Residual Stresses and Nanoindentation Testing of Films and Coatings

In the manufacturing process of various mechanical parts, some processing techniques such as drawing, extrusion, rolling, calibration, casting, welding, cutting, grinding, and heat treatment will introduce various degrees of residual stresses in mechanical parts.

Indentation test is a simple and effective method in the assessment of mechanical properties of materials and has widely been adopted in the latest century. Indentation technique evaluates the mechanical properties of materials by driving an indenter into the material surface and subsequently imaging the impression. Indentation test was firstly used for the measurement of hardness. A hard object with certain shape and size is used as indenter and indented into the tested material under certain pressure keeping for a while before unloaded. Then the hardness of the tested material can be determined from the relationship between the total indentation load and displacement or area. Based on the principle above, there are many traditional methods of hardness testing, such as Vickers hardness method (Vickers), Knoop hardness method (Knoop), and Rockwell hardness method (Rockwell).

Generally, there are two ways that nanoindentation technique can be used to determine the surface residual stresses. One is based on the influence of residual stress on the P-h curve of nanoindentation.

Figure 4.1 shows the variation of pile-up height with indentation depth of the single crystal copper. The pile-up height of the single crystal copper shows a growth of approximate parabola with the increase of the indentation depth (from 300 to 800 nm), and the increase of amplitude is getting larger and larger. The pile-up heights change in the range of 45–95 nm, occupying the proportion of indentation depth about 12–15%.

Laser cladding is a green and potential surface hardening technology, which can be used to prepare high-quality coatings with high bonding strength, high hardness, and high toughness, thereby significantly improving wear resistance, corrosion resistance, and anti-fatigue performance of parts.

The thin film materials with small volume and high reliability have become an integral part of microelectromechanical systems and have a wide range of applications in national defense, aerospace, communication, automobile manufacturing, and other fields. The service reliability and service life are determined by the mechanical properties of thin film materials.

The residual stress in a mechanically polished fused quartz beam was determined to verify the practicability of the Xu model (Xu and Li in Estimation of residual stresses from elastic recovery of nanoindentation 86:2835–2846, 2006 [1]). The used fused quartz is a common homogenous and isotropic amorphous material, with an elastic modulus of 72 GPa and Poisson’s ratio of 0.17. The fused quartz was manufactured into beam samples with the dimension of 70 mm × 13 mm × 5 mm. The fused beam was firmly held in a three-point bending (3 PB) device. The two supports of the bending device were fixed with an interval of 54 mm, and the load is applied by a micrometer with a displacement precision of 0.0254 mm. Nanoindentations were first performed at the end of the fused quartz beam outside the bending zone with a Berkovich indenter using a Hysitron Triboscope. Then the fused beam was strained to generate an applied stress field in the beam. Nanoindentations were subsequently made at the positions in the middle of the beam along the inside or outside edges where maximum bending compressive or tensile stresses occur. At least, five indents were made at each position with a constant peak indentation load of 990 μN.