Research On The Indentation Size Effect Of Hardness Of Porous Composite

Hardness, an important index for evaluating the mechanical performance ofmaterials, often decreases with the increase of the applied test load during the hardnesstest process. This phenomenon is called indentation size effect (ISE). The existence ofthe ISE makes it difficult for hardness to be used as a material selection criterion. Inorder to explain and solve this problem, different empirical or semi-empirical equations,including Meyer’s law, the Hays-Kendall approach, the elastic recovery model, theenergy-balance approach, the proportional specimen resistance (PSR) model and themodified proportional specimen resistance (MPSR) model, were used to describe thevariation of the indentation size with the applied test load. However, which model is themost suitable one now is still a controversial subject. On the other hand, to date, a littlehas been reported on the hardness and the analysis of ISE of composite and porousmaterials.In this study, three kinds of materials comprising NiO/8YSZ composite withdifferent contents of NiO, NiO/8YSZ composite with different porosities and Ni/8YSZcermet with different porosities were prepared via co-precipitation method and addingpore-forming agent of PMMA. Vickers indentation tests were conducted on the polishedsurface of each sample and the variations of the hardness with the applied test load,composition and porosity were studied. Also the PSR model and the modified PSRmodel were compared as the most successful one to describe the hardness data for eachsample.The results show that, for each kind of sample, the measured hardness decreaseswith increasing indentation load, showing a significant indentation size effect.Compared with PSR model, the modified PSR model is the best one to accuratelydescribe the data for the above porous and compound materials. For the NiO/8YSZcomposites with different contents of NiO, the apparent hardness of the compositesincrease as the NiO content decrease and this relationship can be described by a quadratic polynomial which in turn verifies the reliability of the modified PSR model.For the two kinds of porous materials NiO/8YSZ composite and Ni/8YSZ cermet, it’sfound that the resultant load-independent hardness decreases with the increasing porosity while their standard deviation increases with it which can be understood easilyby considering the statistical distribution of the pores in the surface of the test samples.Further analysis shows that the a2value fitted from the modified PSR model increaseswith the decrease of porosity, and the relationship between a2and p2/3is a negative linercorrelation which is consistent with the formula first put forward by Gong.