the other dominated by the interaction of solid-phase species for the TiC-containing sample. For all products synthesized in this study, the XRD analysis identifies formation of Ti₃SiC₂ along with a major impurity TiC and a small amount of Ti₅Si₃. The resulting Ti₃SiC₂ is typically elongated grains. Based upon the XRD profile, the Ti₃SiC₂ content at a level of 71.5 vol.% was obtained in the product from the elemental powder compact. With the addition of TiC, an improvement in the yield of Ti₃SiC₂ was observed and an optimal conversion reaching 85 vol.% was achieved by the sample with 20 mol% of TiC. However, further increase of the TiC amount led to a decrease in the Ti₃SiC₂ content, because of the low reaction temperature around 1150 °C.     

High temperature oxidation behavior of Ti3SiC2-based material in air

The oxidation behavior of Ti₃SiC₂-based material in air has been studied from 900°C to 1200°C. The present work showed that the growth of the oxide scale on Ti₃SiC₂-based material obeyed a parabolic law from 900°C to 1100°C, while at 1200°C it followed a linear rule. The oxide scale was generally composed of an outer layer of coarse-grained TiO₂ (rutile) and an inner layer of fine-grained TiO₂ and SiO₂ (tridymite) above 1000°C. A discontinuous coarse-grained SiO₂ layer was observed within the outer coarse-grained TiO₂ layer on the samples oxidized at 1100°C and 1200°C. Marker experiments showed that the oxidation process was controlled by the inward diffusion of oxygen, outward diffusion of titanium and CO or SiO, and that internal oxidation predominated. The TiC content in Ti₃SiC₂ was deleterious to the oxidation resistance of Ti₃SiC₂.     

Reactions between Ti and Ti3SiC2 in temperature range of 1273-1573 K

The reactions of Ti₃SiC₂ and Ti in the temperature range of 1 273–1 573 K under a pressure of 20 MPa were investigated. The results confirm that Ti reacts with Ti₃SiC₂ above 1 273 K and new phases like TiC_x, Ti₅Si₃ and TiSi₂ are identified. The reactions are closely related to temperature and content of Ti₃SiC₂ in Ti. During the reaction process, Ti₃SiC₂ decomposes in two different modes. The first is caused by the de-intercalation of Si from it and the TiC_x is formed by the remained titanium and carbon; the second is that the carbon is separated from the Ti₃SiC₂ and reacts with titanium furthermore. The diffusing of silicon is believed to be the determinant ingredient of the reaction.     

Improved oxidation resistance of Ti with a thermal sprayed Ti3Al(O)–Al2O3 composite coating

A Ti₃Al(O)–Al₂O₃ in situ composite was explored as a coating system for Ti using thermal spray. Oxidation tests at 700–800 °C showed that this coating remarkably decreased the oxidation rate and increased the scale spallation resistance compared with Ti. The mechanisms for these improvements were then briefly discussed.     

Effect of starting powders size on the Al2O3–TiC composites

Al₂O₃–TiC composites with a content of 30 wt% TiC with various size of starting powders were manufactured by hot pressing. The Vickers hardness, bending strength and fracture toughness were studied. The experiment results show that the starting powder size has a significant effect on the properties of the Al₂O₃–TiC composites. The maximum bending strength of the submicron Al₂O₃ with the fine TiC powders addition is 712 MPa, while the maximum fracture toughness of the same Al₂O₃ matrix with the large TiC powders addition is 6.5 MPa m^1/2.     

微波烧结制备MAX相–金刚石复合材料

采用三元层状导电可加工陶瓷M_n+1AX_n（简称MAX相）材料粉体和金刚石粉体为原料,通过微波烧结制备以MAX相为结合剂的金刚石复合材料,研究MAX相的种类与金刚石含量对该复合材料的物相组成与显微形貌的影响。通过高温微波烧结MAX相-金刚石复合材料,金刚石表面会形成不同的涂层组织。MAX相的种类与金刚石含量对复合材料中基体组成和金刚石的表面涂层状态有显著影响。实验结果表明:在Ti₂SnC-金刚石复合材料中,烧结后Ti₂SnC会发生严重的分解,分解生成Sn与TiC,在含量较低时,表面仍然光滑,金刚石表面黏附少量富锡圆形组织;当金刚石含量较高时,金刚石表面形成许多细小TiC颗粒。在Ti₃AlC₂-金刚石复合材料中,Ti₃AlC₂分解后生成Al与TiC,金刚石的表面受到明显的侵蚀,同时在表面形成Al₄C₃和Al₂O₃二元组织。对于Ti₃SiC₂-金刚石反应体系,基体主相均为Ti₃SiC₂。当金刚石质量分数为10%时,同时还含有少量Si、TiSi₂和SiC;当金刚石质量分数为20%时,基体中还含有少量TiC,金刚石表面形成了SiC和TiSi₂二元涂层组织。      

