Preparation and characterisation of ceramic-faced metal–ceramic interpenetrating composites for impact applications

This article accesses the impact performance of ceramic-faced, metal–ceramic interpenetrating composites (IPCs) produced in situ from infiltrating ceramic foams with a molten aluminium–magnesium alloy. The approach had two variations, viz., the production of a metal bond between a ceramic front face and backing IPC and the creation of a ceramic bond. The impact performance of metal-bonded IPCs was evaluated using both split Hopkinson’s pressure bar (SHPB) and depth of penetration (DoP) techniques. With a 4-mm thick Al₂O₃ front face and an 8-mm thick IPC backing, the DoP was zero. In one case, a sample survived fundamentally intact with only spall damage to the dense Al₂O₃ front face. The resulting damage was thoroughly assessed using a range of techniques, including polarized light microscopy, scanning electron microscopy (SEM), 3D MicroCT and transmission electron microscopy (TEM). The metal phase deformed as a result of the formation of large numbers of dislocations, whilst the ceramic phase accommodated the deformation via localised cracking. Metal bridges across the cracks formed, increasing the damage tolerance of the IPCs. The metal bond between the ceramic front face and the IPC was also observed to withstand the impact of the armour piercing rounds without any sign of debonding occurring.     

High strain rate characteristics of 3-3 metal-ceramic interpenetrating composites

3-3 interpenetrating composites (IPCs) are novel materials with potentially superior multifunctional properties compared with traditional metal matrix composites. The aim of the present work was to evaluate the high strain rate performance of the metal-ceramic IPCs produced using a pressureless infiltration technique through dynamic property testing, viz. the split Hopkinson's pressure bar (SHPB) technique and depth of penetration (DoP) analysis, and subsequent damage assessment. Though the IPCs contained rigid ceramic struts, the samples plastically deformed with only localised fracture in the ceramic phase following SHPB. Metal was observed to bridge the cracks formed during high strain rate testing, this latter behaviour must have contributed to the structural integrity and performance of the IPCs. Whilst the IPCs were not suitable for resisting high velocity, armour piercing rounds on their own, when bonded to a 3 mm thick, dense Al₂O₃ front face, they caused significant deflection and the depth of penetration was reduced.     

Al2O3-5 wt% Al composites by ICP sintering of synthesized precursor

Microstructure developments during the milling of Al₂O₃-5wt% Al composite powder in an attritor and subsequent sintering of the precursor by inductively coupled argon plasma are presented. After 4 h of milling the precursor contained tubular ceramic-metal and uniform ceramic regions. With an increase in the milling period the ceramic-metal regions broke into smaller and almost globular regions, and the smaller regions became dispersed in the ceramic regions. After 8 h of milling the composite powder had a stable microstructure and contained 0.25–0.35 m clusters. The sintered composite was > 99.7% dense and its microstructure consisted of ceramic-metal regions which were dispersed in the matrix of a ceramic region. The sizes of ceramic grains in ceramic-metal regions and the ceramic regions were 0.3–2.2 and 0.8–1.8 m, respectively. Many ceramic grains in ceramic-metal regions were separated by 30–100 nm wide metal layers. The microstructure of the ceramic-metal region showed many features of interpenetrating phase composites. The Knoop and Vickers microhardnesses of the composites at 5–10 N loads were 410–450. Under 10 N loads in Knoop and Vickers microhardness tests the crack length was 11±3 and 3 ± 0.5 m, respectively. The crack propogation mechanisms in the indented areas are discussed.     

Ni–Zr–Ti–Si–Sn/Cu metallic glass composites prepared by magnetic compaction

Ni–Zr–Ti–Si–Sn/Cu metallic glass (BMG) composites were fabricated by magnetic compaction of powder mixtures. A considerable plastic deformation took place without apparent failure during the dynamic compaction even at room temperature and at a high strain rate. The BMG particles retained their amorphous phase after the dynamic magnetic compaction at 450 °C. The resulting Ni_52.65Zr_28.71Ti_13.57Si_1.33Sn_3.74/60% Cu composite exhibited a remarkable tensile ductility at room temperature combined with high strength: tensile elongation of 28% and ultimate tensile stress up to 1.1 GPa.     

