Densification, microstructure, and fracture behavior of TiC/Si3N4 composites by spark plasma sintering

TiC/Si₃N₄ composites were prepared using the β-Si₃N₄ powder synthesized by self-propagating high-temperature synthesis (SHS) and 35 wt.% TiC by spark plasma sintering. Y₂O₃ and Al₂O₃ were added as sintering additives. The almost full sintered density and the highest fracture toughness (8.48 MPa·m^½) values of Si₃N₄-based ceramics could be achieved at 1550°C. No interfacial interactions were noticeable between TiC and Si₃N₄. The toughening mechanisms in TiC/Si₃N₄ composites were attributed to crack deflection, microcrack toughening, and crack impedance by the periodic compressive stress in the Si₃N₄ matrix. However, increasing microcracks easily led to excessive connection of microcracks, which would not be beneficial to the strength.     

Microstructure characteristics of Ti3Al/TiC ceramic layer deposited by laser cladding

Al + TiC laser cladding coatings were prepared on Ti-6Al-4V alloy by CO₂ laser cladding technique. The microstructure, micro-hardness and phase constitutes of the laser cladding layer were investigated by means of scanning electron microscope (SEM), X-ray diffraction (XRD) and microsclermeter. The results indicated that the laser cladding layer solidified into the fine microstructure rapidly, and TiC hard phase was dispersived in the cladding layer. When the mass percent of TiC was 40%, the micro-hardness (1100HV_0.2-1250HV_0.2) of Al + TiC cladding layer was 3 times more than that of the Ti-6Al-4V alloy substrate (350-370HV_0.2). The cladding layer mainly consisted of α-Ti (Al), β-Al (Ti), Ti₃Al, TiAl, Al₃Ti and TiC phase. There phases were beneficial to improve the hardness and wear resistance of the cladding layer.     

Abrasive wear behavior of Ni-P coated Si3N4 reinforced Al6061 composites

Ni-P coated Si₃N₄ reinforced Al6061 composites were fabricated by vortex method. Percentage of reinforcement was varied from 6 wt.% to 10 wt.% in steps of 2. Cast matrix alloy and developed composites were hot forged at a temperature of 500 °C using a 300T hydraulic hammer. Both as cast and hot forged matrix alloy and its composites were subjected to microstructure studies, grain size analysis, microhardness and abrasive wear tests. Microstructure studies reveal uniform distribution of silicon nitride particles with good bond between matrix and reinforcement in both as cast and hot forged condition. It is observed that, increased content of reinforcement in both as cast and hot forged composites do result in significant grain refinement. However, when compared with as cast matrix alloy and its composites hot forged alloy and its composites exhibits higher extent of grain refinement. Both as cast and hot forged composites exhibit improved microhardness and abrasive wear resistance when compared with the unreinforced alloys under identical test conditions. Abraded worn surfaces were examined using scanning electron microscopy (SEM) for possible wear mechanisms. Increased abrasive particle size and load has resulted in larger extent of grooving leading to increased abrasive wear loss for both the matrix alloy and developed composites.     

Ti-coated SiC particle reinforced sintered Fe-Cu-Sn alloy

Ti-coated SiC particles were developed to improve the wear resistance of Fe-Cu-Sn alloy metal matrices designed for diamond tools. The phase structure of the Ti-coated SiC particles was investigated by X-ray diffraction. Ti coating on SiC was composed of Ti₅Si₃, TiC, and Ti. Excellent interfacial bonding between SiC and the matrix was realized. The SiC/iron alloy composites, prepared by hot pressing at 820 °C, were studied by wear and bending strength tests, and scanning electron microscope. For the composites reinforced by uncoated SiC particles, the wear resistance was improved, but the bending strength decreased. The composites with Ti-coated SiC particles outperformed the composites with uncoated SiC particles in both wear resistance and bending strength tests.     

