预应力陶瓷及其抗冲击与穿甲性能

研究陶瓷材料的后期增强技术，提出了“预应力陶瓷”的概念，并将之用于结构材料（抗穿甲复合材料）的设计。通过分析和实验证明，利用巨大压预应力可降低冲击局部的瞬态拉应力并大大提高裂纹扩展阻力，大幅度提高陶瓷的强度和韧性，特别是大幅度提高陶瓷的抗冲击性能。     

残余应力对涂覆Al2O3涂层的ZrO2陶瓷的强度和裂纹扩展阻力的影响

本研究在ZrO₂基体表面涂覆一薄层Al₂O₃涂层, 利用基体与涂层之间热膨胀系数不匹配, 在Al₂O₃-ZrO₂预应力陶瓷(简称A_CZ_S预应力陶瓷)表层引入压应力。采用维氏压痕法评价残余应力对A_CZ_S预应力陶瓷的表层和基体中裂纹扩展阻力的影响。理论分析结合实验结果表明: 表层的压应力使得A_CZ_S预应力陶瓷的裂纹扩展阻力增大, 最终导致强度和损伤容限提高; 且A_CZ_S预应力陶瓷表层的压应力和裂纹扩展阻力随着基体截面积与涂层截面积比值的增加而增大。当ZrO₂基体表层的Al₂O₃涂层厚度为40 μm时, 表层压应力使A_CZ_S预应力陶瓷的弯曲强度达到(1207±20) MPa, 相比于同种工艺下制备的ZrO₂陶瓷强度提高了32%, 同时也是Al₂O₃强度的3倍。此外, A_CZ_S预应力陶瓷也表现出很好的抗热震性能。     

莫来石/氧化铝预应力涂层增强氧化铝的弯曲强度和抗热震性能

在陶瓷表面引入含压应力的涂层是一种有效的增强技术。本研究将氧化铝和石英粉混合浆料涂覆在预烧后的氧化铝坯体上,无压共烧原位合成了热膨胀系数较低的莫来石–氧化铝涂层。利用降温过程中涂层内形成的残余压应力实现了氧化铝陶瓷的预应力强化。结果表明：随着涂层中石英掺量增加,预应力氧化铝的强度出现先增大后减小的趋势;当涂层中掺入石英的质量分数为15%时,预应力增强效果最好,涂层与基体界面结合紧密,预应力氧化铝陶瓷的弯曲强度达到(549.44±27.2)MPa,比普通氧化铝的强度提高了37.19%;当涂层中掺入石英的质量分数增大到15%以上,由于烧结收缩不匹配反而引起强度下降;这种预应力增强效果会随着温度升高逐渐减弱,当测试温度达到并超过1000℃时,预应力氧化铝和普通氧化铝会具有大致相等的抗弯强度。由于表层压应力的存在,预应力氧化铝还展现出更好的抗热震性能和损伤耐受性。     

陶瓷基复合材料疲劳特性的研究

应用相变增韧、相变-晶须复合及相变-颗粒复合三种方式来改善氧化铝陶瓷的力学性能,研究了陶瓷基复合材料的疲劳特性。 在循环压缩载荷作用下,陶瓷材料的应力集中处(如缺口)会产生垂直于压应力轴的疲劳裂纹,随循环周次的增加,裂纹的扩展由快到慢,最终完全停止。循环压缩疲劳裂纹的形成机理是较大的应力集中使材料内出现以微裂纹为主要形式的不可逆损伤,在随后的卸载过程中,不可逆损伤区产生很高的残余拉应力,使疲劳裂纹形核并逐渐扩展。 陶瓷材料在四点弯曲循环载荷作用下,疲劳裂纹具有较长的亚临界扩展过程。裂纹护展速率与循环载荷的最大应力强度因子K_(max)及应力强度因子幅度△K都有关,且随载荷频率的降低及载荷波形由三角波变为正弦波,裂纹扩展速率增加。陶瓷材料四点弯曲疲劳裂纹的亚临界扩展是材料内损伤逐渐累积的结果。疲劳过程中材料通过形成微裂纹及裂纹分叉、克服增强物的阻碍及裂纹表面的桥接与互锁作用、产生裂尖微区内的塑性变形及部分稳定ZrO_2的相变等方式来消耗能量,在材料内造成以微裂纹为主要形式的微观损伤,从而弱化了材料,使疲劳裂纹得以亚临界扩展。 陶瓷材料在1050℃高温下的强度约为其室温强度的一半。陶瓷材料的高温循环疲劳是高温静载效应与循环载荷效应的迭加,1050℃下,循     

