Comprehensive nuclear materials 2 12 properties and characteristics of sic and sic composites

Comprehensive nuclear materials 2 12 properties and characteristics of sic and sic composites Comprehensive nuclear materials 2 12 properties and characteristics of sic and sic composites Comprehensive nuclear materials 2 12 properties and characteristics of sic and sic composites Comprehensive nuclear materials 2 12 properties and characteristics of sic and sic composites Comprehensive nuclear materials 2 12 properties and characteristics of sic and sic composites Comprehensive nuclear materials 2 12 properties and characteristics of sic and sic composites

2.12 Properties and Characteristics of SiC and SiC/SiC Composites J Lamon CNRS/National Institute of Applied Science, Villeurbanne, France ß 2012 Elsevier Ltd All rights reserved 2.12.1 Introduction 324 2.12.2 2.12.2.1 2.12.2.1.1 2.12.2.1.2 2.12.2.1.3 2.12.2.1.4 2.12.2.1.5 2.12.2.1.6 2.12.2.1.7 2.12.2.2 2.12.2.2.1 2.12.2.2.2 2.12.2.2.3 2.12.3 2.12.3.1 2.12.3.2 2.12.3.3 2.12.3.4 2.12.4 2.12.5 2.12.6 2.12.6.1 2.12.6.2 2.12.6.3 2.12.6.4 2.12.6.5 2.12.6.6 2.12.6.7 2.12.6.8 2.12.6.9 2.12.7 References b-SiC Properties23 Mechanical Properties Elastic modulus23 Poisson’s ratio23 Shear modulus23 Hardness23 Fracture toughness23 Fracture strength Thermal creep23 Thermal Properties23 Thermal conductivity Specific heat Thermal expansion SiC/SiC Composite Fibrous Preform Coating of Fibers Infiltration of the SiC Matrix: The CVI Process Infiltration of the SiC Matrix: The NITE Process Properties of CVI SiC/SiC Properties of NITE-SiC/SiC Mechanical Behavior of CVI SiC/SiC Tensile Stress–Strain Behavior Damage Mechanisms Ultimate Failure Reliability Interface Properties: Influence on the Mechanical Behavior Fracture Toughness Fatigue and High-Temperature Behavior Thermal Shock Creep Behavior Concluding Remarks 325 325 325 325 325 325 325 326 326 326 326 326 327 327 327 327 327 328 328 330 330 330 331 333 333 334 335 336 336 336 337 337 Abbreviations C/C C/SiC CMC CVD CVI Carbon matrix composite reinforced by carbon fibers SiC matrix composite reinforced by carbon fibers Ceramic matrix composite Chemical vapor deposition Chemical vapor infiltration LPS MI NITE PIP PyC RS SENB Liquid phase sintering Melt infiltration Nanopowder infiltration and transient eutectic-phase polymer impregnation and pyrolysis Pyrocarbon Reaction sintering Single edge notch bending 323 324 Properties and Characteristics of SiC and SiC/SiC Composites SEP SiC/SiC Socie´te´ Europe´enne de Propulsion SiC matrix composite reinforced by SiC fibers 2.12.1 Introduction Silicon carbide is composed of tetrahedra of carbon and silicon atoms with strong bonds in the crystal lattice This produces a very hard and strong ceramic with outstanding characteristics such as high thermal conductivity, low thermal expansion, and exceptional resistance to thermal shock and to corrosion in aggressive environments at high temperatures However, this implies a few inadequate characteristics for structural applications, such as low fracture toughness, high sensitivity to the presence of microstructural flaws, brittle behavior, and lack of reliability Reinforcing with continuous SiC-based fibers allows these weaknesses to be overcome The composite SiC/SiC that is obtained is damage tolerant, tough, and strong, and it can be insensitive to flaws and notches The concept of composite material is very powerful Composites can be tailored to suit enduse applications through the sound selection and arrangement of the constituents Ceramic matrix composites (CMCs) reinforced with continuous ceramic or carbon fibers are of interest in thermostructural applications.1–4 They are lightweight and damage tolerant and exhibit a much greater resistance to high temperatures and aggressive environments than metals or other conventional engineering materials CMCs can be fabricated by different processing techniques, using either liquid or gaseous precursors The chemical vapor infiltration (CVI) method can produce excellent SiC/SiC composites with a highly crystalline structure and excellent mechanical properties.5 The quality of the material obtained by the polymer impregnation and pyrolysis (PIP) method is insufficient A novel processing technique (nanopowder infiltration and