High-performance grinding of ceramic matrix composites

Ceramic matrix composites(CMCs)are highly promising materials for the next generation of aero-engines.However,machining of CMCs suffers from low efficiency and poor surfacefinish,which presents an obstacle to their wider application.To overcome these problems,this study investigates high-efficiency deep grinding of CMCs,focusing on the effects of grinding depth.The results show that both the sur-face roughness and the depth of subsurface damage(SSD)are insensitive to grinding depth.The material removal rate can be increased sixfold by increasing the grinding depth,while the surface roughness and SSD depth increase by only about 10%.Moreover,it is found that the behavior of material removal is strongly dependent on grinding depth.As the grinding depth is increased,fibers are removed in smaller sizes,with thefiber length in chips being reduced by about 34%.However,too large a grinding depth will result in blockage by chip powder,which leads to a dramatic increase in the ratio of tangential to normal grinding forces.This study demonstrates that increasing the depth of cut is an effective approach to improve the machining efficiency of CMCs,while maintaining a good surfacefin-ish.It provides the basis for the further development of high-performance grinding methods for CMCs,which should facilitate their wider application.

EFFECT OF FIBER FAILURE ON QUASI-STATIC UNLOADING/RELOADING HYSTERESIS LOOPS OF CERAMIC MATRIX COMPOSITES

The two-parameter Weibull model is used to describe the fiber strength distribution.The stress carried by the intact and fracture fibers on the matrix crack plane during unloading/reloading is determined based on the global load sharing criterion.The axial stress distribution of intact fibers upon unloading and reloading is determined based on the mechanisms of fiber sliding relative to matrix in the interface debonded region.The interface debonded length,unloading interface counter slip length,and reloading interface new slip length are obtained by the fracture mechanics approach.The hysteresis loops corresponding to different stresses considering fiber failure are compared with the cases without considering fiber failure.The effects of fiber characteristic strength and fiber Weibull modulus on the fiber failure,the shape,and the area of the hysteresis loops are analyzed.The predicted quasi-static unloading/reloading hysteresis loops agree well with experimental data.

RESEARCH ON MATRIX CRACK EVOLUTION OF CROSS-PLY CERAMIC MATRIX COMPOSITE

The matrix crack evolution of cross-ply ceramic matrix composites under uniaxial tensile loading is investigated using the energy balance method.Under tensile loading,the cross-ply ceramic matrix composites have five damage modes.The cracking mode 3 contains transverse cracking,matrix cracking and fiber/matrix interface debonding.The cracking mode 5 only contains matrix cracking and fiber/matrix interface debonding.The cracking stress of modes 3 and 5 appearing between existing transverse cracks is determined.And the multiple matrix crack evolution of mode 3 is determined.The effects of ply thickness,fiber volume fraction,interface shear stress and interface debonding energy on the cracking stress and matrix crack evolution are analyzed.Results indicate that the cracking mode 3 is more likely to appear between transverse cracks for the SiC/CAS material.

Direct deposition of graphene nanowalls on ceramic powders for the fabrication of a ceramic matrix composite

Uniform mixing of ceramic powder and graphene is of great importance for producing ceramic matrix composite. In this study, graphene nanowalls(GNWs) are directly deposited on the surface of Al2 O3 and Si3 N4 powders using chemical vapor deposition system to realize the uniform mixing. The morphology and the initial stage of the growth process are investigated. It is found that the graphitic base layer is initially formed parallel to the powder surface and is followed by the growth of graphene nanowalls perpendicular to the surface. Moreover, the lateral length of the graphene sheet could be well controlled by tuning the growth temperature. GNWs/Al2 O3 powder is consolidated by using sparking plasma sintering method and several physical properties are measured. Owing to the addition of GNWs, the electrical conductivity of the bulk alumina is significantly increased.

EVOLUTION OF THE MICROCRACKS IN WHISKER TOUGHENING CERAMIC MATRIX COMPOSITES

The modified equivalent inclusion theory by the authors and the internal variable theory are employed to investigate the evolution of the microcracks in whisker toughening ceramics and the influence of the microcracks on the mechanical properties of the material. The effect of residual thermostrain, whisker content and aspect ratio is considered. The modulus, initial nonlinear load, strength and nonlinear constitutive relation are calculated and some important conclusions are given.

Preparation and Mechanical Properties of Al_2O_3/Al Laminated Ceramic Matrix Composites

Three series of Al2O3/Al laminated ceramic matrix composites,named SPA,SPV and HP,were fabricated by different methods.SPA and SPV were prepared using Al2O3 slices and Al slurry via screen printing and subsequent heat treatment in air or vacuum.HP samples were made by hot pressing the layered stack of Al foils and Al2O3 slices.SEM and XRD were applied to analyze the microstructure and the interlayer crystal phase.The bending strength,fracture toughness and fracture work of the samples made by the three methods were measured and compared.The results show that the composites have much better toughness and higher fracture work than the Al2O3 slice.Among the samples made by the three methods,the samples made by hot pressing have the optimum mechanical performance.The displacement-load curves and fracture mechanism were analyzed.