SiC/BN层状陶瓷耐损伤性能

采用压痕法在SiC/BN层状陶瓷试样的表面引入不同尺寸的表面裂纹,利用三点弯曲测量含裂纹试样的极限断裂应力,研究了不同尺寸的表面裂纹对层状陶瓷断裂强度的影响;根据压痕载荷-强度实验结果,测定层状陶瓷的阻力曲线,并与单相SiC陶瓷对比。结果表明,层状陶瓷的压痕强度对压痕裂纹深度的变化不敏感,阻力曲线呈上升型;而单相SiC陶瓷的压痕强度随压痕裂纹深度的增加急剧下降,阻力曲线呈平稳型,说明层状陶瓷具有优异的耐损伤性能和升值R-曲线行为。分析认为,裂纹在弱界面处发生偏折是层状陶瓷具有优良耐损伤性能和升值R-曲线行为的主要原因。这为陶瓷材料在含有一定的制备和加工缺陷以及承受冲击、磨损等接触损伤的条件下保持高强度提供了可能。     

45钢的疲劳裂纹扩展速率及其工程构件的寿命预测

在常温下对调质态和正火态45钢紧凑拉伸试样进行了疲劳裂纹扩展速率试验,并根据Paris公式确定了两种状态试样在稳定扩展阶段的疲劳裂纹扩展速率;分析了含缺陷的45钢工程构件的张开型(I型)裂纹尖端的应力强度因子,然后采用损伤容限设计原则,根据断裂力学试验结果计算了构件的疲劳裂纹扩展寿命,并进行了含缺陷45钢构件的动态弯曲疲劳试验,以验证理论推算的合理性。结果表明:正火态45钢的疲劳裂纹扩展速率略大于调质态45钢的;采用损伤容限设计原则估算的含缺陷45钢构件的疲劳裂纹扩展寿命为2.3×10~6次,与动态弯曲疲劳试验结果相吻合。     

喷丸强化对民用飞机腐蚀损伤构件残余应力及疲劳寿命的影响

以修理过程中去除腐蚀产物后的民用飞机腐蚀损伤构件为研究对象,针对7075铝合金损伤试样,采用玻璃弹丸为介质,研究喷丸强化对损伤构件表面残余应力及疲劳寿命的影响。首先通过试验确定获得所需喷丸强度及覆盖率的工艺参数,进而测量不同参数喷丸强化后的构件表面残余应力,并分析残余应力沿构件表面方向的分布规律。通过对构件损伤表面进行喷丸以及连同相邻侧面进行喷丸的不同范围的试验研究,得到构件的不同强化效果。疲劳试验结果表明:当只对损伤表面进行喷丸强化时,疲劳寿命的改善相对有限,而将喷丸区域扩大至与损伤表面相邻的构件侧表面时,疲劳寿命将有大幅度提高。     

碳化硅陶瓷预应力加工的离散元模拟与实验研究

采用簇单元(Cluster)方法,建立了碳化硅陶瓷材料预应力加工的离散元模型;通过离散元模拟和划痕实验,研究了预应力条件对碳化硅陶瓷材料切削时裂纹的扩展规律的影响,观察了陶瓷材料加工表面/亚表面的裂纹损伤情况.离散元模拟与实验结果均表明:当预应力大小在一定范围内,随着预应力的逐渐增大,径向裂纹的数目逐渐减少,横向裂纹有替代径向裂纹的趋势,并导致材料以较小的碎片的形式被去除;采用预应力加工能有效降低加工损伤,提高加工表面质量,并且进一步证明应用离散元法模拟硬脆材料的加工是可行的.     

陶瓷材料可靠性分析中的统计断裂强度理论(2)其它统计断裂强度理论

评述了当前陶瓷材料统计断裂强度理论的发展概况 ,分析了两种建立强度分布方程的统计方法 ,探讨了它们用于陶瓷材料可靠性分析和陶瓷部件设计的可行性。并基于对缺陷的尺度分布和方位分布以及裂纹扩展路径不规则性的统一考虑 ,理论分析了一般应力状态下陶瓷材料的统计断裂强度     

陶瓷材料可靠性分析中的统计断裂强度理论

评述了当前陶瓷材料统计断裂强度理论的发展概况,分析了两种建立强度分布方程的统计方法,探讨了它们用于陶瓷材料可靠性分析和陶瓷部件设计的可行性.并基于对缺陷的尺度分布和方位分布以及裂纹扩展路径不规则性的统一考虑,理论分析了一般应力状态下陶瓷材料的统计断裂强度.     

Effect of surface finish on strength degradation of glass ceramics

The processing of components manufactured from ceramics, such as machining, induces cracks on or beneath the surface leading to strength degradation. Usually, four-point bending tests are used for measuring the strength of ceramic materials. In this paper, a new approach is proposed to evaluate the loss of strength due to machining. First, beam specimens of glass ceramics are prepared by milling. The surface profile of representative areas of the specimens is determined using a laser profilometer. The surface topography of the representative area is reconstructed to characterize the surface irregularities after machining. A finite element program is then employed to analyze the stress distribution on a rough surface of a beam in four-point bending. A performance index—the stress concentration factor, is introduced to quantify the effect of surface irregularities on determining the critical crack depth related to the machining-induced damage. The flexural strength of machined ceramic specimens is predicted using the fracture mechanics. The strength degradation is compared with the experimentally determined values. A close correspondence between the predicted and measured strength is observed. The potential of using a computer-based evaluation to study the strength degradation of glass ceramics by machining induced surface damage has been established.     

