Characterization and cold compaction of polyether-etherketone powders

This study deals with the characterization and cold compaction of polyetheretherketone (PEEK) powders. Four different types of PEEK powders which are commercially available from Imperial Chemical Industries (ICI) were characterized for density, crystallinity, particle size, particle size distribution, and particle morphology. Fine and coarse size powders of a low viscosity grade (150PF and 150P) and similar powders of a high viscosity grade (450PF and 450P) were processed. Compaction was successful at room temperature using the 150 grade powders but not with the 450 grade powders. Compressibility curves were obtained at room temperature for the 150 grade powders and their post-compaction viscoelastic recoveries were measured. The coarse and fine size 150 grade powders reached relative plateau densities of 95.8% and 95.1%, respectively, when compacted at pressures exceeding 300 MPa. These high densification values at room temperature were associated with minimal post-compaction viscoelastic recoveries and as-compacted strengths as high as 8.9 MPa. A modified densification parameter (DP*) was developed based on the plastic deformation of the crystalline regions alone. This new dimensionless parameter, DP*, demonstrated a better fit of all the compaction data. The transverse rupture strength and the green density data are presented and explained in terms of this DP*.     

Mechanical and Thermal Properties of Poly(phthalazinone biphenyl ether sulfone)/PEEK Blends

A series of blends with various compositions are prepared by melt extrusion on the basis of novel copoly(phthalazinone biphenyl ether sulfone) (PPBES) and poly(ether ether ketone) (PEEK). The melt flowability, mechanical and thermal properties of the blends are studied. The results show that the incorporated PEEK has a large influence on the melt viscosity and thermal stability of blends. The tensile strength of the blends remains at about 90 MPa at room temperature; PPBES improves the mechanical properties of PEEK at 150°C. The flexural strength and modulus of the PPBES/PEEK blends also increase with the addition of PEEK.     

Characterization and comparison of gray cast iron powder produced by target jet milling and high energy ball milling of machining scraps

Target jet milling and conventional ball milling were used to produce powders from gray cast iron scraps. Powders of similar size distribution produced by the two methods were pressed at different compacting pressures. Green compacts were made at the compacting pressures of 500, 600, 700 and 800 MPa. Jet milled powder showed good compaction behavior while ball milled powder showed very poor compressibility. Also, balanced compacts composed of 50% Hoganas SC100.26 iron powder and each of the cast iron powders produced in this work were made at 500 and 800 MPa and their green properties were determined. Results showed that, green properties of the jet milled powder were acceptable and superior compared to ball milled powder. The jet milling process proved to be a much more efficient process compared to ball milling in terms of time and production capacity.     

Compaction and Sintering Behavior of Bimodal Alumina Powder Suspensions by Pressure Filtration

The compaction behavior of fine alumina powders with different particle sizes or bimodal particle-size distributions that are undergoing pressure filtration was investigated. Three alumina powders—average particle sizes of 0.2—0.86 μm—were compacted to a solids fraction of 62—65 vol% from suspensions at pH 3, which was the pH level at which the suspensions showed their lowest viscosity. When the powders of different average sizes were mixed, the suspensions showed better flowability, and the lowest viscosity was obtained when the fraction of fines was ∼30 vol% and pH = 3. The mixed-sized powder suspensions were compacted to higher density than the suspensions of unmixed fine or coarse powders, and the maximum density was obtained for mixed suspensions that had the lowest viscosity, despite the different particle-size ratio. Maximum densities of 72.5% and 75.0% were attained when the size ratios were 2 and 5, respectively. The compacts that were pressure-filtered from mixed suspensions exhibited a single-peaked pore-size distribution and a homogeneous microstructure, whereas the pore-size distributions of dry-pressed compacts were double-peaked. The sintering behavior of the compacts that were pressure-filtrated from bimodal powders exhibited significantly better sinterability and much-less linear shrinkage than the coarser powders and the dry-pressed powder compacts.     

