Hot-pressed SiC-in situ Zr2CN composites were subjected to sliding against SiC counterbody in varying conditions of humidity (<= 20 % RH and >= 90 % RH), and temperature (25 degrees C and 500 degrees C). Results demonstrated that sliding friction and wear decreased with increasing ZrN content from 0 to 10 vol% in the initial powder composition. Wear resistance increased by 37% at dry, 79% at humid and 54% at high temperature condition for the composite with 10 vol% initial ZrN content in comparison to monolithic SiC ceramics. While the presence of Zr2CN phase in SiC ceramics reduced fracture and pull-out, friction and wear are found to be minimum at humid condition due to the formation of thick tribo-oxidative layer at the contact. Results obtained from this study indicate the potential aspects of SiC-10 vol% ZrN composites in tribological applications where material in sliding contacts experiences extreme conditions of humidity and temperature.

Dense SiC-(10-30 vol%) ZrN composites were developed using spark plasma sintering with AlN-Y2O3 additives at 1800 degrees C for 5 min in nitrogen atmosphere. The nascent oxide present on ZrN surface reduces liquid phase formation temperature, facilitating improved densification of SiC ceramics. Mechanical properties were retained, and electrical conductivity increased with ZrN addition. To demonstrate a rapid and low-cost machining alternative for SiC ceramics, SiC-ZrN composites were successfully cut using wire-EDM. The tribological potential of novel electrical discharge machinable SiC-ZrN composite disk against SiC ball at various conditions of reciprocating sliding contacts was investigated. Friction is less affected by reinforcement content at room temperature (25 degrees C) but significant change is observed at high temperature (600 degrees C). Minimum one order of lower wear rate is reported with an increase in temperature from 25 degrees C to 600 degrees C. The major wear mechanism changed from abrasion to tribo oxidation with increase in temperature.

Tantalum carbide (TaC), as ceramic coating, is known for its exceptional wear performance, however, the tribological behavior of bulk TaC ceramics is poorly understood and requires persistent research. In the current study, the tribology of spark plasma sintered TaC-SiC composites in dry sliding contacts against silicon carbide (SiC) counterbody under various sliding loads (5, 10, and 15 N) is investigated with respect to change in SiC content (from 0 to 65 vol%). With changes in reinforcement content and load, the coefficient of friction and wear rate varied in the range of 0.25-0.45 and 4.7 x 10-7-2.1 x 10-6 mm3/Nm, respectively. At 5 N load, dominant abrasion is observed for composites containing SiC content up to 35 vol%, while abrasion and mild fracture are observed for the composites containing above 35 vol% SiC. At 15 N load, abrasion and oxidation are observed on worn surfaces of monolithic TaC. TaC composites with <= 35 vol% SiC content exhibited abrasion, pull-outs, and tribo-oxidation, while composites with >= 50 vol% SiC content experienced severe fracture along with tribo-oxidation. The presence of tribo-oxidation on worn tracks at a higher sliding load (15 N) is confirmed by X-ray photoelectron spectroscopy results.

Friction and wear characteristics of spark plasma sintered SiC-TiB2 composites in reciprocating sliding contacts are evaluated in terms of TiB2 content (0 - 30 vol%), normal load (5, 10 and 15 N) and temperature (25 degrees C and 600 degrees C). With increase in TiB2 content, friction increased, and wear decreased. Scanning electron microscopy, atomic force microscopy and Raman spectroscopy studies confirm protective tribo-oxidation in sliding at 600 degrees C. Effect of wear debris on friction response and wear resistance is emphasised. Wear mechanisms changed from fracture at 25 degrees C to tribo-oxidation at 600 degrees C. The presence of TiB2 reinforcement helps in enhancing wear resistance of SiC ceramics at room temperature, while formation of oxide layer and cylindrical debris are responsible for less wear at 600 degrees C.