Toughened ZrO2 ceramics sintered with a La2O3-rich glass as additive

In this study, the influence of the glass addition and sintering parameters on the densification and mechanical properties of tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO₂ powder and La₂O₃-rich glass were used as starting powders. Two compositions based on ZrO₂ and containing 5 wt.% and 10 wt.% of La₂O₃-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90 °C for 24 h, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200 °C, 1300 °C, 1400 °C for 60 min and at 1450 °C for 120 min, with heating and cooling rates of 10 °C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Hardness and fracture toughness were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Furthermore, only tetragonal-ZrO₂ was identified as crystalline phase in sintered samples, independently of the conditions studied. Samples sintered at 1300 °C for 60 min presented the optimal mechanical properties with hardness and fracture toughness values near to 12 GPa and 8.5 MPa m^1/2, respectively.     

立方氮化硼用量及粒度对其同钛铝碳结合剂的反应程度的影响

为研究CBN用量对Ti₃AlC₂结合剂CBN复合材料的影响,使用不同质量配比的Ti3AlC2粉体和CBN粉体通过放电等离子体烧结的方式制备试样,并对比其物相组成和显微形貌。结果表明:当CBN质量分数为10%时,试样的主相为Ti₃AlC₂、CBN和TiC;当CBN质量分数为20%~40%时,生成了TiC、TiN、AlN、TiB₂等物相。另一方面,当CBN质量分数为10%和20%时,CBN表面会形成厚约10 μm的过渡层;当CBN质量分数为30%和40%时,CBN与基体间没有过渡层。若选用粒度尺寸为10 μm的CBN（质量分数为10%）进行烧结,则复合材料中出现许多气孔,基体主相为TiC等轴晶粒且在CBN表面形成厚度1~2 μm的过渡层。CBN质量分数越大或粒度尺寸越小,其同Ti₃AlC₂的反应越充分、过渡层越薄。      

Fabrication and electrical and thermal properties of Ti2InC, Hf2InC and (Ti,Hf)2InC

In this paper we report on the characterization of predominantly single phase, fully dense Ti₂InC (Ti_1.96InC_1.15), Hf₂InC (Hf_1.94InC_1.26) and (Ti,Hf)₂InC ((Ti_0.47,Hf_0.56)₂InC_1.26) samples produced by reactive hot isostatic pressing of the elemental powders. The a and c lattice parameters in nm, were, respectively: 0.3134; 1.4077 for Ti₂InC; 0.322, 1.443 for (Ti,Hf)₂InC; and 0.331 and 1.472 for Hf₂InC. The heat capacities, thermal expansion coefficients, thermal and electrical conductivities were measured as a function of temperature. These ternaries are good electrical conductors with a resistivity that increases linearly with increasing temperatures. At 0.28 μΩ m, the room temperature resistivity of (Ti,Hf)₂InC is higher than the end members (0.2 μΩ m), indicating a solid solution scattering effect. In the 300 to 1273 K temperature range the thermal expansion coefficients are: 7.6×10⁻⁶ K⁻¹ for Hf₂InC, 9.5×10⁻⁶ K⁻¹ for Ti₂InC, and 8.6×10⁻⁶ K⁻¹ for (Ti,Hf)₂InC. They are all good conductors of heat (20 to 26 W/m K) with the electronic component of conductivity dominating at all temperatures. Extended exposure of Ti₂InC to vacuum (10⁻⁴ atm) at 800 °C, results in the selective sublimation of In, and the conversion of Ti₂InC to TiC_x.     