transient eutectic-phase processing, NITE) was claimed to achieve good material quality.5–7 The SiC/SiC composites prepared using the CVI method and reinforced with the latest nearstoichiometric SiC fibers (such as Hi-Nicalon type S and Tyranno-SA3 fibers) appear to be promising candidates for nuclear applications7–12 because of their high crystallinity, high purity, near stoichiometry and radiation resistance of the b-phase of SiC, as well as excellent resistance at high temperatures to fracture, creep, corrosion, and thermal shock Studies on the b-phase properties suggest that CVI SiC/SiC composites have the potential for excellent radiation stability.3 CVI SiC/SiC is also considered for applications as structural materials in fusion power reactors because of low neutron-induced activation characteristics coupled with excellent mechanical properties at high temperature.10–12 The CVI technique has been studied since the 1960s.13–19 It derives directly from chemical vapor deposition (CVD).13–15 In very simple terms, the SiC-based matrix is deposited from gaseous reactants on to a heated substrate of fibrous preforms (SiC).15 CVI is a slow process, and the obtained composite materials possess some residual porosity and density gradients Despite these drawbacks, the CVI process presents a few advantages: (1) the strength of reinforcing fibers is not affected during the manufacture of the composite; (2) the nature of the deposited material can be changed easily, simply by introducing the appropriate gaseous precursors into the infiltration chamber; (3) a large number of components; and (4) large, complex shapes can be produced in a near-net shape Development of CVI SiC/SiC composites began in the 1980s when SEP (Socie´te´ Europe´enne de Propulsion), Amercorm, Refractory Composites, and others began to develop equipment and processes for producing CVI components for aerospace, defense, and other applications The development of CVI SiC/SiC composites has been inspired by the poor oxidation resistance of their predecessor CVI C/C composites CVI SiC/SiC components have been produced and tested SNECMA (formerly SEP) is at the forefront of this technology and has demonstrated satisfactory component performance in engine and flight tests The mechanical properties of SiC/SiC composites depend on the fiber–matrix interface Pyrocarbon (PyC) has proved to be an efficient interphase to control fiber–matrix interactions and composite mechanical behavior.20 But PyC is sensitive to oxidation at temperatures above 450  C A few versions of high-temperature-resistant CVI SiC/SiC composites have been produced In order to protect the PyC interphase against oxidation, multilayered interphases and matrices have been developed.3,21 Multilayered matrices contain phases that produce sealants at high temperatures, preventing oxygen from reaching the interphase.22 This composite is referred to as CVI SiC/Si–B–C Oxidation-resistant interphases such as BN or multilayered materials can also be coated on the fibers An ‘oxygen getter’ can be Properties and Characteristics of SiC and SiC/SiC Composites added to the matrix to scavenge oxygen that might ingress into the matrix (enhanced CVI SiC/SiC) The mechanical behavior of CMCs displays several typical features that differentiate them from the other composites (such as polymer matrix composites, metal matrix composites, etc.) and from homogeneous (monolithic) materials These features are due to heterogeneous and multiscale composite microstructure and the respective properties of the constituents (interphases, fiber, and matrix) The main characteristics of CVD SiC, CVI SiC/SiC, and NITE-SiC/SiC are reviewed in this chapter Features of mechanical behavior of SiC/SiC are discussed with respect to microstructure, on the basis of the large amount of work done on CVI SiC/SiC E ẳ E0 expCVp ị 325 ẵ1 E0 ẳ 460 GPa for CVD SiC (polycrystalline, highpurity, very dense, and pore-free SiC material) and C ¼ 3.57 No significant difference was obtained between the elastic moduli for a- and b-polycrystalline SiC or among those of hot-pressed, sintered, and CVD materials The elastic modulus at elevated temperatures has been empirically expressed as: E ẳ E0 BT expT0 =T ị ẵ2 with E0 ¼ 460 GPa, B ¼ 0.04 GPa K , and T0 ¼ 962 K 2.12.2.1.2 Poisson’s