Phenomenology of fracture in hot-pressed silicon nitride

Fracture phenomenology in hot-pressed silicon nitride has been studied fractographically as a function of flaw size, temperature and loading rate. Surface cracks of controlled size were introduced using the microhardness indentation technique. At room temperature, the fracture stress was found to depend on initial crack size according to the Griffith relationship and extrapolation of the data indicated that inherent processing flaws of the order of 12 to 24 m are strength-controlling in virgin material. Using a simplified Griffith approach, the fracture surface energy, , at 20° C for hot-pressed Si₃ N₄ is about 22 000 erg cm^–2. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A mixed mode of fracture consisting of transcrystalline and intergranular crack propagation occurred up to 1100° C; at 1200° C and above, subcritical crack growth (SCG) occurred intergranularly and the extent of SCG increased with increasing temperature. Similarly, the extent of SCG decreased with increasing loading rate.     

The compression creep behaviour of silicon nitride ceramics

A comparison has been made of the compression creep characteristics of samples of reaction-bonded and hot-pressed silicon nitride, a sialon and silicon carbide. In addition, the effects of factors such as oxide additions and fabrication variables on the creep resistance of reaction-bonded material and the influence of dispersions of SiC particles on the creep properties of hot-pressed silicon nitride have been considered. For the entire range of materials examined, the creep behaviour appears to be determined primarily by the rate at which the development of grain boundary microcracks allows relative movement of the crystals to take place. Now with the BNF Metals Technology Centre, Wantage.     

Impact damage behaviour of CVD-coated silicon nitride for gas turbines

Impact tests were conducted on the silicon nitride substrates coated with Si₃N₄ and SiC by chemical vapour deposition (CVD). For both 100- and 200-m-thick Si₃N₄-coated silicon nitride, Hertzian crack extension was reduced by debonding at the interface. Although Hertzian crack extension was not reduced for 100-m-thick SiC-coated silicon nitride, it was reduced for 200-m-thick SiC-coated silicon nitride. Theoretical calculations suggest that debonding at the interface consumed the fracture energy of Hertzian crack extension in the case of Si₃N₄ coatings, but it was observed that Hertzian cracks were not arrested at the interface.     

Microstructural effects on properties and new processing techniques of silicon nitride. Part I: Microstructural effects on mechanical,thermal and thermomechanical Properties of Silicon Nitride

Based on the correlation of some essential microstructural parameters and various mechanical, thermal and thermo-mechanical properties of hot-pressed and reaction-bonded silicon nitride, the demand for further improving the properties of silicon nitride and for developing a more economical fabrication of silicon nitride by applying new processing techniques is indicated. Aspects and problem areas of various new techniques, such as sintering of Si₃N₄-powder compacts and post-sintering of reaction-bonded silicon nitride as well as the various routes of hot-isostatic pressing, are discussed. The paper is divided into two parts: Part I: Microstructural effects on mechanical, thermal and thermomechanical properties of silicon nitride. Part II: New processing techniques of silicon nitride. In part I the relationship between microstructure and various mechanical, thermal and thermo-mechanical properties of hot-pressed and reaction-bonded silicon nitride, such as fracture strength, thermal diffusivity and thermal shock resistance, is discussed. On the basis of this correlation today's limits of silicon nitride regarding a broader application, and the possibilities for optimizing this material group are outlined. This discussion demonstrates the necessity of further improving some important properties and of developing new processing techniques. In part II the development lines of these new techniques for silicon nitride processing are discussed and a survey is given of recent progresses in the use of those techniques. As an example of a new processing technique, results of hot-isostatic pressing of reaction-bonded silicon nitride are presented in more detail.     

