The mechanism of reaction sintering of iron-iron boride cermets

The process of reaction sintering of iron and boron in a vacuum has been analysed, as a result of which iron-iron boride cermets have been produced. They constitute a composite material in which iron boride, Fe₂B, with a hardness of about 1800 HV plays the role of the reinforcement, while iron-iron boride, Fe-Fe₂B, with a hardness of about 500 HV plays the role of the matrix. The eutectic filling the spaces between iron boride grains is composed of boron solid solution plates in iron with a hardness of around 250 HV, and iron boride, Fe₂B, plates with a hardness of approximately 1800 HV. The combination of such different materials results in attractive properties of the cermet thus produced: high total hardness (1500 HV), constant over the whole section, and ductility. The properties mentioned, resulting from the cermet structure, depend (apart from the chemical composition) on the phenomena occurring while sintering: the boron diffusion in iron, the formation of the liquid phase and the processes of dissolving powder components in it, and finally upon the crystallization of the boride phase from the liquid. These all determine the unique character of the reaction sintering of iron and boron. The mechanism of this process is reported.     

Mechanical properties of titanium diboride based cermets

Mechanical properties of titanium diboride (TiB₂) cermets critically depend on the composition of the binder phase. Both, fracture toughness and hardness are substantially increased by avoiding the formation of extremely brittle secondary borides which form during sintering by chemical reactions between TiB₂ and the metallic additives. Fractographic observations of TiB₂ cermets without secondary borides show the presence of ductile ligaments of the binder phase bridging the advancing crack tip. The powder metallurgy processing route applied to these materials allows modification of the binder phase structure from the ferritic iron-aluminium phase to Fe-Ni-Al austenite by changing the aluminium content of the powder mixtures. The highest toughness values have been obtained for the TiB₂ cermets with an austenitic binder phase. X-ray diffraction analyses of the fracture surfaces of such samples show that the binder phase is metastable exhibiting stress induced martensitic transformation during fracture. This new family of materials presents an outstanding combination of hardness and toughness, comparable to those obtained with commercial grades of tungsten carbide (WC) hardmetals.     

High tough boride base cermets produced by reaction sintering

Borides, especially transition metal borides are promising candidates for the wear resistant applications. However, poor sinterability and extreme brittleness of borides lead to difficulties in the fabrication of solid structure. A novel sintering technique named “reacting boronizing sintering” has been developed for liquid phase sintering of tough boride base cermets associated with the in situ reaction formation of a ternary boride in a metal matrix. This paper focuses on the criteria for the development of high strength ternary boride base cermets, their sintering behavior and characteristics by employing Mo₂NiB₂ ternary boride base cermets.     

Fascinating microstructural evolution during consolidation and remarkable hardening behaviour of micro alloyed Al-Cu-Ni alloys

Ultrafine B2 NiAl and vacancy ordered phase (Al₃Ni₂ type) (Cu) evolve during consolidation of mechanically alloyed Al-Cu-Ni alloys without and with micro alloying addition of Zr and or Ti and results in high hardness of the alloys. Al-Cu-(Ni15)-Zr alloy consolidated by spark plasma sintering exhibits a remarkable hardness value of 7.2 GPa. Wide range of hardness values of the alloys indicates a significant effect of composition as well as synthesis route. Hardening behaviour through point defect hardening has been demonstrated in terms of constitutional vacancy concentration as well as solute dislocation interaction. Lattice parameter, bulk density, porosity anddislocation density of the alloys have been measured to understand the hardening mechanism further. TEM analysis and wear behaviour are carried out to justify the phase formation and strengthening mechanism of the alloys, respectively.     