A fine cobalt-toughened Al2O3-TiC ceramic and its wear resistance

Mechanical ball milling is the most common method for mixing ceramic powders with a ductile phase such as metal particles. In this paper, a new powder processing way is presented. Al2O3 and TiC powders are coated with a layer of metal cobalt using the chemical deposition process. The thickness of the metal cobalt film can be controlled by adjusting the deposition conditions. The Co-coated Al2O3 (Al2O3–Co) and TiC (Tic–Co) powders are mixed at the rate of 7:3 and hot-press sintered into a fine Al2O3–TiC–Co (ATC) ceramic. The main properties, erosion behaviour, abrasion behaviour, wear mechanism and wear resistance of Al2O3-TiC-Co and Al2O3–30 wt% TiC (AT30) ceramics are determined by transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, etc. It is shown that the ATC ceramic possesses improved mechanical properties. Because of the existence of metal cobalt in the grain boundaries, the bonding strength between grains is increased, and this prevents spalling of grains during wear. Experimentation indicates that ATC is more resistant to wear than Al2O3–TiC ceramic. The relationship between their mechanical properties and wear resistance is also discussed in this paper.     

Preparation of nickel-coated powders as precursors to reinforce MMCs

The preparation of nickel-coated ceramic particles as precursors for MMC fabrication was studied. Al₂O₃ and SiC powders of three different particle sizes were successfully coated with Ni using an electroless metal plating technique. Uniform and continuous nickel films were deposited on both, alumina and silicon carbide powders, with a final composition ranging from 1.6 to 1.9wt% phosphorus, 18–21wt% of metallic nickel and the balance is ceramic. XRD showed that the Ni-P deposit was predominantly amorphous. However, after heat treatment, the metallic deposits crystallize into Ni and Ni₃P phases, as confirmed by DSC analyses. Preliminary results showed that the use of Ni-coated powders enhances the wettability between the matrix and ceramic phase when processing particulate MMCs by infiltration techniques. The coating promoted easy metal flow through the preform, compared to the non-infiltration behavior of the uncoated counterpart samples.     

Joining of zirconia to metal with Cu–Ga–Ti and Cu–Sn–Pb–Ti fillers

The method of brazing by capillary impregnation of Cu–Ga melt through a titanium powder layer situated between brazed details is elaborated. Samples of ZrO₂ ceramic/metal brazed joints using Cu–Ga–Ti filler and Cu–Sn–Pb–Ti filler were fabricated. The joints’ shear strength was 277±37 MPa for the Cu–Ga–Ti and 156±25 MPa for the Cu–Sn–Pb–Ti.     

Mechanical properties of three-dimensional interconnected alumina/steel metal matrix composites

Three-dimensional interconnected alumina/steel metal matrix composites (MMCs) were produced by pressureless Ti-activated melt infiltration method using three types of Al₂O₃ powder with different sizes and shapes. By partial sintering during infiltration an interpenetrating ceramic network was realised. The effect of the ceramic particle size and shape on the resulting ceramic network, volume % fraction and the MMC properties is presented. The MMCs were characterised for mechanical properties at room temperature and elevated temperature. An increase in flexural strength and Young’s modulus with decreasing particle size has been observed. In addition, the effect of the volume of ceramic content and the surface finish of the MMCs on the wear behaviour is shown.     