Microstructural effects on properties and new processing techniques of Silicon Nitride. Part II: New processing techniques of silicon nitride

In part I of this paper the relationship between microstructure and various mechanical, thermal and thermo-mechanical properties of hot-pressed and reaction-bonded silicon nitride, such as fracture strength, thermal diffusivity and thermal shock resistance, was discussed. On the basis of this correlation today's limits of silicon nitride regarding a broader application and the possibilities for optimizing this material group were outlined. This discussion demonstrates the necessity of further improving some important properties and of developing new processing techniques. In part II the development lines of these new techniques for silicon nitride processing are discussed and a survey is given of recent progresses in the use of those techniques. As an example of a new processing technique, results of hot-isostatic pressing of reaction-bonded silicon nitride are presented in more detail.     

Bonding to aged surfaces: A thermally-aged epoxy

It is common experience that aged surfaces are often difficult to bond to. We report an examination of bonding to thermally-aged epoxy surfaces, using as the adhesive the same epoxy as that of the aged surface. The cured and postcured epoxy was aged at 200 ° C, with the ageing time varying from 2 to 8 h. The fracture energy of the bond line was measured by mode I cleavage under conditions of relatively slow crack growth. The bondline fracture energy was found to decrease logarithmically with ageing time. The fracture energies for bonds to surfaces aged for 2, 4, and 8 h at 200 ° C were 0.077, 0.059, and 0.050 kJ M^–2, respectively. These compare to 0.13 kJ M^–2 for a bond to an unaged surface and 0.21 kJ m^–2 for bulk fracture. Fracture surfaces resulting from both slow and rapid fracture were examined by optical and scanning electron microscopy. Fracture features different from those arising from bulk fracture were found. Areas with good adhesion occurred amidst fields of featureless fracture surface; the frequency and size of these areas decreased with increased ageing time. Evidence of plastic deformation was found, always occurring on the new side of the bond: ridges parallel with crack propagation at high crack speeds and subsurface undulations perpendicular to crack propagation at low speeds. The bond has the effect of channelling the crack along the bondline, but fracture does not always remain exactly at the interface. Fracture often occurred a relatively constant distance away from the interface, suggesting that the presence of the interface was felt for some distance.     

Tensile creep of whisker-reinforced silicon nitride

This paper presents a study of the creep and creep rupture behaviour of hot-pressed silicon nitride reinforced with 30 vol% SiC whiskers. The material was tested in both tension and compression at temperatures ranging from 1100 to 1250°C for periods as long as 1000 h. A comparison was made between the creep behaviour of whisker-reinforced and whisker-free silicon nitride. Principal findings were: (i) transient creep due to devitrification of the intergranular phase dominates high-temperature creep behaviour; (ii) at high temperatures and stresses, cavitation at the whisker-silicon nitride interface enhances the creep rate and reduces the lifetime of the silicon nitride composite; (iii) resistance to creep deformation is greater in compression than in tension; (iv) the time to rupture is a power function of the creep rate, so that the temperature and stress dependence of the failure time is determined solely by the temperature and stress dependence of the creep rate; (v) as a consequence of differences in grain morphology and glass composition between whisker-free and whisker-reinforced material, little effect of whisker additions on the creep rate was observed.     

Fracture behavior of notched specimens of TiAl alloys

Fracture behavior of a two-phase TiAl alloy was investigated using notched specimens. Fracture surfaces and metallographic sections of surviving notch in double notched specimens are observed. The fracture process of notched specimens of TiAl alloys was described as that several inter-lamellar cracks initiate and extend directly from the notch root and propagate preferentially along the interfaces between lamellae and stop at various obstacles. With increasing applied load, cracks connect with each other and propagate further by translamellar cracks. The toughening mechanisms, which make the main crack difficult to propagate or cause it to be stopped, could be reducing the driving force for crack propagation. The higher toughness of near fully lamellar microstructure than that of finer duplex microstructure is attributed to the path of crack propagation. On the fracture surfaces of the finer duplex microstructure, more low-energy-spending interlamellar fracture facets are observed, which means that it is easier for crack to bypass a fine duplex lamellar grain with lamellae perpendicular to the main crack and to take a interlamellar path.     