Effect of Relative Humidity on the Viscoelastic and Mechanical Properties of Spray-Dried Powder Compacts

Spray-dried powder compacts exhibit viscoelastic properties such as stress relaxation, creep, and delayed elastic strain. This behavior is attributed to the organic binder, which forms bridges between the particles in spray-dried granules, thereby affecting their deformation characteristics. The viscosity and distribution of the binder within the powder compact can affect its mechanical and viscoelastic properties. In this study, the powder was conditioned at different ambient relative humidity (RH) levels, to vary the binder viscosity. Load deformation, stress relaxation, fracture strength, and fracture toughness behavior of ferrite powder compacts were studied as a function of ambient RH both before and after compaction. The loading rate was found to significantly affect the time-dependent response, and the relaxation times decreased at high humidity levels during compaction. It is proposed that increasing the humidity level during compaction increases the number of particle–particle contacts. This simple mechanism of binder redistribution led to slower relaxation times, increases in fracture strength, and elastic modulus of the green bodies, without significantly altering the fracture toughness when powders were compacted at high humidity to a given density.     

Fabrication and characterization of silica ceramic compact prepared using Aloe-Vera mucilage as a binder

In this study, silica compacts were fabricated through a powder processing route at different compaction pressure, using Aloe-Vera (AV) mucilage as a binder. The silica compacts were prepared at 90, 100, and 110 MPa compaction pressure using 0%–16 wt% of AV binder. The optimum amount of AV binder was 14 wt% for both 90 and 100 MPa and 12 wt% for 110 MPa. The maximum achieved green density and green strength of silica compacts at the optimum binder amount were 62.3% and 4 MPa, respectively at 110 MPa compaction pressure. The green silica compacts prepared at 110 MPa compaction pressure exhibited a minimum porosity of 21% and maximum flexural strength of 15 MPa after sintering at 1400°C. The green silica compacts with the optimum amount of binder were strong enough for machining. The Fourier transform infrared spectroscopy analysis revealed the functional groups present in AV mucilage. The binder burnout characteristic of AV mucilage in the silica compact was determined by thermogravimetric analysis and differential thermal analysis. Additionally, AV gel acted as a binder and solvent simultaneously for ceramic compaction.     

Cold compaction study of Armstrong Process® Ti-6Al-4V powders

The Armstrong Process® developed by Cristal US, Inc./International Titanium Powder, is an innovative, low-cost technology for producing Ti and Ti alloy powders in a one-step, continuous process. In this work, Armstrong Ti-6Al-4V powders were characterized and the cold compaction behavior of the powders were investigated in detail. As-received as well as milled powders were uniaxially die-pressed at designated pressures up to 690 MPa to form disk samples with different aspect ratios. Samples with high aspect ratio exhibited non-uniform density along the pressing axis and the density distribution was consistent with the result predicted by finite element analysis. The model developed from the linear regression analysis on the experimental density data can be used to predict density of compacts with different aspect ratios. In the studied pressure range, an empirical powder compaction equation was applied to linearize the green density — pressure relationship. Cold compaction parameters were obtained for the as-received and milled Armstrong Ti-6Al-4V powders.     

Pressureless sintering of polycarbonate powder compacted at ambient temperature

Solid-state processing of polymer powders through compaction and sintering has potential advantages over conventional polymer processing methods. However, pressureless sintering of compacted polymer powders has been unsuccessful. In a previous paper (2), room temperature compaction of polycarbonate powder was studied to better understand the fundamental mechanisms that control polymer compaction. This paper focuses on the pressureless sintering of room temperature compacted polycarbonate powder. Thermomechanical analysis was used to characterize the dependence of dimensional recovery on time, temperature, and compaction pressure for compacts formed from both aged and unaged polycarbonate. It was found that all polycarbonate compacts exhibited irreversible expansion when heated to relatively low temperatures (～ 50°C), with large-scale expansion occurring near its glass transition temperature. It was concluded that irreversible expansion of polymer compacts is driven by entropic factors. Therefore, parameters that affect the degree of particle deformation during compaction also affect the degree of dimensional recovery that occurs during pressureless sintering.     