Microstructure, mechanical and wear properties of laser processed SiC particle reinforced coatings on titanium

Laser processing of Ti-SiC composite coating on titanium was carried out to improve wear resistance using Laser Engineered Net Shaping (LENS™) — a commercial rapid prototyping technology. During the coating process a Nd:YAG laser was used to create small liquid metal pool on the surface of Ti substrate in to which SiC powder was injected to create Ti-SiC metal matrix composite layer. The composite layers were characterized using X-ray diffraction, scanning and transmission electron microscopy equipped with fine probe chemical analysis. Laser parameters were found to have strong influence on the dissolution of SiC, leading to the formation of TiSi₂, Ti₅Si₃ and TiC with a large amount of SiC on the surface. Detailed matrix microstructural analysis showed the formation of non-stoichiometric compounds and TiSi₂ in the matrix due to non-equilibrium rapid solidification during laser processing. The average Young's modulus of the composite coatings was found to be in the range of 602 and 757 GPa. Under dry sliding conditions, a considerable increase in wear resistance was observed, i.e., 5.91 × 10^− 4 mm³/Nm for the SiC reinforced coatings and 1.3 × 10⁻³ mm³/Nm for the Ti substrate at identical test conditions.     

Spark plasma sintering of TiC particle-reinforced molybdenum composites

In order to improve the recrystallization resistance and the mechanical properties of molybdenum, TiC particle-reinforcement composites were sintered by SPS. Powders with TiC contents between 6 and 25 vol.% were prepared by high energy ball milling. All powders were sintered both at 1600 and 1800 °C, some of sintered composites were annealed in hydrogen for 10 h at 1100 up to 1500 °C. The powders and the composites were investigated by scanning electron microscopy and XRD. The microhardness and the density of composites were measured, and the densification behavior was investigated. It turns out that SPS produces Mo–TiC composites, with relative densities higher than 97%.The densification behavior and the microhardness of all bulk specimens depend on both the ball milling conditions of powder preparation and the TiC content. The highest microhardness was obtained in composites containing 25 vol.% TiC sintered from the strongest milled powders. The TiC particles prevent recrystallization and grain growth of molybdenum during sintering and also during annealing up to 10 h at 1300 °C. Interdiffusion between molybdenum and carbide particles leads to a solid solution transition zone consisting of (Ti_1 −x Mo_x)C_y carbide. This diffusion zone improves the bonding between molybdenum matrix and TiC particles. A new phase, the hexagonal Mo₂C carbide, was detected by XRD measurements after sintering. Obviously, this phase precipitates during cooling from sintering temperature, if (Ti_1 −x Mo_x)C_y or molybdenum, are supersaturated with carbon.     

Selective laser melting additive manufacturing of pure tungsten: Role of volumetric energy density on densification,microstructure and mechanical properties

Due to the intrinsic properties of tungsten, such as high melting point and high thermal conductivity, selective laser melting of pure W parts experiences many challenges. In this study, the effects of volumetric energy density on the densification behavior, microstructure evolution and mechanical performances of SLM-processed pure tungsten parts were investigated. A maximum density of 19.0 g/cm³ (98.4% of the theoretical density) was obtained at the optimal energy density of 1000 J/mm³ and its microstructure was free of pores and balling phenomenon. The formation mechanism of pores and cracks was systematically investigated. The microhardness and compressive strength of SLM-processed pure W parts reached 474 HV and 902 MPa, respectively, which were comparable to the samples produced by conventional manufacturing methods. The morphology of fracture demonstrated that the fracture mechanism of SLM-processed pure W parts was brittle fracture and intergranular fracture was the main fracture mode. Dry sliding wear tests showed that the wear mechanism changed with the energy density. For pure W parts processed by SLM at the optimal parameters, the adhesion of hardened tribolayers was formed. In this case, the reduced coefficient of friction (COF) of 0.45 and a low wear rate of 1.3 × 10⁻⁵ mm³·N⁻¹·m⁻¹ were obtained.     