Effect of different binders and secondary carbides on NbC cermets

Due to the rapidly increasing price of tungsten carbide and the significant health risks associated with the wear products of WC-Co (Co₃O₄ and Wo₃), an alternative is required. Niobium carbide (NbC) is well suited as a cutting tool due to its high melting point and low solubility in iron. Compared to pure NbC, a complete substitution of WC to NbC-Co resulted in an increased toughness and strength. As alternative binders, nickel and iron-based binders were subsequently investigated. Although iron-based cermets would be an economical, low-cost alternative to NbC-Ni cermets, they showed a higher coefficient of friction and wear rate. So far, NbC-Ni cermets best met the requirements of high hardness and toughness. Various secondary carbides such as VC, Mo₂C, TiC, but also WC were added to further improve the hardness. Elemental analyses of NbC-Ni-MeC cermets (Me = metal) showed that the binder is a face-centered cubic solid solution, while the NbC phase is a solid solution of the type (Nb, Me)C.

     

Effect of SiC whisker addition on the microstructures and mechanical properties of Ti(C,N)-based cermets

Ti(C, N)-based cermets with addition of SiC whisker (SiC_w) were prepared by vacuum sintering. The microstructures of the prepared cermets were investigated by using X-ray diffractometry (XRD) and scanning electron microscopy (SEM). Mechanical properties such as transverse rupture strength (TRS), fracture toughness (K_IC) and hardness (HRA) were also measured. It was found that the grain size of the cermets was affected by the SiC whisker addition. The cermets with 1.0 wt.% SiC whisker addition exhibited the smallest grain size. The porosities of the cermets increased with increasing SiC whisker additions. The addition of the SiC whisker had no influence on the phase constituents of the cermets. Compared with the cermets with no whisker addition, the highest TRS and fracture toughness for cermets with 1.0 wt.% SiC whisker addition increased by about 24% and 29%, respectively. The strengthening mechanisms were attributed to finer grain size, homogeneous microstructure and moderate thickness of rim phase. The toughening mechanisms were characterized by crack deflection, whisker bridging and whisker pulling-out.     

Application of Fe-P-B alloys in developing wear-resistant composite materials

Structures and properties of alloys, which belong to the Fe₃(P,B)-Fe₅PB₂-Fe₂P, Fe₂B-Fe₃(P,B)-Fe₅PB₂, Fe₂B-Fe₃(B,P)-Fe₃(P,B), and Fe-Fe₃(P,B)-Fe₃P concentration triangles of the Fe-P-B phase diagram have been studied. To produce composite materials for operation under the conditions of abrasive wear at increased temperatures, a Fe-12P-1B alloy has been recommended as a filler. The formation regularities of the structure and properties of interfaces between the Fe-12P-1B filler and copper-based binders in composite materials produced by infiltration have been defined.     

Phase composition of composite materials produced by high-pressure reaction sintering in the cubic boron nitride—diamond—aluminum system

X-ray diffraction analysis has been used to study the phase composition of composite materials produced by high pressure—high temperature (4.2 GPa, 1750 K) sintering of cBN and Al powders with diamond added to the reaction mixture. It has been shown that as a result of the reaction sintering depending on the relationship among the mixture components, in parallel with cBN and diamond, the composite materials may contain aluminum nitride, diboride, carboboride and carbide as well as solid solutions of boron and/or carbon based on the crystalline lattices of Al, AlN, and cBN. A possibility is shown of dispersion hardening of a composite providing the diamond content is below the threshold percolation. Along with diamond an increase in the resistance to abrasive wear of composites is responsible by the Al₃BC phase, which is located at the phase boundaries.     

Effect of Ni powder characteristics on the consolidation of ultrafine TiMoCN cermets by means of SPS and HIP technologies

Fully dense titanium carbonitride cermets have been consolidated from Ti(C,N)–Ni–Mo₂C–TiAl₃ powder mixtures either by spark plasma sintering or hot isostatic pressing techniques. Carbonyl Ni powders enhance the densification of the cermets produced by SPS (spark plasma sintering), a phenomenon likely related to a more efficient dissolution of Mo₂C additions and the possible precipitation of α″ phase. Both SPS and HIP (hot isostatic pressing) processes lead to materials with a bimodal Ti(C,N) grain size distribution containing a considerable fraction of nanometric grains. Unlike SPS, HIP induces significant graphite precipitation which could be explained by the destabilization of the carbonitride phase under high isostatic pressures at high temperature. Optimized compositions processed by SPS exhibit a combination of hardness and toughness close to the range covered by ultrafine WC–Co hardmetals of similar binder contents.     