Bioinspired Nacre‐Like Alumina with a Metallic Nickel Compliant Phase Fabricated by Spark‐Plasma Sintering

Many natural materials present an ideal “recipe” for the development of future damage‐tolerant lightweight structural materials. One notable example is the brick‐and‐mortar structure of nacre, found in mollusk shells, which produces high‐toughness, bioinspired ceramics using polymeric mortars as a compliant phase. Theoretical modeling has predicted that use of metallic mortars could lead to even higher damage‐tolerance in these materials, although it is difficult to melt‐infiltrate metals into ceramic scaffolds as they cannot readily wet ceramics. To avoid this problem, an alternative (“bottom‐up”) approach to synthesize “nacre‐like” ceramics containing a small fraction of nickel mortar is developed. These materials are fabricated using nickel‐coated alumina platelets that are aligned using slip‐casting and rapidly sintered using spark‐plasma sintering. Dewetting of the nickel mortar during sintering is prevented by using NiO‐coated as well as Ni‐coated platelets. As a result, a “nacre‐like” alumina ceramic displaying a resistance‐curve toughness up to ≈16 MPa m^½ with a flexural strength of ≈300 MPa is produced.     

关于先进结构陶瓷的研究

本文回顾了我国先进陶瓷材料研究四十余年来的进展．我国一贯重视陶瓷学的基础研究和应用基础研究对陶瓷工艺的指导作用,同时亦更注意新材料的工艺研究．本文简要地介绍了这些方面的主要成就．最后,对先进结构陶瓷材料研究需要考虑的问题提出一些拙见,以供讨论．     

Syalon ceramics in metal cutting

A range of ceramic materials has been developed at Lucas Syalon Ltd under the trade name of Syalon ceramics. Some properties of Syalon ceramics related to metal cutting are outlined and a review is given of the machining tests carried out at the Department of Engineering, University of Warwick. The results indicate that the materials ranging from cast-iron to Jncoloy-901 can be machined with Syalon ceramics at speeds and feeds which previously were not achievable with any other tool material, in terms of productivity an increase of six to tenfold has been achieved in different cases. These materials can cut at speeds normally associated with ceramics, but because of their better impact strength approaching those of ceramic coated carbides they are also able to take heavier chip loads.     

A study of the mechanical properties of highly porous ceramics using acoustic emission

In this paper the results of indirect tensile tests on highly porous ceramics are presented. A relation between the mechanical strength of the highly porous ceramic materials and Acoustic Emission (AE) has been established. We have shown that the amplitude distribution of the AE events depends on the crack velocity, which itself depends on the stress intensity of the crack. Apart from the Brazilian (side crushing strength) tests also multi-point loading experiments were carried out. The AE results show the additional damage accumulation due to compressive/shear stresses.     

SiC-BN层状陶瓷复合材料叠层方式优化设计

层状陶瓷复合材料可有效提高纯陶瓷材料的韧性,受到研究者的广泛关注。在材料设计阶段,通过优化叠层方式可显著提高层状陶瓷的力学性能。然而,在现有研究中缺乏叠层方式的优化设计方法。本研究采用基于复合梁模型的遗传算法得到了最优层厚比;针对SiC-BN层状陶瓷复合材料5∶ 1、10∶ 1和梯度体三种铺层形式采用流延成型结合无压烧结法进行材料制备,并进行了完好试件和含缺口试件的三点弯曲试验;基于宏观损伤分析对其增韧机制进行了分析。试验结果表明:通过解析方法计算得到的最优梯度体层状陶瓷的弯曲强度达到434.5 MPa。其力学性能相比于固定层厚比铺层方式有较大提高,同时还保持了较高的缺陷不敏感特性。进一步分析表明:受拉部分分布的较多软层和受压部分分布的较厚硬层是梯度体结构较好性能的重要原因。      

Performance Improvement of Grinding of SiC Using Graphite as a Solid Lubricant

Grinding is a widely employed finishing process for different materials. It is inherently characterized by its high specific energy requirement unlike other machining processes. This leads to a high grinding zone temperature, which impairs the workpiece quality by inducing thermal damage in the form of surface and subsurface cracks, phase transformations, tensile residual stresses, etc. The microcracks and residual stresses induced in the surface of the ceramics during grinding can severely limit the application of ceramic components. This article deals with an investigation on using graphite as a solid lubricant to reduce friction and thereby improve the surface integrity of ground silicon carbide (SiC). An experimental setup has been developed, and experiments have been conducted to study the effect of using a graphite lubricant on the surface roughness, grinding forces, and specific energy while grinding SiC material. Results indicate that there is a considerable improvement in the performance of grinding SiC using graphite as a solid lubricant when compared with dry grinding in terms of specific energy requirements, surface roughness, and damage. The employment of graphite as a solid lubricant in ceramic grinding makes the process more economical and reliable.