Rapid fabrication of the Ti3SiC2 bonded diamond composite by spark plasma sintering

A mixture of Ti/Si/TiC/diamond powders was employed to fabricate the Ti₃SiC₂ bonded diamond composite using the spark plasma sintering-reactive synthesis method. The addition of diamond does not inhibit the synthesis of Ti₃SiC₂ in the sintered product. In the matrix Ti₃SiC₂ grains developed lamellar morphology with an average length size of 5-10 μm. Ti₃SiC₂ matrix displays good pullout strength with diamond, and the Ti₃SiC₂ bonded diamond material exhibits good wear resistance.     

Effect of CuO on the sintering temperature and piezoelectric properties of MnO2-doped 0.75Pb(Zr0.47Ti0.53)O3-0.25Pb(Zn1/3Nb2/3)O3 ceramics

The sintering temperature of 0.75Pb(Zr_0.47Ti_0.53)O₃-0.25Pb(Zn_1/3Nb_2/3)O₃ ceramics containing 1.5 mol% MnO₂ was decreased from 930 to 850 °C with the addition of CuO. The CuO reacted with the PbO and formed a liquid phase during the sintering, which assisted the densification of the specimens. Most of the Cu²⁺ ions existed in the CuO second phase, thereby preventing any possible hardening effect from the Cu²⁺ ions. Variations of the k_p, Q_m, ?₃^T/?₀ and d₃₃ values with CuO were similar to that of the relative density. The specimen containing 0.5 mol% CuO sintered at 850 °C showed the good piezoelectric properties of k_p = 0.5, Q_m = 1000, ?₃^T/?₀ = 1750 and d₃₃ = 300 pC/N.     

Optimization of processing conditions and characterization investigations of ceramic injection molded and sintered ZrO2/WC composites

In this investigation, 3 mol% Y₂O₃ stabilized ZrO₂-based composites reinforced with 10 vol.%, 20 vol.% and 40 vol.% WC (named as 3Y-TZP/10WC, 3Y-TZP/20WC and 3Y-TZP/40WC) were fabricated by using injection molding and sintering. Mechanical properties of these composites varied due to WC addition and dwelling time. Density, strength and toughness decreased with shorter dwelling time and increasing WC content however a significant enhancement in fracture toughness was obtained by 3Y-TZP/20WC composite which had 9.2 MPa m^1/2 toughness. Severe unlubricated wear tests which were performed under 55 N normal load and 45 km sliding distance showed that 3Y-TZP/20WC composite had the lowest wear rate and wear volume values which are 2 × 10⁻⁸ mm³/(N m⁻¹) and 0.05 mm³, respectively.     

Structure and dielectric characterization of xNd(Mg1/2Ti1/2)O3-(1 − x)Ca0.6La0.8/3TiO3 in the microwave frequency range

The crystal structures, phase compositions and the microwave dielectric properties of the xLa(Mg_1/2Ti_1/2)O₃-(1 − x)Ca_0.8Sr_0.2TiO₃ composites prepared by the conventional solid state route have been investigated. The formation of solid solution is confirmed by the XRD patterns. Doping with B₂O₃ (0.5 wt.%) can effectively promote the densification and the dielectric properties of xNd(Mg_1/2Ti_1/2)O₃-(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics. It is found that xNd(Mg_1/2Ti_1/2)O₃-(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics can be sintered at 1375 °C, due to the liquid phase effect of B₂O₃ addition observed by Scanning Electronic Microscopy. At 1375 °C, 0.4Nd(Mg_1/2Ti_1/2)O₃-0.6Ca_0.6La_0.8/3TiO₃ ceramics with 1 wt.% B₂O₃ addition possesses a dielectric constant (?_r) of 49, a Q × f value of 13,000 (at 8 GHz) and a temperature coefficients of resonant frequency (τ_f) of 1 ppm/°C. As the content of Nd(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 20,000 GHz for x = 0.9 is achieved at the sintering temperature 1400 °C.     

