•CoB-Co2B layer was changed to Co2B by diffusion annealing on borided CoCrMo alloy.•The scratch resistance notably improved after a diffusion annealing process.•The accomplished layer avoids severe scratch failure mechanisms.•The scratch residual depth decreased around 60% with this procedure.•The enhanced layer reduced around 50% the volume loss in multipass scratch. This work studies the effect of a diffusion annealing process in cobalt boride layers by progressive load scratch (PLST) and multipass scratch (MPST) tests. First, a double-phase CoB/Co2B layer was formed on CoCrMo alloy at 1000 °C for 6 h. Next, a single-phase Co2B layer was obtained through a diffusion annealing process. The scratch tests were conducted with a progressive load from 5 to 150 N in PLST, whereas in MPST, up to 400 passes were applied with a subcritical load of 38 N. Conducting the diffusion annealing process significantly improved the scratch resistance; in PLST, the achieved layer avoided severe failure mechanisms and decreased residual depth by 60%. In MPST, friction was reduced and the volume loss decreased around 50%.