Arc jet testing and evaluation of Mo–Si–B coated Mo and SiC–ZrB2 ceramics

The transition from blunt leading edges to sharp leading edges on re-entry aircrafts is necessary to increase both maneuverability and safety. However, the oxidation resistance of current materials is inadequate for the extreme conditions experienced by sharp leading edge re-entry vehicles. The Mo–Si–B alloy system has been utilized to design a multilayer coating that has the ability to protect from 800 to 1700 °C. Substrates of Mo and ZrB₂–50 vol% SiC with a flat profile were coated with the Mo–Si–B based coating and evaluated using arc jet testing performed at NASA Langley Research Center. Heat fluxes of 2.5 to nearly 3.5 MW/m² and surface temperatures of 1500–1650 °C were achieved during the 20-min tests. The samples presented in this study showed <3% mass loss and retention of sample shape and integrity, demonstrating the robust environmental protection under a simulated hypersonic environment offered by the Mo–Si–B based coating on refractory metals and ceramics.     

Surface Roughness Investigation in the Hard Turning of Steel Using Ceramic Tools

The present work evaluates the use of conventional and multi-radii ceramic tools in the turning of AISI H13 in terms of surface quality. The selected design factors are the cutting speed, feed rate, and type of tool. To carry out this research, a design of experiments was used. The obtained results were analyzed using the analysis of variance (ANOVA), which allowed recognition of the influence of the feed rate and type of tool on the surface quality. By contrast, the cutting speed was found to be a factor that had no statistical significance. When comparing the experimental with the theoretical values, the results of the conventional and multi-radii tools were found to differ. In addition, it was highlighted that the feed rate has an important influence on the difference between the experimental and theoretical results.     

Introduction of novel kinetic approach to calculation of activation energy and its application to the sinter-crystallization of strontian feldspar

The kinetics, the mechanism and the thermodynamics of activated state of formation of primary strontian feldspar via sinter-crystallization of non-equilibrium melt during the thermal treatment of ceramic body was investigated in this work via differential thermal analysis using isoconversional Kissinger kinetic equation. The process of formation of non-equilibrium melt and subsequent crystallization of primary strontian feldspar requires the activation energy of 631±3 and 664±2 kJ mol⁻¹, respectively. The investigation of mechanism of formation of primary strontian feldspar reveals that the process is driven by the surface nucleation and diffusion controlled growth of the new phase. The nucleation rate decreases with the time of process and non-equilibrium melt can be formed only in metastable equilibrium with activated state of strontian feldspar. Deep consideration of kinetic data leads to the deduction of new kinetic approach that enables single calculation of activation energy and frequency factor of heterogeneous processes as well as the dependence of thermodynamic parameters of activated state on temperature. Further consideration of kinetic data reveals that the activation energy is directly proportional to the function of csch (z)+1. For z=e, this term enables to derive the value for the parameter B(x) in empirical equation for Arrhenius temperature integral p(x) proposed by Doyle to be 1.0642.     

Modeling coupled heat and mass transfer during drying in tape casting with a simple ceramics–water system

In many industrial processes such as in tape casting for electronics or in the food industry, drying is one of the determining physical phenomena. In this study, the evaporation of water from a ceramic–water mixture is investigated with the purpose of understanding the drying rate in the drying process of thin sheets produced by the tape casting process. The rate of mass loss in the drying process is a key factor that often is of interest, as it affects the final properties of the tapes. The 1D heat conduction equation is solved numerically to obtain the temperature field in a ceramic sheet. The change in the concentration of the water content is then used as the driving force for diffusive mass transport of the water. Mass–averaged thermal properties are assumed for the ceramic–water mixture in the initial stage. As the water evaporates, the thermal properties of the solid ceramic become more dominant since the fraction of water approaches zero. The developed model is used to simulate a simple test for the drying process. The drying rate is simply calculated by examining the water content in each time step. It is found that the mass loss due to the evaporation is increasing close to linearly with the drying time corresponding to an almost constant drying rate. However, the rate starts to decrease after some time in the simulation. It is also shown that too extensive surface drying results in a slow diffusion rate from the bottom, which in turn reduces the drying rate in general and hence is not favorable from a process viewpoint.     

Joining partially-sintered alumina ceramics using a mixture slurry of alumina sol and suspension

The joining of advanced ceramics allows the manufacture of components with a range of complex shapes that cannot be achieved in a cost-effective manner using existing techniques, i.e. green state shaping and/or machining. A new technique for joining partially-sintered alumina ceramics was developed by simply using a mixed slurry of Al₂O₃ sol and suspension. The interlayer of the joints had the same composition as the parent bodies, and the mechanical and chemical properties of the joint were comparable to those of the bulk material. This process can be applied to the joining of a variety of advanced ceramics.     

