Rapid sintering of ultra fine WC and WC-Co hard materials by high-frequency induction heated sintering and their mechanical properties

Using a high-frequency induction heated sintering (HFIHS) method, the densification of binderless WC and WC-x wt.%Co (x=8, 10, 12) hard materials were accomplished using an ultra fine powder of WC and WC-Co. The advantages of this process are that it allows very quick densification close to the theoretical density and prohibits grain growth in nano-structured materials. Nearly fully dense WC and WC-Co with a relative density of up to 99.9% could be obtained with a simultaneous application of 60 MPa pressure and induced current (within 2 min) without a significant change in grain size. The average grain size of WC was approximately 270 nm for WC-x wt.%Co. The hardness and fracture toughness of the dense WC and WC-Co composites produced by HFIHS were investigated.     

Comparison of sintering behavior and mechanical properties between WC−8Co and WC−8Ni hard materials produced by high-frequency induction heating sintering

The sintering behavior and mechanical properties of WC-Co and WC-Ni hard materials produced by high-frequency induction-heating sintering (HFIHS) were compared using ultra fine WC, WC-Co, and WC-Ni powders. HFIHS allows very quick densification to near theoretical density and prohibits grain growth in nano-structured materials. Highly dense WC, WC-Co, and WC-Ni with a relative density of up to 99.2% could be obtained with simultaneous application of 60 MPa pressure and induced current within 2 min without significant change in grain size. The hardness and fracture toughness of the dense WC, WC-Co, and WC-Ni composites produced by HFIHS were also investigated.     

放电等离子烧结压力对超细WC-Co硬质合金性能的影响

通过分析放电等离子烧结致密化过程,确定了致密化温度;研究了SPS烧结过程中压力对WC-Co硬质合金致密化、显微组织及性能的影响。结果表明,放电等离子烧结粉末在1 130℃时,达到最大收缩率;烧结压力的增加,样品的致密度、硬度增加;断裂韧性的变化集中在11.5~12.1 MPa.m1/2之间,和硬度的变化呈现相反的趋势;烧结压力相对较小时,样品WC晶粒较粗大且不均匀;在40 MPa和55 MPa时,晶粒相对较小且分布均匀。要得到高性能、高致密度的样品,合理的烧结温度在1 200℃以上,烧结压力为40 MPa。     

Solid-state properties of hot-pressed TiB2 ceramics

Due to the increasing industrial interest in TiB₂ the present work was accomplished to establish various solid-state properties of TiB₂ and to study the influence of sintering aids and different powder conditioning methods on the densification behaviour of TiB₂.

The powders were hot-pressed in graphite dies with various loads up to 45 MPa and the vertex temperature of 1800 °C was held for 1 h. For the pure, non-activated powders theoretical densities between 97.4% and 99.5% were obtained at a pressure of 45 MPa. Hot-pressed activated powders at this pressure led to densities of up to 99.9%. The addition of 0.5 wt.% of various sintering aids also increased the densities. It is shown that pre-alloying TiB₂ with CrB₂ is to favour over mixing the powders. If TiB₂ and the sintering aid were mixed the best result was obtained with Cr₂N. Ceramics with high Young’s moduli and hardnesses were obtained. The Poisson’s ratios of two samples were 0.08 and 0.09, respectively, which are the lowest known values for a ceramic hard material. The heat conductivity of pure TiB₂ is approximately 1/4 of that of copper and the electrical resistivity is only 6 times higher than that of copper.     

Effect of total applied energy density on the densification of copper–titanium slabs produced by a DMLF process

The effect of process parameters (laser scan step, scan speed and furnace initial temperature) on the densification degree of copper–titanium single layer slabs produced by a DMLF process is presented here. For each parameter, varied at three levels, specimens were made. An increasing powder densification degree for increasing initial temperature and decreasing scan step and speed was found, when performed at 63 mTorr of oxygen partial pressure in an argon rich atmosphere. However, decrease in densification at large energy densities (50 J/mm³) occurred. Total applied energy density and powder densification results were fitted to an exponential asymptotic curve using least square error criterion. Reduction in densification at a high energy density plus observation of the asymptotic behavior of the fitted model, are likely because the oxidation of the material inhibits sintering/partial melting of the powder.     

