Foreign Object Damage Resistance of Silicon Nitride and Silicon Carbide

To clarify the foreign object damage (FOD) resistance of ceramics, chipping fracture mode and flexural fracture mode were investigated using several types of Si₃N₄ and Sic. The critical velocity which is the threshold impact velocity of the projectile for chipping fracture and flexural fracture was determined. The critical velocity of the chipping fracture mode is explained as a function of K ^5/2_IC a ^–5/4, and depends on the hardness and the shape of the projectile. The critical velocity of the flexural fracture mode is explained as a function of σ^5/6_C t ^5/3. The mechanisms of impact damage are discussed.     

Anisotropy of Silicon Nitride with Aligned Silicon Nitride Whiskers

A model based on anisotropic sintering shrinkage of silicon nitride with aligned silicon nitride whisker seeds was built in order to provide an easy way to obtain information on how the large elongated grains were aligned. The method requires a simple measuring device for the information. XRD analysis showed a good correlation with predictions of the model. Both predictions of the model and experimental results indicated that the fraction of aligned large elongated grains increased as the whisker content increased.     

Texture and Anisotropy in Silicon Nitride

In this investigation quantitative texture analysis, including calculation of the orientation distribution function, is used to demonstrate the degree of preferred orientation in β -Si₃N₄ which has been hot-pressed or hot-worked. The results indicate that plane strain compression can produce strong textures. The texture is decided by the processing parameters including temperature, sintering additives, and stress state. Grain rotation and preferred grain growth apparently both contribute to texture development in β -Si₃N₄. Basal (00 l ) pole figures obtained from the orientation distribution function are consistent with microstructural observations and are reflected in indentation fracture toughness anisotropy. In plane strain the ratio of maximum to minimum fracture toughness is greater than 2.     

Particle Impact Damage in Silicon Nitride

The evolution of particle-impact-induced fracture damage in hot-pressed (HP) silicon nitride was established by accelerating single 2.4-mm-diameter tungsten carbide spheres against polished HP Si₃N₄ surfaces. Threshold velocities for ring, cone, and radial cracks were determined and the corresponding threshold stress for ring cracking was obtained from an elastic stress analysis. Particle size had significant effects on the threshold velocities for the inelastic impression and the various crack types. Loading rate had little effect on the threshold stress for ring cracks; rate effects on other crack types could not be assessed because the quasistatic indenter failed at stresses less than those required to invoke other crack types. A 20-μm-thick oxide scale had little influence on morphology and extent of damage but was removed easily at low velocities, suggesting higher erosion rates for Si₃N₄ in oxidizing environments. Damage phenomenology in 85% dense reaction-bonded Si₃N₄ was similar to that in HP material; however, all stages of damage occurred at substantially lower velocities.     

Crack-Growth Resistance of in Situ-Toughened Silicon Nitride

The fracture toughness of a commercial, hot-pressed, in situ -toughened silicon nitride with an elongated grain structure is determined by four different testing methods. The fracture toughness is found to be 5.76 ± 0.27, 8.48 ± 0.50, 10.16 ± 0.66, and 10.68 ± 0.39 Mpa.m^1/2, respectively, by indentation crack size measurement, indentation strength, single-edge-precracked-beam, and chevron-notched-beam methods. The discrepancy in fracture toughness between the testing methods is related to R -curve behavior, as measured using the indentation strength technique. These results indicate that there is no unique fracture toughness value and that a fracture toughness testing method with appropriate qualifiers is needed for rising R -curve materials. Therefore, care should be taken in interpreting and utilizing fracture toughness values evaluated from different testing methods if a material exhibits a rising R -curve. Complete characterization of the R -curve may be a prerequisite.     

