Effect of Repeated Sliding Friction on Surface and Interfacial Damage of WC-Co Coating Sprayed by HP-HVOF
Chengwei LI, Bo ZHANG, Masahiko KATO and Keijiro NAKASA
Abstract:Thermal spraying of WC-Co cermet was applied to annealed or quench and tempered tool steel specimens by a high-pressure high-velocity oxygen fuel method. Repeated sliding friction tests were subsequently carried out using a contact load of 1.0 or 1.5kN (the contact area was 6~12mm), and the delamination energy Ed was evaluated by means of an edge-indent test. Many parallel cracks were observed on the coating surface after a certain number of sliding friction cycles and partial shallow delamination of the coating began at 2.5~105 cycles. The delamination energy Ed decreased with increasing number of cycles for both types of substrate. Although the Ed was higher for the annealed than the quench and tempered substrate before the sliding friction tests, the decrease in Ed by repeated friction was greater for the annealed substrate. Two-dimensional finite element analyses revealed that large maximum principal and shear stresses existed in the direction parallel to the coating surface near the contact edge of the sliding block and this was considered to be the reason for crack initiation and the partial shallow delamination of the coating observed. Although the range of shear stress and normal stress were the greatest at the surface, they were still large near the interface and the range of shear stress near the interface was the greatest in the direction parallel to the interface. The reason for the decrease in the delamination energy with increasing number of friction cycles seemed to be the accumulation of interfacial damage resulting from the combination of a range of normal stress and the shear stress along the interface. Key Words:WC-Co coating, High-pressure high-velocity flame spraying, Sliding friction, Edge-indent test, Delamination energy