Mechanical properties study of surface layers of the golf ball structure
In particular, modern golf balls are complex multi-component products of high technology . The properties of the ball should ensure a maximum flight range and trajectory stability, and also good control of short kicks. In the center of the ball is usually dense and hard core from a rubber-like polymer , which ensures the accumulation of potential energy upon impact of the ball and turning it into kinetic energy required for the flight over long distances. Polymer core is encased in hard plastic shell, which reduces the jumping of the ball and making it harder. For control and stability in flight the surface of the ball has the special form, and consists of several layers of plastic (usually polyurethane), coated with a special scratch-resistant ink . Thus, the near-surface volume of the ball, which is in direct contact with the club is a multi-layer structure with complex viscoelastic properties.
To study the mechanical properties of the structural elements forming the near-surface volume of a golf ball, it is reasonable to use the instrumental indentation.
Description of the samples
Two samples were used for research:
sample A – polished cross-section of a golf ball (fig.1a);
sample B – a ball without any preparation.
As shown in fig.1b, the outer shell of the ball consists of a plastic shell, thick polyurethane coating and two thin layers of ink. The main goal of this work was the study of the properties of external coatings (polyurethane layer and two thin layers of ink), but the mechanical properties of the shell and the core were also studied.
The Martens hardness test and the dependence of the indentation depth on the load are most interesting for golf balls developing. These data affect the efficiency of energy transfer when hitting the ball and its durability.
Devices and methods
Mechanical properties of multi-layer Golf ball was investigated using NanoScan-4D scanning nanohardness tester (FSBI TISNCM) [4-8]. General view of the device is presented in fig.2.
Among the measurement methods implemented in this device are sclerometry tests, measurements of hardness and elastic modulus by instrumental indentation, and a number of methods of atomic force microscopy (AFM). The indenting head allows to apply loads in the range from micronewtons to a few newtons, as well as to measure displacement in the range from fractions of nanometer to the millimeter.
Measurement of mechanical properties and surface topography were performed using the same device by moving the sample from one working position to another with the help of motorized tables.
The hardness and elastic modulus were determined by the method of instrumental indentation in accordance with ISO 14577-1:2002 using Berkovich indenter (diamond triangular pyramid).
Surface topography and roughness were measured by AFM Methods.
The surface topography (fig.3) was investigated, primarily to determine the average roughness of the test area. The indentation conditions were chosen based on roughness of the investigated surface. It is established that the thickness of each layer of the ink varies from 14 to 18 μm.
Base for the hardness and elastic modulus profile amounted to 3 mm (fig.4). Profile measurement begins at the core (coordinate 0) and ends within the layer of polyurethane (dotted line in fig.1a). Part of the profile between 1300 and 2400 μm corresponds to the inner layer.
The following indentation parameters were selected:
• loading time of 10 sec.;
• maximum load of 5 mN;
• hold-time at maximum load of 5 sec.
In these conditions, the depth of indentation has ranged from 2 to 8 μm depending on the test area.
The hardness profiles show that the shell has the largest hardness and elastic modulus. The core has the same hardness as the polyurethane layer, but lesser modulus of elasticity. Fig.5 shows typical curves of loading-unloading, obtained by instrumental indentation. The experimental curve for the core (green curve) shows that deformation is mostly elastic in nature, unlike the external layers, which are characterized by a significant share of plastic deformation.
In a separate experiment, the profiles were brought to ink layers, but acquired data showed no significant changes of the properties in the ink area, what corresponds to the results of multicyclic indentation of the surface of the ball in the radial direction.
Instrumental multicyclic indentation with partial unloading of the surface of the sample B (fig.6) was conducted under the following parameters:
time of each part of loading and unloading of 2 sec.;
maximum load of 750 mN;
hold-time at maximum load of 5 sec.;
number of cycles in one measurement of 25;
maximum penetration depth of the indenter of 75 µm.
It turned out that the material of the surface of the ball has considerable fluidity, as during loading and unloading the depth of penetration varies with delay. The hardness increases linearly up to a depth equal to the thickness of two layers of ink, and when it exceeds the hardness value is reduced, but not significantly, as the influence of the upper layers (fig.7).
The mechanical properties of different areas of the near-surface volume of a golf ball were studied by instrumental indentation NanoScan-4D scanning nanohardness tester. The combination of this method with the precision positioning enabled to determine the dependence of mechanical properties on the depth and along the profile. Morphology and surface roughness were studied by AFM. The results of the study are given in the table.
The dependence of hardness on the depth was investigated on the surface of the sample B (a ball) by the instrumental multicyclic indentation, and the results are consistent with the data obtained on sample A (cross-section of a ball).
The experimental results in general correspond to the information of the developer of the balls about the properties of the materials. The most significant are data about the viscoelastic properties of the coatings, whose wear and tear shows that the ball loses its performance.