Powder processing and densification behaviour of alumina–high zirconia nanocomposites using chloride precursors

Al₂O₃–ZrO₂ composites having nominal equal volume fraction of Al₂O₃ and ZrO₂ were prepared from gel-precipitated powder, precipitated powder and washed precipitated powder. These different processing routes affected the crystallization temperature of the amorphous powder as well as the phase evolution of Al₂O₃ and ZrO₂ during calcination. The agglomerate size was largest for gel-precipitated powder (30 μm) and it was smallest for washed precipitated powder (19 μm). While gel-precipitated powder produce hard agglomerated powder (P_j = 110 MPa), washed precipitated powder produce soft agglomerates with low agglomeration strength (P_j = 70 MPa). Thus, washed precipitated powder could sinter to a high density at lower sintering temperature. The bending strength exhibits a semi logarithmic relationship with porosity. The hardness shows an increasing trend with sintering temperature.     

金刚石增强稀土纳米硬质合金复合齿研究

以液相复合—连续还原碳化方法制备的纳米复合WC-Co粉末、共沉淀法制备的Ni-Ce粉末和真空蒸镀W膜金刚石为原料,采用热压活化烧结,在1050℃的烧结温度、70MPa烧结压力、保温(3～5)min烧结条件下,制取了具有良好抗冲击性能和耐磨性的金刚石增强稀土纳米硬质合金复合齿。复合齿基体的维氏硬度HV1≥1800,抗弯强度TRS≥2800MPa,合金晶粒度在(200～400)nm之间,复合齿超硬部分的磨耗比≥80。结果表明共沉淀法制备的Ni-Ce粉末显著降低了合金中微孔和杂质等组织缺陷,大大提高了金刚石增强纳米硬质合金复合齿的整体性能。     

Rapid consolidation of nanostructured WC-FeAl hard composites by high-frequency induction heating and its mechanical properties

In the case of cemented WC, Co or Ni is added as a binder for the formation of composite structures. However, the toxic behavior of both Co and Ni and their cancerogenic classification in Europe and the US, the high cost of Co or Ni, the low hardness and the low corrosion resistance of the WC-Ni and WC-Co cermets have generated interest in recent years for alternative binder phases. In this study, FeAl was used as a novel binder and consolidated by the high-frequency induction sintering (HFIS) method. The advantage of this process is not only rapid densification to near theoretical density but also the prohibition of grain growth in nano-structured materials. Dense WC-FeAl with a relative density of up to 98% was obtained within two minutes by HFIS under a pressure of 80 MPa. The effect of FeAl addition on mechanical properties, consolidation and microstructure of WC was evaluated.     

Properties and rapid consolidation of nanostructured NiTi by pulsed current activated sintering

Highly dense nanostructured TiNi with a relative density of up to 99 % was obtained within two minutes by pulsed current activated sintering under a pressure of 80 MPa. The advantage of this process is that it allows very quick densification to near theoretical density and prohibits grain growth in nano-structured materials. The microstructural and mechanical properties of the dense TiNi produced by PCAS were investigated.     

Toughened ZrO2 ceramics sintered with a La2O3-rich glass as additive

In this study, the influence of the glass addition and sintering parameters on the densification and mechanical properties of tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO₂ powder and La₂O₃-rich glass were used as starting powders. Two compositions based on ZrO₂ and containing 5 wt.% and 10 wt.% of La₂O₃-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90 °C for 24 h, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200 °C, 1300 °C, 1400 °C for 60 min and at 1450 °C for 120 min, with heating and cooling rates of 10 °C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Hardness and fracture toughness were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Furthermore, only tetragonal-ZrO₂ was identified as crystalline phase in sintered samples, independently of the conditions studied. Samples sintered at 1300 °C for 60 min presented the optimal mechanical properties with hardness and fracture toughness values near to 12 GPa and 8.5 MPa m^1/2, respectively.     