Effects of organic binders on the sintering of isostatically compacted zirconia powders

Certain processing-related flaws in cold isostatically pressed ceramic powder compacts may arise from the delayed burn-out of organic binders until the sintering temperature is approached, although the isostatic compaction technique usually gives a higher and much more uniform green density than the conventional die compaction technique. For the 3 mol% Y₂O₃-doped zirconia powder in which 3 wt% PEG 1500 was introduced, the sintered density and sintering shrinkage were found to decrease in a near linear manner with increasing isostatic compaction pressure. The processing-related defects were identified as intergranular pores (1–5 m). It is considered that these processing-related defects are a consequence of incomplete organic burn-out at low and intermediate temperatures in the heating-up period and the swelling of intergranular pores associated with the burn-out of residual organic binders at temperatures close to the sintering temperature. A higher calcination temperature and an extended calcination dwell time may be required to eliminate the organic residuals in the isostatically pressed ceramic powder compacts than in the conventional die-pressed samples.     

Shock-compaction features and shock-induced chemical reaction in some ceramic powders

Shock compaction features are experimentally investigated for some selected ceramic materials and ceramic composite systems, i.e. SiC, AIN, SiC/AIN, and AIN/Al₂O₃. A typical microstructure to support the skin model proposed in a previous paper is obtained by using SiC powder with a particle diameter of several micrometres. AIN transforms into an unknown phase. The transformation needs a small amount of oxygen. The amount of the unknown new phase decreases with the increase of shock temperature. Chemical reaction of AIN/Al₂O₃ mixture into ALON (cubic aluminium oxynitride spinel) occurs under shock loading and proceeds along with increasing shock temperature. Crystallite size and microstrain of the samples are determined by X-ray line broadening analysis. The microstrain for mixture sample and fine powder is larger than that for single-component sample and coarse powder, respectively. As suggested by the skin model, the requirement that the initial particle size is less than 1 m is essential for ceramic powder to be consolidated by a shock compaction technique in order to yield good compacts of optimum strength and to achieve chemical reaction accompanying mass transport.     

Synthesis of Si(N,C) nanostructured powders from an organometallic aerosol using a hot-wall reactor

Current studies show that nanostructured Si(N,C) powders are readily synthesized by rapid condensation of a pyrolytically decomposed silazane precursor, namely [CH₃SiHNH] _n ,n–3 or 4. Basically, the process involves ultrasonic conversion of the liquid-phase precursor to an aerosol, followed by thermal decomposition in a hot reactor. This was followed by the rapid condensation of the gaseous product exiting the reactor, to form ceramic particles of nanoscale dimension. Thermal decomposition was performed at a temperature of 1000 °C, near ambient pressure with a flow rate of 150 standard cm³ min^–1 for NH₃. One critical feature examined in this process was the rapidity of the powder synthesis, in a reaction which involves (i) elimination of ligand groups, (ii) formation of ceramic species, and (iii) condensation of ceramic species into ultrafine ceramic particles. These features have been studied using Fourier transform infrared spectroscopy, transmission electron microscopy, X-ray photo-electron and nuclear magnetic resonance spectroscopies. Additionally, a model is formulated to determine the effect of process parameters on particle size.     

Novel MMC microstructures prepared by melt infiltration of reticulated ceramic preforms

Abstract

Alumina/aluminium composites with an interpenetrating microstructure were made by infiltrating an alumina preform which had the structure of a reticulated ceramic foam. Low density preforms (6% relative density)with porous struts were prepared from a polyurethane suspension of alumina powder which was pyrolysed and partially sintered after foaming. Higher density preforms consisted of ceramic foams with open cells and dense struts. All these preforms were infiltrated with 6061 aluminium alloy using a modified squeeze caster fitted with a vacuum system and fine control of speed and pressure. Such MMCs had a microstructure in which two 3D networks were interpenetrating. One was either a ceramic reinforced alloy (50 vol.-%reinforcement) or a dense ceramic phase while the other was the unreinforced alloy. The microstructure of the preform fitted an established relationship between the ratio of window diameter to cell diameter k and void volume fraction V _p. In addition, a short fibre foam was made to compare with the traditional fibre preforms.     