Ti3AlC2掺杂SPS法制备Ti2AlC/TiAl复合材料的研究

通过2TiC-Ti-1.2Al体系的原位热压反应制备了Ti₃AlC₂陶瓷,然后以59.2Ti-30.8Al-10Ti₃AlC₂(wt%)为反应体系,采用放电等离子烧结技术制备出Ti₂AlC/TiAl基复合材料。借助XRD、SEM分析了产物的相组成和微观结构,并测量了其室温力学性能。结果表明：原位热压烧结产物由Ti₃AlC₂和TiC相组成,Ti₃AlC₂呈典型的层状结构,TiC颗粒分布在其间。SPS法制备的Ti₂AlC/TiAl基复合材料主要由TiAl、Ti₃Al和Ti₂AlC相组成,Ti₂AlC增强相主要分布于基体晶界处,表现为晶界/晶内强化作用。力学性能测试表明：Ti₂AlC/TiAl基复合材料的密度、维氏硬度、断裂韧性和抗弯强度分别为3.85 g/cm₃、5.37 GPa、7.17 MPa?m_1/2和494.85 MPa。     

Combustion synthesis of Ti3Si1−xAlxC2 solid solutions from TiC-, SiC-, and Al4C3-containing powder compacts

Preparation of the Ti₃Si_1−xAl_xC₂ solid solution with x = 0.2-0.8 was investigated by self-propagating high-temperature synthesis (SHS) using TiC-, SiC-, and Al₄C₃-containing powder compacts. Due to the variation of reaction exothermicity with sample stoichiometry, the combustion temperature and reaction front velocity decreased with increasing Al content of Ti₃Si_1−xAl_xC₂ for the TiC- and Al₄C₃-added samples, but increased for the samples with SiC. In contrast to the formation of Ti₃(Si,Al)C₂ as the dominant phase for the TiC- and SiC-added samples, TiC was identified as the major constituent in the final products of samples adopting Al₄C₃. In addition, the evolution of Ti₃(Si,Al)C₂ was improved by increasing the Al content of the TiC- and SiC-added powder compacts, but deteriorated considerably upon the increase of Al₄C₃ in the Al₄C₃-containing sample.     

Investigations on synthesis of ZrB2 and development of new composites with HfB2 and TiSi2

This paper presents the results of experimental investigations carried out on the synthesis of pure ZrB₂ by boron carbide reduction of ZrO₂ and densification with the addition of HfB₂ and TiSi₂. Process parameters and charge composition were optimized to obtain pure ZrB₂ powder. Monolithic ZrB₂ was hot pressed to full density and characterized. Effects of HfB₂ and TiSi₂ addition on densification and properties of ZrB₂ composites were studied. Four compositions namely monolithic ZrB₂, ZrB₂ + 10% TiSi₂, ZrB₂ + 10% TiSi₂ + 10% HfB₂ and ZrB₂ + 10% TiSi₂ + 20% HfB₂ were prepared by hot pressing. Near theoretical density (99.8%) was obtained in the case of monolithic ZrB₂ by hot pressing at 1850 °C and 35 MPa. Addition of 10 wt.% TiSi₂ resulted in an equally high density of 98.9% at a lower temperature (1650 °C) and pressure (20 MPa). Similar densities were obtained for ZrB₂ + HfB₂ mixtures also with TiSi₂ under similar conditions. The hardness of monolithic ZrB₂ was measured as 23.95 GPa which decreased to 19.45 GPa on addition of 10% TiSi₂. With the addition of 10% HfB₂ to this composition, the hardness increased to 23.08 GPa, close to that of monolithic ZrB₂. Increase of HfB₂ content to 20% did not change the hardness value. Fracture toughness of monolithic sample was measured as 3.31 MPa m^1/2, which increased to 6.36 MPa m^1/2 on addition of 10% TiSi₂. With 10% HfB₂ addition the value of K_IC was measured as 6.44 MPa m^1/2, which further improved to 6.59 MPa m^1/2 with higher addition of HfB₂ (20%). Fracture surface of the dense bodies was examined by scanning electron microscope. Intergranular fracture was found to be a predominant mode in all the samples. Crack propagation in composites has shown considerable deflection indicating high fracture toughness. An oxidation study of ZrB₂ composites was carried out at 900 °C in air for 64 h. Specific weight gain vs time plot was obtained and the oxidized surface was examined by XRD and SEM. ZrB₂ composites have shown a much better resistance to oxidation as compared to monolithic ZrB₂. A protective glassy layer was seen on the oxidized surfaces of the composites.     