Dense silicon carbide (SiC) composites with 0-30 vol % (ZrN-TiB2) were fabricated by using liquid phase spark plasma sintering at a relatively lower sintering temperature (1800 °C) and shorter holding time (5min). The effects of co-addition of ZrN and TiB2 on densification, mechanical properties, and electrical conductivity of SiC ceramics were investigated. The reinforcement significantly enhanced density of sintered composites, while the phase analysis and microstructure confirm the suppression of β→α phase transformation in the SiC matrix. The fracture toughness ranged from 4.1 to 4.6MPa·m0.5, hardness from 19.5 to 20.3GPa, and electrical conductivity from 4.85 ×101 to 1.25 ×103 (Ω·cm)-1. Incorporating conductive reinforcements and nitrogen additives enhanced electrical conductivity. Successful wire-EDM machining of SiC-ZrN-TiB2 composites demonstrate the feasible alternative to conventional machining of non-oxide ceramics with a maximum material removal rate of 8.54 mm2/min.

While outstanding properties of boron carbide (B4C)-silicon carbide (SiC) composites make them potential candidates for ballistic and wear resistance applications, the applicability is rather limited due to processing challenges to obtain high density and superior mechanical properties. In the present work, B4C-10wt% SiC composites are spark plasma sintered without additives and with 6wt% (Al2O3 -Y2O3) additives at 1600°C, 1700°C and 1800°C. When sintered at 1600°C, the relative density increased from 94% for the composites without additives to ~99% with Al2O3-Y2O3 additives. The formation of liquid phase comprising Al-Y-Si-O improved density and mechanical properties of the composites. TEM analysis of the composite sintered with (2wt% Al2O3 - 4wt% Y2O3) additives at 1800°C confirmed clean grain boundaries between B4C, SiC and liquid phase precipitates. When sintered at 1700oC, the indentation fracture toughness increased from 4.3MPa.m0.5 for the composite without additive to 6.0MPa.m0.5 for the composite with 6wt%Al2O3 additive due to increased instances of crack splitting, bridging and deflection.

The present investigation demonstrates the feasibility of obtaining a wide range in friction and wear for SiC-hBN composites in sliding conditions: humidity (<= 20 % RH and >= 90 % RH) and temperature (RT and 500 degrees C) against SiC ball. With an increase in hBN content to 10 vol%, wear rate reduced by an order of magnitude irrespective of humidity at RT and up to 55 % reduction at 500 degrees C, and coefficient of friction (COF) reduced by 76 % at highhumidity. Friction and wear results corroborate with the transition in material removal mechanism from fracture of SiC grains and pull-out of hBN platelets at <= 20 % RH to hydroxylative wear at >= 90 % RH and oxidative wear at 500 degrees C. SiC-10 vol% hBN composites are recommended for wear-resistance applications experiencing extreme conditions of humidity and temperature as well as applications where internal lubrication is the only feasible method for reducing friction.

Besides continuous progress in designing superior composition for silicon carbide (SiC) ceramics, a thorough understanding of the relation of material composition with contact surface characteristics is necessary to cater varied requirements of friction and wear for tribological applications. In the present study, the friction and wear behaviour of spark plasma sintered SiC ceramics in sliding contacts against SiC is explained in terms of titanium boride (TiB2) addition (0-30 vol%) and sliding load (5-15 N). Abrasion, fracture, and grain pull-out are observed on worn surfaces. X-Ray photoelectron spectroscopy results confirm the absence of dominant tribo-chemistry and Raman spectroscopy analysis confirms tensile stress accumulation on wear track. The microstructure and me-chanical properties strongly influenced the wear resistance of the composites, while the roughened contact with hard debris increased friction. A combination of low wear rate (2.5 X 10-6 mm3/Nm) and high COF (0.47) obtained at 5 N load for SiC-20 vol% TiB2 composite. Experimental wear results are in agreement with the estimation using lateral fracture model. Results from the present study are believed to be useful in designing new generation SiC based composites for high friction and low wear applications.