Effect of Polyethylene Glycol (PEG) Powder on Compressibility and Microstructural Properties of Sintered α-Alumina

This study examines the effect of various contents of polyethylene glycol (PEG) powders on density, compressibility, and microstructural properties of sintered α-alumina samples. Moreover, the effect of compaction pressure on the green density of the compacts is studied by applying different pressures ranging from 400 to 550 MPa. Samples were prepared by mechanical blending of alumina and various amounts of PEG powders in a Turbula mixer. The binder contents vary from 1 wt.% to 4 wt.%. The as-prepared mixture was compacted in a universal machine at room temperature under the pressure of 6 MPa to produce disk-shaped samples in a pre-compaction step. Experimental results revealed that adding various amounts of PEG powders has a detrimental effect on the green density of alumina pellets and decreases the green density from 1.95 to 1.87 g/cm³. The results also show that sintered density of samples increased by increasing the compaction pressure to pressures higher than 400 MPa. It is observed that a sudden increase in green density has been observed between 450 and 550 MPa.     

Impact of pressure in static and dynamic pressing of zirconia ultradisperse powders on compact density and compaction efficiency during sintering

This paper studies the impact of pressure in static and dynamic pressing on densification of stabilized zirconia ultradisperse powder compacts and on compaction kinetics during sintering. Ultradisperse powders of 97 ZrO₂ + 3 Y₂O₃ zirconia were synthesized using the plasma chemical method. Dry uniaxial static pressing and double-action magnetic pulse compaction were employed. It is shown that double-action magnetic pulse compaction provides the maximum density of the product in comparison to that obtained through static pressing. The dilatometric studies showed that the increased density of compacts from stabilized zirconia powders obtained in dynamic pressing does not make ceramics less compact during isothermal aging as it typically occurs during static pressing. This increases the density of ceramics and improves its mechanical characteristics.     

Dynamic Compaction of Cubic Boron Nitride Powders

Compacts of cubic boron nitride with 94% of theoretical density and a Vickers microhardness of 30.3 GPa were produced from coarse c-BN powder by a shock compaction technique. The density and microhardness of these compacts depend strongly on the grain size of the starting powders.     

Effect of compaction on the sintering of borosilicate glass/alumina composites

The effect of initial compaction on the sintering of borosilicate glass matrix composites reinforced with 25 vol.% alumina (Al₂O₃) particles has been studied using powder compacts that were uniaxially pressed at 74, 200 and 370 MPa. The sintering behaviour of the samples heated in the temperature range 850–1150 °C was investigated by density measurement, axial and radial shrinkage measurement and microstructural observation. The density of the sintered composites increased continuously with temperature for compacts pressed at 74 MPa, while for compacts pressed at 200 and 370 MPa it reached the maximum value at 1050 °C and at higher temperatures it decreased slightly due to swelling. The results showed anisotropic shrinkage behaviour for all the samples, which exhibited an axial shrinkage higher than the radial shrinkage, and the anisotropic character increased with the initial compaction pressure.     

Cold Compaction and Solid-State Sintering of WC-Co-Based Structures: Experiments and Modeling

Cold compaction (200-1900 MPa) and sintering (1250°-1350°C) of cermets based on WC-Co were experimentally studied using die compaction, cold isostatic pressing, sintering, and creep tests. Two different-sized WC powders were used. The cobalt content varied over a range of 10-30 wt%. Cold-compaction behavior has been described by using a Cam-Clay model. Die-wall friction was measured by using green powder compacts that had different aspect ratios. Friction coefficients were 0.28-0.85, depending on the WC particle size and cobalt content. Simple constitutive equations have been used to model the high-temperature behavior (sintering and creep). The constitutive equations were implemented in a finite-element program to model the compaction, ejection, and sintering of bilayer structures that had different cobalt contents. The model can represent the effect of die-wall friction on the average density, as well as deformation inside the green compact. Density gradients were generated; they were revealed during sintering, because the compact does not deform homogeneously. Simulation also can be used to evaluate deformations that are induced by sintering.     

