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This study reports the densification behavior and novel design of core-rim microstructure for different ZrB2–MoSi2-SiCw (ZMSw) composites. Four ZrB2–MoSi2 composites containing various amounts of SiCw (0, 5, 10, and 20 vol%) were densified by multistage spark plasma sintering. Optimal combinations of sintering schedule, temperature, and composition are recognized for producing dense ZrB2-based ceramic-matrix composite with core-rim grain structure in the boride matrix, where ZrB2 core is surrounded by a solid solution of (Mo,W)Si2. Fragmentation, rearrangement, plastic deformation, and grain boundary diffusion mechanisms are responsible for densification of composites sintered by multistage SPS conditions at 1700 °C. Addition of SiCw significantly improves the density and mechanical properties of ZrB2–MoSi2–SiCw composites due to the formation of a core-rim structure.

The effect of SiCw addition on abrasive wear performance of ZrB2-MoSi2 composites were examined. Coefficient of friction, specific wear rate, and surface roughness decrease with the addition of SiCw. The formation of the self-lubricating tribo-oxide layer caused by tribochemical reaction during frictional heating enhances wear resistance of the composites. Tribological performances of the composites are strongly influenced by parametric variation of different experimental conditions which are primarily sliding speed and applied load. The operational wear regime maps reveal that the workable damage tolerance limits for the applied load are 20−25 N for ZMSw-5 composite, whereas that for ZMSw-20 composite is ∼40 N. The wear mechanisms vary from delamination, cracking, and grain pull out at medium applied loads to grain boundary fracture at higher applied loads.

A comparative evaluation has been carried out off the physical and mechanical properties of ZrB2–20 vol% MoSi2 composites reinforced with 5, 10 and 20 vol% SiC whiskers (SiCw). The abovementioned hybrid composites have been densified by spark plasma sintering (SPS) at 1600 °C for 10 min. The present study focuses on the effect of the quantity of SiCw on the densification, electrical conductivity, hardness, and fracture toughness ZrB2–MoSi2–SiCw composites in detail. Microstructural characterization suggests that SiCw restricts the grain growth of ZrB2 during densification. The electrical conductivities of the ZrB2–MoSi2–SiCw composites have been found to vary in the range ~2.4–3.5 × 106 S/m, and it reduce with increasing SiCw content. Results exhibit that the indentation fracture toughness of ZrB2–20 vol% MoSi2 improves ~70%, ~112% and ~130% with an addition of 5, 10 and 20 vol% SiCw, respectively. An increase in SiCw content leads to more amount of crack deflection and crack arrest that successively improves the fracture toughness of the composites.

The present study focuses on the wear behavior of a pressure-less sintered ZrB2–20 vol% SiC composite in dry sliding interaction against the electroplated diamond disc. Pressure-less sintering was carried out at 2000 °C for 2 h in an argon atmosphere. Sintered composite possesses 98% of the theoretical density, and primarily microstructure contains ZrB2 and SiC phases. The sliding wear of the investigated composite has been studied in pin-on-disc equipment at room temperature at different combinations of loads and sliding speeds to examine the influences of the test parameters on wear mechanism. The results show that the elastic modulus, hardness and fracture toughness for ZrB2 20 vol% SiC are 442 ± 4.5 GPa, 16.5 ± 0.5 GPa, and 5.67 MPa√m, respectively. Results also show that the specific wear rate of the ZrB2-SiC composite increases continuously with increasing the applied loads whereas, it decreases with increasing sliding speed. XRD analyses of worn surfaces suggest that the phase transformation from ZrB2 to ZrO2 may occur due to frictional heating during sliding. The specific wear rates are in the order of ~10−6–10−8 cm2/Nm. The post wear test characterization suggests that oxidation dominates mild wear with low roughness values at low load and high sliding speed, whereas at high loads and low sliding speed severe wear mechanism is observed with high roughness values and deep grooves in surfaces. The mixed mode (oxidative–grain pullout-micro-cutting) wear mechanism is controlling the severe wear.

