Effect of Oxygen-fuel Ratio on Impact Resistance of WC-Ni Coatings Deposited by HVOF Spraying

The principal aim of this work is to study the effect of oxygen-fuel ratio (λO/F) on the impact resistance of high-velocity oxy-fuel (HVOF) sprayed WC-12Ni coatings. The value of λO/F=1.1 was used as the starting point to adjust the spray process parameters, and the λO/F values were increased to 1.2 and 1.3, which were regulated by reducing the kerosene flow at a fixed oxygen flow of 811 L/min or increasing the oxygen flow at a fixed kerosene flow of 22.7 L/h. The corresponding five groups of WC-12Ni coatings were prepared at the above different λO/F values. The microstructure, microhardness and elastic modulus of the coatings were analyzed, respectively. The impact behavior resistance of the coatings was studied through a flat-cylinder contact heavy-load impact test. The experimental results indicated that the monotonous relationship between the λO/F value and microstructure and mechanical properties reported in literature was not observed for the WC-12Ni coatings. As the λO/F value increased from 1.1 to 1.3 that was regulated by reducing the kerosene flow at a fixed oxygen flow of 811 L/min, the porosity of the coatings increased from 0.91% to 1.24%, and the microhardness and elastic modulus of the coatings decreased from 10.1 and 344.0 to 8.6 and 313.9 GPa, respectively. In addition, the impact resistance of the coatings was weakened. The crater volume of the coatings increased from 2.1×10−3 to 3.3×10−3 mm3, and the damage features of the coatings changed from WC grains spalling to cracking. When the λO/F value was 1.3, the integral crack length reached the maximum of approximately 54 μm. However, the opposite trend of microstructure, microhardness and elastic modulus of the coatings was observed as the λO/F value increased from 1.1 to 1.3 regulated by increasing the oxygen flow at a fixed kerosene flow of 22.7 L/h. Correspondingly, the porosity of the coatings decreased to 0.85%, and their microhardness and elastic modulus increased to 10.8 and 382.5 GPa, respectively. Moreover, when the λO/F value was 1.2, the crater volume of the coatings increased to 2.4×10−3 mm3, and the coating damage behavior was still the WC grains spalling but with increased spallation as compared with that of λO/F=1.1. Continuing to increase the λO/F value to 1.3, the crater volume reached 2.6×10−3 mm3, and the cracking damage of the coatings occurred with the integral crack length of about 30 μm. By combining the analysis of microhardness and elastic modulus of the coatings, the relationship between the impact damage behavior and H3/E2 of the coatings was determined. Namely, the impact resistance of the coatings was improved with the increase of H3/E2. When H3/E2 was not higher than 8.4 MPa (λO/F=1.3; O 998 L/min, F 22.7 L/h), the surface cracking occurred, and the integral crack length decreased with the increase of H3/E2. When exceeding that value, the main damage feature of the coatings was the WC spalling, and the WC spallation degree reduced with the increase of H3/E2. As H3/E2 increased from 6.4 (λO/F=1.3; O 811 L/min, F 18.4 L/h) to 8.7 MPa (λO/F=1.1), the crater volume and degree of WC spalling was reduced by 36% and 35%, respectively. The ability to resist plastic deformation and cracking of coatings is quantitatively characterized by H3/E2 of HVOF sprayed WC-Ni coatings, which determines the impact resistance of the coatings. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.