<p>The melt pool behavior and defect formation mechanisms during laser brazing of nickel-based/diamond composite coatings were investigated using high-speed camera imaging for in-situ observation of the interactions between Ni60 alloy powder, diamond particles, and the melt pool. The effects of laser power and scanning speed on melt pool morphology and fluidity were systematically examined. Results reveal that the melting process of Ni60 powder involves four distinct stages: falling, contact and causing ripples, being recoiled and floating, and complete fusion. Owing to low density and high melting point, diamond particles do not experience a melting-fusion stage with the melt pool. After undergoing falling, contact and causing ripples, being recoiled and floating, diamond particles migrate under the influence of the melt pool flow field and ultimately float to the coating surface. Appropriate increases in laser power or decreases in scanning speed enhance melt pool fluidity, thereby promoting diamond particle flotation. The formation mechanisms of coating defects such as porosity and cracks were analyzed. Porosity and spatter defects are directly associated with bubble escape behavior within the melt pool, while crack formation is primarily attributed to the substantial difference in thermal expansion coefficients between diamond and nickel-based alloy and residual stresses generated by rapid cooling.</p>