Wu Hao, Dave Foram, Mokhtari Mozaffar, Ali Muhammad Mahmood, Sherlock Richard, McIlhagger Alistair, Tormey David, McFadden Shaun
School of Computing, Engineering and Intelligent Systems, Ulster University, Londonderry BT48 7JL, UK.
Department of Mechanical and Manufacturing Engineering, Centre for Precision Engineering, Materials and Manufacturing Research, Institute of Technology Sligo, F91 YW50 Sligo, Ireland.
Polymers (Basel). 2022 Apr 28;14(9):1804. doi: 10.3390/polym14091804.
Hardness is a useful measure of a material’s resistance to permanent indentation; but for viscoelastic polymers, hardness data are highly dependent on the test type and the parameter set chosen. Vickers microhardness testing is used to leave small indents (<150 µm) and is shown to be applicable to polymers. A detailed investigation of the required steps for microhardness testing in isotactic polypropylene (iPP) is provided. Samples should be mounted in epoxy resin in order to maintain curing temperatures at room temperature. Mounted samples can be ground and polished in a semi-automatic polisher using graduated SiC paper (wet grinding) but progressing onto alumina suspension for polishing. Final polishing should be performed with 0.05-µm alumina suspension. The hardness measured was shown to be dependent on load and dwell time with a much greater dependency on dwell time. Strain recovery was shown to be completed after a time period equal to the dwell time. This study shows that indents can be measured thereafter, and it is recommended that they be measured within a 24 h period after the indent was created. After data fitting, the equation for hardness was shown to follow a power law with load and dwell time as the main variables. Fitting parameters were compared to those found in the literature, and it was found that parameters were significantly different to those reported elsewhere. Therefore, this study highlights the importance of calibrating on a case-by-case basis. Finally, to show the usefulness of the Vickers micro-hardness testing method, the calibrated test method was applied on iPP with additions of carbon black up to 3 wt.%. Comparisons were made with data from the literature, but the hardness data generated in our work were found to be at least twice that reported in the literature. The testing parameters were not cited in the literature: specifically, the dwell time was not provided, and this generated doubt on the usefulness of the cited data. Hence, this work is intended to serve as an exemplar of how to prepare and proceed with hardness testing of polymers.
硬度是衡量材料抗永久压痕能力的一个有用指标；但对于粘弹性聚合物而言，硬度数据高度依赖于测试类型和所选的参数设置。维氏显微硬度测试用于留下小压痕（<150 µm），且已证明适用于聚合物。本文提供了对全同立构聚丙烯（iPP）进行显微硬度测试所需步骤的详细研究。样品应安装在环氧树脂中，以便将固化温度保持在室温。安装好的样品可在半自动抛光机中使用分级的碳化硅砂纸（湿磨）进行研磨和抛光，但之后要使用氧化铝悬浮液进行抛光。最终抛光应使用0.05 µm的氧化铝悬浮液进行。结果表明，所测得的硬度取决于载荷和保压时间，且对保压时间的依赖性更大。应变恢复在等于保压时间的时间段后完成。该研究表明，此后可以测量压痕，建议在压痕形成后的24小时内进行测量。经过数据拟合后，硬度方程显示遵循幂律，其中载荷和保压时间为主要变量。将拟合参数与文献中找到的参数进行比较，发现这些参数与其他地方报道的参数有显著差异。因此，本研究强调了逐案校准的重要性。最后，为了展示维氏显微硬度测试方法的实用性，将校准后的测试方法应用于添加了高达3 wt.%炭黑的iPP上。与文献数据进行了比较，但发现我们工作中产生的硬度数据至少是文献报道数据的两倍。文献中未引用测试参数：具体而言，未提供保压时间，这使得所引用数据的实用性受到质疑。因此，这项工作旨在作为聚合物硬度测试如何准备和进行的一个范例。