Crack propagation and fracture in silicon carbide

Fracture mechanics and strength studies performed on two silicon carbides — a hot-pressed material (with alumina) and a sintered material (with boron) — have shown that both materials exhibit slow crack growth at room temperature in water, but only the hotpressed material exhibits significant high temperature slow crack growth (1000 to 1400? C). A good correlation of the observed fracture behaviour with the crack growth predicted from the fracture mechanics parameters shows that effective failure predictions for this material can be achieved using macro-fracture mechanics data.     

Fracture mechanisms in oxide scale on iron during substrate deformation

Oxide scales of different thickness and structure were grown on iron. Fracture of scales was studied when the underlying iron substrate was torsionally deformed at room temperature. For thin scales (5m) with a porous interface structure, the nucleation and growth of cracks occurred by the successive joining of interface pores. Slowly cooled scales of intermediate thickness (20m) failed by crack growth along oxide grain boundaries and from sharp corners of magnetite cuboids within wustite zones. For thick scales (35m), cracks nucleated from the base of the outermost magnetite crystallites. Rapidly cooled, thick scales exhibited crack nucleation from the sharp edges of voids at the scale/metal interface. Crack spacing in the oxide scale decreased with increasing substrate strain in a parabolic form.     

Fracture phenomenology of a sintered silicon nitride containing oxide additives

Crack propagation mechanisms in a sintered silicon nitride containing various oxide additives (ceria, magnesia, zirconia and strontium oxide) were studied as a function of initial flaw size, temperature, applied stress and time. Surface cracks of controlled size were introduced using the microhardness indentation-induced-flaw technique. At 20° C, the fracture stress was found to depend on initial crack size according to the Griffith relationship and extrapolation of the data indicated that processing flaws of 20 to 35 were strength-controlling. The flexural strength was found to be independent of temperature from 20 to 800° C and the mode of crack propagation was primarily transgranular. At temperatures above 800° C the flexural strength decreased significantly, due to viscous flow of the glassy phase present in the material and resulting in sub-critical crack growth (SCG). The mode of crack propagation during SCG was essentially intergranular. Flexural stress-rupture evaluation in the temperature range 800 to 1000° C has identified the stress levels for time-dependent and time-independent failures.     

Probabilistic models for defect initiated fracture in ceramics

Fracture tests on hot-pressed silicon nitride containing voids and several types of inclusion have been conducted. Fracture models pertinent to each defect type have been proposed and correlated with the data. The specificity of the fracture models is emphasized, and the various trends with defect size that result from the models are described. The resultant fracture probability relations are one of the key inputs to accept/reject decisions for nondestructive failure prediction.     

Effects of temperature and grain size on deformation and fracture in recrystallized Ni3Al doped with boron

The effects of temperature and grain size on the deformation and fracture behaviour of recrystallized Ni₃Al doped with boron were investigated by tensile tests at temperatures up to 973 K as a function of grain sizes from 1.6 to 105m. The yield stress showed a positive temperature dependence to a peak temperature in somewhat different manners depending on the grain size. For coarse-grained specimens, a rapid drop in elongation was observed with increasing temperature. The predominant fracture mode changed with temperature from the transgranular fracture of {1 1 1} cracking to brittle intergranular fracture. This embrittlement at elevated temperatures was considered to occur by a high stress concentration at grain boundaries arising from increased flow stress level and the occurrence of grain boundary sliding (GBS). In contrast, the elongation was not so markedly decreased with temperature for intermediate- and fine-grained specimens which exhibited ductile intergranular fracture and cavitation fracture, respectively, at elevated temperatures, and a slant-type fracture and cup-cone fracture, respectively, at low temperatures. The suppression of serious high-temperature embrittlement for intermediate-grained specimens was explained in terms of the slow propagation of a crack formed by GBS, owing to stress relaxation by dynamic recrystallization (DR) and plastic deformation. In the case of ultra-fine-grained specimens a large elongation was developed at elevated temperatures, which was interpreted as that the further occurrence of DR with increasing volume fraction of grain boundaries reduces the cavitation promoted by GBS, and that the limited sliding length due to extremely small grain diameter raises the stress for cavity formation.     