Synthesis and consolidation of TiN/TiB2 ceramic composites via reactive spark plasma sintering

The simultaneous synthesis and densification of TiN/TiB₂ ceramic composites via reactive spark plasma sintering (RSPS) was investigated. Different component ratios (TiH₂/BN (TiN, B)) and heating rates (112.5-300 °C/min) were used to initiate the chemical reaction for TiN/TiB₂ synthesis. The omit RSPS process was revealed to have three stages, which are described separately. The relationships between the RSPS conditions, the microstructure and the properties of sintered ceramic composites were established. A Vickers hardness of 16-25 GPa and a fracture toughness of 4-6.5 MPa m^1/2 were measured for various compositions. Sintered ceramic composites containing 36 wt% TiB₂ with the highest relative density of 97.4 ± 0.4% and an average grain size of 150-550 nm have been obtained.     

Phase constituents and microstructure of laser cladding Al2O3/Ti3Al reinforced ceramic layer on titanium alloy

Laser cladding of the Fe₃Al + TiB₂/Al₂O₃ pre-placed alloy powder on Ti-6Al-4V alloy can form the Ti₃Al/Fe₃Al + TiB₂/Al₂O₃ ceramic layer, which can greatly increase wear resistance of titanium alloy. In this study, the Ti₃Al/Fe₃Al + TiB₂/Al₂O₃ ceramic layer has been researched by means of electron probe, X-ray diffraction, scanning electron microscope and micro-analyzer. In cladding process, Al₂O₃ can react with TiB₂ leading to formation of amount of Ti₃Al and B. This principle can be used to improve the Fe₃Al + TiB₂ laser cladded coating, it was found that with addition of Al₂O₃, the microstructure performance and micro-hardness of the coating was obviously improved due to the action of the Al-Ti-B system and hard phases.     

Rapid synthesis of TiC/Ti3SiC2 composites by laser melting

TiC/Ti₃SiC₂ composites were successfully synthesized by laser melting (LM) technique using Ti–Si–TiC with a molar ratio of 1:1.2:2 as starting powders in the holding time range of 15–60 s. Phase content and microstructure of the synthesized samples were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and X-ray energy dispersive spectrometer (EDS). The synthesis was extremely fast because the migration of solute mainly depended on the convection mixing of molten pool caused by intense laser beam which was far faster than the solid state diffusion by traditional methods. The apparent density and hardness of the composites are higher than Ti₃SiC₂ because of the existence of TiC phase.     

Structure and properties of selected (Cr–Al–N,TiC–C,Cr–B–N) nanostructured tribological coatings