Chemical vapor processing and applications for nanostructured ceramic powders and whiskers

A new chemical synthetic process for producing non-agglomerated nanostructured ceramic (n-ceramic) powders from metalorganic precursors is described. The process combines rapid thermal decomposition of a precursor/carrier gas stream in a hot tubular reactor with rapid condensation of the product particle species on a cold substrate under a reduced inert gas pressure of 1–50 mbar. A wide variety of metalorganic precursors is available commercially, all of which can be utilized in this process to produce n-ceramic powders, including single phase, multiphase and multicomponent systems. The process has been used to synthesize nonagglomerated n-SiC_xN_y and n-ZrO_xC_y powders, with controlled particle size in the range 2–20 nm. Heat treatment of the as-synthesized n-powders in various high purity gas streams causes compositional and structural modifications, including particle purification and crystallization, as well as transformation to a whisker-like morphology. Non-agglomerated n-ceramic powders form uniformly dense powder compacts by cold pressing, which can be sintered to theoretical density at temperatures as low as 0.5 T_m.     

Interfacial reactions and wetting in Al–Mg/oxide ceramic interpenetrating composites made by a pressureless infiltration technique

The present paper considers the microstructures of Al–Mg/oxide ceramic interpenetrating composites made by a pressureless infiltration technique. The composites were produced using an Al–10 wt.% Mg alloy with two oxide ceramic foams, spinel (MgAl₂O₄) and mullite (Al₆Si₂O₁₃), at 915 °C in a flowing N₂ atmosphere. Full infiltration of the aluminium alloy into the ceramic preform has been achieved with good bonding between the metal and ceramic phases. The composites were characterised by a range of techniques and compared with those for alumina from the literature. It has been found that the metal–ceramic interface of the composite consisted of an oxide layer near the ceramic phase and a nitride layer from Mg₃N₂ to AlN near the metal phase. The improvement of Al wetting and adhesion on the oxide ceramics by the addition of Mg and in the presence of N₂ was studied by a sessile drop technique to clarify which compound that formed at the interface contributed to the spontaneous infiltration.     

Preceramic polymer derived cellular ceramics

Ceramic foams were prepared by a self-blowing process of a poly(silsesquioxane) melt at 270 °C. The cell size, the interconnectivity density and the shape of the foam cells were adjusted by a thermal pre-curing procedure of the polymer at 200 °C. Inorganic fillers were used to modify processing behaviour and properties of the pyrolysed ceramic foam. After pyrolysis in inert atmosphere at 1200 °C ceramic composite foams with a total porosity up to 87% were obtained. The open cell ceramic foams had a mean cell diameter of 1.2 mm and a mean strut thickness of 0.2 mm. Interpenetrating phase composites (IPCs) were fabricated by infiltrating the open cellular ceramic preform with Mg alloy melt at 680 °C and a pressure of 86 MPa. The mechanical properties were found to depend on the reactions between the metal and the ceramic forming MgO, Mg₂Si and Al₁₂Mg₁₇ as the major reaction products. The IPCs showed a significantly higher creep resistance at 135 °C, compression strength and elastic modulus compared to the unreinforced magnesium alloys.     

PZT Material by Spray-Dried Precursors and Solid-State Reaction: Cold Consolidation and Sintering

The Nb-doped Pb(Zr_0.52 Ti_0.48)O₃ (PZT) powder, synthesized by spray drying of the solution followed by calcination, was cold consolidated and sintered under different process conditions. The microstructure and final properties were compared with the material produced by the conventional solid-state reaction of the oxides. The spray-dried powder undergoes a complete reaction in the perovskitic phase at 550°C, while the mixed oxides are converted at temperatures not lower than 800–850°C. Because of the hollow spherical structure of the spray-dried powder, low green densities are obtained. Consequently, a grinding process, as well as high-pressure cold isostatic compaction, were applied. The high reactivity of the powder results in the reduction of the densification temperature of 100°C. The final microstructure differs substantially from that developed with mixed-oxide processing and a different sintering mechanism is proposed.     

