Effect of sintering process on the microstructures and properties of in situ TiB2–TiC reinforced steel matrix composites produced by spark plasma sintering

Spark plasma sintering (SPS) is a new technique to rapidly produce metal matrix composites (MMCs), but there is little work on the production of TiB₂–TiC reinforced steel matrix composites by SPS. In this work, in situ TiB₂–TiC particulates reinforced steel matrix composites have been successfully produced using cheap ferrotitanium and boron carbide powders by SPS technique. The effect of sintering process on the densification, hardness and phase evolution of the composite is investigated. The results show that when the composite is sintered at 1050 °C for 5 min, the maximum densification and hardness are 99.2% and 83.8 HRA, respectively. The phase evolution of the composite during sintering indicates that the in situ TiB₂–TiC reinforcements are formed by a hybrid formation mechanism containing solid–solid diffusion reaction and solid–liquid solution-precipitation reaction. The microstructure investigation reveals that fine TiB₂–TiC particulates with a size of ～2 μm are homogeneously distributed in the steel matrix. The TiB₂–TiC/Fe composites possess excellent wear resistance under the condition of dry sliding with heavy loads.     

Preparation,mechanical properties and microstructure of TiB2 based ceramic cutting tool material toughened by TiC whisker

TiC whiskers were synthesized by carbothermal reduction process. By using these whiskers as the toughening phase, a novel TiB₂ based ceramic cutting tool material was prepared. Due to that the thermal expansion coefficient of TiC is close to that of TiB₂, the addition of no more than 30 vol% TiC whisker not only has little adverse effect on the density and flexure strength of the composite, but also can refine the grains, reduce the defects and improve the grain strength. As a result, both the fracture toughness and flexure strength of the TiB₂ based ceramic composite can be significantly improved. Appropriate sintering temperature and holding time can reduce defects, improve the strength of grains and grain boundaries and enhance the toughening effect of TiC whiskers. Experimental results showed that when the whisker content was 30 vol%, the sintering temperature was 1700 °C and the holding time was 30 min, the flexure strength, fracture toughness and Vickers hardness of the TiB₂ based ceramic cutting tool material was 860 MPa, 7.9 MPa·m^1/2 and 22.6GPa, respectively.     

Mechanical properties and microstructure of TiB2–TiC composite ceramic cutting tool material

TiB₂–TiC composite ceramic cutting tool material was prepared by sintering during hot-pressing in vacuum. The effects of nano-scale Ni and Mo additives and sintering heating rate on mechanical properties and grain characteristics were investigated. TiB₂ and TiC grains exhibited prismatic and equiaxed shapes respectively. The diameter and aspect ratio of prismatic TiB₂ grains were influenced by nano-scale Ni/Mo additives. A higher heating rate could cause a higher aspect ratio of prismatic TiB₂ grains. The good mechanical properties of TN1((TiB₂–TiC)/Ni composite ceramic sintered at a heating rate of 50 °C/min) were ascribed to a relatively fine and homogenous microstructure. And a brittle B₄MoTi solid solution phase and wider distribution of grain size induced the lower flexural strength of TNM2((TiB₂–TiC)/(Ni,Mo) composite ceramic sintered at heating rate of 100 °C/min), but the higher aspect ratio of TiB₂ grains could prevent cracks from propagating and ameliorated the fracture toughness. The optimum resultant mechanical properties were obtained by (TiB₂–TiC)/Ni composite ceramic sintered at a heating rate of 50 °C/min.     

激光原位合成TiB2-TiC颗粒增强铁基涂层

采用B4C,TiO2,石墨以及铁基粉末为激光熔覆材料,利用激光多道搭接熔覆技术在碳钢基体上制备TiB2-TiC颗粒增强铁基复合涂层．利用XRD,SEM对涂层的相结构和显微组织进行了研究．采用显微硬度计和滑动磨损试验机分别测试了涂层的硬度和耐磨性能．结果表明,激光熔覆过程B4C,TiO2和石墨反应生成了TiB2和TiC颗粒,并均匀分布在基体中．随着激光功率密度增加,涂层中TiC含量减少,甚至出现FeB脆性相．TiB2-TiC颗粒增强的涂层其硬度和耐磨性能优于基材45钢．     

Microstructure and properties of Fe–Cr–C hardfacing alloys reinforced with TiC–TiB2