Brazing of reaction-bonded silicon carbide and Inconel 600 with an iron-based alloy

The objective of the present work was to join reaction-bonded silicon carbide to Inconel 600 (IN600, a nickel-based superalloy) for use in high temperature applications by brazing with an Fe-20wt% alloy. This joining method resulted in the molten filler metal reacting with the IN600 to form a Ni-Fe-Si solution, which in turn formed a liquid with the free silicon phase of the RBSC. This liquid reacted vigorously with the SiC component of the RBSC to form low melting point phases in both starting materials and chromium carbides at the metal-ceramic interface. By using solution thermodynamics, it was shown that a Ni-Fe-Si liquid with equimolar nickel and iron contents and silicon content of less than 30 at% Si will decompose -SiC at the experimental brazing temperatures; it was also shown that these predictions agree with the experimentally observed microstructures and line composition profiles.     

Proof-testing of hot-pressed silicon nitride

Proof-testing was investigated as a method for insuring the reliability of hot-pressed silicon nitride in high temperature structural applications. The objective of the study was to determine if the strength distribution of a population of test specimens could be truncated by proof-testing. To achieve this objective the strength of silicon nitride was measured at 25° C and 1200° C, both with and without proof-testing. At 25° C, however, the strength distribution was effectively truncated by proof-test ing. At 1200° C, however, the effectiveness of proof-testing as a means of truncating the strength distribution was determined by the resistance of the silicon nitride to oxidation. Although oxidation removes machining flaws that limit the strength of silicon nitride, long-term exposure to high temperature oxidizing conditions resulted in the formation of surface pits that severely degraded the strength. Provided the effects of high temperature exposure are taken into account, proof-testing is shown to be useful for truncating the strength distribution of hot-pressed silicon nitride at elevated temperatures.     

Effect of selected impurities on the high temperature mechanical properties of hot-pressed silicon nitride

The effect of impurities on the high temperature mechanical properties of hot-pressed silicon nitride has been determined. Selected impurity additions were made to both relatively pure -phase and -phase silicon nitride starting powders. These powders were hot-pressed to full density using 5 wt % MgO as the pressive additive. The silicon nitride hot-pressed from the -phase powder exhibited higher strength at both 25 and 1400 C than that fabricated from the -phase powder. The impurity additions had no effect on the room temperature mechanical properties. The CaO additions had the most significant effect on the high temperature mechanical properties. In both the material hot-pressed from the -phase and -phase powders, increasing CaO additions severely reduced the high temperature strength and increased the amount of non-elastic deformation observed prior to fracture. Although alkali additions (Na₂CO₃, Li₂CO₃, K₂CO₃) also tended to have the same effects as the CaO, the high volatility of these compounds resulted in a much reduced concentration in the hot-pressed material, thus minimizing somewhat their tendency to enhance the high temperature strength degradation. The Fe₂O₃ and Al₂O₃ had no apparent effect on the high temperature mechanical properties.     

Microscopy investigation on fracture mechanisms in hot-isostatically pressed Si3N4/SiC-platelet composites