Dual-beam FIB/FEG-SEM and TEM/HRTEM observations of sub-surfaces of SiC and SiC-50 wt% WC composite disks worn against SiC ball have provided important insights into the micro-mechanisms of material removal and the role of WC towards suppressing the same. Sub-surface analysis reveals the silicon oxide rich tribochemical layer just underneath the worn surfaces of monolithic SiC. Further below this tribochemical layer, dislocations and twins in SiC grains, as well as micro-cracks, are identified. Micro-cracks get extended up to similar to 2 mu m depth below the layer, eventually leading to material removal in SiC ceramics. In case of the SiC-WC composite, neither is the tribochemical layer dominant, nor is twin/dislocation networks seen to extend beyond the SiC/WC interfaces. Overall, the present study highlights that WC particulate reinforcements in SiC-WC composite minimize the wear damage by suppressing oxidative wear and restricting the generation of micro-cracks, which otherwise form extensively due to stress build-up at the tips of dislocations or twins in SiC grains.

In recent times, significant efforts have been put to develop wear-resistant ZrB2-SiC composites for aerospace and automotive applications. The present study details friction and wear of spark plasma sintered ZrB2-(10, 20, 30 vol %) SiC composites in dry unlubricated conditions of sliding against different counterbodies: SiC (120 W/m-K, 22 GPa), WC-Co (100 W/m-K, 15 GPa), Al2O3 (30 W/m-K, 18 GPa), ZrO2 (25 W/m-K, 12.5 GPa). The coefficient of friction (COF) varied in the range of 0.49-0.69 and wear volume in the range of 0.006-0.152 mm(3) with varying SiC content in ZrB2-SiC composites and counterbody. Against a given counterbody, less friction and wear are found for the composites with large SiC content, owing to the high fracture toughness and hardness. Friction and wear of the composites are influenced by thermal conductivity of the counterbody. Worn surface analysis of the composites reveals mechanical aspects of fracture, pull-out, deformation as well as tribo-oxidation, while fracture and pull-out decreased with increased SiC content in the composites or thermal conductivity of the counterbody. Results from the present study recommend ZrB2-30 vol % SiC composites for the use in sliding contacts against SiC counterbody.

In the present study, the effect of simultaneous incorporation of SiC and WC additives on the densification behaviour and microstructural development of TiC-based composites is studied. Four different TiC-SiC-WC (TSW) composites with varying SiC and WC content were synthesized by ultrasonic wet milling followed by spark plasma sintering (SPS) at 1750 ? for 5 min under 40 MPa external pressure. The average particle size of the ultrasonic wet-milled mixture underwent an appreciable refinement from 2.48 mu m (un-milled powder) to between 0.9 and 1.25 mu m. The sintered compacts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and thermodynamic assessment. All TSW sintered specimens exhibited a relative density of greater than 98% with TiC +10 wt% SiC +15 wt% WC reaching the highest value of 99.2%. The XRD analysis and microstructural evaluation confirmed the in-situ formation of Ti3SiC2 compound for specimens TiC +15 wt% SiC +10 wt% WC and TiC +20 wt% SiC +5 wt% WC as suggested by the thermodynamic evaluation. Besides, except for specimen TiC +20 wt% SiC +5 wt% WC, some of the SiC grains with unclean grain boundaries were found to be dissolved partially within the (Ti, W) C solid solution, thereby indicating the formation of (Ti, W, Si) C solid solutions as confirmed by the SEM/EDS analysis. The optimum hardness and indentation fracture toughness of 22.43 GPa and 6.54 MPa m(1/2) were obtained for the samples TS10W15 and TS15W10, respectively. Crack deflection, branching, and bridging induced by the untwine SiC grains, partly un-dissolved WC particles, and (Ti, W) C solid solution phase are among the main toughening mechanisms responsible for improving the fracture toughness of the co-reinforced specimens besides the break of intertwining SiC grains.