Low‐Temperature Rapid Synthesis of Rod‐Like ZrB2 Powders by Molten‐Salt and Microwave Co‐Assisted Carbothermal Reduction

A novel molten‐salt and microwave coassisted carbothermal reduction (termed as MSM‐CTR) method was developed to prepare ZrB₂ powders from raw materials of ZrO₂, B₄C, and amorphous carbon. The results indicated that the carbothermal reduction reaction for synthesizing ZrB₂ was initiated at the temperature as low as 1150°C, and phase pure ZrB₂ powders were obtained after only 20 min at 1200°C, which were significantly milder than that of the conventional CTR method as well as the modified CTR method even using active metal as additional reducing agents. More interestingly, the as‐obtained ZrB₂ powders consisted of well‐defined single‐crystalline nanorods, which had diameters of 40–80 nm and high aspect ratios of >10. These results demonstrated that the MSM‐CTR is a simple and efficient route for preparation of high‐quality ZrB₂ powders.     

Effect of C particle size on the mechanism of self-propagation high-temperature synthesis in the Ni-Ti-C system

Effect of C particle size on the mechanism of self-propagation high-temperature synthesis (SHS) in the Ni-Ti-C system was investigated. Fine C particle resulted in a traditional mechanism of dissolution-precipitation while coarse C particle made the reaction be controlled by a mechanism of the diffusion of C through the TiC_x layer. The whole process can be described: C atoms diffusing through the TiC_x layer dissolved into the Ni-Ti liquid and TiC were formed once the liquid became supersaturated. Simultaneously, the heat generated from the TiC formation made the unstable TiC_x layer break up. However, with the spread of Ti-Ni liquid, a new TiC_x layer was formed again at the interface between spreading liquid and C particle. This process cannot stop until all the C particles are consumed completely.     

异种金属摩擦焊工艺温度场数值模拟研究

基于摩擦焊接过程中热-力学现象,建立摩擦焊接过程中热力耦合模型,对陶瓷/金属焊接过程中温度分布进行数值模拟。所提出的模型能够预测金属陶瓷摩擦焊接过程中随时间增量的温度分布情况;焊接界面区域产生的摩擦热消耗中间层铝,并在氧化铝和低碳钢之间建立焊接层。由于氧化铝和低碳钢具有不同的温度属性,在界面处会产生更多的热应力。数值模拟用来预测氧化铝/低碳钢接口处残余应力的变化情况,进而避免异种金属摩擦焊接过程中不完全联锁、接头强度差的现象。     

Synthesis of hollow sphere-like mesoporous silica with reformer naphtha as a swelling agent

Hollow sphere-like and other shaped mesoporous silicas were prepared using reformer naphtha as a swelling agent via a hydrothermal synthesis process. The mesoporous silicas underwent a complicated process in which their morphology was transformed from a red blood cell-like shape to a golf-ball sphere-like shape, and then from a golf-ball-like shape to a hollow sphere-like shape in response to various synthesis temperatures. Under the control of using reformer naphtha, the mesoporous silica was transformed from a wheat-like shape to a sphere-like shape, and then from a sphere-like shape to a hollow sphere-like shape. The effects of temperature and the swelling agent may have contributed to the formation of reformer naphtha microemulsions in the copolymer aqueous solution, followed by the self-assembly of block copolymer micelles and silica around the emulsions.     

Combining in situ HT-ESEM observations and dilatometry: An original and fast way to the sintering map of ThO2

The sintering of ThO₂ pellets prepared from the initial precipitation of thorium oxalate was investigated by the means of HT-ESEM observations and dilatometric measurements. On the one hand, the use of environmental microscope allowed the in situ observation of the pellet behaviour during heat treatments between 1250 °C and 1400 °C. Subsequent image analysis led to the determination of local (i.e. at the grain scale) and global (i.e. at the pellet scale) kinetic parameters. Particularly, the average grain size was plotted versus the holding time for the three considered temperatures. On the other hand, analogous experiments were performed by dilatometry and led to monitor both linear shrinkage and density of the samples. On this basis, the combination of the two sets of data allowed us to establish for the first time a sintering map for ThO₂. This latter clearly evidenced two different zones driven by densification (d ≤ 92%) then by grain growth (d ≥ 92%) which can be used efficiently to monitor the final microstructure of sintered thorium oxide.