Oxidation behavior of hot isostatically pressed silicon nitride containing Y2O3

Oxidation in the presence of air and water vapor at high temperatures wasstudied for Si₃N₄ ceramics containing Y₂O₃ and Al₂O₃ as sintering aids.The test environments for this study consisted of air with 0, 1.2,and 6.4 v/o H₂O at temperatures from 1000°C to 1350°C. Theoxidation exposure times were up to 500 hours. The presence of water vaporenhances oxidation and crystallization of the oxidation phases. Weight losswas observed for the oxidation in air or dry air because of Na contaminationduring the fabrication processing. The effect of applied stress on the growthof oxide scale is minimal, however, the applied stress resulted in deeperpenetration of oxygen and pit formation in the oxide phase.     

Residual strength of ground hot isostatically pressed silicon nitride

Surface flaws are introduced during grinding of most high strength ceramics. These flaws reduce the strength and it is therefore important to choose grinding parameters such that surface damage is minimized. The assumption that it is the same mechanism that causes cracking beneath both an indenter and a diamond tool made it possible to propose a grinding model. According to this model high wheel speeds, low workpiece velocities and low depths of cut would reduce the grinding forces and thus be beneficial to the strength after grinding. Grinding experiments on hot isostatically pressed silicon nitride showed that this was the case. The experiments also showed that the grinding direction had the strongest influence on the strength, and if possible the direction ought to be parallel with the expected principal stress. Even what can be considered to be mild machine parameters introduce flaws and residual compressive stresses in the surface of the workpiece.     

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.     

Cyclic fatigue of hot isostatically pressed silicon nitride at elevated temperatures

Cyclic fatigue properties of a hot isostatically pressed silicon nitride were investigated at 1150, 1260 and 1370 °C in ambient air. The uniaxial tensile tests were conducted under various cyclic loading wave forms and frequencies. The correlation of stress-life relations between cyclic and static fatigue results was evaluated. At 1150–1370 °C, cyclic loading caused less damage than static loading, as evidenced by the longer failure time under cyclic loading versus static loading with the same maximum applied stresses. The cyclic loading effect was more pronounced in high frequency tests at 1260 and 1370 °C and might be related to the viscoelastic behaviour of the intergranular phase. Microstructural analyses and macroscopic cyclic stress-strain and strain-time relations indicated that cyclic loading/unloading may inhibit the normal accumulation of creep damage.     

Mechanical properties of hot isostatically pressed zirconium nitride materials

The influence of powder properties on the sintering behaviour, the microstructural development and the mechanical properties of hot isostatically pressed (HIPed) zirconium nitrides were investigated. The results show that the densification behaviour is dependent on the powder characteristics, more precisely the grain morphology and size, and oxygen, carbon and metallic impurity contents. The mechanical properties are controlled mainly by the amount of porosity and the presence of a complex intergranular phase in direct relation to the purity of the starting powder. The differences in the fracture strength and toughness between the two grades are perceptible. On the other hand, the thermal shock resistance to fracture initiation, as well as the elastic parameters of both dense materials, are similar.     

Effect of processing conditions on the characteristics of pores in hot isostatically pressed alumina

Effect of processing conditions on the characteristics of residual pores was studied with an optical microscope in hot isostatically pressed translucent alumina ceramics. Green bodies formed by isostatic pressing were sintered at 1300, 1400 and 1600°C and then hot isostatically pressed at a temperature 50°C below the respective sintering temperature for 1 h at 100 MPa. All specimens were fully dense within experimental accuracy (±0.1%), and the grain size increased with increasing sintering/hot isostatic pressing temperatures. A variety of pores were found in all specimens. The distribution of pores was uniform at various locations within the specimen. The pore population decreased with increasing pore size, but was finite in the size range exceeding 84 m. The pores in this range increased with increasing sintering/hot isostatic pressing temperature. Except for these large pores, the pore population was similar under all processing conditions.     

An analysis of the creep of hot pressed silicon nitride in bending

The creep behaviour of hot pressed silicon nitride is investigated in four-point-bend tests at temperatures of about 1200° C. By use of appropriate creep laws for the primary creep range as well as for the secondary range the experimental results can be well described analytically.     

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.     

