Microstructure, mechanical properties and sliding wear behaviour of thermally sprayed cermet coatings

Thermally sprayed tungsten carbide cermet coatings are extensively used in engineering applications, such as cutting tools, rock and earth drilling tools. When coatings are compared, they often differ in a number of ways, such as being deposited from powder feedstock with different characteristics (e.g. particle size, powder porosity, method of manufacture, different carbide grain sizes, different volume fractions of binder, different binder types) and with different thermal spray systems. When a number of factors are varied at once, then it is very difficult to identify the causes of any differences observed in coating microstructure and performance. In the current study, an attempt was made to vary just one variable at a time whilst keeping (as far as possible) the other variables constant. Experiments evaluated the independent effect of three parameters on the microstructure and performance of tungsten-carbide based coatings as follows: (a) metal binder type (comparing WC-12wt%Co and WC-12wt%Ni); (b) WC grain size (coarse and fine carbide size in WC-17wt%Co); and (c) metal binder content (WC-12wt%Co and WC-17wt%Co with similar carbide grain size). All feedstock powders used were agglomerated and sintered with similar particle size (-45+15 µm), deposited on mild steel substrates using a Metallisation MET-JET 4L kerosene-fuelled HVOF system. Coatings were characterized in terms of their microstructure, mechanical properties, and dry sliding wear behaviour. The research also addressed the possibility of using scratch testing as a method for evaluating the fracture mechanism of thick coatings and as a tool for predicting their wear behaviour. The experimental results show more decomposition of WC (into W2C and W) in the WC-12Co coating compared with the WC-12Ni coating. This could be due to melting occurring at lower temperature in the W-Co-C system than in the W-Ni-C system during particle heating in the spray gun. There also appeared to be more amorphous phase formation in the WC-12Co coating. Hardness and fracture toughness are both higher in the WC-12Co coating than in the WC-12Ni coating. Additionally, the WC-12Co coating required higher loads to induce fracture in the scratch test. In sliding wear, both the WC-12Co and WC-12Ni exhibited a similar wear mechanism in mild wear but the WC-12Co coating withstood a higher applied load before the transition to severe wear. The effect of the WC grain size on the WC-17Co coating was evaluated. The results show that fine and coarse carbide coatings have similar microstructural features. However, the fine carbide WC-17Co coating exhibited a higher hardness and lower fracture toughness. Furthermore, the fine carbide WC-17Co coating required higher loads to induce fracture in the scratch test compared to the coarse carbide WC-17Co coating. In sliding wear, both coatings exhibited a similar wear mechanism and rate in mild wear. However, the WC-17Co coating with fine WC grain size withstood higher loads before the transition from mild to severe wear. The coating with lower metal binder content (WC-12Co) exhibited higher levels of decomposition compared with the coating with higher metal binder content (WC-17Co). Increasing the binder content resulted in reduction in the microhardness, increased the fracture toughness and reduced critical loads through all scratch test sections. In sliding wear, the coating with the higher binder content exhibited a wear rate up to four times higher than that of the coating with the lower binder content. Overall, coatings with higher critical load during scratch testing also exhibited higher wear resistance. A linear relationship between the scratch critical load for development of semi-circular cracks and the mild – severe wear transition load is demonstrated. Consequently, it is proposed that scratch testing may be used to predict load bearing capacity in sliding wear