Transient creep behaviour of hot isostatically pressed silicon nitride

Transient creep is shown to dominate the high-temperature behaviour of a grade of hot isostatically pressed silicon nitride containing only 4 wt% Y₂O₃ as a sintering aid. Contributing factors to transient creep are discussed and it is concluded that the most likely cause of longterm transient creep in the present study is intergranular sliding and interlocking of silicon nitride grains. In early stages of creep, devitrification of the intergranular phase, and intergranular flow of that phase may also contribute to the transient creep process. The occurrence of transient creep precluded the determination of an activation energy on the as-received material. However, after creep in the temperature range 1330–1430°C for times exceeding approximately 1100 h, an apparent activation energy of 1260 kJ mol^–1 was measured. It is suggested that the apparent activation energy for creep is determined by the mobility and concentration of diffusing species in the intergranular glassy phase. The time-to-rupture was found to be a power function of the minimum strain rate, independent of applied stress or temperature. Hence, creep-rupture behaviour followed a Monkman-Grant relation. A strain rate exponent of – 1.12 was determined.     

Vapour transport of magnesia into reaction-bonded silicon nitride

Magnesia, as a sintering additive, can be introduced into reaction-bonded silicon nitride (RBSN) via vapours-phase transport. The principal process variables were studied; a controlling factor was found to be the amount of silica on the internal surfaces of the ceramic. Through a controlled oxidation of the RBSN, the amount of magnesia introduced to the compact was increased to 2 wt% allowing a post-sintered density of 93% theoretical to be achieved. Further increases in internal oxidation, and consequent magnesia uptake, were limited by the oxidation of Si₃N₄ whiskers on the surface of the RBSN and in its pore structure.     

Effects of heat treatments on the creep properties of a hot pressed silicon nitride ceramic

Effects of heat treatment in an argon atmosphere at high temperatures for varying times on the creep properties of a Y₂O₃-Al₂O₃ (8-2 wt%) doped hot pressed silicon nitride (HPSN) ceramic were investigated. It was observed from the creep measurements that higher temperature, i.e. 1360C, and longer time, i.e. 8 h, heat treatment in an argon atmosphere improved the creep properties, (e.g. secondary creep rate) of this material. Heat treatment at a lower temperature of 1300C and for a shorter time of 4 h did not change the creep behaviour. Improvement of the creep properties was related to the crystallization of an amorphous grain boundary phase by heat treatment. Secondary creep rate parameters of the as-received material: stress exponent, n (2.95–3.08) and activation energy, Q (634–818 kJ mol^S–1), were in the range of values found by other investigators for various hot pressed silicon nitride ceramics.     

The interpretation of cyclic creep deformation mechanism of copper in terms of the anelastic recovery

The cyclic creep deformation behaviour of copper has been studied in the temperature range of 0.4 to 0.5T _m and under the constant stress range of (/E)=4×10^–4 to 10×10^–4. To see the effect of cyclic stress frequency, stress amplitude and the duration of the unloading time in a fixed frequency on cyclic creep behaviour, static and cyclic creep tests were conducted under the conditions mentioned above. The measured activation energies for static and cyclic creep were analysed in terms of the various experimental parameters. Anelastic behaviour during the unloading period was also studied to find out the possible assistance for the positive creep deformation in cyclic creep. Using the concept of anelastic recovery and the activation energy for the anelastic it is hypothesized that the accelerated cyclic creep deformation is controlled by the anelastic recovery during the unloading period.     

Tensile creep behaviour of a fibre-reinforced SiC-Si3N4 composite

The tensile creep behaviour of a SiC-fibre-Si₃N₄-matrix composite was investigated in air at 1350 C. The unidirectional composite, containing 30 vol % SCS-6 SiC fibres, was prepared by hot pressing at 1700 C. Creep testing was conducted at stress levels of 70, 110, 150 and 190 MPa. An apparent steady-state creep rate was observed at stress levels between 70 and 150 MPa; at 190 MPa, only tertiary creep was observed. For an applied stress of 70 MPa, the steady-state creep rate was approximately 2.5×10^–10 s^–1 with failure times in excess of 790 h. At 150 MPa, the steady-state creep rate increased to an average of 5.6×10^–8 s^–1 with failure times under 40 h. The creep rate of the composite is compared with published data for the steady-state creep rate of monolithic Si₃N₄.     

