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

The high-speed steel (HSS) substrate was coated with CrC, NbC, TiC, and VC layers using the thermo-reactive diffusion (TRD) process at 900 and 1100 °C for 2 and 4 hours. The experimental design was implemented according to the Taguchi L8 orthogonal array. The coating morphology was examined by scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS), while the phase composition was identified by X-ray diffraction (XRD). The effects of coating type, temperature, and time on 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 non-uniform layer thickness, localized porosity, and irregular carbide formation, whereas coatings applied at higher temperatures and longer times resulted in uniform layers, homogeneous elemental distribution, and improved surface morphology. XRD analyses confirmed that the coatings consisted of high-intensity carbide phases and that the chemical composition of the substrate influenced the resulting coating phases. Phase composition analysis revealed the presence of residual phases such as Cr₇C₃ and Cr₂₃C₆ in the coatings. Coating thickness and hardness increased with increasing temperature and time. Analysis of variance revealed that coating type (45.13%) and temperature (42.34%) were the dominant factors affecting coating thickness, whereas temperature (39.92%) and coating type (37.77%) had the greatest contributions to hardness. The friction coefficient was predominantly influenced by coating type (87.98%), whereas the specific wear rate was mainly affected by temperature (34.99%) and coating type (33.37%). NbC coatings exhibited the lowest performance among the investigated coatings across all evaluated properties.