Fracture mechanisms in hot-isostatically pressed (HIP) Si₃N₄/SiC-platelet composites have been investigated by transmission electron (TEM) and scanning electron (SEM) microscopy followed by profilometric analyses. Two composites containing 25 vol% platelets were compared. They were fabricated from the same raw materials and by the same procedure except for the cooling rate from the sintering temperature. The study consists of experimental observations as well as measurements of fractographic parameters which dictates the level of toughening, such as the percentage of intergranular fracture, lengths and angles associated with the debonding process at the matrix/platelet interface. The presence of microcracking in the neighbourhood of the main crack, a higher fraction of intergranular fracture, as well as substantial debonding at the nitride/carbide interface up to high orientation angles were found in the composite cooled at low rates ( 100°Ch^–1) which, despite the unchanged microstructure, was substantially tougher than that cooled at 650°Ch^–1. These trends were not observed in the composite subjected to fast cooling. The stronger interfacial bonding found after fast cooling under high pressure was attributed to an apparent compressive stress remaining stored at the grain boundary, rather than to a weakening of the platelets or the matrix grains. Calculations based on the mechanics analysis of crack/interface interactions and on quantitative profilometric data, indicated a difference of about one order of magnitude in the apparent interface fracture energy of the two composites.     

Mechanical properties of monolithic AIN and SiCw/AIN composites

Flexural, tensile, and high cycle fatigue test data are presented for pressureless sintered aluminium nitride (AIN) and hot-pressed aluminium nitride reinforced with silicon carbide whiskers (SiC_w/AIN). Tests were conducted at ambient temperature. The SiC_w/AIN composites consisting of 30wt% SiC_w produced significant increases in flexural strength, tensile strength, and tensile fatigue strength compared to monolithic AIN. Increases were nearly double in all cases. Corresponding strain-to-failures measured in tensile tests increased from 0.04% in monolithic AIN to 0.10% in the SiC_w reinforced composite. Fracture surfaces showed evidence of whisker-toughening mechanisms due to additions of SiC_w whiskers. High-cycle fatigue results indicated that both materials have the ability to sustain higher stress levels in the cyclic tests compared to the tensile experiments. The improved performance under cyclic testing is explained in terms of strain-rate effects. The times at or near peak stress are considerably less under high-cycle fatigue testing (20 Hz) compared to tensile tests (strain rate = 0.5%min^–1).     

Thermal cycling of siliconized-SiC at high temperatures

Thermal cycling (between 1340 °C and 1480 °C) experiments were conducted using two types of reaction-bonded (siliconized) silicon carbide. A commercial material (Crystar^TM) and various silicon carbide pieces that had been joined together using electrophoretic deposition (EPD) followed by reaction bonding were evaluated. During the thermal cycling, residual free silicon metal rapidly vaporized from the Crystar^TM and cracks developed within its large SiC grains. In contrast, the EPD/reaction-bonded silicon carbide joints did not lose an observable amount of their residual silicon nor develop cracks. The reduced loss was attributed to reduced silicon content with the silicon residing largely in closed pores of the EPD layer. Reduced vaporization of the silicon that resided in surface-connected pores was engineered by applying a thick SiC surface coating. The morphology of the resulting coating was microscopically evaluated and two sequential growth mechanisms were postulated. An implication of this research is that hermetic (gas-tight) joints could be formed using EPD-derived SiC as a filler material.     

Investigation of fracture mechanism of 6063 aluminum alloy under different stress states

Fracture mechanisms in a 6063 aluminum alloy were investigated and analyzed carefully by in-situ tensile tests in SEM with a vacuum chamber. Specimens used were designed to produce different stress states. Studies indicated that with stress triaxiality (σ _m/σ _e) decreasing, the fracture modes changed from normal fracture to shear fracture and the fracture surfaces changed from the dimples and intragranular dominated fracture mode to the shear dominated fracture mode. The grain boundaries of the 6063 aluminum alloy were the weakest positions. In the case of high stress triaxiality, the grain boundary cracks were produced by normal stress or by the incompatibility of deformation between neighboring grains, and the normal stress dominated the crack propagation. In the case of low stress triaxiality, the boundary cracks were produced by the relative slipping of grains against neighboring grains, and the shear stress dominated the crack propagation. The final fracture of the specimens occurred by connections of cracks through transgranular cracking of the ligaments among these cracks.