Abstract

Different amounts of TiB₂ powder were added to flux cores of wear resistant hardfacing flux cored wires for the preparation of new flux cored wires. Fe–Cr–C hardfacing alloys reinforced with TiB₂ were produced by arc hardfacing. The microstructure, hardness and wear resistance behaviour of the hardfacing alloys were investigated using an optical micrograph, scanning electron micrograph (SEM), X-ray diffractometer, macrohardness tester, microhardness tester and abrasive wear tester. The results showed that, among the hardfacing alloys, a new hard phase, i.e. TiC–TiB₂ composite compound particles, was formed and dispersed in the primary carbides and matrix structures. The TiC–TiB₂ reinforced Fe–Cr–C hardfacing alloys imparted greater hardness and better wear resistance. The presence of TiC–TiB₂ hard phase particles is the main reason for the improvement in hardness and wear resistance of Fe–Cr–C hardfacing alloys.     

激光熔覆B4C-TiO2-Al粉末制备原位TiB2+TiC/Fe复合涂层（英文）

采用B4C、TiO2、Al以及Fe基自熔合金粉末为前驱体,利用激光熔覆技术在钢基体上制备TiB2+TiC颗粒增强Fe基复合涂层。结果表明,激光熔覆过程通过B4C-TiO2-Al反应生成了均匀分布于基体的TiB2-TiC复合陶瓷相。TiB2颗粒呈长条块状,TiC以不规则形状分布于基体中。涂层具有比基材1045钢更好的耐磨性能,但涂层的摩擦因数小。     

TIG熔覆原位自生TiC-TiB2/Fe复合涂层

利用氩弧作为热源,以G302铁基合金粉、FeTi70粉和B₄C粉作为原料粉末,在Q235表面原位生成TiC-TiB₂增强的铁基复合涂层. 采用一系列的分析测试方法对涂层进行了表征,结果表明,氩弧熔覆过程冶金反应充分,熔覆层中生成了TiC,TiB₂和M₇C₃等硬质增强相;熔覆层组织呈现出由母材界面到熔覆层表面硬质相逐渐增多的梯度分布特征. 增加FeTi70和B₄C粉末比例提高了熔覆层硬度,质量比为G302:FeTi70:B₄C=6:3:1时,试样最大硬度达到976 HV0.1,是母材硬度的5倍左右. 在与GCr15钢对磨时,熔覆试样磨损量仅为Q235钢的1/30左右,熔覆层磨损表面基本无塑性变形痕迹,涂层中坚硬的TiC,TiB₂陶瓷相起到阻磨作用.     

Microstructure and mechanical properties of B4C-TiB2 composite ceramic fabricated by reactive spark plasma sintering

B₄C-TiB₂ composite ceramic was prepared by reactive spark plasma sintering, using amorphous B, Ti, and graphite as the raw materials. The reaction process and the phase composition in the process of sintering were studied. The effects of the ratio of raw materials and sintering process on the microstructure and mechanical properties of B₄C-TiB₂ composite ceramic were investigated. The composition of the sintered sample was B₄C, TiB₂, and bits of residual unreacted graphite. B and Ti preferentially reacted to form TiB₂ at 800 °C, and then B and graphite reacted to form B₄C at 1250 °C. The 75 vol% B₄C-25 vol% TiB₂ composite ceramic synthesized with 60.6 wt% B, 25.8 wt% Ti, and 13.6 wt% graphite and sintered at 1900 °C for 15 min resulted in nearly full densification and optimal mechanical properties. The relative density, Vickers hardness, fracture toughness, and flexural strength were 98.6 ± 0.01%, 26.6 ± 0.01 GPa, 5.9 ± 0.13 MPa·m^1/2, and 605 MPa, respectively.     