Cold compaction molding and sintering of polystyrene

It is the objective of this paper to demonstrate the applicability of cold compaction molding followed by a sintering treatment to the processing of polystyrene powders. The influence of pressure, compaction speed, and peak pressure dwell time on the green (as compacted) density and the green tensile strength, as well as the effect of sintering on the tensile strength and dimensional change, were evaluated. The resulting data indicate that room temperature compaction alone is insufficient to provide adequate tensile strength for the compacts. Sintering the green compacts at temperatures of 150 to 173°C markedly improves the tensile strength while simultaneously causing a thickness change in the compacts. This thickness change results from gas evolution, pore shrinkage, and viscoelastic recovery of the residual stresses induced by pressure. For compacts of 0.225 in. thickness, an optimum sintering treatment of 173°C for 30 mins is recommended to provide a tensile strength of 4,000 psi and a thickness change of less than + 7 percent. Coining (repressing) the green compacts does not appreciably affect the sintered strength. However, a finer particle size improves the sintered properties. A review of the literature on the flow behavior of polystyrene suggests that a non-Newtonian viscous flow mechanism is followed by a Newtonian one as sintering progresses.     

Studies on the synthesis of nanocrystalline yttria powder by oxalate deagglomeration and its sintering behaviour

Nanocrystalline yttria powders were synthesized from deagglomerated yttrium oxalate. The precipitation of this oxalate was carried in two different modes viz., addition of aqueous oxalic acid into yttrium nitrate solution (forward strike) and vice versa (reverse strike) followed by ultrasonication in acetone and water. Nanocrystalline yttria was obtained by calcining the oxalate in air at 1073 K. The bulk densities, specific surface area, X-ray crystallite size, size distribution of particles as well as the quantity of carbon residue in these powders were determined. The influence of the deagglomeration medium on the powder properties was analyzed. Scanning electron microscopy (SEM) showed that these powders comprised irregular agglomerates while the high resolution transmission electron microscopy (HRTEM) revealed that the constituent units of these agglomerates were randomly oriented cuboidal nanocrystallites (20–40 nm). These powders were compacted at 120 MPa without any lubricant or binder and their sinterability was studied. Pellets with sintered density as high as 97.5% T.D. (theoretical density) could be obtained at a relatively low sintering temperature of 1873 K. Synthesis of nanocrystalline yttria powders by oxalate deagglomeration route as well as the systematic studies of their properties and sinterabilities are being reported for the first time. It was further demonstrated in this study that higher sintered densities could be obtained with less number of process steps and at a much lower compaction pressure. Samples prepared by reverse strike yielded a powder with characteristics most suitable for fabricating high density yttria bodies. 1673 K would be the optimum temperature for sintering the compacts made out of this powder.     

Cold compaction and sintering of ultrahigh-molecular-weight polyethylene containing a segregated iron network

To improve the powder processing behavior of ultrahigh-molecular-weight polyethylene, a conductive iron filler was distributed within the polymer in a segregated network. The filler level was kept at a minimum of 10 volume percent, which was sufficient to coat completely all the polymer particle surfaces. This filler level was low enough to avoid modifying the resin properties to a significant extent. Compaction of these filled samples showed a slower densification, under pressure, similar level of final densification at 80% densification parameter, and a doubling of plateau pressure value to 200 MPa in comparison with the unfilled polymer. The filler was found to reduce drastically the postcompaction relaxation time from 24 h to 6 h. The magnitude of the axial (compaction direction) relaxation was unchanged, but the radial relaxation was one quarter of that for the unfilled polymer. Sintering behavior showed improved densification because of lower dimensional changes during sintering resulting in 80% relative sintered density, higher than the 75% percent value for the unfilled polymer, but yielded a 20% lower sintered strength, An alternative process of rapid sintering by induction heating was explored, its feasibility demonstrated, and a recommendation is made to make powder processing of this polymer commercially attractive.     