Tensile creep behaviour of a silicon carbide-based fibre with a low oxygen content

The high-temperature mechanical behaviour and microstructural evolution of experimental SiC fibres (Hi-Nicalon) with a low oxygen content (<0.5 wt%) have been examined up to 1600 °C. Comparisons have been made with a commercial Si-C-O fibre (Nicalon Ceramic Grade). Their initial microstructure consists of -SiC crystallites averaging 5–10 nm in diameter, with important amounts of graphitic carbon into wrinkled sheet structures of very small sizes between the SiC grains. The fall in strength above 800 °C in air is related to fibre surface degradation involving free carbon. Crystallization of SiC and carbon further develops in both fibres subject to either creep or heat treatment at 1300 °C and above for long periods. The fibres are characterized by steady state creep and greater creep resistance (one order of magnitude) compared to the commercial Nicalon fibre. The experimental fibre has been found to creep above 1280 °C under low applied stresses (0.15 GPa) in air. Significant deformations (up to 14%) have been observed, both in air and argon above 1400 °C. The stress exponents and the apparent activation energies for creep have been found to fall in the range 2–3, both in air and argon, and in the range 200–300 kJ mol^–1 in argon and 340–420 kJ mol^–1 in air. The dewrinkling of carbon layer packets into a position more nearly aligned with the tensile axis, their sliding, and the collapse of pores have been proposed as the mechanisms which control the fibre creep behaviour.     

Static creep behaviour of Al-Zn-Mg and Al-Zn-Mg-Cu alloys

The creep behaviour of Al-Zn-Mg (7039) and Al-Zn-Mg-Cu (7075) alloys is evaluated at elevated temperatures (443T533 K and 483T563 K) under constant stresses between 49 and 123 MPa, respectively, in a custom-built creep testing facility. The measured activation energies of these alloys are 172–185 kJ mo^–1 and 248–272 kJ mol^–1. As the stress increases, the activation energy in both cases decreases due to the high density of dislocations. The average exponent values of these alloys are 7 and 9. The microstructure observation reveals that the dominant fracture mode of 7039 alloy is intergranular and that of 7075 alloy is transgranular.     

Thermodynamics and kinetics of oxidation of hot-pressed silicon nitride

The passive oxidation behaviour of silicon nitride hot-pressed with 1 wt% MgO has been studied in dry oxygen in the temperature range 1000 to 1400 C. The oxidation follows the classical parabolic behaviour with an apparent activation energy of 375 kJ mol^–1. Except for minor amounts of a glass and cristobalite, the oxide film consists predominantly of MgSiO₃ in which various impurity elements, e.g. Ca, Fe, Al, etc., concentrate. The outward diffusion of Mg²⁺ and impurity cations from the grainboundary glass phase through the oxide film appears to be oxidation rate controlling.     

Oxidation protection of porous reaction-bonded silicon nitride

Oxidation kinetics of both as-fabricated and coated reaction-bonded silicon nitride (RBSN) were studied at 900 and 1000°C with thermogravimetry. Uncoated RBSN exhibited internal oxidation and parabolic kinetics. An amorphous Si-C-0 coating provided the greatest degree of protection to oxygen, with a small linear weight loss observed. Linear weight gains were measured on samples with an amorphous Si-N-C coating. Chemically vapour deposited (CVD) Si₃N₄ coated RBSN exhibited parabolic kinetics, and the coating cracked severely. A continuous-SiC-fibre-reinforced RBSN composite was also coated with the Si-C-O material, but no substantial oxidation protection was observed.     

High temperature bending creep of a Sm-α-β Sialon composite

The four-point bending creep behavior of a Sm-- Sialon composite, in which Sm-melilite solid solution (denoted as M) was designed as intergranular phase, was investigated in the temperature range 1260–1350°C and stresses between 85 and 290 MPa. At temperatures less than 1300°C, the stress exponents were measured to be 1.2–1.5, and the creep activation energy was 708 kJ mol^–1, the dominant creep mechanism was identified as diffusion coupled with grain boundary sliding. At temperatures above 1300°C, the stress exponents were determined to be 2.3–2.4, and creep activation energy was 507 kJ mol ^–1, the dominant creep mechanism was suggested to be diffusion cavity growth at sliding grain boundaries. Creep test at 1350°C for pre-oxidation sample showed a pure diffusion mechanism, because of a stress exponent of 1. N^3– diffusing along grain boundaries was believed to be the rate controlling mechanism for diffusion creep. The oxidation and Sialon phase transformation were analyzed and their effect on creep was evaluated.     

