Instrumented Indentation of Carbides in a Martensitic Matrix and in a Sintered Sample

Niobium carbide (NbC) has high wear and corrosion resistance and retains these characteristics at high temperatures. In industrial applications, as well as in academia, NbC can be found in various configurations, such as carbides dispersed in the microstructure of steel or as sintered components. The aim of this study is to use the instrumented indentation (nanoindentation) technique to evaluate the properties of carbides present in the martensitic matrix of a cast steel or as a sintered specimen. In the case of the cast alloy, the sample had an MC-type carbide volume fraction of approximately 5%. In the second case, the NbC specimen was sintered using spark-plasma sintering (SPS). Additional specimen characterization was conducted using X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX). Nanoindentation analyses were conducted with loads up to 10 mN, allowing hardness impressions with dimensions significantly smaller than the size of the carbides in the cast specimens. The results indicated relatively uniform values of 22 GPa and 348 GPa for the hardness and reduced elastic modulus of the sintered specimen, respectively. The EDX analysis results of the cast specimen indicated that the MC-type carbides had regions with different chemical compositions. Differences in the carbide chemical compositions were also observed when different regions of the cast specimen were compared. Nevertheless, similar properties were obtained in all regions, with a trend for the carbides in the cast specimen to present a higher hardness and lower modulus than those in the sintered specimen.