The use of hardness in the study of compaction behaviour and die loading

An experimental relationship between density, hardness and compacting pressure is obtained for the isostatic compaction of Alcoa grade 1202 atomised aluminum powder. These results are used to evaluate the use of hardness measurement in the determination of density contours within compacts and pressure distributions at the compact—die interface. Density contours and pressure distributions are presented for closed die compacts; the results are in general agreement with those reported in the literature for more complex techniques. The technique is shown to be suitable for use in many situations.     

Effect of Externally Applied Plasticizer on Compaction Behavior of Spray-Dried Powders

The effects of an externally applied plasticizer on compaction behavior and green microstructure quality of spray-dried powders was investigated. The plasticizer was applied to the external surfaces of already spray-dried powders by spraying it on tumbling spray-dried granules. The apparent yield point of the spray-dried powder was reduced when the plasticizer was added. Microstructures of compacts made from these granules (with and without the externally applied plasticizer) were compared at different compaction pressures. Better knitting across granule interfaces and fewer defects were obtained for the granules with the externally applied plasticizer.     

The mechanical and tribological properties of PTFE filled with PTFE waste powders

Polytetrafluoroethylene (PTFE) composites filled with PTFE waste offer interesting combination of tribological properties and low cost. PTFE composites waste was mechanically cut and sieved into powders. PTFE composites filled with PTFE waste powders were prepared by compression molding. Friction and wear experiments were carried out in a reciprocating sliding tribotester at a reciprocating frequency of 1.0 Hz, a contact pressure of 5.5 MPa, and a relative humidity of (60 ± 5)%. PTFE materials slid against a 45 carbon steel track. Results showed that a PTFE composite (B) filled with 20 wt % PTFE waste exhibited a coefficient of steady‐state friction slightly higher than that of unfilled PTFE (A), while wear resistance over two orders of magnitude higher than that of unfilled PTFE (A). Another PTFE composite filled with PTFE waste and alumina nanoparticles exhibited the highest wear resistance among the three PTFE materials. This behavior originates from the effective reinforcement of PTFE waste as a filler. It was experimentally confirmed that the low cost recycling of PTFE waste without by‐products is feasible. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1035–1041, 2007     

Consolidation and mechanical properties of reactive nanocomposite powders

Reactive nanocomposite powders with bulk compositions of 8Al·MoO₃, 12Al·MoO₃, and 8Al·3CuO were prepared by arrested reactive milling (ARM) and consolidated into cylindrical and rectangular pellets using a uniaxial die. Pellets were pressed at room temperature without any binder. Reference pellets were prepared from conventional Al powder and from Al-metal oxide powder blends with bulk compositions identical to those of the nanocomposite powders. Materials could be consolidated to densities greater than 90% of the theoretical maximum density while maintaining their high reactivity. Tensile strength and flexural strength of the consolidated materials were measured using diametrical compression and three-point flexural strength tests, respectively. Higher strengths were observed for higher relative densities, and the strength of the composite materials was comparable to that of consolidated aluminum powders. Yield strength of the reactive nanocomposite powders was determined from compaction load vs. die displacement curves using the Heckel equation. It was greater for the nanocomposite powders as compared to the powder blends or pure aluminum. Organic, or low melting point metal binders were added to selected samples to improve strength. Respective pellets were pressed at temperatures above the melting point of the metal binder. The highest density (~ 2.9 g/cm³) and tensile strength (~ 17.5 MPa) was observed with indium as binder. All consolidated samples were found to be highly reactive, and the effect of partial reaction during consolidation remained below the limit quantifiable by differential scanning calorimetry.