Steady state creep and creep recovery behaviours of pre-aging Al–Si alloys

The creep and creep recovery of pre-aging Al–1 wt.%Si and Al–1 wt.%Si–0.1 wt.%Zr–0.1 wt.%Ti alloys have been investigated at room temperature under different constant stresses. The aging temperature dependence of steady creep rate, _st, and the recovery strain rate, π, show that under the same test conditions first alloy yields creep or creep recovery rates much higher as compared with those of second alloy. The stress exponent n was found to change from 2.5 to 7.43 and 4.57 to 11.99 for two alloys, respectively, characterizing dislocation slipping mechanism. The activation energies of steady state creep of the two alloys were found to be 78.4 kJ/mol and 32.8 kJ/mol for Al–Si and Al–Si–Zr–Ti alloys, respectively. The microstructure of the samples studied was investigated by optical and transmission electron microscopy (TEM).     

Transmission electron microscopy analysis of a TiN x substoichiometric CVD coating

The TiN _{x 1–x} substoichiometric chemical vapour deposited titanium nitride coatings have been studied in an early work by means of high-resolution X-ray emission spectroscopy. It was found that a strong vacancy-induced peak was present in the Ti K emission band. Its intensity can be correlated to the 1–x vacancy concentration deduced from nuclear reaction spectroscopy and X-ray diffraction. This relationship is linear if 01–x0.3. If 1–x is higher than 0.3, an anomalous behaviour occurs which is expected to be due to microstructure change. For this purpose, transmission electronic studies of a TiN_{0.57 0.43} layer have been developed. The most striking result of this work is the existence of many stacking faults. These defects are extrinsic ones and the stacking fault energy is about 3.5 mJ m^–2. Their density seems to depend on their distance from the substrate-coating interface. Further investigations are needed to confirm this assumption.     

Creep of (Mg,Fe)O single crystals

The creep behaviour of (Mg, Fe)O single crystals compressed along 1 0 0 has been investigated over the temperature range 1300 to 1500° C, at stresses between 20 and 70 MPa, for oxygen partial pressures between 10^–4 and 10² Pa, and with iron concentrations between 70 and 11 900 p.p.m. Under these conditions, the dependence of the steady-state strain rate on stress, temperature, oxygen partial pressure, and iron concentration can be summarized by the flow law, exp (–445 kJ mol^–1/RT. These results suggest that the steadystate strain rate is controlled by dislocation climb with a jog velocity which is limited by lattice diffusion of oxygen by a vacancy pair mechanism. The activation energy for creep, 445 kJ mol^–1 is larger than that reported for self-diffusion of oxygen, 330 kJ mol^–1, because the formation energy for jogs is relatively large, 115 kJ mol^–1.     

Oxidation mechanism of porous silicon nitride

The oxidation of reaction-bonded silicon nitride (RBSN) has been studied in air between 800 and 1500° C. The extent of internal oxidation is governed by the radius of the pore channels allowing the oxygen to penetrate the specimen. The velocity of oxygen transport into narrow channels is very low compared to the reaction rate of oxygen with Si₃N₄. Because of these two concurrent processes an oxygen gradient is built up along the channel axis leading to SiO₂ formation mainly at the channel mouth. The typical oxidation isotherm of RBSN is represented by an asymptotic law. The mass gain and the penetration depth of oxidation is calculated, based on reaction-rates of Si₃N₄-powder, oxygen-diffusion-data and the pore-characteristics of the RBSN-materials, and compared with the experimental results. The results clearly indicate, that high quality RBSN may wel be used in oxidizing atmosphere without extensive internal oxidation.     

Effect of multiple strain-anneal cycles on the 1000° C creep behaviour of γ/γ′-α

An investigation of the influence of multiple strain-anneal cycles on the 1000° C creep behaviour of the directionally solidified eutectic alloy /- has been undertaken. Cycles consisted of swageing at room temperature or 900° C by about 5 to 10% per pass followed by annealing at 900° C, and were repeated to total strains of approximately 10, 30 and 50%. Transmission electron microscopy (TEM) of strain-annealed materials revealed that three-dimensional dislocation networks were introduced into the matrix, but very little work remained in the fibres. Constant-velocity creep testing indicated that all thermomechanical processing schedules improved the creep strength for strain rates 2 x 10^–6sec^–1; however only strain-annealing with a total of 13% work at room temperature (RT13) improved the behaviour at strain rates 2 x 10^–7sec^–1. The advantage of RT13 processing over as-grown materials at lower strain rates was confirmed by constant-load creep testing. It was also shown that 900° C annealing slightly improves the 1000° C creep properties in comparison to as-grown alloys. TEM of crept materials indicated that the active creep mechanism had been changed from dislocation pile-ups at fibres in as-grown alloys to dislocations being stopped by sub-boundaries in the matrix for RT13.     