Microstructure and mechanical properties of liquid phase sintered Mo2FeB2 based cermets

Mo₂FeB₂ based cermets were fabricated by liquid phase sintering at different sintering temperatures and soaking time. Almost full densification was achieved at 1080 °C without soaking time. However, a lower sintering temperature resulted in the aggregation of binder phase and a higher contiguity of hard phase. An increase of sintering temperature or soaking time promoted an in situ growth of elongated Mo₂FeB₂ grains in the intermediate state of sintering. Abnormally large and faceted Mo₂FeB₂ grains appeared at 1320 °C for 20 min. The growth of the abnormally large grains was associated with grain coalescence. The transverse rupture strength (TRS), hardness (HRA) and fracture toughness (K_IC) were also measured, and attempts were made to relate them to the microstructural development.     

Structural and mechanical behaviour of 5% Al2O3-reinforced Fe metal matrix composites (MMCs) produced by powder metallurgy (P/M) route

The aim of this paper is to investigate the effect of sintering temperature and time on the properties of Fe–Al₂O₃ composite (5 wt% Al₂O₃; 95 wt% Fe) prepared by powder metallurgy process. X-ray diffraction, microstructure, density, hardness and compressive strength of prepared samples have been investigated. XRD studies show the presence of Fe and Al₂O₃ along with iron aluminate phase. Iron aluminate is formed as a result of reactive sintering between iron and alumina particles. Microstructural examination of the specimen showed a dense structure with nanosize dispersion of the reinforcement of ceramic phase. Density as well as hardness of specimens depend on the formation of iron aluminate phase, which in turn depends on sintering temperature and time.     

Preparation, microstructure, and mechanical properties of TiC0.35N0.35-TiNi cermets

TiC_{0.35 ±0.04}N_{0.35 ±0.04}-TiNi cermets were prepared by liquid-phase sintering of TiG_≃0.5N_≃0.5 + TiNi + Ti powder mixtures and characterized by electron microscopy, x-ray diffraction, and chemical analysis. The sintering conditions were optimized for different contents of the TiNi binder. The hardness, flexural strength, and average grain size of the cermets were determined.     

Effect of V content on the microstructure and mechanical properties of Mo2FeB2 based cermets

Four series of cermets with V content between 0 and 7.5 wt.% in 2.5 wt.% increments were studied by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffractometry (XRD). The transverse rupture strength (TRS), hardness (HRA) and fracture toughness (K_IC) were also measured. It was found that the grain size was affected by the V content. The cermets with 2.5 wt.% V addition exhibited the smallest grain size. An increasing V content decreased the wettability of the binder on the Mo₂FeB₂ hard phase, and accordingly resulted in the increase of porosity and aggregation of ceramic grains. EDS results showed that V addition occurred primarily in the hard phase, with a little amount in the Fe alloy binder. In addition, the content of Mo element in the binder decreased with increasing V content. The cermets with 2.5 wt.% V addition showed the highest TRS, hardness and fracture toughness of 2350 MPa, HRA 90.6 and 15.1 MPa m^1/2, respectively.     