Yb2O3-fluxed sintered silicon nitride

Microstructural development and crystallization behaviour of Yb₂O₃-fluxed sintered silicon nitride materials was investigated using CTEM and HREM. The materials contained 5 and 10 vol% Yb₂O₃ as sintering additives. After densification, both compositions were subsequently heat treated to crystallize the residual amorphous secondary phases present at triple-grain regions. In the material doped with 5 vol% Yb₂O₃, only an amorphous secondary phase was observed after sintering, which was about 80% crystalline (Yb₂Si₂O₇) after the post-sintering heat treatment. A metastable phase was formed in the material with 10 vol% additives after sintering, with about 70% crystallinity in the triple-point pockets. Upon postsintering heat treatment, the material could be completely crystallized. During heat treating, the metastable phase combined with the remaining glass to form Yb₂SiO₅ plus Yb₂Si₂O₇ and a small amount of Si₃N₄ which deposited epitaxially on pre-existing Si₃N₄ grains in areas of low-energy within the triple-point pockets. All materials contained thin amorphous films separating the grains. The amorphous intergranular films along grain boundaries (homophase boundaries) revealed excess ytterbium and oxygen. The thickness of the intergranular films was about 1.0 and 2.5 nm for the grain boundaries and the phase boundaries, respectively, independent of additive content and heat-treatment history.     

Si3N4-Al2O3-ZrO3 hot pressed composites

Development of microstructure, thermal expansion behaviour and room temperature MOR, Vickers microhardness and toughness have been studied for 83wt%Si₃N₄-5wt%Al₂O₃-12wt%ZrO₂ composites hot pressed at 34.4MPa for various times and temperatures. The deleterious presence of Zr-O-N phases is avoided in these materials. Microcracking has been observed because of the mismatch between thermal expansion coefficients. A phenomenological model is suggested to account for the variation of cell parameters of the β'-phase with the progress of the α→β' conversion. No detectable improvement in hardness and toughness results from the presence of zirconia. The substantial MOR increase, from ~370 MPa for a Si₃N₄-Al₂O₃ reference material to~630 MPa for the ZrO₂ composite has been ascribed more properly to a grain boundary phase effect, than to specific ZrO₂-induced microcrack strengthening.     

Oxidation resistance and creep behaviour of a silicon nitride ceramic densified with Y2O3

The corrosion resistance and creep behaviour in air of hot-pressed materials with a composition of 70 Si₃N₄-25 SiO₂-5 Y₂O₃ (mol%) has been studied. Kinetics data and microstructural changes in the 1180 to 1650° C temperature range indicate the presence of two oxidation mechanisms. Between 1180 and 1420° C, the preferential oxidation of the intergranular phase containing nitrogen is interpreted in terms of an inward diffusion of ionic oxygen. At temperatures higher than 1420° C, the degradation of the material is the sum of many processes (solution of silicon nitride, migration of oxygen and yttrium and release of nitrogren) but the diffusion to the nitride-oxide interface of a complex combination of yttrium and nitrogen in the boundary phase seems to be the limiting step. The three-point bending creep is discussed in relation to the evolution of the secondary intergranular phases in an oxidizing environment. The creep deformation is the sum of a viscoelastic component and a diffusional component characterized by the same activation energy (720 kJ mol^–1).     

Effect of silicon on the fracture toughness and oxidation behavior of hot pressed NbCr2 alloys

NbCr₂ Laves phase alloyed with 0–7 wt.% Si was fabricated by mechanical alloying followed by hot pressing. The influence of silicon on the mechanical properties and oxidation behavior of NbCr₂ were investigated. It was revealed that Si addition has a beneficial effect on the oxidation resistance and fracture toughness of NbCr₂ alloy. The addition of Si partially occupies the Cr site in the Laves phase and partially forms the hard Nb₅Si₃ phase, which can yield an increase in the hardness of as-HPed NbCr₂ alloys. When alloying with 5 wt.% silicon, the fracture toughness value of NbCr₂ reaches the highest (6.45 MPa √m) which is about 13% more than that of unalloyed NbCr₂ and is 4 times higher than that of cast materials (1.2 MPa √m). Addition of silicon also resulted in a substantial improvement in the oxidation resistance of the NbCr₂ alloys exposed in air at 1373 K and 1473 K.     

