Krishan Kumar Chawla

Ceramic matrix composites

Ceramic materials in general have a very attractive package of properties: high strength and high stiffness at very high temperatures, chemical inertness, low density, and so on. This attractive package is marred by one deadly flaw, namely, an utter lack of toughness. They are prone to catastrophic failures in the presence of flaws (surface or internal). They are extremely susceptible to thermal shock and are easily damaged during fabrication and/or service. It is therefore understandable that an overriding consideration in ceramic matrix composites (CMCs) is to toughen the ceramics by incorporating fibers in them and thus exploit the attractive high-temperature strength and environmental resistance of ceramic materials without risking a catastrophic failure. It is worth pointing out at the very outset that there are certain basic differences between CMCs and other composites. The general philosophy in nonceramic matrix composites is to have the fiber bear a greater proportion of the applied load. This load partitioning depends on the ratio of fiber and matrix elastic moduli, Ef/Em. In nonceramic matrix composites, this ratio can be very high, while in CMCs, it is rather low and can be as low as unity; think of alumina fiber reinforced alumina matrix composite. Another distinctive point regarding CMCs is that because of limited matrix ductility and generally high fabrication temperature, thermal mismatch between components has a very important bearing on CMC performance. The problem of chemical compatibility between components in CMCs has ramifications similar to those in, say, MMCs. We first describe some of the processing techniques for CMCs, followed by a description of some salient characteristics of CMCs regarding interface and mechanical properties and, in particular, the various possible toughness mechanisms, and finally a description of some applications of CMCs.

Microstructure-based simulation of thermomechanical behavior of composite materials by object-oriented finite element analysis

Abstract While it is well recognized that microstructure controls the physical and mechanical properties of a material, the complexity of the microstructure often makes it difficult to simulate by analytical or numerical techniques. In this paper we present a relatively new approach to incorporate microstructures into finite element modeling using an object-oriented finite element technique. This technique combines microstructural data in the form of experimental or simulated microstructures, with fundamental material data (such as elastic modulus or coefficient of thermal expansion of the constituent phases) as a basis for understanding material behavior. The object-oriented technique is a radical departure from conventional finite element analysis, where a “unit-cell” model is used as the basis for predicting material behavior. Instead, the starting point of object-oriented finite element analysis is the actual microstructure of the material being investigated. In this paper, an introduction to the object-oriented finite element approach to microstructure-based modeling is provided with two examples: SiC particle-reinforced Al matrix composites and double-cemented WC particle-reinforced Co matrix composites. It will be shown that object-oriented finite element analysis is a unique tool that can be used to predict elastic and thermal constants of the composites, as well as salient effects of the microstructure on local stress state.

Thermal expansion of metal-matrix composites

Abstract The thermal expansion of various fiber- and particle-reinforced metal-matrix composites has been measured and the experimentally obtained values compared with the predictions of various theoretical models. The particulate composites exhibited some residual strain when cooled down from the peak temperature to room temperature. The magnitude of this strain was a function of the peak temperature and number of thermal cycles. The thermal expansion response of the fiber-reinforced composites was significantly different from that of the particulate composites. These composites exhibited very small residual strains when cooled down from the peak temperature to room temperature. In addition, the thermal expansion response was not linear over the test temperature range but exhibited regions of distinctly different slopes. The observed behavior of these particulate and fibrous composites is described on the basis of the thermal stresses developed in such composites as a result of the differences in the coefficient of thermal expansion between the reinforcement and the matrix.

Use of electrophoretic deposition in the processing of fibre reinforced ceramic and glass matrix composites: a review

Abstract Electrophoretic deposition (EPD) is a simple and cost-effective method for fabricating high-quality ‘green’ composite bodies which, after a suitable high-temperature treatment, can be densified to a composite with improved properties. In this contribution, we describe the use of EPD technique in the fabrication of fibre reinforced composites, with an emphasis on composites with glass and ceramic matrices containing metallic or ceramic fibre fabric reinforcement. EPD has been used to infiltrate preforms with tight fibre weave architectures using different nanosized ceramic particles, including silica and boehmite sols, as well as dual-component sols of mullite composition. The principles of the EPD technique are briefly explained and the different factors affecting the EPD behaviour of ceramic sols and their optimisation to obtain high infiltration of the fibre preforms are considered. In particular, the EPD fabrication of a model alumina matrix composite reinforced by Ni-coated carbon fibres is presented. The pH of the solution and the applied voltage and deposition time are shown to have a strong influence on the quality of the infiltration. Good particle packing and a high solids-loading were achieved in most cases, producing a firm ceramic deposit which adhered to the fibres. Overall, the analysis of the published data and our own results demonstrate that EPD, being simple and inexpensive, provides an attractive alternative for ceramic infiltration and coating of fibre fabrics, even if they exhibit tight fibre weave architectures. The high-quality infiltrated fibre mats are suitable prepregs for the fabrication of advanced glass and ceramic matrix composites for use in heat-resistant, structural components.

Fabrication and characterisation of Ni-coated carbon fibre-reinforced alumina ceramic matrix composites using electrophoretic deposition

The present study explores the feasibility of fabricating Ni-coated carbon fibre-reinforced alumina ceramic matrix composites via a single-infiltration electrophoretic deposition (EPD) process performed in vacuum. The nano-size boehmite sol was seeded using nano-size δ-alumina powder in order to control the final sintered microstructure and then characterised using transmission electron microscopy, differential thermal and thermogravimetric analysis (DTA/TG) and X-ray disc centrifuge system (BI-XDC) in order to determine the sol microstructure, phase transformation temperatures and particle size (also degree of agglomeration), respectively. An EPD manufacturing cell for fabrication of Ni-coated carbon fibre reinforced alumina matrix composites was designed and experiments were conducted under vacuum (first time to date), resulting in full deposition of the sol material throughout the voids within/between the fibre tows. Composites with high green density (67% theoretical density) were produced using an applied voltage of 15 V d.c. and deposition time of 400 s. The sintered density after pressureless sintering at 1250°C for 2 h was 91% theoretical density. Crack path propagation test showed that the metallic Ni coating was able to provide a weak interface, as an indenter induced crack within the alumina matrix was deflected and arrested at the Ni interface.

