Mechanical Testing

Micro hardness and micro flat tensile testing and fretting wear examination of the base materials and generated coatings were conducted in this work. Details of the experimental procedures and equipment are presented below.

4.6.1 Micro Hardness Test

Micro hardness measurements according to the Vickers method (DIN EN ISO 6507) were taken using hardness testing equipment (Zwick Roell Indentec ZHV 2). Indentation lines were drawn horizontally within the coating and vertically to the coating to evaluate the hardness through the heat-affected zone and the unaffected substrate. The measurements were carried out at a distance of 200 µm between each indentation with a load of 2 N (HV 0.2) and a 10 s indentation dwell time.

4.6.2 Tensile Test

The round specimens of the base materials CP titanium Grade 1 and Ti-6Al-4V were machined according to the standard DIN 50125-A 12 × 60. The flat tensile specimens of the Ti-6Al-4V plates were machined according to the standard DIN 50125-E 5 × 16 × 50. The tests were performed using a tensile testing machine (Zwick/Roell, Type: BZ-MM14780.ZW01, Ulm, Germany) and were conducted at room temperature. The testing was accomplished by displacement control with a testing velocity of 1 mm/min.

4.6.3 Micro Flat Tensile Test

Due to the limitation of the coating thickness, conventional tensile test samples cannot be machined out of the current coatings. The micro flat tensile testing method allows the determination of tensile properties in conditions under which insufficient material is available to extract a standardised test sample. The specimen geometry was designed according to the standard (DIN EN ISO 6892-1:2009-12) [63]. The required specimen geometry is 27 × 5 × 0.5 mm with a surface roughness Rt of 25 µm. The specimen geometry and dimensions are shown in (Figure 4.4). Based on the small sample geometry, this tensile testing method has been chosen as an adequate analysis for investigation of the tensile properties of the coatings.

Micro flat tensile tests were also carried out on the base material samples extracted in the rolling direction. The coating samples were tested in the deposition direction. Rod base

material specimens were electro-discharge machined directly from the round bars. Specimens from the coatings were electro-discharge machined from the coating centre and at a height distance of 0.1 mm above the substrate to ensure that the substrate material was not also sampled (Figure 4.5). Micro flat tensile testing equipment (Zwick, Germany) was used to perform the micro tensile tests at ambient temperature with a constant testing velocity of 0.2 mm/min. The displacement was measured with a laser extensometer.

Figure 4.4: Drawing of the micro flat tensile specimen.

Figure 4.5: Schematic of positioning of micro flat tensile specimens within the coating.

4.6.4 Fretting Wear Test

The fretting wear tests were conceived to investigate the wear behaviour of titanium under oscillating motion. A custom-designed testing machine (University Duisburg-Essen, Germany) at the desired specification was used for testing. The tests were carried out in

ball-Substrate

Coating

on-plane configuration at a frequency of 10 Hz under a normal force of 35 N and a displacement of 180 µm.

Force monitoring was carried out in three directions, measured in-situ with a 3-axis dynamometer (Type 9257A, Kistler Instrumente AG, Switzerland). During the experiments, the set and actual displacements and the testing time were recorded to control the movement of the counter body [64].

The desired load of the experiment can be set by weights, and the test duration is established by the number of cycles. The testing installation includes two components: the testing material, defined in tribology as the base body, and a component to enforce the wear evolution, defined as the counter body. High-precision spheres from high-performance ceramics (ZrO2), which are usually made for gears, were used in the experiments with a diameter of 32 mm and a surface roughness of 0.032 µm as a counter body. The ceramic sphere with high wear resistance was chosen to evaluate the wear of the tested material. The mechanical and physical properties of the ceramic sphere are displayed in Table 4.3. The coatings and base materials were used as the base body. Samples measuring 10 × 10 × 15 mm were prepared for the testing. The coating samples were separated from the middle of the deposit. Cross section samples have been prepared from the base material. The sample top surface was ground and polished according to the metallographic preparation presented in subsection 4.5.1. For the tests, the sample was clamped in the movable table and fixed with a screw and the ZrO2 sphere as a counter body was clamped in the upper mechanism and eventually lowered to the sample until contact. The experiments were carried out without lubrication at uncontrolled ambient conditions. The normal force was set to 35 N. The number of cycles was varied from 100 to 10⁶ the stepwise variation and duration of the experiments are presented in Table 4.4. Table 4.5 presents the process parameters used to produce coatings for wear experiments.

Table 4.3: Mechanical properties of the high-precision sphere provided by Hartstofftechnik GmbH.

The wear behaviour of coatings from both titanium alloys produced under two different conditions was characterised by a fretting wear test in a sphere-plane configuration at constant testing conditions with the exception of the testing time. Among the experimental results, the normal and tangential forces were analysed, and the friction coefficient was determined based

on the measured force results. Furthermore, the generated wear volume and maximum wear depth were measured. Eventually, cross sections of the wear tracks were prepared.

Table 4.4: Number of cycles and duration of the fretting wear experiment.

Number of cycles Time

100 10 s

500 50 s

5,000 8 min 20 s

50,000 1 h 23 min

500,000 13 h 50 min

1,000,000 27 h 40 min

Table 4.5: Material conditions investigated by the wear analysis.

Materials Ti-6Al-4V Coating

Ti-Gr.1

Coating Base materials Rotational speed

[min^-1] 400 6000 300 6000 -

Confocal White Light Microscopy

Confocal white light microscopy is based on three-dimensional surface topography measurements. The fretting wear scars were analysed using confocal microscopy (CM) (µSurf, NanoFocus AG). The obtained results were evaluated by the µsoft analysis software package. Images taken from the surfaces of the wear tracks by CM enable profile measurement of the wear depths (cavity) of the coating and transferred material from the coating to the sphere (convexity). The fretting wear profile measurements were conducted in the oscillating direction and perpendicular to the sliding direction. Convex profiles of the ceramic spheres were measured.

Cross sections of the coating wear tracks were prepared perpendicular to the sliding direction to compare the wear profile measured using CM with wear depths of cross sections and to investigate the wear mechanisms and their influence on the microstructure. By means of cross section the deformation mechanisms can be investigated. The coatings were ground and polished according to the standard metallographic procedures previously described.