氮化硅陶瓷抗裂性的研究

讨论了工艺因素对氮化硅陶瓷结构、物理机械性能及其抗裂性的影响。查明，Al2O3 Y2O3复合烧结助剂及与其相配合的硅酸乙酯结合剂对氮化硅陶瓷强受和抗裂性变化有影响。证明用压痕法可以评定陶瓷的抗裂性。     

Fracture Resistance Behavior of Multilayered Silicon Nitride

This paper investigates the fracture resistance behavior of a novel silicon nitride with a unique laminated structure consisting of alternate dense and porous layers, in the latter of which β-silicon nitride whiskers were aligned parallel to the layer. The R -curve was determined by using a chevron–notched–beam technique when a crack propagated in the direction normal to the whisker axis. The resistance markedly increased whenever a crack passed the porous layer, resulting in stepwise rising R–curve behavior. Microscopic study revealed that the aligned whiskers were almost completely pulled out in the porous layer.     

Indentation Crack Length Anisotropy in Silicon Nitride with Aligned Reinforcing Grains

Vickers indentation was performed on surfaces of silicon nitride with an aligned microstructure in order to study the interaction between cracks and the microstructure. Although there was not much evidence of crack bridging, the transverse radial cracks were very short, resulting in high fracture toughness values. The longitudinal radial cracks tended to propagate along the grain boundary of the reinforcements and were much longer than the transverse cracks. As the sintering temperature increased, the lateral cracks on the casting surface led to spalling and consumed more energy for the crack formation, making the longitudinal cracks shorter. On the surface normal to the alignment direction, there was no spalling and the indentation cracks became longer as the sintering temperature increased.     

Contact Damage of Silicon Carbide/Boron Nitride Nanocomposites

To investigate the deformation mechanism of silicon carbide (SiC)/boron nitride (BN) nanocomposites, Hertzian contact tests were performed on monolithic SiC, and nanocomposite and microcomposite SiC/BN. Monolithic SiC had the typical microstructure of hot-pressed SiC with Y₂O₃ and Al₂O₃ additives, composed of slightly large grains in small matrix grains. The microcomposite comprised large BN grains dispersed along the grain boundaries of elongated SiC grains, while the nanocomposite showed a finer microstructure with fine BN particles and small matrix grains. These microstructural differences led to differences in the mechanism of contact damage. The damage of the monolithic SiC and the SiC/BN microcomposite exhibited classical Hertzian cone fracture and many large cracks, whereas the damage observed in the nanocomposites appeared to be quasi-plastic deformation.     

Fracture Resistance Behavior of Highly Anisotropic Silicon Nitride

The R -curve behavior was characterized by the Vickers indentation flaw technique, for highly anisotropic silicon nitride, a silicon nitride whose fibrous grains are highly aligned. The measured crack lengths ranged from 30 to 500 μm. The fracture resistance of a conventional self-reinforced silicon nitride was determined for comparison using the same procedures. While in the self-reinforced material several hundred micrometers of crack extension were required to obtain a high fracture toughness, the highly anisotropic material exhibited a high toughness from the beginning of the measured crack length range with little increase in the following range. It is suggested that the toughness of the highly anisotropic material steeply rises in a very short crack extension, which is advantageous in avoiding catastropic fractures.     

Spherical-Impact Damage and Strength Degradation in Silicon Carbide Whisker/Silicon Nitride Composites

The silicon carbide (SiC) whisker reinforcement of silicon nitride (Si₃N₄) improves fracture strength and toughness, hardness, and Young's modulus, resulting in higher resistance of the composites to sphere penetration and crack initiation at spherical impact. Sintered Si₃N₄ shows an elastic/plastic response and initiates median/radial cracks at 100 m/s impact velocity. SiC-whisker/Si₃N₄ composites, on the other hand, demonstrate an elastic response, with Hertzian cone crack initiation, only when impact velocity exceeds 280 m/s. The SiC-whisker/Si₃N₄ composites thus exhibit improved strength degradation versus critical impact velocity characteristics because of improved mechanical properties provided by the SiC whiskers.     