Spark plasma sintering of SiC-10, 20, or 30 wt% TaC composites was performed at 1800 degrees C. Microstructures of sintered composites revealed uniform dispersion of TaC particles in SiC matrix. With the increase in TaC content, hardness decreased from 25.75 to 23.30 GPa and fracture toughness increased from 3.48 to 3.85 MPa m(1/2). Erosion testing was performed to evaluate the potential of sintered composites at room temperature and 400 degrees C by a stream of SiC particles impinging at different angles (30 degrees, 60 degrees, or 90 degrees). The erosion rate varied from 25 to 166 mm(3)/kg, with change in TaC content, impingement angle, or temperature. The erosion rate increased as the angle of impingement and temperature increased, but reduced when the TaC concentration increased. Worn surfaces revealed that the material was dominantly removed via fracture of SiC grains and TaC particles pull-out. SiC-30 wt% TaC composites exhibited superior erosive wear resistance at low impingement angle and room temperature.

Highly dense electrically conductive silicon carbide (SiC)-(0, 10, 20, and 30 vol%) titanium boride (TiB2) composites with 10 vol% of Y2O3-AlN additives were fabricated at a relatively low temperature of 1800 degrees C by spark plasma sintering in nitrogen atmosphere. Phase analysis of sintered composites reveals suppressed beta ->alpha phase transformation due to low sintering temperature, nitride additives, and nitrogen sintering atmosphere. With increase in TiB2 content, hardness increased from 20.6 to 23.7 GPa and fracture toughness increased from 3.6 to 5.5 MPa m(1/2). The electrical conductivity increased to a remarkable 2.72 x 10(3) (omega cm)(-1) for SiC-30 vol% TiB2 composites due to large amount of conductive reinforcement, additive composition, and sintering in nitrogen atmosphere. The successful electrical discharge machining illustrates potential of the sintered SiC-TiB2 composites toward extending the application regime of conventional SiC-based ceramics.

Friction stir processing (FSP) is an emerging solid-state technique for preparing surface composites using various reinforcements. Ceramics and metallic particles are easily reinforced in a matrix by this technique. Surface composites made from an LM24 alloy reinforced with graphite and tin, with good wettability and material flow owing to the low melting point of tin, were fabricated by FSP at two rotational speeds of 1,400 and 1,000 rpm. Despite its low hardness, the graphite/LM24 surface composite fabricated at a higher rotational speed of 1,400 rpm exhibited better wear resistance. However, its frictional behavior was not significantly affected by the reinforcement. The fabricated surface composites with graphite and tin reinforcement as well as graphite-only reinforcement exhibited contradicting behaviors under sliding wear conditions. The post wear analysis indicated that abrasion, adhesion, layer formation, and delamination occurred on the composite surfaces.

Dense boron carbide (B4C) - silicon carbide (SiC) composites were obtained by spark plasma sintering technique at 1800 degrees C with 3 wt% and 6 wt% aluminium oxide (Al2O3) additives. Addition of sintering additives results in formation of aluminium silicate (Al2SiO5) liquid phase which accelerates sintering kinetics and helps in obtaining high density similar to 99%. Microstructures reveal uniformly distributed SiC particles in B4C matrix. Increase in alumina from 3 wt% to 6 wt% results in decrease in hardness from 35.1 +/- 0.8 to 33.7 +/- 0.9 GPa, and increase in fracture toughness from 5.9 +/- 0.4 to 6.5 +/- 0.4 MPam(0.5). Using a ball-on-disk tribo tester under dry unlubricated conditions at 5, 10 or 15 N load, influence of alumina content on friction and wear properties of B4C-SiC composites was investigated against SiC counterbody with a linear speed of 0.08 m/s for 60 min. The coefficient of friction (COF) increased from 0.25 to 0.65 with load, and the influence of alumina on frictional behaviour appeared to be negligible. With increase in load, wear volume of the composites increased from 7.5 x 10(-2) mm(3) to 16.1 x 10(-2) mm(3) for B4C-10 wt% SiC - 3 wt% Al2O3 and from 4.7 x 10(-2) mm(3) to 14.8 x 10(-2) mm(3) for B4C-10 wt% SiC - 6 wt% Al2O3 composites. Microcracking, abrasion and pull-outs contributed as major wear mechanisms of composites in selected wear conditions. The relation between wear behaviour and mechanical properties of sintered composites is discussed.

