This study investigates the impact of carbon on the kinetics of the spark plasma sintering (SPS) of nano- and submicron powders WC-10 wt.%Co. Carbon, in the form of graphite, was introduced into powders by mixing. The activation energy of solid-phase sintering was determined for the conditions of isothermal and continuous heating. It has been demonstrated that increasing the carbon content leads to a decrease in the fraction of η-phase particles and a shift of the shrinkage curve towards lower heating temperatures. It has been established that increasing the graphite content in nano- and submicron powders has no significant effect on the SPS activation energy for “mid-range” heating temperatures, Q_S(I). The value of Q_S(I) is close to the activation energy of grain-boundary diffusion in cobalt. It has been demonstrated that increasing the content of graphite leads to a significant decrease in the SPS activation energy, Q_S(II), for “higher-range” heating temperatures due to lower concentration of tungsten atoms in cobalt-based γ-phase. It has been established that the sintering kinetics of fine-grained WC-Co hard alloys is limited by the intensity of diffusion creep of cobalt (Coble creep).

The special aspects of spark plasma sintering (SPS) of plasma-chemical nanopowders of tungsten carbide with higher oxygen content were investigated. It was demonstrated that shrinkage rate of the WC nanopowders during SPS is limited by the rate of grain-boundary diffusion with the abnormally low values of activation energy. Decrease in the activation energy of SPS is caused by the influence of oxygen on the diffusion permeability of boundaries of the tungsten carbide grains at the stage of intense densification, as well as by abnormal grain growth. Kinetics of SPS of the WC-W₂C-WO₃-W nanopowder compositions at the stage of severe densification is controlled by the rate of sticking together the oxide particles, with their simultaneous transformation into W₂C particles, and then by the plastic flow of W₂C particles in the presence of the tungsten particles at the stage of high-temperature sintering. Ceramics with a high density ratio (98–99%) and ultra-fine-grained structure (the mean grain size less than 0.3 μm) with higher hardness HV = 30.5 GPa and fracture toughness of ~6.5 MPa m^1/2 were obtained using the SPS method.

The results of studies of the kinetics of high-speed spark plasma sintering (SPS) of plasma-chemical nanosized and industrial micron tungsten carbide powders are described. During SPS, the carburization of the surface layer of tungsten carbide samples occurs, leading to differences in the phase composition and hardness of the surface and central regions of sintered ceramics. The process of SPS of tungsten carbide can be sequentially represented as a change of the following mechanisms: rearrangement of particles at lower temperatures (Stage I) → sintering of particles due to grain boundary diffusion (Stage II) → sintering due to diffusion in the crystal lattice (Stage III-1) → sintering under conditions of intense grain growth with an abnormally low diffusion activation energy (Stage III-2).

The features of spark plasma sintering (SPS) of plasma-chemical nanopowders WC–(0.3, 0.6, 1) wt % Co were studied. The SPS process of ultralow-cobalt hard alloys can be sequentially represented as a change of the following stages: rearrangement of particles at lower temperatures (Stage I) → sintering of WC–Co particles due to Coble creep of γ phase of cobalt, the intensity of which is determined by the grain boundary diffusion rate (Stage II) → sintering due to diffusion creep, the rate of which is limited by the bulk diffusion in cobalt (Stage III-1) → sintering of tungsten carbide particles along the intergranular boundaries of WC/WC under conditions of intense grain growth (Stage III-2). Samples with a high density (96.4–98.4%) and high mechanical properties were obtained (for the WC–0.3% Co hard alloy: H_v ~ 20.5 GPa, K_1C = 7.1 MPa m^1/2).

The features of high-speed spark plasma sintering (SPS) of plasma-chemical nanopowders WC‒(0.3, 0.6, 1) wt % Co with the addition of 0.3 and 0.5 wt % graphite were studied. The structural features of the ultralow-cobalt hard alloys with graphite addition during SPS are due to the simultaneous influence of an increased concentration of oxygen adsorbed on the surface of plasma-chemical WC–Co nanoparticles during mixing with graphite and the effect of graphite, which leads to a decrease in activation energy of sintering owing to a decrease in the intensity of formation of η-phase particles in “oxidized” WC–Co nanopowders, as well as the formation of a fairly uniform fine-grained structure. Samples of fine-grained ultralow-cobalt hard alloys with increased hardness and fracture toughness were obtained (for a WC–0.6 wt % Co–0.3 wt % C hard alloy with an average grain size of ~(1–1.5) μm, the hardness is H_v = 20.2–20.5 GPa with a minimum crack resistance coefficient K_IC = 9.2–10.4 MPa m^1/2).

Spark plasma sintering (SPS) of plasma-chemical nanopowders tungsten carbide with a high oxygen content are studied. The results show that the nanopowder WC shrinkages during SPS is limited by grain boundary diffusion with abnormally low activation energies. It is established that the decrease in the activation energy can be caused by the influence of oxygen on the diffusion permeability of the tungsten carbide grain boundaries at the stage of intense compaction, as well as by abnormal grain growth at the stage of high-temperature sintering. The SPS kinetics of WC-W2C-WO3-W nanopowder compositions at the stage of intense compaction is controlled by the rate of oxide particle sintering with their simultaneous transformation into W2C particles, and then, by the plastic flow of W2C particles in the presence of W particles at the stage of high-temperature sintering. Using the SPS method, we obtained ceramics with a high density (98-99%), ultrafine-grained structure (the average grain size is less than 0.3 mm), having an increased hardness HV = 30.5 GPa with a Palmquist crack resistance of ~6.5 MPa·m1/2.