Microstructural characterization of diamond/CBN grains steel braze joint interface using Cu–Sn–Ti active filler alloy

In order to develop the new generation superhard abrasive tools of diamond and cubic boron nitride (CBN), the brazing joint experiments of diamond/CBN crystals and AISI 1045 steel matrix using Cu–Sn–Ti active filler powder alloy were investigated in vacuum furnace. The brazing temperature was 930 °C and the dwelling time was 20 min. Interfacial characteristics of the brazing joint among the diamond/CBN grains, the active filler layer and the steel substrate were analyzed using scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction techniques. The results indicated that Ti element in the Cu–Sn–Ti alloys diffused preferentially to the surface of diamond/CBN grits to form a Ti-rich reaction layer in the brazed joints by microanalyses. Moreover, the TiC, TiN and TiB₂ phases in diamond/CBN interface and Cu–Ti phase in steel interface were confirmed by X-ray diffraction phase analysis. The wetting and bonding reactions on diamond/CBN by melting Cu–Sn–Ti alloy were realized through the interfacial reaction products like TiC, TiN and TiB₂ compounds during the brazing process. The adhesive strength experiments of the joint interfaces revealed that the grains were not pulled out from the bond interface. The reliable bonding strength of brazed diamond/CBN grains to the steel substrate can meet the application requirements of high efficiency machining in the industrial field.     

HVOF Spraying of Fe-Based MMC Coatings with In Situ Formation of Hard Particles by Hot Isostatic Pressing

Thick (2-3 mm) Fe-base coatings with admixed ferrotitanium (Fe₃₀Ti₇₀) were applied to austenitic steel by a high-velocity oxy-fuel process (HVOF). Hot-isostatic pressing (HIP) was carried out to the decrease porosity and to increase the material strength, wear resistance, and adhesive bond strength of the deposited coating to the substrate material. SEM and XRD investigations confirmed the formation of hard titanium carbide (TiC) particles during HIP treatment as a result of strong carbon diffusion out of the metal matrix and into the Fe₃₀Ti₇₀ particles. The mechanical and wear properties of the densified coatings were investigated by means of shear tests, hardness measurements, and abrasive wear tests. A comparison of the coatings in the as-sprayed and the HIPed state showed a large increase in the wear resistance due to in situ TiC formation.     

Tribological properties of TiC-Fe coatings obtained by plasma spraying reactive powders

Titanium carbide-based coatings have been considered for use in sliding wear resistance applications. Carbides embedded in a metal matrix would improve wear properties, providing a noncontinuous ceramic surface. TiC-Fe coatings obtained by plasma spraying of spray-dried TiC-Fe composite powders containing large and angular TiC particles are not expected to be as resistant as those containing TiC particles formed upon spraying. Coatings containing 60 vol% TiC dispersed in a steel matrix deposited by plasma spraying reactive micropellets, sintered reactive micropellets, and spray-dried TiC-Fe composite powders are compared. The sliding wear resistance of these coatings against steel was measured following the test procedure recommended by the Versailles Advanced Materials and Standards (VAMAS) program, and the inherent surface porosity was evaluated by image analysis. Results show that, after a 1-km sliding distance, TiC-Fe coatings obtained after spraying sintered reactive powders exhibit scar ring three times less deep than sprayed coatings using spray-dried TiC-Fe composite powders. For all coatings considered, porosity is detrimental to wear performance, because it generally lowers the coating strength and provides cavities that favor the adhesion of metal. However, porosity can have a beneficial effect by entrapping debris, thus reducing friction. The good wear behavior of TiC-Fe coatings manufactured by plasma spraying of sintered reactive powders is related to their low coefficient of friction against steel. This is due to the microstructure of these coatings, which consists of 0.3 to 1 μm TiC rounded particles embedded in a steel matrix. Presented at the International Conference on Metallurgical Coatings and Thin Films, ICMCTF-92, Apr 6–10, 1992, San Diego.     

High-temperature compressive properties of TiC–TiB_2/Cu composites prepared by self-propagating high-temperature synthesis

TiC–TiB2 /Cu composites were prepared by self-propagating high-temperature synthesis with pseudo hot isostatic pressing using Ti, B4 C, and Cu powders. The compressive deformation of the composites at high temperature was investigated. It is found that the maximum compressive strength decreases with the increase of temperature and Cu content. The deformation of the composites includes the steps of elastic, stable rheology, and inaction. The maximum strain is in the range of 5 %–10 %. Before fracture, TiC–TiB2 /40Cu becomes drum-shaped at 1123 K; however, TiC–TiB2 /20Cu only has a brittle fracture along the axial direction of 45°. The results show that the compressive strength of TiC–TiB2 /Cu decreases from 823 to 1223 K. However, the maximum compressive strength of TiC–TiB2 /20Cu reaches 1850 MPa at 823 K, which predicts that this series of composites could be applied to high-temperature compressive materials.     