Impact Damage and Strength Degradation in a Silicon Carbide Reinforced Silicon Nitride Composite

Adding SiC particles to Si₃N₄ and subjecting the mixture to a sinter-hot-isostatic-pressing process increases both the strength and elastic modulus. It also decreases the hardness but maintains the fracture toughness, which results in a higher resistance to crack initiation and propagation during spherical particle impact. Sinter-hot-isostatically-pressed composites exhibit elastic response as their dominant behavior. They also display a high resistance to Hertzian cone crack initiation and extension. This is due to the increased degree of inelastic deformation of sinter-hot-isostatically-pressed composites.     

Improved Resistance to Damage of Silicon Carbide-Whisker-Reinforced Silicon Nitride-Matrix Composites by Whisker-Oriented Alignment

The effects of whisker-oriented alignment on resistance to damage of SiC( w )/Si₃N₄ composites have been investigated by the Vickers indentation method and R -curve behavior. It is shown that increasing the degree of whisker-oriented alignment decreases the lengths of Vickers impressions and indentation cracks. The results exhibit rising R -curve behaviors for the SiC( w )/Si₃N₄ composites with different degree of whisker-oriented alignment. Moreover, the initial crack length c _i, the threshold of crack growth resistance K _i, and the upper bound of crack growth resistance K _∞ change regularly with increasing degree of whisker-oriented alignment. All results suggest that the whisker-oriented alignment improves the resistance to damage of the composites, resulting in a more reliable and usable composite.     

Mechanical Properties and Contact Damage Behavior in Aligned Silicon Nitride Materials

Aligned Si₃N₄ microstructures were achieved by seeding, extruding, and laminating methods. The degree of grain alignment was determined by microstructural measurements. Mechanical properties, including toughness, strength, hardness, and elastic modulus, as well as the contact damage response, were addressed and discussed as a function of this anisotropic microstructure. K _{I C} values over 8 MPa·m^1/2 and strengths above 900 MPa were achieved for the most favorable planes in the textured material. Contact damage behavior was influenced by grain orientation in two ways, first, by conferring elliptical shape to the radial surface cracks and second, by the multiple twin/slip formation at the large seeds within the high shear strain region.     

磁控溅射法制备氮化硅薄膜的耐磨性研究

氮化硅(Si_3N_4)薄膜具有化学稳定性高、电阻率高、绝缘性好、光学性能良好(其折射率在2.0左右)等特性。同时氮化硅膜是一种很好的耐磨材料,其铅笔硬度理论上可以达到9H以上,通过在其它的镀膜产品上加镀一层氮化硅膜,可有效改善原有镀膜产品的耐磨性,避免膜层出现膜面划伤而造成的外观不良。本文主要研究采用中频磁控反应溅射制备氮化硅薄膜,氮化硅薄膜的耐磨性能取决于镀膜过程中的各种工艺参数,包括:N_2/Ar比、沉积温度、溅射功率、膜层厚度2等。通过对不同工艺条件下镀制的氮化硅薄膜的耐磨性及膜层结构进行对比,筛选出具有优良耐磨性能的氮化硅薄膜的工艺参数。     

Visualization of Knoop Indentation Damage of Silicon Nitride by Plasma Etching

A visualizing technique for indentation damage of ceramics was developed. Plasma etching was used to enhance the view of cracks and the subsurface microcracking crush zone following Knoop indentation of hot pressed Si₃N₄. The microcracking zone was readily identified from the surface view of the indented surface as a grain-falling-off region (GFOR), defined as a region in which grains were removed by preferential etching using CF₄ gas, followed by ultrasonic cleaning. A fissure-like opening corresponding to the indentation cracks was also observed. It is inferred that the formation of the GFOR region and the fissure-like opening were caused by the etching/cleaning treatment. Meanwhile, the etching on a section which included diagonals of the impression provided a section view of the microcracking zone.     