Sintering characteristic of Al2O3-reinforced 2xxx series Al composite powders

Sintering characteristic of Al₂O₃-reinforced 2xxx series Al composite powders was investigated in order to obtain enhanced densification. In order to confirm the effect of the ceramic phase, Al composite powder, AMB 2905 (Al–3.2Cu–1.0Mg–5.0Al₂O₃), was used as the starting powder. Al blended powder, AMB 2712 (Al–3.8Cu–1.0Mg), was also used for comparison. The sintered density of the blended powder was about 93% of the theoretical value at 620 °C. The sintered density of the composite powder was about 95% at 630 °C. A small decrease in the density of each powder caused by swelling was observed after holding time of 10 min at the sintering temperature. After 20 min, the density slightly increased. The diffusion of the liquid phase was faster in the composite powder sintered specimen than in the blended powder sintered specimen. The liquid phase is thought to have infiltrated into the spaces between ceramic agglomerates. The results show that a greater amount of liquid phase is needed to enhance the sinterability of 2xxx series Al composite materials.     

Densification, mechanical properties and wear behavior of WC–VC–Co–Al hardmetals

The effect of VC and Al additions on the sintering behavior, hardness, toughness, elastic properties and wear characteristics of WC–10 wt% Co has been studied. The amount of VC in the compositions varied up to 18 wt% and the aluminum contents was fixed at 2 wt% with the purpose to promote the in situ formation of the CoAl intermetallic phase. The specimens were prepared by vacuum sintering in the 1350–1500 °C range during 1 h. The sintered samples densification improved both with the temperature and VC contents up to 13 wt%. The heterogeneous microstructure consisting of WC, (W, V)C_1−x and intermetallic Al₅Co₂ phase indicated that the expected reactive sintering induced by the Co and Al could not be properly controlled due to the large Al particle size used, resulting in isolated aluminum enriched pools. Vickers hardness and toughness followed an antagonistic behavior with values ranging from 12.8–17.5 GPa and 7.7–10.5 MPa m^1/2, respectively. The sliding wear performance evaluation showed that friction decreases with VC addition but it could not be established a tendency for the wear rate coefficient though values obtained allow to consider these experimental compositions as promising wear resistant materials.     

Rapid sintering of nanocrystalline 8 mol.%Y2O3-stabilized ZrO2 by high-frequency induction heating method

Using a high-frequency induction-heating sintering (HFIHS) method, nanocrystalline 8 mol.%Y₂O₃-stabilized ZrO₂ was obtained from ultra fine powders. The observed advantages of this process include very quick densification to near theoretical density and prohibition of grain growth in nano-structured materials. Nearly fully dense nanocrystalline 8 mol.%Y₂O₃-stabilized ZrO₂, with a relative density of up to 99.8%, could be obtained with simultaneous application of 100 MPa pressure and an induced current within 10 min of sintering time without significant change in grain size. The influences of the sintering temperature and the mechanical pressure on the final density and grain size of the products were investigated. The hardness and fracture toughness of the dense ZrO₂ ceramics produced by HFIHS were investigated.     

Densification and sintering shrinkage of Zinc Reclaimed WC-Co

Zinc Reclaimed WC-Co powder exhibits faceted WC crystal shape while virgin WC-Co powder can have rounded WC particles. During sintering, the WC grains in Zinc Reclaimed WC-Co powder remain faceted while virgin WC-Co retains its rounded WC shape until the later stages of sintering. The effect of the shape of WC grains in Zinc Reclaimed WC-Co on its densification behavior is not fully understood. The effect of WC crystal shape on the densification of Zinc Reclaimed WC-Co during sintering is investigated. The shrinkage of Zinc Reclaimed WC-Co is measured by dilatometry and found to lag behind the shrinkage of virgin WC-Co after the first densification stage during solid state sintering. There is a greater lag in the shrinkage of coarse grained Zinc Reclaimed WC-Co than fine grained Zinc Reclaimed WC-Co relative to the corresponding virgin WC-Co. The role of packing density on the densification of Zinc Reclaimed WC-Co during solid state sintering is discussed.     

