Characterization and determination of FexB layers’ mechanical properties

The aim of this study is to obtain microstructural characteristics and investigate the mechanical properties such as hardness, Young's modulus and fracture toughness of the boride layer depending on process time and temperature. The produced double layers (FeB and Fe2B) were extensively analyzed with respect to X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD pattern of the boride layers, which were formed on SAE 1020 and 1040 quality steel at 900 °C for 2 h, 4 h and 6 h, include only FeB phase on surface with (1 1 1), (2 1 0), (1 0 1) and (1 1 1) planes. SEM cross-sectional investigations show that double-phase boride layer existence from surface to inside of substrate. The structural compositions of layers consist of boron rich phase (FeB) and iron rich phase (Fe2B), respectively. Surface roughness value of samples is important parameter for micro-indentation test with Dynamic Ultra Micro Hardness Tester. After diffusion controlled boriding process, material surface roughness can be high for micro-indentation test. So, surface polishing process is applied for decreasing roughness of FeB layers. Surface roughness values of 2 h, 4 h and 6 h borided SAE 1020 and SAE 1040 quality steel were decreased from 0.9 μm to 0.05 μm by polishing process. Mechanical properties of layers were examined by Shimadzu Dynamic Ultra-microhardness test machine for estimating Young's modulus due to load–unload sensing analysis and in addition to mechanical investigation hardness–depth curves of the layer were obtained for estimating indentation depth and load dependency of mechanical properties. Load depended elastic modulus (125–624 GPa) and hardness (17–33 GPa) were obtained at 80 mN, 160 mN, 320 mN and 640 mN applied peak loads depending on boriding process time. Fracture toughness properties of FeB surface layers were calculated by Vickers Fracture Toughness method for 1 N applied peak load with measuring crack length after loading stage was finished.