A comparative evaluation of the thermal shock resistance (TSR) of ZrB2-20 MoSi2-20 SiCw (ZMSw20) and ZrB2-20 MoSi2 − 5SiCw (ZMSw5) composites were studied using the indentation quench method. High-dense ZrB2 based composites were prepared by multi-stage spark plasma sintering at 1700°C. The results show that the ZMSw20 composite expressed superior crack shielding and TSR under quenching circumstances. The critical temperature differential ΔTc of ZMSw20 ceramic (ΔTc= 800°C) was higher than that of ZMSw5 ceramic (ΔTc=600°C). The significant enhancement in TSR was imposed residual stresses that improved the resistance to crack progression during thermal shock. Furthermore, an increment in silicon carbide content reduces the crack growth, and increases the TSR of the composites.

Abrasive wear behavior of ZrB2- 20 vol.% MoSi2 composite has been studied using electroplated diamond disk as the counter facing body. The investigated composite has been prepared by hot pressing and it has been found to possess 97% of the theoretical density. The microstructure of the composite has been characterized using scanning electron microscopy (SEM), and results show that MoSi2 particles are uniformly distributed in the ZrB2 matrix. The Young’s modulus, Poisson’s ratio and fracture toughness of the investigated composite are 455 GPa, 0.11 and 4.7MPa√m, respectively. The effect of experimental conditions including applied load, sliding velocity and sliding distance on the tribological behavior of ZrB2- 20 vol.% MoSi2 composite have been investigated. Experimental results show that applied load and sliding velocity plays an important role on specific wear rate and surface roughness of worn surfaces. SEM is used to analyze the wear surface morphology and crack propagation to study the wear mechanism. The dominant mode of wear was identified as abrasion. Grain pullout and micro cracking control the prevailing wear mechanism.

The effects of SiC content (10–40 vol.%) on electrical, thermal and ablation properties of pressureless sintered ZrB 2 -SiC composites showing interfacial segregation of W-rich phases have been studied. The electrical resistivity was measured by four-probe method, whereas thermal diffusivity and coefficient of thermal expansion (CTE) were determined using laser-flash method and thermo-mechanical analyzer, respectively. Whereas thermal conductivities calculated from experimentally obtained thermal diffusivity values are found to be the highest for the ZrB 2 -20 SiC composite, both electrical conductivity and CTE decrease with increasing SiC content. The specimens were subjected to thermal shock by soaking at 800–1200 °C, followed by water-quenching. Further, some specimens were exposed to oxyacetylene flame (2200 °C) for 10 min. The damage was estimated from changes in mass, Young’s modulus, and hardness. The highest thermal shock and ablation resistance have been observed for the ZrB 2 -20 SiC composite, as thermal properties and formation of protective oxide scale play key role.

In this investigation, the hypereutectic in-situ Al-X wt. %Mg2Si (X=15, 20, 25 and 30) composites have been developed through gravity casting technique with the use of commercially pure Al, Si and Mg metals. This composite comprises of three phases, namely the primary Mg2Si phase, which is formed during the pseudo-eutectic transformation at the time of solidification engulfed by the matrix (α-Al) and the binary eutectic (Al-Mg2Si) phase. Microstructure evaluation and the mechanical properties of these composites has been investigated. It is observed that the change Mg2Si concentration has a great impression not only on the morphology and volume fraction but also in the hardness of all the phases that exist. The size and volume percentage of Mg2Si particle increases with the increase in Mg2Si concentration. Bulk hardness also increases with higher Mg2Si concentration. The micro hardness of the reinforcement (primary Mg2Si) particle decreased slightly with the increase in the Mg2Si concentration due to the less compact particle. On the contrary, the micro hardness of the other phases showed a gradual rise with the rise in Mg2Si concentration. The higher values of Quality Index, % elongation, ultimate tensile strength and toughness are achieved in the samples where the Mg2Si concentration is nearer to the pseudo-eutectic (13.9 wt. %) region. The yield strength, firstly shows a rising trend up to 25 wt. % Mg2Si and then decreases at 30 wt. % Mg2Si concentration.

