Fracture behavior of silicon nitride ceramics subjected to hypervelocity impact

A new advanced ceramic thruster made of monolithic silicon nitride ceramics was developed for the planetary exploration spacecraft AKATSUKI (PLANET-C) at Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA). To ensure its operation onboard the spacecraft, the reliability of the ceramic thruster against micrometeoroid hypervelocity impact has been investigated. Silicon nitride plates were impacted by spheres of stainless-steel and other materials with 0.2-0.8-mm diameters in the velocity range up to 8.0 km/s using a two-stage light-gas gun. Using crater depth data under various impact conditions, the penetration equation of silicon nitride was determined. The impacted samples showed fracture patterns of three types: cratering, cratering with spallation, and perforation. These fracture patterns were well categorized by the multiple forms of the penetration equation.     

热冲击作用下的陶瓷材料破裂过程数值分析

运用热传导和热2力耦合的相关理论 , 借助统计分布来考虑陶瓷中存在的微孔洞和微裂隙 ; 建立了一种可以模拟陶瓷遭受热冲击作用下的裂纹萌生、 扩展过程的数值模拟方法 , 并通过材料破坏过程分析系统 (RF2PA , Realistic Failure Process Analysis) 加以实施。该数值方法基于细观非均匀性假设 , 突破了以往连续介质力学视陶瓷为均匀介质的假设 , 并从细观损伤角度考虑陶瓷热冲击破坏演化的过程。运用该方法对三面绝热、 一面受热冲击的平板状陶瓷材料的破裂过程进行了数值试验。结果表明 : 起始裂纹发端于受热冲击表面 , 且在初始的裂纹萌生阶段 , 在受热冲击表面产生一系列无序的裂纹 ; 但随着时间的延续 , 裂纹逐渐演变成多条近乎平行的、沿受冲击表面内法向方向扩展的主裂纹 , 其中一些裂纹的发展受到了屏蔽 , 这一结果与试验结果吻合较好。本数值方法为相关研究提供了新的思路。      

氮化硅陶瓷的研究进展

氮化硅陶瓷是一种有广阔发展前景的高温高强度结构陶瓷.其具有高性能(如强度高、抗热震稳定性好、疲劳韧性高、室温抗弯强度高、耐磨、抗氧化、耐腐蚀性好等).已广泛应用于各行各业.氮化硅的制备方法主要有反应烧结法(RS)、热压烧结法(HPS)、常压烧结法(PLS)和气压烧结法(GPS)等.目前存在的主要问题是氮化硅陶瓷产品韧性低、成本较高.今后应改善制粉、成型和烧结工艺及氮化硅与碳化硅的复合化,研制出更加优良的氮化硅陶瓷.     

Thermal shock resistance of silicon nitride

Journal of Materials Science -     

Fracture toughness modification by using a fibre laser surface treatment of a silicon nitride engineering ceramic

Surface treatment of a silicon nitride (Si₃N₄) engineering ceramic with fibre laser radiation was conducted to identify changes in the fracture toughness as measured by K_1c. A Vickers macro-hardness indentation method was adopted to determine the K_1c of the Si₃N₄ before and after fibre laser surface treatment. Optical and a scanning electron microscopy (SEM), a co-ordinate measuring machine and a focus variation technique were used to observe and measure the dimensions of the Vickers indentation, the resulting crack lengths, as well as the crack geometry within the as-received and fibre laser-treated Si₃N₄. Thereafter, computational and analytical methods were employed to determine the K_1c using various empirical equations. The equation K_1c = 0.016 (E/Hv)^1/2 (P/c^3/2) produced most accurate results in generating K_1c values within the range from 4 to 6 MPa m^1/2. From this it was found that the indentation load, hardness, along with the resulting crack lengths in particular, were the most influential parameters within the K_1c equation used. An increase in the near surface hardness of 4% was found with the Si₃N₄ in comparison with the as-received surface, which meant that the fibre laser-treated surface of the Si₃N₄ became harder and more brittle, indicating that the surface was more prone to cracking after the fibre laser treatment. Yet, the resulting crack lengths from the Vickers indentation tests were reduced by 37% for the Si₃N₄ which in turn led to increase in the K_1c by 47% in comparison with the as-received surface. It is postulated that the fibre laser treatment induced a compressive stress layer by gaining an increase in the dislocation movement during elevated temperatures from the fibre laser surface processing. This inherently increased the compressive stress within the Si₃N₄ and minimized the crack propagation during the Vickers indentation test, which led to the fibre laser-radiated surface of the Si₃N₄ engineering ceramic to have more resistance to crack propagation.     