Oxidation behaviour of the sintered Si3N4-Y2O3-Al2O3 system

The oxidation behaviour of silicon nitride composed of Si₃N₄, Y₂O₃, Al₂O₃, AlN and TiO₂ was investigated in dry and wet air at 1100–1400 °C. The oxidation rates were confirmed to obey the parabolic law. An activation energy of 255 kJ mol^–1 was calculated from the Arrhenius plots of the results of oxidation in an air flow. In still air the oxidation rate was larger than that in an air flow, but the oxidation rate in flowing air was not affected by the air flow rate. -cristobalite and Y₂O₃·2SiO₂ were formed in oxidized surface layers. These crystal phases increased with increasing oxidation temperature. In particular, a higher content of -cristobalite was obtained in still air oxidation. The existence of water vapour in flowing air greatly promoted the oxidation.Concurrent with Kanagawa Academy of Science and Technology.     

The high temperature creep deformation of Si3N4-6Y2O3-2Al2O3

The creep properties of silicon nitride containing 6 wt % yttria and 2 wt% alumina have been determined in the temperature range 1573 to 1673 K. The stress exponent, n, in the equation ⁿ was determined to be 2.00±0.15 and the true activation energy was found to be 692±25 kJ mol^–1. Transmission electron microscopy studies showed that deformation occurred in the grain boundary glassy phase accompanied by microcrack formation and cavitation. The steady state creep results are consistent with a diffusion controlled creep mechanism involving nitrogen diffusion through the grain boundary glassy phase.     

Ultrasonic examination of reaction bonded silicon nitride

An ultrasonic examination has been made of a series of partially nitrided reaction-bonded silicon nitride (RBSN) ceramics whose weight gains varied from 22% to nearly 64% representing full nitridation. A pulse echo overlap technique was used which enabled both the longitudinal and shear velocities of propagation to be measured at 15 MHz; from these measurements values of Young's modulus (E) and the bulk modulus (K) at room temperature were derived. For fully nitrided RBSN the values obtained wereE=160 GN m^–2,K=90 GN m^–2 in good agreement with published values obtained by ultrasonic methods. Both Young's modulus and the bulk modulus were found to be markedly sensitive to the changes in fractional porosity due to changes in weight gain (and green density) each decreasing by over 50% as the fractional porosity increased from 0.16 for a fully nitrided ceramic to 0.26 for 60% weight gain material.     

Fracture strength of low-pressure injection-moulded reaction-bonded silicon nitride

Reaction-bonded silicon nitride (RBSN) samples were fabricated via a low-pressure injection-moulding technique. Sample batches of 58, 68, and 70 vol % silicon solids loading were moulded using a multicomponent, nonaqueous binder. These samples were analysed in terms of their nitrided bulk density, flexural strength, and microstructure. Bulk densities of 2.9 g cm^–3 (91% theoretical density) and three-point moduli of rupture in excess of 304 MPa (44×10³ p.s.i.) were achieved. These results indicate a potential use of low-pressure injection moulding as a forming technique for the fabrication of RBSN components.     

Mössbauer study on the magnetic state of iron particles in Fe-N and Fe-Zr-N soft magnetic thin films

The magnetic state of -Fe particles and the behaviour of nitrogen and zirconium during annealing in Fe₉₆N₄ and Fe_85.6Zr_7.6N_6.8 magnetic thin films have been studied by conversion electron Mössbauer spectroscopy for ⁵⁷Fe. The crystalline phases present in the Fe-N annealed films were -Fe and -Fe₄N, and those in the Fe-Zr-N annealed films were -Fe and ZrN. In the Fe-N films annealed below 300°C, about 60% nitrogen is incorporated interstitially into -Fe and the rest is used for the formation of -Fe₄N. In the Fe-N film annealed at 500°C, almost all nitrogen participates in the formation of -Fe₄N, leading to the grain growth of -Fe particles and an increase in coercive force. The values (291–325 kOe) of internal magnetic field of iron sites in -Fe in the Fe-Zr-N films are much smaller than that (333 kOe) of the iron site in pure -Fe. Even if the Fe-Zr-N films were annealed at 500–700°C, some zirconium and nitrogen is still incorporated substitutionally and interstitially into -Fe, respectively. In particular, the substitutional zirconium depresses the grain growth of -Fe particles, perhaps due to a chemical interaction between zirconium and iron.