In this study, aluminum-based metal matrix composites containing 5, 10, 15, 20, and 25% fly ash particles by weight have been fabricated using a stir casting route. Microstructural observations under an optical microscope suggest that fly ash particles in the cast composites are uniformly distributed throughout the matrix. The mechanical properties such as tensile strength and hardness of the aluminum alloy have enhanced significantly with the addition of fly ash. Wear behavior of base metal and its composite have been studied in dry sliding conditions using a pin-on-disc tribometer. Wear tests have been conducted at three different loads of 9.81, 19.62, and 29.43 N at a constant sliding velocity of 1.3 m s−1. Microstructural observations of worn surfaces and wear debris suggest that wear mechanism in base metal is controlled by adhesion, whereas that for composites is abrasive. The least wear rate has been obtained in Al-10% Fly ash composite. The Al-10% Fly ash is observed to possess an optimum level of mechanical properties and wear behavior.

Compressive creep studies have been carried out on hot-pressed ZrB2–SiC (ZS) and ZrB2–SiC–Si3N4 (ZSS) composites in air under stress and temperature ranges of 93–140 MPa and 1300°C–1425°C, respectively for time durations of ≈20–40 h. The results of these studies have shown the creep resistance of ZS composite to be greater than that of ZSS. As the temperature is increased from 1300°C to 1425°C, the stress exponent of ZS decreases from 1.7 to 1.1, whereas that of ZSS drops from 1.6 to 0.6. The activation energies for these composites have been found as ≈95 ± 32 kJ/mol at temperatures ≤1350°C, and as ≈470 ± 20 kJ/mol in the range of 1350°C–1425°C. Studies of the postcreep microstructures using scanning and transmission electron microscopy have shown the presence of glassy film with cracks at both ZrB2 grain boundaries and ZrB2–SiC interfaces. These results along with calculated values of activation volumes suggest grain-boundary sliding as the major damage mechanism, which is controlled by O2− diffusion through SiO2 at ≤1350°C, and by viscoplastic flow of the glassy interfacial film at temperatures ≥1350°C. Studies by transmission electron microscopy have shown formation of crystalline precipitates of Si2N2O near ZrB2–SiC interfaces in ZSS tested at ≥1400°C, which along with stress exponent values

The effect of SiC content, additives, and process parameters on densification and structure–property relations of pressureless sintered ZrB2–(10–40 vol%) SiC particulate composites have been studied. The ZrB2–SiC composite powders mixed by ball-milling with 1.2 wt% C (added as phenolic resin) and 3 wt% B4C have been uniaxially cold-compacted and sintered in argon environment at 1950–2050 °C for 2 h, or at 2000 °C for durations between 1/2 and 3 h. The amount of densification is found to increase with sintering duration, and by prior holding at 1250 and 1600 °C for reduction of oxide impurities (ZrO2, B2O3 and SiO2) on powder particle surfaces by the aforementioned additives. Presence of SiC with average size smaller than that of ZrB2 appears to aid in densification by enhancing green density, increasing WC content by erosion of milling media, and inhibiting matrix grain growth. Both SiC and WC appear to aid in reduction of oxide impurities. Furthermore, the impurities enriched in W, Fe and Co obtained from milling media are found to be segregated at ZrB2 grain boundaries, and appear to assist in densification by forming liquid phase, which completely wets the ZrB2 grains. Hardness increases with SiC content or with sintering duration till 1 h, but decreases for periods ≥2 h due to grain growth. The experimentally measured elastic moduli approaches corresponding theoretically predicted values with increasing SiC content due to reduction in porosity.