ratio23 2.12.2 b-SiC Properties23 Silicon carbide has a myriad polytypes depending on the varied stacking of closed atomic planes.23 Only CVD SiC material is inherently highly crystalline, pure, and stoichiometric, which is critical to irradiation stability Much emphasis is placed on CVD SiC in this chapter, as it corresponds very closely to the matrix of CVI SiC/SiC The reader will find further details on the SiC structure–property relationships in the excellent comprehensive review by Snead and colleagues.23 Here the main data from Snead’s paper are summarized Only the 3C–SiC crystal, known as b-SiC, has the sequence showing cubic symmetry out of the infinite number of variations All the other polytypes which show noncubic symmetry are classified as a-SiC a-SiC is formed above 2373 K and b-SiC at 1273–1873 K Various fabrication techniques, such as sintering, direct conversion, gas-phase reaction, and polymer pyrolysis, are currently used for the synthesis of SiC The CVD technique is one of the most familiar gasphase reaction methods for the synthesis of highly crystalline, stoichiometric, high-purity b-SiC 2.12.2.1 Mechanical Properties The Poisson ratio of CVD SiC with excess residual silicon yields the lowest value ($0.13) The highest value of 0.21 was typically obtained for pure CVD SiC The temperature dependence is very minor 2.12.2.1.3 Shear modulus23 The shear modulus at room temperature of 191 GPa for CVD SiC has been determined by the four-point bending technique This value was also derived from the elastic modulus and Poisson’s ratio (n), using the conventional formula for isotropic solids: G ¼ E/2(1 þ n) The temperature dependence of shear modulus can be estimated from E by applying this formula 2.12.2.1.4 Hardness23 There appears to be no significant difference between Vicker’s and Knoop hardness: H $ 20.7–24.5 GPa has been reported for CVD b-SiC By contrast, slightly higher values were obtained by nanoindentation Nanoindentation is known to yield local values which depend on microstructural features The aforementioned exponential function of porosity for elastic modulus can be extended to the hardness evaluation: HV ¼ 27:7 exp5:4Vp ị ẵ3 where HV is the Vicker hardness Currently, there is no high-temperature data reported for high-purity CVD SiC 2.12.2.1.1 Elastic modulus23 Generally, a dense and high-purity SiC material, for example, CVD SiC, exhibits the highest elastic modulus; however, the elastic modulus decreases with increasing porosity or impurity concentration The elastic modulus at room temperature is conventionally expressed as an exponential function of porosity (Vp): 2.12.2.1.5 Fracture toughness23 Values between 2.4 and 5.1 MPa√m have been measured for CVD b-SiC, depending on the test technique employed and grain size Fracture toughness of CVD SiC increases slightly at elevated temperatures It does not exceed MPa√m 326 Properties and Characteristics of SiC and SiC/SiC Composites ec ẳ Ap s=Gịn ðt =tÞp 2.12.2.1.6 Fracture strength As is usual with brittle ceramics, fracture data exhibit a significant scatter, as flaws that have a random distribution induce fracture An important consequence is that the fracture stress is not an intrinsic characteristic It is, instead, a statistical variable, which depends on several factors including the test method, the size of test specimens, and the number of test specimens.24 Therefore, a universal reference value of fracture strength cannot be recommended It is widely accepted that the Weibull model satisfactorily describes the statistical distribution of failure strengths:   m ẵ4 P ẳ À exp À ðs=s0 Þ dV =V0 where P is the probability of failure, s is the stress, s0 is the scale factor, m is the Weibull modulus, V is the volume of specimen, and V0 is a reference volume (1 m3 is generally used); m reflects the scatter in data, and s0 is related to the mean value of the strength The strength data for a given geometry and stress state can be determined using eqn [4] However, m, s0, and V0 must be available It is important to note that the estimate of s0 depends on V0.24 It will be substantially different if V0 ¼ m3 or mm3 This dependence is ignored in most publications, even in the work by Snead and coworkers23 in which a number of s0 values are reported When V0 is not given, the