Production of ferroboron powders by solid boronizing method

Ferroboron is an iron-boron alloy containing 10–20% of boron by weight. Commercial ferroboron production is made by two main processes: carbothermic reaction and aluminothermic reaction. Ferroboron also occurs in steel surfaces due to boronizing, which is applied to increase surface hardness in steel. Boronizing is a thermo-chemical surface hardening treatment. The ferroboron phases like Fe₂B, FeB form by diffusing of boron element into iron. These phases are very hard, wear strengths are high, and friction coefficients are low.In this study, ferroboron powder was obtained by boronizing ASC 100.29 iron powder that was used widely in powder metallurgy area. Solid boronizing method was preferred due to its advantages in applications and Ekabor-HM powder was used as the boronizing agent. The 80% ASC 100.29 and 20% Ekabor HM were mixed homogeneously and subjected to boronizing at 850–950 °C for 1–6 h. Formation and development of ferroboron phase on the samples was determined by metallographic studies depending on various treatment conditions. The X-ray diffraction analysis revealed that the Fe₂B phase did form but FeB phase did not. Micro hardness distributions were measured on the powder grains. Eighteen GPa hardness was measured at Fe₂B phase obtained by boronizing while hardness of non-boronized iron powders was 1.06 GPa. The thickness of ferroboron layer formed by boronizing changed with boronizing conditions. The thickness of ferroboron layer increased with boronizing temperature or boronizing time. Depending upon processing parameters, ferroboron layers was formed partially or throughout ferrous powder structure. Since boronizing can be applied to iron powders having any size or shape, ferroboron production with required shape and size is possible.Finally, a new method, namely solid boronizing method, was developed in ferroboron powder production.     

Microstructural study and densification analysis of hot work tool steel matrix composites reinforced with TiB2 particles

Hot work tool steels are characterized by good toughness and high hot hardness but are less wear resistant than other tooling materials, such as high speed steel. Metal matrix composites show improved tribological behavior, but not much work has been done in the field of hot work tool steels. In this paper TiB₂-reinforced hot work tool steel matrix composites were produced by spark plasma sintering (SPS). Mechanical alloying (MA) was proposed as a suited process to improve the composite microstructure. Density measurements and microstructure confirmed that MA promotes sintering and produces a fine and homogeneous dispersion of reinforcing particles. X-ray diffraction patterns of the sintered composites highlighted the formation of equilibrium Fe₂B and TiC, as predicted by thermodynamic calculations using Thermo-Calc® software. Scanning electron microscopy as well as scanning Kelvin probe force microscopy highlighted the reaction of the steel matrix with TiB₂ particles, showing the formation of a reaction layer at the TiB₂-steel interface. Phase investigations pointed out that TiB₂ is not chemically stable in steel matrix because of the presence of carbon even during short time SPS.     

Characterization of microstructure and properties of Al–Al<Subscript>3</Subscript>Zr–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite

Aluminium-based metal matrix composite strengthened by in situ Al₂O₃ and Al₃Zr particles were synthesized by powder metallurgy route. Phase analysis by X-ray diffraction and scanning electron microscopy revealed that the reaction between Al and ZrO₂ produced Al₂O₃ and Al₃Zr phases in the sintered composites. The hardness of the composite is a strong function of sintering temperature as well as the volume fraction of reinforcements. The dry sliding wear test results clearly indicated that increasing the volume fraction of zirconia particles in the composite improved the wear resistance. Microcutting, ploughing, delamination and oxidation were the main mechanisms of wear.     

Sintering behaviour and mechanical properties of Cr3C2–NiCr cermets

Cr₃C₂–NiCr cermets are used as metal cutting tools due to their relatively high hardness and low sintering temperatures. In this study, a powder mixture consisting of 75 wt% Cr₃C₂–25 wt% NiCr was sintered at four different temperatures and characterized for its microstructure and mechanical properties. The highest relative density obtained was 97% when sintered at 1350 °C. As the relative density increased, elastic modulus, transverse rupture strength, fracture toughness and hardness of the samples reached to a maximum of 314 GPa, 810 MPa, 10·4 MPa·m^1/2 and 11·3 GPa, respectively. However, sintering at 1400 °C caused further grain growth and pore coalescence which resulted in decreasing density and degradation of all mechanical properties. Fracture surface investigation showed that the main failure mechanism was the intergranular fracture of ceramic phase accompanied by the ductile fracture of the metal phase which deformed plastically during crack propagation and enhanced the fracture toughness.     