Thermochemical preparation of W–25%Cu nanocomposite powder through a CVT mechanism

In order to produce a W–25%Cu nanocomposite powder manufactured by a thermochemical procedure a novel pair of precursors were used. Cu₂WO₄(OH)₂ and CuWO₄·2H₂O precipitates were first produced by reacting the copper nitrate and sodium tungstate aqueous solutions under certain pH and temperature. The precipitates were then dried and calcined in order to prepare CuWO_4 − x, CuO, and WO₃ oxide powders for the next step reduction. The reduction was carried out under a H₂ atmosphere to form the final W–Cu metal nanocomposite powder. Characteristics of the final powder such as distribution, uniformity and size were then discussed based thoroughly on the dominant mechanism of reduction; Chemical Vapor Transport. It was found that the average particle size of the reduced powder is 35 nm for W and 54 nm for Cu.     

Preparation of ZrO2-Al2O3 powder by thermal decomposition of gels produced from an aluminium chelate compound and zirconium butoxide

Organic precursors containing Al and Zr atoms were synthesized from an aluminium chelate compound and zirconium n-butoxide. A ZrO₂-Al₂O₃ composite powder was prepared by the thermal decomposition of these precursors. An amorphous phase exists to higher temperatures for this ZrO₂-Al₂O₃ powder than for a comparable powder prepared from aluminium sec-butoxide and zirconium n-butoxide. In addition the tetragonal ZrO₂ phase was stabler in this ZrO₂-Al₂O₃ powder than in a comparison powder. The ZrO₂ grains were 50–500 nm in diameter and were homogeneously dispersed in the Al₂O₃ matrix after heating at 1400 °C.     

Fabrication and characterization of hydroxyapatite reinforced with 20 vol % Ti particles for use as hard tissue replacement

Hydroxyapatite(HA)-based composite reinforced with 20 vol % titanium (Ti) particles was fabricated by hot pressing based on the studies of the structural stability of HA phase in HA–Ti composite by means of FTIR spectrometry and X-ray diffractometry. The mechanical properties and biological behaviors of the composite were investigated by mechanical and in vivo studies. The existence of Ti metal phase can promote the dehydration and decomposition of HA ceramic phase into the more stable calcium phosphate phases, such as –Ca₃(PO₄)₂ (–TCP) and Ca₄O(PO₄)₂ at high temperatures. Comparing with pure HA ceramic manufactured under the same conditions, HA–20 vol % Ti composite with higher fracture toughness (0.987 MPa m^1/2), bending strength (78.59 MPa), work of fracture (12.8J/m²), porosity (9.8%) and lower elastic modulus (75.91 GPa) is more suitable for use as hard tissue replacement. Crack deflection is the chief toughening mechanism in the composite. Histological evaluation by light microscope shows HA–20 vol % Ti composite implant could be partially integrated with newborn bone tissues after 3 weeks and fully osteointegrated at 12 weeks in vivo. The excellent biological properties of HA–20 vol % Ti composite may be contributed to the coexistence of high porosity and the decomposition products of HA phase in the composite.     

Effect of glass–ceramic microstructure on its in vitro bioactivity

Two routes were used to obtain a glass–ceramic composed of 43.5 wt % SiO₂ – 43.5 wt % CaO – 13 wt % ZrO₂. Heat treatment of a glass monolith produced a glass–ceramic (WZ1) containing wollastonite-2M and tetragonal zirconia as crystalline phases. The WZ1 did not display bioactivity in vitro. Ceramizing the glass via powder technology routes formed a bioactive glass–ceramic (WZ2). The two glass–ceramics, WZ1 and WZ2, were composed of the same crystalline phases, but differed in microstructure. The in vitro studies carried out on WZ2 showed the formation of an apatite-like layer on its surface during exposure to a simulated body fluid. This paper examined the influence of both chemical and morphological factors on the in vitro bioactivitity. The interfacial reaction product was examined by scanning and transmission electron microscopy. Both instruments were fitted with energy-dispersive X-ray analyzers. Measurements of the pH made directly at the interface of the two glass–ceramics were important in understanding their different behavior during exposure to the same physiological environment.