STUDY ON PREPARATION OF CERAMIC MATRIX COMPOSITE OF WC-Co ULTRAFINE POWDERS

The W -Co compound precursor powders with an average particle sife of 60 nm were prepared by the chemical coprecipitation as the raw materials of Na2WO1 and CoCl2 and as the reagents of HCI and NH3 ?H2O. After re-ducing and carburizing the precursor powders by hydrogen gas and CO-CO 2 mixture gas. the WC-Co composite povvders ivith an average particle size of 0. 18/wi can be obtained. The purity and particle size of powders -were analysed by XRD and TEM. respectively. Meanwhile, the key factors to influ-ence the reducing and carburizing process of powders were also studied.

Nanoparticles Transport in Ceramic Matriсes: A Novel Approach for Ceramic Matrix Composites Fabrication

The results of practical implementation of a new method for porous piezoceramics, and ceramic matrix piezocomposites fabrication were presented. The method was based on nanoparticles transport in ceramic matrices using a polymer nanogranules coated or filled with a various chemicals, with successive porous ceramics fabrication processes. Different types of polymer microgranules filled and coated by metal-containing nanoparticles were used for a pilot samples fabrication. Polymer microgranules were examined using transmission and scanning electron microscopy as well as by EXAFS and X-ray emission spectroscopy. Pilot samples of nano- and microporous ceramics and composites were fabricated using different piezoceramics compositions (PZT, lead potassium niobate and lead titanate) as a ceramic matrix bases. Resulting ceramic matrix piezocomposites were composed by super lattices of closed or open pores filled or coated by nanoparticles of metals, oxides, ferromagnetics etc. embedded in piezoceramic matrix. Dielectric and piezoelectric parameters of pilot samples were measured using piezoelectric resonance analysis method. New family of nano- and microporous piezoceramics and ceramic matrix piezocomposites are characterized by a unique spectrum of the electrophysical properties unachievable for standard PZT ceramic compositions and fabrication methods.

Effect of Interfacial Bonding on the Toughening of Al_2O_3/Ni Ceramic Matrix Composites

The main Iimitation to the toughening of the α-Al2O3/Ni composite is the poor bonding atthe interface. which causes the nickel particles to be pulled-out during crack propagation with-out obvious plastic deformation. A proper control of oxygen content at the Al2O3-Ni interfacecan promote wetting at the intedece, and produce a mechanically interlocked and chemically strengthened intedece, causing most of the nickel particles to be stretched to failure and to expe-rience severe plastic deformation during crack propagation in the composite. Fracture toughnesstesting using a modified double cantilever beam method with in situ observation of crack prop-agation in a scanning electron microscope shows that the composite with the strengthenedinterface has a more desirable R-curve behaviour and a higher fracture toughness value than thenormal composite.

Experimental Investigation on Active Cooling for Ceramic Matrix Composite

Compared with conventional materials,the active cooling ceramic matrix composite used in ramjet or scramjet makes their structures lighter in mass and better in performance.In this paper,an active and a passive cooling refractory composite specimens are designed and tested with an experimental facility composed of multilayer smale scale cooling penel which consists of a water cooling system and a ceramic matrix composite specimen,and a gas generator used for providing lower and higher transfer rate gases to simulate the temperatures in combustion chamber of ramjst.The active cooling specimen can continuously suffer high surface temperature of 2 000 K for 30 s and that of 3 000 K for 9.3 s,respectively.The experiment results show that the active cooling composite structure is available for high-temperature condition in ramjet.

Identification of Growth Promoter to Fabrication SiCp/Al₂O₃Ceramic Matrix Composites Prepared by Directed Metal Oxidation of An Al Alloy

