The effect of substrate roughness, time and temperature on the formation of iron boride coatings onto AISI-1018 steel substrates by paste-boriding thermochemical (PBT) process has been studied. Three parameters were considered: substrate roughness (0.015, 0.2, 0.8 and 1.2 µm), time (1, 2, 4, 6 and 8h) and temperature (700, 800, 900 and 1,000°C). Coatings were characterised by SEM and XRD; Fe₂B and FeB phases were identified. The ANOVA showed that temperature is the parameter with the highest contribution on the iron boride thickness (66.96%), followed by time (15.48%) and substrate roughness (7%). In addition, the interaction of the substrate roughness with temperature and time contributed with 5.7 % to the formation of the iron boride coating. Based on a regression model, an empirical equation and contour levels representing the layer thickness were proposed to estimate the iron boride coatings thickness. The growth kinetic showed that the maximum layer thickness and the minimum activation energy Q = 119.7 kJ/mol are characteristic of a PBT process with substrate roughness of 0.8 µm. Diffusion coefficients and activation energies of each PBT process with different substrate roughness are also given.