Effect of TaC addition on the microstructures and mechanical properties of Ti(C, N)-based cermets

The microstructures of the prepared Ti(C, N)-based cermets with various TaC additions were studied using X-ray diffractometry (XRD) and scanning electron microscopy (SEM). Mechanical properties such as transverse rupture strength (TRS), fracture toughness (K_1C) and hardness (HRA) were also measured. The results showed that the grain size of the cermets decreased with increasing TaC addition, but too high TaC addition resulted in agglomeration of the grains. An increasing TaC addition increased the dissolution of tungsten, titanium, molybdenum and tantalum in the binder phase. The hardness of the cermets decreased slightly with increasing TaC addition. The transverse rupture strength was the highest for the cermets with 5 wt.% TaC addition, which was characterized by fine grains, homogeneous microstructure and the moderate thickness of rim phase in the binder. The fracture toughness of the cermets with TaC addition from 0 to 5 wt.% decreased obviously, which resulting from decreased grain size. The further decreasing of fracture toughness for the cermets with 7 wt.% TaC addition was due to increased porosity and interfacial tensile stress.     

Microstructure and mechanical properties of copper–titanium–nitrogen multiphase layers produced by a duplex treatment on C17200 copper–beryllium alloy

In this study, a copper–titanium–nitrogen multiphase coating was fabricated on the surface of C17200 copper–beryllium alloy by deposition and plasma nitriding in order to improve the surface mechanical properties. The phase composition, microstructure and microhardness profiles of the as-obtained multiphase coating were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and Vickers microhardness measurements, respectively. Pin-on-disk tribometer and SEM equipped with energy dispersive spectrometer (EDS) were applied to measure tribological properties and analyze wear mechanisms involved. The XRD results show that the phase composition changes with nitriding temperature. The Ti₂N layer is replaced by a Cu–Ti intermetallic layer when the nitriding temperature is higher than 700 °C. The Cu/Ti ratio in the multiphase coatings remains at a constant value of 2:1 due to the incorporation of nitrogen atoms. The surface hardness achieves a maximum value of 983 HV at 650 °C, and decreases as the nitriding temperature increases. The increased hardness corresponds to the improved wear resistance and decreased frictional coefficient and the surface hardness is proportional to the wear rates. The wear mechanism depends on the phase composition of the multiphase coatings. With the nitriding temperature increasing, the oxidative wear mechanism changes to adhesive and abrasive mode.     

Electrochemical boriding and characterization of AISI D2 tool steel

D2 is an air-hardening tool steel and due to its high chromium content provides very good protection against wear and oxidation, especially at elevated temperatures. Boriding of D2 steel can further enhance its surface mechanical and tribological properties. Unfortunately, it has been very difficult to achieve a very dense and uniformly thick boride layers on D2 steel using traditional boriding processes. In an attempt to overcome such a deficiency, we explored the suitability and potential usefulness of electrochemical boriding for achieving thick and hard boride layers on this tool steel in a molten borax electrolyte at 850, 900, 950 and 1000 °C for durations ranging from 15 min to 1 h. The microstructural characterization and phase analysis of the resultant boride layers were performed using optical, scanning electron microscopy and X-ray diffraction methods. Our studies have confirmed that a single phase Fe₂B layer or a composite layer consisting of FeB + Fe₂B is feasible on the surface of D2 steel depending on the length of boriding time. The boride layers formed after shorter durations (i.e., 15 min) mainly consisted of Fe₂B phase and was about 30 μm thick. The thickness of the layer formed in 60 min was about 60 μm and composed mainly of FeB and Fe₂B. The cross sectional micro-hardness values of the boride layers varied between 14 and 22 GPa, depending on the phase composition.