The deformation behavior of ceramic crystals subjected to very low load (nano)indentations
The deformation behavior of ceramic crystals subjected to very low load (nano)indentations
复制标题
DOI：
10.1557/jmr.1992.0450
复制
发表时间：
1992-02
影响因子：
2.7
通讯作者：
T. Page;W. Oliver;C. McHargue
中科院分区：
文献类型：
--
作者：
T. Page;W. Oliver;C. McHargue
使用软件控制的硬度计（纳米压痕仪）在 2-60 mN 的负载范围内研究了陶瓷单晶表面（Al_2O_3、SiC、Si）的超低负载压痕响应。在所有情况下，扫描和透射电子显微镜都已用于表征与这些非常小尺寸的硬度压痕相关的变形结构。重点是将观察到的特定压痕的变形行为与记录的载荷-位移曲线中的不规则性相关联。对于仔细退火的蓝宝石，观察到阈值载荷（对于给定压头），低于该阈值，唯一的表面响应是弹性弯曲，超过该阈值，在理论剪切强度或接近理论剪切强度时会发生位错环成核，从而产生压痕。这种塑性的开始被视为载荷-位移响应中的突然位移不连续性。在较高的载荷下，压痕似乎主要是通过位错活动来调节的，尽管观察到微裂纹形成的接触载荷仅为数十毫牛顿。此类裂纹可能是较大的、压痕引起的裂纹的早期滑移引起的核，通常仅在高得多的载荷下在表面上显现出来，并且通常用于估计压痕韧性。相比之下，硅没有表现出这种行为，但在压痕内表现出异常大量的深度恢复，导致卸载期间压头上产生特征性反向推力。对硅压痕的 TEM 研究显示，与蓝宝石相比，明显位错活动的证据较少（特别是在使用的最低载荷下），但确实显示了压痕内残留的高度不完美且通常是非晶态的结构，这与压头下产生的极高静水应力时发生的致密化转变一致。反向推力是由卸载过程中致密材料的松弛引起的。因此，硅的低负载硬度响应似乎是由压敏相变控制的。尽管预计 SiC 会经历与硅类似的致密化转变，但发现其载荷-位移行为与 Al_2O_3 类似，这表明至少对于这些接触实验，在达到致密化的临界静水压力之前，超过了位错成核的临界解析剪切应力。在所有情况下，测量到的残余塑性压痕均小于满载压痕深度，这证实了变形的很大一部分是弹性表面弯曲。然而，即使在非常小的穿透深度下，硅是否也表现出纯弹性变形仍存在一些疑问。使用微观结构研究来补充纳米压痕实验被证明是一条关键途径，不仅可以解释记录的载荷-位移响应，而且可以检查在这些小空间尺度上控制陶瓷与表面接触的机械行为的变形机制。
The ultra-low load indentation response of ceramic single crystal surfaces (Al_2O_3, SiC, Si) has been studied with a software-controlled hardness tester (Nanoindenter) operating in the load range 2–60 mN. In all cases, scanning and transmission electron microscopy have been used to characterize the deformation structures associated with these very small-scale hardness impressions. Emphasis has been placed on correlating the deformation behavior observed for particular indentations with irregularities in recorded load-displacement curves. For carefully annealed sapphire, a threshold load (for a given indenter) was observed below which the only surface response was elastic flexure and beyond which dislocation loop nucleation occurred at, or near, the theoretical shear strength to create the indentation. This onset of plasticity was seen as a sudden displacement discontinuity in the load-displacement response. At higher loads, indentations appeared to be accommodated predominantly by dislocation activity, though microcracks were observed to form ät contact loads of only tens of milliNewtons. Possibly such cracks are the incipient slip-induced nuclei for the much larger, indentation-induced cracks usually apparent only on the surface at much higher loads and often used for estimating indentation toughness. By contrast, silicon did not show this behavior but exhibited unusually large amounts of depth recovery within indentations, resulting in a characteristic reverse thrust on the indenter during unloading. TEM studies of indentations in silicon revealed less evidence of obvious dislocation activity than in sapphire (particularly at the lowest loads used) but did show residual highly imperfect–and often amorphous–structures within the indentations, consistent with a densification transformation occurring at the very high hydrostatic stresses produced under the indenter. The reverse thrust is caused by the relaxation of densified material during unloading. Thus, it appears that the low-load hardness response of silicon is controlled by a pressure-sensitive phase transformation. Though SiC has been predicted to undergo a densification transformation similar to silicon, its load-displacement behavior was found to be similar to Al_2O_3 suggesting that, for these contact experiments at least, the critical resolved shear stress for dislocation nucleation is exceeded before the critical hydrostatic pressure for densification is reached. In all cases, residual, plastically formed indentations were measured to be smaller than the fully loaded indentation depths would suggest, confirming that a significant portion of the deformation is elastic surface flexure. However, there is some doubt as to whether silicon displays elastic-only deformation even at very small penetration depths. The use of microstructural studies to complement nanoindentation experiments is shown to be a key route not only to interpreting the recorded load-displacement responses, but also to examining the deformation mechanisms controlling the mechanical behavior of ceramics to surface contacts at these small spatial scales.