The paper will present the state-of-art in the process, structure and properties of nanostructured multifunctional tribological coatings used in different industrial applications that require high hardness, toughness, wear resistance and thermal stability. The optimization of these coating systems by means of tailoring the structure (graded, superlattice and nanocomposite systems), composition optimization, and energetic ion bombardment from substrate bias voltage control to provide improved mechanical and tribological properties will be assessed for a range of coating systems, including nanocrystalline graded Cr_1−xAl_xN coatings, superlattice CrN/AlN coatings and nanocomposite Cr–B–N and TiC/a-C coatings. The results showed that the superlattice CrN/AlN coating exhibited a super hardness of 45 GPa when the bilayer period Λ was about 3.0 nm. Improved toughness and wear resistance have been achieved in the CrN/AlN multilayer and graded CrAlN coatings as compared to the homogeneous CrAlN coating. For the TiC/a-C coatings, increasing the substrate bias increased the hardness of TiC/a-C coatings up to 34 GPa (at −150 V) but also led to a decrease in the coating toughness and wear resistance. The TiC/a-C coating deposited at a −50 V bias voltage exhibited an optimized high hardness of 28 GPa, a low coefficient of friction of 0.19 and a wear rate of 2.37 × 10⁻⁷ mm³ N⁻¹ m⁻¹. The Cr–B–N coating system consists of nanocrystalline CrB₂ embedded in an amorphous BN phase when the N content is low. With an increase in the N content, a decrease in the CrB₂ phase and an increase in the amorphous BN phase were identified. The resulting structure changes led to both decreases in the hardness and wear resistance of Cr–B–N coatings.     

Synthesis of Ti3SiC2 bulks by infiltration method

T₃SiC₂ bulks have been synthesized by infiltrating Si liquid into porous precursor pellets composed of solid TiC and Ti powders. Silicon pellets were placed at the bottom of the precursor pellets as the liquid source. The starting compositions can be represented by the formula 2TiC + Ti + xSi, where x = 1.0, 1.2, 1.5 and 1.8, respectively. The phase formation and microstructure of the bulks were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS) system. The results demonstrated that the TiC/Ti precursor pellet could only react with Si completely when the x value is 1.8. Impurities SiC, Ti-Si binary compounds and Ti₈C₅ appeared along the silicon diffusion direction. It is found that the compositions of impurities strongly depended on the Si-concentration. Reaction mechanism of this Ti₃SiC₂ infiltration synthesis has also been discussed based on the Si-concentration changes on the diffusion path.     

High rate deposition of thick CrN and Cr2N coatings using modulated pulse power (MPP) magnetron sputtering

As a variation of high power pulsed magnetron sputtering technique, modulated pulse power (MPP) magnetron sputtering can achieve a high deposition rate while at the same time achieving a high degree of ionization of the sputtered material with low ion energies. These advantages of the MPP technique can be utilized to obtain dense coatings with a small incorporation of the residual stress and defect density for the thick coating growth. In this study, the MPP technique has been utilized to reactively deposit thick Cr₂N and CrN coatings (up to 55 μm) on AISI 440C steel and cemented carbide substrates in a closed field unbalanced magnetron sputtering system. High deposition rates of 15 and 10 μm per hour have been measured for the Cr₂N and CrN coating depositions, respectively, using a 3 kW average target power (16.7 W/cm² average target power density), a 50 mm substrate to target distance and an Ar/N₂ gas flow ratio of 3:1 and 1:1. The CrN coatings showed a denser microstructure than the Cr₂N coatings, whereas the Cr₂N coatings exhibited a smaller grain size and surface roughness than those of the CrN coatings for the same coating thickness. The compressive residual stresses in the CrN and Cr₂N coatings increased as the coating thickness increased to 30 μm and 20 μm, respectively, but for thicker coatings, the stress gradually decreased as the coating thickness increased. The CrN coatings exhibited an increase in the scratch test critical load as the thickness was increased. Both CrN and Cr₂N coatings showed a decrease in the hardness and an increase in the sliding coefficient of friction as the coating thickness increased from 2.5 to 55 μm. However, the wear rate of the CrN coatings decreased significantly as the coating thickness was increased to 10 μm or higher. The 10-55 μm CrN coating exhibited low wear rates in the range of 3.5-5 × 10⁻⁷ mm³ N⁻¹ m⁻¹. To the contrary, the Cr₂N coating exhibited relatively low wear resistance in that high wear rates in the range of 3.5 to 7.5 × 10⁻⁶ mm³ N⁻¹ m⁻¹ were observed for different thicknesses.