Fracture analysis for ceramic ball in backflow valve

Ceramic balls have been used as components of devices, such as those found in high‐pressure pumps for automobiles and industrial machines. In the backflow valve, for example, a ceramic ball is in contact with a conical surface. Fractures of ceramic balls are extremely rare. It is important to investigate the cause of these rare fractures to guarantee higher reliability in the backflow valve. In this paper, the fracture mechanism and strength are discussed for an equivalent normal stress σ_eq beneath the contact region and the maximum principal stress σ_p near the contact boundary using stress intensity from fracture mechanics. The fracture surface of the ceramic ball was formed perpendicular to load direction. We assumed that fracture origins (defect/crack) existed on lines through three high stresses that analysed by finite element method. Actual fracture of a ceramic ball was found to be caused by the equivalent normal stress beneath contact region and not to be caused by the Hertz principal stress. Stress intensity factor (SIF) was clarified to depend on pressure, taper angle, CrN‐coating thickness and the friction factor of the inside of the valve hole. A pre‐existing defect size involved in failure was estimated by the SIF using three‐dimensional elliptic defects and equivalent normal stress. Therefore, the actual fracture of a ceramic ball, which rarely occurs, could be evaluated by considering three‐dimensional elliptic defects and the Weibull distribution of defects.     

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.     

Fracture mechanics approach to evaluation of strength in sintered silicon nitride

The evaluation of strength of ceramics is an important problem in their application to engineering components. Although fracture initiating from inherent flaws in ceramics is supposed to be analyzed through linear elastic fracture mechanics, the applicability of a plane-strain fracture toughness criterion is in dispute.In this investigation, fracture tests for two grades of sintered silicon nitride were conducted under plane-bending and ring-compression. As the size of inherent flaw observed at the fracture origin was larger, the critical stress intensity factor for the flaw increased and then almost coincided with the fracture toughness. This suggests that ordinary fracture mechanics data should not always be applied directly for predicting the strength of engineering ceramic components. The flaw-size effect was discussed from the point of the applicability of fracture mechanics and the relation with material microstructure.

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Résumé L'évaluation de la résistance des céramiques est un problème important pour leur application à des composants mécaniques. Bien que l'amorçage d'une fissure dans une céramique au départ de défauts inhérents soit normalement analysable par la mécanique de rupture linéaire et élastique, on n'est pas d'accord sur la pertinence d'un critère de ténacité à la rupture en état plan de déformation.Dans cette recherche, des essais de rupture de deux nuances de nitrure de silicium frittées ont été exécutés en flexion plane et en compression d'anneaux. On a observé que lorsque augmente la taille d'un défaut initial repérable comme origine de la rupture, le facteur critique d'intensité de contrainte pour le défaut considéré augmente et coïncide presque avec la ténacité à la rupture. Ceci suggère que les données habituelles de la mécanique de la rupture ne peuvent pas être toujours appliquées directement pour prédire la résistance de composants mécaniques en céramique.L'effet de la taille du défaut est discuté des points de vue de l'application possible de la mécanique de rupture et de la relation avec la microstructure du matériau.

     

Microstructures and mechanical properties of silicon nitride bonded silicon carbide ceramic foams

Silicon nitride bonded silicon carbide foams have been produced by nitridation of the foamed compacts containing silicon carbide and silicon powders. When no nitridation additive was used the ceramic foams nitrided at all temperatures studied contained a significant amount of whisker phase α-Si₃N₄ formed both inside and outside the cell walls leading to a loose microstructure and a low mechanical strength. When the Al₂O₃ and Y₂O₃ were used as nitridation additives, the ceramic foams nitrided at temperatures of 1360 and 1395 °C containing certain amount of Si₂N₂O and whisker α-Si₃N₄ phases that are bonded by a glassy phase and behave as reinforcements for the ceramic foams exhibited a much higher mechanical strength. At nitridation temperature of 1430 °C, the ceramic foam showed the locally formed β-Si₃N₄ as the main nitrided phase that caused no increase in bonding area between the nitrided phase and the silicon carbide particles. Thus, a relatively lower mechanical strength was observed for the ceramic foam.     

Resistance to crack propagation in ceramics subjected to thermal shock

It is shown that the resistance to crack propagation in alumina, zirconia and porcelain, when subjected to thermal shock, is high when the initial strength is low and, vice versa when the initial strength is high. It is proposed that the high resistance to crack propagation results from discrete microcracking in the highly stressed zone ahead of the major crack, whereas the low resistance results from the unrestrained propagation of major cracks. Evidences of the crack patterns in alumina and zirconia substantiate this hypothesis.     

Fracture toughness of multilayer silicon nitride with crack deflection

The fracture resistance of a multilayer silicon nitride consisting of alternate dense and porous layers was investigated by a single-edge-V-notched beam (SEVNB) technique. Since silicon nitride whiskers were aligned parallel to the laminar direction in the porous layer, the crack deflected macroscopically along the whiskers, resulting in high apparent K_I values, 15–25 MPa m^1/2. The crack then propagated in mode I, and was arrested when K_I was reduced to the fracture resistance without the crack deflection effects. These fracture resistance behaviors were well-explained in terms of the notch-insensitivity and the shielding effects of pull-out of the aligned whiskers.