Frutos E, González-Carrasco J L, Polcar T
Department of Control Engineering, Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, Prague 6, 16627, Czech Republic.
Centro Nacional de Investigaciones Metalúrgicas, CENIM-CSIC, Avda. Gregorio del Amo 8, 28040 Madrid, Spain; Centro de Investigación Biomédica en Red de Bioingeniería, Biomateriales y Nanomedicina CIBER-BBN, Spain.
J Mech Behav Biomed Mater. 2016 Apr;57:310-20. doi: 10.1016/j.jmbbm.2016.01.027. Epub 2016 Feb 1.
This work studies the feasibility of using repetitive-nano-impact tests with a cube-corner tip and low loads for obtaining quantitative fracture toughness values in thin and brittle coatings. For this purpose, it will be assumed that the impacts are able to produce a cracking, similar to the pattern developed for the classical fracture toughness tests in bulk materials, and therefore, from the crack developed in the repetitive impacts it will be possible to evaluate the suitability of the classical indentation models (Anstins and Laugier) for measuring fracture toughness. However, the length of this crack has to be lower than 10% of the total coating thickness to avoid substrate contributions. For this reason, and in order to ensure a small plastic region localized at the origin of the crack tip, low load values (or small distance between the indenter tip and the surface) have to be used. In order to demonstrate the validity of this technique, repetitive-nano-impact will be done in a fine and dense oxide layer (α-Al2O3), which has been developed on the top of oxide dispersion strengthened (ODS) FeCrAl alloys (PM 2000) by thermal oxidation at elevated temperatures. Moreover, it will be shown how it is possible to know with each new impact the crack geometry evolution from Palmqvist crack to half-penny crack, being able to study the proper evolution of the different values of fracture toughness in terms of both indentation models and as a function of the strain rate, ε̇, decreasing. Thereby, fracture toughness values for α-Al2O3 layer decrease from ~4.40MPam , for high ϵ̇ value (10(3)s(-1)), to ~3.21MPam, for quasi-static ϵ̇ value (10(-3)s(-1)). On the other hand, ϵ̇ a new process to obtain fracture toughness values will be analysed, when the classical indentation models are not met. These values are typically found in the literature for bulk α-Al2O3, demonstrating the use of repetitive-nano-impact tests which not only provide qualitative information about fracture resistance of the materials but it also can be used to obtain quantitative information as fracture toughness values in the case of brittle materials.
本研究探讨了使用具有立方角尖端和低载荷的重复纳米冲击试验来获取薄而脆涂层中定量断裂韧性值的可行性。为此，假设这些冲击能够产生裂纹，类似于在块状材料经典断裂韧性试验中形成的裂纹模式，因此，从重复冲击中产生的裂纹可以评估经典压痕模型（安斯汀斯和劳吉尔）用于测量断裂韧性的适用性。然而，该裂纹的长度必须小于涂层总厚度的10%，以避免基体的影响。因此，为了确保裂纹尖端起始处有一个小的塑性区域，必须使用低载荷值（或压头尖端与表面之间的小距离）。为了证明该技术的有效性，将在精细致密的氧化层（α - Al2O3）上进行重复纳米冲击试验，该氧化层是通过在高温下对氧化物弥散强化（ODS）FeCrAl合金（PM 2000）进行热氧化而在其顶部形成的。此外，将展示如何通过每次新的冲击了解裂纹几何形状从帕尔姆奎斯特裂纹到半便士裂纹的演变，从而能够根据压痕模型以及应变速率ε̇减小的函数关系研究不同断裂韧性值的适当演变。由此，α - Al2O3层的断裂韧性值从高ε̇值（10³ s⁻¹）时的约4.40 MPam下降到准静态ε̇值（10⁻³ s⁻¹）时的约3.21 MPam。另一方面，当不满足经典压痕模型时，将分析一种获取断裂韧性值的新方法。这些值通常在文献中针对块状α - Al2O3给出，证明了重复纳米冲击试验的应用，该试验不仅提供了有关材料抗断裂性的定性信息，而且在脆性材料的情况下还可用于获取作为断裂韧性值的定量信息。