Infrared brazing of Ti50Ni50 shape memory alloy using pure Cu and Ti–15Cu–15Ni foils

Infrared brazing of Ti₅₀Ni₅₀ using two brazing filler metals was investigated in the study. Three phases, including Cu-rich, CuNiTi (Δ) and Ti(Ni,Cu), were observed in the Ti₅₀Ni₅₀/Cu/Ti₅₀Ni₅₀ joint after brazing at 1150 °C. The Cu-rich phase was rapidly consumed in the first 10 s of brazing, and the eutectic mixture of CuNiTi and Ti(Ni,Cu) phases were subsequently observed in the joint. Samples brazed for longer time resulted in less CuNiTi and more Ti(Ni,Cu) phases in the joint. The existence of CuNiTi phase deteriorated the shape memory effect of the joint, but Ti(Ni,Cu) could still preserve shape memory behavior even alloyed with a large number of Cu. Therefore, higher shape recovery ratio was observed for specimens brazed for a longer time period. Extensive presence of Ti₂(Ni,Cu) phase was observed in Ti₅₀Ni₅₀/Ticuni^®/Ti₅₀Ni₅₀ joint upon brazing the specimens up to 1150 °C. The bending test could not be performed due to the inherent brittleness of Ti₂(Ni,Cu) matrix. Moreover, the stable Ti₂(Ni,Cu) phase was difficult to be removed completely by increasing either brazing time and/or temperature.     

TIG熔敷Ti-Si-C系统在Ti-5Al-2.5Sn表面形成陶瓷涂层的化学反应分析

采用TIG热源在钛合金表面堆焊Ti-SiC和Ti-SiC-C两种体系的粉末,形成表面陶瓷涂层.利用SEM和XRD等分析手段对两种配方陶瓷涂层的微观组织和物相进行了分析,用热力学方法计算了两种系统中各种可能发生的化学反应的Gibbs自由能变化.结果表明,Ti-SiC系统熔敷层的微观组织主要由树枝状的TiC、针棒状和菊花状的Ti₅Si₃相组成,其反应机理为:8Ti+3SiC→3TiC+Ti₅Si₃.Ti-SiC-C系统熔敷层的微观组织除了树枝状的TiC相之外,还有TiSi₂和Ti₃SiC₂,其反应机理为:6Ti+3SiC+C→Ti₃SiC₂+TiSi₂+2TiC.通过对两种系统的微观组织及化学反应分析可得出,在Ti-SiC系统中适当的添加石墨,可生成具有自润滑性能的Ti₃SiC₂相,避免Ti₅Si₃脆性相的生成.     

Fracture and creep of an Al2O3-SiC (whisker)-TiC (particle) composite

High-temperature fracture strength and compressive creep of an electrodischarge-machinable composite, Al₂O₃-30.9 vol.% SiC whiskers-23 vol.% TiC particles have been studied to 1200 °C and 1450 °C, respectively, in inert atmosphere. Microstructures of fractured and deformed specimens were examined using scanning and transmission electron microscopy. Fast fracture occurred at T ≤ 1200 °C. Steady-state creep was achieved for T > 1350 °C at stresses < 80 MPa, with the rate-controlling mechanism being partially unaccommodated grain-boundary sliding, with a stress exponent of ≈ 1 and an activation energy of≈ 470 kJ/mol.     

Temperature and loading time dependence of hardness of LaB6, YB6 and TiC single crystals

The temperature and loading time dependence of Vickers’ microhardness of LaB₆, YB₆ and TiC single crystals prepared by the floating zone method have been investigated under various temperatures from 25 to 1000 °C and loading time 5–60 s. Clear inflection points at 700 °C for LaB₆ and YB₆, and at 600 °C for TiC can be seen in the plot of log hardness versus the temperature. A series of straight lines with various slopes were obtained in the plot of log hardness versus log loading time of all samples. As the temperature is above the inflection temperature, the present data confirm our previous observation of ZrB₂ single crystals that .     

Mechanical alloying and spark plasma sintering of the intermetallic compound Ti50Al50

This study investigates the microstructures and mechanical properties of Ti₅₀Al₅₀ alloys prepared via mechanical alloying (MA) starting from elemental powders. The process of the spark plasma sintering (SPS) has also been studied. It is found that the nanocrystallization process of the Ti–Al alloy proceeds and the sintering temperature can control the microstructure of alloy. The sintering of the compacts is carried out at the temperatures of 1100–1200 °C with a compaction pressure of 30 MPa and a heating rate of 30 °C min⁻¹. Specimens with high densities and approaching the equilibrium state can be obtained in short time by spark sintering than conventional sintering. Such shorter high temperature is important to prevent grain growth.     