Axial fatigue behavior of binder-treated versus diffusion alloyed powder metallurgy steels

A comparative study has been conducted on the microstructure, tensile, and axial fatigue behavior of two Fe‐0.5Mo‐1.5Cu‐ 1.75Ni alloys, made by binder-treated and diffusion alloying processes. The mechanical properties will be explained in terms of the pore size and morphology, as well as the heterogeneous microstructures typical of ferrous powder metallurgy materials. Binder treatment can provide a variety of advantages in manufacturing, over diffusion alloyed powders, including faster and more consistent flow into the die cavity, increased green strength, and reduction of fine particle dusting. In addition to conventional porosity, smaller, ‘‘copper diffusion’’ pores were observed where copper particles had been prior to forming a liquid phase during sintering and diffusing into the Fe particles. The microstructure in both alloys was typical of P:M alloy steels, with a heterogeneous microstructure consisting of areas of ‘‘divorced pearlite,’’ martensite, and nickel-rich ferrite. The modulus and tensile strength of both types of alloys were equivalent. Yield strength in the binder-treated alloy was higher which coincided with somewhat lower ductility. The fatigue behavior in terms of stress versus cycles (S‐N curves) was almost identical for the two systems. Fractographic observations showed fracture to have initiated primarily at pore clusters in the surface region. Fracture surfaces after fatigue tests showed ductile fracture in the interparticle bridge regions, cleavage facets in pearlitic regions, and striations.

Processing of fibre reinforced thermoplastic composites

Abstract With the advent of high performance thermoplastic polymers, structural applications for thermoplastic composites are increasing rapidly. Thermoplastic matrix composites possess distinct advantages vis-à-vis thermoset matrix composites in terms of recyclability, high specific strength and specific stiffness, corrosion resistance, enhanced impact toughness, cost effectiveness, and flexibility of design. Since 1990s, the number of material forms and combinations in fibre reinforced thermoplastic polymers has increased exponentially, thereby expanding application avenues in transportation, automotive, mass transit, marine, aerospace, military and construction sectors. In this paper we review the state of the art in processing of fibre reinforced thermoplastics. We start with a brief description of thermoplastic polymers used in structural applications followed by material forms and methods of impregnation of the reinforcement with polymer. Long fibre based processing methods are described next. A description of emerging thermoplastic composite processes and products follows. Finally we review process models and representative applications.

Microstructure and properties of monazite (LaPO4) coated saphikon fiber/alumina matrix composites

Abstract The objective of this research was to engineer a weak interfacial bond in single crystal α-alumina (Saphikon) fiber/polycrystalline alumina matrix composites by incorporating a monazite (lanthanum phosphate, LaPO 4 ) coating onto Saphikon fibers via sol–gel dip process. Uniaxial hot pressing was used to densify LaPO 4 -coated Al 2 O 3 fiber in an Al 2 O 3 matrix composites. Characterization of the composites was done by optical microscopy, SEM (scanning electron microscopy), EDS (energy dispersive spectrometer), indentation tests, three-point bend and fiber pushout tests. The results showed that the Saphikon fiber/monazite interface was weaker than the polycrystalline alumina/monazite interface. Crack deflection, interfacial debonding and fiber pullout occurred at this interface. This was attributed to the fact that the Saphikon fiber/monazite interface was smoother than the monazite/polycrystalline alumina matrix interface. Monazite coating obtained by sol–gel dip coating method withstood high fabrication temperatures (1400°C) and was conducive to the toughness properties of the composites.

Interface engineering in oxide fibre/oxide matrix composites

Abstract Oxide fibre/oxide matrix composites form an important and attractive subpart of ceramic matrix composites because of their inherent stability in oxidising atmospheres at high temperatures. An important attribute of such composites, however, is that the interfacial bond between oxide matrix and oxide fibre is generally very strong, and consequently, the toughness and damage tolerance of such composites are low. One way to overcome this problem is to tailor the interface such that the energy dissipating phenomena such as debonding and crack deflection at the fibre/ matrix interface, followed by fibre pullout are brought into play. In this paper, the salient aspects of control of interface characteristics in oxide fibre/oxide matrix composites, with emphasis on composites consisting of alumina and mullite based oxide fibres in a variety of oxide matrixes, are reviewed.

Processing, structure, and properties of mullite fiber/mullite matrix composites

Abstract Oxide fiber/oxide matrix composites form an important and very attractive subpart of ceramic matrix composites because of their inherent stability in oxidizing atmospheres at high temperature. In particular, mullite fiber/mullite matrix composites have the potential of high temperature usage in oxidizing atmospheres. The interface in mullite fiber/mullite matrix was engineered by using thick BN (1 μm) or BN/SiC double coating on mullite fibers, such that deformation mechanisms conducive to toughness enhancement could be brought to play. Significant improvements in the room temperature mechanical properties of these mullite fiber/mullite matrix composites could be achieved by incorporation of these interfacial coatings and by using a colloidal processing route to make dense mullite matrix.