Effects of TiB2+TiC content on microstructure and wear resistance of Ni55-based composite coatings produced by plasma cladding

TiB₂-TiC reinforced Ni55 matrix composite coatings were in-situ fabricated via plasma cladding on steels using Ti, B₄C, and Ni55 as precursor materials at different proportions. Effects of TiB₂+TiC content of ceramics phase on the microstructure and wear resistance were studied. The results showed that ceramic phases TiB₂ and TiC were in-situ synthesized by plasma cladding, and the ceramic phase content significantly affected tribological performance and the wear mechanism of coatings under different loads. The composite ceramics protected coatings from further delamination wear by crack-resistance under a load of 30 N. Severe abrasive wear and adhesive wear were prevented when the load increased to 60 N because of the high hardness and strength of ceramic phases. Moreover, a compacted layer appeared on the wear surface of coatings with high content of ceramic phases, which effectively decreased the friction coefficient and wear rate. The TiB₂-TiC composite ceramics significantly improved the wear performance of metal matrix composite coatings by different mechanisms under loads of 30 and 60 N.     

Thermal Stability and Coefficient of Friction of the Diamond Composites with the Titanium Compound Bonding Phase

In this paper, processes occurring during heat treatment of the diamond-Ti compound composites without Co addition were investigated and compared with commercial PCD. Three types of materials were prepared. The first material was sintered using the mixture containing diamond and 10 mass% of TiC, the second material was prepared using diamond powder and 10 mass% of Ti-Si-C, and the third composite was sintered using the addition of 10 mass% of TiB₂. During the research, it was proved that TiO₂ formation contributes to material swelling and WO₃ (W is present from the milling process) causes a significant increase in coefficient of friction. TiC and Ti-Si-C bonded materials are very susceptible to this process of oxidation; their hardness drops absolutely after wear test at 600 °C. The diamond composite with TiB₂ is the most resistant to oxidation from investigated materials.     

A novel TiC-based composite co-strengthened with AlN particulates and graphene nano-platelets

Fully dense TiC-based composites were produced by spark plasma sintering at 1900 °C with introducing AlN and graphene nano-platelets (GNPs) additives to the TiC matrix. The participation of AlN in removing oxygen-bearing species (i.e., TiO₂) has been found to be explicable for boosting the sinterability of TiC. TiN was formed in-situ as a product of the chemical reaction between AlN and TiO₂ compounds, dissolving within the TiC matrix and producing Ti(C,N) solid solution. Moreover, the co-addition of AlN and GNPs resulted in the creation of Al₂OC as another in-situ phase. This compound together with the initially introduced additives activated grain refinement and dispersion strengthening mechanisms, uplifting the flexural strength of TiC by ~35% (standing at 703 MPa). Although the intragranular fracture was the predominant wear mode in the monolithic TiC, the synergetic impacts of AlN and GNPs additives altered the prevailing mode to intergranular.     

Characterization of hot pressed SiC whisker reinforced TiB2 based composites

TiB₂–SiC ceramic composites, with different contents of SiC whiskers (SiC_w), as a ceramic sinter-additive, were prepared by the hot pressing process at 1850 °C for 2 h under a pressure of 20 MPa. For comparison, a monolithic TiB₂ ceramic was also fabricated under the identical temperature, pressure, atmosphere, and holding time by the hot pressing process. The effects of fabrication process and SiC whiskers on microstructural features, phase evolution and mechanical properties were investigated. Hardness measurements revealed an initial increase in hardness for TiB₂–SiC compared to TiB₂. Also the improvement of the fracture toughness was attributed to the toughening and strengthening effects of SiC whiskers such as crack deflection. The results showed that promoted densification of TiB₂–SiC ceramic composites is due to addition of SiC whiskers and reduction of oxide impurities by reacting with SiC whiskers and removing them from the surface layer of TiB₂ particles. The reaction between TiB₂ particles and SiC whiskers led to in-situ formation of TiC phase in the matrix as well. In general, it is concluded that the sinterability of TiB₂-based composites was remarkably improved by introducing SiC whiskers compared to the single phase TiB₂ ceramic.     