SiC-4 vol% BN composites were hot-pressed at 2050°C for 4 h at 40 MPa in a N2 atmosphere using micron-sized β-SiC and h-BN starting powders with 1 vol% sintering additives. Four batches were prepared using four different types of additive systems, i.e. Y2O3-Sc2O3, Yb2O3-CaO, Yb2O3-MgO, and Al2O3-AlN-Y2O3. The electrical, thermal, and mechanical properties of the SiC-4 vol% BN composites, which are primarily limited by the intrinsic weakness of h-BN and by the point defects (BC and AlSi) created by the dissolution of B and Al in the SiC lattice, were successfully tuned with the use of different additive systems. The electrical conductivity of SiC-4 vol% BN composites improved ~fivefold, i.e. from 2.6 (Ω·cm)−1 for the Al2O3-AlN-Y2O3-containing specimen to 13.9 (Ω·cm)−1 for the Y2O3-Sc2O3-containing specimen, owing to grain-growth-assisted N-doping and the elimination of Al-derived acceptors. Thermal conductivity was altered by 16% with the use of different additive systems. Fracture toughness dramatically increased from 4.3 MPa∙m1/2 in Yb2O3-CaO-containing specimens to 7.3 MPa∙m1/2 in Y2O3-Sc2O3-containing specimens. The electrical conductivity, thermal conductivity, flexural strength, and fracture toughness of SiC-4 vol% BN composite sintered with 1 vol% Y2O3-Sc2O3 were 13.9 (Ω·cm)−1, 82.0 W·m−1 K−1, 505 MPa, and 7.3 MPa∙m1/2, respectively.

This chapter provides a brief description of the key results on the abrasive wear of detonation sprayed tungsten carbide cobalt (WC-Co) coatings using a planned set of dry sand rubber wheel abrasion tests against three abrasives, i.e. silicon dioxide, aluminium oxide, and silicon carbide. Apart from presenting the published quantitative results of the abrasive wear resistance, it focuses on the surface and subsurface material removal mechanisms. In determining the tribological performance, the abrasive wear behavior of thermal sprayed WC-Co coatings has been widely investigated. The results indicated that decarburization hardly affected the abrasive wear rate of WC-17Co when carbon loss was below 60%. The chapter discusses surface and subsurface damage mechanisms. It unambiguously establishes a linear correlation between abrasive wear of detonation sprayed WC-12Co coatings and abrasive hardness.

This research work deals with the investigation of erosive wear of spark plasma sintered ZrB2-SiC composites with variation in angle of incidence (30 degrees, 60 degrees, and 90 degrees), test temperature (room and 800 degrees C) and SiC content (10, 20, and 30 vol.%). Results indicate a large variation in erosion rate from 2.13 to 75.45 mm(3)/kg with change in angle of incidence, test temperature, and SiC content. Erosion rate decreased with the decrease in angle of incidence, increase in temperature, and increase in SiC content. With increase in SiC content from 10 to 30 vol.%, a maximum reduction of 68% in erosion rate obtained at shallow incidence and room temperature, and a maximum reduction of 78% in erosion rate obtained at shallow incidence and 800 degrees C. SEM-EDS and XRD analyses indicate that formation of B2O3 and SiO2-rich protective surface is responsible for high temperature erosion resistance of ZrB2-SiC composites.