Macro-indentation tests

The impact impression of the shot was observed under an optical microscope. After the macro-indentation test, a part of specimen surrounding the indentation was cut out, embedded in epoxy-resin, sectioned and had the sectional face ground and polished with abrasive (Buehler s Colloidal Silica). The surface was observed under the optical microscope in the bright field and through the differential interference, a scanning electron microscope (SEM Hitachi s-4000), and an electron probe surface analyzer (EPSA Elionix ERA8000). 

Figure 8. Scanning electron micrographs of the indentations of (a) pure PEEK sintered at 350°C for 1.5h and (b) HAp-PEEK nano composite sintered at 350°C for 1.5h, specimens after macro hardness test.

Although Vickers and DPH microhardness tests should yield the same numerical results on a given material, such is not always the case. Much of the observed variance maybe a function of differences in the volume of sample material displaced by the macro and micro indentations. 

It is no longer neeessary to demonstrate the influence of oil contamination on Rolling Contact Fatigue (RCF). Previous studies have shown the consequences induced by the travel of solid particles inside the contacts. In fact, when particles enter the contact, they indent the surfaces. The resulting surface defects, called dents, become stress raisers and increase the risk of spalling and consequently of RCF. In order to compare numerical and experimental results, some tests were performed on a two disk machine with artificially dented surfaces. Cross-sections were performed to complete the surface observations, and to analyse the crack initiation sites and the crack propagation directions. This paper proposes an analysis of these cross-sections to identify the cracks that lead to micro and macro spalling. 

Macro hardness of PEEK and HAp-PEEK composite, both sintered at 350°C, was measured by Rockwell hardness tester (Newage Testing Instruments, Inc make, model ME-2 Rockwell Hardness Testing System) using 1/16 steel ball and 60 kg applied load. Minimum five indentations were taken to obtain an average macro hardness value for each specimen. 

The macro hardness value was carried out in order to evaluate the mechanical property of the pure PEEK and HAp-PEEK nano composite. The hardness value of 24.3 for pure PEEK was significantly improved at 64.5 by reinforcing of HAp particles in HAp-PEEK nano composite. However, few micro cracks were foimd beside the indentations of HAp-PEEK composites. This may due to the low bonding between HAp particles and PEEK polymer in sinto-ed HAp-PEEK composites. The indentations for pure PEEK sintered at 350"C and HAp-PEEK nano composite sinto-ed at 350"C specimens after hardness test are shown in Figures 8(a) and 8(b), respectively. 

---