SiC particulates reinforced alumina matrix composites were fabricated using Directed Metal Oxidation (DIMOX) process. Continuous oxidation of an Al-Si-Mg-Zn alloy with different interlayers (dopents) as growth promoters, will encompasses the early heating of the alloy ingot, melting and continued heating to temperature in the narrow range of 950°C to 980°C in an atmosphere of oxygen. Varying interlayers (dopents) are incorporated to examine the growth conditions of the composite materials and to identification of suitable growth promoter. The process is extremely difficult because molten aluminum does not oxidize after prolonged duration at high temperatures due to the formation of a passivating oxide layer. It is known that the Lanxide Corporation had used a combination of dopents to cause the growth of alumina from molten metal. This growth was directed, i.e. the growth is allowed only in the required direction and restricted in the other directions. The react nature of the dopants was a trade secret. Though it is roughly known that Mg and Si in the Al melt can aid growth, additional dopents used, the temperatures at which the process was carried out, the experimental configurations that aided directed growth were not precisely known. In this paper we have evaluated the conditions in which composites can be grown in large enough sizes for evaluation application and have arrived at a procedure that enables the fabrication of large composite samples by determining the suitable growth promoter (dopant). Scanning electron microscopic, EDS analysis of the composite was found to contain a continuous network of Al2O3, which was predominantly free of grain-boundary phases, a continuous network of Al alloy. Fabrication of large enough samples was done only by the inventor company and the property measurements by the company were confirmed to those needed to enable immediate applications. Since there are a large number of variable affecting robust growth of the composite, fabrication large sized samples for measurements is a difficult task. In the present work, to identify a suitable window of parameters that enables robust growth of the composite has been attempted.

Thermal shock fatigue behavior of TiC/Al_2O_3 composite ceramics

The thermal shock fatigue behaviors of pure hot-pressed alumina and 30 wt.% TiC/Al2O3 composites were studied. The effect of TiC and Al2O3 starting particle size on the mechanical properties of the composites was discussed. Indentation-quench test was conducted to evaluate the effect of thermal fatigue temperature difference （ΔT） and number of thermal cycles （Ⅳ） on fatigue crack growth （Δa）. The mechanical properties and thermal fatigue resistance of TiC/Al203 composites are remarkably improved by the addition of TiC. The thermal shock fatigue of monolithic alumina and TiC/Al2O3 composites is due to a ＂true＂ cycling effect （thermal fatigue）. Crack deflection and bridging are the predominant reasons for the improvement of thermal shock fatigue resistance of the composites.

Corrosion behavior of SiC foam ceramic reinforced Al-23Si composites in NaCl solution

SiC foam ceramic reinforced aluminum matrix composites(SFCAMCs)were prepared by squeeze casting aluminum alloy(Al-23Si)into the SiC foam ceramic with different pore sizes,and the corrosion behavior of the SFCAMCs was studied in NaCl solutions.Static immersion corrosion tests were conducted at 20°C,50°C and 80°C,respectively.Corrosion morphology and products were analyzed by scanning electron microscope,energy dispersive system and X-ray diffraction.It was found that the corrosion rate of SFCAMCs increases as the temperature rising,and the bigger pore size of SiC foam ceramic reinforcement,the better corrosion resistance of SFCAMCs.

C_(f)/(CrZrHfNbTa)C-SiC high-entropy ceramic matrix composites for potential multi-functional applications

In this work,C_(f)/(CrZrHfNbTa)C-SiC high-entropy ceramic matrix composites with good load-bearing,elec-tromagnetic shielding and ablation resistance were designed and reported for the first time.The compos-ites were fabricated by an efficient combined processing of slurry infiltration lamination(SIL)and precur-sor infiltration and pyrolysis(PIP).Density and porosity of the as-fabricated composites are 2.72 g/cm^(3) and 12.44 vol.%,respectively,and the flexural strength is 185±13 MPa.Due to the carbon fiber rein-forcement with high conductivity and strong reflection,and high-entropy(CrZrHfNbTa)C ceramic matrix with strong absorbability,the total Electromagnetic interference shielding efficiency(SET)of the compos-ites with a thickness of 3 mm are as high as 88.2 dB and 90 dB respectively in X-band and Ku-band.This means that higher than 99.999999%electromagnetic shielding is achieved at 8-18 GHz,showing excel-lent electromagnetic shielding performance.The C_(f)/(CrZrHfNbTa)C-SiC composites also present excellent ablation resistance,with the linear and mass ablation rates of 0.9μm/s and 1.82 mg/s after ablation at the heat flux of 5 MW/m^(2) for 300 s(∼2450℃).This work opens a new insight for the synergistic de-sign of structural and functional integrated materials with load-bearing,electromagnetic shielding and ablation resistance,etc.

Preparation of Fe-M Intermetallic/TiC-M_2O_3 Ceramic Composites from Ilmenite by SHS

Fe-Al intermetallic/TiC-Al2O3 ceramic composites were successfully prepared by selfpropagating high-temperature synthesis （SHS） from natural ilmenite, aluminium and carbon as the raw materials. The effects of carbon sources, preheating time and heat treatment temperature on synthesis process and products were investigated in detail, and the reaction process of the FeTiO3-Al-C system was also discussed. It is shown that the temperature and velocity of the combustion wave are higher when graphite is used as the carbon source, which can reflect the effect of the carbon source structure on the combustion synthesis; Prolonging the preheating time or heat treatment temperature is beneficial to the formation of the ordered intermetallics; The temperature and velocity of the combustion wave are improved, but the disordered alloys are difficult to eliminate with the preheating time prolonged. The compound powders mainly containing ordered Fe3Al intermetallic can be prepared through heat treatment at 750 ℃.