Electron transport in hot pressed Y3−x Gd x Fe5O12

Electron transport properties of a few hot-pressed garnets of the series Y_3−xGd_xFe₅O₁₂ (wherex=0, 1 and 2·4) have been measured. For comparison, a normal sintered YIG has been studied to see the effect of porosity and microstructure. The electron transport properties have been discussed on the basis of the model suggested by Austin and Mott keeping in view the distortion caused by the substitution.     

Characterization of hot pressed Al2O3-platelet reinforced hydroxyapatite composites

Hydroxyapatite reinforced with monocrystalline Al₂O₃ platelets was densified by hot pressing. The effect of volume fraction and size of platelets on the microstructure, strength and toughness was investigated. It was demonstrated that no phase degradation occured during thermal treatments. A better homogeneity of composite mixtures was achieved when large platelets had been used. In return, the incorporation of small platelets appeared more favourable to increase the mechanical characteristics although limiting effect induced by microstructural defects. The flexural strength can reach 140 MPa with an associated fracture toughness of 2.5 MPa % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaca% qGTbaaleqaaaaa!36F8!\[\sqrt {\text{m}} \] compared to 137 MPa and 1.2 MPa % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaca% qGTbaaleqaaaaa!36F8!\[\sqrt {\text{m}} \] for dense monolithic HAP. The observation of crack propagation allowed us to point out the mechanisms responsible of this toughening. Crack deflection on disk faces should be considered as the initiating phenomenon leading to platelet debonding, crack bridging or branching. Crack deflection and branching may also result in the formation of unbroken ligaments of material which bridge the crack.     

Deformation behaviors of as-built and hot isostatically pressed Ti-6Al-4V alloys fabricated via electron beam rapid manufacturing

The deformation behavior of as-built and hot isostatically pressed(HIP) Ti-6Al-4V alloys fabricated using electron beam rapid manufacturing(EBRM) were investigated in this work.The deformation characteristics were characterized using a laser scanning confocal microscope and electron back-scattered diffraction(EBSD).In the as-built sample,prismatic slip was the main mode of deformation,as well as a small amount of basal slip and cross-slip.Some planar slip lines with large length scales were observed across severalα lamellae.After hot isostatical pressing,prismatic and basal slip were the main mode of deformation,accompanied by abundant cross-slip and multiple slip,and most of the slip lines were blocked within an a lamellae.These differences in deformation behavior were associated with the coarsening of a laths and the more retained p phase after HIP compared to the as-built alloy.More cross-slip and multiple slip can lead to superior elongation-to-failure and a greater strain hardening effect in the HIP alloy compared to the as-built sample.     

Ultrafast deposition of silicon nitride and semiconductor silicon thin films by hot wire chemical vapor deposition

The technology of Hot Wire Chemical Vapor Deposition (HWCVD) or Catalytic Chemical Vapor Deposition (Cat-CVD) has made great progress during the last couple of years. This review discusses examples of significant progress. Specifically, silicon nitride deposition by HWCVD (HW-SiN_x) is highlighted, as well as thin film silicon single junction and multijunction junction solar cells. The application of HW-SiN_x at a deposition rate of 3 nm/s to polycrystalline Si wafer solar cells has led to cells with 15.7% efficiency and preliminary tests of our transparent and dense material obtained at record high deposition rates of 7.3 nm/s yielded 14.9% efficiency. We also present recent progress on Hot-Wire deposited thin film solar cells. The cell efficiency reached for (nanocrystalline) nc-Si:H n-i-p solar cells on textured Ag/ZnO presently is 8.6%. Such cells, used in triple junction cells together with Hot-Wire deposited proto-Si:H and plasma-deposited SiGe:H, have reached 10.9% efficiency. Further, in our research on utilizing the HWCVD technology for roll-to-roll production of flexible thin film solar cells we recently achieved experimental laboratory scale tandem modules with HWCVD active layers with initial efficiencies of 7.4% at an aperture area of 25 cm².     

Effect of Y2O3 on the mechanical properties of open cell aluminum foams

Open cell aluminum foams were synthesized by the space-holder method. NaCl particles as space-holder were used to produce samples. The effect of the Y₂O₃, which was added to the aluminum powders on the mechanical properties of the open cell aluminum foams, has been studied. Adding Y₂O₃ powders in the compacts has important effect on the foams showing better mechanical properties than those without Y₂O₃.