Properties and rapid consolidation of ultra-hard tungsten carbide

Extremely dense WC with a relative density of up to 99% was obtained within 3 min under a pressure of 80 MPa using the high frequency induction heating sintering method (HFIHS) method. The average grain size of the WC was about 87 nm. The advantage of this process is not only rapid densification to obtain a near theoretical density but also the prohibition of grain growth in nanostructured materials. The hardness and fracture toughness of the dense WC produced by the HFIHS were investigated.     

Design of multimaterial processed by powder metallurgy: Processing of a (steel/cemented carbides) bilayer material

A bimaterial, consisting of tough and hard layers, was designed to be processed by powder metallurgy. A materials selection software lead to choose steel and cemented carbide for the tough and hard components. To insure the interlayer compatibility, Fe was selected as base of the tough part and as binder of the hard part. The compositions and the temperature range enabling the simultaneous consolidation of the Fe base by transient liquid phase sintering and of the WC base by liquid phase sintering were defined from thermodynamics data. Experiments lead to define a treatment in two steps: presintering (1080 °C) for the partial densification of the Fe base then sintering (1280–1320 °C) to achieve densification and cohesion of the bimaterial. The sintering temperature increase provided a higher liquid fraction leading to interface cohesion but coarse porosity development. Bimaterial, with fair mechanical properties, were obtained by presintering at 1080 °C then sintering at 1280 °C.     

Tailored sintering of VC-doped WC–Co cemented carbides by pulsed electric current sintering

WC–12 wt.% Co powder mixtures with 0, 0.45 or 0.9 wt.% VC additions were consolidated by solid state pulsed electric current sintering (PECS) for 2 min at 1080–1240 °C. The influence of the sintering condition and VC concentration on the densification, WC grain growth and mechanical properties of the cemented carbides were investigated. Finite element simulation revealed that the radial temperature gradient inside the sintering powder compacts could be homogenised using a carbon felt insulation surrounding the graphite die set-up.     

VC, Cr3C2 and NbC doped WC–Co cemented carbides prepared by pulsed electric current sintering

WC–12 wt.% Co grade cemented carbides doped with 0.9 wt.% VC, NbC or Cr₃C₂ grain growth inhibitor were consolidated by pulsed electric current sintering (PECS), also known as spark plasma sintering (SPS), in the solid state at 1240 °C for 2 min. The microstructure and properties of the PECS material grades are compared with those of pressureless sintered grades, liquid phase sintered at 1420 °C for 1 h. Microstructural and hardness characterization revealed that both the chemical composition and sintering technique play an important role on the WC grain growth and final mechanical properties. To obtain a nanometer sized WC–Co microstructure, it is essential to carefully select the grain growth inhibitor in addition to the application of a fast thermal densification cycle by means of spark plasma sintering.     

Densification of AISI M2 high speed steel by means of capacitor discharge sintering (CDS)

Ball milled AISI M2 high speed steel (HSS) powders with nanometric crystalline grains have been consolidated by means of a new technique called capacitor discharge sintering (CDS) which employs one single high intensity current pulse (up to 100 MA/mm²in 10–20 ms) and high pressure (up to 400 MPa). The consolidated disks with diameter of 10 mm and 2–3 mm thickness, show low residual porosity and very fine microstructures. It is confirmed that the CDS process is well suited for the complete densification of AISI M2 powders without grain growth.     

Study of solid-state sintered fine-grained cemented carbides

Submicron cemented carbides are most often produced by liquid phase sintering. To retard the grain growth during sintering, these materials are sintered at low temperatures and with addition of grain growth inhibitors, e.g. Cr and V. The common hypothesis is that the sintered material would benefit from a more evenly distributed inhibitor in the WC raw material, in order to control the grain growth during both the solid-state and liquid phase part of the sintering.

Aiming to study the distribution of Cr specifically after solid-state sintering, a Cr-doped WC-powder was mixed with Co and excess of carbon, and subsequently hot-pressed for 1 h and 30 MPa at 1200 °C. This transmission electron microscopy study shows that Cr is concentrated to the surfaces of the WC grains in the sample after mentioned solid-state sintering.