Electrical resistivities, thermal conductivities and thermal expansion coefficients of hot-pressed ZrB2–SiC, ZrB2–SiC–Si3N4, ZrB2–ZrC–SiC–Si3N4 and HfB2–SiC composites have been evaluated. Effects of Si3N4 and ZrC additions on electrical and thermophysical properties of ZrB2–SiC composite have been investigated. Further, properties of ZrB2–SiC and HfB2–SiC composites have been compared. Electrical resistivities (at 25 °C), thermal conductivities (between 25 and 1300 °C) and thermal expansion coefficients (over 25–1000 °C) have been determined by four-probe method, laser flash method and thermo-mechanical analyzer, respectively. Experimental results have shown reasonable agreement with theoretical predictions. Electrical resistivities of ZrB2-based composites are lower than that of HfB2–SiC composite. Thermal conductivity of ZrB2 increases with addition of SiC, while it decreases on ZrC addition, which is explained considering relative contributions of electrons and phonons to thermal transport. As expected, thermal expansion coefficient of each composite is reduced by SiC additions in 25–200 °C range, while it exceeds theoretical values at higher temperatures.

Non-isothermal, isothermal and cyclic oxidation behavior of hot pressed ZrB2–20 (vol.%) SiC (ZS) and HfB2–20 SiC (HS) composites have been compared. Studies involving heating in thermogravimetric analyzer have shown sharp mass increases at 740 and 1180 °C for ZS, and mass gain till 1100 °C followed by loss for HS. Isothermal oxidation tests for 1, 24 and 100 h durations at 1200 or 1300 °C have shown formation of partially and completely stable oxide scales after ∼24 h exposure for ZS and HS, respectively. X-ray diffraction, scanning electron microscopy and energy or wavelength dispersive spectroscopy has confirmed presence of ZrO2 or HfO2 in oxide scales of ZS or HS, respectively, besides B2O3–SiO2. Degradation appears more severe in isothermally oxidized ZS due to phase transformations in ZrO2; and is worse in HS on cyclic oxidation at 1300 °C with air cooling, because of higher thermal residual stresses in its oxide scale.

A comparative study has been carried out on the mechanical properties at room temperature, thermal shock and ablation resistance as well as oxidation behaviour of ZrB2-20SiC, ZrB2-20SiC-5Si3N4 and ZrB2-20ZrC-20SiC-5Si3N4 (amounts represent volume percent) composites. Fracture toughness has been determined using either three-point bend tests on single edge notch bend specimens, or by indentation technique. Addition of Si3N4 as sintering aid leads to enhancement in flexural strength and fracture toughness in the composite without ZrC. The specimens were subjected to thermal shock by quenching from temperatures in the range of 800o- 1200oC to ice cold water, and to ablation by exposure to oxy-acetylene flame at 2200oC. The composite having ZrC as constituent, exhibits the highest resistance to damage due to thermal shock and ablation, while the ZrB2-SiC composite shows the least change in mass during ablation. On the other hand, thermogravimetric experiments from room temperature to 1300oC have shown that the presence of ZrC is detrimental for oxidation resistance. Hence, the constituents of the composites need to be selected on the basis of the nature of application. The results of this study show that the investigated ZrB2 based composites bear the potential for multiple use thermal protection of reentry type space vehicles.

The oxidation behavior of ZrB2-20 vol % SiC and ZrB2-20 vol % SiC-5 vol % Si3N4 composites prepared by hot-pressing and subjected to isothermal exposure at 1200 or 1300 °C for durations of 24 or 100 h in air, as well as cyclic exposure at 1300 °C for 24 h, have been investigated. The oxidation resistance of the ZrB2-20 vol % SiC composite has been found to improve by around 20%–25% with addition of 5 vol % Si3N4 during isothermal or cyclic exposures at 1200 or 1300 °C. This improvement in oxidation resistance has been attributed to the formation of higher amounts of SiO2 and Si2N2O, as well as a greater amount of continuity in the oxide scale, because these phases assist in closing the pores and lower the severity of cracking by exhibiting self-healing type behavior. For both the composites, the mass changes are found to be higher during cyclic exposure at 1300 °C by about 2 times compared to that under isothermal conditions.