estimate of s0 is meaningless The strength cannot be determined safely Unfortunately, reliable s0 values (characteristic strength in a few papers) cannot be recommended here until the authors have completed their papers The values of Weibull modulus of CVD SiC at room temperature reported in Snead et al.23 span a large range, from to 12 The following values were measured using tensile tests on CVI SiC/SiC minicomposites: m ¼ 6.1, s0 ¼ 10.5 MPa (V0 ¼ m3).25,26 2.12.2.1.7 Thermal creep23 Primary and secondary creep deformations have been reported in the literature for CVD SiC (high-purity and polycrystalline b-SiC) Creep in SiC is highly dependent on the crystallographic orientation The loading orientation of 45 from the CVD growth axis is the direction in which the most prominent creep strain is observed A review of creep behaviors of stoichiometric CVD SiC has been provided by Davis and Carter.27 Primary creep of CVD SiC occurs immediately upon loading and tends to saturate with time The primary creep strain generally obeys the following relationship: ½5Š where Ap , p, and t are creep parameters, and t is the time elapsed n ¼ 1.63, Ap ¼ 29, p ¼ 0.081, and t ¼ 0.0095 s for the temperature of 1923 K These parameters are for the loading orientation of 45 from the CVD growth axis In severe conditions, primary creep strain in the CVD SiC can reach as high as 1% Steady-state creep rates for polycrystalline materials have been measured only above $1673 K, when the stress axis is 45 inclined from the deposition direction; temperatures as high as 2023 K are required when the stress axis is parallel to the deposition direction The strain rate is given by a powerlaw creep equation: de=dt ẳ As s=Gịn expQ =kb T ị ½6Š where As ¼ 2.0  103, n ¼ 2.3, Q ¼ 174 kJ molÀ1 (activation energy), s is the applied stress, G is the shear modulus, and kb is the Boltzmann constant 2.12.2.2 Thermal Properties23 2.12.2.2.1 Thermal conductivity It is reasonable to assume that the single-crystal form of SiC, compared to the other varieties, exhibits the highest thermal conductivity However, high-purity and dense polycrystalline CVD SiC exhibits practically the same conductivity as the single-crystal material It is worth noting that the impurity content of the very high thermal conductivity CVD SiC materials is negligibly small, and this material has near theoretical density ($3.21 g cmÀ3) The curve-fitting to the single-crystal SiC data above 300 K yields an upper limit of the thermal conductivity of SiC (in W mÀ1 KÀ1): Kp ẳ 0:0003 ỵ 1:05 105 T ị1 ẵ7 2.12.2.2.2 Specific heat The temperature dependence of the specific heat can be treated in two temperature regions: a rapid increase at low temperatures (below 200 K), and a gradual increase at higher temperatures No systematic difference can be distinguished between the structural types The specific heat, Cp (in J kgÀ1 K), over the temperature range 200–2400 K can be approximately expressed as Cp ẳ 925:65 ỵ 0:3772T 7:9259 10À5 T À 3:1946  107 =T ½8Š Properties and Characteristics of SiC and SiC/SiC Composites The specific heat of SiC at room temperature is taken as 671 Æ 47 J kgÀ1 K 2.12.2.2.3 Thermal expansion The coefficient of thermal expansion for b-SiC has been reported over a wide temperature range The average value in the interval from room temperature to 1700 K is a ¼ 4.4  10À6 KÀ1 At higher temperatures  10À6 KÀ1 At lower temperatures a ẳ 2.08 ỵ 4.51 103T (T > 1273 K), (550 < T < 1273 K), It is worth addressing the processing method first because this information is useful for a better understanding of the structure of SiC/SiC The manufacture of long fiber-reinforced composites requires three main steps14,15,28,29: preparation of fibrous preform, fiber coating, which provides an interface material (interphase), and infiltration of the matrix Fibrous Preform The preforms of SiC/SiC composites are made of refractory SiC-based continuous fibers The latest near-stoichiometric SiC fibers (such as Hi-Nicalon type S and Tyranno-SA3 fibers) are the most appropriate for those CVI SiC/SiC foreseen for nuclear applications These fibers exhibit high strength, high stiffness, low density, and high thermal and chemical stability to withstand long exposures at high temperatures.30 Finally, the fiber diameter must be small (