WC-Ni Cermets Research Articles

High-temperature sliding wear, elastic modulus and transverse rupture strength of Ni bonded NbC and WC cermets

The effects of rapid pulse electric current sintering (PECS), substitution of WC by NbC and Co by Ni, and carbide additives (TiC and Mo2C) on the microstructure, elastic modulus, B3B transverse rupture strength (TRS) and high temperature sliding wear on WC-Co, WC-Ni, NbC-Co and NbC-Ni cermets were studied. Additions of x% Mo2C and y% TiC (where x and y were <10 wt%), coupled with PECS, significantly refined the NbC-Ni cermet's carbide grain size from ~5.0 μm to <0.8 μm, giving mechanical properties comparable to WC-Co and WC-Ni cermets: >14 GPa hardness and ~10 MPa.m1/2 fracture toughness (KIC) and ball-on-three-balls (B3B) TRS > 1600 MPa. The sintering techniques had negligible effect on the samples' elastic and shear modulus, and all WC-based samples had higher elastic modulus than all NbC-based samples (by ~120 GPa). High temperature sliding wear tests were carried out using a ball-on-disk tribometer, with a 10 N force, at a sliding speed of 1.34 m/s for 0.8 km (10 min) and 2.4 km (30 min), using 100Cr6 (AISI 52100) steel balls at 400 °C and 0% humidity. For the 2.4 km sliding distance, all the WC cermets had lower wear volumes than NbC cermets, with LPS WC-0.5Cr3C2-10Co having the lowest wear volume. Additions of TiC and Mo2C to NbC-12Ni improved the sliding wear resistance, with TiC having the greater effect, reducing the sample wear rate by over 30% from 15.1 × 10−6 mm3/N·m to 9.4 × 10−6 mm3/N·m after sliding distance of 2.4 km. Generally, the LPS samples had lower wear volumes than the corresponding SPS samples, due to higher K1c and TRS.

Effect of WC or NbC addition on lattice parameter of surrounding structure in Ti(C 0.7N 0.3)–Ni cermets investigated by TEM/CBED

The changes in the lattice parameters of the solid solutions in the Ti(C 0.7N 0.3)–WC–Ni and Ti(C 0.7N 0.3)–NbC–Ni systems were first shown quantitatively by the CBED (Convergent Beam Electron Diffraction) technique together with TEM (Transmission Electron Microscopy) microstructure characterization. The extent of the changes in the lattice parameters between core and rim differs in the case of WC and NbC additions. No change in the lattice parameters is observed in the Ti(C 0.7N 0.3)–WC–Ni cermets, in contrast to the Ti(C,N)–NbC–Ni cermets where significant changes in the lattice parameters are observed. The difference in the parameters is correlated with the core/rim structure, which disappears in the Ti(C,N)–NbC–Ni cermets when a large amount of NbC is added, and is discussed based on thermodynamic arguments. Large strain in the core and rim structure, especially near the core/rim interface, is also observed from the HOLZ (High Order Laue Zone) line splitting.

(Ti,W)C–Ni cermets by laser engineered net shaping

The effect of the laser engineered net shaping (LENS) process on the microstructure and mechanical properties of (Ti,W)C–Ni solid solution cermets was investigated. Nano-(Ti,W)C–Ni powder was synthesised, spray dried and densified by two different processes: sintering and LENS. Furthermore, all results were compared with conventional micrometre sized TiC–WC–Ni cermets. Solid solution (Ti,W)C–Ni cermets made by sintering resulted in a coreless and ultra fine microstructure, while the LENS processed specimens revealed inhomogeneous microstructure and particle coarsening. The hardness values of the LENS processed specimens range from 15 to 20 GPa, which is higher than for the ultra fine and conventional sintered composites (∼12 GPa). The measured toughness values show greater variability.

GI-XRD studies on surface structure of ultrafine Ti(C 0.5N 0.5)-WC-Ni cermets at high temperature

Influence of coarse and ultrafine particle size of Ti(C 0.5N 0.5) on the high-temperature stability of various Ti(C 0.5N 0.5)-WC-Ni cermets under oxidizing atmosphere were compared. The evolved surface microstructure was correlated to the wear performance of the cermet exposed to similar thermophysical conditions. Investigation by glancing incidence X-ray diffraction revealed a considerable fraction of TiO 2, TiC and TiN in the near surface layer of the cermet heat treated at ∼873 K. Ultrafine cermets showed good high-temperature wear resistance, which was ascribed to the homogenous distribution of the ultrafine hard phase grains along with high total hard phase content of ∼82–85 vol.%. Comparable wear performance in coarse cermets with 5 and 15 wt.% WC was also observed in spite of a low total hard phase content (66–72 vol.%), which was attributed to the high TiN in the surface. The presence of high amount of NiO (∼22 vol.%) and low Ni in the surface layer of WC-rich (25 wt.%) coarse grade cermet contributed to its poor high-temperature wear resistance

Rapid sintering of ultrafine WC–Ni cermets

Using high-frequency induction-heated sintering (HFIHS), cermets of WC– xNi ( x = 8, 10, and 12 wt.%) were consolidated to a density of about 98%. The sintered samples had WC grain size of 300 nm. The hardness was significantly higher than that obtained with conventionally sintered WC–Co and WC–Ni cermets without a reduction in fracture toughness.

TRIBOLOGICAL EVALUATION OF ULTRAFINE Ti(CN) CERMETS

High temperature wear performance of ultrafine and coarse grade Ti(CN)–WC–Ni cermets was investigated by surface profilometry, atomic force microscopy, and scanning electron microscopy to ascertain the contribution of particle size effect. The observed wear depths, produced during wear test, showed direct correlation with the surface roughness, which in turn is controlled by the microstructural features. The cermets with ultrafine Ti(CN) exhibit greater homogeneity along with a considerably higher rim fraction in the typical core-rim microstructure when compared to its coarse counterpart. The improved microstructure accounts for the enhanced tribological properties in the ultrafine cermets.

Microstructure and tribo-mechanical properties of ultrafine Ti(CN) cermets

Role of Ti(CN) particle size on the microstructure of the Ti(CN)–WC–Ni cermets has been evaluated. The systems containing ultrafine grade Ti(CN) shows better structural homogeneity and a large volume fraction of rim phase in the core-rim structure when compared to the coarse Ti(CN) cermets. Measurable shift in the lattice parameter of the Ti(CN) core occurs in the ultrafine grade cermet possibly due to the diffusion of W and/or Ni. The improved structural features in the ultrafine grade led to considerably enhanced tribo-mechanical properties of the cermets.