Taguchi-Based Analysis of Microstructural and Tribological Effects of CrC, NbC, TiC, and VC Coatings on High-Speed Steels via the TRD Method

High-speed steel (HSS) substrates were coated with layers of CrC, NbC, TiC, and VC using thermo-reactive diffusion (TRD) at 900 and 1100 °C for 2 and 4 h. The experimental design was implemented according to the Taguchi L8 orthogonal array. Coating morphology was investigated using scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS), while phase composition was determined by X-ray diffraction (XRD). The contributions of coating type, temperature, and time to coating thickness, microhardness, coefficient of friction, and specific wear rate were evaluated using analysis of variance (ANOVA). SEM and EDS analyses showed that coatings produced at lower temperatures and shorter times exhibited irregular layer thickness, localized porosity, and irregular carbide formation, while coatings applied at higher temperatures and longer times were associated with smoother layers, more homogeneous element distribution, and improved surface morphology. XRD analyses confirmed that the coatings consisted of dense carbide phases and that the chemical composition of the substrate affected the resulting coating phases. Phase composition analysis revealed the presence of phases such as Cr7C3 and Cr23C6 in the coatings. It was observed that coating thickness and hardness generally increased with increasing temperature and coating time. Variance analysis showed that the highest additive ratios in terms of coating thickness belonged to coating type (45.13%) and temperature (42.34%), while in terms of microhardness, temperature (39.92%) and coating type (37.77%) had higher additive ratios. The highest additive ratio in terms of friction coefficient was obtained with coating type (87.98%), while temperature (34.99%) and coating type (33.37%) were determined as the parameters with the highest additive ratios in terms of specific wear rate. NbC coatings generally showed lower performance values compared to other coating types under the examined experimental conditions.