CMC-EBC损伤耦合机理及一体化设计研究进展

陶瓷基复合材料与环境障碍涂层组合(CMC-EBC)是目前航空航天领域最具应用前景的热结构材料体系。本文对CMC-EBC失效机理与分析模型的研究进展进行综述。首先,简要回顾了CMC-EBC材料体系的发展及主要制备工艺。然后,综述了CMC-EBC在服役环境下的主要损伤模式与失效机理,总结发现CMC预制体结构、孔洞缺陷和EBC内裂纹等损伤演化相互影响,这种细观损伤模式的耦合是决定其寿命的关键因素之一,但目前的机理研究主要集中于涂层本身性能及其受环境因素的影响,缺乏对涂层和复合材料编制结构在损伤演化过程中协同效应的考虑。接下来,详细分析了CMC-EBC的失效模拟与预测模型研究的历史与现状,指出其中存在的问题,包括环境因素建模方法和损伤耦合演化模拟技术。目前大部分工作致力于分别开发CMC和EBC的失效模型,而对于CMC-EBC构件的失效预测应考虑其损伤演化与微观结构之间的相互耦合影响。最后,对CMC-EBC材料体系研发与服役性能预测方法进行了展望,认为CMC本体和EBC失效模式相互耦合,开展结构功能一体化设计和分析是CMC-EBC构件研究的趋势。

航机CMC热端部件用热喷涂涂层的机遇与挑战

随着航空发动机涡轮前燃气入口温度的不断攀升,陶瓷基复合材料(Ceramic Matrix Composite, CMC)以其轻质、高强、抗氧化、对裂纹不敏感、耐温性能优异等特点,成为新一代航空发动机高温部件的首选基体材料。但CMC存在抗高温水氧侵蚀性能不足等问题,发动机CMC热端部件用热喷涂涂层成为亟待解决的技术瓶颈。本文结合国外航空发动机热端部件选材方案的更迭及工程应用实例,分析了发动机热端部件采用高温合金+气膜冷却+热障涂层方案的技术局限性,明确了CMC+适量气膜冷却+环境障碍涂层方案的技术优势;综述了CMC用热与环境障碍涂层(Thermal and Environmental Barrier Coatings, TEBCs)和环境障碍可磨耗封严涂层(Environmental Barrier Coatings-Abradable Sealing Coatings, EBCs-ASCs)的研究进展、应用情况以及近些年国内外学者的研究成果;辨析了面向更高温燃气来流时热喷涂环境障碍涂层面临的机遇与挑战,为后续TEBCs的组分和结构设计以及制备明确了方向,并对今后研究工作的重点进行了展望。

A Numerical Study of Densification Behavior of Silicon Carbide Matrix Composites in Isothermal Chemical Vapor Infiltration

We studied the characteristics of two-scale pore structure of preform in the deposition process and the mass transfer of reactant gas in dual-scale pores, and observed the physiochemical phenomenon associated with the reaction. Thereby, we established mathematical models on two scales, respectively, preform and reactor. These models were used for the numerical simulation of the process of ceramic matrix composites densified by isothermal chemical vapor infiltration(ICVI). The models were used to carry out a systematic study on the influence of process conditions and the preform structure on the densification behaviors. The most important findings of our study are that the processing time could be reduced by about 50% without compromising the quality of the material, if the processing temperature is 950-1 000 ℃ for the first 70 hours and then raised to 1 100 ℃.

CMC S弯喷管材料-结构多尺度优化设计研究

S弯喷管作为大尺寸薄壁结构,在多场耦合作用下有着复杂的整体和局部刚度特性。本文针对S弯喷管变形特征,提出一种基于陶瓷基复合材料(CMC)的S弯喷管连续筋条加强结构,可以有效抑制S弯喷管的整体变形和局部变形。与GH4169合金相比,CMC材料可以降低S弯喷管45.31%的变形量,同时减重62.77%,在此基础上,采用连续筋条加强结构可以进一步降低CMC S弯喷管43.15%的变形量。基于代理模型和差分进化算法,以CMC材料筋条加强S弯喷管的结构质量为目标,以CMC材料细观编织结构中的经/纬纱比例以及喷管宏观结构中的分区域壁厚和加强筋尺寸为优化变量,开展了材料-结构多尺度优化,在相同的刚度指标要求下进一步优化减重26.21%。