Sand erosion performance of CVD boron carbide coated tungsten carbide

Abstract

The erosion performance and the interaction between the micro-mechanisms of erosion and the microstructure of a chemical vapour deposited boron carbide coating are presented. Samples were tested using both water–sand slurry and air–sand jet impingements at 90° incidence. Tests used angular quartz sand with a mean diameter between 135 and 235 μm and jet impingement velocities between 16 and 268 m s −1. The chemical vapour deposition (CVD) boron carbide coatings were 15–20 μm thick and deposited on a range of substrates of sintered tungsten carbide with 6 to 15 wt.% metal binder. The results, relative to the erosion resistance of the uncoated substrates, show the coatings to have higher resistance (10 times) under lower energy impacts but similar resistance at higher energy impacts. The sintered boron carbide had a similar erosion resistance to that of sintered tungsten carbide except at high energy impacts where it outperforms tungsten carbide and CVD boron carbide by a factor of 2. The performance of these coatings against erodent mass and impact energy are discussed and related to the nano and micro brittle fracture mechanisms identified by detailed microscopy and predicted by Hertzian cone crack theory. Partial concentric spalling of the coating was also evident in regions where circular Hertzian surface cracks are present. These erosion mechanisms, primarily nano-chipping and crack propagation, are also related to the microstructure and composition identified by XRD analysis and Raman spectroscopy. These results, in conjunction with fracture toughness and micro-hardness measurements, suggest that the coating composition is not pure B 13C 2 but has less erosion resistant forms of boron carbide present such as B 50C 2.