Ti3AlC2掺杂SPS法制备Ti2AlC/TiAl复合材料的研究

通过2TiC-Ti-1.2Al体系的原位热压反应制备了Ti₃AlC₂陶瓷,然后以59.2Ti-30.8Al-10Ti₃AlC₂(wt%)为反应体系,采用放电等离子烧结技术制备出Ti₂AlC/TiAl基复合材料。借助XRD、SEM分析了产物的相组成和微观结构,并测量了其室温力学性能。结果表明：原位热压烧结产物由Ti₃AlC₂和TiC相组成,Ti₃AlC₂呈典型的层状结构,TiC颗粒分布在其间。SPS法制备的Ti₂AlC/TiAl基复合材料主要由TiAl、Ti₃Al和Ti₂AlC相组成,Ti₂AlC增强相主要分布于基体晶界处,表现为晶界/晶内强化作用。力学性能测试表明：Ti₂AlC/TiAl基复合材料的密度、维氏硬度、断裂韧性和抗弯强度分别为3.85 g/cm₃、5.37 GPa、7.17 MPa?m_1/2和494.85 MPa。     

Development of moderate temperature CVD Al2O3 coatings

Chemically vapor deposited Al₂O₃ coatings, due to their high hardness and chemical inertness, are currently the state of art in the cutting tool industry. The conventional high deposition temperature of about 1050 °C for Al₂O₃ coatings, based on the water–gas shift process, has to a great extend restricted the development of several hybrid coatings, such as TiC/TiN/TiCN/Al₂O₃. To overcome this limitation, alternate systems to deposit Al₂O₃ at moderate temperatures have been investigated. Systems using NO–H₂, H₂O₂, NO₂–H₂ and HCOOH were identified and thermodynamic calculations were performed to evaluate them as potential sources of oxygen donors to form Al₂O₃ in the moderate temperature range of 700–950 °C. Preliminary results have clearly demonstrated that it is possible to grow moderate temperature alumina (using such alternate sources) on the TiC/TiN coated cemented carbide substrates.     

Microstructure and mechanical properties of mechanically alloyed and spark plasma sintered amorphous–nanocrystalline Al65Cu20Ti15 intermetallic matrix composite reinforced with TiO2 nanoparticles

Multiphase Al₆₅Cu₂₀Ti₁₅ intermetallic alloy matrix composite, dispersed with 10 wt.% of TiO₂ nanoparticles, has been processed by mechanical alloying, followed by spark plasma sintering under pressure in the temperature range of 623–873 K. Differential scanning calorimetry and X-ray diffraction suggest that equilibrium crystalline phases evolve from the amorphous or intermediate crystalline phases. Transmission electron microscopy shows that the composite sintered at 873 K has partially amorphous microstructure, with dispersion of equilibrium, crystalline, intermetallic precipitates of Al₅CuTi₂, Al₃Ti, and Al₂Cu of 25–50 nm size, besides the TiO₂. The composite sintered at 873 K exhibits little porosity, hardness of 5.6 GPa, indentation fracture toughness in the range of 3.1–4.2 MPa√m, and compressive strength of 1.1 GPa. Indentation crack deflection by TiO₂ particle aggregates causes increase in fracture resistance with crack length, and suggests R-curve type behaviour. The study provides guidelines for processing high strength amorphous–nanocrystalline intermetallic composites based on the Al–Cu–Ti ternary system.     

The ternary system Na2O–ZnO–WO3: Compounds and phase relationships

The subsolidus phase relationships of ternary system Na₂O–ZnO–WO₃ have been investigated by X-ray diffraction (XRD) and differential thermal analyzer (DTA). All the samples were synthesized in the temperature range from 530 to 850 °C in air. There are one ternary compound and five binary compounds in the Na₂O–ZnO–WO₃ system, which can be divided into eight three-phase regions. The crystal structure of the ternary compound Na_3.6Zn_1.2(WO₄)₃ is determined by single-crystal structure analysis method. It belongs to triclinic system with space group and lattice constants a = 7.237 (5) Å, b = 9.172 (6) Å, c = 9.339 (6) Å and  = 94.920 (4)°, β = 105.772 (9)°, γ = 103.531 (8)°, Z = 2. DTA analyses indicate that the compound Na₂WO₄ is not suitable to be the flux for ZnO crystal growth below 1250 °C, since no liquidus was observed in the system before 1250 °C.