Scratch Damage in Zirconia Ceramics

Scratch damage modes in zirconia-based ceramics—Mg-PSZ, Y-TZP, and Ce-TZP—are investigated. Precursor indentation tests with a tungsten carbide sphere foreshadow the nature of damage: in Mg-PSZ, extensive (quasi-)plastic deformation in the region outside and beneath the contact; in Y-TZP, less plastic deformation beneath the contact but incipient cone cracking in the region of tension outside the contact; in Ce-TZP, intermediate behavior. Scratch testing is conducted using a conical diamond indenter. In all materials the damage mode changes from smooth plastic deformation to limited cracking with increasing scratch load: in Mg-PSZ, plastic deformation is predominant at lower loads, with microcracking at higher loads; in Y-TZP, plastic deformation is predominant over the range of the test loads—macrocracks initiate only at relatively high loads, but penetrate to a relatively large depth; again, Ce-TZP shows intermediate behavior, but with cracking patterns closer to that of Mg-PSZ. Bending tests on specimens subjected to scratch damage indicate a relatively high damage tolerance in the Mg-PSZ and Ce-TZP; Y-TZP shows the highest initial strength, but suffers relatively large strength loss above the critical load for macrocracking. Implications concerning relative merits of each zirconia type for wear properties, contact fatigue, and machining damage are briefly discussed.     

Contact Damage and Strength Degradation in Brittle/Quasi-Plastic Silicon Nitride Bilayers

A study is made of the damage resistance of silicon nitride bilayers consisting of a hard overlayer (coating) on a soft underlayer (substrate). The two layers are fabricated with different starting powders, to provide distinctive elongate-grain microstructures, and are cosintered, to provide strong interfacial bonding and thus to minimize subsequent delamination. Contact testing with spherical indenters is used to characterize the damage response. The elastic-plastic mismatch between the layers is sufficiently high as to produce distinctive damage modes in the two layers: predominantly cone cracking in the coating, and quasi-plasticity in the substrate. However, the mismatch is also sufficiently low as to preclude secondary transverse cracks of the kind observed in other bilayer systems to initiate immediately beneath the contact at the coating/substrate interface and propagate upward within the coating. The dominant damage mode shifts from coating fracture to substrate quasi-plasticity with increasing contact load and decreasing coating thickness. Significantly, the presence of the soft underlayer inhibits growth of the coating cone cracks as the latter approach and intersect the interface. The underlayer also substantially diminishes strength losses from the contact-induced damage, especially in bilayers with thinner coatings. The implication is that bilayer structures with thin, hard coatings can preserve benefits from the inherent toughness of soft substrate materials, and at the same time afford surface protection (high wear resistance) to the underlayer.     

Coating of Fine-Grained Silicon Nitride Whiskers on Fiber-like Silicon Nitride

Fine-grained silicon nitride (Si3N4) whiskers were coated on Si3N4 fibers through a vapor-liquid-solid mechanism under the following process. After the oxide glass of a Si-Al-Y-O system was coated on seed Si3N4 fibers, the coated fibers were heated at 1490°C and 700° in the vapor of the Si-N system which was generated by decomposition of amorphous Si3N4. The resulting specimens looked just like rose twigs. The Si3N4 whiskers were precipitated by nucleation in a liquid phase generated by melting of the oxide glass layers on the Si3N4 fibers.     

Texture in Silicon Nitride Seeded with Silicon Nitride Whiskers of Different Sizes

Silicon nitride ceramics seeded with 3 wt%β-Si₃N₄ whiskers of two different sizes were prepared by a modified tape casting and gas pressure sintering. The fine whiskers had a higher aspect ratio than the coarse whiskers. Quantitative texture analysis including calculation of the orientation distribution function (ODF) was used for obtaining the degrees of preferred orientation of sintered samples. The maximum multiples of random distribution (mrd) values of samples seeded with the fine and coarse whiskers were large, greater than 15 and 9, respectively. Meanwhile, the mrd value of a sample seeded with fine whiskers was only 9 when it was prepared by conventional tape casting. The microstructures and the XRD data revealed that the well-aligned whiskers grew significantly after sintering and dominated the texture. Differences among the degrees of preferred orientation of the samples were explained using Jeffrey's model on rotation of elliptical particles carried by a viscous fluid.