Sol–gel derived hydroxyapatite coatings on titanium substrates

The oscillatory micromovements at the interface between the implant and the bone induce fretting wear and sometimes, fatigue cracks, causing early failure of the joint prosthesis. Hydroxyapatite films were formed using a sol–gel method from an organic precursor solution. The average film thickness was found to be 1.0 μm. Composite coatings containing HA doped with ZrO₂ were also formed. Hydroxyapatite (HA) and composite films of HA and ZrO₂ formed on commercial titanium substrates using an organic precursor solution by sol–gel route, were tested for fretting wear using a ball-on-flat fretting apparatus. The moderately lower values of the coefficient of friction (0.4–0.5) and morphology of the wear pits for considerably long cycles of fretting indicate strong bonding of the HA coating to the titanium surface. The interface shear strength of a thin hydroxyapatite film on commercial purity titanium has been evaluated using a substrate straining method. The maximum interfacial strength was about 570 and 678 MPa, for the pure HA and composite films, respectively, on the highly polished surface. However, the maximum interfacial strength was found to be about 263 MPa on the oxidized surface.     

Effect of tungsten content on microstructure and mechanical properties of PCBN synthesized in cBN-Ti-Al-W system

Polycrystalline cubic boron nitride (PCBN) compacts were prepared using the mixture of cubic BN and Ti-Al-W powders at 5.5 GPa and 1550 °C for 10 min. The influence of different Tungsten (W) content on composition, microstructure, porosity, mechanical property and cutting performance of the PCBN is investigated. The results show that, with the addition of tungsten, the cubic boron nitride (cBN) crystals are connected with each other by the new product phases TiB_2, TiN, Al₃Ti and W₂B under the pressure of 5.5 GPa and the temperature of 1550 °C. The rod-shaped crystals in the PCBN are expanded from the surface portion of the cBN. As the W content increases, the amount of rod-shaped crystals and the length-diameter ratios decrease in the system. When the tungsten content is 6 wt%, PCBN presents the best comprehensive performance and cutting performance, the porosity, the hardness, the flexural strength and the flank wear are 0.55%, 30.71 GPa, 972.3 MPa and 292 μm, respectively.     

Tribological behavior of hafnium diboride thin films

Transition metal diborides and their coatings offer an excellent combination of high hardness, high chemical stability and high thermal conductivity, thus they are excellent candidates for a wide range of tribological applications. In this work, stoichiometric hafnium diboride films were grown by chemical vapor deposition from a single-source, heteroatom-free precursor Hf(BH₄)₄ under conditions that afford highly conformal and smooth films. HfB₂ films of thickness ∼ 0.6 μm deposited on steel substrates were subjected to pin-on-disk wear testing against a counter face disc of AISI 440C martensitic stainless steel. Based on wear measurements, both as-deposited (X-ray amorphous) and annealed (nanocrystalline) samples showed very high wear resistance compared to uncoated samples. For the annealed samples, SEM analysis indicated the formation of a wear resistant composite body in the wear scar, even at depths far exceeding the film thickness, which appears to dramatically improve wear resistance. No mild-to-severe wear transition was observed which indicates that mild wear occurred throughout the wear test. This ensemble of results, when considered in the light of high contact pressures (∼ 700 MPa) used in the study, makes the HfB₂ material potentially attractive for wear-resistant applications.     

Investigation of Properties and Wear Behavior of HVOF Sprayed TiC-Strengthened Fe Coatings

High-velocity oxyfuel (HVOF) sprayed carbide based coatings (such as Cr₃C₂/NiCr) are industrially well established for wear protection applications. Due to their high carbide content of typically 75 wt.% and more, they provide very high hardness and excellent wear resistance. Unfortunately, costs for matrix materials such as nickel underlie strong fluctuations and are normally well above the prices for iron. Therefore an alternative concept to conventional carbides is based on TiC-strengthened low-cost Fe-base materials, which are already used for sintering processes. Depending on the carbon content, the Fe-base material can additionally offer a temperable matrix for enhanced wear behavior. The sprayability of TiC-strengthened Fe-powders with a gaseous and a liquid fuel driven HVOF system was investigated in this study. The resulting coatings were analyzed with respect to microstructure, hardness, and phase composition and compared with galvanic hard chrome, NiCrBSi, and Cr₃C₂/NiCr (80/20) coatings as well as with sintered Fe/TiC reference materials. Furthermore, the Fe/TiC coatings were heat treated to proof the retained temperability of the Fe matrix after thermal spray process. Tribometer tests (pin-on-disk tests) were conducted to determine wear properties.